MK-801 increases endogenous acetylcholine release in the rat parietal cortex: a study using brain microdialysis

Pro-cognitive effects of dual tacrine derivatives acting as cholinesterase inhibitors and NMDA receptor antagonists

Therapeutic options for Alzheimer's disease are limited. Dual compounds targeting two pathways concurrently may enable enhanced effect. The study focuses on tacrine derivatives inhibiting acetylcholinesterase (AChE) and simultaneously N-methyl-D-aspartate (NMDA) receptors. Compounds with balanced inhibitory potencies for the target proteins (K1578 and K1599) or increased potency for AChE (K1592 and K1594) were studied to identify the most promising pro-cognitive compound. Their effects were studied in cholinergic (scopolamine-induced) and glutamatergic (MK-801-induced) rat models of cognitive deficits in the Morris water maze. Moreover, the impacts on locomotion in the open field and AChE activity in relevant brain structures were investigated. The effect of the most promising compound on NMDA receptors was explored by in vitro electrophysiology. The cholinergic antagonist scopolamine induced a deficit in memory acquisition, however, it was unaffected by the compounds, and a deficit in reversal learning that was alleviated by K1578 and K1599. K1578 and K1599 significantly inhibited AChE in the striatum, potentially explaining the behavioral observations. The glutamatergic antagonist dizocilpine (MK-801) induced a deficit in memory acquisition, which was alleviated by K1599. K1599 also mitigated the MK-801-induced hyperlocomotion in the open field. In vitro patch-clamp corroborated the K1599-associated NMDA receptor inhibitory effect. K1599 emerged as the most promising compound, demonstrating pro-cognitive efficacy in both models, consistent with intended dual effect. We conclude that tacrine has the potential for development of derivatives with dual in vivo effects. Our findings contributed to the elucidation of the structural and functional properties of tacrine derivatives associated with optimal in vivo pro-cognitive efficacy.

Phenoxytacrine derivatives: Low-toxicity neuroprotectants exerting affinity to ifenprodil-binding site and cholinesterase inhibition

Tacrine (THA), a long withdrawn drug, is still a popular scaffold used in medicinal chemistry, mainly for its good reactivity and multi-targeted effect. However, THA-associated hepatotoxicity is still an issue and must be considered in drug discovery based on the THA scaffold. Following our previously identified hit compound 7-phenoxytacrine (7-PhO-THA), we systematically explored the chemical space with 30 novel derivatives, with a focus on low hepatotoxicity, anticholinesterase action, and antagonism at the GluN1/GluN2B subtype of the NMDA receptor. Applying the down-selection process based on in vitro and in vivo pharmacokinetic data, two candidates, I-52 and II-52, selective GluN1/GluN2B inhibitors thanks to the interaction with the ifenprodil-binding site, have entered in vivo pharmacodynamic studies. Finally, compound I-52, showing only minor affinity to AChE, was identified as a lead candidate with favorable behavioral and neuroprotective effects using open-field and prepulse inhibition tests, along with scopolamine-based behavioral and NMDA-induced hippocampal lesion models. Our data show that compound I-52 exhibits low toxicity often associated with NMDA receptor ligands, and low hepatotoxicity, often related to THA-based compounds.

Dual orexin/hypocretin receptor antagonism attenuates NMDA receptor hypofunction-induced attentional impairments in a rat model of schizophrenia

Schizophrenia is a neuropsychiatric condition that is associated with impaired attentional processing and performance. Failure to support increasing attentional load may result, in part, from inhibitory failure in attention-relevant cortical regions, and available antipsychotics often fail to address this issue. Orexin/hypocretin receptors are found throughout the brain and are expressed on neurons relevant to both attention and schizophrenia, highlighting them as a potential target to treat schizophrenia-associated attentional dysfunction. In the present experiment, rats (N = 14) trained in a visual sustained attention task that required discrimination of trials which presented a visual signal from trials during which no signal was presented. Once trained, rats were then co-administered the psychotomimetic N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (MK-801: 0 or 0.1mg/kg, intraperitoneal injections) and the dual orexin receptor antagonist filorexant (MK-6096: 0, 0.1, or 1mM, intracerebroventricular infusions) prior to task performance across six sessions. Dizocilpine impaired overall accuracy during signal trials, slowed reaction times for correctly-responded trials, and increased the number of omitted trials throughout the task. Dizocilpine-induced increases in signal trial deficits, correct response latencies, and errors of omission were reduced following infusions of the 0.1mM, but not 1mM, dose of filorexant. As such, orexin receptor blockade may improve attentional deficits in a state of NMDA receptor hypofunction.

