Septal and Hippocampal Release of Oxytocin, but not Vasopressin, in the Conscious Lactating Rat During Suckling

The central release of both oxytocin and vasopressin within the septum and dorsal hippocampus in response to suckling was studied in conscious, freely-behaving lactating rats. Three consecutive 30-min push-pull perfusions were carried out before, during and after suckling (suckled group) or without suckling (control group). As compared to control levels, suckling resulted in a significantly increased oxytocin release within both limbic brain areas (septum: to 140%, dorsal hippocampus: to 1,600%). After removal of the suckling pups, the oxytocin concentration in the final perfusates remained at the stimulation level (septum) or tended to return to control values (dorsal hippocampus). In contrast to oxytocin, the vasopressin perfusate levels did not differ significantly between unsuckled and suckled rats.     

Role of central oxytocin in the control of the milk ejection reflex

The neuropeptide oxytocin, synthetized by magnocellular neurons in the hypothalamus, is well known for its peripheral action after it is released into the bloodstream from axons in the neurohypophysis. Less familiar is the notion that it is also released centrally to control the activity of oxytocinergic neurons themselves. When injected into the third ventricle of lactating rats during suckling, oxytocin increases the basal firing rate of oxytocinergic neurons as well as their activity at the time of each reflex milk ejection. On the other hand, centrally administered oxytocin engenders the neuronal-glial and synaptic plasticity characteristic of the oxytocin system when it is physiologically activated. From numerous in vivo and in vitro observations, it appears that central oxytocin is released in the hypothalamic nuclei themselves. For example, the use of push-pull cannulae inserted into one supraoptic nucleus of suckled rats shows that oxytocin is released inside the nucleus specifically during milk ejection. Moreover, ultrastructural immunocytochemistry reveals synaptic terminals in the supraoptic nucleus where both the pre- and postsynaptic elements are oxytocinergic. Nevertheless, the mechanism of the central release of the neuropeptide has still to be determined, especially in view of electrophysiological observations indicating that the release process in the hypothalamus is different from that within the neurohypophysis.     

The Glutamatergic Innervation of Oxytocin- and Vasopressin-secreting Neurons in the Rat Supraoptic Nucleus and its Contribution to Lactation-induced Synaptic Plasticity

The present ultrastructural study analysed the distribution of glutamatergic synapses on oxytocin- and vasopressin-secreting neurons in the rat supraoptic nucleus (SON) after post-embedding immunogold labelling for glutamate and GABA, oxytocin or vasopressin. About 20% of SON axo—somatic synapses were enriched in glutamate immunoreactivity, visible over synaptic-like vesicles, mitochondria and synaptic densities. Double labelling for glutamate and GABA showed that putative glutamatergic terminals were distinct from GABAergic terminals. In ultrathin sections stained for glutamate and either oxytocin or vasopressin, the proportion of glutamatergic synapses was similar on oxytocinergic and vasopressinergic somata in virgin rats under basal conditions of peptide release as well as in lactating rats, in which oxytocin secretion is enhanced. Cross-sectional soma areas were significantly increased in lactating rats: oxytocinergic profiles were, on average, ˜40% larger than in virgin rats. However, the incidence of axo—somatic glutamatergic synapses (assessed as mean number of synapses per 100 μm of plasmalemma or proportion of somatic surface apposed to synaptic active zones) did not diminish, indicating that there was a compensatory increase of synapses during lactation. Also, we found an increase in the number of glutamatergic terminals making synaptic contact simultaneously onto two or more oxytocinergic elements in the same plane of section. Our observations therefore indicate that SON oxytocinergic and vasopressinergic neurons are innervated to a similar extent by a relatively large proportion of glutamatergic synapses. They reveal, moreover, that glutamatergic afferents participate in the lactation-induced synaptic plasticity of the oxytocinergic system.     

