A test of the opponent-process theory of motivation using lesions that selectively block morphine reward.

Abstract

The opponent-process theory of motivation postulates that motivational stimuli activate a rewarding process that is followed by an opposed aversive process in a homeostatic control mechanism. Thus, an acute injection of morphine in nondependent animals should evoke an acute rewarding response, followed by a later aversive response. Indeed, the tegmental pedunculopontine nucleus (TPP) mediates the rewarding effects of opiates in previously morphine-naive animals, but not other unconditioned effects of opiates, or learning ability. The aversive opponent process for acute morphine reward was revealed using a place-conditioning paradigm. The conditioned place aversion induced by 16-h spontaneous morphine withdrawal from an acute morphine injection in nondependent rats was abolished by TPP lesions performed prior to drug experience. However, TPP-lesioned rats did show conditioned aversions for an environment paired with the acute administration of the opioid antagonist naloxone, which blocks endogenous opioids. The results show that blocking the rewarding effects of morphine with TPP lesions also blocked the opponent aversive effects of acute morphine withdrawal in nondependent animals. Thus, this spontaneous withdrawal aversion (the opponent process) is induced by the acute rewarding effects of morphine and not by other unconditioned effects of morphine, the pharmacological effects of morphine or endogenous opioids being displaced from opiate receptors.

Full text

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A test of the opponent-process theory of motivation using
lesions that selectively block morphine reward
Hector Vargas-Perez,1Ryan A. Ting-A-Kee,2Andrew Heinmiller,3Jessica E. Sturgess3and Derek van der Kooy1,2,3
1Department of Medical Genetics and Microbiology, University of Toronto, Canada
2Institute of Medical Science, University of Toronto, Canada
3Department of Medical Biophysics, University of Toronto, Canada
Keywords: aversion, morphine, opponent process, pedunculopontine tegmental nucleus, reward, withdrawal
Abstract
The opponent-process theory of motivation postulates that motivational stimuli activate a rewarding process that is followed by an
opposed aversive process in a homeostatic control mechanism. Thus, an acute injection of morphine in nondependent animals
should evoke an acute rewarding response, followed by a later aversive response. Indeed, the tegmental pedunculopontine nucleus
(TPP) mediates the rewarding effects of opiates in previously morphine-naive animals, but not other unconditioned effects of opiates,
or learning ability. The aversive opponent process for acute morphine reward was revealed using a place-conditioning paradigm. The
conditioned place aversion induced by 16-h spontaneous morphine withdrawal from an acute morphine injection in nondependent
rats was abolished by TPP lesions performed prior to drug experience. However, TPP-lesioned rats did show conditioned aversions
for an environment paired with the acute administration of the opioid antagonist naloxone, which blocks endogenous opioids. The
results show that blocking the rewarding effects of morphine with TPP lesions also blocked the opponent aversive effects of acute
morphine withdrawal in nondependent animals. Thus, this spontaneous withdrawal aversion (the opponent process) is induced by the
acute rewarding effects of morphine and not by other unconditioned effects of morphine, the pharmacological effects of morphine or
endogenous opioids being displaced from opiate receptors.
Introduction
After the acute reward triggered by psychoactive drugs, animals
experience a rebound aversive state (Wise, 1996; Koob & Le Moal,
1997; Robinson & Berridge, 2003). This two-sided motivational effect
is predicted by the opponent-process theory of motivation (Solomon &
Corbit, 1974), which states that any motivational stimulus activates
two opposing processes. The first process has a fast onset and end,
similar in timing to the actual stimulus. The second process is slower
to start and slower to end, lasts longer than the stimulus, and opposes
the actions of the first process (Fig. 1A). In this view, positive
motivational stimuli activate a dose-dependent process in brain reward
circuits, which in turn causes the activation of a negative affective
state or opponent second-process, which is part of a homeostatic
balance that brings brain states back to normal. This theory implies
that blocking the first process should also prevent the second process.
Consequently it could be predicted that, if specifically the rewarding
effects of morphine administration were blocked, the aversive
motivational effects of acute opiate withdrawal would not occur in
nondependent animals.
