Reduction of the bradykinin-induced activation of feline group III and IV muscle receptors by acetylsalicylic acid.

1. In chloralose-anaesthetized cats, the influence of systemically or locally applied acetylsalicylic acid (ASA) on the responses of thin-fibre muscle receptors to close-arterial injections of bradykinin was studied. 2. Many of the slowly conducting (group III and IV) muscle afferents had a background activity of low frequency. This discharge was either unaffected or slightly increased by the ASA doses used. In two units which had a very high discharge rate ASA led to a marked decrease in background activity. 3. On local (I.A. or I.M.) injection of ASA, doses below 1 mg were sufficient for reducing the bradykinin-induced activations of group III and IV muscle receptors. The reduction lasted for about 15-30 min. 4. On systemic (I.V.) administration of ASA (50 mg/kg body weight) the reduction in response magnitude to bradykinin became significant 8 min after injection of the analgesic. The effect was maximal about 10 min later and lasted for more than 60 min. 5. Five receptors were found which gave a repeated response to 5-hydroxytryptamine (5-HT) injected at 10 min intervals. The 5-HT-induced activations could not be reduced by ASA (50 mg/kg I.V.). 6. Most of the receptors responding to bradykinin had a high threshold on mechanical stimulation and thus were probably nociceptors. It is concluded that the reduction of their bradykinin-induced activations reflects the suppression of nociceptive information by an analgesic. Since the recordings were obtained from primary afferent units the data constitute direct evidence for a peripheral action of ASA.