Reviewing the ketamine model for schizophrenia

The observation that antagonists of the N-methyl-D-aspartate receptor (NMDAR), such as phencyclidine (PCP) and ketamine, transiently induce symptoms of acute schizophrenia had led to a paradigm shift from dopaminergic to glutamatergic dysfunction in pharmacological models of schizophrenia. The glutamate hypothesis can explain negative and cognitive symptoms of schizophrenia better than the dopamine hypothesis, and has the potential to explain dopamine dysfunction itself. The pharmacological and psychomimetic effects of ketamine, which is safer for human subjects than phencyclidine, are herein reviewed. Ketamine binds to a variety of receptors, but principally acts at the NMDAR, and convergent genetic and molecular evidence point to NMDAR hypofunction in schizophrenia. Furthermore, NMDAR hypofunction can explain connectional and oscillatory abnormalities in schizophrenia in terms of both weakened excitation of inhibitory γ-aminobutyric acidergic (GABAergic) interneurons that synchronize cortical networks and disinhibition of principal cells. Individuals with prenatal NMDAR aberrations might experience the onset of schizophrenia towards the completion of synaptic pruning in adolescence, when network connectivity drops below a critical value. We conclude that ketamine challenge is useful for studying the positive, negative, and cognitive symptoms, dopaminergic and GABAergic dysfunction, age of onset, functional dysconnectivity, and abnormal cortical oscillations observed in acute schizophrenia.

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.

Postnatal NMDA receptor ablation in corticolimbic interneurons confers schizophrenia-like phenotypes

Cortical GABAergic dysfunction may underlie the pathophysiology of psychiatric disorders, including schizophrenia. Here, we characterized a mouse strain in which the essential NR1 subunit of the NMDA receptor (NMDAR) was selectively eliminated in 40-50% of cortical and hippocampal interneurons in early postnatal development. Consistent with the NMDAR hypofunction theory of schizophrenia, distinct schizophrenia-related symptoms emerged after adolescence, including novelty-induced hyperlocomotion, mating and nest-building deficits, as well as anhedonia-like and anxiety-like behaviors. Many of these behaviors were exacerbated by social isolation stress. Social memory, spatial working memory and prepulse inhibition were also impaired. Reduced expression of glutamic acid decarboxylase 67 and parvalbumin was accompanied by disinhibition of cortical excitatory neurons and reduced neuronal synchrony. Postadolescent deletion of NR1 did not result in such abnormalities. These findings suggest that early postnatal inhibition of NMDAR activity in corticolimbic GABAergic interneurons contributes to the pathophysiology of schizophrenia-related disorders.

Research applications of magnetic resonance spectroscopy to investigate psychiatric disorders

Advances in magnetic resonance spectroscopy (MRS) methodology and related analytic strategies allow sophisticated testing of neurobiological models of disease pathology in psychiatric disorders. An overview of principles underlying MRS, methodological considerations, and investigative approaches is presented. A review of recent research is presented that highlights innovative approaches applying MRS, in particular, hydrogen MRS, to systematically investigate specific psychiatric disorders, including autism spectrum disorders, schizophrenia, panic disorder, major depression, and bipolar disorder.

The acute effects of NMDA antagonism: from the rodent to the human brain

In the past decade, the N-methyl-d-aspartate receptor (NMDAR) hypofunction hypothesis of schizophrenia has received support from several lines of clinical evidence, including genetic, postmortem and human psychosis modeling. Recently, superiority of a mGluR2/3 receptor agonist over placebo was demonstrated in a randomized double-blind clinical trial in patients with schizophrenia. Considering the fact that currently available antipsychotics are all dopamine blockers to varying degrees without direct effects on glutamate transmission, this clinical trial highlights the potential utility of glutamatergic agents. In healthy volunteers, the NMDA channel antagonist ketamine induces transient cognitive dysfunction, perceptual aberrations and changes reminiscent of the negative symptoms of schizophrenia. However, how ketamine produces these effects is unclear. Preclinical data on NMDAR hypofunction offer further insights into the pathogenesis of the disorder as it relates to disorganized behavior, stereotypic movements and cognitive dysfunction in the rodent. This review evaluates the existing clinical and preclinical literature in an effort to shed light on the mechanism of action of ketamine as a probe to model NMDAR hypofunction in healthy volunteers. Included in this perspective are direct and indirect effects of ketamine at the neuronal level and in the intact brain. In addition to ketamine's effects on presynaptic and postsynaptic function, effects on glia and other neurotransmitter systems are discussed. While increased extracellular glutamate levels following NMDA antagonist administration stand out as a well replicated finding, evidence suggests that ketamine's effects are not restricted to pyramidal cells, but extend to GABAergic interneurons and the glia. In the glia, ketamine has significant downstream effects on the glutathione metabolism. Further studies are needed to identify the mechanistic connections between ketamine's effects at the cellular and behavioral levels.

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.

Inhibition of neprilysin by thiorphan (i.c.v.) causes an accumulation of amyloid β and impairment of learning and memory

An accumulation of amyloid beta peptide (Abeta) due to an imbalance between anabolism and catabolism triggers Alzheimer's disease (AD). Neprilysin is a rate-limiting peptidase, which participates in the catabolism of Abeta in brain. We investigated whether rats continuously infused with thiorphan, a specific inhibitor for neprilysin, into the cerebral ventricle cause cognitive dysfunction, with an accumulation of Abeta in the brain. Thiorphan-infused rats displayed significant cognitive dysfunction in the ability to discriminate in the object recognition test and spatial memory in the water maze test, but not in other hippocampus-dependent learning and memory tasks. Thiorphan infusion also elevated the Abeta40 level in the insoluble fraction of the cerebral cortex, but not that of the hippocampus. There was no significant difference in the nicotine-stimulated release of acetylcholine in the hippocampus between vehicle- and thiorphan-infused rats. These results indicate that continuous infusion of thiorphan into the cerebral ventricle causes cognitive dysfunction by raising the level of Abeta in the cerebral cortex, and suggest that a reduction of neprilysin activity contribute to the deposition of Abeta and development of AD.

