Autoradiographic imaging of altered synaptic alphabetagamma2 and extrasynaptic alphabeta GABAA receptors in a genetic mouse model of anxiety.

To image the possible alterations in brain regional GABAA receptor subtype properties in a genetic animal model of human anxiety, mice heterozygous for the deletion of GABAA receptor gamma2 subunit (gamma2+/-) were studied using ligand autoradiographic assays on brain cryostat sections. The [35S]TBPS binding assay was designed to reveal impaired GABA and channel site coupling shown to be more prominent in recombinant alpha1/6beta3 than in alpha1/2beta3gamma2 or beta2 subunit-containing GABAA receptors expressed in HEK 293 cells. Increased GABA-insensitive [35 S]TBPS binding in the gamma2+/- mouse brains was evident in the cerebral cortex and in subcortical regions, the alterations being regionally similar to the loss of gamma2 subnunit-dependent benzodiazepine (BZ) sites as revealed by [3H]Ro 15-4513 autoradiography. As the gamma2 subunit protein is needed for synaptic clustering of GABAA receptors, these results indicate that the extrasynaptic alphabeta3 receptors can be visualized in vitro as atypical GABA-insensitive [35S]TBPS binding sites. The results suggest that GABAAergic synaptic inhibition is widely decreased in the brains of anxiety-prone gamma2+/- mice, while extrasynaptic GABAA receptors are increased. These autoradiographic imaging findings further demonstrate the need to develop GABAA receptor subtype-selective in vivo ligands to aid in assessing the contributions of various subcellular receptor populations in anxious and other patient groups.