Differential psychostimulant-induced activation of neural circuits in dopamine transporter knockout and wild type mice

The Evolving Role of Animal Models in the Discovery and Development of Novel Treatments for Psychiatric Disorders

Historically, animal models have been routinely used in the characterization of novel chemical entities (NCEs) for various psychiatric disorders. Animal models have been essential in the in vivo validation of novel drug targets, establishment of lead compound pharmacokinetic to pharmacodynamic relationships, optimization of lead compounds through preclinical candidate selection, and development of translational measures of target occupancy and functional target engagement. Yet, with decades of multiple NCE failures in Phase II and III efficacy trials for different psychiatric disorders, the utility and value of animal models in the drug discovery process have come under intense scrutiny along with the widespread withdrawal of the pharmaceutical industry from psychiatric drug discovery. More recently, the development and utilization of animal models for the discovery of psychiatric NCEs has undergone a dynamic evolution with the application of the Research Domain Criteria (RDoC) framework for better design of preclinical to clinical translational studies combined with innovative genetic, neural circuitry-based, and automated testing technologies. In this chapter, the authors will discuss this evolving role of animal models for improving the different stages of the discovery and development in the identification of next generation treatments for psychiatric disorders.

Heterozygote Dopamine Transporter Knockout Rats Display Enhanced Cocaine Locomotion in Adolescent Females

Cocaine is a powerful psychostimulant that is one of the most widely used illicit addictive. The dopamine transporter (DAT) plays a major role in mediating cocaine's reward effect. Decreases in DAT expression increase rates of drug abuse and vulnerability to comorbid psychiatric disorders. We used the novel DAT transgenic rat model to study the effects of cocaine on locomotor behaviors in adolescent rats, with an emphasis on sex. Female rats showed higher response rates to cocaine at lower acute and chronic doses, highlighting a higher vulnerability and perceived gender effects. In contrast, locomotor responses to an acute high dose of cocaine were more marked and sustained in male DAT heterozygous (HET) adolescents. The results demonstrate the augmented effects of chronic cocaine in HET DAT adolescent female rats. Knockout (KO) DAT led to a level of hyperdopaminergia which caused a marked basal hyperactivity that was unchanged, consistent with a possible ceiling effect. We suggest a role of alpha synuclein (α-syn) and PICK 1 protein expressions to the increased vulnerability in female rats. These proteins showed a lower expression in female HET and KO rats. This study highlights gender differences associated with mutations which affect DAT expression and can increase susceptibility to cocaine abuse in adolescence.

Review of rodent models of attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a polygenic neurodevelopmental disorder that affects 8-12 % of children and >4 % of adults. Environmental factors are believed to interact with genetic predispositions to increase susceptibility to ADHD. No existing rodent model captures all aspects of ADHD, but several show promise. The main genetic models are the spontaneous hypertensive rat, dopamine transporter knock-out (KO) mice, dopamine receptor subtype KO mice, Snap-25 KO mice, guanylyl cyclase-c KO mice, and latrophilin-3 KO mice and rats. Environmental factors thought to contribute to ADHD include ethanol, nicotine, PCBs, lead (Pb), ionizing irradiation, 6-hydroxydopamine, neonatal hypoxia, some pesticides, and organic pollutants. Model validation criteria are outlined, and current genetic models evaluated against these criteria. Future research should explore induced multiple gene KOs given that ADHD is polygenic and epigenetic contributions. Furthermore, genetic models should be combined with environmental agents to test for interactions.

Functional MHCI deficiency induces ADHD-like symptoms with increased dopamine D1 receptor expression

Inappropriate synaptic development has been proposed as a potential mechanism of neurodevelopmental disorders, including attention-deficit hyperactivity disorder (ADHD). Major histocompatibility complex class I (MHCI), an immunity-associated molecule expressed by neurons in the brain, regulates synaptic development; however, the involvement of MHCI in these disorders remains elusive. We evaluated whether functional MHCI deficiency induced by β2m^-/-Tap1^-/- double-knockout in mice leads to abnormalities akin to those seen in neurodevelopmental disorders. We found that functional MHCI deficiency induced locomotor hyperactivity, motor impulsivity, and attention deficits, three major symptoms of ADHD. In contrast, these mice showed normal spatial learning, behavioral flexibility, social behavior, and sensorimotor integration. In the analysis of the dopamine system, upregulation of dopamine D1 receptor (D1R) expression in the nucleus accumbens and a greater locomotor response to D1R agonist SKF 81297 were found in the functional MHCI-deficient mice. Low-dose methylphenidate, used for the treatment of ADHD patients, alleviated the three behavioral symptoms and suppressed c-Fos expression in the D1R-expressing medium spiny neurons of the mice. These findings reveal an unexpected role of MHCI in three major symptoms of ADHD and may provide a novel landmark in the pathogenesis of ADHD.

NURR1 deficiency is associated to ADHD-like phenotypes in mice

The transcription factor NURR1 regulates the dopamine (DA) signaling pathway and exerts a critical role in the development of midbrain dopaminergic neurons (mDA). NURR1 alterations have been linked to DA-associated brain disorders, such as Parkinson's disease and schizophrenia. However, the association between NURR1 defects and the attention-deficit hyperactivity disorder (ADHD), a DA-associated brain disease characterized by hyperactivity, impulsivity and inattention, has never been demonstrated. To date, a comprehensive murine model of ADHD truly reflecting the whole complex human psychiatric disorder still does not exist. NURR1-knockout (NURR1-KO) mice have been reported to exhibit increased spontaneous locomotor activity, but their complete characterization is still lacking. In the present study a wide-ranging test battery was used to perform a comprehensive analysis of the behavioral phenotype of the male NURR1-KO mice. As a result, their hyperactive phenotype was confirmed, while their impulsive behavior was reported for the first time. On the other hand, no anxiety and alterations in motor coordination, sociability and memory were observed. Also, the number of mDA expressing tyrosine hydroxylase, a rate-limiting enzyme of catecholamines biosynthesis, and DA level in brain were not impaired in NURR1-KO mice. Finally, hyperactivity has been shown to be recovered by treatment with methylphenidate, the first line psychostimulant drug used for ADHD. Overall, our study suggests that the NURR1 deficient male mouse may be a satisfactory model to study some ADHD behavioral phenotypes and to test the clinical efficacy of potential therapeutic agents.

Polymorphisms in Dopaminergic Genes in Schizophrenia and Their Implications in Motor Deficits and Antipsychotic Treatment

Dopaminergic system dysfunction is involved in schizophrenia (SCZ) pathogenesis and can mediate SCZ-related motor disorders. Recent studies have gradually revealed that SCZ susceptibility and the associated motor symptoms can be mediated by genetic factors, including dopaminergic genes. More importantly, polymorphisms in these genes are associated with both antipsychotic drug sensitivity and adverse effects. The study of genetic polymorphisms in the dopaminergic system may help to optimize individualized drug strategies for SCZ patients. This review summarizes the current progress about the involvement of the dopamine system in SCZ-associated motor disorders and the motor-related adverse effects after antipsychotic treatment, with a special focus on polymorphisms in dopaminergic genes. We hypothesize that the genetic profile of the dopaminergic system mediates both SCZ-associated motor deficits associated and antipsychotic drug-related adverse effects. The study of dopaminergic gene polymorphisms may help to predict drug efficacy and decrease adverse effects, thereby optimizing treatment strategies.

