Glutamate deficits in Alzheimer's disease

Hippocampal plasticity during the progression of Alzheimer's disease

Neuroplasticity involves molecular and structural changes in central nervous system (CNS) throughout life. The concept of neural organization allows for remodeling as a compensatory mechanism to the early pathobiology of Alzheimer's disease (AD) in an attempt to maintain brain function and cognition during the onset of dementia. The hippocampus, a crucial component of the medial temporal lobe memory circuit, is affected early in AD and displays synaptic and intraneuronal molecular remodeling against a pathological background of extracellular amyloid-beta (Aβ) deposition and intracellular neurofibrillary tangle (NFT) formation in the early stages of AD. Here we discuss human clinical pathological findings supporting the concept that the hippocampus is capable of neural plasticity during mild cognitive impairment (MCI), a prodromal stage of AD and early stage AD.

The neurochemistry of Alzheimer type, vascular type and mixed type dementias compared

We present the results of a meta-analysis of neurochemical changes in human post mortem brains of Alzheimer type (AD), vascular type (VD) and mixed type (MF) dementias, and matched controls based on 275 articles published between January 1980 and February 1994. Severity of degeneration between the different neurochemical systems is as follows, although ranking is difficult with regard to limited numbers of investigations in some neurochemical systems: Cholinergic system > serotonergic system > excitatory amino acids > GABAergic system > energy metabolism > NA > oxidative stress parameters > neuropeptides > DA. But, within a neurochemical system, degeneration is not evenly distributed. Spared parameters, e.g. muscarinic receptors and MAO-B, allow to make some suggestions for future therapeutic strategies.

Excessive glutamine sensitivity in Alzheimer's disease and Down syndrome lymphocytes

In addition to clinical and neuropathological similarities between Alzheimer's disease and Down syndrome there are genetic and biochemical data which suggest common disease mechanism. Using an in vitro assay examining variations of the mitotic index in the presence or absence of various inhibitors or metabolites of purine and/or pyrimidine synthesis, we studied 19 Alzheimer disease patients and 16 patients with both Down syndrome and Alzheimer type dementia. A highly significant decrease in mitotic index in the presence of exogenous glutamine was noted in patients presenting an Alzheimer type dementia with or without associated Down syndrome. These findings suggest that glutamine sensitivity or some dysregulation of the glutamine/glutamate pathway may play a role in the pathogenesis of Alzheimer's disease. If these findings are confirmed, they would have important implications in the development of preventive strategies.

D-aspartate binding to the glutamate uptake site in human brain tissue--effects of leucotomy

Binding of [3H]D-aspartate, as an indicator of glutamate uptake sites, was investigated in post-mortem human brain tissue by use of a centrifugation assay to separate free and bound ligand. Binding was displaceable, apparently saturable and to a single site, with mean KD and Bmax values of 2.3 microM and 40.3 nmol/g tissue in the frontal cortex. The method was applied to the study of tissue from frontal and temporal cortices and the caudate nucleus of five psychiatric patients who had undergone a frontal leucotomy. The effects of this neurosurgical procedure were to diminish by almost 50% the density of D-aspartate binding sites in the frontal cortex and caudate nucleus, while the temporal cortex was less affected. It is concluded that the method provides a potentially useful correlate of glutamatergic innervation in human brain tissue.

Age-related changes in binding to excitatory amino acid uptake sites in human cerebellum

In vitro autoradiography and test-tube assay of the sodium-dependent binding of D-[3H]aspartate were used to localize and quantify the uptake site for the excitatory amino acid neurotransmitters glutamate and aspartate in the cerebellar cortex of human cerebellar hemispheres. Autoradiograms revealed a pronounced heterogeneity in the distribution of D-[3H]aspartate binding in cortex from adult brains, with the highest binding density corresponding to the Purkinje cell layer, high binding in molecular layer and low binding in granule cell layer. In contrast, cerebellar cortex from infants at term (40 weeks gestation) had only low binding of the ligand in both the molecular and the Purkinje cell layers. Both methods employed for measuring D-[3H]aspartate binding showed that the number of binding sites in Purkinje and molecular layers increased rapidly from term to 20 weeks postnatal age and achieved levels higher than those found in adult cerebellum. It is concluded that a substantial increase in the numbers of glutamate/aspartate uptake sites takes place in the human cerebellum during the early postnatal period. It is deduced that the excess uptake sites are eliminated as the cerebellum matures.

