Three types of gonadotrophin-releasing hormone neurones and steroid-sensitive sexually dimorphic kisspeptin neurones in teleosts.

In general, the gonadotrophin-releasing hormone (GnRH) neuronal systems of vertebrates consist of one group of hypothalamic neuroendocrine and one or two group(s) of extrahypothalamic neuromodulatory GnRH neurones. By taking advantage of the brains of dwarf gourami and GnRH-green fluorescent protein transgenic medaka, the spontaneous electrical activities of all three different types of GnRH neurones have now been characterised. The hypophysiotrophic preoptic area-gnrh1 neurones show irregular and episodic spontaneous electrical activities, whereas extrahypothalamic midbrain gnrh2 and terminal nerve-gnrh3 neurones show regular pacemaker potentials. It is suggested that these spontaneous electrical activities are related to their different functions as neuroendocrine hormones (gnrh1 neurones) and neuromodulators (gnrh2 and gnrh3 neurones). On the other hand, recent evidence strongly suggests that the GnRH neurones are regulated by another class of peptidergic neurones, the kisspeptin neurones. The gene encoding kisspeptin (kiss1 gene) has been cloned, and the anatomical distribution of kiss1 mRNA expressing neurones (kiss1 neurones) has recently been studied in brains of several fish species. In medaka, two kiss1 neuronal populations in hypothalamic areas, called the nucleus ventral tuberis (NVT) and nucleus posterioris periventricularis (NPPv), have been identified. The NVT kiss1 neurones are sexually dimorphic in number (male >> female) under breeding conditions and are sensitive to ovarian oestrogens, whereas the NPPv kiss1 neurones are neither sexually dimorphic, nor sensitive to steroids. The steroid-sensitive changes in kiss1 mRNA expression in the NVT occur physiologically, closely linked to the reproductive state. As in the mammalian counterpart, the medaka kiss1 neuronal system is suggested to be involved in the central regulation of reproductive functions. Interestingly, kiss2, another gene paralogous to kiss1, has been cloned in some fish species. The possible regulation of reproduction and other unknown functions by kiss1 and kiss2 neurones may be the focus of future studies.