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

SOAJE M; DEIS RP

EUROPEAN JOURNAL OF PHARMACOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 1999 vol. 371 p. 43 - 49

0014-2999

We examined the role of the opioid system on the regulation of prolactin secretion in neonatally androgenized rats and evaluated theparticipation of ovarian steroids in this regulation. Androgenized rats exhibited an increase of prolactin secretion with higher serumcirculating levels in the afternoon 1800. than in the morning 1000.. The administration of the opioid antagonist naloxone 2 mgrkg, 30min before decapitation. reduced serum prolactin levels in both groups. To identify the opioid receptor subtypes involved in thisregulation, opioid agonists were administered i.c.v. 15 min before the decapitation 1000.. The m-opioid receptor agonist DAMGOwD-Ala2, NMe-Phe4, Gly 5-olx-enkephalin. caused a significant increase in serum prolactin concentration. The selective k-opioid receptoragonist U-50, 488H trans-".-3,4-dichloro-N-w21-pyrrolidinyl.-cyclohexylx-benzene acetamide methane sulfonate salt. induced a smallbut significant increase in serum prolactin levels but no effect was observed after administration of the d-opioid agonist DPDPE wD-Pen2,D-Pen5x-enkephalin.. The role of oestradiol and the opioid system in the continuous secretion of prolactin was also study. Chronicgonadectomy 3?4 weeks. reduced serum prolactin concentrations measured at 1000 but the administration of naloxone had no effect.Three days of oestrogen treatment 2 mgrrat in oil. restored serum prolactin levels compared with ovariectomized animals and this effectwas abolished by naloxone treatment. Interestingly, acute ovariectomy or administration of tamoxifen to intact androgenized rats did notprevent the continuous secretion of prolactin observed in these animals and naloxone treatment reduced serum prolactin levels in bothgroups of rats. We also examine the participation of adrenal progesterone and the endogenous opioid peptides on the regulation ofprolactin levels in androgenized rats. After adrenalectomy, no changes in serum prolactin levels 1000. were observed compared with thecontrol animal and naloxone treatment significantly reduced circulating prolactin levels. Progesterone treatment to intact androgenizedrats significantly increased prolactin levels and the administration of naloxone blocked the stimulatory effect of the steroid. These resultssuggest that the opioid system play a role in the regulation of prolactin secretion in androgenized rats modulated by the persistence ofoestrogen action. Moreover, the presence or absence of progesterone did not modify the regulation of prolactin secretion by the opioids.The m- and k-opioid receptor subtypes are the ones involved in the modulation of pituitary prolactin secretion.