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

Abstract

Evidence is accumulating that voltage-gated channels are distributed nonuniformly throughout neurons and that this nonuniformity underlies regional differences in excitability within the single neuron. Previous reports have shown that Ca2+, Na+, A-type K+, and hyperpolarization-activated, mixed cation conductances have varying distributions in hippocampal CA1 pyramidal neurons, with significantly different densities in the apical dendrites compared with the soma. Another important channel mediates the large-conductance Ca2+-activated K+ current (IC), which is responsible in part for repolarization of the action potential (AP) and generation of the afterhyperpolarization 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 recording techniques. We found no IC activation by back-propagating APs in distal dendritic recordings. Dendritic APs activated IC only in the proximal dendrites, and this activation decayed within the first 100-150 micrometer of distance from the soma. The decay of IC in the proximal dendrites occurred despite AP amplitude, plus presumably AP-induced Ca2+ influx, that was comparable with that at the soma. Thus we conclude that IC activation by action potentials is nonuniform in the hippocampal pyramidal neuron, which may represent a further example of regional differences in neuronal excitability that are determined by the nonuniform distribution of voltage-gated channels in dendrites.

Fig. 1.

Back-propagating APs recorded in the apical dendrites lack a fast AHP. A, Hippocampal CA1 pyramidal neurons were visualized using IR-DIC optics. The large arrow points to cell soma; the small arrowpoints to the apical dendrite ∼150 μm from the soma, where a patch pipette can be faintly seen. B, Top, Back-propagating dendritic action potentials (traceshown) differ from somatic action potentials in their smaller amplitude, slower repolarization, and lack of a fast AHP.Bottom, When APs were repetitively stimulated, dendritic APs decremented in amplitude over the course of the train.C, Top, Somatic action potentials showed much faster repolarization and a fast AHP (arrowhead) after the AP. Bottom, When repetitively stimulated, somatic APs had a nearly constant amplitude. Synaptic blocks were omitted in this experiment.

Fig. 2.

Repolarization of action potentials in the distal dendrites is not affected by Ca²⁺ channel blockade.A, Top, Action potentials recorded at the soma are shown under control and 100 μmCd²⁺ conditions. Right, Superimposed APs show that Ca²⁺ channel blockade significantly slowed AP repolarization and blocked the fast AHP (arrow). Bottom, Repolarization of dendritic APs was not significantly affected by Ca²⁺channel blockade. B, The same responses presented inA (right) are shown at a longer time scale to show block of the fast AHP (arrow) at the soma by 100 μm Cd²⁺. C, Summary of data shows the average decrease in AP repolarization (repol.) slope between control and Ca²⁺ channel blockade conditions (grouped data from 100–200 μm Cd²⁺ and from 0 Ca²⁺ and 6 mm Mg²⁺solutions; n = 8 for soma, n = 9 for dendrite).

Fig. 3.

Blockade of Ca²⁺-activated K⁺ currents does not affect repolarization of dendritic action potentials. A, Top, Action potentials recorded at the soma showed slowing of repolarization and blockade of the fast AHP (arrow) by CTX, an inhibitor of large-conductance Ca²⁺-activated K⁺ conductances. Bottom, Repolarization of dendritic action potentials was not affected by CTX.B, TEA, in low concentrations, blocksI_C to the exclusion of most other K⁺ channels. Application of TEA (1 mm) slowed AP repolarization at the soma (top), while having a minimal effect on dendritic APs (bottom).

Fig. 4.

Repetitive firing does not cause activation of dendritic I_C. A,Top, Short trains of antidromic action potentials were elicited in control and Cd²⁺ solutions.Bottom, Comparison of superimposed APs in control and Cd²⁺ solutions from the beginning (left) and end (right) of the train shows that Ca²⁺ channel blockade had a similar effect on AP repolarization (arrow) at the beginning and end of the train. This suggests that I_Cactivation was nearly maximal with the first AP. B, A similar comparison of dendritic APs shows thatI_C was minimally activated at both the beginning (bottom left) and end (bottom right) of the train of APs.

Fig. 5.

Activation of I_Cdiminishes in the proximal dendrites. A, Action potentials recorded at the soma showed a marked slowing of repolarization in 200 μm Cd²⁺(arrow). B–D, Action potentials recorded in the proximal dendrites at progressively greater distances from the soma showed a gradual decrease in the activation ofI_C (arrows).

Fig. 6.

The decrease in dendriticI_C activation does not depend on action potential amplitude. A, A plot of the decrease in action potential repolarization rate with Ca²⁺ channel blockade versus distance in the dendrites from the soma. In this case, the repolarization rate at both the soma and dendrites was computed as the slope of AP repolarization measured between 80 and 30% of action potential amplitude. Each point represents a single experiment, with the values at the soma (dendritic distance = 0) and at 174 μm (mean distal dendritic recording location) representing the mean values of experiments at that location (n = 8 at the soma;n = 9 in the distal dendrites). Note thatI_C activation decreased continuously from its value at the soma with increasing dendritic distance.B, The decrease in repolarization slope (squares) and AP amplitude (triangles) normalized to these values at the soma and plotted versus dendritic distance. [The SEM of the mean AP amplitude (ampl.) in the distal dendrites was less than the symbol size.] In the proximal dendrites up to ∼70 μm, AP amplitude was nearly invariant, whereas the activation of I_Cdeclined. Thus, the diminishing activation ofI_C in the dendrites was not a function of decreasing AP amplitude.