Effect of Specific Lesion of Non Serotonergic Pathway on Neurons of Nucleus Raphe Magnus Morphology in Rat

Purpose: The nucleus raphe magnus (NRM) is a medullary nucleus containing serotonergic and non serotonergic neurons, both of which densely project to spinal cord. The goal of this study was to determine the role of these non serotonergic neurons in pain perception and their cytological changes after the specific lesion of bulbo-spinal serotonergic pathway. Materials and Methods: Male rats were intrathecally administered 5,6 Dihydroxytryptamine (5,6 DHT). Then animals were perfused 5, 14 and 30 days after the injection of neurotoxin or vehicle. Serial microtom sections (5-8 microns) were Nissl Stained, and the cytological changes of neurons were evaluated by light microscope. Results: The results revealed that the number of all three types of NRM neurons was significantly reduced following the intrathecal injection of 5, 6 DHT. The reduction was not recovered within one month, although; the pain perception returned to normal level. In addition, a significant enlargement (increase in long axis) was also observed in NRM neurons after the chemical lesions of descending 5-HT pathways (p<0.001). Conclusion: It is concluded that no evident recovery will be seen in serotonergic pathways after their select in destruction, although the pain perception becomes normal after one month. Normal threshold of pain indicates a probable function adaptation of other transmission systems, which compensates for the loss of serotonergic neurons.