The Glutamate Cascade: Common Pathways of Central Nervous System Disease States

Speaker Abstracts

Somatosensory primary afferent axons that respond specifically to tissue-damaging stimuli (nociceptors) use glutamate (and aspartate) as a neurotransmitter. Neurons in the spinal cord dorsal horn that respond to nociceptor input express all three subtypes of glutamate receptor.

In inflammation, nociceptors with unmyelinated axons (C-nociceptors) are sensitized such that they discharge spontaneously, respond to normally innocuous stimuli, and have a supernormal response to noxious stimuli. Nociceptor sensitization underlies the spontaneous pain and hypersensitivity of the injured site. The area surrounding the injured site also becomes a source of pain and hypersensitivity, and this is now known to be due, at least in part, to an NMDA receptor-mediated hyperexcitability in spinal neurons evoked by C-nociceptor input from the injured site. NMDA receptor blockade reduces the pain of inflammation, but has little or no effect on an acute pain stimulus. The chronic pain syndromes due to peripheral neuropathies are now believed to be related, in part, to a similar NMDA receptor-mediated hyperexcitability in spinal neurons. Axotomized C-nociceptors begin to discharge ectopically, and this has the same effect as the discharge of sensitized C-nociceptors it evokes NMDA receptor-mediated central hyperexcitability. NMDA receptor antagonists block neuropathic pain in animals, and early clinical trials suggest that they are effective drugs for human painful peripheral neuropathies. High levels of activity at NMDA receptors in the brain are known to lead to neuronal death or dysfunction. There is evidence that pain-evoked glutamate release is excitotoxic to small, presumed inhibitory interneurons, in the spinal dorsal horn.

Since most fast excitatory synaptic neurotransmission in the mammalian brain is mediated by glutamate, it is plausible that many neurological or psychiatric disorders may be associated with abnormalities in the glutamatergic signaling system, and that therapeutic benefits might be attainable through the manipulation of this system. A prominent example of both the potential benefits, and the complex pitfalls, associated with the therapeutic manipulation of the glutamate system can be found in the setting of ischemic brain injury. The excitotoxic overactivation of glutamate receptors likely contributes directly to the neuronal and glial cell loss induced by ischemia, to a large extent reflecting excess Na+ and Ca2+ influx and, at least in the case of neurons, consequent cell necrosis. However, efforts to administer either calcium channel blockers, or pharmacological N-methyl-D-aspartate (NMDA) receptor antagonists acutely to patients suffering focal brain ischemia have so far been disappointing. The latter drugs have produced substantial side effects, and neither class of drugs has exhibited clear protective benefits. While the side effects of glutamate antagonists may be surmountable through improving drug selectivity at the level of receptor subtypes or spatial localization, lack of efficacy may point to a flaw at the level of underlying assumptions. Perhaps in man, reduction of ischemic excitotoxicity will require block of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in addition to block of NMDA receptors. Another possible problem may be lie in the growing body of evidence that suggests that ischemia may trigger substantial amounts of programmed cell death, culminating in apoptosis, in addition to excitotoxic necrosis. One can wonder whether reduction in intracellular free calcium induced by the pharmacological blockade of glutamate receptors or even voltage-gated calcium channels might enhance brain cell susceptibility to apoptosis enough to negate the benefits associated with attenuating excitotoxic necrosis.

The glutamatergic system may be important in the pathogenesis and the pathophysiology of Parkinson's disease (PD). A defect in mitochondrial energy metabolism has been found in PD, and we and others have demonstrated that mitochondrial dysfunction can lead to secondary or "weak" excitotoxicity. Metabolic impairment depletes ATP, depresses Na+/K+-ATPase activity and causes graded neuronal depolarization. This relieves the voltage-dependent Mg++ block of the NMDA receptor and allows innocuous levels of glutamate to become lethal. Mitochondrial impairment also disrupts cellular calcium homeostasis. Increased activation of the NMDA receptor, in turn, leads to further mitochondrial impairment and damage. In large part, this occurs because calcium entering a neuron through NMDA receptors has "privileged" access to mitochondria where it leads to free radical production and mitochondrial depolarization. Thus, there may be a feed-forward cycle wherein mitochondrial dysfunction causes NMDA receptor activation which leads to further mitochondrial impairment. In this scenario, NMDA receptor antagonists may be neuroprotective.

