Neurochemical, behavioral and architectural changes after chronic inactivation of NMDA receptors in mice

MK-801 and cognitive functions: Investigating the behavioral effects of a non-competitive NMDA receptor antagonist

RATIONALE

MK-801 (dizocilpine) is a non-competitive NMDA receptor antagonist originally explored for anticonvulsant potential. Despite its original purpose, its amnestic properties led to the development of pivotal models of various cognitive impairments widely employed in research and greatly impacting scientific progress. MK-801 offers several advantages; however, it also presents drawbacks, including inducing dose-dependent hyperlocomotion or ambiguous effects on anxiety, which can impact the interpretation of behavioral research results.

OBJECTIVES

The present review attempts to summarize and discuss the effects of MK-801 on different types of memory and cognitive functions in animal studies.

RESULTS

A plethora of behavioral research suggests that MK-801 can detrimentally impact cognitive functions. The specific effect of this compound is influenced by variables including developmental stage, gender, species, strain, and, crucially, the administered dose. Notably, when considering the undesirable effects of MK-801, doses up to 0.1 mg/kg were found not to induce stereotypy or hyperlocomotion.

CONCLUSION

Dizocilpine continues to be of significant importance in preclinical research, facilitating the exploration of various procognitive therapeutic agents. However, given its potential undesirable effects, it is imperative to meticulously determine the appropriate dosages and conduct supplementary evaluations for any undesirable outcomes, which could complicate the interpretation of the findings.

Prostacyclin Promotes Degenerative Pathology in a Model of Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is the most common form of dementia in aged populations. A substantial amount of data demonstrates that chronic neuroinflammation can accelerate neurodegenerative pathologies. In AD, chronic neuroinflammation results in the upregulation of cyclooxygenase and increased production of prostaglandin H2, a precursor for many vasoactive prostanoids. While it is well-established that many prostaglandins can modulate the progression of neurodegenerative disorders, the role of prostacyclin (PGI2) in the brain is poorly understood. We have conducted studies to assess the effect of elevated prostacyclin biosynthesis in a mouse model of AD. Upregulated prostacyclin expression significantly worsened multiple measures associated with amyloid-β (Aβ) disease pathologies. Mice overexpressing both Aβ and PGI2 exhibited impaired learning and memory and increased anxiety-like behavior compared with non-transgenic and PGI2 control mice. PGI2 overexpression accelerated the development of Aβ accumulation in the brain and selectively increased the production of soluble Aβ42. PGI2 damaged the microvasculature through alterations in vascular length and branching; Aβ expression exacerbated these effects. Our findings demonstrate that chronic prostacyclin expression plays a novel and unexpected role that hastens the development of the AD phenotype.

Sex-specific behavioral impairments produced by neonatal exposure to MK-801 are partially reversed by adolescent CDPPB treatment

Psychomimetic behaviors manifest in adult rodents long after neonatal exposure to the noncompetitive NMDA receptor antagonist MK-801. In the present study, we used this neurodevelopmental model of schizophrenia to evaluate the therapeutic potential of positive allosteric modulation of metabotropic glutamate receptor 5 (mGluR5) during adolescence. To this end, we randomly assigned male and female C57BL6 mouse littermates to one of three treatment groups: (i) neonatal and adolescent saline, (ii) neonatal MK-801 (0.25 mg/kg) and adolescent saline, and (iii) neonatal MK-801 and adolescent CDPPB (10 mg/kg), a positive allosteric modulator of mGluR5. When animals reached adulthood, a wide range of behavioral tests were conducted including sucrose preference, anxiety assessment in the elevated plus maze, and a series of food-reinforced operant procedures meant to assess motor activity, motivation, learning, and attention. Neonatal MK-801 exposure produced profound motor hyperactivity in both sexes and attenuated sucrose preference in males, effects that were reversed by CDPPB. MK-801 produced other deficits such as impaired set shifting or response inhibition deficits that were not reversed by CDPPB. Overall, female mice were more susceptible to MK-801's behavioral effects than males. These findings further support the use of neonatal MK-801 exposure as an animal model of schizophrenia and suggest that CDPPB can reverse the neurodevelopmental progression of some schizophrenia-like behaviors.

