A subclass of GABAA/benzodiazepine receptor exclusively localized in the limbic system

An in vivo saturation study using 11C-labelled Ro15-4513 with high specific radioactivity (> 70 GBq mumol-1) revealed the presence of very high-affinity and high-affinity binding sites (Kd values in the amygdala approximately 0.4 and 18.7 nM, respectively) in the gamma-aminobutyric acid type A/benzodiazepine (GABAA/BZ) receptor in the living monkey brain. Subtraction of an image obtained using [11C]Ro15-4513 with low specific radioactivity from an image obtained by that with high specific radioactivity, both of which were scanned with a high spatial resolution positron emission tomography camera, demonstrated that the very high-affinity sites are exclusively localized in the limbic system, such as in the amygdala, hippocampus, anterior cingulate, septum, nucleus accumbens and insular cortex. The localization of the very high-affinity binding sites, which may be derived from a subclass of central GABAA/BZ receptors, might account for the anxiolytic effect of BZ analogues.