Blockade of the NMDA receptor increases developmental apoptotic elimination of granule neurons and activates caspases in the rat cerebellum

NMDA receptor-mediated Ca2+ signaling: Impact on cell cycle regulation and the development of neurodegenerative diseases and cancer

NMDA receptors are Ca2+-permeable ligand-gated ion channels that mediate fast excitatory transmission in the central nervous system. NMDA receptors regulate the proliferation and differentiation of neural progenitor cells and also play critical roles in neural plasticity, memory, and learning. In addition to their physiological role, NMDA receptors are also involved in glutamate-mediated excitotoxicity, which results from excessive glutamate stimulation, leading to Ca2+ overload, and ultimately to neuronal death. Thus, NMDA receptor-mediated excitotoxicity has been linked to several neurodegenerative diseases such as Alzheimer's, Parkinson's, Huntington's, dementia, and stroke. Interestingly, in addition to its effects on cell death, aberrant expression or activation of NMDA receptors is also involved in pathological cellular proliferation, and is implicated in the invasion and proliferation of various types of cancer. These disorders are thought to be related to the contribution of NMDA receptors to cell proliferation and cell death through cell cycle modulation. This review aims to discuss the evidence implicating NMDA receptor activity in cell cycle regulation and the link between aberrant NMDA receptor activity and the development of neurodegenerative diseases and cancer due to cell cycle dysregulation. The information presented here will provide insights into the signaling pathways and the contribution of NMDA receptors to these diseases, and suggests that NMDA receptors are promising targets for the prevention and treatment of these diseases, which are leading causes of death and disability worldwide.

Restoration of Sarco/Endoplasmic Reticulum Ca2+-ATPase Activity Functions as a Pivotal Therapeutic Target of Anti-Glutamate-Induced Excitotoxicity to Attenuate Endoplasmic Reticulum Ca2+ Depletion

Glutamate-induced excitotoxicity is a pathological basis of many acute/chronic neurodegenerative diseases. Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA2b) is a membrane-embedded P-type ATPase pump that manages the translocation of calcium ions (Ca²⁺) from cytosol into the lumen of the endoplasmic reticulum (ER) calcium stores. It participates in a wide range of biological functions in the central nervous system (CNS). However, the role of SERCA2b in glutamate-induced excitotoxicity and its mechanism must be elucidated. Herein, we demonstrate that SERCA2b mutants exacerbate the excitotoxicity of hypo-glutamate stimulation on HT22 cells. In this study, SERCA2b mutants accelerated Ca²⁺ depletion through loss-of-function (reduced pumping capacity) or gain-of-function (acquired leakage), resulting in ER stress. In addition, the occurrence of ER Ca²⁺ depletion increased mitochondria-associated membrane formation, which led to mitochondrial Ca²⁺ overload and dysfunction. Moreover, the enhancement of SERCA2b pumping capacity or inhibition of Ca²⁺ leakage attenuated Ca²⁺ depletion and impeded excitotoxicity in response to hypo-glutamate stimulation. In conclusion, SERCA2b mutants exacerbate ER Ca²⁺-depletion-mediated excitotoxicity in glutamate-sensitive HT22 cells. The mechanism of disruption is mainly related to the heterogeneity of SERCA2b mutation sites. Stabilization of SRECA2b function is a critical therapeutic approach against glutamate-induced excitotoxicity. These data will expand understanding of organelle regulatory networks and facilitate the discovery and creation of drugs against excitatory/inhibitory imbalance in the CNS.

The Role of D-Amino Acids in Alzheimer's Disease

Alzheimer's disease (AD), the main cause of dementia worldwide, is characterized by a complex and multifactorial etiology. In large part, excitatory neurotransmission in the central nervous system is mediated by glutamate and its receptors are involved in synaptic plasticity. The N-methyl-D-aspartate (NMDA) receptors, which require the agonist glutamate and a coagonist such as glycine or the D-enantiomer of serine for activation, play a main role here. A second D-amino acid, D-aspartate, acts as agonist of NMDA receptors. D-amino acids, present in low amounts in nature and long considered to be of bacterial origin, have distinctive functions in mammals. In recent years, alterations in physiological levels of various D-amino acids have been linked to various pathological states, ranging from chronic kidney disease to neurological disorders. Actually, the level of NMDA receptor signaling must be balanced to promote neuronal survival and prevent neurodegeneration: this signaling in AD is affected mainly by glutamate availability and modulation of the receptor's functions. Here, we report the experimental findings linking D-serine and D-aspartate, through NMDA receptor modulation, to AD and cognitive functions. Interestingly, AD progression has been also associated with the enzymes related to D-amino acid metabolism as well as with glucose and serine metabolism. Furthermore, the D-serine and D-/total serine ratio in serum have been recently proposed as biomarkers of AD progression. A greater understanding of the role of D-amino acids in excitotoxicity related to the pathogenesis of AD will facilitate novel therapeutic treatments to cure the disease and improve life expectancy.

The Biology and Pathobiology of Glutamatergic, Cholinergic, and Dopaminergic Signaling in the Aging Brain

The elderly population is growing worldwide, with important health and socioeconomic implications. Clinical and experimental studies on aging have uncovered numerous changes in the brain, such as decreased neurogenesis, increased synaptic defects, greater metabolic stress, and enhanced inflammation. These changes are associated with cognitive decline and neurobehavioral deficits. Although aging is not a disease, it is a significant risk factor for functional worsening, affective impairment, disease exaggeration, dementia, and general disease susceptibility. Conversely, life events related to mental stress and trauma can also lead to accelerated age-associated disorders and dementia. Here, we review human studies and studies on mice and rats, such as those modeling human neurodegenerative diseases, that have helped elucidate (1) the dynamics and mechanisms underlying the biological and pathological aging of the main projecting systems in the brain (glutamatergic, cholinergic, and dopaminergic) and (2) the effect of defective glutamatergic, cholinergic, and dopaminergic projection on disabilities associated with aging and neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Detailed knowledge of the mechanisms of age-related diseases can be an important element in the development of effective ways of treatment. In this context, we briefly analyze which adverse changes associated with neurodegenerative diseases in the cholinergic, glutaminergic and dopaminergic systems could be targeted by therapeutic strategies developed as a result of our better understanding of these damaging mechanisms.

d-glutamate and Gut Microbiota in Alzheimer's Disease

Background: An increasing number of studies have shown that the brain–gut–microbiota axis may significantly contribute to Alzheimer’s disease (AD) pathogenesis. Moreover, impaired memory and learning involve the dysfunction neurotransmission of glutamate, the agonist of the N-methyl-d-aspartate receptor and a major excitatory neurotransmitter in the brain. This systematic review aimed to summarize the current cutting-edge research on the gut microbiota and glutamate alterations associated with dementia. Methods: PubMed, the Cochrane Collaboration Central Register of Controlled Clinical Trials, and Cochrane Systematic Reviews were reviewed for all studies on glutamate and gut microbiota in dementia published up until Feb 2020. Results: Several pilot studies have reported alterations of gut microbiota and metabolites in AD patients and other forms of dementia. Gut microbiota including Bacteroides vulgatus and Campylobacter jejuni affect glutamate metabolism and decrease the glutamate metabolite 2-keto-glutaramic acid. Meanwhile, gut bacteria with glutamate racemase including Corynebacterium glutamicum, Brevibacterium lactofermentum, and Brevibacterium avium can convert l-glutamate to d-glutamate. N-methyl-d-aspartate glutamate receptor (NMDAR)-enhancing agents have been found to potentially improve cognition in AD or Parkinson’s disease patients. These findings suggest that d-glutamate (d-form glutamate) metabolized by the gut bacteria may influence the glutamate NMDAR and cognitive function in dementia patients. Conclusions: Gut microbiota and glutamate are potential novel interventions to be developed for dementia. Exploring comprehensive cognitive functions in animal and human trials with glutamate-related NMDAR enhancers are warranted to examine d-glutamate signaling efficacy in gut microbiota in patients with AD and other neurodegenerative dementias.

