Antinociceptive and Nociceptive Actions of Opioids
Michael H. Ossipov, Josephine Lai, Tamara King, Todd W. Vanderah,
T. Philip Malan, Jr., Victor J. Hruby, Frank Porreca
Departments of Pharmacology, Anesthesiology and Chemistry, University of Arizona,
Tucson, Arizona 85724
Received 15 April 2004; accepted 16 June 2004
pal treatment options for moderate to severe pain, their
use is also associated with the development of tolerance,
defined as the progressive need for higher doses to
achieve a constant analgesic effect. The mechanisms
which underlie this phenomenon remain unclear. Recent
studies revealed that cholecystokinin (CCK) is upregu-
lated in the rostral ventromedial medulla (RVM) during
persistent opioid exposure. CCK is both antiopioid and
Although the opioids are the princi-
pronociceptive, and activates descending pain facilita-
tion mechanisms from the RVM enhancing nociceptive
transmission at the spinal cord and promoting hyperal-
gesia. The neuroplastic changes elicited by opioid expo-
sure reflect adaptive changes to promote increased pain
transmission and consequent diminished antinociception
© 2004 Wiley Periodicals, Inc. J Neurobiol 61:
Keywords: antinociception; nociception; opioids
The pain-relieving properties of the opioids, of which
morphine is considered the prototype, have been ex-
tensively studied and are well known. The existence
of endogenous receptors for opioids was reported
almost simultaneously by three different groups in
1973 (Pert and Snyder, 1973a; Simon, 1973; Tere-
nius, 1973). The discovery of receptors selective for
the opioids, suggesting the existence of endogenous
pain-relieving substances, generated great interest in
understanding endogenous pain mechanisms. More-
over, this interest extended into the study of the bio-
logical mechanisms of opiate dependence and with-
drawal. The probability of the existence of multiple
subtypes of the opioid receptor was first proposed
through purely pharmacologic studies reported by
Martin and colleagues (Martin et al., 1976). It was
found that vastly different behavioral syndromes were
elicited in spinalized dogs by morphine, ketocyclazo-
cine and SKF-10,047 leading to the nomenclature of
mu, kappa, and sigma opiate receptors, respectively
(McClane and Martin, 1967; Martin et al., 1976; Mar-
tin, 1983). The observation of differential binding
affinities for [Met5]enkephalin and ?-endorphin
against [3H][Leu5]enkephalin and [3H]naloxone in
guinea pig brain and differential agonistic activity
profiles of [Leu5]enkephalin, [Met5]enkephalin and
morphine in the isolated mouse vas deferens and
guinea pig ileum strongly supported the existence of
an additional receptor subtype, termed the delta (?)-
opioid receptor (Lord et al., 1977; Waterfield et al.,
1978, 1979). Over time, the existence of additional
opioid receptor subtypes has been hypothesized to
include subtypes of these proposed receptors and a
suggested receptor for ?-endorphin, termed the epsi-
lon receptor (Schulz et al., 1981). The advent of
molecular cloning of the opioid receptors, however,
led to the clear identification of only three subtypes of
the opioid receptors (for review, see Massotte and
Correspondence to: F. Porreca (email@example.com).
© 2004 Wiley Periodicals, Inc.
Published online in Wiley InterScience (www.interscience.wiley.
Kieffer, 1998; Ossipov et al., 2004). The relationship
of the cloned opioid receptors to those identified phar-
macologically remain to be elucidated (for review, see
Zaki et al., 1996). Although different opioid receptor
subtypes have not been cloned, it is possible, for
example, that posttranscriptional events may occur to
produce pharmacologically distinct profiles (Zaki et
al., 1996). Because the opioid receptors are widely
distributed throughout the entire nervous system, to
include the peripheral terminations of sensory fibers,
opioids may exert their antinociceptive activity
through numerous mechanisms.
Opioid Receptors Are Prominent in
Spinal Sites of Action. The distribution and anatom-
ical localization of the opioid receptors have been
extensively explored throughout the preceding de-
cades through a variety of means, ranging from auto-
radiography, radioligand binding, in situ hybridization
of mRNA for the receptors and immunohistochemis-
try. The distributions of message coding for the ?
(Delfs et al., 1994; Mansour et al., 1995; Minami et
al., 1994), ? (Mansour et al., 1994a), and ? (Minami
et al., 1993a, 1993b; DePaoli et al., 1994; Mansour et
al., 1994b, 1995) opioid receptors have extensively
reviewed (Satoh and Minami, 1995). Autoradio-
graphic methods demonstrated that opioid receptors
are concentrated in the outer laminae of the dorsal
horns of the spinal cord (Besse et al., 1990a, 1991).
Studies that employed tritiated selective agonists for
the opioid receptors indicated that the ?-opioid recep-
tor is highly concentrated in the outer laminae of the
spinal dorsal horns, whereas the ?-opioid receptor is
diffusely distributed throughout the dorsal horn
(Quirion et al., 1983; Quirion, 1984). The ?-opioid
receptor is believed to be concentrated in the outer
laminae of the dorsal horns of the lumbosacral cord,
and is closely associated with nociceptive inputs from
the viscera (Quirion et al., 1983; Quirion, 1984).
Immunolabeling techniques provided more detailed
insights into the localization of the opioid receptors
and allowed the ultrastructural localization of the re-
ceptors as well as the covisualization of two or more
receptor populations on the same neurons. All three
opioid receptors were found to be expressed primarily
in nociceptive C- and A? fibers of the dorsal root
ganglia (DRG) cells as measured by immunohisto-
chemistry (Dado et al., 1993; Arvidsson et al., 1995;
Ji et al., 1995). Spinal morphine is believed to act in
large part through opioid receptors residing on the
central terminals of these primary afferent C-fibers
(Yaksh and Noueihed, 1985; Lombard et al., 1995;
Mansour et al., 1995). Studies performed with patch-
clamp techniques on isolated nociceptors found that
activation of opioid receptors predictably inhibited
Ca??channels of small-diameter nociceptors and not
of large-diameter cells, and suggest that receptor ac-
tivation selectively inhibits the activity of C-fibers
(Taddese et al., 1995). Autoradiographic inspection of
spinal cord slices after dorsal root rhizotomy from T13
to S2suggested that the great majority of spinal opioid
? (60%) and ? (70%) receptors probably reside on the
central terminals of afferent neurons (Besse et al.,
1990a, 1991). The remaining receptors are believed to
reside on either interneurons or on cell bodies of
second-order neurons that transmit nociceptive inputs
to supraspinal sites that process nociceptive signals.
Such studies also indicated that there is a greater
concentration of ?-opioid receptors in the outer lam-
inae of the dorsal horn relative to the deeper layers
(Dado et al., 1993; Arvidsson et al., 1995; Lai et al.,
1996). This distribution is consistent with the hypoth-
esis that ? opioid agonists suppress the transmission
of pain signals from the primary sensory afferents that
terminate in laminae I and II onto projection neurons
that form the spinothalamic tract. In contrast to the
distribution of the receptor protein, the levels of
mRNA for the ?-opioid receptor were low to moder-
ate throughout laminae I–VI, which would support the
suggestion that the preponderance of the ?-opioid
receptors arise from dorsal root ganglion (DRG) neu-
rons and are transported to the terminals of primary
afferents (Besse et al., 1990, 1991). Presumably,
? receptors synthesized in the DRG are transported to
the central terminal of the primary afferent. The pre-
synaptic ? receptors are contained on the nociceptive
C-fibers, which terminate in laminae I and II. Impor-
tantly, this distribution is consistent with the termina-
tion fields of the unmyelinated C-fiber nociceptors,
and not of the large-diameter, myelinated ? fibers that
do not transmit noxious inputs (Mansour et al., 1995).
Consistent with the presence of opioid receptors in
the spinal cord, spinally administered opiates exert
robust antinociceptive effects in animal models of
pain and in clinical practice as well. Morphine has
produced dose-dependent, naloxone reversible antino-
ciception after direct spinal administration in the rat,
suggesting a modulation of nociceptive input or pro-
cessing at the spinal level (Yaksh and Rudy, 1976,
1977a). The direct spinal administration of [Met5]-
enkephalin and related analogs also produced dose-
dependent antinociception to noxious thermal stimuli
that was reversed by systemic naloxone (Yaksh et al.,
1977a). Advokat and colleagues reported that transec-
tion of the spinal cord caused a loss in potency of
systemic morphine against thermally evoked spinal
Actions of Opioids
nociceptive responses, whereas that of spinally in-
jected morphine remained unchanged (Advokat and
Burton, 1987). These observations supported the pre-
vailing hypothesis that opioids may act directly at
spinal sites to modulate nociceptive spinal reflexes
and nociceptive inputs. Systemically administered
morphine produced an attenuation of spontaneous and
noxious-evoked neuronal activity of lamina V inter-
neurons (Le Bars et al., 1975). Moreover, systemic
morphine blocked the activity of second-order neu-
rons in response to electrical stimulation of ? and
C-fibers of the sural nerve of decerebrate cats (Jurna
and Grossman, 1976). The iontophoretic application
of morphine into the outer laminae of the dorsal horns
of the spinal cord has attenuated the responses of
dorsal horn units to noxious stimuli, even when such
neurons were detected within lamina V of the dorsal
horn (Duggan et al., 1976). These early studies pro-
vided a clear demonstration that opioids may act
spinally to alleviate pain. The spinal administration of
morphine has now become routine medical practice in
the treatment of pain.
Supraspinal Sites of Action
Nociceptive signals entering at the level of the spinal
cord are regulated not only by intrinsic interneurons
but are also modulated by descending inhibitory pro-
jections from supraspinal sites that are activated by
opioid receptors. Autoradiographic studies have dem-
onstrated that there are significant levels of opioid
receptor mRNA in many cortical, diencephalic, and
brainstem regions in addition to spinal loci (Quirion et
al., 1983; Quirion, 1984). Mansour and colleagues
had performed exhaustive surveys of the brain utiliz-
ing [3H]-labeled ligands for the ?, ?, and ?-opioid
receptors and in situ hybridization methods for
mRNA for the receptors (Mansour et al., 1987, 1994a,
1995). Regions that were shown to express the opioid
receptors include the frontal cortex, nucleus accum-
bens, hippocampus, thalamus, and hypothalamus
(Quirion et al., 1983; Quirion, 1984; Mansour et al.,
1987, 1994b, 1985; Schmidt et al., 1994; Svingos et
al., 1996). Two regions prominent in opioid-mediated
antinociception, the periaqueductal gray (PAG) and
the rostral ventromedial medulla (RVM) were identi-
fied as expressing opioid receptors. The PAG was
found to be rich in ?-opioid receptors, whereas levels
of ?- or ?-opioid receptors were low or undetectable
(Mansour et al., 1987). The RVM, which is defined as
the region of the medulla including the nucleus raphe
magnus, the nucleus gigantocellularis pars alpha, and
surrounding reticular neurons ventral to the nucleus
gigantocellularis and extending between the caudal
facial nucleus and the inferior olivary complex, was
found to express all three subtypes of the opioid
receptors (Fields and Heinricher, 1985; Mansour et
al., 1987). Later studies employing in situ hybridiza-
tion for mRNA confirmed this differential distrubu-
tion of the opioiod receptor subtypes within the PAG
and RVM. These regions demonstrated extensive la-
beling for the ?-opioid receptor and light labeling for
the ? or ?- receptors (Mansour et al., 1995).
In addition, opioid receptors have also been iden-
tified in the locus coeruleus, which also plays a role in
modulation of nociceptive inputs (Van Bockstaele et
al., 1996). Many of the neurons that labeled for the
?-opioid receptor also labeled tyrosine hydroxylase,
and these were usually postsynaptic to unlabeled axon
terminals that had characteristics of excitatory syn-
apses (Van Bockstaele et al., 1996). These results
suggest an important postsynaptic role for ? opioid
receptor regulation of excitatory responses to cate-
cholamine-containing neurons in the locus coeruleus.
A critical consideration is that the locus coeruleus is
the source of the majority of the noradrenergic inner-
vation of the brain, and it also expresses high levels of
mRNA for the ?-opioid receptor. The noradrenergic
neurons have terminals that innervate many other
brain regions. These terminals may, in turn, have
? opioid receptors on their surface, a fact that may
relate to the lack of correlation between the distribu-
tion of receptors and their mRNAs.
There is considerable evidence to indicate that these
regions are important for the antinociceptive effects me-
diated by opioids. Importantly, reciprocal connections
between the PAG and the RVM have been identified
(Basbaum et al., 1978; Basbaum and Fields, 1978;
Fields and Anderson, 1978). Opioids like morphine
cause an activation of a population of cells in the PAG
that, in turn, excites neurons of the RVM (Fields and
1988). In 1976, Yaksh, Yeung, and Rudy conducted an
extensive sterotactic survey of the brain by microinject-
ing morphine into 403 sites and measuring nociceptive
responses to noxious pinch (Yaksh et al., 1976). They
found that the ventrolateral aspect of the PAG mediated
a robust, naloxone-reversible antinociceptive effect, in
agreement with previous suggestions (Jacquet and La-
jtha, 1973). Further exploration revealed that the PAG
sites that were responsive to morphine also produced
antinociception in response to electrical stimulation
(Yeung et al., 1977). The microinjection of opioids into
the cerebral ventricles has been shown to produce dose-
dependent antinociception in several species, including
mice and rats (Porreca et al., 1984; Dauge et al., 1987;
Erspamer et al., 1989; Jiang et al., 1990; Miaskowski et
al., 1991). A considerable amount of evidence exists to
Ossipov et al.
in part by descending pathways that originate from the
PAG (Yaksh et al., 1976; Lewis and Gebhart, 1977a;
Yaksh and Rudy, 1978). The microinjection or electrical
stimulation of the ventrolateral aspect of the PAG has
produced robust antinociceptive effects in rats (Lewis
and Gebhart, 1977a, 1977b). Moreover, electrophysi-
ologic studies demonstrated that the microinjection of
morphine into the PAG attenuated the activity of pro-
jection neurons in the dorsal horn in response to periph-
eral nociceptive stimuli (Bennett and Mayer, 1979).The
microinjection of morphine or of the ?-opioid ago-
nists [D-Pen2,D-Pen5]enkephalin (DPDPE) or [D-Ala2,
Glu4]deltorphin into the medullary reticular formation
although the ?-opioid agonists were inactive when ad-
ministered into the PAG (Jensen and Yaksh, 1986a,
1986b; Ossipov et al., 1995a). In addition to the PAG,
the serotonergic nucleus raphe magnus (NRM) has been
shown to communicate with the PAG and have seroto-
nergic projections to the spinal cord, and thus may
function as a relay for descending antinociceptive infor-
mation arising from the PAG (Conrad and Pfaff, 1976a,
1976b; Basbaum et al., 1977).