Once PD is established, it is clear that several glutamatergic pathways, including the corticostriatal and subthalamofugal projections, become overactive. This causes regulatory changes in basal ganglia glutamate receptor subunits. Moreover, it suggests that glutamate receptor antagonists may be useful as antiparkinsonian medications. We have shown in MPTP-treated parkinsonian monkeys that NMDA and non-NMDA antagonists have antiparkinsonian efficacy. They may also suppress the involuntary movements that are associated with chronic dopaminergic therapy. Several clinical trials are now underway to test whether manipulation of the glutamatergic system has beneficial effects in PD. Initial results should be available by the end of the year. (Supported by NS33779, AG11755, AG14648 and the Huntington's Disease Society of America.)

Pharmacological studies in the mammalian brain have revealed three classes of glutamate activated ligand gated receptor channels, AMPA, kainate and NMDA. Because of a lack of specific drugs, it has been difficult to separate synaptic currents mediated by AMPA and kainate receptors. Thus, the role of the kainate receptor system in the brain has not been studied intensively and remains a mystery. We have used a genetic approach to define the function of kainate receptors in the mouse brain. Mutations in the kainate receptor genes have been made in mice in order to elucidate the function of the kainate receptor system in the brain. We have produced a null mutation in the mouse GluR 6 kainate receptor subunit. The homozygous GluR 6 "knock-out" mouse is viable and healthy, although there is about a 10 percent reduction in weight. Both GluR 6 subunit specific RNA and GluR 6 subunit protein are not detected in the mutant mouse brain. Kainate binding is absent in areas of the brain which normally have high levels of GluR 6 specific RNA, i.e. the CA3 region and the dentate gyrus in the hippocampus. Analysis of synaptic transmission at the mossy fiber- CA3 synapse demonstrates that kainate receptors mediate a post-synaptic current which is absent in the GluR6 "knock-out" mouse. Behavioral analysis of the GluR6 "knock-out" mouse showed that they exhibit reduced locomotion activity but that they can learn motor and water maze tasks. The homozygous GluR 6 "knock-out" mouse is resistant to kainate induced seizures as assayed by seizure onset and immediate early gene activation in the hippocampus. These results prove that the kainate receptor subunit GluR 6 is a subunit of a kainate receptor which mediates a post-synaptic current during synaptic transmission in the hippocampus as well as the epileptogenic effects of kainate, a model of human temporal lobe epilepsy. This observation suggests that drugs which target the GluR 6 subunit may have therapeutic value for treatment of epilepsy.

Physical dependence on alcohol, which is one component of the alcohol dependence syndrome, is thought to result from an adaptation of the CNS to the chronic presence of alcohol, and is defined by the appearance of withdrawal signs and symptoms, including seizures (convulsions), upon cessation of alcohol intake. Neuronal systems that undergo adaptation to chronic ethanol exposure may be those that are sensitive to acute perturbation by ethanol. Acutely, ethanol is a potent inhibitor of the function of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. Following chronic exposure of animals to alcohol, there is evidence for an up-regulation of NMDA receptors, and/or a change in NMDA receptor subunit composition, in several brain areas. The time course of this up-regulation parallels the time course of alcohol withdrawal convulsions, and these convulsions can be attenuated by NMDA receptor antagonists. Prevention of the up-regulation of the NMDA receptor by ganglioside treatment also reduces withdrawal seizures, supporting the role of changes in the NMDA receptor in alcohol withdrawal/physical dependence. NMDA receptor function is also increased in neuronal cultures exposed chronically to ethanol in vitro, and these ethanol-exposed/withdrawn neurons show enhanced susceptibility to glutamate-induced excitotoxic death. This toxicity can be blocked with NMDA receptor antagonists, but not with diazepam. Withdrawal-induced glutamate excitotoxicity may underlie the neuronal damage observed in chronic alcoholics. These findings suggest the possibility of development of therapeutic agents to treat both alcohol withdrawal seizures and withdrawal-induced neuronal damage. (Supported by NIAAA and the Banbury Foundation.)