Electrical activity of dorsal horn neurons during the suckling-induced milk ejection reflex in the lactating rat

In lactating rats, suckling elicits the milk ejection reflex which consists of an intermittent synchronous activation of hypothalamic oxytocin neurons which releases oxytocin into the bloodstream. We here investigated the electrophysiological behaviour of spinal cord neurons linked to mammary innervation in relation to suckling and the suckling-induced milk ejection reflex. Experiments were carried out on 58 urethane-anaesthetized rats, paralysed with gallamine triethiodide and artificially ventilated. Fixation of the spinal cord and laminectomy significantly slowed down the reflex, which occurred in only 27% of the animals. In these rats, 31 dorsal horn neurons at the thoraco-lumbar level were found to be excited by nipple stimulation. During suckling by a litter of at least nine pups, they displayed an irregular pattern of brief bursts of activity (peak firing rate 22.0 ± 3.2 Hz, mean ± SD) correlated to the bouts of suckling of the pups. Seventeen out of 19 cells tested by stimulation of at least 2 adjacent nipples received convergent input from different ipsilateral nipples. Out of 11 cells tested, 8 were also activated by stimulation of a contralateral nipple. Fourteen out of 30 units were recorded through at least one reflex milk ejection. Their firing rate was significantly higher than the firing rate of cells recorded in animals which failed to milk eject (4.4 Hz ± 2.8 versus 1.5 Hz ± 0.7). At the moment of the high frequency discharge of action potentials, occurring in oxytocinergic cells 10 to 15 s before each milk ejection, spinal neurons showed no systematic change in electrical activity. In contrast, the stretch reaction of the pups, which corresponds to an intense period of suckling when milk ejection has started, induced, in 12 cells, a considerable increase in electrical activity. One unit was found to be inhibited by suckling and during the stretch reaction. Ten more units, which were not activated by stimulation of the nipples but responded to stimulation of excitatory receptive fields near the last three pairs of nipples, were recorded through reflex milk ejections: 8 remained silent during reflex milk ejections but 2 were activated when the pups stimulated their excitatory receptive field. We conclude that some dorsal horn neurons, able to respond readily to the suckling movements of pups, appear to receive an ungated input from the nipples. At the time of the activation of oxytocin neurons, they present no particular pattern of activation or inhibition which could account in a simple manner for the intermittence of the high frequency discharge in oxytocinergic cells. However, in so far as these dorsal horn neurons may be part of the milk ejection reflex pathway, their activity, showing convergence and summation of input, and being facilitated in milk ejecting animals, indicates that the reflex does undergo a certain degree of processing at the spinal cord level.     

Effects of neurotensin on the organization of activity in supraoptic nucleus cells in virgin and lactating rats

Neurotensin increases the firing rate of supraoptic nucleus oxytocin and vasopressin neurones in vitro and induces Fos protein expression in the supraoptic nucleus in vivo. Here, we used extracellular single-unit electrophysiological recording combined with local microdialysis administration of neurotensin (1 mM at 2 micro l/min) to investigate the effects of locally applied neurotensin on the firing of oxytocin and vasopressin neurones in urethane-anaesthetized virgin and lactating rats. Neurotensin decreased the mean firing rate of oxytocin cells in virgin, but not lactating, rats. In addition, neurotensin increased the index of dispersion (a measure of the variability of firing) in virgin, but not lactating, rats. By contrast to oxytocin cells, neurotensin increased the mean firing rate of vasopressin cells in both virgin and lactating rats, but did not alter the index of dispersion. The increase in firing of phasic vasopressin cells was achieved through an increase in intraburst frequency (rather than an increase in burst duration or decrease in interburst interval), which resulted from a reduction of the spike-frequency adaptation that develops over the course of phasic bursts. Thus, neurotensin has differential effects on activity patterning in oxytocin and vasopressin cells and the effects on oxytocin cells, but not vasopressin cells, depend upon the physiological status of the animal. The increase in the variability of firing of oxytocin cells induced by neurotensin in virgin rats, but not in lactating rats, suggests that neurotensin (or other neurotransmitters/neuromodulators with similar actions) might establish conditions that predispose oxytocin cells to fire in milk-ejection bursts in lactating rats.     

