Modeling bipolar disorder in mice by increasing acetylcholine or dopamine: chronic lithium treats most, but not all features

Affective disorders and the loudness dependence of the auditory evoked potential: Serotonin and beyond

Identifying additional noninvasive biomarkers for affective disorders, such as unipolar major depressive disorder (MDD) and bipolar disorder (BD), could aid in the diagnosis and treatment of these prevalent and debilitating neuropsychiatric conditions. One such candidate biomarker is the loudness dependence of the auditory evoked potential (LDAEP), an event-related potential that measures responsiveness of the auditory cortex to different intensities of sound. The LDAEP has been associated with MDD and BD, including therapeutic response to particular classes of antidepressant drugs, while also correlating with several other neuropsychiatric disorders. It has been suggested that increased values of the LDAEP indicate low central serotonergic neurotransmission, further implicating this EEG measure in depression. Here, we briefly review the literature on the LDAEP in affective disorders, including its association with serotonergic signaling, as well as with that of other neurotransmitters such as dopamine. We summarize key findings on the LDAEP and the genetics of these neurotransmitters, as well as prediction of response to particular classes of antidepressants in MDD, including SSRIs versus noradrenergic agents. The possible relationship between this EEG measure and suicidality is addressed. We also briefly analyze acute pharmacologic studies of serotonin and/or dopamine precursor depletion and the LDAEP. In conclusion, the existing literature suggests that serotonin and norepinephrine may modulate the LDAEP in an opposing manner, and that this event-related marker may be of use in predicting response to chronic treatment with particular pharmacologic agents in the context of affective disorders, such as MDD and BD, including in the presence of suicidality.

Patterns of ingestion of rats during chronic oral administration of lithium chloride

Chronic lithium administration to rodents is used to explore the potential neural mechanisms of mood stabilization, as well as to model the side effects of chronic lithium on multiple organ systems. Oral administration of lithium in the maintenance diet or drinking water is convenient, but lithium can acutely affect intake and it can mediate acquisition of conditioned taste aversions (CTA). We compared ad libitum food and fluid intake by male rats with LiCl or NaCl solutions as their sole source of fluid across 20 days, with a commonly used dosage of LiCl (24 mM: 1 g / L LiCl). To quantify the pattern of intake, rats were housed in cages equipped with lickometers to detect licks and infrared photobeams to detect food access with 6-s resolution. To determine if rats formed a CTA to LiCl, they were subsequently tested with access to NaCl. Rats showed an immediate avoidance of the LiCl solution, as seen on the first day of access by an increased latency to initiate drinking and a decreased size of drinking bouts. Rats showed a differential response to LiCl vs. NaCl after as few as 5 licks. Chronic consumption of LiCl solution led to significantly decreased food and fluid intake compared to baseline, with concomitant weight loss. The decreased intake was realized by marked changes in the pattern of drinking and feeding bouts: a decrease in per-lick volume and a decrease in licks per drinking bout, and an increase in feeding bout duration resulting in an overall decrease in eating rate. Conversely, chronic NaCl access led to an increase in drinking bout number and licks/bout. The avoidance of LiCl was likely a combination of toxic effects of ingested LiCl and rapid acquisition of a learned aversion to the taste of LiCl, as shown by an extinguishable generalized aversion to NaCl solution during subsequent NaCl test days. The marked effect of chronic oral LiCl on ingestion may impact the oral dosing of lithium as well as the rat's metabolic status.

Genetic loss of the dopamine transporter significantly impacts behavioral and molecular responses to sub-chronic stress in mice

Dopaminergic neurotransmission has emerged as a critical determinant of stress susceptibility and resilience. Although the dopamine transporter (DAT) is known to play a key role in maintaining dopamine (DA) homeostasis, its importance for the regulation of stress susceptibility remains largely unknown. Indeed, while numerous studies have examined the neurochemical and behavioral consequences of genetic loss of DAT, very few have compared responses to stress in wild-type and DAT-knockout (KO) animals. The current study compared the responses of male and female WT and DAT-KO mice to a model of sub-chronic stress. Our results reveal that DAT-KO mice are resistant to stress-induced increases in the latency to enter the light chamber of the light-dark emergence test and demonstrate that DAT-KO mice exhibit baseline reductions in forced swim test immobility and grooming time in the splash test of grooming behavior. In addition to these behavioral changes, our results highlight the importance of sex and dopaminergic neurotransmission on stress-induced changes in the expression and phosphorylation of several signal transduction molecules in the nucleus accumbens that have previously been implicated in the regulation of stress susceptibility, including ERK, GSK3β, and ΔFosB. Overall, these results provide further evidence of the importance of dopaminergic neurotransmission in regulating stress susceptibility and suggest that genetic loss of DAT prevents stress-induced increases in anxiety-like behavior.

