Firing Rate Of Oxytocin Neurons Research Articles

Excitatory Effects of Astrocytic Hydrogen Sulfide on the Electrical Activity of Oxytocin Neurons in the Supraoptic Nucleus

Introduction: In the regulation of oxytocin (OT) neuronal activity, hydrogen sulfide (H₂S), a gaseous neurotransmitter, likely exerts an excitatory role. This role is associated with increased expression of astrocytic cystathionine-β-synthase (CBS), the key enzyme for H₂S synthesis. However, it remains unclear whether H₂S is mainly produced in astrocytes and contributes to the autoregulation of OT neurons. Methods: In hypothalamic slices of male rats, OT and H₂S-associated drug effects were observed on the firing activity and spontaneous excitatory postsynaptic currents (sEPSCs) of putative OT neurons in the supraoptic nucleus (SON) in whole-cell patch-clamp recording. Expression of glial fibrillary acidic protein (GFAP) in the SON was analyzed in Western blots. In addition, changes in the length of rat pups’ hypothalamic astrocytic processes were observed in primary cultures. Results: In brain slices, OT significantly increased the firing rate of OT neurons, which was simulated by CBS allosteric agonist S-adenosyl-L-methionine (SAM) and H₂S slow-releasing donor GYY4137 but blocked by CBS inhibitor aminooxyacetic acid (AOAA). L-α-aminoadipic acid (a gliotoxin) blocked SAM-evoked excitation. OT and SAM also increased the frequency and amplitude of sEPSCs; the effect of OT was blocked by AOAA. Both OT and GYY4137 reduced GFAP expression in the SON. Morphologically, OT or GYY4137 time-dependently reduced the length of astrocytic processes in primary cultures. Conclusions: These findings indicate that the auto-excitatory effect of OT on OT neurons is mediated by H₂S from astrocytes at least partially and astrocytic H₂S can elicit retraction of astrocytic processes that subsequently increase OT neuronal excitability.

Local kisspeptin excitation of rat oxytocin neurones in late pregnancy.

The hormone, oxytocin, is synthesised by magnocellular neurones of the supraoptic and paraventricular nuclei and is released from the posterior pituitary gland into the circulation to trigger uterine contractions during parturition. Kisspeptin fibre density increases around the supraoptic nucleus over pregnancy and intracerebroventricular kisspeptin excites oxytocin neurones only in late pregnancy. However, the mechanism of this excitation is unknown. Here, we found that microdialysis administration of kisspeptin into the supraoptic nucleus consistently increased the action potential (spike) firing rate of oxytocin neurones in urethane‐anaesthetised late‐pregnant rats (gestation day 18–21) but not in non‐pregnant rats. Hazard analysis of action potential firing showed that kisspeptin specifically increased the probability of another action potential firing immediately after each action potential (post‐spike excitability) in late‐pregnant rats. Patch‐clamp electrophysiology in hypothalamic slices showed that bath application of kisspeptin did not affect action potential frequency or baseline membrane potential in supraoptic nucleus neurones. Moreover, kisspeptin superfusion did not affect the frequency or amplitude of excitatory postsynaptic currents or inhibitory postsynaptic currents in supraoptic nucleus neurones. Taken together, these studies suggest that kisspeptin directly activates oxytocin neurones in late pregnancy, at least in part, via increased post‐spike excitability.Key points Oxytocin secretion is triggered by action potential firing in magnocellular neurones of the hypothalamic supraoptic and paraventricular nuclei to induce uterine contractions during birth.In late pregnancy, kisspeptin expression increases in rat periventricular nucleus neurones that project to the oxytocin system.Here, we show that intra‐supraoptic nucleus administration of kisspeptin increases the action potential firing rate of oxytocin neurones in anaesthetised late‐pregnant rats, and that the increased firing rate is associated with increased oxytocin neurone excitability immediately after each action potential.By contrast, kisspeptin superfusion of hypothalamic slices did not affect the activity of supraoptic nucleus neurones or the strength of local synaptic inputs to supraoptic nucleus neurones.Hence, kisspeptin might activate oxytocin neurons in late pregnancy by transiently increasing oxytocin neuron excitability after each action potential.

