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The songbird syrinx is sexually dimorphic and responds to changes in blood testosterone levels with changes in muscle size and in activity of cholinergic enzymes (Luine, V., F. Nottebohm, C. Harding, and B.S. McEwen (1980) Brain Res. 192: 89-107). Here, we demonstrate that there is a sex difference in the number of acetylcholine receptors (AChRs) in the syrinx and that alterations in the levels of circulating testosterone can cause changes in AChR number in syringeal muscles. These results suggest that the size or number of endplates in the syringeal muscles may be increased by circulating testosterone. The time course of these effects was examined after increases in blood testosterone in females and decreases in males. We also examined the effect of increased testosterone in syrinx isolated from neural influences by denervation and found that denervated muscle responded to testosterone with an increase in acetylcholinesterase activity but not with increases in protein content or AChR number.

The dorsal bulbocavernosus or "levator ani" muscle of the rat is highly responsive to androgens. Both the muscle and the motoneurons which innervate it contain high concentrations of androgen receptors. The neuromuscular synapses in this muscle are also affected by changing androgen levels. In particular, the total number of ACh receptors (AChRs) in the muscle is lower in males that have been castrated, and it increases after treatment with the androgens, testosterone and 5 alpha-dihydrotestosterone. An examination of individual endplates using histochemistry and quantitative autoradiography suggested that the reduction in AChR number following castration is caused by reductions in both the size of endplates and in the density of AChRs at each synapse.

The "levator ani" muscle of male rats provides a neuromuscular system in which both the muscle and its motoneurons have high levels of androgen receptors. Two weeks of castration caused a 48% loss of acetylcholine receptors in this muscle. One week of testosterone propionate injections initiated on week after castration increased receptor number by 27% over untreated castrate levels. These changes paralleled changes in muscle protein content. In contrast, castration and testosterone treatments of castrates had no effect on total. Triton X-100-extractable acetylcholinesterase activity. This system may provide a useful model of synaptic plasticity.

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