The immunomodulatory effect of lithium as a mechanism of action in bipolar disorder

Bipolar disorder (BD) is a chronic mental disorder characterized by recurrent episodes of mania and depression alternating with periods of euthymia. Although environmental and genetic factors have been described, their pathogenesis is not fully understood. Much evidence suggests a role for inflammatory mediators and immune dysregulation in the development of BD. The first-line treatment in BD are mood-stabilizing agents, one of which is lithium (Li) salts. The Li mechanism of action is not fully understood, but it has been proposed that its robust immunomodulatory properties might be one of the mechanisms responsible for its effectiveness. In this article, the authors present the current knowledge about immune system changes accompanying BD, as well as the immunomodulatory effect of lithium. The results of studies describing connections between immune system changes and lithium effectiveness are often incoherent. Further research is needed to understand the connection between immune system modulation and the therapeutic action of lithium in BD.

LPS-induced impairment of Na+/K+-ATPase activity: ameliorative effect of tannic acid in mice

Acetylcholine is an excitatory neurotransmitter that modulates synaptic plasticity and communication, and it is essential for learning and memory processes. This neurotransmitter is hydrolyzed by acetylcholinesterase (AChE), which plays other cellular roles in processes such as inflammation and oxidative stress. Ion pumps, such as Na⁺/K⁺-ATPase and Ca²⁺-ATPase, are highly expressed channels that derive energy for their functions from ATP hydrolysis. Impairment of the cholinergic system and ion pumps is associated with neuropsychiatric diseases. Major depressive disorder (MDD) is an example of a complex disease with high morbidity and a heterogenous etiology. Polyphenols have been investigated for their therapeutic effects, and tannic acid (TA) has been reported to show neuroprotective and antidepressant-like activities. Animal models of depression-like behavior, such as lipopolysaccharide (LPS)-induced models of depression, are useful for investigating the pathophysiology of MDD. In this context, effects of TA were evaluated in an LPS-induced mouse model of depression-like behavior. Animals received TA for 7 days, and on the last day of treatment, LPS (830 μg/kg) was administered intraperitoneally. In vitro exposure of healthy brain to TA decreased the AChE activity. Additionally, this enzyme activity was decreased in cerebral cortex of LPS-treated mice. LPS injection increased the activity of Ca²⁺-ATPase in the cerebral cortex but decreased the enzyme activity in the hippocampus. LPS administration decreased Na⁺/K⁺-ATPase activity in the cerebral cortex, hippocampus, and striatum; however, TA administration prevented these changes. In conclusion, tannins may affect Na⁺/K⁺-ATPase and Ca²⁺-ATPase activities, which is interesting in the context of MDD.

Effects of Prenatal Nicotine, THC, or Co-Exposure on Cognitive Behaviors in Adolescent Male and Female Rats

INTRODUCTION

Although there has been a decrease in the prevalence of tobacco smoking, exposure to nicotine during pregnancy remains a substantial problem worldwide. Further, given the recent escalation in e-cigarette use and legalization of cannabis, it has become essential to understand the effects of nicotine and cannabinoid co-exposure during early developmental stages.

AIMS AND METHODS

We systematically examined the effects of nicotine and/or THC prenatal exposure on cognitive behaviors in male and female offspring. Dams were exposed to nicotine vape or vehicle, and oral edible THC or vehicle, throughout pregnancy. Adolescent offspring were then tested in the prepulse inhibition test, novel object recognition task, and novelty suppressed feeding task.

RESULTS

At birth, pups from mothers exposed to nicotine vape or oral THC exhibited reduced body weight, compared to control pups. Prenatal nicotine vape exposure resulted in a decreased baseline startle reactivity in adolescent male and female rats, and in females, enhanced sensorimotor gating in the prepulse inhibition test. Prenatal nicotine and THC co-exposure resulted in significant deficits in the prepulse inhibition test in males. Deficits in short-term memory were also found in males prenatally exposed to THC, either alone or with nicotine co-exposure, and in females exposed to THC alone. Finally, in males, a modest increase in anxiety-associated behaviors was found with THC or nicotine exposure in the latency to approach a novel palatable food.

