beta-endorphin: characteristics of binding sites in the rabbit cerebellar and brain membranes

Specific binding of human beta-endorphin to rabbit cerebellar and brain membranes was measured using [3H2-Tyr27]-beta h-endorphin as the primary ligand. In both tissues binding was time dependent and saturable, with apparent equilibrium dissociation constants of 0.275 nM and 0.449 nM in the cerebellum and brain, respectively. The binding capacity of cerebellum is greater than that of brain. Kinetic studies showed that the association rate constants were 2.7 X 10(7) M-1min-1 for cerebellum and 2.4 X 10(7) M-1min-1 for brain. Dissociation of tritiated beta h-endorphin from both cerebellum and brain is not consistent with a first order decay from a single site. In the cerebellum there is a time-dependent increase in slowly dissociating complex. The potency of several opioid peptides and opiates to inhibit the binding of tritiated beta h-endorphin was determined. Ligands with preference for mu, delta, and kappa opiate receptor (morphine, Metenkephalin and ethylketocyclazocine) all have similar affinities toward beta h-endorphin sites in both brain and cerebellar membranes.