westrich & sprouse, 2010 - coid - circadian rhythm dysregulation in bipolar disorder

REVIEW

Current Opinion in Investigational Drugs 2010 11(7):779-787© Thomson Reuters (Scientific) Ltd ISSN 2040-3429

IntroductionThe regulation of mood follows a circadian pattern – an oscillation of physiological and behavioral events over a period of approximately 24 h; disturbances in these endogenous rhythms can contribute to psychiatric disorders. Mood alterations occur when the internal rhythms become desynchronized with external environmental cues, such as daylight. Such a causal link has been postulated for more than 30 years [1]. Historically, evidence of the connection has relied on the successful management of symptoms by use of pharmacological and light therapies that have an impact on circadian rhythms. The changes achieved in the amplitude or phase (ie, timing of peaks and troughs) of various circadian output parameters by such therapies suggest that chronobiological disturbances are involved in both the cause and treatment of mood alterations. This review encompasses aspects of animal and clinical studies of circadian behavior, as well as molecular findings, that provide evidence for the involvement of circadian abnormalities in psychiatric disorders, particularly in bipolar disorder. Therapeutic approaches that can resynchronize internal timekeeping to match the solar day highlight what might be gained by circadian rhythm-centric modalities of treatment. In particular, pharmacological approaches that may be used to achieve such resynchronization are discussed.

The mammalian molecular clock and its physiological roleEndogenous clocks drive numerous physiological and behavioral functions to oscillate with a circadian rhythm.

Such a system allows organisms that experience daily light:dark (L:D) cycles to adapt their internal functions continuously to the prevailing environment. Consequently, one of the primary functions of the circadian clock is to maintain both plants and animals in synchrony with their surroundings, to cause a temporary suspension of the natural homeostatic drive to adjust to the time of day and, as a result, to ensure the health of the organism. In mammals, the principal circadian pacemaker is located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. Although the SCN has an endogenous rhythm that is slightly shorter or longer than 24 h (depending on inter-species as well as intra-species differences), it is synchronized precisely to this time period by dominant light cues. Phase regulation depends on input to the SCN from the retina through the retinohypothalmic tract (RHT), which enables the transmission of light and therefore ensures the entrainment of daily rhythms to 24-h cycles [2,3]. 'Peripheral clocks' with tissue-specific functions are located throughout the body and are under the control of the master clock in the SCN, but may not necessarily have the same phase [4,5]. Together, the central and peripheral clocks comprise the circadian system. When the clocks are in synchrony, normal functioning prevails; when not in proper synchrony, problems may ensue, such as the malaise and fatigue associated with jet lag.

At the molecular level, the rhythms of both central and peripheral clocks are controlled by a primary transcriptional-translational negative feedback network comprised of clock genes, including circadian locomotor

Circadian rhythm dysregulation in bipolar disorder Ligia Westrich* & Jeffrey SprouseAddressLundbeck Research USA Inc, Neuroscience, 215 College Road, Paramus, NJ 07652, USAEmail: [email protected]

*To whom correspondence should be addressed

When circadian rhythms – the daily oscillations of various physiological and behavioral events that are controlled by a central timekeeping mechanism – become desynchronized with the prevailing light:dark cycle, a maladaptative response can result. Animal data based primarily on genetic manipulations and clinical data from biomarker and gene expression studies support the notion that circadian abnormalities underlie certain psychiatric disorders. In particular, bipolar disorder has an interesting link to rhythm-related disease biology; other mood disturbances, such as major depressive disorder, seasonal affective disorder and the agitation and aggression accompanying severe dementia (sundowning), are also linked to changes in circadian rhythm function. Possibilities for pharmacological intervention derive most readily from the molecular oscillator, the cellular machinery that drives daily rhythms.

