Sex-specific changes in the expression of kisspeptin, kisspeptin receptor, gonadotropins and gonadotropin receptors in the Senegalese sole (Solea senegalensis) during a full reproductive cycle

Kisspeptin is thought to have a major role in the control of the onset of puberty in vertebrates. However, our current understanding of its function in fish and how it integrates with other hormones is incomplete due to the high diversity of this group of animals and a still limited amount of available data. This study examined the temporal and spatial changes in expression of kisspeptin, gonadotropins and their respective receptors in the Senegalese sole during a full reproductive cycle. Kiss2 and kiss2r expression was determined by qRT-PCR in the forebrain and midbrain while expression of fshβ and lhβ was determined in the pituitary and fshr and lhr in the gonads. Plasma levels of testosterone (T), 11-ketotestosterone (11-KT) and estradiol-17β were measured by ELISA and gonadal maturation was assessed histologically. In males, kiss2 and kiss2r expression in the brain areas examined was highest towards the end of winter, just before the spawning season, which took place the following spring. This coincided with maximum levels of pituitary fshβ and lhβ, plasma T and 11-KT and the highest number of maturing fish. However, these associations were not evident in females, since the highest expression of kiss2, kiss2r and gonadotropins were observed in the fall, winter or spring, depending upon the variable and tissue considered. Taken together, these data show not only temporal and spatial, but also sex-specific differences in the expression of kisspeptin and its receptor. Thus, while expression of kiss2 in Senegalese sole males agrees with what one would expect according to its proposed role as a major regulator of the onset of reproduction, in females the situation was not so clear, since kiss2 and kiss2r expression was highest either before or during the spawning season.