title: Regulation of behavioral and synaptic plasticity by serotonin release within local modulatory fields in the CNS of Aplysia.
abstract: In Aplysia, serotonergic neurons are widely activated during sensitization training, but the effects of exogenous serotonin (5-HT) on reflex circuits vary, inducing short- or long-term synaptic facilitation or synaptic inhibition, depending on the site of application. During learning, it is possible that specific spatial patterns of 5-HT release evoked by training may produce different phases of sensitization or behavioral inhibition. To test this hypothesis, we examined the modulation of the tail-induced siphon withdrawal reflex by repeated noxious stimuli applied to one of three sites: the (1) ipsilateral or (2) contralateral sides of the tail or (3) the head. Ipsilateral tail shock produced long-term sensitization, whereas contralateral tail shock induced only short-term sensitization, and head shock produced inhibition. In parallel cellular experiments, tail-nerve shock evoked large 5-HT release localized around the ipsilateral tail sensory neurons (SNs) and motor neurons (MNs) but only modest 5-HT release in the contralateral pleural-pedal ganglia and in the abdominal ganglion, in which the siphon MNs are located. Head-nerve shock, in contrast, produced only modest 5-HT release in the pleural, pedal, and abdominal ganglia. Thus, each training protocol evoked a specific pattern of 5-HT release within the CNS. In addition, we found that 5-HT released in the pleural ganglia was correlated with facilitation of SN-MN synapses; however, in the abdominal ganglion, it was associated with inhibition of the synapses between identified interneurons (L29s) and siphon MNs (LFSs). Because 5-HT differentially modulates synaptic efficacy at different synaptic sites, our data can explain how specific spatial patterns of 5-HT release in local modulatory fields can contribute to the induction of short- or long-term sensitization or to behavioral inhibition.
authors.affiliation: Department of Neurobiology and Behavior, Center for the Neurobiology of Learning and Memory, University of California, Irvine, California 92697-4550, USA.
authors.forename: Stéphane
authors.initials: S
authors.lastname: Marinesco
authors.forename: Nimalee
authors.initials: N
authors.lastname: Wickremasinghe
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Carew
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D001048:Aplysia / Q000502:physiology; D001522:Behavior, Animal / Q000502:physiology*; D002490:Central Nervous System / Q000378:metabolism / Q000502:physiology*; D004558:Electric Stimulation / Q000379:methods; D009473:Neuronal Plasticity / Q000502:physiology*; D012701:Serotonin / Q000378:metabolism*; D013569:Synapses / Q000378:metabolism / Q000502:physiology*
chemical_list: D012701:Serotonin