Whole Brain-Pituitary In Vitro Preparation of the Transgenic Medaka (Oryzias latipes) as a Tool for Analyzing the Differential Regulatory Mechanisms of LH and FSH Release

Two types of gonadotropins, luteinizing hormone (LH) and follicle stimulating hormone (FSH), are important pituitary hormones for sexual maturation and reproduction, and both of them are centrally regulated by gonadotropin-releasing hormone (GnRH) from the hypothalamus. In mammals, these two gonadotropins are secreted from a single type of gonadotrope. The mechanisms of differential regulation by GnRH of the release of two types of gonadotropins with different secretory profiles are still unknown. In teleosts, however, LH and FSH are secreted from separate cellular populations, unlike in mammals. This feature makes them useful for studying the regulatory mechanisms of LH and FSH secretions independently. Here, we generated transgenic medaka lines that express Ca(2+) indicator protein, inverse-pericam, specifically in the LH or FSH cells. We performed cell-type-specific Ca(2+) imaging of LH and FSH cells, respectively, using the whole brain-pituitary preparations of these transgenic fish in which all neural circuits and GnRH neuronal projection to the pituitary are kept intact. LH and FSH cells showed different Ca(2+) responses to GnRH. The results suggest differential regulation mechanisms for LH and FSH release by GnRH. Moreover, we also succeeded in detecting the effect on LH cells of endogenous GnRH peptide, which was released by electrical stimulation of the axons of GnRH1 neurons. Thus, our newly developed experimental model system using the whole brain-pituitary in vitro preparation of the transgenic medaka is a powerful tool for analyzing the differential regulatory mechanisms of the release of LH and FSH by multisynaptic neural inputs to the pituitary.