Caspase-3 Mediated Cell Death in the Normal Development of the Mammalian Cerebellum

Caspase-3, onto which there is a convergence of the intrinsic and extrinsic apoptotic pathways, is the main executioner of apoptosis. We here review the current literature on the intervention of the protease in the execution of naturally occurring neuronal death (NOND) during cerebellar development. We will consider data on the most common altricial species (rat, mouse and rabbit), as well as humans. Among the different types of neurons and glia in cerebellum, there is ample evidence for an intervention of caspase-3 in the regulation of NOND of the post-mitotic cerebellar granule cells (CGCs) and Purkinje neurons, as a consequence of failure to establish proper synaptic contacts with target (secondary cell death). It seems possible that the GABAergic interneurons also undergo a similar type of secondary cell death, but the intervention of caspase-3 in this case still remains to be clarified in full. Remarkably, CGCs also undergo primary cell death at the precursor/pre-migratory stage of differentiation, in this instance without the intervention of caspase-3. Glial cells, as well, undergo a process of regulated cell death, but it seems possible that expression of caspase-3, at least in the Bergmann glia, is related to differentiation rather than death.

Role of Glutamate and NMDA Receptors in Alzheimer's Disease

Excitatory glutamatergic neurotransmission via N-methyl-d-aspartate receptor (NMDAR) is critical for synaptic plasticity and survival of neurons. However, excessive NMDAR activity causes excitotoxicity and promotes cell death, underlying a potential mechanism of neurodegeneration occurred in Alzheimer's disease (AD). Studies indicate that the distinct outcomes of NMDAR-mediated responses are induced by regionalized receptor activities, followed by different downstream signaling pathways. The activation of synaptic NMDARs initiates plasticity and stimulates cell survival. In contrast, the activation of extrasynaptic NMDARs promotes cell death and thus contributes to the etiology of AD, which can be blocked by an AD drug, memantine, an NMDAR antagonist that selectively blocks the function of extrasynaptic NMDARs.

Mechanisms responsible for the effect of median nerve electrical stimulation on traumatic brain injury-induced coma: orexin-A-mediated N-methyl-D-aspartate receptor subunit NR1 upregulation

Electrical stimulation of the median nerve is a noninvasive technique that facilitates awakening from coma. In rats with traumatic brain injury-induced coma, median nerve stimulation markedly enhances prefrontal cortex expression of orexin-A and its receptor, orexin receptor 1. To further understand the mechanism underlying wakefulness mediated by electrical stimulation of the median nerve, we evaluated its effects on the expression of the N-methyl-D-aspartate receptor subunit NR1 in the prefrontal cortex in rat models of traumatic brain injury-induced coma, using immunohistochemistry and western blot assays. In rats with traumatic brain injury, NR1 expression increased with time after injury. Rats that underwent electrical stimulation of the median nerve (30 Hz, 0.5 ms, 1.0 mA for 15 minutes) showed elevated NR1 expression and greater recovery of consciousness than those without stimulation. These effects were reduced by intracerebroventricular injection of the orexin receptor 1 antagonist SB334867. Our results indicate that electrical stimulation of the median nerve promotes recovery from traumatic brain injury-induced coma by increasing prefrontal cortex NR1 expression via an orexin-A-mediated pathway.

Defects in the CAPN1 Gene Result in Alterations in Cerebellar Development and Cerebellar Ataxia in Mice and Humans

A CAPN1 missense mutation in Parson Russell Terrier dogs is associated with spinocerebellar ataxia. We now report that homozygous or heterozygous CAPN1-null mutations in humans result in cerebellar ataxia and limb spasticity in four independent pedigrees. Calpain-1 knockout (KO) mice also exhibit a mild form of ataxia due to abnormal cerebellar development, including enhanced neuronal apoptosis, decreased number of cerebellar granule cells, and altered synaptic transmission. Enhanced apoptosis is due to absence of calpain-1-mediated cleavage of PH domain and leucine-rich repeat protein phosphatase 1 (PHLPP1), which results in inhibition of the Akt pro-survival pathway in developing granule cells. Injection of neonatal mice with the indirect Akt activator, bisperoxovanadium, or crossing calpain-1 KO mice with PHLPP1 KO mice prevented increased postnatal cerebellar granule cell apoptosis and restored granule cell density and motor coordination in adult mice. Thus, mutations in CAPN1 are an additional cause of ataxia in mammals, including humans.

Calpains and neuronal damage in the ischemic brain: The swiss knife in synaptic injury

The excessive extracellular accumulation of glutamate in the ischemic brain leads to an overactivation of glutamate receptors with consequent excitotoxic neuronal death. Neuronal demise is largely due to a sustained activation of NMDA receptors for glutamate, with a consequent increase in the intracellular Ca(2+) concentration and activation of calcium- dependent mechanisms. Calpains are a group of Ca(2+)-dependent proteases that truncate specific proteins, and some of the cleavage products remain in the cell, although with a distinct function. Numerous studies have shown pre- and post-synaptic effects of calpains on glutamatergic and GABAergic synapses, targeting membrane- associated proteins as well as intracellular proteins. The resulting changes in the presynaptic proteome alter neurotransmitter release, while the cleavage of postsynaptic proteins affects directly or indirectly the activity of neurotransmitter receptors and downstream mechanisms. These alterations also disturb the balance between excitatory and inhibitory neurotransmission in the brain, with an impact in neuronal demise. In this review we discuss the evidence pointing to a role for calpains in the dysregulation of excitatory and inhibitory synapses in brain ischemia, at the pre- and post-synaptic levels, as well as the functional consequences. Although targeting calpain-dependent mechanisms may constitute a good therapeutic approach for stroke, specific strategies should be developed to avoid non-specific effects given the important regulatory role played by these proteases under normal physiological conditions.

Delineating the factors and cellular mechanisms involved in the survival of cerebellar granule neurons

Cerebellar granule neurons (CGNs) constitute the most abundant neuronal population in the mammalian brain. Their postnatal generation and the feasibility to induce their apoptotic death in vitro make them an excellent model to study the effect of several neurotransmitters and neurotrophins. Here, we first review which factors are involved in the generation and proliferation of CGNs in the external granule layer (EGL) and in the regulation of their differentiation and migration to internal granule layer (IGL). Special attention was given to the role of several neurotrophins and the NMDA subtype of glutamate receptor. Then, using the paradigm of potassium deprivation in cultured CGNs, we address several extracellular factors that promote the survival of CGNs, with particular emphasis on the cellular mechanisms. The role of specific protein kinases leading to the regulation of transcription factors and recent data involving the small G protein family is also discussed. Finally, the participation of some members of Bcl-2 family and the inhibition of mitochondria-related apoptotic pathway is also considered. Altogether, these studies evidence that CGNs are a key model to understand the development and the survival of neuronal populations.

NMDA receptor activity determines neuronal fate: location or number?

