Stimulated Oxytocin Release Research Articles

Local interaction of prostaglandin F 2alpha with endothelin-1 and tumor necrosis factor-alpha on the release of progesterone and oxytocin in ovine corpora lutea in vivo: a possible implication for a luteolytic cascade.

Endothelin-1 (ET-1) and tumor necrosis factor-alpha (TNFalpha) participate in the cascade of luteolysis. Thus, in the present study the interactions of ET-1 and TNFalpha with prostaglandin F(2alpha) (PGF(2alpha)) on the release of progesterone and oxytocin (OT) within the corpus luteum (CL) were investigated. A microdialysis system (MDS) was surgically implanted in ovine CL (one MDS line/CL; 5-10 lines/ewe) formed after super-ovulation. A 4-h perfusion with PGF(2alpha) (0.01-1 micromol l (-1)) induced no clear effect on progesterone release, but acutely stimulated OT release in a dose-dependent manner. A perfusion of PGF(2alpha) (1 micromol l (-1)) increased ET-1 release over a period of 12 h. Two perfusions of ET-1 (0.1 micromol l(-1)) or a perfusion of ET-1 followed by TNFalpha (200 ng ml(-1)) decreased progesterone release (56-64% at 36-48 h). When the CL were pre-perfused with PGF(2alpha) (1 micromol l(-1)), two consecutive perfusions of ET-1 decreased progesterone release more rapidly. Similarly, a pre-perfusion with PGF(2alpha) followed by consecutive perfusions of ET-1 and then TNFalpha rapidly decreased progesterone release, with the inhibition most pronounced (35%) at 36-48 h. The simultaneous infusion of ET-1 with PGF(2alpha) induced a rapid decrease in progesterone release (36% at 36-48 h). In a further study, the possible second messenger systems involved in PGF(2alpha) action on the release of progesterone, OT and ET-1 were investigated. A perfusion with 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 10 micromol l(-1)), A23187 (10 micromol l(-1)), or PGF(2alpha) + A23187 increased progesterone release during infusion, but decreased it after perfusion. All treatments induced a massive release of OT during infusion, and increased ET-1 release after infusion. These results show that ET-1 is capable of suppressing progesterone release in the PGF(2alpha)-primed ovine CL in vivo and thus ET-1 works as a local luteolysin together with PGF(2alpha) during the process of functional luteolysis. During structural luteolysis, TNFalpha may interact with PGF(2alpha) and ET-1 to cause a rapid drop in progesterone release and accelerate the process of luteolysis. This result supports the contention that ET-1 and TNFalpha interact with PGF(2alpha) as local luteolytic mediators in the ewe as previously suggested.

α-Melanocyte-Stimulating Hormone Stimulates Oxytocin Release from the Dendrites of Hypothalamic Neurons While Inhibiting Oxytocin Release from Their Terminals in the Neurohypophysis

The peptides alpha-melanocyte stimulating hormone (alpha-MSH) and oxytocin, when administered centrally, produce similar behavioral effects. alpha-MSH induces Fos expression in supraoptic oxytocin neurons, and alpha-MSH melanocortin-4 receptors (MC4Rs) are highly expressed in the supraoptic nucleus, suggesting that alpha-MSH and oxytocin actions are not independent. Here we investigated the effects of alpha-MSH on the activity of supraoptic neurons. We confirmed that alpha-MSH induces Fos expression in the supraoptic nucleus when injected centrally and demonstrated that alpha-MSH also stimulates Fos expression in the nucleus when applied locally by retrodialysis. Thus alpha-MSH-induced Fos expression is not associated with electrophysiological excitation of supraoptic neurons because central injection of alpha-MSH or selective MC4 receptor agonists inhibited the electrical activity of oxytocin neurons in the supraoptic nucleus recorded in vivo. Consistent with these observations, oxytocin secretion into the bloodstream decreased after central injection of alpha-MSH. However, MC4R ligands induced substantial release of oxytocin from dendrites in isolated supraoptic nuclei. Because dendritic oxytocin release can be triggered by changes in [Ca2+]i, we measured [Ca2+]i responses in isolated supraoptic neurons and found that MC4R ligands induce a transient [Ca2+]i increase in oxytocin neurons. This response was still observed in low extracellular Ca2+ concentration and probably reflects mobilization of [Ca2+]i from intracellular stores rather than entry via voltage-gated channels. Taken together, these results show for the first time that a peptide, here alpha-MSH, can induce differential regulation of dendritic release and systemic secretion of oxytocin, accompanied by dissociation of Fos expression and electrical activity.