CONCLUSIONS

These studies demonstrate differential effects of prenatal exposure to e-cigarette nicotine vape and/or edible THC on cognitive function, with differing effects within male and female groups.

IMPLICATIONS

These studies demonstrate an impact of nicotine, THC, or co-exposure during early developmental stages in utero on behavioral outcomes in adolescence. These findings have important translational implications given the continued use of nicotine and THC containing products by pregnant women worldwide, which can be applied to support healthcare and policy efforts restricting nicotine and THC use during pregnancy.

Chronic lithium administration in a mouse model for Krabbe disease

Krabbe disease (KD; or globoid cell leukodystrophy) is an autosomal recessive lysosomal storage disorder caused by deficiency of the galactosylceramidase (GALC) enzyme. No cure is currently available for KD. Clinical applied treatments are supportive only. Recently, we demonstrated that two differently acting autophagy inducers (lithium and rapamycin) can improve some KD hallmarks in-vitro, laying the foundation for their in-vivo pre-clinical testing. Here, we test lithium carbonate in-vivo, in the spontaneous mouse model for KD, the Twitcher (TWI) mouse. The drug is administered ad libitum via drinking water (600 mg/L) starting from post natal day 20. We longitudinally monitor the mouse motor performance through the grip strength, the hanging wire and the rotarod tests, and a set of biochemical parameters related to the KD pathogenesis [i.e., GALC enzymatic activity, psychosine (PSY) accumulation and astrogliosis]. Additionally, we investigate the expression of some crucial markers related to the two pathways that could be altered by lithium: the autophagy and the β-catenin-dependent pathways. Results demonstrate that lithium has not a significant rescue effect on the TWI phenotype, although it can slightly and transiently improves muscle strength. We also show that lithium, with this administration protocol, is unable to stimulate autophagy in the TWI mice central nervous system, whereas results suggest that it can restore the β-catenin activation status in the TWI sciatic nerve. Overall, these data provide intriguing inputs for further evaluations of lithium treatment in TWI mice.

Differential Expression Patterns of Lynx Proteins and Involvement of Lynx1 in Prepulse Inhibition

Negative allosteric modulators, such as lynx1 and lynx2, directly interact with nicotinic acetylcholine receptors (nAChRs). The nAChRs are integral to cholinergic signaling in the brain and have been shown to mediate different aspects of cognitive function. Given the interaction between lynx proteins and these receptors, we examined whether these endogenous negative allosteric modulators are involved in cognitive behaviors associated with cholinergic function. We found both cell-specific and overlapping expression patterns of lynx1 and lynx2 mRNA in brain regions associated with cognition, learning, memory, and sensorimotor processing, including the prefrontal cortex (PFC), cingulate cortex, septum, hippocampus, amygdala, striatum, and pontine nuclei. Since lynx proteins are thought to play a role in conditioned associations and given the expression patterns across brain regions, we first assessed whether lynx knockout mice would differ in a cognitive flexibility task. We found no deficits in reversal learning in either the lynx1^–/– or lynx2^–/– knockout mice. Thereafter, sensorimotor gating was examined with the prepulse inhibition (PPI) assessment. Interestingly, we found that both male and female lynx1^–/– mice exhibited a deficit in the PPI behavioral response. Given the comparable expression of lynx2 in regions involved in sensorimotor gating, we then examined whether removal of the lynx2 protein would lead to similar behavioral effects. Unexpectedly, we found that while male lynx2^–/– mice exhibited a decrease in the baseline startle response, no differences were found in sensorimotor gating for either male or female lynx2^–/– mice. Taken together, these studies provide insight into the expression patterns of lynx1 and lynx2 across multiple brain regions and illustrate the modulatory effects of the lynx1 protein in sensorimotor gating.

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.