Keywords Biological clock, bipolar disorder, circadian rhythm, desynchronization, entrainment, kinase, light:dark cycle, oscillator, period length

780 Current Opinion in Investigational Drugs 2010 Vol 11 No 7

output cycles kaput (Clock), brain and muscle arnt-like-1 (Bmal1), period 1/2 (Per1/2), cryptochrome 1/2 (Cry1/2), and their protein products, all of which vary in quantity over a 24-h cycle [6] (Figure 1). The BMAL1-CLOCK transcription factor complex activates the transcription of Per and Cry during daylight periods. In the cytoplasm, the PER and CRY proteins heterodimerize and translocate to the nucleus to block their own transcription, by interfering with the BMAL1-CLOCK complex. Concurrently, PER and CRY undergo post-translational modification, specifically phosphorylation, in a precise, rhythmic and light phase-specific manner. The phosphorylated PER-CRY complex degrades during the phase of darkness, thus attenuating its inhibition of the BMAL1-CLOCK complex and allowing activation of Per and Cry transcription to resume. This process underscores the significance of post-translational modifications in determining and

controlling endogenous cyclical rhythms. As part of a second regulatory loop, the BMAL1-CLOCK transcription activator complex also controls the transcription of Rev-Erbα, which subsequently represses the transcription of Bmal1 by competing for the retinoic acid-related orphan receptor β (RORβ) in the Bmal1 promoter region [6]. The timing of a complete cycle is close to the prevailing light:dark cycle; synchronization occurs when light cues align the rhythms.

Endogenous clocks have the ability to respond to various salient changes in the environment – primarily light, food and temperature – allowing mammals to anticipate and coordinate their physiological needs with the available resources and demands of the environment. The most thoroughly characterized response to external cues is the circadian rhythm of the hypothalamic-pituitary-

Figure 1. The mammalian circadian clock: A series of transcriptional-translational feedback loops.

The core clock genes circadian locomotor output cycles kaput (Clock) and brain and muscle arnt-like-1 (Bmal1) are transcribed and form heterodimers in the cytoplasm, and then re-enter the nucleus to activate the transcription of period (Per) and cryptochrome (Cry) genes. PER and CRY protein products form heterodimers in the cytoplasm that, upon phosphorylation, re-enter the nucleus and repress further BMAL1-CLOCK activation of transcription, thus completing one cycle of the oscillator (dotted arrows). Phosphorylation also primes the PER-CRY hetereodimer for degradation, allowing a new cycle of transcription to occur. A secondary autoregulatory loop results from the activation of Rev-Erbα transcription by the BMAL1-CLOCK complex. Phosphorylated Rev-Erbα then re-enters the nucleus to repress Bmal1 transcription by competing with RAR-related orphan receptor β (RORβ). Mouse albumin D element-binding protein (DBP) is located downstream of the molecular oscillator, and controls the expression of other clock-controlled genes. Two key kinases in these loops are casein kinase I δ (CKIδ) for the phosphorylation of the PER-CRY complex and glycogen synthase kinase 3β (GSK3β) for the phosphorylation of Rev-Erbα. Degradation of these proteins is accomplished via the β−TrCP and FBXL3 ubiquitin ligase complexes.(Adapted with permission from Lundbeck Research USA Inc. © 2010 Lundbeck Research USA Inc)

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Circadian rhythms and bipolar disorder Westrich & Sprouse 781 781

adrenocortical (HPA) axis: the SCN-timed release of ACTH controls the peak (early morning) and nadir (midnight) of cortisol secretions in diurnal species. In addition to the HPA axis, the SCN controls the release of melatonin from the pineal gland, and also controls the sleep-wake cycle, feeding behavior, body temperature, locomotor activity and the immune system response. The disruption of normal circadian rhythms in these systems can result in disease, including depression (hypercortisolemia) [7], bipolar disorder (abnormal sleep-wake cycle related to cycling of mood) [8] and severe dementia (increased nocturnal activity) [9].

The biological clock in desynchronyAn organism can entrain to LD cycles of slightly differing lengths from the 24-h day (eg, typically from 23 to 26 h), but this relationship is weakened when internal circadian rhythms are compacted or stretched to 22- or 28-h days. Under these conditions, internal circadian rhythms resort to their endogenous period length and animals 'free-run' independently of the influence of light, the most powerful zeitgeber (an exogenous cue functioning as a synchronizer). This outcome is referred to as 'forced desynchrony', defined as a dissociation of circadian behaviors from the external solar clock, in some cases resulting in multiple dissociated internal rhythms [10]. For example, the rhythms of locomotor activity, core body temperature, sleep stages and melatonin secretion are internally desynchronized in rats exposed to 22-h LD cycles, but the impact of the shortened day is not equal for every function [11]. In particular, melatonin release schedules become the product of two distinct oscillator mechanisms, one driven by the LD cycle, the other by internal timekeeping.

