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

Metformin inhibits Aβ25-35 -induced apoptotic cell death in SH-SY5Y cells

Metformin, a first-line drug for type-2 diabetes, plays a potentially protective role in preventing Alzheimer's disease (AD), but its underlying mechanism is unclear. In this study, Aβ_25-35 -treated SH-SY5Y cells were used as a cell model of AD to investigate the neuroprotective effect of metformin, as well as its underlying mechanisms. We found that metformin decreased the cell apoptosis rate and death, ratio of Bcl-2/Bax, and expression of NR2A and NR2B, and increased the expression of LC3 in Aβ_25-35 -treated SH-SY5Y cells. Metformin also reduced intracellular and extracellular Glu concentrations, as well as the intracellular concentration of Ca²⁺ and ROS in Aβ_25-35 -treated SH-SY5Y cells. These findings suggest that metformin inhibits Aβ_25-35 -treated SH-SY5Y cell death by inhibiting apoptosis, decreasing intracellular Ca²⁺ and ROS by reducing neurotoxicity of excitatory amino acids, and by possibly reversing autophagy disorder via regulating autophagy process.

Neurotransmitter receptors and cognitive dysfunction in Alzheimer's disease and Parkinson's disease

Cognitive dysfunction is one of the most typical characteristics in various neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease (advanced stage). Although several mechanisms like neuronal apoptosis and inflammatory responses have been recognized to be involved in the pathogenesis of cognitive dysfunction in these diseases, recent studies on neurodegeneration and cognitive dysfunction have demonstrated a significant impact of receptor modulation on cognitive changes. The pathological alterations in various receptors appear to contribute to cognitive impairment and/or deterioration with correlation to diversified mechanisms. This article recapitulates the present understandings and concepts underlying the modulation of different receptors in human beings and various experimental models of Alzheimer's disease and Parkinson's disease as well as a conceptual update on the underlying mechanisms. Specific roles of serotonin, adrenaline, acetylcholine, dopamine receptors, and N-methyl-D-aspartate receptors in Alzheimer's disease and Parkinson's disease will be interactively discussed. Complex mechanisms involved in their signaling pathways in the cognitive dysfunction associated with the neurodegenerative diseases will also be addressed. Substantial evidence has suggested that those receptors are crucial neuroregulators contributing to cognitive pathology and complicated correlations exist between those receptors and the expression of cognitive capacities. The pathological alterations in the receptors would, therefore, contribute to cognitive impairments and/or deterioration in Alzheimer's disease and Parkinson's disease. Future research may shed light on new clues for the treatment of cognitive dysfunction in neurodegenerative diseases by targeting specific alterations in these receptors and their signal transduction pathways in the frontal-striatal, fronto-striato-thalamic, and mesolimbic circuitries.

Analysis of cholinergic markers, biogenic amines, and amino acids in the CNS of two APP overexpression mouse models

Two transgenic mouse models expressing mutated human amyloid precursor protein and previously found to display cognitive and behavioural alterations, reminiscent of Alzheimer patients' symptomatology, were scrutinised for putative brain region-specific changes in neurochemical parameters. Brains of NSE-hAPP751m-57, APP23 and wild-type mice were microdissected to perform brain region-specific neurochemical analyses. Impairment of cholinergic transmission, the prominent neurochemical deficit in Alzheimer brain, was examined; acetylcholinesterase and choline acetyltransferase activity levels were determined as markers of the cholinergic system. Since Alzheimer neurodegeneration is not restricted to the cholinergic system, brain levels of biogenic amines and metabolites, and amino acidergic neurotransmitters and systemic amino acids were analysed as well. Cholinergic dysfunction, reflected in reduced enzymatic activity in the basal forebrain nuclei, was restricted to the APP23 model, which also exhibited more outspoken and more widespread changes in other neurotransmitter systems. Significant changes in compounds of the noradrenergic and serotonergic system were observed, as well as alterations in levels of the inhibitory neurotransmitter glycine and systemic amino acids. These observations were clearly in occurrence with the more pronounced histopathological and behavioural phenotype of the APP23 model. As transgenic models often do not represent an end-stage of the disease, some discrepancies with results from post-mortem human Alzheimer brain analyses were apparent; in particular, no significant alterations in excitatory amino acid levels were detected. Our findings of brain region-specific alterations in compound levels indicate disturbed neurotransmission pathways, and greatly add to the validity of APP23 mice as a model for Alzheimer's disease. Transgenic mouse models may be employed as a tool to study early-stage neurochemical changes, which are often not accessible in Alzheimer brain.

