Role of the histidine residue at position 105 in the human α5 containing GABAA receptor on the affinity and efficacy of benzodiazepine site ligands

A histidine residue in the N‐terminal extracellular region of α1,2,3,5 subunits of the human GABAA receptor, which is replaced by an arginine in α4 and α6 subunits, is a major determinant for high affinity binding of classical benzodiazepine (BZ)‐site ligands. The effect of mutating this histidine at position 105 in the α5 subunit to an arginine (α5H105R) on BZ‐site pharmacology has been investigated using radioligand binding on HEK293 and L(tk‐) cells and two electrode voltage clamp recording on Xenopus oocytes in which GABAA receptors of subtypes α5, α5H105R, α4 and α6 were co‐expressed with β3γ2s. The classical BZs, diazepam and flunitrazepam (full agonists on the α5 receptor) showed negligible affinity and therefore negligible efficacy on α5H105R receptors. The β‐carbolines DMCM and βCCE (inverse agonists on the α5 receptor) retained some affinity but did not exhibit inverse agonist efficacy at α5H105R receptors. Therefore, the α5H105R mutation confers an α4/α6‐like pharmacology to the classical BZs and β‐carbolines. Ro15‐4513, flumazenil, bretazenil and FG8094, which share a common imidazobenzodiazepine core structure, retained high affinity and were higher efficacy agonists on α5H105R receptors than would be predicted from an α4/α6 pharmacological profile. This effect was antagonized by DMCM, which competes for the BZ‐site and therefore is likely to be mediated via the BZ‐site. These data indicate that the conserved histidine residue in the α subunit is not only a key determinant in the affinity of BZ‐site ligands on α5 containing GABAA receptors, but also influences ligand efficacy. British Journal of Pharmacology (2002) 135, 248–256; doi:10.1038/sj.bjp.0704459