A chemotherapy drug, oxaliplatin, induces cold and mechanical hypersensitivity, but effective treatments for this neuropathic pain without side effects are still lacking. We previously showed that Cinnamomi Cortex suppresses oxaliplatin-induced pain behaviors in rats. However, it remains unknown which phytochemical of Cinnamomi Cortex plays a key role in that analgesic action. Thus, here we investigated whether and how cinnamic acid or cinnamaldehyde, major components of Cinnamomi Cortex, alleviates cold and mechanical allodynia induced by a single oxaliplatin injection (6 mg/kg, i.p.) in rats. Using an acetone test and the von Frey test for measuring cold and mechanical allodynia, respectively, we found that administration of cinnamic acid, but not cinnamaldehyde, at doses of 10, 20 and 40 mg/kg (i.p.) significantly attenuates the allodynic behaviors in oxaliplatin-injected rats with the strongest effect being observed at 20 mg/kg. Our in vivo extracellular recordings also showed that cinnamic acid (20 mg/kg, i.p.) inhibits the increased activities of spinal wide dynamic range neurons in response to cutaneous mechanical and cold stimuli following the oxaliplatin injection. These results indicate that cinnamic acid has an effective analgesic action against oxaliplatin-induced neuropathic pain through inhibiting spinal pain transmission, suggesting its crucial role in mediating the effect of Cinnamomi Cortex.
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