The RVM is recognized as a critical region with
respect to nociceptive processing and modulation, re-
ceiving inputs from the spinal dorsal horn and from
rostral sites as well (Fields et al., 1983; Fields and
Heinricher, 1985; Fields and Basbaum, 1999). Elec-
trophysiologic studies where the responses of RVM
neurons to noxious thermal stimulation have identi-
fied the existence of “on”-cells and “off”-cells (Fields,
1992; Fields and Basbaum, 1999; Heinricher et al.,
2003). The off-cells are tonically active and pause in
firing immediately before the animal withdraws from
the noxious thermal stimulus, whereas the on-cells
accelerate firing immediately before the nociceptive
reflex occurs. An additional class, the “neutral” cells
were initially characterized by the absence of re-
sponse to noxious thermal stimulation. It is now gen-
erally understood that the activity of the off-cells
correlate with inhibition of nociceptive input and no-
cifensive responses, and these neurons may be the
source of descending inhibition of nociceptive inputs
(Fields, 1992; Fields and Basbaum, 1999; Heinricher
et al., 2003). In contrast, the response characteristics
of the on-cells suggest that these neurons are the
source of descending facilitation of nociception
(Fields, 1992; Fields and Basbaum, 1999; McNally,
1999; Heinricher et al., 2003). Accordingly, manipu-
lations that facilitate responses to nociceptive stimuli
also increase on-cell activity (Heinricher and Roy-
chowdhury, 1997; Fields and Basbaum, 1999; Fields,
2000; Heinricher et al., 2003). For example, pro-
longed delivery of a noxious thermal stimulus pro-
duced increased on-cell along with a facilitation of
nociceptive reflexes (Morgan and Fields, 1994).
Moreover, inactivation of RVM neuronal activity
with lidocaine blocked the facilitated withdrawal re-
sponse (Morgan and Fields, 1994). It is now generally
accepted that a spino-bulbo-spinal loop may be im-
portant to the development and maintenance of exag-
gerated pain behaviors produced by noxious (i.e.,
hyperalgesia) and nonnoxious (i.e., allodynia) periph-
eral stimuli (Urban and Gebhart, 1999; Ossipov et al.,
2001; Porreca et al., 2002; Heinricher et al., 2003).
Morphine microinjection or electrical stimulation in
the NRM has produced naloxone-sensitive antinoci-
ception (Oliveras et al., 1977; Dickenson et al., 1979).
Application of morphine or glutamate into the NRM
or the nucleus gigantocellularis par alpha (NGCpA)
has also produced antinociception in rats by acti-
vating spinopetal mechanisms (Kiefel et al., 1993;
McGowan and Hammond, 1993; Rossi et al., 1993).
Accordingly, spinally administered 5-HT antagonists
have been shown to attenuate antinociception pro-
duced by supraspinal or systemic morphine (Proudfit
and Hammond, 1981). Studies employing selective
antibodies for the ?- and ?-opioid receptors and ret-
rograde tracing methods demonstrated the existence
of opioid-expressing neurons that project from the
PAG or RVM to the spinal cord to provide a descend-
ing inhibition of nociceptive inputs (Kalyuzhny et al.,
1996). It was also proposed that opioids exert indirect
effects on PAG-RVM projection neurons, and have
both direct and indirect effects on bulbospinal neurons
(Kalyuzhny et al., 1996). Centrally administered mor-
phine has also been demonstrated to activate descend-
ing noradrenergic inhibitory pathways. The A5 (locus
coeruleus), A6, A7, and the catecholamine-containing
nuclei in the vicinity of the lateral reticular nucleus
(LRN) are major sources of noradrenergic innervation
of the spinal cord, and have been implicated in the
descending inhibitory modulation of nociception (Ge-
bhart and Ossipov, 1986; Jones and Gebhart, 1986a,
1986b; Yeomans et al., 1992; Clark and Proudfit,
SYNERGISTIC ACTIONS AT SPINAL/
An important aspect of morphine-mediated antinoci-
ception is that antinociceptive efficacy when given
systemically is greater than that which would be pre-
dicted based solely on its in vitro activity in isolated
tissues. The efficacy of morphine is believed to be a
Actions of Opioids
result of a synergistic interaction between its activity
at spinal and supraspinal sites. This spinal/supraspinal
synergy was initially demonstrated by Yeung and
Rudy (1980a) when it was found that naloxone given
either spinally into the cerebral ventricles did not
attenuate the antinociceptive actions of systemic mor-
phine in a manner consistent with an additive effect
between these sites (Yeung and Rudy, 1980a). It was
found that a 1:1 fixed ratio of morphine administered
intrathecally (i.th.) and i.c.v. produced an approxi-
mate 30-fold increase in potency when evaluated in
the tail-flick test and a 45-fold increase in the hot-
plate test compared to i.c.v. morphine alone (Yeung
and Rudy, 1980a). Isobolographic analyses employ-
ing several dose ratios demonstrated a hyperbolic
function with a strong degree of curvature, and it was
concluded that potentiation would occur at all possi-
ble combinations of spinal and supraspinal levels of
morphine after systemic injection. This observation of
a multiplicative interaction between spinal and su-
praspinal morphine has also been observed in mice
(Roerig and Fujimoto, 1988), and been extended
to include the highly selective ?-opioid agonist
DAMGO (Roerig and Fujimoto, 1989). Further, evi-
dence that the synergistic antinociceptive activity of
morphine is the result of a site–site interaction was
confirmed when synergy was determined based on
spinal and supraspinal morphine content, rather than
doses administered, in rats (Miyamoto et al., 1991).
Repeated exposure to morphine produced a loss of
spinal/supraspinal synergy. Rats received systemic
morphine and challenged with PAG and spinal mor-
phine. Those with morphine “tolerance” showed a
loss of the synergistic interaction between spinal and
PAG morphine (Siuciak and Advokat, 1989). A sim-
ilar observation was made with regard to neuropathic
pain. Rats with ligation of the L5and L6spinal nerves
(SNL) demonstrate abnormal, enhanced pain behav-
iors, some of which are resistant to opioids. Specifi-
cally, spinal morphine fails to attenuate tactile hyper-
sensitivity in rats with SNL, and in these animals the
spinal/supraspinal synergy was absent and the po-
tency of systemic morphine was diminished (Bian et
al., 1999). Restoration of the spinal site of morphine
activity with an NMDA antagonist restored supraspi-
nal-spinal synergy and increased the potency of sys-
temically given morphine in animals with nerve injury
(Bian et al., 1999). Critically, these studies showed
that systemic opiates were active in neuropathic con-
ditions, a finding that has now been confirmed in
humans (Rowbotham, 1995; Dworkin et al., 2003;
Rowbotham et al., 2003).
Nociceptive Actions of Opioids
Antinociceptive and Analgesic Tolerance. The opi-
oids continue to hold a prominent position in the
treatment of both acute and chronic pain states. Mor-
phine, given through a variety of drug delivery sys-
tems or fentanyl, typically given in a transdermal
system, represent the most efficacious and aggressive
methods to treat severe pain conditions. A significant
hindrance, however, with regard to the prolonged use
of such opioids is the development of tolerance to
their analgesic effects. Tolerance is defined as a de-
crease in analgesic activity of a drug after a previous
exposure to the same or a similar drug (Way et al.,
1969; Cox, 1990; Foley, 1993, 1995). The repeated
daily systemic injections of morphine to mice or rats
produced significant rightward shifts in the antinoci-
ceptive effect of morphine challenge in two tests of
nociception, the hot-plate test, and acetic acid-induced
writhing (Fernandes et al., 1977a, 1977b). Repeated
systemic or i.th. injections of morphine also produced
a rightward shift in the dose–response curves for i.th.
morphine in the hot-plate and tail-flick tests (Yaksh et
al., 1977b). Clinically, antinociceptive tolerance man-
ifests as a diminished or lost pain relief of a given
opioid dose administered repeatedly or with continu-
ous administration of an opioid over some period of
time. Opioid analgesic tolerance is well recognized
experimentally and clinically, and can occur over a
period of days to weeks (Way et al., 1969; Foley,
1993, 1995). Clinically, the need for increasing doses
of opioids in cases of chronic pain is well documented
and usually presented as a major obstacle to providing
adequate pain relief over a long period of time (Foley,
1993, 1995; Cherney and Portenoy, 1999). By way of
illustration, a review of the clinical experience of over
700 patients that received spinal morphine over an
average of 124 days revealed that analgesic tolerance
to spinal opioid use developed to different degrees
among patients, and appeared to be related to the type
of pain and differences in pharmacokinetics among
patients (Arner et al., 1988). Despite much intensive
research, however, the mechanisms that underlie the
development of tolerance to the analgesic effects of
opioids remain largely unknown. There are some clin-
ical reports that suggest that tolerance to morphine
may not always develop. A 10-year prospective study
that included 2118 patients receiving palliative care,
56% of whom received morphine, was performed to
validate the World Health Organization (WHO)
guidelines (Zech et al., 1995). It was reported that a
need for increased doses of morphine occurred in
one-half the patients, whereas the remainder main-
tained a stable or decreasing dosing requirement
Ossipov et al.
(Zech et al., 1995). However, these patients also re-
ceived, at various times, ancillary treatments that in-
cluded antidepressants, anticonvulsants, antihista-
mines, antiemetics, or corticosteroids, which could
diminish or prevent tolerance development (Zech et
al., 1995). Patients with inflamed joints were found to
have elevated levels of endogenous opioid peptides in
the synovial fluid (Stein et al., 1996). Nevertheless,
the analgesic effect of intraarticular morphine in this
population, leading to the assertion that opioid pep-
tides expressed in inflamed tissue do not produce
tolerance to peripheral morphine (Stein et al., 1996).
In a study of gastrointestinal (GI) transit in mice, it
was found that intestinal inflammation reversed toler-
ance to morphine-induced inhibition of GI motility
(Pol and Puig, 1997). However, the apparent reversal
of tolerance may be related to the observations that
conditions of inflammation are associated with re-
duced availability of CCK and enhanced activity of
morphine (Stanfa et al., 1994; Ossipov et al., 1995a;
Vanderah et al., 1996a). This interpretation is consis-
tent with the fact that CCK antagonists reversed tol-
erance to morphine-induced inhibition of GI motility
(Pol and Puig, 1997).
Many studies have focused on changes occurring
at the cellular level to gain an appreciation of the
mechanisms that drive the development of antinoci-
ceptive tolerance (Sabbe and Yaksh, 1990; Childers,
1991; Collin and Cesselin, 1991). Although alter-
ations in subcellular processes undoubtedly contribute
to changes in physiology that occur during prolonged
exposure to opioids, the present level of understand-
ing of such processes is insufficient to allow for the
direct correlation of intracellular changes to those
occurring at the level of the neuronal circuits mediat-
ing antinociception or analgesia. Further complicating
possible translation of intracellular changes that may
underlie the development of tolerance is the puzzling
observation that many, seemingly unrelated classes of
substances, including antagonists of classical and pep-
tidergic neurotransmitters and enzyme inhibitors have
the ability to “block” antinociceptive tolerance.
Among the substances reported to “block” opioid
antinociceptive tolerance are CGRP antagonists (Me-
nard et al., 1996; Powell et al., 2000), nitric oxide
synthase inhibitors (Powell et al., 1999), calcium
channel blockers (Aley and Levine, 1997), cyclooxy-
genase inhibitors (Powell et al., 1999), protein kinase
C inhibitors (Mao et al., 1995c), competitive and
noncompetitive antagonists of the N-methyl, D-aspar-
tate (NMDA) receptor, AMPA antagonists (Kest et
al., 1997), superoxide dismutase mimics (Dr. D.
Salvemini, personal communication), dynorphin anti-
serum (Vanderah et al., 2000), and CCK antagonists
(Dourish et al., 1988; Xu et al., 1992). Blockade of
opioid tolerance by antagonists of NMDA receptors
has been especially well studied, and proposed mech-
anisms have focused on the likely colocalization of
NMDA and opioid receptors (Trujillo and Akil, 1991;
Mao et al., 1995a, 1996; Lutfy et al., 1996; Manning
et al., 1996). It has been suggested that prolonged
exposure to opioids causes an NMDA-receptor linked
translocation of PKC from cytosol to membrane
where it becomes activated (Mao et al., 1995a; Mao et
al., 1995b; Mayer et al., 1995a,b). Blockade of PKC
translocation with the GM1 ganglioside or blockade
of NMDA receptors have prevented the development
of tolerance to morphine (Mao et al., 1995a, 1995b;
Mayer et al., 1995b). A number of intracellular
changes that may occur after persistent opioid expo-
sure, including changes in second-messenger systems
have been extensively reviewed recently (Williams et
al., 2001; Kieffer and Evans, 2002).
As so many systems can be invoked to modulate
opioid antinociceptive tolerance, it becomes difficult
to implicate a common cellular mechanism for the
actions of all these substances. Efforts to reveal phys-
iological mechanisms that might underlie the phe-
nomenon of opioid antinociceptive tolerance therefore
are justified. There is emerging evidence that peptidic
neuromodulators such as CCK or dynorphin may act
as an endogenous physiologic antagonists of endoge-
nous or exogenous opioid activity. More precisely,
these substances may be classified as “pronocicep-
tive” and by enhancing nociception; these and other
substances may drive the expression of antinocicep-
tive tolerance to opioids. In this regard, it should be
recognized that pain may be considered as a physio-
logical antagonist of analgesia and increased states of
pain require increased levels of pain relieving opiate,
resulting in “opiate tolerance” (Vanderah et al.,
Opioid-Induced Paradoxical Pain. An important
concept that has been gaining considerable experi-
mental validation is that exposure to opioids either
acutely or for prolonged periods paradoxically elicits
hyperalgesia and other signs of abnormal pain. This
paradoxical pain, and the neurobiological adaptations
that underlie this state, may play an important role in
the requirement for increased levels of opioids to
maintain a constant degrees of antinociception, that is,
tolerance (Vanderah et al., 2000, 2001a, 2001b;
Gardell et al., 2002).
Clinical Evidence for Opiate-Induced Abnormal
Pain. A number of clinical reports exist that show
that opioid administration in a clinical setting can
Actions of Opioids
elicit an unexpected paradoxic abnormally heightened
pain sensations. In a review of the clinical experience
of 750 patients receiving epidural morphine, and
18 patients receiving spinal morphine, morphine-in-
duced abnormal pain was documented (Arner et al.,
1988). Many patients developed hyperesthesias (in-
creased sensitivity to sensory stimuli such as light
touch or brush) and allodynia (pain elicited by nor-
mally innocuous sensory stimulation). Severe allo-
dynia unrelated to the original pain complaint oc-
curred in one patient after receiving an intrathecal
infusion of 30 mg of morphine (Arner et al., 1988). In
another clinical report, a patient with an original pain
complaint from advanced epidermoid carcinoma of
the right lung and thoracic pain developed spontane-
ous pain, hyperesthesia, and allodynia of the legs with
a dermatomal distribution of pain corresponding from
the lumbosacral region after receiving continuous
subarachnoid infusion of morphine (De Conno et al.,
1991). Hyperesthesias were also induced by long-
term intrathecal morphine infusion in a cancer patient
where the pain was originally controlled by 1 mg/day
of intrathecal morphine (Ali, 1986). Again, the abnor-
mal pain was manifested in a different way than the
original pain complaint, and was described as a burn-
ing sensation of the whole leg, rather than an inter-
mittent shooting type of pain that was initially de-
scribed (Ali, 1986). More recently, a report was
published where a spinal infusion of sufentanil in a
patient with neuropathic pain secondary to arachnoid-
itis and laminectomy originally alleviated the pain but
then evoked hyperesthesias in the lower extremities
(Devulder, 1997). This abnormal pain state was de-
scribed as being qualitatively different from the orig-
inal complaint and included the back, abdomen, and
both legs. Cancer patients that received high doses of
intrathecal morphine by bolus injections also reported
paradoxical intense pain within one-half hour of the
injections (Stillman et al., 1987). The possibility that
opioids may produce abnormal pain after even short-
term exposures has been suggested as a plausible
reason as to why clinical results of studies on preemp-
tive analgesia have been disappointing (Eisenach,
2000). It is important to note, however, that although
higher doses of intraoperative fentanyl may be asso-
ciated with lower pain thresholds than at lower doses,
patients receiving intraoperative fentanyl still demon-
strate higher pain thresholds than those receiving none
(see Eisenach, 2000).
