Introduction. Carbon monoxide (CO) is a colorless, odorless, and tasteless gas. It is toxic to humans and animals when encountered in higher concentrations, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal biological function. Main source of this gas in body is a reaction catalyzed by heme oxygenas enzyme. Hemoglobin is known as a potent inducer for heme oxygenas and causes exhaustive protection in lethal endotoxemia, on the other hand, the injection of carbon monoxide releasing molecules (CORMs), causes reduction in neuropathic pain sensation. So it would be predictable that intrathecal injection (i.t.) of hemoglobin may increases the amount of carbon monoxide and reduce pain sensation. Materials and Methods. Male Wistar rat (200-250 g) were randomly divided into 3 groups: I. Control with i.t. injection of saline. II. One i.t. treatment of hemoglobin (10µg/10 µl). III. Daily treatments of hemoglobin (10µg/10 µl) for 5 constitutive days. Pain behaviors was assessed using hind paw subplantar injection of 5ml formalin (2.5%) for one hour. Results. Results were shown a significant decrease on pain sensation produced by acute i.t. treatment of hemoglobin both in the first and second phase of the formalin test (p<0.01). Same observation was repeated after chronic treatment of hemoglobin also (P<0.001) which was more potent than acute one (p<0.01). Conclusion. Results of this experiment showed us that hemoglobin reduces pain sensation through increasing the level of CO in spinal cord tissue. This effect was higher in chronic group that is possibly due to effect of hemoglobin on the heme oxygenas gene expression.

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