Dopamine transporter knockdown mice in the behavioral pattern monitor: A robust, reproducible model for mania-relevant behaviors

The emerging neuroimmune hypothesis of bipolar disorder: An updated overview of neuroimmune and microglial findings

Bipolar disorder (BD) is a severe and multifactorial disease, with onset usually in young adulthood, which follows a progressive course throughout life. Replicated epidemiological studies have suggested inflammatory mechanisms and neuroimmune risk factors as primary contributors to the onset and development of BD. While not all patients display overt markers of inflammation, significant evidence suggests that aberrant immune signaling contributes to all stages of the disease and seems to be mood phase dependent, likely explaining the heterogeneity of findings observed in this population. As the brain's immune cells, microglia orchestrate the brain's immune response and play a critical role in maintaining the brain's health across the lifespan. Microglia are also highly sensitive to environmental changes and respond to physiological and pathological events by adapting their functions, structure, and molecular expression. Recently, it has been highlighted that instead of a single population of cells, microglia comprise a heterogeneous community with specialized states adjusted according to the local molecular cues and intercellular interactions. Early evidence has highlighted the contribution of microglia to BD neuropathology, notably for severe outcomes, such as suicidality. However, the roles and diversity of microglial states in this disease are still largely undermined. This review brings an updated overview of current literature on the contribution of neuroimmune risk factors for the onset and progression of BD, the most prominent neuroimmune abnormalities (including biomarker, neuroimaging, ex vivo studies) and the most recent findings of microglial involvement in BD neuropathology. Combining these different shreds of evidence, we aim to propose a unifying hypothesis for BD pathophysiology centered on neuroimmune abnormalities and microglia. Also, we highlight the urgent need to apply novel multi-system biology approaches to characterize the diversity of microglial states and functions involved in this enigmatic disorder, which can open bright perspectives for novel biomarkers and therapeutic discoveries.

Doxycycline reversal of amphetamine-induced mania-like behavior is related to adjusting brain monoamine abnormalities and antioxidant effects in primary hippocampal neurons

Mania is associated with disturbed dopaminergic transmission in frontotemporal regions. D-amphetamine (AMPH) causes increased extracellular DA levels, considered an acknowledged mania model in rodents. Doxycycline (DOXY) is a second-generation tetracycline with promising neuroprotective properties. Here, we tested the hypothesis that DOXY alone or combined with Lithium (Li) could reverse AMPH-induced mania-like behavioral alterations in mice by the modulation of monoamine levels in brain areas related to mood regulation, as well as cytoprotective and antioxidant effects in hippocampal neurons. Male Swiss mice received AMPH or saline intraperitoneal (IP) injections for 14 days. Between days 8-14, mice receive further IP doses of DOXY, Li, or their combination. For in vitro studies, we exposed hippocampal neurons to DOXY in the presence or absence of AMPH. DOXY alone or combined with Li reversed AMPH-induced risk-taking behavior and hyperlocomotion. DOXY also reversed AMPH-induced hippocampal and striatal hyperdopaminergia. In AMPH-exposed hippocampal neurons, DOXY alone and combined with Li presented cytoprotective and antioxidant effects, while DOXY+Li also increased the expression of phospho-Ser133-CREB. Our results add novel evidence for DOXY's ability to reverse mania-like features while revealing that antidopaminergic activity in some brain areas, such as the hippocampus and striatum, as well as hippocampal cytoprotective effects may account for this drug's antimanic action. This study provides additional rationale for designing clinical trials investigating its potential as a mood stabilizer agent.

Dopamine Transporter Deficient Rodents: Perspectives and Limitations for Neuroscience

The key element of dopamine (DA) neurotransmission is undoubtedly DA transporter (DAT), a transmembrane protein responsible for the synaptic reuptake of the mediator. Changes in DAT's function can be a key mechanism of pathological conditions associated with hyperdopaminergia. The first strain of gene-modified rodents with a lack of DAT were created more than 25 years ago. Such animals are characterized by increased levels of striatal DA, resulting in locomotor hyperactivity, increased levels of motor stereotypes, cognitive deficits, and other behavioral abnormalities. The administration of dopaminergic and pharmacological agents affecting other neurotransmitter systems can mitigate those abnormalities. The main purpose of this review is to systematize and analyze (1) known data on the consequences of changes in DAT expression in experimental animals, (2) results of pharmacological studies in these animals, and (3) to estimate the validity of animals lacking DAT as models for discovering new treatments of DA-related disorders.

