Differential Effects of Iodide and Chloride on Allosteric Interactions of the GABAA Receptor

t-35S]Butylbicyclophosphorothionate ( 35S]TBPS) has been shown to bind to the GABAA receptor complex. The binding is modulated allosterically by drugs that interact at components of the receptor complex. The present studies were designed to evaluate the influence of ionic environment and state of equilibrium on the allosteric modification of 35S]TBPS binding. In both I- and Cl- under nonequilibrium conditions, diazepam, gamma-aminobutyric acid (GABA), and pentobarbital (PB) stimulate and methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) inhibits 35S]TBPS binding. In addition, there is an inhibitory component to the effect of GABA and PB at higher drug concentrations. These effects are blocked by the appropriate antagonists for each drug. In Cl-, the stimulation of 35S]TBPS binding by drugs disappears at equilibrium, whereas the inhibition by GABA and PB persists. The inhibitory effect of DMCM in Cl- also disappears at equilibrium. When assayed in I- at equilibrium, however, DMCM stimulates 35S]TBPS binding. In addition, bicuculline, which is without effect under nonequilibrium conditions in either Cl- or I-, stimulates 35S]TBPS binding in I- at equilibrium. The persistent effects of DMCM, bicuculline, and GABA in I- are accompanied by alterations in the affinity of 35S]TBPS for its receptor. In addition, the stimulation of 35S]TBPS binding by GABA is associated with a decreased number of 35S]TBPS binding sites. These data demonstrate that receptor complex interactions with anions influence the responsiveness to drug binding.