Stimulated Oxytocin Secretion Research Articles

Microbial stimulation of oxytocin release from the intestinal epithelium via secretin signaling

ABSTRACT Intestinal microbes impact the health of the intestine and organs distal to the gut. Limosilactobacillus reuteri is a human intestinal microbe that promotes normal gut transit, the anti-inflammatory immune system, wound healing, normal social behavior in mice, and prevents bone reabsorption. Oxytocin impacts these functions and oxytocin signaling is required for L. reuteri-mediated wound healing and social behavior; however, the events in the gut leading to oxytocin stimulation and beneficial effects are unknown. Here we report evolutionarily conserved oxytocin production in the intestinal epithelium through analysis of single-cell RNA-Seq datasets and imaging of human and mouse intestinal tissues. Moreover, human intestinal organoids produce oxytocin, demonstrating that the intestinal epithelium is sufficient to produce oxytocin. We find that L. reuteri facilitates oxytocin secretion from human intestinal tissue and human intestinal organoids. Finally, we demonstrate that stimulation of oxytocin secretion by L. reuteri is dependent on the gut hormone secretin, which is produced in enteroendocrine cells, while oxytocin itself is produced in enterocytes. Altogether, this work demonstrates that oxytocin is produced and secreted from enterocytes in the intestinal epithelium in response to secretin stimulated by L. reuteri. This work thereby identifies oxytocin as an intestinal hormone and provides mechanistic insight into avenues by which gut microbes promote host health.

Measuring oxytocin release in response to gavage: Computational modelling and assay validation.

In the present experiments, we tested the conclusion from previous electrophysiological experiments that gavage of sweet food and systemically applied insulin both stimulate oxytocin secretion. To do so, we measured oxytocin secretion from urethane-anaesthetised male rats, and demonstrated a significant increase in secretion in response to gavage of sweetened condensed milk but not isocaloric cream, and a significant increase in response to intravenous injection of insulin. We compared the measurements made in response to sweetened condensed milk with the predictions from a computational model, which we used to predict plasma concentrations of oxytocin from the published electrophysiological responses of oxytocin cells. The prediction from the computational model was very closely aligned to the levels of oxytocin measured in rats in response to gavage.

Oxytocin Levels in Response to Pituitary Provocation Tests in Healthy Volunteers

Background: Oxytocin, secreted into the circulation through the posterior pituitary, regulates lactation, weight, and socio-behavioral functioning. Oxytocin deficiency has been suggested in patients with hypopituitarism, however, diagnostic testing for oxytocin deficiency has not been developed. Known stimuli used in the diagnosis of pituitary deficiencies - the hypertonic saline and arginine infusion tests stimulating copeptin levels, and the oral macimorelin test stimulating growth hormone levels - have also been shown to stimulate oxytocin secretion in animal models. We hypothesized that these provocation tests would stimulate plasma oxytocin levels in humans. Methods: Basal plasma oxytocin levels were measured for all three tests. Stimulated plasma oxytocin was measured once plasma sodium >150 mmol/l for the hypertonic saline and after 45 minutes for the arginine infusion and the oral macimorelin test, expected peak of copeptin and growth hormone levels, respectively. Primary outcome was change between basal and stimulated oxytocin levels using a paired t-test. Results: Median (IQR) age of all participants was 24 years (22, 28), 51% were female. As expected, copeptin increased in response to hypertonic saline from 4.0 pmol/L [3.3, 6.7] to 34.2 pmol/L [23.2, 45.4] (p-value <0.001) and in response to arginine infusion from 4.6 pmol/L [3.2, 6.2] to 8.3 pmol/L [6.4, 10.8] (p-value <0.001). Growth hormone increased in response to oral macimorelin from 1.6 ng/mL [0.3, 17.2] to 106.0 ng/mL [73.3, 127.2] (p-value <0.001). Oxytocin levels increased in response to hypertonic saline infusion from 0.3 pg/mL [0.3, 0.5] to 0.6 pg/mL [0.4, 0.7] (p-value 0.007), while there was no change in response to arginine infusion (basal 0.4 pg/mL [0.4, 0.6], stimulated 0.4 pg/mL [0.3, 0.6], p-value 0.6), nor to oral macimorelin (basal 38.7 pg/mL [31.1, 66.9], stimulated 34.2 pg/mL [31.2, 48.2], p-value 0.3). Conclusion: We found that hypertonic saline infusion results in doubling of oxytocin levels. Further research will be important to determine whether this test could be used diagnostically to identify patients with oxytocin deficiency. In contrast to animal data, arginine and macimorelin did not stimulate oxytocin.