Responses of paraventricular and supraoptic units to angiotensin II, SAR-ILE-angiotensin II and hypertonic NaCl administered into the cerebral ventricle

Extracellular recordings of action potentials were made from neurones antidromically identified as neurosecretory cells in the paraventricular and supraoptic nuclei of urethane-anesthetized female rats. Eighty-six neurones were examined for their responsiveness to 10 ng of angiotensin II (AII) injected into the third cerebral ventricle and 78 (91%) of them increased their firing rate following the AII injection. None of the neurosecretory cells tested showed a response to the intraventricular (IVT) injection of isotonic NaCl. Thalamic neurones and non-neurosecretory hypothalamic neurones did not respond to the AII given IVT. Firing activity of 13 neurosecretory neurones was recorded during reflex milk ejection induced by suckling pups in the lactating rats. Seven of them were classified as oxytocinergic cells because they showed a burst of activity before reflex milk ejections and the remaining 6 neurones which gave no burst of firing before milk ejections were classified as non-oxytocinergic neurones. The IVT application of AII resulted in activation of all the oxytocinergic neurones and 5 of the 6 non-oxytocinergic neurones. The effect of AII on the firing of the neurosecretory cell was inhibited by the simultaneous application of Sar¹-Ile⁸-AII (1 μg), a competitive AII antagonist. The IVT injection of the antagonist alone inhibited the spontaneous firing of the neurosecretory cells, but it did not affect the firing of thalamic or non-neurosecretory hypothalamic neurones. Hypertonic NaCl (0.85 M NaCl, 1 μ1 IVT) also activated 13 of 20 neurosecretory cells tested. Combined application of AII and hypertonic NaCl elicited a marked potentiation of the response of neurosecretory cells to each of the stimuli. These findings indicate that AII activates neurosecretory cells stimulating specific AII receptors in the brain and that AII has a synergistic action with hypertonic NaCl. Inhibition of spontaneous activity of neurosecretory cells by a competitive AII antagonist suggests that endogenous AII may participate in the maintenance of basal activity of neurosecretory cells.     

Activation of brown adipose tissue thermogenesis by chemical stimulation of the hypothalamic supraoptic nucleus

Glutamate microinjection (1 M, 250 nl) into the hypothalamic supraoptic nucleus (SON) stimulated heat production in brown adipose tissue (BAT) and caused a rapid and sustained increase in interscapular BAT and core temperatures in urethane-anaesthetized rats. This effect was blocked by intraperitoneal pretreatment with a sympathetic ganglionic blocker, chlorisondamine chloride (2.5 mg/kg), or a ß-adrenergic receptor blocker, propranolol (2.5 mg/kg), but not by prior hypophysectomy or intracerebroventricular pretreatment with specific receptor blockers to vasopressin (d(CH₂)5[Tyr(Me)²]AVP, 5 μg) or oxytocin (d(CH₂5[Tyr(Me)²,Thr⁴,Tyr-NH₂⁹]OVT, 5 μg). The results demonstrate that stimulation of SON cells with glutamate elicits a non-vasopressinergic/non-oxytocinergic neural signal that can bring about a sympathetically-mediated increase in BAT thermogenesis. Heat production in BAT is an important mechanism of thermal protection during cold stimulation, and there is evidence that osmotic stimulation can influence thermoregulation. SON neurons play a major role in osmoregulation via release of the peptide hormones vasopressin and oxytocin. The present results suggest the possibility that apart from releasing peptide hormones for osmoregulation, SON neurons might be involved in mediating the effect of osmotic stimulation on thermoregulatory responses involved in thermal adaptation.     

Central interactions between angiotensin II and PGD2 in the regulation of vasopressin and oxytocin secretion in dehydrated rats

Brain-derived angiotensin II (ANG II) and prostaglandins have important roles in the regulation of body fluid and blood pressure homeostasis. In the present studies we investigated the central interactions between these two neurochemical products in regulating the hypothalamo-neurohypophysial system during dehydration. Intracerebroventricular (icv) administration of prostaglandin D₂ (PGD₂; 20 μg/5 μl) to conscious adult male Sprague–Dawley rats deprived of water for 24 h did not alter significantly the already elevated plasma levels of vasopressin or oxytocin. When PGD₂ was administered in combination with losartan, an antagonist of ANG II AT₁-receptor subtype, however, concentrations of both hormones in plasma became further elevated. Icv administration of ANG II (50 ng/5 μl) increased further the enhanced plasma levels of vasopressin and oxytocin, as expected. Pretreatment with indomethacin (200 μg/5 μl; icv), an inhibitor of cyclo-oxygenase, significantly attenuated the ANG II-induced increase in oxytocin secretion only. Independent of the presence of ANG II, however, indomethacin decreased plasma levels of vasopressin, but not oxytocin. These results indicate that a prostaglandin is required for the stimulated release of vasopressin during dehydration and that the elevation of oxytocin secretion in response to ANG II depends largely on activation of cyclo-oxygenase and production of prostaglandins. The oxytocin response to exogenously administered PGD₂, however, can be negatively modulated by a mechanism dependent upon ANG II AT₁ receptors.     