Dissociation of impulsivity and aggression in mice deficient for the ADHD risk gene Adgrl3: Evidence for dopamine transporter dysregulation

Adhesion G protein-coupled receptor L3 (ADGRL3, LPHN3) has putative roles in neuronal migration and synapse function. Various polymorphisms in ADGRL3 have been linked with an increased risk of attention deficit/hyperactivity disorder (ADHD). In this study, we examined the characteristics of Adgrl3-deficient mice in multiple behavioural domains related to ADHD: locomotive activity, impulsivity, gait, visuospatial and recognition memory, sociability, anxiety-like behaviour and aggression. Additionally, we investigated the effect of Adgrl3-depletion at the transcriptomic level by RNA-sequencing three ADHD-relevant brain regions: prefrontal cortex (PFC), hippocampus and striatum. Adgrl3^-/- mice show increased locomotive activity across all tests and subtle gait abnormalities. These mice also show impairments across spatial memory and learning domains, alongside increased levels of impulsivity and sociability with decreased aggression. However, these alterations were absent in Adgrl3^+/- mice. Across all brain regions tested, the numbers of genes found to exhibit differential expression was relatively small, indicating a specific pathway of action, rather than a broad neurobiological perturbation. Gene-set analysis of differential expression in the PFC detected a number of ADHD-relevant pathways including dopaminergic synapses as well as cocaine and amphetamine addiction. The Slc6a3 gene coding for the dopamine transporter was the most dysregulated gene in the PFC. Unexpectedly, several neurohormone/peptides which are typically only expressed in the hypothamalus were found to be dysregulated in the striatum. Our study further validates Adgrl3 constitutive knockout mice as an experimental model of ADHD while providing neuroanatomical targets for future studies involving ADGRL3 modified models. This article is part of the Special Issue entitled 'Current status of the neurobiology of aggression and impulsivity'.

Developmental role of adenosine kinase for the expression of sex-dependent neuropsychiatric behavior

Deficits in social memory, cognition, and aberrant responses to stimulants are common among persons affected by schizophrenia and other conditions with a presumed developmental etiology. We previously found that expression changes in the adenosine metabolizing enzyme adenosine kinase (ADK) in the adult brain are associated with deficits in various cognitive domains. To distinguish between developmental and adult functions of ADK, we used two transgenic mouse lines with widespread disruption of ADK expression in the adult brain, but differences in the onset of ADK deletion. Specifically, we compared Nestin-Cre^+/-:ADK-flox^fl/fl (ADK^ΔBrain) mice with global loss of ADK in the whole brain, beginning in mid-gestation and persisting for life, with Gfa2-Cre^+/-:ADK-flox^fl/fl (ADK^ΔAstro) mice that have normal ADK expression throughout development, but lose astrocyte-specific ADK-expression in young adulthood. Because ADK-expression in adulthood is generally confined to astrocytes, adult ADK^ΔAstro mice show a similar expression profile of ADK in key areas of the brain related to neuropsychiatric behavior, compared to adult ADK^ΔBrain mice. We sought to determine a neurodevelopmental role of ADK on the expression of psychiatric behaviors in adult male and female mice. Adult ADK^ΔBrain mice showed significant deficits in social memory in males, significant contextual learning impairments in both sexes, and a hyper-responsiveness to amphetamine in males. In contrast, ADK^ΔAstro mice showed normal social memory and contextual learning but hypo-responsiveness to amphetamine in males. Our results demonstrate a key developmental role of ADK in mediating behaviors in adulthood related to neuropsychiatric disease and support the greater prevalence of these disorders among males.

N-methyl-d-aspartate receptor dysfunction in the prefrontal cortex of stroke-prone spontaneously hypertensive rat/Ezo as a rat model of attention deficit/hyperactivity disorder

AIM

We previously reported that stroke-prone spontaneously hypertensive rat/Ezo (SHRSP/Ezo) has high validity as an attention deficit/hyperactivity disorder (AD/HD) animal model, based on its behavioral phenotypes, such as inattention, hyperactivity, and impulsivity. Fronto-cortical dysfunction is implicated in the pathogenesis of AD/HD. In this study, we investigated prefrontal cortex (PFC) function in SHRSP/Ezo rats by electrophysiological methods and radioreceptor assay.

METHODS

We recorded excitatory postsynaptic potential in layer V pyramidal neurons in the PFC by intracellular recording method to assess synaptic plasticity in the form of long-term potentiation (LTP). We also performed N-methyl-d-aspartate acid (NMDA) receptor binding assay in the PFC and hippocampus using radiolabeled NMDA receptor antagonist [³ H]MK-801.

RESULTS

Theta-burst stimulation induced LTP in the PFC of genetic control, WKY/Ezo, whereas failed to induce LTP in that of SHRSP/Ezo. The K_d value of [³ H]MK-801 binding for NMDA receptors in the PFC of SHRSP/Ezo was higher than in the WKY/Ezo. Neither the B_max nor K_d of [³ H]MK-801 binding in the SHRSP/Ezo hippocampus was significantly different to WKY/Ezo.

CONCLUSION

These results suggest that the AD/HD animal model SHRSP/Ezo has NMDA receptor dysfunction in the PFC.

Glutamatergic synapses in neurodevelopmental disorders

Neurodevelopmental disorders (NDDs) are a group of diseases whose symptoms arise during childhood or adolescence and that impact several higher cognitive functions such as learning, sociability and mood. Accruing evidence suggests that a shared pathogenic mechanism underlying these diseases is the dysfunction of glutamatergic synapses. We summarize present knowledge on autism spectrum disorders (ASD), intellectual disability (ID), Down syndrome (DS), Rett syndrome (RS) and attention-deficit hyperactivity disorder (ADHD), highlighting the involvement of glutamatergic synapses and receptors in these disorders. The most commonly shared defects involve α-amino-3-hydroxy-5-methyl- 4-isoxazole propionic acid receptors (AMPARs), N-methyl-d-aspartate receptors (NMDARs) and metabotropic glutamate receptors (mGluRs), whose functions are strongly linked to synaptic plasticity, affecting both cell-autonomous features as well as circuit formation. Moreover, the major scaffolding proteins and, thus, the general structure of the synapse are often deregulated in neurodevelopmental disorders, which is not surprising considering their crucial role in the regulation of glutamate receptor positioning and functioning. This convergence of defects supports the definition of neurodevelopmental disorders as a continuum of pathological manifestations, suggesting that glutamatergic synapses could be a therapeutic target to ameliorate patient symptomatology.

Effect of methylphenidate on visual responses in the superior colliculus in the anaesthetised rat: Role of cortical activation

The mechanism of action of psychostimulant drugs in the treatment of Attention Deficit Hyperactivity Disorder is still largely unknown, although recent evidence suggests one possibility is that the drugs affect the superior colliculus (SC). We have previously demonstrated that systemically administered d-amphetamine attenuates/abolishes visual responses to wholefield light flashes in the superficial layers of the SC in anaesthetised rats, and the present study sought to extend this work to methylphenidate (MPH). Anaesthetised rats were administered MPH at a range of doses (or saline) and subjected to monocular wholefield light flashes at two intensities, juxta-threshold and super-threshold. In contrast to d-amphetamine, systemic MPH produced an enhancement of visual activity at both intensities. Methylphenidate was also found to produce activation of the cortical EEG in anaesthetised rats. Furthermore, cortical activation induced by electrical stimulation of the pons was found to enhance visual responses in superficial layers of the SC, and when MPH was paired with pontine-induced cortical activation, the response-enhancing effects of MPH were substantially attenuated. Taken together, the results suggest that the enhancement of visual responses in the superficial layers of the SC by MPH in the anaesthetised rat is an artefact of the drug's interaction with cortical arousal.

