for Vitamins and Hormones, volume 70
Regulation of Oxytocin Secretion
Gareth Leng, Celine Caquineau and Nancy Sabatier
Centre for Integrative Physiology, The University of Edinburgh College of Medicine and Veterinary
Sciences, Edinburgh EH8 9XD, UK
A baby sucks at a mother’s breast for comfort, and of course for milk. Milk is made in specialised cells of the
mammary gland, and for a baby to feed, must be released into a collecting chamber from where it can be
extracted by sucking. Milk “let down” is a reflex response to the suckling and kneading of the nipple, and
sometimes in response to the sight, smell and sound of the baby, and is ultimately effected by secretion of
oxytocin. Oxytocin has many physiological roles, but its only irreplaceable role is to mediate milk let-down:
oxytocin-deficient mice cannot feed their young; the pups suckle but no milk is let down, and they will die unless
cross-fostered. Most other physiological roles of oxytocin, including its role in parturition, are redundant in the
sense that the roles can be assumed by other mechanisms in the absence of oxytocin throughout development and
adult life. Nevertheless, physiological function in these roles can be altered or impaired by acute interventions
that alter oxytocin secretion or change the actions of oxytocin. Here we focus on the diverse stimuli that regulate
oxytocin secretion, and on the apparent diversity of roles for oxytocin.
I. The hypothalamo-neurohypophysial oxytocin system
Oxytocin is secreted into the circulation from the posterior pituitary gland (the neurohypophysis) but is
synthesised by large (magnocellular) neurosecretory neurons in the hypothalamus. These neurons are found
mainly in the paraventricular nucleus (PVN) and in the supraoptic nucleus (SON), although smaller groups are
scattered throughout the medial hypothalamus. Nearly all SON neurons project to the neurohypophysis and
immunostain for either oxytocin or vasopressin (see Leng et al., 1998); the PVN is heterogeneous, containing
neurons involved in a variety of autonomic and other neuroendocrine functions, including smaller
(parvocellular) oxytocin cells that project not to the neurohypophysis but to the brainstem, spinal cord, and
other parts of the hypothalamus. Magnocellular neurons produce either oxytocin or vasopressin, and only very
exceptionally both, at least in appreciable quantities; most express mRNA for both peptides, but in very
different amounts (Xi et al., 1999). Oxytocin and vasopressin are very similar nonapeptides, and each is
synthesised as part of a precursor protein that is packaged into neurosecretory vesicles in the cell body. The
vesicles are transported along the axons to the neurohypophysis, and the precursor proteins are cleaved by
enzymes during transport to yield oxytocin and vasopressin and their related neurophysins.
Oxytocin and vasopressin are encoded by separate genes that appear to be descendants of a single gene,
as only one neurohypophysial peptide gene has so far been identified in lamprey and hagfish; these have
arginine vasotocin, which differs from both oxytocin and vasopressin by just one amino acid (see Hoyle, 1998).
In mammals the genes for oxytocin and vasopressin are on the same chromosome, separated by only a few
kilobases of sequence. Members of the vasopressin/ oxytocin superfamily are widely distributed throughout the
animal kingdom; all vertebrate species except for cyclostomes are thought to express at least two family
members (vasotocin and isotocin in teleost fish and amphibians), and the genetic mechanisms that determine
cell-specificity of expression of the two peptides appear to be conserved among vertebrates (Gilligan et al.,
2003). Invertebrates generally express just one: Lys-conopressin is the only member present in the pond snail
Lymnaea stagnalis (van Kesteren et al., 1995), where it has both oxytocin-like reproductive functions and
vasopressin-like metabolic functions. However, octopressin, a novel superfamily member, was isolated recently
from Octopus vulgaris (Takuwa-Kuroda et al., 2003); another peptide of this superfamily, cephalotocin, has
previously been isolated from this species, so Octopus has two members of the superfamily. Octopressin causes
contractions of the Octopus peripheral tissues including the oviduct, aorta and rectum, whereas cephalotocin
has no effect on these tissues. Octopressin mRNA is expressed in many lobes of the octopus brain, including the
superior buccal lobe, which controls feeding, and the gastric ganglion, which controls digestion.
Most “new” genes arise by the duplication of an existing sequence, with subsequent mutations
accumulating in a copy that is initially “redundant”. The North American opossum and the northern brown
bandicoot exhibit dual gene duplication; they have both arginine vasopressin and lysine vasopressin, and both
oxytocin and mesotocin. We think of oxytocin as a quintessentially mammalian hormone, whose primary
functions are to control milk let down and parturition, but the gene duplication that gave rise to oxytocin and
vasopressin occurred early in evolution, and family members have performed many different functions in a long
evolutionary history. These include reproductive roles, notably in oviposition, that seem analogous to the roles
of oxytocin in parturition, and roles in body fluid/electrolyte homeostasis that seem analogous to that of
vasopressin in antidiuresis, but these peptides have also fulfilled other roles in different species. Lys-
conopressin G is found in neurons that govern sexual behavior in male L stagnalis, being present in the
innervation of the sex organs as well as in the CNS, and in Aplysia it is responsible for gill motility associated
with feeding (see Hoyle, 1998).
Here we start with the only indispensable physiological role for oxytocin – the milk-ejection reflex-
focussing on some recent contributions to our understanding. We then consider oxytocin secretion in some of
its apparently redundant roles in mammals, some of which might be analogous to functions of ancestral
II. The milk-ejection reflex
Oxytocin acts on myoepithelial cells of the mammary gland to cause contraction of the smooth muscle.
In the rat, pups suckle continuously, but are rewarded with milk only every 5-15 min, when they display a
characteristic “stretching” response. During suckling, oxytocin cells fire in a pulsatile manner first observed by
Lincoln and Wakerley (1974); recording in the SON, they noted that, 10-15 s before the pup response, some
neurons showed a brief, high-frequency burst of activity. Nearly all oxytocin cells burst in approximate
synchrony, leading to intermittent bolus secretion of oxytocin into the circulation. Pulsatile secretion is
probably a feature of suckling in all mammals; most recently, imaging of milk-flow in humans has revealed
that milk let-down in response to the continuous sucking of a baby occurs intermittently, even though the
mother might not always be aware of this discrete response patterning (Ramsay et al., 2004).
Oxytocin cells fire in bursts so that oxytocin is secreted in pulses, and pulses lead to effective milk let-
down whereas continuous secretion of oxytocin does not. The mammary gland responds to oxytocin only at
relatively high concentrations, and it rapidly desensitises in response to continued exposure, so pulsatile
delivery of hormone is essential for efficient function. Moreover, bursting is efficient for releasing oxytocin;
action potentials releases more oxytocin if they occur in a burst (see Leng and Brown, 1997; Soldo et al., 2004)
than if they occur at a lower frequency over a longer time. In the neurohypophysis the axons of magnocellular
neurons have numerous swellings, and branch into about 2000 neurosecretory endings per axon (Nordmann,
1977). Only about half of secreted oxytocin is secreted from nerve endings, the rest is secreted from the axonal
swellings (Morris and Pow, 1991). When oxytocin cells fire at high frequency, the action potentials
progressively broaden as voltage-dependent Ca2+ entry is facilitated, and, according to a recent report,
depolarisation can also mobilise intracellular Ca2+ stores (De Crescenzo et al., 2004); the increase in [Ca2+]i
'couples' action potentials to hormone secretion (see Fisher and Bourque, 2001), and the coupling is non-linear,
so higher [Ca2+]i triggers disproportionately more secretion. Moreover, high-frequency firing produces a large
increase in extracellular [K+] around the axons, and the resulting depolarisation reduces the likelihood that
action potentials will fail at branch points on the axon, and so more neurosecretory terminals and swellings are
invaded (Leng and Brown, 1997; Giovannucci and Stuenkel, 1997). In the rat, a burst of activity in oxytocin
cells releases about 1 mU of oxytocin - just right for a maximally effective contraction of the mammary gland.