Neural progenitor cells derived from lithium responsive and non-responsive bipolar disorder patients exhibit distinct sensitivity to cell death following methamphetamine

Bipolar disorder (BD) is characterized by manic and depressive mood episodes and loss of brain gray matter. Lithium has antimanic and neuroprotective properties, but only 30% BD patients respond to lithium pharmacotherapy. Dopamine signaling has been implicated in BD and may contribute to lithium response. Methamphetamine (METH) stimulates dopamine release and models the clinical features of mania but has never been used to study cell death in BD patient neurons. We used BD patient derived neuronal progenitor cells (NPCs) to determine whether the vulnerability to cell death differed in samples from lithium responder (Li-R) and non-responder (Li-NR) BD patients and healthy controls following METH exposure in vitro. We hypothesized that NPCs from Li-R and Li-NR would differ in vulnerability to METH, dopamine signaling and neuroprotection from lithium. Following METH, NPCs from controls and Li-NR showed significantly greater cell loss compared to Li-R. Pre-treatment of NPCs with the D1 dopamine receptor antagonist SCH 23390 reversed the neurotoxic effects of METH. In Li-R NPCs, expression of phosho-ERK1/2 was significantly increased. In Li-NR NPCs, phospho-AKT, D1 and D2 dopamine receptor proteins were significantly increased. Pre-treatment of NPCs with lithium before METH reversed the neurotoxic effects of METH in control NPCs, whereas Li-NR showed less protective benefit. Li-R cells showed decreased levels of cell death after METH and comparatively high viability, and lithium treatment did not increase viability any further. This novel NPC model of mania reveals differences in cell death that could help identify mechanisms of lithium response in BD.

Altered levels of dopamine transporter in the frontal pole and the striatum in mood disorders: A postmortem study

Dopamine dysregulation is known to play a major role in the pathophysiology of major depressive disorders (MDD) and bipolar disorders (BD). The dopamine transporter (DAT) plays a critical role in regulating dopamine concentration at the synaptic cleft and therefore could have an important role in the molecular pathology of MDD and BD. To test this hypothesis, we measured levels of [³H]mazindol binding to DAT in Brodmann's area (BA) 10, BA 17 as well as in the dorsal and ventral striatum from 15 controls, 15 patients with MDD and 15 patients with BD, obtained postmortem, using in situ radioligand binding with autoradiography. Compared to controls, levels of [³H]mazindol binding to DAT was significantly higher in BA10 from patients with MDD but not BD. There was no significant difference in [³H]mazindol binding to DAT in BA 17 or the dorsal and ventral striatum from patients with MDD or BD. In addition, levels of [³H]mazindol binding show no correlation with donor age, postmortem interval, tissue pH, sex or duration of illness. In conclusion, our data suggest that changes in levels of DAT may be selectively affecting dopamine homeostasis in BA 10 in patients with MDD.

iTat transgenic mice exhibit hyper-locomotion in the behavioral pattern monitor after chronic exposure to methamphetamine but are unaffected by Tat expression

Although antiretroviral therapy (ART) has increased the quality of life and lifespan in people living with HIV (PWH), millions continue to suffer from the neurobehavioral effects of the virus. Additionally, the abuse of illicit drugs (methamphetamine in particular) is significantly higher in PWH compared to the general population, which may further impact their neurological functions. The HIV regulatory protein, Tat, has been implicated in the neurobehavioral impacts of HIV and is purported to inhibit dopamine transporter (DAT) function in a way similar to methamphetamine. Thus, we hypothesized that a combination of Tat expression and methamphetamine would exert synergistic deleterious effects on behavior and DAT expression. We examined the impact of chronic methamphetamine exposure on exploration in transgenic mice expressing human Tat (iTat) vs. their wildtype littermates using the behavioral pattern monitor (BPM). During baseline, mice exhibited sex-dependent differences in BPM behavior, which persisted through methamphetamine exposure, and Tat activation with doxycycline. We observed a main effect of methamphetamine, wherein exposure, irrespective of genotype, increased locomotor activity and decreased specific exploration. After doxycycline treatment, mice continued to exhibit drug-dependent alterations in locomotion, with no effect of Tat, or methamphetamine interactions. DAT levels were higher in wildtype, saline-exposed males compared to all other groups. These data support stimulant-induced changes of locomotor activity and exploration, and suggest that viral Tat and methamphetamine do not synergistically interact to alter these behaviors in mice. These findings are important for future studies attempting to disentangle the effect of substances that impact DAT on HAND-relevant behaviors using such transgenic animals.

