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

Abstract

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 μM (D-Ala 2, D-Leu 5) enkephalin (DAL), the total high affinity sites can be resolved in two classes of sites, DAL sensitive sites (DAL s sites) and DAL insensitive sites (DAL I sites). In these conditions, [ 3H]-etorphine binding is completely abolished, and the binding capacity and properties of [ 3H]-etorphine correspond to the DAL s sites. Pharmacological investigations indicated that DAL I sites represent the kappa sites, whereas DAL s sites closely correspond to benzomorphan sites described in rat brain and spinal cord. From these results, a new subclassification of “kappa” sites is proposed.