Stimulus-secretion coupling in the neurohypophysis is also modulated by the autocrine actions of factors co-
released with oxytocin, notably the opioid peptide dynorphin (see Brown et al., 2000), and ATP and its
metabolite adenosine (Noguchi and Yamashita, 2000, Wang et al. 2002,). Dynorphin and adenosine are
activity-dependent feedback inhibitors of oxytocin secretion, and their actions might explain why oxytocin
secretion in response to high-frequency stimulation can be sustained only transiently, though intrinsic activity –
dependent hyperpolarisation is also involved (Greffrath et al., 2004).
Pathways that convey the suckling stimulus to oxytocin cells
Stimulation of mechanoreceptors in the nipple initiate the milk-ejection reflex, but the neurons that
transmit this information through the spinal cord and forward to the hypothalamus are unidentified (see
Higuchi and Okere, 2002). The hypothalamus receives inputs from several brainstem regions; notably the
nucleus of the solitary tract (NST), the locus coeruleus, and the ventrolateral medulla, and adrenergic and
noradrenergic projections are prominent from these areas. Noradrenergic cells that project to the SON are
active during parturition (Meddle et al. 2000), noradrenaline excites oxytocin cells, and noradrenergic
antagonists disrupt the milk-ejection reflex; recently Wang and Hatton (2004) reported that bursting could be
elicited in oxytocin cells in a slice preparation in response to an α1-adrenergic agonist. This is the first
reproduction of milk-ejection like bursts in an in-vitro preparation from adult hypothalamus, but the
experimental conditions were unusual: low extracellular [Ca2+] was necessary, and the bursts are not
synchronised. Nevertheless, noradrenaline, which has presynaptic actions as well as postsynaptic actions in the
SON and PVN (Boudaba and Tasker, 2003), is the current best candidate for being the final mediator of
suckling-induced activation of oxytocin cells.
Another theory is that the final input derives from local glutamate neurons. Israel et al. (2003) reported
that in a monolayer organotypic culture of neonatal rat hypothalamus, oxytocin cells display bursts that are
driven by local glutamate neurons, and they proposed that such a mechanism underlies milk-ejection bursts in
vivo. Bursting in this preparation is similar to bursting in response to suckling: the periodicity is similar, and
the bursts are synchronised, although generally longer and less intense than in vivo. Importantly, bursting is
also modulated by oxytocin itself, as in vivo (Richard et al., 1991). In these cultures the oxytocin cells retain
axon collaterals and synaptic connections with other neurons, connections that are present in early development
but are normally lost by adulthood, so it is not clear whether the network formed in culture operates in the
Glutamate is the main excitatory transmitter influencing oxytocin cells, and is present in many
pathways, including those from anterior circumventricular structures involved in osmoregulation. However,
although osmotic stimuli can excite oxytocin cells strongly, they do not induce bursting, even in lactating rats.
There is no direct evidence of the identity of the afferent transmitters released by suckling, and the best
evidence that glutamate might mediate the milk-ejection reflex is referred to above – the bursting behavior of
organotypic cultures. Even if glutamate is involved in milk-ejection bursts, other factors might be necessary; it
has been suggested that, for oxytocin cells to fire at high frequencies, their membrane properties must be
modified, perhaps in response to neuropeptides released by suckling.
The milk-ejection bursts are synchronised between magnocellular neurons in different nuclei on both
sides of the brain, so either there must be fast and efficient intercommunication between nuclei, or a common
input to the different nuclei must be involved. Knife cuts that sever fibers crossing the midline at the level of
the optic medial chiasm disrupt the synchronisation between nuclei. A recent study has described cells in the
ventrolateral medulla oblongata that project bilaterally (Moos et al., 2004b), and at least some of these fibers
cross the midline at the level of the optic chiasm. The identity of these neurons is not known, but they are not
catecholaminergic. In response to suckling, Fos is expressed in some catecholaminergic neurons and in some
non-catecholaminergic neurons of the ventrolateral medulla (Chen and Smith, 2003, Moos et al., 2004b), but
whether the bilaterally projecting cells are involved is not yet established. (Fos is the protein product of the
immediate-early gene c-fos; Fos is transiently expressed in neurons following depolarisation, and so is
commonly used as an anatomical marker of activation).
Because suckling is continuous while the response is intermittent, the afferent information must be
processed to generate the response pattern. This processing occurs in the hypothalamus: during suckling, the milk-
ejection reflex is facilitated if oxytocin is injected into just one SON, and is blocked by local injection of oxytocin
antagonist, so it is generally believed that oxytocin release from dendrites plays a pivotal role in the generation of
the patterned response. In the rat SON, oxytocin cells typically have two sparsely-branched dendrites, most of
which project to the ventral glial lamina where they form a dense plexus with many presynaptic elements. The
dendrites can release very large amounts of oxytocin: the concentrations of oxytocin in the extracellular fluid of the
SON can exceed circulating concentrations by over 100-fold (see Ludwig, 1998).
The direct actions of oxytocin involve mobilization of intracellular calcium ([Ca2+]i), which stimulates
further dendritic oxytocin release (see Dayanithi et al., 2000). Oxytocin cells express oxytocin receptors, and
oxytocin is depolarising to oxytocin cells in early post-natal life (Chevaleyre et al., 2002), but in the adult SON
oxytocin has complex actions that include inhibition of afferent glutamate release. This presynaptic action
might be mediated by oxytocin-induced production of endocannabinoids (Kombian et al., 2002; Curras-Collazo
et al., 2003; Hirasawa et al., 2004).
Bursting behavior does not seem to be hard-wired into the oxytocin cell circuitry; many different stimuli
excite oxytocin cells but few cause bursting. In lactating rats, suckling has little immediate effect on the firing
rate of oxytocin cells, but progressively induces “disorderliness” of firing activity, and increases the correlation
in the activity of different oxytocin cells (Moos et al., 2004a), culminating in full bursting. Thus bursting seems
to emerge as a specific acute response to suckling, and there has been much interest in the neurotransmitters
that are released by suckling. Some peptides such as neurotensin specifically affect discharge patterning in
supraoptic oxytocin cells (Johnstone et al., 2004), recently, several novel peptides have been identified as
ligands of orphan G-protein coupled receptors present in the SON (Ueta et al., 2004), and some peptides like
PACAP (Jamen et al., 2003) and α-melanocyte stimulating hormone (α-MSH; Sabatier et al., 2003a,b) are able
to stimulate dendritic oxytocin release.
