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Introduction: In female rats sensitivity to antinociceptive treatment varies during estrous cycle stages. The role of GABA through GABAB receptor in nociception has been established. The aim of the present study is to investigate the effect of intercerebroventicular injection of GABAB receptor agonist (baclofen) and GABAB antagonist (CGP35348) on pain sensitivity during different stages of estrous cycle. Methods: Forty adult female rats weighing 200-220g were used. Rats were maintained on 12 h reversed light/dark cycle, and standard temperature 22 ± 2°C. Food and water were available ad libitum. Pain sensitivity was evaluated by formalin test, performing by subcutaneous injection of 50µl formalin solution (2.5%) into the hind paw in each stages of estrous cycle. Data were analyzed by two ways ANOVA measuring and Tucky test as post-hoc test. The level of significance was P<0.05. Results: Our data showed that baclofen significantly decreased pain sensitivity in all stages of estrous cycle (P<0.05). The analgesic effect of baclofen was significantly higher during metestruse and diestruse as compared to proestruse and estrus (P<0.05). CGP35348 significantly increased pain sensitivity in all stages of estrous cycle (P<0.05). The hyperalgesic effect of CGP35348 was significantly lower during metestruse and diestruse than proestruse and estrus (P<0.05). Administration of baclofen in pretreated rats with CGP35348 did not induce any significant change in pain sensitivity (P>0.05). Conclusion: We have demonstrated that GABA through GABAB receptor can modulate pain sensitivity during estrous cycle.

Keywords: GABA; Pain; Baclofen; CGP35348; Estrous cycle

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