Muscimol prevents NMDA antagonist neurotoxicity by activating GABAA receptors in several brain regions

NMDA Antagonists for Treatment-Resistant Depression

Fifteen to thirty percent of patients with major depressive disorder do not respond to antidepressants that target the monoaminergic systems. NMDA antagonists are currently being actively investigated as a treatment for these patients. Ketamine is the most widely studied of the compounds. A brief infusion of a low dose of this agent produces rapid improvement in depressive symptoms that lasts for several days. The improvement occurs after the agent has produced its well characterized psychotomimetic and cognitive side effects. Multiple infusions of the agent (e.g., 2-3× per week for several weeks) provide relief from depressive symptoms, but the symptoms reoccur once the treatment has been stopped. A 96-h infusion of a higher dose using add-on clonidine to mitigate the psychotomimetic effects appears to also provide relief and resulted in about 40% of the subjects still having a good response 8 weeks after the infusion. As this was a pilot study, additional work is needed to confirm and extend this finding. Nitrous oxide also has had positive results. Of the other investigational agents, CERC-301 and rapastinel remain in clinical development. When careful monitoring of neuropsychiatric symptoms has been conducted, these agents all produce similar side effects in the same dose range, indicating that NMDA receptor blockade produces both the wanted and unwanted effects. Research is still needed to determine the appropriate dose, schedule, and ways to mitigate against unwanted side effects of NMDA receptor blockade. These hurdles need to be overcome before ketamine and similar agents can be prescribed routinely to patients.

Catecholamines and cognition after traumatic brain injury

Cognitive problems are one of the main causes of ongoing disability after traumatic brain injury. The heterogeneity of the injuries sustained and the variability of the resulting cognitive deficits makes treating these problems difficult. Identifying the underlying pathology allows a targeted treatment approach aimed at cognitive enhancement. For example, damage to neuromodulatory neurotransmitter systems is common after traumatic brain injury and is an important cause of cognitive impairment. Here, we discuss the evidence implicating disruption of the catecholamines (dopamine and noradrenaline) and review the efficacy of catecholaminergic drugs in treating post-traumatic brain injury cognitive impairments. The response to these therapies is often variable, a likely consequence of the heterogeneous patterns of injury as well as a non-linear relationship between catecholamine levels and cognitive functions. This individual variability means that measuring the structure and function of a person’s catecholaminergic systems is likely to allow more refined therapy. Advanced structural and molecular imaging techniques offer the potential to identify disruption to the catecholaminergic systems and to provide a direct measure of catecholamine levels. In addition, measures of structural and functional connectivity can be used to identify common patterns of injury and to measure the functioning of brain ‘networks’ that are important for normal cognitive functioning. As the catecholamine systems modulate these cognitive networks, these measures could potentially be used to stratify treatment selection and monitor response to treatment in a more sophisticated manner.

[18F]FDG-PET as an imaging biomarker to NMDA receptor antagonist-induced neurotoxicity

Positron emission tomography (PET) is an effective tool for noninvasive examination of the body and provides a range of functional information. PET imaging with [(18)F]fluoro-2-deoxy-d-glucose ([(18)F]FDG) has been used to image alterations in glucose metabolism in brain or cancer tissue in the field of clinical diagnosis but not in the field of toxicology. A single dose of N-methyl-d-aspartate (NMDA) receptor antagonist induces neuronal cell degeneration/death in the rat retrosplenial/posterior cingulate (RS/PC) cortex region. These antagonists also increase local cerebral glucose utilization. Here, we examined the potential of [(18)F]FDG-PET as an imaging biomarker of neurotoxicity induced by an NMDA receptor antagonist, MK-801. Using [(18)F]FDG-PET, we determined that increased glucose utilization involved the neurotoxicity induced by MK-801. The accumulation of [(18)F]FDG was increased in the rat RS/PC cortex region showing neuronal cell degeneration/death and detected before the onset of neuronal cell death. This effect increased at a dose level at which neuronal cell degeneration recovered 24h after MK-801 administration. Scopolamine prevented the neurotoxicity and [(18)F]FDG accumulation induced by MK-801. Furthermore, in cynomolgus monkeys that showed no neuronal cell degeneration/death when treated with MK-801, we noted no differences in [(18)F]FDG accumulation between test and control subjects in any region of the brain. These findings suggest that [(18)F]FDG-PET, which is available for clinical trials, may be useful in generating a predictive imaging biomarker for detecting neurotoxicity against NMDA receptor antagonists with the same pharmacological activity as MK-801.

