Postsynaptic mechanism of depression of GABAergic synapses by oxytocin in the supraoptic nucleus of immature rat

Glutamatergic input governs periodicity and synchronization of bursting activity in oxytocin neurons in hypothalamic organotypic cultures

During suckling, oxytocin (OT) neurons display a bursting electrical activity, consisting of a brief burst of action potentials which is synchronized throughout the OT neuron population and which periodically occurs just before each milk ejection in the lactating rat. To investigate the basis of such synchronization, we performed simultaneous intracellular recordings from pairs of OT neurons identified retrospectively by intracellular fluorescent labelling and immunocytochemistry in organotypic slice cultures derived from postnatal rat hypothalamus. A spontaneous bursting activity was recorded in 65% of OT neurons; the remaining showed only a slow, irregular activity. Application of OT triggered bursts in nonbursting neurons and accelerated bursting activity in spontaneously bursting cells. These cultures included rare vasopressinergic neurons showing no bursting activity and no reaction to OT. Bursts occurred simultaneously in all pairs of bursting OT neurons but, as in vivo, there were differences in burst onset, amplitude and duration. Coordination of firing was not due to electrotonic coupling because depolarizing one neuron in a pair had no effect on the membrane potential of its partner and halothane and proprionate did not desynchronize activity. On the other hand, bursting activity was superimposed on volleys of excitatory postsynaptic potentials (EPSPs) which occurred simultaneously in pairs of neurons. EPSPs, and consequently action potentials, were reversibly blocked by the non-NMDA glutamatergic receptor antagonist CNQX. Taken together, these data, obtained from organotypic cultures, strongly suggest that a local hypothalamic network governs synchronization of bursting firing in OT neurons through synchronous afferent volleys of EPSPs originating from intrahypothalamic glutamatergic inputs.

Somatodendritic secretion in oxytocin neurons is upregulated during the female reproductive cycle

During the female reproductive cycle, hypothalamic oxytocin (OT) neurons undergo sharp changes in excitability. In lactating mammals, bursts of electrical activity of OT neurons result in the release of large amounts of OT in the bloodstream, which causes milk ejection. One hypothesis is that OT neurons regulate their own firing activity and that of nearby OT neurons by somatodendritic release of OT. In this study, we show that OT neuron activity strongly reduces inhibitory synaptic transmission to these neurons. This effect is blocked by antagonists of both adenosine and OT receptors and is mimicked by OT application. Inhibition of soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex formation by tetanus toxin completely blocked the stimulation-induced reduction in inhibitory input, as did the calcium chelator BAPTA. During lactation, the readily releasable pool of secretory vesicles in OT cell bodies was doubled, and calcium currents were upregulated. This resulted in an increased inhibition of GABAergic synaptic transmission by somatodendritic release during lactation compared with the adult virgin stage. These results demonstrate that somatodendritic release is augmented during lactation, which is a novel form of plasticity to change the strength of synaptic transmission.

Vasopressin differentially modulates non-NMDA receptors in vasopressin and oxytocin neurons in the supraoptic nucleus

Magnocellular neurons of the supraoptic nucleus release the neuropeptides oxytocin and vasopressin from their dendrites to regulate their synaptic inputs. This study aims to determine the cellular mechanism by which vasopressin modulates excitatory synaptic transmission. Presumably by electroporation through perforated patch, we were able to successfully introduce biocytin into cells in which we performed an electrophysiological study. This method enabled us to determine that roughly half of the recorded neurons were immunoreactive to oxytocin-associated neurophysin and showed two characteristic features: an inward rectification and a sustained outward rectification. The remaining half showed a linear voltage-current relationship and was immunoreactive to vasopressin-associated neurophysin. Using these electrophysiological characteristics and post hoc immunohistochemistry to identify vasopressin or oxytocin neurons, we found that vasopressin decreased evoked EPSCs in vasopressin neurons while increasing EPSCs in oxytocin neurons. In both types of neurons, EPSC decay constants were not affected, indicating that desensitization of non-NMDA receptors did not underlie the EPSC amplitude change. In vasopressin neurons, both vasopressin and a V1a receptor agonist, F-180, decreased AMPA-induced currents, an effect blocked by a V1a receptor antagonist SR49059. In oxytocin neurons, AMPA-induced currents were facilitated by vasopressin, whereas F-180 had no effect. An oxytocin receptor antagonist blocked the facilitatory effect of vasopressin. Thus, we conclude that vasopressin inhibits EPSCs in vasopressin neurons via postsynaptic V1a receptors, whereas it facilitates EPSCs in oxytocin neurons through oxytocin receptors.

Presence of functional vasopressin receptors in spinal ventral horn neurons of young rats: a morphological and electrophysiological study

The objective of the present work was double. (i) Light microscopic autoradiography was used to determine the distribution of vasopressin and oxytocin binding sites in the spinal cord of rats. (ii) Whole-cell recordings were performed in lumbar spinal cord slices in order to assess whether these receptors are functional, whether they are located pre- or postsynaptically and whether they are present in motoneurons. In newborns, vasopressin binding sites of the V1a type were present in all laminae of the central gray at all segmental levels, whereas oxytocin binding sites were found only in the superficial layers of the dorsal horn. In adults, binding sites for both neuropeptides were also present, but were less dense. The dissociation constants for vasopressin were similar in newborns and adults. Whole-cell recordings showed that in identified motoneurons vasopressin exerted a direct effect, by inducing a membrane depolarization or by generating a sustained inward current, and an indirect effect, by enhancing glycinergic and GABAergic inhibitory transmission. Vasopressin-induced facilitation of inhibitory transmission could also be demonstrated in unidentified ventral horn neurons. All these effects were mediated by V1a but not V1b receptors. In some neurons, glycinergic transmission was also facilitated by a selective oxytocin receptor agonist. Our data, together with data obtained previously in brainstem motor nuclei, suggest that vasopressin of hypothalamic origin could play a role in motricity. The neuropeptide could act as a neuromodulator, because it would not directly activate motoneurons, but rather render them more responsive to incoming excitatory inputs. Vasopressin may thus act as a regulator of muscular force.

Modulation of GABAergic transmission by endogenous glutamate in the rat supraoptic nucleus

The presence of group III metabotropic glutamate receptors on GABAergic terminals in the supraoptic nucleus suggests that the level of glutamate in the extracellular space may regulate synaptic strength at inhibitory synapses. To test this hypothesis we examined the consequences of increasing ambient glutamate on GABA-mediated synaptic activity in supraoptic neurons. The concentration of the excitatory amino acid in the extracellular space was increased pharmacologically by blocking glutamate transporters. Inhibition of the astrocyte-specific GLT-1 glutamate transporter led to a reversible decrease in evoked inhibitory postsynaptic current amplitude. This modulation had a presynaptic origin as revealed by analysis of paired-pulse ratio and miniature inhibitory currents. Furthermore, blocking group III metabotropic glutamate receptors with the specific antagonist MAP4 prevented the depression of GABAergic transmission induced by glutamate transporter blockade. Thus, presynaptic metabotropic glutamate receptors located on inhibitory terminals in the supraoptic nucleus appear to sense changes in ambient glutamate and modify GABA release accordingly. However, it seems that such changes need to reach a certain magnitude because the discrete deficit in glutamate clearance which occurs in the supraoptic nucleus of lactating rats is not sufficient to modulate GABA-mediated transmission. These results suggest that ambient glutamate contributes to the modulation of synaptic efficacy not only at glutamatergic synapses but also at inhibitory GABAergic synapses.

Somatodendritic secretion in oxytocin neurons is upregulated during the female reproductive cycle

During the female reproductive cycle, hypothalamic oxytocin (OT) neurons undergo sharp changes in excitability. In lactating mammals, bursts of electrical activity of OT neurons result in the release of large amounts of OT in the bloodstream, which causes milk ejection. One hypothesis is that OT neurons regulate their own firing activity and that of nearby OT neurons by somatodendritic release of OT. In this study, we show that OT neuron activity strongly reduces inhibitory synaptic transmission to these neurons. This effect is blocked by antagonists of both adenosine and OT receptors and is mimicked by OT application. Inhibition of soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex formation by tetanus toxin completely blocked the stimulation-induced reduction in inhibitory input, as did the calcium chelator BAPTA. During lactation, the readily releasable pool of secretory vesicles in OT cell bodies was doubled, and calcium currents were upregulated. This resulted in an increased inhibition of GABAergic synaptic transmission by somatodendritic release during lactation compared with the adult virgin stage. These results demonstrate that somatodendritic release is augmented during lactation, which is a novel form of plasticity to change the strength of synaptic transmission.

