Multifocal brain damage induced by phencyclidine is augmented by pilocarpine

Estrogens and the cognitive symptoms of schizophrenia: Possible neuroprotective mechanisms

Schizophrenia is a complex neuropsychiatric illness with marked sex differences. Women have later onset and lesser symptoms, which has led to the hypothesis that estrogens are protective in schizophrenia. Cognitive dysfunction is a hallmark of the disease and the symptom most correlated with functional outcome. Here we describe a number of mechanisms by which estrogens may be therapeutic in schizophrenia, with a focus on cognitive symptoms. We review the relationship between estrogens and brain derived neurotrophic factor, neuroinflammation, NMDA receptors, GABA receptors, and luteinizing hormone. Exploring these pathways may enable novel treatments for schizophrenia and a greater understanding of this devastating disease.

Impaired limbic cortico-striatal structure and sustained visual attention in a rodent model of schizophrenia

BACKGROUND

N-methyl-d-aspartate receptor (NMDAR) dysfunction is thought to contribute to the pathophysiology of schizophrenia. Accordingly, NMDAR antagonists such as phencyclidine (PCP) are used widely in experimental animals to model cognitive impairment associated with this disorder. However, it is unclear whether PCP disrupts the structural integrity of brain areas relevant to the profile of cognitive impairment in schizophrenia.

METHODS

Here we used high-resolution magnetic resonance imaging and voxel-based morphometry to investigate structural alterations associated with sub-chronic PCP treatment in rats.

RESULTS

Sub-chronic exposure of rats to PCP (5mg/kg twice daily for 7 days) impaired sustained visual attention on a 5-choice serial reaction time task, notably when the attentional load was increased. In contrast, sub-chronic PCP had no significant effect on the attentional filtering of a pre-pulse auditory stimulus in an acoustic startle paradigm. Voxel-based morphometry revealed significantly reduced grey matter density bilaterally in the hippocampus, anterior cingulate cortex, ventral striatum, and amygdala. PCP-treated rats also exhibited reduced cortical thickness in the insular cortex.

CONCLUSIONS

These findings demonstrate that sub-chronic NMDA receptor antagonism is sufficient to produce highly-localized morphological abnormalities in brain areas implicated in the pathogenesis of schizophrenia. Furthermore, PCP exposure resulted in dissociable impairments in attentional function.

[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.

NMDA receptor function, memory, and brain aging

An increasing level of N-methyl-D-aspartate (NMDA) receptor hypofunction within the brain is associated with memory and learning impairments, with psychosis, and ultimately with excitotoxic brain injury. As the brain ages, the NMDA receptor system becomes progressively hypofunctional, contributing to decreases in memory and learning performance. In those individuals destined to develop Alzheimer's disease, other abnormalities (eg, amyloidopathy and oxidative stress) interact to increase the NMDA receptor hypofunction (NRHypo) burden. In these vulnerable individuals, the brain then enters into a severe and persistent NRHypo state, which can lead to widespread neurodegeneration with accompanying mental symptoms and further cognitive deterioration. If the hypotheses described herein prove correct, treatment implications may be considerable. Pharmacological methods for preventing the overstimulation of vulnerable corticolimbic pyramidal neurons developed in an animal model may be applicable to the prevention and treatment of Alzheimer's disease.

Mouse strain differences in phencyclidine-induced behavioural changes

Administration of phencyclidine (PCP) is acknowledged to generate a model of psychosis in animals. With the identification of genetic susceptibility factors for schizophrenia and bipolar disorder, great efforts have been made to generate genetic animal models for major mental illnesses. As these disorders are multifactorial, comparisons among drug-induced (non-genetic) and genetic models are becoming an important issue in biological psychiatry. A major barrier is that the standard mouse strain used in the generation of genetic models is C57BL/6, whereas almost all studies with PCP-induced models have utilized other strains. To fill this technical gap, we systematically compared the behavioural changes upon PCP administration in different mouse strains, including C57BL/6N, C57BL/6J, ddY, and ICR. We observed strain differences in PCP-induced hyperlocomotion and enhanced immobility in the forced swim test (ddY>>C57BL/6N and 6J>ICR). In contrast, there was no strain difference in the impairment of recognition memory in the novel object recognition memory test after withdrawal of chronic PCP administration. This study provides practical guidance for comparing genetic with PCP-induced models of psychosis in C57BL/6. Furthermore, such strain differences may provide a clue to the biological mechanisms underlying PCP-induced endophenotypes possibly relevant to major mental illnesses.

Low-level lead exposure triggers neuronal apoptosis in the developing mouse brain

While the toxic effects of lead have been recognized for millennia, it has remained a significant public health concern due to its continued use and toxicological potential. Of particular interest is the increased susceptibility of young children to the toxic effects of lead. Although the exact mechanism(s) for lead toxicity is currently not well understood, research has established that it can be a potent NMDA antagonist. Previous research has established that exposure to NMDA antagonists during the brain growth spurt period (first 2 weeks of life in mice) can produce apoptotic neurodegeneration throughout the brain. Based on this information, the ability of lead exposure (two injections of 350 mg/kg lead 4h apart) to produce apoptosis in the neonatal mouse brain was assessed histologically 8-24h after treatment using activated caspase-3 immunohistochemistry, De Olmos silver technique, Nissl staining, and electron microscopy. Lead exposure produced significant neurodegeneration in the caudate/putamen, hippocampus, subiculum, and superficial and deep cortical layers of the frontal cortical regions. Further ultrastructural examination revealed cellular profiles consistent with apoptotic cell death. Statistical results showed that lead exposure significantly increased apoptotic neurodegeneration above that seen in normal controls in animals treated at postnatal day 7, but not on day 14. The results of this study may provide a basis for further elucidation of mechanisms through which the immature nervous system may be particularly susceptible to lead exposure.

Isolation rearing-induced deficits in sensorimotor gating and social interaction in rats are related to cortico-striatal oxidative stress, and reversed by sub-chronic clozapine administration

Social isolation rearing (SIR) in rats induces behavioral and glutamatergic changes akin to schizophrenia. We studied the effects of 8 weeks SIR on cortico-striatal redox and social and cognitive behaviors in rats. SIR increased superoxide dismutase activity, decreased oxidized:reduced glutathione ratio and increased lipid peroxidation in both brain regions, and induced deficits in prepulse inhibition and social and self-directed interactive behaviors. Both behavioral and cortico-striatal redox disturbances were corrected by clozapine (5 mg/kg/day×11days). Behavioral changes evoked by SIR are associated with cortico-striatal oxidative stress that is reversed by clozapine treatment, providing novel insight into the neurobiology and treatment of schizophrenia.