Effect of MK-801-induced impairment of inhibitory avoidance learning in zebrafish via inactivation of extracellular signal-regulated kinase (ERK) in telencephalon

N-Methyl-D-aspartate (NMDA) receptors are implicated in a wide range of complex behavioral functions, including cognitive activity. Numerous studies have shown that using the repetitive administration of a noncompetitive NMDA receptor antagonist, MK-801, induces amnesia in rodents. In this study, the effect of a subchronic MK-801 treatment on the cognitive function of zebrafish was evaluated using a novel inhibitory avoidance task. First, we established a new system to investigate the inhibitory avoidance learning of zebrafish where they were trained to refrain from swimming from a shallow compartment to a deep compartment in order to avoid electric shock. Second, we found that blocking NMDA receptors by MK-801 could significantly attenuate the inhibitory avoidance behavior of the zebrafish and alter the telencephalic extracellular signal-regulated kinase (ERK) phosphorylation level 90 min after the inhibitory avoidance training. These results suggest that the formation of long-term emotional memory is possibly mediated by ERK activation in the telencephalon of zebrafish.

CPB-K mice a mouse model of schizophrenia? Differences in dopaminergic, serotonergic and behavioral markers compared to BALB/cJ mice

Schizophrenia is characterized by disturbances in social behavior, sensorimotor gating and cognitive function, that are discussed to be caused by a termination of different transmitter systems. Beside morphological alterations in cortical and subcortical areas reduced AMPA- NMDA-, 5-HT2-receptor densities and increased 5-HT1-receptor densities are found in the hippocampus.The two inbred mouse strains CPB-K and BALB/cJ are known to display considerable differences in cognitive function and prepulse inhibition, a stable marker of sensorimotor gating. Furthermore, CPB-K mice exhibit lower NMDA-, AMPA- and increased 5-HT-receptor densities in the hippocampus as compared to BALB/cJ mice. We investigated both mouse strains in social interaction test for differences in social behavior and with immuncytochemical approaches for alterations of dopaminergic and serotonergic parameters. Our results can be summarized as follows: compared to BALB/cJ, CPB-K mice showed:(1) significantly reduced traveling distance and number of contacts in social interaction test, (2) differences in the number of serotonin transporter-immunoreactive neurons and volume of raphe nuclei and a lower serotonergic fiber density in the ventral and dorsal hippocampal subfields CA1 and CA3, (3) no alterations of dopaminergic markers like neuron number, neuron density and volume in subregions of substantia nigra and ventral tegmental area, but a significantly higher dopaminergic fiber density in the dorsal hippocampus, the ventral hippocampus of CA1 and gyrus dentatus, (4) no significant differences in serotonergic and dopaminergic fiber densities in the amygdala.Based on our results and previous studies, CPB-K mice compared to BALB/cJ may serve as an important model to understand the interaction of the serotonergic and dopaminergic system and their impact on sensorimotor gating and cognitive function as related to neuropsychiatric disorders like schizophrenia.

Role of the anterior thalamic nucleus in the motor hyperactivity induced by systemic MK-801 administration in rats

Non-competitive N-methyl-D-aspartate receptor (NMDA-R) antagonists have been extensively used in rodents to model psychotic symptoms of schizophrenia. Although the motor syndrome induced by acute and systemic administration of low doses of dizocilpine (MK-801) has been extensively characterized, its neurobiological basis is not fully understood. NMDA-R antagonists can disinhibit excitatory inputs in certain brain areas, but the precise circuitry is not fully known. We examined the involvement of the anterior thalamic nucleus (ATN) in hyperlocomotion and other related behaviors (stereotypies, ataxia signs) induced after acute systemic administration of MK-801. Since GABAergic neurons of the reticular thalamic nucleus (RTN) exert the main inhibitory control on thalamic projection neurons, we hypothesized that systemically injected MK-801 might block NMDA-R on RTN GABAergic neurons. This effect would subsequently result in disinhibition of GABAergic inputs onto ATN projections to cortical motor areas, thereby inducing behavioral effects. We evaluated the behavioral syndrome induced by the systemic administration MK-801 (0.2 mg/kg) in control rats and in rats subjected to a bilateral stereotaxic infusion of the GABA(A) agonist muscimol (0.2 μl of 2.5 and 5.0 mM; 0.5-1 nmol per application, respectively) into the ATN. As previously reported, MK-801-induced hyperlocomotion in parallel with disorganized movements (e.g. not guided by normal exploration) slight ataxia signs and stereotypies. All responses were antagonized by pre-infusion of muscimol but not saline into the ATN. According to our results we suggest that the ATN plays a role on hyperlocomotion evoked by MK-801 and could involve a thalamic GABAergic disinhibition mechanism.

