Release of GABA from the terminals of hippocampal inhibitory neurons is inhibited by activation of GABAB autoreceptors and μ opioid receptors. However, it is not known whether these presynaptic processes affect all inhibitory synapses equally. We examined the effects of the GABAB receptor agonist baclofen and the p opioid receptor agonist DAGO on postsynaptic currents evoked by minimal stimulation of inhibitory fibers (mePSCs) in area CA3. Baclofen reversibly depressed approximately half of the meIPSCs evoked in the stratum pyramidale. The remaining meIPSCs were unaffected despite a coincident depression of spontaneous IPSCs. In contrast, all meIPSCs were depressed by DAGO. In addition, minimal stimulation in the stratum radiatum evoked me1PSCs that were always depressed by baclofen. These results indicate that regulation of GABA release by GABAB autoreceptors occurs at a subset of inhibitory synapses and that GABAB-resistant inhibitory synapses are located on pyramidal neuron somata. Hippocampal inhibitory neurons may be heterogeneous with respect to presynaptic receptor-mediated regulation of GABA release.
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