Colleen A. McClung

Mania-like behavior induced by disruption of CLOCK

Circadian rhythms and the genes that make up the molecular clock have long been implicated in bipolar disorder. Genetic evidence in bipolar patients suggests that the central transcriptional activator of molecular rhythms, CLOCK, may be particularly important. However, the exact role of this gene in the development of this disorder remains unclear. Here we show that mice carrying a mutation in the Clock gene display an overall behavioral profile that is strikingly similar to human mania, including hyperactivity, decreased sleep, lowered depression-like behavior, lower anxiety, and an increase in the reward value for cocaine, sucrose, and medial forebrain bundle stimulation. Chronic administration of the mood stabilizer lithium returns many of these behavioral responses to wild-type levels. In addition, the Clock mutant mice have an increase in dopaminergic activity in the ventral tegmental area, and their behavioral abnormalities are rescued by expressing a functional CLOCK protein via viral-mediated gene transfer specifically in the ventral tegmental area. These findings establish the Clock mutant mice as a previously unrecognized model of human mania and reveal an important role for CLOCK in the dopaminergic system in regulating behavior and mood.

Regulation of gene expression and cocaine reward by CREB and DeltaFosB.

ΔFosB (a truncated form of FosB) and CREB (cAMP response element binding protein) are transcription factors induced in the brains reward pathways after chronic exposure to drugs of abuse. However, their mechanisms of action and the genes they regulate remain unclear. Using microarray analysis in the nucleus accumbens of inducible transgenic mice, we found that CREB and a dominant-negative CREB have opposite effects on gene expression, as do prolonged expression of ΔFosB and the activator protein-1 (AP-1) antagonist ΔcJun. However, unlike CREB, short-term and prolonged ΔFosB induction had opposing effects on gene expression. Gene expression induced by short-term ΔFosB and by CREB was strikingly similar, and both reduced the rewarding effects of cocaine, whereas prolonged ΔFosB expression increased drug reward. Gene expression after a short cocaine treatment was more dependent on CREB, whereas gene expression after a longer cocaine treatment became increasingly ΔFosB dependent. These findings help define the molecular functions of CREB and ΔFosB and identify clusters of genes that contribute to cocaine addiction.

Neuroplasticity Mediated by Altered Gene Expression

Plasticity in the brain is important for learning and memory, and allows us to respond to changes in the environment. Furthermore, long periods of stress can lead to structural and excitatory changes associated with anxiety and depression that can be reversed by pharmacological treatment. Drugs of abuse can also cause long-lasting changes in reward-related circuits, resulting in addiction. Each of these forms of long-term plasticity in the brain requires changes in gene expression. Upon stimulation, second messenger pathways are activated that lead to an enhancement in transcription factor activity at gene promoters. This stimulation results in the expression of new growth factors, ion channels, structural molecules, and other proteins necessary to alter the neuronal circuit. With repeated stimulation, more permanent modifications to transcription factors and chromatin structure are made that result in either sensitization or desensitization of a circuit. Studies are beginning to uncover the molecular mechanisms that lead to these types of long-term changes in the brain. This review summarizes some of the major transcriptional mechanisms that are thought to underlie neuronal and behavioral plasticity.

Two Interdependent TRPV Channel Subunits, Inactive and Nanchung, Mediate Hearing in Drosophila

Hearing in Drosophila depends on the transduction of antennal vibration into receptor potentials by ciliated sensory neurons in Johnstons organ, the antennal chordotonal organ. We previously found that a Drosophila protein in the vanilloid receptor subfamily (TRPV) channel subunit, Nanchung (NAN), is localized to the chordotonal cilia and required to generate sound-evoked potentials (Kim et al., 2003). Here, we show that the only other Drosophila TRPV protein is mutated in the behavioral mutant inactive (iav). The IAV protein forms a hypotonically activated channel when expressed in cultured cells; in flies, it is specifically expressed in the chordotonal neurons, localized to their cilia and required for hearing. IAV and NAN are each undetectable in cilia of mutants lacking the other protein, indicating that they both contribute to a heteromultimeric transduction channel in vivo. A functional green fluorescence protein-IAV fusion protein shows that the channel is restricted to the proximal cilium, constraining models for channel activation.

CREB regulation of nucleus accumbens excitability mediates social isolation-induced behavioral deficits

Here, we characterized behavioral abnormalities induced by prolonged social isolation in adult rodents. Social isolation induced both anxiety- and anhedonia-like symptoms and decreased cAMP response element–binding protein (CREB) activity in the nucleus accumbens shell (NAcSh). All of these abnormalities were reversed by chronic, but not acute, antidepressant treatment. However, although the anxiety phenotype and its reversal by antidepressant treatment were CREB-dependent, the anhedonia-like symptoms were not mediated by CREB in NAcSh. We found that decreased CREB activity in NAcSh correlated with increased expression of certain K+ channels and reduced electrical excitability of NAcSh neurons, which was sufficient to induce anxiety-like behaviors and was reversed by chronic antidepressant treatment. Together, our results describe a model that distinguishes anxiety- and depression-like behavioral phenotypes, establish a selective role of decreased CREB activity in NAcSh in anxiety-like behavior, and provide a mechanism by which antidepressant treatment alleviates anxiety symptoms after social isolation.

How might circadian rhythms control mood? Let me count the ways…..