Neurosteroid regulation of oxytocin and vasopressin release from the rat supraoptic nucleus

In adult rats somato-dendritic release of oxytocin and vasopressin from magnocellular neurones in the supraoptic nucleus of the hypothalamus has important autoregulatory actions on the neuronal electrical activity, and in neonatal rats it plays a role in the development of dendritic arborisation. In the adult, oxytocin effects are modulated by allopregnanolone via an interaction with inhibitory GABAA receptors. This study examined the effects of allopregnanolone, progesterone and 17beta-oestradiol on oxytocin and vasopressin release from intact isolated supraoptic nuclei and from the neurophypophyses in rats of differing ages. In supraoptic nuclei from rats of 3-4 weeks old or less, all three neurosteroids induced oxytocin release from the isolated supraoptic nucleus, but only allopregnanolone induced significant release of vasopressin. Surprisingly, in these very young rats, allopregnanolone-induced oxytocin release was inhibited by GABAA receptor antagonists as well as by an oxytocin receptor antagonist. By contrast, in supraoptic nuclei from adult rats allopregnanolone-induced oxytocin release was much smaller, and was enhanced in the presence of bicuculline. The GABAA receptor agonist muscimol also induced oxytocin release from supraoptic nuclei in young rats, but had no effect in adult rats. Oxytocin cells isolated from young rats showed an increase in [Ca2+]i in response to both allopregnanolone and muscimol. Allopregnanolone had no effect on [Ca2+]i or on the release of oxytocin or vasopressin from neurohypophysial axon terminals in either young or old rats. We conclude that, in very young rats, (i) neurosteroids induce oxytocin release from the supraoptic nucleus by a mechanism that partly depends on the presence of GABA, which in young rats is depolarising to oxytocin cells, and which also partly depends upon endogenous oxytocin, and (ii) the effect of allopregnanolone upon oxytocin release changes with age, as the functional activity of GABAA receptors changes from excitation to inhibition of oxytocin cells.

Oxytocin regulates neurosteroid modulation of GABA(A) receptors in supraoptic nucleus around parturition

In this study, we investigate how neurosteroid sensitivity of GABA(A) receptors (GABA(A)Rs) is regulated. We examined this issue in neurons of the supraoptic nucleus (SON) of the rat and found that, during parturition, the GABA(A)Rs become insensitive to the neurosteroid allopregnanolone attributable to a shift in the balance between the activities of endogenous Ser/Thr phosphatase and PKC. In particular, a constitutive endogenous tone of oxytocin within the SON after parturition suppressed neurosteroid sensitivity of GABA(A)Rs via activation of PKC. Vice versa before parturition, during late pregnancy, application of exogenous oxytocin brings the GABA(A)Rs from a neurosteroid-sensitive mode toward a condition in which the receptors are not sensitive. This indicates that there may be an inverse causal relationship between the extent to which the GABA(A)R or one of its interacting proteins is phosphorylated and the neurosteroid sensitivity of the GABA(A)R. Neurosteroid sensitivity was not affected by changes in subunit composition of GABA(A)Rs known to occur concurrently in these cells.

Oxytocin regulates neurosteroid modulation of GABA(A) receptors in supraoptic nucleus around parturition

In this study, we investigate how neurosteroid sensitivity of GABA(A) receptors (GABA(A)Rs) is regulated. We examined this issue in neurons of the supraoptic nucleus (SON) of the rat and found that, during parturition, the GABA(A)Rs become insensitive to the neurosteroid allopregnanolone attributable to a shift in the balance between the activities of endogenous Ser/Thr phosphatase and PKC. In particular, a constitutive endogenous tone of oxytocin within the SON after parturition suppressed neurosteroid sensitivity of GABA(A)Rs via activation of PKC. Vice versa before parturition, during late pregnancy, application of exogenous oxytocin brings the GABA(A)Rs from a neurosteroid-sensitive mode toward a condition in which the receptors are not sensitive. This indicates that there may be an inverse causal relationship between the extent to which the GABA(A)R or one of its interacting proteins is phosphorylated and the neurosteroid sensitivity of the GABA(A)R. Neurosteroid sensitivity was not affected by changes in subunit composition of GABA(A)Rs known to occur concurrently in these cells.

Vasopressin and oxytocin decrease excitatory amino acid release in adult rat supraoptic nucleus

Oxytocin and vasopressin reduce the amplitude of excitatory postsynaptic responses in magnocellular neuroendocrine cells of the supraoptic nucleus (SON). To test whether synaptic glutamate release is modulated by these neuropeptides, we examined the combined effect of vasopressin and oxytocin on depolarization-induced glutamate and aspartate release from acutely dissected rat SON or fronto-parietal cortex punches. Glutamate release was stimulated with 60 mm K+ for 5-10 min and measured using ion exchange chromatography or high-performance liquid chromatography. During depolarization with high K+, extracellular glutamate levels increased, on average, to 204% of control values. In the presence of vasopressin/oxytocin, K+-stimulated glutamate and aspartate release were significantly reduced by 34% and 62%, respectively, in the SON. Treatment with the aminopeptidase inhibitor amastatin did not mimic the effects of exogenous vasopressin/oxytocin on glutamate or aspartate release, suggesting that, under the conditions tested here, amastatin treatment may produce more complex effects. The effects of exogenous neuropeptides are likely mediated by oxytocin and/or vasopressin receptors, as the oxytocin- and V1a-receptor antagonist, Manning Compound (10-100 micro m), partially reversed the effects of vasopressin/oxytocin on SON glutamate release. In contrast, in cortical punches, glutamate release was enhanced by high K+, but vasopressin/oxytocin did not significantly reduce glutamate/aspartate release, consistent with the relatively sparse distribution of vasopressin/oxytocin receptors in fronto-parietal cortex. These findings suggest that locally released oxytocin and vasopressin may autoregulate SON magnocellular neuroendocrine cell activity in part by modulating the release of excitatory amino acids from afferent terminals targeting these cells and/or from other cellular sources.

Short-term modulation of GABAA receptor function in the adult female rat

Oxytocin neurons in the supraoptic nucleus (SON) exhibit marked neuronal plasticity during each reproductive cycle. We have previously shown that this neuronal plasticity includes GABAA receptor subunit switching around the time of parturition. Here we focus on addition plasticity in short-term regulatory mechanisms of postsynaptic receptor function before and after parturition, i.e. alterations in metabotropic and allosteric modulation of GABAA receptor activity. Both short- and long-term regulation of the GABAA receptor function affects the electrical behaviour of the oxytocin neurons (Brussaard and Herbison, 2000); however, their causal linkage until recently remained unclear. Non-genomic gonadal steroid feedback to oxytocin neurons is mediated via the neurosteroid allopregnanolone (3 alpha-OH-DHP) that is an allosteric modulator of postsynaptic GABAA receptors. We recently found evidence to support the idea that (1) neurosteroids not only potentiate GABAA receptor function but also prevent its suppression by PKC (Brussaard et al., 2000), and (2) that neurosteroid sensitivity of GABAA receptor is not regulated by subunit switching, but instead, is dependent on the balance between endogenous phosphatase and PKC activity (Koksma et al., 2002). Thus, before pregnancy, the GABAA receptors are sensitive to 3 alpha-OH-DHP, due to a constitutively high level of phosphatase activity. At parturition, endogenous release of oxytocin within the SON shifts the intracellular balance towards a higher level of phosphorylation, leading to 3 alpha-OH-DHP insensitivity of the GABAA receptors. Here we discuss the putative mechanisms underlying these changes in receptor physiology, their causal relations and the functional significance for the hormonal output.

The active role of dendrites in the regulation of magnocellular neurosecretory cell behavior

The interactions of the dendritically released neuropeptides vasopressin and oxytocin with co-released neuroactive substances such as opioids and nitric oxide are reviewed. Endogenous opioids regulate magnocellular neurons at the level of the supraoptic nucleus and the relationship of dendritically released peptides and co-released opioids seems to be dependent on the stimulus given and the physiological state of the animal. Nitric oxide has a prominent inhibitory action on supraoptic neurons and these actions are predominantly mediated indirectly by GABA inputs. The role of these co-released neuroactive substances in differentially regulated release of neuropeptides from dendrites versus distant axon terminals has to be determined in more detail. A picture emerges in which release of vasopressin and oxytocin from different anatomical compartments of a single neuron may arise from different intracellular secretory pools and their preparation before release.