Cholinesterase inhibitors ameliorate behavioral deficits induced by MK-801 in mice

Enhancing cholinergic function has been suggested as a possible strategy for ameliorating the cognitive deficits of schizophrenia. The purpose of this study was to examine the effects of acetylcholinesterase (AChE) inhibitors in mice treated with the noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, MK-801, which has been suggested as an animal model of the cognitive deficits of schizophrenia. Three separate experiments were conducted to test the effects of physostigmine, donepezil, or galantamine on deficits in learning and memory induced by MK-801. In each experiment, MK-801 (0.05 or 0.10 mg/kg) or saline was administered i.p. 20 min prior to behavioral testing over a total of 12 days. At 30 min prior to administration of MK-801 or saline, one of three doses of the AChE inhibitor (ie physostigmine-0.03, 0.10, or 0.30 mg/kg; donepezil-0.10, 0.30, or 1.00 mg/kg; or galantamine-0.25, 0.50, or 1.00 mg/kg) or saline was administered s.c. Behavioral testing was performed in all experimental animals using the following sequence: (1) spatial reversal learning, (2) locomotion, (3) fear conditioning, and (4) shock sensitivity. Both doses of MK-801 produced impairments in spatial reversal learning and in contextual and cued memory, as well as hyperlocomotion. Physostigmine and donepezil, but not galantamine, ameliorated MK-801-induced deficits in spatial reversal learning and in contextual and cued memory in a dose-dependent manner. Also, physostigmine, but not donepezil or galantamine, reversed MK-801-induced hyperlocomotion. Galantamine, but not physostigmine or donepezil, altered shock sensitivity. These results suggest that AChE inhibitors may differ in their capacity to ameliorate learning and memory deficits produced by MK-801 in mice, which may have relevance for the cognitive effects of cholinomimetic drugs in patients with schizophrenia.

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 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.

Tyrosine nitration of a synaptic protein synaptophysin contributes to amyloid beta-peptide-induced cholinergic dysfunction

Amyloid beta (Abeta) is a critical factor involved in the pathogenesis of Alzheimer's disease (AD). We have previously demonstrated that continuous intracerebroventricular infusion of Abeta1-40 induced a time-dependent expression of the inducible nitric oxide (NO) synthase (iNOS) and an overproduction of NO in the rat hippocampus. The pathophysiological significance of the overproduction of NO on brain function was manifested by an impairment of nicotine-evoked acetylcholine(ACh) release and memory deficits.(4) Molecular mechanisms by which NO participates in the Abeta-induced brain dysfunction, however, remain to be determined. Here we show that chronic Abeta1-40 infusion caused a robust peroxynitrite formation and subsequent tyrosine nitration of proteins in the hippocampus. Immunoprecipitation and Western blot analyses further revealed that synaptophysin, a synaptic protein, was a main target of tyrosine nitration. Chronic infusion of Abeta1-40 resulted in an impairment of nicotine-evoked ACh release as analyzed by microdialysis. Daily treatment with the iNOS inhibitor aminoguanidine (AG) or the peroxynitrite scavenger uric acid (UA) prevented the tyrosine nitration of synaptophysin as well as the impairment of nicotine-evoked ACh release induced by Abeta. Our findings suggest that the tyrosine nitration of synaptophysin is related to Abeta-induced impairment of ACh release.

NMDA receptor regulation of memory and behavior in humans

N-methyl-D-aspartate (NMDA) receptor hypofunction is associated with a range of effects on cognition and behavior in whole animal and human studies. NMDA receptor hypofunction within the brain, which can be induced experimentally in vivo using NMDA receptor antagonist drugs, produces adverse effects on memory function. The results suggest that NMDA receptor hypofunction can preferentially affect neural mechanisms regulating the efficiency of encoding and consolidation into longer-term storage. More pronounced NMDA receptor hypofunction can produce a clinical syndrome that includes core features of psychosis, as well as dissociation. Finally, sustained and severe underexcitation of NMDA receptors in the adult brain is associated with a neurotoxic process with well-characterized neuropathological features. Progressive increases in severity of NMDA receptor hypofunction within the brain can produce a range of effects on brain function, involving local and distributed circuitry, which may underlie the observed changes in behavior. As the brain ages, the NMDA receptor system becomes progressively hypofunctional, potentially contributing to further age-related decreases in memory and learning performance. Pharmacological and genomic methods for preventing NMDA receptor hypofunction, or for preventing the upstream or downstream consequences modeled by treatment with NMDA antagonists, may be applicable to the prevention and treatment of memory and behavioral dysfunction in a variety of neuropsychiatric disease conditions.