Attentional deficits and altered neuronal activation in medial prefrontal and posterior parietal cortices in mice with reduced dopamine transporter levels

The executive control function of attention is regulated by the dopaminergic (DA) system. Dopamine transporter (DAT) likely plays a role in controlling the influence of DA on cognitive processes. We examined the effects of DAT depletion on cognitive processes related to attention. Mice with the DAT gene genetically deleted (DAT+/- heterozygotes) were compared to wild type (WT) mice on the Attentional Set-Shifting Task (ASST). Changes in neuronal activity during the ASST were shown with early growth response genes 1 and 2 (egr-1 and egr-2) immunohistochemistry in the medial prefrontal cortex (mPFC) and in the posterior parietal cortex (PPC). Heterozygotes were impaired in tasks that tax reversal learning, attentional-set formation and set-shifting. Densities of egr-2 labeled cells in the mPFC were lower in mutant mice when compared with wild-types in intradimensional shift of attention (IDS), extradimensional shift of attention and extradimensional shift of attention-reversal phases of the ASST task, and in PPC in the IDS phase of the task. The results demonstrate impairments of the areas associated with attentional functions in DAT+/- mice and show that an imbalance of the dopaminergic system has an impact on the complex attention-related executive functions.

A new mouse model of ADHD for medication development

ADHD is a major societal problem with increasing incidence and a stagnant track record for treatment advances. A lack of appropriate animal models has partly contributed to the incremental advance of this field. Hence, our goal was to generate a novel mouse model that could be useful for ADHD medication development. We reasoned that hyperactivity is a core feature of ADHD that could easily be bred into a population, but to what extent other hallmark features of ADHD would appear as correlated responses was unknown. Hence, starting from a heterogeneous population, we applied within-family selection over 16 generations to produce a High-Active line, while simultaneously maintaining an unselected line to serve as the Control. We discovered that the High-Active line demonstrated motor impulsivity in two different versions of the Go/No-go test, which was ameliorated with a low dose of amphetamine, and further displayed hypoactivation of the prefrontal cortex and dysregulated cerebellar vermal activation as indexed by c-Fos immunohistochemical staining. We conclude that the High-Active line represents a valid model for the Hyperactive-Impulsive subtype of ADHD and therefore may be used in future studies to advance our understanding of the etiology of ADHD and screen novel compounds for its treatment.

Methylphenidate modulates dorsal raphe neuronal activity: Behavioral and neuronal recordings from adolescent rats

Methylphenidate (MPD) is a widely prescribed psychostimulants used for the treatment of attention deficit hyperactive disorder (ADHD). Unlike the psychostimulants cocaine and amphetamine, MPD does not exhibit direct actions on the serotonin transporter, however there is evidence suggesting that the therapeutic effects of MPD may be mediated in part by alterations in serotonin transmission. This study aimed to investigate the role of the dorsal raphe (DR) nucleus, one of the major sources of serotonergic innervation in the mammalian brain, in the response to MPD exposure. Freely behaving adolescent rats previously implanted bilaterally with permanent electrodes were used. An open field assay and a wireless neuronal recording system were used to concomitantly record behavioral and DR electrophysiological activity following acute and chronic MPD exposure. Four groups were used: one control (saline) and three experimental groups treated with 0.6, 2.5, and 10.0mg/kg MPD respectively. Animals received daily MPD or saline injections on experimental days 1-6, followed by 3 washout days and MPD rechallenge dose on experimental day (ED)10. The same chronic dose of MPD resulted in either behavioral sensitization or tolerance, and we found that neuronal activity recorded from the DR neuronal units of rats expressing behavioral sensitization to chronic MPD exposure responded significantly differently to MPD rechallenge on ED10 compared to the DR unit activity recorded from animals that expressed behavioral tolerance. This correlation between behavioral response and DR neuronal activity following chronic MPD exposure provides evidence that the DR is involved in the acute effects as well as the chronic effects of MPD in adolescent rats.

Object recognition impairment and rescue by a dopamine D2 antagonist in hyperdopaminergic mice

Genetically-modified mice without the dopamine transporter (DAT) are hyperdopaminergic, and serve as models for studies of addiction, mania and hyperactive disorders. Here we investigated the capacity for object recognition in mildly hyperdopaminergic mice heterozygous for DAT (DAT +/-), with synaptic dopaminergic levels situated between those shown by DAT -/- homozygous and wild-type (WT) mice. We used a classical dopamine D2 antagonist, haloperidol, to modulate the levels of dopaminergic transmission in a dose-dependent manner, before or after exploring novel objects. In comparison with WT mice, DAT +/- mice showed a deficit in object recognition upon subsequent testing 24h later. This deficit was compensated by a single 0.05mg/kg haloperidol injection 30min before training. In all mice, a 0.3mg/kg haloperidol injected immediately after training impaired object recognition. The results indicate that a mild enhancement of dopaminergic levels can be detrimental to object recognition, and that this deficit can be rescued by a low dose of a D2 dopamine receptor antagonist. This suggests that novel object recognition is optimal at intermediate levels of D2 receptor activity.

Clock Speed as a Window into Dopaminergic Control of Emotion and Time Perception

Although fear-producing treatments (e.g., electric shock) and pleasure-inducing treatments (e.g., methamphetamine) have different emotional valences, they both produce physiological arousal and lead to effects on timing and time perception that have been interpreted as reflecting an increase in speed of an internal clock. In this commentary, we review the results reported by Fayolle et al. (2015):Behav. Process., 120, 135–140) and Meck (1983: J. Exp. Psychol. Anim. Behav. Process., 9, 171–201) using electric shock and by Maricq et al. (1981: J. Exp. Psychol. Anim. Behav. Process., 7, 18–30) using methamphetamine in a duration-bisection procedure across multiple duration ranges. The psychometric functions obtained from this procedure relate the proportion ‘long’ responses to signal durations spaced between a pair of ‘short’ and ‘long’ anchor durations. Horizontal shifts in these functions can be described in terms of attention or arousal processes depending upon whether they are a fixed number of seconds independent of the timed durations (additive) or proportional to the durations being timed (multiplicative). Multiplicative effects are thought to result from a change in clock speed that is regulated by dopamine activity in the medial prefrontal cortex. These dopaminergic effects are discussed within the context of the striatal beat frequency model of interval timing (Matell & Meck, 2004:Cogn. Brain Res.,21, 139–170) and clinical implications for the effects of emotional reactivity on temporal cognition (Parker et al., 2013:Front. Integr. Neurosci., 7, 75).

NMDA receptor agonists reverse impaired psychomotor and cognitive functions associated with hippocampal Hbegf-deficiency in mice

BACKGROUND

Structural and functional changes of the hippocampus are correlated with psychiatric disorders and cognitive dysfunctions. Genetic deletion of heparin-binding epidermal growth factor-like growth factor (HB-EGF), which is predominantly expressed in cortex and hippocampus, also causes similar psychiatric and cognitive dysfunctions, accompanying down-regulated NMDA receptor signaling. However, little is known of such dysfunctions in hippocampus-specific Hbegf cKO mice.

RESULTS

We successfully developed hippocampus-specific cKO mice by crossbreeding floxed Hbegf and Gng7-Cre knock-in mice, as Gng7 promoter-driven Cre is highly expressed in hippocampal neurons as well as striatal medium spiny neurons. In mice lacking hippocampus Hbegf gene, there was a decreased neurogenesis in the subgranular zone (SGZ) of the dentate gyrus as well as down-regulation of PSD-95/NMDA-receptor-NR1/NR2B subunits and related NMDA receptor signaling. Psychiatric, social-behavioral and cognitive abnormalities were also observed in hippocampal cKO mice. Interestingly, D-cycloserine and nefiracetam, positive allosteric modulators (PAMs) of NMDA receptor reversed the apparent reduction in NMDA receptor signaling and most behavioral abnormalities. Furthermore, decreased SGZ neurogenesis in hippocampal cKO mice was reversed by nefiracetam.