It is widely believed that the proper activation of N-methyl-D-aspartate (NMDA) receptors (NMDARs) promotes neuronal survival, whereas an excessive activation of NMDARs leads to neuronal damage. NMDARs are found at both synaptic and extrasynaptic sites. One current prevailing theory proposes the dichotomy of NMDAR activity. The role of the two population receptors is mutual antagonism. The activation of synaptic NMDARs, such as synaptic activity at physiological levels, promotes neuronal survival. However, the activation of extrasynaptic NMDARs occurring during stroke, brain injury, and chronic neurological diseases contributes to neuronal death. Thus, the location of NMDARs determines the neuronal fate. However, the theory is greatly challenged. Several studies suggested that synaptic NMDARs are involved in neuronal death. Recently, our work further showed that the coactivation of synaptic and extrasynaptic NMDARs contributes to neuronal death under neuronal insults. Therefore, we propose that the magnitude and duration of NMDAR activation determines the neuronal fate. More interestingly, there appears to be some subtle differences in the affinity between synaptic and extrasynaptic NMDARs, shedding light on the development of selective drugs to block extrasynaptic NMDARs.

Impaired working memory by repeated neonatal MK-801 treatment is ameliorated by galantamine in adult rats

Early life blockade of the NMDA receptor by using MK-801, a non-competitive NMDA receptor antagonist, induces behavioral changes that mimic several features of schizophrenia. In the current study, we first examined the effects of neonatal MK-801 treatment in male Sprague-Dawley rats on locomotor activity, prepulse inhibition and spatial working memory in adolescence (postnatal day 35, PND35) and adulthood (PND63). Next, we investigated the effects of an acetylcholinesterase inhibitor, galantamine, on working memory deficits induced by MK-801 treatment. Rats were treated with either saline or MK-801 (0.25mg/kg twice daily) at PND 5-14, and the long-term behavioral effects were investigated. MK-801 treated rats showed moderate working memory impairments in adolescence but a pronounced deficit in adulthood. However, locomotion and prepulse inhibition at two life stages were not affected by this treatment. Systemic administration of galantamine (1mg/kg) 30 min before each training session significantly improved neonatal MK-801-induced working memory deficits in adulthood. In conclusion, these results suggest that the neonatal MK-801 treatment-induced selective working memory deficit is related to a change in brain cholinergic systems.

NMDA Neurotransmission Dysfunction in Behavioral and Psychological Symptoms of Alzheimer's Disease

Dementia has become an all-important disease because the population is aging rapidly and the cost of health care associated with dementia is ever increasing. In addition to cognitive function impairment, associated behavioral and psychological symptoms of dementia (BPSD) worsen patient’s quality of life and increase caregiver’s burden. Alzheimer’s disease is the most common type of dementia and both behavioral disturbance and cognitive impairment of Alzheimer’s disease are thought to be associated with the N-methyl-D-aspartate (NMDA) dysfunction as increasing evidence of dysfunctional glutamatergic neurotransmission had been reported in behavioral changes and cognitive decline in Alzheimer’s disease. We review the literature regarding dementia (especially Alzheimer’s disease), BPSD and relevant findings on glutamatergic and NMDA neurotransmission, including the effects of memantine, a NMDA receptor antagonist, and NMDA-enhancing agents, such as D-serine and D-cycloserine. Literatures suggest that behavioral disturbance and cognitive impairment of Alzheimer’s disease may be associated with excitatory neurotoxic effects which result in impairment of neuronal plasticity and degenerative processes. Memantine shows benefits in improving cognition, function, agitation/aggression and delusion in Alzheimer’s disease. On the other hand, some NMDA modulators which enhance NMDA function through the co-agonist binding site can also improve cognitive function and psychotic symptoms. We propose that modulating NMDA neurotransmission is effective in treating behavioral and psychological symptoms of Alzheimer’s disease. Prospective study using NMDA enhancers in patients with Alzheimer’s disease and associated behavioral disturbance is needed to verify this hypothesis.

Enhancement of social novelty discrimination by positive allosteric modulators at metabotropic glutamate 5 receptors: adolescent administration prevents adult-onset deficits induced by neonatal treatment with phencyclidine

Metabotropic glutamate-5 receptors (mGluR5), which physically and functionally interact with N-methyl-D-Aspartate (NMDA) receptors, likewise control cognitive processes and have been proposed as targets for novel classes of antipsychotic agent. Since social cognition is impaired in schizophrenia and disrupted by NMDA receptor antagonists like dizocilpine, we evaluated its potential modulation by mGluR5. Acute administration (0.63-40 mg/kg) of the mGluR5 positive allosteric modulators (PAMs), 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) and ADX47273, reversed a delay-induced impairment in social novelty discrimination (SND) in adult rats. The action of CDPPB was blocked by the mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (2.5-10 mg/kg), and was also expressed upon microinjection into frontal cortex (0.63-10 μg/side), but not striatum. Supporting an interrelationship between mGluR5 and NMDA receptors, enhancement of SND by CDPPB was blocked by dizocilpine (0.08 mg/kg) while, reciprocally, dizocilpine-induced impairment in SND was attenuated by CDPPB (10 mg/kg). The SND deficit elicited by post-natal administration of phencyclidine (10 mg/kg, days 7-11) was reversed by CDPPB or ADX47273 in adults at week 8. This phencyclidine-induced impairment in cognition emerged in adult rats from week 7 on, and chronic, pre-symptomatic treatment of adolescent rats with CDPPB over weeks 5-6 (10 mg/kg per day) prevented the appearance of SND deficits in adults until at least week 13. In conclusion, as evaluated by a SND procedure, mGluR5 PAMs promote social cognition via actions expressed in interaction with NMDA receptors and exerted in frontal cortex. MGluR5 PAMs not only reverse but also (when given during adolescence) prevent the emergence of cognitive impairment associated with a developmental model of schizophrenia.

A combined bioinformatics and chemoinformatics approach for developing asymmetric bivalent AMPA receptor positive allosteric modulators as neuroprotective agents

PAMs new in town! An effective, combined bioinformatics and chemoinformatics approach was applied to the design of novel asymmetric bivalent α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor positive allosteric modulators (PAMs) with marked potency in vitro and efficacy in vivo for preventing neuroapoptosis. The novel chemotype could provide pharmacological probes and potential therapeutic agents for glutamatergic hypofunction and its related neurological and psychiatric disorders.

Models of calcium dynamics in cerebellar granule cells

Intracellular calcium dynamics is critical for many functions of cerebellar granule cells (GrCs) including membrane excitability, synaptic plasticity, apoptosis, and regulation of gene transcription. Recent measurements of calcium responses in GrCs to depolarization and synaptic stimulation reveal spatial compartmentalization and heterogeneity within dendrites of these cells. However, the main determinants of local calcium dynamics in GrCs are still poorly understood. One reason is that there have been few published studies of calcium dynamics in intact GrCs in their native environment. In the absence of complete information, biophysically realistic models are useful for testing whether specific Ca(2+) handling mechanisms may account for existing experimental observations. Simulation results can be used to identify critical measurements that would discriminate between different models. In this review, we briefly describe experimental studies and phenomenological models of Ca(2+) signaling in GrC, and then discuss a particular biophysical model, with a special emphasis on an approach for obtaining information regarding the distribution of Ca(2+) handling systems under conditions of incomplete experimental data. Use of this approach suggests that Ca(2+) channels and fixed endogenous Ca(2+) buffers are highly heterogeneously distributed in GrCs. Research avenues for investigating calcium dynamics in GrCs by a combination of experimental and modeling studies are proposed.

