Long-Lasting Impairment of Associative Learning Is Correlated with a Dysfunction of N-Methyl-d-aspartate-Extracellular Signaling-Regulated Kinase Signaling in Mice after Withdrawal from Repeated Administration of Phencyclidine

Fingolimod ameliorates schizophrenia-like cognitive impairments induced by phencyclidine in male rats

BACKGROUND AND PURPOSE

Improvement of cognitive deficits in schizophrenia remains an unmet need owing to the lack of new therapies and drugs. Recent studies have reported that fingolimod, an immunomodulatory drug for treating multiple sclerosis, demonstrates anti-inflammatory and neuroprotective effects in several neurological disease models. This suggests its usefulness for ameliorating cognitive dysfunction in schizophrenia. Herein, we assessed the efficacy profile and mechanism of fingolimod in a rat model of phencyclidine (PCP)-induced schizophrenia.

EXPERIMENTAL APPROACH

Male Sprague-Dawley rats were treated with PCP for 14 days. The therapeutic effect of fingolimod on cognitive function was assessed using the Morris water maze and fear conditioning tests. Hippocampal neurogenesis and the expression of astrocytes and microglia were evaluated using immunostaining. Cytokine expression was quantified using multiplexed flow cytometry. Brain-derived neurotrophic factor expression and phosphorylation of extracellular signal-regulated kinase were determined using western blot analysis.

KEY RESULTS

Fingolimod attenuated cognitive deficits and restored hippocampal neurogenesis in a dose-dependent manner in PCP-treated rats. Fingolimod treatment exerted anti-inflammatory effects by inhibiting microglial activation and IL-6 and IL-1β pro-inflammatory cytokine expression. The underlying mechanism involves the upregulation of brain-derived neurotrophic factor protein expression and activation of the ERK signalling pathway.

CONCLUSION AND IMPLICATIONS

This is the first preclinical assessment of the effects of fingolimod on cognitive function in a model for schizophrenia. Our results suggest the immune system plays an crucial role in cognitive alterations in schizophrenia and highlight the potential of immunomodulatory strategies to improve cognitive deficits in schizophrenia.

Targeting α6GABAA receptors as a novel therapy for schizophrenia: A proof-of-concept preclinical study using various animal models

GABA_A receptors containing α6 subunits (α6GABA_ARs) in the cerebellum have -been implicated in schizophrenia. It was reported that the GABA synthesizing enzymes were downregulated whereas α6GABA_ARs were upregulated in postmortem cerebellar tissues of patients with schizophrenia and in a rat model induced by chronic phencyclidine (PCP). We have previously demonstrated that pyrazoloquinolinone Compound 6, an α6GABA_AR-highly selective positive allosteric modulator (PAM), can rescue the disrupted prepulse inhibition (PPI) induced by methamphetamine (METH), an animal model mimicking the sensorimotor gating deficit based on the hyper-dopaminergic hypothesis of schizophrenia. Here, we demonstrate that not only Compound 6, but also its structural analogues, LAU463 and LAU159, with similarly high α6GABA_AR selectivity and their respective deuterated derivatives (DK-I-56-1, DK-I-58-1 and DK-I-59-1) can rescue METH-induced PPI disruption. Besides, Compound 6 and DK-I-56-I can also rescue the PPI disruption induced by acute administration of PCP, an animal model based on the hypo-glutamatergic hypothesis of schizophrenia. Importantly, Compound 6 and DK-I-56-I, at doses not affecting spontaneous locomotor activity, can also rescue impairments of social interaction and novel object recognition in mice induced by chronic PCP treatments. At similar doses, Compound 6 did not induce sedation but significantly suppressed METH-induced hyperlocomotion. Thus, α6GABA_AR-selective PAMs can rescue not only disrupted PPI but also hyperlocomotion, social withdrawal, and cognitive impairment, in both METH- and PCP-induced animal models mimicking schizophrenia, suggesting that they are a potential novel therapy for the three core symptoms, i.e. positive symptoms, negative symptoms, and cognitive impairment, of schizophrenia.

The prediction-error hypothesis of schizophrenia: new data point to circuit-specific changes in dopamine activity

Schizophrenia is a severe psychiatric disorder affecting 21 million people worldwide. People with schizophrenia suffer from symptoms including psychosis and delusions, apathy, anhedonia, and cognitive deficits. Strikingly, schizophrenia is characterised by a learning paradox involving difficulties learning from rewarding events, whilst simultaneously 'overlearning' about irrelevant or neutral information. While dysfunction in dopaminergic signalling has long been linked to the pathophysiology of schizophrenia, a cohesive framework that accounts for this learning paradox remains elusive. Recently, there has been an explosion of new research investigating how dopamine contributes to reinforcement learning, which illustrates that midbrain dopamine contributes in complex ways to reinforcement learning, not previously envisioned. This new data brings new possibilities for how dopamine signalling contributes to the symptomatology of schizophrenia. Building on recent work, we present a new neural framework for how we might envision specific dopamine circuits contributing to this learning paradox in schizophrenia in the context of models of reinforcement learning. Further, we discuss avenues of preclinical research with the use of cutting-edge neuroscience techniques where aspects of this model may be tested. Ultimately, it is hoped that this review will spur to action more research utilising specific reinforcement learning paradigms in preclinical models of schizophrenia, to reconcile seemingly disparate symptomatology and develop more efficient therapeutics.

