Modeling Hermissenda .1. Differential contributions of I-A and I-C to type-B cell plasticity

We developed a multicompartmental Hodgkin-Huxley model of the Hermissenda type-B photoreceptor and used it to address the relative contributions of reductions of two K+ currents, I-A and I-C, to changes in cellular excitability and synaptic strength that occur in these cells after associative learning. We found that reductions of <(g)over bar (C)>, the peak conductance of I-C, substantially increased the firing frequency of the type-B cell during the plateau phase of a simulated light response, whereas reductions of <(g)over bar (A)> had only a modest contribution to the plateau frequency. This can be understood at least in part by the contributions of these currents to the light-induced (nonspiking) generator potential, the plateau of which was enhanced by <(g)over bar (C)> reductions, but not by <(g)over bar (A)> reductions. In contrast, however, reductions of <(g)over bar (A)> broadened the type-B cell action potential, increased Ca2+ influx, and increased the size of the postsynaptic potential produced in a type-A cell, whereas similar reductions of <(g)over bar (C)> had only negligible contributions to these measures. These results suggest that reductions of I-A and I-C play important but different roles in type-B cell plasticity.