Dynamics of molecular mechanisms underlying ovarian oxytocin secretion

In the ruminant ovary, synthesis and secretion of oxytocin begin in the granulosa cells of the preovulatory follicle and are markedly stimulated by the surge of LH and FSH. Luteinization of the granulosa cells results in a further increase in oxytocin gene expression, but translation of mRNA appears to be retarded because the peak concentration of luteal oxytocin occurs later than the maximal accumulation of the message. Several hormones have been shown to stimulate oxytocin secretion from granulosa and luteal cells in vivo or in vitro. However, the role of prostaglandin F2 alpha (PGF2 alpha) in regulating luteal oxytocin secretion has perhaps received more study than other hormones. The mechanism of action of PGF2 alpha has been shown to encompass a phosphoinositide cascade and activation of protein kinase C, events that are associated with luteal secretion of oxytocin. Protein kinase C phosphorylation of the actin-binding protein myristolated alanine-rich C kinase substrate (MARCKS) may be required for exocytosis of oxytocin.

Intranasal Oxytocin Treatment Increases Eye-Gaze Behavior toward the Owner in Ancient Japanese Dog Breeds.

Dogs acquired unique cognitive abilities during domestication, which is thought to have contributed to the formation of the human-dog bond. In European breeds, but not in wolves, a dog’s gazing behavior plays an important role in affiliative interactions with humans and stimulates oxytocin secretion in both humans and dogs, which suggests that this interspecies oxytocin and gaze-mediated bonding was also acquired during domestication. In this study, we investigated whether Japanese breeds, which are classified as ancient breeds and are relatively close to wolves genetically, establish a bond with their owners through gazing behavior. The subject dogs were treated with either oxytocin or saline before the starting of the behavioral testing. We also evaluated physiological changes in the owners during mutual gazing by analyzing their heart rate variability (HRV) and subsequent urinary oxytocin levels in both dogs and their owners. We found that oxytocin treatment enhanced the gazing behavior of Japanese dogs and increased their owners’ urinary oxytocin levels, as was seen with European breeds; however, the measured durations of skin contact and proximity to their owners were relatively low. In the owners’ HRV readings, inter-beat (R-R) intervals (RRI), the standard deviation of normal to normal inter-beat (R-R) intervals (SDNN), and the root mean square of successive heartbeat interval differences (RMSSD) were lower when the dogs were treated with oxytocin compared with saline. Furthermore, the owners of female dogs showed lower SDNN than the owners of male dogs. These results suggest that the owners of female Japanese dogs exhibit more tension during interactions, and apart from gazing behavior, the dogs may show sex differences in their interactions with humans as well. They also suggest that Japanese dogs use eye-gazing as an attachment behavior toward humans similar to European breeds; however, there is a disparity between the dog sexes when it comes to the owners’ oxytocin secretion. Japanese dogs also showed different attachment behaviors from both European breeds and wolves, and they likely use additional strategies to substitute gaze when forming the human–dog bond.

