Evidence for presence and function of kisspeptin/GnRH system in Leydig cells of horses (Equus caballus)

The aim of the present study was to evaluate in the horse Leydig cells (i) the immune-presence of kisspeptin (KiSS) and gonadotropinreleasing hormone (GnRH) and of their cognate receptors (GPR54 and GnRHR, respectively) and (ii) the possible direct interaction between KiSS and GnRH on the cellular endocrine activity in vitro. Immunohistochemistry displayed the presence of KiSS, GPR54, GnRH, and GnRHR in the cytoplasm of Leydig cells. In vitro experiments with isolated Leydig cells showed that KiSS analogue (kisspeptin-10) and GnRH analogue (buserelin) increased testosterone and PGF2a and decreased PGE2 secretions. GnRH antagonist (antide) counteracted the KiSS effects, whereas KiSS antagonist (kisspeptin- 234) did not affect the GnRH effects on Leydig cells. The present study indicates, for the first time, that KiSS/GPR54 and GnRH/GnRHR systems are present in horse Leydig cells and that both peptides affect the endocrine activity. In particular, the in vitro data evidenced that the KiSS effects are mediated by GnRH. In conclusion, this study led us to hypothesize that the hypothalamic interaction between KiSS and GnRH occurs also in the horse testis and that the KiSS/GnRH system directly modulates the Leydig cell endocrine activity.