DECREASED AGONIST AFFINITY AND CHLORIDE CONDUCTANCE OF MUTANT GLYCINE RECEPTORS ASSOCIATED WITH HUMAN HEREDITARY HYPEREKPLEXIA

Hereditary hyperekplexia is a dominant neurological disorder associated with point mutations at the channel-forming segment M2 of the glycine receptor al subunit. Voltage-clamp recordings from the heterologously expressed mutants (alpha 1(R271L) Or alpha 1(R271Q)) revealed 146- to 183-fold decreased potencies of glycine to activate the chloride channel, and significantly reduced maximal whole-cell currents as compared with wild-type receptors. In contrast, the ability of the competitive antagonist strychnine to block glycine-induced currents was similar in all cases. Radioligand binding assays showed a 90- to 1365-fold reduction in the ability of glycine to displace [H-3]strychnine from its binding site on the mutant receptors. Paralleling the reductions in whole-cell current, the elementary main-state conductances of the mutants (alpha 1(R271L), 64 pS; alpha 1(R271Q), 14 pS) were lower than that of the wild-type receptor (86 pS). The decreased agonist affinities and chloride conductances of the mutants are likely to cause neural hyper-excitability of affected patients by impairing glycinergic inhibition. In addition, our data reveal that structural modifications of the ion-channel region can affect agonist binding to the glycine receptor.