Receptor mechanisms and circuitry underlying NMDA antagonist neurotoxicity

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 and schizophrenia. Thus, understanding the mechanism underlying NRHypo-induced neurotoxicity and psychosis could have significant clinically relevant benefits. NRHypo neurotoxicity can be prevented by several classes of agents (e.g. antimuscarinics, non-NMDA glutamate antagonists, and alpha(2) adrenergic agonists) suggesting that the mechanism of neurotoxicity is complex. In the present study a series of experiments was undertaken to more definitively define the receptors and complex neural circuitry underlying NRHypo neurotoxicity. Injection of either the muscarinic antagonist scopolamine or the non-NMDA antagonist NBQX directly into the cortex prevented NRHypo neurotoxicity. Clonidine, an alpha(2) adrenergic agonist, protected against the neurotoxicity when injected into the basal forebrain. The combined injection of muscarinic and non-NMDA Glu agonists reproduced the neurotoxic reaction. Based on these and other results, we conclude that the mechanism is indirect, and involves a complex network disturbance, whereby blockade of NMDA receptors on inhibitory neurons in multiple subcortical brain regions, disinhibits glutamatergic and cholinergic projections to the cerebral cortex. Simultaneous excitotoxic stimulation of muscarinic (m(3)) and glutamate (AMPA/kainate) receptors on cerebrocortical neurons appears to be the proximal mechanism by which the neurotoxic and psychotomimetic effects of NRHypo are mediated.

Inhibition of synaptically evoked cortical acetylcholine release by intracortical glutamate: involvement of GABAergic neurons

Cortical acetylcholine (ACh) has been shown to regulate diverse cognitive processes and its release can be regulated by neuromodulators that act presynaptically at cholinergic terminals. The neocortex receives dense glutamatergic input from thalamocortical and other fibres. The present study used in vivo microdialysis to examine, and pharmacologically characterize, the effect of glutamate on cortical ACh release evoked by electrical stimulation of the pedunculopontine tegmental nucleus in urethane-anaesthetized rats. All drugs were administered locally within the cortex by reverse dialysis. Application of glutamate had no detectable effect on spontaneous ACh release but reduced evoked cortical ACh efflux in a concentration-dependent manner. This effect was mimicked by the glutamate transporter blocker L-trans-pyrrolidine-2,4-dicarboxylic acid, as well as by the ionotropic glutamate receptor agonists N-methyl-D-aspartic acid and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and was blocked by the ionotropic glutamate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione and (+/-)-3-(2-carboxypiperazin-4yl)-propyl-1-phosphonic acid. Glutamate application also increased extracellular adenosine levels but the simultaneous delivery of the broad-spectrum adenosine receptor antagonist caffeine failed to affect the inhibitory action of glutamate on evoked ACh release. However, the effect of glutamate was fully blocked by simultaneous delivery of the GABAA receptor antagonist bicuculline and partially blocked by the GABAB receptor antagonist phaclofen. These results suggest that ionotropic glutamate receptor activation by glutamate inhibits evoked cortical ACh release via an indirect pathway involving GABAergic neurons in the cortex.

NMDA receptor hypofunction model of schizophrenia

Several decades of research attempting to explain schizophrenia in terms of the dopamine hyperactivity hypothesis have produced disappointing results. A new hypothesis focusing on hypofunction of the NMDA glutamate transmitter system is emerging as a potentially more promising concept. In this article, we present a version of the NMDA receptor hypofunction hypothesis that has evolved from our recent studies pertaining to the neurotoxic and psychotomimetic effects of PCP and related NMDA antagonist drugs. In this article, we examine this hypothesis in terms of its strengths and weaknesses, its therapeutic implications and ways in which it can be further tested.

Sigma receptor ligands (+)-SKF10,047 and SA4503 improve dizocilpine-induced spatial memory deficits in rats

This study examined the effects of the sigma receptor ligands (+)-N-allylnormetazocine ((+)-SKF10,047) and 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503) on dizocilpine-induced impairment of working and reference memory in a radial arm maze task in rats. Dizocilpine, a non-competitive NMDA receptor antagonist, significantly impaired both reference and working memory, an effect which was accompanied by ataxia and impairment of food intake. The dizocilpine-induced impairment of reference memory was dose-dependently attenuated by (+)-SKF10,047 and SA4503. SA4503 also attenuated the dizocilpine-induced working memory impairment, although (+)-SKF10,047 had no effect. Neither sigma receptor ligand affected the behavioral symptoms such as ataxia and impairment of food intake induced by dizocilpine. The ameliorating effects of both (+)-SKF10,047 and SA4503 on dizocilpine-induced spatial memory impairment were completely antagonized by a sigma1 receptor antagonist N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine-mon ohydrochloride. These results suggest that the interaction of sigma1 receptors with NMDA receptors modulates spatial memory in rats.

