Hypothalamic Kiss1 but Not Kiss2 Neurons Are Involved in Estrogen Feedback in Medaka (Oryzias latipes)

Kiss2, a paralogous gene for kiss1, has recently been identified in several vertebrates. However, their relative potencies for the regulation of reproductive functions appear to differ among species. Here we used medaka as a model animal to examine the kiss1 and kiss2 expression dynamics by in situ hybridization under different conditions: breeding or nonbreeding and ovariectomized or sham operated. Medaka kiss1-expressing neurons and kiss2-expressing neurons were mainly localized in two hypothalamic nuclei, nucleus ventralis tuberis (NVT) and nucleus recessus lateralis (NRL), respectively. NRL kiss2 expression did not change according to differences in breeding condition, whereas NVT kiss1 expression was strongly correlated with breeding condition. In addition, ovariectomy did not change kiss2 expression but significantly decreased the kiss1 expression. Moreover, double-label in situ hybridization revealed that NVT Kiss1 neurons coexpress estrogen receptor-α, whereas NRL Kiss2 neurons do not. From these results, we conclude that the NVT Kiss1 neurons are positively regulated by ovarian estrogen via their coexpressed estrogen receptor-α and are directly involved in the central regulation of reproduction in medaka. In contrast, we argue that the NRL Kiss2 neurons in medaka may serve nonreproductive functions. These functional differences between Kiss1 and Kiss2 neurons are discussed from a phylogenetic viewpoint. Kiss1- and kiss2-expressing neurons are distributed in discrete hypothalamic nuclei, and only the former regulate reproduction in medaka.