Local stabilization of microtubule assembly improves recovery of facial nerve function after repair

Within a recent study on the recovery of vibrissae motor performance after facial nerve repair in blind (strain SD/RCS) and sighted (strain SD) rats, we found that, despite persisting myotopic disorganization in the facial nucleus, the blind animals fully restored vibrissal whisking. Searching for the morphological substrates of this improved recovery, we compared the amount of cytoskeletal proteins in the leading edge of elongating axons between both strains. Since our results showed an enhanced expression of neuronal class III β-tubulin in the blind rats, we wondered whether this was due to an increased synthesis or to a delayed turnover of microtubules. In the present report, we approached this question applying established pharmacological agents to the transected buccal branch of the facial nerve in sighted Wistar rats perturbing either microtubule assembly towards stabilization (enhanced polymerization with 10 μg/ml taxol) or towards increased synthesis (challenged by destabilization with 100 μg/ml nocodazole and 20 μg/ml vinblastine). Evaluation of the effect(s) 2 months later included estimation of (i) vibrissae motor performance by video-based motion analysis, (ii) the degree of collateral axonal branching by double retrograde neuronal labeling with crystals of Fluoro-Gold and DiI and (iii) the pattern of motor end-plate reinnervation (proportions of mono- and poly-reinnervated) in the largest extrinsic vibrissal muscle, the m. levator labii superioris. We found that only stabilization of microtubules with 10 μg/ml taxol reduced intramuscular axonal sprouting and polyinnervation of the motor end-plates, which was accompanied by improved restoration of function.