ALPHA-[H-3]AMINO-3-HYDROXY-5-METHYLISOXAZOLE-4-PROPIONIC ACID BINDING TO HUMAN CEREBRAL CORTICAL MEMBRANES - MINIMAL CHANGES IN POSTMORTEM BRAINS OF CHRONIC-SCHIZOPHRENICS

The binding of alpha-[H-3]amino-3-hydroxy-5-methylisoxazole-4-propionic acid ([H-3]AMPA), a selective ligand for the ion channel-linked quisqualate receptor, was evaluated in Triton X-100-treated membranes of human cerebral cortex. The presence of chaotropic ions produced divergent effects on specific [H-3]AMPA binding: A twofold increase in the binding was observed with thiocyanide at 100 mM, although iodide (100 mM) and perchlorate (100 mM) reduced the binding. Chemical modifications of the sulfhydryl group with p-chloromercuriphenylsulfonic acid (PCMBS) produced threefold increases in specific [H-3]AMPA binding in the absence of KSCN as well as in the presence of KSCN. Treatment with dithiothreitol restored the enhanced specific [H-3]AMPA binding by PCMBS to the basal level. Although specific [H-3]AMPA binding in the absence of KSCN showed a single site (K(D) = 220 nM, B(max) = 235 fmol/mg of protein), curvilinear Scatchard plots of specific [H-3]AMPA binding in the presence of 100 KSCN can be resolved into two binding sites with the following parameters: K(D1) = 5.82 nM, B(max1) = 247 fmol/mg of protein; K(D2) = 214 nM, B(max2) = 424 fmol/mg of protein. Quisqualate and AMPA were the most potent inhibitors of the [H-3]AMPA binding in the presence of KSCN. Potent inhibitors of the binding included beta-N-oxalylamino-L-alanine (L-BOAA), cysteine-S-sulfate, L-glutamate, 6-cyano-7-nitroquinoxaline-2,3-dione, and 6,7-dinitroquinoxaline-2,3-dione. Kainate, L-homocysteine sulfinic acid, and L-homocysteic acid were active with an IC50 value of a micromolar concentration. whereas L-cysteic acid and L-cysteine sulfinic acid were weakly active. N-Methyl-D-aspartate, L-aspartate, N-acetylaspartylglutamate, quinolinate, and 1-naphthylacetyl spermine, an analogue of Joro spider toxin, were inactive at 1 mM. These results suggest that L-glutamate has an important effect on AMPA receptors in the human cerebral cortex and that L-BOAA and cysteine-S-sulfate exhibit their neurotoxicity through AMPA receptors. Specific [H-3]AMPA binding, using the ligand at 6 nM, in the presence of 100 mM KSCN exhibited a heterogeneous distribution pattern in the human cerebral cortex: [H-3]AMPA binding values were high in the frontal and occipital cortex, whereas they were low in the parietotemporal cortex. However, no statistically significant changes in [H-3]AMPA binding were observed in 22 brain areas of the cerebral cortex of chronic schizophrenics, compared with controls, suggesting that AMPA receptors in the cerebral cortex are minimally involved in an abnormality of excitatory amino acidergic neurons hypothesized in schizophrenic brain.