Collateral input to the paraventricular and supraoptic nuclei in rat. II. Afferents from the ventral lateral medulla and nucleus tractus solitarius

In the rat, medullary afferents to the hypothalamic magnocellular nuclei mediate the baroreceptor reflexes of vasopressinergic neurons and the cholecystokinin- or gastric distention-induced excitation of oxytocinergic neurons. One strategy that reflexes such as these may use to coordinate the activity of magnocellular neuroendocrine neurons is collateral branching of input. Previous work has shown that the distributions of medullary neurons projecting to the paraventricular and the supraoptic nuclei overlap and that their axons branch. Thus, we hypothesized that single neurons in the ventral lateral medulla and/or the nucleus tractus solitarius would project to both the paraventricular and supraoptic nuclei via collateral branches of their axons. Medullary afferent neurons were retrogradely labeled after injection into the paraventricular and the supraoptic nucleus on one side of the brain with two different fluorescent tracers: Fluoro-Gold or rhodamine-labeled latex microspheres. The topographic distribution of labeled cells in the medulla containing either a single fluorescent tracer or both tracers were plotted. Of these labeled neurons, a small percentage (7%) contained both dyes, suggesting that they send collateral branches to both of the magnocellular neuroendocrine nuclei injected. Single labeled cells were both ipsi- and contralateral to the injected side (53% ipsilateral), but most double-labeled cells were ipsilateral (84%). In rats, areas that project to both the paraventricular and the supraoptic nuclei may act upon both nuclei together. Thus, afferent inputs, in conjunction with the known inter- and intracellular changes that take place within the magnocellular nuclei, may be involved with the coordinated responses throughout magnocellular neuroendocrine system during medullary reflexes, i.e., the baroreceptor-mediated reflexes or the gastric distention reflexes.     

Involvement of medullary A2 noradrenergic neurons in the activation of oxytocin neurons after conditioned fear stimuli

Fear-related stimuli activate oxytocin neurons in the hypothalamus and facilitate oxytocin release from the pituitary. Oxytocin neurons in the supraoptic nucleus receive direct noradrenergic innervations from the A1 and A2 cell groups in the medulla oblongata. In the present study, we investigated the role of hypothalamic-projecting noradrenergic neurons in controlling oxytocin cell activity following fear-related stimuli in rats. An unconditioned fear stimulus (intermittently applied footshock) or conditioned fear stimulus induced expression of Fos protein, a protein product of an immediate-early gene, in magnocellular oxytocin neurons in the supraoptic or paraventricular nucleus. A neurotoxin, 5-amino-2,4-dihydroxy-alpha-methylphenylethylamine, microinjected into the vicinity of the supraoptic nucleus, selectively depleted the noradrenaline contents of the nucleus and blocked the Fos expression in the supraoptic nucleus after the unconditioned or conditioned fear stimulus. In the medulla oblongata, the unconditioned fear stimulus induced expression of Fos protein in both A2/C2 and A1/C1 catecholaminergic neurons. On the other hand, the conditioned fear stimulus induced expression of Fos protein preferentially in the A2/C2 neurons. Furthermore, the unconditioned fear stimulus induced Fos expression in the A1/C1 and A2/C2 catecholaminergic neurons labelled with retrograde tracers previously injected into the supraoptic nucleus. The conditioned fear stimulus induced Fos expression preferentially in the A2/C2 catecholaminergic neurons labelled with the retrograde tracers. These data suggest that the conditioned fear-induced oxytocin cell activity is mediated by the A2 noradrenergic neurons projecting to oxytocin neurons, while the unconditioned fear response is mediated by both A2 and A1 noradrenergic neurons.     