Opioid-Induced Abnormal Pain Is Confirmed in An-
imal Studies. A considerable number of animal stud-
ies have clearly demonstrated that opioid administra-
tion may produce an abnormal, paradoxic pain state.
In some instances, opioid-induced abnormal pain has
been likened to the behavioral symptomology ob-
served after peripheral nerve injury (Mao et al.,
1992a, 1994; Mayer et al., 1995a, 1995b; Wegert et
al., 1997; Ossipov et al., 1998 Vanderah, 2001; Ossi-
pov et al., 2000b). Large doses of intrathecal mor-
phine have caused paradoxical algesia and hyperes-
thesias (Woolf, 1981). The spinal administration of a
bolus dose of 50 ?g of morphine to rats elicited pain
behavior that involved intermittent bouts of biting and
scratching at the dermatomes corresponding to the
injection site along with aggressive and nocifensive
behaviors in response to light brushing of the flanks
(Yaksh et al., 1986). Higher doses of i.th. morphine
(90 to 150 ?g) also provoked periodic bouts of spon-
taneous agitation and nocifensive responses to light
touch (Yaksh and Harty, 1988). Moreover, rats that
were made tolerant to either systemic or spinal mor-
phine demonstrated hyperreflexia and extreme sensi-
tivity to handling upon the injection of either spinal or
systemic naloxone (Yaksh et al., 1977b). Subse-
quently, studies employing either continuous infusion
or repeated injections of morphine demonstrated
enhanced pain. The repeated daily injection of spi-
nal morphine produced enhanced responses of the
tail or hindpaw to noxious thermal stimuli within
eight days (Mao et al., 1994; Mayer et al., 1995a).
Likewise, repeated daily systemic injections of
morphine produced a loss of its antinociceptive
effect that progressed to enhanced behavioral re-
sponses to noxious radiant heat (Trujillo and Akil,
An interesting phenomenon has been described
where a single systemic dose of opioid produced signs
of hyperalgesia after the initial opioid-induced antino-
ciceptive action has subsided, suggesting that the hy-
peralgesic component is present after opioid admin-
istration but masked by the analgesia observed in the
initial postopioid period (Larcher et al., 1998; Celerier
et al., 1999, 2000). In one study, the systemic injec-
tion of heroin produced antinociception in the tail-
flick test and the antinociceptive effect of a subse-
quent dose given immediately after the effect of the
initial dose was terminated produced a significantly
lower effect (Larcher et al., 1998). Additionally, the
injection of naloxone after a single dose of heroin
seemed to unmask a hyperalgesia, indicated by short-
ened tail-flick latencies (Larcher et al., 1998). In a
separate study, the s.c. injection of heroin produced
antinociception that was followed by decreased
thresholds to evoke paw-pressure induced vocaliza-
tion, which was interpreted as opioid-induced allo-
dynia (Laulin et al., 1998). The same group also
demonstrated that a single injection of morphine pro-
Ossipov et al.
duced an initial antinociception followed by thermal
hyperalgesia (Celerier et al., 1999). Furthermore, in
the same study, it was reported that the administration
of naloxone during the antinociceptive phase of mor-
phine or fentanyl unmasked an NMDA-mediated hy-
peralgesic effect (Celerier et al., 1999). Most recently,
in an effort to determine if fentanyl elicits a sensiti-
zation to pain, rats received four injections of fentanyl
15 min apart and nociceptive thresholds to paw pres-
sure were determined at selected time intervals after
injection and daily afterwards (Celerier et al., 2000).
Impressively, significant hyperalgesia persisted for up
to 5 days after the fentanyl injections (Celerier et al.,
2000). These studies indicate that even a short-term
exposure to opioids may produce a rebound hyperal-
gesisic effect, and this may manifest behaviorally as
Reports of “opioid-induced” hyperalgesia have
been suggested to be the result of an unmasking of a
compensatory neuronal hyperactivity in response to
morphine-induced inhibition of neuronal function
(Gutstein, 1996). This hyperresponsiveness, or sensi-
tization, becomes evident after the opioid is removed
or occurs intermittently between injections such that
opioid-induced hyperalgesia might be interpreted as a
result of repeated episodes of opioid withdrawal, a
condition identified as “miniwithdrawals” (Gutstein,
1996). Studies of abnormal pain consequent to long-
term opioid administration that depend on paradigms
of repeated injection are generally subject to this
criticism (Trujillo and Akil, 1991; Mao et al., 1994;
Mao et al., 1995a). Recent studies emerging from our
laboratories have demonstrated that continuous expo-
sure to opioids produced behavioral signs of exagger-
ated pain and, importantly, that such pain occurred
while the opioid was continuously present in the sys-
tem (Vanderah et al., 2000, 2001b; Gardell et al.,
2002). The continuous spinal infusion of [D-Ala2,
N-Me-Phe4,Gly-ol5]enkephalin (DAMGO) delivered
through an osmotic minipump to rats produced an-
tinociceptive tolerance to DAMGO or morphine, as
demonstrated by a reduction in their antinociceptive
effect within 6 days, and by a rightward shift in the
morphine dose–response curve against the tail-flick
test (Vanderah et al., 2000). Concurrently, these ani-
mals expressed tactile allodynia and thermal hyperal-
gesia, indicated by significant reductions in paw with-
drawal responses to light tactile or noxious radiant
heat applied to the hindpaws (Vanderah et al., 2000).
Importantly, these behavioral signs of abnormal pain
were present while DAMGO was still being infused
(Vanderah et al., 2000). Similar effects were seen
when prolonged exposure to morphine was produced
through the subcutaneous administration of morphine
(Vanderah et al., 2001b). Morphine was continuously
infused through an osmotic minipump or released
from a pair of subcutaneously implanted pellets con-
taining free-base morphine. Within 7 days, the rats
demonstrated reduced response thresholds to light
tactile or noxious radiant heat stimuli, indicating the
presence of tactile allodynia and thermal hyperalgesia
(Vanderah et al., 2001b). As with spinal DAMGO
infusion, continuous exposure to systemic morphine
also produced a significant rightward shift in the in-
trathecal or systemic morphine dose–response curves
(Vanderah et al., 2001b). In both these studies, abnor-
mal pain was present while the opioid was still being
administered to the animals. The demonstration of
abnormal pain during the continuous delivery of opi-
oids, by multiple routes and through minipumps and
pellets, minimizes concerns that the sensory changes
were due to the development of states of “miniwith-
It should be noted that some studies suggest that
the repeated administration of opioids do not cause
hyperalgesia, but rather a potentiation of antinocicep-
tive activity (Kayser and Guilbaud, 1985; Kayser et
al., 1986; Neil et al., 1990; Gutstein et al., 1995).
Initially based on drug discrimination studies, Col-
paert and colleagues suggest that the phenomenon of
tolerance to opioids simply does not exist, and instead
propose a “Systems Theory” where tolerance does not
develop to the primary physiologic action of opioids
(Colpaert et al., 1976; Colpaert, 1978). They suggest
instead that chronic administration of opioids causes
an adaptive hyperalgesia, resulting in an apparent loss
of antinociceptive effect of morphine (Colpaert, 1995,
1996, 2002). Conversely, a persistent noxious stimu-
lus causes an adaptive hypoalgesia (Colpaert, 1995,
1996, 2002). Consequently, overestimating the mor-
phine dose required to just block chronic pain causes
a mismatch between morphine-induced hyperalgesia
is insufficiently offset by pain-induced hypoalgesia,
and an apparent tolerance to morphine is observed
(Colpaert, 1995, 1996, 2002). The proposed adaptive
mechanisms that might underlie such adaptations,
particularly pain induced hypoalgesia, have not yet
Mechanisms of Opioid-Induced Paradoxical Pain.
Abnormal pain is promoted by descending facilitation
from the RVM: the descending facilitatory mecha-
nism from the RVM (described above) may also sig-
nificantly underlie the development of abnormal pain
states subsequent to opioid exposure. It has been
reported that spontaneous activity of on-cells in-
creases along with facilitated nocisponsive behavior
during naloxone-precipitated withdrawal (Bederson et
Actions of Opioids
al., 1990; Kim et al., 1990). Moreover, these actions
were blocked by microinjection of lidocaine into the
RVM (Kaplan and Fields, 1991). Despite these inves-
tigations, the state of on-cell or off-cell firing during
sustained morphine administration, in the absence of
withdrawal, has not been directly studied. Continuous
exposure to morphine by s.c. pellet implantation or
osmotic minipump has been shown to produce tactile
allodynia and thermal hyperalgesia while the rats
were still receiving morphine (Vanderah et al.,
2001a). The microinjection of lidocaine into the RVM
produced a reversible block of both tactile allodynia
and thermal hyperalgesia in morphine-tolerant rats
(Vanderah et al., 2001b). Such abnormal pain devel-
oped over a period of days, and did not reflect the
acute activity of the opioid (Vanderah et al., 2001b).
Moreover, the s.c. implantation of morphine pellets
produced significant rightward shifts in the antinoci-
ceptive dose–response curves for morphine given ei-
ther i.th. or s.c. (Vanderah et al., 2001b). However, in
the presence of lidocaine in the RVM, these dose–
response curves were shifted to the left such that the
A50values were not significantly different from those
of nontolerant rats (Vanderah et al., 2001b). Finally,
the ability of lidocaine microinjected into the RVM to
restore the antinociceptive potency of i.th. morphine
was reversible and consistent with the duration of
action of lidocaine (Vanderah et al., 2001b).
Similar results were obtained by performing bilat-
eral lesions of the dorsolateral funiculus (DLF), fur-
ther supporting this hypothesis. Spinopetal projec-
tions from RVM neurons make up the majority of
fibers of the dorsolateral funiculus (DLF), and may
contain descending facilitatory fibers from this region
(Fields and Heinricher, 1985). To further investigate
this mechanism, rats made tolerant to the antinocicep-
tive action of morphine by s.c. pellets expressed be-
havioral signs of abnormal pain (Vanderah et al.,
2001b). Bilateral disruption of the DLF prevented the
development of both tactile allodynia and thermal
hyperalgesia resulting from sustained opioid delivery
without affecting normal responses to acute noxious
or innocuous stimuli (Vanderah et al., 2001b). Fur-
thermore, bilateral DLF lesions prior to morphine
pellet implantation prevented the development and
expression of morphine antinociceptive tolerance as
shown by a lack of rightward displacement of the i.th.
dose–response curve compared to rats with sham DLF
lesions (Vanderah et al., 2001b). Normal nocifensive
responses and the antinociceptive action of morphine
in rats implanted with placebo pellets was not affected
by DLF lesions, indicating that these changes were
not due to a disruption of normal sensory processing
(Vanderah et al., 2001b). These observations are sim-
ilar to other situations where abnormal pain was
caused by different means. Tactile allodynia caused
by spinal nerve ligation was abolished by DLF lesion
(Ossipov et al., 2000a). Likewise, the electrical stim-
ulation of the DLF produced clear excitation of neu-
rons in the superficial laminae of the dorsal horn,
demonstrating a clear descending facilitation through
this pathway (McMahon and Wall, 1983, 1988).
Taken together, these data support the hypothesis that
descending facilitation from the RVM serves to pro-
mote a facilitated pain state manifested as opioid
As noted above, the on-cells of the RVM are an
important source of descending pain facilitatory pro-
jections. Evidence exists to show that these cells
might be activated by CCK, because the microinjec-
tion of CCK8into the RVM has enhanced nociceptive
input and attenuated the morphine-induced reduction
of on-cell responses to nociception (Heinricher and
McGaraughty, 1996). Behavioral signs of tactile allo-
dynia and thermal hyperalgesia were produced by
CCK8in the RVM as well (Kovelowski et al., 2000).
In contrast, the microinjection of the CCKBreceptor
antagonist, L365,260, into the RVM blocked both
tactile allodynia and thermal hyperalgesia in rats with
L5/L6SNL (Kovelowski et al., 2000). Lidocaine in the
RVM also reversibly blocked these behavioral signs
of neuropathic pain, presumably by blocking facilita-
tion arising from the RVM (Pertovaara et al., 1996;
Kovelowski et al., 2000). Electrical stimulation of the
RVM at low intensities has facilitated dorsal horn
neuronal activity and the spinal nociceptive tail-flick
reflex, further demonstrating the existence of nocicep-
tive facilitation arising from this region (Zhuo and
Gebhart, 1992, 1997).
Spinal dynorphin and opioid-induced paradoxical
pain: considerable evidence has demonstrated that
enhanced expression of spinal dynorphin is pronoci-
ceptive and appears to promote facilitated pain states.
States of chronic inflammation and peripheral nerve
injury that are accompanied by manifestations of ab-
normal pain, including spontaneous pain, allodynia,
and hyperalgesia, are also associated with elevated
spinal dynorphin content (Kajander et al., 1990;
Draisci et al., 1991; Dubner and Ruda, 1992). Cryo-
neurolytic disruption of the sciatic nerve has led to
elevated spinal dynorphin content, and associated
pain behaviors were blocked by antisera to dynorphin
(Wagner et al., 1993; Wagner and Deleo, 1996).
Dynorphin-like immunoreactivity and prodynorphin
mRNA levels were elevated in the spinal cord perfus-
ate of polyarthritic rats (Pohl et al., 1997). A single
spinal injection dynorphin has produced long-lasting
tactile allodynia in rats and mice (Vanderah et al.,
Ossipov et al.
1996b; Laughlin et al., 1997). Enhanced abnormal
pain states that result from peripheral nerve injury are
associated with enhanced spinal dynorphin levels, and
those procedures or treatments that interfere with el-
evated spinal dynorphin content or function also abol-
ish injury-induced abnormal pain (Malan et al., 2000;
Porreca et al., 2001; Wang et al., 2001; Burgess et al.,
2002; Gardell et al., 2003).
Elevations in spinal dynorphin content are also
seen in animals with prolonged, constant exposure to
opioids either systemically or spinally (Vanderah et
al., 2001a, 2001b; Gardell et al., 2002). Spinal infu-
sion of the mu opioid agonist, DAMGO over 6—
7 days was shown to elicit tactile allodynia and ther-
mal hyperalgesia while the opioid infusion was con-
tinuing (Vanderah et al., 2000). This treatment also
produced elevated dynorphin content in the lumbar
cord as well as immunoreactivity for prodynorphin
(Vanderah et al., 2000). The spinal injection of anti-
serum to dynorphin blocked tactile allodynia and ther-
mal hyperalgesia in the DAMGO-treated rats, but did
not elicit any changes in nontolerant rats. More im-
portantly, antiserum to dynorphin unmasked the an-
tinociceptive action of the DAMGO that was still
infused (Vanderah et al., 2000) and blocked the right-
ward displacement of the dose–effect curve for spinal
morphine in DAMGO infused rats, indicating a block-
ade of antinociceptive tolerance (Vanderah et al.,
2000). Antiserum to dynorphin did not alter the an-
tinociceptive activity and potency of spinal morphine
in vehicle-infused rats (Vanderah et al., 2000). Bilat-
eral lesions of the DLF, which were shown to block
abnormal pain and tolerance to the antinociceptive
effect of morphine, also prevented the upregulation of
spinal dynorphin (Gardell et al., 2003). Thus, manip-
ulations that block opioid-induced pain, in this case
due to spinal infusion of opioid, also block the behav-
ioral manifestation of antinociceptive tolerance. The
data show that sustained opioid administration leads
to elevated spinal dynorphin content, which in turn,
promotes an abnormal pain state. This enhanced pain
states increases the requirement for opioid dose pro-
duce a comparable antinociceptive effect seen in an-
imals without enhanced nociception, resulting in an
apparent manifestation of antinociceptive tolerance. It
should be emphasized that such pain occurred while
the opioid was continually delivered to the spinal
cord, arguing against opioid withdrawal as an expla-
nation of altered sensory level.
