Student Theses and Dissertations

Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Androgen-sensitive muscles are specialized muscles in which both muscle and motoneurons are highly sensitive to androgens. Previous studies have shown that alterations in the levels of circulating androgens affect the activities of cholinergic enzymes in two androgen-sensitive muscles; the syringeal muscles of songbirds and the levator ani muscle of the rat. This study sought to characterize the nature of the effects of androgens on the neuromuscular junctions in these muscles.

I measured the relative abundance of the various molecular forms of acetylcholinesterase (AChE) in muscles of the zebra finch. Castration causes a large decrease in AChE activity but has little or no effect on the relative abundance of AChE forms. This is in marked contrast to the effect of denervation, which causes changes in the relative abundance of the forms of AChE, as well as in total activity.

To quantify the postsynaptic effects of androgens, I measured acetylcholine receptor number (AChRs). AChR number was androgen-sensitive in both the syrinx and levator ani, decreasing after castration of males, and increasing in response to treatment with testosterone.

I performed a detailed examination of the time course of the effect of alterations in androgens was examined in detail in the syrinx. I also examined the effect of increased androgens in syrinx isolated from neural influences by denervation and found that isolated muscle could respond with a small increase in AChE activity.

Since the number of fibers in the levator ani does not change after castration or testosterone treatment, and each fiber has a single endplate, changes in AChR number in this muscle are strong evidence for the effects of androgens on endplates. In the syrinx, no evidence of sex differences in the number of muscle fibers was seen. However, some fibers have multiple endplates. Therefore, changing hormone levels may cause changes in the number of endplates per fiber, as well as in the number of AChRs per endplate. An examination of individual endplates in these muscles visualized histochemically indicates that androgens exert at least part of their effects on synaptic components by causing alterations at individual endplates. Androgen-sensitive neuromuscular junctions such as these may provide powerful systems in which to study the mechanism and adaptive significance of synaptic plasticity in vertebrates.