NMDA receptor antagonists ketamine and PCP have direct effects on the dopamine D(2) and serotonin 5-HT(2)receptors-implications for models of schizophrenia

The putative antipsychotic alstonine reverses social interaction withdrawal in mice

Negative symptoms of schizophrenia are particularly problematic due to their deleterious impact on a patient's social life. The indol alkaloid alstonine, the major component of traditional remedies used for treating mental illnesses in Nigeria, presents a clear antipsychotic-like profile in mice, as well as anxiolytic properties. Considering that social interaction is the core of negative symptoms, and that anxiolytic drugs can improve social interaction behavior, the aim of this study was to evaluate the effects of alstonine in the social interaction and MK801-induced social withdrawal models in mice. Sub-chronic (but not acute) treatment with alstonine 0.5 mg/kg (but not 1.0 mg/kg) significantly increased social interaction in mice. Moreover, MK801-induced social withdrawal was completely prevented by sulpiride (10 mg/kg) and alstonine 1.0 mg/kg, and partially prevented by alstonine 0.5 mg/kg. The study indicates that alstonine not only increases social interaction in normal mice, but also averts social deficits attributable to negative symptoms of schizophrenia. This study reinforces and complements the antipsychotic-like profile of alstonine, and emphasizes its potential as a drug useful for the management of negative symptoms in schizophrenia.

Dopaminergic and glutamatergic regulation of effort- and delay-based decision making

Cost/benefit decisions regarding the relative effort or delay costs associated with a particular response are mediated by distributed dopaminergic and glutamatergic neural circuits. The present study assessed the contribution of dopamine and NMDA glutamate receptors in these different forms of decision making using novel effort- and delay-discounting procedures. In the effort-discounting task, rats could either emit a single response on a low-reward lever to receive two pellets, or make 2, 5, 10, or 20 responses on a high-reward (HR) lever to obtain four pellets. In the delay-discounting task, one press of the HR lever delivered four pellets after a delay (0.5-8 s). A third task (effort-discounting with equivalent delays) was similar to the effort-discounting procedure, except that the relative delay to reward delivery was equalized across response options. The dopamine receptor antagonist flupenthixol reduced choice of the HR lever under all three testing conditions, indicating that dopamine antagonism alters effort-based decision making independent of any contribution of delay. Amphetamine exerted dose-dependent, biphasic effects; a higher dose (0.5 mg/kg) increased effort discounting, whereas a lower dose (0.25 mg/kg) reduced delay discounting. The noncompetitive NMDA antagonist ketamine (5 mg/kg) increased effort and delay discounting, but did not affect choice on the effort with equivalent delays task, indicating a reduced tolerance for delayed rewards. These findings highlight the utility of these procedures in pharmacologically dissociating the neurochemical mechanisms underlying these different, yet interrelated forms of decision making. Furthermore, they suggest that dopamine and NMDA receptors make dissociable contributions to these different types of cost-benefit analyses.

Visual-procedural memory consolidation during sleep blocked by glutamatergic receptor antagonists

Visual cortex plasticity is enhanced by sleep. It is hypothesized that a reactivation of glutamatergic synapses is essential for this form of plasticity to occur after learning. To test this hypothesis, human subjects practiced a visual texture discrimination skill known to require post-training sleep for improvements to occur. During sleep, glutamatergic transmission was inhibited by administration of the two glutamate antagonists, caroverine and ketamine, targeting the ionotropic NMDA and AMPA receptors. Both substances given during consolidation sleep in a placebo controlled crossover design were able to prevent improvement of the skill measured the next morning. An off-line activation of glutamatergic synapses therefore seems to play a critical part in the consolidation of plastic changes in the visual cortex.

Lack of persistent effects of ketamine in rodent models of depression

RATIONALE

We investigated the immediate and enduring effects of ketamine in behavioral and neurochemical assays predictive of antidepressant activity.

MATERIALS AND METHODS

One week after a single administration of ketamine (50-160 mg/kg), otherwise experimentally naive rats and mice were tested either in the forced swim test (FST) or the tail suspension test (TST). Other mice were also tested twice in the FST: immediately and 2 weeks after a single dose (1.25-50 mg/kg) of ketamine. In the next series of experiments, rats treated for 2 weeks twice daily with ketamine (50 or 160 mg/kg) or desipramine (10 mg/kg) were challenged with apomorphine and scored for locomotor activity and assayed for the density of cortical beta-adrenoceptors. The latter test was also carried out in rats that had received a single dose of ketamine (50 mg/kg) 1 week before the assay. The antidepressant-like (FST) and locomotor effects of ketamine (50 mg/kg) and desipramine (10 mg/kg) were assessed after their chronic (2 weeks, twice daily) administration as well.

RESULTS

We report the lack of enduring antidepressant-like effect of ketamine in both rats and mice. A 2-week treatment with ketamine neither changed apomorphine-evoked locomotor hyperactivity nor did it decrease the density of cortical beta-adrenoceptors. However, some tolerance to the antidepressant-like effect of ketamine was noted in the FST, but it was accompanied by sensitization to its locomotor stimulatory effects.

CONCLUSIONS

These data indicate that ketamine neither produces enduring antidepressant-like effects in rodents nor does it display antidepressant-like behavioral or neurochemical effects after chronic treatment.

N-methyl d-aspartate receptor antagonists ketamine and MK-801 induce wake-related aberrant gamma oscillations in the rat neocortex

BACKGROUND

Single subanesthetic doses of ketamine, a non-competitive NMDA receptor (NMDAr) antagonist, induce cognitive impairment, schizophreniform psychosis, hallucinations, and exacerbate schizophrenia symptoms. The neuronal mechanisms underlying transient disruption in NMDAr function are unknown. Disorders of cognition-related coherences of gamma frequency (30-80 Hz) oscillations between cortical areas are a major functional abnormality in schizophrenic patients. Does a single subanesthetic dose of ketamine or MK-801 alter properties of cortical gamma oscillations?

METHODS

Properties of spontaneously occurring gamma oscillations in the electrocorticogram of the neocortex of freely moving rats (n = 16) were measured before and after subcutaneous administration of a single dose of ketamine (< or = 10 mg/kg), MK-801 (< or = .16 mg/kg), d-amphetamine (< or = 1 mg/kg), apomorphine (< or = 1.6 mg/kg), or vehicle (sodium chloride, .9%).

RESULTS

The present study gives the first evidence that ketamine and MK-801, both of which induce NMDAr-dependent functional disconnections, dose-dependently increase the power (200%-400%) of wake-related gamma oscillations in the neocortex. Substances that modulate dopaminergic neurotransmission could also increase the gamma power but to a lesser degree.

CONCLUSIONS

The present findings suggest that abnormal increased synchronization in ongoing gamma oscillations in cortical-related networks might cause dysfunctions of conscious integration, as seen in patients with schizophrenia.

Relationship between ketamine-induced psychotic symptoms and NMDA receptor occupancy: a [(123)I]CNS-1261 SPET study

RATIONALE

Ketamine induces effects resembling both positive and negative psychotic symptoms of schizophrenia. These are thought to arise through its action as an uncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor.

OBJECTIVES

We used [(123)I]CNS-1261 to study ketamine binding to NMDA receptors in healthy human controls in vivo and its relationship to positive and negative psychotic symptom induction.

MATERIALS AND METHODS

Ten healthy controls underwent two single-photon emission tomography scans with [(123)I]CNS-1261. On each occasion, they received a bolus infusion of either ketamine or saline. The Brief Psychiatric Rating Scale (BPRS) was administered at the end of each scan. Predefined regions of interest were used to estimate change in volume of distribution of [(123)I]CNS-1261 following ketamine administration. Two normalised-to-cortex binding indices were also used in order to study effects of ketamine on NMDA receptor availability by region, after correction for global and nonspecific effects.

RESULTS

Ketamine-induced reduction in [(123)I]CNS-1261 volume of distribution in all regions showed the strongest correlation with BPRS negative subscale (p < 0.01). With the normalised-to-cortex measures, NMDA receptor binding in middle inferior frontal cortex showed a significant correlation with BPRS negative subscale (BI1 r = 0.88, BI2 r = 95.9, p < 0.001).

CONCLUSIONS

[(123)I]CNS-1261 binding was modulated by ketamine, a drug known to compete for the same site on the NMDA receptor in vitro. Ketamine may induce negative symptoms through direct inhibition of the NMDA receptor, and positive symptoms may arise through a different neurochemical pathway.

