The role of capsaicin-sensitive muscle afferents in fatigue-induced modulation of the monosynaptic reflex in the rat

1. The role of group III and IV afferent fibres of the lateral gastrocnemio us muscle (LG) in modulating the homonymous monosynaptic reflex was investi gated during muscle fatigue in spinalized rats. 2. Muscle fatigue was induced by a series of increasing tetanic electrical stimuli (85 Hz, 600 ms) delivered to the LG muscle nerve. Series consisted of increasing train numbers from 1 to 60. 3. Potentials from the spinal cord LG motor pool and from the ventral root were recorded in response to proprioceptive afferent stimulation and analys ed before and during tetanic muscle activations. Both the pre- and postsyna ptic waves showed an initial enhancement and, after a '12-train' series, an increasing inhibition. 4. The enhancement of the responses to muscle fatiguing stimulation disappe ared after L3-L6 dorsal root section, while a partial reflex inhibition was still present. Conversely, after section of the corresponding ventral root , there was only a reduction in the inhibitory effect. 5. The monosynaptic reflex was also studied in animals in which a large num ber of group III and IV muscle afferents were eliminated by injecting capsa icin (10 mM) into the LG: muscle. As a result of capsaicin treatment, the f atigue-induced inhibition of the pre- and postsynaptic waves disappeared, w hile the response enhancement remained. 6. We concluded that the monosynaptic reflex inhibition, but not the enhanc ement, was mediated by those group III and IV muscle afferents that are sen sitive to the toxic action of capsaicin. The afferents that are responsible for the response enhancement enter the spinal cord through the dorsal root , while those responsible for the inhibition enter the spinal cord through both the ventral and dorsal roots.