Neuromedin U facilitates oxytocin release from the pituitary via beta adrenoceptors.

Neuromedin U activates noradrenergic neurones in the medulla oblongata and oxytocin neurones in the hypothalamus. Here we examined roles of noradrenergic transmission in oxytocin release from the pituitary after intracerebroventricular administration of neuromedin U in rats. Neuromedin U administration facilitated noradrenaline release in the supraoptic nucleus. Administration of a beta1 adrenoceptor antagonist, metoprolol, or a beta2 antagonist, ICI 118551 but not an alpha1 antagonist, benoxathian, reduced increases in plasma oxytocin concentrations observed after administration of neuromedin U, but plasma ACTH concentrations were not significantly changed. All theses data suggest that neuromedin U stimulates oxytocin release from the pituitary, at least in part, via activation of beta adrenoceptors.

5α-Reduced androgens block estradiol–BSA-stimulated release of oxytocin

In this study we test the postulate that estradiol conjugated to bovine serum albumin (E-BSA) acts via receptors for the steroid-binding protein sex hormone binding globulin (SHBG) by attempting to block E-BSA-stimulated release of oxytocin with two antagonists of SHBG receptor actions: the 5α-reduced androgens dihydrotestosterone (DHT) and 3α-diol. Simultaneous superfusion with either DHT or 3α-diol significantly blocked E-BSA-stimulated release of oxytocin. We also found that a wide range of free 17β-estradiol was unable to stimulate oxytocin release, suggesting that E-BSA stimulates receptors other than those for free estradiol to release oxytocin, perhaps SHBG receptors.

Facilitative role of prolactin-releasing peptide neurons in oxytocin cell activation after conditioned-fear stimuli

Emotional stress activates oxytocin neurons in the hypothalamic supraoptic and paraventricular nuclei and stimulates oxytocin release from the posterior pituitary. Oxytocin neurons in the hypothalamus have synaptic contact with prolactin-releasing peptide (PrRP) neurons. Intracerebroventricular administration of PrRP stimulates oxytocin release from the pituitary. These observations raise the possibility that PrRP neurons play a role in oxytocin response to emotional stress. To test this hypothesis, we first examined expression of Fos protein, an immediate early gene product, in the PrRP neurons in the medulla oblongata after conditioned-fear stimuli. Conditioned-fear stimuli increased the number of PrRP cells expressing Fos protein especially in the dorsomedial medulla. In order to determine whether PrRP cells projecting to the supraoptic nucleus are activated after conditioned-fear stimuli, we injected retrograde tracers into the supraoptic nucleus. Conditioned-fear stimuli induced expression of Fos protein in retrogradely labeled PrRP cells in the dorsomedial medulla. Finally we investigated whether immunoneutralization of endogenous PrRP impairs oxytocin release after emotional stimuli. An i.c.v. injection of a mouse monoclonal anti-PrRP antibody impaired release of oxytocin but not of adrenocorticotrophic hormone or prolactin and did not significantly change freezing behavior in response to conditioned-fear stimuli. From these data, we conclude that PrRP neurons in the dorsomedial medulla that project to the hypothalamus play a facilitative role in oxytocin release after emotional stimuli in rats.