Early postnatal inhibition of GLAST causes abnormalities of psychobehaviors and neuronal morphology in adult mice

The importance of glutamate transporters in learning, memory, and emotion remains poorly understood; hence, in the present study, we investigated whether deficiency of pharmacological GLAST in neurodevelopmental processes affects cognitive and/or emotional behaviors in mice. The mice were injected with a glutamate transporter inhibitor, dl-threo-β-benzyloxyaspartate (dl-TBOA), during the early postnatal period. At 8 weeks of age, they showed impairments in cognitive or emotional behaviors; dysfunction of glutamatergic neurotransmission (increased expressions of GLAST, GLT-1, or GFAP protein, and decreased ability of glutamate release) in the cortex or hippocampus; morphological changes (decreased cell size in the cortex and thickness of the pyramidal neuronal layer of the CA1 area in the hippocampus). Such behavioral and morphological changes were not observed in adult mice injected with dl-TBOA. These results suggest that GLAST plays an important role in the regulation of cognitive and emotional behaviors. Early postnatal glutamatergic facilitation by GLAST dysfunction leads to cognitive and emotional abnormalities due to neurodevelopmental abnormalities such as morphological changes.

Relationships and Interactions between Ionotropic Glutamate Receptors and Nicotinic Receptors in the CNS

Although ionotropic glutamate receptors and nicotinic receptors for acetylcholine (ACh) have usually been studied separately, they are often co-localized and functionally inter-dependent. The objective of this review is to survey the evidence for interactions between the two receptor families and the mechanisms underlying them. These include the mutual regulation of subunit expression, which change the NMDA:AMPA response balance, and the existence of multi-functional receptor complexes which make it difficult to distinguish between individual receptor sites, especially in vivo. This is followed by analysis of the functional relationships between the receptors from work on transmitter release, cellular electrophysiology and aspects of behavior where these can contribute to understanding receptor interactions. It is clear that nicotinic receptors (nAChRs) on axonal terminals directly regulate the release of glutamate and other neurotransmitters, α7-nAChRs generally promoting release. Hence, α7-nAChR responses will be prevented not only by a nicotinic antagonist, but also by compounds blocking the indirectly activated glutamate receptors. This accounts for the apparent anticholinergic activity of some glutamate antagonists, including the endogenous antagonist kynurenic acid. The activation of presynaptic nAChRs is by the ambient levels of ACh released from pre-terminal synapses, varicosities and glial cells, acting as a 'volume neurotransmitter' on synaptic and extrasynaptic sites. In addition, ACh and glutamate are released as CNS co-transmitters, including 'cholinergic' synapses onto spinal Renshaw cells. It is concluded that ACh should be viewed primarily as a modulator of glutamatergic neurotransmission by regulating the release of glutamate presynaptically, and the location, subunit composition, subtype balance and sensitivity of glutamate receptors, and not primarily as a classical fast neurotransmitter. These conclusions and caveats should aid clarification of the sites of action of glutamate and nicotinic receptor ligands in the search for new centrally-acting drugs.

Glutathione peroxidase-1 and neuromodulation: Novel potentials of an old enzyme

Glutathione peroxidase (GPx) acts in co-ordination with other signaling molecules to exert its own antioxidant role. We have demonstrated the protective effects of GPx,/GPx-1, a selenium-dependent enzyme, on various neurodegenerative disorders (i.e., Parkinson's disease, Alzheimer's disease, cerebral ischemia, and convulsive disorders). In addition, we summarized the recent findings indicating that GPx-1 might play a role as a neuromodulator in neuropsychiatric conditions, such as, stress, bipolar disorder, schizophrenia, and drug intoxication. In this review, we attempted to highlight the mechanistic scenarios mediated by the GPx/GPx-1 gene in impacting these neurodegenerative and neuropsychiatric disorders, and hope to provide new insights on the therapeutic interventions against these disorders.

Preventive role of regular low-intensity exercise during adolescence in schizophrenia model mice with abnormal behaviors

Schizophrenia is probably ascribed to perinatal neurodevelopmental deficits, and its onset might be affected by environmental factors. Hypofrontality with glutamatergic and dopaminergic neuronal dysfunction are known factors, but a way to mitigate abnormalities remains unfound. An early enriched environment such as a wheel running in rodents may contribute to the prevention, but its clinical applicability is very limited. From our studies, low-intensity exercise training (LET) based on physiological indices, such as lactate threshold, easily translates to humans and positively affects the brains. Hence, LET during adolescence may ameliorate abnormalities in neurodevelopment and prevent the development of schizophrenia. In the current study, LET prevented sensitization to phencyclidine (PCP) treatment, impairment of cognition, and affective behavioral abnormalities in an animal model of schizophrenia induced by prenatal PCP treatment. Further, LET increased dopamine turnover and attenuated the impairment of phosphorylation of ERK1/2 after exposure to a novel object in the prenatal PCP-treated mice. These results suggest that LET during adolescence completely improves schizophrenia-like abnormal behaviors associated with improved glutamate uptake and the dopamine-induced ERK1/2 signaling pathway in the PFC.

Functional roles of the glial glutamate transporter (GLAST) in emotional and cognitive abnormalities of mice after repeated phencyclidine administration

Alterations of the glutamatergic system components, including N-methyl-d-aspartate (NMDA) receptors are relevant to the pathophysiology of schizophrenia. Repeated phencyclidine (PCP) administration induces several schizophrenia-like psychobehavioral abnormalities and decreases extracellular glutamate levels, which are associated with increased levels of glial glutamate and aspartate transporter (GLAST) in the prefrontal cortex (PFC) of mice. In the present study, we investigated the functional roles of GLAST in the emotional and cognitive abnormalities in mice following repeated PCP administration by using GLAST heterozygous (+/-) mice, since GLAST mutant mice are a useful tool for elucidating the contribution of glutamate dysfunction to the pathophysiology of schizophrenia. PCP-administered GLAST wild-type (+/+) mice showed enhancement of immobility in a forced swimming test, impairments of visual recognition memory in a novel object recognition test, decrease in high potassium (K⁺)-induced extracellular glutamate release, and overexpression of GLAST and S100 proteins in the PFC, compared to saline-administered GLAST^+/+ mice. Such behavioral and neurochemical abnormalities were not observed in PCP-administered GLAST^+/- mice. In conclusion, these results clearly suggest that genetic GLAST dysfunction and glial activation play important roles in the development of emotional and cognitive abnormalities in PCP-administered GLAST^+/+ mice.

