Effects of testosterone on a cross-facial nerve graft model

A cross-facial nerve graft (CFNG) can be used to restore the blink reflex following facial paralysis. However, the efficacy of a CFNG depends on the ability of regenerating axons to breach two nerve coaptations and reinnervate endplates in denervated muscle. The neurons involved, facial motor neurons, are androgen-dependent. Testosterone enhances facial-nerve regeneration and accelerates blink-reflex recovery following nerve crush. The current study tested the hypothesis that testosterone administered to castrated or normal adult male rats would enhance axonal regeneration through a CFNG, and thereby enhance recovery of the blink reflex. To test this hypothesis, 20 adult male rats were randomly assigned to four groups, (1) normal, intact; (2) castrated; (3) castrated with testosterone proprionate (TP) administration; and (4) normal, intact with TP Each rat underwent transection of the left facial nerve, and a CFNG using the saphenous nerve as an interpositional graft, with coaptations to the ipsilateral and contralateral zygomatic branches, was carried out. Assessments included return of blink reflex, electrophysiology, quantification of motor endplates, and axonal numbers. The data from the blink reflex evaluation, the electrophysiologic assessment, and the endplate quantification suggested that TP may have an effect on regeneration through a CFNG, but that the differences between groups were not statistically significant. In contrast, exogenously administered TP significantly enhanced the number of axons that entered the nerve graft. These data suggest that pharmacologic doses of testosterone may enhance recovery following a CFNG.