Dopamine partial agonist actions of the glutamate receptor agonists LY 354,740 and LY 379,268

Because glutamate compounds alter the release of dopamine and prolactin, the present study examined whether group II metabotropic receptor agonists, LY 354,740 and LY 379,268, had any direct in vitro action on dopamine D2 receptors on rat striatal tissue, cloned D2Long receptors, and prolactin release from anterior pituitary cells. In competition versus the D2-specific ligand [(3)H]domperidone, LY 354,740 had a dissociation constant of 24 nM at D2(High) (the functional high-affinity state of dopamine D2 receptors), while the value for LY 379,268 was 21 nM. LY 354,740 also stimulated by 50% the incorporation of [(35)S]-GTP-gamma-S at a concentration of 120 nM, but its maximal stimulation was only 22% of the maximum elicited by dopamine. LY 379,268 stimulated by 50% the incorporation of [(35)S]-GTP-gamma-S at 280 nM, but its maximal stimulation was also only 22% of the maximum elicited by dopamine. However, both LY 354,740 and LY 379,268 potently inhibited the dopamine-induced incorporation of [(35)S]-GTP-gamma-S with inhibitory Ki values of 43 nM and 30 nM, respectively. The release of prolactin from rat isolated anterior pituitary cells in culture was 50% inhibited by 20 nM LY 379,268 and by 100 nM LY 354,740. These Ki values are similar to those known for the mGluR II receptor, suggesting that these compounds may have both glutamate and dopamine actions in vivo. The dopamine agonist and antagonist actions of these compounds indicate that these drugs have properties of a dopamine partial agonist, and may, therefore, have antipsychotic action.

Nitric oxide synthase inhibition attenuates phencyclidine-induced disruption of cognitive flexibility

Schizophrenia encompasses, amongst other symptoms, a heavy load of cognitive dysfunctionality. Using the psychotomimetic agent, phencyclidine (PCP), we have previously found that PCP-induced disruptions of cognitive function in translational rodent models of schizophrenia are dependent on nitric oxide (NO) production. In the present study, male Sprague-Dawley rats were subjected to a Morris water maze task designed to assess cognitive flexibility (i.e. the ability to cope with an increasingly demanding cognitive task) by means of a "constant reversal learning paradigm". Experiments were conducted to evaluate the effects of the NO synthase inhibitor, L-NAME (10 mg/kg), on PCP-induced (2 mg/kg) impairments. Control animals significantly improved their learning over the first 3 consecutive days, whereas PCP-treated animals failed to show any significant learning. Pretreatment with L-NAME normalized the PCP-induced disruption of learning to control levels. These findings suggest that PCP-induced disruptions of cognitive flexibility (i.e. ability to modify behaviour according to an increasingly demanding cognitive task) are dependent upon NO production. These observations, together with accumulated clinical findings, suggest that the NO system is a potential treatment target for cognitive dysfunctions in schizophrenia.

Food restriction markedly increases dopamine D2 receptor (D2R) in a rat model of obesity as assessed with in-vivo muPET imaging ([11C] raclopride) and in-vitro ([3H] spiperone) autoradiography

INTRODUCTION

Dopamine (DA) regulates food intake by modulating food reward and motivation but its involvement in obesity is much less understood. Recent evidence points to the involvement of leptin in the DA-related modulation of food intake. Here we assess DA D2 receptors (D2R) in a genetic rodent obesity model characterized by leptin-receptor deficiency and assess the influence of food restriction on these receptors.

METHODS

We compared D2R levels between Zucker Obese (fa/fa) and Lean (Fa/Fa) rats at 1 and 4 months of age and in two different feeding conditions (restricted and unrestricted food access) using in-vivo muPET imaging ([11C] raclopride, which is a method sensitive to competition with endogenous DA) and in-vitro ([3H] spiperone washed to ensure no competition with endogenous DA) autoradiography (ARG).

RESULTS

Both ARG and muPET showed that D2R were higher at 1 month than at 4 months of age and that food restricted animals had higher D2R than unrestricted animals. However there were significant differences in the results obtained at 4 months between ARG and muPET. ARG showed that at 1 month and at 4 months unrestricted lean rats (Le U) had significantly higher D2R binding than obese unrestricted rats (Ob U) but showed no differences between restricted obese (Ob R) and restricted lean rats (Le R). It also showed that D2R decline between 1 and 4 months of age was significantly attenuated in food restricted rats [both obese and lean]. In contrast, muPET showed that at 4 months of age, Ob U showed greater D2R availability than Le U rats but like ARG showed no differences between Ob R and Le R rats.

CONCLUSION

The lower D2R binding in Ob U than Le U rats observed with ARG most likely reflects decreases in striatal D2 receptors levels whereas the increased availability observed with muPET is likely to reflect reduced DA release (resulting in decreased competition with endogenous DA). Lack of a significant difference between Ob R and Le R suggests that the differences in dopamine activity and D2R levels between Ob and Le Zucker rats are modulated by access to food. The ARG finding of an attenuation of the age-related loss of D2R binding corroborates previous studies of the salutary effects of food restriction in the aging process. Because [11C] raclopride is sensitive to competition with endogenous DA, the higher D2R binding in obese rats with raclopride despite the lower D2R levels shown with spiperone could reflect lower extracellular DA in the Ob rats and merits further investigation.

Effect of common anesthetics on dendritic properties in layer 5 neocortical pyramidal neurons

Understanding the impact of active dendritic properties on network activity in vivo has so far been restricted to studies in anesthetized animals. However, to date no study has been made to determine the direct effect of the anesthetics themselves on dendritic properties. Here, we investigated the effects of three types of anesthetics commonly used for animal experiments (urethane, pentobarbital and ketamine/xylazine). We investigated the generation of calcium spikes, the propagation of action potentials (APs) along the apical dendrite and the somatic firing properties in the presence of anesthetics in vitro using dual somatodendritic whole cell recordings. Calcium spikes were evoked with dendritic current injection and high-frequency trains of APs at the soma. Surprisingly, we found that the direct actions of anesthetics on calcium spikes were very different. Two anesthetics (urethane and pentobarbital) suppressed dendritic calcium spikes in vitro, whereas a mixture of ketamine and xylazine enhanced them. Propagation of spikes along the dendrite was not significantly affected by any of the anesthetics but there were various changes in somatic firing properties that were highly dependent on the anesthetic. Last, we examined the effects of anesthetics on calcium spike initiation and duration in vivo using high-frequency trains of APs generated at the cell body. We found the same anesthetic-dependent direct effects in addition to an overall reduction in dendritic excitability in anesthetized rats with all three anesthetics compared with the slice preparation.

Characterization of interactions between phencyclidine and amphetamine in rodent prefrontal cortex and striatum: Implications in NMDA/glycine-site-mediated dopaminergic dysregulation and dopamine transporter function

N-Methyl-D-aspartate (NMDA) antagonists induced behavioral and neurochemical changes in rodents that serve as animal models of schizophrenia. Chronic phencyclidine (PCP, 15 mg/(kg day) for 3 weeks via Alzet osmotic pump) administration enhances the amphetamine (AMPH)-induced dopamine (DA) efflux in prefrontal cortex (PFC), similar to that observed in schizophrenia. NMDA/glycine-site agonists, such as glycine (GLY), administered via dietary supplementation, reverse the enhanced effect. The present study investigated mechanisms of glycine-induced reversal of PCP-induced stimulation of AMPH-induced DA release, using simultaneous measurement of DA and AMPH in brain microdialysate, as well as peripheral and tissue AMPH levels. PCP treatment, by itself, increased peripheral and central AMPH levels, presumably via interaction with hepatic enzymes (e.g. cytochrome P450 CYP2C11). GLY (16% diet) had no effect on peripheral AMPH levels in the presence of PCP. Nevertheless, GLY significantly reduced extracellular/tissue AMPH ratios in both PFC and striatum (STR), especially following PCP administration, suggesting a feedback mediated effect on the dopamine transporter. GLY also inhibited acute AMPH (5 mg/kg)-induced DA release in PFC, but not STR. These findings suggest that GLY may modulate DA release in brain by producing feedback regulation of dopamine transporter function, possibly via potentiation of NMDA-stimulated GABA release and presynaptic GABAB receptor activation. The present studies also demonstrate pharmacokinetic interaction between AMPH and PCP, which may be of both clinical and research relevance.

