Inhibition Of Oxytocin Release Research Articles

External stress increases sympathoadrenal activity and prolongs the expulsive phase of foaling in pony mares

Mares usually give birth when they perceive their environment as safe and therefore disturbance at foaling may inhibit labor. In this study, foaling mares were transferred to an unfamiliar environment at rupture of the allantochorion (stress, n = 6) or were left undisturbed (control, n = 5). The progress of foaling, heart rate, heart rate variability (HRV) and plasma catecholamine, oxytocin and cortisol concentration were determined. In stressed mares, time from rupture of the allantochorion to appearance of the fetal feet (5.3 ± 1.1 vs. 1.6 ± 0.4 min) and total length of fetal expulsion were longer than in controls (both p < 0.05). Heart rate decreased during the expulsive phase of foaling in control mares (p < 0.01) but increased transiently in stressed mares. Heart rate calculated as percentage of the baseline was higher in stressed than in control mares (p = 0.05). HRV variables SDRR (standard deviation of the beat-to-beat interval) and RMSSD (root mean square of successive beat-to-beat differences) increased during foal expulsion (SDRR p < 0.01 and RMSSD p < 0.05). The increase in HRV was delayed in stressed compared to control mares (SDRR and RMSSD time x group p < 0.05). Plasma epinephrine and norepinephrine concentrations calculated as area under the curve for the expulsive phase of foaling were higher in stressed than control mares (p < 0.05). Concentrations of oxytocin and cortisol were elevated during foal expulsion (both p < 0.001) but not different between groups. In conclusion, disturbance of mares during expulsion of the foal prolonged foaling. This effect is most likely mediated via increased sympathetic activity and not inhibition of oxytocin release.

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.

The hypothalamic endocannabinoid system participates in the secretion of oxytocin and tumor necrosis factor-alpha induced by lipopolysaccharide

This study investigated the participation of the hypothalamic endocannabinoid system in the response to lipopolysaccharide (LPS) challenge evaluating oxytocin (OXT) and tumor necrosis factor-alpha (TNF- a) plasma levels in vivo and their release from hypothalamic fragments in vitro. LPS increased OXT and TNF-α release through anandamide-activation of hypothalamic cannabinoid receptor CB 1, since the antagonist AM251 blocked this effect. Anandamide, through its receptors, also increased hypothalamic nitric oxide (NO) which inhibited OXT release, ending the stimulatory effect of the endocannabinoid. Our findings reveal a hypothalamic interaction between oxytocin, endocannabinoid and NO-ergic systems providing a regulation of the hypothalamic–neurohypophyseal axis under basal and stress conditions.

Insulin and release of neurohypophysial hormones: in vivo and in vitro studies.

No significant effect of intracerebroventricular administration of insulin on the bioassayed neurohypophysial storage of vasopressin and oxytocin could be found. This finding, however, only indirectly reflects possible changes of vasopressin and oxytocin release. Incubation of neurointermediate lobes in Locke's solution containing 3.3 mumol/l insulin resulted in an inhibition of oxytocin release under resting conditions as well as in a decrease of both vasopressin and oxytocin release during depolarization due to excess potassium. These results seem to suggest a possible regulatory role of brain insulin in the mechanisms of release of neurohypophysial hormones.

Mechanism of CCK‐induced oxytocin release from hypothalamic synaptosomal preparation

Previously we have reported that both KCl and CCK‐8 cause oxytocin release from spinal cord synaptosomes. We have also demonstrated the involvement of mitochondria in this release process. Mitochondria could modulate OT release either as a source of Ca2+ or as a source of ATP or both. The current experiments were designed to determine if ATP has some modulatory role in this release mechanism. Present experiments were conducted with hypothalamic synaptosomes prepared from hypothalamic block obtainedfrom freshly sacrificed rats containing both PVN and SON. Synaptosomes were purified by discontinuous percoll gradient (23%,15%,10% &amp;3%). The interface between 23% &amp;15 % was collected and used for the release study. For the current experiments ATP was used as a stimulant. ATP in a concentration of 10nM, 50 nM and 100 nM caused release of oxytocin 90%,364% &amp; 77% rtespectively.. Thus, these results suggest that inhibition of oxytocin release from spinal cord synaptosomes following treatment with electron transport chain inhibitors was probably due to lack of ATP and not due to lack of Ca2+