Elevated spinal dynorphin levels are the result of
descending facilitation: as noted above, systemic ad-
ministration of opioids elicit an increased expression
of spinal dynorphin, and this may be the result of
tonic descending facilitation arising in brainstem
sites. The precise mechanisms through which in-
creased spinal dynorphin expression promotes pain,
and consequently, the manifestation of opioid toler-
ance, remains to be elucidated. However, there is
evidence that increased spinal dynorphin promotes the
further release of excitatory transmitters from primary
afferent neurons, in this way provoking a positive
feedback loop that amplifies further sensory input.
Microdialysis studies have demonstrated localized,
dose-dependent release of glutamate and aspartate
elicited by exogenous dynorphin in the hippocampus
and spinal cord (Faden, 1992; Skilling et al., 1992).
More recently, the capsaicin-stimulated release of cal-
citonin gene-related peptide (CGRP) was potentiated
by dynorphin A(2–13), a nonopioid fragment, in spinal
cord slices in vitro (Claude et al., 1999; Gardell et al.,
2002, 2003). In these studies, CGRP was employed as
a marker for release of excitatory transmitter from
primary afferent neurons (Claude et al., 1999; Gardell
et al., 2002, 2003).These observations are consistent
with previous reports of dynorphin facilitation of cap-
saicin-evoked substance P release from trigeminal
nuclear slices, an effect blocked by MK-801 but not
by opioid antagonists (Arcaya et al., 1999). Most
recently, it was demonstrated that persistent exposure
to morphine pellets implanted subcutaneously pro-
duced enhanced capsaicin-evoked release of CGRP
from spinal tissue (Gardell et al., 2002, 2003). This
enhanced evoked release was blocked by the addition
of antiserum to dynorphin in the perfusion medium.
Moreover, the disruption of descending facilitation
from suprsapinal sites by selective ablation of RVM
neurons that express the mu-opioid receptor or by
surgical lesions of the DLF prevented opioid-induced
abnormal pain, spinal dynorphin upregulation, and
enhanced capsaicin-evoked release of CGRP (Gardell
et al. 2002, 2003). Finally, enhanced capsaicin-
evoked release of CGRP was also blocked by the
NMDA antagonist MK-801 (Gardell et al., 2002,
Enhanced pain from increased spinal excitatory
amino acid activity: the loss of antinociceptive activ-
ity after sustained spinal opioid administration is qual-
itatively similar to the diminished effect of spinal
opioids in animal models of neuropathic pain. Both
states show diminished opioid analgesic potency and
efficacy, and both states are associated with abnormal
pain including thermal hyperalgesia and tactile allo-
dynia, suggesting the possibility of common mecha-
nisms in the postnerve injury state and in spinal ?
opioid tolerance (Mao et al., 1995a; Mao et al., 1995b;
Mayer et al., 1995a; Wegert et al., 1997; Ossipov et
al., 1998, 2000b). A prominent similarity between
opioid tolerance and nerve injury is that abnormal
Actions of Opioids
pain is likely the result of central sensitization conse-
quent to the release of glutamate acting upon the
NMDA receptor complex. In support of this concept,
it has been found that the i.th. injection of NMDA
antagonists reversed established thermal hyperalgesia
elicited by peripheral nerve injury (Mao et al., 1992a,
1993, 1995a; Wegert et al., 1997; Bian et al., 1999).
Similarly, the loss of antinociceptive potency of mor-
phine to suppress the tail-flick response in animals
with an nerve injury to lumbar spinal nerves was
restored by i.th. MK-801 (Wegert et al., 1997). Sim-
ilar to peripheral nerve injury, blockade of NMDA
receptors has also prevented opioid tolerance or ab-
normal pain due to prolonged opioid exposure (Tru-
jillo and Akil, 1991, 1994; Tiseo and Inturrisi, 1993;
Mao et al., 1994, 1996, 1998; Tiseo et al., 1994).
Further, once daily spinal injection of morphine to
normal rats produced antinociceptive tolerance along
with thermal hyperalgesia of the hindpaws (Mao et
al., 1994), and both of these effects were prevented by
coadministration of MK801. Continuous i.th. infusion
of morphine also reliably produced tolerance to its
antinociceptive effect, and the development of toler-
ance was prevented by the coinfusion of the NMDA
antagonists MK801 or dextromethorphan (Manning et
al., 1996). Competitive and noncompetitive blockers
of the NMDA receptor both blocked antinociceptive
tolerance to morphine as determined in the formalin-
induced flinch model (Lutfy et al., 1996). Thus, the
NMDA receptor complex modulates hyperalgesia as-
sociated with neuropathic pain states as well as opioid
tolerance. In further support of the role of NMDA
receptors, Simonnet and colleagues (Larcher et al.,
1998; Celerier et al., 2000) also demonstrated that the
long-term allodynia occurring after a single dose of
heroin or fentanyl was prevented by MK-801. Collec-
tively, these studies suggest that exposure to opioids
result in an enhanced sensitivity of the spinal cord to
nociceptive inputs, and that this spinal sensitization is
mediated, at least in part, through NMDA receptors.
Blocking Opioid-Induced Enhanced
Abnormal Pain Restores Morphine
The condition of elevated nociceptive input causing
increases in the requirement for opioid dose in order
maintain analgesic activity may result in expression of
antinociceptive tolerance. The s.c. presence of mor-
phine pellets over 7 days has resulted decreased paw
withdrawal latencies to noxious radiant heat (Van-
derah et al., 2001b). Correspondingly, these animals
also demonstrated a significant shift to the right of the
i.th. morphine antinociceptive dose–response curve
when compared to that of placebo-implanted rats
(Vanderah et al., 2001b). However, when the intensity
of the noxious radiant heat source is adjusted so that
the paw withdrawal latencies of the placebo-im-
planted and morphine-implanted rats are not signifi-
cantly different, then the antinociceptive dose–re-
sponse curves for i.th. morphine between the two
groups of animals do not differ statistically (Porreca
and Vanderah, unpublished observations). These ob-
servations support the hypothesis that opioid antino-
ciceptive “tolerance” may be construed as arising as a
consequence of increased sensitivity to nociceptive
input after exposure to opioids. This view is also
supported by more detailed analysis of the substances
that have been demonstrated to “block” opioid toler-
ance. Some of these are considered individually
NMDA Antagonists. It has long been appreciated
that activation of the N-methyl, D-aspartate (NMDA)
receptor by glutamate results in the “sensitization” of
neurons (Wilcox, 1991; Ma and Woolf, 1995;
Baranauskas and Nistri, 1998). NMDA receptor-me-
diated central sensitization has been associated with
the development of hyperalgesia in conditions of
chronic pain (Haley and Wilcox, 1992; Mao et al.,
1995a; Wegert et al., 1997). Similarly, it has been
argued that opioid-induced abnormal pain is depen-
dent upon NMDA-mediated pain facilitation (Mao et
al., 1994, 1995b; Larcher et al., 1998; Laulin et al.,
1998; Celerier et al., 2000). Repeated daily injections
of i.th. morphine to rats produced tolerance to the
antinociceptive effect of morphine along with thermal
hyperalgesia of the hindpaws, and both of these ef-
fects were prevented by concurrent injections of
MK801 (Mao et al., 1994). In another study, the
systemic coadministration of morphine with MK801
to rats prevented in a dose-dependent manner the
development of tolerance to the antinociceptive effect
of morphine (Trujillo and Akil, 1991). In these stud-
ies, MK801 did not produce antinociception alone,
nor did it increase antinociceptive action of morphine
in nontolerant rats. Continuous i.th. infusion of mor-
phine also reliably produced tolerance to its antinoci-
ceptive effect, and the development of tolerance was
prevented by the coinfusion of the NMDA antagonists
MK801 or dextromethorphan (Manning et al., 1996).
Furthermore, dextromethorphan blocked the manifes-
tations of tolerance to the antinociceptive effects of
morphine in rats (Elliott et al., 1995). Hyperalgesia
that was evoked by short-term administration of her-
oin or fentanyl was blocked by the injection of the
NMDA antagonists MK-801 or ketamine (Laulin et
al., 1998; Celerier et al., 1999, 2000). Likewise, hy-
Ossipov et al.
peralgesia provoked by naloxone after heroin injec-
tion was also blocked by MK-801 (Larcher et al.,
1998; Laulin et al., 1999). The presence of presynap-
tic NMDA receptors on central terminals of primary
afferent fibers has been demonstrated, suggesting an
anatomical link between opioid and NMDA receptor
systems (Liu et al., 1994, 1997). Based on these
observations, it may be speculated that increased
NMDA receptor activity mediates spinal sensitiza-
tion, resulting in increased nociceptive input.
Dynorphin Antiserum. Manipulations that block the
pronociceptive action of dynorphin also abolish ab-
normal pain. A single spinal injection dynorphin has
produced long-lasting tactile allodynia (Vanderah et
al., 1996b; Laughlin et al., 1997). The i.th. injection of
antiserum to dynorphin has blocked tactile allodynia
and thermal hyperalgesia in rats receiving continuous
i.th. infusion of DAMGO (Vanderah et al., 2000). In
the same study, dynorphin antiserum unmasked the
antinociceptive effect of DAMGO still present in the
animal (Vanderah et al., 2000). Moreover, the antino-
ciceptive dose-response curve for spinal morphine
was restored to equal potency with that of nontolerant
rats (Vanderah et al., 2000). As noted above, the
enhanced evoked release of CGRP was blocked
by manipulations that prevented morphine-induced
dynorphin upregulation such as DLF lesion or by
dynorphin antiserum (Gardell et al., 2002, 2003). In a
recent study employing microdialysis, it was found
that the introduction of NMDA, dynorphin A(1–17)
or of dynorphin A(2–17) into the lumar spinal cord
elicited a long-lasting release of prostaglandin E2 and
of excitatory amino acids (Koetzner et al., 2004).
Because these substances are associated with en-
hanced sensitivity of the spinal cord to noxious inputs,
this observation provides a mechanism through which
pathologically elevated levels of spinal dynorphin
may promote enhanced pain (Koetzner et al., 2004).
CGRP. Calcitonin gene-related peptide (CGRP) is a
neuropeptide that has been found to be partially co-
localized with substance P and glutamate in central
terminals of primary afferent neurons, and is thought
to promote nociceptive processing in the spinal cord
(Gibson et al., 1984). The ability of CGRP to antag-
onize the pharmacologic actions of opioids has been
demonstrated in several studies. CGRP has been as-
sociated with increased glutamate release and en-
hanced NMDA receptor activity, both of which would
oppose the antinociceptive action of opioids (Murase
et al., 1989; Kangrga et al., 1990). Exogenously ad-
ministered CGRP has also elicited significant right-
ward shifts in the antinociceptive dose–response
curves for morphine or [D-Pen2, D-Pen5]-enkephalin
(DPDPE), and this effect was not mediated through
actions of CGRP at the opioid receptor sites (Welch et
al., 1989). Long-term (up to 14 days) exposure of rats
to i.th. morphine infusion has produced antinocicep-
tive tolerance along with increased spinal expression
of CGRP and a concomitant decrease in CGRP recep-
tor densities (Menard et al., 1995). Importantly, there
was no corresponding increase in other spinal neu-
ropeptides, including substance P, galanin, neuroten-
sin, and neuropeptide Y (Menard et al., 1995).
The coadministration of the CGRP antagonist,
CGRP(8–37), prevented the development of antinoci-
ceptive tolerance to morphine (Menard et al., 1996).
Additionally, elevations in endogenous spinal CGRP
levels were prevented by this treatment as well (Me-
nard et al., 1996). It was concluded that a critical
interaction may develop between opioidergic systems
and CGRP to promote the development of tolerance to
opioids (Menard et al., 1996). In a more recent study,
rats were rendered tolerant to morphine by repeated
i.th. injections of morphine (Powell et al., 2000). The
coadministration of CGRP(8–37)or of the nonpeptidic
CGRP antagonist BIBN4096BS prevented the devel-
opment of antinociceptive tolerance (Powell et al.,
2000). More importantly, however, CGRP(8–37)re-
stored the antinociceptive efficacy and potency of i.th.
morphine in rats where antinociceptive tolerance was
already established (Powell et al., 2000). The possi-
bility that the apparent reversal of tolerance was due
to potentiation of morphine by CGRP(8–37) or
BIBN4096BS was discounted by the fact that these
doses alone were not antinociceptive, and that in
naive rats, CGRP(8–37)slightly reduced the antinoci-
ceptive action of morphine (Powell et al., 2000). It has
been shown that CGRP increases the release of glu-
tamate and of substance P, both of which exert an
excitatory influence on postsynaptic second order
neurons of the dorsal horns of the spinal cord (Oku et
al., 1987; Kangrga et al., 1990). Thus, it has been
suggested that increased CGRP release may promote
nociceptive input by amplification of excitatory amino
acid activity in the spinal cord (Oku et al., 1987;
Powell et al., 2000). Furthermore, CGRP given spi-
nally has enhanced mechanically evoked nociception
induced by i.th. substance P (Kangrga et al., 1990). As
stated above, development of tolerance to morphine
was also associated with increased capsaicin-evoked
release of CGRP from spinal cord tissue in vitro.
Furthermore, spinal cord tissue taken from animals
with bilateral lesions of the DLF, which prevented the
development of abnormal pain and tolerance, do not
present with an increase in capsaicin-evoked CGRP
release (Gardell et al., 2003). The block of descending
Actions of Opioids
facilitation through the DLF is believed to have pre-
vented spinal plasticity leading to the manifestation of
abnormal pain along with the attendant modulation of
primary afferent activity. It is likely that increased
nociceptive input, which may be promoted in part by
elevated CGRP levels, would lead to increased input
to supraspinal sites. Ultimately, activation of tonic
descending facilitation, which along with elevated
spinal dynorphin levels, would further promote en-
hanced pain and tolerance to the antinociceptive ac-
tion of morphine. Therefore, the development of a
CGRP antagonist given along with an opioid, or of a
mixed CGRP antagonist/opioid agonist, is expected to
prevent the development of opioid-induced abnormal
pain and tolerance, and would represent a significant
advance in analgesic therapeutics.
Substance P. Because one of the primary mecha-
nisms through which opioids may exert their antino-
ciceptive effects is through the inhibition of release of
substance P from primary afferent terminals, it stands
to reason that prolonged exposure to opioids may alter
substance P functionality in some fashion (Jhaman-
das, 1984; Gouarderes et al., 1993). However, long-
term spinal infusion with either morphine or naloxone
did not produce any changes in NK1 receptor densi-
ties in the dorsal horns of the spinal cord, suggesting
rather that alterations in substance P availability may
be expected instead (Gouarderes et al., 1993). Cul-
tured DRG neurons that are exposed to morphine for
6 days show enhanced release of substance P as well
as increased numbers of neurons immunoreactive for
substance P (Ma et al., 2000). Likewise, long-term
exposure of cultured DRG neurons to agonists acting
at the mu, delta, and kappa opioid receptors produced
dose-dependent increases in substance P immunore-
activity (Belanger et al., 2002). Moreover, long-term
spinal administration of morphine produced increased
substance P immunoreactivity in the spinal dorsal
horns (Powell et al., 2000). Most recently, it was
shown that coadministration of an NK1 antagonist
SR140333 with spinal morphine prevented the devel-
opment of antinociceptive tolerance (Powell et al.,
2003). Furthermore, SR140333 reversed tolerance to
spinal morphine that was firmly established (Powell et
COX Inhibitors. The coadministration of either ke-
torolac or ibuprofen with spinal morphine over a
period of 7 days prevented the development of an-
tinociceptive tolerance to spinal morphine (Powell et
al., 1999). Moreover, spinal ketorolac reversed estab-
lished tolerance to morphine (Powell et al., 1999).
