Thermodynamic analysis of binding to the cerebellar type I benzodiazepine receptor

The temperature dependence of binding to the type I benzodiazepine receptor in rat cerebellum was determined using [3H]Ro15-1788 and regression analysis techniques. The ligands chosen were from diverse chemical families and display different pharmacological properties. Included were the agonists flunitrazepam, CL 218,872, zolpidem and alpidem; the antagonists Ro15-1788 and propyl-beta-carboline-3-carboxylate (beta-CCP); the inverse agonists ethyl-beta-carboline-3-carboxylate (beta-CCE) and methyl-6,7-dimethoxy-beta-carboline-3-carboxylate (DMCM); and the selective muscle relaxant AHR-11797. Assays were performed at 0 degrees C, 20 degrees C and 37 degrees C and the K(i) at each temperature was used to construct a van't Hoff plot for each compound. The binding of all ligands, with the exception of DMCM, was enthalpy-driven. However, enthalpy alone does not determine the rank order of affinity. There was no relationship between the thermodynamic behavior of binding and the observation of agonism, antagonism or inverse agonism, indicating that activation and recognition are distinct steps in this receptor system.