William F. Maragos

Glutamate dysfunction in Alzheimer's disease: an hypothesis

Abstract Glutamate is a major excitatory neurotransmitter that has been implicated in memory formation and learning. This acidic amino acid also has neurotoxic properties, and in animals produces lesions reminiscent of human neurodegenerative diseases. Here we present evidence that supports the hypothesis that glutamate dysfunction is involved in the pathophysiology of Alzheimers disease and can account for many of the neurochemical and behavioral deficits observed in this disease.

GLUTAMATE TRANSMISSION AND TOXICITY IN ALZHEIMER'S DISEASE

1. Despite intensive research, the cause of Alzheimers disease is unknown. 2. Glutamate is the major excitatory transmitter of the cerebral cortex and hippocampus and it appears to have an important role in learning and memory. In addition to its transmitter function, glutamate is a neurotoxin which has been implicated in the pathogenesis of a variety of neurodegenerative disorders. 3. Glutamate toxicity may play a role in the pathogenesis of Alzheimers disease. 4. Disruption of glutamatergic neurotransmission may account, in part, for the learning and memory deficits of Alzheimers disease. 5. Labeling of the glutamate receptor complex may allow in vivo diagnosis by positron emission tomography. 6. Glutamate receptor ligands may provide a means of therapeutic intervention in Alzheimers disease.

Excitatory amino acid binding sites in the hippocampal region of Alzheimer's disease and other dementias

Quantitative receptor autoradiography was used to measure muscarinic cholinergic, benzodiazepine, kainate, phencyclidine (PCP), N-methyl-D-aspartate (NMDA) (measured in Tris acetate), quisqualate-sensitive, non-quisqualate-sensitive and total glutamate (measured in Tris chloride buffer) binding sites in adjacent sections of the hippocampal region of 10 Alzheimers disease, nine control, and six demented, non-Alzheimers disease postmortem human brains. The measurements were compared to the number of neurofibrillary tangles as revealed by Congo red staining of adjacent sections. All assays and measurements were done by observers blinded to the clinical diagnoses. Binding was decreased significantly for all ligands except quisqualate in stratum pyramidale of CA1 of the Alzheimers disease brains. The binding loss was significantly greater for the non-quisqualate and NMDA sites than for the muscarinic, benzodiazepine and kainate sites with the total glutamate and PCP site losses being intermediate. Only the loss of benzodiazepine binding was significantly correlated with the number of neurofibrillary tangles. Lesser binding losses were seen in adjacent areas. This difference in the degree of binding decrease is consistent with the hypothesis that NMDA receptors are located on more distal dendrites of hippocampal neurons. There they may be relatively more vulnerable than the other receptors to the pathological process.

The mitochondrial uncoupler 2,4-dinitrophenol attenuates tissue damage and improves mitochondrial homeostasis following transient focal cerebral ischemia.

Ischemic stroke is caused by acute neuronal degeneration provoked by interruption of cerebral blood flow. Although the mechanisms contributing to ischemic neuronal degeneration are myriad, mitochondrial dysfunction is now recognized as a pivotal event that can lead to either necrotic or apoptotic neuronal death. Lack of suitable ‘upstream’ targets to prevent loss of mitochondrial homeostasis has, so far, restricted the development of mechanistically based interventions to promote neuronal survival. Here, we show that the uncoupling agent 2,4 dinitrophenol (DNP) reduces infarct volume approximately 40% in a model of focal ischemia–reperfusion injury in the rat brain. The mechanism of protection involves an early decrease in mitochondrial reactive oxygen species formation and calcium uptake leading to improved mitochondrial function and a reduction in the release of cytochrome c into the cytoplasm. The observed effects of DNP were not associated with enhanced cerebral perfusion. These findings indicate that compounds with uncoupling properties may confer neuroprotection through a mechanism involving stabilization of mitochondrial function.

Loss of hippocampal (3H)TCP binding in Alzheimer's disease

We have previously demonstrated a marked loss in N-methyl-D-aspartate (NMDA) receptors in the hippocampus and cerebral cortex of patients dying with dementia of the Alzheimer type (DAT). In addition, we have found that the dissociative anesthetic N-(1-[2-thienyl]cyclohexyl)3,4-piperidine ([3H]TCP) binds to a site whose regional distribution is highly correlated with that of NMDA receptor sites. We studied the binding of [3H]TCP to sections of hippocampi from 8 controls, 12 patients with DAT and 7 patients with other dementias. [3H]TCP binding was significantly reduced in strata pyramidalia of CA1/CA2, CA3 and subiculum of DAT hippocampal formation compared to that of control. Labelled dissociative anesthetics could potentially be used with positron emission tomography in the diagnosis of DAT.