µ‐Opioid Receptor Preferentially Inhibits Oxytocin Release from Neurohypophysial Terminals by Blocking R‐type Ca2+ Channels

Oxytocin release from neurophypophysial terminals is particularly sensitive to inhibition by the micro-opioid receptor agonist, DAMGO. Because the R-type component of the neurophypophysial terminal Ca2+ current (ICa) mediates exclusively oxytocin release, we hypothesised that micro-opioids could preferentially inhibit oxytocin release by blocking this channel subtype. Whole-terminal recordings showed that DAMGO and the R-type selective blocker SNX-482 inhibit a similar ICa component. Measurements of [Ca2+]i levels and oxytocin release confirmed that the effects of DAMGO and SNX-482 are not additive. Finally, isolation of the R-type component and its associated rise in [Ca2+]i and oxytocin release allowed us to demonstrate the selective inhibition by DAMGO of this channel subtype. Thus, micro-opioid agonists modulate specifically oxytocin release in neurophypophysial terminals by selectively targeting R-type Ca2+ channels. Modulation of Ca2+ channel subtypes could be a general mechanism for drugs of abuse to regulate the release of specific neurotransmitters at central nervous system synapses.

Inhibition of oxytocin release during repeated milking in unfamiliar surroundings: the importance of opioids and adrenal cortex sensitivity.

The aim of this study was to test if the opioid antagonist naloxone has a beneficial effect on normalization of oxytocin (OT) release during repeated milking of cows in unfamiliar surroundings. One control milking without naloxone treatment in all cows was performed in the familiar parlour. For four successive evening milkings, cows were transported to, and milked in, the operating theatre of the research station without (control group) or with naloxone administration (1 mg/kg BW) (naloxone group) before milking. After cessation of spontaneous milk flow, but not before 3 min of milking, vaginal stimulation was applied for 2 min. After milk flow ceased again, 10 IU of OT was injected intravenously to remove the remaining milk including residual milk. Milk flow was recorded continuously and blood samples were collected via a jugular vein cannula at 1-min intervals from 1 min before the start of milking until i.v. injection of OT. The inhibition of milk ejection and its normalization during repeated milking in unfamiliar surroundings was not influenced by naloxone treatment. Concentrations of cortisol and beta-endorphin during control milking and all relocations were similar in the naloxone and control groups, although their concentrations were higher after relocations than in the control. Therefore, a role of endogenous opioids in the inhibition of milk ejection in unfamiliar surroundings could not be demonstrated. In addition, the effect of exogenous ACTH1-24 (8 IU, i.v.) on the release of cortisol related to the response of cows milked in unfamiliar surroundings was studied. Cows with totally inhibited milk ejection in response to vaginal stimulation during milking after first relocation had numerically, but not significantly lower cortisol levels (8.8+/-3.4 ng/ml; AUC/min) in response to ACTH than did cows with at least partial milk ejection (38.7+/-12.9 ng/ml). Thus animals with a higher adrenal response to ACTH seemed to have less severe inhibition of milk ejection.

Inhibition of oxytocin release and milk let-down in postpartum primiparous cows is not abolished by naloxone.