Blonanserin ameliorates social deficit through dopamine-D3 receptor antagonism in mice administered phencyclidine as an animal model of schizophrenia

Blonanserin differs from other antipsychotic drugs, such as risperidone and olanzapine, and exhibits a higher affinity for dopamine-D_2/3 receptors than for serotonin 5-HT_2A receptors. We investigated the involvement of dopamine-D₃ receptors in the effect of blonanserin on the social deficit observed in an animal model of schizophrenia and sought to elucidate the molecular mechanism underlying its action. Mice received phencyclidine (PCP: 10 mg/kg/day, s.c.), a non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, once a day for 14 consecutive days. We then evaluated the sociability, using a social interaction test, and the expression of GluN1 subunit, an essential subunit of the NMDA receptors, in these mice. Blonanserin significantly ameliorated the PCP-induced social deficit, whereas olanzapine and haloperidol did not. This effect of blonanserin was antagonized by 7-OH-DPAT, a dopamine-D₃ receptor agonist, and SCH23390, a dopamine-D₁ receptor antagonist. However, the ameliorating effect of blonanserin was not inhibited by DOI, a serotonin 5-HT_2A receptor agonist. The PCP-induced social deficit was also ameliorated by U99194, a dopamine-D₃ receptor antagonist and SKF38393, a dopamine-D₁ receptor agonist, being effects antagonized by 7-OH-DPAT or SCH23390. Blonanserin significantly inhibited the decrease in the phosphorylation levels of GluN1 at Ser⁸⁹⁷ by protein kinase A (PKA) in the prefrontal cortex (PFC) in PCPadministered mice. These results suggest that activation of NMDA receptors due to Ser⁸⁹⁷-phosphorylation of GluN1 subunit, which is a step linked to dopamine-D₁ receptor-PKA signaling through dopamine-D₃ receptor antagonism in the PFC, is required for the ameliorating effect of blonanserin on the PCP-induced social deficit. These findings also provide in vivo evidence that blonanserin antagonism of the dopamine-D₃ receptors may be useful as a novel treatment strategy and that the dopamine-D₃ receptors can be a novel therapeutic target molecule for the social deficit observed in schizophrenia.

The effects of donepezil on phencyclidine-induced cognitive deficits in a mouse model of schizophrenia

Donepezil is the first-line of treatment for Alzheimer's disease (AD), which improves cognitive impairment effectively, but whether it has an impact on cognitive impairment in schizophrenia remains unknown. In this study, we evaluated the effects and mechanisms of donepezil on schizophrenia-like cognitive deficits induced by phencyclidine (PCP). The cognitive deficits model of schizophrenia was established by injecting PCP into mice. Risperidone, an atypical antipsychotic drug, served as positive control drug. Three behavioral tests including novel object recognition (NOR) test, Morris Water Maze (MWM) and passive avoidance (PA) test were performed to evaluate the effect of donepezil on PCP-induced cognitive deficits. Furthermore, the content of BDNF and NGF in the hippocampus and cortex of mice was determined using ELISA. Expressions of p-GSK-3β/GSK-3β, p-Akt/Akt, Bcl-2/Bax and Caspase-3 in the hippocampus and cortex were detected by Western blot. Results revealed that donepezil has a protective effect on PCP-induced cognitive dysfunction. Moreover, donepezil can also improve PCP-induced schizophrenia-like cognitive deficits by inhibiting neuronal apoptosis and regulating synaptic plasticity, which was possible through the up-regulation of p-Akt, p-GSK-3β, Bcl-2 and the down-regulation of Bax, Caspase-3. The results indicated that donepezil might exhibit a beneficial effect on the treatment of cognitive dysfunction in schizophrenia.

Ginsenoside Re protects against phencyclidine-induced behavioral changes and mitochondrial dysfunction via interactive modulation of glutathione peroxidase-1 and NADPH oxidase in the dorsolateral cortex of mice

We investigated whether ginsenoside Re (Re) modulates phencyclidine (PCP)-induced sociability deficits and recognition memory impairments to extend our recent finding. We examined the role of GPx-1 gene in the pharmacological activity of Re against mitochondrial dysfunction induced by PCP in the dorsolateral cortex of mice. Since mitochondrial oxidative stress activates NADPH oxidase (PHOX), we applied PHOX inhibitor apocynin for evaluating interactive modulation between GPx-1 and PHOX against PCP neurotoxicity. Sociability deficits and recognition memory impairments induced by PCP were more pronounced in GPx-1 knockout (KO) than in wild type (WT) mice. PCP-induced mitochondrial oxidative stress, mitochondrial dysfunction, and membrane translocation of p47phox were more evident in GPx-1 KO than in WT. Re treatment significantly attenuated PCP-induced neurotoxic changes. Re also significantly attenuated PCP-induced sociability deficits and recognition memory impairments. The attenuation by Re was comparable to that by apocynin. The attenuation was more obvious in GPx-1 KO than in WT. Importantly, apocynin did not show any additional positive effects on the neuroprotective activity of Re, indicating that PHOX is a molecular target for therapeutic activity of Re. Our results suggest that Re requires interactive modulation between GPx activity and PHOX (p47phox) to exhibit neuroprotective potentials against PCP insult.

