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SUMMARY1. The present study investigated the mechanisms by which endogenous opioids regulate oxytocin secretion at the level of the posterior pituitary gland. Effects of the selective K-agonist U50,488 on oxytocin secretion were studied in urethaneanaesthetized lactating rats. Oxytocin secretion in response to electrical stimulation (0 5 mA, matched biphasic 1 ms pulses, 50 Hz, 60-180 pulses) of the neurohypophysial stalk was bioassayed on-line by measuring increases in intramammary pressure, calibrated with exogenous oxytocin. Intravenous (i.v.) U50,488 inhibited electrically stimulated oxytocin secretion, without affecting mammary gland sensitivity to oxytocin. The inhibition was dose related, with an ID50 of 441 (+ 194, -136) ,ug/kg and was naloxone reversible. Antagonism of endogenous fl-adrenoceptor activation by propranolol (1 mg/kg) reduced the potency of U50,488. The selective ,u-agonist morphine (up to 5 mg/kg), had no effect on electrically stimulated oxytocin secretion, but depressed the mammary response to oxytocin.2. In lactating rats given intracerebroventricular (I.c.v.) morphine infusion for 5 days to induce tolerance and dependence, i.v. U50,488 still inhibited electrically stimulated oxytocin secretion, but the ID50 was reduced to 170 (+ 78, -54) #g/kg; thus at the posterior pituitary the sensitivity of K-receptors is enhanced rather than reduced in morphine-tolerant rats, indicating the absence of cross-tolerance. In these rats, naloxone produced a large, sustained, fluctuating increase in intramammary pressure indicating morphine-withdrawal excitation of oxytocin secretion; i.v. U50,488 diminished this response, confirmed by radioimmunoassay, demonstrating the independence of #u-and K-receptors regulating oxytocin secretion.3. In pregnant rats, i.c.v. infusion of morphine from day 17-18 of pregnancy delayed the start of parturition by 4 h, but did not significantly affect the progress of parturition once established, indicating tolerance to the inhibitory actions of morphine on oxytocin secretion in parturition, and lack of cross-tolerance to endogenous opioids restraining oxytocin in parturition.4. Neurointermediate lobes from control and i.c.v. morphine-infused virgin rats were impaled on electrodes and perifused in vitro. Vasopressin and oxytocin release from the glands was measured by radioimmunoassay. Each gland was exposed to two periods of electrical stimulation (13 Hz, for 3 min). Naloxone (5 x 10-6 M) was MS 1308 J. A. RUSSELL AND OTHERS added before the second stimulation; half the lobes from each I.c.v. treatment were exposed to 5 x 10-M morphine throughout. Oxytocin secretion in response to the first stimulation was similar in the four groups, indicating no acute effect of morphine, and no cross-tolerance from morphine to endogenous opioids released by electrical stimulation. In the presence of naloxone, stimulated oxytocin secretion was potentiated in all four treatment groups, again indicating lack of cross-tolerance to the endogenous opioids that restrain oxytocin secretion...

