Effects of testosterone on a cross-facial nerve graft model

A cross-facial nerve graft (CFNG) can be used to restore the blink reflex f ollowing facial paralysis. However, the efficacy of a CFNG depends on the a bility of regenerating axons to breach two nerve coaptations and reinnevate endplates in denervated muscle. The neurons involved, facial motor neurons , are androgen-dependent. Testosterone enhances facial-nerve regeneration a nd accelerates blink-reflex recovery following nerve crush. The current stu dy tested the hypothesis that testosterone administered to castrated or nor mal adult male rats would enhance axonal regeneration through a CFNG, and t hereby enhance recovery of the blink reflex. To test this hypothesis, 20 ad ult male rats were randomly assigned to four groups, (1) normal, intact; (2 ) castrated; (3) castrated with testosterone proprionate (TP) administratio n: and (4) normal, intact with TP. Each rat underwent transection of the le ft facial nerve, and a CFNG using the saphenous nerve as an interpositional graft, with coaptations to the ipsilateral and contralateral zygomatic bra nches, was carried out. Assessments included return of blink reflex, electr ophysiology, quantification of motor endplates, and axonal numbers. The dat a from the blink reflex evaluation, the electrophysiologic assessment, and the endplate quantification suggested that TP may have an effect on regener ation through a CFNG, but that the differences between groups were not stat istically significant. In contrast, exogenously administered TP significant ly enhanced the number of axons that entered the nerve graft. These data su ggest that pharmacologic doses of testosterone may enhance recovery followi ng a CFNG.