Effects of background mutations and single nucleotide polymorphisms (SNPs) on the Disc1 L100P behavioral phenotype associated with schizophrenia in mice.

BACKGROUND: Disrupted-in-schizophrenia 1 (DISC1) is a promising candidate susceptibility gene for psychiatric disorders, including schizophrenia, bipolar disorder and major depression. Several previous studies reported that mice with N-ethyl-N-nitrosourea (ENU)-induced L100P mutation in Disc1 showed some schizophrenia-related behavioral phenotypes. This line originally carried several thousands of ENU-induced point mutations in the C57BL/6 J strain and single nucleotide polymorphisms (SNPs) from the DBA/2 J inbred strain. METHODS: To investigate the effect of Disc1 L100P, background mutations and SNPs on phenotypic characterization, we performed behavioral analyses to better understand phenotypes of Disc1 L100P mice and comprehensive genetic analyses using whole-exome resequencing and SNP panels to map ENU-induced mutations and strain-specific SNPs, respectively. RESULTS: We found no differences in spontaneous or methamphetamine-induced locomotor activity, sociability or social novelty preference among Disc1 L100P/L100P, L100P/+ mutants and wild-type littermates. Whole-exome resequencing of the original G1 mouse identified 117 ENU-induced variants, including Disc1 L100P per se. Two females and three males from the congenic L100P strain after backcrossing to C57BL/6 J were deposited to RIKEN BioResource Center in 2008. We genotyped them with DBA/2 J × C57BL/6 J SNPs and found a number of the checked SNPs still remained. CONCLUSION: These results suggest that causal attribution of the discrepancy in behavioral phenotypes to the Disc1 L100P mutant mouse line existing among different research groups needs to be cautiously investigated in further study by taking into account the effect(s) of other ENU-induced mutations and/or SNPs from DBA/2 J.

Activation of prefrontal parvalbumin interneurons ameliorates working memory deficit even under clinically comparable antipsychotic treatment in a mouse model of schizophrenia.

One of the critical unmet medical needs in schizophrenia is the treatment for cognitive deficits. However, the neural circuit mechanisms of them remain unresolved. Previous studies utilizing animal models of schizophrenia did not consider the fact that patients with schizophrenia generally cannot discontinue antipsychotic medication due to the high risk of relapse. Here, we used multi-dimensional approaches, including histological analysis of the prelimbic cortex (PL), LC-MS/MS-based in vivo dopamine D2 receptor occupancy analysis for antipsychotics, in vivo calcium imaging, and behavioral analyses of mice using chemogenetics to investigate neural mechanisms and potential therapeutic strategies for working memory deficit in a chronic phencyclidine (PCP) mouse model of schizophrenia. Chronic PCP administration led to alterations in excitatory and inhibitory synapses, specifically in dendritic spines of pyramidal neurons, vesicular glutamate transporter 1 (VGLUT1) positive terminals, and parvalbumin (PV) positive GABAergic interneurons located in layer 2-3 of the PL. Continuous administration of olanzapine, which achieved a sustained therapeutic window of dopamine D2 receptor occupancy (60-80%) in the striatum, did not ameliorate these synaptic abnormalities and working memory deficit in the chronic PCP-treated mice. We demonstrated that chemogenetic activation of PV neurons in the PL, as confirmed by in vivo calcium imaging, ameliorated working memory deficit in this model even under clinically comparable olanzapine treatment which by itself inhibited only PCP-induced psychomotor hyperactivity. Our study suggests that targeting prefrontal PV neurons could be a promising therapeutic intervention for cognitive deficits in schizophrenia in combination with antipsychotic medication.

Animal models of attention-deficit/hyperactivity disorder.

Attention-deficit hyperactivity disorder (AD/HD) is a clinically heterogenous disorder including hyperactivity, impulsivity, and inattention. Both psychostimulant and non-psychostimulant drugs such as methylphenidate and atomoxetine, respectively, to modulate catecholeamine neurotransmission are used as current pharmacotherapies for AD/HD. Multiple lines of evidence suggest that genetic factors play major roles in the etiology of AD/HD. meta-Analyses and pooled data analyses have suggested associations between AD/HD and polymorphisms in genes encoding monoamine neurotransmission molecules. There has been considerable research on this disorder using genetic, pharmacological, and neuroimaging approaches, and several animal models of AD/HD such as spontaneously hypertensive rat (SHR), dopamine transporter (DAT) knockout mice, coloboma mutant mouse, and Grin1 mutant mouse have been reported. These animal models are valuable tools for investigating molecular, cellular, and behavioral mechanisms as well as the neural development and circuit mechanisms of AD/HD. Here, we review the recent literature on animal models of AD/HD and discuss their advantages and limitations.

