Calcium influx and release from intracellular stores contribute differentially to activity-dependent neuronal facilitation in Hermissenda photoreceptors.

A series of experiments is described that elucidates the sources of Ca2+ that contribute to activity-dependent neuronal facilitation in Hermissenda B photoreceptors during associative conditioning. In an in vitro preparation, pairings of a 4-s light with a 3-s mechanical stimulation of presynaptic hair cells increased the input resistance and elicited spike rate (i.e., excitability) of the B photoreceptors in the Hermissenda eye, indicative of a Ca(2+)-dependent process that is analogous to associative conditioning in the intact animal. This increase in excitability was reduced but not eliminated when hyperpolarizing current was applied to the B cell during the pairings, suggesting that voltage-dependent influx of Ca2+ contributed only a portion of the total calcium signal necessary for facilitation. Moreover, no increase in excitability was observed when a comparable current-induced depolarization of the photoreceptor was substituted for light-induced depolarization. In other experiments, Ca(2+)-dependent inactivation of a light-induced Na+ current was used as an index of intracellular Ca2+ concentration. It was determined that light caused a large increase in intracellular Ca2+ concentration regardless of whether the photoreceptor was allowed to freely depolarize in response to light or was voltage clamped at its resting membrane potential. Current-induced depolarization produced a smaller increase, while presynaptic stimulation had no measurable effect. Intracellular injections of either heparin, an antagonist of intracellular Ca2+ release, or EGTA, a general Ca2+ chelator, induced comparable reductions of light-induced Ca2+ accumulation. Finally, intracellular injections of heparin blocked the pairing-induced increases in B cell excitability as effectively as injections of EGTA. Taken as a whole, these data suggest that Ca2+ release from intracellular stores may be sufficient for the induction of facilitation in this preparation, while Ca2+ influx through voltage-dependent channels may have an additive effect and provide further evidence for the ubiquitous role of Ca2+ in learning-related forms of neuronal plasticity.