The role of gonadal hormones in neuromuscular synapse elimination in rats. I. Androgen delays the loss of multiple innervation in the levator ani muscle.

The normal period of synapse elimination in the androgen-sensitive levator ani (LA) muscle occurs between 2 and 4 weeks after birth, well after the period of synapse elimination for most other rat muscles. To evaluate whether gonadal androgen might be involved in the delayed development of single innervation in the LA, we compared the time course of synapse elimination in LA muscles that lacked endogenous gonadal androgen or were exposed to exogenous androgen. Tetranitroblue tetrazolium was used to stain neuromuscular connections. Our results suggest that both endogenous and exogenous androgen delay the normal process of synapse elimination. Removing endogenous androgen resulted in lower levels of multiple innervation in the LA, suggesting that androgen may normally influence synapse elimination. Moreover, androgen treatment prevented much of the normal loss of multiple innervation in the LA. Androgen treatment during the normal period of synapse elimination also increased the diameter of LA muscle fibers, enhanced the development of preterminal branching, and increased the number of junctional sites on some LA fibers. Because androgen did not appear to induce the formation of new synapses through sprouting, we conclude that androgen maintained multiple innervation in the LA by preventing the normal ontogenetic process of synapse elimination.