In situ voltammetric microelectrodes: Application to the measurement of median eminence catecholamine release during simulated suckling

Catechol-sensitive microelectrodes (10–30 μm) were developed and then used to study the dynamic regulatory role of the prolactin inhibiting factor, dopamine, under conditions of simulated suckling. Current flow resulting from the electrochemical oxidation of catecholamines at the microelectrode surface was linearly related to the concentration of catecholamines present in solution over the range of 5–100 μM. Endogenous catecholamine levels in the rat median eminence were readily detectable and the electrochemical signal corresponding to dopamine release responded in an appropriate manner to various pharmacologic manipulations. We then implanted carbon microelectrodes into the medial median eminence region among capillaries of the primary portal plexus of urethane anesthetized lactating rats. Catecholamine release into the extracellular fluid was electrochemically measured once each minute before, during and after electrical stimulation (15 Hz, 5–30 V, 15 min) of a surgically isolated mammary nerve trunk. This simulated suckling paradigm reliably evoked prolactin secretory episodes qualitatively similar to those observed during suckling of the nipples by the young. During the period of nerve stimulation, a transient (3–5 min) 65% decline in electrochemically detectable catecholamine release was observed. Following cessation of nerve stimulation an oscillatory pattern of catecholamine release was observed with an overall trend toward an increased level of release. This latter observation corresponds with previous reports of increased hypothalamic dopamine turnover during or following suckling and with the increased levels of dopamine measured in hypophysial portal blood following mammary nerve stimulation. The transient nature of the decline of catecholamine release during the nerve stimulation period may explain why a similar observation has not been forthcoming from experiments utilizing the stalk blood collection technique (unless the standard collection periods are considerably shortened). These observations lead us to reject the hypothesis of a mirror image relationship between stalk blood dopamine and peripheral prolactin levels. Instead, we suggest that a transient decline in dopamine secretion coincident with the onset of suckling acts to prepare the pituitary lactotrophs to respond to a prolactin releasi ng factor which then facilitates prolactin secretion.     

The role of central adrenergic receptors in the reflex release of oxytocin

The effects of the α-adrenergic receptor antagonists, phentolamine and phenoxybenzamine, and the β-antagonists, propranolol, oxprenolol and practolol on the milk-ejection reflex of the rat were studied. The mother rats, at day 8–11 or lactation, were anaesthetized with urethane (1.2–1.4 g/kg, i.p.) and litters of 9–11 hungry pups were placed on the nipples to suckle for 2–6 h. With 65% of the 425 rats studied, a regular pattern of milk ejection was observed, from the changes in pup behaviour and intramammary pressure, with milk ejection recurring at intervals of 3–15 min throughout the nursing period.Propranolol (1.0–1.5 mg/kg, i.v.) and the other β-antagonists failed to either facilitate or inhibit the suckling-induced reflex when given to animals that were already milk ejecting. When given to animals which were not milk ejecting or which had ceased to milk eject in response to the suckling stimulus, both propranolol and oxprenolol at doses as low as 30 μg/kg promoted a normal pattern of reflex milk ejection. Practolol (1.5 mg/kg, i.v.), a β-antagonist with limited access to the brain, did not facilitate the reflex when given to these refractory animals. Both the racemate and the l-isomer of propranolol were effective in promoting reflex milk ejection when given by either the intravenous or intraventricular route; the d-isomer was ineffective even at 1.5 mg/kg, i.v. The rats which had failed to milk eject when suckled had mammary glands that were as sensitive to oxytocin as other animals, and the abrupt onset of the milk-ejection reflex following propranolol was apparently not related to the lowered threshold of mammary gland sensitivity to oxytocin promoted by the β-antagonists. Slow infusions of adrenaline and isoprenaline (0.1–0.2 μg/min) failed to inhibit the milk-ejection reflex even at doses that reduced the gland sensitivity to oxytocin.The α-adrenergic antagonists had an opposite effect and produced a dose-dependent inhibition of the reflex when given to animals that were milk ejecting at regular intervals. Thus, 1 mg/kg phentolamine, i.v., increased the milk ejection interval 6-fold and 2 mg/kg increased the interval 13-fold. Both phentolamine and phenoxybenzamine caused a marked fall in blood pressure, but a similar fall in blood pressure was induced with hexamethonium (2 mg/kg, i.v.) without altering the pattern of reflex milk ejection. Two other centrally active drugs were also examined, picrotoxin (2–4 mg/kg, i.v.) and hyoscine (90 mg/kg, i.v.), but neither were found to influence the reflex.These results suggest that both α- and β-adrenergic receptors are involved in the central control of the milk-ejection reflex in the anaesthetized rat, and both are probably activated by neuronally released noradrenaline. The α-receptors would appear to form a component of the natural reflex and are excitatory. The β-receptors are external to the reflex and are inhibitory.     