Excitatory amino acids and memory: evidence from research on Alzheimer's disease and behavioral pharmacology

The excitatory amino acid transmitter (EAA) system is believed to play a crucial role in a variety of physiological processes related to neuronal plasticity. Substantial neurophysiological evidence suggests that, under normal physiological conditions. EAAs may be involved in mechanisms underlying learning and memory. However, overactivity of this system produces excitotoxic damage to neurons which is believed to be an etiological factor in various neurological conditions, such as epilepsy, and stroke-induced impairments. The fact that EAAs have been implicated in both, normal cognitive function and in degenerative neurological conditions suggests that they may contribute to the etiology of Alzheimer's disease (AD), because AD is characterized by memory deficits and specific histopathological signs of neuronal damage. This paper summarizes information regarding 1) the involvement of EAAs in Alzheimer's disease and 2) results from psychopharmacological studies of EAAs in laboratory animal models of learning. Investigations of the pathophysiology of AD indicate that glutamatergic deficits are associated with this syndrome. However, there is controversy concerning the nature of this defect. As a result, it is unclear whether it is a consequence of excitotoxic changes produced by glutamatergic overactivity or result from a decrease in glutamatergic function. Evidence from behavioral studies is consistent with the conclusion that EAAs may be involved in the acquisition of conditioned responses. However, in parallel with the clinical findings, learning impairments have been produced by treatments which either increase or decrease activity within this transmitter system. Therefore, although present results suggest that the EAAs play a role in cognition and in clinical syndromes in which such function is compromised, the specific nature of that role needs to be elucidated by future research.

Age-related changes in binding to excitatory amino acid uptake site in temporal cortex of human brain

The binding of D-[3H]aspartate to the specific uptake site for the excitatory amino acids glutamate and aspartate was measured in homogenates of temporal lobe cortex taken at postmortem from 76 human infant and adult brains. Binding levels were very low in brains of preterm and term infants but increased rapidly during the first 20 postnatal weeks to reach levels which exceeded those in adult brains. Linear regression analysis which compared the amount of D-[3H]aspartate binding with the age of the infant, showed a positive correlation up to 25 postnatal weeks. Saturation analysis showed that the maximum number of D-[3H]aspartate binding sites (Bmax) in temporal cortex from infants aged 20 postnatal weeks was 3 times greater than the number of sites in adult brain. The findings show that the number of excitatory amino acid uptake sites, which may be associated in part with presynaptic terminals, increase in number rapidly after birth. Furthermore, the data may indicate that a slow regression of excitatory amino acid terminals occurs during the later stages of brain development.

Binding to the glycine site of the NMDA receptor complex in brains of patients with Alzheimer's disease

Ligand binding and in vitro receptor autoradiography were used to measure [3H]glycine binding to the modulatory site associated with the glutamate N-methyl-D-aspartate (NMDA) receptor in 4 areas of control and Alzheimer's disease brains. Binding of [3H]glycine to membranes prepared from temporal cortex was significantly reduced in Alzheimer's disease brains. Saturation analysis showed reductions in both the affinity of binding and the maximum number (Bmax) of binding sites. Autoradiograms of [3H]glycine binding in temporal cortex sections did not show a statistically significant reduction in binding in Alzheimer's disease. The findings suggest that in Alzheimer's disease the glycine modulatory site of the NMDA receptor complex is affected to a small extent by the loss of glutamatergic terminals which has previously been demonstrated.

Sodium-dependent D-aspartate 'binding' is not a measure of presynaptic neuronal uptake sites in an autoradiographic assay

The binding of D-[3H]aspartate to sections of rat brain was examined in an autoradiographic assay. Binding was entirely dependent on the presence of sodium ions, but not chloride ions, and was optimal at 2 degrees C. D-Aspartate bound rapidly, reached equilibrium within 20 min and remained stable for 45 min. The rate of dissociation was relatively rapid with a t1/2 of 56 s, but was not as fast as anticipated, perhaps because of some sequestration of ligand. Binding had a Kd of 6.8 +/- 1.2 microM and a Bmax of 49.4 +/- 8.6 pmol/mg protein. The high Bmax value may further indicate some sequestration of D-aspartate. L-Glutamate, unlabeled D-aspartate, and D,L-threo-hydroxyaspartate, a potent inhibitor of synaptosomal uptake, each competed for D-[3H]aspartate binding with IC50s of 7.0 +/- 4.3 microM, 5.4 +/- 1.5 microM, and 2.5 +/- 1.0 microM, respectively. N-methyl-D-aspartate (NMDA), quisqualate, and kainate had no affinity for this site. The regional distribution of D-aspartate binding sites was unique and did not conform to the distribution of neuronal uptake sites described by others. Striatal D-aspartate binding was unaffected by unilateral decortication or striatal quinolinic acid lesions. In contrast, binding to NMDA, quisqualate, and kainate receptors was reduced by 80-90% by quinolinate lesions of the striatum. The results of D-aspartate binding after lesions strongly suggest that this site is not associated with either lesioned glutamatergic afferents or intrinsic neurons of the striatum; it may be associated with glia.