Astrocytic Hydrogen Sulfide Regulates Supraoptic Cellular Activity in the Adaptive Response of Lactating Rats to Chronic Social Stress.

Maternal social stress among breastfeeding women can be adapted in chronic process. However, neuroendocrine mechanisms underlying such adaptation remain to be identified. Here, we report the effects of 2 hr/day unfamiliar male rat invasion (UMI) stress on maternal behaviors in lactating rats during postpartum day 8 (UMI8) to postpartum day 12 (UMI12). Rat dams at UMI8 presented signs of maternal anxiety, depression, and attacks toward male intruder. These changes partially reversed at UMI12 except the sign of anxiety. In the supraoptic nucleus (SON), UMI12 but not UMI8 significantly increased the expression of c-Fos and phosphorylated extracellular signal-regulated protein kinase 1/2. At UMI8 but not UMI12, length of glial fibrillary acidic protein (GFAP, astrocytic cytoskeletal element) filaments around oxytocin (OT) neurons was significantly longer than that of their controls; the amount of GFAP fragments at UMI12 was significantly less than that at UMI8. Expression of cystathionine β-synthase (CBS, enzyme for H2S synthesis) at UMI12 was significantly higher than that at UMI8. CBS expression did not change significantly in the somatic zone of the SON but decreased significantly at the ventral glia lamina at UMI8. In brain slices of the SON, aminooxyacetate (a CBS blocker) significantly increased the expression of GFAP proteins that were molecularly associated with CBS. Aminooxyacetate also reduced the firing rate of OT neurons whereas Na2S, a donor of H2S, increased it. The adaptation during chronic social stress is possibly attributable to the increased production of H2S by astrocytes and the subsequent retraction of astrocytic processes around OT neurons.

Oxytocin Modulation of Maternal Behavior and Its Association With Immunological Activity in Rats With Cesarean Delivery.

Oxytocin (OT), a neuropeptide produced in the supraoptic (SON) and paraventricular (PVN) nuclei, is not only essential for lactation and maternal behavior but also for normal immunological activity. However, mechanisms underlying OT regulation of maternal behavior and its association with immunity around parturition, particularly under mental and physical stress, remain unclear. Here, we observed effects of OT on maternal behavior in association with immunological activity in rats after cesarean delivery (CD), a model of reproductive stress. CD significantly reduced maternal interests to the pups throughout postpartum day 1-8. On postpartum day 5, CD decreased plasma OT levels and thymic index but increased vasopressin, interleukin (IL)-1β, IL-6 and IL-10 levels. CD had no significant effect on plasma adrenocorticotropic hormone and corticosterone levels. In the hypothalamus, CD decreased corticotropin-releasing hormone contents in the PVN but increased OT contents in the PVN and SON and OT release from hypothalamic implants. CD also increased c-Fos expression, particularly in the cytoplasm of OT neurons. Lastly, CD depolarized resting membrane potential and increased spike width while increasing the variability of the firing rate of OT neurons in brain slices. Thus, CD can increase hypothalamic OT contents and release but reduce pituitary release of OT into the blood, which is associated with depressive-like maternal behavior, increased inflammatory cytokine release and decreased relative weight of the thymus.

