An allosteric model for benzodiazepine receptor function

FREDERICK J. EHLERT*, WILLIAM R. ROESKE, KELVIN W. GEE and HENRY I. YAMAMURA# Departments of Pharmacology, Biochemistry, Psychiatry and Internal Medicine, and the Arizona Research Laboratories, University of Arizona Health Sciences Center, Tucson, AZ 85724, U.S.A. The assignment of specific 3H-labeled ligand binding to a pharmacologically relevant receptor is not a trivial matter. Nevertheless, evidence has accumu- lated during the past few years that benzodiazepines mediate their pharmacological effects by interacting with neuronally localized receptors within the brain. An intriguing question is how the recognition of a benzodiazepine by its receptor initiates a chain of events leading to the pharmacological response, e.g. prevention of seizures, tranquility and sedation. The conventional approach has been to rationalize the initial recognition step within the context of a lock-and-key model. This hypothesis can readily account for the pharmacological specificity and stereospecificity of receptors as being a consequence of their geometry. It is with this recognition process that the implications of receptor binding data have their most immediate application. In addition, ligand binding methods can yield some insight into the activation of the receptor-effector system. It has been pointed out that part of the intrinsic binding energy of an agonist is used to induce a conforma- tional change in the receptor [1]; in other words, the observed affinity of a drug for a receptor depends on both the bimolecular association of the ligand with the receptor and the subsequent conformational change of the ligand-receptor complex. Although this idea is not a new one, the importance of this phenomenon in regard to interpreting binding data is not generally recognized. It is useful to consider how the binding properties of drugs for other neurotransmitter receptors are affected by the intrinsic activity of the ligand. With regard to beta-adrenergic and muscarinic receptors, there are striking differences in the way agonists and antagonists bind to these receptors [2-4]. In general, agonists tend to display heterogeneous binding properties [5]. The magnitude of this heterogeneity can be estimated by computing the relative differ- ences in the affinities of a drug for subclasses of the receptor or, in some instances, by estimating the relative densities of the high and low affinity states of the receptor. Strong correlations between the * Present address: Department of Pharmacology, School of Medicine, University of California, Los Angeles, CA 90024, U.S.A. t Address correspondence to: Henry I. Yamamura, Ph.D., Department of Pharmacology, Arizona Health Sci- ences Center, Tucson, AZ 85724, U.S.A. heterogeneity of binding and the efficacy ligand have been noted for muscarinic and beta- adrenergic receptors [2-4]. Also, the negatively cooperative effects of guanine nucleotides on binding properties show a strong correlation with efficacy [5, 6]. For opiate receptors, a correlation between the influence of Na + on binding and efficacy opiate analogues has been noted [7]. It is not our intent to assess the models that have been proposed to account for these phenomena but merely to emphasize the fundamental principle providing basis for these relationships: the intrinsic activity of an agonist is related to its ability to induce a confor- mational change in the receptor. During the course of our studies of the benzodi- azepine receptor binding activity of the beta-car- boline compounds which are structurally unrelated to benzodiazepines (see Fig. 1), we noticed certain correlations between the influence of gamma-ami- nobutyric acid (GABA) on binding and the intrinsic activity of the drug [8, 9]. These differences have been rationalized in terms of an allosteric model for benzodiazepine receptor function which views the benzodiazepine receptor as a heterotropic site on the GABA receptor-chloride ionophore complex [10], The model is an attempt to define the rela- tionships between the behavioral effects of benzo- diazepines and the influence of GABA on binding. Presently, it is our goal to describe the model in greater detail and to demonstrate that it can account for most of the pharmacological effects of ligands that interact specifically with the recognition site of the benzodiazepine receptor. We shall begin our discussion by reviewing some of the complex binding characteristics of the benzodiazepine receptor. Benzodiazepine receptor heterogeneity Initially, it seemed as if benzodiazepine receptors were homogeneous [11-13]. This assumption was based on the results of numerous binding studies which have shown that the equilibrium binding iso- therms for [3H]diazepam and [3H]flunitrazepam ([3H]FLU) in the range of concentrations studied were consistent with the simple Langmuir isotherm [11-15]. Also, when the specific binding of [3H]FLU or [3H]diazepam in low concentrations is inhibited by other nonlabeled benzodiazepines, the competi- tion curves usually have Hill coefficients of approx- imately 1. However, evidence soon emerged which suggested that benzodiazepine receptors are not sim- ply a homogeneous class of binding sites. In 1979, 2375