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.