Kisspeptin Regulation of Oxytocin Neuron Activity in Late Pregnancy

Oxytocin is secreted from the posterior pituitary gland by oxytocin neurons to trigger uterine contractions during birth. We have recently shown that the oxytocin neuron firing rate increases in response to intracerebroventricular (ICV) kisspeptin only in late‐pregnant rats. Furthermore, the kisspeptin projection from the periventricular nucleus of the hypothalamus (PeN) to the perinuclear zone (PNZ) surrounding the supraoptic nucleus (SON) increases in late pregnancy. This study aimed to determine the site and mechanism of central kisspeptin activation of oxytocin neuron activity in late pregnant rats. First, in vivo extracellular single unit recordings were made from SON neurons in urethane‐anaesthetised non‐pregnant and late‐pregnant rats during microdialysis application of kisspeptin (100 μM in the dialysate) into the SON. Intra‐SON kisspeptin consistently increased the firing rate of oxytocin neurons in late‐pregnant rats but not in non‐pregnant rats. Two‐way repeated measures (RM) ANOVA revealed a significant interaction between REPRODUCTIVE STATUS and TIME (F(6,88) = 10.31, p < 0.001); post hoc all pairwise Holm‐Sidak tests revealed a significant increase in firing rate in late‐pregnant rats at 30 (p < 0.05), 40, 50 and 60 minutes (all p < 0.001 versus pre‐kisspeptin), and a significantly higher firing rate than in time‐matched recordings from non‐pregnant rats at 50 and 60 minutes (p < 0.05).Then, to examine whether excitatory effects of central kisspeptin emerges directly within the SON and paraventricular nucleus (PVN) containing oxytocin neurons, we performed single label diaminobenzidine (DAB) immunostaining against phosphorylated extracellular regulated kinase (pERK), a second messenger activated by kisspeptin. ICV cannulae were implanted on day 13 or 14‐pregnant and non‐pregnant rats and one week later, day 21 pregnant and non‐pregnant rats were perfused 15 min after ICV administration of kisspeptin (2 μg/2 μl) or vehicle under pentobarbital anaesthesia. ICV kispeptin injection did not alter pERK expression in the SON or PVN. Two‐way RM ANOVA revealed a significant effect of REPRODUCTIVE STATUS (F(1,28) = 4.78, p = 0.03) but no significant effect of TREATMENT (kisspeptin/vehicle) (F(1,28) = 0.04, p = 0.82) and no REPRODUCTIVE STATUS X TREATMENT interaction (F(1,28) = 0.81, p = 0.37 ) in the SON. There was no significant effect of REPRODUCTIVE STATUS (F(1,27) = 0.93, p = 0.34, two‐way RM ANOVA) or TREATMENT (F(1,27) = 0.93, p = 0.34) and no REPRODUCTIVE STATUS X TREATMENT interaction (F(1,27) = 0.29, p = 0.59) in the PVN. Similarly, there was no significant effect of REPRODUCTIVE STATUS (F(1,27) = 2.28, p = 0.14, two‐way RM ANOVA) or TREATMENT (F(1,27) = 0.28, p = 0.59) and no REPRODUCTIVE STATUS X TREATMENT interaction (F(1,27) = 0.07, p = 0.78) in the magnocellular division of PVN. Hence, it appears that kisspeptin excites oxytocin neurons at the end of pregnancy by a presynaptic action at local nerve terminals to stimulate peripheral oxytocin release and so might contribute to oxytocin neuron activation for parturition.

Development of an excitatory kisspeptin projection to the oxytocin system in late pregnancy.

Oxytocin release from the posterior pituitary gland stimulates uterine contraction during birth but the central mechanisms that activate oxytocin neurones for birth are not well characterized. We found that that kisspeptin fibre density around oxytocin neurones increases in late-pregnant rats. These kisspeptin fibres originated from hypothalamic periventricular nucleus neurones that upregulated kisspeptin expression in late pregnancy. Oxytocin neurones were excited by central kisspeptin administration in late-pregnant rats but not in non-pregnant rats or early- to mid-pregnant rats. Our results reveal the emergence of a new excitatory kisspeptin projection to the oxytocin system in late pregnancy that might contribute to oxytocin neurone activation for birth. The hormone oxytocin promotes uterine contraction during parturition. Oxytocin is synthesized by magnocellular neurones in the hypothalamic supraoptic and paraventricular nuclei and is released into the circulation from the posterior pituitary gland in response to action potential firing. Systemic kisspeptin administration increases oxytocin neurone activity to elevate plasma oxytocin levels. Here, immunohistochemistry revealed that rats on the expected day of parturition (day21 of gestation) had a higher density of kisspeptin-positive fibres in the perinuclear zone surrounding the supraoptic nucleus (which provides dense glutamatergic and GABAergic innervation to the supraoptic nucleus) than was evident in non-pregnant rats. Retrograde tracing showed the kisspeptin projections to the perinuclear zone originated from the hypothalamic periventricular nucleus. Quantitative RT-PCR showed that kisspeptin receptor mRNA, Kiss1R mRNA, was expressed in the perinuclear zone-supraoptic nucleus and that the relative Kiss1R mRNA expression does not change over the course of pregnancy. Finally, intracerebroventricular administration of kisspeptin increased the firing rate of oxytocin neurones in anaesthetized late-pregnant rats (days 18-21 of gestation) but not in non-pregnant rats, or in early- or mid-pregnant rats. Taken together, these results suggest that kisspeptin expression is upregulated in the periventricular nucleus projection to the perinuclear zone of the supraoptic nucleus towards the end of pregnancy. Hence, this input might activate oxytocin neurones during parturition.