Contribution of serine racemase/d-serine pathway to neuronal apoptosis

Recent data indicate that age-related N-methyl-d-aspartate receptor (NMDAR) transmission impairment is correlated with the reduction in serine racemase (SR) expression and d-serine content. As apoptosis is associated with several diseases and conditions that generally occur with age, we investigated the modulation of SR/d-serine pathway during neuronal apoptosis and its impact on survival. We found that in cerebellar granule neurons (CGNs), undergoing apoptosis SR/d-serine pathway is crucially regulated. In the early phase of apoptosis, the expression of SR is reduced, both at the protein and RNA level through pathways, upstream of caspase activation, involving ubiquitin proteasome system (UPS) and c-Jun N-terminal kinases (JNKs). Forced expression of SR, together with treatment with NMDA and d-serine, blocks neuronal death, whereas pharmacological inhibition and Sh-RNA-mediated suppression of endogenous SR exacerbate neuronal death. In the late phase of apoptosis, the increased expression of SR contribute to the last, NMDAR-mediated, wave of cell death. These findings are relevant to our understanding of neuronal apoptosis and NMDAR activity regulation, raising further questions as to the role of SR/d-serine in those neuro-pathophysiological processes, such as aging and neurodegenerative diseases characterized by a convergence of apoptotic mechanisms and NMDAR dysfunction.

Nurr1 protein is required for N-methyl-D-aspartic acid (NMDA) receptor-mediated neuronal survival

NMDA receptor (NMDAR) stimulation promotes neuronal survival during brain development. Cerebellar granule cells (CGCs) need NMDAR stimulation to survive and develop. These neurons differentiate and mature during its migration from the external granular layer to the internal granular layer, and lack of excitatory inputs triggers their apoptotic death. It is possible to mimic this process in vitro by culturing CGCs in low KCl concentrations (5 mm) in the presence or absence of NMDA. Using this experimental approach, we have obtained whole genome expression profiles after 3 and 8 h of NMDA addition to identify genes involved in NMDA-mediated survival of CGCs. One of the identified genes was Nurr1, a member of the orphan nuclear receptor subfamily Nr4a. Our results report a direct regulation of Nurr1 by CREB after NMDAR stimulation. ChIP assay confirmed CREB binding to Nurr1 promoter, whereas CREB shRNA blocked NMDA-mediated increase in Nurr1 expression. Moreover, we show that Nurr1 is important for NMDAR survival effect. We show that Nurr1 binds to Bdnf promoter IV and that silencing Nurr1 by shRNA leads to a decrease in brain-derived neurotrophic factor (BDNF) protein levels and a reduction of NMDA neuroprotective effect. Also, we report that Nurr1 and BDNF show a similar expression pattern during postnatal cerebellar development. Thus, we conclude that Nurr1 is a downstream target of CREB and that it is responsible for the NMDA-mediated increase in BDNF, which is necessary for the NMDA-mediated prosurvival effect on neurons.

Calpain-mediated degradation of myocyte enhancer factor 2D contributes to excitotoxicity by activation of extrasynaptic N-methyl-D-aspartate receptors

Synaptic and extrasynaptic NMDA receptors (NMDARs) appear to play opposite roles in neuronal survival and death. Here we report the new findings on the dysregulation of survival factor, myocyte enhancer factor 2D (MEF2D), by extrasynaptic NMDARs. Excitotoxicity led to the NMDAR-dependent degradation of MEF2D protein and inhibition of its transactivation activity in mature cortical neurons. The activation of extrasynaptic NMDARs alone was sufficient for degradation of MEF2D. Calpain directly cleaved MEF2D in vitro and blocking this protease activity greatly attenuated NMDAR signaled degradation of MEF2D in neurons. Consistently, inhibition of calpain protected cortical neurons from NMDA-induced excitotoxicity. Furthermore, knockdown of MEF2D sensitized neurons to NMDA-induced excitotoxicity, which was not protected by calpain inhibition. Collectively, these findings suggest that dysregulation of MEF2D by calpain may mediate excitotoxicity via an extrasynaptic NMDAR-dependent manner.

Cell signaling in NMDA preconditioning and neuroprotection in convulsions induced by quinolinic acid

The search for novel, less invasive therapeutic strategies to treat neurodegenerative diseases has stimulated scientists to investigate the mechanisms involved in preconditioning. Preconditioning has been report to occur in many organs and tissues. In the brain, the modulation of glutamatergic transmission is an important and promising target to the use of effective neuroprotective agents. The glutamatergic excitotoxicity is a factor common to neurodegenerative diseases and acute events such as cerebral ischemia, traumatic brain injury and epilepsy. In this review we focus on the neuroprotection and preconditioning by chemical agents. Specially, chemical preconditioning models using N-methyl-d-aspartate (NMDA) pre-treatment, which has demonstrated to lead to neuroprotection against seizures and damage to neuronal tissue induced by quinolinic acid (QA). Here we attempted to gather important results obtained in the study of cellular and molecular mechanisms involved in NMDA preconditioning and neuroprotection.

CNS peroxiredoxins and their regulation in health and disease

Oxidative stress is thought to be a contributing factor in many chronic neurodegenerative pathologies, as well as acute cerebrovascular disorders such as stroke. Peroxiredoxins are a family of antioxidant enzymes that reduce peroxides directly through the use of a redox active cysteine within their active site, which in the process becomes oxidized. In order to cycle back to the reduced state, many peroxiredoxins rely on thiol-dependent reduction by the ubiquitous antioxidant enzyme thioredoxin. Peroxiredoxins, together with thioredoxin and thioredoxin's own 'recycling enzyme', thioredoxin reductase, represent an antioxidant enzymic system of growing significance in the context of neuronal physiology and pathology. Overexpression, knockdown, and knockout approaches have demonstrated an important role for peroxiredoxins in protecting neurons from oxidative insults. It is also becoming clear that neuronal peroxiredoxins are subjected to post-translational modifications that impair function as part of disease pathology. Conversely, components of this pathway are also subject to dynamic upregulation such as via endogenous synaptic activity-dependent signaling and induction of the Nrf2-dependent Phase II response. As such, the thioredoxin-peroxiredoxin system represents a potential therapeutic target for central nervous system disorders associated with oxidative stress.

Synaptic versus extrasynaptic NMDA receptor signalling: implications for neurodegenerative disorders

There is a long-standing paradox that NMDA (N-methyl-D-aspartate) receptors (NMDARs) can both promote neuronal health and kill neurons. Recent studies show that NMDAR-induced responses depend on the receptor location: stimulation of synaptic NMDARs, acting primarily through nuclear Ca(2+) signalling, leads to the build-up of a neuroprotective 'shield', whereas stimulation of extrasynaptic NMDARs promotes cell death. These differences result from the activation of distinct genomic programmes and from opposing actions on intracellular signalling pathways. Perturbations in the balance between synaptic and extrasynaptic NMDAR activity contribute to neuronal dysfunction in acute ischaemia and Huntington's disease, and could be a common theme in the aetiology of neurodegenerative diseases. Neuroprotective therapies should aim to both enhance the effect of synaptic activity and disrupt extrasynaptic NMDAR-dependent death signalling.

