DECREASE IN GROWTH CONE NEURITE FASCICULATION BY SENSORY OR MOTOR CELLS IN-VITRO ACCOMPANIES DOWN-REGULATION OF APLYSIA CELL-ADHESION MOLECULES BY NEUROTRANSMITTERS

Cell adhesion molecules play important roles in axon guidance and syna pse formation. Recent studies suggest that the expression of some of t hese molecules can be regulated either by electrical activity or by sp ecific neurotransmitters. The expression of neural cell adhesion molec ule (NCAM)like molecules in Aplysia, designated apCAM, is downregulate d from the surface of sensory neurons by 5-HT, a transmitter known to evoke long-term changes in the structure and function of these neurons . We tested whether the distribution of apCAM on the surface of other neurons can be regulated by treatments with other neurotransmitters kn own to evoke long-term functional and structural changes in Aplysia ne urons, and we examined the consequences of treatments with the neurotr ansmitters on the pattern of growth cone-neurite interactions. We repo rt that applications of the neuropeptide Phe-Met-Arg-Phe-amide (FMRFam ide) that evoke long-term synaptic depression also reduce apCAM expres sion on the surface of motor cell L7 via a mechanism that appears to b e similar to the mechanism mediating the B-HT-induced change in the se nsory cells. Specific treatments that affect apCAM distribution on the surface of their respective cells, 5-HT on sensory cells or FMRFamide on motor cell L7, mimic treatment with monoclonal antibodies against apCAM by evoking a significant reduction in the fasciculation of growt h cones with other neurites extending from homologous cells. Thus, one way that activity-dependent mechanisms might influence cell-cell inte ractions during development, regeneration, or following stimuli that e voke structural changes in the mature nervous system is via the releas e of neurotransmitters or neuropeptides that have multiple actions on receptive cells, including the regulation of expression or distributio n of cell adhesion molecules.