Mutation at the putative GABA(A) ion-channel gate reveals changes in allosteric modulation.
We have mutated a conserved leucine in the putative membrane-spanning domain to serine in human GABA(A) beta2 and investigated the actions of a number of GABA(A) agonists, antagonists and modulators on human alpha1beta2deltaL259Sgamma2s compared to wild type alpha1beta2gamma2s GABA(A) receptors, expressed in Xenopus oocytes. The mutation resulted in smaller maximum currents to gamma-aminobutyric acid (GABA) compared to alpha1beta2gamma2s receptors, and large leak currents resulting from spontaneous channel opening. As reported, this mutation significantly decreased the GABA EC50 (110 fold), and reduced desensitization. Muscimol and the partial agonists 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) and piperidine-4-sulphonic acid (P4S) also displayed a decrease in EC50. In addition to competitively shifting GABA concentration response curves, the antagonists bicuculline and SR95531 both inhibited the spontaneous channel activity on alpha1beta2deltaL259Sgamma2s receptors, with different degrees of maximum inhibition. The effects of a range of allosteric modulators, including benzodiazepines and anaesthetics were examined on a submaximal GABA concentration (EC20). Compared to wild type, none of these modulators potentiated the EC20 response of alpha1beta2deltaL259Sgamma2s receptors, however they all directly activated the receptor in the absence of GABA. To conclude, the above mutation resulted in receptors which exhibit a degree of spontaneous activity, and are more sensitive to agonists. Benzodiazepines and other agents modulate constitutive activity, but positive modulation of GABA is lost. The competitive antagonists bicuculline and SR95531 can also act as allosteric channel modulators through the same GABA binding site.
Thompson
S A
SA
Merck Sharp & Dohme Research Laboratories, Harlow, Essex.
Smith
M Z
MZ
Wingrove
P B
PB
Whiting
P J
PJ
Wafford
K A
KA
D000494:Allosteric Regulation / Q000502:physiology*; D000777:Anesthetics / Q000494:pharmacology; D000818:Animals; D001569:Benzodiazepines / Q000494:pharmacology; D001665:Binding Sites / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D005260:Female; D018755:GABA Agonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects; D007473:Ion Channels / Q000235:genetics / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects; D009154:Mutation; D009865:Oocytes / Q000187:drug effects / Q000502:physiology; D010424:Pentobarbital / Q000494:pharmacology; D011963:Receptors, GABA-A / Q000235:genetics / Q000502:physiology*; D014981:Xenopus
D000777:Anesthetics; D018755:GABA Agonists; D018756:GABA Antagonists; D007473:Ion Channels; D011963:Receptors, GABA-A; D001569:Benzodiazepines; D010424:Pentobarbital