Neuromuscular synapse formation was studied using nerve and muscle cells dissociated from Xenopus embryos and kept in culture for 1 to 3 days. Within a few minutes of manipulated contact with isolated cholinergic neurons, miniature endplate potential-like depolarizations (MEPPs) due to spontaneous release of acetylcholine (ACh) from the neurons were detected in the muscle cells. 2. Addition of an antibody to a frog neural cell adhesion molecule (anti-NCAM) into the culture medium of nerve-muscle co-cultured for 1-3 days decreased the percentage of functional nerve-muscle contacts. 3. Acute exposure to anti-NCAM (1 hour) inhibited significantly muscle cell contact-triggered ACh release from initially identified cholinergic neurons. 4. Lysed muscle cells manipulated into contact with neurons induced ACh release, whereas lysed neurons did not, suggesting the presence of specific molecules on the muscle cell membrane capable of triggering ACh release from the cholinergic neuron. 5. Transient appearance of electrical coupling was detected between neuronal soma and muscle cell, suggesting the possibility of exchange of modulators for the formation and maintenance of neuromuscular synapses. 6. Neuromuscular synaptogenesis constitutes a complex process where at least two different types of direct cell-cell interaction seem to occur: a) cell surface molecule contact (and binding) for cell recognition and triggering of ACh release, and b) transient intercytoplasmic communication between the cells for possible passage of modulatory molecules.
