Estrogen-induced alterations of spinal cord enkephalin gene expression

Enkephalin-synthesizing neurons in the superficial laminae of the spinal an d trigeminal dorsal horn are critical components of the endogenous pain-mod ulatory system. We have previously demonstrated that these neurons display intracellular estrogen receptors, suggesting that estrogen can potentially influence their enkephalin expression. By using Northern blot, we now show that a bolus injection of estrogen results in a rapid increase in spinal co rd enkephalin mRNA levels in ovariectomized female rats. Thus, 4 h after es trogen administration the enkephalin mRNA-expression in the lumbar spinal c ord was on average 68% higher (P < 0.05) than in control animals injected w ith vehicle only. A small increase in the amount of enkephalin mRNA was als o seen after 8 h (P < 0.05), whereas no difference between estrogen-injecte d and control animals was found after 24 h or at time periods shorter than 4 h. Taken together with the previous anatomical data, the present findings imply that estrogen has an acute effect on spinal opioid levels in areas i nvolved in the transmission of nociceptive information. (C) 1999 Internatio nal Association for the Study of Pain. Published by Elsevier Science B.V.