About 10% of primiparous cows have no milk ejection during the first milkings after delivery. Therefore, 17 Brown Swiss dairy cows in their first lactation were used to evaluate the extent of disturbed milk let-down and the corresponding oxytocin (OT) plasma values in the 1st 5 days after delivery. The first milking was 9-22 h after parturition and served for classification of the cows to groups with inhibited (INH), bimodal (BIMO), or normal (NOR) milk let-down. The OT plasma levels before the start of manual teat stimulation and machine milking were comparably high during the first milking especially in NOR and BIMO cows. Ten minutes before the second milking (M2), 300 mg of the opioid antagonist naloxone was injected to test whether the disturbance was affected by the action of endogenous opioids on the neurohypophysis. The milk yield was not influenced by the naloxone treatment, and the INH cows had milk ejection only after a vaginal stimulation. Afterwards, the cows were milked twice every day, until the milk let-down and the OT release were unaffected (equal to control milking). Then, at the next milking, the cows were injected with 300 mg morphine 10 min before milking. The central OT release in response to manual teat stimulation and machine milking was completely blocked in all cows, but a vaginal stimulation was able to abolish this block, at least partially, in 16 cows. Thus, morphine produced a milk let-down characteristic as in the INH cows during the first three milkings. For the following milking, the cows were pre-treated with 300 mg naloxone (-15 min) plus 300 mg morphine (-10 min) before milking. The OT release and the milk yields were unaffected when compared with the control milking. This experiment demonstrates that exogenous opioids can affect the central release of OT in a naloxone-reversible manner even very soon after parturition. However, endogenous opioids are probably not the main mediators of disturbed central OT release and alveolar milk ejection in post-partum primiparous cows.

Central and peripheral inhibition of milk ejection

Milk ejection can be inhibited as a result of a lack of release of oxytocin (OT) from the pituitary gland, or, when OT is released normally, as a lack of effect on the mammary gland. The former can be overcome by administration of exogenous OT, whereas the latter cannot. Central inhibition of OT release can be caused by several factors, such as relocating cows to unfamiliar surroundings for milking, oestrus, change from suckling to machine milking. In such cases peripheral blood concentrations of β-endorphin and cortisol are elevated. Administration of exogenous cortisol does not cause inhibition of milk ejection. On the other hand, the administration of morphine inhibits OT release from the pituitary, and, therefore, milk ejection. The effect of morphine can be abolished by naloxone (opioid antagonist) administration. However, premilking administration of naloxone does not abolish spontaneous inhibition of milk ejection in cows during milking in unfamiliar surroundings. The role of β-endorphin in central inhibition of milk ejection remains unclear. Peripheral inhibition of milk ejection is caused by exogenous α-adrenergic receptor agonists or OT receptor blocking agents, while OT release from the pituitary is normal. Administered β-adrenergic agonists cause elevated milk flow rates as compared with normal milking. Inhibition of milk ejection by catecholamines cannot be abolished by administration of OT but α-adrenergic receptor blocking agents. Effects of OT receptor blocking agents can be abolished by administration of high dosages of exogenous OT. However, milk flow rates are not reduced as long as milk is present in the cistern. The presence of high concentrations of adrenergic receptors in the teat and lower amounts in more proximal regions of the bovine udder has been demonstrated. Their influence on milkability has been reported and relations between adrenergic receptor concentrations in the udder or on blood platelets and milking characteristics are most likely. Under stress free conditions the sympathetic tone in the mammary gland is reduced during milking and increases between milkings. All these investigations reflect the importance of the sympathic nervous system in the process of milk removal. Milk removal seems to be regulated by the adrenergic system, although the mechanism is not clear.

The transfection-induced overexpression of IGF-binding protein-4 affects the secretory activity of porcine ovarian granulosa cells and their response to hormones and IGF-I