Mood disorders are serious diseases that affect a large portion of the population. There have been many hypotheses put forth over the years to explain the development of major depression, bipolar disorder, and other mood disorders. These hypotheses include disruptions in monoamine transmission, hypothalamus-pituitary-adrenal axis function, immune function, neurogenesis, mitochondrial dysfunction, and neuropeptide signaling (to name a few). Nearly all people suffering from mood disorders have significant disruptions in circadian rhythms and the sleep/wake cycle. In fact, altered sleep patterns are one of the major diagnostic criteria for these disorders. Moreover, environmental disruptions to circadian rhythms, including shift work, travel across time zones, and irregular social schedules, tend to precipitate or exacerbate mood-related episodes. Recent studies have found that molecular clocks are found throughout the brain and body where they participate in the regulation of most physiological processes, including those thought to be involved in mood regulation. This review will summarize recent data that implicate the circadian system as a vital regulator of a variety of systems that are thought to play a role in the development of mood disorders.

Orexin Signaling Mediates the Antidepressant-Like Effect of Calorie Restriction

During periods of reduced food availability, animals must respond with behavioral adaptations that promote survival. Despite the fact that many psychiatric syndromes include disordered eating patterns as a component of the illness, little is known about the neurobiology underlying behavioral changes induced by short-term calorie restriction. Presently, we demonstrate that 10 d of calorie restriction, corresponding to a 20–25% weight loss, causes a marked antidepressant-like response in two rodent models of depression and that this response is dependent on the hypothalamic neuropeptide orexin (hypocretin). Wild-type mice, but not mice lacking orexin, show longer latency to immobility and less total immobility in the forced swim test after calorie restriction. In the social defeat model of chronic stress, calorie restriction reverses the behavioral deficits seen in wild-type mice but not in orexin knock-out mice. Additionally, chronic social defeat stress induces a prolonged reduction in the expression of prepro-orexin mRNA via epigenetic modification of the orexin gene promoter, whereas calorie restriction enhances the activation of orexin cells after social defeat. Together, these data indicate that orexin plays an essential role in mediating reduced depression-like symptoms induced by calorie restriction.

Knockdown of Clock in the Ventral Tegmental Area Through RNA Interference Results in a Mixed State of Mania and Depression-Like Behavior

BACKGROUND Circadian rhythm abnormalities are strongly associated with bipolar disorder; however the role of circadian genes in mood regulation is unclear. Previously, we reported that mice with a mutation in the Clock gene (ClockDelta19) display a behavioral profile that is strikingly similar to bipolar patients in the manic state. METHODS Here, we used RNA interference and viral-mediated gene transfer to knock down Clock expression specifically in the ventral tegmental area (VTA) of mice. We then performed a variety of behavioral, molecular, and physiological measures. RESULTS We found that knockdown of Clock, specifically in the VTA, results in hyperactivity and a reduction in anxiety-related behavior, which is similar to the phenotype of the ClockDelta19 mice. However, VTA-specific knockdown also results in a substantial increase in depression-like behavior, creating an overall mixed manic state. Surprisingly, VTA knockdown of Clock also altered circadian period and amplitude, suggesting a role for Clock in the VTA in the regulation of circadian rhythms. Furthermore, VTA dopaminergic neurons expressing the Clock short hairpin RNA have increased activity compared with control neurons, and this knockdown alters the expression of multiple ion channels and dopamine-related genes in the VTA that could be responsible for the physiological and behavioral changes in these mice. CONCLUSIONS Taken together, these results suggest an important role for Clock in the VTA in the regulation of dopaminergic activity, manic and depressive-like behavior, and circadian rhythms.

Regulation of Gene Expression by Chronic Morphine and Morphine Withdrawal in the Locus Ceruleus and Ventral Tegmental Area

Morphine dependence is associated with long-term adaptive changes in the brain that involve gene expression. Different behavioral effects of morphine are mediated by different brain regions, for example, the locus ceruleus (LC), a noradrenergic nucleus, is implicated in physical dependence and withdrawal, whereas the ventral tegmental area (VTA), a dopaminergic nucleus, contributes to rewarding and locomotor responses to the drug. However, the global changes in gene expression that occur in these brain regions after morphine exposure and during withdrawal remain unknown. Using DNA microarray analysis in both mice and rats, we now characterize gene expression changes that occur in these brain regions with chronic morphine and antagonist-precipitated withdrawal. In the LC, numerous genes display common regulation between mouse and rat, including tyrosine hydroxylase, prodynorphin, and galanin. Furthermore, we identify clusters of genes that are regulated similarly by chronic morphine and by withdrawal, as well as clusters that show opposite regulation under these two conditions. Interestingly, most gene expression changes that occur in the VTA in response to chronic morphine are different from those seen in the LC, but the gene expression patterns in the two brain regions are very similar after withdrawal. In addition, we examined two genes (prodynorphin and FK506 binding protein 5) that are strongly regulated by chronic morphine or morphine withdrawal in the LC for their role in regulating withdrawal-associated behaviors. Inhibition of either protein profoundly affects withdrawal responses, demonstrating that the genes identified in this study have important functional roles in mediating opiate-induced behaviors.

Circadian rhythms and mood regulation: Insights from pre-clinical models

Affective disorders such as major depression, bipolar disorder, and seasonal affective disorder are associated with major disruptions in circadian rhythms. Indeed, altered sleep/wake cycles are a critical feature for diagnosis in the DSM IV and several of the therapies used to treat these disorders have profound effects on rhythm length and stabilization in human populations. Furthermore, multiple human genetic studies have identified polymorphisms in specific circadian genes associated with these disorders. Thus, there appears to be a strong association between the circadian system and mood regulation, although the mechanisms that underlie this association are unclear. Recently, a number of studies in animal models have begun to shed light on the complex interactions between circadian genes and mood-related neurotransmitter systems, the effects of light manipulation on brain circuitry, the impact of chronic stress on rhythms, and the ways in which antidepressant and mood-stabilizing drugs alter the clock. This review will focus on the recent advances that have been gleaned from the use of pre-clinical models to further our understanding of how the circadian system regulates mood.