Differential interaction of opiates to multiple "kappa" binding sites in the guinea-pig lumbo-sacral spinal cord

Binding characteristics of [3H]-etorphine and [3H]-ethylketocyclazocine are different in the lumbo-sacral spinal cord of guinea-pig. [3H]-etorphine binds to a single class of high affinity sites whereas [3H]-ethylketocyclazocine interacts with two components, a high affinity and a low affinity components. In the presence of 5 microM (D-Ala2, D-Leu5) enkephalin (DAL), the total high affinity sites can be resolved in two classes of sites, DAL sensitive sites (DALs sites) and DAL insensitive sites (DALI sites). In these conditions, [3H]-etorphine binding is completely abolished, and the binding capacity and properties of [3H]-etorphine correspond to the DALs sites. Pharmacological investigations indicated that DALI sites represent the kappa sites, whereas DALs sites closely correspond to benzomorphan sites described in rat brain and spinal cord. From these results, a new subclassification of "kappa" sites is proposed.