Secretory function of ovarian cells and myometrial contractions in cow are affected by chlorinated insecticides (chlordane, heptachlor, mirex) in vitro

The aim of the study was to investigate the effect of chlordane, heptachlor and mirex, on hormonal regulation of the force of myometrial contractions.Myometrial, endometrial, granulosa and luteal cells as well as strips of myometrium from non-pregnant cows were incubated with three insecticides at environmentally relevant doses (0.1, 1 or 10ng/ml).None of the insecticides affected the viability of studied cells. Chlordane stimulated, while heptachlor and mirex inhibited, secretion of testosterone and estradiol from granulosa cells as well as secretion of progesterone from luteal cells, respectively. Secretion of oxytocin (OT) from granulosa cells was increased after incubation with all studied insecticides. Only mirex stimulated OT secretion from luteal cells, while heptachlor inhibited this effect. None of them affected synthesis of OT in luteal cells and prostaglandins (PGF2 and PGE2) secretion from uterine cells, except PGE2 secretion from endometrial cells was decreased when the cells were incubated with 0.1ng/ml of chlordane. Basal and OT-stimulated myometrial contractions were increased by mirex and decreased by heptachlor.The data show that the insecticides altered secretory function of ovarian cells. Heptachlor and mirex affected also myometrial contractions in vitro, but uterine secretion of prostaglandins were not involved in the mechanism of that adverse effect of insecticides. The data indicate on potential of these insecticides to disturb fertilisation, blastocyst implantation or even the length of gestation.

Salsolinol Up-Regulates Oxytocin Expression and Release During Lactation in Sheep.

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline) is a dopamine-derived compound present in the central nervous system and pituitary gland. Several previous studies on lactating sheep and rats have reported that salsolinol plays a crucial role in the regulation of prolactin secretion. The present study investigated the effects of salsolinol, which was infused into the third ventricle of the brain, on oxytocin expression and release in lactating sheep, 48 h after weaning of 8-week-old lambs. Serial 30-min infusions of salsolinol and vehicle were performed at 30-min intervals from 10.00 to 15.00 h. Blood samples were collected every 10 min. The supraoptic nucleus (SON), paraventricular nucleus (PVN) and posterior pituitary were collected immediately after the experiment. Expression levels of mRNAs for oxytocin and peptidylglycine α-amidating monooxygenase (PAM), the terminal enzyme in the oxytocin synthesis pathway, were measured using a real-time polymerase chain reaction. Oxytocin peptide content in the posterior pituitary was measured by an enzyme-linked immunosorbent assay, and plasma oxytocin concentration was measured by radioimmunoassay. Salsolinol treatment significantly up-regulated oxytocin and PAM gene expression in the SON (P < 0.01 and P < 0.05, respectively), PVN (P < 0.01 and P < 0.05, respectively) and posterior pituitary (P < 0.05 and P < 0.05, respectively). Oxytocin peptide content in the posterior pituitary and the area under the response curve of plasma oxytocin were significantly (P < 0.05 and P < 0.01, respectively) higher in salsolinol-treated sheep than in control animals. The present study shows for the first time that salsolinol stimulates oxytocin secretion during lactation in sheep.

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.

Oxytocin response to an experimental psychosocial challenge in adults exposed to traumatic experiences during childhood or adolescence

Long-term implications of the exposure to traumatizing experiences during childhood or adolescence, such as sexual abuse, or cancer, have been documented, namely the subjects' response to an acute stress in adulthood. Several indicators of the stress response have been considered (e.g. cortisol, heart rate). Oxytocin (OT) response to an acute stress of individuals exposed to trauma has not been documented. Eighty subjects ( n=26 women who had experienced episodes of child abuse, n=25 men and women healthy survivors of cancer in childhood or adolescence, and 29 controls) have been submitted to a laboratory session involving an experimental stress challenge, the Trier social stress test. Overall, there was a clear OT response to the psychosocial challenge. Subjects having experienced a childhood/adolescence life-threatening illness had higher mean levels of OT than both abused and control subjects. There was a moderate negative relationship between OT and salivary cortisol. It is suggested that an acute stress stimulates OT secretion, and that the exposure to enduring life-threatening experiences in childhood/adolescence has long-lasting consequences regarding the stress system and connected functions, namely the activation of OT secretion. Better knowledge of such long-term implications is important so that to prevent dysregulations of the stress responses, which have been shown to be associated to the individual's mental health.

