MODELING HERMISSENDA .1. DIFFERENTIAL CONTRIBUTIONS OF I-A AND I-C TOTYPE-B CELL PLASTICITY

We developed a multicompartmental Hodgkin-Huxley model of the Hermisse nda type-B photoreceptor and used it to address the relative contribut ions of reductions of two K+ currents, I-A and I-C, to changes in cell ular excitability and synaptic strength that occur in these cells afte r associative learning. We found that reductions of <(g)over bar (C)>, the peak conductance of I-C, substantially increased the firing frequ ency 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 co ntribution to the plateau frequency. This can be understood at least i n part by the contributions of these currents to the light-induced (no nspiking) generator potential, the plateau of which was enhanced by <( g)over bar (C)> reductions, but not by <(g)over bar (A)> reductions. I n contrast, however, reductions of <(g)over bar (A)> broadened the typ e-B cell action potential, increased Ca2+ influx, and increased the si ze of the postsynaptic potential produced in a type-A cell, whereas si milar reductions of <(g)over bar (C)> had only negligible contribution s to these measures. These results suggest that reductions of I-A and I-C play important but different roles in type-B cell plasticity.