Pharmacology of sensory stimulation-evoked increases in frontal cortical acetylcholine release

Recent research has demonstrated that a variety of sensory stimuli can increase acetylcholine release in the frontal cortex of rats. The aim of the present experiments was to investigate the pharmacological regulation of sensory stimulation-induced increases in the activity of basal forebrain cholinergic neurons. To this end, the effects of agonists and antagonists at a variety of neurotransmitter receptors on basal and tactile stimulation-evoked increases in frontal cortical acetylcholine release were studied using in vivo brain microdialysis. Tactile stimulation, produced by gently stroking the rat's neck with a nylon brush for 20 min, significantly increased frontal cortical acetylcholine release by more than 100% above baseline. The noradrenergic alpha2 agonist clonidine (0.1 or 0.2 mg/kg) and alpha1 antagonist prazosin (1 mg/kg) failed to affect basal cortical acetylcholine release; however, both compounds significantly reduced the increases evoked by sensory stimulation. In contrast, the alpha2 antagonist yohimbine (3 mg/kg) increased basal cortical acetylcholine release, thereby preventing meaningful investigation of its effects on tactile stimulation-evoked increases. The benzodiazepine agonist diazepam (5 mg/kg) reduced, and the GABA(A) receptor antagonist picrotoxin (2 mg/kg) increased basal cortical acetylcholine release; in addition, diazepam attenuated the increases in cortical acetylcholine release evoked by tactile stimulation. While dopaminergic D1 (SCH 23390, 0.15 mg/kg) and D2 (raclopride, 1 mg/kg) receptor antagonists did not by themselves significantly influence the increases evoked by tactile stimulation, their co-administration produced a significant reduction. The opioid receptor antagonist naltrexone (1.5 mg/kg) failed to affect either basal or tactile stimulation-evoked increases in acetylcholine overflow. Finally, the non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (MK-801; 0.025 and 0.05 mg/kg) increased basal cortical acetylcholine release. These results confirm that cortically projecting cholinergic neurons are activated by sensory stimuli, and indicate that the increases in cortical acetylcholine release produced by tactile stimulation are inhibited by stimulation of alpha2 or blockade of alpha1 noradrenergic receptors, and by enhanced GABAergic transmission. In addition, simultaneous blockade of dopamine D1 and D2 receptors appears necessary to achieve a significant reduction of sensory stimulation-evoked acetylcholine release in the frontal cortex. The results are consistent with the hypothesis that cortical acetylcholine release is a component of the neurochemistry of arousal and/or attention and indicate that this is modulated by GABAergic, noradrenergic and dopaminergic systems. In contrast, endogenous opioid actions do not appear to be involved.

M1 muscarinic receptor stimulation decreases aspartate release in the rat neostriatum

This study investigates the effects of different muscarinic receptor agonists on extracellular glutamate and aspartate concentrations in the rat neostriatum. In vivo intracerebral perfusions were undertaken in the conscious rat using a concentric push-pull cannulae system. Amino acid concentrations in samples were determined by HPLC with fluorometric detection. The intrastriatal perfusion of arecoline, a M1-M2 muscarinic receptor agonist, produced a significant decrease in extracellular [ASP] (45% of decrease) but not in extracellular [GLU]. These effects were blocked by scopolamine, a M1-M2 muscarinic receptor antagonist. McN-A-343, a M1 muscarinic receptor agonist, but not the M2 muscarinic receptor agonist, oxotremorine, produced a significant decrease in extracellular [ASP] (40% of decrease) but not in extracellular [GLU]. The effects of McN-A-343 on extracellular [ASP] were blocked by pirenzepine, a M1 muscarinic receptor antagonist. These results suggest that the decrease in extracellular [ASP] could be mediated, at least in part, by M1 muscarinic receptor activation in the rat neostriatum.

Glutamatergic modulation of cortical acetylcholine release in the rat: a combined in vivo microdialysis, retrograde tracing and immunohistochemical study

The microdialysis technique with one or two probes was used to investigate the modulation of cortically projecting cholinergic neurons by glutamatergic input in the rat in vivo. Male albino Wistar rats (250-300 g) were used. Under chloral hydrate anaesthesia microdialysis membranes were positioned in the parietal cortex, nucleus basalis magnocellularis (NBM) or medial septum. Acetylcholine was assayed using high-performance liquid chromatography (HPLC) with electrochemical detection while GABA was detected using HPLC with fluorimetric detection after derivatization of the amino acid with o-phthalaldehyde. Septo-cortical neurons were retrogradely labelled with fluoro-gold. Double labelling with choline acetyltransferase (ChAT) immunoreactivity was performed to identify these neurons. Our main findings were that: (i) i.c.v. administration of the NMDA antagonist 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 1-5 nmol) increased cortical acetylcholine outflow; (ii) local administration of CPP (100 microM) to the cortex had no effect on cortical acetylcholine outflow; (iii) local administration of CPP (100 microM) to the NBM decreased cortical acetylcholine outflow; (iv) local administration of CPP (100-200 microM) to the septum increased cortical GABA and acetylcholine outflow; (v) administration of muscimol to the septum prevented the effect of CPP on cortical acetylcholine outflow; (vi) retrograde tracing with fluoro-gold labelled cell bodies in the medial septum; (vii) septal fluoro-gold-positive neurons were not ChAT-immunoreactive. Our in vivo neurochemical results, in combination with retrograde tracing and immunohistochemistry, indicate that the cortically projecting cholinergic system is indirectly regulated by a glutamatergic input via a polysynaptic GABAergic circuitry located in the septum.