GABAergic mediation of stress-induced secretion of corticosterone and oxytocin, but not prolactin, by the hypothalamic paraventricular nucleus

The hypothalamus–pituitary–adrenal axis (HPA) participates in mediating the response to stressful stimuli. Within the HPA, neurons in the medial parvocellular region of paraventricular nucleus (PVN) of the hypothalamus integrate excitatory and inhibitory signals triggering secretion of corticotropin-releasing hormone (CRH), the main secretagogue of adrenocorticotropic hormone (ACTH). Stressful situations alter CRH secretion as well as other hormones, including prolactin and oxytocin. Most inputs to the PVN are of local origin, half of which are GABAergic neurons, and both GABA-A and GABA-B receptors are present in the PVN. The objective of the present study was to investigate the role of GABA-A and GABA-B receptors in the PVN's control of stress-induced corticosterone, oxytocin and prolactin secretion. Rats were microinjected with saline or different doses (0.5, 5 and 50 pmol) of GABA-A (bicuculine) or GABA-B (phaclofen) antagonists in the PVN. Ten minutes later, they were subjected to a stressor (ether inhalation) and blood samples were collected 30 min before and 10, 30, 60, 90 and 120 min after the stressful stimulus to measure hormone levels by radioimmunoassay. Our results indicate that GABA acts in the PVN to inhibit stress-induced corticosterone secretion via both its receptor subtypes, especially GABA-B. In contrast, GABA in the PVN stimulates oxytocin secretion through GABA-B receptors and does not alter prolactin secretion.

Effect of polychlorinated biphenyls on the secretion of oxytocin from luteal and granulosa cells in cow: possible involvement of glucocorticoid receptors

Polychlorinated biphenyls (PCBs) stimulate oxytocin secretion from bovine granulosa and luteal cells. Since oxytocin on the one hand is released from ovarian cells by cortisol and on the other hand PCBs can be bound by glucocorticoid receptors (GCr), we have tested the hypothesis that PCBs acting via GCr can stimulate oxytocin secretion. In preliminary studies the effect of RU486 (GCr blocker) on cells viability was tested. Thereafter, a selected dose of RU486 (10&lt;sup&gt;5&lt;/sup&gt;M), which did not affect cell viability, was used in further experiments. It appears that this dose of RU486 completely blocked GCr against cortisol-stimulated oxytocin secretion, in both types of cells. Furthermore, granulosa cells (10&lt;sup&gt;5&lt;/sup&gt;) from follicles of two sizes (&amp;gt;1 cm &amp;lt; in diameter) and luteal cells (10&lt;sup&gt;5&lt;/sup&gt;) from day 5&amp;ndash;10 of estrous cycle were incubated for 72 h with congeners of PCB (126, 77 or 153) at doses of 1, 10 or 100&amp;nbsp;ng/ml each, separately or jointly with RU486. The effect of PCB 77 and 153 on oxytocin secretion was blocked by RU486, but it did not change the effect evoked by PCB126 in both granulosa and luteal cells. We assume that some PCB congeners can affect oxytocin secretion from granulosa and luteal cells acting via GCr.

Involvement of the cytoskeleton in oxytocin secretion by cultured bovine luteal cells.

