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Background: Pain is an unpleasant feeling which humans experience. It is a warning sign of the damaged tissue. Due to the awful sense of pain, scientists always attempt to relieve it. Retinoic acid (RT), an active metabolite of natural vitamin A has important roles in modulation of the inflammatory responses. The aim of the present study was to analyze the pain threshold of rats which had microinjections of RT, applying acute and chronic models. Methods and Materials: In this study, the tail flick and formalin tests were used to determine pain threshold. In each test, the acute and chronic pain thresholds of 252 Wistar male rats (275 ± 25 gr) were assayed. The druge were injected in the acute model one-dose30 minutes before behavioral testing and in chronic model two-dose for one or two-weeks. The rats of both models divided randomly into six groups (n=7). In four treatment groups retinoic acid (RT) intra cerebro ventricular (i.C.V) were injected as dosagc of 0.5, 3 and 6 (µg/kg) micrograms per kilogram. In control group, was microinjected by ACSF. In vehicle group injected RT solvent (DMSO+ Distil water). Results: The resuits Showed acute injection of RT did not change pain thresholds in the tail-flick methd, but the chronic administration of RT (0.5, 1, 3, 6 µg/kg) reduced tail-flick latencies of the rats (p<0.05) in compare to DMSO group. The threshold of pain in the first phase of formalin test was reduced after injection of 3µg/kg of RT for two weeks. Conclusion: It was concluded that chronic i.c.v. injections of RT can induce significant hyperalgesia in rat.

Keywords: Pain, Retinoic Acid, Tail flick test, Formalin test, Intera cerebral ventricular

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