A thalamo-hippocampal-ventral tegmental area loop may produce the positive feedback that underlies the psychotic break in schizophrenia

The N-methyl-D-aspartate receptor (NMDAR) hypofunction model of schizophrenia is based on the ability of NMDAR antagonists to produce many symptoms of the disease. Recent work in rats shows that NMDAR antagonist works synergistically with dopamine to produce delta frequency bursting in the thalamus. This finding, together with other results in the literature, suggests a mechanism for the sudden onset of schizophrenia. Among the thalamic nuclei most activated by NMDAR antagonist is the nucleus reuniens. This nucleus excites the cornu ammonis area 1 (CA1) region of the hippocampus. Experiments indicate that such activation can lead to excitation of dopaminergic cells of the ventral tegmental area by a polysynaptic pathway. The resulting elevation of dopamine in the thalamus will enhance thalamic bursting, thereby creating a loop with the potential for positive feedback. We show through computer simulations that in individuals with susceptibility to schizophrenia (e.g., because of partially compromised NMDAR function), an event that stimulates the dopamine system, such as stress, can cause the system to reach the threshold for thalamic bursting. When this occurs, positive feedback in the loop will cause all components to become highly active and to remain active after the triggering stimulus is removed. This is a physiologically specific hypothesis for the sudden and lasting transition that underlies the psychotic break in schizophrenia. Furthermore, the model provides an explanation for the observed selective activation of the CA1 hippocampal region in schizophrenia. The model also predicts an increase of basal activity in the dopamine system and thalamus; the relevant evidence is reviewed.

Neuropeptide S attenuates neuropathological, neurochemical and behavioral changes induced by the NMDA receptor antagonist MK-801

Neuropeptide S (NPS) and its cognate receptor were reported to mediate anxiolytic-like and arousal effects. NPS receptors are predominantly expressed in the brain, especially in limbic structures, including amygdala, olfactory nucleus, subiculum and retrosplenial cortex. In contrast, the NPS precursor is expressed in only a few brainstem nuclei where it is co-expressed with various excitatory transmitters, including glutamate. The current study investigates interactions of the NPS system with glutamatergic neurotransmission. It has been suggested that dysfunctions in glutamatergic neurotransmission via N-methyl-D-aspartate (NMDA) receptors might be involved in the pathophysiology of schizophrenia since NMDA receptor antagonists, such as MK-801, have been shown to induce psychotic-like behavior in humans and animal models. Also, MK-801 is known to produce histological changes such as cytoplasmic vacuoles in retrosplenial cortex neurons where NPS receptors are highly expressed. In this study we show that NPS is able to alleviate neuropathological, neurochemical and behavioral changes produced by NMDA receptor antagonists. NPS treatment attenuated MK-801-induced vacuolization in the rat retrosplenial cortex in a dose-dependent manner that can be blocked by an NPS receptor-selective antagonist. NPS also suppressed MK-801-induced increases of extracellular acetylcholine levels in the retrosplenial cortex. In the prepulse inhibition (PPI) assay, animals pretreated with NPS recovered significantly from MK-801-induced disruption of PPI. Our study suggests that NPS may have protective effects against the neurotoxic and behavioral changes produced by NMDA receptor antagonists and that NPS receptor agonists may elicit antipsychotic effects.