An animal model of emotional blunting in schizophrenia

Schizophrenia is often associated with emotional blunting—the diminished ability to respond to emotionally salient stimuli—particularly those stimuli representative of negative emotional states, such as fear. This disturbance may stem from dysfunction of the amygdala, a brain region involved in fear processing. The present article describes a novel animal model of emotional blunting in schizophrenia. This model involves interfering with normal fear processing (classical conditioning) in rats by means of acute ketamine administration. We confirm, in a series of experiments comprised of cFos staining, behavioral analysis and neurochemical determinations, that ketamine interferes with the behavioral expression of fear and with normal fear processing in the amygdala and related brain regions. We further show that the atypical antipsychotic drug clozapine, but not the typical antipsychotic haloperidol nor an experimental glutamate receptor 2/3 agonist, inhibits ketamine's effects and retains normal fear processing in the amygdala at a neurochemical level, despite the observation that fear-related behavior is still inhibited due to ketamine administration. Our results suggest that the relative resistance of emotional blunting to drug treatment may be partially due to an inability of conventional therapies to target the multiple anatomical and functional brain systems involved in emotional processing. A conceptual model reconciling our findings in terms of neurochemistry and behavior is postulated and discussed.

Acute dopamine D(1) and D(2) receptor stimulation does not modulate mismatch negativity (MMN) in healthy human subjects

RATIONALE

Schizophrenia is commonly associated with an impairment in pre-attentive change detection, as represented by reduced mismatch negativity (MMN), an auditory event related potential. While the neurochemical basis of MMN has been linked to the integrity of the glutamatergic system involving N-methyl-D-aspartate (NMDA) receptors, the role of the dopaminergic system and in particular, the role of D(1) and D(2) receptors on MMN is yet to be determined.

OBJECTIVES

The aim of the present project was to investigate the acute effects of dopamine D(2) (bromocriptine) and D(1)/D(2) (pergolide) receptor stimulation on the human MMN in healthy subjects.

METHODS

Fifteen healthy male subjects participated in a double-blind, placebo-controlled, cross-over design in which each subject was tested under three acute treatment conditions separated by a 1-week wash out period; placebo, bromocriptine (2.5 mg) and pergolide (0.1 mg). The subjects were exposed to a duration-MMN paradigm with 50 ms standard tones (91%) and 100 ms deviant tones (9%).

RESULTS

The results showed that neither D(2) receptor stimulation with bromocriptine, nor simultaneous D(1) and D(2) receptor stimulation with pergolide, modulated MMN.

CONCLUSIONS

These findings suggest that acute D(1) and D(2) receptor stimulation does not modulate MMN. While the role of dopamine cannot be completely ruled out, the findings support the view that the aberrant MMN reported in schizophrenia may be linked primarily to glutamate dysfunction involving NMDA receptors.

Baseline temperature in an animal model of schizophrenia: long-term effects of perinatal phencyclidine administration

Phencyclidine (PCP), a dissociative anaesthetic, acts as a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist. PCP is a psychostimulant capable of producing both positive and negative symptoms of schizophrenia, including cognitive dysfunction in normal humans. Perinatal phencyclidine administration to rats has been widely accepted as an animal model of schizophrenia. It has been known for a long time that schizophrenia patients may develop various thermoregulatory disturbances. The aim of this study was to assess the acute effects of phencyclidine administration on the temperature of newborn rats, the long-term effects on the baseline temperature of perinatal phencyclidine administration and the effects of a PCP challenge on the temperature of adult perinatally treated rats. The animals were treated on the 2nd, 6th, 9th and 12th postnatal (PN) days with either phencyclidine (10 mg/kg) or saline. The interscapular skin temperature was measured during the first 40 postnatal days and subsequently the colonic temperature until PN day 62. The immediate effect of phencyclidine administration to pups was a significant decrease of the body temperature, while the application of PCP to adult rats perinatally treated with either saline or PCP caused a significant increase of the baseline temperature. Perinatal phencyclidine administration to rat pups produced a long lasting effect on the baseline temperature. It can be concluded that the nature of the response to acute phencyclidine administration differs between newborn and adult rats. Further experiments are necessary in order to clarify the role of specific neurotransmitter systems in the changes of temperature regulation provoked by phencyclidine administration.

Ketamine: a controversial drug for neonates

Ketamine is widely used for anesthesia and analgesia in neonates and children. It provides potent sedation, analgesia, and amnesia, a short duration of action, supporting hemodynamic and respiratory stability. Noncompetitive antagonism of NMDA receptors produces its primary therapeutic effects, but it also alters receptor function at dopaminergic, serotonergic, cholinergic, and opioidergic sites. Recent interest in ketamine stems from its potential to block excitotoxic cell death, although concerns have been raised about anesthetic neurotoxicity in neonatal animal models. The development of ketamine, its clinical profile, toxic effects in the immature brain, and future applications in neonates and children are reviewed in this article.

Effects of chronic oral methylphenidate on cocaine self-administration and striatal dopamine D2 receptors in rodents

BACKGROUND

Methylphenidate (MP) and amphetamine, which are the mainstay for the treatment of ADHD, have raised concerns because of their reinforcing effects and the fear that their chronic use during childhood or adolescence could induce changes in the brain that could facilitate drug abuse in adulthood.

METHODS

Here we measured the effects of chronic treatment (8 months) with oral MP (1 or 2 mg/kg), which was initiated in periadolescent rats (postnatal day 30). Following this treatment, rats were tested on cocaine self-administration. In addition at 2 and 8 months of treatment we measured dopamine D2 receptor (D2R) availability in the striatum using [(11)C]raclopride microPET (microPET) imaging.

RESULTS

Animals treated for 8 months with 2 mg/kg of MP showed significantly reduced rates of cocaine self-administration at adulthood than vehicle treated rats. D2R availability in the striatum was significantly lower in rats after 2 months of treatment with MP (1 and 2 mg/kg) but significantly higher after 8 months of MP treatment than in the vehicle treated rats. In vehicle treated rats D2R availability decreased with age whereas it increased in rats treated with MP. Because low D2R levels in the striatum are associated with a propensity for self-administration of drugs both in laboratory animals and in humans, this effect could underlie the lower rates of cocaine self-administration observed in the rats given 8 months of treatment with MP.

CONCLUSIONS

Eight month treatment with oral MP beginning in adolescence decreased cocaine-self administration (1 mg/kg) during adulthood which could reflect the increases in D2R availability observed at this life stage since D2R increases are associated with reduced propensity for cocaine self administration. In contrast, two month treatment with MP started also at adolescence decreased D2R availability, which could raise concern that at this life stage short treatments could possibly increase vulnerability to drug abuse during adulthood. These findings indicate that MP effects on D2R expression in the striatum are sensitive not only to length of treatment but also to the developmental stage at which treatment is given. Future studies evaluating the effects of different lengths of treatment on drug self-administration are required to assess optimal duration of treatment regimes to minimize adverse effects on the propensity for drug self administration.

Acute S-ketamine application does not alter cerebral [18F]altanserin binding: a pilot PET study in humans

Modeling short-term psychotic states with subanaesthetic doses of ketamine provides substantial experimental evidence in support of the glutamate hypothesis of schizophrenia. Ketamine exerts its pharmacological effects both directly via interactions with glutamate receptors and indirectly by stimulating presynaptic release of endogenous serotonin (5-HT). The aim of this feasibility study was to examine whether acute ketamine-induced 5-HT release interferes with the binding of the 5-HT(2A) receptor (5-HT(2A)R) radioligand [(18)F]altanserin and positron emission tomography (PET). Two subjects treated with ketamine and one subject treated with placebo underwent [(18)F]altanserin PET at distribution equilibrium conditions. Robust physiological, psychopathological and cognitive effects were present at ketamine plasma concentrations exceeding 100 microg/l during >70 min. Notwithstanding, we observed stable radioligand binding (changes +/-95% CI of -1.0 +/- 1.6% and +4.1 +/- 1.8% versus -1.2 +/- 2.6%) in large cortical regions presenting high basal uptake of both, [(18)F]altanserin and ketamine. Marginal decreases of 4% of radioligand binding were observed in the frontal lobe, and 8% in a posteriorily specified frontomesial subregion. This finding is not compatible with a specific radioligand displacement from 5-HT(2A)R which should occur proportionally throughout the whole brain. Instead, the spatial pattern of these minor reductions was congruent with ketamine-induced increases in cerebral blood flow observed in a previous study using [(15)O]butanol PET. This may caused by accelerated clearance of unspecifically bound [(18)F]altanserin from cerebral tissue with increased perfusion. In conclusion, this study suggests that [(18)F]altanserin PET is not sensitive to acute neurotransmitter fluctuations under ketamine. Advantageously, the stability of [(18)F]altanserin PET towards acute influences is a prerequisite for its future use to detect sub-acute and chronic effects of ketamine.