Atrial natriuretic peptide mediates oxytocin secretion induced by osmotic stimulus

Atrial natriuretic peptide (ANP), first discovered in the heart, has been also detected in various brain regions involved in the control of cardiovascular function and water and sodium balance. The anteroventral region of the third ventricle (AV3V) and the subfornical organ (SFO) have ANP-immunoreactive projections towards the paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamus. Extracellular fluid (ECF) hyperosmolality stimulates the secretion of oxytocin (OT) which induces ANP release by the atrium. On the other hand, passive immunoneutralization of ANP reduces OT secretion in response to ECF hypertonicity. Previous studies have shown the co-localization of ANP and OT in PVN and SON neurons and in the periventricular region, as well as the presence of ANPergic and oxytocinergic neurons in the median eminence. The aim of the present study was to investigate the OT and ANP content in the SON and PVN of the hypothalamus and in the posterior pituitary (PP) after an osmotic stimulus that induces OT secretion. The results showed that intracerebroventricular microinjection of normal rabbit serum (NRS) or of ANP antiserum followed or not by an intraperitoneal injection of isotonic saline did not alter OT secretion or OT content in the PVN, SON, and PP; passive ANP immunoneutralization reduced the basal content of ANP in the PVN, SON, and PP of animals in a situation of isotonicity; the ANP antiserum inhibited the increase of OT secretion and content of OT and ANP in the PVN, SON and PP induced by the osmotic stimulus. Thus, the increase in plasma OT and oxytocinergic neurons of the hypothalamus–posterior pituitary system in response to hypertonicity depends on the action of endogenous ANP, i.e., ECF hypertonicity must activate ANPergic neurons which directly or indirectly stimulate OT release.

Endogenous opioid regulation of stress-induced oxytocin release within the hypothalamic paraventricular nucleus is reversed in late pregnancy: a microdialysis study

Oxytocin secretion into blood in response to swim stress is differentially regulated by endogenous opioids in virgin and pregnant rats. Here, the influence of endogenous opioids on oxytocin release within the hypothalamic paraventricular and supraoptic nuclei was investigated using microdialysis in virgin and pregnant (day 19–21) rats. Rats fitted with a U-shaped microdialysis probe 3 days before testing were injected with naloxone (5 mg/kg body weight, s.c.) or vehicle (sterile saline) and, 3 min later, were forced to swim (10 min at 19°C). Within the paraventricular nucleus, basal and stimulated oxytocin release did not significantly differ between vehicle-treated virgin and pregnant rats. After naloxone, local oxytocin release in response to swimming was lowered in virgin rats ( P<0.01), whereas it was further increased in pregnant rats ( P<0.01). Within the supraoptic nucleus, basal oxytocin release was significantly lower in pregnant compared to virgin rats ( P<0.01). Forced swimming induced a similar rise in intranuclear oxytocin release in both vehicle-treated virgin and pregnant rats, but peak levels were still higher in the virgin controls. In contrast to the paraventricular nucleus, naloxone did not alter swim-induced oxytocin release within the supraoptic nucleus either in virgin or pregnant rats. Vasopressin release in the paraventricular nucleus was also increased by forced swimming but there was no effect of pregnancy or naloxone on it. In summary, in pregnancy, basal and stress-induced oxytocin release within the paraventricular nucleus was not changed, whereas it was blunted within the supraoptic nucleus. Further, within the paraventricular nucleus the excitatory effect of endogenous opioids on local oxytocin release seen in virgins was switched into an inhibitory action in pregnancy. In contrast, endogenous opioids were evidently not involved in the regulation of swim-induced oxytocin release within the supraoptic nucleus either in virgin or pregnant rats. Thus, pregnancy-related neuroendocrine plasticity also includes site-specific functional alterations in opioid receptor-mediated actions in the hypothalamus.

The Metastasis Suppressor Gene KiSS-1 Encodes Kisspeptins, the Natural Ligands of the Orphan G Protein-coupled Receptor GPR54

Natural peptides displaying agonist activity on the orphan G protein-coupled receptor GPR54 were isolated from human placenta. These 54-, 14,- and 13-amino acid peptides, with a common RF-amide C terminus, derive from the product of KiSS-1, a metastasis suppressor gene for melanoma cells, and were therefore designated kisspeptins. They bound with low nanomolar affinities to rat and human GPR54 expressed in Chinese hamster ovary K1 cells and stimulated PIP(2) hydrolysis, Ca(2+) mobilization, arachidonic acid release, ERK1/2 and p38 MAP kinase phosphorylation, and stress fiber formation but inhibited cell proliferation. Human GPR54 was highly expressed in placenta, pituitary, pancreas, and spinal cord, suggesting a role in the regulation of endocrine function. Stimulation of oxytocin secretion after kisspeptin administration to rats confirmed this hypothesis.

