Hyperpolarization following activation of K+ channels by excitatory postsynaptic potentials

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Abstract
We have postulated that an excitatory postsynaptic potential (e.p.s.p.) may open voltage-sensitive K+ (‘M’) channels1, in an appropriate depolarizing range, and that this could alter the e.p.s.p. waveform. Consequently, the fast e.p.s.p. in neurones of sympathetic ganglia, elicited by a nicotinic action of acetylcholine (ACh)2, could be followed by a hyperpolarization, produced by the opening of M channels during the depolarizing e.p.s.p. and their subsequent slow closure (time constant∼150 ms)1. This introduces the concept that transmitter-induced p.s.ps may trigger voltage-sensitive conductances other than those initiating action potentials, and that in the present case this could produce a true post-e.p.s.p. hyperpolarization. (Some hyperpolarizations other than inhibitory postsynaptic potentials (i.p.s.ps) have been reported to follow e.p.s.ps3,4.) We show here that this is so.