Nuri B. Farber

NMDA receptor hypofunction model of schizophrenia

Several decades of research attempting to explain schizophrenia in terms of the dopamine hyperactivity hypothesis have produced disappointing results. A new hypothesis focusing on hypofunction of the NMDA glutamate transmitter system is emerging as a potentially more promising concept. In this article, we present a version of the NMDA receptor hypofunction hypothesis that has evolved from our recent studies pertaining to the neurotoxic and psychotomimetic effects of PCP and related NMDA antagonist drugs. In this article, we examine this hypothesis in terms of its strengths and weaknesses, its therapeutic implications and ways in which it can be further tested.

Ketamine-Induced NMDA Receptor Hypofunction as a Model of Memory Impairment and Psychosis

N-methyl-D-aspartate (NMDA) glutamate receptor antagonists are reported to induce schizophrenia-like symptoms in humans, including cognitive impairments. Shortcomings of most previous investigations include failure to maintain steady-state infusion conditions, test multiple doses and/or measure antagonist plasma concentrations. This double-blind, placebo-controlled, randomized, within-subjects comparison of three fixed subanesthetic, steady-state doses of intravenous ketamine in healthy males (n = 15) demonstrated dose-dependent increases in Brief Psychiatric Rating Scale positive (F[3,42] = 21.84; p < 0.0001) and negative symptoms (F[3,42] = 2.89; p = 0.047), and Scale for the Assessment of Negative Symptoms (SANS) total scores (F[3,42] = 10.55; p < 0.0001). Ketamine also produced a robust dose-dependent decrease in verbal declarative memory performance (F[3,41] = 5.11; p = 0.004), and preliminary evidence for a similar dose-dependent decrease in nonverbal declarative memory, occurring at or below plasma concentrations producing other symptoms. Increasing NMDA receptor hypofunction is associated with early occurring memory impairments followed by other schizophrenia-like symptoms.

Isoflurane-induced Neuroapoptosis in the Neonatal Rhesus Macaque Brain

Background:Brief isoflurane anesthesia induces neuroapoptosis in the developing rodent brain, but susceptibility of non-human primates to the apoptogenic action of isoflurane has not been studied. Therefore, we exposed postnatal day 6 (P6) rhesus macaques to a surgical plane of isoflurane anesthesia for 5 h, and studied the brains 3 h later for histopathologic changes. Method:With the same intensity of physiologic monitoring typical for human neonatal anesthesia, five P6 rhesus macaques were exposed for 5 h to isoflurane maintained between 0.7 and 1.5 end-tidal Vol% (endotracheally intubated and mechanically ventilated) and five controls were exposed for 5 h to room air without further intervention. Three hours later, the brains were harvested and serially sectioned across the entire forebrain and midbrain, and stained immunohistochemically with antibodies to activated caspase-3 for detection and quantification of apoptotic neurons. Results:Quantitative evaluation of brain sections revealed a median of 32.5 (range, 18.0-48.2) apoptotic cells/mm3 of brain tissue in the isoflurane group and only 2.5 (range, 1.1-5.2) in the control group (difference significant at P = 0.008). Apoptotic neuronal profiles were largely confined to the cerebral cortex. In the control brains, they were sparse and randomly distributed, whereas in the isoflurane brains they were abundant and preferentially concentrated in specific cortical layers and regions. Conclusion:The developing non-human primate brain is sensitive to the apoptogenic action of isoflurane and displays a 13-fold increase in neuroapoptosis after 5 h exposure to a surgical plane of isoflurane anesthesia.

Neuronal vacuolization and necrosis induced by the noncompetitive N-methyl-D-aspartate (NMDA) antagonist MK(+)801 (dizocilpine maleate): a light and electron microscopic evaluation of the rat retrosplenial cortex.

