Comparison of mu, delta, and kappa opiate binding sites in rat brain and spinal cord

The binding characteristics of mu, delta, and kappa opiate sites were studied in rat brain and spinal cord membrane homogenates. Scatchard analysis of 3H-Dihydromorphine, 3H-D-Ala2-D-Leu5-Enkephalin (in the presence of morphiceptin), and 3H-Ethylketocyclazocine (in the presence of morphiceptin and D-Ala2-D-Leu5-Enkephalin) binding sites revealed similar high affinities of these ligands for their respective sites in brain and spinal cord. The majority of binding in brain and spinal cord was attributed to mu and delta sites, with only about 10% of the combined total binding capacity being kappa.