Adult plasticity in hormone-sensitive motoneuron morphology: Methodological/behavioral confounds

Changes in androgen levels can alter the structure of motoneurons in the sp inal nucleus of the bulbocavernosus (SNB), a motor nucleus that innervates perineal muscles involved in copulatory behavior. While sexual activity can alter androgen levels in normal males, it has no effect on SNB motoneuron soma size or dendritic morphology (Beversdorf, Kurz, and Sengelaub, 1990). However, Breedlove (1997) reported reductions in the size of SNB somata, nu clei, and target muscles of copulating versus noncopulating castrated rats maintained on subphysiological testosterone. To reconcile the results obtai ned using intact versus implant paradigms, we tested the hypothesis that th e implant/behavior paradigm could produce differences in hormone levels, po tentially confounding sexual behavior effects on the morphology of this and rogen-sensitive neuromuscular system. Young adult male rats were castrated and immediately given 5-mm Silastic implants containing crystalline testost erone. One week later, blood samples were drawn and the males were housed w ith receptive females (copulators) or nonreceptive females (noncopulators) or housed alone (singles). After 27 days, blood samples were drawn again, a nd SNB target muscles and spinal cords removed. No differences in target mu scle weight or SNB somata and nuclei size were observed between copulators, noncopulators, or singles; as expected, all measures were significantly re duced relative to intact males. Radioimmunoassay showed that testosterone d eclined differentially over the course of the behavioral manipulation acros s groups, being greatest in copulators and least pronounced in single males . These data indicate that differences in sexual or housing experience can alter testosterone titers under these implant conditions, potentially confo unding hormone-sensitive measures of morphology. (C) 2000 Academic Press.