Nonetheless, many suspect that oxytocin cells must be “conditioned” by pregnancy to enable them to
display synchronised bursting, perhaps by ovarian steroids. This is an attractive hypothesis, but even during
lactation oxytocin cells do not burst in response to any stimulus other than suckling; they respond to most
stimuli just as they do in virgin rats. Magnocellular oxytocin cells express estrogen receptor β (Somponpon and
Sladek, 2003; Somponpon et al., 2004), but not estrogen receptor α or progesterone receptors, although many
inputs to them do (Francis et al., 2002). Many changes have been described in the intrinsic properties of
oxytocin cells in pregnancy and lactation (e.g. Armstrong and Stern, 1998; Teruyama and Armstrong 2002;
Carter et al., 2003; de Kock et al., 2003), and changes in the neuronal networks that regulate them (e.g.
Cosgrave and Wakerley 2002; Jankowski et al., 2004), but what these changes mean is less clear. For instance,
there are more GABA synapses on oxytocin cells by the end of pregnancy, but the tonic GABA current density
in each oxytocin cell is unaltered, so GABA must be proportionately less effective, balancing out the
consequences of synaptic proliferation. There are marked changes in the glial and synaptic architecture of the
SON at parturition which might have important consequences for neuronal function (Langle et al., 2003; and
see Theodosis, 2002; Miyata and Hatton, 2003). For example, in a virgin rat the dendrites are normally
enveloped by glial cell processes, but in lactation these processes are retracted, allowing dendrites to be directly
apposed to each other. This might favour dendro-dendritic interactions, and might also favour afferent
excitation by reducing extracellular glutamate uptake (Oliet and Piet, 2004), and might facilitate electrotonic
interactions (Hatton and Yang, 2002). However, similar changes accompany chronic osmotic stimulation, a
state not characterised by bursting, and many similar changes affect vasopressin cells; yet vasopressin cells
Parturition and lactation are times of high secretory demand for oxytocin, and many changes in the SON
and PVN might reflect consequences of chronic neuronal hyperactivation, and the metabolic demands of
elevated synthesis. The reason why we don’t see bursting in oxytocin cells in a virgin rat or in a male rat might
simply be because they’re not giving birth or being suckled; as stimuli from the birth canal and the nipples are
present only during parturition and lactation, the behavior of oxytocin cells might be a specific response to
these stimuli, rather than reflecting an organisational state peculiar to lactation.
Oxytocin neurons display bursting only during parturition and suckling, so what is special about these
stimuli? One clue is that suckling elicits oxytocin release in the SON apparently before any milk-ejection
bursts, and indeed before any change in electrical activity of the oxytocin cells (Moos et al., 1989). Dendritic
oxytocin is not normally released in vivo in direct response to electrical discharge activity (although activity-
dependent release has been demonstrated in vitro), but agents that mobilize Ca2+ from intracellular stores –
including oxytocin itself – can prime the dendritic stores of oxytocin, making them available for activity-
dependent release (Ludwig et al., 2002a). Priming involves relocation of neurosecretory vesicles to sites close to
the dendritic membrane, into a recognisable readily-releasable pool (Tobin et al., 2004). One thing that might
be special about the suckling input is that it potently causes dendritic oxytocin release at a time when the
neuronal circuitry can respond to this priming signal in such a way as to result in bursting.
III. Control of oxytocin secretion at parturition.
In the rat, parturition normally begins on the afternoon of day 21 of pregnancy, when strong co-
ordinated contractions of the uterus deliver 8-14 pups at intervals of approximately 10 min. The two main
uterotonic agents, oxytocin and prostaglandins; induce contraction of uterine myometrial cells via specific cell
surface receptors which trigger an increase in [Ca2+]i that leads to phosphorylation of the cell contractile
apparatus (see Sanborn, 2001). In the rat, two days before parturition, increased production of prostaglandin F2α
(PGF2α) by the uterus causes a drop in production of ovarian progesterone (functional luteolysis). PGF2α induces
expression of the ovarian enzyme 20α-hydroxysteroid dehydrogenase that converts progesterone into a
biologically inactive steroid, so the corpus luteum loses its ability to secrete progesterone. As a consequence of
the fall in progesterone, expression of oxytocin receptors in the uterus increases sharply (Lefebvre et al., 1992)
and myometrial contractility increases. Signals from the contracting uterus and cervix are transmitted via vagal
and pelvic nerves; these pathways relay in the NTS, from which A2 noradrenergic cells project directly to the
oxytocin cells (Douglas et al., 2001). During parturition, noradrenaline is released in the SON (Herbison et al.,
1997) and depolarises oxytocin cells via α1 receptors, leading to oxytocin secretion. At parturition, a positive-
feedback loop is thus established between the contracting uterus and oxytocin cells.
Oxytocin is the most powerful endogenous uterotonic agent known, but is not essential for parturition as
oxytocin knock-out mice give birth normally (Young et al., 1996; Nishimori et al., 1996; and see Russell and
Leng, 1998), although oxytocin clearly plays a major role in normal parturition in the mouse as in other
mammals (Douglas et al., 2002b). Conversely, mice deficient in cyclooxygenase-1, an enzyme essential for
PGF2α synthesis, have delayed parturition (Gross et al., 1998), and PGF2α receptor-deficient mice fail to give
birth (Sugimoto et al., 1997); the absence of PGF2α leads to a failure of the corpus luteum to cease progesterone
production. As a result, there is no induction of oxytocin receptor expression in the uterus and no increase in
uterine contractions. So, despite the apparent importance of oxytocin for parturition, and although a major role
of PGF2α is to initiate the actions of oxytocin through induction of oxytocin receptor expression, PGF2α, can
drive uterine contractions and parturition in the absence of oxytocin.
Nonetheless, in normal parturition in rats, acute administration of an oxytocin antagonist will interrupt
delivery of the young (Antonijevic et al., 1995). Normal parturition is associated with very high levels of
oxytocin secretion from the neurohypophysis. In anticipation of this demand, neurohypophysial stores of
oxytocin expand by ~ 50% during pregnancy, and this increase mainly reflects reduced secretion rather than
increased synthesis, reflecting pregnancy-specific mechanisms to restrain secretion (see Russell et al., 2003). It
might be suspected that the build-up of stores is in preparation for lactation, but the entire accumulated excess
is secreted during the 90 min of pup delivery in rats, so the build-up appears to be in anticipation of the
secretory demands of parturition. Uterine contractions triggered by prostaglandins begin several days before
parturition, so well before the onset of parturition oxytocin secretion has to be restrained to prevent depletion of
stored oxytocin. In fact, despite the increase in excitation from the uterus, in the last two days of pregnancy
oxytocin secretion is low because the oxytocin cells are actively inhibited at this time.