A number of substances have been implicated in the regulation of oxytocin (OT) secretion from bovine corpus luteum in vivo. However, isolated bovine luteal cells cultured in a monolayer lose the ability to secrete OT in response to stimulatory substances. The present study investigated how cell-to-cell contact and the cytoskeleton affect OT secretion by isolated bovine luteal cells. In experiment 1, bovine midluteal cells (Days 8-12 of the estrous cycle) were stimulated with prostaglandin F2alpha (PGF2alpha; 1 microM), noradrenaline (NA; 10 microM), or growth hormone (GH; 5 nM) in two culture systems: In one system, cell monolayers were incubated in 24-well culture plates, and in the other system, aggregates of cells were incubated in glass tubes in a shaking water bath. The cells cultured in a monolayer underwent considerable spreading and showed a variety of shapes, whereas the cells cultured in glass tubes remained fully rounded during the experimental period and soon formed aggregates of cells. Although PGF2alpha, NA, and GH did not stimulate OT secretion by the monolayer cells, all tested substances stimulated OT secretion by the aggregated cells (P < 0.01). In experiment 2, the monolayer cells were pre-exposed for 1 h to an antimicrofilament agent (cytochalasin B; 1 microM) or two antimicrotubule agents (colchicine or vinblastine; 1 microM) before stimulation with PGF2alpha, NA, or GH. Although PGF2alpha, NA, and GH did not stimulate OT secretion by the monolayer cells in the presence of colchicine or vinblastine, they all stimulated OT secretion in the presence of cytochalasin B (P < 0.001). The overall results show that OT secretion by bovine luteal cells depends on microfilament function and cell shape. Moreover, the aggregate culture system that allows three-dimensional, cell-to-cell contact seems to be a good model for studying OT secretion by isolated bovine luteal cells.

Neural pathways controlling central and peripheral oxytocin release during stress.

Oxytocin is released from the pituitary gland in response to a variety of stressful stimuli, including noxious stimuli, conditioned fear and exposure to novel environments. These responses are believed to be mediated, at least in part, by noradrenergic projections from the medulla oblongata, and some of these noradrenergic neurones also contain prolactin-releasing peptide (PrRP). Central administration of either PrRP or noradrenaline stimulates oxytocin secretion into the circulation. Stressful stimuli activate PrRP-containing noradrenergic neurones in the medulla oblongata, and it is thus possible that PrRP/noradrenergic projections to the hypothalamus mediate oxytocin responses to stressful stimuli. Here, the roles of brainstem PrRP/noradrenergic projections to the hypothalamus in oxytocin responses to different kinds of stressful stimuli are reviewed, with a particular emphasis on conditioned fear. Roles of dendritic oxytocin release during stress and metabolic factors affecting stress pathways are also discussed.

The role of nitric oxide in morphine dependence and withdrawal excitation of rat oxytocin neurons.

Magnocellular oxytocin neurons develop morphine dependence after intracerebroventricular infusion for 5 days as revealed by their profound excitation following naloxone-induced withdrawal. Oxytocin neurons strongly express nitric oxide synthase (NOS) and nitric oxide (NO) inhibits their activity. This study investigated whether excitation of oxytocin neurons during morphine withdrawal involves reduced activity of NOS and NO. Neuron activity was measured in urethane-anaesthetized rats with blood sampling for oxytocin radioimmunoassay and extracellular single unit firing rate recording of supraoptic nucleus oxytocin neurons. To compare morphine-dependent and -naive rats oxytocin secretion was measured during stimulation by intravenous hypertonic saline infusion. Prior treatment with Nomega-nitro-l-arginine methyl ester, a NOS inhibitor, facilitated osmotically stimulated oxytocin secretion in both morphine-dependent and -naive rats. The facilitation was not different between these groups when corrected for the slower responses observed in morphine-dependent rats. Treatment of morphine-dependent rats with Nomega-nitro-l-arginine methyl ester also enhanced oxytocin secretion during naloxone-precipitated withdrawal. Oxytocin neurons excited by withdrawal were recorded during microdialysis application to the supraoptic nucleus of the NO donor sodium nitroprusside alone and in combination with the GABAA antagonist bicuculline. Sodium nitroprusside inhibited oxytocin neurons during naloxone-precipitated morphine withdrawal and, while bicuculline alone increased firing rate, it did not reduce the inhibition by sodium nitroprusside, in contrast with previous findings in naive rats. Together, these findings indicate that NO restraint of oxytocin secretion is not curtailed during morphine dependence and remains a potent inhibitor of withdrawal excitation despite reduced effectiveness on GABA innervation of the supraoptic nucleus. Hence there is no evidence that changes in NO regulation underlie excitation of oxytocin neurons during opiate withdrawal in morphine dependence.

