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 Ca²⁺-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, Ca²⁺- 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.