Opiate receptor binding profile in the rabbit cerebellum and brain membranes

The equilibrium dissociation constants (Kd) and maximal binding capacities of tritiated dihydromorphine (DHM), [D-Ala2,D-Leu5]enkephalin (DADLE), ethylketocyclazocine (EKC) and human beta-endorphin (beta-EP) in rabbit cerebellum and brain membranes have been investigated. Binding of tritiated DHM and DADLE was adequately described by a single affinity class of binding sites, while that of EKC required two affinity sites. Binding of tritiated beta-EP was also consistent with a single affinity class of binding sites by Scatchard analysis, but inhibition of the binding with type selective opiate receptor ligands revealed multiple sites. Sequential displacement of a broad spectrum opiate ligand, diprenorphine (DIP), by type selective ligands showed that cerebellum membranes are relatively rich in mu (40%) and deficient in K (12%) binding sites, while brain membranes are relatively rich in K (32%) and deficient in mu (12%) binding sites. beta-EP displaces 88 and 73% of tritiated DIP from cerebellum and brain membranes, respectively, suggesting multiple sites of beta-EP binding.