APLYSIA CELL-ADHESION MOLECULES AND SEROTONIN REGULATE SENSORY CELL-MOTOR CELL-INTERACTIONS DURING EARLY STAGES OF SYNAPSE FORMATION IN-VITRO

Long-term facilitation of sensorimotor synapses of Aplysia in culture by serotonin (5-HT) is accompanied by two changes: an increase in the number of sensory cell branches and varicosities contacting the major axons of the target motor cell L7, and a downregulation of Aplysia cel l adhesion molecules (apCAM) from the surface of the presynaptic senso ry cell. We tested the hypothesis that the two changes may be linked; the 5-HT-induced decrease of apCAM levels from sensory neurites may de fasciculate sensory neurites from each other and make the surface of t he motor axons a more attractive substrate for new growth and synapses . We used developing cultures to examine the relationship of neuritic branching, varicosity formation, and efficacy of the connections forme d by sensory cells to levels of apCAM expression on the motor cell. We then determined the consequences of 5-HT applied during the early per iod of interaction between sensory and motor cells (day 1 or 2 in cult ure) on the pattern of sensory cell growth and synapse formation. We r eport that the number of sensory cell branches and varicosities, and t he ability of sensory growth cones to fasciculate with L7 axons and fo rm chemical connections correlate with the level of apCAM expression o n different regions of L7. Early exposure to 5-HT increased the number of sensory cell branches and varicosities contacting newly regenerate d distal neurites of L7 to levels that would normally occur when the s ensory neurites interact with the major proximal axons of L7. Treatmen t with 5-HT also modulated the efficacy of the developing synaptic con nections. The change in synapse efficacy was accompanied by an increas e in the formation of new sensory varicosities and branches with pione ering growth cones extending on the major axons of L7. The results are consistent with the hypothesis that treatment with 5-HT modulates loc al differences in the expression of cell adhesion molecules on the sur face of the interacting cells making motor neurites more attractive fo r sensory growth cones, thereby affecting new sensory neuritic growth and synapse formation.