Intracerebroventricular injection of choline increases plasma oxytocin levels in conscious rats

In the present study, we examined the effect of intracerebroventricularly (i.c.v.) injected choline on both basal and stimulated oxytocin release in conscious rats. I.c.v. injection of choline (50–150 μg) caused time- and dose-dependent increases in plasma oxytocin levels under normal conditions. The increase in plasma oxytocin levels in response to i.c.v. choline (150 μg) was greatly attenuated by the pretreatment of rats with atropine (10 μg; i.c.v.), muscarinic receptor antagonist. Mecamylamine (50 μg; i.c.v.), a nicotinic receptor antagonist, failed to suppress the effect of 150 μg choline on oxytocin levels. Pretreatment of rats with 20 μg of hemicholinium-3 (HC-3), a specific inhibitor of choline uptake into nerve terminals, greatly attenuated the increase in plasma oxytocin levels in response to i.c.v. choline injection. Osmotic stimuli induced by either oral administration of 1 ml hypertonic saline (3 M) following 24-h dehydration of rats (type 1) or an i.c.v. injection of hypertonic saline (1 M) (type 2) increased plasma oxytocin levels significantly, but hemorrhage did not alter basal oxytocin concentrations. The i.c.v. injection of choline (50, 150 μg) under these conditions caused an additional and significant increase in plasma oxytocin concentrations beyond that produced by choline in normal conditions. These data show that choline can increase plasma oxytocin concentrations through the stimulation of central cholinergic muscarinic receptors by presynaptic mechanisms and enhance the stimulated oxytocin release.     

A suspected infrared-recipient nucleus in the brainstem of the vampire bat,Desmodus rotundus

The effects of suckling on the metabolic activity both of pituitary tissue and of subnuclei in the supraoptic and paraventricular nuclei of the hypothalamus were assessed by means of the [14C]deoxyglucose (2-DG) method of autoradiography. Three groups of female rats were deprived of their pups and of food for 6 h on the fifth or sixth day after parturition. Metabolic activity was assessed either before, during, or after a bout of suckling. The anterior pituitary is more active in suckled than in either non-suckled or postsuckled females. These differences may be a consequence of depletion/transformation/repletion of prolactin during a bout of suckling. The posterior pituitary is metabolically more active in suckled females and in postsuckled females than in non-suckled rats. These differences may reflect metabolic activity needed for the dynamics of oxytocin release and the restoration of ionic gradients. The dorsal and ventral subdivisions of the supraoptic nucleus and the lateral, dorsal, and posterior subnuclei of the paraventricular nucleus were more active in postsuckled females than in non-suckled females. These metabolic changes in the subnuclei of the magnocellular neurohypophysial system may reflect increased resynthesis of oxytocin (and vasopressin) in the cell bodies following a bout of suckling.     

Taurine selectively modulates the secretory activity of vasopressin neurons in conscious rats.

