NEUROPEPTIDE LOCALIZATION IN VARICOSITIES OF APLYSIA SENSORY NEURONS IS REGULATED BY TARGET AND NEUROMODULATORS EVOKING LONG-TERM SYNAPTIC PLASTICITY

The synapses between the sensory neuron (SN) and motor neuron of Aplys ia undergo long-term functional and structural modulation with appropr iate behavioral training or with applications of specific neuromodulat ors. Expression of molecules within the presynaptic terminals may be r egulated in parallel with the changes evoked by the neuromodulators. W e examined with immunocyto-chemical methods whether the level of senso rin, the SN-specific neuropeptide, is modulated in SN varicosities by the location of interaction with the target motor cell L7 and by appli cations of either 5-HT that evoke long-term facilitation or FMRFamide that evoke long-term depression of Aplysia sensorimotor connections in vitro. A significantly higher proportion of SN varicosities are senso rin positive when they are in contact with the proximal axons of L7 co mpared to varicosities of the same SNs in contact with distal L7 neuri tes. Both 5-HT and FMRFamide evoked changes in the efficacy and struct ure of sensorimotor connections that are accompanied by changes in the frequency of sensorin-positive varicosities contacting the axons of L 7. More preexisting SN varicosities are stained after 5-HT, and fewer preexisting SN varicosities are stained after FMRFamide. These results suggest that the postsynaptic target and the neuromodulators not only regulate overall structure but also regulate the level of SN neuropep tide at synaptic sites. (C) 1996 John Wiley & Sons, Inc.