The aim of our studies was to examine whether IGF-binding protein (IGFBP)-4 is involved in the control of the secretion of various ovarian substances and also the mediation of the effects of several hormones and growth factors on this secretion. For this purpose, we carried out the transfection of porcine granulosa cells with a cDNA sense construct, increasing IGFBP-4 synthesis. We then compared the release of IGFBP-3, progesterone, oxytocin and IGF-I by control and transfected cells cultured with and without porcine LH (100 ng/ml), porcine GH (100 ng/ml), IGF-I (10 ng/ml), oxytocin (10 ng/ml) and estradiol-17beta (100 ng/ml). The concentration of IGFBP-4 produced was assessed using ligand blotting, and the release of progesterone, oxytocin, IGF-I and IGFBP-3 was evaluated using RIA/IRMA techniques. It was observed that GH, IGF-I, estradiol, LH and oxytocin alter the progesterone, oxytocin, IGF-I and IGFBP-3 release by porcine ovarian granulosa cells. Transfection of these cells with an IBFBP-4 cDNA expression construct significantly increased the IGFBP-4 accumulation in cell-conditioned medium. Furthermore, this transfection significantly reduced progesterone, oxytocin and IGFBP-3 release, and increased IGF-I output in cells cultured in the absence or presence of GH, IGF-I, estradiol and LH. The addition of oxytocin, but not of other tested substances, fully or partially prevented the effects of IGFBP-4 overexpression on IGFBP-3, IGF-I, but not on progesterone release. The present results suggested that IGFBP-4, as well as GH, IGF-I, estradiol, LH and oxytocin, is a potent regulator of porcine ovarian steroid (progesterone), nonapeptide hormone (oxytocin), growth factor (IGF-I) and growth factor-binding protein (IGFBP-3) release. IGFBP-4 is an inhibitor of basal progesterone, oxytocin and IGFBP-3 release and a stimulator of IGF-I output by porcine ovarian cells. The action of IGFBP-4 on the ovary can be mediated by (1) inhibition of oxytocin release, (2) suppression of receptor/postreceptor events induced by other hormones and IGF-I and (3) stimulation of IGF-I release.

Factors affecting milk ejection and removal during milking and suckling of dairy cows

The release of oxytocin and milk ejection occurrence in response to teat stimulation are crucial for fast and complete milk removal during milking or suckling. The milk ejection reflex can be disturbed at central or peripheral level under different experimental and practical conditions. The central disturbance results in the lack or insufficient ejection of the alveolar milk into the cistern due to inhibited oxytocin release from pituitary into the blood circulation. The important role in the pathophysiological regulation of the inhibited release of oxytocin is played by an opioid system. Endogenous opioids have suppressive effects on oxytocin release under the normal conditions of milk removal. However under the conditions of disturbed milk ejection their role in dairy cows was not confirmed. Other possible mechanisms involved in the central inhibition of oxytocin release are discussed. In dairy cows as compared with rats the mechanisms involved in the regulation of oxytocin release at the central level remain unclear. The central inhibition of oxytocin release has often been observed in dairy practice during milking of primiparous cows after parturition, suckling by alien calf, calf removal before milking, milking of cows in the presence of own calf, relocation and milking in an unknown milking place. If sufficient released oxytocin cannot induce the transfer of milk from alveoli to cistern, peripheral mechanisms are involved. Peripheral mechanisms are related to the increased levels of catecholamines and/or activation sympathetic nervous system at the udder level. In conclusion, the release of oxytocin and milk ejection efficiency can be very easily suppressed by many factors. The effect of milking conditions on regulation of milk ejection has to be considered. Thus the physiological requirements of dairy cows have to be respected.

Chronic morphine treatment inhibits oxytocin release from the supraoptic nucleus slices of rats

Effect of chronic morphine treatment on oxytocin (OT) release from the long term-cultured organotypic slice of the supraoptic nucleus (SON) was investigated using radioimmunoassay. The co-localization of oxytocin and mu-opioid receptor in neurons within the SON was observed with the double-labeled methods of in situ hybridization combined with immunohistochemistry. After exposure to morphine for 6 days, the OT levels in culture media were significantly decreased. Naloxone caused much greater release of OT in chronic morphine treatment group than in controls. Naloxone has no effect after acute morphine treatment. 90% of OT-ir (immunoreactive) neurons expressed μ-opioid receptor mRNA in the SON and 45% of the neurons that expressed μ-opioid receptor mRNAs were OT-ir neurons. These results indicated that the neurons within SON could develop dependence on morphine in vitro, and these effects might be exerted via μ-opioid receptor in oxytocin neurons of the SON.

Effect of four cGMP analogues with different mechanisms of action on hormone release by porcine ovarian granulosa cells in vitro.

