EFFECTS OF TESTOSTERONE ON SYNAPTIC EFFICACY AT NEUROMUSCULAR-JUNCTIONS IN A SEXUALLY DIMORPHIC MUSCLE OF MALE FROGS

1. The effects of testosterone on synaptic efficacy were studied in th e 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 be tween the elbow and wrist. These regions are mainly innervated by spin al nerve 2 (SN2) and spinal nerve 3 (SN3), respectively. 3. In CT musc les, the percentage of fibres that failed to generate an action potent ial in response to a single nerve stimulus was greater than in C muscl es. The percentage of such fibres was greater among SN2 fibres than am ong SN3 fibres. 4. The input resistance and membrane time constant wer e 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 i n 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 junctio ns than at SN3 junctions. The amplitude of MEPPs was lower in CT muscl es than in C muscles only for SN2 junctions. 6. Facilitation of transm itter release with 70 or 10 Hz nerve stimulation was greater at CT jun ctions than at C junctions. Also, SN2 junctions showed greater facilit ation than SN3 junctions. 7. We hypothesize that the more androgen-sen sitive SN2 junctions, which have lower quantal content and greater fac ilitation, belong to motor units that are tonically active, fatigue re sistant, and contract more slowly. The less androgen-sensitive SN3 jun ctions, which have higher quantal content and less facilitation, may b elong to motor units that are phasically active, fatigable and contrac t more rapidly. Testosterone enhances these differences such that flex or carpi radialis junctions are better adapted to;the functional deman ds of clasping.