Pradeep K. Banerjee

Memantine Improves Cognition and Reduces Alzheimer’s-Like Neuropathology in Transgenic Mice

Memantine is an N-methyl-d-aspartate receptor antagonist that is approved for the treatment of moderate to severe Alzheimers disease (AD). In this study, three groups of triple-transgenic (3xTg-AD) mice with differing levels of AD-like pathology (6, 9, and 15 months of age) were treated for 3 months with doses of memantine equivalent to those used in humans. After the treatment, memantine-treated mice had restored cognition and significantly reduced the levels of insoluble amyloid-beta (Abeta), Abeta dodecamers (Abeta*56), prefibrillar soluble oligomers, and fibrillar oligomers. The effects on pathology were stronger in older, more impaired animals. Memantine treatment also was associated with a decline in the levels of total tau and hyperphosphorylated tau. Finally, memantine pre-incubation prevented Abeta-induced inhibition of long-term potentiation in hippocampal slices of cognitively normal mice. These results suggest that the effects of memantine treatment on AD brain include disease modification and prevention of synaptic dysfunction.

Memantine lowers amyloid-β peptide levels in neuronal cultures and in APP/PS1 transgenic mice

Memantine is a moderate‐affinity, uncompetitive N‐methyl‐D‐aspartate (NMDA) receptor antagonist that stabilizes cognitive, functional, and behavioral decline in patients with moderate to severe Alzheimers disease (AD). In AD, the extracellular deposition of fibrillogenic amyloid‐β peptides (Aβ) occurs as a result of aberrant processing of the full‐length Aβ precursor protein (APP). Memantine protects neurons from the neurotoxic effects of Aβ and improves cognition in transgenic mice with high brain levels of Aβ. However, it is unknown how memantine protects cells against neurodegeneration and affects APP processing and Aβ production. We report the effects of memantine in three different systems. In human neuroblastoma cells, memantine, at therapeutically relevant concentrations (1–4 μM), decreased levels of secreted APP and Aβ1–40. Levels of the potentially amylodogenic Aβ1–42 were undetectable in these cells. In primary rat cortical neuronal cultures, memantine treatment lowered Aβ1–42 secretion. At the concentrations used, memantine treatment was not toxic to neuroblastoma or primary cultures and increased cell viability and/or metabolic activity under certain conditions. In APP/presenilin‐1 (PS1) transgenic mice exhibiting high brain levels of Aβ1–42, oral dosing of memantine (20 mg/kg/day for 8 days) produced a plasma drug concentration of 0.96 μM and significantly reduced the cortical levels of soluble Aβ1–42. The ratio of Aβ1–40/Aβ1–42 increased in treated mice, suggesting effects on the γ‐secretase complex. Thus, memantine reduces the levels of Aβ peptides at therapeutic concentrations and may inhibit the accumulation of fibrillogenic Aβ in mammalian brains. Memantines ability to preserve neuronal cells against neurodegeneration, to increase metabolic activity, and to lower Aβ level has therapeutic implications for neurodegenerative disorders.

Memantine leads to behavioral improvement and amyloid reduction in Alzheimer's-disease-model transgenic mice shown as by micromagnetic resonance imaging.

Memantine, an N‐methyl‐D‐aspartate (NMDA) receptor antagonist, has been shown to improve learning and memory in several preclinical models of Alzheimers disease (AD). Memantine has also been shown to reduce the levels of amyloid β (Aβ) peptides in human neuroblastoma cells as well as to inhibit Aβ oligomer‐induced synaptic loss. In this study, we assessed whether NMDA receptor inhibition by memantine in transgenic mice expressing human amyloid‐beta precursor protein (APP) and presenilin 1 (PS1) is associated with cognitive benefit and amyloid burden reduction by using object recognition, micromagnetic resonance imaging (μMRI), and histology. APP/PS1 Tg mice were treated either with memantine or with vehicle for a period of 4 months starting at 3 months of age. After treatment, the mice were subjected to an object recognition test and analyzed by ex vivo μMRI, and histological examination of amyloid burden. μMRI was performed following injection with gadolinium‐DTPA‐Aβ1–40. We found that memantine‐treated Tg mice performed the same as wild‐type control mice, whereas the performance of vehicle‐treated Tg mice was significantly impaired (P = 0.0081, one‐way ANOVA). Compared with vehicle‐treated animals, memantine‐treated Tg mice had a reduced plaque burden, as determined both histologically and by μMRI. This reduction in amyloid burden correlates with an improvement in cognitive performance. Thus, our findings provide further evidence of the potential role of NMDA receptor antagonists in ameliorating AD‐related pathology. In addition, our study shows, for the first time, the utility of μMRI in conjunction with gadolinium‐labeled Aβ labeling agents to monitor the therapeutic response to amyloid‐reducing agents.

