Memantine inhibits [3H]MK-801 binding to human hippocampal NMDA receptors

Bipolar disorder: involvement of signaling cascades and AMPA receptor trafficking at synapses

There is increasing evidence that severe mood disorders are associated with impairment of structural plasticity and cellular resilience. Cumulative data demonstrate that mood stabilizers regulate intracellular signaling cascades, including protein kinase C (PKC), PKA, mitogen-activated protein (MAP) kinase, glycogen synthase kinase 3-beta (GSK3-beta) and intracellular calcium, which are signaling pathways that regulate synaptic plasticity. In this context, it is noteworthy that a growing body of data indicates that the glutamatergic system, has a major role in neuronal plasticity and cellular resilience, might be involved in the pathophysiology and treatment of mood disorders. AMPA glutamate-receptor trafficking is important in synaptic plasticity and might play crucial roles in maintaining critical neuronal circuits associated with mood. Two clinically effective, structurally dissimilar, antimanic agents, lithium and valproate (VPA), down-regulate synaptic expression of AMPA receptor subunit GluR1 in hippocampus in chronically treated rats. This reduction in synaptic GluR1 by lithium and VPA is due to attenuated phosphorylation of GluR1 at a specific PKA site (residue 845 of GluR1), which is crucial for AMPA receptor insertion. By contrast,imipramine, which can provoke mania, increases synaptic expression of GluR1 in the hippocampus in vivo. Furthermore, there is ample evidence from preclinical and clinical research that the glutamatergic system is involved in the pathophysiology of mood disorders and that many of the somatic treatments used for mood disorders including antidepressants, mood stabilizers, atypical antipsychotic drugs and electroconvulsive therapy have both direct and indirect effects on the glutamatergic system. Given these findings, further research with medications that specifically affect the glutamatergic system is warranted. Recent studies in our lab have shown that riluzole, a FDA approved medicine that regulates the glutamatergic system, shows antidepressant efficacy in unipolar and bipolar depression. These studies indicate that regulation of glutamate-mediated synaptic plasticity might play a role in the treatment of mood disorders, and raise new avenues for novel therapies for this devastating illness.

From benefit to damage. Glutamate and advanced glycation end products in Alzheimer brain

The glutamatergic system is the most widespread neurotransmitter system in the mammalian brain. It is connected to the acetylcholinergic neurotransmitter system to form the glutamatergic/aspartatergic-acetylcholinergic circuit, which is the morphobiochemical basis of learning, memory and cognition assisted by the glutamatergic N-methyl-D-aspartate receptor, which mediates long-term potentiation as the fundamental molecular mechanisms of these mental capacities. Glutamate and acetylcholine as ligands of the two neurotransmitter systems are products of the neuronal glucose metabolism as holds true also for advanced glycation end products (AGEs), which are markers of damaged and/or aged proteins. During normal aging, both the neurotransmitters glutamate and acetylcholine undergo strong functional variations. Their synthesis was found to be reduced as a common feature. In contrast, basal release of acetylcholine and receptor number decrease, whereas basal release of glutamate and receptor number increase. AGEs increase during aging obviously preferentially in glutamatergic pyramidal neurons in cerebral cortical layers prone to neurodegeneration. In sporadic Alzheimer disease (SAD), glutamate concentration was shown to fall since it may serve as a substitute for lacking glucose in the beginning of the disease. In contrast, glutamate receptor density was found to be much less involved indicating an excessive activation of the glutamatergic neurotransmitter system particularly via the NMDA receptor, mediating endogenous excitotoxicity. The morphological hallmarks of SAD neuritic plaques and neurofibrillary tangles have been demonstrated to crosslink with AGEs causing an increased rate of free radical production. First data from animal studies and investigations on human beings may indicate that the NMDA receptor antagonist memantine may have beneficial effects on the course of SAD and its clinical symptoms.