Milk Ejection Bursts of Supraoptic Oxytocin Neurones during Bilateral and Unilateral Suckling in the Rat

Extracellular recordings of the electrical activity of oxytocin neurones were made from the supraoptic nuclei (SON) of lactating rats, and the milk-ejection bursts and the background activity of oxytocin neurones were investigated during unilateral and bilateral suckling. When application of pups was limited to the nipples on either the same side (ipsilateral suckling) or the side opposite (contralateral suckling) to the oxytocin neurone recorded, the burst amplitude and background firing rate were significantly (P<0.05) lower and the inter-burst interval was significantly (P<0.05) longer than during bilateral suckling. Furthermore, the burst amplitude was significantly (P<0.05) lower during ipsilateral suckling than during contralateral suckling. The majority of the oxytocin neurones showed a gradual increase in the burst amplitude during bilateral (88.9%) and contralateral (77.3%) suckling, but during ipsilateral suckling only 40% of the neurones did. The inter-burst interval became shorter with the progress of the milk ejection reflex during any mode of suckling. Three pairs of oxytocin neurones recorded simultaneously from both SON were successfully tested for the effect of bilateral and unilateral suckling on the electrical activity, and the results showed the same direction of change in the burst amplitude, background activity and burst interval as shown in single side recordings. These findings indicate that the burst amplitude mainly depends on the amount of afferent suckling signals arising from the nipples on the side opposite to the recording side, and that there may exist bilateral summation centres coordinating with the synchronization mechanism of milk-ejection bursts of oxytocin neurones.     

The role of the AV3V region in the control of magnocellular oxytocin neurons

The AV3V region is important in the control of body fluid and Na⁺ regulation and projects to the supraoptic and paraventricular nuclei. Oxytocin from the neurohypophysis mediates milk ejection and is involved in parturition, but has also been recently implicated as a candidate natriuretic hormone. We have studied the role of the AV3V region in the control of magnocellular oxytocin neurons in rats. Electrical stimulation of the AV3V region increased the firing rate of supraoptic oxytocin neurons and evoked a concomitant release of oxytocin. Acute electrolytic AV3V lesions silenced supraoptic neurons and abolished their excitation by hyperosmotic stimulation. The lesions also abolished osmotically-induced release of oxytocin. Re-activation of supraoptic neurons by local glutamate restored their osmoresponsiveness to about 50% normal. Thus, while supraoptic neurons are directly osmosensitive, the AV3V region is essential for their normal osmoresponsiveness. Electrolytic AV3V lesions did not affect suckling-induced oxytocin secretion or, in conscious rats, the release of oxytocin secretion during parturition. Thus the AV3V region is not involved in the activation of oxytocin neurons during suckling or parturition.     

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.     

Colocalization of vasopressin and oxytocin in hypothalamic magnocellular neurons in water-deprived rats

The posterior lobe hormones vasopressin and oxytocin are expressed in mutually-exclusive sets of magnocellular hypothamalic neurons. However, under certain functional conditions a partial coexpression has been observed. In the present study we subjected adult rats to long-term osmotic stress by water deprivation for up to 3 days.After 3 days, a marked reduction of vasopressin immunostaining was observed in the paraventricular and supraoptic nuclei as compared with controls. Coexistence of oxytocin and vasopressin occurred in a portion of the magnocellular neurons. Many fibers of the hypothalamic-neurohypophyseal tract contained both peptides. Rehydration for 24 h after 3 days of thirsting resulted in a light recovery of vasopressin immunoreactivity with almost none magnocellular neurons containing both nonapeptides. Our findings indicate that magnocellular hypothalamo neurohypophysial neurons are capable of oxytocin and vasopressin coexpression upon extended osmotic stress.     

Localization of kappa opioid receptors in oxytocin magnocellular neurons in the paraventricular and supraoptic nuclei

It has been demonstrated previously that kappa opioid receptor agonists, such as dynorphin, inhibit oxytocin secretion in the rat. To determine whether kappa agonists act directly on oxytocin-containing magnocellular neurons to inhibit hormone secretion, we utilized immunofluorescence to examine the cellular localization of kappa opioid receptors in the rat paraventricular and supraoptic nuclei. kappa Opioid receptor immunoreactivity co-localized with oxytocin-containing cell bodies, their axons and axon terminals. Thus, our results suggest that kappa opioid receptor agonists can exert direct inhibitory actions on oxytocin magnocellular neurons.     

