Hyperpolarization following activation of K(+) channels by excitatory postsynaptic potentials

We have postulated that an excitatory postsynaptic potential (e.p.s.p.) may open voltage-sensitive K(+) (‘M’) channels(1), 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.ps(3,4).) We show here that this is so.