Prolonged deficits of associative motor learning in cynomolgus monkeys after long-term administration of phencyclidine

Phencyclidine (PCP) is a potent drug of abuse that induces sustained schizophrenia-like symptoms in humans by blocking neurotransmission at N-methyl-d-aspartate (NMDA)-type glutamate receptors. Alterations in NMDA receptor function have been linked to numerous behavioral deficits and cognitive dysfunction. Classical eye-blink conditioning (EBC), including delay (dEBC) and trace (tEBC) paradigms, provides an effective means to study the neurobiology of associative motor learning in rodents, mammals and primates. To assess whether administration of low-dosage PCP for extended periods has prolonged effect to alter associative motor learning, in this study 19 adult cynomolgus monkeys were administered PCP (0.3mg/kg, intramuscularly) or saline twice a day for 14days. Twelve-fifteen months after PCP or saline injection, monkeys received dEBC, tEBC, or pseudo-paired training for 6 or 12 successive daily sessions, respectively. The results of this study show that percentage of conditioned response (CR) in dEBC increased as a function of training sessions in both PCP-treated and control monkeys and there was no significant CR% difference between the two groups. However, the CR timing in dEBC of PCP-treated monkeys was significantly impaired, as manifested by shorter CR peak latencies than those of the control group. PCP-treated animals showed significantly lower percentage of CR in tEBC compared to controls. PCP-treated animals were also more sensitive to outside stimuli in tEBC because the UR peak latency of PCP-treated group was significantly lower than the control group. These results indicated that cynomolgus monkeys manifested prolonged deficits in associative motor learning after long-term administration of phencyclidine.

Cannabinoids and Vanilloids in Schizophrenia: Neurophysiological Evidence and Directions for Basic Research

Much of our knowledge of the endocannabinoid system in schizophrenia comes from behavioral measures in rodents, like prepulse inhibition of the acoustic startle and open-field locomotion, which are commonly used along with neurochemical approaches or drug challenge designs. Such methods continue to map fundamental mechanisms of sensorimotor gating, hyperlocomotion, social interaction, and underlying monoaminergic, glutamatergic, and GABAergic disturbances. These strategies will require, however, a greater use of neurophysiological tools to better inform clinical research. In this sense, electrophysiology and viral vector-based circuit dissection, like optogenetics, can further elucidate how exogenous cannabinoids worsen (e.g., tetrahydrocannabinol, THC) or ameliorate (e.g., cannabidiol, CBD) schizophrenia symptoms, like hallucinations, delusions, and cognitive deficits. Also, recent studies point to a complex endocannabinoid-endovanilloid interplay, including the influence of anandamide (endogenous CB₁ and TRPV₁ agonist) on cognitive variables, such as aversive memory extinction. In fact, growing interest has been devoted to TRPV₁ receptors as promising therapeutic targets. Here, these issues are reviewed with an emphasis on the neurophysiological evidence. First, we contextualize imaging and electrographic findings in humans. Then, we present a comprehensive review on rodent electrophysiology. Finally, we discuss how basic research will benefit from further combining psychopharmacological and neurophysiological tools.

Blonanserin ameliorates phencyclidine-induced visual-recognition memory deficits: the complex mechanism of blonanserin action involving D₃-5-HT₂A and D₁-NMDA receptors in the mPFC

Blonanserin differs from currently used serotonin 5-HT₂A/dopamine-D₂ receptor antagonists in that it exhibits higher affinity for dopamine-D₂/₃ receptors than for serotonin 5-HT₂A receptors. We investigated the involvement of dopamine-D₃ receptors in the effects of blonanserin on cognitive impairment in an animal model of schizophrenia. We also sought to elucidate the molecular mechanism underlying this involvement. Blonanserin, as well as olanzapine, significantly ameliorated phencyclidine (PCP)-induced impairment of visual-recognition memory, as demonstrated by the novel-object recognition test (NORT) and increased extracellular dopamine levels in the medial prefrontal cortex (mPFC). With blonanserin, both of these effects were antagonized by DOI (a serotonin 5-HT₂A receptor agonist) and 7-OH-DPAT (a dopamine-D₃ receptor agonist), whereas the effects of olanzapine were antagonized by DOI but not by 7-OH-DPAT. The ameliorating effect was also antagonized by SCH23390 (a dopamine-D₁ receptor antagonist) and H-89 (a protein kinase A (PKA) inhibitor). Blonanserin significantly remediated the decrease in phosphorylation levels of PKA at Thr(197) and of NR1 (an essential subunit of N-methyl-D-aspartate (NMDA) receptors) at Ser(897) by PKA in the mPFC after a NORT training session in the PCP-administered mice. There were no differences in the levels of NR1 phosphorylated at Ser(896) by PKC in any group. These results suggest that the ameliorating effect of blonanserin on PCP-induced cognitive impairment is associated with indirect functional stimulation of the dopamine-D₁-PKA-NMDA receptor pathway following augmentation of dopaminergic neurotransmission due to inhibition of both dopamine-D₃ and serotonin 5-HT₂A receptors in the mPFC.

Neonatal phencyclidine administration and post-weaning social isolation as a dual-hit model of 'schizophrenia-like' behaviour in the rat

RATIONALE

Schizophrenia is a debilitating disorder comprising positive, negative and cognitive deficits with a poorly defined neurobiological aetiology; therefore, animal models with greater translational reliability are essential to develop improved therapies.

OBJECTIVES

This study combines two developmental challenges in rats, neonatal phencyclidine (PCP) injection and subsequent rearing in social isolation from weaning, to attempt to produce more robust behavioural deficits with greater translational relevance to schizophrenia than either challenge alone.

METHODS

Forty-two male Lister-hooded rat pups received the N-methyl-D-aspartate (NMDA) receptor antagonist, phencyclidine (PCP, 10 mg/kg, s.c.), or vehicle on post-natal day (PND) 7, 9 and 11 and were weaned on PND 23 into group housing (saline-treated n = 11 or PCP-treated n = 10) or isolation (saline n = 10 or PCP n = 11). Six weeks post-weaning, novelty- and PCP-induced (3.2 mg/kg) locomotor activity, novel object discrimination, prepulse inhibition of acoustic startle and contextual memory in a conditioned emotion response (CER) were recorded.