Glutamate and dopamine dysregulation in schizophrenia--a synthesis and selective review

The dopamine hypothesis of schizophrenia is the principal explanatory model of antipsychotic drug action. Recent discoveries extend our understanding of the neurochemistry of schizophrenia, with increasing evidence of dysfunction in glutamate and GABA as well as dopamine systems. In this review, we study the evidence for dopaminergic dysfunction in schizophrenia, drawing data from neurochemical imaging studies. We also review the NMDA receptor hypofunction hypothesis of schizophrenia as a supplementary explanatory model for the illness. We examine predictions made by the NMDA receptor hypofunction hypothesis and consider how they fit with current neurochemical findings in patients and animal models. We consider the case for glutamatergic excitotoxicity as a key process in the development and progression of schizophrenia, and suggest ways in which glutamate and dopamine dysregulation may interact in the condition.

Novel pharmacotherapy for fibromyalgia

Fibromyalgia is a common disorder that is characterized by chronic widespread pain, tenderness to light palpation, fatigue and sleep disturbances. The present lack of a well-accepted model of the disorder has hampered progress towards adequate treatment. A review of potential models to explain the pathophysiology underlying its primary symptom (i.e., chronic widespread pain) lends insight on the therapeutic potential of novel therapies. Following this, a mechanistic evaluation of those medications that are under consideration for the treatment of the disorder is offered. Adequate treatment will be likely to involve the identification of biologic subgroups within the greater fibromyalgia construct. Key insights from basic research are the basis for increased optimism for effective relief among patients and clinicians.

Do NMDA receptor antagonist models of schizophrenia predict the clinical efficacy of antipsychotic drugs?

N-methyl-D-aspartate (NMDA) receptor antagonists, such as ketamine and phencyclidine, induce perceptual abnormalities, psychosis-like symptoms, and mood changes in healthy humans and patients with schizophrenia. The similarity between NMDA receptor antagonist-induced psychosis and schizophrenia has led to the widespread use of the drugs to provide models to aid the development of novel treatments for the disorder. This review investigates the predictive validity of NMDA receptor antagonist models based on a range of novel treatments that have now reached clinical trials. Furthermore, it considers the extent to which the different hypotheses that have been proposed to account for the psychotomimetic effects of NMDA receptor antagonist have been validated by the results of these trials. Finally, the review discusses some of the caveats associated with use of the models and some suggestions as to how a greater use of translational markers might ensure progress in understanding the relationship between the models and schizophrenia.

Low dose ketamine: a therapeutic and research tool to explore N-methyl-D-aspartate (NMDA) receptor-mediated plasticity in pain pathways

Ketamine is a dissociative anaesthetic that has been used in the clinic for many years. At low, sub-anaesthetic doses, it is a relatively selective and potent antagonist of the N-methyl-D-aspartate (NMDA) receptor. It belongs to the class of uncompetitive antagonists and blocks the receptor by binding to a specific site within the NMDA receptor channel when it is open. Like other compounds of this class, ketamine can cause hallucinations or other untoward central effects which limit its use in the clinic. Nevertheless, because of the evidence on the importance of NMDA receptor-mediated pLasticity in chronic pain, low doses of ketamine have been explored in a wide range of pain conditions. The majority of studies with ketamine have shown efficacy; however, it has not been possible to separate safely the pain relief from the side effects of the drug. Hence, clinical use of ketamine as a pain treatment is very limited. Nevertheless, ketamine has served as a useful tool to provide a compelling rationale for developing other NMDA antagonists. Some of the new compounds of this class, particularly those acting at the NR2B subtype of the NMDA receptor, have shown promise in preclinical and clinical studies.

Imaging of striatal dopamine release elicited with NMDA antagonists: is there anything there to be seen?

Increased release of striatal dopamine, indexed with in vivo imaging of the D(2) receptor, has been reported following an acute challenge with N-methyl-D-aspartate (NMDA) antagonist ketamine in humans. Replications of this result have not been consistently successful. The aim of this manuscript is to evaluate in vivo imaging examination of NMDA antagonism on striataL dopamine release in published reports of humans and pre-clinical species. The literature is evaluated in conjunction with insights on the effect of NMDA antagonism on dopamine release, elicited from microdialysis and tissue turnover studies, and suggestions for future studies are made.

Gating information by two-state membrane potential fluctuations

Two-state voltage fluctuations between a hyperpolarized down-state and a depolarized up-state have been observed experimentally in a wide variety of neurons across brain regions. Using a biophysical model, we show that synaptic input by NMDA receptors can cause such membrane potential fluctuations. In this model, when a neuron is driven by two input pathways with different AMPA/NMDA receptor content, the NMDA-rich input causes up-state transitions, whereas the AMPA-rich input generates spikes only in the up-state. Therefore the NMDA-rich pathway can gate input from an AMPA pathway in an all-or-none fashion by switching between different membrane potential states. Furthermore, once in the up-state, the NMDA-rich pathway multiplicatively increases the gain of a neuron responding to AMPA-rich input. This proposed mechanism for two-state fluctuations directly suggests specific computations, such as gating and gain modulation based on the distinct receptor composition of different neuronal pathways. The dynamic gating of input by up- and down-states may be an elementary operation for the selective routing of signals in neural circuits, which may explain the ubiquity of two-state fluctuations across brain regions.

Ketamine dose-dependently induces high-frequency oscillations in the nucleus accumbens in freely moving rats

BACKGROUND

In humans, subanesthetic doses of ketamine and recovery from ketamine anesthesia are associated with psychotic-like behavior. In rodents, ketamine produces hyperactivity, stereotypies, and abnormal social interaction used to model certain features of schizophrenia. Increasing evidence has implicated aberrant activity in the nucleus accumbens (NAc) with the pathophysiology of schizophrenia.

METHODS

Here, we examined the effect of an IP injection of ketamine (10, 25, 50, and 200 mg/kg) and d-amphetamine (3 mg/kg) on local field potentials in the rodent NAc. Locomotor activity was recorded simultaneously.

RESULTS

Spontaneous high-frequency oscillations (HFO) (140-180 Hz) were present in local field potentials recorded from the NAc. Ketamine dose-dependently induced rapid and substantial increases in HFO that correlated with behavioral hyperactivity. Similarly, large increases in HFO occurred during recovery from ketamine anesthesia. In contrast, d-amphetamine, which induced locomotor activity, produced only small increases in HFO.

CONCLUSIONS

We propose that ketamine-induced abnormal increases in HFO form part of the complex neurological changes in this model of schizophrenia. Ketamine-induced increases in HFO, although sharing similar temporal dynamics to hyperactivity, may not be functionally related to increased movement.

Dopaminergic control of sleep-wake states

Dopamine depletion is involved in the pathophysiology of Parkinson's disease, whereas hyperdopaminergia may play a fundamental role in generating endophenotypes associated with schizophrenia. Sleep disturbances are known to occur in both schizophrenia and Parkinson's disease, suggesting that dopamine plays a role in regulating the sleep-wake cycle. Here, we show that novelty-exposed hyperdopaminergic mice enter a novel awake state characterized by spectral patterns of hippocampal local field potentials that resemble electrophysiological activity observed during rapid-eye-movement (REM) sleep. Treatment with haloperidol, a D2 dopamine receptor antagonist, reduces this abnormal intrusion of REM-like activity during wakefulness. Conversely, mice acutely depleted of dopamine enter a different novel awake state characterized by spectral patterns of hippocampal local field potentials that resemble electrophysiological activity observed during slow-wave sleep (SWS). This dopamine-depleted state is marked by an apparent suppression of SWS and a complete suppression of REM sleep. Treatment with D2 (but not D1) dopamine receptor agonists recovers REM sleep in these mice. Altogether, these results indicate that dopamine regulates the generation of sleep-wake states. We propose that psychosis and the sleep disturbances experienced by Parkinsonian patients result from dopamine-mediated disturbances of REM sleep.