Multiple cholinesterase inhibitors have antidepressant-like properties in the mouse forced swim test

There is high clinical interest in improving the pharmacological treatment of individuals with Major Depressive Disorder (MDD). This neuropsychiatric disorder continues to cause significant morbidity and mortality worldwide, where existing pharmaceutical treatments such as selective serotonin reuptake inhibitors often have limited efficacy. In a recent publication, we demonstrated an antidepressant-like role for the acetylcholinesterase inhibitor (AChEI) donepezil in the C57BL/6J mouse forced swim test (FST). Those data added to a limited literature in rodents and human subjects which suggests AChEIs have antidepressant properties, but added the novel finding that donepezil only showed antidepressant-like properties at lower doses (0.02, 0.2 mg/kg). At a high dose (2.0 mg/kg), donepezil tended to promote depression-like behavior, suggesting a u-shaped dose-response curve for FST immobility. Here we investigate the effects of three other AChEIs with varying molecular structures: galantamine, physostigmine, and rivastigmine, to test whether they also exhibit antidepressant-like effects in the FST. We find that these drugs do exhibit therapeutic-like effects at low but not high doses, albeit at lower doses for physostigmine. Further, we find that their antidepressant-like effects are not mediated by generalized hyperactivity in the novel open field test, and are also not accompanied by anxiolytic-like properties. These data further support the hypothesis that acetylcholine has a u-shaped dose-response relationship with immobility in the C57BL/6J mouse FST, and provide a rationale for more thoroughly investigating whether reversible AChEIs as a class can be repurposed for the treatment of MDD in human subjects.

Repurposing Cholinesterase Inhibitors as Antidepressants? Dose and Stress-Sensitivity May Be Critical to Opening Possibilities

When stress becomes chronic it can trigger lasting brain and behavioral changes including Major Depressive Disorder (MDD). There is conflicting evidence regarding whether acetylcholinesterase inhibitors (AChEIs) may have antidepressant properties. In a recent publication, we demonstrated a strong dose-dependency of the effect of AChEIs on antidepressant-related behavior in the mouse forced swim test: whereas the AChEI donepezil indeed promotes depression-like behavior at a high dose, it has antidepressant-like properties at lower doses in the same experiment. Our data therefore suggest a Janus-faced dose-response curve for donepezil in depression-related behavior. In this review, we investigate the mood-related properties of AChEIs in greater detail, focusing on both human and rodent studies. In fact, while there have been many studies showing pro-depressant activity by AChEIs and this is a major concept in the field, a variety of other studies in both humans and rodents show antidepressant effects. Our study was one of the first to systematically vary dose to include very low concentrations while measuring behavioral effects, potentially explaining the apparent disparate findings in the field. The possibility of antidepressant roles for AChEIs in rodents may provide hope for new depression treatments. Importantly, MDD is a psychosocial stress-linked disorder, and in rodents, stress is a major experimental manipulation for studying depression mechanisms, so an important future direction will be to determine the extent to which these depression-related effects are stress-sensitive. In sum, gaining a greater understanding of the potentially therapeutic mood-related effects of low dose AChEIs, both in rodent models and in human subjects, should be a prioritized topic in ongoing translational research.

[Lithium in the psychopharmacology of affective disorders and mechanisms of its effects on cellular physiology]

However, despite successful use of lithium in the treatment of affective disorders for almost 40 years, the mechanisms of its therapeutic action are still poorly understood. This review presents and summarizes the current literature about the use of lithium in treatment of affective disorders, as well as its effects on cellular physiology, with a separate description of the effect of this ion on the functioning of nerve tissue and ion-molecular mechanisms.

Converging evidence that short-active photoperiod increases acetylcholine signaling in the hippocampus

Seasonal variations in environmental light influence switches between moods in seasonal affective disorder (SAD) and bipolar disorder (BD), with depression arising during short active (SA) winter periods. Light-induced changes in behavior are also seen in healthy animals and are intensified in mice with reduced dopamine transporter expression. Specifically, decreasing the nocturnal active period (SA) of mice increases punishment perseveration and forced swim test (FST) immobility. Elevating acetylcholine with the acetylcholinesterase inhibitor physostigmine induces depression symptoms in people and increases FST immobility in mice. We used SA photoperiods and physostigmine to elevate acetylcholine prior to testing in a probabilistic learning task and the FST, including reversing subsequent deficits with nicotinic and scopolamine antagonists and targeted hippocampal adeno-associated viral administration. We confirmed that physostigmine also increases punishment sensitivity in a probabilistic learning paradigm. In addition, muscarinic and nicotinic receptor blockade attenuated both physostigmine-induced and SA-induced phenotypes. Finally, viral-mediated hippocampal expression of human AChE used to lower ACh levels blocked SA-induced elevation of FST immobility. These results indicate that increased hippocampal acetylcholine neurotransmission is necessary for the expression of SA exposure-induced behaviors. Furthermore, these studies support the potential for cholinergic treatments in depression. Taken together, these results provide evidence for hippocampal cholinergic mechanisms in contributing to seasonally depressed affective states induced by short day lengths.

