Oxytocin Release In Rats Research Articles

Sex Specific Effects of Salt Loading in Oxytocin Neurons From The Supraoptic Nucleus

Arginine Vasopressin (AVP) and oxytocin (OXY) contribute to body fluid balance homeostasis. Salt loading (2% NaCl for 7 days) increases both AVP and OXY release in rats. The chronic increase in AVP release is associated with a change in the sensitivity of AVP neurons in the supraoptic nucleus (SON) to GABA so that GABAA receptor activation becomes excitatory. It is not clear if a similar mechanism is associated with chronic OXY release in this model. Our hypothesis is that changes in chloride homeostasis associated with salt loading occur in OXY neurons. To test this hypothesis, we used a chloride imaging approach with a ratiometric chloride sensitive dye, ClopHensorN (Addgene #50758) combined with an AAV with an oxytocin specific promoter (pFBOT563, Addgene # 40864). Adult, intact, Sprague Dawley rats of both sexes were anesthetized with isoflurane (2-3%) and were bilaterally injected with the AAV2-pFBOT--ClophensorN virus directly into the SON. After a two-week recovery period, the animals were sacrificed and the brains were rapidly removed. The SON was dissected away from the brain and the cells were dissociated, plated on cover slips, and incubated for two hours. After incubation, recordings were taken using ratiometric live cell imaging on an inverted microscope. Selected neurons were sequentially excited at 445nm and 556nm and then emission data was collected between 500-550nm and 580-653nm respectively. After 40 cycles of 3-second recordings, muscimol (100nM), a GABAA receptor agonist was transiently applied to the cells and then allowed to wash off. Background fluorescence was subtracted. In cells from males, muscimol resulted in chloride influx in 50% of the OXY cells tested while chloride influx was observed in all OXY cells tested from females. The results suggest that salt loading may influence GABA responses of OXY neurons in males but not females.

Function of the hypothalamo-neurohypophysial system in rats with myocardial infarction is modified by melatonin

BackgroundPineal and melatonin interactions with the hypothalamo-neurohypophysial system are well documented. In addition, vasopressin and oxytocin secretion are known to be part of the neuroendocrine response to chronic heart failure evoked by myocardial infarction. The present study was undertaken to evaluate the possible regulatory role of melatonin in the vasopressin and oxytocin release in rats with myocardial infarction. MethodsThe vasopressin and oxytocin content of the hypothalamus and neurohypophysis, as well as their plasma levels, were radioimmunoassayed in sham-operated or pinealectomized rats with left coronary artery ligation (CAL)-evoked myocardial infarction as well as under melatonin treatment. ResultsInfarcted rats demonstrated increased vasopressin and oxytocin plasma levels, but melatonin restricted the release of both neurohormones in these rats. Experimental myocardial infarction in pinealectomized rats caused a distinct inhibition of vasopressin release but intensified oxytocin secretion. In pinealectomized rats substituted with melatonin, pineal indole amine was seen to inhibit oxytocin release and stimulate vasopressin secretion. Conclusions(i) CAL-induced myocardial infarction is the reason for increased hypothalamo-neurohypophysial system activity in rats; melatonin plays the role of inhibitory neuromodulator of vasopressin and oxytocin release in this state. (ii) Myocardial infarction evoked in pinealectomized rats is characterized by the inversion of the neurohumoral response pattern in respect of inhibited vasopressin release. (iii) Melatonin stimulates vasopressin (but decreases oxytocin) release in pinealectomized rats with myocardial infarction.

Tachykinin neurokinin 3 receptor signaling in cholecystokinin-elicited release of oxytocin and vasopressin

Neurokinin 3 receptor (NK3R) signaling has an integral role in the stimulated oxytocin (OT) and vasopressin (VP) release in response to hyperosmolarity and hypotension. Peripheral injections of cholecystokinin (CCK) receptor agonists for the CCK-A (sulfated CCK-8) and CCK-B (nonsulfated CCK-8) receptors elicit an OT release in rat. It is unknown whether NK3R contributes to this endocrine response. Freely behaving male rats were administered an intraventricular pretreatment of 250 or 500 pmol of SB-222200, a specific NK3R antagonist, or 0.15 M NaCl before an intraperitoneal or intravenous injection of CCK-8 (nonsulfated or sulfated) or 0.15 M NaCl. Blood samples were taken before intraventricular treatment and 15 min after intraperitoneal or intravenous injection, and plasma samples were assayed for OT and VP concentration. Intraperitoneal injection of both nonsulfated and sulfated CCK-8 significantly increased plasma OT levels and had no effect on plasma VP levels. Intravenous injection of sulfated CCK-8 stimulated an increase in plasma OT levels and did not alter plasma VP levels. However, intravenous injection of nonsulfated CCK-8 stimulated a significant increase in plasma levels of both OT and VP. No other studies have demonstrated CCK-8-stimulated release of VP in rat. NK3R antagonist did not alter baseline levels of either hormone. However, pretreatment of NK3R antagonist significantly blocked the CCK-stimulated release of OT in all CCK treatment groups and blocked VP release in response to intravenous injection of nonsulfated CCK-8. Therefore, central NK3R signaling is required for OT and VP release in response to CCK administration.