Nuclei of origin of monoaminergic, peptidergic, and cholinergic afferents to the cat trigeminal motor nucleus: a double-labeling study with cholera-toxin as a retrograde tracer.

The aim of the present study was to determine the brainstem afferents and the location of neurons giving rise to monoaminergic, cholinergic, and peptidergic inputs to the cat trigeminal motor nucleus (TMN). This was done in colchicine treated animals by using a very sensitive double immunostaining technique with unconjugated cholera-toxin B subunit (CT) as a retrograde tracer. After CT injections in the TMN, retrogradely labeled neurons were most frequently seen bilaterally in the nuclei reticularis parvicellularis and dorsalis of the medulla oblongata, the alaminar spinal trigeminal nucleus (magnocellular division), and the adjacent pontine juxtatrigeminal region and in the ipsilateral mesencephalic trigeminal nucleus. We further observed that inputs to the TMN arise from the medial medullary reticular formation (the nuclei retricularis magnocellularis and gigantocellularis), the principal bilateral sensory trigeminal nucleus, and the dorsolateral pontine tegmentum. In addition, the present study demonstrated that the TMN received 1) serotonergic afferents, mainly from the nuclei raphe obscurus, pallidus, and dorsalis; 2) catecholaminergic afferent projections originating exclusively in the dorsolateral pontine tegmentum, including the K?lliker-Fuse, parabrachialis lateralis, and locus subcoeruleus nuclei; further, that 3) methionin-enkephalin-like inputs were located principally in the medial medullary reticular formation (nuclei reticularis magnocellularis and gigantocellularis and nucleus paragigantocellularis lateralis), in the caudal raphe nuclei (Rpa and Rob) and the dorsolateral pontine tegmentum; 4) substance P-like immunoreactive neurons projecting to the TMN were present in the caudal raphe and Edinger-Westphal nuclei; and 5) cholinergic afferents originated in the whole extent of the nuclei reticularis parvicellularis and dorsalis including an area located ventral to the nucleus of the solitary tract at the level of the obex. In the light of these anatomical data, the present report discusses the possible physiological involvement of TMN inputs in the generation of the trigeminal jaw-closer muscular atonia occurring during the periods of paradoxical sleep in the cat.     

Lesions of Hypothalamic Mammillary Body Desynchronise Milk‐Ejection Bursts of Rat Bilateral Supraoptic Oxytocin Neurones

Successful milk ejection depends on a bolus release of oxytocin, which results from the synchronised burst firing of magnocellular oxytocin neurones in several hypothalamic nuclei. Despite extensive studies of the mechanism underlying the burst synchrony of oxytocin neurones in the same nucleus, brain regions controlling burst synchronisation among different nuclei remain elusive. We hypothesised that some structures in the ventroposterior hypothalamus may function as the major component of neural circuits controlling burst synchronisation of bilateral oxytocin neurones. To test this hypothesis, we recorded burst firing of bilateral oxytocin neurones in the two supraoptic nuclei after microsurgical disconnection of different hypothalamic regions in anaesthetised lactating rats. The results obtained showed that the interhemispheric section of the caudal part of the hypothalamus but not the rostral hypothalamus resulted in burst desynchronisation. The difference in burst onset time between paired bursts of bilateral oxytocin neurones was 129.2 ± 34.7 s, which is significantly (P < 0.01) longer than that of sham‐lesioned controls (0.24 ± 0.02 s). Hypothalamic lesions leading to the desynchronisation involved the mammillary body, supramammillary nucleus and tuberomammillary nucleus in the ventroposterior hypothalamus. Consistently, electrolytic lesion of the median part of this mammillary body region also desynchronised the burst of bilateral oxytocin neurones and disrupted milk ejections. These results indicate that the mammillary body region is critically involved in the burst synchronisation of bilateral oxytocin neurones during suckling and possibly functions as the major component of a putative synchronisation centre.     

Distribution of glutamic acid decarboxylase mRNA-containing neurons in rat medulla projecting to thoracic spinal cord in relation to monoaminergic brainstem neurons.