The cholinesterase inhibitor donepezil has antidepressant-like properties in the mouse forced swim test

Finding new antidepressant agents is of high clinical priority given that many cases of major depressive disorder (MDD) do not respond to conventional monoaminergic antidepressants such as the selective serotonin reuptake inhibitors (SSRIs), tricyclic antidepressants, and monoamine oxidase inhibitors. Recent findings of effective fast-acting antidepressants indicate that there are biological substrates to be taken advantage of for fast relief of depression and that we may find further treatments in this category. In this vein, the cholinergic system may be a relatively overlooked target for antidepressant medications, given its major role in motivation and attention. Furthermore, the classically engaged monoaminergic neurotransmitter systems in depression treatment-serotonin, norepinephrine, and dopamine-interact directly at times with cholinergic signaling. Here we investigate in greater detail how the cholinergic system may impact depression-related behavior, by administering widely ranging doses of the cholinesterase inhibitor drug, donepezil, to C57BL/6J mice in the forced swim test. First, we confirm prior findings that this drug, which is thought to boost synaptic acetylcholine, promotes depression-like behavior at a high dose (2.0 mg/kg, i.p.). But we also find paradoxically that it has an antidepressant-like effect at lower doses (0.02 and 0.2 mg/kg). Further this antidepressant-like effect is not due to generalized hyperactivity, since we did not observe increased locomotor activity in the open field test. These data support a novel antidepressant-like role for donepezil at lower doses as part of an overall u-shaped dose-response curve. This raises the possibility that donepezil could have antidepressant properties in humans suffering from MDD.

Cholinergic regulation of mood: from basic and clinical studies to emerging therapeutics

Mood disorders are highly prevalent and are the leading cause of disability worldwide. The neurobiological mechanisms underlying depression remain poorly understood, although theories regarding dysfunction within various neurotransmitter systems have been postulated. Over 50 years ago, clinical studies suggested that increases in central acetylcholine could lead to depressed mood. Evidence has continued to accumulate suggesting that the cholinergic system has a important role in mood regulation. In particular, the finding that the antimuscarinic agent, scopolamine, exerts fast-onset and sustained antidepressant effects in depressed humans has led to a renewal of interest in the cholinergic system as an important player in the neurochemistry of major depression and bipolar disorder. Here, we synthesize current knowledge regarding the modulation of mood by the central cholinergic system, drawing upon studies from human postmortem brain, neuroimaging, and drug challenge investigations, as well as animal model studies. First, we describe an illustrative series of early discoveries which suggest a role for acetylcholine in the pathophysiology of mood disorders. Then, we discuss more recent studies conducted in humans and/or animals which have identified roles for both acetylcholinergic muscarinic and nicotinic receptors in different mood states, and as targets for novel therapies.

Bipolar Disorder: Its Etiology and How to Model in Rodents

Characterized by the switch of manic and depressive phases, bipolar disorder was described as early as the fifth century BC. Nevertheless up to date, the underlying neurobiology is still largely unclear, assuming a multifactor genesis with both biological-genetic and psychosocial factors. Significant process has been achieved in recent years in researching the causes of bipolar disorder with modern molecular biological (e.g., genetic and epigenetic studies) and imaging techniques (e.g., positron emission tomography (PET) and functional magnetic resonance imaging (fMRI)). In this chapter we will first summarize our recent knowledge on the etiology of bipolar disorder. We then discuss how several factors observed to contribute to bipolar disorder in human patients can be manipulated to generate rodent models for bipolar disorder. Finally, we will give an overview on behavioral test that can be used to assess bipolar-disorder-like behavior in rodents.