Systemic Leptin Increases the Electrical Activity of Supraoptic Nucleus Oxytocin Neurones in Virgin and Late Pregnant Rats

In the rat hypothalamus, fasting attenuates the expression of oxytocin and this can be reversed by exogenous leptin administration. In the present study, we investigated the effects of systemically administered leptin on the electrical activity of magnocellular neurones in the supraoptic nucleus of urethane-anaesthetised rats. In virgin female rats, systemic leptin significantly excited identified oxytocin neurones with no detected effects on the patterning of activity, as reflected by hazard function analyses. The lowest dose that was consistently effective was 100 μg/i.v., and this dose had no significant effect on vasopressin neurones. In virgin rats fasted overnight, the spontaneous firing rate of oxytocin neurones was significantly lower than in unfasted rats, although leptin had a similar excitatory effect as in unfasted rats. In late pregnant rats (days 19-21 of pregnancy), spontaneous firing rates of oxytocin neurones were higher than in virgins, and the initial response to leptin was similar to that in virgin rats, although the increase in activity was more persistent. In fasted pregnant rats, the mean spontaneous firing rate of oxytocin neurones was again lower than in unfasted rats, although leptin had no significant effect even at the higher dose of 1 mg/rat. Thus, fasting reduced the spontaneous firing rates of oxytocin neurones in nonpregnant rats, and this effect could be reversed by the excitatory effects of leptin. Pregnant rats showed some evidence of leptin resistance but only after an overnight fast.

Allopregnanolone and Induction of Endogenous Opioid Inhibition of Oxytocin Responses to Immune Stress in Pregnant Rats

In virgin rats, systemic administration of interleukin (IL)-1β (i.e. to mimic infection), increases oxytocin secretion and the firing rate of oxytocin neurones in the supraoptic nucleus (SON). However, in late pregnancy, stimulated oxytocin secretion is inhibited by an endogenous opioid mechanism, preserving the expanded neurohypophysial oxytocin stores for parturition and minimising the risk of preterm labour. Central levels of the neuroactive metabolite of progesterone, allopregnanolone, increase during pregnancy and allopregnanolone acting on GABA(A) receptors on oxytocin neurones enhances inhibitory transmission. In the present study, we tested whether allopregnanolone induces opioid inhibition of the oxytocin system in response to IL-1β in late pregnancy. Inhibition of 5α-reductase (an allopregnanolone-synthesising enzyme) with finasteride potentiated IL-1β-evoked oxytocin secretion in late pregnant rats, whereas allopregnanolone reduced the oxytocin response in virgin rats. IL-1β increased the number of magnocellular neurones in the SON and paraventricular nucleus (PVN) expressing Fos (an indicator of neuronal activation) in virgin but not pregnant rats. In immunoreactive oxytocin neurones in the SON and PVN, finasteride increased IL-1β-induced Fos expression in pregnant rats. Conversely, allopregnanolone reduced the number of magnocellular oxytocin neurones activated by IL-1β in virgin rats. Treatment with naloxone (an opioid antagonist) greatly enhanced the oxytocin response to IL-1β in pregnancy, and finasteride did not enhance this effect, indicating that allopregnanolone and the endogenous opioid mechanisms do not act independently. Indeed, allopregnanolone induced opioid inhibition over oxytocin responses to IL-1β in virgin rats. Thus, in late pregnancy, allopregnanolone induces opioid inhibition over magnocellular oxytocin neurones and hence on oxytocin secretion in response to immune challenge. This mechanism will minimise the risk of preterm labour and prevent the depletion of neurohypophysial oxytocin stores, which are required for parturition.

