Chronic central infusion of apelin decreased LH levels but did not affect gene expression in KNDy neurons
Résumé
Central control of reproduction relies on the hypothalamic integration of several exogenous and endogenous stimuli allowing LH and FSH release from pituitary cells controlled by GnRH secretion. This latter is driven by modulatory neurons that relay steroid feedback and metabolic status. Among them, kisspeptin (KP) neurons both sensitive to metabolic status and major players in eliciting GnRH release are found in the arcuate nucleus (ARC) as well as apelin neurons. This latter hormone is known to exert a prominent role in metabolic regulation, and was reported to decrease LH secretion. In the present study, we sought to check for an influence of apelin on KP neurons both anatomically and functionally through several experiments. Regarding KP neurons, we observed using immunocytochemistry neuroanatomical contacts between apelin fibers and KP neurons of the ARC of male rats. In a functional experiment, we treated male mice for 2 weeks with intracerebroventricular infusion of apelin and found a decrease of LH levels in treated mice but no effect on gene expression levels of KP, neurokinin and dynorphin in microdissected ARC between treated and control animals. Similarly, no effect was noted on testicular weight, glycemia and body weights. These results suggest that apelin effect on LH secretion do not pass through a modulation of genes expressed in KP neurons in male rodents but we cannot rule out a modulation of the secretory activity of these neurons. Further studies are underway to assess whether downstream populations are also affected by apelin.