Translational cognitive systems: focus on attention

Cognitive dysfunction, particularly attentional impairment, is a core feature of many psychiatric disorders, yet is inadequately addressed by current treatments. Development of targeted therapeutics for the remediation of attentional deficits requires knowledge of underlying neurocircuit, cellular, and molecular mechanisms that cannot be directly assayed in the clinic. This level of detail can only be acquired by testing animals in cross-species translatable attentional paradigms, in combination with preclinical neuroscience techniques. The 5-choice continuous performance test (5C-CPT) and rodent continuous performance test (rCPT) represent the current state of the art of preclinical assessment of the most commonly studied subtype of attention: sustained attention, or vigilance. These tasks present animals with continuous streams of target stimuli to which they must respond (attention), in addition to non-target stimuli from which they must withhold responses (behavioral inhibition). The 5C-CPT and rCPT utilize the same measures as gold-standard clinical continuous performance tests and predict clinical efficacy of known pro-attentional drugs. They also engage common brain regions across species, although efforts to definitively establish neurophysiological construct validity are ongoing. The validity of these tasks as translational vigilance assessments enables their use in characterizing the neuropathology underlying attentional deficits of animal models of psychiatric disease, and in determining therapeutic potential of drugs ahead of clinical testing. Here, we briefly review the development and validation of such tests of attentional functioning, as well as the data they have generated pertaining to inattention, disinhibition, and impulsivity in psychiatric disorders.

Enlightened: addressing circadian and seasonal changes in photoperiod in animal models of bipolar disorder

Bipolar disorders (BDs) exhibit high heritability and symptoms typically first occur during late adolescence or early adulthood. Affected individuals may experience alternating bouts of mania/hypomania and depression, with euthymic periods of varying lengths interspersed between these extremes of mood. Clinical research studies have consistently demonstrated that BD patients have disturbances in circadian and seasonal rhythms, even when they are free of symptoms. In addition, some BD patients display seasonal patterns in the occurrence of manic/hypomanic and depressive episodes as well as the time of year when symptoms initially occur. Finally, the age of onset of BD symptoms is strongly influenced by the distance one lives from the equator. With few exceptions, animal models useful in the study of BD have not capitalized on these clinical findings regarding seasonal patterns in BD to explore molecular mechanisms associated with the expression of mania- and depression-like behaviors in laboratory animals. In particular, animal models would be especially useful in studying how rates of change in photoperiod that occur during early spring and fall interact with risk genes to increase the occurrence of mania- and depression-like phenotypes, respectively. Another unanswered question relates to the ways in which seasonally relevant changes in photoperiod affect responses to acute and chronic stressors in animal models. Going forward, we suggest ways in which translational research with animal models of BD could be strengthened through carefully controlled manipulations of photoperiod to enhance our understanding of mechanisms underlying seasonal patterns of BD symptoms in humans. In addition, we emphasize the value of incorporating diurnal rodent species as more appropriate animal models to study the effects of seasonal changes in light on symptoms of depression and mania that are characteristic of BD in humans.

Altered Grooming Syntax and Amphetamine-Induced Dopamine Release in EAAT3 Overexpressing Mice