Importantly, the doses of the COX inhibitors em-
ployed did not produce a potentiation of the antino-
ciceptive effect of morphine in nontolerant animals
(Powell et al., 1999). More recently, the coadminis-
tration of the COX-2 inhibitor nimesulide both pre-
vented and reversed antinociceptive tolerance to spi-
nal morphine given over a period of 7 days (Powell et
al., 2003). In addition, nimesulide prevented the up-
regulation of spinal CGRP induced by persistent ex-
posure to morphine (Powell et al., 2003). A complex
interaction among the NMDA receptors, nitric oxide,
and PGE2 has been proposed to promote morphine
tolerance (Wong et al., 2000). Nitric oxide has been
shown to directly enhance the enzymatic activity of
COX-2, resulting in increased synthesis of PGE2
(Salvemini et al., 1993; Wong et al., 2000). In turn,
PGE2 may promote nitric oxide release through EP1-
mediated activation of NMDA receptors, in effect
promoting a self-perpetuating sensitized state (Sakai
et al., 1998; Wong et al., 2000). These studies further
suggest that prolonged exposure to opioids produce
an enhanced nociception through increased spinal
scribed as an endogenous “antiopioid” (Stanfa et al.,
1994) has been well established to have an important
role in promoting tolerance to opioids. Substantial
overlap is seen between the distributions of CCK and
of CCK receptors with those of endogenous opioid
peptides and the opioid receptors within the central
nervous system (Stengaard-Pedersen and Larsson,
1981; Ghilardi et al., 1992; Verge et al., 1993). Fur-
thermore, many of the sites associated with CCK are
also involved in the modulation of nociception by
opiates, further strengthening the possibility of a mod-
ulatory interaction between opioid-induced antinoci-
ception and CCK activity. In addition, CCK coexists
with substance P in neurons of the dorsal root ganglia,
where it is well poised to promote nociceptive inputs
into the spinal cord (Baber et al., 1989). The ability of
CCK to act as an endogenous antiopioid has been well
documented (Faris et al., 1983; Watkins et al., 1985a,
1985b; Kellstein et al., 1991). Exogenously adminis-
tered CCK attenuates and CCK antagonists potentiate,
morphine-induced antinociception in vivo and electro-
physiologically (Faris et al., 1983; Stanfa et al., 1992,
CCK may act as a modulator of tonic spinal an-
tinociceptive activity, because morphine increases the
release of CCK in the spinal cord of the rat, which
serves to attenuate its own activity (Zhou et al., 1992,
1993; Noble et al., 1993; Stanfa et al., 1994). For
example, a single s.c. injection of morphine produced
a doubling of CCK mRNA content in the hypothala-
Ossipov et al.
mus and a threefold increase in the spinal cord (Ding
and Bayer, 1993). Moreover, prolonged exposure to
morphine has caused the upregulation of CCK in the
brain and the spinal cord, and produced a tripling of
proCCK mRNA in the hypothalamus and spinal cord,
and a 97% increase in whole brain proCCK mRNA
(Zhou et al., 1992; Ding and Bayer, 1993; Pu et al.,
1994). Repeated injections of morphine was associ-
ated with a 2.6-fold increase immunoreactive CCK in
the hypothalamus, a 2.1-fold increase in the spinal
cord, and a 1.6-fold increase in the brainstem (Ding
and Bayer, 1993). These results demonstrated a re-
gion-specific increase in CCK content elicited by
chronic morphine administration. Repeated morphine
administration also provokes an increase in CCK-like
content from spinal cord perfusate of morphine-toler-
ant rats (Zhou et al., 1993; Hoffmann and Wiesenfeld-
Consistent with such observations, behavioral and
electrophysiological studies had shown that the CCK
antagonists proglumide and lorglumide elicited an
enhancement of morphine-induced antinociception
while producing no antinociceptive activity when
given alone (Watkins et al., 1985a, 1985b; Suh and
Tseng, 1990). Furthermore, the CCKBselective an-
tagonists L365,260 and CI-988 (PD134308) enhanced
the antinociceptive effects of systemic or intrathecal
morphine (Dourish et al., 1990; Hughes et al., 1991;
Ossipov et al., 1994; Vanderah et al., 1996a). The
behavioral signs of antinociceptive tolerance to mor-
phine have been reversed by CCK antiserum or pre-
vented by CI-988 (Ding et al., 1986; Hoffmann and
Wiesenfeld-Hallin, 1994). Moreover, repeated coad-
ministration of CI 988 with morphine prevented the
development of antinociceptive tolerance to morphine
(Xu et al., 1992). These data suggested that a true
reversal of tolerance occurred, rather than a potentia-
tion of the effect of morphine, because the same dose
of CI988 did not potentiate morphine in drug naive
rats. Increased CCK activity within the RVM may
also drive descending facilitation of nociception (see
above) because the direct application of antagonists to
the CCKB receptor into the RVM blocked opioid-
induced abnormal pain and antinociceptive tolerance
(Porreca and Vanderah, unpublished observations).
Furthermore, evidence exists to show that opioid ex-
posure promotes the release of CCK in the RVM
(unpublished observations). These studies suggest
that persistent opioid exposure may induce neuroplas-
tic changes that include increased CCK availability in
the RVM, resulting in enhanced activation of a tonic
spinopetal pain facilitation arising from the RVM.
This enhanced pronociceptive system elicits an up-
regulation of spinal dynorphin, which serves to pro-
mote nociceptive inputs at the spinal level, and per-
petuate enhanced pain through a positive feedback
The opioid analgesics have been employed through-
out our history for the treatment and control of pain.
The opioids, exemplified by the prototype morphine,
represent the most efficacious means of controlling
pain at the present time. Paradoxically, the very same
substances that are so efficacious against pain may
actually cause an abnormal pain phenomenon them-
selves by eliciting the activation of endogenous
pronociceptive systems. Persistent exposure to the
opioids cause an increase in the presence of the prono-
ciceptive neurotransmitter CCK, which acts as an
endogenous modulator of antinocieptive activity. One
means through which this is acheived is through the
activation of a tonic descending facilitation from the
RVM. Such descending facilitation may represent the
missing link relating to the puzzling observation that
the many substances which block opioid “tolerance”
are substances which block excitation. Unlike the
situation with states of pain arising from inflammation
or nerve injury, however, the source of such excitation
following prolonged exposure to opioids was un-
known. It is now clear that descending pain facilita-
tion arising from the RVM may represent this excita-
tory input to the spinal cord, and may lead to the
neuroplastic changes that could reflect a state analo-
gous to the “central sensitization” characterized in
injury states. The descending pain modulatory system,
spinal plasticity, and modulation of primary afferent
activity represent a point of intersection of different
parts of the nervous system and reflect mechanisms by
which multiple mediators contribute to increased ex-
citation and hyperalgesia. Critically, enhanced opioid-
induced pain may require increased opioid doses to
elicit antinociception or analgesia, manifesting as
“tolerance.” It might be argued that increased opioid
doses would elicit a further sensitization, and this
could explain why in long-term opioid use, doses are
constantly adjusted upward. It is also possible that
there is a ceiling to the extent that facilitation and
sensitization may occur, which would correspond to
an eventual stabilization of opioid dose. Certainly,
further explorations into the mechanisms driving an-
tinociceptive tolerance to opioids are warrented. An
important consequence of opioid-induced nociceptive
processes is that treatment of severe pain states with
opioids may result in unintentional harm to patients.
Understanding the neurobiology of prolonged expo-
sure to opioids may allow the design of approaches to
Actions of Opioids
limit these adapations and may change the way in
which opioids are used clinically.
Advokat C, Burton P. 1987. Antinociceptive effect of sys-
temic and intrathecal morphine in spinally transected rats.
Eur J Pharmacol 139:335–343.
Aley KO, Levine JD. 1997. Different mechanisms mediate
development and expression of tolerance and dependence
for peripheral mu-opioid antinociception in rat. J Neuro-
Ali NM. 1986. Hyperalgesic response in a patient receiving
high concentrations of spinal morphine. Anesthesiology
Arcaya JL, Cano G, Gomez G, Maixner W, Suarez-Roca H.
1999. Dynorphin A increases substance P release from
trigeminal primary afferent C-fibers. Eur J Pharmacol
Arner S, Rawal N, Gustafsson LL. 1988. Clinical experi-
ence of long-term treatment with epidural and intrathecal
opioids—a nationwide survey. Acta Anaesthesiol Scand
Arvidsson U, Dado RJ, Riedl M, Lee JH, Law PY, Loh HH,
Elde R, Wessendorf MW. 1995. delta-opioid receptor
immunoreactivity: distribution in brainstem and spinal
cord, and relationship to biogenic amines and enkephalin.
J Neurosci 15:1215–1235.
Baber NS, Dourish CT, Hill DR. 1989. The role of CCK
caerulein, and CCK antagonists in nociception. Pain 39:
Baranauskas G, Nistri A. 1998. Sensitization of pain path-
ways in the spinal cord: cellular mechanisms. Prog Neu-
Basbaum AI, Clanton CH, Fields HL. 1978. Three bulbospi-
nal pathways from the rostral medulla of the cat: an
autoradiographic study of pain modulating systems.
J Comp Neurol 178:209–224.
Basbaum AI, Fields HL. 1978. Endogenous pain control
mechanisms: review and hypothesis. Ann Neurol 4:451–
Basbaum AI, Fields HL. 1984. Endogenous pain control
systems: brainstem spinal pathways and endorphin cir-
cuitry. Annu Rev Neurosci 7:309–338.
Basbaum AI, Marley NJ, O’Keefe J, Clanton CH. 1977.
Reversal of morphine and stimulus-produced analgesia
by subtotal spinal cord lesions. Pain 3:43–56.
Bederson JB, Fields HL, Barbaro NM. 1990. Hyperalgesia
during naloxone-precipitated withdrawal from morphine
is associated with increased on-cell activity in the rostral
ventromedial medulla. Somatosens Mot Res 7:185–203.
Belanger S, Ma W, Chabot JG, Quirion R. 2002. Expression
of calcitonin gene-related peptide, substance P and pro-
tein kinase C in cultured dorsal root ganglion neurons
following chronic exposure to mu, delta and kappa opi-
ates. Neuroscience 115:441–453.
Bennett GJ, Mayer DJ. 1979. Inhibition of spinal cord
interneurons by narcotic microinjection and focal electri-
cal stimulation in the periaqueductal central gray matter.
Brain Res 172:243–257.
Besse D, Lombard MC, Besson JM. 1991. Autoradio-
graphic distribution of mu, delta and kappa opioid bind-
ing sites in the superficial dorsal horn, over the rostro-
caudal axis of the rat spinal cord. Brain Res 548:287–291.
Besse D, Lombard MC, Zajac JM, Roques BP, Besson JM.
1990a. Pre- and postsynaptic location of mu, delta and
kappa opioid receptors in the superficial layers of the
dorsal horn of the rat spinal cord. Prog Clin Biol Res
Besse D, Lombard MC, Zajac JM, Roques BP, Besson JM.
1990b. The use of (3H) DAGO to label projections of thin
primary afferent fibres at the superficial dorsal horn level
of the rat spinal cord. Prog Clin Biol Res 328:179–182.
Bian D, Ossipov MH, Ibrahim M, Raffa RB, Tallarida RJ,
Malan TP Jr, Lai J, Porreca F. 1999. Loss of antiallodynic
and antinociceptive spinal/supraspinal morphine synergy
in nerve-injured rats: restoration by MK-801 or dynor-
phin antiserum. Brain Res 831:55–63.
Burgess SE, Gardell LR, Ossipov MH, Malan TP Jr, Van-
derah TW, Lai J, Porreca F. 2002. Time-dependent de-
scending facilitation from the rostral ventromedial me-
dulla maintains, but does not initiate, neuropathic pain.
J Neurosci 22:5129–5136.
Celerier E, Laulin J, Larcher A, Le Moal M, Simonnet G.
1999. Evidence for opiate-activated NMDA processes
masking opiate analgesia in rats. Brain Res 847:18–25.
Celerier E, Rivat C, Jun Y, Laulin JP, Larcher A, Reynier P,
Simonnet G. 2000. Long-lasting hyperalgesia induced by
fentanyl in rats: preventive effect of ketamine. Anesthe-
Cherney NI, Portenoy RK. 1999. Practical issues in the
management of cancer pain. In: Wall PD, Melzack R,
Wall PD, Melzack RS, ediotrs. Textbook of pain.
Edinburgh: Churchill Livingstone, p 1479–1522.
Childers SR. 1991. Opioid receptor-coupled second mes-
senger systems. Life Sci 48:1991–2003.
Clark FM, Proudfit HK. 1993. The projections of noradren-
ergic neurons in the A5 catecholamine cell group to the
spinal cord in the rat: anatomical evidence that A5 neu-
rons modulate nociception. Brain Res 616:200–210.
Claude P, Gracia N, Wagner L, Hargreaves KM. 1999.
Effect of dynorphin on ICGRP release from capsaicin-
sensitive fibers. Abstracts of the 9th World Congress on
Pain, vol. 9, p 262.
Collin E, Cesselin F. 1991. Neurobiological mechanisms of
opioid tolerance and dependence. Clin Neuropharmacol
Colpaert FC. 1978. Long-term suppression of pain by nar-
cotic drugs in the absence of tolerance development. Arch
Int Pharmacodyn Ther 236:293–295.
Colpaert FC. 1995. Drug discrimination: no evidence for
tolerance to opiates. Pharmacol Rev 47:605–629.
Colpaert FC. 1996. System theory of pain and of opiate
analgesia: no tolerance to opiates. Pharmacol Rev 48:
Ossipov et al.
Colpaert FC. 2002. Mechanisms of opioid-induced pain and
antinociceptive tolerance: signal transduction. Pain 95:
Colpaert FC, Kuyps JJ, Niemegeers CJ, Janssen PA. 1976.
Discriminative stimulus properties of fentanyl and mor-
phine: tolerance and dependence. Pharmacol Biochem
Conrad LC, Pfaff DW. 1976a. Efferents from medial basal
forebrain and hypothalamus in the rat. I. An autoradio-
graphic study of the medial preoptic area. J Comp Neurol
Conrad LC, Pfaff DW. 1976b. Efferents from medial basal
forebrain and hypothalamus in the rat. II. An autoradio-
graphic study of the anterior hypothalamus. J Comp Neu-
Cox BM. 1990. Drug tolerance and physical dependence.
In: Pratt WB, Taylor P, Pratt WB, Taylor P, editors.
Principles of drug action: the basis of pharmacology.
New York: Churchill Livingstone, p 639–690.
Dado RJ, Law PY, Loh HH, Elde R. 1993. Immunofluo-
rescent identification of a delta (delta)-opioid receptor on
primary afferent nerve terminals. Neuroreport 5:341–344.