Effects of excitatory amino acids on cerebral oxygen consumption and blood flow in rat

This investigation tested the importance of excitatory amino acids' effects on regional cerebral O2 consumption and the concomitant changes in cerebral blood flow (rCBF) in isoflurane anesthetized rats. In the glutamate or N-methyl-D-aspartate (NMDA) groups, 10(-2) M glutamate or NMDA was topically applied to the right cortex and the left cortex was used as a control. One mg/kg dizocilpine maleate (MK-801), a non-competitive NMDA receptor antagonist, was administered (iv) to the MK-801 group and saline was given to the control group. Cortical rCBF was determined using 14C-iodoantipyrine and regional O2 extraction was measured microspectrophotometrically. Cerebral O2 consumption increased 77% after glutamate (contralateral cortex: 9.0 +/- 1.1 ml O2/min/100 g, glutamate treated cortex: 15.9 +/- 3.9), while a 46% increase was observed with the same concentration of NMDA (contralateral cortex: 9.8 +/- 2.0, NMDA treated cortex: 14.3 +/- 5.5). After MK-801, the O2 consumption decreased to 37% of the control value (control cortex: 7.0 +/- 1.3, MK-801 treated cortex: 2.6 +/- 3.9). MK-801 significantly decreased cerebral O2 extraction from 7.1 +/- 1.3 ml O2/100 ml (control cortex) to 5.3 +/- 0.6 (MK-801 treated cortex). However, there was no significant difference in cerebral O2 extraction between treated and contralateral cortex in either the glutamate or NMDA groups. The increase in O2 consumption caused by glutamate or NMDA was coupled with increased rCBF. Glutamate increased rCBF from 95 +/- 5 ml/min/100 g (contralateral cortex) to 165 +/- 31 (treated cortex), while NMDA increased rCBF from 114 +/- 12 (contralateral cortex) to 178 +/- 60 (treated cortex). MK-801 decreased O2 consumption with a lesser decrease of rCBF. The rCBF was 48 +/- 9 in the MK-801 treated cortex and 99 +/- 22 in the control cortex. Some substances produced by the activation of NMDA receptors may be related to the coupling of cerebral metabolism and blood flow, since after blockade of NMDA receptors with MK-801, this relationship is uncoupled. These findings suggest that glutamatergic processes have a major effect on cerebral O2 consumption and that this is at least partly due to NMDA receptors.

Effects of metrifonate on memory impairment and cholinergic dysfunction in rats

Metrifonate is an organophosphorous compound that has been used in the treatment of schistosomiasis. In this study, we investigated the effects of metrifonate on the impairment of learning and on central cholinergic dysfunction in scopolamine-treated and basal forebrain-lesioned rats. Oral administration of metrifonate (5.0-15.0 mg/kg) ameliorated the scopolamine- and basal forebrain. lesion-induced learning impairment in the water maze and passive avoidance tasks. Metrifonate (50 and 100 mg/kg) also significantly increased extracellular acetylcholine levels but decreased choline levels in the cerebral cortex of the basal forebrain-lesioned rats. The basal forebrain lesion decreased the cholinesterase activity in the cerebral cortex, and metrifonate (100 mg/kg) further reduced the cholinesterase activity. However, cholinesterase inhibition was not observed at the dose that ameliorated learning impairments. These results indicated that metrifonate ameliorated the impairment of learning in both scopolamine-treated and basal forebrain-lesioned rats by not only increasing extracellular acetylcholine levels by inhibiting cholinesterase, but also by undefined other mechanism(s). This finding suggests the usefulness of metrifonate for the therapy of Alzheimer's disease.

The effect of the loss of molar teeth on spatial memory and acetylcholine release from the parietal cortex in aged rats

It has been demonstrated that a loss of teeth is a troublesome problem among age-related pathological phenomena of the oral cavity, which influences the entire body, due to the impairment of mastication. The present studies investigated the abilities of learning and memory and acetylcholine (ACh) release in the parietal cortex in aged rats without molar teeth (hereafter referred to as 'teethless'). After the molar teeth of rats were extracted, the rats were fed with powdered food for 135 weeks. Although the performance in the radial arm maze was progressively acquired by daily training, an increase in the number of errors and a decrease in the initial correct responses were observed in the teethless aged rats compared to the control aged rats, indicating impaired acquisition of spatial memory in the teethless aged rats. The basal level of extracellular ACh in the parietal cortex was not different between the teethless aged rats and the control aged rats. However, the extracellular ACh level of the teethless aged rats under high-concentration of K+ and atropine sulfate stimulation was significantly low compared to that of the control aged rats. These results suggest that the impairment of spatial memory in the teethless aged rats may be due to the functional deterioration of the cholinergic neuronal system induced by tooth loss and that there is a possibility that the loss of teeth may be one of the risk factors for senile dementia.

