Pharmacological study of formalin-induced inflammatory pain response

Abstract

An injection of 25μEl of 0.5% formalin was made into the right hindpaws of mice. The times spent in licking or biting response were measured as indicators of pain response. The response curve was biphasic, having two peaks, from 0 to 5 min (first phase) and from 15 to 20 min (second phase). Using this method, the effects of various analgesic and anti-inflammatory drugs were assessed. Furthermore, the properties of this biphasic pattern and the mechanism of this pain response were investigated and the following results were obtained: 1) Centrally acting drugs such as morphine and pentazocine inhibited pain response at the first phase and the second phase equally. Naloxone and levallorphan reversed morphine analgesia completely. 2) Peripherally acting drugs, aspirin and hydrocortisone inhibited only the second phase significantly. Aminopyrine which acts on both central and peripheral sites inhibited both phases, but the inhibition was stronger in the second phase more than in the first phase. 3) From these results, it is suggested that pain response at the first phase is evoked by direct chemical stimulation of the nerve endings and the second phase is due to inflammatory reaction. Furthermore, this method enables to distinguish roughly the action site of analgesics between the central nervous system and the periphery. 4) The first phase was inhibited by capsaicin pretreatment for sciatic nerve and (D-Pro², D-Trp^{7, 9}) substance P (substance P antagonist) treatment. 5) Compound 48/80 pretreatment inhibited only the second phase significantly. 6) Des-Arg⁹-(Leu⁸)-bradykinin, a specific inhibitor of bradykinin, inhibited both first and second phases. 7) It was recognized that there was a higher correlation between the pain response and vascular permeability especially at the first phase. It is believed that this pain model is useful as a screening method not only for a assessing analgesic effect but for a explanation of the mechanism of analgesic action and for a classification according to the site of action as well.