CONCLUSIONS

The present study demonstrates that PAMs of NMDA receptor have pharmacotherapeutic potentials to reverse down-regulated NMDA receptor signaling, neuro-socio-cognitive abnormalities and decreased neurogenesis in hippocampal cKO mice.

Investigating the underlying mechanisms of aberrant behaviors in bipolar disorder from patients to models: Rodent and human studies

Psychiatric patients with bipolar disorder suffer from states of depression and mania, during which a variety of symptoms are present. Current treatments are limited and neurocognitive deficits in particular often remain untreated. Targeted therapies based on the biological mechanisms of bipolar disorder could fill this gap and benefit patients and their families. Developing targeted therapies would benefit from appropriate animal models which are challenging to establish, but remain a vital tool. In this review, we summarize approaches to create a valid model relevant to bipolar disorder. We focus on studies that use translational tests of multivariate exploratory behavior, sensorimotor gating, decision-making under risk, and attentional functioning to discover profiles that are consistent between patients and rodent models. Using this battery of translational tests, similar behavior profiles in bipolar mania patients and mice with reduced dopamine transporter activity have been identified. Future investigations should combine other animal models that are biologically relevant to the neuropsychiatric disorder with translational behavioral assessment as outlined here. This methodology can be utilized to develop novel targeted therapies that relieve symptoms for more patients without common side effects caused by current treatments.

Synaptic Cytoskeletal Plasticity in the Prefrontal Cortex Following Psychostimulant Exposure

Addiction is characterized by maladaptive decision-making, a loss of control over drug consumption and habit-like drug seeking despite adverse consequences. These cognitive changes may reflect the effects of drugs of abuse on prefrontal cortical neurobiology. Here, we review evidence that amphetamine and cocaine fundamentally remodel the structure of excitatory neurons in the prefrontal cortex. We summarize evidence in particular that these psychostimulants have opposing effects in the medial and orbital prefrontal cortices ('mPFC' and 'oPFC', respectively). For example, amphetamine and cocaine increase dendrite length and spine density in the mPFC, while dendrites are impoverished and dendritic spines are eliminated in the oPFC. We will discuss evidence that certain cytoskeletal regulatory proteins expressed in the oPFC and implicated in postnatal (adolescent) neural development also regulate behavioral sensitivity to cocaine. These findings potentially open a window of opportunity for the identification of novel pharmacotherapeutic targets in the treatment of drug abuse disorders in adults, as well as in drug-vulnerable adolescent populations. Finally, we will discuss the behavioral implications of drug-related dendritic spine elimination in the oPFC, with regard to reversal learning tasks and tasks that assess the development of reward-seeking habits, both used to model aspects of addiction in rodents.

Acute oral administration of low doses of methylphenidate targets calretinin neurons in the rat septal area

Methylphenidate (MPD) is a commonly administered drug to treat children suffering from attention deficit hyperactivity disorder (ADHD). Alterations in septal driven hippocampal theta rhythm may underlie attention deficits observed in these patients. Amongst others, the septo-hippocampal connections have long been acknowledged to be important in preserving hippocampal function. Thus, we wanted to ascertain if MPD administration, which improves attention in patients, could affect septal areas connecting with hippocampus. We used low and orally administered MPD doses (1.3, 2.7 and 5 mg/Kg) to rats what mimics the dosage range in humans. In our model, we observed no effect when using 1.3 mg/Kg MPD; whereas 2.7 and 5 mg/Kg induced a significant increase in c-fos expression specifically in the medial septum (MS), an area intimately connected to the hippocampus. We analyzed dopaminergic areas such as nucleus accumbens and striatum, and found that only 5 mg/Kg induced c-fos levels increase. In these areas tyrosine hydroxylase correlated well with c-fos staining, whereas in the MS the sparse tyrosine hydroxylase fibers did not overlap with c-fos positive neurons. Double immunofluorescence of c-fos with neuronal markers in the septal area revealed that co-localization with choline acethyl transferase, parvalbumin, and calbindin with c-fos did not change with MPD treatment; whereas, calretinin and c-fos double labeled neurons increased after MPD administration. Altogether, these results suggest that low and acute doses of methylphenidate primary target specific populations of caltretinin medial septal neurons.

The effects of atomoxetine and methylphenidate on the prepulse inhibition of the acoustic startle response in mice

Atomoxetine (ATM) and methylphenidate (MPD) have been used for the treatment of attention deficit hyperactivity disorder (ADHD). ATM is a selective norepinephrine reuptake inhibitor, whereas MPD is a psychostimulant and acts as a norepinephrine and dopamine reuptake inhibitor. In the present study, we investigated the effects of ATM (1, 3 or 10mg/kg) and MPD (5, 10 or 20mg/kg) on pharmacological mouse models of sensorimotor gating measured by prepulse inhibition (PPI) using the acoustic startle response test. MK-801, a non-competitive N-methyl-d-aspartate receptor antagonist, or apomorphine, a non-competitive dopamine receptor agonist, was used to induce PPI deficits. ATM (3 or 10mg/kg, s.c.) significantly attenuated the MK-801-, but not apomorphine-, induced PPI deficits. In contrast to ATM, MPD did not reverse the PPI deficits induced by either MK-801 or apomorphine. Immunostaining revealed that the number of c-Fos-immunopositive cells was increased in the nucleus accumbens following MK-801 treatment, and this was reversed by the administration of ATM (3mg/kg), but not MPD (10mg/kg). However, neither ATM nor MPD reversed the increased number of c-Fos-immunopositive cells in the nucleus accumbens following apomorphine treatment. These results suggest that the attenuating effect of ATM on the increased c-Fos immunoreactivity in the nucleus accumbens induced by MK-801 may be attributed to the PPI deficit-ameliorating effects of ATM and that ATM would be useful to treat sensorimotor gating-related disorders by improving the patient's attention span or cognitive function.

Attention as an effect not a cause

Attention is commonly thought to be important for managing the limited resources available in sensory areas of the neocortex. Here we present an alternative view that attention arises as a byproduct of circuits centered on the basal ganglia involved in value-based decision making. The central idea is that decision making depends on properly estimating the current state of the animal and its environment and that the weighted inputs to the currently prevailing estimate give rise to the filter-like properties of attention. After outlining this new framework, we describe findings from physiological, anatomical, computational, and clinical work that support this point of view. We conclude that the brain mechanisms responsible for attention employ a conserved circuit motif that predates the emergence of the neocortex.

Postmortem brain: an underutilized substrate for studying severe mental illness

We propose that postmortem tissue is an underutilized substrate that may be used to translate genetic and/or preclinical studies, particularly for neuropsychiatric illnesses with complex etiologies. Postmortem brain tissues from subjects with schizophrenia have been extensively studied, and thus serve as a useful vehicle for illustrating the challenges associated with this biological substrate. Schizophrenia is likely caused by a combination of genetic risk and environmental factors that combine to create a disease phenotype that is typically not apparent until late adolescence. The complexity of this illness creates challenges for hypothesis testing aimed at understanding the pathophysiology of the illness, as postmortem brain tissues collected from individuals with schizophrenia reflect neuroplastic changes from a lifetime of severe mental illness, as well as treatment with antipsychotic medications. While there are significant challenges with studying postmortem brain, such as the postmortem interval, it confers a translational element that is difficult to recapitulate in animal models. On the other hand, data derived from animal models typically provide specific mechanistic and behavioral measures that cannot be generated using human subjects. Convergence of these two approaches has led to important insights for understanding molecular deficits and their causes in this illness. In this review, we discuss the problem of schizophrenia, review the common challenges related to postmortem studies, discuss the application of biochemical approaches to this substrate, and present examples of postmortem schizophrenia studies that illustrate the role of the postmortem approach for generating important new leads for understanding the pathophysiology of severe mental illness.

