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The Journal of Neuroscience, September 15, 1998, 18(18):7328-7335

Synaptic Competition during the Reformation of a Neuromuscular Map

Michael B. Laskowski¹, Howard Colman², Carla Nelson², and Jeff W. Lichtman²

¹ WWAMI Medical Program, University of Idaho, Moscow, Idaho 83844-4207, and ² Department of Anatomy and Neurobiology, Washington University, School of Medicine, St. Louis, Missouri 63110

We have been studying the mechanisms whereby pools of motor neurons establish a rostrocaudal bias in the position of their synapses in some skeletal muscles. The serratus anterior (SA) muscle of the rat displays a rostrocaudal topographic map before birth, and the topography is re-established after denervation. In this report, we explore the potential 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 enhanced the likelihood of observing synaptic competition. This was accomplished by forcing caudal axons to regenerate ahead of rostral axons onto a surgically reduced SA muscle. In this way, caudal (C₇) motor neurons had unopposed access to vacated synaptic sites on the remaining rostral half of the SA before the return of the rostral (C₆) axons. Intracellular recording revealed that 2 d after the second denervation, most of the reinnervated end plates contained only axons from the C₇ branch; the remaining reinnervated end plates received input from C₆ only or were multiply innervated by C₆ and C₇ axons. After 6 d, the pattern was reversed, with most end plates innervated exclusively by C₆. After 17 d, axons from C₆ were the sole input to reinnervated end plates. During the transition from C₇- to C₆-dominated input, at end plates coinnervated by C₆ and C₇ axons, the average quantal content from C₆ was the same as that from C₇; after 7 d, the quantal content of C₆ was greater than that of C₇. We have thus developed an experimental situation in which the outcome of synaptic competition is predictable and can be influenced by the positional labels associated with axons from different levels in the spinal cord.

Key words: synaptic competition; topography; neuromuscular junction denervation; motor neurons; synapse elimination; end plates

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Copyright © 1998 Society for Neuroscience  0270-6474/98/18187328-08$05.00/0

This article has been cited by other articles:

				H. Wang, S. R. Chadaram, A. S. Norton, R. Lewis, J. Boyum, W. Trumble, J. R. Sanes, and M. B. Laskowski Positionally Selective Growth of Embryonic Spinal Cord Neurites on Muscle Membranes J. Neurosci., June 15, 1999; 19(12): 4984 - 4993. [Abstract] [Full Text] [PDF]

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