SULFHYDRYL-GROUPS MODULATE THE ALLOSTERIC INTERACTION BETWEEN GLYCINE BINDING-SITES AT THE INHIBITORY GLYCINE RECEPTOR

We have investigated the effect of chemical reagents that modify sulfhydryl groups on the ligand binding properties of the glycine receptor (GlyR). The Hill coefficient (n(H)) for the displacement of [H-3]-strychnine binding by glycine was increased from approximately 0.8 to values significantly above (approximately 1.2-1.4) in membranes pretreated with the disulfide-reducing agent dithiothreitol or glutathione. However, the affinity of strychnine or glycine for the GlyR was not affected by these treatments. This indicates that several glycine binding sites interact cooperatively for displacing bound strychnine under such experimental circumstances. A similar increase in the n(H) for glycine has been observed when the temperature of the binding assay was increased to 37-degrees-C. Combination of dithiothreitol pretreatment and increased binding temperature led to n(H) variations similar to those observed with either of these treatments alone, a finding suggesting that their mechanisms of action are not independent. Conversely, modification of rat spinal cord membranes or of purified and reconstituted GlyR preparations with the sulfhydryl-alkylating agent N-ethylmaleimide or fluorescein-maleimide decreased n(H) values to approximately 0.5, without affecting glycine or strychnine affinities. This effect may be caused by an increased heterogeneity of GlyR populations. It is interesting that occupancy of the receptor by glycine or beta-alanine (but not by antagonists) specifically protects from the effects of the different sulfhydryl reagents. Moreover, the presence of some of the Eccles' anions, i.e., anions that permeate through the channels associated with GlyRs and gamma-aminobutyric acid(A) receptors, seems to be required for the action of both dithiothreitol and N-ethylmaleimide. Our results suggests the existence of thiol groups at the GlyR that are involved in the interaction between several glycine binding sites for displacing bound strychnine and that may also be functionally related to ionic and agonist sites. Furthermore, they raise the possibility that GlyR function could be regulated in vivo by reduction and oxidation.