Channel opening locks agonist onto the GABAC receptor

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 [H-3]ligand binding assay that enables us to make repeated binding measurements from the same intact oocyte expressing recombinant human rho 1 GABA(C) receptors and directly correlate the binding kinetics with electrophysiological measurements. We have determined an association rate for GABA of about 10(5) M(-1)s(-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.