Effects of metrifonate on impairment of learning and dysfunction of cholinergic neuronal system in basal forebrain-lesioned rats

Several studies have indicated the possibility of using cholinesterase (ChE) inhibitors as therapeutic drugs for Alzheimer's disease. Metrifonate (MTF) is an organophosphorus compound that has been used in the treatment of schistosomiasis. In this study, we investigated the effects of MTF on the impairment of learning and memory, decreased ChE activity and extracellular acetylcholine (ACh) levels in basal forebrain (BF)-lesioned rats. The oral administration of MTF improved the BF-lesion-induced impairment of performance on passive avoidance task. Further, MTF reduced ChE activity in the cerebral cortex. In vivo brain microdialysis studies showed that MTF significantly increased the release of ACh, but decreased that of choline (Ch) in the cerebral cortex of BF-lesioned rats. These results indicated that MTF ameliorates the impairment of performance on passive avoidance task in BF-lesioned rats by increasing the extracellular ACh levels by inhibiting ChE. This suggested that MTF may be useful as a therapeutic drug for Alzheimer's disease.

Effects of lobeline and dimethylphenylpiperazinium iodide (DMPP) on N-methyl-D-aspartate (NMDA)-evoked acetylcholine release in vitro: evidence for a lack of involvement of classical neuronal nicotinic acetylcholine receptors

Biochemical, behavioral and electrophysiological evidence suggests interactions between pathways containing neuronal nicotinic acetylcholine receptors (NAChRs) and excitatory amino acid receptors. Recently, protective effects of nicotine against N-methyl-D-aspartate (NMDA)-induced toxicity in primary cortical cultures were reported. To address possible interactions between NAChR and NMDA receptor containing pathways, several NAChR agonists were evaluated for their effects on NMDA-evoked [3H]acetylcholine ([3H]ACh) release from slices of rat striatum. Nicotine, cytisine and epibatidine had no effect on NMDA-evoked release or basal release of [3H]ACh over a wide range of concentrations. Lobeline and dimethylphenylpiperazinium iodide (DMPP), however, decreased basal [3H]ACh release and attenuated NMDA-evoked [3H]ACh release with EC50 values of 35 and 155 microM, respectively. The NAChR antagonists, dihydro-beta-erythroidine (DH beta E) and d-tubocurarine had no effect on NMDA-evoked [3H]ACh release, whereas mecamylamine attenuated the NMDA-evoked [3H]ACh evoked release with an EC50 value of 144 microM. Methyllycaconitine (MLA), a highly selective and potent antagonist of the alpha-bungarotoxin-sensitive alpha 7 NAChR subtype, also had no effect on NMDA-evoked [3H]ACh release at concentrations upto 10 microM. The inhibitory effects of DMPP and lobeline on NMDA-evoked [3H[ACh release were relatively insensitive to mecamylamine, d-tubocurarine, MLA and DH beta E. In addition, DMPP or lobeline-induced attenuation of basal [3H]ACh release was insensitive to blockade by sulpiride, a dopamine (D2) receptor antagonist. In contrast to their effects on NMDA-evoked striatal [3H]ACh release, both DMPP and lobeline increased basal release of striatal [3H]DA and hippocampal [3H]norepinephrine ([3H]NE) and did not attenuate NMDA-evoked release of these two transmitters. Instead, DMPP and lobeline appeared to have an additive effect on both NMDA-evoked hippocampal [3H]NE release and striatal [3H]DA release. These pharmacological results suggest that the inhibitory effects on lobeline and DMPP on striatal [3H]ACh release are independent of their interactions with classical NAChRs or the NMDA receptor complex itself.

Role of dopaminergic neuronal system in dizocilpine-induced acetylcholine release in the rat brain

The effects of dopaminergic receptor antagonists on dizocilpine-induced increase in extracellular acetylcholine (ACh) levels in the rat parietal cortex were examined in freely-moving rats, using an in vivo brain microdialysis method. Dizocilpine (0.5 mg/kg) significantly increased extracellular ACh levels in the rat parietal cortex and hippocampus, but not in the striatum. Pretreatment with alpha-methyl-p-tyrosine methyl ester (alpha MpT) delayed the onset but prolonged the duration of the dizocilpine-induced increases in extracellular ACh levels. The dopamine D2 receptor antagonist, haloperidol, showed dual effects similarly to alpha MpT, while the dopamine D1 receptor antagonist, SCH23390, prolonged, but did not delay, the onset of the dizocilpine-induced increases in ACh levels. These results suggest that the dopaminergic system is involved in the dizocilpine-induced increase in the extracellular ACh level in the parietal cortex in two ways, through both dopamine D1 and D2 receptors.

Effects of and interactions between antagonists for different sites on the NMDA receptor complex on hippocampal and striatal acetylcholine efflux in vivo

Intraperitoneal administration of the non-competitive NMDA receptor antagonists (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801, 0.25 and 0.5 mg/kg) and 1-(1-phenylcyclohexyl)piperidine (PCP, 5 and 10 mg/kg) increased the extracellular concentration of acetylcholine in rat hippocampus but not striatum. In contrast, R-(+)-3-amino-1-hydroxypyrrolid-2-one (R(+)-HA-966, 30 and 60 mg/kg), an antagonist at the glycine modulatory site of the NMDA receptor, did not affect acetylcholine efflux in either region. (+/-)-3-[2-Carboxypiperazin-4-yl]-propyl-1-phosphonic acid ((+/-)CPP, 10 mg/kg) and cis-4-(phosphonomethyl)piperidine-2-carboxylic acid (CGS19755, 5 mg/kg), competitive antagonists at the glutamate agonist site of the NMDA receptor, marginally increased hippocampal acetylcholine efflux. Pretreatment with R(+)-HA-966 (60 mg/kg) or (+/-)CPP (10 mg/kg) attenuated the increase of hippocampal acetylcholine efflux by MK-801 (0.5 mg/kg). However, prior administration of CGS19755 (5 mg/kg) prolonged the MK-801-induced increase of hippocampal acetylcholine efflux. Results demonstrate differential effects on hippocampal and striatal acetylcholine efflux of antagonists at different sites on the NMDA receptor complex and are discussed in relation to previously described effects of these drugs on mesolimbic dopamine function.

