Exogenous application of acetylcholine elicits inward currents in hippocampal interneurons that are mediated via a-bungarotoxin-sensitive nicotinic acetylcholine receptors, but synaptic responses mediated via such receptors have never been reported in mammalian brain. In the present study, EPSCs were evoked in hippocampal interneurons in rat brain slices by electrical stimulation and were recorded by using whole-cell voltage-clamp techniques. Nicotinic EPSCs were isolated pharmacologically, using antagonists to block other known types of ligand-gated ion channels, and then were tested with either a-bungarotoxin or methyllycaconitine, which are selective antagonists for nicotinic acetylcholine receptors that contain the a7 receptor subunit. Each antagonist proved highly effective at reducing the remaining synaptic current. Evoked a7-mediated nicotinic EPSCs also were desensitized by superfusion with 1 mM nicotine, had extrapolated reversal potentials near 0 mV, and showed strong inward rectification at positive potentials. In several interneurons, methyllycaconitine-sensitive spontaneous EPSCs also were observed that exhibited a biphasic decay rate very similar to that of the a7-mediated evoked response. These studies provide the first demonstration of a functional cholinergic synapse in the mammalian brain, in which the primary postsynaptic receptors are a-bungarotoxin-sensitive nicotinic acetylcholine receptors.
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