Withdrawal is a major unpleasant side-effect of opiate administra-
tion that occurs when opiates are cleared from an animal (Koob & Le
Moal, 1997). The dramatic somatic manifestations of opiate with-
drawal (including diarrhea, body shakes and jumping) are the best
known aspects of the withdrawal syndrome in dependent animals
(Way et al., 1969; Koob et al., 1992; Koob & Le Moal, 2005). In
nondependent animals the effects of opiate withdrawal are less striking
(Bozarth & Wise, 1984; Bechara et al., 1995). However, the aversive
motivational effects of withdrawal can be revealed (albeit to a lesser
degree than in dependent animals) when morphine withdrawal is
paired with visual and textural environmental stimuli, such as in
conditioned place-aversion studies (Mucha et al., 1982; van der Kooy
et al., 1982; Bechara et al., 1995; Azar et al., 2003).
Although the motivational effects of withdrawal are seen in both
opiate-dependent and -nondependent rats, they are fundamentally
different. The acute withdrawal aversions in nondependent animals
can only be detected at 11–16 h post-morphine, whereas in dependent
animals the morphine withdrawal aversions last longer (11–24 h) and
are more prominent (Bechara et al., 1995). Furthermore, there is
evidence to suggest that the conditioned place aversions produced by
morphine withdrawal in the drug-dependent vs. -nondependent states
depend on different brain mechanisms (Bozarth & Wise, 1984;
Bechara et al., 1995). The aversion produced by morphine withdrawal
in opiate-dependent animals is mediated by a dopaminergic pathway
(Di Chiara & Imperato, 1988; Koob et al., 1992; Nestler, 1993;
Bechara et al., 1995; Wise, 1996; Dockstader et al., 2001; Laviolette
et al., 2002). In particular, disrupting dopaminergic signalling with
either neuroleptics or mutations on dopamine D2 receptors blocks the
conditioned place aversions produced by morphine withdrawal in
opiate-dependent animals (Bechara et al., 1995; Dockstader et al.,
2001). However, the same manipulations do not block the aversive
effects of acute morphine withdrawal in nondependent animals
(Bechara et al., 1995; Dockstader et al., 2001). The tegmental
Correspondence: Dr Hector Vargas-Perez, as above.
Email: vargashector@yahoo.com
Received 31 January 2007, revised 29 March 2007, accepted 19 April 2007
European Journal of Neuroscience, Vol. 25, pp. 3713–3718, 2007
doi:10.1111/j.1460-9568.2007.05599.x
ª The Authors (2007). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd

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pedunculopontine nucleus (TPP) of the brainstem is a critical neural
substrate underlying the motivational effects of opiates in nondepend-
ent animals (Bechara & van der Kooy, 1992; Olmstead & Franklin,
1993). However, the TPP is not involved in the regulation of other
effects of acute morphine administration in nondependent animals,
such as conditioned taste aversions (Bechara et al., 1993) or analgesia
(Bechara & van der Kooy, 1992). According to the opponent-process
theory, TPP lesions should block the aversive response shown by
nondependent animals to spontaneous opiate withdrawal due to their
ability to block the acute rewarding effects of morphine. Conversely, if
withdrawal aversions are the result of other unconditioned effects of
morphine, such as the pharmacological effect of morphine being
displaced from its opiate receptor and leaving the brain and blood,
then TPP lesions should not block withdrawal aversions.
In the present study, a possible role of the TPP in the aversive
effects of spontaneous withdrawal from acute morphine administration
was assessed in nondependent rats. Using an unbiased place-
conditioning paradigm, we examined the effects of bilateral lesions
of the TPP on the motivational properties of spontaneous withdrawal.
As in the case of opiate reward, TPP lesions blocked the conditioned
place aversion to morphine withdrawal in nondependent animals but,
in concordance with previous studies (Bechara et al., 1995), did not
block the general ability to learn conditioned place aversions.
Materials and methods
Animals
Subjects were male Wistar rats (Charles River) weighing 350–500 g
during experimental training. Rats were housed individually in
Plexiglas cages in a room maintained at 22 ?C and lit from 07.00 to
19.00 h. Rats were given food and water ad libitum throughout the
experiment. All experimental protocols were approved by and
conformed to the Institutional and Governmental Animal Care
Committee guidelines and coincided with principles of animal care
(Faculty Advisory committee on animal services).
Excitotoxic TPP lesions
Rats were anaesthetized with sodium pentobarbital (30 mg/kg i.p.)
