L-homocysteic acid: an endogenous excitotoxic ligand of the NMDA receptor.
L-Homocysteic acid (L-HCA) has been proposed as a natural transmitter at the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptor based on recent evidence that L-HCA occurs L-HCA occurs naturally in the mammalian CNS, is released from K+ stimulated brain slices in a calcium-dependent manner and may be contained in nerve terminals located in certain brain regions that have a high density of NMDA receptors. Here we report that L-HCA potently induces a pattern of cytopathology in the ex vivo chick retina which mimics the pattern of NMDA but not kainic acid (KA) neurotoxicity. We also show that known NMDA antagonists, including Mg++, D-aminophosphonopentanoate and certain anesthetics, analgesics, and sedative hypnotics block the neurotoxic actions of L-HCA in direct proportion to their efficacy in blocking NMDA neurotoxicity. While there is a perfect correspondence between agents that block NMDA and L-HCA neurotoxicity, only a few such agents are active against KA neurotoxicity. We find that 3H-Glu binding is inhibited more potently by L-HCA (Ki = 67 microM). Moreover the patterns with which L-HCA and NMDA displace 3H-Glu binding in autoradiograms appear essentially identical. These findings are consistent with the proposal that L-HCA is an endogenous ligand at NMDA receptors.
D000818:Animals; D001224:Aspartic Acid / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism; D002642:Chick Embryo; D005971:Glutamates / Q000378:metabolism / Q000494:pharmacology; D018698:Glutamic Acid; D006710:Homocysteine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D007608:Kainic Acid / Q000494:pharmacology; D008024:Ligands / Q000378:metabolism; D016202:N-Methylaspartate; D016194:Receptors, N-Methyl-D-Aspartate; D017981:Receptors, Neurotransmitter / Q000378:metabolism*; D012160:Retina / Q000187:drug effects / Q000473:pathology
D005971:Glutamates; D008024:Ligands; D016194:Receptors, N-Methyl-D-Aspartate; D017981:Receptors, Neurotransmitter; D006710:Homocysteine; C007956:homocysteic acid; D001224:Aspartic Acid; D018698:Glutamic Acid; D016202:N-Methylaspartate; D007608:Kainic Acid