The aim of our in-vitro experiments was to examine the role of cGMP-dependent intracellular mechanisms in control of ovarian hormone secretion, as well as to understand, whether cGMP effect on the ovary may be mediated by either protein kinase G (PKG), cGMP-gated ion channels (CGI) or cGMP-specific phosphodiesterases (PDE). We compared the effects of the cGMP analogues 8-pCPT-cGMP, an activator of PKG 1-alpha, 1-beta and type II and of CGI, but not of PDE: Rp-8-pCPT-cGMPS and Rp-8-Br-cGMPS, inhibitors of PKG, stimulators of CGI with no effect of PDE, and Rp-8-Br-PET-cGMPS, an inhibitor of both, PKG and CGI and stimulator of PDE (all at 0.01, 0.1, 1, 10 or 100 nM), on the release of oxytocin (OT) and progesterone (P) by cultured porcine granulosa cells. It was observed, that Rp-8-pCPT-cGMPS significantly (p<0.05) suppressed OT release when given at 1 or 10 nM. Rp-8-Br-cGMPS increased OT output, when given at 1-10 nM too, but decreased it at 100 nM. Rp-8-Br-PET-cGMPS inhibited OT release at 1 nM. No influence of 8-pCPT-cGMP on OT output was found. 8-pCPT-cGMP stimulated P release at 0.1, 10 or 100 nM. All other cGMP analogues studied suppressed P release at all doses used. The present observations suggest the involvement of cGMP-dependent intracellular mechanisms in control of ovarian steroid and nonapeptide hormone release. The lack of association between patterns of influence of cGMP analogues on CGI and PDE, and the coincidence of the majority of effects of cGMP analogues on P, OT and PKG may indirectly indicate that cGMP action on release of ovarian hormones is mediated mainly by PKG, but not by CGI or PDE.

Oxytocin Release and Milk Removal in Ruminants

Before milking, less than 20% of the milk yielded by dairy cows is stored within the cistern, where it is immediately available for removal. Most of the milk is available for the milking machine only after milk ejection, which occurs in response to tactile teat stimulation and oxytocin release. For complete milk removal, milk ejection is necessary throughout the entire milking process. The continuation of stimulatory effect of the milking machine until the end of milking is, therefore, essential. Premilking teat stimulation causes induction of alveolar milk ejection before the start of milking. Thus, bimodal milk flow curves (i.e., interruption of milk flow after removal of the cisternal milk) are avoided. Continual ejection of milk is dependent on the presence of elevated oxytocin concentrations during the entire milking. Any interruption of the milk ejection process can disturb milk removal.Disruption of milk removal can be caused by peripheral inhibition of oxytocin effects on the mammary gland or by inhibition of oxytocin release by the central nervous system. Peripheral inhibition is induced by elevated concentrations of catecholamines through stimulation of α-adrenergic receptors in the mammary gland, likely via changes in ductal resistance. Inhibition of oxytocin release by the central nervous system has been observed in primiparous cows immediately after parturition, during peak estrus, and during milking in unfamiliar surroundings; concentrations of β-endorphin and cortisol are elevated in this situation. However, the role of endogenous opioid peptides in the inhibition of oxytocin release in cows remains unclear. In conclusion, during machine-milking, the physiological requirements of the cows need to be considered, and, most importantly, stressors must be minimized.

Effects of L-NAME on cerebral metabolic, vasopressin, oxytocin, and blood pressure responses in hemorrhaged rats.

NG-nitro-L-arginine methyl ester (L-NAME; 250 micrograms/5 microliters), an inhibitor of NO synthase, or the vehicle artificial cerebrospinal fluid (aCSF; 5 microliters) was administered intracerebroventricularly to conscious rats hemorrhaged (0.7 ml/min) to a 20% volume depletion. Hypotension was maximal 5 min after hemorrhage ended, with compensatory recovery to basal levels 20 min later, regardless of drug treatment. L-NAME, however, elevated (P < 0.05) blood pressure (vs. aCSF controls) 40-45 min after intracerebroventricular administration. In normovolemic rats, L-NAME produced a significant pressor response and increased plasma levels of vasopressin (VP) and oxytocin (OT). After hemorrhage, both hormone levels increased, but only OT was further enhanced by L-NAME. Thus centrally produced NO tonically inhibits OT and VP secretion under basal normovolemic conditions and selectively inhibits OT release during hypovolemia. Hemorrhage increased the rates of glucose utilization in the neural lobe, indicative of enhanced efferent neural functional activity. L-NAME further enhanced the metabolic activity in the entire hypothalamoneurohypophysial system of hemorrhaged animals. Several other brain structures involved in the regulation of blood pressure and the stress response were also metabolically affected by the hemorrhage and L-NAME.