Neonatal blockade of NR2A-containing but not NR2B-containing NMDA receptor induces spatial working memory deficits in adult rats

The immature brain is highly sensitive to disturbances in the functioning of N-methyl-d-aspartate (NMDA) receptor in rodents, and blockade of the receptor during postnatal brain development period causes schizophrenia-like behavior in adulthood. During the postnatal period, NR2A- and NR2B-containing NMDA receptors are highly expressed, and these two subunits show different expression patterns in the brain. However, the functions of these two NMDA receptors are unknown. In this study, we treated rats with an NR2A-preferring NMDA receptor antagonist (PEAQX, 10 mg/kg), an NR2B-selective NMDA receptor antagonist (ifenprodil, 7.5 mg/kg), or a nonselective blocker of the NMDA receptor (MK-801, 0.4 mg/kg) during the neonatal period. Rats neonatally treated with MK-801 or PEAQX showed spatial working memory deficits in the Y-maze test. PEAQX-treated rats also showed greater reactivity to acoustic stimuli and hypersensitivity to acute MK-801 challenge. However, ifenprodil treatment did not cause any detectable behavioral changes. These results suggest that the NR2A-containing NMDA receptor is indispensable for proper brain development in rats, and functional disturbances in this subunit impair hippocampus-dependent spatial working memory in adulthood.

Placental transfer of NMDAR antibodies causes reversible alterations in mice

Objective

To determine whether maternofetal transfer of NMDA receptor (NMDAR) antibodies has pathogenic effects on the fetus and offspring, we developed a model of placental transfer of antibodies.

Methods

Pregnant C57BL/6J mice were administered via tail vein patients' or controls' immunoglobulin G (IgG) on days 14–16 of gestation, when the placenta is able to transport IgG and the immature fetal blood-brain barrier is less restrictive to IgG crossing. Immunohistochemical and DiOlistic (gene gun delivery of fluorescent dye) staining, confocal microscopy, standardized developmental and behavioral tasks, and hippocampal long-term potentiation were used to determine the antibody effects.

Results

In brains of fetuses, patients' IgG, but not controls' IgG, bound to NMDAR, causing a decrease in NMDAR clusters and cortical plate thickness. No increase in neonatal mortality was observed, but offspring exposed in utero to patients' IgG had reduced levels of cell-surface and synaptic NMDAR, increased dendritic arborization, decreased density of mature (mushroom-shaped) spines, microglial activation, and thinning of brain cortical layers II–IV with cellular compaction. These animals also had a delay in innate reflexes and eye opening and during follow-up showed depressive-like behavior, deficits in nest building, poor motor coordination, and impaired social-spatial memory and hippocampal plasticity. Remarkably, all these paradigms progressively improved (becoming similar to those of controls) during follow-up until adulthood.

Conclusions

In this model, placental transfer of patients' NMDAR antibodies caused severe but reversible synaptic and neurodevelopmental alterations. Reversible antibody effects may contribute to the infrequent and limited number of complications described in children of patients who develop anti-NMDAR encephalitis during pregnancy.

Kynurenine Pathway as a New Target of Cognitive Impairment Induced by Lead Toxicity During the Lactation

The immature brain is especially vulnerable to lead (Pb²⁺) toxicity, which is considered an environmental neurotoxin. Pb²⁺ exposure during development compromises the cognitive and behavioral attributes which persist even later in adulthood, but the mechanisms involved in this effect are still unknown. On the other hand, the kynurenine pathway metabolites are modulators of different receptors and neurotransmitters related to cognition; specifically, high kynurenic acid levels has been involved with cognitive impairment, including deficits in spatial working memory and attention process. The aim of this study was to evaluate the relationship between the neurocognitive impairment induced by Pb²⁺ toxicity and the kynurenine pathway. The dams were divided in control group and Pb²⁺ group, which were given tap water or 500 ppm of lead acetate in drinking water ad libitum, respectively, from 0 to 23 postnatal day (PND). The poison was withdrawn, and tap water was given until 60 PND of the progeny. The locomotor activity in open field, redox environment, cellular function, kynurenic acid (KYNA) and 3-hydroxykynurenine (3-HK) levels as well as kynurenine aminotransferase (KAT) and kynurenine monooxygenase (KMO) activities were evaluated at both 23 and 60 PND. Additionally, learning and memory through buried food location test and expression of KAT and KMO, and cellular damage were evaluated at 60 PND. Pb²⁺ group showed redox environment alterations, cellular dysfunction and KYNA and 3-HK levels increased. No changes were observed in KAT activity. KMO activity increased at 23 PND and decreased at 60 PND. No changes in KAT and KMO expression in control and Pb²⁺ group were observed, however the number of positive cells expressing KMO and KAT increased in relation to control, which correlated with the loss of neuronal population. Cognitive impairment was observed in Pb²⁺ group which was correlated with KYNA levels. These results suggest that the increase in KYNA levels could be a mechanism by which Pb²⁺ induces cognitive impairment in adult mice, hence the modulation of kynurenine pathway represents a potential target to improve behavioural alterations produced by this environmental toxin.