Chronic nicotine and dizocilpine effects on regionally specific nicotinic and NMDA glutamate receptor binding

Chronic nicotine administration has long been known to increase the number of high-affinity alpha4beta2 nicotinic receptors with lesser effects on low-affinity alpha7 nicotinic receptors. Nicotine has been shown to promote the release of a variety of neurotransmitters including glutamate. Nicotine may also interact directly with the glutamatergic receptors. Nicotinic-glutamate interactions may be critical to the long-term effects of nicotine. Conversely, glutamatergic drugs may interact with the nicotinic system. Such interactions have important implications in interpretation of the mechanism of drug actions, especially when the drugs are given together. The current study examined the effects of chronic administration of nicotine (5 mg of the nicotine base/kg/day for 28 days), dizocilpine (MK-801) (0.3 mg/kg/day for 28 days), an NMDA receptor antagonist, as well as the combination of the two drugs on nicotinic and NMDA receptor densities in discrete brain regions. The chronic dose of dizocilpine used was behaviorally active causing a dramatic reduction in prepulse inhibition (PPI) of acoustic startle response. The nicotine dose used did not significantly affect PPI but previously we have found it to be behaviorally active in improving working memory function. High-affinity nicotinic receptor binding, as has been seen previously, was significantly increased by chronic nicotine in most areas. Chronic dizocilpine alone did not affect high-affinity nicotinic receptor binding, but it did modify the effects of chronic nicotine, attenuating nicotine-induced increases in the frontal cortex and striatum. Low-affinity nicotinic binding was significantly increased by chronic nicotine in only one area, the cerebellum. Chronic dizocilpine significantly increased low-affinity nicotinic binding in several brain areas, the colliculi, hippocampus, and the hypothalamus. The combination of nicotine and dizocilpine attenuated the effects of each with diminished nicotine-induced increased nicotinic low-affinity binding in the cerebellum and diminished dizocilpine-induced increased nicotinic low-affinity binding in the hippocampus and hypothalamus. In contrast, chronic nicotine and dizocilpine had a mutually potentiating effect of increasing nicotinic low-affinity binding in the frontal cortex. NMDA receptor binding was affected only in the hippocampus, where both dizocilpine and nicotine significantly increased binding. Chronic nicotine effects on receptor regulation are significantly affected by concurrent blockade of NMDA glutamate receptors.

Memantine: targeting glutamate excitotoxicity in Alzheimer's disease and other dementias

The management of dementia has changed since the development of new antidementia drugs. The benefits observed in Alzheimer's disease (AD) with selective cholinergic transmission treatments are mainly symptomatic, without clear evidence of neuroprotection. The hypothesis that glutamate-mediated neurotoxicity is involved in the pathogenesis of AD is finding increasingly more acceptance in the scientific community. Glutamate receptors are overactive, and N-methyl-D-aspartate (NMDA) receptor antagonists have therapeutic potential for the treatment of AD and other neurological disorders. Memantine is a noncompetitive NMDA antagonist that is considered a neuroprotective drug. Memantine's capacity has been demonstrated in preclinical studies, and it is considered a useful symptomatic treatment for AD. Memantine has been shown to benefit cognition, function, and global outcome in patients with moderate to severe AD, and it is currently approved by the US Food and Drug Administration (FDA) for the treatment of moderate to severe AD. Recently, memantine has also demonstrated efficacy in the initial stages of AD, although FDA authorization is pending. This review highlights the important pharmacological and clinical aspects of memantine, as well as some basic mechanisms mediating glutamatergic neurodegeneration.

