Alzheimer's disease and dementia.
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Acquired Immunodeficiency Syndrome↗
Biomedical subjects
D W Choi
Publications and source records attributed to D W Choi.
Phenelzine toxicity: case report of an electroencephalographically defined biphasic syndrome.
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Adult↗
Vulnerability of cultured cortical neurons to damage by excitotoxins: differential susceptibility of neurons containing NADPH-diaphorase.
Quantitative concentration-toxicity relationships were determined for the injury of cultured murine cortical neurons by several excitatory amino acid (EAA) agonists. All tested agonists produced concentration-dependent neuronal injury at concentrations between 1 and 1000 microM. With 5 min exposure, glutamate, aspartate, N-methyl-D-aspartate (NMDA), L-homocysteate (HCA), and quisqualate all had similar potencies, destroying half of the neuronal population (LD50) at concentrations of 50-200 microM, and similar efficacies, with 88-92% neuronal loss produced by exposure to high agonist concentrations. Quinolinate and kainate were substantially weaker toxins, producing only 20-30% neuronal loss after 5 min exposure to 3 mM concentrations; with prolonged (24 hr) exposure, 85-95% neuronal loss could be attained. The comparative EAA vulnerability of a specific cortical neuronal subpopulation containing high concentrations of the enzyme, reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), was also examined. Glutamate had no differential toxicity on these cells, damaging them at all concentrations in proportion to the general population; however, other, more selective, agonists produced strikingly differential injuries. These NADPH-d-containing [NADPH-d(+)]neurons were selectively resistant to damage by low concentrations of the NMDA agonists quinolinate, HCA, aspartate, or NMDA itself. By contrast, NADPH-d(+)neurons were selectively destroyed by concentrations of quisqualate or kainate too low to produce much general neuronal injury. The differential susceptibility of these neurons was not absolute, as high concentrations of all tested agonists produced nonselective neuronal injury. In light of recent evidence that forebrain NADPH-d(+)neurons are selectively spared in Huntington's disease, the present study continues to support the hypothesis that neuronal loss in Huntington's disease might result from excessive NMDA-receptor stimulation by any selective NMDA agonist. Furthermore, the demonstration that the differential susceptibility of NADPH-d(+)neurons is agonist concentration-dependent, rather than absolute, could provide a basis for explaining some existing conflicting experimental data.
Animals↗
Zinc alters excitatory amino acid neurotoxicity on cortical neurons.
Recent studies have suggested that large amounts of free zinc may be coreleased during excitatory synaptic transmission at glutamatergic synapses, and may act postsynaptically to decrease actions mediated by N-methyl-D-aspartate (NMDA) receptors, while often increasing neuroexcitation mediated by quisqualate receptors. The present study examined the ability of zinc to alter excitatory amino acid (EAA) neurotoxicity. Murine cortical cell cultures were exposed to EAAs for 5 min in defined solutions, and neuronal cell injury was examined the following day both morphologically and by lactate dehydrogenase assay. Inclusion of 30-500 microM zinc in the exposure solution produced a zinc concentration-dependent, noncompetitive attenuation of NMDA-induced neuronal injury, with an ED50 of about 80 microM. In contrast, zinc produced the same concentration-dependent potentiation of quisqualate neurotoxicity; and with 500 microM zinc, a small potentiation of kainate neurotoxicity was suggested. The effect of zinc on the neurotoxicity of the broad-spectrum agonist glutamate was consistent with these effects on specific agonists, as well as with a previous study showing that glutamate neurotoxicity normally depends predominantly on NMDA-receptor activation. Zinc produced a concentration-dependent reduction in glutamate-induced neuronal injury in a fashion similar to that seen with NMDA, but less effectively. In addition, despite this overall protective effect, zinc paradoxically increased the glutamate-induced destruction of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d)-containing neurons, a subpopulation that was shown in the preceding paper (Koh and Choi, 1988) to exhibit resistance to NMDA receptor-mediated neurotoxicity, and vulnerability to non-NMDA receptor-mediated neurotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)
Amino Acids↗
L-homocysteate is a potent neurotoxin on cultured cortical neurons.