Chronic MK-801 Application in Adolescence and Early Adulthood: A Spatial Working Memory Deficit in Adult Long-Evans Rats But No Changes in the Hippocampal NMDA Receptor Subunits

The role of NMDA receptors in learning, memory and hippocampal function has long been recognized. Post-mortem studies have indicated that the expression or subunit composition of the NMDA glutamate receptor subtype might be related to the impaired cognitive functions found in schizophrenia patients. NMDA receptor antagonists have been used to develop animal models of this disorder. There is accumulating evidence showing that not only the acute but also the chronic application of NMDA receptor antagonists may induce schizophrenia-like alterations in behavior and brain functions. However, limited evidence is available regarding the consequences of NMDA receptor blockage during periods of adolescence and early adulthood. This study tested the hypothesis that a 2-week treatment of male Long-Evans and Wistar rats with dizocilpine (MK-801; 0.5 mg/kg daily) starting at postnatal days (PD) 30 and 60 would cause a long-term cognitive deficit and changes in the levels of NMDA receptor subunits. The working memory version of the Morris water maze (MWM) and active place avoidance with reversal on a rotating arena (Carousel) requiring cognitive coordination and flexibility probed cognitive functions and an elevated-plus maze (EPM) was used to measure anxiety-like behavior. The western blot method was used to determine changes in NMDA receptor subunit levels in the hippocampus. Our results showed no significant changes in behaviors in Wistar rats. Slightly elevated anxiety-like behavior was observed in the EPM in Long-Evans rats with the onset of treatment on PD 30. Furthermore, Long-Evans rats treated from PD 60 displayed impaired working memory in the MWM. There were; however, no significant changes in the levels of NMDA receptor subunits because of MK-801 administration. These findings suggest that a 2-week treatment starting on PD 60 in Long-Evans rats leads to long-term changes in working memory, but this deficit is not paralleled by changes in NMDA receptor subunits. These results support the face validity, but not construct validity of this model. We suggest that chronic treatment of adolescent and adult rats does not constitute a plausible animal model of schizophrenia.

The myelinated fiber loss in the corpus callosum of mouse model of schizophrenia induced by MK-801

Previous magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) investigations have shown that the white matter volume and fractional anisotropy (FA) were decreased in schizophrenia (SZ), which indicated impaired white matter integrity in SZ. However, the mechanism underlying these abnormalities has been less studied. The current study was designed to investigate the possible reasons for white matter abnormalities in the mouse model of SZ induced by NMDA receptor antagonist using the unbiased stereological methods and transmission electron microscope technique. We found that the mice treated with MK-801 demonstrated a series of schizophrenia-like behaviors including hyperlocomotor activity and more anxiety. The myelinated fibers in the corpus callosum (CC) of the mice treated with MK-801 were impaired with splitting lamellae of myelin sheaths and segmental demyelination. The CC volume and the total length of the myelinated fibers in the CC of the mice treated with MK-801 were significantly decreased by 9.4% and 16.8% when compared to those of the mice treated with saline. We further found that the loss of the myelinated fibers length was mainly due to the marked loss of the myelinated nerve fibers with the diameter of 0.4-0.5 μm. These results indicated that the splitting myelin sheaths, demyelination and the loss of myelinated fibers with small diameter might provide one of the structural bases for impaired white matter integrity of CC in the mouse model of SZ. These results might also provide a baseline for further studies searching for the treatment of SZ through targeting white matter.

Cannabidiol attenuates sensorimotor gating disruption and molecular changes induced by chronic antagonism of NMDA receptors in mice

BACKGROUND

Preclinical and clinical data suggest that cannabidiol (CBD), a major non-psychotomimetic compound from Cannabis sativa, induces antipsychotic-like effects. However, the antipsychotic properties of repeated CBD treatment have been poorly investigated. Behavioral changes induced by repeated treatment with glutamate N-methyl-D-aspartate receptor (NMDAR) antagonists have been proposed as an animal model of schizophrenia-like signs. In the present study, we evaluated if repeated treatment with CBD would attenuate the behavioral and molecular modifications induced by chronic administration of one of these antagonists, MK-801.