Phencyclidine (PCP)-induced disruption in cognitive performance is gender-specific and associated with a reduction in brain-derived neurotrophic factor (BDNF) in specific regions of the female rat brain

Phencyclidine (PCP), used to mimic certain aspects of schizophrenia, induces sexually dimorphic, cognitive deficits in rats. In this study, the effects of sub-chronic PCP on expression of brain-derived neurotrophic factor (BDNF), a neurotrophic factor implicated in the pathogenesis of schizophrenia, have been evaluated in male and female rats. Male and female hooded-Lister rats received vehicle or PCP (n=8 per group; 2 mg/kg i.p. twice daily for 7 days) and were tested in the attentional set shifting task prior to being sacrificed (6 weeks post-treatment). Levels of BDNF mRNA were measured in specific brain regions using in situ hybridisation. Male rats were less sensitive to PCP-induced deficits in the extra-dimensional shift stage of the attentional set shifting task compared to female rats. Quantitative analysis of brain regions demonstrated reduced BDNF levels in the medial prefrontal cortex (p<0.05), motor cortex (p<0.01), orbital cortex (p<0.01), olfactory bulb (p<0.05), retrosplenial cortex (p<0.001), frontal cortex (p<0.01), parietal cortex (p<0.01), CA1 (p<0.05) and polymorphic layer of dentate gyrus (p<0.05) of the hippocampus and the central (p<0.01), lateral (p<0.05) and basolateral (p<0.05) regions of the amygdaloid nucleus in female PCP-treated rats compared with controls. In contrast, BDNF was significantly reduced only in the orbital cortex and central amygdaloid region of male rats (p<0.05). Results suggest that blockade of NMDA receptors by sub-chronic PCP administration has a long-lasting down-regulatory effect on BDNF mRNA expression in the female rat brain which may underlie some of the behavioural deficits observed post PCP administration.

Effects of postnatal dietary choline manipulation against MK-801 neurotoxicity in pre- and postadolescent rats

Prenatal supplementation of rat dams with dietary choline has been shown to provide their offspring with neuroprotection against N-methyl-d-aspartate (NMDA) antagonist-mediated neurotoxicity. This study investigated whether postnatal dietary choline supplementation exposure for 30 and 60 days of rats starting in a pre-puberty age would also induce neuroprotection (without prenatal exposure). Male and female Sprague-Dawley rats (postnatal day 30 of age) were reared for 30 or 60 concurrent days on one of the four dietary levels of choline: 1) fully deficient choline, 2) 1/3 the normal level, 3) the normal level, or 4) seven times the normal level. After diet treatment, the rats received one injection of MK-801 (dizocilpine 3mg/kg) or saline control. Seventy-two hours later, the rats were anesthetized and transcardially perfused. Their brains were then postfixed for histology with Fluorojade-C (FJ-C) staining. Serial coronal sections were prepared from a rostrocaudal direction from 1.80 to 4.2mm posterior to the bregma to examine cell degeneration in the retrosplenial and piriform regions. MK-801, but not control saline, produced significant numbers of FJ-C positive neurons, indicating considerable neuronal degeneration. Dietary choline supplementation or deprivation in young animals reared for 30-60days did not alter NMDA antagonist-induced neurodegeneration in the retrosplenial region. An interesting finding is the absence of the piriform cortex involvement in young male rats and the complete absence of neurotoxicity in both hippocampus regions and DG. However, neurotoxicity in the piriform cortex of immature females treated for 60days appeared to be suppressed by low levels of dietary choline.

α7 nicotinic receptor agonism mitigates phencyclidine-induced changes in synaptophysin and Arc gene expression in the mouse prefrontal cortex

Repeated phencyclidine (PCP) administration in mice reproduces several histopathological features of schizophrenia, such as reduced synaptophysin and parvalbumin mRNA expression in the frontal cortex. These changes can be prevented by co-administering the α7 nicotinic acetylcholine receptor (nAChR) agonist SSR180711 with PCP, but it is not known to what extent PCP-induced changes can be normalized once they have already occurred. Here we use semi-quantitative in situ hybridization to show that repeated administration of SSR180711 (3 mg/kg b.i.d. for 5 days) subsequent to repeated PCP administration (10 mg/kg/day for 10 days) is able to mitigate the reduction of synaptophysin mRNA expression induced by PCP in two prefrontal cortical regions, the medial prefrontal cortex (mPFC) and the ventrolateral orbitofrontal cortex (VLO). This effect is accompanied by a normalization of the PCP-induced increase in Arc mRNA expression in the same regions. In contrast, subsequent administration of SSR180711 does not affect PCP-induced decreases in parvalbumin mRNA in the mPFC, and glutamate decarboxylase 67 mRNA in the mPFC or VLO. These data demonstrate that it is possible to restore some, but not all, of the molecular dysregulations induced by repeated PCP administration with an α7 nAChR agonist. They also suggest that the previously demonstrated cognitive improvement with SSR180711 subsequent to PCP treatment does not require normalization of parvalbumin expression, but may instead be related to a restoration of synaptophysin and/or Arc levels in the frontal cortex. These data lend support to the potential for development of α7 nAChR agonists for the treatment of cognitive deficits in schizophrenia.

Targeted knock-down of neuronal nitric oxide synthase expression in basal forebrain with RNA interference

Nitric oxide (NO) is a gas messenger with diverse physiological roles in the nervous system, from modulation of synaptic plasticity and neurogenesis to the mediation of neuronal death. NO production in the brain is catalyzed by three isoforms of NO synthase (NOS) including neuronal NOS (nNOS), inducible NOS and endothelial NOS. In this report, we demonstrate a method for in vitro and in vivo silencing of nNOS using RNAi strategies. Because of their efficiency in infecting postmitotic cells like neurons, lentiviral vectors were used as nNOS shRNA carriers. Of the siRNA sequences screened, one corresponding to exon 10 of the rat nNOS specifically and efficiently inhibited nNOS expression at the mRNA and protein level. In vitro experiments using rat cortical neurons showed the general efficacy of shRNA vectors in silencing constitutively expressed nNOS. To demonstrate the anatomical specificity of nNOS silencing in vivo, vectors were used to selectively knock-down the endogenous nNOS expression in cortical GABAergic interneurons of rat piriform cortex. Our findings show that the method reported here can achieve stable and highly effective nNOS suppression in an anatomically defined brain region. The ability of our nNOS silencing vectors to effectively and precisely silence nNOS expression shows their value as research tools for further studies of the role of nNOS in specific brain circuits. Furthermore, our findings raise the possibility for future considerations of lentiviral strategies as therapies for diseases of the nervous system involving NO neurotoxic cascades.

Neuroprotective effect of atypical antipsychotics in cognitive and non-cognitive behavioral impairment in animal models

Antipsychotic drugs are divided into two groups: typical and atypical. Recent clinical studies show atypical antipsychotics have advantages over typical antipsychotics in a wide variety of neuropsychiatric conditions, in terms of greater efficacy for positive and negative symptoms, beneficial effects on cognitive functioning, and fewer extra pyramidal side effects in treating schizophrenia. As such, atypical antipsychotics may be effective in the treatment of depressive symptoms associated with psychotic and mood disorders, posttraumatic stress disorder and psychosis in Alzheimer disease. In this paper, we describe the effects and potential neurochemical mechanisms of action of atypical antipsychotics in several animal models showing memory impairments and/or non-cognitive behavioral changes. The data provide new insights into the mechanisms of action of atypical antipsychotics that may broaden their clinical applications.