(Somnotol; MTC pharmaceuticals, Canada) and placed in a stereotaxic
device. Bilateral TPP lesions (n ¼ 12) were induced by microinfusing
NMDA (0.1 m; Sigma–Aldrich, St Louis, MO) in a volume of
0.20 lL of physiological saline. Sham-lesioned control rats (n ¼ 8)
received bilateral injections of the physiological saline vehicle. The
injection coordinates for the TPP (taken from Paxinos & Watson,
1986) were, bilaterally, 7.8 mm posterior to bregma, 1.6 mm lateral to
the midline and 6.6 mm below the dura. Microinfusions were
performed with a 1-lL Hamilton (Reno, NV, USA) microsyringe for
a 20-min period per hemisphere. The infusion rate for NMDA was
0.01 lL⁄min, after which the injector was left in place for an
additional 5 min to allow diffusion of the solution from the injector tip
(Kippin & van der Kooy, 2003). At least 2 weeks were allowed for
postsurgical recovery prior to behavioural training.
Histological analysis
At the end of experiments, rats were deeply anaesthetized with sodium
pentobarbital (Somnotol, 0.8 mL⁄kg, i.p.) and were perfused tran-
scardially with 200 mL of 0.9% saline followed by 400 mL of 10%
formalin. Brains were rapidly removed and stored for 12 h in 25%
sucrose in a 10% formalin solution. Brains were then flash-frozen at
)70 ?C, sliced in a freezing microtome at )20 ?C into 40-lm-thick
sections, and mounted on gelatine-coated slides. Sections of the TPP
were processed for cresyl violet staining and subsequently examined
by light microscopy.
Fig. 1. The opponent-process model for morphine motivation in nondependent rats and photomicrograph of a representative TPP lesion. (A) Drug intake arouses a
positive first process (reward) that in turn elicits a negative second ‘opponent process’ (aversion). (B) Coronal section (10· magnification) of a cresyl violet-stained
section in the region of the TPP. Inset, schematic of the approximate anatomical region from which the section displayed was taken; Aq, aqueduct.
3714 H. Vargas-Perez et al.
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European Journal of Neuroscience, 25, 3713–3718

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Place-conditioning apparatus
The place-conditioning apparatus was identical to that described
previously (Mucha et al., 1982; Bechara & van der Kooy, 1992), as
was the conditioning procedure (Bechara & van der Kooy, 1992;
Bechara et al., 1992). Briefly, conditioning took place in one of two
(41 · 41 · 38 cm) distinct boxes, which differed in colour, texture
and smell. During testing, each rat was placed in a narrow neutral grey
zone (41 · 10 cm) that separated the two compartments and was
allowed to explore both environments freely for a period of 10 min.
This place-conditioning apparatus has been shown to yield equal
preferences for the two environments (Mucha et al., 1982; Mucha &
Iversen, 1984). When uninjected or vehicle-injected nondependent rats
were repeatedly placed in only one environment of this place-
conditioning apparatus, these rats showed equal preferences for the
familiar environment and the unfamiliar novel environment when later
allowed to explore both environments freely for 10 min (Mucha et al.,
1982; Mucha & Iversen, 1984; Bechara & van der Kooy, 1992;
Bechara et al., 1992).
Conditioning and training
To assess the effect of TPP lesions on the aversive motivational effect
of spontaneous withdrawal from morphine, we have employed a
modified place-conditioning procedure for which conditioning took
place in only one compartment (Procedure Wor withdrawal-paired) of
the place-conditioning apparatus (Bechara et al., 1992, 1995). This
single-side-withdrawal procedure of place conditioning has been
shown to measure only the aversive motivational effects of morphine
withdrawal, separate from the rewarding value of morphine itself
(Bechara et al., 1992, 1995). Briefly, each nondependent rat was given
morphine (20 mg⁄kg i.p.) and returned immediately to its home cage.
Approximately 16 h later, the rat was injected with saline vehicle and
then confined immediately to a distinct conditioning environment for
40 min. This procedure was repeated four times over 8 days. Testing,
as previously described, took place 2 days after the end of condition-
ing; all rats were tested drug-free. One of the two compartments was
previously paired with the absence of morphine and the other was an
unfamiliar neutral environment. The time spent in each environment
was recorded over a 10-min test period. Times spent in each
environment were scored separately for each animal.
To behaviourally verify the TPP lesions, we then tested the same
group of rats using the standard place-conditioning procedure. Both
groups of rats were administered intraperitoneal injections of
morphine (20 mg⁄kg). The rats received four drug–environment and
four saline–environment conditioning sessions for 40 min over eight
consecutive days. Exposure to environments and order of saline vs.
morphine injections was fully counterbalanced. The pairing of each rat
with an environment was counterbalanced with the previous experi-
ment; thus, half of each group received morphine in the previously
neutral environment and half in the previously withdrawal-paired
environment. Two days later, rats were tested drug-free and the time
spent in each environment was recorded over a 10-min test period.