Dynorphin, an opioid peptide co-secreted with oxytocin, has an auto-inhibitory action on oxytocin nerve
terminals via κ-opioid receptors, so blocking these actions enhances oxytocin secretion in response to all
stimuli in all known circumstances (see Brown et al., 2000). The effect of dynorphin is stronger in mid-
pregnancy, reducing the ability of stimuli to release oxytocin and enabling oxytocin stores to accumulate, but
the effect of dynorphin declines near term, allowing the stores to be readily releasable during parturition. By
contrast, central opioid inhibition is maximal at the end of pregnancy; then, enkephalins co-secreted with
noradrenaline auto-inhibit the nerve terminals of noradrenergic neurons via µ-receptors (see Russell et al.,
2003), and this acts like a “brake” to repress excitation. β-endorphin from pro-opiomelanocortin neurons that
project from the arcuate nucleus to the SON (Douglas et al., 2002a) might also contribute to the restraint of
oxytocin cell activity.
Several other inputs to oxytocin cells are active during pregnancy. Oxytocin secretion contributes to
maintaining the larger plasma volume required in pregnancy, and this role involves the organum vasculosum of
the lamina terminalis (OVLT) and the subfornical organ (SFO). These structures are activated by relaxin,
which is secreted by the corpora lutea into the circulation from mid pregnancy onwards, and they in turn
activate oxytocin cells (McKinley et al., 1997). Relaxin production falls 12-24 h before parturition, and this
might contribute to the decline in the activity of oxytocin cells at that time. Oxytocin cells are also modulated
by inputs from the olfactory bulbs, the suprachiasmatic nucleus, and the tuberomammillary nucleus, which
provides a histamine input (Hatton and Yang, 2001; Knigge et al., 2003). Olfactory inputs are powerful during
parturition: the mother rat inspects and cleans each young as it is born and eats the afterbirth, and the mitral
layer of the main olfactory bulb contains cells that project to the SON and which express Fos during parturition
(Meddle et al., 2000). Circadian rhythms also are important; the onset of parturition is strongly linked to time
of day (see Russell and Leng, 1998), and the SON receives direct GABA and glutamate inputs from the
suprachiasmatic nucleus that change during the light/dark cycle (Saeb-Parsy and Dyball, 2004).
Pregnancy is accompanied by high plasma concentrations of progesterone until shortly before term, when, in
most mammals, progesterone secretion falls abruptly, and oestrogen secretion surges. The oestrogen surge is only
transient; suckling suppresses the hypothalamo-gonadal axis, and in rats ovarian cyclicity is restored only after the
pups are weaned. The fall in progesterone might indeed influence oxytocin cells; the progesterone metabolite
allopregnanalone stimulates dendritic oxytocin secretion (Widmer et al., 2003) and alters the sensitivity of oxytocin
cells to GABA (Koksma et al., 2003). Post-synaptic actions of oxytocin attenuate the efficacy of GABA, but this
action is blocked by allopregnanalone. In late pregnancy, oxytocin cells express GABAA receptors that are sensitive
to allopregnanalone; this was thought to depend on a particular subunit composition of the GABAA receptor
(Brussard et al., 1999; 2000), but new evidence indicates that it is determined by phosphorylation events regulated
by oxytocin itself. During pregnancy, application of oxytocin abolishes GABAA receptor sensitivity to
allopregnanalone; conversely, during lactation, sensitivity to allopregnanalone can be restored by administration of
an oxytocin antagonist; Koksma et al. (2003) proposed that the sensitivity of GABAA receptors to allopregnanalone
depends on PKC-phosphorylating activity following oxytocin receptor activation. The fall in progesterone might
thus precipitate a state of “positive-feedback disinhibition” in the oxytocin cells that favours bursting. However,
parturition begins only about 24 h after the fall in progesterone, and so the fall in progesterone does not directly
GABA is an important neurotransmitter for oxytocin cells; about one third of synapses onto them
contain GABA, and there are many more GABA synapses in the SON in lactating animals than in virgins (see
Burbach et al. 2001). GABAB receptors as well as GABAA receptors are involved in the actions of GABA (Li
and Stern, 2004), and GABA acts in the neurohypophysis as well as in the hypothalamus (Jackson and Zhang
1995). GABA has a role in the pulsatile discharge of oxytocin cells both during parturition and during suckling
(Voisin et al., 1994; Moos, 1995).
If bursting is facilitated by the fall in progesterone at term, is it terminated by the increase in oestrogen
at weaning? Probably not. Israel and Poulain (2000) recorded in hypothalamic slices from rats in early
lactation, and at the end of lactation. The electrophysiological characteristics of oxytocin cells differed at these
two stages, and at the end of lactation, but not in early lactation, these characteristics were influenced by
oestrogen. The authors speculated that increasing oestrogen might allow the milk-ejection reflex to persist
despite a diminishing frequency of suckling.
Oxytocin cells express neuronal nitric oxide synthase (NOS) in large amounts; expression is regulated in
different physiological states (Popeski et al., 2003; Srisawat et al., 2004), and activity-dependent production of
NO acts both presynaptically (by activating GABA release, Ozaki et al., 2000)-and post synaptically to inhibit
oxytocin cells. This mechanism appears to moderate intense activation of oxytocin cells. However NO
production by oxytocin cells is altered in late pregnancy (Srisawat et al., 2000); expression of NOS mRNA is
reduced and NOS inhibitors lose their efficacy, reflecting a dramatic down-regulation in NO activity that might
make oxytocin cells more excitable.
In the last few hours before parturition, expression of oxytocin receptors in the uterus increases sharply.
The positive-feedback loop between the uterus and oxytocin cells via the brainstem is then strongly reinforced,
and eventually overcomes the opioid restraint, and parturition can start. The background electrical activity of
oxytocin cells rises shortly before the first birth, and is sustained throughout parturition; each birth is then
associated with a pulse of oxytocin secretion triggered by brief, intense, high-frequency bursts, like milk-
ejection bursts. Pulsatile secretion, associated with the increased expression of oxytocin receptors in the uterus,
provides a very effective way for promoting the delivery of pups. Administration of morphine, an agonist of
central µ-opioid receptors, can interrupt parturition in rats by inhibiting the oxytocin cells, and in morphine-
treated rats exogenous pulses of oxytocin can restore the progress of parturition, whereas the same amount of
oxytocin infused in a steady pattern is ineffective (Luckman et al., 1993).
IV. Oxytocin secretion and stress
Generally, a stressor can be considered to be any stimulus that disrupts or threatens homeostasis in
response to which the organism activates a specific, adaptive, compensatory response. Many stressors used
experimentally can be classified as physical stressors, like immobilization and pain; emotional stressors, like
fear, novel environment, and social defeat; or as a combination of both, like restraint, forced swimming, and
shaker platform stress. Stressors can be applied acutely or chronically.
Neuroendocrine responses involving the hypothalamo-pituitary-adrenal axis are crucial for the organism
to survive stressful situations. Corticotrophin-releasing factor (CRF) neurons in the PVN receive inputs from
stress-integrating brain areas and secrete CRF into the hypophysial portal blood. At corticotroph cells in the
anterior pituitary CRF triggers adrenocorticotropin (ACTH) secretion that in turn stimulates the adrenal cortex
to secrete glucocorticoids, which restore homeostasis, and which feed back to switch off CRF secretion. Both
vasopressin and oxytocin, released at the periphery and within the brain, are involved in stress responses.
