Activity-dependent depression of GABAergic IPSCs in cultured hippocampal neurons

Short term depression of monosynaptic GABAergic inhibitory postsynaptic currents (IPSCs) evoked between pairs of cultured rat hippocampal neurons was investigated using dual whole cell patch-clamp recordings. Paired stimuli applied to the GABAergic neuron resulted in paired-pulse depression (PPD) of the second IPSC (IPSC2) at interpulse intervals from 25 to 2,000 ms. CGP 55845A, but not CGP 35348, reduced PPD marginally. Brief paired-pulse applications of exogenous GABA indicated that postsynaptic factors made only minimal contribution to PPD of IPSCs. IPSC, and PPD was reduced on lowering [Ca2+](o) and enhanced on increasing [Ca2+](o). The potassium-channel blocker 4-aminopyridine (4-AP), which increases presynaptic Ca2+ influx, enhanced IPSCI and PPD. Chelation of residual Ca2+ in the GABAergic boutons with EGTA-AM enhanced PPD. Stimulation of the presynaptic neuron at frequencies (f) ranging from 2.5 to 80 Hz resulted in tetanic depression of IPSCs, which declined rapidly and reached a plateau depending on f and [Ca2+](o). CGP 55845A decreased tetanic depression in the first part of the train, but this could be overcome with continued stimulation. We show that GABAergic IPSCs are robustly depressed by paired-pulse stimulation in cultured hippocampal neurons. The depression of IPSCs is mainly independent of presynaptic GABA(B) receptors and could be caused by depletion of releasable vesicles. Depleted synapses recover with a slow time course, depending on factors that regulate [Ca2+](i) in the GABAergic boutons.