[Neuronal development of the hyperdopaminergic animal model].

Dopamine transporter knockout (DAT KO) mice exhibited hyperdopaminergic tone in the nucleus accumbens and striatum, whereas they showed normal levels of extracellular dopamine in the prefrontal cortex. DAT KO mice showed numerous behavioral alterations that can be linked to abnormal dopaminergic function, including hyperlocomotion, deficits of prepulse inhibition (1PI) and impairment of working memory. PPI deficits were also shown in schizophrenic patients and hyperlocomotion was observed in AD/HD patients; therefore DAT KO mice had face validity for these psychiatric disorders. Impairment of neuronal development such as brain volume loss and decrease in spine density was reported especially in the prefrontal cortex of schizophrenia and AD/HD patients. We therefore investigated the neuronal development of DAT KO mice. Our results indicated that DAT KO mice had deficits of neuronal development in the prefrontal cortex similar to schizophrenia and AD/HD patients at least in part. These findings suggest that DAT KO mice are one of the useful models to investigate the impairment of neuronal development observed in psychiatric disorders including schizophrenia and AD/HD.

[Trend in the study of cocaine addiction].

Psychostimulant drugs including cocaine increase extracellular levels of monoamines by blocking the neuronal plasma membrane transporters. Increased extracellular dopamine levels in mesocorticolimbic dopamine systems have been postulated to mediate the rewarding effects of cocaine. Studies in genetically modified mice models, particularly knockout mice have contributed a great deal to our understanding of the mechanisms underlying psychostimulant actions. Phenotypic analysis of genetically modified mice models has been instrumental in identifying the role of specific molecular targets of cocaine. In this article, we summarize studies that have reported the effects of cocaine using genetically modified mice especially gene knockouts of the monoamine transporters and receptors.

Abnormal neural activation patterns underlying working memory impairment in chronic phencyclidine-treated mice.

Working memory impairment is a hallmark feature of schizophrenia and is thought be caused by dysfunctions in the prefrontal cortex (PFC) and associated brain regions. However, the neural circuit anomalies underlying this impairment are poorly understood. The aim of this study is to assess working memory performance in the chronic phencyclidine (PCP) mouse model of schizophrenia, and to identify the neural substrates of working memory. To address this issue, we conducted the following experiments for mice after withdrawal from chronic administration (14 days) of either saline or PCP (10 mg/kg): (1) a discrete paired-trial variable-delay task in T-maze to assess working memory, and (2) brain-wide c-Fos mapping to identify activated brain regions relevant to this task performance either 90 min or 0 min after the completion of the task, with each time point examined under working memory effort and basal conditions. Correct responses in the test phase of the task were significantly reduced across delays (5, 15, and 30 s) in chronic PCP-treated mice compared with chronic saline-treated controls, suggesting delay-independent impairments in working memory in the PCP group. In layer 2-3 of the prelimbic cortex, the number of working memory effort-elicited c-Fos+ cells was significantly higher in the chronic PCP group than in the chronic saline group. The main effect of working memory effort relative to basal conditions was to induce significantly increased c-Fos+ cells in the other layers of prelimbic cortex and the anterior cingulate and infralimbic cortex regardless of the different chronic regimens. Conversely, this working memory effort had a negative effect (fewer c-Fos+ cells) in the ventral hippocampus. These results shed light on some putative neural networks relevant to working memory impairments in mice chronically treated with PCP, and emphasize the importance of the layer 2-3 of the prelimbic cortex of the PFC.

Methamphetamine increases locomotion and dopamine transporter activity in dopamine d5 receptor-deficient mice.

