Action of α‐dendrotoxin on K + currents in nerve terminal regions of axons in rat olfactory cortex

1 In the rat olfactory cortex, unmyelinated axons give rise to synapses en passant . This tissue was used to study the pharmacology of axonal K + ‐currents. Responses were measured from a group of these axons as unclamped field currents, with a polarizable suction electrode. 2 A single stimulus to the axons elicited a tetrodotoxin‐sensitive Na + ‐dependent transient. K + ‐currents were revealed by positive polarization of the suction electrode and were manifest as a negative current following the Na + ‐component. 3 In the presence of tetraethylammonium (TEA, 5 m m ) and Cd 2+ (100 μ m ), the K + ‐component was depressed by 3,4‐diaminopyridine (3,4‐DAP; 1 to 20 μ m ; IC 50 2.0 ± 0.4 μ m ). α‐Dendrotoxin (DTX; 15–1500 n m ) also attenuated the aminopyridine‐sensitive component (IC 50 93 ± 4 n m ). At the highest DTX concentration, depression of the K + ‐current was incomplete, the residual K + current being reduced by 3,4‐DAP (0.1 to 5 μ m ). 4 These results indicate the presence of two aminopyridine‐sensitive K + currents in this preparation distinguished by their susceptibility to DTX.