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Mutation at the putative GABA(A) ion-channel gate reveals changes in allosteric modulation
S A Thompson1, M Z Smith, P B Wingrove
1Merck Sharp & Dohme Research Laboratories, Harlow, Essex.
British Journal of Pharmacology
|August 24, 1999
Summary
A mutation in GABA(A) receptors increases sensitivity to agonists and causes spontaneous activity. Benzodiazepines and anesthetics modulate this activity but lose positive GABA modulation, with antagonists acting as allosteric modulators.
Area of Science:
- Neuroscience
- Molecular Biology
- Pharmacology
Background:
- GABA(A) receptors are crucial inhibitory neurotransmitter receptors in the central nervous system.
- Understanding the structure-function relationship of GABA(A) receptor subunits is vital for drug development.
- Specific mutations can alter receptor function and drug interactions.
Purpose of the Study:
- To investigate the functional consequences of mutating a conserved leucine to serine in the human GABA(A) beta2 subunit (L259S).
- To characterize the activity of agonists, antagonists, and modulators on the mutated alpha1beta2deltaL259Sgamma2s GABA(A) receptors.
- To compare the mutated receptor's response to wild-type alpha1beta2gamma2s GABA(A) receptors.
Main Methods:
- Site-directed mutagenesis of the human GABA(A) beta2 subunit.
- Expression of wild-type and mutated receptors in Xenopus oocytes.
- Electrophysiological recordings to measure GABA-induced currents, spontaneous activity, and responses to various ligands.
Main Results:
- The L259S mutation in the beta2 subunit led to spontaneous channel opening and large leak currents.
- Mutated receptors showed significantly decreased GABA EC50 (110-fold increase in sensitivity) and reduced desensitization.
- Agonists (muscimol, THIP, P4S) and antagonists (bicuculline, SR95531) displayed altered EC50 values and inhibition patterns.
- Allosteric modulators (benzodiazepines, anesthetics) directly activated mutated receptors in the absence of GABA, but failed to potentiate GABA-induced currents.
Conclusions:
- The L259S mutation confers spontaneous activity and increased agonist sensitivity to GABA(A) receptors.
- Positive allosteric modulation of GABA is lost, while constitutive activity is modulated by benzodiazepines and anesthetics.
- Competitive antagonists bicuculline and SR95531 can also function as allosteric channel modulators via the GABA binding site.