Modulation of synaptic transmission by oxytocin and vasopressin in the supraoptic nucleus

It is now generally accepted that magnocellular neurons of the supraoptic and paraventricular nuclei release the neuropeptides oxytocin and vasopressin from their dendrites. Peptide release from their axon terminals in the posterior pituitary and dendrites differ in dynamics suggesting that they may be independently regulated. The dendritic release of peptide within the supraoptic nucleus (SON) is an important part of its physiological function since the local peptides can regulate the electrical activity of magnocellular neurons (MCNs) which possess receptors for these peptides. This direct postsynaptic action would affect the output of peptide in the neurohypophysis. Another way that these peptides can regulate MCN activity would be to modulate afferent inputs unto themselves. Although the influence of afferent inputs (inhibitory and excitatory) on SON magnocellular neuron physiology has been extensively described in the last decade, a role for these locally released peptides on synaptic physiology of this nucleus has been difficult to show until recently, partly because of the difficulty of performing stable synaptic recordings from these cells in suitable preparations that permit extensive examination. We recently showed that under appropriate conditions, oxytocin acts as a retrograde transmitter in the SON. Oxytocin, released from the dendrites of MCNs, decreased evoked excitatory synaptic transmission by inhibiting glutamate release from the presynaptic terminals. It modulated voltage-dependent calcium channels, mainly N-type and to a lesser extent P/Q-type channels, located on glutamatergic terminals. Although evidence is less conclusive, it is possible that vasopressin has similar actions to reduce excitatory transmission. This synaptic depressant effect of oxytocin and/or vasopressin, released from dendrites, would ensure that MCNs regulate afferent input unto themselves using their own firing rate as a gauge. Alternatively, it may only be a subset of afferent terminals that are sensitive to these peptides, thereby providing a means for the MCNs to selectively filter their afferent inputs. Indeed its specificity is partly proven by our observation that oxytocin does not affect spontaneous glutamate release, or GABA release from inhibitory terminals (Brussaard et al., 1996). Thus, the dendrites of MCNs of the supraoptic nucleus serve a dual role as both recipients of afferent input and regulators of the magnitude of afferent input, allowing them to directly participate in the shaping of their output. This adds to a rapidly growing body of evidence in support of the concept of a two-way communication between presynaptic terminals and postsynaptic dendrites, and shows the potential of this nucleus as a model to study such form of synaptic transmission.

Cholinergic modulation of nucleus accumbens medium spiny neurons

The rat nucleus accumbens contains acetylcholine-releasing interneurons, presumed to play a regulatory role in the electrical activity of medium spiny output neurons. In order to examine this issue in detail, we made electrophysiological recordings in rat nucleus accumbens slices. These experiments showed that gamma-aminobutyric acid-mediated inhibition of the output neurons might be facilitated by activation of nicotinic acetylcholine receptors, in addition to being suppressed via activation of muscarinic acetylcholine receptors. In contrast, glutamatergic excitation of output neurons appeared to be inhibited by activation of muscarinic acetylcholine receptors and to be insensitive to activation of nicotinic acetylcholine receptors. The spontaneous firing frequency of cholinergic neurons appeared to be under control of both a muscarinic and a nicotinic pathway in a bi-directional manner. Finally, we made paired recordings in which the functional connection between cholinergic neurons and output neurons was monitored. Driving the cholinergic neurons at physiological firing frequencies stimulated gamma-aminobutyric acid-mediated inhibition of the output neurons, via activation of nicotinic acetylcholine receptors. The onset of this effect was slow and lacked a fixed delay. These data indicate that activation of nicotinic acetylcholine receptors in rat nucleus accumbens may mediate the facilitation of gamma-aminobutyric acid-mediated inhibition of medium spiny output neurons. Possible mechanisms of neurotransmission, mediating this cholinergic modulation are discussed.

Neonatal development of the rat visual cortex: synaptic function of GABAA receptor alpha subunits

Each GABA(A) receptor consists of two alpha and three other subunits. The spatial and temporal distribution of different alpha subunit isomeres expressed by the CNS is highly regulated. Here we study changes in functional contribution of different alpha subunits during neonatal development in rat visual cortex. First, we characterized postsynaptic alpha subunit expression in layer II-III neurons, using subunit-specific pharmacology combined with electrophysiological recordings in acutely prepared brain slices. This showed clear developmental downregulation of the effects of bretazenil (1 microm) and marked upregulation of the effect of 100 nM of zolpidem on the decay of spontaneous inhibitory postsynaptic currents (sIPSCs). Given the concentrations used we interpret this as downregulation of the synaptic alpha3 and upregulation of alpha1 subunit. Furthermore, the effect of furosemide, being indicative of the functional contribution of alpha4, was increased between postnatal days 6 and 21. Our second aim was to study the effects of plasticity in alpha subunit expression on decay properties of GABAergic IPSCs. We found that bretazenil-sensitive IPSCs have the longest decay time constant in juvenile neurons. In mature neurons, zolpidem- and furosemide-sensitive IPSCs have relatively fast decay kinetics, whereas bretazenil-sensitive IPSCs decay relatively slowly. Analysis of alpha1 deficient mice and alpha1 antisense oligonucleotide deletion in rat explants showed similar results to those obtained by zolpidem application. Thus, distinct alpha subunit contributions create heterogeneity in developmental acceleration of IPSC decay in neocortex.

Plasticity in the electrophysiological properties of oxytocin neurons

In mammals, the neurohypophysial hormone oxytocin (OT) is released into the bloodstream during labor and lactation to promote uterine contraction and milk ejection, respectively. Electrophysiological studies have established that OT neurons fire in brief, synchronized bursts during this release. During pregnancy and lactation, the intrinsic membrane and synaptic properties of OT, and to a lesser extent vasopressin (VP) neurons, are altered as a part of the adaptation to these specialized states. During lactation OT neurons specifically exhibit an enhanced rebound depolarization which could assist in instigating bursts and an increased gating of firing frequency which is correlated with an enhanced Ca(2+)-dependent after hyperpolarization. Spike broadening occurs in both VP and OT neurons, but in OT neurons this and other changes are present during late pregnancy, suggesting involvement of steroidal hormones in programming neuronal adaptations. Excitatory and inhibitory synaptic activity also are altered by reproductive state. There is a doubling of glutamatergic activity specific to OT neurons which is consistent with an increase in terminal numbers, but this is accompanied by an increase in paired-pulse facilitation, suggesting an increase in the probability of glutamate release during lactation as well. Together with profound changes in both pre- and postsynaptic GABAergic synaptic activity, these data suggest that neurosecretory, and particularly OT neuronal, properties are state-dependent. These modifications may adjust the responsiveness of these neurons to afferent stimulation during periods of increased hormone demand and thereby enhance stimulus-secretion coupling.

Involvement of postsynaptic EP4 and presynaptic EP3 receptors in actions of prostaglandin E2 in rat supraoptic neurones

We have reported that supraoptic nucleus (SON) neurones are excited by prostaglandin E2 (PGE2) presumably via dual postsynaptic PG receptors, FP receptors and unidentified EP receptors, and that presynaptic EP receptors may also be involved in the excitation. In the present study, to clarify the receptor mechanism of the PGE2-mediated actions on SON neurones, we studied the pre- and postsynaptic effects of four newly developed EP agonists that are selective for each of the four EP receptors, EP1-4, on rat SON neurones using extracellular recording and whole-cell patch-clamp techniques. The EP4 agonist ONO-AE1-329 mimicked the excitatory effects of PGE2, whereas the EP1 agonist ONO-DI-004, the EP2 agonist ONO-AE1-257 and the EP3 agonist ONO-AE-248 had little or no effect. The effects of ONO-AE1-329 were unaffected by the EP1/FP/TP antagonist, ONO-NT-012, which potently suppressed the excitation caused by the FP agonist fluprostenol and PGE2. ONO-AE1-329 caused marked excitation when responses to fluprostenol were desensitized by repeated applications of fluprostenol. Patch-clamp analysis in SON neurones showed that ONO-AE1-329 induced inward currents at a holding potential of -70 mV and the reversal potential of the currents was -35.1 +/- 2.3 mV. On the other hand, the frequency of spontaneous inhibitory postsynaptic currents recorded from SON slice preparations was suppressed by ONO-AE-248, but unaffected by the other three EP agonists. These results suggest that SON neurones possess postsynaptic EP4 receptors and that gamma-aminobutyric acid neurones innervating SON neurones possess presynaptic EP3 receptors in their terminals. Activation of the two EP receptors may be involved in the excitatory regulation of SON neurones by PGE2.

