L-740093

Several studies using radioligand binding assays, have shown that measurement of thermodynamic parameters can allow discrimination of agonists and antagonists (Weilandet al., 1979;Boreaet al., 1996a). Here we investigate whether agonists and antagonists can be thermodynamically discriminated at CCK2receptors in rat cerebral cortex.

The pKLof [3H]‐JB93182 in rat cerebral cortex membranes was determined at 4, 12, 21 and 37°C in 50 mMTris‐HCl buffer (buffer B pH 6.96; containing 0.089 mMbacitracin). pKIvalues 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.

[3H]‐JB93182 labelled a homogeneous population of receptors in rat cerebral cortex at 4, 12, 21 and 37°C and the pKLand Bmaxwere not altered by incubation temperature. [3H]‐JB93182 binding reached equilibrium after 10, 50, 90 and 220 min at 37, 21, 12 and 4°C, respectively. pKIvalues forR‐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 pKIvalues for pentagastrin, CCK‐8S,S‐L‐365,260, YM022, PD140,376 and JB93242.

CCK2receptor agonists and antagonists at rat CCK2receptors cannot be discriminated by thermodynamic analysis using [3H]‐JB93182 as the radioligand.British Journal of Pharmacology(2007)151, 1352–1367; doi:10.1038/sj.bjp.0707355

The alternatively spliced, short and long cholecystokinin receptors (CCK2S and CCK2L) were expressed in NIH3T3 cells, and compared using radioligand‐binding assays with identical buffer and incubation conditions.As judged by a saturation analysis, the selective CCK2‐receptor antagonist radioligand [3H]‐JB93182 did not discriminate between the CCK2S or CCK2L receptors.A global analysis of competition studies, using a range of structurally diverse, CCK‐receptor selective ligands, provided further evidence that these receptor subtypes were pharmacologically indistinguishable. However, when analysed individually a number of small, yet significant differences were observed with some of the compounds.These data are consistent with previous study that suggested a possible pharmacological difference between these isoforms, at least in terms of the CCK2‐receptor antagonist, L‐365,260. However, it would appear that the pharmacological profile of these compounds is not consistent with their affinity at the putative G1/G2 receptors previously described by Harper et al.British Journal of Pharmacology (2003) 140, 218–224. doi:10.1038/sj.bjp.0705423