STARTLE DISEASE MUTATIONS REDUCE THE AGONIST SENSITIVITY OF THE HUMANINHIBITORY GLYCINE RECEPTOR

The receptor for the inhibitory neurotransmitter glycine is a member o f the ligand gated ion channel recep tor superfamily. Point mutations in the gene encoding the alpha 1 subunit of the glycine receptor chann el complex (GlyR) have recently been identified in pedigrees with the autosomal dominant neurological disorder, startle disease (hyperekplex ia). These mutations result in the substitution of leucine or glutamin e for arginine 271. This charged residue is located near the ion chann el region and is predicted to affect chloride permeation through the G lyR. We found little evidence for this role from the anion/cation sele ctivity and lack of pronounced rectification of currents flowing throu gh recombinant human alpha 1 subunit GlyRs containing the startle dise ase mutations. We reveal, however, that the startle disease mutations profoundly disrupt GlyR func tion by causing 230-410-fold decreases in the sensitivity of receptor currents activated by the agonist glycine . Additionally, we report corresponding 56- and 120-fold reductions in the apparent binding affinity (K-i) of glycine to the mutant GlyRs, b ut no change in the binding affinity of the competitive antagonist, st rychnine. Thus, startle disease reduces the efficacy of glycinergic in hibitory neurotransmission by producing GlyRs with diminished agonist responsiveness. Our results show that startle disease mutations define a novel receptor activation site.