Androgenic, but not estrogenic, protection of motoneurons from somal and dendritic atrophy induced by the death of neighboring motoneurons.

Motoneuron loss is a significant medical problem, capable of causing severe movement disorders or even death. We have been investigating the effects of motoneuron loss on surviving motoneurons in a lumbar motor nucleus, the spinal nucleus of the bulbocavernosus (SNB). SNB motoneurons undergo marked dendritic and somal atrophy following the experimentally induced death of other nearby SNB motoneurons. However, treatment with testosterone at the time of lesioning attenuates this atrophy. Because testosterone can be metabolized into the estrogen estradiol (as well as other physiologically active steroid hormones), it was unknown whether the protective effect of testosterone was an androgen effect, an estrogen effect, or both. In the present experiment, we used a retrogradely transported neurotoxin to kill the majority of SNB motoneurons on one side of the spinal cord only in adult male rats. Some animals were also treated with either testosterone, the androgen dihydrotestosterone (which cannot be converted into estradiol), or the estrogen estradiol. As seen previously, partial motoneuron loss led to reductions in soma area and in dendritic length and extent in surviving motoneurons. Testosterone and dihydrotestosterone attenuated these reductions, but estradiol had no protective effect. These results indicate that the neuroprotective effect of testosterone on the morphology of SNB motoneurons following partial motoneuron depletion is an androgen effect rather than an estrogen effect.