Evaluation of models for analysis of radioligand binding data

In the presence of agonists, many neurotransmitter receptors interact with regulatory components, resulting in the formation of a ternary complex composed of agonist, receptor, and a regulatory component, and in biphasic or shallow dose response curves for inhibition of the binding of a radiolabeled antagonist by agonists. Complex dose response curves are often analyzed using equations that describe a model that assumes the presence of two independent populations of receptors (two-independent-receptor model) or equations that describe a model that assumes the reversible interaction of agonist-occupied receptors with a regulatory component (ternary complex model). In this study, the ability of these models to provide good estimates of the concentration of the regulatory component and of the affinities involved in formation of the ternary complex was evaluated. Dose response curves were generated by a computer using an equation that describes the ternary complex model. Analysis of the dose response curves with the two-independent-receptor model resulted in good estimates of the concentration of the regulatory component and of the affinity of the receptor for the agonist. Reliable estimates of the other affinity constants that relate to formation of the ternary complex could not be obtained. Analysis of the dose response curves with the ternary complex model resulted in good estimates of the concentration of the regulatory component and of the affinity constants that relate to formation of the ternary complex. Random error was added to the data points that made up the dose response curves to simulate error observed in experiments with biological systems. Analysis of the dose response curves that contained random error with the two-independent-receptor model yielded results similar to those obtained from analysis of dose response curves that did not contain random error. Analysis of the dose response curves that contained random error with the ternary complex model resulted in good estimates of the concentration of the regulatory component and of the affinity of the receptor for the agonist. However, reliable estimates of the other affinity constants involved in formation of the ternary complex could not be obtained. Thus, caution should be exercised when interpreting results of the analysis of dose response curves with either the two-independent-receptor model or the ternary complex model.