The excitatory amino acid transporter EAAT3 plays an important role in the neuronal uptake of glutamate regulating the activation of glutamate receptors. Polymorphisms in the gene-encoding EAAT3 have been associated with obsessive-compulsive disorder (OCD), although the mechanisms underlying this relationship are still unknown. We recently reported that mice with increased EAAT3 expression in forebrain neurons (EAAT3 ^{g lo} /CMKII) display behavioral and synaptic features relevant to OCD, including increased grooming, higher anxiety-like behavior and altered cortico-striatal synaptic function. The dopamine neurotransmitter system is implicated in ritualistic behaviors. Indeed, dopaminergic neurons express EAAT3, and mice lacking EAAT3 exhibit decreased dopamine release and decreased expression of the dopamine D1 receptor. Moreover, EAAT3 plays a role on the effect of the psychostimulant amphetamine. As such, we sought to determine if the OCD-like behavior in EAAT3 ^{g lo} /CMKII mice is accompanied by altered nigro-striatal dopaminergic transmission. The aim of this study was to analyze dopamine transmission both in basal conditions and after an acute challenge of amphetamine, using behavioral, neurochemical, molecular, and cellular approaches. We found that in basal conditions, EAAT3 ^{g lo} /CMKII mice performed more grooming events and that they remained in phase 1 of the grooming chain syntax compared with control littermates. Administration of amphetamine increased the number of grooming events in control mice, while EAAT3 ^{g lo} /CMKII mice remain unaffected. Interestingly, the grooming syntax of amphetamine-control mice resembled that of EAAT3 ^{g lo} /CMKII mice in basal conditions. Using in vivo microdialysis, we found decreased basal dopamine levels in EAAT3 ^{g lo} /CMKII compared with control mice. Unexpectedly, we found that after acute amphetamine, EAAT3 ^{g lo} /CMKII mice had a higher release of dopamine compared with that of control mice, suggesting that EAAT3 overexpression leads to increased dopamine releasability. To determine postsynaptic effect of EAAT3 overexpression over dopamine transmission, we performed Western blot analysis of dopaminergic proteins and found that EAAT3 ^{g lo} /CMKII mice have higher expression of D2 receptors, suggesting a higher inhibition of the indirect striatal pathway. Together, the data indicate that EAAT3 overexpression impacts on dopamine transmission, making dopamine neurons more sensitive to the effect of amphetamine and leading to a disbalance between the direct and indirect striatal pathways that favors the performance of repetitive behaviors.

Chronic antipsychotic treatment exerts limited effects on the mania-like behavior of dopamine transporter knockdown mice

BACKGROUND

Bipolar disorder is a life-threatening disorder linked to dopamine transporter (DAT) polymorphisms, with reduced DAT levels seen in positron emission tomography and postmortem brains.

AIMS

The purpose of this study was to examine the effects of approved antipsychotics on DAT dysfunction-mediated mania behavior in mice.

METHODS

DAT knockdown mice received either D2-family receptor antagonist risperidone or asenapine and mania-related behaviors were assessed in the clinically-relevant behavioral pattern monitor to assess spontaneous exploration.

RESULTS

Chronic risperidone did not reverse mania-like behavior in DAT knockdown mice. Chronic asenapine reduced mania behavior but this effect was more pronounced in wild-type littermates than in DAT knockdown mice.

CONCLUSION

Taken together, these findings suggest that while acute antipsychotic treatment may be beneficial in management of bipolar mania, more targeted therapeutics may be necessary for long-term treatment. Specific investigation into DAT-targeting drugs could improve future treatment of bipolar mania.

Cannabis Use and Mental Illness: Understanding Circuit Dysfunction Through Preclinical Models

Patients with a serious mental illness often use cannabis at higher rates than the general population and are also often diagnosed with cannabis use disorder. Clinical studies reveal a strong association between the psychoactive effects of cannabis and the symptoms of serious mental illnesses. Although some studies purport that cannabis may treat mental illnesses, others have highlighted the negative consequences of use for patients with a mental illness and for otherwise healthy users. As epidemiological and clinical studies are unable to directly infer causality or examine neurobiology through circuit manipulation, preclinical animal models remain a valuable resource for examining the causal effects of cannabis. This is especially true considering the diversity of constituents in the cannabis plant contributing to its effects. In this mini-review, we provide an updated perspective on the preclinical evidence of shared neurobiological mechanisms underpinning the dual diagnosis of cannabis use disorder and a serious mental illness. We present studies of cannabinoid exposure in otherwise healthy rodents, as well as rodent models of schizophrenia, depression, and bipolar disorder, and the resulting impact on electrophysiological indices of neural circuit activity. We propose a consolidated neural circuit-based understanding of the preclinical evidence to generate new hypotheses and identify novel therapeutic targets.