A novel immunocytochemistry technique to measure hsp70 induction in L929 cells exposed to cadmium chloride

The heat shock response has been suggested as a potential biomarker in toxicology. A vast amount of stimuli have been shown to induce heat shock proteins and new techniques to measure the response are constantly being assessed. In this study we use a novel immunocytochemistry technique to measure heat shock protein 70 (hsp70) induction in L929 cells exposed to cadmium chloride. Hsp70 induction was quantifiably measured using a soluble coloured substrate and qualitatively measured using a coloured substrate that precipitated at the location of hsp70. Using the insoluble coloured substrate hsp70 was identified predominantly within the cytoplasm of control cells. At intermediate cadmium concentrations hsp70 was observed to translocate to the nucleus. At these intermediate concentrations a heterogeneous heat shock response was observed. At lethal concentrations a strong heat shock response was observed with a widespread cellular response. This study demonstrates the potential of this immunocytochemistry technique to measure toxicological effects in cells by identifying the response quantitatively and qualitatively.

Altered Excitatory-Inhibitory Balance in the NMDA-Hypofunction Model of Schizophrenia

Schizophrenia is a common psychiatric disorder of high incidence, affecting approximately 1% of the world population. The essential neurotransmitter pathology of schizophrenia remains poorly defined, despite huge advances over the past half-century in identifying neurochemical and pathological abnormalities in the disease. The dopamine/serotonin hypothesis has originally provided much of the momentum for neurochemical research in schizophrenia. In recent years, the attention has, however, shifted to the glutamate system, the major excitatory neurotransmitter in the CNS and towards a concept of functional imbalance between excitatory and inhibitory transmission at the network level in various brain regions in schizophrenia. The evidence indicating a central role for the NMDA-receptor subtype in the aetiology of schizophrenia has led to the NMDA-hypofunction model of this disease and the use of phencyclidines as a means to induce the NMDA-hypofunction state in animal models. The purpose of this review is to discuss recent findings highlighting the importance of the NMDA-hypofunction model of schizophrenia, both from a clinical perspective, as well as in opening a line of research, which enables electrophysiological studies at the cellular and network level in vitro. In particular, changes in excitation–inhibition (E/I) balance in the NMDA-hypofunction model of the disease and the resulting changes in network behaviours, particularly in gamma frequency oscillatory activity, will be discussed.

Donepezil markedly potentiates memantine neurotoxicity in the adult rat brain

The NMDA antagonist, memantine (Namenda), and the cholinesterase inhibitor, donepezil (Aricept), are currently being used widely, either individually or in combination, for treatment of Alzheimer's disease (AD). NMDA antagonists have both neuroprotective and neurotoxic properties; the latter is augmented by drugs, such as pilocarpine, that increase cholinergic activity. Whether donepezil, by increasing cholinergic activity, might augment memantine's neurotoxic potential has not been investigated. In the present study, we determined that a dose of memantine (20mg/kg, i.p.), considered to be in the therapeutic (neuroprotective) range for rats, causes a mild neurotoxic reaction in the adult rat brain. Co-administration of memantine (20 or 30 mg/kg) with donepezil (2.5-10mg/kg) markedly potentiated this neurotoxic reaction, causing neuronal injury at lower doses of memantine, and causing the toxic reaction to become disseminated and lethal to neurons throughout many brain regions. These findings raise questions about using this drug combination in AD, especially in the absence of evidence that the combination is beneficial, or that either drug arrests or reverses the disease process.

Ketamine exposure in adult mice leads to increased cell death in C3H, DBA2 and FVB inbred mouse strains

BACKGROUND

Drug abuse is common among adolescents and young adults. Although the consequences of intoxication are known, sequelae of drugs emerging on campuses and in clubs nationwide are not. We previously demonstrated that ketamine exposure results in lasting physiological abnormalities in mice. However, the extent to which these deficits reflect neuropathologic changes is not known.

METHODS

The current study examines neuropathologic changes following sub-anesthetic ketamine administration (5mg/kg i.p. x 5) to three inbred mouse strains. Stereologic quantification of silver stained nuclear and linear profiles as well as activated caspase-3 labeling was used to address: (1) whether or not ketamine increases excitotoxic and apoptotic cell death in hippocampal CA3 and (2) whether or not ketamine-induced cell death varies by genetic background.

RESULTS

Ketamine increased cell death in hippocampal CA3 of adult C3H, DBA2 and FVB mice. Neither silver staining nor activated caspase-3 labeling varied by strain, nor was there an interaction between ketamine-induced cell death and strain.

CONCLUSIONS

Ketamine exposure among young adults, even in limited amounts, may lead to irreversible changes in both brain function and structure. Loss of CA3 hippocampal cells may underlie persistent ERP changes previously shown in mice and possibly contribute to lasting cognitive deficits among ketamine abusers.

Glutamate antagonists are neurotoxins for the developing brain

Neurotransmitters and neuromodulators are essential for normal nervous system development. Disturbances in the expression timetable or intensity of neurotransmitter signalling during critical periods of brain development can lead to permanent damage. Neuroactive drugs and environmental toxins interfere with neurotransmitter signalling and may thereby provide one mechanism underlying neurological abnormalities. Glutamate is the main excitatory neurotransmitter in the mammalian central nervous system and mediates neurotransmission across most excitatory synapses. In this article we review the timely expression of the excitatory neurotransmitter glutamate and its receptors during brain development, briefly review glutamate receptor antagonists and present clinical and experimental evidence describing their adverse effects in the developing brain.

Oxidative stress in prefrontal cortex of rat exposed to MK-801 and protective effects of CAPE

MK-801 was shown to be one of the most neurotoxic non-competitive NMDA receptor antagonists. It is known that repeated injection of MK-801 was proposed in an animal model in psychosis. The aims of this study are to investigate the contributing effect of oxidative stress in MK-801-induced experimental psychosis model, and to show that prevention of oxidative stress may improve prognosis. Furthermore, there is evidence that oxygen free radicals play an important role in the pathophysiology of schizophrenia. In this study, Wistar Albino rats were divided into three groups: 1st group: Control, 2nd group: MK-801, 3rd group: MK-801+CAPE (Caffeic acid phenethyl ester) group. MK-801 was given intraperitoneally at the dose of 0.5 mg/kg/day for 5 days. CAPE was given to the treatment group while exposed to MK-801. In control group, saline was given intraperitoneally at the same time. After 7 days, rats were killed by decapitation. Prefrontal cortex (PFC) of rats was removed for biochemical and histological analyses. As a result, malondialdehyde (MDA), protein carbonyl (PC), nitric oxide (NO) levels and superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and xanthine oxidase (XO) and adenosine deaminase (AD) enzyme activities were found to be increased significantly in prefrontal cortex (PFC) of MK-801 group (p<0.0001) compared to control group. In CAPE treated rats, prefrontal tissue MDA, PC, NO levels and, GSH-Px, XO, AD enzyme activities were significantly decreased when compared to MK-801 groups (p<0.0001) whereas catalase (CAT) enzyme activity was not changed. Moreover, in the light of microscopic examination of MK-801 groups, a great number of apoptotic cells were observed. CAPE treatment decreased the apoptotic cell count in PFC. The results of this study showed that MK-801-induced neurotoxicity caused oxidative stress in PFC of rats. This experimental study may also provide some evidences for the new treatment strategies with antioxidants in schizophrenia.