Serotonin receptors involved in vasopressin and oxytocin secretion.

Serotonin (5-HT), 5-HT agonists, the 5-HT precursor 5-hydroxytryptophan, 5-HT-releasers and -reuptake inhibitors stimulate the release of vasopressin and oxytocin. We investigated the involvement of 5-HT receptors in the serotonergic regulation of vasopressin and oxytocin secretion. Vasopressin and oxytocin secretion was stimulated by 5-HT, the 5-HT(1A+1B+5A+7) agonist 5-carboxamidotryptamine (5-CT), the 5-HT(2A+2C) agonist DOI, the 5-HT(2C+2A) agonist mCPP, the 5-HT(2C) agonist MK-212, the 5-HT(3) agonist SR 57277 and the 5-HT(4) agonist RS 67506. The 5-HT(1A) agonist 8-OH-DPAT, which had no effect on vasopressin secretion, stimulated oxytocin secretion. The 5-HT-induced release of vasopressin and oxytocin was inhibited by central infusion of the 5-HT antagonists WAY 100635 (5-HT(1A)), LY 53857 (5-HT(2A+2C)), ICS 205-930 (5-HT(3+4)) and RS 23597 (5-HT(4)). The 5-HT2+6+7 antagonist metergoline in combination with the 5-HT1A+2+7 antagonist methysergide inhibited the stimulatory effect of 5-CT on both hormones, whereas the 5-HT1A+1B antagonist cyanopindolol only inhibited the oxytocin response. The 5-HT(2A) antagonist 4-(4-flourobenzoyl)-1-(4-phenylbutyl)-piperidine oxalate had no effect on DOI-induced hormone response. The 5-HT(2C) antagonist Y 25130 partly inhibited the stimulating effect of MK-212. ICS 205-930 and RS 23597 inhibited vasopressin and oxytocin secretion induced by RS 67506. WAY 100635 inhibited 8-OH-DPAT-induced oxytocin secretion. We conclude that 5-HT-induced vasopressin secretion primarily is mediated via 5-HT(2C), 5-HT(4) and 5-HT(7) receptors, whereas 5-HT(2A), 5-HT(3) and 5-HT(5A) receptors seem to be of minor importance. 5-HT-induced oxytocin secretion involves 5-HT(1A), 5-HT(2C) and 5-HT(4) receptors; in addition an involvement of 5-HT(1B), 5-HT(5A) and 5-HT(7) receptors seems likely, whereas 5-HT(2A) and 5-HT(3) receptors seem to be less important.

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.

Evidence for opioidergic inhibition of oxytocin release in periparturient mares

In the horse mare, the onset of parturition is associated with an increase in oxytocin secretion, and it has been suggested that the onset of parturition may be triggered by endogenous oxytocin release. To test the hypothesis that oxytocin secretion is regulated by endogenous opioids in the periparturient period, we have 1) characterized oxytocin secretion in response to vaginocervical stimulation and 2) determined the effect of naloxone, an opioid antagonist, on oxytocin secretion induced by vaginocervical stimulation in prepartum mares and in postpartum mares at estrus and diestrus. During the last 2 months of pregnancy, the first diestrus and subsequent estrus post partum, a total of 66 vaginocervical stimulations were performed. Mares were pretreated with naloxone (0.5 mg/kg iv) or saline, administered 20 min beforevaginocervical stimulation on subsequent days, using a randomized switchback design in which mares served as their own controls. Plasma was collected from 30 min before until 30 min after stimulation and was analyzed for oxytocin concentrations. Vaginocervical stimulation resulted in a significant increase in oxytocin secretion in all mares. Between Days 30 and 20 prepartum, the total amount of oxytocin secreted (calculated as area under the curve for 0 to 10 min after vaginocervical stimulation) was significantly greater in naloxone-treated than in saline-treated mares. From Day 20 prepartum until parturition, the differences between naloxone and saline-treated mares tended to decrease with approaching parturition, and were no longer statistically different. Peak plasma oxytocin concentrations were greater in naloxone-treated mares than in saline-treated mares during the entire prepartum period. During the postpartum period, total amount of oxytocin secreted following vaginocervical stimulation tended to be greater than during the prepartum period, and stimulated oxytocin secretion was significantly greater in naloxone-treated mares than in saline-treated mares. In conclusion, these data suggest that endogenous opioids suppress oxytocin secretion pre and post partum. It appears that opioid inhibition is not limited to the prepartum period, tends to decrease gradually towards parturition and is reinstated after foaling.

