Seasonal variation in mammalian striated muscle mass and motoneuron morphology

Feral white-footed mice are seasonal breeders that undergo predictable cycles of reproductive function. Photoperiod-induced fluctuations in gonadal function of white-footed mice were associated with morphological changes in perineal muscles and their motoneurons. Exposure to short daylengths resulted in testicular regression, decreased perineal muscle mass, and shrinkage of somata and nuclei of motoneurons of the spinal nucleus of the bulbocavernosus (SNB). These effects were reversed by reinstatement of long daylengths. Similar reductions in muscle mass and SNB soma size were seen following gonadectomy of white-footed mice. In addition, dendritic trees of SNB motoneurons were reduced in gonadectomized mice compared with dendritic arbors of intact mice or castrates provided with testosterone capsules. Androgen-mediated annual changes in muscle mass and motoneuron morphology appear to be a natural part of this species' physiology.