Histamine H 3-receptor agonists and imidazole-based H 3-receptor antagonists can be thermodynamically discriminated

Abstract

Background and purpose: Studies suggest that measurement of thermodynamic parameters can allow discrimination of agonists and antagonists. Here we investigate whether agonists and antagonists can be thermodynamically discriminated at histamine H 3 receptors. Experimental approach: The pK L of the antagonist radioligand, [ 3H]-clobenpropit, in guinea-pig cortex membranes was estimated at 4, 12, 21 and 30°C in 20mM HEPES-NaOH buffer (buffer A), or buffer A containing 300mM CaCl 2, (buffer A Ca). pK I′ values for ligands with varying intrinsic activity were determined in buffer A and A Ca at 4, 12, 21 and 30°C. Key results: In buffer A, the pK L of [ 3H]-clobenpropit increased with decreasing temperature while it did not change in buffer A Ca. The Bmax was not affected by temperature or buffer and n H values were not different from unity. In buffer A, pK I′ values for agonists remained unchanged or decreased with decreasing temperature, while antagonist pK I values increased with decreasing temperature; agonist binding was entropy-driven while antagonist binding was enthalpy and entropy-driven. In buffer A Ca, temperature had no effect on antagonist and agonist pK I values; both agonist and antagonist binding were enthalpy and entropy-driven. Conclusions and implications: The binding of H 3-receptor agonists and antagonists can be thermodynamically discriminated under conditions where agonist pK I′ values are over-estimated (pK I′≠pK app). However, under conditions when agonist pK I ∼pK app, the thermodynamics underlying the binding of agonists are not different to those of antagonists. © 2007 Nature Publishing Group All rights reserved.