Involvement of Opioid Receptor Subtypes in Both Stimulatory and Inhibitory Effects of the Opioid Peptides on Prolactin Secretion During Pregnancy

SOAJE M; DEIS RP

CELLULAR AND MOLECULAR NEUROBIOLOGY.

SPRINGER/PLENUM PUBLISHERS

Lugar: New York; Año: 2004 vol. 24 p. 193 - 204

0272-4340

regulationof prolactin secretion by the opioid system. In the present work, we evaluated theopioid receptor subtypes involved in both the stimulatory and the inhibitory regulation ofprolactin secretion in pregnant rats.2. Specific opioid agonists and antagonists were administered intracerebro ventricular(i.c.v.) to rats on day 3 and on day 19 pregnancy in rats of pretreated with mifepristone.Blood samples were obtained after decapitation at 12.00 and 18.00 h. Serum prolactin levelswere measured by RIA.3. The ¹-selective agonist DAMGO and ¯-endorphin caused a significant increase inserum prolactin secretion on day 3 of pregnancy, during the diurnal surge and intersurgeperiod. Pretreatment with naloxone prevented the increase on prolactin levels induced byDAMGO. The administration of U-50,488, a ·-selective agonist or DPDPE, a ±-selectiveagonist, did not modify serum prolactin concentration while the ¹1-antagonist naloxonazinereduced significantly serum prolactin levels.On day 19 of pregnancy, the release of prolactininduced by mifepristone was significantly increase by naloxonazine, while the ·-antagonistnor-binaltorfimine induced only a small but significant increase. No effect was observedafter administration of the ±-antagonist naltrindole.4. We conclude that the ¹-opioid receptor seems to be more specifically involved inboth the stimulatory and inhibitory regulation by the opioid system on prolactin secretionduring pregnancy. The increase on serum prolactin levels on day 3 after administration ofDAMGO and ¯-endorphin may suggest the participation of other regulatory mechanismsas the dopaminergic and serotoninergic systems. On day 19, only the endogenous ligands± did not participate in the regulation of prolactin secretion, while the participation of the·-opioid receptor was significantly less effective than the endogenous ligand ¹. Our resultsprovide evidences of an important role of the opioid system through specific receptors onthe regulation of prolactin secretion during early and late pregnancy.