Kisspeptin Activation of Supraoptic Nucleus Neurons in Vivo

Oxytocin and vasopressin are synthesized by magnocellular neurosecretory cells in the hypothalamic supraoptic and paraventricular nuclei and are released from the posterior pituitary gland into the circulation. Intravenous administration of the ligand for the G protein-coupled receptor 54 receptor, kisspeptin-10, increases plasma oxytocin levels and intracerebroventricular kisspeptin-10 increases vasopressin levels, indicating that kisspeptin might play a role in various physiological functions via stimulation of oxytocin and vasopressin secretion. Because posterior pituitary hormone secretion is dependent on action potential (spike) discharge, we used in vivo extracellular single unit recording to determine the effects of kisspeptin-10 on supraoptic nucleus neurons in urethane-anaesthetized female rats. Intravenous kisspeptin-10 (100 μg) increased the firing rate of oxytocin neurons from 3.7 ± 0.8 to 4.7 ± 0.8 spikes/sec (P = 0.0004), but only a quarter of vasopressin neurons responded to iv kisspeptin-10, showing a short (<3 sec) high-frequency (>15 spikes/sec) burst of firing. By contrast, intracerebroventricular kisspeptin-10 (2 and 40 μg) did not alter oxytocin or vasopressin neuron firing rate. To investigate the pathway involved in the peripheral action of kisspeptin-10, we used i.p. capsaicin to desensitize vagal afferents, which prevented the i.v. kisspeptin-10-induced increase of oxytocin neuron firing rate. This is the first report to show that peripheral, but not central, kisspeptin-10 increases the activity of oxytocin neurons and a proportion of vasopressin neurons and that endogenous kisspeptin regulation of supraoptic nucleus neurons is likely via vagal afferent input, with kisspeptin acting as a hormone rather than as a neuropeptide in this system.

Apamin increases post-spike excitability of supraoptic nucleus neurons in anaesthetized morphine-naïve rats and morphine-dependent rats: consequences for morphine withdrawal excitation

Supraoptic nucleus (SON) oxytocin neurons develop morphine dependence when chronically exposed to this opiate and undergo excitation when morphine is subsequently withdrawn. Morphine withdrawal excitation is evident as an increased action potential (spike) firing rate and is associated with an increased post-spike excitability that is consistent with the expression of an enhanced post-spike afterdepolarization (ADP) during withdrawal. Here, we administered apamin (which inhibits the medium afterhyperpolarization [mAHP] in vitro and unmasks an ADP) into the SON of urethane-anaesthetized rats to determine its effects on oxytocin neurons in vivo. As predicted, intra-SON apamin administration increased the propensity to fire a spike soon (<100ms) after each spike (post-spike excitability) more in oxytocin neurons recorded from morphine-treated rats than in morphine-naïve rats. However, intra-SON apamin did not alter the overall firing rate of oxytocin neurons recorded from morphine-treated rats or morphine-naïve rats, indicating that an increase in post-spike excitability alone is not sufficient to trigger withdrawal excitation of oxytocin neurons. Nevertheless, bilateral intra-SON apamin infusion increased oxytocin secretion (which depends on firing pattern as well as firing rate) by 90±46% in morphine-dependent rats (P<0.01 compared to aCSF). Hence, an increase in post-spike excitability does not appear to drive morphine withdrawal-induced increases in oxytocin neuron firing rate, but does contribute to withdrawal-induced hyper-secretion of oxytocin.

Circulating Secretin Activates Supraoptic Nucleus Oxytocin and Vasopressin Neurons via Noradrenergic Pathways in the Rat

Secretin is a 27-amino acid brain-gut peptide from duodenal S-cells. We tested the effects of systemic administration of secretin to simulate its postprandial release on neuroendocrine neurons of the supraoptic nucleus (SON) in urethane-anesthetized female rats. Secretin dose-dependently increased the firing rate of oxytocin neurons, more potently than cholecystokinin, and dose-dependently increased plasma oxytocin concentration. The effect of secretin on SON vasopressin neurons was also predominantly excitatory, in contrast to the inhibitory actions of cholecystokinin. To explore the involvement of noradrenergic inputs in secretin-induced excitation, benoxathian, an alpha1-adrenoceptor antagonist, was infused intracerebroventricularly. Benoxathian intracerebroventricular infusion blocked the excitation by secretin of both oxytocin and vasopressin neurons. To test the role of local noradrenaline release in the SON, benoxathian was microdialyzed onto the SON. The basal firing rate of oxytocin neurons was slightly reduced and the secretin-induced excitation was attenuated during benoxathian microdialysis. Hence, noradrenergic pathways mediate the excitation by systemic secretin of oxytocin neurons via alpha1-adrenoceptors in the SON. As both systemic secretin and oxytocin are involved in regulating gastrointestinal functions and natriuresis, systemically released secretin might act partly through oxytocin.

