In the mollusk Aplysia the neurotransmitter serotonin (5HT) has a fundamental modulatory role in several forms of learning and memory that involve an increase in the efficacy of synaptic transmission between tail sensory neurons (SNs) and motor neurons. The classical 5HT antagonist cyproheptadine (CYP) permits dissociation of three forms of serotonergic modulation in these SNs: (i) CYP reversibly blocks spike-broadening induced either by exogenous application of 5HT or by sensitizing stimulation of a tail nerve, (ii) CYP does not block 5HT-induced or tail input-induced increases in SN somatic excitability. (iii) Concomitant with its block of spike-broadening, CYP reversibly blocks 5HT-induced facilitation of synaptic transmission from SNs. These results suggest that endogenously released 5HT may act at different receptor subtypes that are coupled to different forms of neuromodulation in tail SNs of Aplysia.
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