One explanation for the observed desynchrony in rats exposed to shortened days could reflect the disparate components of the circadian rhythm that are under the control of different subregions of the SCN. The RHT projects primarily to the ventrolateral aspect of the SCN (vlSCN); therefore, systems receiving input from the efferent projections of this subregion entrain to the LD cycle. The vlSCN projects to the dorsomedial division (dmSCN). Abrupt changes in the LD cycle may induce dissociations in rhythmic clock gene expression between the vlSCN and dmSCN [11]. For example, under forced desynchrony conditions, rapid eye movement sleep (REMS) – directly controlled via dmSCN – free-runs with the endogenous circadian period, while slow-wave sleep (SWS) is synchronized with the sleep episode via direct input from vlSCN [12]. Investigators have considered whether these delicately balanced connections might be dysregulated in patients with psychiatric disorders, thereby resulting in rhythms that are desynchronized with the 24-h day. Further research is required, but an understanding of the mechanisms underlying the effects of lithium may provide some insight. A commonly prescribed mood stabilizer, lithium delays circadian rhythms as a consequence of its ability to prolong period length. This impact is observed in some rhythmic

functions in rats (eg, wheel running, body temperature, corticosteroid levels and REM sleep), but has not been observed in others (eg, pineal serotonin, melatonin and liver glycogen) [13].

Circadian rhythm dysregulation in bipolar disorderHuman perceptions of mood occur in a cyclical pattern; misalignments between internal and external rhythms may result in mood alterations, such as major depressive disorder (MDD), depressive states linked to seasonality or seasonal affective disorder (SAD), and mood swings that occur on a cyclical basis or with bipolar disorder. In neurodegenerative states, the loss of SCN function can manifest as agitation or aggression in patients with severe dementia – the 'sundowning' effect observed in Alzheimer's disease. In this review, discussions focus on bipolar disorder, as multiple lines of evidence suggest a link for the condition with rhythmic disturbances (for other circadian rhythm-based disease states, see references [14-17]).

Bipolar disorder is a chronically relapsing condition that is characterized by a spontaneous cycling of mood between depression and mania [18]. Such mood swings can be rapid, or there can be extended periods of euthymia (ie, neutral mood) between episodes. The modulation of various circadian signaling pathways has been described, but little is known regarding the mechanisms that underlie the manic and depressive states and the factors that initiate the switch between states. Abnormalities observed in patients with bipolar disorder include a shortening of the normal 24-h period length of endogenous rhythms and/or a blunting of the rhythms (loss of amplitude). Several non-invasive approaches are available to monitor such changes (eg, rest:activity rhythms measured by actigraphy, melatonin cycling and self-assessment questionnaires). In practice, these biomarkers reveal that phase advances occur in patients with bipolar disorder, resulting from the period shortening that generally precedes the switch to a hypomanic state (occurring prior to the manifestation of full mania). For example, shifts in the temporal distribution of REM sleep and the circadian rhythm of body temperature have been observed in patients with bipolar disorder during manic-depressive cycles. The peak or acrophase of REM sleep and body temperature occurs progressively earlier during the hypomanic period and progressively later during depressive episodes [18].

A consideration of a causal link between altered internal phase relationships and bipolar disorder has not been evaluated extensively. One framework, established in the late 1970s, depicts a 'cyclic beat' that occurs when the phase of the shortened internal rhythms periodically coincides with that of the prevailing day. The frequency of this 'beat' reflects the disparity in period length between the internal and external rhythms; in some patients with bipolar disorder, the rate of this beat predicts the frequency of mood swings [1,19]. Data from animal models


are limited but supportive of this correlation. Rats placed in a 28-h forced desynchrony paradigm exhibited a range of rest-activity patterns. In some animals, bouts of hyperlocomotor activity (ie, a manic-like state) occurred with a frequency that was predicted using the cyclic beat phenomenon [20]. Additional studies are required to confirm this relationship, but the notion that rhythmic disparities control mood in bipolar disorder should encourage further research. Potential areas of study include assessing whether patients with bipolar disorder experience symptom improvement in a time-shortened day that is designed to match their period length and, conversely, whether healthy individuals experience mood swings in the presence of long-day forced desynchrony schedules.