Brevetoxin-induced neural insult in the retrosplenial cortex of mouse brain

Brevetoxins (polyether breve toxins; PbTx) are polyether neurotoxins produced by the marine dinoflagellate Karenia brevis, an organism associated with red tide blooms in the Gulf of Mexico and along the Atlantic coast from Florida to North Carolina. Brevetoxin-3 (PbTx-3) is a major component of the array of brevetoxins found in marine aerosols measured along red tide affected beaches. Humans exposed to aerosolized brevetoxins for short periods of time often suffer a variety of adverse health effects. It was consequently of interest to assess the potential for aerosolized brevetoxin to produce a neurotoxic response. Female BALB/c mice were exposed nose-only for 2 consecutive days to PbTx-3 aerosol, with a 2-h exposure on the first day and a 4-h exposure on the second day. The average PbTx-3 exposure concentrations on days 1 and 2 were 312 +/- 113 mug brevetoxin 3/m3 and 278 +/- 24 mug brevetoxin 3/m3, respectively. The brevetoxin-containing aerosol had a mass median aerodynamic diameter of 0.92 mum with a geometric standard deviation of 1.38. Coronal sections of mouse brains were evaluated for neuronal damage using both silver and Fluoro-Jade B staining to identify degenerating neuronal elements. PbTx-3 inhalation exposure produced neuronal degeneration in the posterior cingulate/retrosplenial cortex of mice as evidenced by silver-positive degenerating neurons in this region. No staining was found in other regions of the PBTx-3-exposed mouse brains or in brains of control, sham-exposed mice. The existence of a neurotoxic insult in PbTx-3-exposed mice was confirmed using Fluoro-Jade B to label degenerating neurons. Fluro-Jade-positive neurons were observed in the retrosplenial cortex of PBTx-3 exposed, but not control, mice. These results suggest that subacute exposure to PbTx-3 for 2 days is sufficient to induce neuronal degeneration in a discrete region of the mouse cerebral cortex.

NMDA inhibitors cause apoptosis of pyramidal neurons in mature piriform cortex: evidence for a nitric oxide-mediated effect involving inhibitory interneurons

Pyramidal relay neurons in limbic cortex are vulnerable to denervation lesions, i.e. pyramidal neurons in layer IIalpha of piriform cortex undergo transsynaptic apoptosis after lesions that interrupt their inputs from the olfactory bulb. We have previously established the role of inhibitory interneurons in elaborating signals that lead to the apoptosis of projection neurons in these lesion models, i.e. the upregulation of neuronal NOS and release of nitric oxide. Thus, we have proposed that cortical interneurons play an essential role in transducing injury to degenerative effects for nearby pyramidal neurons. In the present study, we extend the previous findings to a toxic model of degeneration of pyramidal neurons in the adult paralimbic cortex, i.e. after exposure to the NMDA channel blocker MK801. Our findings indicate that treatment of adult rats with MK801 in doses previously found to cause alterations in pyramidal neurons of the retrosplenial cortex (5mg/kg) results in an active caspase 3 (+), ultrastructurally apoptotic type of cell death involving the same projection neurons of layer IIalpha that are also vulnerable to bulbotomy lesions. Interneurons of layer I are induced by MK801 treatment to higher levels of nNOS expression and the selective nNOS inhibitor BRNI ameliorates pyramidal cell apoptosis caused by MK801. Our results indicate that certain pyramidal neurons in piriform cortex are very sensitive to NMDA blockade as they are to disconnection from modality-specific afferents and that inhibitory interneurons play significant roles in mediating various types of pro-apoptotic insults to cortical projection neurons via nNOS/NO signaling.

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.

Basal forebrain cholinergic lesions reduce heat shock protein 72 response but not pathology induced by the NMDA antagonist MK-801 in the rat cingulate cortex

Non-competitive N-methyl-D-aspartate (NMDA) antagonists, in addition to their neuroprotective potential, possess neurotoxic properties and induce seizures and psychosis. MK-801 induces cytoplasmic vacuoles and heat shock protein in pyramidal neurones in the rodent posterior cingulate and retrosplenial cortex. The mechanism of this neurotoxicity is unclear, involving many neurotransmitter systems. The aim of this study was to investigate the role of cholinergic pathways from the nucleus basalis of Meynert in mediating MK-801-induced neurotoxicity. Cholinergic projections from the nucleus basalis of Meynert were lesioned by focal injection of 192-IgG-saporin (80 ng), which after 7 days reduced the number of cholinergic cell bodies by 70% in the lesioned nucleus compared to the uninjected nucleus. Following a unilateral cholinergic lesion, MK-801 (5 mg/kg s.c.) induced expression of hsp72 mRNA (6 h) and HSP72 protein immunoreactivity (24 h) was reduced by 42 and 60%, respectively in the ipsilateral compared to the contralateral posterior cingulate. Despite this apparent protective effect, the unilateral cholinergic lesion did not affect the degree of neuronal vacuolation (6 h), necrosis (24 h) or the large and prolonged increase in cerebral blood flow which occurred over the first 9h following MK-801 administration. These results demonstrate that cholinergic neurones in the nucleus basalis of Meynert play an important role in the heat shock response to NMDA antagonist-induced neurotoxicity but also reveal an unexpected divergence between the heat shock response and the pathophysiological response. This suggests that other cholinergic pathways or non-cholinergic mechanisms are responsible for the pathological changes induced by MK-801.

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.

The protective effects of omega-3 fatty acids against MK-801-induced neurotoxicity in prefrontal cortex of rat

The aims of this study are to investigate the contribution effect of oxidative stress in MK-801-induced experimental psychosis model, and to show that prevention of oxidative stress may improve prognosis. Because oxidative damage has been suggested in the neuropathophysiology of schizophrenia, the possible protecting agents against lipid peroxidation are potential target for the studies in this field. For this purpose, Wistar Albino rats were divided into three groups: the first group was used as control, MK-801 was given to the rats in the second group and MK-801+omega-3 essential fatty acids (EFA) was given to the third group. MK-801 was given intraperitoneally at the dose of 0.5mg/(kgday) once a day for 5 days in experimental psychosis group. In the second group, 0.8g/(kgday), omega-3 FA (eicosapentaenoic acid, 18%, docosahexaenoic acid, 12%) was given to the rats while exposed MK-801. In control group, saline was given intraperitoneally at the same time. After 7 days, rats were killed by decapitation. Prefrontal brain area was removed for histological and biochemical analyses. As a result, malondialdehyde (MDA), as an indicator of lipid peroxidation, protein carbonyl (PC), as an indicator of protein oxidation, nitric oxide (NO) levels and superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) activities as antioxidant enzymes, and xanthine oxidase (XO) and adenosine deaminase (AD) activities as an indicator of DNA oxidation was found to be increased significantly in prefrontal cortex (PFC) of MK-801 group (P<0.0001) compared to control group. In omega-3 FA treated rats, prefrontal tissue MDA, PC and NO levels as well as SOD, GSH-Px, XO, and AD enzyme activities were significantly decreased when compared to MK-801 groups (P<0.0001) whereas catalase (CAT) enzyme activity was not changed. Moreover, in the light of microscopic examination of MK-801 groups, a great number of apoptotic cells were observed. omega-3 FA supplementation decreased the apoptotic cell count in PFC. The results of this study revealed that oxidative stress and apoptotic changes in PFC may play an important role in the pathogenesis of MK-801-induced neuronal toxicity. This experimental study also provides some evidences for the protective effects of omega-3 FA on MK-801-induced changes in PFC of rats.

