EVIDENCE FOR A CONTRIBUTION OF I-CA TO SEROTONERGIC MODULATION OF I-K,I-CA IN HERMISSENDA PHOTORECEPTORS

1. The Ca2+-dependent K+ current (I-K,I-Ca) contributes to both the pl ateau phase of light-elicited generator potentials and enhanced excita bility of identified type B photoreceptors of Hermissenda detected aft er classical conditioning Serotonergic modulation of membrane conducta nces mimics some of the effects of conditioning. Serotonin (5-HT) redu ces the magnitude of I-K,I-Ca and decreases the sustained voltage-acti vated Ca2+ current (I-Ca) in type B photoreceptors. We have examined t he modulatory role of 5-HT in regulation of I-K,I-Ca by I-Ca using a C a2+ ionophore in conjunction with the whole cell patch-clamp technique in isolated photoreceptors. 2. The 40-50% reduction of I-K,I-Ca by 5- HT was voltage independent, Cd2+ blocked I-Ca and reduced I-K,I-Ca by 70-80%. The remaining 20-30% of I-K,I-Ca may result from Ca2+ release from intracellular stores, because I-K,I-Ca was further reduced to 5-1 0% as the pipette ethylene glycol-bis (beta-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+ infl ux independent of the voltage-activated Ca2+ channels, restored I-K,I- Ca. 3. The application of A23187 reversed the effects of 5-HT and Cd2 on I-K,I-Ca for experiments lasting 15-20 min. However, for longer ti me periods (>25 min), complete restoration of I-K,I-Ca by A23187 was o btained in the presence of Cd2+ but not 5-HT. These results suggest th at for 15 to 20 min exposures the reduction of I-K,I-Ca by 5-HT is a c onsequence of modulation of I-Ca by 5-HT and not a direct effect of 5- HT on I-K,I-Ca.