[Formalin-induced minor tremor response as an indicator of pain]

Formalin which was said to produce prolonged pain and inflammation was injected subcutaneously into the back of guinea pigs, and minor tremor pain response (MTP-response) was measured using the MT-pick up, integrator and digital volt meter. The MTP-response curve showed a biphasic pattern. Immediately after injection, the MTP-response curve showed a significant peak which lasted for about 2 min (the first phase) and subsequently dipped rapidly, and after 5 min, it began to rise slowly again and had a peak at 30 min (the second phase). Morphine (6 mg/kg, s.c.) inhibited completely the first and second phases. Levallorphan (1.2 mg/kg), however, reversed the inhibitory effect of morphine at the first phase, but not at the second phase. Aspirin (200 mg/kg, i.p.), aminopyrine (100 mg/kg, s.c.) and pentazocine (5 mg-10 mg/kg, s.c.) inhibited significantly the formalin-induced MTP-response at both phases. Pyridinol carbamate (200 mg/kg, i.p.) and hydrocortisone (25 mg/kg, i.p.) had no effect on the MTP-response at the first phase, but inhibited it at the second phase. There was a parallelism between the time course of the vascular permeability induced by formalin and that of the second phase of MTP-response. From these results, it is suggested that the first phase of MTP-response is derived from the direct effect of formalin on free nerve endings, while the second phase is derived from the inflammation. Since two kinds of pain features were differentiated in this method, the relationships with so-called "immediate pain" and "delayed pain" were discussed. Furthermore, this method can be utilized to assess pain and the action of analgesics objectively and quantitatively.