L-Homocysteate (L-HCA) neurotoxicity was quantitatively studied in dissociated cell cultures prepared from the fetal mouse neocortex. Five minute exposure to 3 microM-1 mM L-HCA was associated with neuronal cell loss, but not glial cell loss; the extent of neuronal damage was dependent on the concentration of L-HCA, with an ED50 of approximately 40 microM. The stereoisomer D-HCA was a somewhat weaker neurotoxin than L-HCA. Ion substitution experiments suggested that L-HCA neurotoxicity can be separated into two components on the basis of differences in time course and ionic dependence: an acute, sodium-dependent 'excitotoxic' component, marked by rapid early cell swelling; and a late, calcium-dependent component, marked by delayed cell degeneration. L-HCA neurotoxicity could be attenuated by 2-amino-5-phosphonovalerate (APV), ketamine, and kynurenate, but not by L-glutamate diethyl ester or gamma-D-glutamylaminomethyl sulfonate, consistent with a predominant involvement of N-methyl-D-aspartate receptors. APV and ketamine produced different effects on the L-HCA concentration-toxicity relation, the former drug consistent with a competitive, and the latter drug consistent with a non-competitive, mechanism of antagonism.
Animals↗
Dextrorphan and dextromethorphan attenuate hypoxic injury in neuronal culture.
The dextrorotatory opioid derivatives, dextrorphan and dextromethorphan, can attenuate hypoxic injury in cortical cell cultures. This effect is concentration-dependent in the micromolar range, and not strongly stereospecific, as it can also be demonstrated with the levorotatory enantiomer of dextrorphan, levorphanol. The possibility that these clinically available compounds may have therapeutic utility in hypoxic or ischemic encephalopathy warrants further investigation.
Animals↗
Quinolinate is a weak excitant of cortical neurons in cell culture.
Defined concentrations of quinolinate (QUIN) were administered to murine cortical neurons in culture impaled for intracellular recording. In physiological recording medium containing 1 mM Mg, concentrations of QUIN up to 2 mM had minimal effect on membrane potential and input resistance; only at higher concentrations did QUIN produce consistent depolarizations, which were accompanied by apparent increases in membrane resistance. In the absence of Mg, responses to QUIN were larger and were accompanied by decreases in membrane resistance, but QUIN was still a weak neuroexcitant, exhibiting an ED50 of greater than 1 mM. Phthalic, dipicolinic and nicotinic acids, structural analogues of QUIN, were even less potent neuroexcitants. The relationship between QUIN depolarization amplitude and membrane potential was linear in the absence of Mg, but in the presence of 1 mM Mg showed a non-linearity consistent with the voltage-dependent Mg block of N-methyl-D-aspartate (NMDA) receptor-mediated responses described by others. QUIN responses had a marked dependence on the presence of extracellular Na, and an extrapolated reversal potential of +12 mV, consistent with the large involvement of an Na influx. The responses were attenuated by the selective NMDA receptor antagonists, DL-2-amino-5-phosphonovalerate and ketamine, as well as by the broad spectrum antagonist kynurenate, but not by L-glutamate diethylester or gamma-D-glutamylaminomethyl sulfonate, compounds reported to block quisqualate or kainate receptors. The present study is consistent with the suggestion of other workers that QUIN neuroexcitation is mediated in large part by an Na influx through cation permeable NMDA-activated channels, but provides new quantitative data suggesting that the potency of QUIN as a cortical neuroexcitant is low. This low potency may argue against a role for QUIN as a traditional fast excitatory neurotransmitter.
2-Amino-5-phosphonovalerate↗
High concentrations of naloxone attenuate N-methyl-D-aspartate receptor-mediated neurotoxicity.