The Effects of Apelin on the Electrical Activity of Hypothalamic Magnocellular Vasopressin and Oxytocin Neurons and Somatodendritic Peptide Release

Apelin, a novel peptide originally isolated from bovine stomach tissue extracts, is widely but selectively distributed throughout the nervous system. Vasopressin and oxytocin are synthesized in the magnocellular neurons of the hypothalamic supraoptic nucleus (SON) and paraventricular nucleus, which are apelin-rich regions in the central nervous system. We made extracellular electrophysiological recordings from the transpharyngeally exposed SON of urethane-anaesthetized rats to assess the role of apelin in the control of the firing activity of identified magnocellular vasopressin and oxytocin neurons in vivo. Apelin-13 administration onto SON neurons via microdialysis revealed cell-specific responses; apelin-13 increased the firing rates of vasopressin cells but had no effect on the firing rate of oxytocin neurons. A direct excitatory effect of apelin-13 on vasopressin cell activity is also supported by our in vitro studies showing depolarization of membrane potential and increase in action potential firing. To assess the effects of apelin-13 on somatodendritic peptide release, we used in vitro release studies from SON explants in combination with highly sensitive and specific RIA. Apelin-13 decreases basal (by 78%; P < 0.05; n = 6) and potassium-stimulated (by 57%; P < 0.05; n = 6) vasopressin release but had no effect on somatodendritic oxytocin release. Taken together, our data suggest a local autocrine feedback action of apelin on magnocellular vasopressin neurons. Furthermore, these data show a marked dissociation between axonal and dendritic vasopressin release with a decrease in somatodendritic release but an increase in electrical activity at the cell bodies, indicating that release from these two compartments can be regulated wholly independently.

Rapid Estradiol-17β Modulation of Opioid Actions on the Electrical and Secretory Activity of Rat Oxytocin Neurons In vivo

During pregnancy, emergence of endogenous opioid inhibition of oxytocin neurons is revealed by increased oxytocin secretion after administration of the opioid receptor antagonist, naloxone. Here we show that prolonged estradiol-17beta and progesterone treatment (mimicking pregnancy levels) potentiates naloxone-induced oxytocin secretion in urethane-anesthetized virgin female rats. We further show that estradiol-17beta alone rapidly modifies opioid interactions with oxytocin neurons, by recording their firing rate in anesthetized rats sensitized to naloxone by morphine dependence. Naloxone-induced morphine withdrawal strongly increased the firing rate of oxytocin neurons in morphine dependent rats. Estradiol-17beta did not alter basal oxytocin neuron firing rate over 30 min, but amplified naloxone-induced increases in firing rate. Firing pattern analysis indicated that acute estradiol-17beta increased oxytocin secretion in dependent rats by increasing action potential clustering without an overall increase in firing rate. Hence, rapid estradiol-17beta actions might underpin enhanced oxytocin neuron responses to naloxone in pregnancy.

Suppressed oxytocin neuron responses to immune challenge in late pregnant rats: a role for endogenous opioids

