DIFFERENTIAL DISTRIBUTION OF FUNCTIONAL RECEPTORS FOR NEUROMODULATORSEVOKING SHORT-TERM HETEROSYNAPTIC PLASTICITY IN APLYSIA SENSORY NEURONS

Synaptic transmission and excitability in Aplysia sensory neurons (SNs ) are bidirectionally modulated by 5-HT and FMRF-amide. To explore the regional distribution of different functional receptors that modulate SN properties, we examined changes in synaptic efficacy and excitabil ity with brief focal applications of the neuromodulators to different regions of SNs that have established connections with motor cell L7 in culture. Short-term changes in synaptic efficacy were evoked only whe n 5-HT or FMRFamide was applied to regions with SN vancosities along t he surface of L7 axons. Applications to adjacent SN neurites with few varicosities in contact with L7 axons failed to evoke a significant ch ange in synaptic efficacy. The distribution of functional receptors me diating changes in excitability differed for 5-HT and FMRFamide. Where as excitability increases were evoked only when 5-HT was applied to SN cell bodies, excitability decreases in SNs were evoked only when FMRF -amide was applied to regions along the L7 axon with SN varicosities. Without the target cell, cell bodies of SNs expressed both 5-HT and FM RFamide receptors that modulate excitability. These results indicate t hat functional G-protein-coupled receptors for two neuromodulators are distributed differentially along the surface of a presynaptic neuron that forms chemical connections in vitro. This differential distributi on of receptors on the presynaptic neuron is regulated by a target and does not require the physical presence of neurons that release the ne uromodulators.