RESULTS

Isolation rearing alone significantly elevated baseline locomotor activity and induced visual recognition memory impairment in novel object discrimination. Neonatal PCP treatment did not induce locomotor sensitisation to a subsequent acute PCP injection, but it impaired prepulse inhibition when combined with isolation rearing. CER freezing behaviour was significantly reduced by isolation rearing but an even greater effect occurred when combined with neonatal PCP treatment.

CONCLUSIONS

Neonatal PCP and isolation rearing both produce behavioural deficits in adult rats, but combined treatment caused a wider range of more severe cognitive impairments, providing a more comprehensive preclinical model to determine the neurobiological aetiology of schizophrenia than either treatment alone.

Severe cross-modal object recognition deficits in rats treated sub-chronically with NMDA receptor antagonists are reversed by systemic nicotine: implications for abnormal multisensory integration in schizophrenia

Schizophrenia is a complex and debilitating disorder, characterized by positive, negative, and cognitive symptoms. Among the cognitive deficits observed in patients with schizophrenia, recent work has indicated abnormalities in multisensory integration, a process that is important for the formation of comprehensive environmental percepts and for the appropriate guidance of behavior. Very little is known about the neural bases of such multisensory integration deficits, partly because of the lack of viable behavioral tasks to assess this process in animal models. In this study, we used our recently developed rodent cross-modal object recognition (CMOR) task to investigate multisensory integration functions in rats treated sub-chronically with one of two N-methyl-D-aspartate receptor (NMDAR) antagonists, MK-801, or ketamine; such treatment is known to produce schizophrenia-like symptoms. Rats treated with the NMDAR antagonists were impaired on the standard spontaneous object recognition (SOR) task, unimodal (tactile or visual only) versions of SOR, and the CMOR task with intermediate to long retention delays between acquisition and testing phases, but they displayed a selective CMOR task deficit when mnemonic demand was minimized. This selective impairment in multisensory information processing was dose-dependently reversed by acute systemic administration of nicotine. These findings suggest that persistent NMDAR hypofunction may contribute to the multisensory integration deficits observed in patients with schizophrenia and highlight the valuable potential of the CMOR task to facilitate further systematic investigation of the neural bases of, and potential treatments for, this hitherto overlooked aspect of cognitive dysfunction in schizophrenia.

Prolonged exposure to NMDAR antagonist induces cell-type specific changes of glutamatergic receptors in rat prefrontal cortex

N-methyl-d-aspartic acid (NMDA) receptors are critical for both normal brain functions and the pathogenesis of schizophrenia. We investigated the functional changes of glutamatergic receptors in the pyramidal cells and fast-spiking (FS) interneurons in the adolescent rat prefrontal cortex in MK-801 model of schizophrenia. We found that although both pyramidal cells and FS interneurons were affected by in vivo subchronic blockade of NMDA receptors, MK-801 induced distinct changes in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and NMDA receptors in the FS interneurons compared with pyramidal cells. Specifically, the amplitude, but not the frequency, of AMPA-mediated miniature excitatory postsynaptic currents (mEPSCs) in FS interneurons was significantly decreased whereas both the frequency and amplitude in pyramidal neurons were increased. In addition, MK-801-induced new presynaptic NMDA receptors were detected in the glutamatergic terminals targeting pyramidal neurons but not FS interneurons. MK-801 also induced distinct alterations in FS interneurons but not in pyramidal neurons, including significantly decreased rectification index and increased calcium permeability. These data suggest a distinct cell-type specific and homeostatic synaptic scaling and redistribution of AMPA and NMDA receptors in response to the subchronic blockade of NMDA receptors and thus provide a direct mechanistic explanation for the NMDA hypofunction hypothesis that have long been proposed for the schizophrenia pathophysiology.

ERK activation in the amygdala and hippocampus induced by fear conditioning in ethanol withdrawn rats: modulation by MK-801

The extracellular signal-regulated kinase (ERK) pathway, which can be activated by NMDA receptor stimulation, is involved in fear conditioning and drug addiction. We have previously shown that withdrawal from chronic ethanol administration facilitated the formation of contextual fear memory. In order to explore the neural substrates and the potential mechanism involved in this effect, we examined: 1) the ERK1/2 activation in the central (CeA) and basolateral (BLA) nuclei of the amygdala and in the dorsal hippocampus (dHip), 2) the effect of the NMDA receptor antagonist MK-801 on fear conditioning and ERK activation and 3) the effect of the infusion of U0126, a MEK inhibitor, into the BLA on fear memory formation in ethanol withdrawn rats. Rats made dependent via an ethanol-containing liquid diet were subjected to contextual fear conditioning on day 3 of ethanol withdrawal. High basal levels of p-ERK were found in CeA and dHip from ethanol withdrawn rats. ERK activation was significantly increased both in control (60min) and ethanol withdrawn rats (30 and 60min) in BLA after fear conditioning. Pre-training administration of MK-801, at a dose that had no effect on control rats, prevented the increase in ERK phosphorylation in BLA and attenuated the freezing response 24h later in ethanol withdrawn rats. Furthermore, the infusion of U0126 into the BLA, but not the CeA, before fear conditioning disrupted fear memory formation. These results suggest that the increased fear memory can be linked to changes in ERK phosphorylation, probably due to NMDA receptor activation in BLA in ethanol withdrawn rats.