MK-801 administration during neonatal ethanol withdrawal attenuates interpositus cell loss and juvenile eyeblink conditioning deficits

Binge-level doses of ethanol have been demonstrated to severely disrupt the cerebellum and cerebellum-dependent tasks when administered to rodent subjects during the early postnatal period. N-methyl-d-aspartic acid (NMDA) receptor-mediated excitotoxicity associated with ethanol withdrawal has been implicated as a significant component contributing to neurotoxic effects resulting from early ethanol exposure, and studies using MK-801 (dizocilpine) have reported protection from ethanol-induced damage. The present study examined whether the administration of MK-801 during ethanol withdrawal would ameliorate ethanol-associated cell death in the interpositus nucleus of the cerebellum and behavioral deficits in a cerebellar dependent task. Long Evans rat pups were treated with ethanol (5.25 g/kg) in a binge-like manner on postnatal day 6 using intragastric intubation. Subjects then received an injection of MK-801 (0.5mg/kg) or vehicle during withdrawal, 30h after ethanol exposure. Rats were then trained on an eyeblink classical conditioning task as juveniles (40 days of age), and cerebellar interpositus nucleus numbers were assessed after conditioning. Ethanol-exposed subjects exhibited reductions in neuronal populations and behavioral deficits during eyeblink conditioning. However, MK-801 administration significantly attenuated observed deficiencies, suggesting a protective effect resulting from MK-801 treatment during ethanol withdrawal. These results support the role of NMDA receptor-mediated excitotoxicity as a component mechanism by which ethanol produces teratogenicity. Additionally, our findings support previous reports that have shown correlations between dependent measures of eyeblink classical-conditioning behavior and unbiased cell counts in the interpositus nucleus.

Reduced blockade by extracellular Mg(2+) is permissive to NMDA receptor activation in cerebellar granule neurons that model a migratory phenotype

NMDA receptors (NMDAR) contribute to neuronal development throughout the CNS. However, their mode(s) of activation preceding synaptic maturation is unclear, as they are not co-localized with alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPARs) which normally provide sufficient depolarization to relieve voltage-dependent blockade by Mg(2+). We used cerebellar granule neurons (CGNs) cultured at a near-physiological KCl concentration to examine maturation-dependent changes in NMDAR responses. In contrast, most studies use KCl-supplemented medium to promote survival. At 2-4 days in vitro CGNs: (i) express developmental markers resembling the in vivo migratory phenotype; (ii) maintain a basal amount of calcium responsive element-binding protein phosphorylation that requires NMDARs and calcium/calmodulin-dependent kinases, but not AMPARs; (iii) exhibit NMDA-mediated Ca(2+) influx not effectively blocked by ambient Mg(2+) (0.75 mM) or AMPARs; (iv) maintain a more depolarized resting membrane potential and increased resistance compared to synaptically-connected CGNs. Moreover, migrating CGNs in explant cultures demonstrate NMDA-mediated Ca(2+) influx not effectively blocked by 0.75 mM Mg(2+), and NMDAR but not AMPAR antagonists slow migration. These data suggest the biophysical properties of immature CGNs render NMDARs less sensitive to Mg(2+) blockade, enhancing the likelihood of activation in the absence of AMPAR depolarization.

Suppression of the intrinsic apoptosis pathway by synaptic activity

Synaptic activity promotes resistance to diverse apoptotic insults, the mechanism behind which is incompletely understood. We show here that a coordinated downregulation of core components of the intrinsic apoptosis pathway by neuronal activity forms a key part of the underlying mechanism. Activity-dependent protection against apoptotic insults is associated with inhibition of cytochrome c release in most but not all neurons, indicative of anti-apoptotic signaling both upstream and downstream of this step. We find that enhanced firing activity suppresses expression of the proapoptotic BH3-only member gene Puma in a NMDA receptor-dependent, p53-independent manner. Puma expression is sufficient to induce cytochrome c loss and neuronal apoptosis. Puma deficiency protects neurons against apoptosis and also occludes the protective effect of synaptic activity, while blockade of physiological NMDA receptor activity in the developing mouse brain induces neuronal apoptosis that is preceded by upregulation of Puma. However, enhanced activity can also confer resistance to Puma-induced apoptosis, acting downstream of cytochrome c release. This mechanism is mediated by transcriptional suppression of apoptosome components Apaf-1 and procaspase-9, and limiting caspase-9 activity, since overexpression of procaspase-9 accelerates the rate of apoptosis in active neurons back to control levels. Synaptic activity does not exert further significant anti-apoptotic effects downstream of caspase-9 activation, since an inducible form of caspase-9 overrides the protective effect of synaptic activity, despite activity-induced transcriptional suppression of caspase-3. Thus, suppression of apoptotic gene expression may synergize with other activity-dependent events such as enhancement of antioxidant defenses to promote neuronal survival.

Coupling of the NMDA receptor to neuroprotective and neurodestructive events

NMDA (N-methyl-D-aspartate) receptors are a subtype of ionotropic glutamate receptor with an important role in the physiology and pathophysiology of central neurons. Inappropriate levels of Ca(2+) influx through the NMDA receptor can contribute to neuronal loss in acute trauma such as ischaemia and traumatic brain injury, as well as certain neurodegenerative diseases such as Huntington's disease. However, normal physiological patterns of NMDA receptor activity can promote neuroprotection against both apoptotic and excitotoxic insults. As a result, NMDA receptor blockade can promote neuronal death outright or render neurons vulnerable to secondary trauma. Thus responses to NMDA receptor activity follow a classical hormetic dose-response curve: both too much and too little can be harmful. There is a growing knowledge of the molecular mechanisms underlying both the neuroprotective and neurodestructive effects of NMDA receptor activity, as well as the factors that determine whether an episode of NMDA receptor activity is harmful or beneficial. It is becoming apparent that oxidative stress plays a role in promoting neuronal death in response to both hyper- and hypo-activity of the NMDA receptor. Increased understanding in this field is leading to the discovery of new therapeutic targets and strategies for excitotoxic disorders, as well as a growing appreciation of the harmful consequences of NMDA receptor blockade.

Paradoxical effect of ethanol on potassium channel currents and cell survival in cerebellar granule neurons

Transient exposure to ethanol (EtOH) results in a massive neurodegeneration in the developing brain leading to behavioral and cognitive deficits observed in fetal alcohol syndrome. There is now compelling evidence that K+ channels play an important role in the control of programmed cell death. The aim of the present work was to investigate the involvement of K+ channels in the EtOH-induced cerebellar granule cell death and/or survival. At low and high concentrations, EtOH evoked membrane depolarization and hyperpolarization, respectively. Bath perfusion of EtOH (10 mM) depressed the I(A) (transient K+ current) potassium current whereas EtOH (400 mM) provoked a marked potentiation of the specific I(K) (delayed rectifier K+ current) current. Pipette dialysis with GTPgammaS or GDPbetaS did not modify the effects of EtOH (400 mM) on both membrane potential and I(K) current. In contrast, the reversible depolarization and slowly recovering inhibition of I(A) induced by EtOH (10 mM) became irreversible in the presence of GTPgammaS. EtOH (400 mM) induced prodeath responses whereas EtOH (10 mM) and K+ channel blockers promoted cell survival. Altogether, these results indicate that in cerebellar granule cells, EtOH mediates a dual effect on K+ currents partly involved in the control of granule cell death.