Memantine Normalizes Several Phenotypic Features in the Ts65Dn Mouse Model of Down Syndrome

Ts65Dn (TS) mice exhibit several phenotypic characteristics of human Down syndrome, including an increased brain expression of amyloid-beta protein precursor (AbetaPP) and cognitive disturbances. Aberrant N-methyl-D-aspartate (NMDA) receptor signaling has been suspected in TS mice, due to an impaired generation of hippocampal long-term potentiation (LTP). Memantine, an uncompetitive NMDA receptor antagonist approved for the treatment of moderate to severe Alzheimers disease, is known to normalize LTP and improve cognition in transgenic mice with high brain levels of AbetaPP and amyloid-beta protein. It has recently been demonstrated that acute injections of memantine rescue performance deficits of TS mice on a fear conditioning test. Here we show that oral treatment of aged TS mice with a clinically relevant dose of memantine (30 mg/kg/day for 9 weeks) improved spatial learning in the water maze task and slightly reduced brain AbetaPP levels. We also found that TS mice exhibited a significantly reduced granule cell count and vesicular glutamate transporter-1 (VGLUT1) labeling compared to disomic control mice. After memantine treatment, the levels of hippocampal VGLUT1 were significantly increased, reaching the levels observed in vehicle treated-control animals. Memantine did not significantly affect granule cell density. These data indicate that memantine may normalize several phenotypic abnormalities in TS mice, many of which--such as impaired cognition--are also associated with Down syndrome and Alzheimers disease.

The basal forebrain cholinergic system in aging and dementia. Rescuing cholinergic neurons from neurotoxic amyloid-β42 with memantine

The dysfunction and loss of basal forebrain cholinergic neurons and their cortical projections are among the earliest pathological events in the pathogenesis of Alzheimers disease (AD). The evidence pointing to cholinergic impairments come from studies that report a decline in the activity of choline acetyltransferase (ChAT) and acetylcholine esterase (AChE), acetylcholine (ACh) release and the levels of nicotinic and muscarinic receptors, and loss of cholinergic basal forebrain neurons in the AD brain. Alzheimers disease pathology is characterized by an extensive loss of synapses and neuritic branchings which are the dominant scenario as compared to the loss of the neuronal cell bodies themselves. The appearance of cholinergic neuritic dystrophy, i.e. aberrant fibers and fiber swelling are more and more pronounced during brain aging and widely common in AD. When taking amyloid-β (Aβ) deposition as the ultimate causal factor of Alzheimers disease the role of Aβ in cholinergic dysfunction should be considered. In that respect it has been stated that ACh release and synthesis are depressed, axonal transport is inhibited, and that ACh degradation is affected in the presence of Aβ peptides. β-Amyloid peptide 1-42, the principal constituent of the neuritic plaques seen in AD patients, is known to trigger excess amount of glutamate in the synaptic cleft by inhibiting the astroglial glutamate transporter and to increase the intracellular Ca(2+) level. Based on the glutamatergic overexcitation theory of AD progression, the function of NMDA receptors and treatment with NMDA antagonists underlie some recent therapeutic applications. Memantine, a moderate affinity uncompetitive NMDA receptor antagonist interacts with its target only during states of pathological activation but does not interfere with the physiological receptor functions. In this study the neuroprotective effect of memantine on the forebrain cholinergic neurons against Aβ42 oligomers-induced toxicity was studied in an in vivo rat dementia model. We found that memantine rescued the neocortical cholinergic fibers originating from the basal forebrain cholinergic neurons, attenuated microglial activation around the intracerebral lesion sides, and improved attention and memory of Aβ42-injected rats exhibiting impaired learning and loss of cholinergic innervation of neocortex.

Cognition-enhancing and anxiolytic effects of memantine.

Memantine, a moderate-affinity NMDA receptor antagonist, is clinically used for the treatment of Alzheimers disease (AD). Both clinical and preclinical studies have shown that memantine, at doses producing a steady-state plasma level of 0.5-1 microM, is well tolerated and improves cognition. Here we tested the effects of chronic oral administration of memantine (10, 30 and 100mg/kg per day) producing steady state plasma drug levels ranging between approximately 0.5 and 6 microM on motor, social, emotional and cognitive behavior in normal C57BL/6J mice. Memantine dose-dependently reduced escape latency (hidden platform) and decreased wall swimming tendency in the Morris water maze test, increased time spent in open arms in the elevated plus-maze test, and reduced the number of isolation-induced aggressive attacks, but did not affect exploratory activity in the open field. These data indicate that high, stable doses of memantine improved cognition and exhibited a potential anxiolytic response in normal mice.

