Kappa opioid receptor density is consistent along the rostrocaudal axis of the female rat spinal cord.

Kappa opioid receptors (KORs) were immunocytochemically localized at four different levels of the spinal cord of normally-cycling female rats in estrus or diestrus. KOR labeling was primarily observed 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 there were no significant differences in KOR densities among spinal segments C1--C2, T2, T13--L1, and L6--S1 in either the estrus or diestrus phases. These results suggest that the potential for KOR-mediated antinociceptive responses is consistent along the rostrocaudal axis of the female rat spinal cord.