Estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta) mRNAs are both expressed in rat dorsal root ganglion (DRG) neurons, but the distribution of these two mRNAs differs markedly. Radiolabeled probes highly specific to ERalpha or ERbeta mRNAs were used for in situ hybridization studies; two antibodies specific to ERalpha protein were used for immunocytochemistry and specific primers were used for reverse transcription polymerase chain reaction (RT-PCR) studies. These revealed that ERbeta mRNA is widely expressed in the DRG of both male and female rats, with virtually all neurons showing positive signals. In contrast, ERalpha mRNA, as well as nuclear localized ERalpha protein, is more restricted in its localization and is present in many, but not all, of the small-sized (<600 microm(2)) DRG neurons, but is only rarely present in larger neurons. The L6-S1 DRG levels, which contain sensory neurons that innervate reproductive tissues, are relatively enriched in ERalpha compared to L3-L5 DRG levels, which contain sensory neurons that innervate hind limb regions. Long-term estrogen treatment of ovariectomized rats (21-28 days) dramatically reduces immunocytochemically detectable ERalpha protein in the DRG relative to that in ovariectomized controls. RT-PCR studies also showed that long-term estrogen treatment of ovariectomized rats downregulates the levels of ERalpha mRNA, but upregulates the levels of ERbeta mRNA in the DRG. Interestingly, in intact cycling female rats, ERalpha and ERbeta mRNA levels in the DRG were both higher during proestrus compared to metestrus. These findings suggest that the changes in expression of estrogen receptors which occur dynamically during the estrus cycle differ from those induced by long-term estrogen treatment of ovariectomized animals.

While a number of chronic pain conditions are much more prevalent in women than men, the role of estrogen in regulating nociception remains unclear. Estrogen receptors (ER) are known to be expressed in various parts of the nociceptive pathway, including in the small-sized primary sensory neurons of the dorsal root ganglion (DRG). This study evaluated the effects of long-term estrogen replacement on pain sensitivity and neuropeptide expression in the DRG of female Sprague Dawley rats. The goal was to evaluate whether estrogen modulates nociceptive neuropeptides in the DRG in a manner consistent with its effects on pain sensitivity. Our results show that long term (28 days) ovariectomy (ovx) of adult rats induces a profound thermal and mechanical hyperalgesia of the hindpaw and tail compared to ovariectomized animals that were continuously estrogen-treated (ovx+E). Significant changes in the expression of two neuropeptides, substance P (SP) and calcitonin gene related peptide (CGRP), were observed using immunocytochemistry and in situ hybridization (ISH) in the small lumbar DRG neurons which contain ER. CGRP and SP were differentially regulated by estrogen, with SP showing a significant downregulation at both the peptide and mRNA level while CGRP and its mRNA were increased in the DRG of estrogen-treated animals. We also evaluated the development of mechanical allodynia after partial sciatic nerve injury and found that both ovx and ovx+E animals developed significant allodynia within a week of the partial nerve injury, which continued for at least one month. The estrogen treated animals showed a partial amelioration of the extent of the allodynia at 2 weeks post injury. Overall, the results suggest that estrogen has significant anti-nociceptive actions that can be directly correlated with changes in expression of two peptides in the small nociceptive ERα expressing neurons of the DRG.

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