Antagonists of the NMDA receptor-channel complex and motor coordination

Many structurally different, centrally active antagonists of the NMDA receptor-channel complex induce phencyclidine-like side effects in mammals which include head weaving, body rolling, sniffing and disturbances of motor coordination. The ability of these compounds to cause disturbances of motor coordination correlates directly with their ability to antagonize the NMDA receptor-channel complex in vivo. Although noncompetitive antagonists increase motility in rodents, whereas competitive antagonists do not, both classes of compounds appear to induce schizophrenia-like psychosis in human beings, and cause similar changes in a variety of different biogenic amine neurotransmitter systems in the limbic and motoric areas of the brain. The complex spectrum of behavioural effects observed after the administration of antagonists of the NMDA receptor-channel complex probably reflects the intricate nature of the interaction with positive and negative feedback loops of the motor circuit. Recent research indicates that the site of integration of this interaction could be the striatal medium spiny GABAergic neuron.

Glutamatergic regulation of acetylcholine output in different brain regions: a microdialysis study in the rat

The glutamatergic regulation of cortical and striatal cholinergic neurons was investigated by measuring ACh output from the parietal cortex and striata of freely moving rats after administration of the competitive NMDA-receptor antagonist 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). It has been shown that intracerebroventricular administration of 5 nmol of CPP brings about a long lasting 100% increase in ACh output from the parietal cortex but does not affect ACh output from the striatum. Conversely, local perfusion of the striata with 50 microM CPP results in a 45% decrease in ACh output from the striatum but has no effect on parietal ACh output. The decrease in striatal ACh output induced by CPP is antagonized by concurrent perfusion with NMDA. In conclusion, glutamate may exert both inhibitory and excitatory modulatory effects on ACh output, through NMDA receptors, according to the neuronal circuitry existing in different brain regions.

Effect of dizocilpine (MK-801) on the catalepsy induced by delta 9-tetrahydrocannabinol in mice

Mice treated with delta 9-tetrahydrocannabinol (THC; 5 and 10 mg/kg i.v.) showed the catalepsy in high bar test, and median descent latencies of catalepsy were about 150 sec. Dizocilpine (MK-801, 0.05 and 0.1 mg/kg), non-competitive N-methyl-D-aspartate (NMDA) antagonist, significantly attenuated THC-induced catalepsy. Furthermore, the anticataleptic effect of MK-801 on THC-induced catalepsy was blocked by acetylcholine agonist oxotremorine (0.005 mg/kg) and dopamine antagonist haloperidol (0.01 mg/kg), but not by NMDA. Oxotremorine, haloperidol, and NMDA themselves did not affect THC-induced catalepsy at the doses used. These results suggest that the anticataleptic effect of MK-801 on THC-induced catalepsy may be developed through dopaminergic and acetylcholinergic neuronal systems.

Effects of chronic treatment with (+)- and (-)-nicotine on nicotinic acetylcholine receptors and N-methyl-D-aspartate receptors in rat brain

In this study, the effects of chronic treatment with (+)-nicotine on brain nicotinic acetylcholine receptors (nAChRs) and N-methyl-D-aspartate (NMDA) receptors, as well as on animal body weight were compared with those of chronic treatment with (-)-nicotine. Male Sprague-Dawley rats were s.c. injected with saline, (+)-nicotine (2.0 mg free base/kg b.w.) or (-)-nicotine (0.45 mg free base/kg b.w.) for 18 days. Brain nAChRs were investigated by (-)-[3H]nicotine binding. A significant increase in the high-affinity (-)-[3H]nicotine (5 nM)-binding sites was observed in the cortex, hippocampus, midbrain and striatum but not in the cerebellum of the rats treated with either (+)- or (-)-nicotine. The displacement curves of (-)-[3H]nicotine/(-)-nicotine in the cortices of rats treated with either (+)- or (-)-nicotine showed only one population of high-affinity binding sites, whereas both high- and low-affinity binding sites were observed in the cortices of control animals. Brain NMDA receptors were studied by [3H]MK-801, which binds to the NMDA receptor-ion channel complex. A significant decrease in the Bmax, but not in the KD for [3H]MK-801 binding in the cortices of rats treated with either (+)- or (-)-nicotine was only detected under certain experimental conditions where the NMDA receptors seem not to be maximally activated. The body weight of the animals treated with (-)-nicotine was significantly lower than that of the control animals, whereas there was no difference in body weight between (+)-nicotine- and saline-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)