Seasonal variation in mammalian striated muscle mass and motoneuron morphology

Abstract

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.

The spinal nucleus of the bulbocavernosus: Firsts in androgen-dependent neural sex differences

“…Similar observations have been made in white-footed mice (Forger and Breedlove, 1987), gerbils (Ulibarri et al, 1995), and several strains of Mus musculus (Wee and Clemens, 1987;Park et al, 2002;Monks et al, 2003;Zuloago et al, 2007). Larger somata likely reflect differences in electrophysiological properties, protein synthesis, and more robust dendritic organization, all of which are androgen sensitive in SNB motoneurons (see below).…”

“…For example, a naturally occurring analog of hormone depletion and replacement can be seen in seasonally breeding mammals such as white-footed mice. As in rats, castration results in decreased dendritic arbors in these mice (Forger and Breedlove, 1987). Similarly, circulating testosterone levels decline with advanced aging, and these declines are accompanied by reductions in sexual behavior.…”

“…In whitefooted mice (Peromyscus leucopus), SNB somata and nuclei, and the BC/LA muscles all shrink in response to short daylengths mimicking late fall, and regrow when mice are transferred to long daylengths characteristic of spring (Forger and Breedlove, 1987). These changes in the SNB system parallel the regression and growth of the testes, and likely result from photoperioddependent changes in circulating testosterone (Forger and Breedlove, 1987). Similarly, transfer to short daylengths leads to testicular regression and a reduction in SNB soma size in seasonally-breeding Siberian hamsters (Phodopus sungorus; Hegstrom and Breedlove, 1999).…”

Functional differentiation of adult neural circuits from a single embryonic network

“…However, it appears that, in Homarus, the maturation of the adult neural networks involves neither the elimination of certain neuronal elements by programmed cell death (Truman, 1983;Oppenheim, 1991) nor the addition of elements by postembryonic neurogenesis (Thomas et al, 1984), because the final neuronal population in the STG occurs very early and remains stable during the course of subsequent development (Garzino and Reichert, 1994). Growth or regression of some dendritic structures can modify neuronal circuitry during development (Levine and Truman, 1985;Jacobs and Weeks, 1990) and in some adult nervous systems under the influence of steroid hormones (Nottebohm, 1981;Forger and Breedlove, 1987), but such dendritic remodelling has not been examined in the STNS. Developmental alterations in neural network activity may occur at three main levels involving (1) synaptic connections, (2) cellular properties, (3) descending modulatory inputs.…”

Androgen regulation of axon growth and neurite extension in motoneurons

“…For example, it has long been established that androgens can increase soma size and dendritic arborization in spinal motoneurons (Breedlove and Arnold, 1981;Forger and Breedlove, 1987;Kurz et al, 1986). The fact that androgens increase cell size and process outgrowth in MN hybrid cells, but only in those expressing AR, provides evidence that androgens are capable of acting directly on motoneurons to affect these properties in a cell autonomous fashion in vitro (Brooks et al, 1998).…”