Kappa opiate receptors inhibit release of oxytocin from the magnocellular system during dehydration.

Magnocellular neurons synthesize vasopressin (VP) or oxytocin (OT) and release these hormones preferentially from the neural lobe during physiological stimulation. In the rat, VP is secreted preferentially during dehydration and hemorrhage, whereas OT is released without VP by suckling, parturition, stress, and nausea. Vasopressinergic neurons also synthesize and release dynorphin-related peptides--alpha- and beta-neoendorphin, dynorphin A (1-8) or (1-17), dynorphin B--which are agonists selective for kappa opiate receptors in the neural lobe. We proposed that one mechanism for preferential secretion of neurohypophysial hormones is that a dynorphin-related peptide(s) coreleased with VP inhibits selectively OT secretion from magnocellular neurons. We tested this hypothesis in conscious adult male Sprague-Dawley rats which were stimulated by either hypertonic saline administered intraperitoneally (2.5%, 20 ml/kg) or subcutaneously (1 M, 15 ml/kg) or by 24 h of water deprivation. Two approaches were used: (1) dynorphin-related peptides (0.02-20.4 mM) were injected intracerebroventricularly 1 min before decapitating the animal, and (2) the action of endogenous opioid peptides was blocked by injecting subcutaneously or intracerebroventricularly either naloxone or a selective kappa receptor antagonist, Mr 2266 or nor-binaltorphimine. VP and OT were measured by radioimmunoassay. After 24 h of water deprivation, the elevation in plasma [OT] but not [VP] was attenuated (p less than 0.05) by alpha-neoendorphin. Dynorphin A (1-8) also inhibited the release of OT and not VP after intraperitoneal administration of hypertonic saline. Blocking the action of endogenous opioid peptides at kappa receptors with Mr 2266 given peripherally (s.c.) elevated plasma [OT] but not [VP] after stimulation with hypertonic saline administered intraperitoneally or subcutaneously.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous opioid peptides inhibit oxytocin release in the lactating rat after dehydration and urethane.

One physiological role for endogenous opioid peptides is to attenuate the release of oxytocin (OT) from the hypothalamo-neurohypophysial system during dehydration and hemorrhage when vasopressin maintains fluid balance and blood pressure. During lactation, OT, which stimulates milk ejection, is released without vasopressin. The influence of endogenous opioid peptides on OT release during suckling has been studied primarily in animals anesthetized with urethane. In addition to anesthesia, urethane dehydrates the animal by elevating plasma osmolality and reducing cardiovascular volume. Thus, we examined in lactating rats the response of the magnocellular neuroendocrine system to dehydration and the role of endogenous opioid peptides in regulating OT release during suckling under conditions of altered fluid balance in conscious and urethane-anesthetized rats. Release of OT in response to an increase in plasma osmolality or a decrease in blood volume was attenuated during lactation in both conscious and anesthetized rats. Blockade of opiate receptors with naloxone (5 mg/kg) did not alter suckling-induced release of immunoreactive OT in conscious, normally hydrated rats, but did augment hormone release after urethane (1.1 g/kg, ip) or after osmotic stimulation with hypertonic sodium chloride (2.5%; 20 ml/kg, ip). During dehydration, the combination of decreased responsiveness of oxytocinergic neurons to osmotic stimulation and inhibition of OT release by opioid peptides may be important in the lactating rat for conserving pituitary stores of OT needed for milk ejection.

Inhibition of VP and OT release by water in hypovolemia is independent of opioid peptides.

Overhydration inhibits release of vasopressin (VP) and oxytocin (OT) from the hypothalamo-neurohypophysial system during hypovolemia. We investigated whether opioid peptides mediate the inhibitory effect of water on secretion of these hormones. Conscious male rats were made hypovolemic by hemorrhage (HEM, 0.51 ml/min) of 20 and 35% of the blood volume or by injection of either subcutaneous polyethylene glycol (PEG, 20,000 mol wt, 35 ml/kg) or intraperitoneal histamine (HIS, 15 mg/kg, 1 ml/kg). Animals were intubated orally 1-4 min (HEM, HIS) or 6.75 h (PEG) later with or without administration of water (40 ml/kg). Four to seven min after intubation rats were injected with saline (1 ml/kg) or naloxone (2 or 5 mg/kg) and then decapitated 6-10 min later. Control animals were treated similarly but were not stimulated by hypovolemia. VP and OT were extracted from plasma and quantified by radioimmunoassay. Data were analyzed by analysis of variance. In HEM animals blood pressure fell and plasma osmolality increased, both of which correlated positively with the rise in plasma [VP] and [OT]. Overhydration lowered the plasma osmolality, attenuated the fall in blood pressure, and reduced [VP] and [OT] in plasma of HEM animals. The opiate receptor antagonist, naloxone, did not alter these changes in blood pressure or plasma osmolality, or the plasma [VP] after HEM in rats treated with or without water. Plasma [OT] was, however, increased by naloxone in both normally hydrated and overhydrated rats. Thus, regardless of the hydrational state of the animal, opioid peptides inhibited release of OT but not VP during hemorrhage. Data consistent with this interpretation were also obtained from rats made hypovolemic with PEG or HIS.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of release of neurohypophysial hormones by endogenous opioid peptides in pregnant and parturient rats.

