[3H]muscimol photolabels the gamma-aminobutyric acid receptor binding site on a peptide subunit distinct from that labeled with benzodiazepines.

Affinity column-purified GABA-benzodiazepine receptor protein from bovine brain was photoaffinity labeled with both [3H]flunitrazepam and [3H]muscimol. Gel electrophoresis in sodium dodecyl sulfate revealed that the benzodiazepine binding site labeled with [3H]flunitrazepam was primarily associated with a major peptide subunit revealed by protein staining with Mr = 52 kiloDaltons, with minor labeling of a second peptide of Mr = 57 kiloDaltons, corresponding to a second major stained band. Covalent incorporation of [3H]muscimol was limited to the 57 kiloDalton band, with no labeling of the 52 kiloDalton peptide, showing that the GABA binding site is carried by a subunit distinct from that carrying the benzodiazepine binding site.