Dissociation of the central and peripheral effects of naloxone on PRL secretion

Participation of the opioid system in the regulation of prolactin secretion in androgenized rats: effect of ovarian steroids

We examined the role of the opioid system on the regulation of prolactin secretion in neonatally androgenized rats and evaluated the participation of ovarian steroids in this regulation. Androgenized rats exhibited an increase of prolactin secretion with higher serum circulating levels in the afternoon (1800) than in the morning (1000). The administration of the opioid antagonist naloxone (2 mg/kg, 30 min before decapitation) reduced serum prolactin levels in both groups. To identify the opioid receptor subtypes involved in this regulation, opioid agonists were administered i.c.v. 15 min before the decapitation (1000). The mu-opioid receptor agonist DAMGO ([D-Ala2, NMe-Phe4, Gly5-ol]-enkephalin) caused a significant increase in serum prolactin concentration. The selective kappa-opioid receptor agonist U-50, 488H (trans-(+/-)-3,4-dichloro-N-[2(1-pyrrolidinyl)-cyclohexyl]-benzene acetamide methane sulfonate salt) induced a small but significant increase in serum prolactin levels but no effect was observed after administration of the delta-opioid agonist DPDPE ([D-Pen2, D-Pen5]-enkephalin). The role of oestradiol and the opioid system in the continuous secretion of prolactin was also study. Chronic gonadectomy (3-4 weeks) reduced serum prolactin concentrations measured at 1000 but the administration of naloxone had no effect. Three days of oestrogen treatment (2 microg/rat in oil) restored serum prolactin levels compared with ovariectomized animals and this effect was abolished by naloxone treatment. Interestingly, acute ovariectomy or administration of tamoxifen to intact androgenized rats did not prevent the continuous secretion of prolactin observed in these animals and naloxone treatment reduced serum prolactin levels in both groups of rats. We also examine the participation of adrenal progesterone and the endogenous opioid peptides on the regulation of prolactin levels in androgenized rats. After adrenalectomy, no changes in serum prolactin levels (1000) were observed compared with the control animal and naloxone treatment significantly reduced circulating prolactin levels. Progesterone treatment to intact androgenized rats significantly increased prolactin levels and the administration of naloxone blocked the stimulatory effect of the steroid. These results suggest that the opioid system play a role in the regulation of prolactin secretion in androgenized rats modulated by the persistence of oestrogen action. Moreover, the presence or absence of progesterone did not modify the regulation of prolactin secretion by the opioids. The mu- and kappa-opioid receptor subtypes are the ones involved in the modulation of pituitary prolactin secretion.

Opioid influences on pituitary function in sheep under basal conditions and during psychological stress

The effects of intravenous injections of naloxone (2 mg/kg), morphine (0.3 mg/kg) and saline vehicle on plasma concentrations of cortisol, prolactin, vasopressin and oxytocin were assessed in sheep (N = 10) when in their social groups (basal conditions) and during a period of isolation (psychological stress). Blood samples were collected by jugular venipuncture before and during the 60-min period following drug administration. Plasma hormone concentrations were determined by radioimmunoassay. Under basal conditions, cortisol levels were increased after naloxone (36-48%), but not after morphine or saline, and concentrations of prolactin, vasopressin and oxytocin did not change. Under stress conditions, (1) cortisol concentrations were elevated throughout the 60-min sampling period after naloxone or saline but for only 20 min after morphine; maximum increases observed were 161% (naloxone), 150% (saline) and 112% (morphine); (2) prolactin levels were raised after saline (85-129%) and morphine (55-61%) but were unchanged after naloxone; (3) vasopressin concentrations decreased transiently (43%) after saline but not following naloxone or morphine; and (4) oxytocin levels did not change after any treatment. These results indicate that endogenous and exogenous opioids modulate cortisol release in nonstressed sheep, and cortisol and prolactin secretion in sheep subjected to psychological stress. The nature of the anterior pituitary responses induced, together with the absence of a discernible effect on posterior pituitary function, suggest that the central opioid systems involved are similar in sheep and primates but different from those in the rat.

Plasma levels of beta-endorphin, prolactin and gonadotropins in male athletes after an international nordic ski race

Plasma beta-endorphin, prolactin (PRL), FSH and LH were measured in 17 volunteer male subjects at rest and under the stress caused by a long-distance nordic ski race. The race induced increased levels of beta-endorphin and PRL in all skiers. The changes in PRL with exercise were significantly related to the changes in beta-endorphin (r = 0.69, p less than 0.001). Furthermore, the highly trained skiers training over 150 km.week-1 of nordic ski showed consistently higher post-exercise beta-endorphin and PRL levels than the moderately trained skiers who trained for 20 km.week-1. In addition the race induced slight falls in FSH and LH; however plasma gonadotropin levels did not show any correlation with plasma beta-endorphin concentrations and did not differ between the two groups of skiers. These results suggest that endogenous opioid peptides may modulate PRL secretion in heavy exercise, since they are of minor importance in the release of FSH and LH in such a situation. The observations also suggest that the degree of previous training and the exercise intensity do seem to be responsible for the hormonal changes.