Effect of galanin-like peptide on hypothalamic kisspeptin expression in female Zucker fatty rats

Kisspeptin and galanin-like peptide (GALP) neurons in the hypothalamic arcuate nucleus (ARC) are involved in gonadotropin-releasing hormone (GnRH) neuron-mediated pulsatile luteinizing hormone (LH) secretion. Zucker fatty (ZF) rats display a leptin receptor gene abnormality and suppressed pulsatile LH secretion. ZF rats reportedly exhibit low hypothalamic GALP and kisspeptin expression, and GALP administration induces LH release in ZF rats. Therefore, we performed a histochemical analysis to determine whether GALP-induced LH release is mediated by kisspeptin neurons in ZF rats. All ZF rats were ovariectomized and subcutaneously implanted with an estradiol tube before the central injection of GALP or vehicle. GALP administration increased the plasma LH concentration. However, no significant difference was observed in the number of Kiss1 cells and the proportion of Fos-positive Kiss1 cells. The number of c-Fos-positive GnRH neurons significantly increased after GALP administration. Our results suggest that hypothalamic GALP neurons promote LH release by activating GnRH neurons without the activation of kisspeptin neurons in the ARC.