Glutamate receptor expression is developmentally regulated and data suggest that this is likely to contribute to age-dependent differences in vulnerability to hypoxic/ischemia and seizure-induced neuronal death. Glutamate neurotransmission is generally enhanced in the immature brain at ages when certain glutamate receptors are transiently overexpressed and/or receptor subunit composition differs compared to the adult. Glutamate receptors play a critical role in neuronal plasticity and activity mediated growth during brain development. However, the immature brain appears to be more vulnerable than the adult to certain forms of excitotoxic neuronal injury, suggesting that the maturational state of glutamate receptors modifies response to injury. Furthermore, clinical data demonstrate that seizures are most common in the neonatal brain, and animal models suggest that this heightened excitability correlates with transiently enhanced glutamate receptor activity. We have shown that glutamate receptor expression can be modified by brain injury during early development. Both hypoxia and seizures alter glutamate receptor subunit expression and in some cases chronically alter neuronal excitability. Given the maturational differences in excitatory amino acid mediated injury, age specific therapeutic strategies represent an important future research direction. The immature brain represents a unique challenge because therapeutic strategies will have to be devised that minimize adverse effects on physiological roles of the glutamate receptor in plasticity and synaptic development.

Neuronal injury in a number of neurodegenerative disorders, including AIDS dementia and stroke, is mediated by excitotoxic pathways leading to free radical generation. Apoptosis results if the initial insult is relatively mild, but necrosis intervenes if the initial insult is fulminant [ref. 1-6]. How can HIV-1 result in neuronal damage if the predominant cell type infected in the CNS is the macrophage/microglia? Experiments from several laboratories have lent support for the existence of HIV- and immune-related excitotoxins secreted by activated microglia and astrocytes. These substances may include arachidonic acid, platelet-activating factor, free radicals (NO. and O2.), glutamate, quinolinate, cysteine, amines, cytokines (TNF-, IL1-, IL-6), and as yet unidentified factors (Fig. 1). A final common pathway for neuronal susceptibility appears to underlie AIDS dementia, similar in many ways to that observed in focal cerebral ischemia or stroke. This mechanism involves overactivation of glutamate receptors, predominantly the N-methyl-d-aspartate (NMDA) subtype, followed by excessive influx of calcium and the generation of free radicals. After outlining these pathways, this lecture will discuss the development of clinically-tolerated NMDA antagonists, including (i) the open-channel blocker memantine, and (ii) redox-active, NO-related species such as nitroglycerin (see Fig. 2) [ref. 7].

References

[1] Ankarcrona, M.; Dypbukt, J.M.; Bonfoco, E.; Zhivotovsky, B.; Orrenius, S.; Lipton, S.A.; and Nicotera, P. Glutamate-induced neuronal death: A succession of necrosis or apoptosis depending on mitochondrial function. Neuron 15:961-973, 1995.

[2] Bonfoco, E.; Krainc, D.; Ankarcrona, M.; Nicotera, P.; and Lipton, S.A. Apoptosis and necrosis: Two distinct events induced respectively by mild and intense insults with NMDA or nitric oxide/superoxide in cortical cell cultures. Proc Natl Acad Sci USA 92:7162-7166, 1995.

[3] Lipton, S.A. Neurobiology: HIV displays its coat of arms. Nature 367:13-114, 1994.