Activation of dopamine D1 receptors blocks phencyclidine-induced neurotoxicity by enhancing N-methyl-D-aspartate receptor-mediated synaptic strength

Early postnatal blockade of NMDA receptors by phencyclidine (PCP) causes cortical apoptosis in animals. This is associated with the development of schizophrenia-like behaviors in rats later in life. Recent studies show that the mechanism involves a loss of neurotrophic support from the phosphoinositol-3 kinase/Akt pathway, which is normally maintained by synaptic NMDA receptor activation. Here we report that activation of dopamine D1 receptors (D1R) with dihydrexidine (DHX) prevents PCP-induced neurotoxicity in cortical neurons by enhancing the efficacy of NMDAergic synapses. DHX increases serine phosphorylation of the NR1 subunit through protein kinase A activation and tyrosine phosphorylation of the NR2B subunit via Src kinase. DHX enhances recruitment of NR1 and NR2B, but not NR2A, into synapses. DHX also facilitated the synaptic response in cortical slices and this was blocked by an NR2B antagonist. DHX pre-treatment of rat pups prior to PCP on postnatal days 7, 9 and 11 inhibited PCP-induced caspase-3 activation on PN11 and deficits in pre-pulse inhibition of acoustic startle measured on PN 26-28. In summary, these data demonstrate that PCP-induced deficits in NMDA receptor function, neurotoxicity and subsequent behavioral deficits may be prevented by D1R activation in the cortex and further, it is suggested that D1R activation may be beneficial in treating schizophrenia.

Inverse relationship between adult hippocampal cell proliferation and synaptic rewiring in the dentate gyrus

Adult neurogenesis is a key feature of the hippocampal dentate gyrus (DG). Neurogenesis is accompanied by synaptogenesis as new cells become integrated into the circuitry of the hippocampus. However, little is known to what extent the embedding of new neurons rewires the pre-existing network. Here we investigate synaptic rewiring in the DG of gerbils (Meriones unguiculatus) under different rates of adult cell proliferation caused by different rearing conditions as well as juvenile methamphetamine treatment. Surprisingly, we found that an increased cell proliferation reduced the amount of synaptic rewiring. To help explain this unexpected finding, we developed a novel model of dentate network formation incorporating neurogenesis and activity-dependent synapse formation and remodelling. In the model, we show that homeostasis of neuronal activity can account for the inverse relationship between cell proliferation and synaptic rewiring.

Riluzole in the treatment of mood and anxiety disorders

Recent advances implicate amino acid neurotransmission in the pathophysiology and treatment of mood and anxiety disorders. Riluzole, which is approved and marketed for the treatment of amyotrophic lateral sclerosis, is thought to be neuroprotective through its modulation of glutamatergic neurotransmission. Riluzole has multiple molecular actions in vitro; the two that have been documented to occur at physiologically realistic drug concentrations and are therefore most likely to be clinically relevant are inhibition of certain voltage-gated sodium channels, which can lead to reduced neurotransmitter release, and enhanced astrocytic uptake of extracellular glutamate.Although double-blind, placebo-controlled trials are lacking, several open-label trials have suggested that riluzole, either as monotherapy or as augmentation of standard therapy, reduces symptoms of obsessive-compulsive disorder, unipolar and bipolar depression, and generalized anxiety disorder. In studies of psychiatrically ill patients conducted to date, the drug has been quite well tolerated; common adverse effects include nausea and sedation. Elevation of liver function tests is common and necessitates periodic monitoring, but has been without clinical consequence in studies conducted to date in psychiatric populations. Case reports suggest utility in other conditions, including trichotillomania and self-injurious behaviour associated with borderline personality disorder. Riluzole may hold promise for the treatment of several psychiatric conditions, possibly through its ability to modulate pathologically dysregulated glutamate levels, and merits further investigation.

Neonatal PCP is more potent than ketamine at modifying preweaning behaviors of Sprague-Dawley rats

Treatment with N-methyl-D-aspartate (NMDA) receptor antagonists, such as ketamine (KET) or phencyclidine (PCP), can trigger apoptotic neurodegeneration in neonatal rodents; however, little is known about the behavioral alterations resulting from such treatment. Here, rats were sc treated with saline; 10 mg/kg PCP on postnatal days (PNDs) 7, 9, and 11; 20 mg/kg KET (six injections every 2 h on PND 7); or a regimen of ketamine and 250 mg/kg L-carnitine (KLC) both administered on PND 7 with additional 250 mg/kg doses of L-carnitine given on PNDs 8-11. Postinjection, the home cage behavior of each pup was categorized on PNDs 7-11. Slant board and forelimb hang behaviors were examined on PNDs 8-11 and 12-16, respectively. The initial KET or KLC injections on PND 7 elevated abnormal home cage activity (i.e., paresis and paddling); however, KLC pup behavior returned to normal by the fourth injection, indicating the protective effects of L-carnitine against NMDA antagonist toxicity. PCP treatment caused substantial abnormal home cage activity on each injection day (PNDs 7, 9, and 11). Latencies to turn on the slant board were significantly longer on PND 8 for KET- and PCP-treated pups and PND 10 for PCP-treated pups. On PND 12, the forelimb hang time of PCP-treated pups was significantly shorter. Body weight was decreased on PNDs 8-18 in PCP-treated pups and PNDs 8-10 in KET-treated pups. These data indicate that developmental NMDA antagonist treatment causes short-term behavioral alterations which appear related to motor coordination and may be cerebellar in nature. Furthermore, single PCP injections appear more potent at altering behavior than multiple injections of KET.

Altered dopamine receptor and dopamine transporter binding and tyrosine hydroxylase mRNA expression following perinatal NMDA receptor blockade

This study examined how perinatal phencyclidine (PCP) treatment would affect dopamine D2 receptor and dopamine transporter (DAT) binding at different stages after treatment cessation. Female rat pups received injections of PCP (10 mg/kg, s.c.) or saline on postnatal day (PN)7, 9 and 11. D2 receptor and transporter binding was examined at four time-points (PN12, 18, 32 and 96) following injections. PCP treatment altered D2 receptor binding throughout development, with a final end-point of 22-33% decreased binding at adulthood in the nucleus accumbens and caudate putamen (P < 0.01), accompanied by a small but significant increase in DAT binding in the caudate putamen. Tyrosine hydroxylase mRNA expression was also significantly increased by 25% (P < 0.05) in the ventral tegmental area of adult rats, suggesting that this model may produce a long-term increase in dopamine output. This study demonstrates that early insult to the brain from NMDA receptor hypofunction alters the dopaminergic system at different stages of development.

The dichotomy of NMDA receptor signaling

The N-methyl-D-aspartate (NMDA) subtype of ionotropic glutamate receptors plays a Jekyll and Hyde role in the mammalian central nervous system. In pathological scenarios such as ischemia, Ca2+ influx through the NMDA receptor is a key mediator of cell death. However, physiological levels of NMDA-receptor activity can promote neuronal survival and resistance to trauma and play important roles in synaptic plasticity and transmission. This dichotomy may explain the poor tolerance and efficacy of NMDA-receptor antagonists in clinical trials for excitotoxic trauma. There is a growing understanding of the signaling events that mediate the opposing effects of NMDA-receptor activity and the factors that determine whether an episode of NMDA-receptor activity will promote survival or death. This knowledge may lead to therapeutic strategies that enable the selective blockade of prodeath signaling cassettes while sparing physiological signaling to survival and plasticity.

