Properties of 3H-cimetidine binding in rat brain membrane fractions

In an attempt to characterize the brain histamine H₂ receptor, experiments were undertaken to study the binding properties of (N-methyl-³H) -cimetidine, an H₂ receptor antagonist, in rat brain membranes. Using a centrifugation assay, ³H-cimetidine binding having a K_d of 0.40μM and a Bmax of 3.9 pmoles/mg protein was detected. Of fourteen anions and cations tested, one, Cu⁺⁺, dramatically increased specific ³H-cimetidine binding, the increase being due mainly to a change in Bmax. Studies of substrate specificity for ³H-cimetidine binding revealed that Cu⁺⁺, while not significantly affecting the potency of H₂ receptor agonists and antagonists, dramatically decreases the potency of H₁ receptor substances on the ³H-cimetidine binding site. In addition, both the relative and absolute potencies of various H₂ receptor agonistsv and antagonists in displacing the ligand in the presence of Cu⁺⁺ parallels their potencies in biological systems. These findings suggest that, under these conditions, ³H-cimetidine may be labelling a biologically relevant H₂ binding site in brain and that Cu⁺⁺ may regulate the substrate specificity for this site. The brain regional distribution and kinetic analysis of the binding suggest that it is not localized solely to the synaptic receptor for histamine, but may also be associated with histamine receptors at other neuronal, glial or vascular sites.