Kiss1 Neurons Drastically Change Their Firing Activity in Accordance With the Reproductive State: Insights From a Seasonal Breeder

Kisspeptin (Kiss) neurons show drastic changes in kisspeptin expression in response to the serum sex steroid concentration in various vertebrate species. Thus, according to the reproductive states, kisspeptin neurons are suggested to modulate various neuronal activities, including the regulation of GnRH neurons in mammals. However, despite their reproductive state-dependent regulation, there is no physiological analysis of kisspeptin neurons in seasonal breeders. Here we generated the first kiss1-enhanced green fluorescent protein transgenic line of a seasonal breeder, medaka, for histological and electrophysiological analyses using a whole-brain in vitro preparation in which most synaptic connections are intact. We found histologically that Kiss1 neurons in the nucleus ventralis tuberis (NVT) projected to the preoptic area, hypothalamus, pituitary, and ventral telencephalon. Therefore, NVT Kiss1 neurons may regulate various homeostatic functions and innate behaviors. Electrophysiological analyses revealed that they show various firing patterns, including bursting. Furthermore, we found that their firings are regulated by the resting membrane potential. However, bursting was not induced from the other firing patterns with a current injection, suggesting that it requires some chronic modulations of intrinsic properties such as channel expression. Finally, we found that NVT Kiss1 neurons drastically change their neuronal activities according to the reproductive state and the estradiol levels. Taken together with the previous reports, we here conclude that the breeding condition drastically alters the Kiss1 neuron activities in both gene expression and firing activities, the latter of which is strongly related to Kiss1 release, and the Kiss1 peptides regulate the activities of various neural circuits through their axonal projections.