An autoradiographical saturation kinetic study of the different benzodiazepine binding sites in rat brain by using [ 3H] flunitrazepam as a radioligand

Abstract

The present study reports on the equilibrium association constant ( K D ) and receptor density ( B max) values of a number of brain areas from the mesencephalon, cerebral cortex, hippocampus, and cerebellum for the overall benzodiazepine (BZ) binding sites as well as for benzodiazepine binding site subtype 1 (BZ1) and subtype 2 (BZ2), determined by autoradiographical procedures using [ 3H] flunitrazepam. The differences between BZ1 ind BZ2 binding sites were analyzed using the specific BZ1 agonist zolpidem as inhibitor of the radioligand. Statistically significant differences in the affinities of BZ2 with respect to BZ and BZ1 binding sites were mainly found in cortical layers when pK D (negative logarithms of K D values) values were compared (ANOVA-SNK test). The distribution of B max, as well as the percentages of BZ1 and BZ2 and Hill coefficients which, surprisingly, are always close to 1 (> 0.9) for all the saturation kinetics analyzed, are also described. The possibility of heterogeneity related to anatomical distribution in the different subtypes is discussed.