Vasopressin differentially modulates noradrenaline release in the rat supraoptic nucleus

Vasopressin is released not only from axon terminals in the neurohypophysis but also from soma/dendrite regions in the supraoptic nucleus. In order to investigate presynaptic roles of dendritically released vasopressin, we examined effects of local application of vasopressin upon noradrenaline release within the supraoptic nucleus by a microdialysis method. Noradrenaline release within the supraoptic nucleus was facilitated by local perfusion with high K+ or an NMDA receptor antagonist. Vasopressin augmented noradrenaline increase after high K+ but reduced it after an NMDA receptor antagonist, AP-5. The results suggest that dendritically released vasopressin modulates noradrenaline release within the supraoptic nucleus in a bimodal fashion.

Neurotransmitter/neuropeptide interactions in the regulation of neurohypophyseal hormone release

Regulation of neurohypophyseal hormone release reflects the convergence of a large number of afferent pathways on the vasopressin (VP)- and oxytocin-producing neurons. These pathways utilize a broad range of neurotransmitters and neuropeptides. In this review, the mechanisms by which this information is coordinated into appropriate physiological responses is discussed with a focus on the responses to agents that are coreleased from A1 catecholamine nerve terminals in the supraoptic nucleus. The A1 pathway transmits hemodynamic information to the vasopressin neurons by releasing several neuroactive agents including ATP, norepinephrine, neuropeptide Y, and substance P. These substances stimulate VP release from explants of the hypothalamo-neurohypophyseal system and certain combinations of these agents elicit potent but selective synergism. Evaluation of the signal cascades elicited by these agents provides insights into mechanisms underlying these synergistic interactions and suggests mechanisms responsible for coordinated responses of the VP neurons to activation of a range of ion-gated ion channel and G-protein-coupled receptors.

Vasopressin- and oxytocin-induced activity in the central nervous system: electrophysiological studies using in-vitro systems

During the last two decades, it has become apparent that vasopressin and oxytocin, in addition to playing a role as peptide hormones, also act as neurotransmitters/neuromodulators. A number of arguments support this notion: (i) vasopressin and oxytocin are synthesized not only in hypothalamo-neurohypophysial cells, but also in other hypothalamic and extrahypothalamic cell bodies, whose axon projects to the limbic system, the brainstem and the spinal cord. (ii) Vasopressin and oxytocin can be shed from central axons as are classical neurotransmitters. (iii) Specific binding sites, i.e. membrane receptors having high affinity for vasopressin and oxytocin are present in the central nervous system. (iv) Vasopressin and oxytocin can alter the firing rate of selected neuronal populations. (v) In-situ injection of vasopressin and oxytocin receptor agonists and antagonists can interfere with behavior or physiological regulations. Morphological studies and electrophysiological recordings have evidenced a close anatomical correlation between the presence of vasopressin and oxytocin receptors in the brain and the neuronal responsiveness to vasopressin or oxytocin. These compounds have been found to affect membrane excitability in neurons located in the limbic system, hypothalamus, circumventricular organs, brainstem, and spinal cord. Sharp electrode intracellular recordings and whole-cell recordings, done in brainstem motoneurons or in spinal cord neurons, have revealed that vasopressin and oxytocin can directly affect neuronal excitability by opening non-specific cationic channels or by closing K(+) channels. These neuropeptides can also influence synaptic transmission, by acting either postsynaptically or upon presynaptic target neurons or axon terminals. Whereas, in cultured neurons, vasopressin and oxytocin appear to mobilize intracellular Ca(++), in brainstem slices, the action of oxytocin is mediated by a second messenger that is distinct from the second messenger activated in peripheral target cells. In this review, we will summarize studies carried out at the cellular level, i.e. we will concentrate on in-vitro approaches. Vasopressin and oxytocin will be treated together. Though acting via distinct receptors in distinct brain areas, these two neuropeptides appear to exert similar effects upon neuronal excitability.

The oxytocin receptor system: structure, function, and regulation

The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.

Oxytocin receptor agonists enhance inhibitory synaptic transmission in the rat hippocampus by activating interneurons in stratum pyramidale

Oxytocin probably plays a role as a neurotransmitter/neuromodulator in the hippocampus of the rat. Oxytocin binding sites are present in the subiculum and CA1 region and oxytocin can excite a class of CA1 nonpyramidal neurons. In the present work we characterized the effect of oxytocin on hippocampal synaptic transmission. Whole-cell recordings were obtained from pyramidal neurons, in conditions of nearly symmetrical chloride concentrations. The selective oxytocin receptor agonist, [Thr4,Gly7]-oxytocin (TGOT), caused an increase in the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) in virtually all neurons. These peptide-enhanced IPSCs were blocked by bicuculline, but not by strychnine, and reversed near 0 mV, indicating that they were mediated by gamma-aminobutyric acid (GABA)A receptors. On average, TGOT caused a nearly threefold increase in the frequency and almost a doubling in the amplitude of spontaneous IPSCs. TGOT did not influence the frequency and the amplitude of miniature IPSCs or spontaneous excitatory postsynaptic currents (EPSCs), and had no effect on evoked IPSCs. The peptide did not affect the basic membrane properties of pyramidal neurons or their GABA sensitivity. Thus, TGOT facilitated inhibitory transmission by exerting an excitatory action on the soma and/or dendrites of GABAergic interneurons. Extracellular recordings were performed in interneurons located in various hippocampal strata. Their sensitivity to TGOT was compared to that of substance P (SP). Interneurons in stratum pyramidale were excited both by TGOT and by SP. By contrast, stratum radiatum interneurons responded to SP but not to TGOT. In stratum oriens, half of the interneurons responded to SP, but only a minority to TGOT. Thus, oxytocin-responsive interneurons appear to be preferentially located in close vicinity of pyramidal neurons.

Permissive effect of centrally administered oxytocin on the excitatory response of oxytocin neurones to ventral tegmental stimulation in the suckled lactating rat

The mesencephalic ventral tegmentum has been implicated in the milk-ejection reflex and modulation of inputs from this region could provide a mechanism whereby central oxytocin facilitates synchronous bursting of oxytocin neurones during suckling. Experiments were therefore undertaken to investigate the effect of intracerebroventricular (i.c.v.) oxytocin on the response of oxytocin neurones to ventral tegmental stimulation. Oxytocin neurones were recorded in the supraoptic nucleus of urethane-anaesthetized lactating rats during suckling, and their response to single shock stimulation of the ventral tegmentum was monitored using peri-stimulus time-interval histograms. Before i.c.v. oxytocin, oxytocin neurones were either unresponsive to ventral tegmental stimulation, or displayed a small inhibition. However, after administration of oxytocin (2.2 ng i.c.v.), seven out of eight neurones tested displayed a pronounced excitatory response (onset latency 78.4 +/- 4.8 ms, duration 73.4 +/- 8.3 ms). This permissive effect on the excitatory response was only observed in the presence of suckling, and followed the same time course as facilitation of the milk-ejection reflex, being maximal immediately before each facilitated bursting response in the oxytocin neurones. The response to ventral tegmental stimulation remained unaltered after intraperitoneal administration of hypertonic saline to cause a generalized increase in the excitability of the oxytocin neurones. Moreover, i.c.v. oxytocin had no effect on the response of oxytocin neurones to stimulation of a descending input from the medial septum. In conclusion, administration of i.c.v. oxytocin has a selective permissive effect on the excitation of oxytocin neurones from the ventral tegmentum, and this supports previous in vitro studies suggesting that centrally released oxytocin may act as a modulator of afferent transmission to the magnocellular nuclei. This effect on the afferent excitation of oxytocin neurones may provide a mechanism whereby i.c.v. oxytocin facilitates suckling-evoked bursting activity.

Oxytocin depresses spontaneous gamma-aminobutyric acid-ergic inhibitory postsynaptic currents in cultured mitral cells of the rat olfactory bulb by a presynaptic mechanism

Oxytocin (OT) modulation of synaptic transmission between olfactory bulb neurones has been implicated in the induction of maternal behaviour, but the mechanism of action is unknown. We examined the action of OT on gamma-aminobutyric acid(A) (GABA(A)) receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) in cultured mitral/tufted (M/T) cells with the use of whole-cell patch-clamp recordings. OT reversibly reduced the frequency of sIPSCs without affecting the amplitudes. The effect of OT on sIPSCs was mimicked by the OT receptor agonist [Thr(4), Gly(7)]-OT in a reversible manner and blocked by the OT receptor antagonist desGly-NH(2)(9), d(CH(2))(5)-[Tyr(Me)(2), Thr(4)]-ornithine-vasotocin. OT has no effect, however, on the membrane currents evoked by exogenous application of GABA. These results demonstrate that OT depresses GABA(A) receptor-mediated sIPSCs in M/T cells by a presynaptic mechanism.