Dauge V, Petit F, Rossignol P, Roques BP. 1987. Use of mu
and delta opioid peptides of various selectivity gives
further evidence of specific involvement of mu opioid
receptors in supraspinal analgesia (tail-flick test). Eur
J Pharmacol 141:171–178.
De Conno F, Caraceni A, Martini C, Spoldi E, Salvetti M,
Ventafridda V. 1991. Hyperalgesia and myoclonus with
intrathecal infusion of high-dose morphine. Pain 47:337–
Delfs JM, Kong H, Mestek A, Chen Y, Yu L, Reisine T,
Chesselet MF. 1994. Expression of mu opioid receptor
mRNA in rat brain: an in situ hybridization study at the
single cell level. J Comp Neurol 345:46–68.
DePaoli AM, Hurley KM, Yasada K, Reisine T, Bell G.
1994. Distribution of kappa opioid receptor mRNA in
adult mouse brain: an in situ hybridization histochemistry
study. Mol Cell Neurosci 5:327–335.
Devulder J. 1997. Hyperalgesia induced by high-dose intra-
thecal sufentanil in neuropathic pain. J Neurosurg Anes-
Dickenson AH, Fardin V, Le Bars D, Besson JM. 1979.
Antinociceptive action following microinjection of me-
thionine-enkephalin in the nucleus raphe magnus of the
rat. Neurosci Lett 15:265–270.
Ding XZ, Bayer BM. 1993. Increases of CCK mRNA and
peptide in different brain areas following acute and
chronic administration of morphine. Brain Res 625:139–
Ding XZ, Fan SG, Zhou JP, Han JS. 1986. Reversal of
tolerance to morphine but no potentiation of morphine-
induced analgesia by antiserum against cholecystokinin
octapeptide. Neuropharmacology 25:1155–1160.
Dourish CT, Hawley D, Iversen SD. 1988. Enhancement of
morphine analgesia and prevention of morphine tolerance
in the rat by the cholecystokinin antagonist L-364,718.
Eur J Pharmacol 147:469–472.
Dourish CT, O’Neill MF, Coughlan J, Kitchener SJ, Haw-
ley D, Iversen SD. 1990. The selective CCK-B receptor
antagonist L-365,260 enhances morphine analgesia and
prevents morphine tolerance in the rat. Eur J Pharmacol
Draisci G, Kajander KC, Dubner R, Bennett GJ, Iadarola
MJ. 1991. Up-regulation of opioid gene expression in
spinal cord evoked by experimental nerve injuries and
inflammation. Brain Res 560:186–192.
Dubner R, Ruda MA. 1992. Activity-dependent neuronal
plasticity following tissue injury and inflammation.
Trends Neurosci 15:96–103.
Duggan AW, Hall JG, Headley PM. 1976. Morphine, en-
kephalin and the substantia gelatinosa. Nature 264:456–
Dworkin RH, Backonja M, Rowbotham MC, Allen RR,
Argoff CR, Bennett GJ, Bushnell MC, Farrar JT, Galer
BS, Haythornthwaite JA, et al. 2003. Advances in neu-
ropathic pain: diagnosis, mechanisms, and treatment rec-
ommendations. Arch Neurol 60:1524–1534.
Eisenach JC. 2000. Preemptive hyperalgesia, not analgesia?
Elliott KJ, Brodsky M, Hyanansky A, Foley KM, Inturrisi
CE. 1995. Dextromethorphan shows efficacy in experi-
mental pain (nociception) and opioid tolerance. Neurol-
Erspamer V, Melchiorri P, Falconieri-Erspamer G, Negri L,
Corsi R, Severini C, Barra D, Simmaco M, Kreil G. 1989.
Deltorphins: a family of naturally occurring peptides with
high affinity and selectivity for delta opioid binding sites.
Proc Natl Acad Sci USA 86:5188–5192.
Faden AI. 1992. Dynorphin increases extracellular levels of
excitatory amino acids in the brain through a non-opioid
mechanism. J Neurosci 12:425–429.
Faris PL, Komisaruk BR, Watkins LR, Mayer DJ. 1983.
Evidence for the neuropeptide cholecystokinin as an an-
tagonist of opiate analgesia. Science 219:310–312.
Fernandes M, Kluwe S, Coper H. 1977a. The development
of tolerance to morphine in the rat. Psychopharmacology
Fernandes M, Kluwe S, Coper H. 1977b. Quantitative as-
sessment of tolerance to and dependence on morphine in
mice. Naunyn Schmiedebergs Arch Pharmacol 297:53–
Fields HL. 1992. Is there a facilitating component to central
pain modulation? APS J 1:71–78.
Fields HL. 2000. Pain modulation: expectation, opioid an-
algesia and virtual pain. Prog Brain Res 122:245–253.
Fields HL, Anderson SD. 1978. Evidence that raphe-spinal
neurons mediate opiate and midbrain stimulation-pro-
duced analgesias. Pain 5:333–349.
Fields HL, Barbaro NM, Heinricher MM. 1988. Brain stem
neuronal circuitry underlying the antinociceptive action
of opiates. Prog Brain Res 77:245–257.
Fields HL, Basbaum AI. 1978. Brainstem control of spinal
pain-transmission neurons. Annu Rev Physiol 40:217–
Fields HL, Basbaum AI. 1999. Central nervous system
Actions of Opioids
mechanisms of pain modulation. In: Wall PD, Melzack R,
Wall PD, Melzack R, editors. Textbook of pain. Edin-
burgh: Churchill Livingstone, p 309–329.
Fields HL, Bry J, Hentall I, Zorman G. 1983. The activity of
neurons in the rostral medulla of the rat during with-
drawal from noxious heat. J Neurosci 3:2545–2552.
Fields HL, Heinricher MM. 1985. Anatomy and physiology
of a nociceptive modulatory system. Philos Trans R Soc
Lond B Biol Sci 308:361–374.
Foley KM. 1993. Opioids. Neurol Clin 11:503–522.
Foley KM. 1995. Misconceptions and controversies regard-
ing the use of opioids in cancer pain. Anticancer Drugs
Gardell LR, Vanderah TW, Gardell SE, Wang R, Ossipov
MH, Lai J, Porreca F. 2003. Enhanced evoked excitatory
transmitter release in experimental neuropathy requires
descending facilitation. J Neurosci 23:8370–8379.
Gardell LR, Wang R, Burgess SE, Ossipov MH, Vanderah
TW, Malan TP Jr, Lai J, Porreca F. 2002. Sustained
morphine exposure induces a spinal dynorphin-dependent
enhancement of excitatory transmitter release from pri-
mary afferent fibers. J Neurosci 22:6747–6755.
Gebhart GF, Ossipov MH. 1986. Characterization of inhi-
bition of the spinal nociceptive tail-flick reflex in the rat
from the medullary lateral reticular nucleus. J Neurosci
Ghilardi JR, Allen CJ, Vigna SR, McVey DC, Mantyh PW.
1992. Trigeminal and dorsal root ganglion neurons ex-
press CCK receptor binding sites in the rat, rabbit, and
monkey: possible site of opiate–CCK analgesic interac-
tions. J Neurosci 12:4854–4866.
Gibson SJ, Polak JM, Bloom SR, Sabate IM, Mulderry PM,
Ghatei MA, McGregor GP, Morrison JF, Kelly JS, Evans
RM, et al. 1984. Calcitonin gene-related peptide immu-
noreactivity in the spinal cord of man and of eight other
species. J Neurosci 4:3101–3111.
Gouarderes C, Jhamandas K, Cridland R, Cros J, Quirion R,
Zajac JM. 1993. Opioid and substance P receptor adap-
tations in the rat spinal cord following sub-chronic intra-
thecal treatment with morphine and naloxone. Neuro-
Gutstein HB. 1996. The effects of pain on opioid tolerance:
how do we resolve the controversy? Pharmacol Rev
Gutstein HB, Trujillo KA, Akil H. 1995. Does chronic
nociceptive stimulation alter the development of mor-
phine tolerance? Brain Res 680:173–179.
Haley JE, Wilcox GL. 1992. Involvement of excitatory
amino acids and peptides in the spinal mechanisms un-
derlying hyperalgesia. In: Willis WD, editor. Hyperalge-
sia and allodynia. New York: Raven Press, p 281–293.
Heinricher MM, McGaraughty S. 1996. CCK modulates the
antinociceptive actions of opioids by an action within the
rostral ventromedial medulla: a combined electrophysio-
logical and behavioral study. In: Abstracts of the 8th
World Congress on Pain, Vancouver. Seattle: IASP Press,
Heinricher MM, Pertovaara A, Ossipov MH. 2003. De-
scending modulation after injury. In: Dostrovsky DO,
Carr DB, Koltzenburg M, Dostrovsky DO, Carr DB,
Koltzenburg M, editors. Proceedings of the 10th World
Congress on Pain. Seattle: IASP Press, p 251–260.
Heinricher MM, Roychowdhury SM. 1997. Reflex-related
activation of putative pain facilitating neurons in rostral
ventromedial medulla requires excitatory amino acid
transmission. Neuroscience 78:1159–1165.
Hoffmann O, Wiesenfeld-Hallin Z. 1994. The CCK-B re-
ceptor antagonist Cl 988 reverses tolerance to morphine
in rats. Neuroreport 5:2565–2568.
Hughes J, Hunter JC, Woodruff GN. 1991. Neurochemical
actions of CCK underlying the therapeutic potential of
CCK-B antagonists. Neuropeptides 19(Suppl):85–89.
Jacquet YF, Lajtha A. 1973. Morphine action at central
nervous system sites in rat: analgesia or hyperalgesia
depending on site and dose. Science 182:490–492.
Jensen TS, Yaksh TL. 1986a. Comparison of antinocicep-
tive action of morphine in the periaqueductal gray, medial
and paramedial medulla in rat. Brain Res 363:99–113.
Jensen TS, Yaksh TL. 1986b. Comparison of the antinoci-
ceptive action of mu and delta opioid receptor ligands in
the periaqueductal gray matter, medial and paramedial
ventral medulla in the rat as studied by the microinjection
technique. Brain Res 372:301–312.
Jhamandas KH. 1984. Opioid-neurotransmitter interactions:
significance in analgesia, tolerance and dependence. Prog
Neuropsychopharmacol Biol Psychiatry 8:565–570.
Ji RR, Zhang Q, Law PY, Low HH, Elde R, Hokfelt T.
1995. Expression of mu-, delta-, and kappa-opioid recep-
tor-like immunoreactivities in rat dorsal root ganglia after
carrageenan-induced inflammation. J Neurosci 15:8156–
Jiang Q, Mosberg HI, Porreca F. 1990. Antinociceptive
effects of [D-Ala2]deltorphin II, a highly selective delta
agonist in vivo. Life Sci 47:PL43–PL47.
Jones SL, Gebhart GF. 1986a. Characterization of coeru-
leospinal inhibition of the nociceptive tail-flick reflex in
the rat: mediation by spinal alpha 2-adrenoceptors. Brain
Jones SL, Gebhart GF. 1986b. Quantitative characterization
of ceruleospinal inhibition of nociceptive transmission in
the rat. J Neurophysiol 56:1397–1410.
Jurna I, Grossman W. 1976. The effect of morphine on the
activity evoked in ventrolateral tract axons of the cat
spinal cord. Exp Brain Res 24:473–484.
Kajander KC, Sahara Y, Iadarola MJ, Bennett GJ. 1990.
Dynorphin increases in the dorsal spinal cord in rats with
a painful peripheral neuropathy. Peptides 11:719–728.
Kalyuzhny AE, Arvidsson U, Wu W, Wessendorf MW.
1996. mu-Opioid and delta-opioid receptors are ex-
pressed in brainstem antinociceptive circuits: studies us-
ing immunocytochemistry and retrograde tract-tracing.
J Neurosci 16:6490–6503.
Kangrga I, Larew JS, Randic M. 1990. The effects of
substance P and calcitonin gene-related peptide on the
efflux of endogenous glutamate and aspartate from the rat
spinal dorsal horn in vitro. Neurosci Lett 108:155–160.
Ossipov et al.
Kaplan H, Fields HL. 1991. Hyperalgesia during acute
opioid abstinence: evidence for a nociceptive facilitating
function of the rostral ventromedial medulla. J Neurosci
Kayser V, Guilbaud G. 1985. Can tolerance to morphine be
induced in arthritic rats? Brain Res 334:335–338.
Kayser V, Neil A, Guilbaud G. 1986. Repeated low doses of
morphine induce a rapid tolerance in arthritic rats but a
potentiation of opiate analgesia in normal animals. Brain
Kieffer BL, Evans CJ. 2002. Opioid tolerance—in search of
the holy grail. Cell 108:587–590.
Kellstein DE, Price DD, Mayer DJ. 1991. Cholecystokinin
and its antagonist lorglumide respectively attenuate and
facilitate morphine-induced inhibition of C-fiber evoked
discharges of dorsal horn nociceptive neurons. Brain Res
Kest B, McLemore G, Kao B, Inturrisi CE. 1997. The
4-propionate receptor antagonist LY293558 attenuates
and reverses analgesic tolerance to morphine but not to
delta or kappa opioids. J Pharmacol Exp Ther 283:1249–
Kiefel JM, Rossi GC, Bodnar RJ. 1993. Medullary mu and
delta opioid receptors modulate mesencephalic morphine
analgesia in rats. Brain Res 624:151–161.
Kim DH, Fields HL, Barbaro NM. 1990. Morphine analge-
sia and acute physical dependence: rapid onset of two
opposing, dose-related processes. Brain Res 516:37–40.
Koetzner L, Hua XY, Lai J, Porreca F, Yaksh T. 2004.
Nonopioid actions of intrathecal dynorphin evoke spinal
excitatory amino acid and prostaglandin E2 release me-
diated by cyclooxygenase-1 and -2. J Neurosci 24:1451–
Kovelowski CJ, Ossipov MH, Sun H, Lai J, Malan TP,
Porreca F. 2000. Supraspinal cholecystokinin may drive
tonic descending facilitation mechanisms to maintain
neuropathic pain in the rat. Pain 87:265–273.
Lai J, Riedl M, Stone LS, Arvidsson U, Bilsky EJ, Wilcox
GL, Elde R, Porreca F. 1996. Immunofluorescence anal-
ysis of antisense oligodeoxynucleotide-mediated “knock-
down” of the mouse delta opioid receptor in vitro and in
vivo. Neurosci Lett 213:205–208.
Larcher A, Laulin JP, Celerier E, Le Moal M, Simonnet G.
1998. Acute tolerance associated with a single opiate
administration: involvement of N-methyl-D-aspartate-de-
pendent pain facilitatory systems. Neuroscience 84:583–
Laughlin TM, Vanderah TW, Lashbrook J, Nichols ML,
Ossipov M, Porreca F, Wilcox GL. 1997. Spinally ad-
ministered dynorphin A produces long-lasting allodynia:
involvement of NMDA but not opioid receptors. Pain
Laulin JP, Celerier E, Larcher A, Le Moal M, Simonnet G.
1999. Opiate tolerance to daily heroin administration: an
apparent phenomenon associated with enhanced pain sen-
sitivity. Neuroscience 89:631–636.
Laulin JP, Larcher A, Celerier E, Le Moal M, Simonnet G.
1998. Long-lasting increased pain sensitivity in rat fol-
lowing exposure to heroin for the first time. Eur J Neu-
Le Bars D, Menetrey D, Conseiller C, Besson JM. 1975.
