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NMDA receptors mediate several important functions in the CNS; however, little is known about the pharmacology, biochemistry, and function of distinct NMDA receptor subtypes in brain tissue. To facilitate the study of native NMDA receptor subpopulations, we have determined the radioligand binding properties of [3H]homoquinolinate, a potential subtype-selective ligand. Using quantitative receptor autoradiography, NMDA-specific [3H]homoquinolinate binding selectively labeled brain regions expressing NR2B mRNA (layers I-III of cerebral cortex, striatum, hippocampus, and septum). NMDA-specific [3H]homoquinolinate binding was low in brain regions that express NR2C and NR2D mRNA (cerebellar granular cell layer, NR2C; glomerular layer of olfactory bulb, NR2C/NR2D; and midline thalamic nuclei, NR2D). In forebrain, the pattern of NMDA-specific [3H]homoquinolinate binding paralleled NR2B and not NR2A distribution. In addition to NMDA-displaceable binding, there was a subpopulation of [3H]homoquinolinate binding sites in the forebrain, cerebellum, and choroid plexus that was not displaced by NMDA or L-glutamate. In contrast, we found that the derivative of homoquinolinate, 2-carboxy-3-carboxymethylquinoline, markedly inhibited the NMDA-insensitive binding of [3H]homoquinolinate without inhibiting the NMDA-sensitive population. [3H]Homoquinolinate may be useful for selectively characterizing NR2B-containing NMDA receptors in a preparation containing multiple receptor subtypes and for characterizing a novel binding site of unknown function.

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N-Methyl-D-aspartate (NMDA) receptors are a calcium-conducting class of excitatory amino acid receptors that are involved in neuronal development and migration. Certain well known teratogens (e.g. homocysteine, ethanol, and chloroform) that induce congenital neural tube and neural crest defects also have the capacity to act as NMDA receptor antagonists. We hypothesized that teratogenicity was a general property of NMDA receptor antagonists, and that high affinity NMDA receptor antagonists would induce neural tube and neural crest defects. Chicken embryos were given 5, 50, or 500 nmol/d of selected NMDA receptor antagonists for 3 consecutive days during the process of neural tube closure, beginning 4 h after the beginning of incubation. Selected NMDA receptor antagonists represented three classes of antagonists: ion channel blockers, glycine site antagonists, and glutamate site agonists and antagonists. All classes of NMDA receptor antagonists induced embryonic death and congenital defects of the neural crest and neural tube; however, the channel blockers were the most potent teratogens. Dextromethorphan at 500 nmol/embryo/d killed more than half the embryos and induced congenital defects in about one-eighth of the survivors; dextromethorphan was also highly lethal at 50 nmol/embryo/d. Glutamate site NMDA receptor agonists (NMDA and homoquinolinic acid) displayed weak toxicity relative to their known NMDA receptor potency. Taken together, these data indicate that NMDA receptor antagonists, particularly channel blockers, are potent teratogens in the chicken embryo model. Because dextromethorphan is a widely used nonprescription antitussive, its strong teratogeneticity using this model is particularly noteworthy.

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The phosphoinositide system plays a critical role in mesangial cell contraction. myo-Inositol depletion occurs in glomeruli from diabetic animals and may result in mesangial cell dysfunction. The hypothesis that mesangial cell exposure to high concentrations of glucose could lead to abnormalities in phosphoinositide metabolism and receptor-mediated inositol phosphate release was tested. When compared with controls (5 mM glucose), inositol phosphate release in mesangial cells exposed to 28 mM glucose was decreased by 27% after maximal stimulation with angiotensin II, by 41% after arginine vasopressin, and by 63% after the thromboxane A2 analog, U46619. Increasing the concentration of glucose to 50 mM caused a further reduction (from 27 to 54%) in maximal angiotensin II stimulation of inositol phosphate release. High glucose decreased incorporation of myo-inositol into phospholipids but did not change phosphoinositide mass. High glucose also resulted in increased de novo synthesis of diacylglycerol which was associated with membrane translocation of protein kinase C. myo-inositol supplementation prevented the reduction in phosphoinositide hydrolysis whereas sorbinil did not. It was concluded that high concentrations of glucose cause abnormalities in myo-inositol metabolism in mesangial cells which lead to reduced receptor-mediated phosphoinositide hydrolysis. These abnormalities appear to be related to desensitization of receptor-mediated phosphoinositide responses due to negative feedback by protein kinase C which becomes activated as a result of enhanced de novo diacylglycerol formation from glucose. These changes are unrelated to the polyol pathway and can be prevented by myo-inositol supplementation.

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