The variability of basal action potential firing is positively correlated with bursting in hypothalamic oxytocin neurones

Magnocellular oxytocin neurones are proposed as a suitable system for studying the mechanisms involved in the regulation of neuronal bursting activity. They display high frequency (50 sp./s) bursts of spikes (approximately every 300 s), in response to specific stimuli, which are superimposed on a variable level of basal activity and are tightly co-ordinated as a result of network interactions. The relationship between the strength of the bursting activity (as quantified by burst amplitude and interburst interval) and the characteristics of the interburst basal activity were assessed. During control conditions, mean basal activity and variability of firing increased just before bursts. During experimental conditions leading to burst facilitation, burst amplitude increased and interburst interval decreased while a sustained increase in mean firing rate occurred. Variability of firing (measured by both the standard deviation of firing rate, and the index of dispersion which corrected this standard deviation for differences in mean firing rate), increased demonstrating an increase in spike clustering greater than expected as a result of increased basal activity. When bursting was restrained (i.e. interburst interval increased), mean basal activity increased substantially, but index of dispersion decreased. A narrowing of the interspike interval distribution occurred, indicating increased regularity of firing. The aspect of basal activity most strongly correlated with bursting was variability of firing rate. The strongest correlate of burst amplitude was the standard deviation of mean firing rate, whereas the strongest and most consistent correlate of interburst interval was the index of dispersion. In conclusion, bursting behaviour is most strongly related to the irregularity rather than the level of basal activity.

Vasopressin regulation of noradrenaline release within the supraoptic nucleus

The effect of electrically evoked dendritic vasopressin release on noradrenaline release into the hypothalamic supraoptic nucleus was assessed by in vivo microdialysis in conjunction with high pressure liquid chromatography and electrochemical detection. Electrical activation of magnocellular supraoptic neurones by stimulation of their axons at the level of the neural lobe significantly increased noradrenaline release into the nucleus (2.5-fold, P<0.03). This increase was completely blocked by administration of a nonpeptide vasopressin V1a receptor antagonist via the microdialysis probe. These data suggest that dendritically released vasopressin facilitates noradrenaline release into the hypothalamic nucleus.

Long-term plasticity of postsynaptic GABAA-receptor function in the adult brain: insights from the oxytocin neurone

The subunit switching of ligand-gated receptors is a potentially important mechanism through which synaptic plasticity can be achieved in the nervous system. Although established in an activity-dependent manner for neurotransmission that is mediated by excitatory amino acids, there is much less direct evidence for a role of subunit switching in long-term plasticity of GABAA receptors in the adult. We argue that the hypothalamic oxytocin neurones, which exhibit marked plasticity through each reproductive cycle, provide an excellent model of both presynaptic and postsynaptic long-term plasticity of GABA-mediated transmission in the mature nervous system. The postsynaptic plasticity involves GABAA-receptor-subunit switching in an activity-independent manner. It also has profound effects on the electrical behaviour of the oxytocin neurones and, thus, the neural control of pregnancy and lactation.

Vasopressin preferentially depresses excitatory over inhibitory synaptic transmission in the rat supraoptic nucleus in vitro

Endogenous arginine-vasopressin (AVP) in the supraoptic nucleus is known to decrease the firing rate of some supraoptic nucleus neurones. To determine a possible mechanism by which this locally released AVP produces this change in neuronal excitability, we investigated the effects of AVP on evoked excitatory (e.p.s.c.) and inhibitory post-synaptic (i.p.s.c.) responses recorded in magnocellular neurones in a hypothalamic slice preparation, using the perforated-patch recording technique. Our data show that AVP produces a dose-dependent decrease in the evoked e.p.s.c. in about 80% of magnocellular neurones tested with an estimated EC50 of about 0.9 microM. The maximum decrease in e.p.s.c. amplitude was about 31% of control and was obtained with an AVP concentration of 2 microM. The AVP-induced synaptic depression was blocked by Manning Compound (MC), a non-selective antagonist of oxytocin (OXT) and vasopressin (AVP) receptors, but not by a selective OXT receptor antagonist. It was not mimicked by desmopressin (ddAVP), a V2-receptor subtype agonist. By contrast, AVP used at the same concentration (2 microM), had no global effect on pharmacologically isolated i.p.s.c.s in the majority of magnocellular neurones tested. These results show that AVP acts in the supraoptic nucleus to reduce excitatory synaptic transmission to magnocellular neurones by activating a non-OXT receptor, presumably the V1 receptor subtype.

Progesterone-metabolite prevents protein kinase C-dependent modulation of gamma-aminobutyric acid type A receptors in oxytocin neurons

Gonadal steroid feedback to oxytocin neurons during pregnancy is in part mediated via the neurosteroid allopregnanolone (3alpha-OH-DHP), acting as allosteric modulator of postsynaptic gamma-aminobutyric acid type A (GABA(A)) receptors. We describe here a form of nongenomic progesterone signaling by showing that 3alpha-OH-DHP not only potentiates GABA(A) receptor-channel activity but also prevents its modulation by protein kinase C (PKC). Application of oxytocin or stimulation of PKC suppressed the postsynaptic GABA responses of oxytocin neurons in the absence, but not in the presence of 3alpha-OH-DHP. This finding was true at the juvenile stage and during late pregnancy, when the GABA(A) receptor is sensitive to 3alpha-OH-DHP. In contrast, after parturition, when the GABA(A) receptors expressed by oxytocin neurons are less sensitive to 3alpha-OH-DHP, this neurosteroid no longer counteracts PKC. The change in GABA(A)-receptor responsiveness to 3alpha-OH-DHP helps to explain the onset of firing activity and thus the induction of oxytocin release at parturition.

Progesterone-metabolite prevents protein kinase C-dependent modulation of gamma-aminobutyric acid type A receptors in oxytocin neurons

Gonadal steroid feedback to oxytocin neurons during pregnancy is in part mediated via the neurosteroid allopregnanolone (3alpha-OH-DHP), acting as allosteric modulator of postsynaptic gamma-aminobutyric acid type A (GABA(A)) receptors. We describe here a form of nongenomic progesterone signaling by showing that 3alpha-OH-DHP not only potentiates GABA(A) receptor-channel activity but also prevents its modulation by protein kinase C (PKC). Application of oxytocin or stimulation of PKC suppressed the postsynaptic GABA responses of oxytocin neurons in the absence, but not in the presence of 3alpha-OH-DHP. This finding was true at the juvenile stage and during late pregnancy, when the GABA(A) receptor is sensitive to 3alpha-OH-DHP. In contrast, after parturition, when the GABA(A) receptors expressed by oxytocin neurons are less sensitive to 3alpha-OH-DHP, this neurosteroid no longer counteracts PKC. The change in GABA(A)-receptor responsiveness to 3alpha-OH-DHP helps to explain the onset of firing activity and thus the induction of oxytocin release at parturition.

Brain oxytocin: differential inhibition of neuroendocrine stress responses and anxiety-related behaviour in virgin, pregnant and lactating rats

The involvement of brain oxytocin in the attenuated responsiveness of the hypothalamo-pituitary-adrenal axis and the oxytocin systems to external stressors found in pregnant and lactating rats has been studied, including both neuroendocrine and behavioural aspects. Intracerebroventricular infusion of an oxytocin receptor antagonist (0.75 microg/5 microl), but not of vehicle, elevated basal corticotropin and corticosterone secretion into blood of virgin female, but not of late pregnant or lactating rats. Oxytocin antagonist treatment further elevated the stress-induced (exposure to the elevated plus-maze or forced swimming) secretion of both corticotropin and corticosterone, but only in virgin and not in pregnant or lactating rats. Thus, corticotropin and corticosterone plasma concentrations remained attenuated in antagonist-treated pregnant and lactating animals. In contrast, infusion of the oxytocin antagonist significantly elevated the stress-induced secretion of oxytocin into blood in pregnant and lactating, but not in virgin, animals, indicating an autoinhibitory influence of intracerebral oxytocin on neurohypophysial oxytocin secretion induced by non-reproduction-related stimuli. Treatment with oxytocin antagonist 10 min prior to behavioural testing on the elevated plus-maze significantly reduced the anxiety-related behaviour in both pregnant and lactating rats, without exerting similar effects in virgin female rats. The results demonstrate a tonic inhibitory effect of endogenous oxytocin on corticotropin and, consequently, corticosterone secretion in virgin female rats, an effect which is absent in the peripartum period. In contrast, an anxiolytic action of endogenous oxytocin was detectable exclusively in pregnant and lactating rats. Therefore, we conclude that the actions of intracerebral oxytocin include independent effects on the responses of the hypothalamo-pituitary-adrenal axis and oxytocin systems to stressors and the anxiety-related behaviour which are modulated by the reproductive state of the animals.