Naloxone, an opiate receptor antagonist, was used to determine whether opioid peptides modulate release of oxytocin (OT) or vasopressin (AVP) in the rat after expulsion of the fetus, i.e. parturition. We measured the concentrations of AVP and OT in plasma and in the neurointermediate lobe of the pituitary of pregnant rats given naloxone (5 mg/kg, s.c.) or saline on day 20 of gestation, and on day 21 either before or during the expulsive stage of labor. Non-pregnant rats in diestrus were given naloxone for comparison. On days 20 and 21 of gestation, before the onset of parturition, plasma [AVP] but not [OT] was elevated, compared to the non-pregnant controls. After delivery of the first two pups, plasma [OT] approximately doubled, whereas plasma [AVP] remained unchanged. Blocking the action of endogenous opioid peptides with naloxone caused an elevation of plasma [OT] in pregnant animals on days 20 and 21 of gestation and during parturition. Naloxone, however, did not alter plasma [AVP] in either parturient or preparturient animals. In contrast, [AVP], but not [OT], was increased in plasma of non-pregnant rats given naloxone. The content of OT in the neuro-intermediate lobe was similar in pregnant and non-pregnant rats and was unaffected by delivery of the first two pups. However, AVP content and the ratio of AVP/OT in the pituitary were lower in pregnant animals before and during delivery than in the non-pregnant controls. The content of neither hormone was altered by naloxone. Thus, AVP release apparently increases and pituitary stores of this peptide are decreased by day 20 of gestation, when labor has not yet begun.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of opioid inhibition of vasopressin and oxytocin release in response to osmotic stimulation.

We have shown, using the opiate receptor antagonist naltrexone, that endogenous opioid peptides inhibit the release of oxytocin (OT), but not of vasopressin (AVP), from the hypothalamo-neurohypophysial system during dehydration. The stimulus for the release of neurohypophysial hormones during dehydration is both hypovolemia and increased plasma osmolality. The aims of this study were to determine whether opioid peptides inhibit OT secretion during an osmotic stimulus alone and, if so, to study the ontogeny of opiate inhibition of OT and AVP release during osmotic stimulation. Effects of endogenous opioid peptides were evaluated by injecting naloxone into immature and adult rats. Hypertonic saline was used as the osmotic stimulus. Adult male rats were injected sc with normal saline (0.85%; 1 ml/kg BW) or naloxone (5 mg/kg BW), followed 5 min later by normal or hypertonic (1 M) saline (15 ml/kg BW). After 170 min, a second injection of saline or naloxone was given; animals were decapitated 10 min later. Immature male and female rats at 2, 8, 21, and 30 days of age received 0.85% saline (1 ml/kg BW) or naloxone (5 mg/kg BW) ip 5 min before normal or hypertonic (2.5%) saline (20 ml/kg BW, ip). Pups were decapitated 15 min later. AVP and OT were measured by RIA in extracts of plasma, pituitaries, and hypothalami. In control rats, the contents of AVP and OT increased with age in both the pituitary and hypothalamus, attaining adult levels by day 21 for AVP and by day 30 for OT. In contrast, plasma concentrations of both AVP and OT were highest in 8-day-old rats and decreased thereafter to adult levels by 30 days of age. Hypertonic saline raised plasma osmolality 9-16 mosmol/kg H2O, increased AVP and OT concentrations in plasma of adults and immature rats at 2, 8, 21, and 30 days of age, and reduced pituitary stores of OT in adult animals. Blocking the action of opioid peptides with naloxone during osmotic stimulation augmented the rise in plasma OT in rats of all ages but further elevated plasma AVP only in immature rats. In adult animals, blocking opiate receptors with naloxone enhanced the depletion of OT stores from the pituitary, but did not affect the AVP content. We conclude that in the adult rat, endogenous opioid peptides inhibit OT release during osmotic stimulation, thereby allowing preferential release of AVP.(ABSTRACT TRUNCATED AT 400 WORDS)

Naloxone effects on plasma vasopressin and oxytocin concentrations elevated by histamine, nicotine, isoproterenol and an acute increase in [NaCl] in cerebrospinal fluid.

