Thermodynamic analysis of ligands at cholecystokinin CCK 2 receptors in rat cerebral cortex

Background and purpose: Several studies using radioligand binding assays, have shown that measurement of thermodynamic parameters can allow discrimination of agonists and antagonists ( Weiland et al ., 1979 ; Borea et al ., 1996a ). Here we investigate whether agonists and antagonists can be thermodynamically discriminated at CCK 2 receptors in rat cerebral cortex. Experimental approach: The pK L of [ 3 H]‐JB93182 in rat cerebral cortex membranes was determined at 4, 12, 21 and 37°C in 50 m M Tris‐HCl buffer (buffer B pH 6.96; containing 0.089 m M bacitracin). pK I values of ligands of diverse chemical structure and with differing intrinsic activity ( α ), as defined by the lumen‐perfused rat and mouse stomach bioassays, were determined in buffer B at 4, 12, 21 and 37°C. Key results: [ 3 H]‐JB93182 labelled a homogeneous population of receptors in rat cerebral cortex at 4, 12, 21 and 37°C and the pK L and B max were not altered by incubation temperature. [ 3 H]‐JB93182 binding reached equilibrium after 10, 50, 90 and 220 min at 37, 21, 12 and 4°C, respectively. pK I values for R ‐L‐365,260, R ‐L‐740,093, YM220, PD134,308 and JB95008 were higher at 4°C than at 37°C. There was no effect of temperature on pK I values for pentagastrin, CCK‐8S, S ‐L‐365,260, YM022, PD140,376 and JB93242. Conclusions and implications: CCK 2 receptor agonists and antagonists at rat CCK 2 receptors cannot be discriminated by thermodynamic analysis using [ 3 H]‐JB93182 as the radioligand. British Journal of Pharmacology (2007) 151 , 1352–1367; doi: 10.1038/sj.bjp.0707355