Serotonin stimulation of cAMP-dependent plasticity in Aplysia sensory neurons is mediated by calmodulin-sensitive adenylyl cyclase.

Calmodulin (CaM)-sensitive adenylyl cyclase (AC) in sensory neurons (SNs) in Aplysia has been proposed as a molecular coincidence detector during conditioning. We identified four putative ACs in Aplysia CNS. CaM binds to a sequence in the C1b region of AC-AplA that resembles the CaM-binding sequence in the C1b region of AC1 in mammals. Recombinant AC-AplA was stimulated by Ca(2+)/CaM. AC-AplC is most similar to the Ca(2+)-inhibited AC5 and AC6 in mammals. Recombinant AC-AplC was directly inhibited by Ca(2+), independent of CaM. AC-AplA and AC-AplC are expressed in SNs, whereas AC-AplB and AC-AplD are not. Knockdown of AC-AplA demonstrated that serotonin stimulation of cAMP-dependent plasticity in SNs is predominantly mediated by this CaM-sensitive AC. We propose that the coexpression of a Ca(2+)-inhibited AC in SNs, together with a Ca(2+)/CaM-stimulated AC, would enhance the associative requirement for coincident Ca(2+) influx and serotonin for effective stimulation of cAMP levels and initiation of plasticity mediated by AC-AplA.