It has been shown that TPP lesions do not block the aversive effects
of naloxone in nondependent rats (Bechara et al., 1995). To test for
possible learning impairments in our TPP-lesioned rats, we next tested
the same groups of sham and TPP-lesioned rats in the conditioned
place preference paradigm with naloxone (5.0 mg⁄kg, i.p.). Both
groups of rats were conditioned for 40 min each with four drug–
environment and four saline–environment sessions spread equally
over 8 days. Exposure to these environments was fully counterbal-
anced. The pairing with the environment was also counterbalanced
with the previous experiment. Thus, half of each group that got
morphine in the white environment now got naloxone in the black
environment, and vice versa. Two days later, rats were tested drug-free
and the time spent in each environment was recorded over a 10-min
test period.
After each experiment was completed, an extinction conditioning
procedure was performed (Laviolette & van der Kooy, 2004). Briefly,
at least one month after each experiment was completed, rats were
placed in a drug-free state into the conditioning boxes, in a fully
counterbalanced order. On day 1, the rats were placed into one of the
conditioning environments for 2 h then returned to their home cages
for 1 h. After that, the rats were placed into the alternate conditioning
environment for 2 h, and then returned to their home cages. This cycle
was repeated over 2 days. Thus, rats were exposed to each environ-
ment for a total of 4 h over the 2-day extinction period. On the third
day, the rats were given an extinction test and were given free access
to both environments for a 10-min period. Time spent in each of the
environments was scored separately for each animal. Rats that
displayed no preference for either of the environments were consid-
ered extinguished.
The place conditioning produced by spontaneous withdrawal
aversion was completely extinguished and rats displayed no aversion
prior to retraining (mean ± SEM in sham animals: time spent in
previous unconditioned environment, 242.68 ± 39.17 vs. previous
conditioned environment, 210.36 ± 35.52 s (t7¼ 0.49, P > 0.05).
Mean ± SEM in TPP-lesioned animals: previous unconditioned,
227.13 ± 36.29 vs.previous
251 ± 42.37 s (t8¼ 0.44, P > 0.05). In the same way, place condi-
tioning produced by morphine was completely extinguished and rats
displayed no preference prior to retraining (mean ± SEM in sham
animals: previous morphine-paired environment, 221.9 ± 68.52 vs.
previous saline-paired environment, 298.46 ± 68.90 s (t7¼ 0.75,
P > 0.05). Mean ± SEM in TTP-lesioned animals: previous mor-
phine-paired side, 208.89 ± 51.25 vs. previous saline paired side,
267.62 ± 55.09 s (t8¼ 0.75, P > 0.05).
To test whether TPP lesions altered general reward value perception
(i.e. anhedonia), rats were tested for sucrose preference (Shimura
et al., 2002; Martinez-Hernandez et al., 2006). Non-water-deprived
rats were given simultaneous access to one bottle containing water and
another containing 0.1 m sucrose in water. The location (left or right
side) of the bottles containing each liquid was randomly assigned for
each animal. We measured the intake of water and sucrose solution
during a 48-h period by weighing both bottles before and after the test
(Martinez-Hernandez et al., 2006).
conditioned environment,
Statistical analyses
All data were analysed by two-tailed paired Student’s t-tests. Due to
the multiple comparisons made on each experiment, the Bonferroni
correction was performed.
Results
Histological analysis
All TPP-lesioned rats included in statistical analyses of the beha-
vioural data were verified histologically to confirm that the lesion
destroyed the majority of the neurons in the TPP nucleus bilaterally.
Three rats were excluded following histological analysis because of
either incomplete lesions or lesion damage extending beyond the
boundaries of the TPP. Figure 1B shows a photomicrograph of a
representative TPP lesion.