MK(+)801 (dizocilpine maleate) is a noncompetitive antagonist at the N-methyl-D-aspartate (NMDA) receptor, the major glutamate receptor at excitatory synapses in the central nervous system. Since NMDA antagonists are neuroprotective, there is interest in their development for treatment of cerebral ischemia. Unfortunately, many of these compounds also induce vacuole formation in neurons of the rat retrosplenial cortex (Olney et al., Science 244: 1360-1362, 1989). Although vacuolization was initially reported to be reversible with MK(+)801, preliminary data later suggested that higher doses might produce neuronal necrosis. To explore this issue, young male Sprague-Dawley rats were given a single subcutaneous dose of vehicle or 1, 5, or 10 mg/kg MK(+)801. At 4 h and 1, 2, 3, 4, 7, and 14 days postdose (DPD), the retrosplenial cortex was examined by light and electron microscopy. At 4 h, vacuoles occurred in neurons of retrosplenial cortical layers 3 and 4 in all rats given MK(+)801. Mitochondria and endoplasmic reticulum contributed to vacuole formation. At 1 DPD, vacuoles or necrotic neurons were rarely observed. At all subsequent time points, necrotic neurons were readily evident in rats given 5 or 10 mg/kg MK(+)801, but only rarely evident in rats given 1 mg/kg. Necrotic neurons were associated with reactive microglial cells that contained electron-dense debris ultrastructurally. If similar dose-dependent neuronal necrosis proves to be a feature of other NMDA antagonists, such effects might raise concerns for the development and use of these compounds in human cerebrovascular diseases.

Drug-induced apoptotic neurodegeneration in the developing brain.

Physiological cell death (PCD), a process by which redundant or unsuccessful neurons are deleted by apoptosis (cell suicide) from the developing central nervous system, has been recognized as a natural phenomenon for many years. Whether environmental factors can interact with PCD mechanisms to increase the number of neurons undergoing PCD, thereby converting this natural phenomenon into a pathological process, is an interesting question for which new answers are just now becoming available. In a series of recent studies we have shown that 2 major classes of drugs (those that block NMDA glutamate receptors and those that promote GABAA receptor activation), when administered to immature rodents during the period of synaptogenesis, trigger widespread apoptotic neurodegeneration throughout the developing brain. In addition, we have found that ethanol, which has both NMDA antagonist and GABAmimetic properties, triggers a robust pattern of apoptotic neurodegeneration, thereby deleting large numbers of neurons from many different regions of the developing brain. These findings provide a more likely explanation than has heretofore been available for the reduced brain mass and lifelong neurobehavioral disturbances associated with the human fetal alcohol syndrome (FAS). The period of synaptogenesis, also known as the brain growth spurt period, occurs in different species at different times relative to birth. In rats and mice it is a postnatal event, but in humans it extends from the sixth month of gestation to several years after birth. Thus, there is a period in pre‐ and postnatal human development, lasting for several years, during which immature CNS neurons are prone to commit suicide if exposed to intoxicating concentrations of drugs with NMDA antagonist or GABAmimetic properties. These findings are important, not only because of their relevance to the FAS, but because there are many agents in the human environment, other than ethanol, that have NMDA antagonist or GABAmimetic properties. Such agents include drugs that may be abused by pregnant mothers (ethanol, phencyclidine [angel dust], ketamine [Special K], nitrous oxide [laughing gas], barbiturates, benzodiazepines), and many medicinals used in obstetric and pediatric neurology (anticonvulsants), and anesthesiology (all general anesthetics are either NMDA antagonists or GABAmimetics).

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.

NMDA Antagonists as Neurotherapeutic Drugs, Psychotogens, Neurotoxins, and Research Tools for Studying Schizophrenia

Antagonists of the N-methyl-D-aspartate (NMDA) subtype of glutamate (Glu) receptor have become the focus of considerable attention as potential neurotherapeutic agents in view of mounting evidence implicating NMDA receptors in acute central nervous system (CNS) injury syndromes such as stroke, trauma, and status epilepticus. In addition, NMDA receptor antagonists are of potential interest for the clinical management of neuropathic pain and preventing the development of tolerance to opiate analgesics. A potentially serious obstacle to the development of NMDA antagonists as neurotherapeutic drugs is the paradoxical fact that whereas these agents do have significant neurotherapeutic potential, they also have psychotogenic and neurotoxic properties. We have been intensively investigating the mechanisms underlying these adverse properties and have discovered several methods of suppressing or preventing their expression. In addition, we have been exploring the possibility that a common mechanism may underlie the psychotogenic and neurotoxic actions of these agents and that this mechanism may have relevance to the pathogenesis of idiopathic psychotic processes such as schizophrenia. In this chapter, we will review our findings pertaining to NMDA antagonists in the dual context of their value as tools for exploring mechanisms underlying neuropsychiatric disturbances, particularly schizophrenia, and their potential promise as therapeutic agents. For additional references and a more complete elaboration of our hypothesis pertaining to NMDA receptor dysfunction and schizophrenia, please see a recent review (Olney and Farber 1995).

