Effects of morning compared with evening bright light administration to ameliorate short-photoperiod induced depression- and anxiety-like behaviors in a diurnal rodent model

Circadian rhythms-related disorders in diurnal fat sand rats under modern lifestyle conditions: A review

Modern lifestyle reduces environmental rhythmicity and may lead to circadian desynchrony. We are exposed to poor day-time lighting indoors and excessive night-time artificial light. We use air-conditioning to reduce ambient temperature cycle, and food is regularly available at all times. These disruptions of daily rhythms may lead to type 2 diabetes mellitus (T2DM), obesity, cardiometabolic diseases (CMD), depression and anxiety, all of which impose major public health and economic burden on societies. Therefore, we need appropriate animal models to gain a better understanding of their etiologic mechanisms, prevention, and management.We argue that the fat sand rat (Psammomys obesus), a diurnal animal model, is most suitable for studying the effects of modern-life conditions. Numerous attributes make it an excellent model to study human health disorders including T2DM, CMD, depression and anxiety. Here we review a comprehensive series of studies we and others conducted, utilizing the fat sand rat to study the underlying interactions between biological rhythms and health. Understanding these interactions will help deciphering the biological basis of these diseases, which often occur concurrently. We found that when kept in the laboratory (compared with natural and semi-wild outdoors conditions where they are diurnal), fat sand rats show low amplitude, nocturnal or arrhythmic activity patterns, dampened daily glucose rhythm, glucose intolerance, obesity and decreased survival rates. Short photoperiod acclimation exacerbates these pathologies and further dampens behavioral and molecular daily rhythms, resulting in CMD, T2DM, obesity, adipocyte dysfunction, cataracts, depression and anxiety. Increasing environmental rhythmicity by morning bright light exposure or by access to running wheels strengthens daily rhythms, and results in higher peak-to-trough difference in activity, better rhythmicity in clock genes expression, lower blood glucose and insulin levels, improved glucose tolerance, lower body and heart weight, and lower anxiety and depression. In summary, we have demonstrated that fat sand rats living under the correspondent of “human modern lifestyle” conditions exhibit dampened behavioral and biological rhythms and develop circadian desynchrony, which leads to what we have named “The Circadian Syndrome”. Environmental manipulations that increase rhythmicity result in improvement or prevention of these pathologies. Similar interventions in human subjects could have the same positive results and further research on this should be undertaken.

Effects of photoperiod and diet on BDNF daily rhythms in diurnal sand rats

Brain-derived neurotrophic factor (BDNF), its receptors and epigenetic modulators, are implicated in the pathophysiology of affective disorders, T2DM and the circadian system function. We used diurnal sand rats, which develop type 2 diabetes (T2DM), anxiety and depressive-like behavior under laboratory conditions. The development of these disorders is accelerated when animals are maintained under short photoperiod (5:19L:D, SP) compared to neutral photoperiod (12:12L:D, NP). We compared rhythms in plasma BDNF as well as BDNF and PER2 expression in the frontal cortex and suprachiasmatic nucleus (SCN) of sand rats acclimated to SP and NP. Acclimation to SP resulted in higher insulin levels, significantly higher glucose levels in the glucose tolerance test, and significantly higher anxiety- and depression-like behaviors compared with animals acclimated to NP. NP Animals exhibited a significant daily rhythm in plasma BDNF levels with higher levels during the night, and in BDNF expression levels in the frontal cortex and SCN. No significant BDNF rhythm was found in the plasma, frontal cortex or SCN of SP acclimated animals. We propose that in sand rats, BDNF may, at least in part, mediate the effects of circadian disruption on the development of anxiety and depressive-like behavior and T2DM.

Light therapy improved depression-like behavior induced by chronic unpredictable mild stress in Mongolian gerbils

Progress has been made in elucidating the mechanism by which light modulates depressive-like behaviors. However, almost all of these studies ignore an important issue, namely, that examining the effects of light therapy in nocturnal animals may be difficult because the influences of light on behavioral responses differ between nocturnal and diurnal animals. To date, few diurnal rodents have been utilized to establish animal models that closely mimic clinical depression. Herein, the chronic unpredictable mild stress model, which is the most representative, reliable, and effective rodent model of depression, was implemented in diurnal Mongolian gerbils for the first time. The gerbils were subjected to two hours of light therapy or fluoxetine treatment for 2 weeks. Our work revealed that Mongolian gerbils subjected to chronic unpredictable mild stress showed depression-like behaviors. Interestingly, we also found that light therapy improved anhedonic behavior more effectively than fluoxetine after two weeks of treatment. In summary, our study is the first to use diurnal Mongolian gerbils, which have the same circadian rhythm as humans, to establish an effective, economical, and practical animal model of depression and confirmed that light therapy could improve depression-like behavior more effectively than fluoxetine to some extent in diurnal Mongolian gerbils, which establishes a good foundation for clarifying the neural mechanism of light therapy for depression.