Opponent-process theory of motivation3715
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The aversive effects of acute morphine withdrawal were blocked
by TPP lesions
In sham-lesioned rats (n ¼ 8), 16 h abstinence from morphine
(20 mg⁄kg) produced a robust conditioned place aversion for the
spontaneous withdrawal-paired environment. In contrast, TPP-
lesioned rats (n ¼ 9) did not exhibit conditioned place aversion for
the withdrawal-paired compartment (Fig. 2A). A significant difference
between the times spent during testing on the unfamiliar neutral vs.
withdrawal-paired (saline vehicle) compartments was seen in the sham
rats (t7¼ 7.056, P < 0.05). Conversely, there was no significant
difference between the times spent during testing on the unfamiliar
neutral vs. withdrawal-paired compartments in TPP-lesioned rats
(t8¼ 0.999, P > 0.05). Thus sham, but not TPP-lesioned, rats showed
significant aversions for places paired with acute morphine with-
drawal.
TPP lesions blocked acute morphine reward
Morphine (20 mg⁄kg, i.p.) produced conditioned place preferences in
rats with sham but not with excitotoxic lesions of the TPP (Fig. 2B).
There was a significant difference between the times spent in the
previously morphine- vs. saline vehicle-paired compartments in sham
rats (t7¼ 5.034, P < 0.05), but not in TPP-lesioned rats (t8¼ 0.768,
P > 0.05).
TPP lesions did not block acute naloxone aversion
The acute aversive properties of naloxone in place conditioning were
not blocked by TPP lesions (Fig. 2C). There was a significant
difference between the times spent during testing in the previously
naloxone- vs. saline vehicle-paired compartments in sham rats
(t7¼ 7.317, P < 0.05) and in TPP-lesioned rats (t8¼ 5.837,
P < 0.05).
TPP lesions did not block sucrose consumption
The preference for sucrose consumption was not blocked by TPP
lesions (Fig. 2D). A significant difference between tap water and
sucrose consumption was seen in both sham rats (t7¼ 4.448,
P < 0.05) and in TPP-lesioned rats (t8¼ 12.737, P < 0.05).
Discussion
Even the first administration of morphine produces a biphasic pattern
of motivational changes in animals. The opponent-process theory of
motivation posits that the onset of the drug produces a reward state
and then, after the drug has been cleared from the organism, an
aversive state emerges that is inextricably linked to the first rewarding
state (Solomon & Corbit, 1974). We tested the opponent-process
theory using lesions known to disrupt the rewarding properties of
acute opioid administration in nondependent rats. Indeed, TPP lesions
blocked both the acute rewarding and the aversive withdrawal effects
produced by morphine administration in nondependent rats. In a
behavioural verification of the TPP lesions, acute morphine condi-
tioned-place preferences were blocked by the TPP lesions in the same
rats. The data support the conclusion that the same neuronal substrate
(the TPP) that mediates acute opioid reward in nondependent rats
plays a key role in spontaneous withdrawal aversion. In other words,
when the rats were unable to sense the reward state related to acute
morphine administration, the aversive second opponent process did
Fig. 2. Effects of bilateral sham (n ¼ 8) or excitotoxic lesions (n ¼ 9) of the TPP on motivated behaviours in nondependent rats. (A) TPP lesions blocked the
aversions for places paired with acute morphine withdrawal. Data represent the mean ± SEM of the absolute time spent on the previously unfamiliar novel and
morphine withdrawal-paired compartments (*P < 0.05). (B) Morphine-conditioned place preferences were blocked by TPP lesions. Data represent mean ± SEM of
the absolute time spent in the previously morphine- and previously saline-paired compartments during testing (*P < 0.05). (C) TPP lesions did not block
conditioned place aversions induced by naloxone. Data represent the mean ± SEM of the absolute time spent in the previously saline- and previously naloxone-
paired compartments (*P < 0.05). (D) TPP lesions did not block the preference for sucrose vs. water consumption over 48 h (*P < 0.05).
3716H. Vargas-Perez et al.
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not emerge. This suggests that the aversive state related to acute opiate
withdrawal depends on the previous rewarding state evoked by acute
morphine administration.
It is possible that TPP-lesioned rats have an altered perception of
general reward value, producing an acute decrease in sensitivity to
reward (Zacharko & Anisman, 1991). As a consequence, a disruption
of the aversive second opponent process could be due to the blockade
of the general mechanism of reward in the TPP-lesioned rats.
However, TPP lesions did not interfere with a preference for sweet
tastes, showing that TPP lesions do not interfere with rats’ general
perception of reward (Keating et al., 2002). Furthermore, it has been
shown that TPP lesions do not block opiate reward in drug-dependent
rats (Bechara & van der Kooy, 1992; Bechara et al., 1992, 1998), nor
do they block the rewarding effects of other nonopiate drugs such as
cocaine (Parker & van der Kooy, 1995). Contrary to producing a
general anhedonic state, TPP lesions interfere with the reward
mediated by morphine only in nondependent rats. Thus, we conclude
that the blockade of the opiate withdrawal aversion in these rats is
probably the result of blocking acute opiate reward.