Vasopressin secretion into hypothalamo-hypophysial portal vessels from parvocellular PVN neurons acts
synergistically with CRF as a secretagogue for ACTH (see Landgraf, 2001), and the CRF neurons co-synthesise
vasopressin in conditions of chronic stress (Sawchenko et al., 1984).
In rats, stress increases Fos expression and expression of oxytocin mRNA in magnocellular oxytocin
neurons (Wotjak et al., 2001; Miyata et al., 1995). Many stressors trigger oxytocin secretion into the blood,
including immobilization; foot shock; restraint; forced swimming; and shaker platform (see Douglas, 2005).
However, stressors described as emotional only, such as social defeat, do not induce oxytocin secretion
(Engelmann et al., 2004). Oxytocin secretion also depends on species; it does not change in humans exposed to
stress, or in horses exposed to a novel environment (Hada et al., 2003).
Stress-induced oxytocin secretion in rats is partly mediated by A2 and A1 noradrenergic cells of the
brainstem (Onaka, 2004), as oxytocin response to fear-related stimuli or noxious stimuli is impaired after
central noradrenaline depletion by neurotoxin treatment or by central injection of a α1 adrenergic antagonist.
Also, Fos expression induced by fear-related stimuli in the SON is reduced after destruction of noradrenergic
afferents by injection of a selective neurotoxin into the vicinity of the SON (Onaka, 2004). However, not all
stressors are mediated by noradrenergic transmission; its disruption does not affect oxytocin secretion following
exposure to novel environment. Recently, a subpopulation of A2 cells that contains prolactin-releasing peptide
(PrRP) was shown to be involved, as immunoneutralization of PrRP reduced oxytocin secretion in response to
conditioned-fear stress (Zhu and Onaka, 2003; and see Honda et al., 2004).
Although stress induces oxytocin secretion, it is not clear which peripheral target oxytocin acts upon or
what its effect is. Oxytocin might act on corticotrophs, as it can be released from collateral axons of
magnocellular neurons into the portal blood circulation (Brownstein et al., 1980); a pharmacological study has
shown that oxytocin can induce ACTH secretion from dispersed rat anterior pituitary cells in synergy with CRF
(Watanabe et al., 1989). The ACTH-secreting activity is not mediated by oxytocin receptors since oxytocin
receptor mRNA is not expressed in corticotrophs (Breton et al., 1995) and specific oxytocin agonists have little
effect on ACTH secretion (Evans and Catts, 1989); this effect might be mediated by vasopressin receptors as it
can be inhibited by a vasopressin V1 receptor antagonist. The action of oxytocin on corticotrophs suggested by
in vitro studies has yet to be confirmed to be physiologically relevant in vivo in conditions of stress.
The thymus is responsible for selection of the peripheral T-cell repertoire. Oxytocin, like many
neuropeptides, has been identified in the human thymus by immunoreactivity and at the transcriptional level; it
is present in the thymus in surprisingly large amounts in particular subtypes of thymic epithelial cells. Oxytocin
is co-localized with the cytokeratin network of thymic epithelial cells rather than in secretory granules, so is not
secreted but behaves like an antigen presented at the cell surface. Thymic oxytocin also behaves as a cryptocrine
signal, interacting with receptors expressed by pre-T cells. Oxytocin receptors are expressed by cytotoxic CD8+
lymphocytes (Elands et al., 1988), and oxytocin induces phosphorylation of focal adhesion kinase in pre-T
cells. Thus oxytocin might intervene in T-cell differentiation as a neuroendocrine self-antigen and as a
promoter of T-cell focal adhesion. Interestingly, Caldwell et al. (1993) reported that sexual activity leads to a
decrease in oxytocin receptor density in the rat thymus; sexual activity triggers neurohypophysial oxytocin
secretion, so this might reflect receptor down-regulation. However, the adaptive value of actions of circulating
oxytocin at the thymus is unclear.
Central oxytocin and stress
Many stressors induce central release of oxytocin that produces high extracellular concentrations in
various regions of the brain including the PVN, the SON, the septum and the amygdala. Central oxytocin is
thought to be an antistress and anti-anxiety factor; in rats, central injection of an oxytocin antagonist increases
basal plasma concentrations of ACTH and facilitates ACTH secretion in response to various stressors (see
Neumann, 2002), and central injection of oxytocin attenuates glucocorticoid secretion in response to stress.
Rats previously exposed to central or peripheral administration of oxytocin are less anxious when challenged in
an elevated-plus maze, a behavioral test of anxiety. Central oxytocin also attenuates stress-induced CRF mRNA
expression in the PVN and attenuates expression of c-fos mRNA in stress-activated neural circuits including
the PVN, the ventrolateral septum (LSV), and the dorsal hippocampus (Windle et al., 2004). Finally, according
to a recent report, early exposure to oxytocin produces changes in adrenoceptor activity in brain circuits
involved in stress responses that persist into adulthood (Diaz-Cabiale et al., 2004)
The effect of endogenous oxytocin on stress and anxiety has been investigated in oxytocin-deficient
mice. In these mice, CRH mRNA expression in the PVN in response to restraint stress is stronger than in wild-
type mice, and Fos expression in the bed nucleus of the stria terminalis and the medial amygdala is reduced,
(Nomura et al., 2003). Female oxytocin-deficient mice display more anxiety-related behavior as measured in an
elevated plus maze, but also secrete more corticosterone in response to a psychogenic stressor, and have a larger
hyperthermic response when exposed to a novel environment (Amico et al., 2004). However, another study has
reported that male oxytocin-deficient mice are less fearful in the elevated-plus maze and show more aggressive
behavior, suggesting that oxytocin is necessary during development for normal emotional behavior in adult
animals (Winslow et al., 2000).
In the SON and the PVN, forced swimming, shaker stress, and social defeat induce dendritic release of
oxytocin from magnocellular neurons (see Neumann, 2002). The role for dendritic oxytocin during stress is
unclear, but endogenous oxytocin modulates the activity of supraoptic oxytocin cells and their noradrenergic
inputs in response to noxious stimuli (Onaka et al., 2003). An oxytocin antagonist applied directly onto the
SON or injected centrally, attenuated footshock-induced noradrenaline release in the SON and oxytocin
secretion. This suggests that stress activates brainstem noradrenergic neurons that excite oxytocin cells,
resulting in increased dendritic and plasma release of oxytocin, as well as presynaptic facilitation of
V. Osmotic regulation of oxytocin secretion
In some mammals, oxytocin functions with vasopressin to control plasma osmolality. In the rat, oxytocin
cells are osmoresponsive; they respond to elevated plasma osmolality with a graded increase in electrical
activity, not the bursting behavior that is characteristic of milk ejection. Oxytocin secreted in response to this
stimulus acts synergistically with vasopressin at the kidney to promote natriuresis (Verbalis et al., 1991; Ozaki
et al., 2004), reflecting an action at oxytocin receptors and vasopressin V2 receptors in the inner medullary
collecting duct, and an action at oxytocin receptors in the heart to induce release of atrial natriuretic peptide
(see Antunes-Rodrigues et al., 2004). In the rat, at least as much oxytocin as vasopressin is secreted in response
to osmotic challenge.
