Temporal asymmetry in activation of Aplysia adenylyl cyclase by calcium and transmitter may explain temporal requirements of conditioning

Cellular experiments have suggested that during classical conditioning of the gill and siphon withdrawal reflex of Aplysia, adenylyl cyclase may serve as a molecular site of convergence for Ca²⁺ and serotonin (5-hydroxytryptamine; 5-HT), the cellular representations of the conditioned and unconditioned stimuli (CS and US). We explored the possible molecular basis of the behavioral requirement that the CS and US be paired within a narrow time window and in the appropriate order. To examine the temporal interactions of brief pulses of Ca²⁺ and 5-HT in stimulating Aplysia neural cyclase, we used a perfused-membrane cyclase assay. When brief pulses of Ca²⁺ and 5-HT were paired, cyclase activation depended upon the sequence of the pulses: peak cyclase activation was greater when the Ca²⁺ pulse immediately preceded the 5-HT pulse. Examination of the rising phase of 5-HT stimulation suggested that a Ca²⁺ prepulse might accelerate the onset of activation by 5-HT, without affecting the final level of activation achieved with prolonged 5-HT exposure. The observed interactions between Ca²⁺ and transmitter in activating cyclase could contribute importantly to the temporal requirements of conditioning for CS-US pairing.