Abstract: AIM: To evaluate the analgesic effect of agmatine on the model of formalin-evoked nociception;To investigate the changes in the expression of phosphorylated extracellular signal-regulated protein kinase (pERK) in the spinal dorsal horn of the formalin-induced inflammatory pain and the effect of agmatine on the pERK expression.METHODS: Male SD rats weighing 180-220 g were randomly classified into saline control group, formalin group, and agmatine group n=20.Rats were injected subcutaneously with 50 μL of 5% formalin solution into the plantar surface of the right hind paw.During the next 60 min, the weighted pain score was recorded in 5-min intervals for 12 rats group.Another 8 rats group were decapitated at 8 min after intraplantar injection and L_4,5 spinal cords were dissected and coronal sections were cut. Immunohistochemistry was used to observe the changes of pERK expression in the dorsal horn of spinal cord.RESULTS: Intraplantar injection of formalin produced a typical biphasic nociceptive response.Intraperitoneally pretreatment with agmatine inhibited the nociceptive and hyperalgesic response.Injection of 5% formalin subcutaneously in the hindpaw of rats induced an increase in pERK expression in neurons in the superficial dorsal horn on the ipsilateral side of the L_4,5 spinal cord.Pretreatment with 160 mg/kg agmatine significantly inhibited formalin-induced spinal pERK expression.CONCLUSION :Agmatine induces analgesia in the formalin-induced nociception.pERK activation may underlie spinal nociceptive processing and secondary hyperalgesia after formalin inflammation and may contribute to the analgesic effects of agmatine.

Key words: agmatine, analgesia, hyperalgesia, phosphorylated extracellular signal-regulated protein kinase