Oxytocin cells, like vasopressin cells, are directly sensitive to the osmotic pressure of the extracellular
medium by which they are surrounded, and as the SON and PVN are very densely vascularised, the
extracellular osmotic pressure closely matches that of the plasma. This osmosensitivity has been studied in
detail by Bourque and co-workers (Bourque and Chakfe, 2000; Bourque et al., 2002), who showed that
magnocellular neurons are much more sensitive to osmotic pressure than other neurons, and that their response
is reflected in a change in the current/voltage characteristics of the neuronal membrane due to enhancement of
a voltage-independent inward cationic current, resulting in a direct membrane depolarisation. The conductance
increase is a consequence of osmotically-induced cell shrinkage: the cell membrane contains mechanosensitive
ion channels that are inactivated when the membrane is stretched. As the SON is inside the blood-brain barrier,
increased plasma concentrations of small non-permeant molecules result in osmotic withdrawal of water from
the extracellular fluid, and hence an increase in extracellular [Na+]. However, it is increased osmolarity that
activates the neurons, and infusions of small permeant molecules such as mannitol cause a similar activation
even without any change in [Na+].
Although magnocellular neurons are osmosensitive, when deafferented they apparently lose the ability to
respond to osmotic pressure changes (see Leng et al., 1998). Magnocellular neurons in vivo are barraged by a
huge amount of synaptic input, producing a fluctuating membrane potential. Action potentials (spikes) arise
when fluctuations exceed the spike threshold, so when the average membrane potential is depolarised, more
fluctuations exceed the spike threshold (Leng et al., 2001). However if the neuron is deprived of its normal
tonic activation, the fluctuations of membrane potential around the mean are smaller and less frequent, and
osmotically-induced depolarisation becomes ineffective in increasing spike frequency. In addition, some
afferent inputs derive from other osmoreceptors located in the SFO and the OVLT. Paradoxically, these include
some inputs that are inhibitory as well as some that are excitatory (Leng et al., 2001). Moreover, the
osmoregulation of magnocellular neurons involves interactions with local glial cells that release taurine in
response to osmotic stimulation (Hussy et al., 2001).
The hypothalamus is bounded in part by specialized regions that lack a blood-brain barrier. The SFO
and the OVLT are circumventricular organs, and cells here are in intimate connection with both blood and
CSF. These structures are densely vascularised; they contain osmoreceptive and Na-receptive cells; and cells
with receptors for angiotensin II, atrial natriuretic factor, endothelin and relaxin, all of which are important in
fluid and electrolyte balance. The OVLT and SFO project directly to the SON and PVN and also indirectly via
the nucleus medianus, which is a co-ordinating center for osmoregulatory functions including thirst; these
structures comprise the `osmoregulatory complex'. The OVLT is a particularly critical site; lesions here result
in a virtual absence of drinking in response to hyperosmolarity; a natriuretic deficit leading to progressive
accumulation of sodium; and a deficit in osmotically regulated secretion of vasopressin and oxytocin, although
secretion in response to other stimuli is normal (see Leng et al., 1998; McKinley et al., 2004).
The brain structures activated by osmotic stimuli have been mapped using c-fos as a marker of neuronal
excitation. Within 15 min of intraperitoneal injection of hypertonic saline, c-fos mRNA appears in most
magnocellular neurons. Lower levels of c-fos mRNA also appear within the SFO, the OVLT, and in the nucleus
medianus. The pharmacology of these afferent pathways is complex, including an inhibitory GABA component
as well as excitatory components mediated by excitatory amino acids and several peptides, including
angiotensin II, which is present in a projection from the SFO to the magnocellular nuclei. None of the pathways
can be characterised as simply excitatory or simply inhibitory, but if they are disconnected, the SON falls silent,
and becomes relatively inexcitable (see Leng et al., 1998).
Oxytocin secretion is also stimulated by gastric distension, and probably the response of oxytocin cells to
cholecystokinin (CCK) reflects this physiological pathway. Systemic injections of CCK transiently activate
oxytocin cells while inhibiting or having no effect on vasopressin cells (Sabatier et al., 2004), and this response
reflects activation by CCK of CCKA receptors on the gastric vagus that project via A2 cells of the NTS to both
magnocellular and parvocellular oxytocin cells (see Leng et al., 1998). Gastric distension, which similarly
activates oxytocin cells, initiates a reflex closure of the gastric sphincter via a pathway from the ascending
vagus via the brainstem to the PVN, to activate parvocellular oxytocin cells that project back to the brainstem to
activate vagal motoneurons. However magnocellular oxytocin cells are separate from this central reflex
pathway, and their activation might thus reflect reflex natriuresis in response to food ingestion. Centrally,
oxytocin suppresses food intake and salt appetite in particular (Amico et al., 2001; Puryear et al., 2001; Fitts et
al., 2003; Stricker and Verbalis, 2004), though whether this reflects oxytocin released from dendrites of
magnocellular neurons or from the nerve endings of parvocellular neurons is not known.
A recent study explored gene expression in the rat SON using a 35,319-element mouse cDNA
microarray; 95% of these genes were expressed in the SON at above background levels (Mutsuga et al., 2004).
To a reasonable approximation therefore, every gene expressed in the rat is expressed in the rat SON. Of these,
1385 genes were expressed in the SON at substantially greater levels than in the rest of the hypothalamus in
basal conditions; a sample of ten preferentially-expressed genes and four other genes were studied in conditions
of hyperosmotic stimulation, and all increased their expression level by at least 75% above control levels. Even
if a gene is expressed at a relatively low level in the SON compared to the rest of the hypothalamus it might
nevertheless be important in the SON, and many genes that are not preferentially expressed in the SON in basal
conditions might be preferentially expressed in other physiological states; a much smaller scale micro-array
study (with an array of 152 gene sequences) identified 9 genes whose expression was strongly influenced by
dehydration (Murphy and Wells, 2003), but none of these appear in the list of genes that are preferentially
expressed in the SON compared to the whole hypothalamus.
These and related studies (see also Zhang et al., 2001; Davies et al., 2003) indicate that the expression
of a very large number of genes indeed is likely to be influenced by chronic osmotic stimulation and perhaps by
chronic activation of any kind.