Tacrine, a drug with therapeutic potential for dementia: post-mortem biochemical evidence

A review of biochemical findings is presented which support the idea that Alzheimer's disease represents a condition for which tetrahydroaminoacridine (tacrine) may have a beneficial effect. There is evidence that clinical and histopathologic hallmarks of the disease relate to cholinergic and serotonergic dysfunction, with less obvious abnormalities in other neurotransmitters (aspartate, dopamine, gamma-aminobutyrate, glutamate, noradrenaline and somatostatin). Clinically relevant concentrations of tacrine may ameliorate the above presynaptic deficits without producing harmful (neurotoxic) effects of aspartate and glutamate. The disease seems to be associated with an early and clinically relevant degeneration of some neurons with cortical perikarya that release these amino acid transmitters. Studies are now required on the effect of tacrine on postulated harmful peptide-bond hydrolase activity within and around such cells.

Excitatory amino acids and Alzheimer's disease

Excitatory amino acids (EAA) such as glutamate and aspartate are major transmitters of the cerebral cortex and hippocampus, and EAA mechanisms appear to play a role in learning and memory. Anatomical and biochemical evidence suggests that there is both pre- and postsynaptic disruption of EAA pathways in Alzheimer's disease. Dysfunction of EAA pathways could play a role in the clinical manifestations of Alzheimer's disease, such as memory loss and signs of cortical disconnection. Furthermore, EAA might be involved in the pathogenesis of Alzheimer's disease, by virtue of their neurotoxic (excitotoxic) properties. Circumstantial evidence raises the possibility that the EAA system may partially determine the distribution of pathology in Alzheimer's disease and may be important in producing the neurofibrillary tangles, RNA reductions and dendritic changes which characterize this devastating disorder. In this article, we will review the evidence suggesting a role for EAA in the clinical manifestations and pathogenesis of Alzheimer's disease.

Reduced D-[3H]aspartate binding in Down's syndrome brains

The binding of D-[3H]aspartate to glutamate uptake sites was measured in post-mortem brains from subjects with Down's syndrome (DS) and age-matched controls. DS brains had substantially reduced D-[3H]aspartate binding in the frontal and temporal cortex, hippocampus and caudate nucleus. There was no correlation between the numbers of Alzheimer-like plaques and tangles or clinically-assessed dementia and D-[3H]aspartate binding in DS brains. The binding of [3H]N-(1-[2-thienyl]cyclohexyl)piperidine ([3H]TCP) to postsynaptic N-methyl-D-aspartate sites was normal in DS brains. This study suggests that the reduction in glutamate uptake sites in DS is more substantial and widespread than in Alzheimer's disease.

Amino acid neurotransmitter deficits in adult Down's syndrome brain tissue

Neurotransmitter amino acids have been measured in several brain regions taken postmortem from 5 adults cases of Down's syndrome and 6 age-matched control subjects. A significant deficit of glutamate was found in the hippocampus in Down's syndrome, and gamma-aminobutyric acid was reduced in the hippocampus and temporal cortex in those patients with neocortical neurofibrillary tangles. These results are consistent with losses of cortical neurones containing these neurotransmitters, and resemble the deficits exhibited by the more severely affected cases of Alzheimer's disease.

Regional changes in [3H]D-aspartate and [3H]TCP binding sites in Alzheimer's disease brains

The specific binding of [3H]D-aspartate, a marker for the presynaptic glutamate uptake site, and [3H]N-(1-[2-Thienyl]cyclohexyl)-piperidine [( 3H]TCP), a high affinity ligand for the N-methyl-D-aspartate (NMDA)-associated phencyclidine binding site, was measured in homogenates of brain from normal subjects and from subjects with neuropathologically confirmed Alzheimer's disease. Alzheimer's disease was associated with a reduction in [3H]D-aspartate binding density in temporal cortex and caudate nucleus. By contrast, a reduction in the receptor density for [3H]TCP binding was only recorded in the frontal cortex. Thus, glutamate-containing nerve terminals are severely reduced in Alzheimer's disease, whilst the postsynaptic NMDA-phencyclidine receptor complex is much less affected. These findings have implications for theories of glutamate neurotoxicity in Alzheimer's disease.

