Resumo

The Leptin (Lep), a hormone secreted mainly by adipose tissue, has been suggested as possible mediator of interaction between energy homeostasis and reproduction. However, the gonadotropin releasing hormone (GnRH) neurons do not express leptin receptor and nitric oxide is a possible candidate to mediate the leptin action on GnRH neurons. Moreover, previous studies showed that leptin action on reproductive function would be modulated by estrogen. The aim of this work was verify if the effect of leptin on the luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) secretion at proestrous depends on interaction between estrogen and nitric oxide in the medial preoptic area (MPOA). Methods: Wistar female rats with regular estrous cycles received a cannula in the lateral ventricle. The animals that recovered estrous cycles were treated with estrogen antagonist, tamoxifen s.c. (TMX, 3mg/rat) during metestrous and diestrous. At proestrous day, these animals received leptin (0,3; 1; 3 and 10ug/ul) or vehicle (veh) i.c.v. and were decapitated at 5:00 pm to withdraw blood and brain. Plasma hormones were measured by radioimmunoassay. Brains were processed for NOS western blotting from MPOA. Results: Lep-3ìg increased plasma LH (veh 28.7 + 7.1 / Lep 78.9 + 24.7ng/ml) and Lep-10ìg increased plasma PRL (veh 234.8 + 35 / Lep 377.2 + 71.9 ng/ml). On the other hand, TMX reduced plasma LH (veh 28.7 + 7.1 / TMX 4.7 + 0.5 ng/ml), FSH (veh 3.1 + 0.5 / TMX 1.1 + 0.1 ng/ml) and PRL (veh 234.8 + 35 / TMX 15.1 + 1.7 ng/ml) and hindered the effects of LEP i.c.v. described previously. Furthermore, NOS expression in the MPOA was reduced by Lep-10ug (veh 0.75 + 0.02 / Lep 0.66 + 0.01) and ovariectomy (veh 0.75 + 0.02 / ovx 0.58 + 0.02) but not by TMX. Conclusion: Leptin action on LH and PRL secretion depends on estrogen since it was blocked by tamoxifen. However, the interaction with nitric oxide in the MPOA seems be different for secretion of each hormone. Leptin dose that increased PRL secretion also reduced NOS expression in the MPOA corroborated previous results that showed an increase of plasma PRL after inhibition of NOS in the MPOA. On the other hand, leptin dose that increased LH secretion did no change NOS expression in the MPOA.