Differential effects of glutamate agonists and D-aspartate on oxytocin release from hypothalamus and posterior pituitary of male rats.

In order to determine whether ionotropic (iGluRs) and metabotropic (mGluRs) glutamate receptor activation modulates oxytocin release in male rats, we investigated the effect of agonists of both types of glutamate receptors on oxytocin release from hypothalamus and posterior pituitary. Kainate and quisqualate (1 mM) increased hypothalamic oxytocin release. Their effects were prevented by selective AMPA/kainate receptor antagonists. NMDA (0.01-1 mM) did not modify hypothalamic oxytocin release. Group I mGluR agonists, such as quisqualate and 3-HPG, significantly increased hypothalamic oxytocin release. These effects were blocked by AIDA (a selective antagonist of group I mGluRs). In the posterior pituitary, oxytocin release was not modified by kainate, quisqualate, trans-ACPD (a broad-spectrum mGluR agonist) and L-SOP (a group III mGluR agonist). However, NMDA (0.1 mM) significantly decreased oxytocin release from posterior pituitary. D-Aspartate significantly increased oxytocin release from the hypothalamus, while it decreased oxytocin release from posterior pituitary. AP-5 (a specific NMDA receptor antagonist) reduced the D-Aspartate effect in the hypothalamus, but not in the posterior pituitary. Our data indicate that the activation of non-NMDA receptors and group I mGluRs stimulates oxytocin release from hypothalamic nuclei, whereas NMDA inhibits oxytocinergic terminals in the posterior pituitary. D-Aspartate also has a dual effect on oxytocin release: stimulatory at the hypothalamus and inhibitory at the posterior pituitary. These results suggest that excitatory amino acids differentially modulate the secretion of oxytocin at the hypothalamic and posterior pituitary levels.

Verapamil prevents withdrawal excitation of oxytocin neurones in morphine-dependent rats

We investigated whether the full expression of morphine withdrawal excitation by supraoptic nucleus (SON) oxytocin neurones is a property of the neurones themselves or a partial function of their afferent inputs, by interrupting synaptic input activity via central administration of the L-type Ca 2+ channel blocker verapamil. In morphine-dependent rats, withdrawal-induced release of oxytocin from the posterior pituitary was suppressed by prior administration of intracerebroventricular (i.c.v.) verapamil (160 μg), as was release of oxytocin within the SON measured by microdialysis. During morphine withdrawal the increased electrical activity of SON neurones was also reduced both by i.c.v. verapamil and microdialysis application of verapamil or nifedipine into the SON. Oxytocin secretion evoked by electrical stimulation of the pituitary stalk was unaffected by i.c.v. verapamil suggesting a central site of action. To determine whether the inhibitory actions of verapamil were specific to morphine withdrawal, we also investigated the effects of verapamil on other oxytocin-secreting stimuli. I.C.V. verapamil given to morphine-naı̈ve rats abolished pituitary oxytocin release in response to activation of brainstem or rostral excitatory inputs by cholecystokinin (20 μg kg −1, i.v.) and 1.5 M saline (4 ml kg −1, i.p.) respectively, whilst in lactating rats, i.c.v. verapamil reduced suckling-induced release of oxytocin within the SON. These results suggest that verapamil has a central site of action on stimulated oxytocin release (including an action within the SON) and that both pre and post-synaptic L-type Ca 2+ channels are required for the full expression of morphine withdrawal in SON oxytocin neurones.