(-)-Naloxone, 1 mM, partially reduced neuronal loss induced by exposure of murine cortical cell cultures to N-methyl-D-aspartate (NMDA) or quinolinate, but produced little or no attenuation of kainate or quisqualate neurotoxicity. Antagonism of NMDA neurotoxicity was (-)-naloxone concentration-dependent between 100 microM and 3 mM. (+)-Naloxone produced a slightly greater reduction of NMDA neurotoxicity, arguing against mediation by opioid receptors. Although this novel neuron-protective action of (-)-naloxone was weak, it may contribute to reported beneficial effects in CNS ischemia.
Animals↗
Zinc selectively blocks the action of N-methyl-D-aspartate on cortical neurons.
Large amounts of zinc are present in synaptic vesicles of mammalian central excitatory boutons and may be released during synaptic activity, but the functional significance of the metal for excitatory neurotransmission is currently unknown. Zinc (10 to 1000 micromolar) was found to have little intrinsic membrane effect on cortical neurons, but invariably produced a zinc concentration-dependent, rapid-onset, reversible, and selective attenuation of the membrane responses to N-methyl-D-aspartate, homocysteate, or quinolinate. In contrast, zinc generally potentiated the membrane responses to quisqualate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate and often did not affect the response to kainate. Zinc also attenuated N-methyl-D-aspartate receptor-mediated neurotoxicity but not quisqualate or kainate neurotoxicity. The ability of zinc to specifically modulate postsynaptic neuronal responses to excitatory amino acid transmitters, reducing N-methyl-to-aspartate receptor-mediated excitation while often increasing quisqualate receptor-mediated excitation, is proposed to underlie its normal function at central excitatory synapses and furthermore could be relevant to neuronal cell loss in certain disease states.
Animals↗
Dextrorphan and dextromethorphan attenuate glutamate neurotoxicity.
The dextrorotatory morphinan opioid, dextrorphan, which has recently been reported to block the excitation of cortical neurons by N-methyl-D-aspartate, was found at 10-100 microM concentrations to attenuate both morphological and chemical evidence of glutamate neurotoxicity in murine neocortical cell cultures; a similar effect was found with its methyl ester derivative, dextromethorphan. Given other data suggesting that glutamate neurotoxicity may participate in the pathogenesis of the central neuronal loss associated with certain human neurological diseases, the present observations raise the possibility that these clinically tested opioids, or related compounds, may eventually prove to have some clinical therapeutic utility.
Animals↗
Cortical neurons exposed to glutamate rapidly leak preloaded 51chromium.
The acute toxic effects of excess glutamate exposure on cortical neurons in culture was followed using a novel adaptation of the 51chromium efflux assay. Although the acute, sodium-dependent phase of glutamate neurotoxicity may contribute to several acute disease settings, including sustained seizures and stroke, functional aspects of the phenomenon have not been previously studied. We report here that the earliest morphologic sign of glutamate neurotoxicity, neuronal swelling, is accompanied by a large efflux of complexed 51chromium from preloaded neurons in the first hour after exposure, and that this efflux is detectable as early as 15 min after the onset of glutamate exposure. We suggest that this pathological burst of 51chromium may result from glutamate-induced "leakiness" of neuronal cell membranes.
Animals↗
Quantitative determination of glutamate mediated cortical neuronal injury in cell culture by lactate dehydrogenase efflux assay.
Measurement of lactate dehydrogenase (LDH) activity released to the extracellular bathing media has been found to be a simple yet quantitative method for assessing glutamate mediated central neuronal cell injury in cortical cell culture. Extracellular LDH is both chemically and biologically stable; the magnitude of LDH efflux in the cultures correlates in a linear fashion with the number of neurons damaged by glutamate exposure.
Animals↗
Quinolinate neurotoxicity in cortical cell culture.
