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Introduction: 17β-estradiol modulates nociception by binding to estrogenic receptors and also by allosteric interaction with other membrane-bound receptors like glutamate and GABAA receptors. Beside its autonomic functions, paragigantocellularis lateralis (LPGi) nucleus is also involved in pain modulation. The aim of the current study was to investigate the role of the intracellular estrogenic receptors in the pain modulation by the LPGi nucleus of male rats. Methods: In this study, male Wistar rats in the range of 200-270 gr were used. In order to study the effect of intra- LPGi microinjection of 17β-estradiol on both acute and persistent pain modulation, cannulation of LPGi nucleus was performed. At first, drugs were injected and 15 minutes later 50 μl of 4% formalin was injected into the rat's hind paw. Then formalin-induced flexing and licking behaviours were recorded for 60 min. Results: The results of current study showed that intra-LPGi injection of 17β-estradiol attenuated the flexing and the licking behaviours both in the first phase (P<0.01) and in the second phase (P<0.001) of formalin test. The estrogen receptor antagonist (ICI182,780) prevented 17β-estradiol-induced analgesic effect but could not reverse this effect to the control condition, and it had significant difference with the control group, yet. Conclusion: It may be concluded some part of the analgesic effect of 17β-estradiol in the LPGi nucleus on the formalin-induced inflammatory pain is probably mediated by estrogenic receptors.

Keywords: 17-β-estradiol, paragigantocellularis lateralis nucleus, ICI182, 780, pain modulation, Rat

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