Recent neurophysiological work has suggested the existence of monosynaptic gamma-aminobutyric acidergic (GABAergic) projections from the medulla oblongata to sympathetic preganglionic neurons. The purpose of the present study was to identify the possible anatomical location of these neurons. The location of GABAergic neurons with projection to the thoracic spinal cord was studied by using in situ hybridization for both 65-kD and 67-kD isoforms of glutamic acid decarboxylase (GAD) mRNA (GAD-65 and GAD-67, respectively) combined with midthoracic spinal cord injections of the tracer Fast Blue. Tyrosine hydroxylase (TH) or tryptophan hydroxylase immunohistochemistry was combined with GAD mRNA detection and Fast Blue to determine whether any bulbospinal catecholaminergic or serotonergic cell groups in the medulla also are GABAergic. GAD-67 and GAD-65 mRNA-containing neurons had similar distribution patterns in the medulla oblongata, with some areas exhibiting lighter labeling for GAD-65 mRNA. GABAergic bulbospinal neurons were located in the caudal part of the solitary nucleus, the parasolitary nucleus, the vestibular nuclei, the ventral medial medulla, the raphe nuclei, and parapyramidal areas. TH-immunoreactive neurons in the A1, A2, C1, and C2 areas or the area postrema did not contain either GAD-67 or GAD-65 mRNA. GAD mRNA-positive bulbospinal cells were present medial to theA1 and C1 catecholaminergic cell groups, with little or no overlap. Serotonergic neurons positive for GAD mRNAwere found in the parapyramidal area and just dorsal to the pyramidal tract in the raphe magnus. This population included bulbospinal neurons. In conclusion, GABAergic neurons with projections to the thoracic spinal cord exist in a restricted number of medullary nuclei from which inhibitory sympathetic control may originate.     

Projections of the paratrigeminal nucleus to the ambiguus, rostroventrolateral and lateral reticular nuclei, and the solitary tract

The paratrigerminal nucleus (Pa5), a constituent of the spinal interstitial system, was linked to the pressor effect caused by bradykinin injected in the dorsal lateral medulla of the rat. The nucleus receives primary afferent sensory fibers contained in branches of the trigeminal, glossopharyngeal and vagus nerves. In this investigation connections of the paratrigeminal nucleus to other medullary structures were studied with the use of retrograde and anterograde neuronal tracers. Fluorescent light microscopy analyses of medullary sections of rats injected with the retrograde transport tracer Fluoro-gold in the nucleus of the solitary tract (NTS) or in the pressor area of the rostral ventrolateral medulla (RVLM) revealed labeled neuronal cell bodies in the ipsi- and contralateral Pa5. FluoroGold microinjections in the caudal ventrolateral medulla (CVLM) did not produce fluorescent labeling of Pa5 neurons. Microinjection of the anterograde transport neuronal tracer biocytin in the Pa5 produced bilateral labeling of the solitary tract (sol). rostroventrolateral reticular nucleus (RVL), ambiguus nucleus (Amb), lateral reticular nucleus (LRt) and ipsilateral parabrachial nuclei, but not the contralateral Pa5. Confocal laser microscopy showed fluorescence labeling of fibers and presumptive terminal varicosities in the NTS, RVL, Amb and LRt. The present findings showing the paratrigeminal nucleus interposed between sensory afferent and stuctures associated to cardiovascular and respiratory functions, suggest that the structure may act as a medullary relay nucleus for sensory stimuli directly connecting primary afferents to structures mediating cardiovascular and respiratory reflexes.     