The central neurotoxicity of the endogenous tryptophan metabolite, quinolinate, has been postulated to participate in the pathogenesis of the neuronal cell loss associated with several neurological disease states. In the present study, quinolinate neurotoxicity was quantitatively studied in dissociated cell cultures prepared from the fetal mouse neocortex. Sufficient exposure of cortical cultures to quinolinate was associated with considerable neuronal cell loss, but no glial cell loss; this neurotoxicity could be blocked by 2-amino-5-phosphonovalerate and kynurenate, drugs known to block N-methyl-D-aspartate receptors. The quinolinate dose-toxicity relationship showed that the potency of quinolinate as a neurotoxin is relatively low, especially with brief (20 min) exposure times, where an ED50 of 2 mM was observed. However, with longer exposure times of 24 and 96 h, quinolinate is more potent: the latter exposure was characterized by an ED50 of 250-400 microM. Ion substitution experiments suggested that quinolinate neurotoxicity can be separated into two distinct components on the basis of differences in time course and ionic dependence: an acute, sodium-dependent "excitotoxic" component, marked by early cell swelling; and a late, calcium-dependent component, marked by delayed cell degeneration. Acute neuronal swelling was seen only with exposure to quinolinate concentrations in excess of 1 mM, so under actual pathophysiological conditions, quinolinate neurotoxicity might be nearly completely related to the calcium-dependent component, with little or no "excitotoxic" contribution.
2-Amino-5-phosphonovalerate↗
Effect of anticonvulsant drugs on glutamate neurotoxicity in cortical cell culture.
Pretreatment with anticonvulsants partially protects animals against the brain damage induced by intraparenchymal injection of kainate, an analogue of the neurotransmitter glutamate. In murine cortical cell culture, high concentrations of phenobarbital, diazepam, phenytoin, or GABA itself did not prevent glutamate-induced neuronal loss. Addition of a glutamate receptor antagonist (gamma-D-glutamyl glycine) did reduce glutamate neurotoxicity. The in vivo protective effect of anticonvulsant drugs against the toxicity of excitatory amino acids must be indirect.
Animals↗
Glutamate neurotoxicity in cortical cell culture.
The central neurotoxicity of the excitatory amino acid neurotransmitter glutamate has been postulated to participate in the pathogenesis of the neuronal cell loss associated with several neurological disease states, but the complexity of the intact nervous system has impeded detailed analysis of the phenomenon. In the present study, glutamate neurotoxicity was studied with novel precision in dissociated cell cultures prepared from the fetal mouse neocortex. Brief exposure to glutamate was found to produce morphological changes in mature cortical neurons beginning as quickly as 90 sec after exposure, followed by widespread neuronal degeneration over the next hours. Quantitative dose-toxicity study suggested an ED50 of 50-100 microM for a 5 min exposure to glutamate. Immature cortical neurons and glia were not injured by such exposures to glutamate. Uptake processes probably do not limit GNT in culture, as the uptake inhibitor dihydrokainate did not potentiate GNT. Possibly reflecting the lack of uptake limitation, glutamate was found to be actually more potent than kainate as a neurotoxin in these cultures, a dramatic reversal of the in vivo potency rank order. Some neurons regularly survived brief glutamate exposure; these possibly glutamate-resistant neurons had electrophysiologic properties, including chemosensitivity to glutamate, that were grossly similar to those of the original population.
Animals↗
Ionic dependence of glutamate neurotoxicity.
The cellular mechanisms by which excess exposure to the excitatory neurotransmitter glutamate can produce neuronal injury are unknown. More than a decade ago it was hypothesized that glutamate neurotoxicity (GNT) is a direct consequence of excessive neuronal excitation ("excitotoxicity" hypothesis); more recently, it has been hypothesized that a Ca influx triggered by glutamate exposure might mediate GNT (Ca hypothesis). A basic test to discriminate between these hypotheses would be to determine the dependence of GNT on the extracellular ionic environment. The excitotoxicity hypothesis predicts that GNT should depend critically on the presence of extracellular Na, since that ion appears to mediate glutamate neuroexcitation in the CNS; the Ca hypothesis predicts that GNT should depend critically on the presence of extracellular Ca. The focus of the present experiments was to determine the effects of several alterations in the extracellular ionic environment upon the serial morphologic changes that occur after mouse neocortical neurons in cell culture receive toxic exposure to glutamate. The results suggest that GNT in cortical neurons can be separated into 2 components distinguishable on the basis of differences in time course and ionic dependence. The first component, marked by neuronal swelling, occurs early, is dependent on extracellular Na and Cl, can be mimicked by high K, and is thus possibly "excitotoxic." The second component, marked by gradual neuronal disintegration, occurs late, is dependent on extracellular Ca, can be mimicked by A23187, and is thus possibly mediated by a transmembrane influx of Ca. While either component alone is ultimately capable of producing irreversible neuronal injury, the Ca-dependent mechanism predominates at lower exposures to glutamate. Glutamate exposure likely leads to a Ca influx both through glutamate-activated cation channels and through voltage-dependent Ca channels activated by membrane depolarization. Addition of 20 mM Mg, however, did not substantially block GNT; this finding, together with the observation that GNT is largely preserved in sodium-free solution, supports the notion that the activation of voltage-dependent Ca channels may not be required for lethal Ca entry. The possibility that N-methyl-D-aspartate receptors may play a dominant role in mediating glutamate-induced lethal Ca influx is discussed.
