Presynaptic cholinergic inhibition in hippocampal cultures.

Activity of hippocampal neurons was recorded in a dissociated culture under patch-clamp conditions. Excitatory and inhibitory postsynaptic currents (PSCs) were evoked in response to short pulse application of glutamate to other neurons in the culture dish. These PSCs were suppressed by topical application of acetylcholine (ACh) near the recorded neuron. The dose-dependent effect of the muscarinic antagonist pirenzepine indicates that the effect of ACh is mediated by an M2 receptor. ACh did not affect inward current responses to direct application of glutamate onto postsynaptic neurons. This indicates that ACh may interfere with the release process and not with the postsynaptic response to the neurotransmitter. In some cells, ACh reduced inward Ca currents recorded in the presence of Na and K channel blockers. This effect was atropine sensitive and may underly the reduced PSCs. It is suggested that ACh modulates release of neurotransmitters by reducing presynaptic Ica and thereby reducing evoked PSCs.