Both diffusible and surface-bound molecules are thought to induce sprouting of motor nerve terminals in response to paralysis. Here we report that the sprouting induced by ciliary neurotrophic factor (CNTF) is qualitatively different from the sprouting induced by botulinum toxin (BoTX). We show first that subcutaneous application of CNTF to levator auris longus muscles of adult mice evokes sprouting from nearly all nerve terminals. Surprisingly, however, most terminal sprouts remain within the boundaries of the endplate region and rarely grow extrasynaptically even if CNTF is administered chronically. In contrast, terminal sprouts induced by BoTX extend vigorously along the extrasynaptic muscle surface. The different patterns of sprout elongation are attributable in part to different patterns of initiation: whereas CNTF-induced sprouts emerge randomly from the surface of terminal branches, BoTX-induced sprouts emerge exclusively along the perimeter of terminal branches in direct apposition to muscle fiber membranes. Combined treatment with CNTF and BoTX produces exceptionally robust extraterminal sprouting with little if any intrasynaptic growth of terminal sprouts. We interpret these results as showing that paralysis induces sprouting primarily by muscle-associated, surface-bound molecules rather than by diffusible factors. Our findings may be useful in defining the physiological role of the numerous candidate sprouting-inducers and in promoting compensatory sprouting after nerve injury for therapeutic benefit. J. Comp. Neurol. 504:1-16, 2007. © 2007 Wiley-Liss, Inc.
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