SYNAPTIC COMPETITION DURING THE REFORMATION OF A NEUROMUSCULAR MAP

We have been studying the mechanisms whereby pools of motor neurons es tablish a rostrocaudal bias in the position of their synapses in some skeletal muscles. The serratus anterior (SA) muscle of the rat display s a rostrocaudal topographic map before birth, and the topography is r e-established after denervation. In this report, we explore the potent ial role of synaptic competition between innervating axons as a means of generating topographic specificity. We followed the progress of the reformation of this map in neonatal animals under conditions that enh anced the likelihood of observing synaptic competition. This was accom plished by forcing caudal axons to regenerate ahead of rostral axons o nto a surgically reduced SA muscle. In this way, caudal (C-7) motor ne urons had unopposed access to vacated synaptic sites on the remaining rostral half of the SA before the return of the rostral (C-6) axons. I ntracellular recording revealed that 2 d after the second denervation, most of the reinnervated end plates contained only axons from the C-7 branch; the remaining reinnervated end plates received input from C-6 only or were multiply innervated by C-6 and C-7 axons. After 6 d, the pattern was reversed, with most end plates innervated exclusively by C-6. After 17 d, axons from C-6 were the sole input to reinnervated en d plates. During the transition from C-7- to C-6-dominated input, at e nd plates coinnervated by C-6 and C-7 axons, the average quantal conte nt from C-6 was the same as that from C-7; after 7 d, the quantal cont ent of C-6 was greater than that of C-7. We have thus developed an exp erimental situation in which the outcome of synaptic competition is pr edictable and can be influenced by the positional labels associated wi th axons from different levels in the spinal cord.