β‐N‐Oxalylamino‐L‐Alanine Action on Glutamate Receptors

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Abstract
β-N-OxalyJamino-L-aJanine (L-BOAA) is a nonprotein excitatory amino acid present in the seed of Lathyrus sativus L. This excitotoxin has been characterized as the causative agent of human neurolathyrism; an upper motor neuron disease producing corticospinal dysfunction from excessive consumption of the lathyrus pea. Previous behavioral, tissue-culture, and in vitro receptor binding investigations revealed that L-BOAA might mediate acute neurotoxicity through quisqualate (QA)-preferring glutamate receptors. The present study demonstrates the stereospecific action of L-BOAA on glutamate receptor binding in whole mouse brain synaptic membranes. L-BOAA was most active in displacing thiocyanate (KSCN)-sensitive specific tritiated (RS)-α-3-hy-droxy-5-methylisoxazole-4-propionic acid (AMPA) binding (i.e., QA receptor) (Ki= 0.76 μM) with a rank-order potency of QA > kainate > N-methyl-D-aspartate (NMDA). By contrast, the nonneurotoxic D-BOAA isomer (100 μM) was essentially inactive in displacing radioligands for glutamate receptors, except the NMDA site, where it was equipotent with L-BOAA. Scatchard analysis of L-BOAA displacement of specific [3H]AMPA binding indicated competitive antagonism (KD: control, 135 nM; L-BOAA, 265 nM) without a significant change in QA-receptor density, and Hill plots yielded coefficients approaching unity. Differential L-BOAA concentration-dependent decreases in specific [3H]AMPA binding were observed in synaptic membranes, indicating that the neurotoxin was more potent in displacing specific binding from frontal cortex membranes, followed by that for corpus striatum, hippocampus, cerebellum, and spinal cord. Comparative experiments for inhibition of specific [3H]-AMPA binding in cortex revealed that L-BOAA was approximately five- and 10-fold less potent than QA and AMPA, respectively, but twofold greater than the endogenous neurotransmitter, glutamate. Parallel studies with KSCN-treated spinal cord synaptic membranes (compared to cortex controls) indicated that specific [3H]AMPA binding in untreated tissue was (a) approximately 5% of cortex and pharmacologically distinct and (b) differentially sensitive to L-BOAA and stimulation of binding by KSCN. These data suggest that low levels of L-BOAA in vivo might initially exert excitotoxicity through sensitive cortex QA-preferring neurons while eliciting little interference with QA sites in the spinal cord. Although the precise role(s) of the excitatory amino acid synapse in neurodegenerative diseases remains unclear, the cortex QA receptor could be preferentially sensitive to L-BOAA attack and might represent the initial molecular recognition site responsible for the pathogenesis of this upper motor neuron disease.