Alterations of noradrenalin and one of its metabolites in the Locus Coeroleus nucleus in formalin induced pain in anesthetized rat

Introduction: Pain as a complex process in central nervous system (CNS) has been studied by many researchers. Pain is controlled by several CNS pathways, one of the most important of which, is the descending noradrenergic system. This system begins from locus coeruleus (LC) nucleus in pons and ends in the spinal cord. In this research, the effect of pain induced by formalin was studied. Methods: Male Sprague-Dawley rats weighing 280-320 g were categorized into two groups of control (injection of 50 μl normal saline) and test (injection of 50 μl 2.5% formalin). Rats were anesthetized by pentobarbital sodium (50 mg/kg i.p.). Microdialysis probes were inserted 24 hrs before the test was done. Rats were anesthetized by urethane and formalin test for induction of chemical and tonic pain was performed on the hind paw of the animals. Micro dialysis samples were taken in 15 minutes intervals and noradrenaline (NA) and its metabolite, 3-methoxy 4-hydroxy phenylglycol (MHPG), were measured by HPLC-ECD. Results: The NA and MHPG concentration in the first and second phases of formalin test did not change significantly in neither test nor control groups. Conclusion: LC has no role in perception of pain induced by formalin test during anesthesia.