Endogenous opioid peptides inhibit secretion of oxytocin during dehydration, hemorrhage and parturition and attenuate release of vasopressin by tail electroshock. Diverse agents were used to stimulate the hypothalamo-neurohypophysial system to investigate the hypothesis that if oxytocin (or vasopressin) release were inhibited by opioid peptides regardless of the stimulus, the site of opiate action may be in the final common pathway (i.e. the magnocellular neuron) or on pituicytes in the neural lobe. Using male Sprague-Dawley rats, we therefore investigated the effect of an opiate receptor antagonist, naloxone (5 mg/kg s.c.), on the plasma concentrations of oxytocin and vasopressin elevated by various pharmacologic stimuli, including histamine (10 mg/kg i.p.), nicotine (0.15 or 1.5 mg/kg i.p.), isoproterenol (30 or 120 micrograms/kg i.m.) and increased [NaCl] in cerebrospinal fluid (CSF; 10 microliter artificial CSF containing 1 M NaCl i.v.t.). Control animals received saline (0.85%) or artificial CSF (containing 0.16 M NaCl). Animals were decapitated 60 s (increases[NaCl] in CSF) or 10 min after the stimulus or vehicle. Vasopressin and oxytocin were extracted from plasma and quantified by RIA. The concentrations of oxytocin and vasopressin in plasma were elevated (p less than 0.05) by histamine, isoproterenol (30 and 120 micrograms/kg), increases[NaCl] in CSF, and nicotine at the higher (1.5 mg/kg) but not lower (0.15 mg/kg) dose. Naloxone increased further (p less than 0.05) the concentration of oxytocin in plasma after histamine, nicotine (0.15 and 1.5 mg/kg), isoproterenol (30 and 120 micrograms/kg) and increases[NaCl] in CSF. Naloxone also increased (p less than 0.05) oxytocin concentration in controls receiving CSF or saline.(ABSTRACT TRUNCATED AT 250 WORDS)

Dexamethasone differentially alters naltrexone effects on vasopressin and oxytocin release during tail electroshock.

The origin of endogenous opioid peptides that inhibit release of vasopressin (VP) and oxytocin (OT) into the bloodstream after tail electroshock was investigated. We hypothesized that endogenous opioid peptides derived from the anterior pituitary reduced secretion of VP and OT during this stimulus. To test this hypothesis, dexamethasone (DEX) was used to preferentially suppress release of endorphins with ACTH from the anterior pituitary. We evaluated the effects of an opiate receptor antagonist, naltrexone, on the rise in plasma [VP] and [OT] after tail electroshock in male Sprague-Dawley rats given DEX either chronically or acutely before the shock. In the chronic study rats were injected SC daily with saline (3.2 ml/kg) or DEX (0.2 mg/kg) for 17 days. In the short term study, rats were injected IP with saline (5 ml/kg) or DEX (0.5 mg/kg) the day before and again 105 min prior to tail electroshock. Thirty min (chronic study) or 90 min (acute study) after saline or DEX was given on the last day, rats were injected SC with saline (1 ml/kg) or naltrexone (1 mg/kg). Fifteen min later, animals received tail electroshock (41 V, 30 sec) and were decapitated 15 sec after shock was completed. Control animals were treated similarly but not shocked. Amounts of VP and OT in plasma and the neurointermediate lobe were quantified by RIA. [VP] and [OT] were elevated in plasma of all rats given tail electroshock. Greater increases (p less than 0.05) in hormone concentrations were measured in plasma of shocked rats treated with DEX.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of naloxone on the release of neurohypophysial hormones in normotensive and spontaneously hypertensive rats.

