Evidence for a contribution of I(Ca) to serotonergic modulation of I(K,Ca) in Hermissenda photoreceptors

1. The Ca²⁺-dependent K⁺ current (I(K,Ca)) contributes to both the plateau phase of light-elicited generator potentials and enhanced excitability of identified type B photoreceptors of Hermissenda detected after classical conditioning. Serotonergic modulation of membrane conductances mimics some of the effects of conditioning. Serotonin (5-HT) reduces the magnitude of I(K,Ca) and decreases the sustained voltage- activated Ca²⁺ current (I(Ca)) in type B photoreceptors. We have examined the modulatory role of 5-HT in regulation of I(K,Ca) by I(C) using a Ca²⁺ ionophore in conjunction with the whole cell patch-clamp technique in isolated photoreceptors. 2. The 40-50% reduction of I(K,Ca) by 5-HT was voltage independent. Cd²⁺ blocked I(Ca) and reduced I(K,Ca) by 70-80%. The remaining 20-30% of I(K,Ca) may result from Ca²⁺ release from intracellular stores, because I(K,Ca) was further reduced to 5-10% as the pipette ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) was raised from 0.5 to 5 mM. The application of the Ca²⁺ ionophore A23187, which was designed to produce Ca²⁺ influx independent of the voltage-activated Ca²⁺ channels, restored I(K,Ca). 3. The application of A23187 reversed the effects of 5-HT and Cd²⁺ on I(K,Ca) for experiments lasting 15-20 min. However, for longer time periods (>25 min), complete restoration of I(K,Ca) by A23187 was obtained in the presence of Cd²⁺ but not 5-HT. These results suggest that for 15 to 20 min exposures the reduction of I(K,Ca) by 5-HT is a consequence of modulation of I(Ca) by 5-HT and not a direct effect of 5-HT on I(K,Ca).