Abstract
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.
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