OPIOIDS MODIFY THE RELEASE OF LH AT THE PITUITARY LEVEL - IN-VITRO STUDIES WITH ENTIRE RAT PITUITARIES

It is currently accepted that opioids modify the secretion of LH by af fecting the release of GnRH in the hypothalamus. A direct action of op ioids at the pituitary level is not yet fully established. To this end , we tested the effects of opioids on the release of LH by the entire pituitary in adult male rats. Opioid agonists with mu (DAGO), delta (D S-LET) and kappa (U-50488H) specificity were tested at 0.01 to 10 mu M in static incubations. DAGO inhibited dose-dependently the spontaneou s and GnRH-induced release of LH. DSLET inhibited only the GnRH-induce d release of LH. On the other hand, U-50488H increased spontaneous LH release dose-dependently. The opioid antagonists naloxone, diallyl-G ( delta antagonist) and MR 2266 (kappa antagonist) blocked the effects i nduced by DAGO, DSLET or U-50488H respectively, implying an opioid rec eptor-mediated effect. The above results showed that opioids with mu, delta and kappa specificity act on the entire pituitary and modify dif ferentially the release of LH. In this study we also compared spontane ous and GnRH-induced LH release by anterior and entire pituitaries and found that the amount of LH released by the anterior lobe was twofold higher, suggesting that inhibitory factors present in the neurointerm ediate part may affect the release of LH.