Role of NO on vasopressin and oxytocin release and blood pressure responses during osmotic stimulation in rats.

NG-nitro-L-arginine methyl ester (L-NAME, 250 micrograms/5 microliters), an inhibitor of nitric oxide (NO) synthase, or artificial cerebrospinal fluid (5 microliters) was administered intracerebroventricularly to conscious naive rats or to rats treated subcutaneously (15 microliters/kg) with NaCl (0.15, 0.45, or 1.0 M) or given a needle prick only. Intracerebroventricular injection of L-NAME increased plasma concentration of vasopressin (VP) and oxytocin (OT) in control naive rats, indicating that NO tonically inhibits basal secretion of both hormones during isosmotic isovolemic conditions. Osmotic stimulation with hypertonic saline (0.45 and 1.0 M NaCl) elevated plasma levels of both hormones as expected. Central blockade of NO synthase further enhanced secretion of OT during mild, but not strong, osmotic stimulation, whereas the high levels of VP remained unaffected by L-NAME. In animals treated with the needle prick or 0.15 M NaCl, only OT levels were increased after L-NAME. Therefore, NO selectively inhibits OT release in response to a painful stimulus (needle prick) and moderate osmotic stimulation to promote a preferential release of VP. A transient pressor response was observed after subcutaneous injection of 0.15 and 0.45 M NaCl, but a sustained response was obtained after 1.0 M NaCl. Regardless of whether the animals received NaCl solutions, however, treatment with L-NAME elevated blood pressure in all animals. Thus NO-induced vasodilation maintains basal arterial blood pressure and limits the pressor response to osmotic stimulation.

NO and angiotensin II effects on blood pressure and fluid homeostasis.

Angiotensin II (50 ng/5 microl) and L-NAME (250 microg/5 microl), an inhibitor of NO synthase (NOS), were administered intracerebroventricularly alone or in combination to conscious rats. Mean arterial blood pressure (MABP) increased reaching a peak within 5 min in all groups compared to controls treated with the vehicle, artificial CSF (5 microl). MABP returned to basal levels at 30 min after angiotensin II and remained stable for the following 90 min. In animals treated with L-NAME alone, after the initial pressor response, MABP declined but began to increase progressively from 30 min until the end of the experiment at 120 min. When administered with angiotensin II, however, the initial pressor response was prolonged. Angiotensin II-induced drinking was significantly attenuated by L-NAME. In control rats, inhibiting NOS elevated plasma levels of oxytocin and vasopressin but in angiotensin II-stimulated animals, only oxytocin was further elevated after L-NAME. Thus, NO formed centrally inhibits basal secretion of oxytocin and vasopressin as well as the resting blood pressure. During stimulation with angiotensin II, NO facilitates drinking, limits the pressor response and selectively inhibits oxytocin release.

Oxytocin stimulates the release of luteinizing hormone-releasing hormone from medial basal hypothalamic explants by releasing nitric oxide.