Inhibition of the dopamine transporter as an animal model of bipolar disorder mania: Locomotor response, neuroimmunological profile and pharmacological modulation

Inhibition of dopamine transporter (DAT) by GBR12909 has been proposed as a pharmacological model of mania related to bipolar disorder (BD). Here we tested the hypothesis that GBR12909 injection impairs habituation and induces hyperlocomotion in mice, along with changes in cytokines and neurotrophic factors levels, as observed in BD patients. We also tested if lithium carbonate, sodium valproate and aripiprazole prevent GBR12909-induced locomotion. Male Swiss mice received GBR12909 (15 mg/kg) injections and locomotor responses were quantified in an open field. Cytokines and neurotrophic factors levels were assessed in the prefrontal cortex, striatum and hippocampus 30 min and 24 h after injections. Pre-treatments with lithium, valproate or aripiprazole were performed with single and repeated injection protocols. GBR12909 prevented motoric habituation and increased basal locomotion in habituated mice in the open field. This compound also induced changes in IL-2 and BDNF levels in prefrontal cortex; IL-2, IL-4 and IL-10 in striatum; and IL-10, IL-4, IFN-γ and NGF in hippocampus. GBR12909-induced hyperlocomotion was attenuated by lithium (12.5-100 mg/kg), but not valproate (75-300 mg/kg), and prevented by aripiprazole (0.1-10 mg/kg). Repeated injections of these drugs (twice a day for 3 days), however, failed to inhibit hyperlocomotion. The main limitations of the protocols in this study are the analysis of locomotion as the only behavioral parameter, changes in immune factors that may overlap with other psychiatric disorders and the lack chronic drug injections. Despite of these limitations, this study adds to previous literature suggesting DAT inhibition as a potential animal model of mania related to BD.

Progress in bipolar disorder drug design toward the development of novel therapeutic targets: a clinician's perspective

INTRODUCTION

Bipolar disorder (BD) is a considerable burden to the affected individual. The need for novel drug targets and improved drug design (DD) in BD is therefore clear. Areas covered: The following article provides a brief, narrative, clinician-oriented overview of the most promising novel pharmacological targets for BD along with a concise overview regarding the general DD process and the unmet needs relevant to BD. Expert opinion: A number of novel potential drug targets have been investigated. With the notable exception of the kynurenine pathway, available evidence is too scarce to highlight a definitive roadmap for forthcoming DD in BD. BD itself may present with different facets, as it is a polymorphic clinical spectrum. Therefore, promoting clinical-case stratification should be based on precision medicine, rather than on novel biological targets. Furthermore, the full release of raw study data to the scientific community and the development of uniform clinical trial standards (including more realistic outcomes) should be promoted to facilitate the DD process in BD.

An Oldie but Goodie: Lithium in the Treatment of Bipolar Disorder through Neuroprotective and Neurotrophic Mechanisms

Lithium has been used for the treatment of bipolar disorder (BD) for the last sixty or more years, and recent studies with more reliable designs and updated guidelines have recommended lithium to be the treatment of choice for acute manic, mixed and depressive episodes of BD, along with long-term prophylaxis. Lithium’s specific mechanism of action in mood regulation is progressively being clarified, such as the direct inhibition on glycogen synthase kinase 3β, and its various effects on neurotrophic factors, neurotransmitters, oxidative metabolism, apoptosis, second messenger systems, and biological systems are also being revealed. Furthermore, lithium has been proposed to exert its treatment effects through mechanisms associated with neuronal plasticity. In this review, we have overviewed the clinical aspects of lithium use for BD, and have focused on the neuroprotective and neurotrophic effects of lithium.

Blockade of dopamine D1-family receptors attenuates the mania-like hyperactive, risk-preferring, and high motivation behavioral profile of mice with low dopamine transporter levels

BACKGROUND

Patients with bipolar disorder mania exhibit poor cognition, impulsivity, risk-taking, and goal-directed activity that negatively impact their quality of life. To date, existing treatments for bipolar disorder do not adequately remediate cognitive dysfunction. Reducing dopamine transporter expression recreates many bipolar disorder mania-relevant behaviors (i.e. hyperactivity and risk-taking). The current study investigated whether dopamine D1-family receptor blockade would attenuate the risk-taking, hypermotivation, and hyperactivity of dopamine transporter knockdown mice.

METHODS

Dopamine transporter knockdown and wild-type littermate mice were tested in mouse versions of the Iowa Gambling Task (risk-taking), Progressive Ratio Breakpoint Test (effortful motivation), and Behavioral Pattern Monitor (activity). Prior to testing, the mice were treated with the dopamine D1-family receptor antagonist SCH 23390 hydrochloride (0.03, 0.1, or 0.3 mg/kg), or vehicle.

RESULTS

Dopamine transporter knockdown mice exhibited hyperactivity and hyperexploration, hypermotivation, and risk-taking preference compared with wild-type littermates. SCH 23390 hydrochloride treatment decreased premature responding in dopamine transporter knockdown mice and attenuated their hypermotivation. SCH 23390 hydrochloride flattened the safe/risk preference, while reducing activity and exploratory levels of both genotypes similarly.

