Two Groups of Amino Acids Interact with GABA-A Receptors Coupled to t-[35S]Butylbicyclophosphorothionate Binding Sites: Possible Involvement with Seizures Associated with Hereditary Amino Acidemias

Abstract

Seven L-amino acids (Trp, Arg, Lys, Met, Ile, Val, and Phe) partially (28-81%) reversed the inhibitory action of 1 .mu.M .gamma.-aminobutyric acid (GABA) on t-[35S]-butylbicyclophosphorothionate ([35S]TBPS) binding to rat brain membranes, with EC50 values ranging from 5 to 120 mM. D-Trp, D-Arg, D-Lys, D-Met, D-Val, and D-Phe were approximately equipotent with their L-isomers. Tyramine, phenethylamine, and tryptamine, the decarboxylation products of the aromatic amino acids (Tyr, Phe, and Trp, respectively), reversed the inhibitory action of 1 .mu.M GABA on [35S]TBPS binding more potently than the parent amino acids (EC50 values 1.5-3.0 mM). Human hereditary amino acidemias involving Arg, Lys, Ile, Val, and Phe are associated with seizures, and these amino acids and/or their metabolites may block GABA-A receptors. Five other L-amino acids (ornithine, His, Glu, Pro, and Ala) as well as Gly and .beta.-Ala inhibited [35S]TBPS binding with IC50 values ranging from 0.1 to 37 mM, and these inhibitions were reversed by the GABA-A receptor blocker R 5135 in all cases. The inhibitory effects of L-ornithine, L-Ala, L-Glu, and L-Pro were stereospecific, because the corresponding D-isomers were considerably less inhibitory. L-His, D-His, and L-Glu gave incomplete (plateau) inhibitions. Human hereditary amino acidemias involving L-ornithine, His, Pro, Gly, and .beta.-Ala are also associated with seizures, and we speculate that these GABA-mimetic amino acids may desensitize GABA-A receptors. L-Asn, L-Ser, L-Thr, L-Cys, L-Asp, L-Leu, and L-Gln at 100 mM and L-Tyr at 10 mM had no significant inhibitory effect on [35S]TBPS binding, nor did they reverse the inhibitory effect of 1 .mu.M GABA. Amino acidemias involving the latter eight amino acids do not appear to be associated with seizures.