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.
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