Calretinin is differentially localized in magnocellular oxytocin neurons of the rat hypothalamus. A double-labeling immunofluorescence study

By use of a double-labeling immunofluorescence method with a confocal laser scanning microscope, we have examined whether a calcium-binding protein, calretinin, is localized in magnocellular oxytocin and vasopressin neurons of the rat hypothalamus. In the supraoptic nucleus, all oxytocin-labeled cells were stained for calretinin. However, in the magnocellular part of the paraventricular nucleus, almost all oxytocin-stained cells were devoid of calretinin immunoreactivity. All vasopressin-positive cells of both the supraoptic nucleus and the magnocellular part of the paraventricular nucleus lacked calretinin immunoreactivity. No calretinin immunoreactivity was found in oxytocin-labeled cells of the the anterior commissural nucleus or in vasopressin-labeled cells of the suprachiasmatic nucleus. We previously showed that another calcium-binding protein, calbindin-D28k, was localized in magnocellular oxytocin neurons of the supraoptic nucleus but not in those of the paraventricular nucleus. These findings suggest that, in general, magnocellular oxytocin neurons of the supraoptic nucleus and those of the paraventricular nucleus can be chemically distinguished, that is, the former contain both calretinin and calbindin-D28k but the latter lack the two calcium-binding proteins.     

Tyrosine hydroxylase-containing neurons in the supraoptic and paraventricular nuclei of the adult human

We have studied the distribution of tyrosine hydroxylase-containing neurons in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the adult human hypothalamus. Large numbers of these neurons were seen in these hypothalamic nuclei; approximately 40% of all the cells within the SON and PVN were immunoreactive for tyrosine hydroxylase (TH-ir). Most of these cells were magnocellular. Their distribution was compared to that of arginine-vasopressin-immunoreactive (AVP-ir) cells. In the SON a greater proportion of magnocellular TH-ir cells was found caudally compared to AVP-ir cells. In the PVN the magnocellular TH-ir cells were larger in mean diameter compared to AVP-ir cells. In double-immunofluorescence experiments some TH-ir cells contained oxytocin immunoreactivity but none contained AVP-ir. In the adult human a large number of PVN and SON magnocellular cells appear to synthesize a catecholamine. A subclass of these neurons also synthesize oxytocin but most cells are distinct from the classically described neurosecretory neurons.     

Chronic stress elevates enkephalin expression in the rat paraventricular and supraoptic nuclei.

Numerous studies have implicated opioids in the regulation of hypothalamic functions. Dynorphin, which is co-expressed with vasopressin in the magnocellular neurons of the paraventricular and supraoptic nuclei, is co-regulated with vasopressin in response to hyperosmolality and appears to inhibit vasopressin and oxytocin release from the posterior pituitary. Enkephalin is present in paraventricular parvocellular neurons and its expression is elevated in response to various stresses. However, enkephalin's presence and roles in paraventricular and supraoptic magnocellular neurons are uncertain. By giving rats daily intraperitoneal injections of hypertonic saline for up to 12 days, we induced a marked increase in enkephalin expression in magnocellular neurons of the paraventricular and supraoptic nuclei, beyond what develops from drinking hypertonic saline. Our results suggest that enkephalin expression in both vasopressin and oxytocin neurons may increase in response to chronic stresses and provide another source of enkephalin in addition to the parvocellular neurons.     

Transient induction of c-fos in rat magnocellular hypothalamic neurons after hypophysectomy.

Using immunofluorescence histochemistry, the paraventricular and supraoptic hypothalamic nuclei of normal control and hypophysectomized rats were studied in double labelling experiments with antibodies against the protein c-fos (Fos) and against vasopressin or oxytocin in order to characterize the activated neurons chemically. Normal controls showed no expression of Fos, whereas in hypophysectomized animals an intense induction of Fos-like immunoreactivity (-LI) was observed 12 h and 24 h post hypophysectomy but not beyond this survival time. Both vasopressinergic and oxytocinergic magnocellular neurons were labelled with Fos-LI. Thus Fos-LI can be induced in magnocellular hypothalamic neurons by injury, suggesting that this protein may be involved in adaptive mechanisms following axotomy.     

