Utilizing the flexor response to intra-arterial bradykinin in rats, as a pain-response, the activity of certain analgesics has been assessed. By increasing the intensity of the chemo-algesic (bradykinin) stimulus and measuring this against the blocking effect of a constant dose of certain analgesics, an attempt was made to assess analgesic activity quantitatively. Representatives of peripherally and centrally acting analgesics were employed. It has been shown in the rat: 1. (1) that a similar maximal analgesia is attained for all the substances tested, at non-central depressant doses excepting for morphine which was superior. Depth of analgesia could be increased (excepting with aspirin) but only at dose-levels showing central depression. Possible mechanisms and sites of action to explain these phenomena are discussed; 2. (2) that a supra-additive effect and increased duration of action result from combining phenylbutazone with paracetamol (1: 10). It is suggested that the method is sensitive and useful for quantitating analgesia, may distinguish central from peripherally-acting analgesics, and gives reproducible results. © 1969.
