Structural requirements for ligand interactions at the benzodiazepine recognition site of the GABA(A) receptor

His101 of the GABA(A) receptor alpha1 subunit is an important determinant of benzodiazepine recognition and a major site of photolabeling by [3H]flunitrazepam. To investigate further the chemical specificity of the residue in this position, we substituted it with phenylalanine, tyrosine, lysine, glutamate, glutamine, or cysteine. The mutant alpha subunits were coexpressed with the rat beta2 and gamma2 subunits in TSA201 cells, and the effects of the substitutions on the binding of benzodiazepine site ligands were examined. [3H]Ro 15-4513 bound to all mutant receptors with equal or greater affinity than to the wild-type receptor. However, flunitrazepam and ZK93423 recognition was adversely affected by substitutions of the amino acid in this position. The binding of the antagonists, Ro 15-1788 and ZK93426, was also sensitive to the mutations, with the largest decreases in affinity occurring with the tyrosine, lysine, and glutamate substitutions. In all mutants that recognized flunitrazepam, GABA potentiated the binding of this ligand to a similar extent, suggesting that it is a full agonist at these receptors. The effects of GABA on the binding of Ro 15-1788 and Ro 15-4513 suggest that their efficacies may have been changed by some of the substitutions. This study further emphasizes the importance of the residue at position 101 in both ligand recognition and pharmacological effect.