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Department of Biological Sciences, University of Southern California, Los Angeles 90089-2520, USA.

1. The effects of testosterone on synaptic efficacy were studied in the flexor carpi radialis, a sexually dimorphic forelimb muscle involved in frog clasping behaviour. Male Xenopus laevis frogs were castrated and then given either testosterone-filled implants (CT frogs) or empty implants (C frogs) for 2, 8 or 16 weeks. 2. Intracellular recordings were made from fibres in the shoulder region and in a region midway between the elbow and wrist. These regions are mainly innervated by spinal nerve 2 (SN2) and spinal nerve 3 (SN3), respectively. 3. In CT muscles, the percentage of fibres that failed to generate an action potential in response to a single nerve stimulus was greater than in C muscles. The percentage of such fibres was greater among SN2 fibres than among SN3 fibres. 4. The input resistance and membrane time constant were lower in CT muscles than in C muscles for SN2 fibres but not for SN3 fibres. The action potential threshold was lower in CT muscles than in C muscles. In SN2 fibres, the action potential threshold was higher than in SN3 fibres. 5. Quantal content with 1 Hz nerve stimulation and miniature endplate potential (MEPP) frequency did not differ between CT and C junctions. However, both parameters were lower at SN2 junctions than at SN3 junctions. The amplitude of MEPPs was lower in CT muscles than in C muscles only for SN2 junctions. 6. Facilitation of transmitter release with 70 or 10 Hz nerve stimulation was greater at CT junctions than at C junctions. Also, SN2 junctions showed greater facilitation than SN3 junctions. 7. We hypothesize that the more androgen-sensitive SN2 junctions, which have lower quantal content and greater facilitation, belong to motor units that are tonically active, fatigue resistant, and contract more slowly. The less androgen-sensitive SN3 junctions, which have higher quantal content and less facilitation, may belong to motor units that are phasically active, fatigable and contract more rapidly. Testosterone enhances these differences such that flexor carpi radialis junctions are better adapted to the functional demands of clasping.

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