Distributions of transmitter receptors in the macaque cingulate cortex

The primate cingulate cortex is structurally and functionally complex. Although no studies have investigated the regional densities of multiple neurotransmitter receptor systems, such information would be useful for assessing its functions and disease vulnerabilities. We quantified nine different receptors in five transmitter systems by in vitro autoradiographic mapping of the cingulate cortex of macaque monkeys with the aim to link cytoarchitectonic regions and functional specialization. Receptor mapping substantiated the subdivision of the cingulate cortex into anterior versus posterior regions. In anterior cingulate cortex (ACC) AMPA glutamatergic receptors and GABA(A) inhibitory receptors were present in significantly higher concentrations than the modulatory alpha-adrenergic and muscarinic receptors. These differences were absent in the posterior cingulate cortex (PCC). By contrast, NMDA receptor densities were significantly higher than AMPA receptor densities in PCC, but not in ACC. The midcingulate area 24' shared more features with ACC than PCC. This area was characterized by the highest ratios of NMDA receptors to alpha-adrenergic, muscarinic and 5-HT2 receptors among all cingulate regions. Compared to rostrocaudal divisions, the differences between dorsoventral subdivisions a-c were small in all regions of cingulate cortex, and only muscarinic and alpha-adrenergic receptor densities followed the degree of cytoarchitectonic differentiation. We conclude that multiple receptor mapping reveals a highly differentiated classification of cingulate cortex with a characteristic predominance of fast ionotropic excitatory and inhibitory receptors in ACC, but a strong and varied complement of NMDA and metabotropic receptors in PCC.

Emerging experimental therapeutics for bipolar disorder: clues from the molecular pathophysiology

Bipolar affective disorder (manic-depressive illness) is a common, severe, chronic, and often life-threatening illness, associated with significant comorbidity. The recognition of the significant morbidity and mortality of patients with bipolar disorder, as well as the growing appreciation that a high percentage of patients respond poorly to existing treatments, has made the task of discovering new therapeutic agents, that are both efficacious and have few side effects increasingly more important. Most recent agents introduced into the pharmacopeia for the treatment of bipolar disorder have been anticonvulsants and atypical antipsychotics. We propose that novel treatments developed specifically for bipolar disorder will arise from (1) understanding more precisely the molecular mechanisms of treatments that are clearly efficacious or (2) developing medications based on the knowledge obtained of the underlying pathophysiology of bipolar disorder. Knowledge with regard to the underlying pathophysiology of bipolar disorder is increasing at a rapid pace, including alterations in intracellular signaling cascades as well as impairments of cellular plasticity and resilience in critical neuronal circuits. We propose that therapeutics designed to enhance cellular plasticity and resilience and that counter maladaptive stress-responsive systems may have considerable utility for the treatment of bipolar disorder. Therapeutic strategies designed to address cellular resilience and plasticity include the regulation of neurotrophic pathways, glucocorticoid signaling, phosphodiesterase activity, and glutamatergic throughput and mitochondrial function. While the task of developing novel medications for bipolar disorder is truly daunting, these and similar approaches will ultimately lead to better medications for the millions who suffer from this devastating illness.

Regulation of cellular plasticity cascades in the pathophysiology and treatment of mood disorders: role of the glutamatergic system

There is increasing evidence from a variety of sources that mood disorders are associated with regional reductions in brain volume, as well as reductions in the number, size, and density of glia and neurons in discrete brain areas. Although the precise pathophysiology underlying these morphometric changes remains to be fully elucidated, the data suggest that severe mood disorders are associated with impairments of structural plasticity and cellular resilience. In this context, it is noteworthy that a growing body of data suggests that the glutamatergic system--which is known to play a major role in neuronal plasticity and cellular resilience--may be involved in the pathophysiology and treatment of mood disorders. Preclinical studies have shown that the glutamatergic system represents targets (often indirect) for the actions of antidepressants and mood stabilizers. There are a number of glutamatergic "plasticity enhancing" strategies that may be of considerable utility in the treatment of mood disorders. Among the most immediate ones are NMDA antagonists, inhibitors of glutamate-release agents, and AMPA potentiators; this research progress holds much promise for the development of novel therapeutics for the treatment of severe, refractory mood disorders.