Dendritic secretion of peptides from hypothalamic magnocellular neurosecretory neurones: a local dynamic control system and its functions

The role of the dendrites of magnocellular neurones in the release of neurosecretory peptides and the synthesis of many proteins locally is reviewed. Oxytocin and vasopressin contained in dense-cored neurosecretory vesicles are released from magnocellular dendrites not only by excitatory transmitters such as glutamate acting through well-established receptors, but also by a rapid action of oestradiol acting by a mechanism which appears to involve NMDA receptors. Magnocellular dendrites also contain substantial amounts of the synthetic machinery which could synthesise proteins for local use. The presence in dendrites of polysomes and of mRNAs encoding microtubule-associated protein 2, calcium calmodulin kinase II, alpha-synapsin-associated protein, and components of the GABA(A) and NMDA receptors strongly suggests that these proteins can be translated in the dendrites, close to the sites at which they function. Mechanism(s) which control the translation of these dendritic mRNAs and the insertion into the dendritic membranes of proteins translated by dendritic ribosomes remain to be determined. However, an overall picture emerges of magnocellular dendrites as active secretory and synthetic components of the neurosecretory neurones.

Neurohypophysial peptides as retrograde transmitters in the supraoptic nucleus of the rat

A possible role for vasopressin and oxytocin in the physiology of the supraoptic nucleus was investigated using nystatin-perforated patch recording in acute brain slices from the rat containing the supraoptic nucleus. We observed that exogenously applied oxytocin reduced glutamate-mediated synaptic transmission by acting at a presynaptic oxytocin receptor. Endogenous oxytocin, released either by afferent excitation (tetanus) or by postsynaptic depolarization of the recorded magnocellular neurone caused a similar reduction of excitatory input and this could be blocked with an oxytocin antagonist. Thus endogenous oxytocin, released from magnocellular dendrites, acts as a retrograde transmitter to reduce afferent excitation. We discuss the possible significance of these results in terms of the physiological role of oxytocin in the intact animal and suggest possible avenues for further experimentation.

Vasopressin increases GABAergic inhibition of rat hypothalamic paraventricular nucleus neurons in vitro

This investigation used an in vitro hypothalamic brain slice preparation and whole cell and perforated-patch recording to examine the response of magnocellular neurons in hypothalamic paraventricular nucleus (PVN) to bath applications of vasopressin (VP; 100-500 nM). In 22/38 cells, responses were characterized by an increase in the frequency of bicuculline-sensitive inhibitory postsynaptic potentials or currents with no detectable influence on excitatory postsynaptic events. Perforated-patch recordings confirmed that VP did not have an effect on intrinsic membrane properties of magnocellular PVN neurons (n = 17). Analysis of intrinsic membrane properties obtained with perforated-patch recording (n = 23) demonstrated that all of nine VP-sensitive neurons showed a rebound depolarization after transient membrane hyperpolarization from rest. By contrast, 12/14 nonresponding neurons displayed a delayed return to resting membrane potentials. Recordings of reversed inhibitory postsynaptic currents with chloride-loaded electrodes showed that responses to VP persisted in media containing glutamate receptor antagonists but were abolished in the presence of tetrodotoxin. In addition, responses were mimicked by vasotocin [Phe(2), Orn(8)], a selective V(1a) receptor agonist, and blocked by [beta-Mercapto-beta, beta-cyclopentamethylenepropionyl(1),O-Me-Tyr(2), Arg(8)]-VP (Manning compound), a V(1a)/OT receptor antagonist. Neither [deamino-Cys(1),Val(4),D-Arg(8)]-VP, a selective V(2) receptor agonist, nor oxytocin were effective. Collectively, the results imply that VP acts at V(1a) receptors to excite GABAergic neurons that are presynaptic to a population of magnocellular PVN neurons the identity of which features a unique rebound depolarization. Endogenous sources of VP may be VP-synthesizing neurons in suprachiasmatic nucleus, known to project toward the perinuclear regions of PVN, and/or the magnocellular neurons within PVN.

Opioid modulation of magnocellular neurosecretory cell activity

Magnocellular neurosecretory cells of the hypothalamic supraoptic and paraventricular nuclei secrete the hormones, oxytocin and vasopressin, into the systemic circulation from the posterior pituitary gland. Oxytocin is important for parturition and is essential for lactation. Vasopressin regulates body fluid homeostasis. The secretion of these hormones is altered in response to peripheral stimuli that are conveyed via projections from other parts of the brain. Endogenous opioid peptide systems interact with the magnocellular neurosecretory system at several levels to restrain the basal secretion of these hormones as well as their secretory responses to various physiological stimuli. The inhibition of basal secretion can occur at the level of the neurosecretory terminals where endogenous opioids inhibit the release of oxytocin, and at the cell bodies of magnocellular cells to modulate the activity pattern of vasopressin cells. The responses of the magnocellular neurosecretory system to physiological stimuli are also regulated by these mechanisms but in addition probably also by pre-synaptic inhibition of afferent inputs to magnocellular cells as well as direct effects on the cell bodies of afferent input cells to modulate their activity. Here, we review the mechanisms and functional consequences of opioid interactions with oxytocin and vasopressin cells.

Oxytocin induces preservation of social recognition in male rats by activating alpha-adrenoceptors of the olfactory bulb

In this report, a series of four experiments was performed to evaluate the relationship between the olfactory bulb norepinephrine system and intra-olfactory bulb infusion of oxytocin in the preservation of social memory responses. The present data indicate that oxytocin exerts this preservation of social recognition through a specific, receptor-mediated mechanism within the olfactory bulb (experiment 1). The involvement of the olfactory bulb norepinephrine system is revealed by the demonstration that retrodialysis of oxytocin into the olfactory bulb increases norepinephrine release (experiment 4). Our data suggest that the increased output of olfactory bulb norepinephrine resulting from oxytocin appears to activate alpha-adrenoceptors to produce this preservation in recognition because infusions of clonidine into the olfactory bulb preserve recognition responses in a manner similar to that observed with oxytocin (experiment 2). In addition, a co-infusion of oxytocin with phentolamine abolishes recognition responses (experiment 3). Accordingly, this model affords the opportunity to study neuropeptide-catecholamine interactions, link these interactions with a specific behavioural outcome and identify a novel function/site of action for oxytocin in the male.

Selective modulation of excitatory transmission by mu-opioid receptor activation in rat supraoptic neurons

Opioid peptides have profound inhibitory effects on the production of oxytocin and vasopressin, but their direct effects on magnocellular neuroendocrine neurons appear to be relatively weak. We tested whether a presynaptic mechanism is involved in this inhibition. The effects of mu-opioid receptor agonist D-Ala(2), N-CH(3)-Phe(4), Gly(5)-ol-enkephalin (DAGO) on excitatory and inhibitory transmission were studied in supraoptic nucleus (SON) neurons from rat hypothalamic slices using whole cell recording. DAGO reduced the amplitude of evoked glutamatergic excitatory postsynaptic currents (EPSCs) in a dose-dependent manner. In the presence of tetrodotoxin (TTX) to block spike activity, DAGO also reduced the frequency of spontaneous miniature EPSCs without altering their amplitude distribution, rising time, or decaying time constant. The above effects of DAGO were reversed by wash out, or by addition of opioid receptor antagonist naloxone or selective mu-antagonist Cys(2)-Tyr(3)-Orn(5)-Pen(7)-NH(2) (CTOP). In contrast, DAGO had no significant effect on the evoked and spontaneous miniature GABAergic inhibitory postsynaptic currents (IPSCs) in most SON neurons. A direct membrane hyperpolarization of SON neurons was not detected in the presence of DAGO. These results indicate that mu-opioid receptor activation selectively inhibits excitatory activity in SON neurons via a presynaptic mechanism.