[4] Lipton, S.A.; Choi, Y.-B.; Pan, Z.-H.; Lei, S.Z.; Chen, H.-S.V.; Sucher, N.J.; Loscalzo, J.; Singel, D.J.; and Stamler, J.S. A redox-based mechanism for the neuroprotective and neurodestructive effects of nitric oxide and related nitroso-compounds. Nature 364:626-632, 1993.

[5] Lipton, S.A., and Gendelman, H.E. The dementia associated with the acquired immunodeficiency syndrome. N Engl J Med 332:934-40, 1995.

[6] Lipton, S.A., and Rosenberg, P.A. Mechanisms of disease: Excitatory amino acids as a final common pathway for neurologic disorders. N Engl J Med 330:613-622, 1994.

[7] Das, S.; Sasaki, Y.F.; Rothe, T.; Premkumar, L.S.; Takasu, M.; Crandall, J.E.; Dikkes, P.; Connor, D.A.; Rayudu, P.V.; Cheung, W.; Chen, H.-S.V.; Lipton, S.A.; and Nakanishi, N. Increased NMDA current and spine density in mice lacking the NMDAR subunit, NR3A. Nature, in press.

Animal studies have shown injury can sensitize spinal sensory neurons and cause prolonged and severe pain through excitation mediated via glutamate receptors. N-methyl-D-aspartate (NMDA) receptors are the subclass of glutamate receptors that have been most extensively studied. Although ketamine reduces a number of types of experimental and clinical pain in humans, the drug also causes severe cognitive and affective side effects, making it unattractive for chronic treatment. We have carried out clinical trials of the orally available NMDA receptor antagonist and anti-tussive, dextromethorphan in patients with painful diabetic neuropathy and postherpetic neuralgia. Dextromethorphan, 400 mg/day, reduced pain in 32 patients with diabetic neuropathy by about 25 percent relative to placebo, but had little effect on post-herpetic neuralgia.

Non-NMDA glutamate receptors may also be involved in pain and hyperalgesia. We obtained LY293558 from Eli Lilly to assess its analgesic and anti-hyperalgesic effects in human volunteers. This was the first administration of this compound or any AMPA/kainate blocker to humans, so we carried out extensive Phase 1 safety testing as well as pain studies in 26 human volunteers. During rising-dose studies in each subject, we assessed responses to brief painful electrical and heat stimuli administered to undamaged skin. After the highest dose not producing significant side effects (maximal tolerated dose, MTD) was determined for each subject, he or she entered a 3-session, randomized, double-blind crossover study comparing the effects of the MTD, 1/3 MTD, or placebo on capsaicin-evoked pain, and areas of mechanical hyperalgesia and allodynia. There was a dose-related reduction of pain, hyperalgesia, and allodynia.Allodynia and hyperalgesia were reduced by about half by the 1/3 MTD, a dose that had no greater side effects than placebo. This promising result led Eli Lilly to carry out further studies with superselective compounds, which suggested that the anti-hyperalgesic effect was due to blockade of GluR5 kainate-type receptors, while sedative effects result from GluR1-4 AMPA receptors. If current studies of LY 293558 in patients with clinical pain confirm these results, kainate antagonists may offer a powerful and nontoxic treatment for pain.

Recent immunohistochemical studies have demonstrated both a high degree of specificity in hippocampal circuits with respect to representation of glutamate receptor families and subunits, as well as a surprising degree of glutamate receptor plasticity in response to aging, deafferentation, and neuroendocrine status. We have hypothesized that such glutamate receptor plasticity underlies age-related functional impairment, as well as modifications in excitatory transmission and morphology that can be related to circulating estrogen levels. Three examples of such plasticity will be presented: 1) A subunit and circuit-specific decrease in NMDAR1 in the perforant path terminal zone of the dentate gyrus molecular layer in aged primates; 2) An increase in NMDAR1 protein and mRNA in the dentate gyrus molecular layer following perforant path lesions in rats; and 3) an estrogen-induced increase in NMDAR1 protein, but not mRNA, in CA1 and dentate gyrus following ovariectomy. These data will be discussed in the context of age-related memory impairment in the absence of neuron loss, as well as their relevance to neurodegenerative disorders such as Alzheimer's disease.

