Excitatory and modulatory effects of inflammatory cytokines and neurotrophins on mechanosensitive group IV muscle afferents in the rat

In inflamed tissue--including skeletal muscle--the concentrations of cytokines and neurotrophins are known to increase. However, nothing is known about a possible contribution of these agents to muscle pain and hyperalgesia. The present study investigated acute effects of cytokines and neurotrophins on response properties of slowly conducting muscle afferents. In anaesthetised rats, the impulse activity of single mechanosensitive group IV fibres innervating the gastrocnemius-soleus muscle was recorded and tumour necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), nerve growth factor (NGF), or brain-derived neurotrophic factor (BDNF) were injected into the muscle. Changes in the mechanosensibility of the endings following administration of the agents were tested with repeated pressure stimuli of defined forces. A low mechanical threshold in the innocuous range was found in 44.4% of the units tested, 55.6% required strong, potentially tissue-damaging pressure stimuli for activation. NGF excited only units that had a high mechanical threshold, while IL-6 was a stimulant for low-threshold mechanosensitive units only. TNF-alpha and BDNF did not excite group IV units but had a desensitising action: after TNF-alpha or BDNF, the response magnitudes to pressure stimuli decreased significantly. The data indicate that cytokines and neurotrophins influence the impulse activity and mechanosensitivity of group IV muscle afferent units. These effects could be of functional significance when the agents are released from muscle cells under pathophysiological circumstances.