Androgenic regulation of tubulin gene expression in axotomized hamster facial motoneurons

Abstract

We have previously demonstrated that systemic administration of testosterone increases the rate of axonal regeneration following facial nerve crush in adult male hamsters. In this investigation, the molecular mechanisms by which androgens may enhance axonal regeneration were examined. Specifically, the following question was addressed using Northern blot and in situ hybridization with three cytoskeletal cDNA probes complementary to β(II)-, β(III)-, and α1-tubulin mRNA: does exogenous testosterone augment axotomy-induced changes in tubulin mRNA expression in hamster facial motoneurons (FMN)? Adult male hamsters were subjected to unilateral facial nerve severance, with the opposite side serving as an internal control. One- half of the animals were subcutaneously implanted with Silastic capsules containing crystalline testosterone propionate and the other half implanted with blank capsules. Postoperative survival times were 2 and 7 d. At 2 d after axotomy alone, no changes in levels of any of the three tubulin mRNAs were observed in the injured FMN. However, by 7 d after axotomy, significant increases in all three tubulin mRNAs were observed. This time course of axotomy-induced changes in tubulin gene expression is consistent with findings in other injured neuronal populations. Administration of testosterone at the time of injury had two major effects on the cytoskeletal response pattern in axotomized FMN. First, testosterone differentially regulated the set of tubulin mRNAs examined, in that β(II)-tubulin mRNA levels were selectively altered by the steroid, whereas β(III)- or α1- tubulin mRNAs were not. Second, administration of testosterone at the time of injury resulted in a more rapid induction of β(II)-tubulin mRNA changes in axotomized neurons relative to injury alone. These findings lend support to the hypothesis that testosterone enhances the rate of axonal regeneration through an augmentation in the neuronal cytoskeletal response pattern after axon disconnection.