STUDIES OF THE ROLE OF GPR54-KISSPEPTIN SIGNALING IN ENDOCRINE FUNCTION OF PRIMATE TESTES

Abstract

Reproductive functions are tightly regul ated by the hormones of hypothalamus and anterior pituitary; together with gonadal hormones they form thc so called hypothalamic pituitary gonadal axis. Recently, kisspeptin peptide along with its seven transmembrane G protein coupled receptor, GPR54, was identified in mammals as a central gatekeeper of the reproductive cascade. However all recent work in rodents and primates has focused on central effects of kisspeptin administration. Demonstration of presence of GPR54 receptor in testes raises the possibility of direct action of kisspeptin on the distal component of the reproductive axis. Therefore, in the present study we analyzed direct testicular action of kisspeptin in the adult intact male rhesus monkey, a representative higher primate. The paradigm we used to examine the hypothesis was that of pituitary gonadotropin-clamped monkey model with pretreatment with acyline, a GnRH receptor antagonist. Since effect of kisspeptin administration on testosterone levels in adult male monkey has not been reported, a corollary obj ective of the study was to characteri ze changes in testosterone levels fo llowing peri pheral kisspeptin administration. Four adult intact male rhesus monkeys (NIacaca mulatta), maintained under standard colony conditions of feeding and management, were used in this study. The animals were habituated to chair restraint prior to experiments in order to study them without sedation. Animals were implanted with iv cannula to gain continuous access to venous circulation for drug administration and blood sampling. Animals were assigned to receive iv saline (0.9 % NaCl), kisspeptin-lO 1 (50 ug) and kisspeptin-l0 (50 ug) with acyline pretreatment (60 ug/kg and 120 ug/kg BW, sc, morning and evening, respectively). Endocrine effects of kisspeptin on the testes were examined by monitoring plasma testosterone levels. In addition effect of kisspeptin administration on plasma glucose and cortisol levels was also studied because of presence of GPR54 receptor on pancreas and adrenal gland. Testosterone and cortisol concentration were measured by specific radioimmunoassays. Plasma glucose levels were measured by using blood glucose strip test in sensocard blood monitoring system. The peripheral administration of kisspeptin but not vehicle caused a robust increase in plasma testosterone levels 30 minutes post inj ection that lasted for the next 180 minutes. However, this dramatic increase of testosterone was abolished when kisspeptin was administered to acyline pretreated animals. Plasma cortisol levels of kisspeptin treated animals were moderately low as compared to the vehicle treated animals. Plasma glucose levels were not affected by kisspeptin administration. These studies suggest that peripheral kisspeptin administration induces a robust acute stimu lation of testosterone secretion in adult intact male monkeys. However, such an effect is not produced directly at the testicular Leydig cell level. Rather our results demonstrate that the primate hypothalamic-pituitarytesticular axis is strongly stimulated by kisspeptin, through an action of the peptide at a site afferent to GnRH neurons. A role of kisspeptin on other testicular functions like spermatogenesis, however, still cannot be excluded. Also present study provides a rationale to f'urther asses the involvement of kisspeptin-GPR54 signal i ng in affecting primate hypothalamic response to stress.