Chronic nicotine, but not suramin or resveratrol, partially remediates the mania-like profile of dopamine transporter knockdown mice

Bipolar disorder (BD) is a severe mental illness affecting 2% of the global population. Current pharmacotherapies provide incomplete symptom remediation, highlighting the need for novel therapeutics. BD is characterized by fluctuations between mania and depression, likely driven by shifts between hyperdopaminergia and hypercholinergia, respectively. Hyperdopaminergia may result from insufficient activity of the dopamine transporter (DAT), the primary mediator of synaptic dopamine clearance. The DAT knockdown (DAT KD) mouse recreates this mechanism and exhibits a highly reproducible hyperexploratory profile in the cross-species translatable Behavioral Pattern Monitor (BPM) that is: (a) consistent with that observed in BD mania patients; and (b) partially normalized by chronic lithium and valproate treatment. The DAT KD/BPM model of mania therefore exhibits high levels of face-, construct-, and predictive-validity for the pre-clinical assessment of putative anti-mania drugs. Three different drug regimens - chronic nicotine (nicotinic acetylcholine receptor (nAChR) agonist; 40 mg/kg/d, 26 d), subchronic suramin (anti-purinergic; 20 mg/kg, 1 × /wk, 4 wks), and subchronic resveratrol (striatal DAT upregulator; 20 mg/kg/d, 4 d) - were administered to separate cohorts of male and female DAT KD- and wildtype (WT) littermate mice, and exploration was assessed in the BPM. Throughout, DAT KD mice exhibited robust hyperexploratory profiles relative to WTs. Nicotine partially normalized this behavior. Resveratrol modestly upregulated DAT expression but did not normalize DAT KD behavior. These results support the mania-like profile of DAT KD mice, which may be partially remediated by nAChR agonists via restoration of disrupted catecholaminergic/cholinergic equilibrium. Delineating the precise mechanism of action of nicotine could identify more selective therapeutic targets.

Reduced Plasma Dopamine-β-Hydroxylase Activity Is Associated With the Severity of Bipolar Disorder: A Pilot Study

Dopamine-β-hydroxylase (DβH) is an enzyme converting dopamine to norepinephrine, a key neurotransmitter in mood disorders, such as major depressive disorder (MDD) and bipolar disorder (BD). Due to overlapping symptomology of unipolar and bipolar depression, the present study attempted to explorer if the plasma DβH activity could discriminate the depressive episodes of BD from MDD. The aim of this study was to compare the plasma DβH activity among MDD patients (n = 104), BD patients (n = 101), and healthy controls (n = 160). Clinical characteristics and cognitive function were assessed using the Young Mania Rating Scale (YMRS), Hamilton Depression Scale (HAM-D), Hamilton Anxiety Scale (HAM-A), Patient Health Questionnaire-9 (PHQ-9), and Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). Our data showed a lower plasma DβH activity in patients with BD, not MDD, than that in controls. For the BD patients, the plasma DβH activities were negatively correlated with HAM-D scores and HAM-A scores. However, there was no significant correlation between plasma DβH activity and severity of depressive symptoms in MDD patients. No significant correlation between DβH activities and cognitive assessments neither in BD nor in MDD patients. The present study provides evidence that BD is associated with decreased circulating DβH activity.

Convergent neural substrates of inattention in bipolar disorder patients and dopamine transporter-deficient mice using the 5-choice CPT

OBJECTIVES

Bipolar disorder (BD) is a debilitating psychiatric illness affecting 2%-5% of the population. Although mania is the cardinal feature of BD, inattention and related cognitive dysfunction are observed across all stages. Since cognitive dysfunction confers poor functional outcome in patients, understanding the relevant neural mechanisms remains key to developing novel-targeted therapeutics.

METHODS

The 5-choice continuous performance test (5C-CPT) is a mouse and fMRI-compatible human attentional task, requiring responding to target stimuli while inhibiting responding to nontarget stimuli, as in clinical CPTs. This task was used to delineate systems-level neural deficits in BD contributing to inattentive performance in human subjects with BD as well as mouse models with either parietal cortex (PC) lesions or reduced dopamine transporter (DAT) expression.

RESULTS

Mania BD participants exhibited severe 5C-CPT impairment. Euthymic BD patients exhibited modestly impaired 5C-CPT. High impulsivity BD subjects exhibited reduced PC activation during target and nontarget responding compared with healthy participants. In mice, bilateral PC lesions impaired both target and nontarget responding. In the DAT knockdown mouse model of BD mania, knockdown mice exhibited severely impaired 5C-CPT performance versus wildtype littermates.

CONCLUSIONS

These data support the role of the PC in inattention in BD-specifically regarding identifying the appropriate response to target vs nontarget stimuli. Moreover, the findings indicate that severely reduced DAT function/hyperdopaminergia recreates the attentional deficits observed in BD mania patients. Determining the contribution of DAT in the PC to attention may provide a future target for treatment development.