CONCLUSIONS

Dopamine transporter knockdown mice exhibited mania-relevant behavior compared to wild-type mice. Systemic dopamine D1-family receptor antagonism attenuated these behaviors in dopamine transporter knockdown, but not all effects were specific to only the knockdown mice. The normalization of behavior via blockade of dopamine D1-family receptors supports the hypothesis that D1 and/or D5 receptors could contribute to the mania-relevant behaviors of dopamine transporter knockdown mice.

Brexpiprazole reduces hyperactivity, impulsivity, and risk-preference behavior in mice with dopamine transporter knockdown-a model of mania

RATIONALE

Bipolar disorder (BD) is a unique mood disorder defined by periods of depression and mania. The defining diagnosis of BD is the presence of mania/hypomania, with symptoms including hyperactivity and risk-taking. Since current treatments do not ameliorate cognitive deficits such as risky decision-making, and impulsivity that can negatively affect a patient's quality of life, better treatments are needed.

OBJECTIVES

Here, we tested whether acute treatment with brexpiprazole, a serotonin-dopamine activity modulator with partial agonist activity at D_2/3 and 5-HT_1A receptors, would attenuate the BD mania-relevant behaviors of the dopamine transporter (DAT) knockdown mouse model of mania.

METHODS

The effects of brexpiprazole on DAT knockdown and wild-type littermate mice were examined in the behavioral pattern monitor (BPM) and Iowa gambling task (IGT) to quantify activity/exploration and impulsivity/risk-taking behavior respectively.

RESULTS

DAT knockdown mice exhibited hyper-exploratory behavior in the BPM and made fewer safe choices in the IGT. Brexpiprazole attenuated the mania-like hyper-exploratory phenotype and increased safe choices in risk-preferring DAT knockdown mice. Brexpiprazole also reduced safe choices in safe-preferring mice irrespective of genotype. Finally, brexpiprazole reduced premature (impulsive-like) responses in both groups of mice.

CONCLUSIONS

Consistent with earlier reports, DAT knockdown mice exhibited hyper-exploratory, risk-preferring, and impulsive-like profiles consistent with patients with BD mania in these tasks. These behaviors were attenuated after brexpiprazole treatment. These data therefore indicate that brexpiprazole could be a novel treatment for BD mania and/or risk-taking/impulsivity disorders, since it remediates some relevant behavioral abnormalities in this mouse model.

Striatal dopamine D1 receptor suppression impairs reward-associative learning

Dopamine (DA) is required for reinforcement learning. Hence, disruptions in DA signaling may contribute to the learning deficits associated with psychiatric disorders. The DA D1 receptor (D1R) has been linked to learning and is a target for cognitive/motivational enhancement in patients with schizophrenia. Separating the striatal D1R contribution to learning vs. motivation, however, has been challenging. We suppressed striatal D1R expression in mice using a D1R-targeting short hairpin RNA (shRNA), delivered locally to the striatum via an adeno-associated virus (AAV). We then assessed reward- and punishment-associative learning using a probabilistic learning task and motivation using a progressive-ratio breakpoint procedure. We confirmed suppression of striatal D1Rs immunohistochemically and by testing locomotor activity after the administration of (+)-doxanthrine, a full D1R agonist, in control mice and those treated with the D1RshRNA. D1RshRNA-treated mice exhibited impaired reward-associative learning, while punishment-associative learning was spared. This deficit was unrelated to general learning impairments or amotivation, because the D1shRNA-treated mice exhibited normal Barnes maze learning and normal motivation in the progressive-ratio breakpoint procedure. Suppression of striatal D1Rs selectively impaired reward-associative learning whereas punishment-associative learning, aversion-motivated learning, and appetitive motivation were spared. Because patients with schizophrenia exhibit similar reward-associative learning deficits, D1R-targeted treatments should be investigated to improve reward learning in these patients.

The effects of reduced dopamine transporter function and chronic lithium on motivation, probabilistic learning, and neurochemistry in mice: Modeling bipolar mania

BACKGROUND

Bipolar disorder (BD) mania patients exhibit poor cognition and reward-seeking/hypermotivation, negatively impacting a patient's quality of life. Current treatments (e.g., lithium), do not treat such deficits. Treatment development has been limited due to a poor understanding of the neural mechanisms underlying these behaviors. Here, we investigated putative mechanisms underlying cognition and reward-seeking/motivational changes relevant to BD mania patients using two validated mouse models and neurochemical analyses.