Collateral input to the paraventricular and supraoptic nuclei in rat. I. Afferents from the subfornical organ and the anteroventral third ventricle region

Injections of two fluorescent retrograde tracers were used to investigate the existence of collateral branching of input to the hypothalamic magnocellular neuroendocrine neurons. Injection of one tracer (either Fluoro-Gold or rhodamine-labeled microspheres) into the supraoptic nucleus and the other tracer into the ipsilateral paraventricular nucleus produced labeled neurons within the subfornical organ and the anteroventral third ventricle area. Some labeled cells were found to contain both fluorescent tracers (double-labeled cells), suggesting that they project to both the paraventricular and supraoptic nuclei via branching axons. Most double-labeled cells were found within the subfornical organ. Fewer of these cells were located within the nucleus medianus preopticus, and still fewer were distributed in the organum vasculosum lamina terminalis, the bed nucleus of the stria terminalis, and the medial and the lateral preoptic areas. These data present the first direct evidence that single cells may provide input to more than one magnocellular neuroendocrine nucleus. Hypothetically, hormonal release would require coordinated firing of many magnocellular cells. Thus, the branched input to these neurons may assist in the organization and the timely activation of this system in response to physiological stimuli.     

Localization of κ opioid receptors in oxytocin magnocellular neurons in the paraventricular and supraoptic nuclei

It has been demonstrated previously that κ opioid receptor agonists, such as dynorphin, inhibit oxytocin secretion in the rat. To determine whether κ agonists act directly on oxytocin-containing magnocellular neurons to inhibit hormone secretion, we utilized immunofluorescence to examine the cellular localization of κ opioid receptors in the rat paraventricular and supraoptic nuclei. κ Opioid receptor immunoreactivity co-localized with oxytocin-containing cell bodies, their axons and axon terminals. Thus, our results suggest that κ opioid receptor agonists can exert direct inhibitory actions on oxytocin magnocellular neurons.     

Periventricular dopaminergic neurons terminating in the neuro-intermediate lobe of the cat hypophysis

The aim of the present study was to determine the exact origins of the dopaminergic hypothalamohypophyseal projections in the cat brain. For this purpose, we used a retrograde tracer technique with horseradish peroxidase (HRP) in conjunction with tyrosine hydroxylase (TH) immunohistochemistry as a marker for the dopaminergic neurons. After injections of the tracer into the neuro-intermediate lobe, a substantial number of HRP-labeled neurons was observed in the supraoptic and paraventricular neurosecretory nuclei. Furthermore, a cluster of HRP-positive neurons was found in the tuberal component of the periventricular nucleus where few, if any, neurosecretory magnocellular cells are identified. TH immunohistochemistry on the same sections further revealed that virtually all these HRP-containing neurons showed TH immunoreactivity. These double-labeled neurons were medium in size and fusiform or ovoid and appeared to belong to the A14 dopamine cell group. In addition to these medium-sized double-labeled neurons, a magnocellular type of double-labeled cell body was identified just adjacent to the organum vasculosum of the lamina terminalis and in and around the supraoptic and paraventricular nuclei. These double-labeled cells appeared to be members of the A14 and A15 dopamine cell groups. In conclusion, the present study indicated that the dopaminergic projections to the cat neurointermediate lobe might originate mainly in the medium-sized cells located in the tuberal periventricular nucleus and partly in the large-sized cells located in and around the supraoptic and paraventricular neurosecretory nuclei.     

Physiological Regulation of Magnocellular Neurosecretory Cell Activity: Integration of Intrinsic,Local and Afferent Mechanisms

The hypothalamic supraoptic and paraventricular nuclei contain magnocellular neurosecretory cells (MNCs) that project to the posterior pituitary gland where they secrete either oxytocin or vasopressin (the antidiuretic hormone) into the circulation. Oxytocin is important for delivery at birth and is essential for milk ejection during suckling. Vasopressin primarily promotes water reabsorption in the kidney to maintain body fluid balance, but also increases vasoconstriction. The profile of oxytocin and vasopressin secretion is principally determined by the pattern of action potentials initiated at the cell bodies. Although it has long been known that the activity of MNCs depends upon afferent inputs that relay information on reproductive, osmotic and cardiovascular status, it has recently become clear that activity depends critically on local regulation by glial cells, as well as intrinsic regulation by the MNCs themselves. Here, we provide an overview of recent advances in our understanding of how intrinsic and local extrinsic mechanisms integrate with afferent inputs to generate appropriate physiological regulation of oxytocin and vasopressin MNC activity.