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.

A comparison between effects of intraventricular insulin and intraperitoneal lithium chloride on three measures sensitive to emetic agents.

When low doses of insulin are infused directly into the third ventricle, rats reduce their food intake and lose weight. To determine whether these effects could be due to malaise induced by the treatment, the effects of intraventricular insulin were compared to the effects of the emetic agent lithium chloride to condition a taste aversion, to stimulate oxytocin secretion, and to reduce sodium appetite in response to furosemide treatment. For all three of these measures, lithium chloride treatment had a predictable effect compared to controls. Specifically, lithium caused a significant taste aversion, elevated plasma oxytocin, and attenuated sodium appetite. However, lithium did not produce a significant change in food intake or body weight. On the other hand, intraventricular insulin treatment did cause a significant reduction in body weight yet had no effect on these indices of malaise in the rat. These data are consistent with the hypothesis that intraventricular insulin does not reduce food intake and body weight by producing malaise but rather serves as a negative feedback signal of body adiposity to the central nervous system.

Involvement of oxytocin in histamine- and stress-induced ACTH and prolactin secretion.

The neurohypophysial peptide oxytocin (OT) is released in response to different stressors and has been suggested as a 'stress hormone'. In addition, we have recently shown that centrally administered histamine (HA), which is a mediator of stress-induced release of ACTH and prolactin (PRL), stimulates OT secretion. The aim of the present investigation was to further characterize the HA-induced OT secretion with respect to the type of postsynaptic receptor involved and to investigate the possible role of OT in HA- and stress-induced ACTH and PRL secretion. We studied (1) the effect of HA, HA agonists and HA antagonists on OT secretion in normal male rats, (2) the secretion of OT in response to HA stimulation and to restraint stress, endotoxin stress [lipopolysaccharide (LPS) administration] and insulin/hypoglycemia stress and compared the OT response to that of arginine vasopressin (AVP), (3) the OT response to restraint stress or HA in normal and AVP-deficient Brattleboro rats (DI) suffering from diabetes insipidus and (4) the effect of inhibiting the oxytocinergic system by immunoneutralization or receptor blockade on HA- and stress-induced ACTH and PRL release in normal as well as in DI rats. HA, an H1 and an H2 receptor agonist, stimulated the OT secretion dose-dependently. The HA-induced release of OT was inhibited by pretreatment with an H1 or an H2 receptor antagonist. Restraint stress and HA but not LPS or insulin stress induced an increase in peripheral OT levels, whereas only LPS stress and HA caused an increase in circulating AVP levels. Neither an OT antiserum nor an OT antagonist inhibited the HA- or restraint-stress-induced ACTH or PRL release in normal rats. AVP-deficient DI rats showed, in comparison with their nondeficit counterparts, an increased basal level of OT and no increase in OT levels following restraint stress, whereas the OT response to HA was similar in the two rat types. In AVP-deficient rats immunoneutralization of OT had no inhibitory effect on the HA- and restraint-stress-induced ACTH and PRL response. The ACTH and PRL response to restraint stress and the ACTH response to HA was impaired in DI rats compared to their healthy controls. We conclude (1) that HA stimulates OT secretion via activation of postsynaptic H1 and H2 receptors, and (2) that, in contrast to AVP, OT does not seem to be involved in HA- and restraint-stress-induced ACTH and PRL secretion.