Effects of repeated injection of kisspeptin-10 on the initiation of egg-laying in juvenile quail
The kisspeptin (Kp) signaling pathway plays an essential role in the onset of reproduction in mammals. To investigate the effects of Kp on the initiation of egg laying in birds, juvenile female quail were given daily intraperitoneal injections of 300 µl saline (control, Con), or 10 nmol (low dosage, L) or 100 nmol (high dosage, H) kisspeptin-10 (Kp-10) dissolved in 300 µl saline for 3 weeks. The ratio of egg laying of quail in the L and H groups was notably increased compared to that of the Con group (P < 0.01), which paralleled earlier ovarian growth and increases in circulating estrogen (E 2 ) concentrations. In the hypothalamus, gonadotropin-releasing hormone-I (GnRH-I) mRNA expression was markedly up-regulated, whereas the level of gonadotropin-inhibiting hormone mRNA was down-regulated by high-dose Kp-10 (P < 0.05). In the pituitary gland, expression of GnRH receptor type II, but not type I mRNA was significantly up-regulated by high-dose Kp-10 administration (P < 0.05). Moreover, compared with the Con group, follicle-stimulating hormone (FSH) gene expression in the pituitary was significantly decreased in the L and H groups (P < 0.05), whereas luteinizing hormone (LH) mRNA expression was significantly increased in the H, but not the L group (P < 0.05). These results indicate that repeated peripheral Kp-10 injections can advance the sexual maturation of female quail by regulating the activities of the hypothalamus-pituitary-gonadal axis.