The effects of selective antagonists of serotonin 5-HT7 and 5-HT1A receptors on MK-801-induced impairment of learning and memory in the passive avoidance and Morris water maze tests in rats: mechanistic implications for the beneficial effects of the novel atypical antipsychotic lurasidone

We have previously reported that lurasidone, a novel atypical antipsychotic with potent serotonin 5-HT(7) antagonist and 5-HT(1A) partial agonist activities, is superior to other antipsychotics in improving the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801-induced learning and memory impairment in the passive avoidance (PA) and Morris water maze (MWM) tests in rats. In this study, we investigated the effects of selective antagonists of 5-HT(7) and 5-HT(1A) receptors (SB-656104-A and WAY-100635, respectively) on MK-801-induced learning and memory impairment in the same tests. In the PA test, either pre-training (3 and 10mg/kg, p.o.) or post-training (0.3mg/kg, i.v.) administration of lurasidone significantly reversed the test response impaired by MK-801, consistent with our previous reports. Pre-training administration of either SB-656104-A (10 and 30 mg/kg, i.p.) or WAY-100635 (1mg/kg, s.c.) also significantly reversed MK-801-induced memory impairment. Furthermore, post-training administration of either SB-656104-A (0.3mg/kg, i.v.) or WAY-100635 (0.01 mg/kg, i.v.) counteracted the effect of MK-801, which suggested that both 5-HT receptor subtype-selective antagonists could restore the memory consolidation process. In the MWM test, SB-656104-A (3mg/kg, i.p.) reversed learning impairment induced by MK-801. On the other hand, WAY-100635 (0.3 and 1mg/kg, i.p.) did not have any effect on the MK-801-induced learning impairment. Taken together, our results showed that 5-HT(7) and 5-HT(1A) receptor antagonists mimic the effect of lurasidone in whole or in part, respectively, to reverse MK-801-induced learning and memory impairment, which warrants further investigation of the interaction of lurasidone with these serotonin receptors as a possible mechanism underlying its procognitive effects in these animal models.

Prenatal exposure to phencyclidine produces abnormal behaviour and NMDA receptor expression in postpubertal mice

Several studies have shown the disruptive effects of non-competitive N-methyl-d-aspartate (NMDA) receptor antagonists on neurobehavioural development. Based on the neurodevelopment hypothesis of schizophrenia, there is growing interest in animal models treated with NMDA antagonists at developing stages to investigate the pathogenesis of psychological disturbances in humans. Previous studies have reported that perinatal treatment with phencyclidine (PCP) impairs the development of neuronal systems and induces schizophrenia-like behaviour. However, the adverse effects of prenatal exposure to PCP on behaviour and the function of NMDA receptors are not well understood. This study investigated the long-term effects of prenatal exposure to PCP in mice. The prenatal PCP-treated mice showed hypersensitivity to a low dose of PCP in locomotor activity and impairment of recognition memory in the novel object recognition test at age 7 wk. Meanwhile, the prenatal exposure reduced the phosphorylation of NR1, although it increased the expression of NR1 itself. Furthermore, these behavioural changes were attenuated by atypical antipsychotic treatment. Taken together, prenatal exposure to PCP produced long-lasting behavioural deficits, accompanied by the abnormal expression and dysfunction of NMDA receptors in postpubertal mice. It is worth investigating the influences of disrupted NMDA receptors during the prenatal period on behaviour in later life.

Disruptions in spatial working memory, but not short-term memory, induced by repeated ketamine exposure

Treatment with non-competitive N-methyl-D-aspartate (NMDA) antagonists such as phencyclidine or ketamine have been shown to induce schizophrenia-like psychotic and cognitive symptoms in humans and animals. However, there have been a number of contradictory findings regarding the effects of repeated treatment with these drugs on working memory in experimental animals. We hypothesized that processes dependent on dopamine transmission in the medial prefrontal cortex (PFC) may be more sensitive to disruption following these treatment. We assessed the effects of repeated treatment with ketamine on working memory performance using a delayed spatial win-shift procedure conducted on a radial-arm maze, dependent on a neural circuit linking hippocampal and dopamine inputs to the medial PFC. Rats were trained on the task prior to drug exposure, after which they were subjected to one of two dosing regimes of ketamine (30 mg/kg twice a day for either 5 or 10 days). After a 10 day withdrawal period, they were re-tested on the task for 15 days. Ketamine treatment for 10 days, but not 5 days, increased the number of errors and days to re-achieve the criterion on the delayed task. However, in a separate group of rats, subchronic ketamine treatment (10 days) did not affect performance of the non-delayed random foraging task, dependent on the hippocampus, but not the PFC. These results indicate that working memory performance assessed with these procedures is sensitive to disruption following repeated exposure to ketamine. Impairments in working memory induced by these treatments are not attributable to dysfunction of motivational, motor, short-term or spatial memory processes. The use of these procedures may prove useful in modeling impairments in this executive function observed in schizophrenia.

Aripiprazole ameliorates phencyclidine-induced impairment of recognition memory through dopamine D1 and serotonin 5-HT1A receptors

RATIONALE

Cognitive deficits, including memory impairment, are regarded as a core feature of schizophrenia. Aripiprazole, an atypical antipsychotic drug, has been shown to improve disruption of prepulse inhibition and social interaction in an animal model of schizophrenia induced by phencyclidine (PCP); however, the effects of aripiprazole on recognition memory remain to be investigated.

OBJECTIVES

In this study, we examined the effect of aripiprazole on cognitive impairment in mice treated with PCP repeatedly.

MATERIALS AND METHODS

Mice were repeatedly administered PCP at a dose of 10 mg/kg for 14 days, and their cognitive function was assessed using a novel-object recognition task. We investigated the therapeutic effects of aripiprazole (0.01-1.0 mg/kg) and haloperidol (0.3 and 1.0 mg/kg) on cognitive impairment in mice treated with PCP repeatedly.

