ABSTRACT
The KISS1/Kiss1/kiss1 gene product kisspeptin is suggested to be involved in the steroid feedback system in vertebrates. In addition to kiss1, kiss2 has been identified in many vertebrates, including some mammals, suggesting that the both genes were originally expressed in the common ancestor of teleosts and tetrapods. Moreover, peptides from both genes have been shown to activate the kisspeptin receptors. To investigate the involvement of kiss1 or kiss2 neurones in steroid feedback, we used a seasonal breeder, the goldfish (Carassius auratus). We found that kiss2 is expressed in the preoptic area (POA), nucleus lateralis tuberis and nucleus recessus lateralis, and that kiss1 is expressed in the habenula. Greater mRNA expression in breeding than in nonbreeding condition animals and conspicuous up-regulation of gene expression by gonadal steroids was seen only in the kiss2 neurones of the POA. Furthermore, double in situ hybridisation suggested that these neurones express oestrogen receptors. Given that amphibians express kiss2 in POA and mammalian anteroventral periventricular nucleus/POA Kiss1 neurones show similar expression dynamics as goldfish POA Kiss2 neurones, we hypothesise that kiss1 and kiss2 share the same evolutionary origin; and, after the loss of kiss2, kiss1 became active for steroid feedback in mammals.