4-Aminopyridine and dendrotoxin induce repetitive firing in rat visceral sensory neurones by blocking a slowly inactivating outward current.

In a subpopulation of rat visceral afferent neurones we have identified a potassium (K) current which is novel to mammalian neurones. It activates rapidly at potentials positive to - 70 mV but shows only slow and incomplete inactivation and is inhibited by 1-30 microM 4-aminopyridine (4-AP) or 3-10 nM dendrotoxin (DTX). Inhibition of this slowly inactivating current suppresses spike adaptation and leads to pronounced repetitive firing. In contrast, other visceral afferent neurones possessing the normal transient A-current were insensitive to 4-AP at concentrations below 100 microM. We suggest that inhibition of the slowly inactivating current may contribute to the convulsant actions of 4-AP and DTX.