Biochemical and molecular studies of NMDA receptor subunits NR1/2A/2B in hippocampal subregions throughout progression of Alzheimer's disease pathology

Alzheimer's disease (AD) is characterized by loss of specific cell populations within selective subregions of the hippocampus. Excitotoxicity, mediated via ionotropic glutamate receptors, may play a crucial role in this selective neuronal vulnerability. We investigated whether alterations in NMDA receptor subunits occurred during AD progression. Employing biochemical and in situ hybridization techniques in subjects with a broad range of AD pathology, protein levels, and mRNA expression of NR1/2A/2B subunits were assayed. With increasing AD neuropathology, protein levels and mRNA expression for NR1/2B subunits were significantly reduced, while the NR2A subunit mRNA expression and protein levels were unchanged. Cellular analysis of neuronal mRNA expression revealed a significant increase in the NR2A subunit in subjects with moderate neurofibrillary tangle neuropathology. This investigation supports the hypothesis that alterations occur in the expression of specific NMDA receptor subunits with increasing AD pathologic severity, which is hypothesized to contribute to the vulnerability of these neurons.

[3H]MK-801 binding and the mRNA for the NMDAR1 subunit of the NMDA receptor are differentially distributed in human and rat forebrain

The distributions of [3H]MK-801 binding and the NMDA NR1 subunit mRNA were studied using receptor autoradiography and in-situ hybridization in rat and human brain whole-hemisphere coronal sections. Receptor protein detected by radioligand autoradiography and the mRNA for the key subunit of the receptor presented similar distributions in the forebrain, with a few areas showing an imbalance between the levels of mRNA and receptor protein. Human frontal cortex showed a relative abundance of NMDAR1 mRNA as compared to [3H]MK-801 binding. The same area in rat brain did not show any difference in the two distributions. In comparison, the rat claustrum presented a relative excess of NMDAR1 mRNA which was not detected in human sections. Human caudate nucleus exhibited relatively high levels of [3H]MK-801 binding that were unmatched in rat caudate. The hippocampi of either species presented similar levels of [3H]MK-801 binding and NMDAR1 mRNA, but when the two signals were measured in specific subfields of the hippocampal formation, the differential distribution of the two signals reflected the anatomy of hippocampal connections assuming a preferential dendritic distribution for MK-801 binding. Interestingly, rat and human hippocampi also showed some important species-dependent difference in the relative distribution of the receptor protein and mRNA. The data presented show an overall good correlation between the mRNA for the key subunit of the NMDA receptor and the functional receptor detected with radioligand binding and highlight the presence of local differences in their ratio. This may reflect different splicing of the mRNA for the NMDAR1 subunit in specific brain areas of rat and human. The species-dependent differences in the relative distribution of the mRNA for the key subunit of the NMDA receptor and that of a marker of functional receptors also highlights important differences in the NMDA function in rat and human brain.

Free D-serine in post-mortem brains and spinal cords of individuals with and without neuropsychiatric diseases

We have measured the concentrations of free D-serine post-mortem in the prefrontal cortex, parietal cortex, cerebellum and spinal cord from individuals with and without (controls) neuropsychiatric diseases using high-performance liquid chromatography with fluorometric detection. The levels of D-serine were found to be high in the prefrontal and parietal cortex (around 100 nmol/g wet weight) and very low in the cerebellum and spinal cord (below 10 nmol/g wet weight). The uneven distribution of the D-amino acid in the human central nervous system (CNS) resembles that observed in rodents, suggesting that, as shown in the rat CNS, the regional variation of D-serine content in the human brain might also be closely correlated with those of the N-methyl-D-aspartate (NMDA) type excitatory amino acid receptor. In the prefrontal cortex, the gray and white matter had a similar concentration of D-serine. These findings, together with the selective action of D-serine at the NMDA-related glycine site and the non-neurogenic nature of extracellular D-serine release, add further support to the view that D-serine could be an intrinsic modulator of the NMDA receptor liberated from certain glial cells in the mammalian brain. Despite the anti-psychotogen activity of D-serine in the rat, there were no statistically significant differences between the D-serine contents in the prefrontal or parietal cortex of controls and those of patients with schizophrenia or dementia of the Alzheimer type.

The distribution of excitatory amino acid receptors in the normal human midbrain and basal ganglia with implications for Parkinson's disease: a quantitative autoradiographic study using [3H]MK-801, [3H]glycine, [3H]CNQX and [3H]kainate

