GABA induces GABAergic MSCs in cultured embryonic rat thalamic neurons

Application of 0.1-10 microM GABA in the vicinity of cultured embryonic rat thalamic neurons recorded with patch pipettes in the presence of 2 microM TTX induced or increased the frequency of miniature synaptic currents (MSCs) that reversed polarity at the Cl- equilibrium potential. These MSCs were blocked by the GABAA receptor antagonist bicuculline and exhibited exponential decay kinetics that closely paralleled those estimated from fluctuation analysis of Cl- channels activated pharmacologically by applying 1-10 microM GABA to the same cells. We conclude that the MSCs are mediated by GABA. Application of the GABAA receptor agonist muscimol activated Cl- current but failed to induce GABAergic MSCs while submicromolar concentrations of GABA evoked GABAergic MSCs but did not activate Cl- channels. The GABAB receptor agonist (-)baclofen did not mimic GABA in inducing MSCs. Induction of GABAergic MSCs by GABA required extracellular Ca2+. Verapamil and Co2+, which block voltage-dependent calcium channels, completely blocked GABA-induced MSCs independent of their effects on the direct activation of a Cl- current response. The results indicate that GABA can trigger GABAergic Cl(-)-dependent MSCs in a Cao(2+)-dependent manner. The mechanism may involve a novel receptor and/or signal transduction pathway.