Central administration of prolactin-releasing peptide stimulates oxytocin release in rats

The prolactin-releasing peptide (PrRP) is a novel hypothalamic peptide that has been purified as a ligand of an orphan receptor which is expressed in pituitary cells, and is known to stimulate prolactin release both in vitro and in vivo. We previously determined the immunocytochemical localization of PrRP neurons in the rat brain and our results suggest that PrRP takes part in a variety of brain functions. Additionally, in rats we have demonstrated the synaptic contact of PrRP neurons with oxytocin cell bodies in the paraventricular hypothalamic nucleus (PVH) and supraoptic nucleus (SON). This observation indicates that PrRP may regulate oxytocin secretion. In the present study, we performed intra-cerebroventricular administration of PrRP to conscious rats, and examined the effect of PrRP on the plasma levels of oxytocin and vasopressin. Our results show that central administration of PrRP increased the plasma oxytocin and vasopressin levels in female rats, but in male rats only oxytocin was increased. These results suggest that the PrRP acts as a neuro-modulator of the function of magnocellular neurons, especially oxytocin neurons, in the brain.

Suppression of ovarian oxytocin secretion after intra-luteal administration of the arachidonate 5-lipoxygenase inhibitor BWA4C in the ewe

The effect of an intra-luteal injection of the 5-lipoxygenase (5-LO) inhibitor BWA4C (2 mg in 50 μ1 dmso) on the secretion of oxytocin (OT) from the corpus luteum in response to a close-arterial infusion of prostaglandin (PG)-F 2α (5 ng mint) was examined in anaesthetised sheep. Within 30 minutes of administration both basal (pre-infusion) and PGF 2 α stimulated OT release into the posterior vena cava were significantly (P<0·01) reduced. These results are consistent with the proposition that 5-LO products of arachidonic acid may modulate OT secretion from the ovine corpus luteum.

Oxytocin release is inhibited by the generation of carbon monoxide from the rat hypothalamus – further evidence for carbon monoxide as a neuromodulator

Recent evidence suggests that the gas nitric oxide can modulate the secretion of a number of hypothalamic hormones, and may be co-localized particularly to oxytocin-containing neurons. Another gas, carbon monoxide (CO), has also been suggested to play a role in neural signaling in the brain, and the synthetic enzyme responsible for the generation of carbon monoxide has been reported to be present in the rat hypothalamus. In this study, we have therefore investigated whether CO has the ability to modify the release of oxytocin from acute rat hypothalamic explants. Hemin, a specific CO precursor through the enzyme heme oxygenase (the enzymatic pathway synthesizing endogenous CO, was found to inhibit KCl-stimulated oxytocin release, with a maximal effect at 10 –5 M, while showing no effect on basal oxytocin secretion. The stimulation of oxytocin by serotonin 10 ng/ml was also significantly antagonized by hemin 10 –7 M. An inhibitor of heme oxygenase, zinc-protoporphyrin-9, had no effect on basal or stimulated oxytocin release. When hemin and zinc-protoporphyrin-9 were given together, the hemin-induced inhibition of oxytocin was completely antagonized by the enzyme inhibitor. Ferrous hemoglobin A 0, a substance known to bind CO with high affinity, had no effect on either basal or stimulated oxytocin release, but when hemin and ferrous hemoglobin A 0 were given together the hemin-induced inhibition of oxytocin was completely blocked. These findings provide evidence that endogenous CO may play a role in the control of oxytocin release and that, by analogy with nitric oxide, CO may represent a major new neuroendocrine modulator.

Mechanisms of the effect of Icv IL-1β on oxytocin release in the anesthetized, lactating rat

Interleukin-1β stimulates the release of many hypothalamic hormones, including oxytocin. Experiments were done to examine the effect of icv IL-1β on circulating oxytocin levels in rats throughout lactation, and to determine if alpha-adrenergic mechanisms and/or prostaglandins were involved as mediators. Blood samples were taken from urethane-anesthetized, nonlactating and lactating rats before and after icv treatment with either IL-1β or PBS-BSA; or icv treatment with IL-1β following pretreatment with either phentolamine or indomethacin. Plasma was assayed using a specific oxytocin radioimmunoassay. Interleukin-1β stimulated oxytocin release in all rats tested resulting in an approx 2- to 2.5-fold increase in plasma hormone levels. Throughout the first half of lactation, oxytocin responsiveness to IL-1β increased. On d 20 of lactation, the plasma oxytocin response to IL-1β was depressed. Adrenergic mechanisms may be involved in this differential effect on oxytocin release in lactation as phentolamine pretreatment attenuated IL-1β-induced oxytocin release in early lactation and reversed the depression in IL-1β-stimulation of oxytocin release observed in late lactation. It is possible that central IL-1β is involved in the weaning process: oxytocin responsiveness to IL-1β increases throughout the first half of lactation when oxytocin demands are high and is depressed in late lactation when weaning is occurring.

