INTRACELLULAR CALCIUM STORES MODULATE MINIATURE GABA-MEDIATED SYNAPTIC CURRENTS IN NEONATAL RAT HIPPOCAMPAL-NEURONS

The whole-cell configuration of the patch clamp technique was used to record miniature gamma-aminobutyric acid(A) (GABA(A)) receptor-mediate d currents (in tetrodotoxin, 1 mu M and kynurenic acid 1 mM) from CA3 pyramidal cells in thin hippocampal slices obtained from postnatal (P) day (P6-9) old rats. Switching from a Ca2+-containing to a nominally Ca2+-free medium (in which Ca2+ was substituted with Mg2+, in the pres ence or in the absence of 100 mu M EGTA) did not change significantly the frequency or amplitude of miniature events. Superfusion of thapsig argin induced a concentration-dependent increase in frequency but not in amplitude of tetrodotoxin-resistant currents that lasted for the en tire period of drug application. Mean frequency ratio (thapsigargin 10 mu M over control) was 1.8 +/- 0.5, (n = 9). In nominally Ca2+-free s olutions thapsigargin was ineffective. When bath applied, caffeine (10 mM), reversibly reduced the amplitude of miniature postsynaptic curre nts whereas, if applied by brief pressure pulses, it produced an incre ase in frequency but not in amplitude of spontaneous GABAergic current s. Superfusion of caffeine (10 mM) reversibly reduced the amplitude of the current induced by GABA (100 mu M) indicating a clear postsynapti c effect on GABA(A) receptor. Superfusion of ryanodine (30 mu M), in t he majority of the cells (n = 7) did not significantly modify the ampl itude or frequency of miniature events. In two of nine cells it induce d a transient increase in frequency of miniature postsynaptic currents . These results indicate that in neonatal hippocampal neurons, mobiliz ation of calcium from caffeine-ryanodine-sensitive stores facilitates GABA release.