Previous experiments have shown that a 10-min forced swimming session triggers the release of vasopressin from somata and dendrites, but not axon terminals, of neurons of the hypothalamic-neurohypophysial system. To further investigate regulatory mechanisms underlying this dissociated release, we forced male Wistar rats to swim in warm (20 degrees C) water and monitored release of the potentially inhibitory amino acids gamma amino butyric acid (GABA) and taurine into the hypothalamic supraoptic nucleus using microdialysis. Forced swimming caused a significant increase in the release of taurine (up to 350%; P < 0.05 vs. prestress release), but not GABA. To reveal the physiological significance of centrally released taurine, the specific taurine antagonist 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide was administered into the supraoptic nucleus via retrodialysis. Administration of this antagonist caused a significant increase in the release of vasopressin within the supraoptic nucleus and into the blood both under basal conditions and during stress (up to 800%; P < 0.05 vs. basal values), without affecting hypothalamic or plasma oxytocin. Local administration of the GABA(A) receptor antagonist bicuculline, in contrast, failed to influence vasopressin secretion at either time point. In a separate series of in vivo electrophysiological experiments, administration of the same dosage of the taurine antagonist into the supraoptic nucleus via microdialysis resulted in an increased electrical activity of identified vasopressinergic, but not oxytocinergic, neurons. Taken together our data demonstrate that taurine is released within the supraoptic nucleus during physical/emotional stress. Furthermore, at the level of the supraoptic nucleus, taurine inhibits not only the electrical activity of vasopressin neurons but also acts as an inhibitor of both central and peripheral vasopressin secretion during different physiological states.     

Comparison of firing patterns in oxytocin- and vasopressin-releasing neurones during progressive dehydration

The electrical activity of neurosecretory cells in the supraoptic nucleus of the urethane-anaesthetized lactating rat was examined after periods of water deprivation ranging from 0-24 h. Supraoptic units were identified by antidromic activation following stimulation of the neurohypophysis, and classified as oxytocin or vasopressin cells according to their response during reflex milk ejection. In 65 vasopressin cells, dehydration increased the mean firing rate from 2.1 spikes/sec at 0 h to 6.8 spikes/sec at 24 h and caused a change from a slow irregular to a phasic firing pattern. Thus, after 6 h or more of dehydration, 84-100% of the vasopressin cells fired phasically, compared to 12% under normal conditions. In phasic vasopressin cells , the intraburst firing rates were closely related to the stages of dehydration, rising from a mean of 6.3 spikes/sec at 6 h to 12.0 spikes/sec at 24 h. However, no systematic relationship was observed between the stages of dehydration and the mean burst or silence durations. In 77 identified oxytocin units, dehydration increased the firing rate from 0.9 spikes/sec to 2.8 spikes/sec after 24 h, but only 3 (4%) of the cells showed phasic firing. Instead, the oxytocin units changed from a slow irregular to a fast continuous discharge. In conclusion, both vasopressin and oxytocin neurones are activated during chronic dehydration, but there is a marked difference in the pattern of their response. The phasic firing of the vasopressin cells may be important in increasing the occurrence of short interspike intervals and thus facilitating hormone release.     

Indomethacin prevents the -NAME-induced increase in plasma levels of oxytocin in dehydrated rats

Inhibiting NO synthase (NOS) with N^G-nitro- -arginine methyl ester ( -NAME, 250 μg/5 μl of artificial cerebrospinal fluid (aCSF)) injected intracerebroventricularly (i.c.v.) increased already enhanced levels of oxytocin, but not vasopressin, in conscious adult male Sprague–Dawley rats dehydrated for 24 h. Intracerebroventricular pretreatment with indomethacin (200 μg/5 μl aCSF), an inhibitor of cyclo-oxygenase, but not with losartan (25 μg/5 μl aCSF), an antagonist of angiotensin II (ANG II) AT₁-receptor subtype, nearly prevented the elevation in oxytocin levels after -NAME. Thus, NO inhibits prostaglandin (but not ANG II) mediated the modulatory actions of NO on oxytocin secretion from the hypothalamo-neurohypophysial system (HNS) during water deprivation.     

