Madelon T. Price

The role of specific ions in glutamate neurotoxicity

When the chick embryo retina is incubated in balanced salt solution containing glutamate (Glu) in 1 mM concentration, a neurodegenerative reaction occurs within 30 min. Here we report that the neurotoxic action of Glu on retinal neurons is dependent on the presence of Na+ and Cl-, but not Ca2+, in the incubation medium. Also, we report that depolarizing concentrations of K+ can induce a severe cytotoxic reaction in chick retina which, like the depolarization-linked neurotoxicity of Glu, is a Cl- dependent phenomenon.

Age-specific neurotoxicity in the rat associated with NMDA receptor blockade: Potential relevance to schizophrenia?

Agents that block the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor induce a schizophrenialike psychosis in adult humans and injure or kill neurons in several corticolimbic regions of the adult rat brain. Susceptibility to the psychotomimetic effects of the NMDA antagonist, ketamine is minimal or absent in children and becomes maximal in early adulthood. We examined the sensitivity of rats at various ages to the neurotoxic effects of the powerful NMDA antagonist, MK-801. Vulnerability was found to be age dependent, having onset at approximately puberty (45 days of age) and becoming maximal in early adulthood. This age-dependency profile (onset of susceptibility in late adolescence) in the rat is similar to that for ketamine-induced psychosis or schizophrenia in humans. These findings suggest that NMDA receptor hypofunction, the mechanism underlying the neurotoxic and psychotomimetic actions of NMDA antagonists, may also play a role in schizophrenia.

A benzodiazepine recognition site associated with the non-NMDA glutamate receptor

GYKI 52466 is a benzodiazepine molecule that has muscle relaxant and anticonvulsant properties not attributable to a gamma-aminobutyric acid receptor-mediated mechanism. Here it is shown that GYKI 52466 exerts no blocking action at N-methyl-D-aspartate (NMDA) glutamate receptors, but acts noncompetitively to block ion currents and associated excitotoxicity, including ischemic neuronal degeneration, mediated through non-NMDA glutamate receptors. The inhibition of non-NMDA responses by GYKI 52466 is antagonized by cyclothiazide, hydrochlorothiazide, and diazoxide, benzothiadiazide drugs that inhibit non-NMDA receptor desensitization. These results suggest that non-NMDA receptor-ion channel complexes may contain a novel benzodiazepine recognition site where receptor desensitization is regulated; this postulated site represents a promising new target for rational development of drugs to treat neurological disorders.

Behavioral effects of MK-801 in the rat

Several experiments were conducted to study the effects of the noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, MK-801, on learning and memory in the rat. Rats displayed impaired performance on several sensorimotor tests and appeared grossly intoxicated when treated IP with 0.2 mg/kg MK-801, but not when treated with lower doses (0.05 or 0.1 mg/kg). Postacquisition performance on two spatial learning tasks involving working memory protocols (reinforced alternation and radial arm maze) was impaired by MK-801 at intoxicating doses (≥0.2 mg/kg) but not at lower doses (0.05 or 0.1 mg/kg). Using a position habit reversal task, we found that rats could learn to reverse a position habit while under the influence of a nonintoxicating dose of MK-801 (0.1 mg/kg), but when tested on the following day performed as if they did not recall what they had learned. Thus, acute administration of a nonintoxicating dose of MK-801 disrupts the retention of new information learned under the influence of the drug but does not interfere with the performance of tasks that are well learned before the drug is administered. Whether the performance deficits on the spatial learning tasks observed only following intoxicating doses of MK-801 reflect an effect on memory is not clear.

The anti-excitotoxic effects of certain anesthetics, analgesics and sedative-hypnotics

Various agents were tested for their ability to antagonize the acute excitotoxic action of N-methyl-DL-aspartate (NMA) and kainic acid (KA) on neurons in the in vitro chick embryo retina. The following compounds (in order of descending potencies) were effective in completely blocking the neurotoxic activity of NMA: phencyclidine, ketamine, (+/-)-SKF 10,047, pentazocine, D-aminophosphonovalerate, D-amino-phosphonoheptanoate, D-alpha-aminoadipate, OH-quinoxaline carboxylate, kynurenate, (+/-)-cis-2,3-piperidine dicarboxylate, secobarbital, amobarbital and pentobarbital. The latter 6 agents also protected against KA toxicity but complete protection was observed only from relatively high concentrations. At 20 mM, Mg2+ blocked NMA toxicity but at concentrations up to 30 mM did not block KA toxicity. Compounds that failed to block either NMA or KA toxicity include D- and L-aminophosphonobutyrate, L-glutamic acid diethyl ester, xanthurenate, GABA and taurine. The chick embryo retina is a useful preparation for identifying agents that have either excitotoxic or anti-excitotoxic activity.

Blockade of both NMDA and non-NMDA receptors is required for optimal protection against ischemic neuronal degeneration in the in vivo adult mammalian retina

Under ischemic conditions, the excitatory amino acids (EAA), glutamate and aspartate, accumulate in the extracellular compartment of brain and, by excessive stimulation of EAA receptors, trigger excitotoxic degeneration of CNS neurons. Since glutamate and aspartate exert excitotoxic activity through both of the generally recognized classes of EAA receptors [N-methyl-D-aspartate (NMDA) and non-NMDA], it follows that both receptor classes may play a role in ischemic neuronal degeneration. Although several laboratories have reported that NMDA receptor antagonists confer protection in vivo against ischemic neuronal degeneration, very little is known about the ability of non-NMDA antagonists to confer such protection, a major reason being that non-NMDA antagonists that penetrate blood-brain barriers have not been available. In the present study, we examined the ability of NMDA or non-NMDA antagonists, either individually or in combination, to prevent neuronal degeneration in vivo in the adult rat retina rendered ischemic by dye/photothrombotic occlusion of retinal blood vessels. In this model, delivery of drugs to the ischemic tissue is assured by intravitreal administration. Intravitreal administration of the NMDA antagonist, MK-801, reduced the severity of ischemic damage approximately 50% (a ceiling effect that could not be increased by administering higher doses). The predominantly non-NMDA antagonist, CNQX, when administered in the highest dose permitted by its solubility limitations, provided equivocal (statistically nonsignificant) protection, but the two drugs combined provided greater than 80% protection.(ABSTRACT TRUNCATED AT 250 WORDS)

