Estrogen-Inducible Progesterone Receptors in the Rat Lumbar Spinal Cord: Regulation by Ovarian Steroids and Fluctuation across the Estrous Cycle

Ovarian hormones influence the physiology of the spinal cord through incompletely understood cellular mechanisms. To date, there has been little compelling evidence for progesterone receptors in spinal cord neurons. Using two antibodies specific for progesterone receptors in an immunohistochemical investigation, we now report the presence of estrogen-inducible progesterone receptors in the spinal cord. Estrogen-inducible progesterone receptors were observed in the neurons of lamina X and the interomedialateral cell column, which are also known to express estrogen receptors. Estrogen-inducible progesterone receptors similar to those observed in females were also apparent in lamina X and interomediolateral cell column neurons in the spinal cords of males treated with estradiol. Furthermore, the density of progesterone receptors in lamina X was observed to fluctuate across the estrous cycle in female rats, with the highest progesterone receptor expression levels occurring late in proestrus, following the estradiol surge and coincident with high circulating progesterone levels. The lowest progesterone receptor expression levels were observed late in estrus following the progesterone surge. Together, these results demonstrate that estrogen-sensitive progestin targets exist in the spinal cord, and their possible role in the nervous control of reproduction and ovarian steroid modulation of nociception is discussed.