Effects of repeated phencyclidine administration on adult hippocampal neurogenesis in the rat

Dysfunctional maturation of neural networks, particularly hippocampus-prefrontal networks, may be of particular interest in determining the pathophysiology of schizophrenia. Phencyclidine (PCP)-induced symptoms in humans appear to offer a more complete model of schizophrenia than do amphetamine-induced symptoms. This study investigated the effects of intermittent i.p. injections of PCP (7.5 mg/kg) on cell proliferation and survival of granule cells in the dentate gyrus of the rat brain using quantitative immunohistochemical techniques for 5-bromo-2'-deoxyuridine (BrdU)-positive cells. After repeated PCP injection for 14 days, mean scores for stereotyped behavior increased with the number of injections, while scores for ataxia and backpedaling as serotonergic behaviors gradually decreased. The number of BrdU-positive cells decreased by 23% in the subgranular zone of the dentate gyrus by 24 h after repeated injections. However, decreased levels of BrdU-positive cells returned to control levels within 1 week. Differentiation of newly formed cells was not influenced. Repeated PCP administration after BrdU injection did not exert any effects on survival of newly generated cells. These findings suggest that transient disturbances of cell proliferation in the dentate gyrus occur under PCP-related behavioral abnormalities. Whether disturbed cell proliferation would thus be closely implicated in the development of behavioral sensitization induced by PCP administration is unclear, but this would possibly result from adaptation to new pharmacological conditions under behavioral sensitization or stressful conditions of PCP-related abnormal behaviors. Further studies are required to elucidate the biological significance of hippocampal neurogenesis in the mechanisms underlying the development of cognitive dysfunctions and the psychosis of schizophrenia.

Drug-induced apoptotic neurodegeneration in the developing brain

Physiological cell death (PCD), a process by which redundant or unsuccessful neurons are deleted by apoptosis (cell suicide) from the developing central nervous system, has been recognized as a natural phenomenon for many years. Whether environmental factors can interact with PCD mechanisms to increase the number of neurons undergoing PCD, thereby converting this natural phenomenon into a pathological process, is an interesting question for which new answers are just now becoming available. In a series of recent studies we have shown that 2 major classes of drugs (those that block NMDA glutamate receptors and those that promote GABAA receptor activation), when administered to immature rodents during the period of synaptogenesis, trigger widespread apoptotic neurodegeneration throughout the developing brain. In addition, we have found that ethanol, which has both NMDA antagonist and GABAmimetic properties, triggers a robust pattern of apoptotic neurodegeneration, thereby deleting large numbers of neurons from many different regions of the developing brain. These findings provide a more likely explanation than has heretofore been available for the reduced brain mass and lifelong neurobehavioral disturbances associated with the human fetal alcohol syndrome (FAS). The period of synaptogenesis, also known as the brain growth spurt period, occurs in different species at different times relative to birth. In rats and mice it is a postnatal event, but in humans it extends from the sixth month of gestation to several years after birth. Thus, there is a period in pre- and postnatal human development, lasting for several years, during which immature CNS neurons are prone to commit suicide if exposed to intoxicating concentrations of drugs with NMDA antagonist or GABAmimetic properties. These findings are important, not only because of their relevance to the FAS, but because there are many agents in the human environment, other than ethanol, that have NMDA antagonist or GABAmimetic properties. Such agents include drugs that may be abused by pregnant mothers (ethanol, phencyclidine [angel dust], ketamine [Special K], nitrous oxide [laughing gas], barbiturates, benzodiazepines), and many medicinals used in obstetric and pediatric neurology (anticonvulsants), and anesthesiology (all general anesthetics are either NMDA antagonists or GABAmimetics).

The effects of chronic administration of quetiapine on the phencyclidine-induced reference memory impairment and decrease of Bcl-XL/Bax ratio in the posterior cingulate cortex in rats

Quetiapine, a new atypical antipsychotic drug, effectively alleviates positive and negative symptoms, as well as cognitive impairment that may be caused by neurodegeneration, in schizophrenia patients. Earlier in vivo and in vitro studies have demonstrated that quetiapine may be a neuroprotectant. The present study was designed to examine the beneficial effects of quetiapine on the possible cognitive impairment and changes of brain apoptotic regulation proteins induced by phencyclidine (PCP) in rats. Rats were treated with quetiapine (10 mg/kg/day; intraperitoneal (i.p.)) or vehicle for 16 days. On day 14, 1 h after the administration of quetiapine, the rats were given PCP (50 mg/kg; subcutaneous (s.c.)) or vehicle. Then quetiapine was administrated for an additional 2 days. One day after the last quetiapine injection (3 days after the PCP injection), the rats were trained on a spatial memory task in a radial arm maze. After the behavioural test, the rats were decapitated for Western blot analysis. PCP induced reference memory impairment, and a decrease of the ratio of an anti-apoptotic Bcl-2 family member (Bcl-XL) to a pro-apoptotic analogue (Bax) in the posterior cingulate cortex. Chronic administration of quetiapine counteracted the PCP-induced reference memory impairment and decrease of Bcl-XL/Bax ratio in the posterior cingulate cortex. These results suggest that quetiapine may have ameliorating effects on the cognitive impairment and brain apoptotic processes induced by PCP.

Potential of ketamine and midazolam, individually or in combination, to induce apoptotic neurodegeneration in the infant mouse brain

Recently, it was reported that anesthetizing infant rats for 6 h with a combination of anesthetic drugs (midazolam, nitrous oxide, isoflurane) caused widespread apoptotic neurodegeneration in the developing brain, followed by lifelong cognitive deficits. It has also been reported that ketamine triggers neuroapoptosis in the infant rat brain if administered repeatedly over a period of 9 h. The question arises whether less extreme exposure to anesthetic drugs can also trigger neuroapoptosis in the developing brain. To address this question we administered ketamine, midazolam or ketamine plus midazolam subcutaneously at various doses to infant mice and evaluated the rate of neuroapoptosis in various brain regions following either saline or these various drug treatments. Each drug was administered as a single one-time injection in a dose range that would be considered subanesthetic, and the brains were evaluated by unbiased stereology methods 5 h following drug treatment. Neuroapoptosis was detected by immunohistochemical staining for activated caspase-3. It was found that either ketamine or midazolam caused a dose-dependent, statistically significant increase in the rate of neuroapoptosis, and the two drugs combined caused a greater increase than either drug alone. The apoptotic nature of the neurodegenerative reaction was confirmed by electron microscopy. We conclude that relatively mild exposure to ketamine, midazolam or a combination of these drugs can trigger apoptotic neurodegeneration in the developing mouse brain.

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.

The anesthetics nitrous oxide and ketamine are more neurotoxic to old than to young rat brain

Nitrous oxide (N2O) and ketamine are common general anesthetics and antagonists of N-methyl-D-aspartate (NMDA) glutamate receptors. In clinically relevant concentrations, they induce a psychotomimetic reaction in humans and pathomorphological changes in the rat brain. We have previously shown that ketamine and N2O in combination cause the neurotoxic reaction in young adult rat brain that is apparently synergistic. Ketamine and N2O are occasionally used in geriatric anesthesia since they do not suppress cardiorespiratory function and thus are beneficial for frail elderly patients. However, in view of the evidence that N2O and ketamine have potentially serious neurotoxic effects, and that they potentiate one another's neurotoxicity, their neurotoxic potential in the aging brain needs to be evaluated. In this study we compared the neurotoxicity of ketamine and N2O, alone or in combination, in aging (18- and 24-month-old) rats and in young adult (6-month-old) rats and found that the aging brain is substantially more sensitive than the young adult brain to the neurotoxic reaction induced by either ketamine alone or the ketamine + N2O combination, but equally sensitive to the neurotoxicity induced by N2O alone.