Reduced presynaptic dopamine activity in fibromyalgia syndrome demonstrated with positron emission tomography: a pilot study

Although the pathophysiology underlying the pain of fibromyalgia syndrome (FMS) remains unknown, a variety of clinical and investigational findings suggests a dysregulation of dopaminergic neurotransmission. We therefore investigated presynaptic dopaminergic function in 6 female FMS patients in comparison to 8 age- and gender-matched controls as assessed by positron emission tomography with 6-[(18)F]fluoro-L-DOPA as a tracer. Semiquantitative analysis revealed reductions in 6-[(18)F]fluoro-L-DOPA uptake in several brain regions, indicating a disruption of presynaptic dopamine activity wherein dopamine plays a putative role in natural analgesia. Although the small sample size makes these findings preliminary, it appears that FMS might be characterized by a disruption of dopaminergic neurotransmission.

PERSPECTIVE

An association between FMS and reduced dopamine metabolism within the pain neuromatrix provides important insights into the pathophysiology of this mysterious disorder.

Dopamine D2 receptor-mediated G protein activation assessed by agonist-stimulated [35S]guanosine 5'-O-(gamma-thiotriphosphate) binding in rat striatal membranes

In order to investigate the functional interaction between the native dopamine receptors and their coupled guanine nucleotide-binding regulatory (G) proteins, dopamine-stimulated [(35)S]guanosine 5'-O-(gamma-thiotriphosphate) ([(35)S]GTPgammaS) binding was pharmacologically characterized in rat striatal membranes. Following optimizing the experimental conditions as to the concentrations of GDP, MgCl(2) and NaCl in the assay medium, the agonist and antagonist properties for a series of dopamine receptor ligands were determined mainly under the standard assay condition. The pharmacological profile of this response clearly indicated the involvement of dopamine D(2)-like receptors, but not of dopamine D(1)-like receptors. Among the types of dopamine D(2)-like receptors, dopamine D(2) receptors most likely appeared to be involved in dopamine-stimulated [(35)S]GTPgammaS binding in rat striatal membranes, because the affinities of agonists and antagonists determined in the present study were significantly correlated with those reported in the previous literature only for dopamine D(2) receptors, but not for dopamine D(3) or D(4) types. Though the concentration-dependent inhibition curves of dopamine-stimulated [(35)S]GTPgammaS binding by spiperone and S(-)-raclopride were apparently biphasic, the origin of the low-affinity minor components was not fully determined. The antiparkinsonian drugs with the properties of dopamine receptor agonism were shown to behave as stimulants with varied affinities and relative efficacies in the current assay system. On the other hand, neither phencyclidine (PCP) nor ketamine stimulated the specific [(35)S]GTPgammaS binding, in contrast with the previous report demonstrating that these two N-methyl-D-aspartic acid (NMDA) receptor antagonists behaved as agonists at human dopamine D(2) receptors expressed in Chinese hamster ovary (CHO) cells. These results provide important information about the functional activation of G proteins coupled with dopamine D(2) receptors as well as agonist actions of various compounds at native dopamine D(2) receptors, which are potentially involved in pathophysiology and pharmacotherapy of neuropsychiatric diseases such as Parkinson's disease, schizophrenia and depression.

Psychosis pathways converge via D2high dopamine receptors

The objective of this review is to identify a target or biomarker of altered neurochemical sensitivity that is common to the many animal models of human psychoses associated with street drugs, brain injury, steroid use, birth injury, and gene alterations. Psychosis in humans can be caused by amphetamine, phencyclidine, steroids, ethanol, and brain lesions such as hippocampal, cortical, and entorhinal lesions. Strikingly, all of these drugs and lesions in rats lead to dopamine supersensitivity and increase the high-affinity states of dopamine D2 receptors, or D2High, by 200-400% in striata. Similar supersensitivity and D2High elevations occur in rats born by Caesarian section and in rats treated with corticosterone or antipsychotics such as reserpine, risperidone, haloperidol, olanzapine, quetiapine, and clozapine, with the latter two inducing elevated D2High states less than that caused by haloperidol or olanzapine. Mice born with gene knockouts of some possible schizophrenia susceptibility genes are dopamine supersensitive, and their striata reveal markedly elevated D2High states; suchgenes include dopamine-beta-hydroxylase, dopamine D4 receptors, G protein receptor kinase 6, tyrosine hydroxylase, catechol-O-methyltransferase, the trace amine-1 receptor, regulator of G protein signaling RGS9, and the RIIbeta form of cAMP-dependent protein kinase (PKA). Striata from mice that are not dopamine supersensitive did not reveal elevated D2High states; these include mice with knockouts of adenosine A2A receptors, glycogen synthase kinase GSK3beta, metabotropic glutamate receptor 5, dopamine D1 or D3 receptors, histamine H1, H2, or H3 receptors, and rats treated with ketanserin or aD1 antagonist. The evidence suggests that there are multiple pathways that convergetoelevate the D2High state in brain regions and that this elevation may elicit psychosis. This proposition is supported by the dopamine supersensitivity that is a common feature of schizophrenia and that also occurs in many types of genetically altered, drug-altered, and lesion-altered animals. Dopamine supersensitivity, in turn, correlates with D2High states. The finding that all antipsychotics, traditional and recent ones, act on D2High dopamine receptors further supports the proposition.

Subchronic administration of LY354740 does not modify ketamine-evoked behavior and neuronal activity in rats

Acute treatment with LY354740 {1S,2S,5R,6S-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylate monohydrate}, a potent and selective agonist for group II metabotropic glutamate receptors (mGlu2/3), has previously been shown to block some schizophrenia-like effects of N-methyl-D-aspartate (NMDA) receptor antagonists, suggesting a novel therapeutic strategy for schizophrenia. The present study examined the effects of subchronic pretreatment with LY354740 (0.3, 3 and 10 mg/kg i.p.) on ketamine-evoked (12 mg/kg s.c.) prepulse inhibition deficits, hyperlocomotion and c-fos expression. At all doses, LY354740 failed to reverse both behavioral and neuronal effects of the ketamine. These results therefore do not support the putative antipsychotic role of LY354740.

Effects of phencyclidine on spatial learning and memory: Nitric oxide-dependent mechanisms

Cognitive deficits of schizophrenia constitute a disabling part of the disease predicting treatment success as well as functional outcome. Phencyclidine (PCP), a non-competitive NMDA receptor antagonist was used to model schizophrenic cognitive dysfunctions of learning and memory using the Morris water maze paradigm for reference memory. In experiment 1 male Sprauge-Dawley rats were acutely administered PCP (0.5, 1.0 and 2.0 mg/kg s.c.) before the first swim session on each of the four acquisition days. Probe test for reference memory was performed 2 days after the last acquisition day; the first probe without drug treatment to assess reference memory and a second probe with prior drug treatment to control for state dependency effects of PCP. In experiment 2 the effects of pre-treatment (10 min before PCP) with the nitric oxide synthase inhibitor, L-NAME (10 mg/kg s.c.), on the PCP (2 mg/kg)-induced spatial memory deficit was evaluated in the Morris water maze paradigm for reference memory. The results showed that PCP in a dose of 2 mg/kg disrupts spatial learning as estimated by prolonged search time to find platform during acquisition as well as the reference memory test as measured by less time spent in target quadrant during probe trial. No state dependency effects of PCP were found. Pre-treatment with L-NAME completely reversed the PCP-induced disruption of acquisition learning. The reference memory disruption was, however, not completely restored as measured by probe trial.

In vitro biochemical evidence that the psychotomimetics phencyclidine, ketamine and dizocilpine (MK-801) are inactive at cloned human and rat dopamine D2 receptors

Dopamine potently increased calcium mobilization in Chinese hamster ovary cells expressing human dopamine D2Long receptors (CHO-D2L cells), and increased guanosine-5'-O-(3-[35S]thio)-triphosphate binding to CHO-D2L cell and rat striatal membranes. These effects of dopamine were blocked by the dopamine D2 receptor antagonist (-)raclopride. In contrast to the findings of a recent controversial study, phencyclidine, ketamine and dizocilpine (MK-801) lacked dopamine D2 receptor full agonist, partial agonist and antagonist activity in these assays, suggesting their psychotomimetic effects, and activity in rodent models of schizophrenia, are associated with N-methyl-d-aspartate receptor blockade rather than a direct interaction with dopamine D2 receptors.

