Characterization and distribution of kisspeptins, kisspeptin receptors, GnIH, and GnRH1 in the brain of the protogynous bluehead wrasse (Thalassoma bifasciatum)

The kisspeptin and gonadotropin-inhibitory hormone (GnIH) systems regulate the hypothalamic-pituitary-gonadal (HPG) axis in a broad range of vertebrates through direct or indirect effects on hypothalamic/preoptic gonadotropin-releasing hormone (GnRH) neurons and pituitary gonadotropes. These systems are sensitive to environmental factors, including social conditions, and may assist in relaying environmental signals to the HPG axis in a potentially broad range of taxa. In this study, we characterized expression of kisspeptin-system genes (kiss1, kiss2, kissr1, and kissr2), gnih, and gnrh1 in the brain of the bluehead wrasse (Thalassoma bifasciatum), an important teleost model of socially-controlled sex change. We analyzed cDNA sequences and examined transcript distributions in the brain using in situ hybridization (ISH) to determine if expression occurs in reproductively-relevant and conserved regions. Expression of kiss1 was detected in the habenula, lateral hypothalamic nucleus (LHn), and preoptic area (POA), while kiss2 was expressed in the dorsal hypothalamus, with sporadic signal in the POA. Expression of kissr1 was detected in the POA, habenula, and LHn, while kissr2 expression was widespread. Gnih mRNA was detected in the posterior periventricular nucleus (NPPv), and gnrh1 neurons localized to the POA. Neurons expressing kissr2 and gnih co-regionalized in the NPPv, while kissr1, kissr2, and gnrh1 co-regionalized in the POA. Double-label ISH revealed very close proximity between kissr1 and gnrh1 neurons, suggesting potential communication between the kisspeptin and GnRH1 systems through these interneurons. These expression patterns are generally conserved and suggest that if kisspeptins do signal GnRH1 neurons, the interaction is indirect, possibly through neurons adjacent to GnRH1. With this foundation in place, future studies can help determine the interactions among these systems and whether these peptides assist in transducing social changes into a shift from female to male sexual function.