The hypothalamo-neurohypophysial system is altered in the spontaneously hypertensive rat (SHR). We hypothesized that an aberrant regulation of vasopressin (VP) and oxytocin (OT) release by endogenous opioid peptides alters this neuroendocrine system in the SHR. Concentrations of the neurohypophysial hormones in plasma and the pituitary were measured in 17-week-old SHRs and two strains of normotensive controls. Wistar Kyoto (WKY) and Sprague-Dawley rats. Animals were decapitated 20 min after s.c. injection of saline (1 ml/kg) or naloxone hydrochloride (1 or 10 mg/kg). In addition, neurohypophysial hormones excreted during the day (08.00-17.30 h) and night (17.30-08.00 h) were determined in urine from 16-week-old animals kept in metabolic cages for 5 days. VP at extrahypothalamic sites was also measured as [VP] in acid extracts of the subfornical organ area, hippocampal commissure-fornix and choroid plexus. Hormones were quantified by radioimmunoassay. The pituitary content, plasma concentration, and urinary excretion of OT were reduced (P less than 0.05) in SHRs, whereas VP content was increased (P less than 0.05) in the pituitary and plasma, but unchanged in urine, of hypertensive animals. In extrahypothalamic tissues, [VP] in the hippocampal commissure-fornix was increased in the SHR. Naloxone elevated (P less than 0.05) the plasma concentration of OT in WKY animals and VP in SHRs. Neither [VP] nor [OT] in plasma was changed by naloxone in Sprague-Dawley rats. Pituitary stores of the neurohypophysial hormones were not altered by naloxone in either hypertensive or normotensive rats. In conclusion, endogenous opioid peptides tonically inhibit OT release in WKY rats, whereas VP release is decreased by opioid peptides in SHRs, 16-17 weeks of age. The neuromodulatory role of opioid peptides in the release of neurohypophysial hormones appears to be altered in the SHR such that VP release is suppressed and OT release is augmented.

A functional role for opioid peptides in the differential secretion of vasopressin and oxytocin.

The presence of opioid peptides and opiate receptors in the hypothalamo-neurohypophysial system, as well as the inhibitory effects of enkephalins and beta-endorphin on release of oxytocin and vasopressin have been well documented. The physiological importance of opioid peptides in this classical neurosecretory system, however, has remained illusive. In the present study we tested the effects of naltrexone on the plasma concentrations of oxytocin and vasopressin during dehydration, hemorrhage and suckling in the conscious rat. We obtained evidence supporting the hypothesis that opioid peptides inhibit oxytocin release and thereby promote the preferential secretion of vasopressin when it is of functional importance to maintain homeostasis during dehydration and hemorrhage. Our data support the concept that the coexistence of a neuromodulator and a neurohormone in the same neuron, as demonstrated for vasopressin with dynorphin or leucine-enkephalin, serves to regulate the differential release of two biologically different, yet evolutionarily-related, neurohormones, e.g. oxytocin and vasopressin, from the same neuroendocrine system.

Enkephalin inhibition of angiotensin-stimulated release of oxytocin and vasopressin.

The effect of leucine5 -enkephalin on angiotensin II (AII)-stimulated release of oxytocin and vasopressin (VP) was investigated in the conscious male rat. Changes in the plasma concentration ([]) of both oxytocin and VP were measured in animals: (1) 60 s after intracerebroventricular (i.v.t.) administration of either artificial cerebrospinal fluid (CSF) or CSF with AII (10, 50, 100 ng/5 microliter); (2) 30, 60, 90 and 300 s after single injection of AII (50 ng/5 microliter; i.v.t.) or CSF and (3) 60 s after AII (50 ng/5 microliter) or CSF in animals pretreated with leucine5 -enkephalin (100 ng/5 microliter; i.v.t.) or CSF (5 microliter). Oxytocin and VP were quantified by radioimmunoassay and values corrected for 100% recovery. The development of a sensitive radioimmunoassay for oxytocin is described. The antiserum for oxytocin enabled detection of greater than or equal to 0.8 pg/ml oxytocin with cross-reactivity of 0.01% with arginine vasopressin and 0.10% with arginine vasotocin. The inter- and intra-assay coefficients of variation were 3-9% and 3.2%, respectively. The hypotheses being tested were that i.v.t. injection of AII stimulates release of both neurohypophysial hormones non-selectively and that leucine5 -enkephalin inhibits both AII-stimulated oxytocin and VP release. Angiotensin II at doses ranging from 10 to 100 ng/5 microliter, i.v.t. increased the plasma concentration of both oxytocin and VP. Plasma levels of both neurohypophysial hormones were elevated 30 s after AII administration i.v.t. and remained elevated 300 sec later.(ABSTRACT TRUNCATED AT 250 WORDS)