Quantitative receptor autoradiography using [3H]MK-801, [3H]glycine, [3H]CNQX and [3H]kainate was employed to determine the distribution and density of excitatory amino acid (EAA) binding sites in the midbrain and basal ganglia of the normal human nervous system. Detailed knowledge of the anatomy and subtype specificity of glutamate receptors is important both in understanding the normal physiology of basal ganglia neurotransmission and the pathophysiological changes occurring in diseases affecting the basal ganglia such as Parkinson's disease (PD). In PD, glutamate receptor activation may contribute to cell death of dopaminergic neurones in the substantia nigra. In addition, perturbation of glutamate neurotransmission resulting from dopamine depletion in the basal ganglia is likely to contribute to the clinical manifestations of motor dysfunction. The distribution and density of ligand binding representing N-methyl-D-aspartate (NMDA), AMPA (2-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and kainate receptors has a heterogeneous distribution in the human midbrain and basal ganglia. In the substantia nigra relatively high densities of [3H]MK-801 and strychnine-insensitive [3H]glycine binding sites representing NMDA receptors were present, whereas only moderate densities of [3H]CNQX and [3H]kainate binding sites were present, compared to other regions. In both the medial globus pallidus and subthalamic nucleus, binding sites representing NMDA, AMPA and kainate receptors were all present at low density. These findings suggest that the clinical usefulness of modifying glutamatergic neurotransmission in these basal ganglia nuclei may be limited by the relatively low density of EAA binding sites present.

Increases in strychnine-insensitive glycine binding sites in cerebral cortex of chronic schizophrenics: evidence for glutamate hypothesis

Strychnine-insensitive glycine binding sites, an absolute requirement of the responses mediated by N-methyl-D-aspartate (NMDA) receptors, were measured in the postmortem brains of 13 chronic schizophrenics and 10 controls, using a radiolabeled receptor assay. Specific [3H]glycine binding was significantly increased in six of the 16 areas of the cerebral cortex that were investigated. Scatchard analysis performed in these areas showed a significant increase in the maximum number of binding sites, with no change in the affinity of binding. Multiple regression analysis confirmed that the increase was not due to age at death or interval from death to freezing. The increase was also observed in the off-drug cases of schizophrenics who had not taken antipsychotics for more than 40 days before death. These results suggest that the increases in NMDA-associated glycine binding sites, possibly ascribed to the postsynaptic compensation for impaired glutamatergic neurotransmission, might be implicated in the pathophysiology of schizophrenia.

Preservation of redox, polyamine, and glycine modulatory domains of the N-methyl-D-aspartate receptor in Alzheimer's disease

This study used [3H]dizocilpine ([3H]MK-801) binding to the N-methyl-D-aspartate (NMDA) receptor to examine redox, polyamine, and glycine modulatory sites in membranes derived from the superior frontal and the superior temporal cortex of patients with Alzheimer's disease. In control subjects the competitive polyamine site antagonist arcaine inhibited [3H]dizocilpine binding in a dose-dependent fashion and this curve was shifted to the right by the addition of 50 microM spermidine. Arcaine inhibition of binding was more potent in the temporal cortex than in the frontal cortex, in both the absence and presence of 50 microM spermidine. In Alzheimer's disease, arcaine inhibition of [3H]dizocilpine binding (in both the absence and the presence of spermidine) was not different from control in either of the two brain areas examined. The sulfhydryl redox site of the NMDA receptor was assessed using the oxidizing agent 5,5'-dithio-bis(2-nitrobenzoic acid), which inhibited binding in a dose-dependent fashion. This inhibition was similar in patients with Alzheimer's disease and control subjects. Glycine-stimulated [3H]dizocilpine binding was also unaffected in patients with Alzheimer's disease. However, in the temporal cortex there was a significant age-associated decline in [3H]dizocilpine binding in the presence of 100 microM glutamate (Rs = -0.71) and 100 microM glutamate plus 30 microM glycine (Rs = -0.90). There was also an age-related increase in arcaine IC50 (which reflects an age-related decrease in arcaine affinity) in the frontal cortex, determined both in the absence (Rs = 0.83) and the presence (Rs = 0.79) of spermidine. These data indicate that the NMDA receptor and its modulatory redox, polyamine, and glycine subsites are intact in patients with Alzheimer's disease and that the modulatory activity of polyamine and glycine sites decline with aging.

Presence of the N-methyl-D-aspartate-associated glycine receptor agonist, D-serine, in human temporal cortex: comparison of normal, Parkinson, and Alzheimer tissues

The amino acid D-serine (D-Ser), previously recognized as a pharmacological tool for potentiating neuronal activity mediated by the N-methyl-D-aspartate (NMDA) receptor complex, in vitro and in vivo, has been observed in several brain regions of the rat and mouse, most prominently in cortex. In addition to reconfirming the presence and distribution of D-Ser in rat brain, we have observed, for the first time, endogenous, free D-Ser in temporal cortex of normal human brains at a level of 2.18 +/- 0.12 nmol/mg of protein, representing 15 +/- 2% of the free L-Ser pool. The D- and L-Ser specific content and the D/L-Ser ratio obtained from temporal cortex of Parkinson and Alzheimer brains did not differ significantly from those of controls. However, at the levels observed here, and considering its specificity and affinity for the NMDA-associated glycine receptor, endogenous D-Ser is a plausible NMDA receptor glycine site agonist.