Control of number and distribution of synapses during ectopic synapse formation in adult rat soleus muscles

Changes in the number and distribution of synaptic inputs and acetylcholine receptor clusters were studied during the formation of ectopic nerve-muscle junctions between the transplanted fibular nerve and the denervated soleus muscle of adult rats. The tibial nerve was cut 3 weeks after implanting the fibular nerve. New sites of transmission were first detected 3 days after the cut. These sites were located electrophysiologically, marked by dye and found to coincide with clusters of acetylcholine receptors. There were no ectopic clusters away from fibular nerve sprouts and no clusters on muscles which had not been denervated. Three days after cutting the tibial nerve, the acetylcholine receptor clusters, and probably also the sites of transmission, were randomly distributed along individual muscle fibres. Six days after the cut, the clusters continued to be randomly distributed whereas the synaptic inputs were either close together (within 300 microns) or more than 600 microns apart. Two weeks later the spatial distributions of both clusters and inputs were similar with peaks around 100-300 microns, 1200-1400 microns and 2000-2600 microns. No ectopic clusters were closer than 0.5 mm to the original endplate. We conclude that nerve-muscle contacts and associated acetylcholine receptor clusters initially form at random. One or a few of these contacts develop further and, as a result, the surrounding regions undergo changes that prevent the contacts initially formed there from being maintained. Apparently, in this preparation, approximately 1.5 mm length of fibre is needed to support the maturation and maintenance of each ectopic endplate (mean length 111 micron).