Document Type: Original Article


Department of Physiology, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran



Several studies have demonstrated that nitric oxide (NO) is involved in hyperalgesia induced by NMDA activity in Morphine tolerance and NO synthase inhibitors can attenuate hyperalgesia in morphine tolerance. In addition, one of the possible mechanisms in morphine tolerance is reduction of GABA inhibitory effect. For this reason, we used GABA agonists (muscimol and baclofen) to investigate the effect of GABA agonists on analgesic effect of morphine and NO level of serum. In this study, experimental rats were divided into 4 groups as follows: 1. Control group, 2. Morphine tolerance group, 3. Morphine tolerance + muscimol, and 4-morphine tolerance + baclofen. To induce morphine tolerance in rats, the animals received 10 mg/kg of morphine sulfate intraperitoneal (ip) once a day for 8 days. In the treatment group, GABA agonist was injected on the 1st, 3rd, 5th, and 8th day before morphine injection. Finally, to evaluate GABA agonists treatment on morphine analgesia and hyperalgesia on the 8th day of the study, thermal hyperalgesia was used to evaluate tolerance and hyperalgesia. Then, NO level of serum was investigated as an important factor in OIH. The results indicated that GABA agonists reduced thermal hyperalgesia in the morphine tolerance group (p


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