Mapping the central effects of ketamine in the rat using pharmacological MRI

RATIONALE

Ketamine induces, in both humans and rodents, behaviours analogous to some of the symptoms of schizophrenia.

OBJECTIVES

To utilise pharmacological magnetic resonance imaging (phMRI) techniques that identify changes in blood-oxygenation-level-dependent (BOLD) contrast to determine the temporal and spatial neuronal activation profile of ketamine in the rat brain.

METHOD

To obtain a pharmacodynamic profile of the drug, we assessed changes in locomotor activity after vehicle and 10 and 25 mg/kg ketamine. Separate animals were then anaesthetised and placed in a 4.7-T magnetic resonance (MR) system before receiving the same doses of ketamine during serial MR image acquisition. Subsequent statistical parametric mapping of the main effect of the drug was then undertaken to identify changes in BOLD contrast. Levels of gamma-aminobutyric acid (GABA) and dopamine (DA) in brain areas showing localised changes in BOLD contrast were then assessed via microdialysis.

RESULTS

Both doses of ketamine produced increases in BOLD image contrast in frontal, hippocampal, cortical and limbic areas. A further investigation of the release of DA and its metabolites in the nucleus accumbens, both in anaesthesised and freely moving rats, corroborated these findings. However, an investigation of GABA and DA levels in the ventral pallidum gave no indication of changes in activity.

CONCLUSIONS

Ketamine produced localised dose-dependent alterations in BOLD MR signal, which correlate with the pharmacodynamic profile of the drug. These results can be, at least, partially substantiated with complementary techniques but consideration must be given to the input function applied to the MR signal and the use of anaesthesia during phMRI experimentation.

Ketamine enhances the expression of serine racemase and D-amino acid oxidase mRNAs in rat brain

We have evaluated the effects of the acute administration of noncompetitive N-methyl-D-aspartate receptor antagonist, ketamine, on the expression of serine racemase and D-amino acid oxidase mRNAs in several brain areas of rats. The ketamine administration produced a dose-dependent and transient elevation in the levels of serine racemase and D-amino acid oxidase mRNAs in all the brain areas. These findings suggest that there is a relationship between the gene expression of the d-serine-related enzymes and the blockade of the N-methyl-D-aspartate receptors.

Comparison of psychic emergence reactions after (±)-ketamine and (+)-ketamine in mice

Ketamine is a racemic mixture containing equal parts of (+)-ketamine and (-)-ketamine. The ketamine enantiomorphs are different in anesthesia and psychic emergence reactions after anesthesia. Therefore, (+)-ketamine was compared with racemic ketamine in a number of randomized studies in volunteers and patients. However, their relations remain controversial. In the present studies, the psychic emergence reactions after injection of (+/-)-ketamine and (+)-ketamine were compared in mice. At equimolar doses, the (+)-isomers elicited episodes of hypnosis nearly 1.4-fold more potent than those of the racemic ketamine. After the administration of equihypnotic doses of (+)-ketamine and (+/-)-ketamine, the posthypnotic stimulation of locomotor activity, stereotype behavior and 5-HT-induced head-twitch response by the (+)-enantiomorph was significantly less intense than that of racemic ketamine. In receptor binding test, (+)-ketamine showed a higher affinity for NMDA receptor than that of (+/-)-ketamine, while (+)-ketamine and (+/-)-ketamine showed no affinity for dopamine D2 and serotonin 5-HT2 receptor. These results suggest that the (+)-ketamine has fewer posthypnotic side effects than (+/-)-ketamine when (+)-ketamine and (+/-)-ketamine were administered at equihypnotic dosages and that dopamine D2 and serotonin 5-HT2 receptor were not involved in the effects of (+)-ketamine and (+/-)-ketamine.

The role of the extrapersonal brain systems in religious activity

The neuropsychology of religious activity in normal and selected clinical populations is reviewed. Religious activity includes beliefs, experiences, and practice. Neuropsychological and functional imaging findings, many of which have derived from studies of experienced meditators, point to a ventral cortical axis for religious behavior, involving primarily the ventromedial temporal and frontal regions. Neuropharmacological studies generally point to dopaminergic activation as the leading neurochemical feature associated with religious activity. The ventral dopaminergic pathways involved in religious behavior most closely align with the action-extrapersonal system in the model of 3-D perceptual-motor interactions proposed by . These pathways are biased toward distant (especially upper) space and also mediate related extrapersonally dominated brain functions such as dreaming and hallucinations. Hyperreligiosity is a major feature of mania, obsessive-compulsive disorder, schizophrenia, temporal-lobe epilepsy and related disorders, in which the ventromedial dopaminergic systems are highly activated and exaggerated attentional or goal-directed behavior toward extrapersonal space occurs. The evolution of religion is linked to an expansion of dopaminergic systems in humans, brought about by changes in diet and other physiological influences.

Effects of phencyclidine on acoustic startle and prepulse inhibition in neuronal nitric oxide synthase deficient mice

Prepulse inhibition of acoustic startle is a behavioural response, which is used to estimate sensorimotor gating deficits in schizophrenia. Recent studies show that several behavioural effects of the psychotomimetic drug, phencyclidine (PCP), in rodents are blocked by nitric oxide synthase (NOS) inhibitors suggesting that NO plays an important role in the pharmacological effects of PCP. The present study was conducted to examine the effects of PCP on prepulse inhibition in neuronal NOS (nNOS) deficient mice. PCP treatment caused a significant and dose-related increase in prepulse inhibition in nNOS-/- mice whereas prepulse inhibition was not significantly affected in +/+ and +/- mice. Basal prepulse inhibition level did not differ significantly between the groups. Furthermore, PCP caused a dose-related decrease in startle response reactivity in +/+ mice but did not significantly affect this measure in +/- and -/- mice. Basal startle response level did not differ between +/+ and +/- but was significantly lower in -/- mice. It is concluded that nNOS plays a role in the NO-sensitive effects of PCP.

Dopamine receptor contribution to the action of PCP, LSD and ketamine psychotomimetics

Although phencyclidine and ketamine are used to model a hypoglutamate theory of schizophrenia, their selectivity for NMDA receptors has been questioned. To determine the affinities of phencyclidine, ketamine, dizocilpine and LSD for the functional high-affinity state of the dopamine D2 receptor, D2High, their dissociation constants (Ki) were obtained on [3H]domperidone binding to human cloned dopamine D2 receptors. Phencyclidine had a high affinity for D2High with a Ki of 2.7 nM, in contrast to its low affinity for the NMDA receptor, with a Ki of 313 nM, as labeled by [3H]dizocilpine on rat striatal tissue. Ketamine also had a high affinity for D2High with a Ki of 55 nM, an affinity higher than its 3100 nM Ki for the NMDA sites. Dizocilpine had a Ki of 0.3 nM at D2High, but a Kd of 1.8 nM at the NMDA receptor. LSD had a Ki of 2 nM at D2High. Because the psychotomimetics had higher potency at D2High than at the NMDA site, the psychotomimetic action of these drugs must have a major contribution from D2 agonism. Because these drugs have a combined action on both dopamine receptors and NMDA receptors, these drugs, when given in vivo, test a combined hyperdopamine and hypoglutamate theory of psychosis.

Ketamine induces dopamine-dependent depression of evoked hippocampal activity in the nucleus accumbens in freely moving rats

Noncompetitive NMDA receptor antagonists, such as ketamine, induce a transient schizophrenia-like state in healthy individuals and exacerbate psychosis in schizophrenic patients. In rodents, noncompetitive NMDA receptor antagonists induce a behavioral syndrome that represents an experimentally valid model of schizophrenia. Current experimental evidence has implicated the nucleus accumbens in the pathophysiology of schizophrenia and the psychomimetic actions of ketamine. In this study, we have demonstrated that acute systemic administration of ketamine, at a dose known to produce hyperlocomotion and stereotypy, depressed the amplitude of the monosynaptic component of fimbria-evoked field potentials recorded in the nucleus accumbens. A similar effect was observed using the more selective antagonist dizocilpine maleate, indicating the depression was NMDA receptor dependent. Paired-pulse facilitation was enhanced concomitantly with, and in proportion to, ketamine-induced depressed synaptic efficacy, indicative of a presynaptic mechanism of action. Notably, the depression of field potentials recorded in the nucleus accumbens was markedly reduced after a focal 6-hydroxydopamine lesioning procedure in the nucleus accumbens. More specifically, pretreatment with the D2/D4 antagonist haloperidol, but not the D1 antagonist SCH23390 blocked ketamine-induced depression of nucleus accumbens responses. Our findings provide supporting evidence for the contemporary theory of schizophrenia as aberrant excitatory neurotransmission at the level of the nucleus accumbens.

