Structural determinants of the pharmacological properties of the GABAA receptor alpha6 subunit

GABA(A) receptors are responsible for fast inhibitory neurotransmission in the mammalian brain and are the targets for many clinical drugs that act as antiepileptics, anxiolytics, and sedatives. The pharmacological characteristics of the receptor are largely determined by its subunit composition. Compared with all other alpha subtypes, the alpha6 subtype confers unique pharmacological properties. In particular, alpha6-containing receptors are more sensitive to both the agonist GABA and the antagonist amiloride. Results from chimeric constructs of the alpha1 and alpha6 subunits suggested that structural differences within the extracellular N-terminal domain were responsible for both these characteristics. Within this domain, we examined 15 amino acid residues unique to the alpha6 subtype. Each of these sites was individually mutated in the alpha6 subunit to the corresponding residue of the alpha1 subunit. The mutated subunits were expressed in human embryonic kidney-293T cells along with wild-type beta3 and gamma2L subunits and sensitivity to GABA and amiloride determined with whole-cell electrophysiological recordings. Serine83 in the alpha6 subunit influenced sensitivity to both GABA and amiloride. Leucine174 and tyrosine175 were also found to contribute to inhibition by amiloride but did not affect GABA sensitivity. These structural differences are at least partly responsible for the unique pharmacological properties associated with the alpha6 subunit.