Vestibular insights into cognition and psychiatry

The vestibular system has traditionally been thought of as a balance apparatus; however, accumulating research suggests an association between vestibular function and psychiatric and cognitive symptoms, even when balance is measurably unaffected. There are several brain regions that are implicated in both vestibular pathways and psychiatric disorders. The present review examines the anatomical associations between the vestibular system and various psychiatric disorders. Despite the lack of direct evidence for vestibular pathology in the key psychiatric disorders selected for this review, there is a substantial body of literature implicating the vestibular system in each of the selected psychiatric disorders. The second part of this review provides complimentary evidence showing the link between vestibular dysfunction and vestibular stimulation upon cognitive and psychiatric symptoms. In summary, emerging research suggests the vestibular system can be considered a potential window for exploring brain function beyond that of maintenance of balance, and into areas of cognitive, affective and psychiatric symptomology. Given the paucity of biological and diagnostic markers in psychiatry, novel avenues to explore brain function in psychiatric disorders are of particular interest and warrant further exploration.

Behavioral and dorsal raphe neuronal activity following acute and chronic methylphenidate in freely behaving rats

Concomitant behavioral and dorsal raphe (DR) neuronal activity were recorded following acute and chronic dose response of methylphenidate (MPD) in freely moving rats previously implanted with permanent semi-microelectrodes using telemetric (wireless) technology. On experimental day (ED) 1, the neuronal and locomotor activity were recorded after saline (baseline) and MPD (0.6, 2.5 or 10.0mg/kg) injection (i.p.). Animals were injected daily with a single dose of MPD for five consecutive days (ED 2-6) to elicit behavioral sensitization or tolerance. After three washout days, the neuronal and locomotor activity recording was resumed on ED 10 followed by saline and MPD rechallenge injection. The main findings were: (1) the same dose of chronic MPD administration elicited behavioral sensitization in some animals and behavioral tolerance in others. (2) 46%, 56% and 73% of DR units responded to acute 0.6, 2.5 and 10.0mg/kg MPD respectively. (3) 89%, 70% and 86% of DR units changed their baseline activity on ED 10 compared to that on ED 1 in the 0.6, 2.5 and 10.0mg/kg MPD groups respectively. (4) A significant difference in ED 10 baseline activity was observed in the DR neuronal population recording from animals expressing behavioral sensitization compared to that of animals expressing behavioral tolerance. (5) 89%, 78% and 88% of DR units responded to chronic 0.6, 2.5 and 10.0mg/kg MPD respectively. (6) The DR neuronal population recording following acute MPD on ED 1 and rechallenge MPD on ED 10 from animals expressing behavioral sensitization was significantly different from the neuronal population recorded from animals exhibited behavioral tolerance. The correlation between the DR neuronal activity and animal's behavior following chronic MPD exposure suggested that the DR neuronal activity may play an important role in the expression of behavioral sensitization and tolerance induced by chronic MPD administration.

Effects of N-phenylpropyl-N'-substituted piperazine sigma receptor ligands on cocaine-induced hyperactivity in mice

The present study examined N-phenylpropyl-N'-substituted piperazine sigma receptor ligands on cocaine-induced changes in locomotor activity in mice. Previous reports indicate that N-phenylpropyl-N'-(4-methoxybenzyl)piperazine (Nahas-3h), N-phenylpropyl-N'-(4-methoxyphenethyl)piperazine (YZ-067), and N-phenylpropyl-N'-(3-methoxyphenethyl)piperazine (YZ-185) bind with high affinity (Ki values≈1 nM) to σ1 sigma receptors. YZ-067 and YZ-185 are known to attenuate cocaine-induced convulsions, while Nahas-3h has not been tested in behavioral studies. Nahas-3h significantly attenuated cocaine-induced hyperactivity. YZ-067 decreased the effect of cocaine in a dose-dependent manner. Interestingly, YZ-185 inhibited cocaine's effect at higher doses, but enhanced cocaine's effect at a low dose. The YZ-185 inhibition of cocaine-induced hyperactivity was not surmounted by increasing the cocaine dose. Overall, this study is consistent with previous work showing the ability of certain sigma receptor ligands to affect cocaine-induced hyperactivity. Further, subtle alterations of ligand structure and the specific dosage levels employed influence the behavioral effects observed, with a 3-methoxy substituent apparently conferring the ability of a ligand to enhance cocaine's locomotor stimulatory effects.

Inhibition of VTA neurons activates the centrally projecting Edinger-Westphal nucleus: evidence of a stress-reward link?

The primary site of urocortin 1 (Ucn1) expression in the brain is the centrally projecting Edinger-Westphal nucleus. The EWcp is innervated by dopaminergic neurons of the ventral tegmental area (VTA). To investigate whether activity of EWcp is regulated by the VTA, we investigated the effects of local pharmacological inhibition of VTA activity on the induction of Fos immunoreactivity in the EWcp of male C57BL/6J mice. A unilateral intracranial administration of the GABA agonist muscimol aimed at the VTA resulted in increased number of Fos-positive cells in the EWcp. This induction was lower than that produced by an intraperitoneal injection of 2.5 g/kg of ethanol. To investigate whether inhibition of dopaminergic neurons was responsible for induction of Fos, a second experiment was performed where the dopamine agonist quinpirole was unilaterally injected targeting the VTA. Injections of quinpirole also significantly induced Fos in the EWcp neurons. The induction occurred only on the side of the EWcp ipsilateral to the VTA injection. These results indicate that activity of EWcp is inhibited by tonic activity of dopaminergic VTA neurons, and that unilateral projections of VTA onto Ucn1-containing EWcp neurons provide a link between systems regulating approach and avoidance behaviors.

Transgenic mouse models for ADHD

Attention-deficit hyperactivity disorder (ADHD) is a developmental disorder characterized by symptoms of inattention, impulsivity and hyperactivity that adversely affect many aspects of life. Whereas the etiology of ADHD remains unknown, growing evidence indicates a genetic involvement in the development of this disorder. The brain circuits associated with ADHD are rich in monoamines, which are involved in the mechanism of action of psychostimulants and other medications used to treat this disorder. Dopamine (DA) is believed to play a major role in ADHD but other neurotransmitters are certainly also involved. Genetically modified mice have become an indispensable tool used to analyze the contribution of genetic factors in the pathogenesis of human disorders. Although rodent models cannot fully recapitulate complex human psychiatric disorders such as ADHD, transgenic mice offer an opportunity to directly investigate in vivo the specific roles of novel candidate genes identified in ADHD patients. Several knock-out and transgenic mouse models have been proposed as ADHD models, mostly based on targeting genes involved in DA transmission, including the gene encoding the dopamine transporter (DAT1). These mutant models provided an opportunity to evaluate the contribution of dopamine-related processes to brain pathology, to dissect the neuronal circuitry and molecular mechanisms involved in the antihyperkinetic action of psychostimulants and to evaluate novel treatments for ADHD. New transgenic models mouse models targeting other genes have recently been proposed for ADHD. Here, we discuss the recent advances and pitfalls in modeling ADHD endophenotypes in genetically altered animals.

