Mechanisms of GABA- and Glycine-Induced Increases of Cytosolic Ca2+ Concentrations in Chick Embryo Ciliary Ganglion Cells

Abstract: We used fura-2 microfluorometry and the gramicidin-perforated patch clamp technique in an attempt to clarify the mechanisms underlying the GABA-and glycine-induced increases in the cytosolic Ca²⁺ concentration (Ca]_in) in acutely isolated chick embryo ciliary ganglion neurons. GABA, glycine, and isoguvacine, but not baclofen, increased Ca]_in in a dose- and a Ca²⁺-dependent manner. The GABA-induced Ca]_in increase was inhibited by bicuculline and picrotoxin, and potentiated by pentobarbital, flunitrazepam, and alphaxalone, whereas the glycine-induced Ca]_in increase was inhibited by strychnine but not by bicuculline or picrotoxin. L-and N-type Ca²⁺ channel blockers inhibited the GABA-and glycine-induced Ca]_in increases, whereas Bay K-8644 potentiated these responses. These responses were also substantially potentiated by blockers of various K⁺ channels and by lowering the external Cl^? concentrations. The high KCI- and nicotine-induced Ca]_in increases were substantially reduced during continuous stimulation with either 2 µ M GABA or 1 m M glycine. Electrophysiological studies indicated that the reversal potential of the GABA-induced current exhibited a more depolarized value than the resting membrane potential in 17 of the 25 cells examined. Taken together, these results suggest that both GABA and glycine depolarize the membrane potentials by increasing Cl^? conductance via respective receptors and thus increase the Ca²⁺ influxes through L- and N-type voltage-dependent Ca²⁺ channels.