Abstract: Objective To investigate the regulation of suprachiasmatic nucleus (SCN) on the inflammatory pain in mice at behavioral level. Methods Wheel running of circadian behavioral monitor system, the surgery of SCN lesion and the formalin induced nociceptive responses test were employed to analyze if the synchronize and/or maintain circadian function of SCN could modulate the rhythm of inflammatory pain induced by formalin in mice. Results Our results showed that the formalin induced inflammatory nociceptive responses were shown a circadian oscillation pattern in resting phase (ZT4) and active phase (ZT20), and after SCN lesion, the mice were shown an overturned pattern of nociceptive responses between ZT4 and ZT20 compared with normal control. Conclusions The circadian synchronized function is an important regulator for maintaining normal daily fluctuation patterns of inflammatory pain induced by formalin.

Key words: pain, formalin, inflammatory pain, SCN, circadian rhythm