The role of GABAA receptors in the analgesic effect of intra-paragigantocellularis lateralis injection of 17B-estradiol in male rat

Introduction: 17B-Estradiol modulates nociception by binding to the estrogenic receptors and also by allosteric interaction with other membrane-bound receptors like glutamate and GABAA receptors. In addition to its autonomic functions, paragigantocellularis lateralis (LPGi) is involved in the pain modulation, too. The aim of the current study was to investigate the involvement of the membrane-bound GABAA receptors in the pain modulatory effect of intra- LPGi injection of 17B-estradiol of male rats. Materials and Methods: This study was performed using male Wistar rats in the range of 200-270 g. In order to investigate the antinociceptive effect of intra-LPGi microinjection of 17B-estradiol, cannulation into the LPGi nucleus was performed. 500 nl of drugs were administered 15 minutes prior to formalin injection (50 µl of 4%). Then, formalin-induced paw jerking behaviour was recorded for 60 min. For assessing the role of the GABAA receptors in the estradiol induced pain modulation, 17B-estradiol was administered into the LPGi nucleus 15 min after the injection of 25 ng/µl bicuculline; and paw jerking frequency was recorded for 1 h. Results: The results of the current study showed that intra-LPGi injection of 0.8 µmol of 17B-estradiol attenuated the chronic phase (P < 0.001) of paw jerking behaviour. Bicuculine -the GABAA receptor antagonist- significantly reduced the antinociceptive effect of intra-LPGi 17B-estradiol in the chronic phase (P < 0.001). Conclusion: It may be concluded that the analgesic effect of intra-LPGi injection of 17B-estradiol on the formalin-induced inflammatory pain might be mediated via GABAA receptors.