LAUSR

Motoneuron loss is a severe medical problem that can result in loss of motor control and eventually death. We have previously demonstrated that partial motoneuron loss can result in dendritic atrophy and functional deficits in nearby surviving motoneurons, and that an androgen‐dependent effect of exercise following injury can be neuroprotective against this dendritic atrophy. In this study, we explored where the necessary site of androgen action is for exercise‐driven neuroprotective effects on induced dendritic atrophy. Motoneurons innervating the vastus medialis muscles of adult male rats were selectively killed by intramuscular injection of cholera toxin‐conjugated saporin. Simultaneously, some saporin‐injected animals were given implants of the androgen receptor antagonist hydroxyflutamide, either directly at the adjacent vastus lateralis musculature ipsilateral to the saporin‐injected vastus medialis or interscapularly as a systemic control. Following saporin injections, some animals were allowed free access to a running wheel attached to their home cages. Four weeks later, motoneurons innervating the same vastus lateralis muscle were labeled with cholera toxin‐conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions. Dendritic arbor lengths of saporin‐injected animals allowed to exercise were significantly longer than those not allowed to exercise. Androgen receptor blockade locally at the vastus lateralis muscle prevented the protective effect of exercise. These findings indicate that exercise following neural injury exerts a protective effect on motoneuron dendrites, which acts via androgen receptor action at the target muscle.