AXONAL ACTION-POTENTIAL INITIATION AND NA+ CHANNEL DENSITIES IN THE SOMA AND AXON INITIAL SEGMENT OF SUBICULAR PYRAMIDAL NEURONS

A long-standing hypothesis is that action potentials initiate first in the axon hillock/initial segment (AH-IS) region because of a locally high density of Na+ channels, We tested this idea in subicular pyramid al neurons by using patch-clamp recordings in hippocampal slices. Simu ltaneous recordings from the soma and IS confirmed that orthodromic ac tion potentials initiated in the axon and then invaded the soma. Howev er, blocking Na+ channels in the AH-IS with locally applied tetrodotox in (TTX) did not raise the somatic threshold membrane potential for or thodromic spikes. TTX applied to the axon beyond the AH-IS (30-60 mu m from the soma) raised the apparent somatic threshold by similar to 8 mV, We estimated the Na+ current density in the AH-IS and somatic memb ranes by using cell-attached patch-clamp recordings and found similar magnitudes (3-4 pA/mu m(2)). Thus, the present results suggest that or thodromic action potentials initiate in the axon beyond the AH-IS and that the minimum threshold for spike initiation of the neuron is not d etermined by a high density of Na+ channels in the AH-IS region.