METHODS

The effects of reducing dopamine transporter (DAT) functioning via genetic (knockdown vs. wild-type littermates), or pharmacological (GBR12909- vs. vehicle-treated C57BL/6J mice) means were assessed in the probabilistic reversal learning task (PRLT), and progressive ratio breakpoint (PRB) test, during either water or chronic lithium treatment. These tasks quantify reward learning and effortful motivation, respectively. Neurochemistry was performed on brain samples of DAT mutants ± chronic lithium using high performance liquid chromatography.

RESULTS

Reduced DAT functioning increased reversals in the PRLT, an effect partially attenuated by chronic lithium. Chronic lithium alone slowed PRLT acquisition. Reduced DAT functioning increased motivation (PRB), an effect attenuated by lithium in GBR12909-treated mice. Neurochemical analyses revealed that DAT knockdown mice exhibited elevated homovanillic acid levels, but that lithium had no effect on these elevated levels.

CONCLUSIONS

Reducing DAT functioning recreates many aspects of BD mania including hypermotivation and improved reversal learning (switching), as well as elevated homovanillic acid levels. Chronic lithium only exerted main effects, impairing learning and elevating norepinephrine and serotonin levels of mice, not specifically treating the underlying mechanisms identified in these models.

Therapeutic Mechanisms of Lithium in Bipolar Disorder: Recent Advances and Current Understanding

Lithium is the most effective and well established treatment for bipolar disorder, and it has a broad array of effects within cellular pathways. However, the specific processes through which therapeutic effects occur and are maintained in bipolar disorder remain unclear. This paper provides a timely update to an authoritative review of pertinent findings that was published in CNS Drugs in 2013. A literature search was conducted using the Scopus database, and was limited by year (from 2012). There has been a resurgence of interest in lithium therapy mechanisms, perhaps driven by technical advancements in recent years that permit the examination of cellular mechanisms underpinning the effects of lithium-along with the reuptake of lithium in clinical practice. Recent research has further cemented glycogen synthase kinase 3β (GSK3β) inhibition as a key mechanism, and the inter-associations between GSK3β-mediated neuroprotective, anti-oxidative and neurotransmission mechanisms have been further elucidated. In addition to highly illustrative cellular research, studies examining higher-order biological systems, such as circadian rhythms, as well as employing innovative animal and human models, have increased our understanding of how lithium-induced changes at the cellular level possibly translate to changes at behavioural and clinical levels. Neural circuitry research is yet to identify clear mechanisms of change in bipolar disorder in response to treatment with lithium, but important structural findings have demonstrated links to the modulation of cellular mechanisms, and peripheral marker and pharmacogenetic studies are showing promising findings that will likely inform the exploration for predictors of lithium treatment response. With a deeper understanding of lithium's therapeutic mechanisms-from the cellular to clinical levels of investigation-comes the opportunity to develop predictive models of lithium treatment response and identify novel drug targets, and recent findings have provided important leads towards these goals.

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.

Locomotor Profiling from Rodents to the Clinic and Back Again

The quantification of unconditioned motoric activity is one of the oldest and most commonly utilized tools in behavioral studies. Although typically measured in reference to psychiatric disorders, e.g., amphetamine-induced hyperactivity used as a model of schizophrenia, bipolar disorder (BD), and Tourette's syndrome, the motoric behavior of psychiatric patients had not been quantified similarly to rodents until recently. The rodent behavioral pattern monitor (BPM) was reverse-translated for use in humans, providing the quantification of not only motoric activity but also the locomotor exploratory profile of various psychiatric populations. This measurement includes the quantification of specific exploration and locomotor patterns. As an example, patients with BD, schizophrenia, and those with history of methamphetamine dependence exhibited unique locomotor profiles. It was subsequently determined that reducing dopamine transporter function selectively recreated the locomotor profile of BD mania patients and not any other patient population. Hence, multivariate locomotor profiling offers a first-step approach toward understanding the neural mechanism(s) underlying abnormal behavior in patients with psychiatric disorders. Advances in wearable technology will undoubtedly enable similar multivariate assessments of exploratory and locomotor behavior in "real-world" contexts. Furthermore, trans-diagnostic studies of locomotor activity profiles will inform about essential brain-based functions that cut across diagnostic nosologies.