Calcium-activated potassium conductances contribute to action potential repolarization at the soma but not the dendrites of hippocampal CA1 pyramidalneurons

Evidence is accumulating that voltage-gated channels are distributed nonuni formly throughout neurons and that this nonuniformity underlies regional di fferences in excitability within the single neuron. Previous reports have s hown that Ca2+, Na+, A-type K+, and hyperpolarization-activated, mixed cati on conductances have varying distributions in hippocampal CA1 pyramidal neu rons, with significantly different densities in the apical dendrites compar ed with the soma. Another important channel mediates the large-conductance Ca2+-activated K+ current (I-C) which is responsible in part for repolariza tion of the action potential (AP) and generation of the afterhyperpolarizat ion that follows the AP recorded at the soma. We have investigated whether this current is activated by APs retrogradely propagating in the dendrites of hippocampal pyramidal neurons using whole-cell dendritic patch-clamp rec ording techniques. We found no I-C activation by back-propagating APs in di stal dendritic recordings, Dendritic APs activated I-C only in the proximal dendrites, and this activation decayed within the first 100-150 mu m of di stance from the soma. The decay of I-C in the proximal dendrites occurred d espite AP amplitude, plus presumably AP-induced Ca2+ influx, that was compa rable with that at the soma. Thus we conclude that I-C activation by action potentials is nonuniform in the hippocampal pyramidal neuron, which may re present a further example of regional differences in neuronal excitability that are determined by the nonuniform distribution of voltage-gated channel s in dendrites.