Receptores RF-Amida envolvidos na ação estimulatória da kisspeptina sobre a secreção de prolactina em fêmeas

Abstract

Kisspeptin (Kp), a member of RF-amide family, is recognized as a key component controlling the luteinizing hormone (LH) secretion. Kp also induces prolactin (PRL) secretion through inhibition of the tuberoinfundibular dopaminergic neurons (TIDA), which release dopamine (DA) in the median eminence (ME). The Kp receptor (Kiss1r) is expressed in the rat hypothalamus but not in TIDA neurons. On the other hand, Kp is able to bind with high affinity to the receptor for neuropeptide FF (NPFFR). We investigated the roles of Kiss1r and NPFFR in the effect of Kp on PRL secretion and on the activity of TIDA neurons in females. Initially, we evaluated the effect of the Kiss1r antagonist Kp-234, administrated intracerebroventricularly (i.c.v.), on the genesis of PRL and LH surges in ovariectomized (OVX) rats treated with estradiol (OVX+E). The Kp-234 attenuated the peak phase of the PRL surge and totally prevented the LH surge induced by estradiol, demonstrating the involvement of the kisspeptin-Kiss1r signaling in these physiological events. In subsequent experiments, we evaluated the effect of i.c.v. Kp-234 on secretion of PRL and LH induced by i.c.v. Kp-10 (biological active fragment of Kp) in OVX+E rats. Kp-234 abolished the increase in PRL and LH release induced by Kp-10. However, Kp-234 did not prevent the Kp-10 effect in reducing the activity of TIDA neurons, as determined by the 3,4-dihydroxyphenylacetic acid (DOPAC)/DA ratio in the ME. To further confirm the role of Kiss1r, PRL secretion and the expression of tyrosine hydroxylase (TH) and Ser40-phosphorylated TH (S40pTH) were evaluated in Kiss1r knockout mice, in which Kiss1r was reinserted only in GnRH neurons (Kiss1r-/-R). As result, the PRL increase stimulated by i.c.v. Kp-10 in wild type (WT) females was absent in Kiss1r-/-R mice, while the S40pTH/TH ratio in the ME was not changed by Kp-10 in any of the experimental groups. The participation of NPFFR in the effect of i.c.v. Kp-10 on PRL secretion was evaluated using the i.c.v. injection of the RF-9 antagonist, which inhibited the PRL response while potentiated the LH response to Kp-10. Nevertheless, unlike Kp-234, RF-9 prevented the effects of Kp-10 on DA and DOPAC/DA ratio in the ME. Furthermore, the treatment with the RF-amide related peptide 3 (RFRP-3), agonist of NPFFR, mimicked the Kp-10 actions, increasing the PRL secretion, enhancing DA and decreasing the DOPAC/DA ratio in the ME. These results show that Kp and its receptor Kiss1r play a role in the genesis of the physiological PRL surge induced by estradiol. Moreover, Kp seems to regulate the PRL secretion in two different ways: suppressing the activity of TIDA neurons through activation of NPFFR and in a DA-independent manner through Kiss1r.