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Introduction: Pain as a complex process in the CNS has been extensively studied by many researchers. It has been found that pain is controlled by certain pathways in the CNS, one of most important of which is the descending noradrenergic system. The pathway begins with Locus Coeruleus (LC) nucleus and ends in the spinal cord. In this study, formalin test was used as a chemical and tonic pain test to determine the role of LC in modulating pain in this pathway. Methods: 30 Sprague-Dawley rats with mean±SD weight of 280±30 g were divided into 6 groups (3 test and 3 control groups). In the test groups, 0.5 μL lidocaine was injected in the left LC (group 1), right LC (group 2) or both LC (group 3) to make local anesthesia. Half μL of normal saline was injected to controls (groups 4–6). After 15 min of injection, 50 μL of 2.5% of formalin was injected SQ to the left hind paw of rats and the level of nociception was recorded every 15 sec for one hour. Results: The induction of unilateral pain (right hind paw) in rats, can affect not only the contralateral but also the ipsilateral LC nucleus. Conclusion: The left and right LC nuclei have significant role on unidirectional nociception in formalin test in rats. The ipsilateral LC, however, has a minor effect on nociception.

Keywords: Pain, Formalin test, Locus Coeruleus, laterality, rat

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