The NMDA receptor has been implicated in a wide variety of neurological functions, acting in many cases through the activation of neuronal nitric oxide synthase (nNOS). Work from a number of laboratories has implicated NMDA receptors in the production of tolerance to opioid analgesics, as shown by the prevention of morphine tolerance by noncompetitive NMDA antagonists such as MK801. Subsequent work has now extended these observations to various other classes of NMDA antagonists, including competitive antagonists and glycine-site antagonists. Tolerance to delta drugs also is sensitive to these same agents. The loss of analgesic potency by delta drugs with repeated administration is prevented by these NMDA antaognists. Kappa analgesics, however, yield a different picture. In our studies, we have not observed an effect of NMDA agents on either kappa1 or kappa3 tolerance, although there are some suggestive reports in the literature. In many situations, stimulation of NMDA receptors leads to the activation of neuronal nitric oxide synthase (nNOS). Inhibition of this enzyme also interferes with the production of mu and delta tolerance, without altering kappa tolerance. Many splice variants of nNOS exist, raising the question of whether the role of nNOS in morphine tolerance may be limited to a single isoform. Using antisense approaches, we have selectively downregulated the major form of nNOS and a minor nNOS splice variant and looked at their pharmacology. The predominant nNOS isoform is important in the development of morphine tolerance, resulting in a loss of morphine analgesic activity. However, the minor nNOS variant has an opposite action. Unlike the major isoform, the minor isoform facilitates morphine analgesia. These observations illustrate the complexity of the NMDA/nNOS system in pain perception and opioid tolerance.

Traditional "anti-addictive" pharmacotherapies are "drug-targeted," despite the fact that all of the drugs of abuse produce the same reinforcing (rewarding) effects and their long term administration results in drug addiction. However, the same inhibitory actions of N-methyl-D-aspartate (NMDA) receptor antagonists on phenomena related to drug seeking behavior produced by all addictive substances suggest that glutamatergic receptors may be a final common pathway to all addictions and a possible therapeutic target. Presented are the data illustrating inhibitory effects of NMDA receptor antagonists on the expression and maintenance of physical and motivational aspects of morphine dependence in rats and mice. Next, the data concerning inhibitory effects of several (uncompetitive: memantine, glycine/NMDA: L-701,324 and competitive: NPC 17742) NMDA receptor antagonists on the acquisition, expression and extinction of morphine-induced conditioned reward are discussed. Brain areas sensitive to the treatment with NMDA receptor antagonists have been identified as the administration of NPC 17742 into the nucleus accumbens and ventral tegmental area produced the same, inhibitory effects on the expression of morphine-conditioned reward. These preclinical data are followed by presentation of preliminary clinical findings suggesting that the uncompetitive NMDA antagonist, dextromethorphan may facilitate detoxification from heroin and inhibit craving for this drug in human addicts. Despite the "bad reputation" of some NMDA receptor antagonists that produce ataxia, memory disturbances and psychotomimetic effects, some low affinity, highly voltage dependent uncompetitive antagonists like memantine and dextromethorphan as well as glycine/NMDA site antagonists may have beneficial effects on drug dependence and addiction.