Dorsal raphe neuronal activities are modulated by methylphenidate

This study investigated the electrophysiological properties of the dorsal raphe nucleus (DR) neurons in response to the acute and repetitive administration of methylphenidate (MPH). Activities of DR neurons were recorded from non-anesthetized, freely behaving rats previously implanted bilaterally with permanent semi microelectrodes. The main findings were: (1) after initial (acute) administration of MPH (2.5 mg/kg i.p.) on experimental day one (ED1), 56 % of DR units significantly changed their firing rates. The majority of the responsive units (88 %) exhibited increased firing rate; (2) daily MPH injections were given on ED2 through ED6 followed by 3 washout days. On ED10, 83 % of the DR units significantly changed their baseline activity compared to the baseline activity on ED1; (3) after rechallenge MPH administration on ED10, 63 % of DR units exhibited significant change in their firing rate; the majority of the responsive units (76 %) exhibited a significant increase in their firing rate; (4) The effect of rechallenge MPH administration on ED10 was compared to the effect of initial MPH on ED1, 47 % DR units exhibited a further significant increase in their firing rate while 53 % DR units exhibited decrease or non-change in their firing rate which can be interpreted as electrophysiological sensitization or tolerance. In conclusion, this study demonstrated that acute MPH administration modulated the DR neuronal activities. Repetitive MPH administration modulated the baseline activities of DR units and elicited neurophysiological sensitization or tolerance. The results indicated that MPH affects DR neuronal activity.

Chronic D-amphetamine administered from childhood to adulthood dose-dependently increases the survival of new neurons in the hippocampus of male C57BL/6J mice

Adderall is widely prescribed for attention deficit hyperactivity disorder (ADHD) though long term neurological effects of the main ingredient d-amphetamine are not well understood. The purpose of this study was to examine effects of clinically prescribed doses of d-amphetamine and one abuse dose administered from childhood to adulthood on adult hippocampal neurogenesis and activation of the granule layer of the dentate gyrus. Beginning in early adolescence (age 28 days) to adulthood (age 71), male C57BL/6J mice were administered twice daily i.p. injections of vehicle, 0.25, 0.5 or 2mg/kg d-amphetamine. Locomotor activity was measured in home cages by video tracking. At age 53-56, mice received bromodeoxyuridine (BrdU) injections to label dividing cells. Immunohistochemical detection of BrdU, neuronal nuclear protein (NeuN), doublecortin (DCX) and Ki67 was used to measure neurogenesis and cell proliferation at age 71. ΔFosB was measured as an indicator of repeated neuronal activation. An additional cohort of mice was treated similarly except euthanized at age 58 to measure activation of granule neurons from d-amphetamine (by detection of c-Fos) and cell proliferation (Ki67) at a time when the fate of BrdU cells would have been determined in the first cohort. d-Amphetamine dose-dependently increased survival and differentiation of BrdU cells into neurons and increased number of DCX cells without affecting the number of Ki67 cells. Low doses of d-amphetamine decreased c-Fos and ΔFosB in the granule layer. Only the high dose induced substantial locomotor stimulation and sensitization. Results suggest both therapeutic and abuse doses of d-amphetamine increase the number of new neurons in the hippocampus when administered from adolescence to adulthood by increasing survival and differentiation of cells into neurons not by increasing progenitor cell proliferation. Mechanisms for amphetamine-induced neurogenesis are unknown but appear activity independent. Results suggest part of the beneficial effects of therapeutic doses of d-amphetamine for ADHD could be via increased hippocampal neurogenesis.

Addiction-related gene regulation: risks of exposure to cognitive enhancers vs. other psychostimulants

The psychostimulants methylphenidate (Ritalin, Concerta), amphetamine (Adderall), and modafinil (Provigil) are widely used in the treatment of medical conditions such as attention-deficit hyperactivity disorder and narcolepsy and, increasingly, as "cognitive enhancers" by healthy people. The long-term neuronal effects of these drugs, however, are poorly understood. A substantial amount of research over the past two decades has investigated the effects of psychostimulants such as cocaine and amphetamines on gene regulation in the brain because these molecular changes are considered critical for psychostimulant addiction. This work has determined in some detail the neurochemical and cellular mechanisms that mediate psychostimulant-induced gene regulation and has also identified the neuronal systems altered by these drugs. Among the most affected brain systems are corticostriatal circuits, which are part of cortico-basal ganglia-cortical loops that mediate motivated behavior. The neurotransmitters critical for such gene regulation are dopamine in interaction with glutamate, while other neurotransmitters (e.g., serotonin) play modulatory roles. This review presents (1) an overview of the main findings on cocaine- and amphetamine-induced gene regulation in corticostriatal circuits in an effort to provide a cellular framework for (2) an assessment of the molecular changes produced by methylphenidate, medical amphetamine (Adderall), and modafinil. The findings lead to the conclusion that protracted exposure to these cognitive enhancers can induce gene regulation effects in corticostriatal circuits that are qualitatively similar to those of cocaine and other amphetamines. These neuronal changes may contribute to the addiction liability of the psychostimulant cognitive enhancers.

Reward circuitry dysfunction in psychiatric and neurodevelopmental disorders and genetic syndromes: animal models and clinical findings

This review summarizes evidence of dysregulated reward circuitry function in a range of neurodevelopmental and psychiatric disorders and genetic syndromes. First, the contribution of identifying a core mechanistic process across disparate disorders to disease classification is discussed, followed by a review of the neurobiology of reward circuitry. We next consider preclinical animal models and clinical evidence of reward-pathway dysfunction in a range of disorders, including psychiatric disorders (i.e., substance-use disorders, affective disorders, eating disorders, and obsessive compulsive disorders), neurodevelopmental disorders (i.e., schizophrenia, attention-deficit/hyperactivity disorder, autism spectrum disorders, Tourette's syndrome, conduct disorder/oppositional defiant disorder), and genetic syndromes (i.e., Fragile X syndrome, Prader-Willi syndrome, Williams syndrome, Angelman syndrome, and Rett syndrome). We also provide brief overviews of effective psychopharmacologic agents that have an effect on the dopamine system in these disorders. This review concludes with methodological considerations for future research designed to more clearly probe reward-circuitry dysfunction, with the ultimate goal of improved intervention strategies.

Epigenetics: genetics versus life experiences

Epigenetics is the field of research that examines alterations in gene expression caused by mechanisms other than changes in DNA sequence. ADHD is highly heritable; however, epigenetics are considered relevant in potentially explaining the variance not accounted for by genetic influence. In this chapter, some of the well-known processes of epigenetics, such as chromosome organization, DNA methylation, and effects of transcriptional factors are reviewed along with studies examining the role of these processes in the pathophysiology of ADHD. Potential epigenetic factors conferring risk for ADHD at various developmental stages, such as alcohol, tobacco, toxins, medications, and psychosocial stressor are discussed. Animal studies investigating ADHD medications and changes in CNS Gene/Protein Expression are also explored since they provide insight into the neuronal pathways involved in ADHD pathophysiology. The current limited data suggest that identification of the epigenetic processes involved in ADHD is extremely important and may lead to potential interventions that may be applied to modify the expression of deleterious, as well as protective, genes.

