Benzomorphan binding sites in rat lumbo-sacral spinal cord.

The rat lumbo-sacral spinal cord contains a homogeneous population of opiate binding sites labelled with high affinity (KD = 0.21 +/- 0.04 nM) by [3H]etorphine and lower affinity (KD = 2.2 +/- 0.4 nM) by [3H]ethylketocyclazocine. Benzomorphan drugs are potent competitors for these binding sites while morphine and enkephalin display a low affinity. These binding sites have binding properties which are distinct from the mu-, delta-, and also kappa-sites but are very similar to those of the benzomorphan sites characterized in rat brain.