[The depressing effect of GABA on synaptic excitation in isolated rat hippocampal slices]

The action of various gamma-aminobutyric (GABA) concentrations on the amplitude of population EPSPs (PEPSPs) recorded from dendrites was examined in hippocampal slices. GABA produced reversible dose-dependent diminishing of the amplitude of PEPSPs without features of desensitization. CA1 hippocampal PEPSPs were much more sensitive to GABA (threshold concentrations--3.10(-5)-2.10(-4)mol/l; IC50--5.10(-4)-1.10(-3) mol/l) the action of GABA on PEPSPs was not depressed by bicuculline, picrotoxin or penicillin. The inhibitory action of GABA on dendritic antidromic population spike (DAPS) (post-synaptic inhibitory effect) was greatly reduced by these drugs. Baclofen was more potent inhibitor of CA1 PEPSPs than GABA (threshold concentration--1.10(-6)mol/l; IC50--3.10(-6)mol/l), but even in high concentration it induced only slight reduction of DAPS amplitude. It is concluded that GABA inhibitory effects on CA1 hippocampal PEPSPs are caused in the main by its presynaptic action mediated by GABA(B)-receptors located probably on axon terminals of the Schaffer collaterals.