VI. Oxytocin secretion and behaviour
Oxytocin released from the neurohypophysis does not re-enter the brain in significant amounts because
of the blood-brain barrier, so if central actions of oxytocin reflect physiological rather than pharmacological
actions, they must reflect actions of oxytocin released within the brain. The axons of magnocellular neurons
have few collateral branches within the brain, and the profile of oxytocin secretion into the blood does not
always fit well with putative behavioral actions, so it has generally been assumed that these actions reflect the
functions of centrally-projecting parvocellular oxytocin cells. Oxytocin receptors are present in many brain
areas, including the ventromedial nucleus of the hypothalamus, the olfactory bulbs, the septum, the
hippocampus, and many sites in the brainstem and spinal cord, and some of these do receive projections from
parvocellular oxytocin cells. However, the major source of oxytocin released within the brain is not the
parvocellular system, but the magnocellular system (see Sabatier et al., 2003b)
For over 20 years it has been recognised that the dendrites of magnocellular neurons can release very
large amounts of peptide. The dendrites seem to possess the machinery for local peptide synthesis (Mohr and
Richter, 2004); peptide release in the SON has been implicated in the anatomical remodelling of the nucleus
that is apparent in lactation (see Theodosis, 2002), dendro-dendritic communication has been implicated in the
synchronous burst firing of the milk-ejection reflex (see Ludwig et al., 2002b), and dendritic release influences
afferent inputs to the SON by presynaptic actions (Kombian et al., 2002; Pittman and Bains, 2003). Very
recently it has been established that dendritic release is regulated semi-independently of axonal release (Morris
and Ludwig, 2004). Afferent transmitters that mobilise intracellular Ca2+ in oxytocin cells can release dendritic
oxytocin without increasing the electrical activity of oxytocin cells and hence without accompanying secretion
into the circulation. Indeed, electrical activity in the oxytocin cells does not normally cause significant oxytocin
release from dendrites in vivo (Sabatier et al., 2003a,b). However, some peptides, including oxytocin itself,
have the capacity to “prime” activity-dependent dendritic release of oxytocin (Ludwig et al., 2002a). Priming
involves making dendritic stores of oxytocin available for subsequent activity-dependent release, and may
involve a physical relocation of granules to release sites on the dendrites; priming can last for at least 90 min.
These developments present challenges for the understanding of information processing that have
ramifications far beyond the magnocellular system. The dendrites of oxytocin cells are organised into what look
like hormone-secreting structures within the brain itself. Oxytocin release from these structures – the ventral
glial lamina of the SON and the circumventricular dendritic plexus of the PVN – can be regulated
independently of oxytocin secretion into the blood. Oxytocin is released in very large amounts at these sites,
and is available to diffuse extensively throughout the brain. At oxytocin receptors on oxytocin cells, and
possibly at receptors on other neurons, oxytocin can act to “prime” the post-synaptic neuron. Priming involves
changing the availability of vesicles for activity-dependent release at the post-synaptic target; this implies a
functional re-wiring of neuronal circuits, and priming can last a very long time after an initial exposure to
oxytocin. As oxytocin can prime its own release, once dendritic release is first initiated it can be self-sustaining,
leading to a further prolongation of the effects of the initial priming agent. Here then is a possible framework
for understanding long-lasting behavioral effects of peptides.
Oxytocin and male sexual behavior
The early indications that vasopressin and oxytocin were involved in learning and memory have largely
gave way, in what made be described as a “paradigm shift,” to richer stories of their widespread involvement in
social and reproductive behaviors (see Pedersen and Boccia, 2002a,b; Turner et al., 2002, Choleris et al.,
2004). Central oxytocin facilitates female sexual behavior; injected into the MPOA, oxytocin increases sexual
receptivity, and oxytocin injected into the ventromedial nucleus facilitates lordosis. The effect of oxytocin
depends on both oestrogen and progesterone as infusions of oxytocin into the ventromedial nucleus increase
lordosis in females treated with oestrogen and progesterone but not in females treated with oestrogen alone.
Central oxytocin also has a well established role in maternal behaviour, (Francis et al., 2002, Lipschitz et al.,
2003) and in the formation of pair bonds in monogamous species (Carter, 2003; Insel, 2003).
Here though we focus on actions on male sexual behavior. The functions now ascribed to oxytocin in
mammals have analogs in species that diverged from vertebrates up to 400 million years ago. The earliest
functions of these ancestor peptides were behavioral, and were linked, amongst other things, to sexual function
in males and to feeding. In the nematode C. elegans, insulin signaling coordinates sexual behavior with
metabolic and reproductive status (Lipton et al., 2004), and absence of food affects several behaviors in C.
elegans, including egg laying. Serotonin is one important food-associated signal mediating these effects; and is
also an important transmitter mediating synthesis and secretion of oxytocin in rats (Vacher et al., 2002;
Jorgensen et al., 2003a,b).
Oxytocin and penile erection
Melin and Kihlstrom (1963) first reported that i.v. injection of oxytocin reduced ejaculation latency in
rabbits, and similar actions were later reported in rats. Oxytocin secretion is stimulated during copulation in
rats (Hillegaart et al., 1998) and there is evidence for a pulse of secretion at ejaculation (Ivell, 1997); oxytocin
secretion is also elevated during erection in humans (Uckert et al. 2003). Oxytocin receptors are expressed in
the male reproductive tract including in the testis and in the prostate (Gimpl and Fahrenholz, 2001), but how
oxytocin facilitates sexual behavior remains to be identified; and it is now clear that central actions are
important. Electrical stimulation of the rat dorsal penile nerve excites many oxytocin cells in the PVN and SON
(Honda, 1999; Andersson, 2001) and high concentrations of oxytocin have been measured in the cerebrospinal
fluid at ejaculation (Hughes, 1987). Jirikowski (1992) described that, in repeatedly-mated male rats, oxytocin
immunoreactivity was increased in many brain areas including the MPOA, the anterior hypothalamic nucleus,
the SON, and the PVN. In 1986, Argiolas et al. described oxytocin as “the most potent agent able to induce
penile erection so far” in rats, rabbits and monkeys. and subsequently reported that i.c.v. injection of an
oxytocin antagonist inhibited male copulation by decreasing the mount and intromission frequencies and by
abolishing ejaculation (Argiolas et al., 1999; Melis et al., 2000).
The PVN is the most sensitive brain area for oxytocin injection to induce penile erection (Melis et al.,
1986) and has been the main focus of recent efforts to identify pathways involved in regulating male sexual
behavior (e.g. Melis et al., 2003; 2004). Lesions of the PVN impaired oxytocin–induced penile erection
(Argiolas et al., 1987), and lesions of the catecholaminergic inputs to the hypothalamus attenuate penile
erection in parallel with effects on oxytocin mRNA expression (Fraley, 2002). Witt and Insel (1994) reported
that Fos expression was increased in oxytocin cells in all parvocellular subdivisions of the PVN during sexual
contact. Parvocellular oxytocin cells project from the PVN to many intra- and extra- hypothalamic areas
including the brainstem and the spinal cord, and hippocampus, and Chen and Chang (2001) showed that
injection of oxytocin into the hippocampus of male rats increased penile intracavernous pressure. The
thoracolumbar and lumbosacral segments of the spinal cord contain oxytocin binding sites (Veronneau-
Longueville et al., 1999), and this is one other site of action of oxytocin in promoting penile erection (Giuliano
et al., 2001).
When injected i.c.v., oxytocin and apomorphine have very similar effects on male sexual behavior: both
induce repeated episodes of penile erection and yawning, and the effects of both can be blocked by an oxytocin
antagonist, whereas oxytocin -induced penile erection is not blocked by dopamine antagonists (Argiolas, 1999).
The increase in oxytocin release facilitating penile erection correlates with an increase NO production in the
PVN and in the spinal cord (Ferrini et al., 2003); i.c.v. injection of NOS inhibitors inhibit penile erection
induced by oxytocin or by apomorphine; and NO donors injected i.c.v. induced penile erection in a dose-
dependent manner (Melis and Argiolas, 1997). NOS mRNA expression in the PVN is twice as abundant in
sexually potent rats as in impotent rats (Benelli, 1995).