The natural history of drug dependence is characterized by phases of acquisition, maintenance, extinction and relapse of drug taking. All these phases have now been successfully mimicked using a rodent model of intravenous drug self-administration and the psychostimulant addiction cycle has been extensively investigated. Much evidence suggests that glutamate may play a critical role. During the first days of acquisition of cocaine-seeking behavior a glutamate-dependent enhancement of synaptic efficacy was found using evoked field responses in the rat nucleus accumbens, a critical neural substrate mediating cocaine self-administration. Furthermore, intact glutamatergic neurotransmission was shown to be essential for the full expression of the reinforcing properties of cocaine during the maintenance phase of cocaine self-administration. Later phases of cocaine dependence were also found to be affected by indirect glutamatergic manipulation. Inhibition of nitric oxide synthesis reduced the reinforcing properties of cocaine, cocaine craving during the extinction phase and later relapse into cocaine-seeking behavior. Therefore, a potential for medication development targeting glutamate neurotransmission exists and is further supported by preclinical observations showing that dextromethorphan, a clinically safe medication with glutamate antagonistic properties, reduced the absolute reinforcing properties of cocaine. Glutamate-dependent changes of synaptic efficacy within critical limbic circuits may therefore be part of the early events of the addiction cycle leading to the development of abuse and part of the long-lasting behavioral changes of the natural history of dependence, warranting further studies on pharmacological intervention at the glutamate level for the therapy of drug abuse.

Several lines of evidence establish the importance of glutamate-mediated neurotransmission in epilepsy and epileptogenesis. (i) Massive release of glutamate has been demonstrated in human brain during seizures. (ii) Glutamate and other agonists of glutamate receptors are powerful convulsants. (iii) Manipulations that enhance glutamate release or potentiate glutamate receptor function can result in epileptiform activity and seizures. (iv) Modification of glutamate receptor function in transgenic animals can lead to the development of epilepsy. Most importantly, however, antagonists of glutamate receptors have broad spectrum antiseizure activity in in vitro and in vivo model systems. The possible use of NMDA receptor antagonists in the treatment of seizures has been extensively investigated, but results to date have not been encouraging mainly because the NMDA receptor antagonists evaluated have had significant toxicities. There are a variety of novel approaches currently under investigation that may lead to NMDA receptor antagonists with improved side effect profiles, including selective antagonists of NMDA receptors containing the NR2B subunit, glycine site antagonists and low affinity channel blockers. Recently, selective antagonists of non-NMDA (AMPA/kainate) receptors have become available and provide a new direction for the development of epilepsy therapies. Potential approaches include 2,3-benzodiazepine selective allosteric AMPA antagonists, modulators of AMPA receptor desensitization and antagonists of Ca2+-permeable AMPA receptors (i.e., those lacking the GluR2 subunit). Recent studies have suggested that kainate receptors may be particularly promising targets for epilepsy therapy as they seem to play a dual role as inhibitors of GABA release as well as in mediating postsynaptic excitation. Emerging work on kainate receptor-selective antagonists has focused on drugs that target kainate receptors containing the GluR5 subunit.

Most currently prescribed antianxiety agents act by enhancing GABAergic neurotransmission. Moreover, converging lines of evidence suggest GABAergic dysfunction can contribute to the pathophysiology of anxiety. Nonetheless, both anxiolytic and anxiogenic actions can be produced by agents that affect other neurotransmitter systems indicating that like other psychiatric disorders, anxiety (and anxiolysis) is not mediated by a single neurotransmitter system. It has been proposed that disruption of the homeostatic balance between fast-acting, ligand-gated ion channels utilizing glutamate and GABA as transmitters can result in the development and expression of anxiety. Strategies that reduce transmission at the N-methyl-D-aspartate (NMDA) subtypes of glutamate receptor are effective in preclinical tests that predict anxiolytic activity in humans. For example, functional NMDA antagonists acting at the multiple, allosterically linked sites on this family of ligand-gated ion channels mimic the effects of clinically active agents in the elevated plus maze, ultrasonic vocalization, potentiated startle, and thirsty rat (Vogel's) conflict tests. Moreover, compounds that reduce glutamate release via presynaptic metabotropic glutamate receptors exhibit a similar, but not identical spectrum of activity. I will discuss the potential pitfalls and advantages of achieving an anxiolytic action through modulating glutamatergic transmission.