Opioid actions on oxytocin secretion into blood and cerebrospinal fluid (CSF) were investigated in urethane-anaesthetized female rats after intracerebroventricular (icv) infusion of morphine sulphate or vehicle for 5 days. Serial femoral arterial blood samples and cisterna magna CSF samples were collected for radioimmunoassay. Naloxone was given to assess endogenous opioid tone in icv vehicle-infused rats and to precipitate withdrawal in morphine-dependent animals.Initial plasma oxytocin concentration was not affected by icv morphine infusion. In control rats receiving icv vehicle, naloxone increased plasma oxytocin 11-fold within 5 min. and in icv morphine-infused rats, naloxone increased plasma oxytocin 80-fold within 5 min. In both groups, 90 min after naloxone plasma oxytocin was still 5 and 10 times, respectively, the initial concentration. Without naloxone, neither plasma nor CSF oxytocin concentration changed significantly with time (up to 90min) in either icv treatment group.In the icv vehicle group, there was a 2-fold increase in CSF oxytocin 90 min after naloxone. In the icv morphine-infused group, CSF oxytocin was increased 5-fold 40 min after naloxone.In another group of icv morphine-infused rats, intravenous infusion of oxytocin to achieve plasma levels similar to those seen after naloxone, did not significantly increase CSF oxytocin. In a further group of icv morphine-infused rats, (3H]oxytocin was infused intravenously immediately after naloxone was given; in these rats oxytocin transfer from blood to CSF could account at most for only 20% of the increase in CSF oxytocin after naloxone.A further group of rats underwent bilateral microknife ablation of the paraventricular nuclei (PVN) 9 days before icv vehicle or morphine infusions were started; blood and CSF samples were collected under urethane anaesthesia. Initial concentrations of oxytocin in CSF and in plasma were similar in both groups with PVN ablation. In all PVN-lesioned rats initial plasma concentrations of oxytocin were undetectable ( < 5 pg/ml) and thus less than in intact rats. In contrast, initial levels of oxytocin in CSF were 8-fold greater in PVN-lesioned rats than in intact animals. Naloxone increased plasma oxytocin concentration in the icv vehicle group at least 10-fold within 30 min and in the icv morphine group at least 100-fold within 5 min. CSF oxytocin in the icv vehicle group was not altered by naloxone, but in the icv morphine group CSF oxytocin was increased 5-fold 40 min after naloxone.There were no consistent differences between the icv vehicle-and icv morphine-treated groups in the initial plasma levels of vasopressin, growth hormone and adrenocorticotrophin; PVN ablation did not affect adrenocorticotrophin levels. After naloxone growth hormone levels did not change, vasopressin concentration rose moderately only after 90 min and only in the icv vehicletreated group, and adrenocorticotrophin concentrations decreased with time whether or not naloxone was given.The results imply an endogenous opioid tone on neur...

The effects of morphine dependence and abrupt opiate withdrawal on the release of oxytocin and corticotrophin-releasing factor-41 (CRF-41) into hypophysial portal vessel blood in rats anaesthetized with urethane were investigated. Adult female Sprague-Dawley rats were made dependent upon morphine by intracerebroventricular infusion of morphine for 5 days; abrupt opiate withdrawal was induced by injection of the opiate antagonist naloxone. The basal concentrations of oxytocin in portal or peripheral plasma from morphine-dependent rats did not differ significantly from those in control, vehicle-infused rats. In rats in which the pituitary gland was not removed after stalk section, the i.v. injection of naloxone hydrochloride (5 mg/kg) resulted in a large and sustained increase in the concentration of oxytocin in both portal and peripheral plasma in control and morphine-dependent rats. The i.v. injection of naloxone resulted in a threefold increase in the secretion of oxytocin into portal blood in acutely hypophysectomized rats infused with morphine, but did not alter oxytocin secretion in vehicle-infused hypophysectomized rats. The concentration of oxytocin in peripheral plasma in both vehicle- and morphine-infused hypophysectomized rats was at the limit of detection of the assay and was unchanged by the administration of naloxone. There were no significant differences in the secretion of CRF-41 into portal blood in vehicle- or morphine-infused hypophysectomized rats either before or after the administration of naloxone. These data show that, as for oxytocin release from the neurohypophysis into the systemic circulation, the mechanisms which regulate oxytocin release into the portal vessel blood can also be made morphine dependent. The lack of effect of morphine or naloxone on the release of CRF-41 or other stress neuro-hormones suggests that the effect of opiate dependence and withdrawal is selective for oxytocin and is not simply a non-specific response to 'stress'.

Pethidine (also known as meperidine and as Demerol) injected subcutaneously at 10 mg/kg into parturient rats on the birth of the second pup resulted in a marked slowing of the progress of parturition, associated with reduced plasma oxytocin concentrations. Injection of the opiate antagonist naloxone counteracted the inhibition of oxytocin secretion and largely prevented the slowing of parturition. In vitro, pethidine inhibited spontaneous, oxytocin-induced and acetylcholine-induced contractions of uteri from rats immediately post partum, and these effects were not reversed by naloxone. In anesthetized lactating rats, pethidine inhibited the suckling-induced milk-ejection reflex and attenuated oxytocin-induced contractions of mammary myoepithelium. Finally, pethidine depressed plasma oxytocin concentrations in rats given 2% saline to drink for 24 h to stimulate oxytocin secretion. Thus pethidine inhibits oxytocin secretion in all three conditions; this inhibition is probably mediated by central opioid receptors. In addition, however, pethidine depresses the oxytocin responsiveness both of mammary myoepithelium and of myometrium. The latter effect at least is not opioid mediated.