Excitatory effects of 5-hydroxytryptamine, histamine and potassium ions on muscular group IV afferent units: a comparison with bradykinin.

The excitation of muscular group IV afferent units by the pain-producing substances 5-hydroxytryptamine (5-HT), histamine, and potassium ions was studied in cats and compared with the bradykinin effects of an earlier investigation. The substances were injected into the sural artery and the action potentials of single group IV afferent fibres from the gastrocnemius-soleus muscle were recorded extracellularly. About half of the 180 units tested with chemical stimulation could be excited by amounts of the above agents which are likely to be present in pathologically altered tissues. Of the tested substances, 5-HT was the most and potassium the least effective stimulant; in comparison with bradykinin, 5-HT was by far less potent, though. The molar ratios of approximately equivalent doses of bradykinin, 5-HT, histamine, and potassium were found to be about 1:30:66:4000. The threshold injection doses of KC1 for activation of muscular group IV units were such that an unspecific depolarizaing mode of action seems probable. Differences in the chemosensitivity of individual group IV units might be indicative of the existence of different receptor sites at the nerve endings. The sensitivity of muscle receptors with group IV afferent fibres to pain-producing substances--especially to bradykinin and 5-HT--supports the view that these units might participate in the mediation of muscle pain.