Endogenous opioid control of somatodendritic oxytocin release from the hypothalamic supraoptic and paraventricular nuclei in vitro

Oxytocin release was measured in a perifusion system from microdissected supraoptic (SO) and paraventricular (PV) nuclei of ovariectomised female rats. An initial period of electrical stimulation (S1) applied through a pair of platinum electrodes evoked an increase in peptide release, however, subsequent periods of stimulation (S2, S3, S4) were increasingly less effective, suggesting depletion of releasable stores. However, addition of the opioid antagonist, naloxone (5 × 10 −5 M), during periods S2 and S3 potentiated this stimulated oxytocin release, indicating the presence of an endogenous opioid inhibition. Tissue from ovariectomised animals pre-treated with progesterone for 3 days showed increased basal secretion but no naloxone-induced potentiation of electrically-stimulated release. However, increasing the naloxone concentration (5 × 10 −4 M) again revealed a potentiation, indicating that progesterone had caused a shift in the effective dose of the antagonist. These data demonstrate that, like their axon terminals in the neurohypophysis, the dendrites of magnocellular oxytocin neurones are under control of endogenous opioids, and that progesterone causes an increase in this opioid tone. This may function to regulate intranuclear oxytocin secretion in the pregnant and periparturient animal.

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.

Effects of local growth factors on the secretory function of bovine corpus luteum during the oestrous cycle and pregnancy in vitro.

The impact of insulin-like growth factor-I (IGF-I) basic fibroblast growth factor (bFGF), endothelin-1 (ET-1), tumour necrosis factor-alpha (TNF-alpha), transforming growth factor-alpha (TGF-alpha) and platelet-derived growth factor (PDGF) on the release of progesterone (P4) and oxytocin (OT) from individual bovine corpora lutea at different stages of the oestrous cycle and pregnancy was evaluated with a microdialysis system (MDS) in vitro. IGF-I (1 microgram mL-1) induced significantly the acute effects on P4 release at the late luteal stage (Days 15-18) and early pregnancy (Days 60-120), whereas bFGF (100 ng mL-1) was extremely effective in stimulating P4 release particularly during the mid-luteal stage (Days 8-12). Both peptides stimulated (P < 0.05) the release of OT throughout the three luteal stages and during early and late pregnancy (Days 30-60 and Days 150-210). ET-1 (100 ng mL-1) clearly inhibited P4 release during the early (Days 5-7) and mid-luteal phase and stimulated OT release only during the mid-luteal stage (P < 0.001). TNF-alpha (100 ng mL-1) stimulated the release of P4 exclusively at the early luteal phase (P < 0.05), whereas OT secretion was increased by TNF-alpha during all stages of the oestrous cycle (P < 0.001). TGF-alpha and PDGF (100 ng mL-1) were effective in stimulating P4 release particularly during late pregnancy (P < 0.05). In contrast, stimulation of OT secretion by TGF-alpha was maximal during the late-luteal stage (P < 0.001), whereas PDGF significantly increased OT secretion during the oestrous cycle (except the early luteal stage) and pregnancy (P < 0.001). The data demonstrate distinct and stage-specific effects of growth factors on P4 and OT secretion in vitro. IGF-I, bFGF and TGF-alpha may play an important role in corpus luteum (CL) function during the oestrous cycle and pregnancy since they are locally expressed and synthesized, there are receptors for these growth factors, and they have been demonstrated to exert biological effects on the CL.

