Autoradiographic localization of alpha5 subunit-containing GABAA receptors in rat brain

Multiple subtypes of GABAA receptors are expressed in the rat central nervous system (CNS). To determine the distribution and proportion of alpha5 subunit containing receptors, quantitative autoradiographic analyses were performed with both [3H]L-655,708 and [3H]Ro15-1788, an alpha5 selective and a non selective benzodiazepine binding site ligand, respectively. High densities of [3H]L-655,708 binding sites were observed in hippocampus and olfactory bulb, where alpha5 receptors accounted for 20-35% of total [3H]Ro15-1788 binding sites. Low levels of [3H]L-655,708 sites were associated with the cortex as well as amygdala, thalamic, hypothalamic and midbrain nuclei. These observations indicate that although [3H]L-655,708 binding sites have an overall low expression in rat CNS, they may contribute significantly to GABAergic inhibition in specific brain regions.