Channel opening locks agonist onto the GABAC receptor
Yongchang Chang
& David S. Weiss
Departments of Neurobiology and Physiology and Biophysics,
University of Alabama at Birmingham School of Medicine, 1719
Sixth Avenue So., Birmingham, Alabama 35294-0021
, USA
Determination of the activation mechanism of neurotransmitter-operated
ion channels has been hindered by a limited understanding of the relationship
between agonist binding and the gating of the integral ion pore. Here we describe
a [3H]ligand binding assay that enables us to make repeated
binding measurements from the same intact oocyte expressing recombinant human 1
GABAC receptors and directly correlate the binding kinetics with
electrophysiological measurements. We have determined an association rate
for GABA of about 105 M−1s−1
; this is four orders of magnitude slower than diffusion, indicating
GABA has restricted access to its binding site. We also demonstrate that GABA
dissociates at two rates. Our data are consistent with the faster rate being
the true microscopic dissociation rate of GABA, with the slower rate occurring
because the opening of the pore detains agonist release.
1
GABAC receptors and directly correlate the binding kinetics with
electrophysiological measurements. We have determined an association rate
for GABA of about 105 M−1s−1
; this is four orders of magnitude slower than diffusion, indicating
GABA has restricted access to its binding site. We also demonstrate that GABA
dissociates at two rates. Our data are consistent with the faster rate being
the true microscopic dissociation rate of GABA, with the slower rate occurring
because the opening of the pore detains agonist release.