The role of caspase-3 activation in phencyclidine-induced neuronal death in postnatal rats

This study determined the role of caspase-3 in phencyclidine (PCP)-induced neurodegeneration in postnatal rats. PCP administration to postnatal day 7 rats induced a dose-dependent increase in caspase-3 enzymatic activity in frontal cortex, striatum, and hippocampus. Enzymatic activation was present at 4 h, peaked between 6 and 12 h, and disappeared by 24 h. Further, cleaved caspase-3-immunoreactive neurons were detected as early as 2 h in the cortex, and were found throughout the brain, including, in addition, the thalamus and striatum. Within the cingulate, frontal, parietal, and retrosplenial cortices, immunoreactivity was specific for layers II-IV (especially layer II). Neurons positive for both silver staining and terminal deoxynucleotidyl transferase biotin-d-UTP nick-end labeling (TUNEL) were found in the same brain regions and subregions. Double labeling experiments confirmed that cleaved caspase-3 and TUNEL were coexpressed in many neurons in all brain regions and subregions studied. Temporal studies revealed that procaspase-3 cleavage preceded TUNEL staining by about 3 h, with many neurons being positive for both caspase-3 and TUNEL 9 h after PCP treatment. In organotypic corticostriatal slices, PCP caused a concentration- and time-dependent cleavage of procaspase-3 that was also colocalized with TUNEL staining in layers II-IV of the parietal cortex. Caspase-3 activation again preceded PCP-induced DNA damage assessed by TUNEL. PCP-induced neuronal death in vitro as measured by TUNEL staining was blocked 85% by Ac-AAVALLPAVLLALLAPDEVD-CHO, a cell-permeable selective caspase-3 inhibitor. These data demonstrate that caspase-3 activation plays a necessary role in the regionally selective neuronal death induced by PCP in the developing rat brain.

Role of brain-derived neurotrophic factor in the protective action of N-methyl-D-aspartate in the apoptotic death of cerebellar granule neurons induced by low potassium

Several neurotrophic factors, including brain-derived neurotrophic factor (BDNF), and neurotransmitters, such as glutamate, may influence neuronal apoptotic death. Rat cerebellar granule neurons (CGN) cultured in low potassium (5 or 10 mM KCl) for more than 5 days in vitro (DIV) die apoptotically. These cells survive in the presence of high potassium (25 mM KCl, K25) or N-methyl-D-aspartate (NMDA), an agonist of glutamatergic receptors. CGN transferred from high to low potassium die apoptotically. Here, we characterized the effect of BDNF and NMDA on the apoptotic death induced by low potassium in CGN. Cell death of CGN by culturing in low potassium for 6 DIV was inhibited by BDNF and NMDA. When CGN were cultured in K25 and transferred to a low-potassium medium, 65% of neurons died after 48 hr. Under these conditions, BDNF, NMDA, or BDNF + NMDA increased CGN survival. Both BDNF and NMDA decreased caspase-9 activity and mRNA caspase-3 levels and activity induced by low potassium. CGN survival induced by BDNF is mediated by TrkB activation, whereas that induced by NMDA is mediated by NMDA receptor and TrkB activation. NMDA, but not BDNF, raised [Ca(2+)](i), which was reduced by low-potassium treatment. These results suggest that NMDA receptor stimulation induces CGN survival through the influx of extracellular Ca(2+) that may evoke the release of BDNF and the activation of TrkB. Complementary mechanisms induced by depolarization and changes in Ca(2+) levels would also contribute to the neuroprotection exerted by NMDA and potassium.

NMDA receptors promote survival in somatosensory relay nuclei by inhibiting Bax-dependent developmental cell death

Naturally occurring cell death is a universal feature of developing nervous systems that plays an essential role in determining adult brain function. Yet little is known about the decisions that select a subset of CNS neurons for survival and cause others to die. We report that postnatal day 0 NMDA receptor subunit 1 (NMDAR1) knockout mice display an approximately 2-fold increase in cell death in the brainstem trigeminal complex (BSTC), including all four nuclei that receive somatosensory inputs from the face (principalis, oralis, interpolaris, and caudalis). Treatment with the NMDA receptor antagonist dizocilpine maleate (MK-801) for 24 h before birth also caused an increase in cell death that reached statistical significance in two of the four nuclei (oralis and interpolaris). The neonatal sensitivity to NMDA receptor hypofunction in the BSTC, and in its main thalamic target, the ventrobasal nucleus (VB), coincides with the peak of naturally occurring cell death and trigeminothalamic synaptogenesis. At embryonic day 17.5, before the onset of these events, NMDAR1 knockout does not affect cell survival in either the BSTC or the VB. Immunostaining for active caspase-3 and the neuronal marker Hu specifically confirms the presence of dying neurons in the BSTC and the VB of NMDAR1 knockout neonates. Finally, genetic deletion of Bax rescues these structures from the requirement for NMDA receptors to limit naturally occurring cell death. Taken together, the results indicate that NMDA receptors play a survival role for somatosensory relay neurons during synaptogenesis by inhibiting Bax-dependent developmental cell death.

Early brain development disruption from NMDA receptor hypofunction: relevance to schizophrenia

Disruption to brain development at an early stage can potentially alter chemically coded neural networks and can affect behavior in later life. During early brain development antagonism of glutamate NMDA receptors, which play an important role in neuronal outgrowth and survival, leads to neuronal damage in several brain regions and causes behavioral alterations in rodents that mimic schizophrenia symptoms and endophenotypes. There are several lines of evidence implicating involvement of a dysfunctional glutamate system in schizophrenia. In normal subjects, NMDA receptor antagonists produce behavioral and neurochemical changes that mimic schizophrenia symptoms better than any other psychotomimetic drug. Moreover, these drugs worsen symptoms in schizophrenia patients and can trigger a recrudescence of the acute psychotic state in patients with stable chronic schizophrenia. In addition, genes consistently reported as being altered in schizophrenia play roles in development, neuroplasticity and glutamate/GABAergic neurotransmission. Perinatal NMDA receptor antagonist treatment is a useful model for studying the neurodevelopmental and NMDA receptor hypofunction hypotheses of schizophrenia because neurochemical and behavioral changes, reminiscent of those seen in schizophrenia, are present long after cessation of drug administration, which suggests that a permanent change in brain structure and organization has occurred during brain development.

Effect of N-methyl-d-aspartate receptor blockade on caspase activation and neuronal death in the developing rat cerebellum

In vitro studies have demonstrated that N-methyl-D-aspartate (NMDA) receptor activation rescue cerebellar granule neurons (CGN) from apoptotic death. It has been suggested that this effect mimics the transient glutamate receptors activation by mossy fibers during cerebellar development. We reported previously that CGN from postnatal days 2-4 (P2-P4) rats increased cell survival in response to NMDA treatment. In this study, we evaluated the effect of dizocilpine (MK-801) administrated for three consecutive days on the apoptotic death of CGN during development. MK-801 treatment decreased the large number of CGN condensed nuclei found at P8, but this drug increased the proportion of condensed nuclei at P16. We also found a high activity of caspases during the first postnatal week that decreased during development. MK-801 treatment did not modify the activity of caspase-8 at any age, but decreased caspase-9 activity at P8 and increased the activity of caspase-1 (76%) at P8, caspase-3 (160%) at P16 and caspase-9 (50%) at P12. These results suggest that NMDA receptor stimulation regulates the activity of caspases in a differential way and plays an important role in the in vivo CGN death during postnatal development.

