OBJECTIVETo investigate the role of heme oxygenase and carbon monoxide (HO/CO) in the development of spontaneous pain and hyperalgesia of rats induced by formalin injection.

METHODSZinc protoporphyrin Znpp (the inhibitor of HO) was intrathecally injected to the rats with formalin inflammatory pain. Hemin (the agonist of HO) was intrathecally injected to the normal rats. The weighted pain scores were used to evaluate the degree of pain response. Thermal withdrawal latency and mechanical withdrawal threshold were observed to assess the degree of thermal hyperalgesia and mechanical allodynia.

RESULTSAfter the intrathecal injection of Znpp, the weighted pain score obviously reduced in a dose-dependent manner compared with the rats with formalin inflammatory pain. Intrathecal injection of Znpp had no obvious effect on thermal withdrawal latency and mechanical withdrawal threshold in injected feet compared with formalin group. But there was a prolongation in a dose-dependent manner in non injected feet. Intrathecal injection of Hemin to normal rats could shorten the thermal withdrawal latency and reduce the mechanical withdrawal threshold on both sides of hindpaws.

CONCLUSIONIntrathecal injection of the HO inhibitor produced prominent inhibition to pain related behavior and thermal and mechanical hyperalgesia induced by formalin injection. Intrathecal injection of HO inductor could induce thermal and mechanical hyperalgesia in normal rats. The results indicated that HO/CO took part in the processes of spinal cord nociceptive information transmission and the development of thermal and mechanical hyperalgesia.