The Role of Estrogen Receptors of Paragigantocellularis Lateralis Nucleus in the Formalin-Induced Pain Modulation in Female Rats

Since intra-paragigantocellularis lateralis (LPGi) injection of 17β-estradiol induces strong analgesia in male rats which might be mediated via estrogen receptors, this study investigated the role of estrogen receptors of LPGi nucleus in the modulation of formalin-induced pain (while being stimulated by 17β-estradiol) during estrus phase in female rats. In this experimental study, 48 female Wistar rats (200-270 gr) were randomly divided into 8 groups including control, sham, normal saline (17β-estradiol solvent), DMSO (ICI182,780 solvent), 17β-estradiol, estrogen receptor antagonist (ICI182,780; 15&50 nmol) and ICI182,780/17β-estradiol. After entrance into the estrus phase, drugs were injected into the LPGi nucleus; 15 minutes later, formalin injection (5%, 50 μl) was done into the left rat's hind paw and formalin-induced paw jerking behavior was recorded for 1 hr. Data were analyzed using one-way ANOVA analysis followed by Tukey multiple comparison test. The results of this research showed that during estrus phase, the treatment of LPGi nucleus with 17β-estradiol significantly attenuated paw jerking behavior in the both phases of formalin test (p<0.001). Pretreatment of the LPGi nucleus with ICI182,780 prevented the 17β-estradiol-induced antinociception (p<0.001). Based on these results, it can be suggested that intra-LPGi injection of 17β-estradiol induces robust analgesia during the estrus phase in female rats; and this effect may be mediated via estrogen receptors