Chronic phencyclidine administration induces schizophrenia-like changes in N-acetylaspartate and N-acetylaspartylglutamate in rat brain

Administration of phencyclidine (PCP) to both humans and animals models the symptoms of schizophrenia. Brain concentrations of N-acetylaspartate (NAA) are reduced in this disease, reflecting neuronal dysfunction. This study investigates the effects in rats of a chronic intermittent regime of PCP on NAA and its precursor N-acetylaspartylglutamate (NAAG) in rat frontal and temporal cortex, hippocampus and striatum, determined by HPLC. We found significant PCP-induced deficits of NAA and NAAG only in the temporal cortex; NAAG was significantly elevated in the hippocampus. These changes closely reflect postmortem findings reported in schizophrenia.

Subanesthetic ketamine infusion therapy: a retrospective analysis of a novel therapeutic approach to complex regional pain syndrome

Complex Regional Pain Syndrome (CRPS) is a disorder that can be accompanied by severe pain that is often both chronic and resistant to conventional therapy. Harbut and Correll previously reported the successful treatment of a 9-year case of intractable Type I CRPS with an intravenous inpatient infusion of ketamine in an adult female patient.

OBJECTIVE

The purpose of this study was to ascertain if indeed the use of subanesthetic inpatient infusions of ketamine provide meaningful improvements in pain scores, and thus, quality of life, in patients suffering from CRPS. To achieve this objective we focused our analysis on the relief of pain obtained by patients undergoing this novel treatment option developed at Mackay Base Hospital, Queensland, Australia.

METHODS

Case notes of 33 patients whose CRPS pain was treated by the inpatient administration of a continuous subanesthetic intravenous infusion of ketamine were reviewed. The dose and duration of ketamine therapy and the degree and duration of relief obtained were recorded. Notable side effects were also recorded. The degree of relief obtained (immediately after the infusion) was assessed using pre- and posttreatment numeric pain scores. The duration of relief obtained (throughout the follow-up period) was analyzed using a Kaplan-Meier cumulative survival curve analysis.

RESULTS

A total of 33 patients with diagnoses of CRPS who had undergone ketamine treatment at least once were identified. Due to relapse, 12 of 33 patients received a second course of therapy, and two of 33 patients received a third. The degree of relief obtained following the initial course of therapy was impressive (N=33); there was complete pain relief in 25 (76%), partial relief in six (18%), and no relief in two (6%) patients. The degree of relief obtained following repeat therapy (N=12) appeared even better, as all 12 patients who received second courses of treatment experienced complete relief of their CRPS pain. The duration of relief was also impressive, as was the difference between the duration of relief obtained after the first and after the second courses of therapy. In this respect, following the first course of therapy, 54% of 33 individuals remained pain free for >/=3 months and 31% remained pain free for >/=6 months. After the second infusion, 58% of 12 patients experienced relief for >/=1 year, while almost 33% remained pain free for >3 years. The most frequent side effect observed in patients receiving this treatment was a feeling of inebriation. Hallucinations occurred in six patients. Less frequent side effects also included complaints of lightheadedness, dizziness, and nausea. In four patients, an alteration in hepatic enzyme profile was noted; the infusion was terminated and the abnormality resolved thereafter.

CONCLUSION

This retrospective review suggests that limited subanesthetic inpatient infusions of ketamine may offer a promising therapeutic option in the treatment of appropriately selected patients with intractable CRPS. More study is needed to further establish the safety and efficacy of this novel approach.

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.

(+) MK-801 and Phencyclidine Induced Neurotoxicity do Not Cause Enduring Behaviours Resembling the Positive and Negative Symptoms of Schizophrenia in the Rat

Studies in rats and primates have demonstrated that repeated phencyclidine treatment can produce enduring cognitive deficits that may resemble the cognitive deficits seen in schizophrenia, suggesting that neurodegeneration resulting from NMDA-receptor dysfunction may be a valid model of schizophrenia. The purpose of the present experiments was to expand these findings and to determine if medium and high doses of the NMDA-antagonists phencyclidine and (+)MK-801 could produce permanent behavioural changes in animal tests with face validity for some aspects of the positive and negative symptoms of schizophrenia. Rats were treated with dose regimens of (+)MK-801 and phencyclidine known to produce mild and severe irreversible levels of neurotoxicity, and were tested 7 or 10 days after the last drug administration in the social interaction test and in standard activity cages. The rats did not show any enduring behavioural changes as a result of the treatment. The present study could therefore not provide additional evidence for the face validity of this model of schizophrenia.

Glutamate receptor subunits are altered in forebrain and cerebellum in rats chronically exposed to the NMDA receptor antagonist phencyclidine

Phencyclidine (PCP) is a noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype. It produces transient psychoses in normal individuals and exacerbates psychoses in schizophrenics. When administered to rodents, PCP elicits stereotypic behaviors including unrelenting head swaying, hyperlocomotion, and social withdrawal. In this study, we examined the relative distribution of the NMDA receptor subunits, as well as the subunits of its modulating receptor, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) in the forebrain, hippocampus, and cerebellum of rats chronically exposed to PCP. Rats were injected for 30 days with PCP (10 mg/kg) and age/sex-matched controls were injected for 30 days with saline vehicle. Brain NMDA and AMPA receptor subunit distribution patterns and protein levels were then analyzed by immunocytochemistry and Western blot analysis. Chronic PCP-treated animals showed significant alterations in glutamate receptor subunits, particularly for the NR1, NR2B, NR2C, and NR2D components of the NMDA receptor. AMPA receptor subunits demonstrated few significant changes in subunit availabilities. Western blot analysis largely confirmed the immunocytochemical findings. These results support the conclusion that subunits of the NMDA receptor are selectively altered by chronic PCP antagonism, with minimal to no changes observed in AMPA receptor subunits. Our findings are consistent with the interpretation that a dysfunctional NMDA receptor complex may mediate abnormal glutamatergic neurotransmission and potentially contribute to the complex etiology of cognitive disorders.

