Temperature dependence and GABA modulation of beta-carboline binding to rat cerebellum benzodiazepine receptors

The temperature dependence of the binding of beta-carboline derivatives to the central benzodiazepine receptors was determined using [3H]-Ro 15-1788, as a selective radioligand. The compounds chosen display a wide spectrum of efficacies ranging from inverse agonists to agonists through antagonists. Assays were performed at 0, 10, 20, 25, 30, 35 degrees C in the absence and in the presence of 10 microM GABA. The temperature dependence of the affinity constants K(A)=1/K(D) or 1/Ki is shown in the van't Hoff plots (In K(A) versus 1/T) for each compound. Thermodynamic parameters deltaG degrees, deltaH degrees and deltaS degrees were determined by regression analysis of the plots which were linear in the range of temperatures investigated. Moreover, their slopes were systematically positive indicating that the binding of the compounds analyzed to benzodiazepine receptors is essentially enthalpy-driven both in the presence and in the absence of GABA. We verified that the ratio of affinity constant values in the presence and absence of GABA 10 microM (GABA ratio) (<1 for inverse agonists, =1 for antagonists, >1 for agonists), strongly correlates with the corresponding differences of deltaH degrees and deltaS degrees values obtained for each compound in the absence and in the presence of GABA. These results suggest that binding thermodynamic analysis of BDZ receptor ligands, in the presence and in the absence of GABA, permits to discriminate inverse agonists from antagonists, and agonists.