Differential alternation of the antinociceptive effect of narcotic analgesics on the inflammatory pain state

- Mechanical allodynia is observed in adjuvant-induced inflammatory pain state.
- Antinociception of narcotic analgesics against mechanical allodynia was measured.
- Antinociception of fentanyl was reduced bilaterally in inflammatory pain state.
- Antinociception of oxycodone was reduced bilaterally in inflammatory pain state.
- Antinociception of methadone was reduced unilaterally in inflammatory pain state.

Antinociceptive effect of narcotic analgesics, fentanyl, oxycodone and methadone in inflammatory pain state was described in the von Frey filament test using the complete Freund's adjuvant (CFA)-induced mouse inflammatory pain model. After the i.pl. injection of CFA, mechanical allodynia (decrease of mechanical threshold) was observed in ipsilateral paw. The antinociceptive effect of fentanyl and oxycodone injected s.c. against mechanical allodynia in inflammatory pain state was reduced bilaterally at 1 day after CFA pretreatment. However, the antinociceptive effect of methadone injected s.c. against mechanical allodynia in inflammatory pain state was reduced unilaterally at 1 day after CFA pretreatment. Moreover, the reduction of the antinociceptive effect of methadone in ipsilateral paw on inflammatory pain state was smaller than those of fentanyl or oxycodone. In conclusion, the alternation of the antinociceptive effect of narcotic analgesics in inflammatory pain state is variable among the narcotic analgesics used.