RESULTS

Single (1.0 mg/kg) and repeated (0.03 and 0.1 mg/kg, for 7 days) treatment with aripiprazole ameliorated PCP-induced impairment of recognition memory, although single treatment significantly decreased the total exploration time during the training session. In contrast, both single and repeated treatment with haloperidol (0.3 and 1.0 mg/kg) failed to attenuate PCP-induced cognitive impairment. The ameliorating effect of aripiprazole on recognition memory in PCP-treated mice was blocked by co-treatment with a dopamine D1 receptor antagonist, SCH23390, and a serotonin 5-HT1A receptor antagonist, WAY100635; however, co-treatment with a D2 receptor antagonist raclopride had no effect on the ameliorating effect of aripiprazole.

CONCLUSIONS

These results suggest that the ameliorative effect of aripiprazole on PCP-induced memory impairment is associated with dopamine D1 and serotonin 5-HT1A receptors.

Synergistic effects of selegiline and donepezil on cognitive impairment induced by amyloid beta (25-35)

Selegiline, an irreversible inhibitor of monoamine oxidase B used in the treatment of Parkinson's disease, has been demonstrated to have a potential cognition-improving effect in patients with Alzheimer's disease (AD) undergoing treatment with an acetylcholinesterase inhibitor donepezil. To confirm such clinical events, we investigated whether co-administration of donepezil with selegiline had a synergistic cognition-improving effect in an animal model of AD. Intracerebroventricular injection of amyloid beta protein fragment 25-35 [Abeta(25-35)] induced impairment of learning and memory in a Y-maze, novel object recognition and contextual fear conditioning tests. Either donepezil or selegiline alone improved the cognitive impairments in the Y-maze and conditioned fear learning tasks in Abeta(25-35)-injected mice, whereas donepezil, but not selegiline, failed to improve the impairment in a novel object recognition task. Co-administration of donepezil with selegiline, at doses that do not exert efficacy individually, significantly improved the deficits in all three tests, indicating a synergistic cognition-improving effect. These alleviating effects were antagonized by pretreatment with a muscarinic receptor antagonist scopolamine and a dopamine receptor antagonist haloperidol. These results suggest that selegiline potentiates the effect of donepezil on the cognitive impairment, and that the synergistic effect may be mediated through both the cholinergic and dopaminergic systems.

Expression of Kruppel-like factor 5 gene in human brain and association of the gene with the susceptibility to schizophrenia

Genome-wide gene expression analysis using DNA microarray technology is a potential tool to search for unexpected genes that have a susceptibility to schizophrenia. We carried out a microarray analysis in the postmortem prefrontal cortex and found that the expression of the KLF5 gene, whose locus is on 13q21, was down-regulated in schizophrenia patients. This result was confirmed by a Western blot analysis. In a genetic study, we found that a polymorphism of the KLF5 gene (-1593T>C) was associated with schizophrenia. We identified neurons in the prefrontal cortex of human brain as sites of KLF5 expression by in situ hybridization and immunohistochemistry. KLF5 was immunohistochemically localized in granular and pyramidal cells in the hippocampus, which are the principal source of glutamatergic neurotransmission. These findings suggest that the KLF5 gene is a novel schizophrenia-susceptibility gene, and that the expression of the gene is involved in the pathophysiology of schizophrenia via glutamatergic neurotransmission.

Phencyclidine animal models of schizophrenia: Approaches from abnormality of glutamatergic neurotransmission and neurodevelopment

In humans, phencyclidine (PCP), a non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, reproduces a schizophrenia-like psychosis including positive symptoms, negative symptoms and cognitive dysfunction. Thus, the glutamatergic neuronal dysfunction hypothesis is one of the main explanatory hypotheses and PCP-treated animals have been utilized as an animal model of schizophrenia. The adult rodents treated with PCP repeatedly exhibit hyperlocomotion as an index of positive symptoms, a social behavioral deficit in a social interaction test and enhanced immobility in a forced swimming test as indices of negative symptoms. They also show a sensorimotor gating deficits and cognitive dysfunctions in several learning and memory tests. Some of these behavioral changes endure after withdrawal from repeated PCP treatment. Furthermore, repeated PCP treatment induces some neurochemical and neuroanatomical changes. On the other hand, the exposure to viral or environmental insult in the second trimester of pregnancy increases the probability of subsequently developing schizophrenia as an adult. NMDA receptor has been implicated in controlling the structure and plasticity of developing brain circuitry. Based on neurodevelopment hypothesis of schizophrenia, schizophrenia model rats treated with PCP at the perinatal stage is developed. Perinatal PCP treatment impairs neuronal development and induces long-lasting schizophrenia-like behaviors in adult period. Many findings suggest that these PCP animal models would be useful for evaluating novel therapeutic candidates and for confirming pathological mechanisms of schizophrenia.

Hypofunctional glutamatergic neurotransmission in the prefrontal cortex is involved in the emotional deficit induced by repeated treatment with phencyclidine in mice: Implications for abnormalities of glutamate release and NMDA–CaMKII signaling

In the present study, we investigated the involvement of prefrontal glutamatergic neurotransmission in the enhancement of immobility (emotional deficit) in a forced swimming test in mice treated with phencyclidine (PCP: 10mg/kg/day for 14 days) repeatedly, which is regarded as an animal model for negative symptoms. A decrease in spontaneous extracellular glutamate release and increase in levels of the glutamate transporter GLAST, were observed in the prefrontal cortex (PFC) of PCP-treated mice, compared to saline-treated mice. NMDA receptor subunit 1 (NR1) and Ca(2+)/calmoduline kinase II (CaMKII) were markedly activated in the PFC of saline-treated mice, but not PCP-treated mice, immediately after the forced swimming test. The facilitation of the function of NMDA receptors by d-cycloserine (30mg/kg i.p.), an NMDA receptor glycine-site partial agonist, reversed the enhancement of immobility in the forced swimming test and impairment of CaMKII activation in the PCP-treated mice. Microinjection of dl-threo-beta-benzyloxyaspartate (10nmol/site/bilaterally), a potent blocker of glutamate transporters, into the PFC of PCP-treated mice also had an attenuating effect. In addition, activation of glial cells and a decrease of neuronal cell size were observed in the PFC of PCP-treated mice. These results suggest that repeated PCP treatment disrupts pre- and post-synaptic glutamatergic neurotransmission and induces morphological changes in the PFC and that such changes cause the emotional deficits exhibited in PCP-treated mice.

