The mechanism of SR95531 inhibition at GABA receptors examined in human alpha1beta1 and alpha1beta1gamma2S receptors.
We examined the interaction of GABA and the competitive inhibitor SR95531 at human alpha1beta1gamma2S and alpha1beta1 GABA(A) receptors expressed in Sf9 cells. The efficacy and potency of inhibition depended on the relative timing of the GABA and SR95531 applications. In saturating (10 mM) GABA, the half-inhibitory concentrations of SR95531 (IC50) when coapplied with GABA to alpha1beta1gamma2S or alpha1beta1 receptors were 49 and 210 microM for the peak and 18 and 130 microM for the plateau current, respectively. Our data are explained by an inhibition mechanism in which SR95531 and GABA bind to two sites on the receptor where the binding of GABA allows channel opening but SR95531 does not. The SR95531 affinity for both receptor types was approximately 200 nM and the binding rate was found to be 10-fold faster than that for GABA. The dual binding-site model gives insights into the differential effects of GABA and SR95531 on the peak and plateau currents. The model predicts the effect of SR95531 on GABA currents in the synapse (GABA concentration approximately mM) and at extrasynaptic (GABA concentration < or = microM) sites. The IC50 (50-100 nM) for the synaptic response to SR95531 was insensitive to the GABA affinity of the receptors whereas the IC50 (50-800 nM) for extrasynaptic inhibition correlated with the GABA affinity.
School of Medicine, Lund University, Lund, Sweden.
Catarina E L
CE
Lindquist
Derek R
DR
Laver
Bryndis
B
Birnir
D000818:Animals; D002460:Cell Line; D003001:Cloning, Molecular / Q000379:methods; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation / Q000379:methods; D018756:GABA Antagonists / Q000494:pharmacology*; D018014:Gene Transfer Techniques; D006801:Humans; D020128:Inhibitory Concentration 50; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D046911:Macromolecular Substances / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D008954:Models, Biological; D018408:Patch-Clamp Techniques / Q000379:methods; D021122:Protein Subunits / Q000378:metabolism* / Q000502:physiology; D011724:Pyridazines / Q000494:pharmacology*; D011963:Receptors, GABA-A / Q000737:chemistry / Q000187:drug effects* / Q000378:metabolism; D018411:Spodoptera / Q000166:cytology; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D018756:GABA Antagonists; D046911:Macromolecular Substances; D021122:Protein Subunits; D011724:Pyridazines; D011963:Receptors, GABA-A; D005680:gamma-Aminobutyric Acid; C049853:gabazine