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The Journal of Neuroscience, October 1, 2002, 22(19):8411-8421
Coupled and Uncoupled Gating and Desensitization Effects by Pore
Domain Mutations in GABAA Receptors
Michaela
Scheller1 and
Stuart A.
Forman2
1 Klinik fuer Anaesthesiologie der Technischen
Universitaet Muenchen, Klinikum rechts der Isar, D-81675 Munich,
Germany, and 2 Department of Anesthesia and Critical
Care, Massachusetts General Hospital, Boston, Massachusetts 02114
GABAA receptors are allosteric ligand-gated ion
channels. Agonist-induced gating and desensitization have been proposed
to be coupled via pore domain structures. Mutations at two
 1 subunit pore-domain (transmembrane domain 2)
residues enhance GABA sensitivity, leucine-to-threonine at position 264 (9'), and serine-to-isoleucine at position 270 (15'). We investigated
the role of these residues in gating, desensitization, and deactivation
of 1 2 2L GABAA receptors using rapid GABA concentration jumps and patch-clamp electrophysiology. GABA EC50 values for
1(L264T)22L and
1(S270I)22L currents were,
respectively, ~80-fold and 13-fold lower than the wild-type
EC50. Unlike wild type, both mutant receptors displayed significant picrotoxin-sensitive currents in the absence of GABA, indicating that they enhance gating efficacy. Both mutants displayed current activation rates that matched wild type at 1 µM
GABA and above. Desensitization of wild-type and
1(S270I)22L currents displayed indistinguishable rates and amplitudes, whereas
1(L264T)22L currents
desensitized extremely slowly. Deactivation of wild-type currents
displayed two rates and slowed after partial desensitization, whereas
currents from both mutants deactivated slowly with single rate
constants that were unaffected by desensitization. These results
indicate that both 1(L264T) and 1(S270I)
mutations increase the gating efficacy of receptors by slowing channel
closing, which accounts for nearly all of the similar changes that they
produce in macrocurrent dynamics. Because the 1(S270I)
mutation uncouples its gating effects from those on rapid
desensitization, these two processes are necessarily associated with
movements of distinct receptor structures (gates). The effects of the
1(L264T) mutation suggest that the conserved leucines
may play a role in gating-desensitization coupling.
Key words:
GABA receptor; acetylcholine receptor; ion channel; gating; desensitization; pore; electrophysiology
Copyright © 2002 Society for Neuroscience 0270-6474/02/22198411-11$05.00/0
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