The NMDA Receptor Hypofunction Model of Psychosis

Abstract: Antagonists of the NMDA glutamate receptor, including phencyclidine (PCP), ketamine, and CGS‐19755, produce cognitive and behavioral changes in humans. In rodents these agents produce a myriad of histopathological and neurochemical changes. Several lines of evidence suggest that a large number of these drug‐induced effects are dose‐dependent manifestations of the same general disinhibition process in which NMDA antagonists abolish GABAergic inhibition, resulting in the simultaneous excessive release of acetylcholine and glutamate. Progressive increases in the severity of NMDA receptor hypofunction (NRHypo) within the brain produce an increasing range of effects on brain function. Underexcitation of NMDA receptors, induced by even relatively low doses of NMDA antagonist drugs, can produce specific forms of memory dysfunction without clinically evident psychosis. More severe NRHypo can produce a clinical syndrome very similar to a psychotic schizophrenic exacerbation. Finally, sustained and severe NRHypo in the adult brain is associated with a form of neurotoxicity with well‐characterized neuropathological features. In this paper several of these effects of NMDA antagonists and a likely mechanism responsible for producing them will be reviewed. In addition the possible role of NRHypo in the pathophysiology of idiopathic psychotic disorders will be considered.

Architecture and Neurocytology of Monkey Cingulate Gyrus

Human functional imaging and neurocytology have produced important revisions to the organization of the cingulate gyrus and demonstrate four structure/function regions: anterior, midcingulate (MCC), posterior (PCC), and retrosplenial. This study evaluates the brain of a rhesus and 11 cynomolgus monkeys with Nissl staining and immunohistochemistry for neuron‐specific nuclear binding protein, intermediate neurofilament proteins, and parvalbumin. The MCC region was identified along with its two subdivisions (a24′ and p24′). The transition between areas 24 and 23 does not involve a simple increase in the number of neurons in layer IV but includes an increase in neuron density in layer Va of p24′, a dysgranular layer IV in area 23d, granular area 23, with a neuron‐dense layer Va and area 31. Each area on the dorsal bank of the cingulate gyrus has an extension around the fundus of the cingulate sulcus (f 24c, f 24c′, f 24d, f 23c), whereas most cortex on the dorsal bank is composed of frontal motor areas. The PCC is composed of a dysgranular area 23d, area 23c in the caudal cingulate sulcus, a dorsal cingulate gyral area 23a/b, and a ventral area 23a/b. Finally, a dysgranular transition zone includes both area 23d and retrosplenial area 30. The distribution of areas was plotted onto flat maps to show the extent of each and their relationships to the vertical plane at the anterior commissure, corpus callosum, and cingulate sulcus. This major revision of the architectural organization of monkey cingulate cortex provides a new context for connection studies and for devising models of neuron diseases. J. Comp. Neurol. 485:218–239, 2005.

Chapter 28 The glutamate synapse in neuropsychiatric disorders: Focus on schizophrenia and Alzheimer's disease

Here we have described a novel excitotoxic process in which hypofunctional NMDA receptors cease driving GABA ergic neurons which cease inhibiting excitatory transmitters in the brain. These disinhibited excitatory transmitters then act in concert to slowly hyperstimulate neurons in corticolimbic brain regions. We have discussed how such an abnormality could exist in the brains of individuals with schizophrenia or AD and could account for the clinical stigmata of the two disorders. In addition, we have highlighted how other disorder-specific factors would account for the differences in the clinical presentation of AD and schizophrenia. In an animal model, pharmacological methods have been developed for preventing the overstimulation of these vulnerable corticolimbic pyramidal neurons and at least some of these methods may be applicable for treating AD and schizophrenia.