During synapse elimination, adult patterns of synaptic connections are established through an elimination of excess neural connections. For example, although adult mammalian muscle fibers are each typically innervated by only a single motoneuron, in newborns, muscle fibers receive innervation from several different motoneurons. During synapse elimination, all but one motoneuronal input is retracted from each muscle fiber, thus establishing the adult pattern of single innervation. In the hormone-sensitive perineal levator ani (LA) muscle of the rat, synapse elimination is regulated by androgen. Testosterone (T) treatment of castrated males prevents some synapse elimination. Notable, T treatment has no effect on synapse elimination in the extensor digitorum longus muscle (EDL), a limb muscle generally less sensitive to androgen. The primary goal of the proposed experiments is to determine the site where androgen acts to alter synapse elimination. Three candidate sites will be tested: the brain, the spinal cord, and the LA muscle. One experiment will examine the effect of 3 different androgen metabolites (testosterone, dihydrotestosterone, and estradiol) on synapse elimination in the LA and EDL. Results from this experiment will identify the active metabolite that influences synapse elimination in the LA and given our current knowledge about the steroid binding and/or accumulating characteristics of rat motoneurons, and their target muscles, may also suggest where androgen acts. A second experiment will examine the effects of systemic androgen on synapse elimination in the LA of spinally transected animals to test whether androgen acts in the brain to alter synapse elimination in the LA muscle. 3,4) Two additional experiments will assess whether androgen acts on the muscle or in the spinal cord to influence synapse elimination in the LA by locally exposing each site to androgen. Information about where androgen acts to prevents synapse elimination in the LA is necessary for directing future experiments aimed at identifying the mechanisms underlying synapse elimination that are regulated by androgen.