Diurnal variation in the effect of melatonin on neurohypophysial hormone release from the rat hypothalamus

Secretion of neurohypophysial hormones can show a diurnal variation. This has been investigated further in rats maintained on 14 h light:10 h dark using a previously vaildated in vitro technique employing hypothalami obtained at three different times, 2–3 h after lights on (group A), 13–14 h after lights on (group B), and at 4–5 h after lights off (group C). Hormone release under basal conditions and following stimulation with 40 mM KCl was monitored with or without added melatonin in the concentration range 4.3–43 nM. Basal release of hormone was not influenced by the time of day when the animals were taken, although stimulated release was elevated at midnight. In groups A and B both doses of melatonin significantly reduced basal and stimulated release of vasopressin and basal release of oxytocin ( p < 0.01), although no effect was seen in group C animals. Inhibition of stimulated oxytocin release was only produced in group B. These findings suggest that the inhibitory effect of melatonin depends on the time of day and are consistent with the suggestion that melatonin secretion during the dark period may acutely downregulate binding sites in the brain.

Central stimulation of oxytocin release in the lactating rat by N-methyl-D-aspartate: requirement for coactivation through non-NMDA glutamate receptors or the glycine coagonist site.

Excitatory amino acid (EAA) neurotransmitters participate in the regulation of secretion of several neuropeptides, including oxytocin (OT), via actions at different receptors. In earlier studies, release of OT could be achieved reliably by injection into the supraoptic nucleus (SON) of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor agonists, but not by treatment with N-methyl-D-aspartate (NMDA) alone. This prompted further examination of the possible role of NMDA receptors in OT release following central coapplication of NMDA and AMPA-site agonists, or of NMDA and agonists active at the glycine coagonist site. The agonists were injected into the right SON, the right paraventricular nucleus (PVN), or into the third ventricle (3V) of nonsuckled lactating rats. Cotreatment with NMDA and AMPA (using doses that alone did not include OT release) elicited a strong OT release in all animals by either the SON or the PVN route, and this was attenuated by pretreatment/cotreatment with specific antagonists of either the NMDA or the AMPA receptor. The SON area or 3V coinjection of NMDA and the NMDA/glycine site agonists glycine or D-serine also induced OT discharges in all animals, while cotreatment in the PVN did not result in uniform OT discharges. This release was potently reduced by cotreatment with the specific NMDA/glycine site antagonist 5, 7-dichlorokynurenate (DCK). L-Serine somewhat increased the frequency of discharge-type response to NMDA, while intra-SON coinjection of L-leucine did not stimulate OT release. D-Serine alone stimulated the release of OT much less than in combination with NMDA, and with no obvious dose dependence. The suckling-induced release of OT was attenuated, but not abolished, by DCK, while PRL release was briefly stimulated by this agent. A physiological role for the NMDA receptor in OT release is clearly supported by these studies. NMDA receptor activation in the lactating rat may result from either an allosteric stimulation by glycine site agonists, or a synergistic interaction with the AMPA/kainate group of excitatory amino acid receptors.

Stimulation of oxytocin release within the supraoptic nucleus and into blood by CCK-8.

Simultaneous microdialysis in brain and blood was used to monitor the effects of systemic and central cholecystokinin octapeptide (CCK-8) on the release of oxytocin and vasopressin within the hypothalamic supraoptic nucleus (SON) as well as into blood of urethan-anesthetized female rats. Administration of CCK-8 (20 micrograms/kg iv) increased oxytocin contents in 30-min microdialysates sampled simultaneously within the SON (1.8-fold) and blood (2.4-fold, both P < 0.05) compared with prestimulation levels. In another experiment, after bilateral administration of CCK-8 directly into the SON (10 ng/0.5 microliter) via a microdialysis/infusion probe, oxytocin contents in dialysates sampled from the left and right SON were increased 2.3- and 1.7-fold (P < 0.05), respectively. In simultaneously sampled dialysates from the jugular vein, oxytocin content increased 2.3-fold (P < 0.05). In contrast, oxytocin in dialysates sampled outside the hypothalamic nuclei was not altered by systemic or central CCK-8. The direct infusion of CCK-8 into both SON increased the release of vasopressin within the SON 1.7-fold (P < 0.05) but failed to significantly change vasopressin release into blood. The present findings show a coordinated regulation of intranuclear and systemic release of oxytocin in response to systemic and central CCK-8 and provide further evidence for a possible involvement of endogenous oxytocin in the complex regulation of ingestive and reproductive behaviors induced by CCK-8 at the brain level.