Background
The gamma-aminobutyric acid type A receptor (GABAA-R) beta subunits are critical targets for the actions for several intravenous general anesthetics, but the precise nature of the anesthetic binding sites are unknown. In addition, little is known about the role the fourth transmembrane (M4) segment of the receptor plays in receptor function. The aim of this study was to better define the propofol binding site on the GABAA-R by conducting a tryptophan scan in the M4 segment of the beta2 subunit.
Methods
Seven tryptophan mutations were introduced into the C-terminal end of the M4 segment of the GABAA-R beta2 subunit. GABAA-R subunit complementary DNAs were transfected into human embryonic kidney 293 cells grown on glass coverslips. After transfection (36-72 h), coverslips were transferred to a perfusion chamber to assay receptor function. Cells were whole cell patch clamped and exposed to GABA, propofol, etomidate, and pregnenolone. Chemicals were delivered to the cells using two 10-channel infusion pumps and a rapid solution exchanger.
Results
All tryptophan mutations were well tolerated, and with one exception, all resulted in minimal changes in receptor activation by GABA. One mutation, beta2(Y444W), selectively suppressed the ability of propofol to enhance receptor function while retaining normal sensitivity to etomidate and pregnenolone.
Conclusions
This is the first report of a mutation that selectively reduces propofol sensitivity without altering the action of etomidate. The reduction in propofol sensitivity is consistent with the loss of a hydrogen bond within the propofol binding site. These results also suggest a possible orientation of the propofol molecule within its binding site.
Mapping General Anesthetic Binding Site(s) in Human α1β3 γ-Aminobutyric Acid Type A Receptors with [3H]TDBzl-Etomidate, a Photoreactive Etomidate Analogue