Modulation of pacemaker activity by salmon gonadotropin-releasing hormone (sGnRH) in terminal nerve (TN)-GnRH neurons

The terminal nerve (TN)-gonadotropin-releasing hormone (GnRH) neurons project widely in the brain instead of the pituitary and show endogenous pacemaker activity that is dependent on the physiological conditions of the animal. We suggest that the TN-GnRH system may act as a putative neuromodulator that is involved in the regulation of many long-lasting changes in the animal's behavior. In the present study, we find that the pacemaker activity of TN-GnRH neurons is modulated by salmon GnRH (sGnRH), which is the same molecular species of GnRH peptide produced by TN-GnRH neurons themselves. Bath application of sGnRH (2-200 nM) transiently decreased (early phase) and then subsequently increased (late phase) the frequency of pacemaker activity of TN-GnRH neurons in a dose-dependent manner. These biphasic changes of pacemaker activities were suppressed by intracellular application of guanosin 5'-0-(2-thiodi-phosphate) (GDP-P-S). The results suggest that G-protein coupled receptors are present on the cell surface and play a triggering role in modulating the frequency of pacemaker activities in TN-GnRH neurons. Because the TN-GnRH neurons make tight cell clusters with no intervening glial cells, it may be further suggested that GnRH released from GnRH neurons regulates the activities of their own (autocrine) and/or neighboring GnRH neurons (paracrine).