Quantitative autoradiography of [3H]t-butylbicycloorthobenzoate binding to the gamma-aminobutyric acid receptorA complex.

The picrotoxinin ligand [3H]t-butylbicycloorthobenzoate ([3H] TBOB) was evaluated as an autoradiographic ligand to study gamma-aminobutyric acidA (GABAA) receptors. Specific [3H]TBOB binding was approximately 80% of total binding, was saturable and identified a single population of binding sites with regional Kd approximating 30 nM. [3H]TBOB binding was inhibited by picrotoxin, isoguvacine and pregnenolone sulfate. The benzodiazepines clonazepam and zolpidem produced complex effects with concentration-dependent inhibition and enhancement of [3H] TBOB binding. [3H]TBOB binding was regionally heterogeneous with high levels of binding in globus pallidus and layer IV of neocortex, intermediate levels of binding in other neocortical laminae and the cerebellar molecular layer and low levels of binding in the striatum and septum. The regional distribution of [3H]TBOB binding correlated poorly with the regional distribution of [3H]muscimol binding, somewhat better with the regional distribution of [3H]flunitrazepam binding and [35S]t-butylbicyclophosphorothionate binding and quite well with the regional distribution of [3H]zolpidem binding. [3H]TBOB binding site autoradiography is a convenient technique for studying GABAA receptor pharmacology in a regionally specific manner.