Internal calcium modulates apparent affinity of metabotropic GABA receptors

The metabotropic GABA receptor (GABA(B)R) regulates calcium influx in neurons. Whole cell voltage-clamp techniques were employed to determine the effects of internal calcium on the activity of GABA(B)Rs. GABA(B)R receptor apparent affinity was maximal when bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) maintained internal calcium below 70 nM. Apparent affinity was reduced as internal calcium increased. EGTA did not produce similar effects, suggesting that localized increases in calcium influenced GABA(B)R apparent affinity. Confocal imaging disclosed relatively high internal calcium just below the plasma membrane of isolated neurons. BAPTA, but not EGTA, reduced this ring of high calcium. Heparin, dantrolene, and ryanodine increased GABA(B)R apparent affinity, effects similar to that of BAPTA. Calmodulin inhibitors also increased receptor apparent affinity. These results suggest that internally released calcium activates calmodulin, which reduces GABA(B)R apparent affinity. This identifies a reciprocal system in which the metabotropic GABA receptor can reduce calcium influx, but internal calcium can suppress this receptor pathway. Metabotropic glutamate receptors linked to inositol 1,4,5 trisphosphate (InsP(3)) raised internal calcium and suppressed the action of GABA(B)Rs. Thus negative feedback systems control the balance between excitatory and inhibitory metabotropic receptor pathways in retinal neurons.

Changes in properties and neurosteroid regulation of GABAergic synapses in the supraoptic nucleus during the mammalian female reproductive cycle

1. GABAA receptor-mediated synaptic innervation of oxytocin neurones in the supraoptic nucleus (SON) was analysed in adult female rats going through their first reproductive cycle by recording the spontaneous inhibitory postsynaptic currents (sIPSCs) at six stages of female reproduction. 2. During pregnancy we observed a reduction in the interval between monoquantal sIPSCs. The synaptic current amplitude, current decay and neurosteroid sensitivity of postsynaptic GABAA receptors observed at this stage were not distinguishable from those measured in virgin stage SON. 3. Upon parturition an increase in monoquantal synaptic current decay occurred, whereas potentiation by the progesterone metabolite allopregnanolone (3alpha-OH-DHP) was suppressed. 4. Throughout a substantial part of the lactation period the decay of synaptic currents remained attenuated, whilst the potentiation by 3alpha-OH-DHP remained suppressed. 5. Several weeks after the end of lactation sIPSC intervals, their current decay velocity as well as the potentiation by 3alpha-OH-DHP were restored to pre-pregnancy levels, which is indicative of the cyclical nature of synaptic plasticity in the adult SON. 6. Competitive polymerase chain reaction (PCR) analysis showed that virgin animals expressed alpha1 and alpha2 GABAA receptor subunit mRNA at a relative ratio of 2 : 1 compared with beta-actin. After pregnancy both alpha1 and alpha2 subunit mRNA levels were transiently increased, although at a relative ratio of 1 : 4, in line with the hypothesis that alpha2 plays a large role in postsynaptic receptor functioning. During post-lactation both alpha subunits were downregulated. 7. We propose that synaptic remodelling in the SON during pregnancy includes changes in the putative number of GABA release sites per neurone. At parturition, and during the two consecutive weeks of lactation, a subtype of postsynaptic GABAA receptors was observed, distinct from the one being expressed before and during pregnancy. Synaptic current densities, calculated in order to compare the impact of synaptic inhibition, showed that, in particular, the differences in 3alpha-OH-DHP potentiation of these two distinct GABAA receptor subtypes produce robust shifts in the impact of synaptic inhibition of oxytocin neurones at the different stages of female reproduction.

Vasopressin and oxytocin action in the brain: cellular neurophysiological studies

During the last two decades it has become apparent that vasopressin (VP) and oxytocin (OT), in addition to playing a role as peptide hormones, also act as neurotransmitters. Morphological studies and electrophysiological recordings have shown a close anatomical correlation between the presence of these receptors and the neuronal responsiveness to VP or OT. These compounds have been found to affect membrane excitability in neurons located in the hippocampus, hypothalamus, lateral septum, brainstem, spinal cord and superior cervical ganglion. Sharp electrode intracellular and whole-cell recordings, done in brainstem motoneurons, have revealed that VP and OT can directly affect neuronal excitability by opening non-specific cationic channels. These neuropeptides can also influence synaptic transmission, by acting either postsynaptically or upon presynaptic target neurons or axon terminals. Whereas in some hypothalamic neurons OT appears to mobilize intracellular calcium, as revealed by calcium imaging techniques, in the brainstem the action of this neuropeptide is mediated by a second messenger which is distinct from the second messenger activated in peripheral target cells. Future studies should be aimed at elucidating the properties of the cationic channels responsible for the neuronal action of VP and OT, at identifying the brain-specific second messengers activated by these neuropeptides and at determining whether endogenous VP and OT can exert neuronal effects similar to those elicited by exogenous neuropeptides.

Electrophysiological studies of neurohypophysial neurons and peptides

We have used hypothalamic slices of the supraoptic nucleus (SON) to investigate synaptic control of magnocellular vasopressinergic and oxytocinergic neurons. With the use of perforated patch recording techniques we identified and isolated excitatory or inhibitory postsynaptic currents elicited by electrical stimulation of afferent fibers. Both inhibitory and excitatory afferent fibers displayed presynaptic GABAB receptors; the GABAB agonist, baclofen caused a dose-dependent suppression of the evoked potentials in the absence of any effects on postsynaptic input resistance. Further evidence for a presynaptic locus included an increase in paired pulse ratio and a lack of effect on currents elicited by exogenously applied muscimol (a GABAA receptor agonist) or AMPA (a glutamate agonist). With the use of an GABAB receptor antagonist we demonstrated an action of endogenously released GABA, acting at GABAB receptors on excitatory terminals, to reduce excitatory transmission. In addition to presynaptic modulation by GABA of afferent inputs, we also observed actions of vasopressin and oxytocin, released from dendrites of magnocellular SON neurons, to gate afferent, excitatory transmission in the SON. Exogenously applied vasopressin and oxytocin, or these peptides when released by depolarizing stimuli of magnocellular neurons, reduced the size of evoked excitatory postsynaptic potentials at a presynaptic locus. We have also observed actions of arginine vasopressin to modulate the action of glutamate in slices of the ventral septal area and to attenuate a glutamate-mediated excitatory postsynaptic current in slices of the parabrachial nucleus.

Evidence that multiple P2X purinoceptors are functionally expressed in rat supraoptic neurones

1. The expression, distribution and function of P2X purinoceptors in the supraoptic nucleus (SON) were investigated by reverse transcription-polymerase chain reaction (RT-PCR), in situ hybridization, and Ca2+-imaging and whole-cell patch-clamp techniques, respectively. 2. RT-PCR analysis of all seven known P2X receptor mRNAs in circular punches of the SON revealed that mRNAs for P2X2, P2X3, P2X4, P2X6 and P2X7 receptors were expressed in the SON, and mRNAs for P2X3, P2X4 and P2X7 were predominant. 3. In situ hybridization histochemistry for P2X3 and P2X4 receptor mRNAs showed that both mRNAs were expressed throughout the SON and in the paraventricular nucleus (PVN). 4. ATP caused an increase in [Ca2+]i in a dose-dependent manner with an ED50 of 1.7 x 10-5 M. The effects of ATP were mimicked by ATPgammaS and 2-methylthio ATP (2MeSATP), but not by AMP, adenosine, UTP or UDP. alphabeta-Methylene ATP (alphabetaMeATP) and ADP caused a small increase in [Ca2+]i in a subset of SON neurones. 5. The P2X7 agonist 2'- & 3'-O-(4-benzoylbenzoyl)-ATP (BzATP) at 10-4 M increased [Ca2+]i, but the potency of BzATP was lower than that of ATP. In contrast, BzATP caused a more prominent [Ca2+]i increase than ATP in non-neuronal cells in the SON. 6. The effects of ATP were abolished by extracellular Ca2+ removal or by the P2 antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), and inhibited by extracellular Na+ replacement or another P2 antagonist, suramin, but were unaffected by the P2X7 antagonist oxidized ATP, and the inhibitor of Ca2+-ATPase in intracellular Ca2+ stores cyclopiazonic acid. 7. Two patterns of desensitization were observed in the [Ca2+]i response to repeated applications of ATP: some neurones showed little or moderate desensitization, while others showed strong desensitization. 8. Whole-cell patch-clamp analysis showed that ATP induced cationic currents with marked inward rectification. The ATP-induced currents exhibited two patterns of desensitization similar to those observed in the [Ca2+]i response. 9. The results suggest that multiple P2X receptors, including P2X3, are functionally expressed in SON neurones, and that activation of these receptors induces cationic currents and Ca2+ entry. Such ionic and Ca2+-signalling mechanisms triggered by ATP may play an important role in the regulation of SON neurosecretory cells.