Sex-specific differences in the dynamics of cocaine- and amphetamine-regulated transcript and nesfatin-1 expressions in the midbrain of depressed suicide victims vs. controls

An intriguing novel pathophysiological insight into mood disorders is the notion that one's metabolic status influences mood. In rodents, cocaine- and amphetamine-regulated transcript (CART) and nesfatin-1/NUCB2 have not only been implicated in metabolism, but in the pathobiology of anxiety and depressive-like behaviour, however they have not previously been investigated in depressed subjects. Both peptides are highly expressed in centrally projecting neurons in the Edinger-Westphal nucleus (EWcp) in the midbrain. The EWcp has been implicated in stress adaptation and stress-related mood disorders like major depressive disorder in a sex-specific manner. This is intriguing, given the fact that females have higher prevalence of mood disorders. Here, we hypothesized that the expression of CART and nesfatin-1 in EWcp would exhibit a sex-specific difference between depressed suicide victims vs. controls. We found that CART and nesfatin/NUCB2 colocalized in the human EWcp, and that CART mRNA content was much higher in both male (×3.8) and female (×5.9) drug-free suicide victims than in controls (persons who died without any diagnosed neurodegenerative or psychiatric disorder). Similarly, NUCB2 mRNA content was also higher (×1.8) in male suicides, whereas in female suicide victims, these contents were ×2.7 lower compared to controls. These observations are the first to show changes in the dynamics of CART and nesfatin/NUCB2 expressions in the midbrain of drug-free depressed suicide victims vs. controls. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Differential effect of amphetamine on c-fos expression in female aromatase knockout (ArKO) mice compared to wildtype controls

Estrogen may be involved in psychosis by an interaction with central dopaminergic activity. Aromatase knockout mice are unable to produce estrogen and have been shown to display altered behavioural responses and effects of the dopamine releaser, amphetamine. This study investigates the effect of gonadal status on amphetamine-induced c-fos expression in the brains of female aromatase knockout and wildtype mice. Six groups of mice were treated intraperitoneally with saline or 5mg/kg amphetamine. Fos immunoreactivity was assessed in the cingulate cortex, caudate putamen and nucleus accumbens. Aromatase knockout mice showed markedly reduced amphetamine-induced Fos immunoreactivity compared to wildtype mice. However, the amphetamine response was restored in aromatase-knockout mice after ovariectomy, which reduced this effect in wildtype controls. Estrogen supplementation reversed the effect of ovariectomy in wildtype mice but had no additional significant effect in aromatase-knockout mice. These results indicate that mechanisms involved in amphetamine-induced c-fos expression are altered in aromatase knockout mice and that the primary hormone involved in this effect is not estrogen, but may be another factor released from the ovaries, such as an androgen. These results provide new insight into the effect of gonadal hormones on amphetamine induced c-fos expression in this mouse model of estrogen deficiency. These results could be important for our understanding of the role of sex steroid hormones in psychosis.

Overview of animal models of attention deficit hyperactivity disorder (ADHD)

Attention-deficit/hyperactivity disorder (ADHD) is a heterogeneous, highly heritable, behavioral disorder that affects ∼5% to 10% of children worldwide. Although animal models cannot truly reflect human psychiatric disorders, they can provide insight into the disorder that cannot be obtained from human studies because of the limitations of available techniques. Genetic models include the spontaneously hypertensive rat (SHR), the Naples High Excitability (NHE) rat, poor performers in the 5-choice serial reaction time (5-CSRT) task, the dopamine transporter (DAT) knock-out mouse, the SNAP-25 deficient mutant coloboma mouse, mice expressing a human mutant thyroid hormone receptor, a nicotinic receptor knock-out mouse, and a tachykinin-1 (NK1) receptor knock-out mouse. Chemically induced models of ADHD include prenatal or early postnatal exposure to ethanol, nicotine, polychlorinated biphenyls, or 6-hydroxydopamine (6-OHDA). Environmentally induced models have also been suggested; these include neonatal anoxia and rat pups reared in social isolation. The major insight provided by animal models was the consistency of findings regarding the involvement of dopaminergic, noradrenergic, and sometimes also serotonergic systems, as well as more fundamental defects in neurotransmission.

Association between the dopamine transporter gene and the inattentive subtype of attention deficit hyperactivity disorder in Taiwan

Attention deficit hyperactivity disorder (ADHD) is a common heritable childhood psychiatric disorder. Since methylphenidate, one of the main drugs used to treat ADHD, targets the dopamine transporter, this study examined the linkage disequilibrium (LD) structure of the dopamine transporter gene (DAT1) and investigated whether the DAT1 gene was associated with ADHD. This Chinese family-based association sample consisted of 273 DSM-IV diagnosed ADHD probands and their family members (n=906). We screened 15 polymorphisms across the DAT1 gene, including 14 single nucleotide polymorphism (SNP) markers and the variable number of tandem repeat (VNTR) polymorphism in 3'-untranslated region (3'UTR). Calculations of pairwise LD revealed three main haplotype blocks (HBs): HB1 (intron 2 through intron 6), HB2 (intron 8 through intron 11), and HB3 (3'UTR). Family-Based Association Tests showed that no allele was significantly more transmitted than expected to the ADHD children for these 15 markers. Haplotype-Based Association Tests showed that a haplotype rs27048 (C)/rs429699 (T) was significantly associated with the inattentive subtype (P=0.008). In quantitative analyses, this haplotype also demonstrated significant association with the inattention severity (P=0.012). Our finding of the haplotype rs27048 (C)/rs429699 (T) as a novel genetic marker in the inattentive ADHD subtype suggests that variation in the DAT1 gene may primarily affect the inattentive subtype of ADHD.

Excessive novelty-induced c-Fos expression and altered neurogenesis in the hippocampus of GluA1 knockout mice

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluA1 subunit-deficient (GluA1-/-) mice display novelty-induced hyperactivity, cognitive and social defects and may model psychiatric disorders, such as schizophrenia and depression/mania. We used c-Fos expression in GluA1-/- mice to identify brain regions responsible for novelty-induced hyperlocomotion. Exposure to a novel cage for 2 h significantly increased c-Fos expression in many brain regions in both wild-type and knockout mice. Interestingly, the clearest genotype effect was observed in the hippocampus and its main input region, the entorhinal cortex, where the novelty-induced c-Fos expression was more strongly enhanced in GluA1-/- mice. Their novelty-induced hyperlocomotion partly depended on the activity of AMPA receptors, as it was diminished by the AMPA receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulphonamide (NBQX) and unaffected by the AMPA receptor potentiator 2,3-dihydro-1,4-benzodioxin-6-yl-1-piperidinylmethanone (CX546). The hyperlocomotion of GluA1-/- mice was normalised to the level of wild-type mice within 5-6 h, after which their locomotion followed normal circadian rhythm and was not affected by acute or chronic treatments with the selective serotonin reuptake inhibitor escitalopram. We propose that hippocampal dysfunction, as evidenced by the excessive c-Fos response to novelty, is the major contributor to novelty-induced hyperlocomotion in GluA1-/- mice. Hippocampal dysfunction was also indicated by changes in proliferation and survival of adult-born dentate gyrus cells in the knockout mice. These results suggest focusing on the functions of hippocampal formation, such as novelty detection, when using the GluA1-/- mouse line as a model for neuropsychiatric and cognitive disorders.

Withdrawal from methylphenidate increases neural reactivity of dorsal midbrain

Ritalin (methylphenidate hydrochloride, MP) is a non-amphetamine psychostimulant and is the drug of choice to treat children and adults diagnosed with the attention deficit hyperactivity disorder (ADHD). Several studies have demonstrated that rats treated with MP during early developmental stage exhibit alterations in anxiety-related processes such as an increased response to stressful stimuli and elevated plasma levels of corticosterone. Accordingly, the present study was designed to further characterize the neural and behavioral consequences of withdrawal from MP in adult rats and its influence on the neural reactivity of the dorsal midbrain. After initial exposure to an elevated plus-maze (EPM), brainstem neural activation, elicited by exposure to EPM aversive cues, was analyzed using a Fos-protein immunolabeling technique. Additional independent groups of animals were submitted to electrical stimulation of the dorsal column (DPAG) or the startle response procedure, in order to verify the influence of withdrawal from MP on the expression of unconditioned fear induced by DPAG activation and the effects of or withdrawal from MP on motor response, respectively. Our results provide new findings about the influence of MP treatment in adult rats, showing that, after a sudden MP treatment-break, increased anxiety, associated with the neural sensitization of anxiety-related regions, ensues.