METHODS

Male C57BL/6J mice received daily i.p. injections of MK-801 (0.1, 0.5, or 1mg/kg) for 14, 21, or 28 days. Twenty-four hours after the last injection, animals were submitted to the prepulse inhibition (PPI) test. After that, we investigated if repeated treatment with CBD (15, 30, and 60mg/kg) would attenuate the PPI impairment induced by chronic treatment with MK-801 (1mg/kg; 28 days). CBD treatment began on the 6th day after the start of MK-801 administration and continued until the end of the treatment. Immediately after the PPI, the mice brains were removed and processed to evaluate the molecular changes. We measured changes in FosB/ΔFosB and parvalbumin (PV) expression, a marker of neuronal activity and a calcium-binding protein expressed in a subclass of GABAergic interneurons, respectively. Changes in mRNA expression of the NMDAR GluN1 subunit gene (GRN1) were also evaluated. CBD effects were compared to those induced by the atypical antipsychotic clozapine.

RESULTS

MK-801 administration at the dose of 1mg/kg for 28 days impaired PPI responses. Chronic treatment with CBD (30 and 60mg/kg) attenuated PPI impairment. MK-801 treatment increased FosB/ΔFosB expression and decreased PV expression in the medial prefrontal cortex. A decreased mRNA level of GRN1 in the hippocampus was also observed. All the molecular changes were attenuated by CBD. CBD by itself did not induce any effect. Moreover, CBD effects were similar to those induced by repeated clozapine treatment.

CONCLUSIONS

These results indicate that repeated treatment with CBD, similar to clozapine, reverses the psychotomimetic-like effects and attenuates molecular changes observed after chronic administration of an NMDAR antagonist. These data support the view that CBD may have antipsychotic properties.

MK-801-induced and scopolamine-induced hyperactivity in rats neonatally treated chronically with MK-801

This study investigated the effects of chronic neonatal blockade of N-methyl-D-aspartate (NMDA) receptors on NMDA and muscarinic acetylcholine receptor-mediated neurotransmission in adulthood. Rats neonatally treated chronically with MK-801/saline were tested for 40 min, at the age of 14-16 weeks, for locomotor activity in an open field immediately after acute administration of MK-801 (0.2 mg/kg) or scopolamine (0.4-2.0 mg/kg). Rats neonatally treated with MK-801 showed significantly higher locomotor activity than those treated with saline. Acute MK-801 administration caused hyperlocomotion regardless of neonatal treatment, but the effect was more potent in rats neonatally treated with MK-801. In contrast, acute scopolamine administration did not cause hyperlocomotion in rats neonatally treated with saline, but significantly increased locomotion in those neonatally treated with MK-801. The results suggest that chronic neonatal NMDA receptor blockade causes changes in glutamatergic and cholinergic transmission in adulthood long after the cessation of treatment.

White matter injuries induced by MK-801 in a mouse model of schizophrenia based on NMDA antagonism

The etiology of schizophrenia (SZ) is complex and largely unknown. Neuroimaging and postmortem studies have suggested white matter disturbances in SZ. In the present study, we tested the white matter deficits hypothesis of SZ using a mouse model of SZ induced by NMDA receptor antagonist MK-801. We found that mice with repeated chronic MK-801 administration showed increased locomotor activity in the open field test, less exploration of a novel environment in the hole-board test, and increased anxiety in the elevated plus maze but no impairments were observed in coordination or motor function on accelerating rota-rod. The total white matter volume and corpus callosum volume in mice treated with MK-801 were significantly decreased compared to control mice treated with saline. Myelin basic protein and 2', 3'-cyclic nucleotide 3'-phosphodiesterase were also significantly decreased in the mouse model of SZ. Furthermore, we observed degenerative changes of myelin sheaths in the mouse model of SZ. These results provide further evidence of white matter deficits in SZ and indicate that the animal model of SZ induced by MK-801 is a useful model to investigate mechanisms underlying white matter abnormalities in SZ.

Elevated prostacyclin biosynthesis in mice impacts memory and anxiety-like behavior

Prostacyclin is an endogenous lipid metabolite with properties of vasodilation and anti-platelet aggregation. While the effects of prostacyclin on the vascular protection have been well-documented, the role of this eicosanoid in the central nervous system has not been extensively studied. Recently, a transgenic mouse containing a hybrid enzyme, of cyclooxygenase-1 linked to prostacyclin synthase, was developed that produces elevated levels of prostacyclin in vivo. The goal of this study was to investigate whether increased prostacyclin biosynthesis could affect behavioral phenotypes in mice. Our results uncovered that elevated levels of prostacyclin broadly affect both cognitive and non-cognitive behaviors, including decreased anxiety-like behavior and improved learning in the fear-conditioning memory test. This study demonstrates that prostacyclin plays an important, but previously unrecognized, role in central nervous system function and behavior.