Such questions may be addressed initially in animal models, such as Clock mutants and D-box binding protein (Dbp) knockouts (Table 1). Clock mutants have a free-running period of 25 to 27 h and have completely arrhythmic activities after several weeks in constant darkness [21], while Dbp mutants have a short period length of approximately 20 to 22 h [22,23]. Both animal models display mania-like behavior, but only the Dbp model possesses a depressive phenotype (hypolocomotor behavior) [22-24]. Although Dbp is not essential for the generation of circadian rhythms, the gene controls downstream clock-controlled gene expression (Figure 1), regulating clock outputs such as circadian sleep consolidation and the time course of slow-wave sleep δ power [23]. Interestingly, the typically depressive phenotype of these animals switched to the manic state following exposure to a stressor (ie, chronic isolation plus acute exposure to forced swim, tail suspension or tail flick tests); reversal to the basal phenotype was achieved

with the mood stabilizer valproate [25]. Despite such face validity, no single animal model captures all aspects of bipolar disorder (Table 1); instead, multiple approaches are needed.

Beyond information gained from the use of biomarkers related to clinical state, gene expression studies have revealed rare abnormalities that suggest that a poorly functioning body clock results in a poorly functioning host. For example, a SNP of T → C in the 3' flanking region of the human Clock gene has been associated with a diurnal preference for 'eveningness' (ie, greater alertness and function in evening hours) in patients with bipolar disorder who carry at least one copy of the 3111 C allele. More importantly, this substitution appears to predict the number of manic and depressive episodes accurately [26]. Other circadian genetic links to bipolar disorder have also been reported, including genes encoding vasoactive intestinal peptide (VIP), RORβ, glycogen synthase kinase 3β (GSK3β), PER, casein kinase I (CKI) and Rev-Erbα [27-33], cumulatively suggesting that circadian effects may be causal factors in bipolar disorder.

An interesting area of research is whether bipolar disorder in all patients results from circadian rhythm abnormalities. Such uniformity would be unlikely, given the heterogeneous nature of psychiatric disorders. The available data are both intriguing and puzzling. For example, one study suggested a differential association of clock genes in mood disorders, with Cry1 and neuronal PAS domain-containing protein-2 (NPAS2; a paralog of Clock) possessing a closer link to unipolar depression, and VIP and Clock yielding more bipolar disorder-specific effects [34]. Other studies, however, failed to detect

Genetic manipulation Phenotype/similarities to bipolar disorder Missing components/disadvantages of model References

Dbp knockout Period length < 24 hDecreased locomotor activity and sleep EEG abnormalitiesSwitch to hyperactivity following exposure to stress

Pharmacological and/or light treatments to alter the phenotype unavailable

[22,23,25]

Clock mutants Increased overall locomotor activityHyperactivity in a novel environmentReduced anxiety Increased preference for cocaine (mania-like behavior)

Period length > 24 hNo evidence of depressive-like behavior

[21,24]

Vipr2 knockout Period length < 24 hIncreased wheel-running in DD compared with LD

Arrhythmicity in absence of environmental cues (in a subset of animals)Limited assessment of mood- and stress-related behaviors available

[65]

VPAC2R transgenics(overexpression)

Period length < 24 hResynchronization more quickly than in wild type to phase advance

Unknown receptor compensation because of overexpression

[66]

Cry1 knockout Period length < 24 hPoorly synchronized circadian rhythmicity

Limited assessment of mood- and stress-related behaviors available

[67]

Clock Circadian locomotor output cycles kaput gene, Cry1 cryptochrome 1 gene, Dbp D-box binding protein gene, DD dark:dark, LD light:dark, Vipr2 vasoactive intestinal peptide receptor 2 gene, VPAC2R vasoactive intestinal peptide receptor 2 protein gene

Table 1. Selected animal models of bipolar disorder.


similar changes in patients with bipolar disorder; for example, Clock was not observed to play a key role in the pathophysiology of Japanese patients with bipolar disorder [35]. Overall, there is an insufficient number of positive findings that provide compelling links for clock genes to bipolar disorder. A direct association between genetic abnormality and behavioral outcome is necessary and, by extension, a symptom profile resulting from these abnormalities should coincide with the clinical picture of the condition. If further research identifies such links to bipolar disorder, then a determination of causality will be clearer for some patients.

