SITE-SPECIFIC AND SENSORY NEURON-DEPENDENT INCREASES IN POSTSYNAPTIC GLUTAMATE SENSITIVITY ACCOMPANY SEROTONIN-INDUCED LONG-TERM FACILITATION AT APLYSIA SENSORIMOTOR SYNAPSES

Long-term changes in the efficacy oi Aplysia sensory neuron (SN) conne ctions accompany behavioral training or applications with 5-HT. The ch anges evoked by training or 5-HT include formation of new SN varicosit ies and transmitter release sites. Because new synapse formation requi res proper alignment of presynaptic structures with postsynaptic zones containing a high density of transmitter receptors, we examined wheth er changes in postsynaptic sensitivity to the presumed SN transmitter (glutamate) were correlated with formation and distribution of new SN varicosities in contact with motor cell LT in cell culture. The format ion of stable SN connections after 4 d in culture did not significantl y change overall responses to focal applications of glutamate. However , specific sites along L7's axon apposed to SN varicosities expressed larger responses to glutamate compared with adjacent sites with few SN varicosities. After treatments with 5-HT that evoked long-term change s in both the structure and the function of SN-L7 synaptic interaction , glutamate responses increased selectively at sites along the surface of L7's axon with preexisting or new SN varicosities. increases in po stsynaptic response to glutamate 24 hr after 5-HT treatment required i nteraction with an SN. These results suggest that new synapse formatio n between neurons, either with regeneration or after external stimuli that evoke increases in synaptic efficacy, involves site-specific chan ges in expression of functional neurotransmitter receptors on the post synaptic cell that is regulated by interaction with the presynaptic ne uron.