Androgen regulation of neuromuscular junction structure and function in a sexually dimorphic muscle of the frog Xenopus laevis.

Specific forelimb muscles in anurans are sexually dimorphic and underlie the androgen-dependent clasping response of males during amplexus. Previous studies have reported that androgen treatment slows the contractile properties of these sexually dimorphic forelimb muscles. In amphibians, the expression of functionally distinct acetylcholine (ACh) receptors, the levels of acetylcholinesterase (AChE), the extent of multiple innervation, and the structure of individual end plates vary with the contractile properties of the muscle fibers. In higher vertebrates, androgens have been reported to alter the expression of ACh receptors, AChE, and the neuromodulator, calcitonin gene-related peptide (CGRP). To determine whether the known androgen-dependent changes in contraction of androgen-sensitive forelimb muscles are accompanied by concomitant changes in synaptic structure or function, we have compared functional neuromuscular transmission, the pattern of innervation, and CGRP immunoreactivity in nerve or muscle preparations from castrated (C) and castrated and testosterone-treated (CT) adult male Xenopus laevis. CGRP expression in androgen receptor (AR)-immunopositive neurons was increased in CT animals. However, no significant differences were found in ACh-mediated single channel or macroscopic currents, the extent of multiple end plates, or end plate morphology for forelimb fibers isolated from C and CT Xenopus. In contrast, analysis of forelimb fibers from gonadally intact adult females and juvenile animals of both sexes revealed that macroscopic synaptic currents were significantly shorter in these animals than in either C or CT adult males. Our data suggest that forelimb fibers in sexually dimorphic muscles of Xenopus do show significant differences in synaptic transmission; however, neither end-plate organization nor functional neuromuscular transmission are subject to activational effects of androgens in adult male frogs.