Memantine: an antiglutamatergic option for dementia

Alzheimer's disease (AD) is the most common form of dementia in occidental countries. Currently approved treatments for AD provide mainly symptomatic benefits without clear evidence of neuroprotection. N-methyl-D-aspartate (NMDA) receptor antagonists have therapeutic potential in several central nervous system disorders, including neuroprotective treatment in chronic neurodegenerative diseases, and symptomatic treatment in other neurologic diseases. Memantine, an NMDA antagonist, has been recently approved for the treatment of advanced AD. Due to its mechanism of action, memantine is considered a neuroprotective drug, whose utility has been demonstrated in preclinical studies. In addition, memantine is a useful symptomatic treatment for AD and vascular dementia. This paper reviews both aspects of memantine as well as some basic mechanisms mediating cognition and glutamatergic neurodegeneration.

Targeting the N-methyl-D-aspartate receptor for chronic pain management. Preclinical animal studies, recent clinical experience and future research directions

A 1967-1999 MEDLINE search of published reports evaluating the role of the glutamate N-methyl-D-aspartate (NMDA) receptor in pain identified 378 animal studies and 132 human studies. There is convincing evidence in these studies that the NMDA receptor mediates prolonged nociceptive behaviors in animal models and various chronic pain symptoms in the clinical population. Administration of older compounds, such as ketamine, dextromethorphan, and amantadine, which are now known to act as NMDA receptor antagonists, have recently been shown to alleviate chronic pain. For years, the pharmaceutical industry has been attempting to produce novel compounds that modulate NMDA receptor activity; however, the adverse effects associated with this class of drugs have prevented their widespread clinical use. Collaborative studies between basic researchers, clinical scientists, and clinicians are needed to delineate characteristics of NMDA receptor antagonism that predict optimal analgesic activity and an acceptable toxicity profile in patients with chronic pain.

Evaluation of memantine for neuroprotection in dementia

Memantine, a non-competitive NMDA antagonist, has been approved for use in the treatment of dementia in Germany for over ten years. The rationale for use is excitotoxicity as a pathomechanism of neurodegenerative disorders. Memantine acts as a neuroprotective agent against this pathomechanism, which is also implicated in vascular dementia. HIV-1 proteins Tat and gp120 have been implicated in the pathogenesis of dementia associated with HIV infection and the neurotoxicity caused by HIV-1 proteins can be blocked completely by memantine. Memantine has been investigated extensively in animal studies and following this, its efficacy and safety has been established and confirmed by clinical experience in humans. It exhibits none of the undesirable effects associated with competitive NMDA antagonists such as dizocilpine. The efficacy of memantine in a variety of dementias has been shown in clinical trials. Memantine is considered to be a promising neuroprotective drug for the treatment of dementias, particularly Alzheimer's disease for which there is no neuroprotective therapy available currently. It can be combined with acetylcholinesterase inhibitors which are the mainstay of current symptomatic treatment of Alzheimer's disease. Memantine has a therapeutic potential in numerous CNS disorders besides dementias which include stroke, CNS trauma, Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), epilepsy, drug dependence and chronic pain. If memantine is approved by the FDA for some of these indications by the year 2005, it can become a blockbuster drug by crossing the US$1 billion mark in annual sales.

Receptor autoradiographic mapping of the mesial motor and premotor cortex of the macaque monkey

