Papel dos neurônios KNDY na regulação estrogênica da secreção de hormônio luteinizante e prolactina em ratas

Abstract

Hyperprolactinemia, a highly prevalent dysfunction of the hypothalamus-pituitary axis, is frequently associated with reproductive disorders in women. An important stimulator of the hypothalamus-pituitary-gonadal axis is kisspeptin, a key neuropeptide for fertility. Hypothalamic arcuate nucleus (ARC) neurons that coexpress kisspeptin (Kp), neurokinin B and dynorphin A (KNDy) play an important role in the control of luteinizing hormone (LH) and prolactin (PRL) secretions, but the neuroendocrine mechanisms involved are to be elucidated. The current study aimed to determine the effects of selective neurochemical ablation of KNDy neurons on the estrogenic regulation of LH and PRL secretion in female rats. For this purpose, neurokinin B receptor 3 agonist conjugated with saporin (NK3-SAP) was used to cause selective lesion of KNDy neurons. Regular cycling rats received intra-ARC stereotaxic injections of NK3-SAP (NK3-SAP; n = 5) or vehicle (Veh; n = 6). Estrous cyclicity was monitored over 21 days before and after injections followed by ovariectomy. Ovaries were collected and processed for histological analysis. Rats were treated with estradiol (E2) cypionate (OVX+E2 model). E2 treatment lasted three days and on the fourth day, blood samples were withdrawn from the tail tip at each 30 min from 13:00 h to 19:00 h, for whole blood LH and PRL measurement. After 6 days, animals received corn oil treatment for 3 days and on the fourth day blood samples were taken from the tail again following the same protocol described above (OVX model). Shortly after the last sampling, animals were anaesthetized and perfused. Brains were processed for immunohistochemical staining of Kp, tyrosine hydroxylase (TH) and phosphorylated TH (pTH). NK3-SAP animals displayed an average loss of 66% in the number of Kp-immunoreactive (ir) neurons in the ARC. This lesion induced a decrease in body weight gain, irregularity of estrous cycles, and ovarian analysis revealed an increase in 50% of follicular atresia compared to Veh group. This effect was accompanied by a decrease in the number of small healthy follicles. However, ovulation was not affected, as revealed by the unchanged number of corpora lutea. In the OVX+E2 model, E2-induced LH surge was amplified and advanced by 1 hour compared to Veh. In turn, the PRL surge was suppressed in NK3-SAP OVX+E2 rats. In the OVX model, basal LH secretion was unaffected by the NK3-SAP lesion, and PRL levels were undetectably low in both groups. NK3-SAP and Veh rats displayed a similar number of TH-ir neurons in the ARC but the immunoreactivity to pTH and pTH/TH ratio were increased in the 8 median eminence of NK3-SAP rats. Thus, selective loss of KNDy neurons differently affects the E2 regulation of LH and PRL secretion in female rats. Our results support an inhibitory role of KNDy neurons in the generation of the LH surge. The substantial, but not absolute, loss of KNDy neurons seems to facilitate the LH surge and ovulation even in a condition of irregular estrous cyclicity. The reduction in PRL surge, associated with the increased activity of tuberoinfundibular dopaminergic neurons, reveals that KNDy neurons inhibit neuroendocrine dopamine and this effect seems to be implicated in the E2-induced increase in PRL secretion. These findings suggest a new pathway for the neuroendocrine control of PRL secretion, in which KNDy peptides orchestrate changes in PRL release through dopaminergic neurons. Thus, KNDy neurons seem to exert multiple hypothalamic effects impacting the female reproductive function.