Activation of a nitric-oxide-sensitive cAMP pathway with phencyclidine: elevated hippocampal cAMP levels are temporally associated with deficits in prepulse inhibition

RATIONALE

Schizophrenic patients show deficits in pre-attentive information processing as evidenced, for example, by disrupted prepulse inhibition, a measure of sensorimotor gating. A similar disruption can be observed in animals treated with the psychotomimetic agent, phencyclidine (PCP). However, the mechanism by which PCP alters brain function has not been fully elucidated. Recent studies have demonstrated that certain behavioural and neurochemical effects of PCP in rats and mice are blocked by nitric oxide (NO) synthase inhibition, suggesting an important role for NO in the effects of PCP.

OBJECTIVE

The aim of the present study was to investigate the effects of PCP on cAMP production in the ventral hippocampus and the role of NO in these effects using in vivo microdialysis in rats. Furthermore, the effects of PCP on acoustic startle reactivity and prepulse inhibition of acoustic startle were compared with changes in cAMP levels in the ventral hippocampus.

RESULTS

Significant increases in cAMP levels were observed in the ventral hippocampus following both local infusion (10(-4) mol/l and 10(-3) mol/l) and systemic administration (2 mg/kg) of PCP. The PCP-induced changes in prepulse inhibition and startle reactivity were associated in magnitude and duration with the increase in cAMP levels in the hippocampus. Furthermore, systemic administration of the NO synthase inhibitor, L: -NAME (10 mg/kg), blocked both the changes in cAMP levels and the behavioural responses induced by PCP.

CONCLUSIONS

These findings indicate that the effects of PCP on prepulse inhibition and startle reactivity are associated with an increase in cAMP levels in the ventral hippocampus, and that this change in cAMP response may be linked to the production of NO.

Selective interaction of nitric oxide synthase inhibition with phencyclidine: behavioural and NMDA receptor binding studies in the rat

The psychotomimetic drugs, phencyclidine (PCP) and MK-801, are non-competitive antagonists of the N-methyl-d-aspartate (NMDA) receptor and used as pharmacological tools to mimic a possible NMDA receptor hypofunction in schizophrenia. These drugs were tested in two behavioural paradigms in the present study: prepulse inhibition (PPI) of acoustic startle and locomotor activity (LMA) in an open field. Recent studies show that several behavioural and biochemical effects of PCP are blocked by nitric oxide synthase (NOS) inhibition. Hence, it is likely that some effects of PCP are mediated via an increase in NO production, an assumption not in accordance with the NMDA receptor antagonistic effect of PCP. Experiments were conducted in rats to further elucidate the involvement of NO-dependent mechanisms in the effects of PCP and MK-801, and how these effects may involve the NMDA receptor. The NOS inhibitor N(G)-nitro-l-arginine methyl ester (L-NAME) (10 mg/kg) normalised the disruptive effect of PCP (2 mg/kg) on PPI and the stimulatory effect of PCP (4 mg/kg) on LMA. In contrast to these observations, the deficit in PPI induced by MK-801 (0.1 mg/kg) was not affected by L-NAME (10, 20 or 40 mg/kg). MK-801 (0.15 mg/kg)-induced hyperlocomotion was not affected by L-NAME (10 mg/kg), but attenuated by L-NAME (40 mg/kg). Furthermore, receptor binding studies aimed at investigating the influence of L-NAME on the binding of PCP to the MK-801-sensitive NMDA receptor binding site failed to show such an influence. These results suggest that the NO-sensitive effects of PCP are not sufficiently explained by its antagonistic effect at the NMDA receptor channel complex.

N-Methyl-D-aspartate receptors as a target for improved antipsychotic agents: novel insights and clinical perspectives

RATIONALE

Activation of "co-agonist" N-methyl-D-aspartate (NMDA) and Glycine(B) sites is mandatory for the operation of NMDA receptors, which play an important role in the control of mood, cognition and motor function.

OBJECTIVES

This article outlines the complex regulation of activity at Glycine(B)/NMDA receptors by multiple classes of endogenous ligand. It also summarizes the evidence that a hypoactivity of Glycine(B)/NMDA receptors contributes to the pathogenesis of psychotic states, and that drugs which enhance activity at these sites may possess antipsychotic properties.

RESULTS

Polymorphisms in several genes known to interact with NMDA receptors are related to an altered risk for schizophrenia, and psychotic patients display changes in levels of mRNA encoding NMDA receptors, including the NR1 subunit on which Glycine(B) sites are located. Schizophrenia is also associated with an overall decrease in activity of endogenous agonists at Glycine(B)/NMDA sites, whereas levels of endogenous antagonists are elevated. NMDA receptor "open channel blockers," such as phencyclidine, are psychotomimetic in man and in rodents, and antipsychotic agents attenuate certain of their effects. Moreover, mice with genetically invalidated Glycine(B)/NMDA receptors reveal similar changes in behaviour. Finally, in initial clinical studies, Glycine(B) agonists and inhibitors of glycine reuptake have been found to potentiate the ability of "conventional" antipsychotics to improve negative and, albeit modestly, cognitive and positive symptoms. In contrast, therapeutic effects of clozapine are not reinforced, likely since clozapine itself enhances activity at NMDA receptors.

CONCLUSIONS

Reduced activity at NMDA receptors is implicated in the aetiology of schizophrenia. Correspondingly, drugs that (directly or indirectly) increase activity at Glycine(B) sites may be of use as adjuncts to other classes of antipsychotic agent. However, there is an urgent need for broader clinical evaluation of this possibility, and, to date, there is no evidence that stimulation of Glycine(B) sites alone improves psychotic states.

NMDA hypofunction in the posterior cingulate as a model for schizophrenia: an exploratory ketamine administration study in fMRI

BACKGROUND

Based on animal data, NMDA receptor hypofunction has been suggested as a model for positive symptoms in schizophrenia. NMDA receptor hypofunction affects several corticolimbic brain regions, of which the posterior cingulate seems to be the most sensitive. However, empirical support for a crucial role of posterior cingulate NMDA hypofunction in the pathophysiology of positive symptoms is still missing in humans. We therefore conducted an fMRI study using the NMDA antagonist ketamine in healthy human subjects during episodic memory retrieval, which is supposed to activate the posterior cingulate.

METHODS

We investigated 16 healthy subjects which were assigned to either placebo (n = 7; saline) or ketamine (n = 9; 0.6 mg/kg/h) group in a double-blind study design. All subjects received their infusion while performing an episodic memory retrieval task in the scanner. Immediately after the fMRI session, psychopathological effects of ketamine were measured using the Altered States of Consciousness Questionnaire.

RESULTS

The placebo group showed BOLD signal increases in the posterior and anterior cingulate during retrieval. Signal increases were significantly lower in the ketamine group. Lower signal increases in the posterior cingulate correlated significantly with positive (i.e. psychosis-like) symptoms induced by ketamine.

CONCLUSION

The present study for the first time demonstrates a relationship between NMDA receptors, posterior cingulate and positive (i.e. psychosis-like) symptoms in humans. Confirming findings from animal studies, it supports the hypothesis of a pathophysiological role of NMDA receptor hypofunction in the posterior cingulate in schizophrenia.

Dopamine agonist action of phencyclidine

Although the psychotomimetic action of phencyclidine is often used to model a hypoglutamate theory of psychosis or schizophrenia, work also exists showing that phencyclidine has a significant affinity for the dopamine D2 receptor. The present study was done to determine whether phencyclidine has a direct functional dopamine-like action on cells. The effect of phencyclidine was tested on the release of prolactin from rat cultured anterior pituitary cells. It was found that the release of prolactin was 50% inhibited by 4 nM phencyclidine. This strong dopamine-like agonist action at the functional high-affinity state of the dopamine D2 receptor by the phencyclidine psychotomimetic is consistent with the dopamine hypothesis of psychosis.