Pharmacological interventions targeting circadian rhythm dysregulationExtensive sleep and circadian literature documents the utility of environmental approaches in mood disorders [36]. Sleep deprivation, bright light and behavioral phase shifts, alone or in combination, have demonstrated impressive effectiveness in improving symptoms of MDD, SAD and bipolar disorder [37-39]. In a classic case study, a depressed patient with bipolar disorder housed in isolation displayed a sustained improvement in self-ratings of mood following a 6-h phase advance [40]. This effect was sustained for 3 weeks before mood returned to the baseline level, in accordance with predicted interactions of circadian rhythm and sleep debt drivers. However, a reliance on phase therapy is not practical in everyday life and thus drug treatments that mimic the changed circadian state provide an attractive alternative approach. Methods exist that bridge the two approaches, combining environmental therapy with pharmacological interventions; indeed, the addition of 'chronotherapeutic augmentation' to traditional mood stabilizers has yielded improved efficacy in patients and has strengthened the theory of a common path for the two approaches [41,42].

Drug therapies currently available for bipolar disorder generally have a narrow therapeutic index and do not treat the entire range of symptoms. Lithium is the benchmark mood stabilizer, and is most effective at reducing the frequency of manic episodes. The precise mechanism of action of the drug is unknown, but a link to circadian regulation has been noted because of its reported activity on GSKβ, the kinase thought to be responsible for Rev-Erbα phosphorylation in the secondary feedback loop [43,44] (Figure 1). A lithium-induced shift to the inactive form of GSK3β has been suggested, and would be consistent with a slowing of transcriptional feedback and thus a slowing or lengthening of period [45]. Thus, lithium improves the symptoms of bipolar disorder by lengthening the circadian parameter that is shortened in the disease state: the drug slows the circadian period of patients to enable improved internal:external synchrony. The effect of other mood stabilizers is less clear. Valproate and carbamazepine do not appear to alter the circadian period in hamsters at tolerated doses [46]. Side effects limit the use of higher doses of these drugs that may affect circadian parameters; in addition, these agents are sedative to rodents.

Post-translational modifications have the potential to impact the circadian clock mechanism and thereby impact disease. The efficacy of such modifications may also suggest a basis for causality. With respect to GSK3β, levels of phosphorylated (inactive) enzyme were significantly reduced in fibroblasts of patients with bipolar disorder, implying that an increase in the active enzyme subsequently leads to a dysregulation of downstream genes [47]. Moreover, GSK3β expression levels were significantly reduced in platelets of patients with bipolar disorder, but not in patients with MDD; levels were normalized following treatment with mood-stabilizing agents, such as lithium and valproic acid [48]. A SNP of T → C in the GSK3β promoter gene has been associated with several aspects of the bipolar disease state, including age at onset, therapeutic response to lithium, total sleep deprivation and psychotic symptoms [49,50]. Interestingly, various direct GSK3β inhibitors have been evaluated, with the expectation of a similar action of lengthening

Figure 2A. Wheel-running activity of a rat treated with a CKIδ/ε inhibitor.

Daily records of wheel-running activity are shown for a rat maintained in a 12:12 light:dark (LD) cycle before and after treatment with PF-670462, a selective casein kinase I δ/ε (CKIδ/ε) inhibitor. Each horizontal line represents the rest-activity pattern over 2 days, with the second day re-plotted on subsequent lines. The shaded records indicate a period of constant darkness (DD) with an absence of environmental cues to indicate time of day. PF-670462 (50 mg/kg sc; red dot) administered at ZT 11 (ie, 11 h after lights-on or 1 h before lights-off) resulted in a lengthening of period, which was manifested as approximately a 2-h shift in activity onset. This phase delay was maintained until the animal was returned to the LD condition, following which activity onsets gradually synchronized to the 12:12 cycle. The red line indicates the occurrence of activity onsets before drug administration; the yellow line denotes the impact of drug treatment. The change in the slope of the yellow line in DD reveals the endogenous period length, which was somewhat longer than 24 h in this animal.(Adapted with permission from Lundbeck Research USA Inc. © 2010 Lundbeck Research USA Inc)

PF-670462 at ZT11A


the circadian period as with lithium; however, all of these compounds appeared to shorten period length [51,52]. Consequently, a direct inhibitor does not appear to be equivalent to a modulator, suggesting that research efforts aimed at mimicking the effects of lithium may be unsuccessful.

