Background and purpose: The paragigantocellularis lateralis nucleus (LPGi) is one of the important nuclei involved in the noradrenergic descending pain modulation. 17β-Estradiol modulates nociception by binding to the estrogen receptors and also by allosteric interaction with other membrane-bound receptors like the glutamate receptors. In this study, the role of AMPA receptors of the LPGi nucleus was investigated in the 17β-estradiol-induced pain modulation in ovariectomized rats.
Materials and methods: In this study, female Wistar rats weighing 200-270 gr were used. To investigate the role of the AMPA receptors of the LPGi nucleus, in both acute and persistent pain modulation, cannulation of the LPGi nucleus was performed. Primarily, drugs were injected and 15 minutes later 50 μl of 5% formalin was injected into the rat's hind paw. Then, the paw jerking behavior was recorded for 60 min.
Results: The results showed that intra-LPGi injection of 17β-estradiol significantly reduced the paw jerking behavior in both phases of formalin test (P< 0.01). Pretreatment of LPGi nucleus by AMPA receptor antagonist (CNQX) neutralized the antinociceptive effect of 17β-estradiol on paw jerking frequency in both phases of formalin test (P< 0.001), and even, the CNQX induced hyperalgesia in both phases of paw jerking behavior.
Conclusion: According to this study, intra-LPGi 17β-estradiol produces modest analgesia on formalin-induced pain in ovariectomized female rats, which is probably mediated via the AMPA receptors of this nucleus.