Prolonged exposure to inhalational anesthetic nitrous oxide kills neurons in adult rat brain

Short-term exposure of adult rats to nitrous oxide (N2O), an inhalational anesthetic and NMDA (N-methyl-D-aspartate) antagonist, causes a reversible neurotoxic vacuole reaction in neurons of the posterior cingulate/retrosplenial cortex (PC/RSC) which resembles that caused by low doses of other NMDA antagonists. Since high doses or prolonged exposure to other NMDA antagonists can cause neurons to die, we assessed whether prolonged N2O exposure might also cause neuronal cell death. Adult female Sprague-Dawley rats were exposed to 150-vol% N2O (approximately EC50 for N2O anesthesia in rats) for various durations from 1 to 16 h. The time course for onset and disappearance of the reversible vacuole reaction was studied, as was the time course and dose requirement for triggering cell death. A maximum vacuole reaction was observed in PC/RSC neurons in brains examined immediately after 3 h of 150-vol% N2O exposure and the same magnitude of vacuole reaction was observed when brains were examined immediately after a longer period of N2O exposure. When N2O was terminated at 3 h and the rats were killed 1 h later, the vacuole reaction was markedly diminished and if the rats were killed 3 h later the vacuole reaction had completely disappeared. Prolonged exposure to 150-vol% N2O (for 8 h or more) caused neuronal cell death which was detectable by silver staining 32 h later. Concurrently administered GABAergic agents, diazepam (an i.v. anesthetic), or isoflurane (an inhalational anesthetic), prevented this cell death reaction. Our findings demonstrate that short-term exposure of adult rats to N2O causes injury to PC/RSC neurons that is rapidly reversible, and prolonged N2O exposure causes neuronal cell death. These neurotoxic effects, including the cell death reaction, can be prevented by coadministration of GABAmimetic anesthetic agents. Duration of NMDA receptor blockade appears to be an important determinant of whether neurons are reversibly injured or are driven to cell death by an NMDA antagonist drug.

The NMDA receptor hypofunction model of psychosis

Antagonists of the NMDA glutamate receptor, including phencyclidine (PCP), ketamine, and CGS-19755, produce cognitive and behavioral changes in humans. In rodents these agents produce a myriad of histopathological and neurochemical changes. Several lines of evidence suggest that a large number of these drug-induced effects are dose-dependent manifestations of the same general disinhibition process in which NMDA antagonists abolish GABAergic inhibition, resulting in the simultaneous excessive release of acetylcholine and glutamate. Progressive increases in the severity of NMDA receptor hypofunction (NRHypo) within the brain produce an increasing range of effects on brain function. Underexcitation of NMDA receptors, induced by even relatively low doses of NMDA antagonist drugs, can produce specific forms of memory dysfunction without clinically evident psychosis. More severe NRHypo can produce a clinical syndrome very similar to a psychotic schizophrenic exacerbation. Finally, sustained and severe NRHypo in the adult brain is associated with a form of neurotoxicity with well-characterized neuropathological features. In this paper several of these effects of NMDA antagonists and a likely mechanism responsible for producing them will be reviewed. In addition the possible role of NRHypo in the pathophysiology of idiopathic psychotic disorders will be considered.

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

N-Methyl-D-aspartate (NMDA) glutamate receptor antagonists are being developed as therapeutic agents for several clinical conditions. However, the ability of these agents to produce neurotoxicity and psychosis can compromise their clinical usefulness. In addition, an NMDA receptor hypofunction (NRHypo) state may play a role in neurodegenerative and psychotic disorders. A better understanding of the mechanism underlying these adverse effects should allow for the safer use of these agents and might clarify mechanisms underlying certain clinical disorders. NRHypo neurotoxicity is mediated by a complex disinhibition mechanism in which NMDA antagonists abolish GABAergic inhibition, resulting in the simultaneous excessive release of acetylcholine and glutamate onto the vulnerable retrosplenial cortex (RSC) neurons. Systemically administered GABAergic agents are potent protectors against NRHypo neurotoxicity. To determine where in brain GABAergic agents could be acting to protect against NRHypo neurotoxicity, we injected the GABAergic agonist, muscimol, into different brain regions of rats treated systemically with a neurotoxic dose of the potent NMDA antagonist, MK-801. We report that muscimol injections into the anterior thalamus or diagonal band of Broca provide substantial protection, suggesting that disinhibition of neurons in these regions underlies NRHypo neurotoxicity. Muscimol injections into the RSC also provide substantial protection possibly by directly inhibiting the vulnerable RSC neuron. Injections of muscimol into other areas known to project to the RSC (ventral orbital cortex, anterior cingulate cortex and subiculum) provide only minimal protection. We conclude that GABAergic agents prevent NRHypo neurotoxicity mainly by activating GABA receptors in the anterior thalamus, diagonal band of Broca and RSC.

Dose-dependent effect of MK-801 on the levels of neuropeptides processing enzymes in rat brain regions

The appropriate levels of neuropeptides and their processing enzyme activities are required to continue a normal cell life, and the dysfunction of these peptides and enzymes are responsible for many neuronal abnormalities. Systemic administration of (+) MK-801 (dizocilpine maleate), a noncompetitive N-methyl-[D]-aspartate (NMDA) receptor antagonist, causes both neuroprotective and neurotoxic activities depending on doses and conditions. In the present study, we investigated the dose dependent effect of (+) MK-801 on prolyl endopeptidase (PEP), endopeptidase EC 24.15 (EP 24.15) and beta-D-glucuronidase activities as well as the protein levels of EP 24.15 and neuron specific enolase (NSE) in the posterior cingulate/retrosplenial cortices (PC/RSC), hippocampus, frontal cortex and striatum of female rats 3 days after the treatment. The activity of PEP was significantly increased compared with controls (saline) in the PC/RSC at 1.0 and 5.0 mg/kg doses, and in the frontal cortex at 5.0 mg/kg dose. beta-D-Glucuronidase activity was dose-dependently increased in all brain regions examined. The activity of EP 24.15 was unchanged in all regions after the treatment, whereas the Western blot analysis for EP 24.15 showed the increased protein level in the PC/RSC. These results suggest that a low dose treatment with MK-801 causes neurotoxicity in the PC/RSC and hippocampus, and the high dose treatment causes neurotoxicity in all the brain regions examined.

Differential effects of phencyclidine application on secretogranin II expression in organotypic slices of rat prefrontal cortex

Phencyclidine (PCP) is a non-competitive NMDA glutamate receptor antagonist that induces psychotomimetic effects in humans and experimental animals. Chronic PCP exposure elicits signs of persistently altered frontal brain activity and related behaviors which are also seen in patients with schizophrenia. Secretogranin II (sg II) belongs to the chromogranin family of proteins that exist in large dense core vesicles in nervous tissue. In the brain, 90% of sg II is processed to the small peptide secretoneurin. We previously detected differential effects of single-dose and subchronic PCP administration on sg II expression in the rat prefrontal cortex (PFC). In the present study, we applied PCP to organotypic PFC slices. PCP application for 28 h induced decreased tissue and culture medium secretoneurin content. In contrast, incubation with the adenylate cyclase activator forskolin caused significantly increased secretoneurin levels after 8 h. PCP for 4 h followed by 24 h without PCP resulted in increased culture medium secretoneurin content but no change in tissue levels. sg II mRNA expression was decreased after 28 h PCP application in cortical neurons. Immunohistochemical and TUNEL staining profiles indicated that the alterations were not due to neurodegeneration. PCP for 5 days changed neither the secretoneurin tissue or culture medium levels, nor the sg II mRNA expression. These results demonstrate that PCP modulates sg II expression in PFC tissue in the absence of afferent inputs and that the nature of these changes is dependent upon the duration of exposure to and/or withdrawal from PCP.

