Kappa opioid receptors in rat spinal cord vary across the estrous cycle

Kappa opioid receptors (KORs) were immunocytochemically localized in the lu mbosacral spinal ford of female rats in different stages of the estrous cyc le to examine the influence of hormonal status on receptor density. KOR lab eling was primarily in fine processes and a few neuronal cell bodies in the superficial dorsal horn and the dorsolateral funiculus, Quantitative light microscopic densitometry of the superficial dorsal horn revealed that rats in diestrus had significantly lower KOR densities than those in proestrus or estrus. This suggests that female reproductive hormones regulate spinal KOR levels, which may contribute to variations in analgesic effectiveness o f KOR agonists across the estrous cycle. (C) 2000 Elsevier Science B.V. All rights reserved.