Thermodynamics of 5-HT3 receptor binding discriminates agonistic from antagonistic behaviour

Thermodynamic parameters Delta G degrees, Delta H degrees and Delta S degrees of the binding equilibrium of eleven ligands (seven agonists and four antagonists) to the serotonin 5-HT3 receptor subtype have been determined by affinity measurements carried out on mt cortex membranes at six different temperatures (0, 10, 20, 25, 30, 35 degrees C) and van't Hoff plots. Affinity constants were obtained from saturation experiments of [H-3]endo-N-(9-methyl-9-azabicyclo[3.3.1]non-3-yl)-1-methyl-1-H-indazole-3-carboxamide ([H-3]BRL 43694, a selective 5-HT3 ligand) or by its displacement in inhibition assays for the other compounds. Van't Hoff plots were essentially linear in the temperature range investigated, showing that the Delta C-p degrees of the binding equilibrium is nearly zero. Thermodynamic parameters are in the range 18 less than or equal to Delta H degrees less than or equal to 53 kJ mol(-1) and 202 less than or equal to Delta S degrees less than or equal to 320 J K-1 mol(-1) for agonists and -16 less than or equal to H degrees less than or equal to 0 kJ mol(-1) and 70 less than or equal to Delta S degrees less than or equal to 179 J K-1 mol(-1) for antagonists indicating that agonistic binding is totally entropy-driven while antagonistic binding is relatively less entropy- and more enthalpy-driven in the -T Delta S degrees versus Delta H degrees plot the thermodynamic data are clearly arranged in separate clusters for agonists and antagonists, which, therefore, turn out to be thermodynamically discriminated. Experimental results are discussed according to the following main points: (i) the approximate linearity of the Delta H degrees versus Delta S degrees plot in terms of enthalpy-entropy compensation and (ii) the fact that Delta C-p degrees similar or equal to 0 for practically all membrane receptors at variance with most reactions involving biomacromolecules in solution. Finally, the phenomenon of thermodynamical discrimination is reviewed and found to occur in five distinct membrane receptorial systems.