Dendritic release of vasopressin and oxytocin

In addition to the release of neurotransmitters from their axon terminals, several neuronal populations are able to release their products from their dendrites. The cell bodies and dendrites of vasopressin- and oxytocin-producing neurones are mainly located within the hypothalamic supraoptic and paraventricular nuclei and neuropeptide release within the magnocellular nuclei has been shown in vitro and in vivo. Local release is induced by a range of physiological and pharmacological stimuli, and is regulated by a number of brain areas; locally released peptides are mainly involved in pre- and postsynaptic modulation of the electrical activity of magnocellular neurones. Spatial and temporal differences between peptide release within the nuclei and that from the distant axonal varicosities indicate that the release mechanisms are at least partially independent, supporting the hypothesis of locally regulated dendritic release of vasopressin and oxytocin. In this respect, magnocellular neurones show similarities to other neuronal populations and thus autoregulation of neuronal activity by dendritic neuromodulator release may be a general phenomenon within the brain.

Inhibition of spontaneous inhibitory postsynaptic currents (IPSC) by noradrenaline in rat supraoptic neurons through presynaptic alpha2-adrenoceptors

It has been shown that noradrenergic activation has great influence on the activities of hypothalamic supraoptic neurons. No direct evidence has been reported on the presynaptic effects of adrenoceptors in the actions of noradrenaline on supraoptic neurons, although postsynaptic mechanisms have been studied extensively. In the present study, we explored presynaptic effects of noradrenaline on the supraoptic neurons by measuring spontaneous inhibitory postsynaptic currents (IPSC) with the whole-cell patch-clamp technique. Noradrenaline reduced the frequency of IPSCs in a dose-dependent (10(-9) to 10(-3) M) and reversible manner. Noradrenaline did not affect the amplitude of IPSCs at concentrations of 10(-9) to 10(-5) M, but reduced the amplitude of IPSCs at high concentrations (10(-4) and 10(-3) M). The inhibitory effects of noradrenaline were mimicked by the alpha2-agonist clonidine (10(-4) M), but not by the alpha1-agonist methoxamine (10(-4) M) nor by the beta-agonist isoproterenol (10(-4) M). Moreover, the inhibitory effects of noradrenaline on IPSCs were blocked by the non-selective alpha antagonist phentolamine (10(-4) M) or the selective alpha2-antagonist yohimbine (10(-4) M), but not by the alpha1-antagonist prazosin (10(-4) M). These results suggest that noradrena-line inhibits release of GABA from the presynaptic GABAergic terminals of the supraoptic neurons by activating presynaptic alpha2-adrenoceptors and such presynaptic mechanisms may play a role in the excitatory control of SON neurons by noradrenergic neurons.

Evidence for a hypothalamic oxytocin-sensitive pattern-generating network governing oxytocin neurons in vitro

During lactation and parturition, magnocellular oxytocin (OT) neurons display a characteristic bursting electrical activity responsible for pulsatile OT release. We investigated this activity using hypothalamic organotypic slice cultures enriched in magnocellular OT neurons. As shown here, the neurons are functional and actively secrete amidated OT into the cultures. Intracellular recordings were made from 23 spontaneously bursting and 28 slow irregular neurons, all identified as oxytocinergic with biocytin and immunocytochemistry. The bursting electrical activity was similar to that described in vivo and was characterized by bursts of action potentials (20.1 +/- 4.3 Hz) lasting approximately 6 sec, over an irregular background activity. OT (0.1-1 microM), added to the medium, increased burst frequency, reducing interburst intervals by 70%. The peptide also triggered bursting in 27% of nonbursting neurons. These effects were mimicked by the oxytocin receptor (OTR) agonist [Thr4, Gly7]-OT and inhibited by the OTR antagonist desGly-NH2d(CH2)5[D-Tyr2,Thr4]OVT. Burst rhythmicity was independent of membrane potential. Hyperpolarization of the cells unmasked volleys of afferent EPSPs underlying the bursts, which were blocked by CNQX, an AMPA/kainate receptor antagonist. Our results reveal that OT neurons are part of a hypothalamic rhythmic network in which a glutamatergic input governs burst generation. OT neurons, in turn, exert a positive feedback on their afferent drive through the release of OT.

Evidence for a hypothalamic oxytocin-sensitive pattern-generating network governing oxytocin neurons in vitro

During lactation and parturition, magnocellular oxytocin (OT) neurons display a characteristic bursting electrical activity responsible for pulsatile OT release. We investigated this activity using hypothalamic organotypic slice cultures enriched in magnocellular OT neurons. As shown here, the neurons are functional and actively secrete amidated OT into the cultures. Intracellular recordings were made from 23 spontaneously bursting and 28 slow irregular neurons, all identified as oxytocinergic with biocytin and immunocytochemistry. The bursting electrical activity was similar to that described in vivo and was characterized by bursts of action potentials (20.1 +/- 4.3 Hz) lasting approximately 6 sec, over an irregular background activity. OT (0.1-1 microM), added to the medium, increased burst frequency, reducing interburst intervals by 70%. The peptide also triggered bursting in 27% of nonbursting neurons. These effects were mimicked by the oxytocin receptor (OTR) agonist [Thr4, Gly7]-OT and inhibited by the OTR antagonist desGly-NH2d(CH2)5[D-Tyr2,Thr4]OVT. Burst rhythmicity was independent of membrane potential. Hyperpolarization of the cells unmasked volleys of afferent EPSPs underlying the bursts, which were blocked by CNQX, an AMPA/kainate receptor antagonist. Our results reveal that OT neurons are part of a hypothalamic rhythmic network in which a glutamatergic input governs burst generation. OT neurons, in turn, exert a positive feedback on their afferent drive through the release of OT.

Ionotropic and metabotropic glutamate receptor agonist-induced [Ca2+]i increase in isolated rat supraoptic neurons

In the present study, the effects of glutamate and of agonists for ionotropic and metabotropic glutamate receptors on intracellular Ca2+ concentration ([Ca2+]i) were investigated in neurons of the rat supraoptic nucleus (SON). We used the intracellular Ca2+ imaging technique with fura-2, in single magnocellular neurons dissociated from the SON of rats. Glutamate (10(-6)-10(-4) M) evoked a dose-dependent increase in [Ca2+]i. The glutamate agonists exerted similar effects, although with some differences in the characteristics of their responses. The [Ca2+]i response to NMDA was smaller than those of glutamate or the non-NMDA receptor agonists, AMPA and kainate, but was significantly enhanced by the removal of extracellular Mg2+. Glutamate, as well as quisqualate, an agonist for both ionotropic and metabotropic glutamate receptors, evoked a [Ca2+]i increase in a Ca2+-free condition, suggesting Ca2+ release from intracellular Ca2+ stores. This was further evidenced by [Ca2+]i increases in response to a more selective metabotropic glutamate receptor agonist, t-ACPD, in the absence of extracellular Ca2+. Furthermore, the quisqualate-induced Ca2+ release was abolished by the selective metabotropic glutamate receptor antagonist, (S)-4-carboxyphenylglycine. The results suggest that metabotropic glutamate receptors as well as non-NMDA and NMDA receptors are present in the SON neurons, and that activation of the first leads to Ca2+ release from intracellular Ca2+ stores and the activation of the latter two types induces Ca2+ entry. These dual mechanisms of Ca2+ signalling may play a role in the regulation of SON neurosecretory cells by glutamate.

Oxytocin modulates glutamatergic synaptic transmission between cultured neonatal spinal cord dorsal horn neurons

The functional characteristics of binding sites for the neuropeptide oxytocin (OT) detected by radioautography in laminae I and II of the dorsal horn (DH) and on cultured neonatal DH neurons were studied on the latter using perforated patch-clamp recordings. The neurons were identified by their spike discharge properties and on the basis of the presence of met-enkephalin-like and glutamate decarboxylase-like immunoreactivities. OT (100 nM) never induced any membrane current at a holding potential of -60 mV but increased the frequency of spontaneously occurring AMPA receptor-mediated EPSCs or the mean amplitude of electrically evoked EPSCs in a subset (35%) of neurons. The frequency of miniature EPSCs (m-EPSCs) recorded in the presence of 0.5 microM tetrodotoxin was also increased by OT (100 nM) without any change in their mean amplitude, indicating an action at a site close to the presynaptic terminal. The decay kinetics of any type of EPSC were never modified by OT. The effect of OT was reproduced by [Thr4, Gly7]-OT (100 nM), a selective OT receptor agonist, and blocked by d(CH2)5-[Tyr(Me)2,Thr4,Tyr-NH29]-ornithine vasotocin (100 nM), a specific OT receptor antagonist. Reducing the extracellular Ca2+ concentration from 2.5 to 0.3 mM in the presence of Cd2+ (100 microM) reversibly blocked the effect of OT on m-EPSCs. The OT receptors described here may represent the substrate for modulatory actions of descending hypothalamo-spinal OT-containing pathways on the nociceptive system.