The 5-HT receptor antagonist M100,907 prevents extracellular glutamate rising in response to NMDA receptor blockade in the mPFC

We recently found that intracortical injection of the selective and competitive N-methyl-D-aspartate (NMDA) receptor antagonist 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid (CPP) impaired attentional performance in rats and blockade of 5-hydroxytryptamine (5-HT)2A receptors antagonized this effect. Here, we used the microdialysis technique in conscious rats to study the effect of CPP on extracellular glutamate (GLU) in the medial prefrontal cortex (mPFC) and the regulation of this effect by 5-HT2A receptors. Intraperitoneal injection of 20 mg/kg CPP increased extracellular GLU in the mPFC (201% of basal levels) but had no effect on 5-HT. Intracortical infusion of 100 microm CPP increased extracellular GLU (230% of basal values) and 5-HT (150% of basal values) in the mPFC, whereas 30 microm had no significant effect. The effect of 100 microm CPP on extracellular GLU was abolished by tetrodotoxin, suggesting that neuronal activity is required. Subcutaneous injection of 40 microg/kg M100,907 completely antagonized the effect of 100 microm cpp on extracellular GLU, whereas 10 microg/kg caused only partial attenuation. Likewise, intracortical infusion of 0.1 microm M100,907 completely reversed the increase of extracellular GLU induced by CPP. These findings show that blockade of NMDA receptors in the mPFC is sufficient to increase extracellular GLU locally. The increase of cortical extracellular GLU may contribute to CPP-induced cognitive deficits and blockade of 5-HT2A receptors may provide a molecular mechanism for reversing these deficits caused by dysfunctional glutamatergic transmission in the mPFC.

Glutamate as a therapeutic target in psychiatric disorders

Glutamate is the primary excitatory neurotransmitter in the mammalian brain. Glutamatergic neurotransmission may be modulated at multiple levels, only a minority of which are currently being exploited for pharmaceutical development. Ionotropic receptors for glutamate are divided into N-methyl-D-aspartate receptor (NMDAR) and AMPA receptor subtypes. NMDAR have been implicated in the pathophysiology of schizophrenia. The glycine modulatory site of the NMDAR is currently a favored therapeutic target, with several modulatory agents currently undergoing clinical development. Of these, the full agonists glycine and D-serine have both shown to induce significant, large effect size reductions in persistent negative and cognitive symptoms when added to traditional or newer atypical antipsychotics in double-blind, placebo-controlled clinical studies. Glycine (GLYT1) and small neutral amino-acid (SNAT) transporters, which regulate glycine levels, represent additional targets for drug development, and may represent a site of action of clozapine. Brain transporters for D-serine have recently been described. Metabotropic glutamate receptors are positively (Group I) or negatively (Groups II and III) coupled to glutamatergic neurotransmission. Metabotropic modulators are currently under preclinical development for neuropsychiatric conditions, including schizophrenia, depression and anxiety disorders. Other conditions for which glutamate modulators may prove effective include stroke, epilepsy, Alzheimer disease and PTSD.

Habituation of acoustic startle is disrupted by psychotomimetic drugs: differential dependence on dopaminergic and nitric oxide modulatory mechanisms

RATIONALE

A deficit in attention and information processing has been considered a central feature in schizophrenia, which might lead to stimulus overload and cognitive fragmentation. It has been shown that patients with schizophrenia display a relative inability to gate incoming stimuli. Thus, patients repeatedly subjected to acoustic startle-eliciting stimuli habituate less to these stimuli than controls. Furthermore, schizophrenia-like symptoms can be induced by pharmacological manipulations in humans by psychotomimetic drugs, e.g. phencyclidine (PCP) and D-amphetamine (D-AMP). Recent studies show that the behavioural and biochemical effects of PCP in rodents are blocked by nitric oxide synthase (NOS) inhibitors, suggesting that NO plays an important role in at least the pharmacological effects of PCP.

OBJECTIVES

The first aim of the present study was to investigate if PCP, MK-801 and D-AMP impair habituation of acoustic startle in mice. Secondly, we examine the effect of the NOS inhibitor, L-NAME, and the dopamine receptor antagonist, haloperidol, on drug-induced deficit in habituation.

RESULTS

PCP (4 mg/kg), MK-801 (0.4 mg/kg) and D-AMP (5.0 mg/kg), impaired habituation of the acoustic startle response in mice. This effect was reversed by the NOS inhibitor, L-NAME. The typical antipsychotic, haloperidol, reversed the effects of PCP and D-AMP, but not that of MK-801.

CONCLUSIONS

The finding that PCP, MK-801 and D-AMP impair habituation in mice is consistent with the idea that these treatments model certain filter deficits seen in schizophrenic patients. Furthermore, the present results suggest that NO is critically involved in these effects on habituation, whereas that of dopamine is less clear.

Repeated application of ketamine to rats induces changes in the hippocampal expression of parvalbumin, neuronal nitric oxide synthase and cFOS similar to those found in human schizophrenia

Treatment with the phencyclidine derivative ketamine, a non-competitive N-methyl-D-aspartate receptor antagonist and a well known anesthetic, has recently been introduced to mimic schizophrenia in animals. Using rats repeatedly treated with sub-anesthetic doses we demonstrate in the hippocampal formation the cellular distribution patterns of proteins being relevant to the pathogenesis of schizophrenia. Compared with controls an increase in the density of reduced nicotinamide adenine dinucleotide phosphate diaphorase-, neuronal nitric oxide synthase- and cFOS-positive hippocampal interneurons was found, whereas the density of parvalbumin expressing cells was decreased. Our experiments show that repeated injections of sub-anesthetic doses of ketamine induce significant changes in the nitrergic and GABAergic system which, in part, resemble those described in postmortem brains of human schizophrenics indicating that sub-chronic treatment with sub-anesthetic doses of ketamine might be a useful animal model to study schizophrenia.

Phencyclidine-induced changes in rat cortical gene expression identified by microarray analysis: implications for schizophrenia

Acute phencyclidine induces schizophrenia-like symptoms in healthy humans and psychotic episodes in schizophrenics. Although phencyclidine is known as a N-methyl d-aspartate receptor antagonist (NMDA-R), the molecular events underlying the behavioral symptoms remain largely unknown. Statistical analysis of oligonucleotide microarray data was used to identify phencyclidine-induced alterations in rat cortical gene expression. Acute phencyclidine produced a statistically significant change in 477 genes in rat prefrontal cortex (PFC), a brain area associated with cognitive dysfunction in schizophrenics. Real-time quantitative PCR (RTQ-PCR) confirmed a subset of these changes ranging from -59% to 255% (smallest confirmation: -19%). Subsequent time-course and dose-response studies using RTQ-PCR confirmed and extended the original microarray results. At the molecular level, genes altered by phencyclidine are related to diverse biological processes including stress, inflammatory response, growth and development, neural plasticity and signal transduction. Further analysis, aimed at assessing the relevance of our results to schizophrenia, revealed dysregulation of genes related to: (i) thalamocortical projections, (ii) neurotransmission and neuromodulation, (iii) thyroid hormone activity, (iv) oligodendrocyte linage, (v) brain lipid metabolism, (vi) sleep architecture and (viii) the velocardiofacial syndrome.

Neonatal quinpirole treatment impairs Morris water task performance in early postweanling rats: relationship to increases in corticosterone and decreases in neurotrophic factors

BACKGROUND

Past studies from this laboratory have shown that quinpirole administration from postnatal day (P) 1-21 produces persistent supersensitization of the dopamine D2 receptor that persists throughout the animal's lifetime.

METHODS

In Experiment 1, both male and female rats were treated with quinpirole or saline from P1-21 and tested on the place and match-to-place versions of the Morris water task (MWT) from P22-28. In Experiment 2, both male and female rats were administered either acute or chronic injections of quinpirole (1 mg/kg) or saline beginning on P1 until analysis for corticosterone (CORT) on P7, 14, or 21.

RESULTS

Neonatal quinpirole treatment produced deficits on both versions of the MWT compared with saline control. One day after behavioral testing, brain tissue was harvested, and the hippocampus was analyzed for nerve growth factor (NGF) and brain-derived nerve growth factor (BDNF); NGF was found to be significantly decreased by neonatal quinpirole treatment. Acute or chronic quinpirole treatment on P14 produced a larger increase in CORT than controls and produced larger increases in CORT than control rats on P21.

CONCLUSIONS

These results demonstrate that neonatal quinpirole treatment produces cognitive deficits that could be related to decreases in hippocampal NGF and increases in CORT, resulting in abnormalities in hippocampal development.