Memantine treatment decreases levels of secreted Alzheimer’s amyloid precursor protein (APP) and amyloid beta (Aβ) peptide in the human neuroblastoma cells

Memantine, an uncompetitive NMDA receptor antagonist, is a FDA-approved drug used for the treatment of moderate-to-severe Alzheimers disease (AD). Several studies have documented protective roles of memantine against amyloid beta (A beta) peptide-mediated damage to neurons in both in vitro and in vivo models. Memantine is also effective in reducing amyloid burden in the brain of APP transgenic mice. However, the exact mechanism by which memantine provides protection against A beta-mediated neurodegenerative cascade, including APP metabolism, remains to be elucidated. Herein, we investigated the effect of memantine on levels of the secreted form of A beta precursor protein (APP), secreted A beta and cell viability markers under short/acute conditions. We treated neuronal SK-N-SH cells with 10 microM memantine and measured levels of secreted total APP (sAPP), APP alpha isoform and A beta((1-40)) in a time dependent manner for up to 24h. Memantine significantly decreased the levels of the secreted form of sAPP, sAPP alpha and A beta((1-40)) compared to vehicle treated cells. This change started as early as 8h and continued for up to 24h of drug treatment. Unlike sAPP, a slight non-significant increase in total intracellular APP level was observed in 24-h treated memantine cells. Taken together, these results suggest a role for memantine in the transport or trafficking of APP molecules away from the site of their proteolytic cleavage by the secretase enzymes. Such a novel property of memantine warrants further study to define its therapeutic utility.

Behavioral Characterization of GLT1 (+/-) Mice as a Model of Mild Glutamatergic Hyperfunction

GLT1 is one of the major transporters responsible for maintenance of glutamate homeostasis in the brain. In the present study, glutamate transporter 1-deficient GLT1 homozygous (-/-) and heterozygous (+/-) mice were investigated with the intention that they may provide a model of hyperglutamatergic state resulting in various behavioral alterations. The GLT1 (-/-) mice had lower body and brain weight, mild neuronal loss in CA1 hippocampal region as well as focal gliosis and severe focal neuronal paucity in layer II of the neocortex. The short life-span of GLT1 (-/-) precluded us from systematic behavioral studies in these mice. In contrast, GLT1 (+/-) mice exhibiting a 59% decrease in GLT1 immunoreactivity in their brain tissue, showed no apparent morphological brain abnormalities, and their life-span was not markedly different from controls. Behavior ally, GLT1 (+/-) presented moderate behavioral alterations compared to their wildtype littermates, such as: mild sensorimotor impairment, hyperlocomotion (at 3 month of age only), lower anxiety (at 6 months), better learning of cue-based fear conditioning but worse context-based fear conditioning. Our results suggest that GLT1 (+/-) mice may serve as a potentially useful model to study neurodegenerative disease conditions with mild hyperglutamatergic activity.

Memantine reduces mania-like symptoms in animal models

Memantine, a selective antagonist of the N-methyl-D-aspartate receptor, is approved for the treatment of moderate to severe Alzheimers disease. Ion dysregulation is thought to be involved in the pathophysiology of bipolar illness, suggesting that memantine may be effective in treating bipolar manic and/or depressive episodes. We utilized two preclinical models of mania that mimic pathophysiologic changes seen in bipolar illness to examine the potential efficacy of memantine in the treatment of this disorder. Locomotor hyperactivity of male Sprague-Dawley rats in an open field was induced with intracerebroventricular (ICV) administration of 10(-3) M ouabain. Memantine (2.5, 5 or 7.5mg/kg), lithium (6.75 mEq/kg), or vehicle were administered acutely via intraperitoneal injection immediately prior to ouabain, then chronically for 7 days (oral memantine 20, 30, and 40 mg/kg/day in water; lithium 2.4 g/kg food). In a second model of bipolar disorder, cycling between population spikes and epileptiform bursts was investigated in rat hippocampal slices treated with ouabain (3.3 μM) alone or in combination with memantine (0.5, 1.0, and 5.0 μM). Ouabain-induced hyperlocomotion was normalized with acute and chronic lithium and chronic use of memantine. Memantine delayed the onset of ouabain-induced-cycling in hippocampal slices. Memantine may have antimanic properties.

Effects of memantine and donepezil on cortical and hippocampal acetylcholine levels and object recognition memory in rats

This preclinical study investigated the ability of memantine (MEM) to stimulate brain acetylcholine (ACh) release, potentially acting synergistically with donepezil (DON, an acetylcholinesterase inhibitor). Acute systemic administration of either MEM or DON to anesthetized rats caused dose-dependent increases of ACh levels in neocortex and hippocampus, and the combination of MEM (5 mg/kg) and DON (0.5 mg/kg) produced significantly greater increases than either drug alone. To determine whether ACh release correlated with cognitive improvement, rats with partial fimbria-fornix (FF) lesions were treated with acute or chronic MEM or DON. Acute MEM treatment significantly elevated baseline hippocampal ACh release but did not significantly improve task performance on a delayed non-match-to-sample (DNMS) task, whereas chronic MEM treatment significantly improved DNMS performance but only marginally elevated baseline ACh levels. Acute or chronic treatment with DON (in the presence of neostigmine to allow ACh collection) did not significantly improve DNMS performance or alter ACh release. In order to investigate the effect of adding MEM to ongoing DON therapy, lesioned rats pretreated with DON for 3 weeks were given a single intraperitoneal dose of MEM. MEM significantly elevated baseline hippocampal ACh levels, but did not significantly improve DNMS task scores compared to chronic DON-treated animals. These data indicate that MEM, in addition to acting as an NMDA receptor antagonist, can also augment ACh release; however, in this preclinical model, increased ACh levels did not directly correlate with improved cognitive performance.