GABAergic spill-over transmission onto hippocampal mossy fiber boutons

Presynaptic ionotropic GABA(A) receptors have been suggested to contribute to the regulation of cortical glutamatergic synaptic transmission. Here, we analyzed presynaptic GABAA receptor-mediated currents (34 degrees C) recorded from mossy fiber boutons (MFBs) in rat hippocampal slices. In MFBs from young and adult animals, GABA puff application activated currents that were blocked by GABA(A) receptor antagonists. The conductance density of 0.65 mS center dot cm(2) was comparable to that of other presynaptic terminals. The single-channel conductance was 36 pS (symmetrical chloride), yielding an estimated GABAA receptor density of 20-200 receptors per MFB. Presynaptic GABA(A) receptors likely contain alpha(2)-subunits as indicated by their zolpidem sensitivity. In accordance with the low apparent GABA affinity (EC50 = 60 mu M) of the receptors and a tight control of ambient GABA concentration by GABA transporters, no tonic background activation of presynaptic GABA(A) receptors was observed. Instead, extracellular high-frequency stimulation led to transient presynaptic currents, which were blocked by GABAA receptor antagonists but were enhanced by block of GAT1 (GABA transporter 1), indicating that these currents were generated by GABA spill-over and subsequent presynaptic GABAA receptor activation. Presynaptic spill-over currents were depressed by pharmacological cannabinoid 1 (CB1) receptor activation, suggesting that GABA was released predominantly by a CB1 receptor-expressing interneuron subpopulation. Because GABA(A) receptors in axons are considered to act depolarizing, high activity of CB1 receptor-expressing interneurons will exert substantial impact on presynaptic membrane potential, thus modulating action potential-evoked transmitter release at the mossy fiber-CA3 synapse.