ANALYSIS OF SEQUENCE-DEPENDENT INTERACTIONS BETWEEN TRANSIENT CALCIUMAND 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 associ ative properties function as critical sites of stimulus convergence. D uring conditioning in Aplysia, the Ca2+/calmodulin-sensitive adenylylc yclase (AC) in mechanosensory neurons serves as a molecular site of in teraction bem een Ca2+ and serotonin [5-hydroxytryptamine (5-HT)]-two signals that represent the CS and US in these cells. Conditioning requ ires that the CS and US be paired within a narrow time window and in t he appropriate sequence. AC shows an analogous sequence preference: It is more effectively activated when a pulse of Ca2+ precedes a pulse o f 5-HT than when the 5-HT precedes Ca2+. One mechanism that contribute s to this sequence preference is that Ca2+/calmodulin binding to AC ac celerates the rate of AC activation by receptor-G(s). We have identifi ed two additional properties of AC activation that would cause pairing with Ca2+ preceding 5-HT to be more effective than simultaneous pairi ng 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-H T pulse. (2) A late pulse of Ca2+, which arrived after 5-HT, acted, vi a calmodulin, to accelerate the decay of AC activation by receptor-G(s ). Together, these activation properties of AC may contribute to the C S-US sequence requirement of classical conditioning.