Cytokine challenge (mimicking infection) with systemic interleukin-1beta (IL-1beta) stimulates oxytocin neurons via a noradrenergic brainstem pathway similar to that involved in parturition. As the responses of oxytocin neurons to several stimuli are reduced in late pregnancy, we have investigated whether responses to IL-1beta are also suppressed. In virgin Sprague-Dawley rats, IL-1beta (500 ng/kg i.v.) rapidly increased oxytocin secretion (3.2-fold), via a central action as the firing rate of oxytocin neurons in the supraoptic nucleus was increased. In contrast, IL-1beta had no significant effect on the electrical or secretory activity of oxytocin neurons in late pregnant rats. In pregnancy activation of a central inhibitory opioid mechanism restrains oxytocin neuron responses to various stimuli. Accordingly, we tested the effects of the opioid antagonist, naloxone, on oxytocin neuron responses to IL-1beta in pregnancy. Naloxone (5 mg/kg i.v.) did not affect the oxytocin secretory response to IL-1beta in virgin rats, whereas in late pregnant rats naloxone revealed a greater oxytocin secretory response to IL-1beta (3.5-fold) than in virgin rats. In virgin rats, naloxone decreased oxytocin neuron firing rate after IL-1beta, however, in pregnant rats naloxone increased the firing rate response to IL-1beta to the level seen in virgin rats. Thus, systemic IL-1beta acts centrally to increase oxytocin secretion. In pregnancy this response is suppressed by endogenous opioids, thus preserving neurohypophysial oxytocin stores for parturition and minimizing the risk of preterm labour. The exaggerated oxytocin secretory response to IL-1beta in pregnancy after naloxone reflects increased oxytocin stores and/or increased efficiency of excitation-secretion coupling at the posterior pituitary.

Local opioid inhibition and morphine dependence of supraoptic nucleus oxytocin neurones in the rat in vivo

1. Single neurones of the rat supraoptic nucleus were recorded during microdialysis of naloxone onto the ventral surface of the nucleus in anaesthetized rats. We used this combination of techniques to test whether the acute or chronic effects of systemically or centrally applied opioids upon oxytocin cell activity were due to actions of the opioids within the nucleus itself. 2. Supraoptic nucleus oxytocin neurones were identified antidromically and by an excitatory response to intravenously injected cholecystokinin. Acute intravenous injection of the kappa-agonist U50488H or the mu-agonist morphine (1-5 mg kg-1) reduced the firing rate of identified oxytocin neurones by 97.7 +/- 4.8% (n = 6) and 94.1 +/- 4.1% (n = 7), respectively. The inhibition by each of these opioids was completely reversed after administration by microdialysis (retrodialysis) of the opioid antagonist naloxone (0.1-1.0 microgram microliter-1 at 2 microliters min-1) onto the exposed ventral surface of the supraoptic nucleus. 3. Retrodialysis of naloxone (0.1-10.0 micrograms microliter-1) onto the supraoptic nucleus of rats made dependent by intracerebroventricular morphine infusion for 5 days increased the firing rate of oxytocin neurones from 0.9 +/- 0.4 to 3.1 +/- 0.7 spikes s-1 (P < 0.05, n = 6). This increase in firing rate from basal was 58.5 +/- 15.1% of that following subsequent intravenously injected naloxone (5 mg kg-1). 4. Thus, the acute inhibition of supraoptic nucleus oxytocin neurones which results from systemic administration of opioid agonists primarily occurs within the supraoptic nucleus itself, since the antagonist naloxone was effective when given into the supraoptic nucleus. Furthermore, oxytocin neurones develop morphine dependence by a mechanism which is distinct from an action on their distant afferent inputs. Nevertheless, withdrawal excitation of these afferent inputs may enhance the magnitude of oxytocin neurone withdrawal excitation.

Opioids influence neurohypophysial but not central oxytocin release following direct hyperosmotic stimulation of the supraoptic nucleus in urethane-anaesthetised rats

Microdialysis was used to apply an osmotic stimulus (0.5 M NaCl-aCSF) into both supraoptic nuclei (SON) to investigate the role of endogenous opioid peptides in the control of both central and peripheral oxytocin release in response to this stimulus. There were no differences in central peptide release during direct hyperosmotic stimulation between groups of rats given either vehicle, morphine (5 mg/kg) or naloxone (5 mg/kg) intravenously. Naloxone potentiated oxytocin release into blood; this suggests that endogenous opioid peptides at the level of the neurohypophysis, but not in the SON are important modulators of oxytocin release to this stimulus. However morphine blocked oxytocin release into blood indicative of a central inhibitory action on the firing rate of oxytocin neurones, contrasted with insensitivity to morphine of oxytocin secretion from the dendrites stimulated directly by hyperosmolarity.