GABA HAS EXCITATORY ACTIONS ON GNRH-SECRETING IMMORTALIZED HYPOTHALAMIC (GT1-7) NEURONS

The effects of gamma-aminobutyric acid (GABA) on clonal gonadotropin-r eleasing hormone (GnRH)-secreting hypothalamic (GT1-7) neurons were in vestigated using patch-clamp and fura-2 imaging techniques. Local appl ication of GABA (100 mu M) to GT1-7 cells voltage-clamped in the whole -cell configuration immediately increased membrane conductance and noi se consistent with activation of the GABA(A) receptor-Cl- channel comp lex. Depolarization activated transient Na+ currents which were abolis hed by tetrodotoxin (TTX; 0.5 mu M), and more sustained Ca2+ currents. Under constant current conditions, GT1-7 cells fired spontaneous acti on potentials, and depending on the Cl- equilibrium potential, GABA ei ther depolarized cells, causing a rapid activation of action potential s, or hyperpolarized cells. In order to determine the effect of GABA ( 100 mu M), but not the GABA(B) receptor agonist baclofen (10 mu M), im mediately evoked multiple action potentials. Measurement of [Ca2+](i) using fluorescence video microscopy and fura-2 revealed spontaneous, t ransient, repetitive increases in [Ca2+](i) which had a periodicity ra nging from 1 to 60 s. These Ca2+ oscillations were abolished by TTX (1 mu M) and by the removal of extracellular Ca2+. Application of GABA ( 1 and 10 mu M) induced an immediate increase in [Ca2+](i) in all cells and increased the frequency of Ca2+ oscillations in a dose-dependent manner. The GABA-induced increase in [Ca2+](i) was abolished by bicucu lline and by the removal of extracellular Ca2+, and was inhibited by T TX. Baclofen (1 mu M) had no effect on [Ca2+](i). These results sugges t that activation of GABA(A) receptors has an excitatory action on GnR H-secreting immortalized hypothalamic neurons caused by a Cl--dependen t depolarization. GABA has been reported to increase GnRH secretion; a direct stimulatory action of the neurotransmitter on GABA(A) receptor s of GnRH-secreting hypothalamic neurons may be responsible for this e ffect.