Withdrawal from chronic phencyclidine treatment induces long-lasting depression in brain reward function

Phencyclidine (PCP) is a drug of abuse that has rewarding and dysphoric effects in humans. The complex actions of PCP, and PCP withdrawal in particular, on brain reward function remain unclear. The purpose of the present study was to characterize the effects of withdrawal from acute and chronic PCP treatment on brain reward function in rats. A brain stimulation reward procedure was used to evaluate the effects of acute PCP injection (0, 5, or 10 mg/kg) or chronic PCP treatment (0, 10, 15, or 20 mg/kg/day for 14 days delivered via subcutaneous osmotic minipumps) on brain reward function. Withdrawal from acute administration of 5 and 10 mg/kg PCP produced a decrease in brain reward function as indicated by a sustained elevation in brain reward thresholds. When administered chronically, 10, 15, or 20 mg/kg/day PCP induced a progressive dose-dependent potentiation of brain stimulation reward, while cessation of the treatment resulted in significant elevations in reward thresholds reflecting diminished reward. Specifically, withdrawal from 15 or 20 mg/kg/day PCP induced a depression in brain reward function that lasted for the entire month of observation. These results indicate that prolonged continuous administration of high PCP doses facilitates brain stimulation reward, while withdrawal from acute high PCP doses or chronic PCP treatment results in a protracted depression of brain reward function that may be analogous to the dysphoric and anhedonic symptoms observed in PCP dependence, depression, and schizophrenia.

Effect of repeated administration of phencyclidine on spatial performance in an eight-arm radial maze with delay in rats and mice

Phencyclidine (PCP) is an N-methyl-D-aspartate (NMDA) glutamate receptor channel noncompetitive antagonist that produces some of the symptoms of schizophrenia, including delusions, hallucinations, and negative symptoms as well as cognitive impairment. Thus, administration of PCP to rodents and nonhuman primates has been suggested to provide a potential animal model for schizophrenia. There have been some reports that 7-14 days of PCP administration can bring about enduring impairments in working memory in rodents but not all studies have been consistent in this regard. The present study determined whether repeated PCP administration impaired spatial performance in rats or mice trained to make minimal errors in an eight-arm radial maze task with a delay. Male Sprague-Dawley rats and C57BL/6J mice received 14 daily injection of vehicle or PCP (10 mg/kg, s.c.) followed by a withdrawal period of 1 week. The number of arm reentry errors and the distance traveled to complete the task were not significantly different between PCP-treated and vehicle-treated rats on 2, 8, and 14 days of PCP administration or 8 days following withdrawal of PCP. Mice treated with PCP for up to 2 weeks also had no significant differences in the number of arm reentry errors, travel distances, the numbers of visits to different arms during the first eight choices, or latencies to take all eight pellets compared to the vehicle-treated group. Thus, the present study failed to demonstrate that repeated administration of PCP to rats or mice produces enduring memory impairment. Factors potentially contributing to the discrepancies between various studies are discussed.

Antiepileptic drugs and agents that inhibit voltage-gated sodium channels prevent NMDA antagonist neurotoxicity

N-methyl-D-aspartate (NMDA) glutamate receptor antagonists are used in clinical anesthesia and are being developed as therapeutic agents for preventing neurodegeneration in stroke, epilepsy, and brain trauma. However, the ability of these agents to produce neurotoxicity in adult rats and psychosis in adult humans compromises their clinical usefulness. In addition, an NMDA receptor hypofunction (NRHypo) state might play a role in neurodegenerative and psychotic disorders, like Alzheimer's disease, bipolar disorder and schizophrenia. Thus, developing pharmacological means of preventing these NRHypo-induced effects could have significant clinically relevant benefits. NRHypo neurotoxicity appears to be mediated by a complex disinhibition mechanism that results in the excessive stimulation of certain vulnerable neurons. Here we report our findings that five agents (phenytoin, carbamazepine, valproic acid, lamotrigine, and riluzole), thought to possess anticonvulsant activity because they inhibit voltage-gated sodium channels, prevent NRHypo neurotoxicity. The ability of tetrodotoxin, a highly selective inhibitor of voltage-gated sodium channels, to prevent the same neurotoxicity suggests that inhibition of this ion channel is the likely mechanism of action of these five agents. We also found that three other anticonvulsants (felbamate, gabapentin and ethosuximide), whose mechanism is less clear, also prevent NRHypo neurotoxicity, suggesting that inhibition of voltage-gated sodium channels is not the only mechanism via which anticonvulsants can act to prevent NRHypo neurotoxicity. Several of these agents have been found to be of clinical use in bipolar disorder. It would be of interest to determine whether these agents might have therapeutic benefits for conditions in which a NRHypo state may exist.

Early exposure to common anesthetic agents causes widespread neurodegeneration in the developing rat brain and persistent learning deficits

Recently it was demonstrated that exposure of the developing brain during the period of synaptogenesis to drugs that block NMDA glutamate receptors or drugs that potentiate GABA(A) receptors can trigger widespread apoptotic neurodegeneration. All currently used general anesthetic agents have either NMDA receptor-blocking or GABA(A) receptor-enhancing properties. To induce or maintain a surgical plane of anesthesia, it is common practice in pediatric or obstetrical medicine to use agents from these two classes in combination. Therefore, the question arises whether this practice entails significant risk of inducing apoptotic neurodegeneration in the developing human brain. To begin to address this problem, we have administered to 7-d-old infant rats a combination of drugs commonly used in pediatric anesthesia (midazolam, nitrous oxide, and isoflurane) in doses sufficient to maintain a surgical plane of anesthesia for 6 hr, and have observed that this causes widespread apoptotic neurodegeneration in the developing brain, deficits in hippocampal synaptic function, and persistent memory/learning impairments.

New insights and new issues in developmental neurotoxicology

Here, I will review accumulating evidence that during the developmental period of synaptogenesis, also known as the brain growth spurt period, neurons are very sensitive to specific disturbances in their synaptic environment. During this period, abnormal increases in NMDA glutamate (Glu) receptor activity triggers excitotoxic neurodegeneration, and abnormal inhibition of neuronal activity (by blockade of NMDA Glu receptors or excessive activation of GABAA receptors) triggers neuronal suicide (apoptosis). Only a transient disturbance, lasting for a few hours, is sufficient to trigger either excitotoxic or apoptotic neurodegeneration during this developmental period. Ethanol, which has both NMDA antagonist and GABAmimetic properties, triggers widespread apoptotic neurodegeneration in the developing rat, mouse or guinea pig brain, and this provides a likely explanation for the reduced brain mass and lifelong neurobehavioral disturbances associated with the human fetal alcohol syndrome (FAS). The brain growth spurt occurs in different species at different times relative to birth. In rats and mice it is a postnatal event, but in humans it extends from the 6th month of gestation to several years after birth. Thus, there is a period in fetal and neonatal human development, lasting for several years, during which immature central nervous system (CNS) neurons are exquisitely sensitive to environmental agents (the specific number and variety of which remains to be established) that can trigger widespread neurodegeneration by inducing specific abnormal changes in the synaptic environment. Agents identified thus far include drugs that may be abused by pregnant mothers (ethanol, phencyclidine (PCP) (angel dust), ketamine (Special K), nitrous oxide (laughing gas), barbiturates, benzodiazepines) and many medicinals used in obstetric and pediatric medicine as sedatives, anti-convulsants or anesthetics (all general anesthetics are either NMDA antagonists or GABAmimetics). Many other chemicals in the human environment remain to be evaluated for their ability to cause developing CNS neurons to commit suicide, and this provides an exciting challenge for the field of developmental neurotoxicology.