MK-801 powerfully protects against N-methyl aspartate neurotoxicity

Using the ex vivo chick embryo retina to study the efficacy of antagonists in blocking the excitotoxic effects of excitatory amino acid agonists, we previously identified phencyclidine as the most powerful known anti-excitotoxin. Here we show that MK-801 is 5 times more powerful than phencyclidine as an anti-excitotoxin, that its antagonism is specific for N-methyl-asparate toxicity, is non-competitive and does not entail inhibition of excitatory amino acid receptor binding.

Domoic acid: A dementia-inducing excitotoxic food poison with kainic acid receptor specificity

Domoic acid (Dom), a rigid analog of the excitotoxic amino acids, glutamate and kainic acid, is believed to be the mussel neurotoxin responsible for a recent food poisoning incident in Canada that killed some people and left others with memory impairment. Since the literature contains very little information pertaining to Dom excitotoxicity, we have systematically evaluated the neuroexcitatory properties of Dom in vitro (cultured hippocampal neurons) and its neurotoxic properties both in vitro (chick embryo retina) and in vivo (adult rat). In the in vitro experiments, the properties of Dom were compared with those of kainic acid, N-methyl-D-aspartate (NMDA) and quisqualate, each of which is a prototypic agonist at a different subtype of glutamate receptor. Currents induced in hippocampal neurons by Dom and kainic acid were identical and displayed a linear current/voltage relationship (in contrast to NMDA currents) and were nondesensitizing (in contrast to quisqualate currents). Dom currents were not blocked by NMDA antagonists but were blocked by CNQX, an antagonist of non-NMDA receptors. In the chick embryo retina, Dom induced a lesion pattern having the same distinctive characteristics as a kainic acid lesion which differs from that induced by either NMDA or quisqualate, and the Dom lesion was blocked by CNQX but not by NMDA antagonists. Subcutaneous administration of Dom (2.5-3 mg/kg) to adult rats resulted in an acute seizure-brain damage syndrome almost identical to that induced in rats by KA (12 mg/kg) and having important features analogous to the neurotoxic syndrome observed in the human food poison victims.

Excitotoxicity of l-DOPA and 6-OH-DOPA: Implications for Parkinson's and Huntington's diseases ☆

Despite several decades of research aimed at elucidating the mechanisms underlying neuronal degeneration in Parkinsons and Huntingtons diseases, these mysteries remain unfathomed. The brain contains high concentrations of the putative transmitters, glutamate and aspartate, which have neurotoxic (excitotoxic) potential and are thought to cause neuronal degeneration in certain acute neurological disorders. However, no mechanism has been identified by which these diffusely distributed agents might cause the regionally selective patterns of neuronal degeneration characterizing Parkinsons and Huntingtons diseases. Here we report that L-DOPA, the natural precursor to dopamine, is a weak excitotoxin and its ortho-hydroxylated derivative, 6-OH-DOPA, is a powerful excitotoxin. We propose that an excitotoxic process mediated by L-DOPA or an acidic derivative such as 6-OH-DOPA might be responsible for degeneration of nigral neurons in Parkinsons disease or striatal neurons in Huntingtons disease.

Excitatory Amino Acid Receptor Potency and Subclass Specificity of Sulfur-Containing Amino Acids

Abstract: The sulfur‐containing amino acids, l‐and d‐cysteate, l‐cysteine, l‐and d‐cysteine sulfinate, l‐and d‐cysteine‐S‐sulfate, l‐cystine, l‐and d‐homocysteate, l‐and d‐homocysteine sulfinate, l‐homocysteine, l‐serine‐O‐sulfate, and taurine were tested in two excitatory amino acid receptor functional assays and in receptor binding assays designed to label specifically the AAl/N‐methyl‐d‐aspartate (NMDA), AA2/quisqualate, and AA3/kainate receptor recognition sites, as well as a CaCla‐dependent l‐2‐amino‐4‐phosphonobutanoate site, and a putative glutamate uptake site. Agonist efficacies were determined by chick retinal excitotoxicity and stimulated sodium efflux from rat brain slices. d‐Homocysteine sulfinate, l‐homocysteate, and l‐serine‐O‐sulfate had affinities most selective for the NMDA binding site, whereas the binding affinities of d‐cysteate, d‐cysteine sulfinate, d‐homocysteate, and l‐homocysteine sulfinate were less selective. However, the correlation of agonist activity sensitive to blockade by d‐2‐amino‐7‐phosphonoheptanoate or d‐2‐amino‐5‐phosphonopentanoate in the functional assays with affinity in the NMDA binding assay (r= 0.87, p < 0.005 and r= 0.98, p < 0.005 for excitotoxicity and sodium efflux, respectively) allows characterization of these sulfur‐containing amino acids as acting at NMDA subclass receptors. l‐Homocysteate, which has been found in the brain, and l‐serine‐O‐sulfate are selective agonists and could serve as endogenous neurotransmitters at the NMDA receptor.