Work from our laboratory and others has demonstrated that the development of sensitization requires glutamate transmission, suggesting mechanistic similarities to other forms of neuronal plasticity. More recently, we have shown that sensitization involves adaptations in glutamate transmission in the mesoaccumbens DA system. After repeated amphetamine, VTA DA cells are supersensitive to the excitatory effects of glutamate and AMPA. This is transient, present after 3 but not 14 days of withdrawal, and does not reflect increased expression of AMPA receptor subunits (measured at protein or mRNA levels). NAc neurons show subsensitivity to glutamate, NMDA and AMPA at both withdrawals. Decreased levels of mRNA and immunoreactivity for GluR1, GluR2 and NR1 subunits were found in NAc at the 14 day withdrawal time, which may in part explain the electrophysiological findings. We also examined PFC, a major source of glutamate projections to VTA. After 3 days' withdrawal, we found increased GluR1 expression and increased sensitivity of PFC cells to glutamate. This may contribute to transient increases in excitatory drive to VTA DA cells thought to mediate induction of sensitization. Increases in excitatory drive may also reflect effects of amphetamine within the VTA, since microdialysis studies have found a delayed increase in glutamate efflux after systemic or local amphetamine. This is prevented by agents that also prevent sensitization (MK-801 and SCH 23390), suggesting a link between increased glutamate efflux and induction. The link could reflect LTP-like mechanisms or mild excitotoxic damage to DA neurons, either of which could result in transient hyperexcitability. (Supported by DA09621.)

There is no indication from the current assessment of humans of any psychotropic side effects of acamprosate or any potential for abuse or dependence. Animals will self-administer MK-801, but not acamprosate, and cannot be trained to discriminate subjective effects of acamprosate or to substitute acamprosate for alcohol in a drug discrimination paradigm. Acamprosate has been shown to be devoid of hypnotic, anxiolytic or muscle relaxant properties. There is no evidence of any antidepressant or other psychotropic effect of acamprosate (see Spanagel and Zieglgänsberger, TIPS 18, 1997).

In the late 1950s the ability of the dissociative anesthetic drug phencyclidine (1-(1-phenylcyclohexyl) piperidine; PCP) to elicit a prolonged psychotic state in surgical patients was recognized. Experimental studies then revealed the ability of single small doses of PCP to elicit brief schizophrenia-like symptoms and cognitive abnormalities in normal volunteers. Similar experiments in previously stabilized chronic schizophrenic subjects led to prompt rekindling and exacerbation of disease-specific symptoms persisting up to many weeks. In the 1970s PCP became a prominent drug of abuse; in many cases patients with PCP-induced psychotic reactions could not be clinically distinguished from patients with naturally-occurring schizophrenia. In 1979 a unique brain binding site, selective for drugs with PCP-like behavioral properties, was discovered and characterized. In the 1980s whole-animal electrophysiological evidence demonstrated that PCP-like drugs were potent antagonists of the N-methyl-D-aspartate (NMDA) class of glutamate receptors. This interaction was confirmed at the cellular and biochemical levels. Characterization of the mode of activation of the NMDA receptor complex revealed multiple regulatory and modulatory sites, including a PCP binding site located within the ligand-gated ionophore, and a non-strychnine-sensitive site at which glycine served as an obligatory co-agonist, the local glycine concentration determining the degree of agonist-induced channel activation. Several lines of evidence support the hypothesis that NMDA receptor-mediated neurotransmission is abnormal in schizophrenia. These include observations that 1) large oral doses of glycine diminish negative-symptom ratings in neuroleptic-treated schizophrenic patients; 2) neuroanatomical abnormalities reported in schizophrenia correspond to areas rich in NMDA receptors; 3) PCP-like drugs induce abnormalities in cortical evoked potentials similar to those seen in schizophrenia; 4) neuropsychological abnormalities in schizophrenia involve glutamatergic mechanisms; and 5) a variety of additional neurochemical, neuroanatomical and physiological findings.