Analysis of sequence-dependent interactions between transient calcium and transmitter stimuli in activating adenylyl cyclase in Aplysia: Possible contribution to CS-US sequence requirement during conditioning

An important recent insight in a number of neurobiological systems is that during learning, individual dually regulated proteins with associative properties function as critical sites of stimulus convergence. During conditioning in Aplysia, the Ca²⁺/calmodulin-sensitive adenylyl cyclase (AC) in mechanosensory neurons serves as a molecular site of interaction between Ca²⁺ and serotonin [5-hydroxytryptamine (5-HT)]-two signals that represent the CS and US in these cells. Conditioning requires that the CS and US be paired within a narrow time window and in the appropriate sequence. AC shows an analogous sequence preference: It is more effectively activated when a pulse of Ca²⁺ precedes a pulse of 5-HT than when the 5-HT precedes Ca²⁺. One mechanism that contributes to this sequence preference is that Ca²⁺/calmodulin binding to AC accelerates the rate of AC activation by receptor-G(s). We have identified two additional properties of AC activation that would cause pairing with Ca²⁺ preceding 5-HT to be more effective than simultaneous pairing or pairing with the reciprocal sequence: (1) Activation of Aplysia AC by a Ca²⁺ pulse rose with a delay compared with activation by a 5-HT pulse. (2) A late pulse of Ca²⁺, which arrived after 5-HT, acted, via calmodulin, to accelerate the decay of AC activation by receptor-G(s). Together, these activation properties of AC may contribute to the CS-US sequence requirement of classical conditioning.