Loss of cortical GABA uptake sites in Alzheimer's disease

[3H]Nipecotic acid bound to membranes of human brain in a saturable, reversible manner which was totally dependent on the presence of sodium ions. The potencies of compounds in inhibiting the specific binding of [3H]nipecotic acid were closely correlated with their potencies in inhibiting the neuronal uptake of [3H]GABA. Compounds selective for GABA receptors were inactive. [3H]Nipecotic acid appears to label neuronal high affinity GABA uptake sites. The binding of [3H]nipecotic acid was substantially reduced in the temporal cortex of brains from subjects with Alzheimer-type dementia, but not in other brain regions. It is concluded that some loss of GABA terminals occurs in this disease.

Regional distribution of pre- and postsynaptic glutamatergic function in Alzheimer's disease

Ca2+/Cl- -independent L-[3H]glutamate- and sodium-dependent D-[3H]aspartate binding assays were used to assess the integrity of N-methyl-D-aspartate (NMDA) receptors and glutamate uptake sites in a number of areas from control and Alzheimer's diseased brain. NMDA receptor densities were unchanged in all areas tested. In contrast, significant reductions in the number of glutamate terminals were seen in the hippocampus of the Alzheimer's samples. A smaller reduction was seen in the caudate nucleus but this failed to reach significance. No such reductions were seen in either the frontal or parietal cortices.

Characterisation of Na+-independent L-[3H]glutamate binding sites in human temporal cortex

The binding of L-[3H]glutamate to membranes from human temporal cortex was studied in the absence of Na+, Ca2+, and Cl- ions. Pharmacological characterisation revealed that approximately 35% of specific binding at 50 nM L-[3H]glutamate was sensitive to a combination of kainate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid. The remaining approximately 65% of specific binding was to a single population of sites with a KD of 844 nM and a Bmax of 0.92 pmol/mg protein. The pharmacological characteristics were consistent with an interaction at the N-methyl-D-aspartate subclass of excitatory amino acid receptor. The inclusion of Cl- ions revealed additional glutamate binding; this was sensitive to quisqualate and DL-2-amino-4-phosphonobutyrate, but not to kainate, DL-2-amino-7-phosphonoheptanoate, or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid.

Presynaptic and postsynaptic glutamatergic function in Alzheimer's disease

Sodium dependent D-aspartate and Ca2+/Cl-independent L-glutamate binding assays were used to assess the integrity of glutamate uptake sites and postsynaptic NMDA receptor levels in control and Alzheimer's disease temporal cortex. The number of glutamate uptake sites was significantly and substantially reduced in accordance with previous findings. However, NMDA receptor levels were unchanged. This finding suggests that glutamatergic presynaptic elements are severely reduced in Alzheimer's disease, whilst postsynaptic elements remain intact.

Evidence of glutamatergic denervation and possible abnormal metabolism in Alzheimer's disease

Excitatory dicarboxylic amino acids previously have been ascribed several functions in the brain. Here their total concentration and proposed neurochemical markers of neurotransmitter function have been measured in brain from patients with Alzheimer's disease (AD) and controls. Specimens were obtained antemortem (biopsy) approximately 3 years after emergence of symptoms and promptly (less than 3 h) postmortem some 10 years after onset. Early in the disease a slight elevation in aspartic acid concentration of cerebral cortex was observed in the patients with AD. A reduction in glutamic acid concentration of a similar magnitude was found. It is argued that this, together with a decrease in CSF glutamine content and lack of change in the phosphate-activated brain glutaminase activity of tissue, reflects an early metabolic abnormality. Later in the disease evidence of glutamatergic neurone loss is provided by the finding that in many regions of the cerebral cortex the Na+-dependent uptake of D-[3H]aspartic acid was almost always lowest in AD subjects compared with control when assessed by a method designed to minimise artifacts and epiphenomena. Release of endogenous neurotransmitters from human brain tissue postmortem did not appear to have the characteristics of that from human tissue antemortem and rat brain.

Sodium dependent D-[3H]aspartate binding in cerebral cortex in patients with Alzheimer's and Parkinson's diseases

The sodium dependent binding of D-[3H]aspartate to the high-affinity glutamate uptake system was used as a marker of glutamate-releasing terminals in the cerebral cortex of brains from patients with Alzheimer-type dementia (ATD) and Parkinson's disease (PD). Sodium-dependent D-[3H]aspartate binding was reduced in the ATD patients but not in the PD patients. Within the PD patients no association was observed between sodium-dependent D-[3H]aspartate binding and the presence of dementia. In contrast choline acetyltransferase activity was reduced in both the ATD and the PD patients. The present results suggest that changes in the cortical cholinergic system can occur independently of the cortical glutamate system. The glutamatergic deficit in ATD may contribute to some of the clinical differences between the dementia of ATD and PD.