Antioxidants attenuate MK-801-induced cortical neurotoxicity in the rat

Oxidative stress has been implicated in the pathogenesis of several neurodegenerative diseases and may result from excessive free radical production due to increased local metabolism. Non-competitive N-methyl-D-aspartate (NMDA) antagonists (MK-801 and phencyclidine) increase glucose metabolism in many brain areas and induce cytoplasmic vacuoles, heat shock protein and necrotic cell death in neurones of the rodent posterior cingulate and retrosplenial cortex. We have investigated the effect of several antioxidants with differing properties on MK-801-induced neuronal loss. Free radical scavengers (dimethyl sulfoxide (DMSO) and alpha-tocopherol) and spin traps (N-tert-butyl-alpha-(2-sulfophenyl)-nitrone (S-PBN) and 5-(diethoxyphosphoryl)-5-methyl-1-pyrrole N-oxide (DEPMPO)), produced marked attenuation of MK-801-induced neuronal necrosis in the rat posterior cingulate and retrosplenial cortex. Further, administration of DMSO could be delayed by up to 4 h after MK-801 dosing and still achieve between 80 and 86% reduction in neuronal loss. We also show that MK-801 administration rapidly induced a four-fold and prolonged increase in cerebral blood flow in the posterior cingulate. This elevated regional blood flow was only transiently reduced by DMSO administration. The anterior cingulate, a region which undergoes no neuronal loss, showed only a two-fold increase in regional blood flow following MK-801 administration. These results support a hypothesis that oxidative stress plays a role in MK-801-induced neuronal necrosis since pathological changes can be attenuated by several antioxidants.

Possible Mechanisms of Neurodegeneration in Schizophrenia

Brain morphological alterations in schizophrenic patients have led to the neurodevelopmental hypothesis of schizophrenia. On the other hand, a progressive neurodegenerative process has also been suggested and some follow-up studies have shown progressive morphological changes in schizophrenic patients. Several neurotransmitter systems have been suggested to be involved in this disorder and some of them could lead to neuronal death under certain conditions. This review discusses some of the biochemical pathways that could lead to neurodegeneration in schizophrenia showing that neuronal death may have a role in the etiology or natural course of this disorder.

Acute D2/D3 dopaminergic agonism but chronic D2/D3 antagonism prevents NMDA antagonist neurotoxicity

BACKGROUND

Antagonists of the N-methyl-D-aspartate (NMDA) glutamate receptor, most likely by producing disinhibtion in complex circuits, acutely produce psychosis and cognitive disturbances in humans, and neurotoxicity in rodents. Studies examining NMDA Receptor Hypofunction (NRHypo) neurotoxicity in animals, therefore, may provide insights into the pathophysiology of psychotic disorders. Dopaminergic D2 and/or D3 agents can modify psychosis over days to weeks, suggesting involvement of these transmitter system(s).

METHODS

We studied the ability of D2/D3 agonists and antagonists to modify NRHypo neurotoxicity both after a one-time acute exposure and after chronic daily exposure.

RESULTS

Here we report that D2/D3 dopamine agonists, probably via D3 receptors, prevent NRHypo neurotoxicity when given acutely. The protective effect with D2/D3 agonists is not seen after chronic daily dosing. In contrast, the antipsychotic haloperidol does not affect NRHypo neurotoxicity when given acutely at D2/D3 doses. However, after chronic daily dosing of 1, 3, or 5 weeks, haloperidol does prevent NRHypo neurotoxicity with longer durations producing greater protection.

CONCLUSIONS

Understanding the changes that occur in the NRHypo circuit after chronic exposure to dopaminergic agents could provide important clues into the pathophysiology of psychotic disorders.

Atypical antipsychotics and a Src kinase inhibitor (PP1) prevent cortical injury produced by the psychomimetic, noncompetitive NMDA receptor antagonist MK-801

Noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine, ketamine, and MK-801 produce schizophrenia-like psychosis in humans. The same NMDA antagonists injure retrosplenial cortical neurons in adult rats. We examined the effects of atypical antipsychotics and an inhibitor of nonreceptor tyrosine kinase pp60 (Src) on the cortical injury produced by MK-801. An atypical antipsychotic (either clozapine, ziprasidone, olanzapine, quetiapine, or risperidone) or vehicle was administered to adult female Sprague-Dawley rats. PP1 (Src inhibitor), PP3 (nonfunctional analog of PP1) or vehicle (DMSO) was administered to another group of animals. After pretreatment, animals were injected with MK-801, killed 24 h after the MK-801, and injury to retrosplenial cortex assessed by neuronal Hsp70 protein expression. All atypical antipsychotics examined significantly attenuated MK-801-induced cortical damage. PP1 protected compared to vehicle, whereas PP3 did not protect. The ED50s (decrease injury by 50%) were as follows: PP1 <0.1 mg/kg; olanzapine 0.8 mg/kg; risperdal 1 mg/kg; clozapine 3 mg/kg; ziprasidone 32 mg/kg; and quetiapine 45 mg/kg. The data show that the atypical antipsychotics tested as well as a Src kinase inhibitor prevent the injury produced by the psychomimetic MK-801, and the potency of the atypical antipsychotics for preventing cortical injury was roughly similar to the potency of these drugs for treating psychosis in patients.

Age has a similar influence on the susceptibility to NMDA antagonist-induced neurodegeneration in most brain regions

NMDA antagonists are of potential therapeutic benefit for several conditions. However, their ability to produce neurotoxicity and psychosis has hampered their clinical use. A better understanding of these side effects and the mechanism underlying them could result in their safer use and in improving our understanding of psychotic illnesses. By disinhibiting certain multisynaptic circuits, moderate doses of NMDA antagonists produce reversible neurotoxicity in the retrosplenial cortex in rats older than 1 month. Higher doses of these same agents result in the death of neurons in the retrosplenial cortex and several other brain regions. It is unknown whether susceptibility to this irreversible neurodegeneration has a similar age dependency profile. We, therefore, examined the sensitivity of rats of various ages (PND20-60) to the irreversible neurodegenerative effect of the selective NMDA antagonist, MK-801. Quantification of the severity of neurodegeneration with stereology revealed that the retrosplenial cortex, induseum griseum, and dentate gyrus had decreasing amounts of damage with decreasing age and onset of sensitivity around PND30. The piriform cortex also displayed a decreased amount of degeneration in younger age groups. However, a low level of degeneration continued to occur in the posterior piriform cortex in the PND20-25 animals. The stage of degeneration appeared to be more advanced, suggesting that these neurons were dying by a different mechanism. We conclude that for most neuronal populations, susceptibility to the irreversible and reversible neurodegenerative effects of NMDA antagonists has a similar age dependency profile, consistent with the proposal that the same disinhibitory mechanism underlies both neurotoxicities.

In the adult CNS, ethanol prevents rather than produces NMDA antagonist-induced neurotoxicity

Single doses of an NMDA antagonist cause an adult or a prepubertal form of neurodegeneration, depending on the age of the animal. Single doses of ethanol (EtOH) by blocking NMDA receptors produce apoptotic neurodegeneration in young animals. This capability could account, in part, for the ability of EtOH to produce the fetal alcohol syndrome. We investigated whether EtOH could produce NMDA antagonist-induced neurotoxicity (NAN), a different neurotoxicity that is seen only in adult animals. In spite of producing blood EtOH levels (30 to 600 mg/dl) known to block NMDA receptors, EtOH was unable to produce neurotoxicity in the adult central nervous system (CNS). Moreover, EtOH in a dose-dependent fashion (ED(50)=138 mg/dl) prevented the selective and powerful NMDA antagonist, MK-801, from producing NAN in adult animals, suggesting that activity at another site might be negating the neurotoxic effect of EtOH's inherent NMDA antagonistic activity. Because GABA(A) agonism and non-NMDA glutamate antagonism, properties which EtOH possesses, can prevent NAN, we proceeded to study whether GABA(A) antagonists (or agents capable of reversing EtOH's GABAergic effects) and non-NMDA agonists could reverse EtOH's protective effect. Bicuculline, Ro15-4513, finasteride, kainic acid or AMPA, alone or in combination, did not significantly reverse EtOH's protective effect. Given that EtOH has effects on a wide range of ion channels and receptors, determining the precise mechanism of EtOH's protective effect will take additional effort. The inability of EtOH to acutely produce NAN in the adult CNS indicates that, in contrast to fetuses, brief exposure of the adult CNS to EtOH is non-toxic for neurons.