GONADOTROPIN-RELEASING-HORMONE MODULATES GAMMA-AMINOBUTYRIC ACID-EVOKED INTRACELLULAR CALCIUM INCREASE IN IMMORTALIZED HYPOTHALAMIC GONADOTROPIN-RELEASING-HORMONE NEURONS

To examine the functional role of calcium signaling in the interactive modulation of gonadotropin releasing hormone (GnRH) neurons by gamma- aminobutyric acid (GABA) and GnRH itself, we analyzed the intracellula r calcium level ([Ca2+](i)), using fura-2AM fluorescent dye in immorta lized hypothalamic GTl-1 cells. GTl-1 cells showed spontaneous [Ca2+]( i) oscillations, which were dependent on extracellular Ca2+ level, L-t ype Ca2+ channel and SK-type K+ channel. When GABA or a specific GABA( A) type receptor agonist, muscimol was applied to the media, [Ca2+](i) rapidly increased through L-type Ca2+ channel in a dose-dependent man ner, and subsequently decreased below the basal level without any osci llation. However, a specific GABA(B) type receptor agonist, baclofen s howed no effect. On the other hand, application of GnRH or its potent agonist buserelin, rapidly abolished the spontaneous [Ca2+](i) oscilla tions. Interestingly, a prior treatment with buserelin abolished GABA- evoked increase in [Ca2+](i) in a noncompetitive manner. Since buserel in also blocked K+-evoked increase in [Ca2+](i), we suggest that GnRH may block spontaneous [Ca2+](i) oscillation through modulating the L-t ype [Ca2+](i) channel activity. These results show that GABAergic agen ts may exert both stimulatory and inhibitory controls over the GnRH ne uronal activity, and GnRH can block the stimulatory effect of GABA, im plicating the possible existence of an ultrashort feedback circuit.