This study analyzes regional and laminar distribution patterns of neurotransmitter binding sites in the motor areas of the macaque mesial frontal cortex. Differences in distribution patterns are compared with the cytoarchitectonic parcellation. Binding sites were analyzed with quantitative in vitro receptor autoradiography in unfixed brains of five macaque monkeys. Alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid (AMPA), kainate, and N-methyl-D-aspartate (NMDA) binding sites were labeled with [3H]AMPA, [3H]kainate, and [3H]MK-801, respectively, muscarinic binding sites with [3H]pirenzepine or [3H]oxotremorine-M, noradrenergic binding sites with [3H]prazosin or [3H]UK-14304, gamma-aminobutyric acid (GABA)A binding sites with [3H]muscimol, and serotoninergic binding sites with [3H]ketanserine. Adjacent sections were stained with a modified Nissl method for cytoarchitectonic analysis. In the motor areas F1, F3, and F6, [3H]AMPA, [3H]pirenzepine, and [3H]oxotremorine-M binding was maximal in layers II, III, and V, and [3H]kainate binding was maximal in layers V and VI. Clear-cut changes in laminar distribution patterns of [3H]AMPA, [3H]kainate, and [3H]oxotremorine-M binding sites very closely matched corresponding cytoarchitectonic borders. Mean areal binding densities of all ligands to F1, F3, and F6 were plotted as polar plots for each area. A polygon was obtained for each area ("neurochemical fingerprint") when all the density values belonging to one area were connected with each other. The "neurochemical fingerprints" of F1, F3, and F6 were virtually identical in shape but increased in size from F1 to F6. This result reflects the functional similarity of these motor-related areas and possibly correlates with their differential involvement in motor control. Areas F1, F3, and F6 can thus be grouped into one "neurochemical family" of areas.

The antiseizure efficacies of MK-801, phencyclidine, ketamine, and memantine are altered selectively by stress

Adaptive changes in the NMDA receptor complex occur in response to exposure to stress. We have previously shown that the ability of MK-801, an uncompetitive NMDA receptor antagonist, to antagonize electrically precipitated tonic hind-limb extension is reduced 24 h after mice are forced to swim for up to 10 min in cold water. The stress-induced reduction of the antiseizure efficacy of MK-801 stimulated the proposal that mice exposed to swim stress may serve as "an intact animal model" of altered or diminished NMDA-mediated neural transmission. In the current investigation, the dose-dependent abilities for the antagonism of electrically precipitated seizures in mice were determined for MK-801, phencyclidine, ketamine, and memantine. Interestingly, a single session of cold water swim stress reduced the antiseizure efficacies of MK-801 and memantine without affecting phencyclidine and ketamine when tested 24 h later. The data do not suggest that stress results in a simple reduction in the number of activated or open channels, but rather alters their size or charge characteristics.

Different binding affinities of NMDA receptor channel blockers in various brain regions--indication of NMDA receptor heterogeneity

The N-methyl-D-aspartate (NMDA) receptor-channel complex exists in multiple forms which probably have different physiological and pharmacological properties. To further evaluate this concept of different NMDA receptor subtypes, receptor binding and autoradiographic techniques were used to study the phencyclidine (PCP) binding site of the NMDA receptor ion-channel complex. [3H]MK-801 was employed to characterize binding properties of (+)-MK-801, (-)-MK-801, phencyclidine (PCP), (+/-)-ketamine, amantadine (1-amino-adamantane) and memantine (3,5-dimethyl-1-amino-adamantane) in different brain regions. Saturation experiments on homogenized membranes revealed the existence of single classes of binding sites in cortex and cerebellum but with significant different affinities between these regions (KD/Cortex = 4.59 nM, Bmax/Cortex = 0.836 pmol/mg protein; KD/Cereb. = 25.99 nM, Bmax/Cereb. = 0.573 pmol/mg protein) suggesting that the lower affinity in cerebellum indicates another population of NMDA receptor channels. In contrast, in striatum there was clear evidence for two binding sites (KD/high = 1.43 nM, Bmax/high = 0.272 pmol/mg protein; KD/low = 12.15 nM, Bmax/low = 1.76 pmol/mg protein). Displacement studies (autoradiography and binding) revealed a lower affinity for unlabeled (+)-MK-801 in striatum which was clearly not the case for memantine. In cerebellar membranes there was a significant decrease in the affinity for both MK-801 enantiomers and PCP but not for the 1-amino-adamantanes. In contrast, all compounds showed lowered affinity in the dentate gyrus. These findings support NMDA receptor heterogeneity which may be of particular relevance for the development of subtype-selective drugs.