The allosteric potentiation of nicotinic acetylcholine receptors by galantamine ameliorates the cognitive dysfunction in beta amyloid25-35 i.c.v.-injected mice: involvement of dopaminergic systems

Galantamine, a drug for Alzheimer's disease, is a novel cholinergic agent with a dual mode of action, which inhibits acetylcholinesterase and allosterically modulates nicotinic acetylcholine receptors (nAChRs), as a result stimulates catecholamine neurotransmission. In the present study, we investigated whether galantamine exerts cognitive improving effects through the allosteric modulation of nAChR in the intracerebroventricular beta amyloid (Abeta)(25-35)-injected animal model of Alzheimer's disease. Galantamine (3 mg/kg p.o.) significantly increased the extracellular dopamine release in the hippocampus of saline- and Abeta(25-35)-injected mice. The effects of nicotine on the extracellular dopamine release were potentiated by galantamine, but antagonized by mecamylamine, a nAChR antagonist. Abeta(25-35)-injected mice, compared with saline-injected mice, could not discriminate between new and familiar objects in the novel object recognition test and exhibited less freezing response in the fear-conditioning tasks, suggesting Abeta(25-35) induced cognitive impairment. Galantamine improved the Abeta(25-35)-induced cognitive impairment in the novel object recognition and fear-conditioning tasks. These improving effects of galantamine were blocked by the treatment with mecamylamine, SCH-23390, a dopamine-D1 receptor antagonist, and sulpiride, a dopamine-D2 receptor antagonist, but not by scopolamine, a muscarinic acetylcholine receptor antagonist. This study provides the first in vivo evidence that galantamine augments dopaminergic neurotransmission within the hippocampus through the allosteric potentiation of nAChRs. The improving-effects of galantamine on the Abeta(25-35)-induced cognitive impairment may be mediated through the activation of, at least in part, dopaminergic systems, and the enhancement of dopamine release may be one of multiple mechanisms underlying the therapeutic benefit of galantamine.

Oral vaccination with a viral vector containing Abeta cDNA attenuates age-related Abeta accumulation and memory deficits without causing inflammation in a mouse Alzheimer model

Immunotherapy with Abeta is expected to bring great improvement for Alzheimer disease (AD). However, clinical trials have been suspended because of meningoencephalitics, which accompanied lymphocytic infiltration. We have developed an oral vaccine for AD with a recombinant adeno-associated viral vector carrying Abeta cDNA (AAV/Abeta). The vaccine reduces the amount of Abeta deposited without lymphocytic infiltration in APP transgenic (Tg2576) mice. In the present study, Tg2576 mice showed progressive cognitive impairments in the novel object recognition test, Y-maze test, water maze test, and contextual conditioned fear learning test. A single oral administration of AAV/Abeta to Tg2576 mice at the age of 10 months alleviated progressive cognitive impairment with decreased Abeta deposition, insoluble Abeta, soluble Abeta oligomer (Abeta*56), microglial attraction, and synaptic degeneration induced in the brain regions at the age of 13 months. A histological analysis with hematoxylin and eosin and an immunohistochemical analysis with antibodies against CD3, CD4, CD8, and CD19 suggested there was no lymphocytic infiltration or microhemorrhage in the brain of AAV/Abeta-vaccinated Tg2576 mice at 13 months of age. Taken together, these results suggest that immunotherapy with AAV/Abeta is a safe and effective treatment for AD.

Clozapine Prevents a Decrease in Neurogenesis in Mice Repeatedly Treated With Phencyclidine

It has recently been suggested that neurogenesis in the dentate gyrus is decreased in schizophrenia and this phenomenon may contribute to the pathogenesis of the disorder. Since repeated administration of psychostimulants such as phencyclidine (PCP), MK-801, and methamphetamine (METH) induces schizophrenia-like behavioral changes in animals, we investigated whether repeated administration of these psychostimulants affects neurogenesis in the dentate gyrus of mice. Newborn cells were labeled by bromodeoxyuridine (BrdU) and detected by immunohistochemistry. Repeated administration of PCP and MK-801, but not METH, resulted in a decrease in the number of BrdU-labeled cells in the dentate gyrus. PCP-induced decrease in the number of BrdU-labeled cells was negated by co-administration of clozapine, but not haloperidol, although repeated antipsychotics treatment by themselves had no effect. Furthermore, co-administration of D-serine and glycine, but not L-serine, inhibited the PCP-induced decrease in the number of BrdU-labeled cells. These results suggest that chronic dysfunction of NMDA receptors causes a decrease in neurogenesis in the dentate gyrus.

Phencyclidine-induced cognitive deficits in mice are improved by subsequent subchronic administration of tropisetron: Role of α7 nicotinic receptors

We examined the effects of tropisetron, a 5-hydroxytryptamine (5-HT(3)) receptor antagonist and alpha7 nicotinic receptor agonist, on cognitive deficits in mice after repeated administration of the NMDA receptor antagonist phencyclidine (PCP). PCP (10 mg/kg/day for 10 days)-induced cognitive deficits were significantly improved by subsequent subchronic (2 weeks) administration of tropisetron, but not ondansetron. Effects of tropisetron were significantly antagonized by co-administration of the alpha7 nicotinic receptor antagonist methyllycaconitine, suggesting the role of alpha7 nicotinic receptors in the active mechanisms of tropisetron. These findings suggest that tropisetron could be a potential therapeutic drug for cognitive deficits in schizophrenic patients.