TPP lesions block spontaneous morphine withdrawal in nondepen-
dent rats but the same lesions do not block an aversion to naloxone.
These results confirm that TPP lesions do not cause general attentional
or learning deficits (Bechara & van der Kooy, 1992; Bechara et al.,
1992 1995, 1998; Parker & van der Kooy, 1995). These experiments
also reveal that the aversions produced by the acute termination of
endogenous opioids mediated by naloxone are not blocked by TPP
lesions. Indeed, these acute naloxone aversions are blocked by lesions
of the mediobasal arcuate hypothalamus (Mucha et al., 1985). This
suggests that the aversive morphine withdrawal state in nondependent
rats is not due to the pharmacological effect of endogenous opioids
directly being displaced from opiate receptors by naloxone. Indeed,
the aversion to acute morphine withdrawal in nondependent rats only
can be observed 11–16 h after the last injection of morphine, many
hours after morphine has already been cleared from the brain and
blood (Bechara et al., 1995). Thus, this aversion is probably related to
the neurobiological changes (the opponent process) caused by the
reward during morphine administration, and is not related to the
neurobiological effects caused by endogenous opioids and presumably
morphine leaving the brain and blood. We suggest that the opponent
process is produced by morphine acting on opiate receptors and not by
either morphine or endogenous opioids being displaced from their
binding to opiate receptors.
In nondependent rats, the aversive morphine-induced opponent
process appears to be mediated by the TPP and is not dependent on the
acute aversive effects of morphine as measured by the conditioned
taste aversion paradigm (Bechara et al., 1993). The acute aversive
effects produce by morphine in the conditioned taste aversion
paradigm are mediated by dopamine neurons and not the TPP (Zito
et al., 1988; Bechara et al., 1993). TPP lesions, however, are sufficient
to disrupt the opponent process related to morphine motivation in
nondependent rats. In contrast, in drug-dependent rats the aversive
opponent process appears to involve more neuronal systems than those
that are directly affected by the rewarding effects of morphine, or the
neurobiological substrate(s) mediating this process are more diffusely
distributed (Koob & Bloom, 1988; Koob et al., 1992; Wise, 1996;
Robinson & Berridge, 2003). Indeed, dopamine neurons, and not the
TPP, are involved in the aversive effects of withdrawal in opiate-
dependent rats (Bechara et al., 1995). These data suggest that the two
different opponent processes mediating withdrawal aversions in
nondependent vs. opiate-dependent rats are fundamentally different.
The TPP nucleus is directly involved in mediating the opponent
process triggered by morphine administration in nondependent rats.
However, the cellular and neurobiological mechanisms that underlie
the activation of the aversive second process after morphine reward in
these rats have not been elucidated. One possibility is that the neurons
in the TPP adapt in order to neutralize the acute effects of the
morphine. Consequently, any of the main cellular groups of the TPP,
including the cholinergic cell group Ch5, glutamatergic, GABAergic
and peptidergic cell types (Mena-Segovia et al., 2004), and⁄or the
interaction among them, could be being affected during the acute
morphine administration, and then adapt to produce the opponent
process. Thus, the persistence of the adaptation in the TPP after the
morphine clears the organism would be the cause of the aversive
withdrawal response. This could be tested by reversibly inactivating
the TPP 16 h after acute morphine administration to see whether this
manipulation blocks the acute morphine withdrawal aversions. An
alternate explanation is that a different system, triggered by the
changes in the TPP after the primary drug response, produces the
aversion after the acute morphine reward. Although it is not clear
which other system (or systems) might be involved in the withdrawal
effect of morphine in nondependent animals, the dopaminergic system
is unlikely to be involved as disruption of midbrain dopaminergic
transmission has no effect on spontaneous morphine withdrawal in the
nondependent condition (Bechara et al., 1995).
In summary, the data presented here show that in nondependent rats
the TPP serves as a nodal brainstem substrate for the mechanism
underlying the motivational effects of both acute morphine reward and
spontaneous withdrawal aversion, its opponent process. As such, these
findings support the opponent-process theory.
Abbreviation
TPP, tegmental pedunculopontine nucleus.
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ª The Authors (2007). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd
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