Depressive effects of morphine upon lamina V cells
activities in the dorsal horn of the spinal cat. Brain Res
Lewis VA, Gebhart GF. 1977a. Evaluation of the periaque-
ductal central gray (PAG) as a morphine-specific locus of
action and examination of morphine-induced and stimu-
lation-produced analgesia at coincident PAG loci. Brain
Lewis VA, Gebhart GF. 1977b. Morphine-induced and
stimulation-produced analgesias at coincident periaque-
ductal central gray loci: evaluation of analgesic congru-
ence, tolerance, and cross-tolerance. Exp Neurol 57:934–
Liu H, Mantyh PW, Basbaum AI. 1997. NMDA-receptor
regulation of substance P release from primary afferent
nociceptors. Nature 386:721–724.
Liu H, Wang H, Sheng M, Jan LY, Jan YN, Basbaum AI.
1994. Evidence for presynaptic N-methyl-D-aspartate au-
toreceptors in the spinal cord dorsal horn. Proc Natl Acad
Sci USA 91:8383–8387.
Lombard MC, Besse D, Besson JM. 1995. Opioid receptors
in the superficial layers of the rat spinal cord: functional
implications in pain processing. Prog Brain Res 104:77–
Lord JA, Waterfield AA, Hughes J, Kosterlitz HW. 1977.
Endogenous opioid peptides: multiple agonists and recep-
tors. Nature 267:495–499.
Lutfy K, Shen KZ, Woodward RM, Weber E. 1996. Inhi-
bition of morphine tolerance by NMDA receptor antag-
onists in the formalin test. Brain Res 731:171–181.
Ma QP, Woolf CJ. 1995. Noxious stimuli induce an N-
methyl-D-aspartate receptor-dependent hypersensitivity
of the flexion withdrawal reflex to touch: implications for
the treatment of mechanical allodynia. Pain 61:383–390.
Ma W, Zheng WH, Kar S, Quirion R. 2000. Morphine
treatment induced calcitonin gene-related peptide and
substance P increases in cultured dorsal root ganglion
neurons. Neuroscience 99:529–539.
Malan TP, Ossipov MH, Gardell LR, Ibrahim M, Bian D,
Lai J, Porreca F. 2000. Extraterritorial neuropathic pain
correlates with multisegmental elevation of spinal dynor-
phin in nerve-injured rats. Pain 86:185–194.
Manning BH, Mao J, Frenk H, Price DD, Mayer DJ. 1996.
Continuous co-administration of dextromethorphan or
MK-801 with morphine: attenuation of morphine depen-
dence and naloxone-reversible attenuation of morphine
tolerance. Pain 67:79–88.
Mansour A, Fox CA, Akil H, Watson SJ. 1995. Opioid-
receptor mRNA expression in the rat CNS: anatomical
and functional implications. Trends Neurosci 18:22–29.
Mansour A, Fox CA, Burke S, Meng F, Thompson RC, Akil
H, Watson SJ. 1994a. Mu, delta, and kappa opioid recep-
tor mRNA expression in the rat CNS: an in situ hybrid-
ization study. J Comp Neurol 350:412–438.
Actions of Opioids
Mansour A, Fox CA, Thompson RC, Akil H, Watson SJ.
1994b. mu-Opioid receptor mRNA expression in the rat
CNS: comparison to mu-receptor binding. Brain Res 643:
Mansour A, Khachaturian H, Lewis ME, Akil H, Watson
SJ. 1987. Autoradiographic differentiation of mu, delta,
and kappa opioid receptors in the rat forebrain and mid-
brain. J Neurosci 7:2445–2464.
Mansour A, Thompson RC, Akil H, Watson SJ. 1993. Delta
opioid receptor mRNA distribution in the brain: compar-
ison to delta receptor binding and proenkephalin mRNA.
J Chem Neuroanat 6:351–362.
Mao J, Price DD, Caruso FS, Mayer DJ. 1996. Oral admin-
istration of dextromethorphan prevents the development
of morphine tolerance and dependence in rats. Pain 67:
Mao J, Price DD, Hayes RL, Lu J, Mayer DJ, Frenk H.
1993. Intrathecal treatment with dextrorphan or ketamine
potently reduces pain-related behaviors in a rat model of
peripheral mononeuropathy. Brain Res 605:164–168.
Mao J, Price DD, Lu J, Mayer DJ. 1998. Antinociceptive
tolerance to the mu-opioid agonist DAMGO is dose-
dependently reduced by MK-801 in rats. Neuroscience
Mao J, Price DD, Mayer DJ. 1994. Thermal hyperalgesia in
association with the development of morphine tolerance
in rats: roles of excitatory amino acid receptors and
protein kinase C. J Neurosci 14:2301–2312.
Mao J, Price DD, Mayer DJ. 1995a. Experimental monon-
europathy reduces the antinociceptive effects of mor-
phine: implications for common intracellular mechanisms
involved in morphine tolerance and neuropathic pain.
Mao J, Price DD, Mayer DJ. 1995b. Mechanisms of hyper-
algesia and morphine tolerance: a current view of their
possible interactions. Pain 62:259–274.
Mao J, Price DD, Mayer DJ, Hayes RL. 1992a. Pain-related
increases in spinal cord membrane-bound protein kinase
C following peripheral nerve injury. Brain Res 588:144–
Mao J, Price DD, Mayer DJ, Lu J, Hayes RL. 1992b.
Intrathecal MK-801 and local nerve anesthesia synergis-
tically reduce nociceptive behaviors in rats with experi-
mental peripheral mononeuropathy. Brain Res 576:254–
Mao J, Price DD, Phillips LL, Lu J, Mayer DJ. 1995c.
Increases in protein kinase C gamma immunoreactivity in
the spinal cord of rats associated with tolerance to the
analgesic effects of morphine. Brain Res 677:257–267.
Martin WR. 1983. Pharmacology of opioids. Pharmacol
Martin WR, Eades CG, Thompson JA, Huppler RE, Gilbert
PE. 1976. The effects of morphine- and nalorphine-like
drugs in the nondependent and morphine-dependent
chronic spinal dog. J Pharmacol Exp Ther 197:517–532.
Massotte D, Kieffer BL. 1998. A molecular basis for opiate
action. Essays Biochem 33:65–77.
Mayer DJ, Mao J, Price DD. 1995a. The association of
neuropathic pain, morphine tolerance and dependence,
and the translocation of protein kinase C. NIDA Res
Mayer DJ, Mao J, Price DD. 1995b. The development of
morphine tolerance and dependence is associated with
translocation of protein kinase C. Pain 61:365–374.
McClane TK, Martin WR. 1967. Effects of morphine, na-
lorphine, cyclazocine, and naloxone on the flexor reflex.
Int J Neuropharmacol 6:89–98.
McGowan MK, Hammond DL. 1993. Antinociception pro-
duced by microinjection of L-glutamate into the ventro-
medial medulla of the rat: mediation by spinal GABAA
receptors. Brain Res 620:86–96.
McMahon SB, Wall PD. 1983. A system of rat spinal cord
lamina 1 cells projecting through the contralateral dorso-
lateral funiculus. J Comp Neurol 214:217–223.
McMahon SB, Wall PD. 1988. Descending excitation and
inhibition of spinal cord lamina I projection neurons.
J Neurophysiol 59:1204–1219.
McNally GP. 1999. Pain facilitatory circuits in the mam-
malian central nervous system: their behavioral signifi-
cance and role in morphine analgesic tolerance. Neurosci
Biobehav Rev 23:1059–1078.
Menard DP, van Rossum D, Kar S, Jolicoeur FB, Jhaman-
das K, Quirion R. 1995. Tolerance to the antinociceptive
properties of morphine in the rat spinal cord: alteration of
calcitonin gene-related peptide-like immunostaining and
receptor binding sites. J Pharmacol Exp Ther 273:887–
Menard DP, van Rossum D, Kar S, St Pierre S, Sutak M,
Jhamandas K, Quirion R. 1996. A calcitonin gene-related
peptide receptor antagonist prevents the development of
tolerance to spinal morphine analgesia. J Neurosci 16:
Miaskowski C, Taiwo YO, Levine JD. 1991. Contribution
of supraspinal mu- and delta-opioid receptors to antino-
ciception in the rat. Eur J Pharmacol 205:247–252.
Minami M, Hosoi Y, Toya T, Katao Y, Maekawa K, Kat-
sumata S, Yabuuchi K, Onogi T, Satoh M. 1993a. In situ
hybridization study of kappa-opioid receptor mRNA in
the rat brain. Neurosci Lett 162:161–164.
Minami M, Onogi T, Toya T, Katao Y, Hosoi Y, Maekawa
K, Katsumata S, Yabuuchi K, Satoh M. 1994. Molecular
cloning and in situ hybridization histochemistry for rat
mu-opioid receptor. Neurosci Res 18:315–322.
Minami M, Toya T, Katao Y, Maekawa K, Nakamura S,
Onogi T, Kaneko S, Satoh M. 1993b. Cloning and ex-
pression of a cDNA for the rat kappa-opioid receptor.
FEBS Lett 329:291–295.
Morgan MM, Fields HL. 1994. Pronounced changes in the
activity of nociceptive modulatory neurons in the rostral
ventromedial medulla in response to prolonged thermal
noxious stimuli. J Neurophysiol 72:1161–1170.
Murase K, Ryu PD, Randic M. 1989. Excitatory and inhib-
itory amino acids and peptide-induced responses in
acutely isolated rat spinal dorsal horn neurons. Neurosci
Neil A, Kayser V, Chen YL, Guilbaud G. 1990. Repeated
Ossipov et al.
low doses of morphine do not induce tolerance but in-
crease the opioid antinociceptive effect in rats with a
peripheral neuropathy. Brain Res 522:140–143.
Noble F, Derrien M, Roques BP. 1993. Modulation of
opioid antinociception by CCK at the supraspinal level:
evidence of regulatory mechanisms between CCK and
enkephalin systems in the control of pain. Br J Pharmacol
Oku R, Satoh M, Fujii N, Otaka A, Yajima H, Takagi H.
1987. Calcitonin gene-related peptide promotes mechan-
ical nociception by potentiating release of substance P
from the spinal dorsal horn in rats. Brain Res 403:350–
Oliveras JL, Hosobuchi Y, Redjemi F, Guilbaud G, Besson
JM. 1977. Opiate antagonist, naloxone, strongly reduces
analgesia induced by stimulation of a raphe nucleus (cen-
tralis inferior). Brain Res 120:221–229.
Ossipov MH, Hong Sun T, Malan P, Jr., Lai J, Porreca F.
2000a. Mediation of spinal nerve injury induced tactile
allodynia by descending facilitatory pathways in the dor-
solateral funiculus in rats. Neurosci Lett 290:129–132.
Ossipov MH, Kovelowski CJ, Nichols ML, Hruby VJ,
Porreca F. 1995a. Characterization of supraspinal antino-
ciceptive actions of opioid delta agonists in the rat. Pain
Ossipov MH, Kovelowski CJ, Porreca F. 1995b. The in-
crease in morphine antinociceptive potency produced by
carrageenan-induced hindpaw inflammation is blocked
by naltrindole, a selective delta-opioid antagonist. Neu-
rosci Lett 184:173–176.
Ossipov MH, Kovelowski CJ, Vanderah T, Porreca F. 1994.
Naltrindole, an opioid delta antagonist, blocks the en-
hancement of morphine-antinociception induced by a
CCKB antagonist in the rat. Neurosci Lett 181:9–12.
Ossipov MH, Lai J, Malan TP, Jr., Porreca F. 2000b. Spinal
and supraspinal mechanisms of neuropathic pain. Ann N
Y Acad Sci 909:12–24.
Ossipov MH, Lai J, Malan TP Jr, Vanderah TW, Porreca F.
2001. Tonic descending facilitation as a mechanism of
neuropathic pain. In: Hansson PT, Fields HL, Hill RG,
Marchettini P, Hansson PT, Fields HL, Hill RG, Mar-
chettini P, editors. Neuropatic pain: pathophysiology and
treatment. Seattle: IASP Press, p 107–124.
Ossipov MH, Lai J, Malan TP, Porreca F. 1998. Recent
advances in the pharmacology of opioids. In: Sawynok J,
Cowan A, Sawynok J, Cowan A, editors. Novel aspects
of pain management: opioids and beyond. New York:
John Wiley & Sons.
Ossipov MH, Lai J, Vanderah TW, Porreca F. 2004. The
opioid delta receptor in pain modulation. In: Chang kj,
Porreca F, Woods JH, Chang kj, Porreca F, Woods JH,
editors. The delta receptor. New York: Marcel Dekker,
Inc., p 297–329.
Pert CB, Snyder SH. 1973a. Opiate receptor: demonstration
in nervous tissue. Science 179:1011–1014.
Pert CB, Snyder SH. 1973b. Properties of opiate-receptor
binding in rat brain. Proc Natl Acad Sci USA 70:2243–
Pertovaara A, Wei H, Hamalainen MM. 1996. Lidocaine in
the rostroventromedial medulla and the periaqueductal
gray attenuates allodynia in neuropathic rats. Neurosci
Pohl M, Ballet S, Collin E, Mauborgne A, Bourgoin S,
Benoliel JJ, Hamon M, Cesselin F. 1997. Enkephalin-
ergic and dynorphinergic neurons in the spinal cord and
dorsal root ganglia of the polyarthritic rat—in vivo re-
lease and cDNA hybridization studies. Brain Res 749:
Pol O, Puig MM. 1997. Reversal of tolerance to the anti-
transit effects of morphine during acute intestinal inflam-
mation in mice. Br J Pharmacol 122:1216–1222.
Porreca F, Ossipov MH, Gebhart GF. 2002. Chronic pain
and medullary descending facilitation. Trends Neurosci
Porreca F, Burgess SE, Gardell LR, Vanderah TW, Malan
TP Jr, Ossipov MH, Lappi DA, Lai J. 2001. Inhibition of
neuropathic pain by selective ablation of brainstem med-
ullary cells expressing the mu-opioid receptor. J Neurosci
Porreca F, Mosberg HI, Hurst R, Hruby VJ, Burks TF.
1984. Roles of mu, delta and kappa opioid receptors in
spinal and supraspinal mediation of gastrointestinal tran-
sit effects and hot-plate analgesia in the mouse. J Phar-
macol Exp Ther 230:341–348.
Powell KJ, Hosokawa A, Bell A, Sutak M, Milne B, Quirion
R, Jhamandas K. 1999. Comparative effects of cyclo-
oxygenase and nitric oxide synthase inhibition on the
development and reversal of spinal opioid tolerance. Br J
Powell KJ, Ma W, Sutak M, Doods H, Quirion R, Jhaman-
das K. 2000. Blockade and reversal of spinal morphine
tolerance by peptide and non-peptide calcitonin gene-
related peptide receptor antagonists. Br J Pharmacol 131:
Powell KJ, Quirion R, Jhamandas K. 2003. Inhibition of
neurokinin-1-substance P receptor and prostanoid activity
prevents and reverses the development of morphine tol-
erance in vivo and the morphine-induced increase in
CGRP expression in cultured dorsal root ganglion neu-
rons. Eur J Neurosci 18:1572–1583.
Proudfit HK, Hammond DL. 1981. Alterations in nocicep-
tive threshold and morphine-induced analgesia produced
by intrathecally administered amine antagonists. Brain
Pu S, Zhuang H, Lu Z, Wu X, Han J. 1994. Cholecystokinin
gene expression in rat amygdaloid neurons: normal dis-
tribution and effect of morphine tolerance. Brain Res Mol
Brain Res 21:183–189.