Oxytocin induces mating behavior in rats of both sexes. Previous experiments revealed that progesterone-induced sex behavior in ovariectomized, estrogen-primed rats was caused by release of NO from NOergic neurons that stimulated the release of luteinizing hormone-releasing hormone (LHRH). The LHRH activated brain-stem neurons that initiated the lordosis reflex. We hypothesized that oxytocin might similarly release NO in the medial basal hypothalamic region that would stimulate release of LHRH into the hypophyseal portal vessels to release luteinizing hormone. To investigate this hypothesis, medial basal hypothalamic explants were preincubated in Krebs-Ringer bicarbonate buffer for 30 min, followed by a 30-min incubation in fresh Krebs-Ringer bicarbonate buffer containing the compounds to be tested. Oxytocin stimulated LHRH release 3- to 4-fold at the lowest concentration tested (10(-10) M). Values remained at a plateau as the concentration was increased to 10(-7) M and then declined in a concentration-dependent manner, so that there was no stimulation with a concentration of 10(-5) M. Oxytocin (10(-7) M) stimulated release of prostaglandin E2 into the medium, a finding consistent with a role of NO in the response. That NO indeed mediated the action of oxytocin was supported by blockade of the action of oxytocin by the competitive inhibitor of NO synthase (NOS), N(G)-monomethyl-L-arginine (300 microM). Furthermore, oxytocin (10(-9) to 10(-7) M) activated NOS as measured at the end of the experiments. Oxytocin appeared to act to stimulate norepinephrine terminals in the medial basal hypothalamus, which activated NOS by alpha1-adrenergic receptors, because prazocine, an alpha1 receptor blocker, inhibited the LHRH-releasing action of oxytocin. Finally, incubation of neural lobe explants with sodium nitroprusside, a NO releasor, revealed that nitroprusside (300-600 microM, but not 900 microM) inhibited oxytocin release. Therefore, the NO released by oxytocin also diffuses into the oxytocin neuronal endings and inhibits oxytocin release, forming a negative feedback loop. The results indicate that oxytocin is important not only in induction of mating, but also in stimulating LHRH release with subsequent luteinizing hormone discharge that plays a crucial role in reproduction.

Milk yield, oxytocin and beta-endorphin gradually normalize during repeated milking in unfamiliar surroundings.

procedure previously shown to inhibit oxytocin release and milk ejection. Two control milkings were performed in familiar surroundings. After milk flow had ceased, two i.v. injections of 1 i.u. oxytocin were given to remove the remaining milk. Milk flow was recorded continuously and blood samples were taken every minute during milking and 10 min after milking. During the first milking in unfamiliar surroundings, no oxytocin was released. Thereby, only 13 percent of the total milk yield, the cisternal milk, was available and the alveolar milk fraction could only be removed after injection of oxytocin. During subsequent relocations oxytocin release steadily increased toward the control level, although the timing of oxytocin release remained delayed as compared with controls. However, the milk fraction available before oxytocin injection increased with increasing number of removals, following an asymptotic approach to control levels. The concentrations of beta-endorphin, cortisol (and perhaps also of prolactin) gradually declined with the number of times the animal was moved to unfamiliar surroundings, i.e. hormone concentrations gradually adjusted to control level. During milking, concentrations of prolactin and cortisol increased, while beta-endorphin concentrations decreased (except for the first relocation). We conclude that milking-related oxytocin release and therefore milk ejection adapted gradually to repeated relocations to unfamiliar surroundings. This adaptation was inversely related to beta-endorphin concentrations, so it is possible that oxytocin release was suppressed by high circulating beta-endorphin concentrations.

Changes in pain threshold during pregnancy and parturition in the sow and the involvement of opioids

Endogenous opioids are known to inhibit oxytocin release from the neurohypopysis (Bicknell et al, 1982). This mechanism has been suggested to be involved in the timing of parturition and delivery of young (Leng et al, 1985). Environmental disturbance in the rat (Leng et al, 1988) and pig (Lawrence et al, 1992) during parturition resulted in a naloxone reversible increase in birth interval, and an increase in plasma oxytocin concentration after naloxone treatment suggesting stress-induced opioid inhibition of oxytocin. Pain, which is associated with parturition, results in increased opioid activity and may be involved in the inhibition of oxytocin. Pregnancy-induced analgesia has been shown in the human (Whipple et al, 1990) and the rat (Gintzler, 1980), with the latter suggesting involvement of an opioid mechanism. This study aimed to determine whether an increase in pain threshold occurred over pregnancy and parturition in the pig and whether this was opioid-mediated.