Animals↗
N-methyl-D-aspartate receptors mediate hypoxic neuronal injury in cortical culture.
The ability of several glutamate receptor antagonists to reduce hypoxic cortical neuronal injury was quantitatively examined in cell cultures derived from fetal mice. Cultures exposed to hypoxia for 8 hr showed by the following day widespread neuronal injury, which was substantially attenuated by addition of the specific N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (APV). The protective effect of APV was concentration dependent (ED50 about 2 microM) and stereospecific (D-APV approximately 100 times more potent that L-APV). Neuron-protective effects were also observed with several other NMDA antagonists: 2-amino-7-phosphonoheptanoate, phencyclidine and (+)-SKF 10,047 [(+)-N-allylnormetazocine]--as well as with the nonspecific glutamate antagonists D-glutamylglycine and kynurenate. In addition, a similar antagonist profile was observed with a chemical model of hypoxic neuronal injury, produced by brief exposure to high concentrations of cyanide. In contrast, 1 mM concentrations of glutamate diethylester and gamma-aminomethyl sulfonate, compounds reported in some studies to preferentially antagonize non-NMDA glutamate receptors, failed to protect neurons against either hypoxia or cyanide. These results are consistent with the hypothesis that NMDA receptors are preferentially involved in the pathogenesis of hypoxic cortical neuronal injury and suggest that cortical cell culture may be a useful system in which to quantitatively characterize the pharmacology of that injury.
2-Amino-5-phosphonovalerate↗
Dextrorphan and levorphanol selectively block N-methyl-D-aspartate receptor-mediated neurotoxicity on cortical neurons.
Neocortical neurons in cell cultures prepared from fetal mice were impaled for intracellular recording. Dextrorphan (DX), a clinically tested dextrorotatory morphinan lacking opioid action, did not alter neuronal membrane potential or conductance, but produced a selective attenuation of N-methyl-D-aspartate (NMDA) responses; kainate and quisqualate responses were not affected. DX also antagonized morphological and chemical (lactate dehydrogenase efflux) evidence of cortical neuronal cell injury produced by toxic bath exposure to NMDA, quinolinate or glutamate, but did not affect toxic exposure to quisqualate or kainate. This selective antagonism of neurotoxicity was apparent at micromolar concentrations of DX with an ED50 of 13 to 17 microM. A similar, but less potent neuron-protective effect, was seen with the opioid levorotatory enantiomer of DX, levorphanol (ED50, 40 microM). The O-methyl derivative of DX, dextromethorphan, also antagonized NMDA and glutamate neurotoxicity, but with possibly lower efficacy than DX. The higher potency of DX over levorphanol suggests that this novel neuron-protective action is not mediated by classic opiate receptors; it may be mediated at the "sigma opiate"/phencyclidine site. If further studies establish that DX and related compounds retain neuron-protective efficacy in appropriate animal models, the established clinical safety record of DX and dextromethorphan may allow prompt investigation of the NMDA receptor-blockade strategy in certain neurological disease states.
Animals↗