Quirion R. 1984. Pain, nociception and spinal opioid
receptors. Prog Neuropsychopharmacol Biol Psychia-
Quirion R, Zajac JM, Morgat JL, Roques BP. 1983. Auto-
radiographic distribution of mu and delta opiate receptors
in rat brain using highly selective ligands. Life Sci
Roerig SC, Fujimoto JM. 1988. Morphine antinociception
Actions of Opioids
in different strains of mice: relationship of supraspinal-
spinal multiplicative interaction to tolerance. J Pharmacol
Exp Ther 247:603–608.
Roerig SC, Fujimoto JM. 1989. Multiplicative interaction
between intrathecally and intracerebroventricularly ad-
ministered mu opioid agonists but limited interactions
between delta and kappa agonists for antinociception in
mice. J Pharmacol Exp Ther 249:762–768.
Rossi GC, Pasternak GW, Bodnar RJ. 1993. Synergistic
brainstem interactions for morphine analgesia. Brain Res
Rowbotham MC. 1995. Chronic pain: from theory to prac-
tical management. Neurology 45:S5–S10; discussion
Rowbotham MC, Twilling L, Davies PS, Reisner L, Taylor
K, Mohr D. 2003. Oral opioid therapy for chronic periph-
eral and central neuropathic pain. N Engl J Med 348:
Sabbe MB, Yaksh TL. 1990. Pharmacology of spinal opi-
oids. J Pain Symptom Manage 5:191–203.
Sakai M, Minami T, Hara N, Nishihara I, Kitade H, Ka-
miyama Y, Okuda K, Takahashi H, Mori H, Ito S. 1998.
Stimulation of nitric oxide release from rat spinal cord by
prostaglandin E2. Br J Pharmacol 123:890–894.
Salvemini D, Misko TP, Masferrer JL, Seibert K, Currie
MG, Needleman P. 1993. Nitric oxide activates cycloox-
ygenase enzymes. Proc Natl Acad Sci USA 90:7240–
Satoh M, Minami M. 1995. Molecular pharmacology of the
opioid receptors. Pharmacol Ther 68:343–364.
Schmidt P, Schroder H, Maderspach K, Staak M. 1994.
Immunohistochemical localization of kappa opioid recep-
tors in the human frontal cortex. Brain Res 654:223–233.
Schulz R, Wuster M, Herz A. 1981. Pharmacological char-
acterization of the epsilon-opiate receptor. J Pharmacol
Exp Ther 216:604–606.
Simon EJ. 1973. In search of the opiate receptor. Am J Med
Siuciak JA, Advokat C. 1989. The synergistic effect of
concurrent spinal and supraspinal opiate agonisms is re-
duced by both nociceptive and morphine pretreatment.
Pharmacol Biochem Behav 34:265–273.
Skilling SR, Sun X, Kurtz HJ, Larson AA. 1992. Selective
potentiation of NMDA-induced activity and release of
excitatory amino acids by dynorphin: possible roles in
paralysis and neurotoxicity. Brain Res 575:272–278.
Stanfa L, Dickenson A, Xu XJ, Wiesenfeld-Hallin Z. 1994.
Cholecystokinin and morphine analgesia: variations on a
theme. Trends Pharmacol Sci 15:65–66.
Stanfa LC, Sullivan AF, Dickenson AH. 1992. Alterations
in neuronal excitability and the potency of spinal mu,
delta and kappa opioids after carrageenan-induced in-
flammation. Pain 50:345–354.
Stein C, Pfluger M, Yassouridis A, Hoelzl J, Lehrberger K,
Welte C, Hassan AH. 1996. No tolerance to peripheral
morphine analgesia in presence of opioid expression in
inflamed synovia. J Clin Invest 98:793–799.
Stengaard-Pedersen K, Larsson LI. 1981. Comparative im-
munocytochemical localization of putative opioid ligands
in the central nervous system. Histochemistry 73:89–114.
Stillman MJ, Moulin DE, Foley KM. 1987. Paradoxical
pain following high-dose spinal morphine. Pain 4.
Suh HH, Tseng LF. 1990. Differential effects of sulfated
cholecystokinin octapeptide and proglumide injected in-
trathecally on antinociception induced by beta-endorphin
and morphine administered intracerebroventricularly in
mice. Eur J Pharmacol 179:329–338.
Svingos AL, Moriwaki A, Wang JB, Uhl GR, Pickel VM.
1996. Ultrastructural immunocytochemical localization
of mu-opioid receptors in rat nucleus accumbens: extra-
synaptic plasmalemmal distribution and association with
Leu5-enkephalin. J Neurosci 16:4162–4173.
Taddese A, Nah SY, McCleskey EW. 1995. Selective opi-
oid inhibition of small nociceptive neurons. Science 270:
Terenius L. 1973. Stereospecific interaction between nar-
cotic analgesics and a synaptic plasm a membrane frac-
tion of rat cerebral cortex. Acta Pharmacol Toxicol
Tiseo PJ, Cheng J, Pasternak GW, Inturrisi CE. 1994. Mod-
ulation of morphine tolerance by the competitive N-
methyl-D-aspartate receptor antagonist LY274614: as-
sessment of opioid receptor changes. J Pharmacol Exp
Tiseo PJ, Inturrisi CE. 1993. Attenuation and reversal of
morphine tolerance by the competitive N-methyl-D-as-
partate receptor antagonist, LY274614. J Pharmacol Exp
Trujillo KA, Akil H. 1991. Inhibition of morphine tolerance
and dependence by the NMDA receptor antagonist MK-
801. Science 251:85–87.
Trujillo KA, Akil H. 1994. Inhibition of opiate tolerance by
non-competitive N-methyl-D-aspartate receptor antago-
nists. Brain Res 633:178–188.
Urban MO, Gebhart GF. 1999. Supraspinal contributions to
hyperalgesia. Proc Natl Acad Sci USA 96:7687–7692.
Van Bockstaele EJ, Colago EE, Cheng P, Moriwaki A, Uhl
GR, Pickel VM. 1996. Ultrastructural evidence for prom-
inent distribution of the mu-opioid receptor at extrasyn-
aptic sites on noradrenergic dendrites in the rat nucleus
locus coeruleus. J Neurosci 16:5037–5048.
Vanderah TW, Bernstein RN, Yamamura HI, Hruby VJ,
Porreca F. 1996a. Enhancement of morphine antinocicep-
tion by a CCKB antagonist in mice is mediated via opioid
delta receptors. J Pharmacol Exp Ther 278:212–219.
Vanderah TW, Gardell LR, Burgess SE, Ibrahim M, Dogrul
A, Zhong CM, Zhang ET, Malan TP Jr, Ossipov MH, Lai
J, et al. 2000. Dynorphin promotes abnormal pain and
spinal opioid antinociceptive tolerance. J Neurosci 20:
Vanderah TW, Laughlin T, Lashbrook JM, Nichols ML,
Wilcox GL, Ossipov MH, Malan TP Jr, Porreca F. 1996b.
Single intrathecal injections of dynorphin A or des-Tyr-
dynorphins produce long-lasting allodynia in rats: block-
ade by MK-801 but not naloxone. Pain 68:275–281.
Vanderah TW, Ossipov MH, Lai J, Malan TP Jr, Porreca F.
Ossipov et al.
2001a. Mechanisms of opioid-induced pain and antinoci-
ceptive tolerance: descending facilitation and spinal
dynorphin. Pain 92:5–9.
Vanderah TW, Suenaga NM, Ossipov MH, Malan TP Jr,
Lai J, Porreca F. 2001b. Tonic descending facilitation
from the rostral ventromedial medulla mediates opioid-
induced abnormal pain and antinociceptive tolerance.
J Neurosci 21:279–286.
Verge VM, Wiesenfeld-Hallin Z, Hokfelt T. 1993. Chole-
cystokinin in mammalian primary sensory neurons and
spinal cord: in situ hybridization studies in rat and mon-
key. Eur J Neurosci 5:240–250.
Wagner R, Deleo JA. 1996. Pre-emptive dynorphin and
N-methyl-D-aspartate glutamate receptor antagonism al-
ters spinal immunocytochemistry but not allodynia fol-
lowing complete peripheral nerve injury. Neuroscience
Wagner R, DeLeo JA, Coombs DW, Willenbring S, Fromm
C. 1993. Spinal dynorphin immunoreactivity increases
bilaterally in a neuropathic pain model. Brain Res 629:
Wang Z, Gardell LR, Ossipov MH, Vanderah TW, Brennan
MB, Hochgeschwender U, Hruby VJ, Malan TP Jr, Lai J,
Porreca F. 2001. Pronociceptive actions of dynorphin
maintain chronic neuropathic pain. J Neurosci 21:1779–
Waterfield AA, Leslie FM, Lord JA, Ling N, Kosterlitz
HW. 1979. Opioid activities of fragments of beta-endor-
phin and of its leucine65-analogue. Comparison of the
binding properties of methionine- and leucine-enkepha-
lin. Eur J Pharmacol 58:11–18.
Waterfield AA, Lord JA, Hughes J, Kosterlitz HW. 1978.
Differences in the inhibitory effects of normorphine and
opioid peptides on the responses of the vasa deferentia of
two strains of mice. Eur J Pharmacol 47.
Watkins LR, Kinscheck IB, Kaufman EF, Miller J, Frenk H,
Mayer DJ. 1985a. Cholecystokinin antagonists selec-
tively potentiate analgesia induced by endogenous opi-
ates. Brain Res 327:181–190.
Watkins LR, Kinscheck IB, Mayer DJ. 1985b. Potentiation
of morphine analgesia by the cholecystokinin antagonist
proglumide. Brain Res 327:169–180.
Way EL, Loh HH, Shen FH. 1969. Simultaneous quantita-
tive assessment of morphine tolerance and physical de-
pendence. J Pharmacol Exp Ther 167:1–8.
Wegert S, Ossipov MH, Nichols ML, Bian D, Vanderah
TW, Malan TP Jr, Porreca F. 1997. Differential activities
of intrathecal MK-801 or morphine to alter responses to
thermal and mechanical stimuli in normal or nerve-in-
jured rats. Pain 71:57–64.
Welch SP, Singha AK, Dewey WL. 1989. The antinocicep-
tion produced by intrathecal morphine, calcium, A23187,
U50,488H, [D-Ala2, N-Me-Phe4, Gly-ol]enkephalin and
[D-Pen2, D- Pen5]enkephalin after intrathecal adminis-
tration of calcitonin gene- related peptide in mice. J Phar-
macol Exp Ther 251:1–8.
Wilcox GL. 1991. Excitatory neurotransmitters and pain. In:
Bond MR, Woolf CJ, Bond MR, Woolf CJ, editorss.
Proceedings of the VIth World Congress on Pain. Am-
sterdam: Elsevier, p 97–117.
Williams JT, Christie MJ, Manzoni O. 2001. Cellular and
synaptic adaptations mediating opioid dependence.
Physiol Rev 81:299–343.
Wong CS, Hsu MM, Chou R, Chou YY, Tung CS. 2000.
Intrathecal cyclooxygenase inhibitor administration at-
tenuates morphine antinociceptive tolerance in rats. Br J
Woolf CJ. 1981. Intrathecal high dose morphine produces
hyperalgesia in the rat. Brain Res 209:491–495.
Xu XJ, Wiesenfeld-Hallin Z, Hughes J, Horwell DC, Hok-
felt T. 1992. CI988, a selective antagonist of cholecysto-
kininB receptors, prevents morphine tolerance in the rat.
Br J Pharmacol 105:591–596.
Yaksh TL, Harty GJ. 1988. Pharmacology of the allodynia
in rats evoked by high dose intrathecal morphine. J Phar-
macol Exp Ther 244:501–507.
Yaksh TL, Harty GJ, Onofrio BM. 1986. High dose of
spinal morphine produce a nonopiate receptor-mediated
hyperesthesia: clinical and theoretic implications. Anes-
Yaksh TL, Huang SP, Rudy TA. 1977a. The direct and
specific opiate-like effect of met5-enkephalin and ana-
logues on the spinal cord. Neuroscience 2:593–596.
Yaksh TL, Kohl RL, Rudy TA. 1977b. Induction of toler-
ance and withdrawal in rats receiving morphine in the
spinal subarachnoid space. Eur J Pharmacol 42:275–284.
Yaksh TL, Noueihed R. 1985. The physiology and pharma-
cology of spinal opiates. Annu Rev Pharmacol Toxicol
Yaksh TL, Rudy TA. 1976. Analgesia mediated by a direct
spinal action of narcotics. Science 192:1357–1358.
Yaksh TL, Rudy TA. 1977. Studies on the direct spinal
action of narcotics in the production of analgesia in the
rat. J Pharmacol Exp Ther 202:411–428.
Yaksh TL, Rudy TA. 1978. Narcotic analgestics: CNS sites
and mechanisms of action as revealed by intracerebral
injection techniques. Pain 4:299–359.
Yaksh TL, Yeung JC, Rudy TA. 1976. Systematic exami-
nation in the rat of brain sites sensitive to the direct
application of morphine: observation of differential ef-
fects within the periaqueductal gray. Brain Res 114:83–
Yeomans DC, Clark FM, Paice JA, Proudfit HK. 1992.
Antinociception induced by electrical stimulation of spi-
nally projecting noradrenergic neurons in the A7 cate-
cholamine cell group of the rat. Pain 48:449–461.
Yeung JC, Rudy TA. 1980a. Multiplicative interaction be-
tween narcotic agonisms expressed at spinal and su-
praspinal sites of antinociceptive action as revealed by
concurrent intrathecal and intracerebroventricular injec-
tions of morphine. J Pharmacol Exp Ther 215:633–642.
Yeung JC, Rudy TA. 1980b. Sites of antinociceptive action
of systemically injected morphine: involvement of su-
praspinal loci as revealed by intracerebroventricular in-
jection of naloxone. J Pharmacol Exp Ther 215:626–632.
Yeung JC, Yaksh TL, Rudy TA. 1977. Concurrent mapping
Actions of Opioids
of brain sites for sensitivity to the direct application of Download full-text
morphine and focal electrical stimulation in the produc-
tion of antinociception in the rat. Pain 4:23–40.
Zaki PA, Bilsky EJ, Vanderah TW, Lai J, Evans CJ, Porreca F.
1996. Opioid receptor types and subtypes: the delta receptor
as a model. Annu Rev Pharmacol Toxicol 36:379–401.
Zech DF, Grond S, Lynch J, Hertel D, Lehmann KA. 1995.
Validation of World Health Organization Guidelines for
cancer pain relief: a 10-year prospective study. Pain 63:
Zhou Y, Sun YH, Zhang ZW, Han JS. 1992. Accelerated
expression of cholecystokinin gene in the brain of rats
rendered tolerant to morphine. Neuroreport 3:1121–1123.
Zhou Y, Sun YH, Zhang ZW, Han JS. 1993. Increased
release of immunoreactive cholecystokinin octapeptide
by morphine and potentiation of mu-opioid analgesia by
CCKB receptor antagonist L-365,260 in rat spinal cord.
Eur J Pharmacol 234:147–154.
Zhuo M, Gebhart GF. 1992. Characterization of descending
facilitation and inhibition of spinal nociceptive transmis-
sion from the nuclei reticularis gigantocellularis and gi-
gantocellularis pars alpha in the rat. J Neurophysiol 67:
Zhuo M, Gebhart GF. 1997. Biphasic modulation of spinal
nociceptive transmission from the medullary raphe nuclei
in the rat. J Neurophysiol 78:746–758.
Ossipov et al.