Translational research in bipolar disorder: emerging insights from genetically based models

Bipolar disorder (BPD) is characterized by vulnerability to episodic depression and mania and spontaneous cycling. Because of marked advances in candidate-gene and genome-wide association studies, the list of risk genes for BPD is growing rapidly, creating an unprecedented opportunity to understand the pathophysiology of BPD and to develop novel therapeutics for its treatment. However, genetic findings are associated with major unresolved issues, including whether and how risk variance leads to behavioral abnormalities. Although animal studies are key to resolving these issues, consensus is needed regarding how to define and monitor phenotypes related to mania, depression and mood swing vulnerability in genetically manipulated rodents. In this study we discuss multiple facets of this challenging area, including theoretical considerations, available tests, limitations associated with rodent behavioral modeling and promising molecular-behavioral findings. These include CLOCK, glycogen synthase kinase 3beta (GSK-3beta), glutamate receptor 6 (GluR6), extracellular signal-regulated kinase-1 (ERK1), p11 (or S100A10), vesicular monoamine transporter 2 (VMAT2 or SLC18A2), glucocorticoid receptors (GRs), Bcl-2-associated athanogene-1 (BAG1) and mitochondrial DNA polymerase-gamma (POLG). Some mutant rodent strains show behavioral clusters or activity patterns that cross-species phenocopy objective/observable facets of mood syndromes, and changes in these clustered behaviors can be used as outcome measures in genetic-behavioral research in BPD.

Altered neurocircuitry in the dopamine transporter knockout mouse brain

The plasma membrane transporters for the monoamine neurotransmitters dopamine, serotonin, and norepinephrine modulate the dynamics of these monoamine neurotransmitters. Thus, activity of these transporters has significant consequences for monoamine activity throughout the brain and for a number of neurological and psychiatric disorders. Gene knockout (KO) mice that reduce or eliminate expression of each of these monoamine transporters have provided a wealth of new information about the function of these proteins at molecular, physiological and behavioral levels. In the present work we use the unique properties of magnetic resonance imaging (MRI) to probe the effects of altered dopaminergic dynamics on meso-scale neuronal circuitry and overall brain morphology, since changes at these levels of organization might help to account for some of the extensive pharmacological and behavioral differences observed in dopamine transporter (DAT) KO mice. Despite the smaller size of these animals, voxel-wise statistical comparison of high resolution structural MR images indicated little morphological change as a consequence of DAT KO. Likewise, proton magnetic resonance spectra recorded in the striatum indicated no significant changes in detectable metabolite concentrations between DAT KO and wild-type (WT) mice. In contrast, alterations in the circuitry from the prefrontal cortex to the mesocortical limbic system, an important brain component intimately tied to function of mesolimbic/mesocortical dopamine reward pathways, were revealed by manganese-enhanced MRI (MEMRI). Analysis of co-registered MEMRI images taken over the 26 hours after introduction of Mn²⁺ into the prefrontal cortex indicated that DAT KO mice have a truncated Mn²⁺ distribution within this circuitry with little accumulation beyond the thalamus or contralateral to the injection site. By contrast, WT littermates exhibit Mn²⁺ transport into more posterior midbrain nuclei and contralateral mesolimbic structures at 26 hr post-injection. Thus, DAT KO mice appear, at this level of anatomic resolution, to have preserved cortico-striatal-thalamic connectivity but diminished robustness of reward-modulating circuitry distal to the thalamus. This is in contradistinction to the state of this circuitry in serotonin transporter KO mice where we observed more robust connectivity in more posterior brain regions using methods identical to those employed here.

Transgenic mice in the study of drug addiction and the effects of psychostimulant drugs

The first transgenic models used to study addiction were based upon a priori assumptions about the importance of particular genes in addiction, including the main target molecules of morphine, amphetamine, and cocaine. This consequently emphasized the importance of monoamine transporters, opioid receptors, and monoamine receptors in addiction. Although the effects of opiates were largely eliminated by mu opioid receptor gene knockout, the case for psychostimulants was much more complex. Research using transgenic models supported the idea of a polygenic basis for psychostimulant effects and has associated particular genes with different behavioral consequences of psychostimulants. Phenotypic analysis of transgenic mice, especially gene knockout mice, has been instrumental in identifying the role of specific molecular targets of addictive drugs in their actions. In this article, we summarize studies that have provided insight into the polygenic determination of drug addiction phenotypes in ways that are not possible with other methods, emphasizing research into the effects of psychostimulant drugs in gene knockouts of the monoamine transporters and monoamine receptors.

Stress-induced potentiation of cocaine reward: a role for CRF R1 and CREB

Both clinical and preclinical research have shown that stress can potentiate drug use; however, the underlying mechanisms of this interaction are unknown. Previously, we have shown that a single exposure to forced swim (FS) reinstates extinguished conditioned place preference (CPP) to cocaine and that cAMP response element binding protein (CREB) is necessary for this response. CREB can be activated by corticotropin releasing factor (CRF) receptor type 1 (CRF(R1)) binding, which mediates neuroendocrine and behavioral responses to stress as well as to drugs of abuse. The present experiments investigate whether changes in cocaine reward elicited by previous exposure to stress are mediated by CREB and/or CRF(R1). Chronic exposure to FS in advance of conditioning enhances cocaine CPP in wild-type mice, but this is blocked in CREB-deficient mice. In addition, pretreatment with the CRF(R1) antagonist, antalarmin, before FS exposure blocks this stress-induced enhancement of cocaine CPP. Furthermore, FS-induced increase in phosphorylated CREB (pCREB), specifically in the lateral septum (LS) and nucleus accumbens (NAc) is also blocked by antalarmin. Taken together, these studies suggest that both CREB and CRF(R1) activation are necessary for stress-induced potentiation of drug reward.

Intrauterine environment-genome interaction and children's development (4): Brain-behavior phenotypying of genetically-engineered mice using a comprehensive behavioral test battery on research of neuropsychiatric disorders

Despite massive research efforts, the exact pathogenesis and pathophysiology of psychiatric disorders, such as schizophrenia and bipolar disorder, remain largely unknown. Animal models can serve as essential tools for investigating the etiology and treatment of such disorders. Some mutant mouse strains were found to exhibit behavioral abnormalities reminiscent of human psychiatric disorders. Here we outline our unique approach of extrapolating findings in mice to humans, and present studies on alpha-CaMKII heterozygous knockout (alpha-CaMKII+/-) mice as examples. Alpha-CaMKII+/- mice have profoundly dysregulated behavior and impaired neuronal development in the dentate gyrus (DG). The behavioral abnormalities include a severe working memory deficit and an exaggerated infradian rhythm, which are similar to symptoms seen in schizophrenia, bipolar mood disorder and other psychiatric disorders. By conducting a series of experiments, we discovered that almost all the neurons in the mutant DG were very similar to the immature DG neurons of normal rodents. In other words, alpha-CaMKII+/- mice have an "immature DG". We proposed that an "immature DG" in adulthood might induce alterations in behavior and serve as a promising candidate endophenotype of schizophrenia and other human psychiatric disorders. The impact of a large-scale mouse phenotyping on studies of psychiatric disorders and the potential utility of an "animal-model-array" of psychiatric disorders for the development of suitable therapeutic agents is also discussed.