BINDING OF SEROTONIN TO RECEPTORS AT MULTIPLE SITES IS REQUIRED FOR STRUCTURAL PLASTICITY ACCOMPANYING LONG-TERM FACILITATION OF APLYSIA SENSORIMOTOR SYNAPSES

Long-term changes in the efficacy of Aplysia sensorimotor synapses acc ompany nonassociative and associative forms of behavioral plasticity. This synapse expresses long-term facilitation either with repeated app lications of 5-hydroxytryptamine (5-HT) or with a single pairing of te tanus in the sensory neuron (SN) and bath application of 5-HT. We exam ined whether structural changes in the SN accompany all forms of long- term synaptic enhancement and the locations at which 5-HT must bind re ceptors to evoke long-term functional and/or structural changes. Pairi ng tetanus with one application of 5-HT evoked both functional and str uctural changes after 24 hr only when 5-HT application was temporally paired with the tetanus and activated receptors on both the SN cell bo dy and terminal region. Repeated application of 5-HT to the terminal r egion alone failed to evoke any long-term change. Repeated application s of 5-HT to the SN cell body alone evoked a change in synaptic effica cy at 24 hr but failed to increase SN varicosities. Repeated applicati ons of 5-HT to both the SN cell body and the terminal region evoked in creases in both synaptic efficacy and the number of SN varicosities at 24 hr. The results indicate that different external stimuli can evoke equivalent forms of longterm synaptic facilitation with or without st ructural changes in the SNs. Changes in the number of SN varicosities can accompany different forms of long-term facilitation and require th e activation of 5-HT receptors at multiple sites.