Kinetics of binding to opiate receptors in vivo predicted from in vitro parameters

In order to determine the relationship between in vitro and in vivo binding of opiates in the CNS, the in vitro and in vivo binding properties of 9 opiates, including agonists and antagonists, were studied in mouse brain. Equilibrium dissociation constants (KD) were measured in the presence and absence of 100 mM NaCl and 100 microM 5-guanylylimidodiphosphate (GMP-PNP) at 37 degrees C in mouse brain homogenates. The rate constants describing the exponential clearance of the specifically-bound (i.e. receptor-bound) fraction of ligand in vivo were measured in the striatum, thalamus and cortex. A mathematical model was formulated which relates the in vivo clearance rate for the specifically-bound fraction to the association and dissociation rate constants, the local density of receptors and a parameter which determines the rate at which ligand can diffuse out of the local receptor compartment. The diffusion parameter takes on two forms depending on whether ligand is transported out of the local receptor compartment by simple diffusion or by permeation of a non-polar membrane barrier. In the case of membrane permeation, the model is dependent on the diffusion constant and the lipophilicity of the ligand. A correlation between the in vitro and in vivo measurements is obtained only when KD is measured in the presence of 100 mM NaCl and 100 microM GMP-PNP and when the lipophilicity of the ligand is incorporated into the model. The model corresponds to the physical situation where continuous rebinding of ligand to receptors takes place in the vicinity of a high local density of receptors (possibly at a synapse), thereby retarding the clearance of the specifically-bound ligand from the brain. The rebinding effect is accentuated by: (1) a high local density of receptors; (2) a small KD (measured in the presence of 100 mM NaCl and 100 microM GMP-PNP); (3) a low lipophilicity; and (4) a high molecular weight.