Reduced Oxytocin Release from the Neural Lobe of Lactating Rats is Associated with Reduced Pituitary Content and does not Reflect Reduced Excitability of Oxytocin Neurons

Lactating rats show reduced oxytocin release compared with virgin female rats in response to a variety of stimuli, including stress and osmotic stimulation. We sought to establish whether this is a consequence of a reduced response in the oxytocin cells, or of a change in stimulus-secretion coupling at the level of the neurosecretory terminals in the neural lobe. Blood sampling experiments in anaesthetized rats showed that systemic administration of cholecystokinin resulted in significantly less oxytocin release in lactating rats than in virgin female rats. Electrophysiological recordings of single cells in the supraoptic nucleus, however, showed no difference in the responsiveness of oxytocin cells to this stimulus. Oxytocin release evoked by electrical stimulation or by depolarization with high potassium solutions was lower in isolated neural lobes from lactating rats than in glands from non-lactating rats, whereas evoked vasopressin release was similar in the two groups. The lactating rat neural lobes had a reduced oxytocin content: to study the consequences of depletion we compared hormone release evoked by electrical stimulation in vitro in neural lobes from normal male rats, and from male rats given 2% NaCI to drink for 2 or 4 days. Saline drinking resulted in a reduction in gland content of both oxytocin and vasopressin, and the evoked release of both hormones was also significantly reduced when expressed as a percentage of the gland content, as was also seen for oxytocin release for glands from lactating rats. Finally, measurement of the extracellular potassium response to stimulation of the isolated neural lobe as an index of the excitability of neural lobe neurosecretory axons was unchanged in lactating rats compared with virgin female rats. Together, the data indicate that reduced oxytocin release observed in lactating rats is a simple consequence of reduced oxytocin content in the neural lobe rather than of a reduced excitability of the oxytocin neurons.     

Metabolic responses to suckling in postpartum lactating rats

The effects of suckling on the metabolic activity both of pituitary tissue and of subnuclei in the supraoptic and paraventricular nuclei of the hypothalamus were assessed by means of the [¹⁴C]deoxyglucose (2-DG) method of autoradiography. Three groups of female rats were deprived of their pups and of food for 6 h on the fifth or sixth day after parturition. Metabolic activity was assessed either before, during, or after a bout of suckling.The anterior pituitary is more active in suckled than in either non-suckled or postsuckled females. These difference may be a consequence of depletion/transformation/repletion of prolactin during a bout of suckling. The posterior pituitary is metabolically more active in suckled females and in postsuckled females than in non-suckled rats. These differences may reflect metabolic activity needed for the dynamics of oxytocin release and the restoration of ionic gradients.The dorsal and ventral subdivisions of the supraoptic nucleus and the lateral, dorsal, and posterior subnuclei of the paraventricular nucleus were more active in postsuckled females than in non-suckled females. These metabolic changes in the subnuclei of the magnocellular neurohypophysial system may reflect increased resynthesis of oxytocin (and vasopressin) in the cell bodies following a bout of suckling.     

The Noradrenergic Innervation of Identified Hypothalamic Magnocellular Somata and its Contribution to Lactation-Induced Synaptic Plasticity

Despite several studies showing that the rat supraoptic (SON) and paraventricular (PVN) nuclei are innervated by noradrenergic afferents, the respective contribution of these inputs to the oxytocinergic and vasopressinergic neuronal populations remains to be clearly defined. In the present study, we used the unbiased disector method to estimate the numerical density of noradrenergic varicosities on identified oxytocinergic and vasopressinergic somata in the rat SON and PVN. The analysis was carried out on semithin (1  μm) plastic sections cut from vibratome slices (50  μm) of the SON and PVN which had been double-labelled for noradrenaline (NA) and oxytocin- or vasopressin-related neurophysin. These preparations displayed many noradrenergic varicosities which electron microscopy showed to represent, in the main, synaptic boutons. Our quantitative analysis revealed that noradrenergic varicosities contacted oxytocinergic and vasopressinergic somata to a similar extent in male and female rats, under basal conditions of hormone secretion. The incidence of these axo-somatic contacts was similar in the SON and PVN. In contrast, in lactating rats, in which oxytocin secretion is enhanced, there was a significant increase in the density of noradrenergic varicosities apposed to oxytocinergic somata, in both nuclei. Our observations indicate that, in male and female rats under normal conditions, noradrenergic afferents innervate each type of neurosecretory somata, in both magnocellular nuclei, in a similar fashion. They reveal, moreover, that noradrenergic afferents participate in lactation-induced structural plasticity of synapses impinging on oxytocinergic somata.