Dependence of allosteric modulation of glycine receptor function on agonist efficacy
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The glycine receptor (GlyR) is a ligand-gated chloride channel that mediates inhibitory neurotransmission in the brain and spinal cord. Numerous allosteric modulators act on the GlyR including divalent cations, such as zinc, as well as drugs of abuse including ethanol, inhalants, anesthetics and cannabinoids. GlyRs mediate some of the rewarding effects of addictive drugs and modulate drug related behaviors through activity in the mesolimbic dopamine reward pathway. GlyR activity, however, can differ depending on whether the receptor is activated by the high-efficacy agonist, glycine, or taurine which has much lower efficacy at wild-type GlyRs. As glycine and taurine are believed to activate the GlyR in vivo, it is crucial that we understand receptor function and allosteric modulation of receptors in response to both agonists. We used two-electrode voltage-clamp electrophysiology in Xenopus laevis oocytes to study the effects of zinc and ethanol on wildtype glycine receptors activated by glycine or taurine. We determined that the magnitude of allosteric modulation was higher, overall, at taurine-gated receptors and hypothesized that this may be related to the difference in efficacies. Considering that GlyR mutants can affect agonist sensitivity and the response to allosteric modulators, we wondered whether changes in allosteric modulation at mutant receptors could be due to changes in agonist efficacy caused by these mutations. We tested this hypothesis by characterizing ethanol and zinc modulation of taurine currents at GlyR mutants that showed an increase ([alpha]1[superscript W170S]) or decrease ([alpha]1[superscript A52S]) in taurine efficacy. We found that the W170S mutation increases the relative efficacy of taurine to a level that is comparable with glycine and abolishes ethanol enhancement of maximally-effective taurine currents. We determined that the difference in ethanol potentiation of low taurine currents between W170S and WT receptors is due to zinc enhancement of WT currents. Ethanol modulation of these receptors was equal in the presence of tricine. This suggests that ethanol also increases taurine affinity at W170S. A52S, on the other hand, displayed reduced taurine efficacy and increased ethanol modulation. This work provides evidence of a mechanism by which the degree of allosteric modulation of glycine receptor function is dependent on agonist efficacy.