Abstract: Objective To investigate the role of peripheral N-methyl-D-aspartate (NMDA) receptors in the long-term hyperalgesia induced by peripheral injection of formalin in rats. Methods Formalin was injected in the paws of the SD rats with or without the MK-801 pre-injection. OX-42 expressions in the dorsal horn were observed, and the paw withdrawal thermal latency was measured. Results The OX-42 expression decreased and the paw withdrawal thermal latency prolonged in rats with MK-801 pre-injection. Conclusion Peripheral NMDA receptors may be involved in the long-term hyperalgesia.

Key words: long-term hyperalgesia, formalin test, microglia, N-methyl-D-aspartate (NMDA) receptors, MK-801, OX-42, rats