Dopamine regulates the psychomotor stimulant activities of amphetamine-like substances in the brain. The effects of dopamine are mediated through five known dopamine receptor subtypes in mammals. The functional relevance of D5 dopamine receptors in the central nervous system is not well understood. To determine the functional relevance of D5 dopamine receptors, we created D5 dopamine receptor-deficient mice and then used these mice to assess the roles of D5 dopamine receptors in the behavioral response to methamphetamine. Interestingly, D5 dopamine receptor-deficient mice displayed increased ambulation in response to methamphetamine. Furthermore, dopamine transporter threonine phosphorylation levels, which regulate amphetamine-induced dopamine release, were elevated in D5 dopamine receptor-deficient mice. The increase in methamphetamine-induced locomotor activity was eliminated by pretreatment with the dopamine transporter blocker GBR12909. Taken together, these results suggest that dopamine transporter activity and threonine phosphorylation levels are regulated by D5 dopamine receptors.

Cortico-subcortical neuromodulation involved in the amelioration of prepulse inhibition deficits in dopamine transporter knockout mice.

Prepulse inhibition (PPI) deficits are among the most reproducible phenotypic markers found in schizophrenic patients. We recently reported that nisoxetine, a selective norepinephrine transporter (NET) inhibitor, reversed the PPI deficits that have been identified in dopamine transporter (DAT) knockout (KO) mice. However, the mechanisms underlying nisoxetine-induced PPI recovery in DAT KO mice were unclear in previous experiments. To clarify these mechanisms, PPI was tested after microinjections of nisoxetine into the medial prefrontal cortex (mPFc) or nucleus accumbens (NAc) in wildtype (WT) and DAT KO mice. c-Fos immunohistochemistry provided an indicator of neural activation. Multiple-fluorescent-labeling procedures and the retrograde tracer fluorogold were employed to identify nisoxetine-activated neurons and circuits. Systemic nisoxetine activated the mPFc, the NAc shell, the basolateral amygdala, and the subiculum. Infusions of nisoxetine into the mPFc reversed PPI deficits in DAT KO mice, but produced no changes in WT mice, while infusion of nisoxetine into the NAc had no effect on PPI in both WT and DAT KO mice. Experiments using multiple-fluorescent labeling/fluorogold revealed that nisoxetine activates presumed glutamatergic pyramidal cells that project from the mPFc to the NAc. Activated glutamatergic projections from the mPFc to the NAc appear to have substantial roles in the ability of a NET inhibitor to normalize PPI deficits in DAT KO. Thus, this data suggest that selective NET inhibitors such as nisoxetine might improve information processing deficits in schizophrenia via regulation of cortico-subcortical neuromodulation.

Impaired spatial working memory and decreased frontal cortex BDNF protein level in dopamine transporter knockout mice.

Brain-derived neurotrophic factor (BDNF), one of the key brain neurotrophins, has been implicated in neuronal plasticity and memory. Recent studies document the importance of BDNF for normal long-term memory functions. However, there are few studies of the roles of BDNF in short-term memory. Dopamine is likely to play important roles in BDNF gene expression in specific brain regions, including frontal cortical regions that are implicated in short-term working memory processes that include spontaneous alternation. We have thus tested spatial working memory in dopamine transporter knockout (DAT KO) and wild-type mice. Spontaneous alternation in the Y-maze, an index of short-term spatial working memory in mice, was significantly decreased in DAT KO mice compared to wild-type mice. BDNF protein was significantly decreased in frontal cortex, though not in striatum or hippocampus, of the DAT KO mice. The data support the hypothesis that impaired spatial working memory in DAT KO mice may be related to decreased frontal cortical BDNF in these animals, and document apparent roles for BDNF in a short-term memory process.

Monoamine transporter as a target molecule for psychostimulants.

Methamphetamine (MAP), a drug of abuse known worldwide for its addictive effects and neurotoxicity, causes somatic and psychiatric disorders. MAP enters terminals/neurons via monoamine transporters, displaces both vesicular and intracellular monoamines, and facilitates the release of monoamines into the extraneuronal space through synaptic transport via the monoamine transporters. Chronic psychostimulant abusers exhibit psychotic features, including delusions and auditory hallucinations. The dopamine transporter (DAT) and the vesicular monoamine transporter 2 (VMAT2) play pivotal roles in the action of MAP, including locomotor effects. The deletion of DAT attenuates the locomotor effects of MAP and may play larger role in behavioral responses to MAP compared to the deletion of VMAT2. MAP produces hyperthermia and/or neuronal toxicity in most species. The effects of MAP in DAT or serotonin transporter (SERT) single knockout (KO) mice and DAT/SERT double KO mice suggested that DAT and SERT are key molecules for hyperthermia and neuronal toxicity of MAP.