Selective fasciculation as a mechanism for the formation of specific chemical connections between Aplysia neurons in vitro.

Selective fasciculation of growth cones along preestablished axon pathways expressing matching or complementary adhesion molecules is thought to be an important strategy in axon guidance. Growth cone inhibiting factors also appear to influence pathfinding decisions. We have used identified Aplysia neurons in vitro to explore the hypothesis that similar mechanisms could be involved in target selection. Co-cultures of L10 neurons with RB neuron targets or R2 neurons with RUQ neuron targets reliably formed chemical connections. In contrast, co-cultures of L10 with RUQ targets usually failed to form detectable chemical connections unless cell-cell contact was forced during plating by intertwining the major axons. These data suggested that differences in the ability to form cell-cell contacts might underlie the observed synaptic specificity. This notion was supported when fluorescent dye fills of L10 and R2 revealed a positive correlation between the amount of target contact and the frequency of synapse formation: L10-RUQ cultures showed much less target contact than L10-RB or R2-RUQ cultures. To examine the cellular mechanisms of these differences in target contact, presynaptic growth cones were observed as they interacted with target processes. L10-RUQ cultures showed much less fasciculation and more avoidance behavior compared to L10-RB and R2-RUQ cultures. This initial specificity suggested that the differences in amount of target contact arose through selective fasciculation and avoidance rather than through selective elimination after indiscriminate fasciculation. Selective fasciculation and avoidance might, therefore, aid in target selection by regulating the amount of contact between presynaptic processes and potential target cells.