Chronic treatment with DCPCX, an adenosine A(1) antagonist, worsens long-term memory

Alzheimer's disease is characterized by progressive cognitive disturbances and neurotransmitter dysfunction. Previous studies targeting the adrenergic A1 pathway suggest that this plays a role in cognitive impairment in Alzheimer's disease. Previous studies have reported that acute treatment with A1 antagonists appears to improve behavioral deficits in rodent models of memory and behavioral impairment. In this study, we addressed whether the chronic administration of 8-cyclopentyl-1,3-dipropylxanthine, a potent and selective adenosine A1 antagonist, could reverse the memory deficits found in aged APPswe/PS1dE9 mice. Chronic treatment did not improve memory in the APPswe/PS1dE9 mouse model and resulted in reduced exploratory behavior, suggestive of reduced anxiety, and a worsening of long-term memory in nontransgenic mice. These results have important implications for understanding the mechanisms of A1 receptor modulation as a target in Alzheimer's disease therapy.

NMDA receptors in GABAergic synapses during postnatal development

GABA (gamma-aminobutyric-acid), the main inhibitory neurotransmitter in the adult brain, exerts depolarizing (excitatory) actions during development and this GABAergic depolarization cooperates with NMDARs (N-methyl-D-aspartate receptors) to drive spontaneous synchronous activity (SSA) that is fundamentally important for developing neuronal networks. Although GABAergic depolarization is known to assist in the activation of NMDARs during development, the subcellular localization of NMDARs relative to GABAergic synapses is still unknown. Here, we investigated the subcellular distribution of NMDARs in association with GABAergic synapses at the developmental stage when SSA is most prominent in mice. Using multiple immunofluorescent labeling and confocal laser-scanning microscopy in the developing mouse hippocampus, we found that NMDARs were associated with both glutamatergic and GABAergic synapses at postnatal day 6–7 and we observed a direct colocalization of GABA_A- and NMDA-receptor labeling in GABAergic synapses. Electron microscopy of pre-embedding immunogold-immunoperoxidase reactions confirmed that GluN1, GluN2A and GluN2B NMDAR subunits were all expressed in glutamatergic and GABAergic synapses postsynaptically. Finally, quantitative post-embedding immunogold labeling revealed that the density of NMDARs was 3 times higher in glutamatergic than in GABAergic synapses. Since GABAergic synapses were larger, there was little difference in the total number of NMDA receptors in the two types of synapses. In addition, receptor density in synapses was substantially higher than extrasynaptically. These data can provide the neuroanatomical basis of a new interpretation of previous physiological data regarding the GABA_AR-NMDAR cooperation during early development. We suggest that during SSA, synaptic GABA_AR-mediated depolarization assists NMDAR activation right inside GABAergic synapses and this effective spatial cooperation of receptors and local change of membrane potential will reach developing glutamatergic synapses with a higher probability and efficiency even further away on the dendrites. This additional level of cooperation that operates within the depolarizing GABAergic synapse, may also allow its own modification triggered by Ca²⁺-influx through the NMDA receptors.

Pharmacological inactivation of the small GTPase Rac1 impairs long-term plasticity in the mouse hippocampus

Neuronal development involves several discrete morphological steps requiring migration of newborn neurons to characteristic locations, extension of axons and dendrites into proper target regions, and formation of synapses with appropriate partners. Small GTPases such as Rac1, are believed to be critical regulators of these processes. We have previously reported that Rac1 is highly expressed in mouse hippocampus, where NMDA receptor activation causes Rac1 to translocate to the membrane in a manner similar to that observed in other non-neuronal cells. Additionally Rac1 has been seen to play a role in activation of signal transduction pathways associated with hippocampal learning and memory. Because of the established role of LTP and LTD in learning and memory processes, in this study we investigate whether Rac1 plays also an active and critical role in these types of long-term synaptic plasticity. We found that activation of Rac1 is associated with long-term plasticity, both LTP and LTD. Rac1 appears to have a transient role during the induction of NMDA receptor-dependent LTP, but does not have an effect on LTP maintenance and expression. Similar results were found for NMDA receptor-dependent induction of LTD, while mGluR-dependent LTD was shown to be significantly altered but not abolished. The results of these experiments provide essential knowledge regarding the signaling mechanisms that underlie synaptic plasticity, as well as learning and memory processes, which in turn offers insights into the basis of diseases involving memory impairment, such as Fragile X syndrome, Alzheimer's disease, William's syndrome, Angelman syndrome (AS), and schizophrenia.