Recovery from Facial Paralysis Following Crush Injury of the Facial Nerve in Hamsters: Differential Effects of Gender and Androgen Exposure

Abstract

In recent studies, we have shown that administration of androgens to male hamsters enhances functional recovery from facial paralysis induced by facial nerve crush at the level of the stylomastoid foramen. Furthermore, we have established that the mechanism behind this enhancement of regeneration involves an acceleration of the rate of regeneration, without a shortening in the delay of sprout formation. From those studies, several unexpected findings of inherent sex differences emerged. First, intact (nongonadectomized) females have a faster facial nerve regeneration rate than males and, second, testosterone has a less dramatic effect on the rate of regeneration in females compared to males. In the present study, we explored these novel findings of sex differences in the response of motor neurons to injury. Adult intact female hamsters were subjected to right facial nerve crush axotomy at the level of stylomastoid foramen and either implanted with two or four silastic capsules containing 5α-dihydrotestosterone, or sham-implanted for controls. A group of adult intact male hamsters was also subjected to right facial nerve crush axotomy. Animals were observed daily, beginning on Day 1 postoperatively and continuing throughout a 3-week recovery period for signs of functional recovery from facial paralysis. The average day of return of each of four behavioral components (semi-blink, blink reflex, full vibrissae movement, and complete recovery) was calculated for all four experimental groups. The results indicate that intact females recover functional return of movement following crush injury significantly faster than intact males. In contrast to our previous findings in male hamsters, administration of exogenous steroids does not accelerate recovery from facial paralysis in adult female hamsters. Why these sex differences in both inherent recovery from peripheral nerve injury and in the ability of androgens to augment regeneration occur is unknown, but will be interesting to explore from a molecular perspective in future studies.