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Background: Paragigantocellularis lateralis (LPGi) nucleus plays a key role in the processing of pain information related to the descending pain modulation. Also, 17β-estradiol is involved in the pain modulation. The aim of the present study was to investigate the role of estrogen receptors of LPGi nucleus in the 17β-estradiol-induced pain modulation in the ovariectomized rats. Materials and Methods: In this study, the female Wistar rats (180-250 gr) were used. In order to study the role of the NMDA receptors in the 17β-estradiol-induced pain modulation in the ovariectomized rats, primarily, rats were bilaterally ovariectomized and immediately cannulation of LPGi nucleus was performed. First, drugs were injected and 15 minutes later 50 μl of 5% formalin was injected into the rat's hind paw; and then, formalin-induced paw jerking behavior was recorded for 60 min. Results: Our results indicated that intra-LPGi injection of 17β-estradiol significantly attenuated paw jerking behavior in the both phases of formalin test. Intra-LPGi injection of estrogen receptor antagonist (ICI 182,780), had no effect on the paw jerking behavior. Pre-treatment of LPGi nucleus by ICI 182,780 counteracted the anti-nociceptive effect of 17β-estradiol both in the acute and in the chronic phases of formalin-induced paw jerking behaviour. Conclusion: Based on the results of this study, it can be concluded that the intra-LPGi 17βestradiol produces modest analgesia on the formalin-induced inflammatory pain in the ovariectomized rats, which is probably mediated via the esterogen receptors of this nucleus. [ABSTRACT FROM AUTHOR]

Background: 17β-Estradiol modulates nociception by binding to the estrogen receptors and also by allosteric interaction with other membrane-bound receptors like the NMDA receptors. The paragigantocellularis lateralis nucleus (LPGi) is also involved in the pain modulation. In this study, the role of NMDA receptors of the LPGi nucleus has been investigated in the 17β-estradiol-induced pain modulation in the ovariectomized rats. Materials and Methods: In this study, the female Wistar rats in the range of 200-270 gr were used. In order to study the role of the NMDA receptors in the 17β-estradiol-induced pain modulation in the ovariectomized rats, primarily, rats were bilaterally ovariectomized and immediately cannulation of the LPGi nucleus was performed. Then, drugs were injected and 15 minutes later 50 μl of 5% formalin was injected into the rat's hind paw; and formalin-induced paw jerking behaviour was recorded for 60 min. Results: The results of the present study showed that the intra-LPGi injection of 17β-estradiol significantly reduced the paw jerking behavior both in the first and in the second phases of formalin test. Pretreatment of the LPGi nucleus by NMDA receptor antagonist (AP5) neutralized the antinociceptive effect of 17β-estradiol on the paw jerking frequency in the both phases of formalin test; and induced hyperalgesia in the both phases of this behavior. Conclusion: These results indicated that the intra-LPGi injection of 17β-estradiol produces modest analgesia on the formalin-induced inflammatory pain. Therefore, it can be concluded that the NMDA receptors of the paragigantocellularis lateralis nucleus are probably involved in the antinociceptive effect of 17β-estradiol in the ovariectomized rats. [ABSTRACT FROM AUTHOR]