Mitochondrial uncoupling as a potential therapeutic target in acute central nervous system injury

Mitochondrial dysfunction, resulting from the disruption of calcium homeostasis and the generation of toxic reactive oxygen species, is a central process leading to neuronal injury and death following acute CNS insults. Interventions aimed at preventing disturbances in mitochondrial function have therefore become targets of intense investigation. Mitochondrial uncoupling is a condition in which electron transport is disconnected from the production of ATP. As a consequence, there is a decrease in the mitochondrial membrane potential, which can temporarily decrease calcium influx and attenuate free radical formation. The potential use of pharmacological agents with uncoupling properties may provide a novel therapeutic approach for the treatment of acute neuronal injury.

Quantitative autoradiographic localization of NMDA, quisqualate and PCP receptors in the frog tectum

An organizing role for the N-methyl-D-aspartate (NMDA) receptor/channel has been suggested in the development of the retinotectal projection in Rana pipiens. The regional distributions of NMDA, phencyclidine (PCP) and quisqualic acid (QA) receptors were quantified using in vitro autoradiography in the tectum of normal and surgically produced 3-eyed juvenile frogs. NMDA and QA receptor binding was highest in the pretectum. Of the tectal layers, the superficial retinotectal synaptic zone, layer 9, had the highest amount of NMDA and QA receptor binding. Moderate binding was observed in layer 5, with little binding in the cellular layer 6. No specific [3H]N-(1-[2-thienyl]cyclohexyl) piperidine ([3H]TCP) binding was observed in any of the tectal regions.

Inhibition of Nitric Oxide Synthase Activity Attenuates Striatal Malonate Lesions in Rats

Abstract: Mitochondrial inhibitors such as malonate are potent neurotoxins in vivo. Intrastriatal injections of malonate result in neuronal damage reminiscent of “excitotoxic” lesions produced by compounds that activate NMDA receptors. Although the mechanism of cell death produced by malonate is uncertain, overactivation of NMDA receptors may be involved; pretreatment of animals with NMDA antagonists provides neuroprotection against malonate lesions. NMDA receptor activation stimulates the enzyme nitric oxide (NO) synthase (NOS). Elevated tissue levels of NO may generate highly reactive intermediates that impair mitochondrial function. We hypothesized that NO may be a mediator of malonate toxicity. We investigated whether in vivo inhibition of NO production by the NOS inhibitor Nω‐nitro‐l‐arginine (NLA) would attenuate lesions produced by intrastriatal injections of malonate. We found that systemic injections of 3 mg/kg of NLA significantly reduced the extent of histologic damage elicited by intrastriatal injections of 1.5 µmol of malonate in adult rats.

A study of cortical and hippocampal NMDA and PCP receptors following selective cortical and subcortical lesions.

The neuronal localization of glutamate and phencyclidine (PCP) receptors was evaluated in the cerebral cortex and hippocampal formation of rat CNS using quantitative autoradiography. Scatchard analysis of [3H]glutamate binding in the cortex (layers I and II and V and VI) showed no difference in the total number of binding sites (Bmax) or apparent affinity (Kd) 1 week, 1 month and 2 months following unilateral ibotenate lesions to nucleus basalis of Meynert (nbM) compared to the non-lesioned side. Quisqualic acid displacement of [3H]glutamate in layers I and II, 1 week following nbM destruction, revealed both high- and low-affinity binding sites (representing the quisqualate (QA) and N-methyl-D-aspartate (NMDA) sites, respectively). Compared to the control side, there was no difference in binding parameters for either of the receptor sites. In similarly lesioned animals, the NMDA receptor was specifically labelled with [3H]glutamate and the associated PCP receptor labelled with [3H]N-(1-[2-thienyl]cyclohexyl)3,4-piperidine ([3H]TCP) in adjacent brain sections. For both receptors, there was no change in the total number of binding sites in the cortex following destruction of nbM. On the other hand, virtually all binding to NMDA and PCP receptors was eliminated following chemical destruction of intrinsic cortical neurons. These results suggest that the NMDA/PCP receptor complex does not exist on the terminals of cortical cholinergic afferents. One week after knife cuts of the glutamatergic entorhinal pathway to the hippocampal formation only an approximate 10% reduction of NMDA and PCP receptors was seen in the dentate gyrus. Conversely, selective destruction of the dentate granule cells using colchicine caused a near identical loss of NMDA and PCP receptors (84% vs 92% respectively). It is concluded from these experiments that glutamate and PCP receptors exist almost exclusively on neurons intrinsic to the hippocampal formation and that no more than 10% of NMDA and PCP receptors exist as autoreceptors on glutamatergic terminals.

The mitochondrial inhibitor malonate enhances NMDA toxicity in the neonatal rat striatum

Intra-striatal injections of the mitochondrial inhibitor malonate elicit age-dependent neuronal damage in rat brain; injury is more extensive in older animals than in young adults. We investigated the neurotoxic potential of malonate in the immature rat brain. We found that 7-day-old (P7) rats were highly resistant to malonate neurotoxicity. Yet, although intra-striatal injections of 1 mumol malonate did not elicit overt tissue injury in P7 rats, co-administration of this dose of malonate with a dose of NMDA close to its toxicity threshold (2.5 nmol) doubled the severity of resulting excitotoxic injury.