Physiological and Biochemical Studies on Neuromodulatory Action of Terminal Nerve GnRH System

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The gonadotropin-releasing hormone (GnRH) was originally purified and identified as a hypophysiotrophic decapeptide hormone that facilitates gonadotropin release from the pituitary. We have demonstrated the existence of three anatomically as well as functionally different GnRH neuronal systems in several teleost species. We then used the brain of a tropical fish (Colisa lalia) because the terminal nerve (TN) GnRH neurons of this fish are large and make a tight cell cluster so that one can readily record the activities of a single GnRH neuron in a whole brain in vitro preparation using sharp microelectrodes or patch pipettes. Because the GnRH neuronal cell bodies in most vertebrate species are small and diffusely distributed, it has been extremely difficult to record from a single GnRH neurons. From the results obtained thus far, we propose a tentative model of modulator neurons, TN-GnRH cells, including some hypotheses. The TN-GnRH cells have spontaneous oscillatory activities which or … More iginate from the intrinsic ionic channel properties of their own membranes. A TTX-resistant persistent Na^+ channel, I_<Na(slow)>, and some kind of unidentified K^+ channel play important roles in the generation of pacemaker activities. Changes in the physiological conditions of the fish that are triggered by environmental, pheromonal, hormonal factors, etc., probably act on the modulator neurons via hormones and/or neurotransmitters. The properties of the ionic channels that underlie pacemaker activities of modulator neurons may be modified by some kind of signal transduction mechanisms. This will result in the ability of modulator neurons to alter their own pacemaker frequencies or to switch among different discharge modes. In other words, the animal's physiological conditions may be encoded in the frequency and/or pattern of discharge of modulator neurons. These modulator neurons, in turn, can regulate simultaneously the excitability of target neurons in a wide variety of brain regions via extensive, multiple axonal branches. Less