Serotonin induces temporally and mechanistically distinct phases of persistent PKA activity in Aplysia sensory neurons.

The cAMP signaling cascade has been implicated in several stages of memory formation. We have examined activation of this cascade by serotonin (5-HT) in the sensory neurons of Aplysia. We find that different patterns of 5-HT exposure induce three distinct modes of PKA activation. First, a single 5 min pulse induces transient (5 min) PKA activation that requires neither transcription nor translation. Second, 4-5 pulses induce intermediate-term persistent activation (3 hr duration) that requires translation but not transcription. Third, 5 pulses of 5-HT, as well as continuous (90 min) exposure, induce long-term persistent activation 20 hr later, which requires both transcription and translation. Thus, in the sensory neurons, different patterns of 5-HT give rise to three independent phases of PKA activation that differ in their induction requirements, their temporal profiles, and their molecular mechanisms.