Interactions of oxytocin and –α-MSH
The involvement of the melanocortin system in male sexual function has been known since Ferrari et al.
(1963) reported that i.c.v injection of ACTH, α-MSH and related peptides in common laboratory animals
including rats, induced the “stretching-yawning syndrome” associated with repeated episodes of penile erection
and ejaculation. I.c.v injection of α-MSH and ACTH reduce ejaculation latencies and the number of mounts
and intromissions required to achieve ejaculation in sexually experienced rats (Argiolas, 1999). The behavioral
effects of α-MSH and oxytocin are remarkably similar: both reduce food intake when injected centrally; both
induce grooming behaviors; and both induce the “stretching-yawning reflex” (Richard et al., 1991; Vergoni
and Bertolini, 2000; Argiolas et al., 2000). Finally, oxytocin and α-MSH have similar effects on sexual
behavior; they enhance female sexual behavior, and induce penile erection and stimulate sexual performance in
males (Mizusawa et al., 2002).
The PVN is a common site of action for oxytocin and α –MSH (Argiolas, 1999) as both induce penile
erection when injected into the PVN, and both actions are inhibited by opioids and by NOS inhibitors (Argiolas
and Melis, 1995). α -MSH fibers innervate the SON and the PVN (O’Donohue, 1980), mRNA for the
melanocortin MC4 receptor is strongly expressed in the SON and PVN (Mountjoy et al., 1994), and MC4
receptor agonists induce penile erection in rats (Martin et al., 2002).
Injection of α -MSH i.c.v induces Fos expression in oxytocin cells in the SON and in the PVN (McMinn,
2000; Olszewski et al., 2001; Sabatier et al., 2003a,b). Fos expression is generally assumed to be a marker of
neuronal excitation; but i.c.v. administration of α-MSH or of a highly specific MC4R agonist, produces a rapid,
reversible inhibition of the electrical activity of magnocellular oxytocin cells in the SON (Sabatier et al.,
2003b). α-MSH inhibits oxytocin cells when applied either i.c.v. or directly onto the SON, and also does not
trigger oxytocin secretion into the circulation. α-MSH increases [Ca2+]i in oxytocin cells, apparently by
mobilizing intracellular Ca2+; this action involves MC4 receptors, which are G-protein coupled receptors and
activate the adenylyl cyclase/protein kinase A signalling pathway, so α-MSH-induced activation of cAMP
might account for the induction of Fos expression in supraoptic neurons. However, although α-MSH inhibits
oxytocin cells, and inhibits oxytocin secretion into the circulation, α-MSH stimulates dendritic release of
oxytocin from the SON. In oxytocin cells, mobilisation of [Ca2+] induced by oxytocin can make dendritic stores
of oxytocin available for subsequent release in response to electrical activation (“priming”; Ludwig et al.,
2002a). Thus α-MSH-induced dendritic oxytocin release might potentiate subsequent activity-dependent
oxytocin release, leading to a self-sustaining, long-lasting potentiation of activity-dependent dendritic release.
This might indicate how acute central application of α-MSH can have long-lasting consequences for neuronal
network function, reflected in altered behavior, even when direct effects on neuronal excitability are relatively
Oxytocin participates in diverse physiological functions. If oxytocin had many independently regulated
sources, with focal sites of action linked to these independent sources, this would be easy to understand.
However, peripheral actions of oxytocin mainly, if not exclusively, reflect secretion from the neurohypophysis,
and the magnocellular neurons that project to the neurohypophysis are also involved, through dendritic release,
in many if not all of the central actions of oxytocin. The challenge is therefore to understand how a single
source of oxytocin can fulfil a very wide repertoire of functions effectively.
The potential conflicts are fewer than they might be. Most mammals are not sexually active during
pregnancy and lactation, (except for a post-partum estrus), so sexual activity in females will not trigger
inappropriate oxytocin secretion at these times. In lactating rats, oxytocin must serve both natriuresis and milk
let-down; this potential conflict of roles has an elegant resolution: only bursts of oxytocin secretion are effective
for milk let-down; only sustained, continuous secretion is effective for natriuresis, and the oxytocin cells can
combine both patterns of activity without compromising either (see Leng and Brown, 1997). Finally, the
behavioural roles of oxytocin can be fulfilled independently of the peripheral hormonal roles, since dendritic
oxytocin release can be regulated independently of axonal release (Sabatier et al., 2003b).
We must nevertheless ask, are these many roles of oxytocin all physiologically important, or do some
reflect vestigial remnants of the roles of ancestor peptides, or do they reflect a rather widespread and loosely
regulated expression of oxytocin receptors? Steven Jay Gould (2002) noted that species typically seem to appear
quite suddenly (in geological time) and then remain relatively stable for long periods. He argued that the
potential for sudden change must therefore be present in the ancestral species of a new species, and that
changes might occur in four targets for constructive mutations: 1), features with a physiological purpose, but
which might, given some minor change, serve a different purpose; 2), “architectural” consequences of other
features: he named these “spandrels” by analogy with the triangular vaults formed when a dome is mounted
above arches; spandrels are simple consequences of building a dome above arches, but they are a natural site for
decoration and embellishment; 3), features that have, in the course of evolution, lost an original function
without gaining a new one; and 4), features that arise by neutral random drift. Gould invites us to consider that
complex organisms evolved over millions of years accumulate a huge diversity of actual and potential
functions, and only detrimental features risk being actively eliminated by natural selection; many features that
are incidental by-products of other functions of genes, embellished by neutral mutations, will include some with
a latent functionality. In an established species or in an individual these features, preserved through lack of
strong selection against neutrality, might appear as redundancies, but are not actively selected for as “back-up”
We can recognise that many apparently novel behavioral functions of oxytocin have echoes in
evolutionary history. We can also recognise, from studies of transgenic animals, redundancy in these roles and
in the apparently well-established roles for oxytocin. Russell and Leng (1998) have previously considered the
paradox that oxytocin and its analogs appear deeply important for parturition (or egg laying) in all vertebrate
species, yet oxytocin-deficient mice appear to give birth normally. They suggested that other mechanisms might
have evolved convergent roles by a process of exclusion: “When everything that opposes the actions of
oxytocin in parturition is excluded, the things that remain are neutral, assist oxytocin or, in dogging the
footsteps of oxytocin, can substitute for it.”
While evolution might thus provide an accumulation of mechanisms that support an existing
mechanism, perhaps to the point where the original mechanism is technically redundant, equally, lack of
absolute control of gene expression might provide for scope for new function. The homogeneity of the SON has
made it an outstanding model for the understanding of neuronal function, yet almost every gene that is
expressed in the brain is expressed there (Mutsuga et al., 2004). It is not conceivable that everything is
important: to say so would be to invite despair at ever achieving a workable understanding, and to wantonly
deny the evident truth that we have, by a physiological approach that has largely neglected genetic and
mechanistic complexity, in fact achieved a very effective understanding of the oxytocin system. In the end, the
wonder of the oxytocin system is not in how complex it is, but in how it does simple things, so effectively and
so robustly, despite this complexity, not because of it.
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