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.

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.

Central Somatostatin Inhibits Cholecystokinin‐lnduced Oxytocin Secretion in Conscious Rats

Recent studies have demonstrated the presence of fibres immunoreactive for somatostatin-28 (SS-28), which originate in the brainstem and selectively innervate the magnocellular oxytocin (OT) cells of the supraoptic nucleus. The potential physiological relevance of this pathway was investigated in the present study by measuring plasma OT levels in response to intraperitoneal administration of cholecystokinin in conscious male rats pretreated with intracerebroventricular injections of either SS-28 or artificial cerebrospinal fluid. Cholecystokinin treatment produced the expected marked rise in plasma OT levels in control rats pretreated intracerebroventricularly with artificial cerebrospinal fluid. However, this response was markedly blunted by prior intracerebroventricular administration of SS-28, even though SS-28 itself had no effect on basal plasma OT levels, nor did it impair OT release in response to hypertonic saline injection. These results demonstrate that centrally injected SS-28 can selectively block cholecystokinin-stimulated release of OT in rats, and support an inhibitory role for this peptide in brainstem-mediated neurohypophysial hormone secretion. Central SS-28 administration also elicited up to 3-fold increases in the amount of food ingested in 1 h by previously sated rats. These observations suggest the possibility that endogenous SS-28 may be involved in stimulating food intake in rats, and establish the basis for future studies to clarify the role of this neuropeptide in controlling ingestive behaviours.

Neurohypophyseal secretion in response to cholecystokinin but not meal-induced gastric distention in humans.

Exogenous administration of cholecystokinin octapeptide (CCK) is known to decrease food intake and slow gastric emptying in humans and animals. Recent studies have shown that CCK stimulates neurohypophyseal secretion of oxytocin (OT) in rats and arginine vasopressin (AVP) in monkeys, and that gastric distention also stimulates OT release in rats. We therefore studied AVP and OT secretion in 14 normal subjects in response to meal-induced gastric distention and administration of CCK, both separately and in combination, to assess whether these stimuli similarly activated central neurohypophyseal pathways in humans. Neither plasma AVP nor OT concentrations increased after gastric distention produced by ingestion of a large meal. However, a dose-related increase in plasma AVP, but not OT levels, occurred after CCK administration, the threshold CCK dose being 0.05 micrograms/kg body weight. The AVP secretion in response to CCK administration was significantly correlated with subjective aversive symptoms quantified by use of a numeric scale (r = 0.61, P less than 0.001). In 12 of the 14 subjects plasma AVP levels increased in association with symptoms of epigastric pressure and discomfort before the onset of overt nausea or emesis. The combination of CCK and meal-induced gastric distention did not stimulate increases in plasma AVP levels in excess of those produced by CCK administration alone. The results demonstrate that AVP secretion resulting from emetic center activation often is a graded response that can begin in association with milder degrees of visceral discomfort before symptoms of overt nausea or emesis. In addition, the stimulation of AVP secretion by CCK administration, but not by meal-induced gastric distention in association with physiological satiety, suggests that some component of the anorectic effects of exogenous CCK in man likely results from activation of brainstem emetic centers.

Recurrent mammary gland contractions induced by a low tonic release of oxytocin in rats.

In urethane-anaesthetized lactating rats, intramammary pressure occasionally displayed recurrent variations or oscillations having a slow rise time, low amplitude, long duration and a periodicity of 1-4 min. These oscillations differed from changes in intramammary pressure characteristic of reflex milk ejections induced by suckling, and were also observed in unsuckled rats. They were suppressed by lesions of the pituitary stalk or by stimulating the septum, a structure that inhibits the activity of the magnocellular system. They could be induced by long-term low frequency stimulation of the pituitary stalk, lesions of the septum or long-term infusions of oxytocin at a low rate of 0.05-0.3 mu./min. We suggest that the recurrent oscillations in intramammary pressure constitute a particular mode of response of the mammary gland to a tonic release of oxytocin resulting from a moderate but sustained increase in the basal level of electrical activity of the oxytocin-secreting neurones.