Casein kinase I δ/ε (CKIδ/ε) has also been proposed as a therapeutic target for psychiatric disorders, given its role in PER phosphorylation and possibly modulation of other clock gene products (Figure 1). A small-molecule inhibitor of this kinase, PF-670462 (Figure 2A), which does not distinguish between the two subtypes, increased period length in a dose-dependent manner in rats [53] and in non-human primates [54]. Similar outcomes observed in both nocturnal and diurnal species underscore the notion that the circadian oscillator is upstream of any behavioral manifestations of rhythm; for the purposes of drug discovery, this observation enables the use of common laboratory rodents with application to human research. Only a small degree of CKIδ/ε inhibition appears to be required for a behavioral effect to occur. Efforts to measure clock protein phosphorylation in vivo have failed to reveal a detectable change, while such an effect is clearly observable in vitro [Westrich L, Sprouse J: unpublished data] (Figure 2B). While the ε isoform of CKI was initially believed to be a key component of oscillator function [55], recent studies with more refined tools implicate CKIδ [56,57]. Chronic once-daily dosing with PF-670462 in rats yielded a cumulative

effect on period, with phase delays in activity patterns increasing progressively in magnitude (despite the short pharmacokinetic half-life of the compound) [58]. Perhaps most importantly, all of these effects of CKIδ/ε inhibition occur in the presence of a normal LD cycle, suggesting that PF-670462 was a critical determinant of rhythm, with physiologies ceding to light only after the elimination of the compound. In addition to GSK3β and CKIδ, a number of other potential targets have emerged on inspection of oscillator function (Figure 1), including additional kinases, phosphatases and ligases; however, little is known regarding the potential value of these targets.

Membrane-bound targets (eg, melatonin, serotonin, and VIP receptors) on oscillator cells constitute an entirely separate category of therapeutic targets. Of particular interest is agomelatine (Valdoxan), a synthetic melatonin agonist launched in Europe for the treatment of MDD. Based on a number of clinical trials, agomelatine appears to yield an improved symptom profile for patients with MDD and bipolar disorder [59-61]. Further research is required to link the agomelatine-induced changes in circadian function to changes in symptom profile, and to reconcile the apparent absence of published research for agomelatine regarding potential antidepressant effects, as observed with other synthetic melatonin agonists. Some controversy exists regarding the rhythm-related mechanism of action of agomelatine; some researchers have suggested a combined effect of melatonin agonism

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Blocked spleen tissue explants from mPER2:LUC mice were maintained in culture at 37°C in the presence or absence of PF-670462 (0.1 mM), a selective casein kinase I δ/ε (CKIδ/ε) inhibitor; bioluminescence oscillations were monitored using photomultiplier tubes (LumiCycle) for several days. In this example, treatment with PF-670462 lengthened the circadian period by approximately 2 h, as indicated by the progressive delays in bioluminescence peaks.(Adapted with permission from Lundbeck Research USA Inc. © 2010 Lundbeck Research USA Inc)

Figure 2B. Bioluminescence readings of PER2:LUC fusion protein, demonstrating the effect of PF-670462.


with 5-HT2C blockade [60,62,63]. Such a possibility seems unlikely, however, given the weak affinity of the compound for this serotonin receptor subtype (pKi = 6.39) [63]. Studies examining non-synthetic melatonin in MDD have yielded mixed results. In the most comprehensive assessment of non-seasonal depression, published in 2010, a trend toward improvement was noted, although statistical significance was lacking as a result of the small sample size (n = 31) [64]. Thus, further research is required to gain a fuller understanding of the roles of agomelatine and melatonin on circadian rhythm-related disorders.

ConclusionThe mechanistic link connecting circadian rhythm dysfunction to psychiatric disorders is supported by several sources of evidence, although a causal link remains elusive. Those correlations that are known to exist – between the clinical phenotype and the circadian state, and between animal models of rhythm function and animal models of disease – still await the development of successful investigational drugs that can serve as proof of mechanism. Many areas of research are possible beyond bipolar disorder. Studies might focus on MDD, SAD and sundowning to assess the application of investigational drugs in the circadian field. Beyond agomelatine and melatonin, the search for second-generation chronobiotics (eg, CKIδ inhibitors or lithium-like GSK3β modulators) should also continue, given that preclinical and clinical research has highlighted the potential benefits of such drugs to maintaining a properly functioning circadian rhythm in humans.

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