Nociceptin receptors in the rat spinal cord during morphine tolerance

The anatomical localization of nociceptin receptors was examined by in vitro quantitative autoradiography techniques in rat spinal cord sections by using [(125)I-Tyr(14)]nociceptin. [(125)I-Tyr(14)]nociceptin appeared to interact with a single class of binding sites (K(D)=0.1 nM) present in the grey matter in all laminae of the spinal cord from cervical to sacral levels. Pre-incubation of sections in the presence of 150 mM NaCl, did not modify the radioligand affinity but significantly augmented the number of accessible binding sites and increased specific binding of [(125)I-Tyr(14)]nociceptin differentially on each laminae. In particular, the superficial layers of the dorsal horn exhibited the highest density of sites after pre-wash. Continuous intrathecal infusion of morphine produced a tolerance accompanied by a significant increase in nociceptin site density in the superficial layers. Thus, nociceptin binding sites may have different properties dependent upon the layer and may be up-regulated during the process of opioid-induced tolerance.