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Introduction: Previously, we demonstrated that i.p. injection of aqueous extract prepared from aerial parts of Drosera Spatulata (Droseraceae) can induced remarkable analgesia in both phases of formalin test in rats. Because, analgesia induced in acute phase of formalin test mainly mediated by activation of central analgesic mechanisms and also paragigantocellularis (PGi) nucleus is part of brain descending pain control systems the role of this nucleus in analgesic action of the extract on acute pain studied by its effect on PGi neural activity that augmented by injection of formalin to contralateral hind paw of anesthetized rats. Methods: Firing rate of PGi neurons by extracellular single unit recording technique were recorded in 20 urethane anesthetized (1.2- 1.5 g/kg, i.p.) male rats in control, Drosera extract (0.05 mg/kg, i.p.) treated and sodium salicylate (300 mg/kg, i.p.) receiving groups. Formalin (50µl, 2%, s.c.) injected 30 min after each of the above treatments to intraplantar surface of hind paw of rats and change in response of opposite PGi neurons was recorded. Data was analyzed by Wilcoxon ranking test. Results: Extract injection increased firing rates in 66.6% of PGi neurons and decreased 16.6% of neurons (p<0.001), but had no effect on 16.6% of neurons in formalin injected rats. Sodium salicylate treatment decreased significantly firing rates in 50% PGi neurons in formalin pretreated rats (p<0.001). Conclusion: The relation between PGi neuronal augmentation and analgesia was reported previously. Thus, the augmentations of firing rates in high percent of PGi neurons after the extract injection provide a document about the role of PGi nucleus in reduction of pain in acute phase of formalin test.

Keywords: Aqueous extract of Drosera Spatulata- Formalin induced pain- Paragigantocellularis nucleus- Single unit recording- Rat

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