Proliferation of cerebellar precursor cells is negatively regulated by nitric oxide in newborn rat

The diffusible messenger, nitric oxide plays multiple roles in neuroprotection, neurodegeneration and brain plasticity. Its involvement in neurogenesis has been disputed, on the basis of results on models in vivo and in culture. We report here that pharmacological blockade of nitric oxide production in rat pups resulted, during a restricted time window of the first three postnatal days, in increased cerebellar proliferation rate, as assessed through tritiated thymidine or BrdU incorporation into DNA. This was accompanied by increased expression of Myc, a transcription factor essential for cerebellar development, and of the cell cycle regulating gene, cyclin D1. These effects were mediated downstream by the nitric oxide-dependent second messenger, cGMP. Schedules of pharmacological NO deprivation targeted to later developmental stages (from postnatal day 3 to 7), no longer increased proliferation, probably because of partial escape of the cGMP level from nitric oxide control. Though limited to a brief temporal window, the proliferative effect of neonatal nitric oxide deprivation could be traced into adulthood. Indeed, the number of BrdU-labeled surviving cells, most of which were of neuronal phenotype, was larger in the cerebellum of 60-day-old rats that had been subjected to NO deprivation during the first three postnatal days than in control rats. Experiments on cell cultures from neonatal cerebellum confirmed that nitric oxide deprivation stimulated proliferation of cerebellar precursor cells and that this effect was not additive with the proliferative action of sonic hedgehog peptide. The finding that nitric oxide deprivation during early cerebellar neurogenesis, stimulates a brief increase in cell proliferation may contribute to a better understanding of the controversial role of nitric oxide in brain development.

Preconditioning doses of NMDA promote neuroprotection by enhancing neuronal excitability

Neuroprotection can be induced by low doses of NMDA, which activate both synaptic and extrasynaptic NMDA receptors. This is in apparent contradiction with our recent findings that extrasynaptic NMDA receptor signaling exerts a dominant inhibitory effect on prosurvival signaling from synaptic NMDA receptors. Here we report that exposure to low preconditioning doses of NMDA results in preferential activation of synaptic NMDA receptors because of a dramatic increase in action potential firing. Both acute and long-lasting phases of neuroprotection in the face of apoptotic or excitotoxic insults are dependent on this firing enhancement. Key mediators of synaptic NMDA receptor-dependent neuroprotection, phosphatidylinositol 3 kinase-Akt (PI3 kinase-Akt) signaling to Forkhead box subgroup O (FOXO) export and glycogen synthase kinase 3beta (GSK3beta) inhibition and cAMP response element-binding protein-dependent (CREB-dependent) activation of brain-derived neurotrophic factor (BDNF), can be induced only by low doses of NMDA via this action potential-dependent route. In contrast, NMDA doses on the other side of the toxicity threshold do not favor synaptic NMDA receptor activation because they strongly suppress firing rates below baseline. The classic bell-shaped curve depicting neuronal fate in response to NMDA dose can be viewed as the net effect of two antagonizing (synaptic vs extrasynaptic) curves: via increased firing the synaptic signaling dominates at low doses, whereas firing becomes suppressed and extrasynaptic signaling dominates as the toxicity threshold is crossed.

Anesthetics and brain toxicity

PURPOSE OF REVIEW

Recent experimental data from rodent studies have demonstrated accelerated neurodegeneration in rat pups exposed to commonly used anesthetic drugs. These provocative findings certainly question and undermine the safe use of anesthetic drugs, particularly in pediatric anesthesia, and have prompted many to investigate the neurotoxic effect of anesthetic drugs on the developing brain. This review will address the scientific evidence for the anesthetic-induced neurotoxicity and its applicability in humans.

RECENT FINDINGS

Several investigators have shown that prolonged administration of anesthetic drugs, including ketamine, isoflurane, nitrous oxide and midazolam, produced increased neurodegeneration in 7-day-old rat pups. The combination of the latter three drugs led to altered learning behavior in adulthood. Despite these unequivocal findings in rodents, similar changes cannot be reproduced in other species. Furthermore, withholding anesthesia during painful procedures in neonatal rats resulted in significant long-term aberrant responses to sensory stimulation and pain thresholds.

SUMMARY

Taken together, these studies question the applicability of these data to the anesthetic management of the neonate. Further investigations in this area are needed before withholding anesthetics in the anesthetic management of pediatric surgical patients.

NMDA receptor activation modulates programmed cell death during early post-natal retinal development: a BDNF-dependent mechanism

Glutamate is a classical excitotoxin of the central nervous system (CNS), but extensive work demonstrates neuroprotective roles of this neurotransmitter in developing CNS. Mechanisms of glutamate-mediated neuroprotection are still under scrutiny. In this study, we investigated mediators of glutamate-induced neuroprotection, and tested whether this neurotransmitter controls programmed cell death in the developing retina. The protective effect of N-methyl-d-aspartate (NMDA) upon differentiating cells of retinal explants was completely blocked by a neutralizing antibody to brain-derived neurotrophic factor (BDNF), but not by an antibody to neurotrophin-4 (NT-4). Consistently, chronic activation of NMDA receptor increased the expression of BDNF and trkB mRNA, as well as BDNF protein content, but did not change the content of NT-4 mRNA in retinal tissue. Furthermore, we showed that in vivo inactivation of NMDA receptor by intraperitoneal injections of MK-801 increased natural cell death of specific cell populations of the post-natal retina. Our results show that chronic activation of NMDA receptors in vitro induces a BDNF-dependent neuroprotective state in differentiating retinal cells, and that NMDA receptor activation controls programmed cell death of developing retinal neurons in vivo.

Postnatal cerebellar granule cells of the white rat (Rattus norvegicus): a quantitative study, using design-based stereology

A stereological study was carried out on postnatal cerebellar granule cells of rats aged 6 and 10 days, for detecting whether and how much they would differ from those of young adult rats. The following parameters were estimated: number-weighted mean volume of the nucleus and of the soma; mean total surface area of the soma; mean absolute volumes per cell of total cytoplasm, mitochondria, Golgi apparatus, and cytosol; mean surface density of the rough endoplasmic reticulum (RER); mean total surface area of the RER. These values were compared between the two postnatal ages. In addition, those values were also analysed in comparison to the ones depicted in young adult rats (60 days), already published by our team, in order to detect similarities between them. It was noticed that, between 6 and 10 days, the mean surface density of the RER was the only parameter that did not change significantly. The comparison of each of the postnatal ages with 60 days revealed that, with the exception of the absolute volume of Golgi apparatus, significant differences were displayed concerning other organelles and cellular compartments. It was concluded that, although fine structural differences have been disclosed, from the stereological point of view postnatal granule cells at 10 days were practically similar to the young adult ones at 60 days. Some potential physiological implications have been considered.

Pronounced cell death in the absence of NMDA receptors in the developing somatosensory thalamus

Genetic deletion of NMDA glutamate receptors disrupts development of whisker-related neuronal patterns in the somatosensory system. Independent studies have shown that NMDA receptor antagonists increase cell death among developing neurons. Here, we report that a dramatic feature of the developing somatosensory system in newborn NMDA receptor 1 (NMDAR1) knock-out mice is increased cell death in the ventrobasal nucleus (VB) of the thalamus. Sections were subject to terminal deoxynucleotidyl transferase dUTP nick end labeling staining for apoptotic DNA fragmentation, thionine staining for pyknotic nuclei, silver staining for degenerating cells, and immunostaining for caspase-3. All four methods demonstrated that deletion of NMDAR1 causes a large (on the order of threefold to fivefold) increase in cell death in the VB. The NMDA receptor antagonists dizocilpine maleate (MK-801) and phencyclidine also increase cell death in this structure. The onset of increased cell death in the VB in the absence of NMDA receptor function is approximately the time of birth, overlaps with naturally occurring cell death and synaptogenesis, and displays some anatomical specificity. For example, there was no increase in cell death in the hippocampus or neocortex of NMDAR1 knock-out mice at any of the time points examined: embryonic day 15.5 (E15.5), E17.5, and postnatal day 0. We also report a significant reduction in the size of the VB that is evident starting at E17.5. The results indicate that NMDA receptors play a major role in cell survival during naturally occurring cell death in the VB and demonstrate that cell death is a consideration in NMDA receptor knock-out studies.