mRNA levels of kisspeptins, kisspeptin receptors, and GnRH1 in the brain of chub mackerel during puberty

Kisspeptin (Kiss) and its cognate receptor (Kiss1R), implicated in the neuroendocrine control of GnRH secretion in mammals, have been proposed to be the key factors in regulating puberty. However, the mechanisms underlying the initiation of puberty in fish are poorly understood. The chub mackerel Scomber japonicus expresses two forms of Kiss (kiss1 and kiss2) and two Kiss receptor (kissr1 and kissr2) genes in the brain, which exhibit sexually dimorphic changes during the seasonal reproductive cycle. This indicates that the kisspeptin system plays an important role in gonadal recrudescence of chub mackerel; however, the involvement of the kisspeptin system in the pubertal process has not been identified. In the present study, we examined the mRNA expression of kiss1, kiss2, kissr1, kissr2, and gnrh1 (hypophysiotropic form) in the brain of a chub mackerel during puberty. In male fish, kiss2, kissr1 and kissr2 levels increased significantly at 14 weeks post-hatch (wph), synchronously with an increase in type A spermatogonial populations in the testis; kiss2 and gnrh1 levels significantly increased at 22 wph, just before the onset of meiosis in the testes. In female fish, kiss2 increased significantly at 14 wph, synchronously with an increase in the number of perinucleolar oocytes in the ovary; kiss1 and kiss2 levels significantly increased concomitantly with an increase in the kissr1, kissr2, and gnrh1 levels at 24 wph, just before the onset of vitellogenesis in oocytes. The present results suggest positive involvement of the kisspeptin–GnRH system in the pubertal process in the captive reared chub mackerel.