Measurement of central nervous system activity of systemically administered CCKB receptor antagonists by ex vivo binding

In the present study we have described an ex vivo binding assay in mice to measure the central nervous system (CNS) activity of systemically administered CCKB receptor antagonists. This assay incorporated a transcardiac perfusion step to remove the residual blood from the brain, which otherwise may result in an overestimation of CNS activity. The benzodiazepine CCKB receptor antagonist L-365,260 had marked CNS activity in this assay following i.v. (ED50 12.0 mg/kg) and p.o. (ED50 20.0 mg/kg) administration, whereas the dipeptoid CCKB receptor antagonist, CI988 exhibited relatively weak CNS activity following i.v. injection (ED50 > 30.0 mg/kg). In contrast, following i.c.v. administration, CI988 potently inhibited ex vivo binding of [125I]Bolton Hunter-CCK-8S to mouse brain. The recently described acidic tetrazole CCKB receptor antagonist, L-368,935 had potent CNS activity with an ED50 of 5.6 mg/kg i.v. and an ED50 of 1.9 micrograms/kg i.c.v. These studies suggest that the weak CNS activity of CI988 following systemic injection may, in part, be due to poor brain penetration and that the ex vivo binding assay is a useful way of assessing the brain penetration of CCKB receptor antagonists.