The Role of the Estrogen Receptors of Paragigantocellularis Lateralis Nucleus in the Antinociceptive Effect of 17β-Estradiol during Proestrus Phase of Estrous Cycle of Female Rats

Intra-paragigantocellularis lateralis (LPGi) injection of 17β-estradiol produces robust antinociceptive effect on the inflammatory pain in the both male and ovariectomized female rats which is possibly mediated through estrogen receptors of this nucleus. This study aimed to examine the role of estrogen receptors in the pain modulatory effect of 17β-estradiol during proestrus phase of female rats. In this study, the female Wistar rats in the range of 200-270 gr were used. For studying the influence of intra-LPGi injection of 17β-estradiol on the acute inflammatory pain modulation, cannulation into the LPGi nucleus was performed after entrance into the proestrus cycle. After entrance in the proestrus phase once again, drugs were injected and 15 minutes later, formalin was injected into the rat's hind paw. Then, formalin-induced paw jerking behavior was recorded for 60 min. This study with research ethics code IR.TBZMED.VCR.REC.1397.385 has been approved by research ethics committee at Tabriz University of Medical Sciences. The results of this study showed that intra-LPGi injection of 17β-estradiol during proestrus phase significantly attenuated paw jerking frequency both in the first (p<0.01) and in the second (p<0.001) phases of formalin test. Pretreatment of the LPGi nucleus with estrogen receptor antagonist (ICI182,780) neutralized the 17β-estradiol-induced analgesia. Our results indicated that intra-LPGi injection of 17β-estradiol induces robust analgesia on the inflammatory pain during the proestrus phase. Thus, it can be concluded that the antinociceptive effect of 17β-estradiol is probably mediated via estrogen receptors.