Opioid binding sites in different levels of rat spinal cord.

Opioid receptor binding sites were analyzed in various segments of rat spinal cord. Mu and delta types were labelled with [3H]-DHM or [3H]-DAGO and [3H]-DADLE or [3H]-DSLET respectively. Kappa 1 (kappa) and kappa 2 (benzomorphan) binding sites were individually detected by the overall labeling of opioid binding sites with [3H]-etorphine followed by the elimination of binding to particular sites by the use of appropriate selective unlabelled ligands. Whereas lumbo sacral region contained mainly the kappa 2 site, thoracic membranes had a proportion of approximately 20% mu, 20% kappa 1 and 60% kappa 2 and cervical region contained much less kappa 2 sites (25% mu, 20% delta, 28% kappa 1 and 32% kappa 2).