Sex differences in the response to circadian disruption in diurnal sand rats

Most animal model studies on physiological functions and pathologies are conducted in males. However, diseases such as depression, type 2 diabetes (T2DM) and cardiovascular disease, all show different prevalence and characteristics in females and males. Moreover, most mammal studies are conducted in nocturnal mice and rats, while modelling diurnal humans. We therefore used male and female fat sand rats (Psammomys obesus), which are diurnal in the wild, as an animal model for T2DM, to explore the effects of mild circadian disruption on behavior, glucose tolerance, cholesterol and heart weight. We found significant differences between the sexes: on average, in response to short photoperiods (SP) acclimation, males showed higher levels of depression-like behavior, lower glucose tolerance, and increased plasma cholesterol levels compared with females, with no effect on heart/body weight ratio. Females, however did show an increase in heart/body weight ratio in response to SP acclimation. We also found that regardless of sex, arrhythmic animals showed higher blood glucose levels, cholesterol levels, heart/body weight ratio, and depressive-like behavior compared with rhythmic animals. Hence, we suggest that the expression of the Circadian Syndrome could be different between males and females. Additional work with females is required to clearly delineate the specific effects in both sexes, and promote sex-based health care, prevention measures and therapies.

Beneficial effects of daytime high-intensity light exposure on daily rhythms, metabolic state and affect

While the importance of the circadian system to health and well-being is extensively studied, the role of daylight exposure in these interactions is relatively poorly understood. Here we show, using a diurnal animal model naturally exposed to daylight, that daily morning exposure to 3000 lux, full spectrum electric light has beneficial health effects. Compared with controls, sand rats (Psammomys obesus) subjected to morning light treatment demonstrate daily rhythms with high peak to trough difference in activity, blood glucose levels and per2 gene expression in the suprachiasmatic nucleus, pre-frontal cortex, kidney and liver. The treated animals were also healthier, being normoglycemic, having higher glucose tolerance, lower body and heart weight and lower anxiety- and depression-like behavior. Our results suggest that exposure to high intensity light is important for the proper function of the circadian system and well-being, and are important in face of human's low exposure to daylight and extensive use of artificial light at night.

Linking type 2 diabetes mellitus, cardiac hypertrophy and depression in a diurnal animal model

It was recently suggested that the Metabolic Syndrome should be renamed to "Circadian Syndrome". In this context, we explored the effects of living under standard laboratory conditions, where light is the only cycling variable (relevant to human modern life), in a diurnal mammal, on the relationships between affective-like pathology, type 2 diabetes mellitus (T2DM), and cardiac hypertrophy. After 20 weeks, some of the animals spontaneously developed T2DM, depressive and anxiety-like behavior and cardiac hypertrophy. There were significant correlations between levels of anxiety-like behavior and glucose tolerance, and between heart/total body weight ratio and glucose tolerance. Our data suggest a relationship between the development of T2DM, emotional and cardiac pathology as seen in diurnal humans. Furthermore, our data show a possible relationship between reduced daily cycling cues in the laboratory and what has been regularly termed "Metabolic Syndrome" and recently proposed by us to be renamed to "Circadian Syndrome".

The Circadian Syndrome: is the Metabolic Syndrome and much more!

The Metabolic Syndrome is a cluster of cardio-metabolic risk factors and comorbidities conveying high risk of both cardiovascular disease and type 2 diabetes. It is responsible for huge socio-economic costs with its resulting morbidity and mortality in most countries. The underlying aetiology of this clustering has been the subject of much debate. More recently, significant interest has focussed on the involvement of the circadian system, a major regulator of almost every aspect of human health and metabolism. The Circadian Syndrome has now been implicated in several chronic diseases including type 2 diabetes and cardiovascular disease. There is now increasing evidence connecting disturbances in circadian rhythm with not only the key components of the Metabolic Syndrome but also its main comorbidities including sleep disturbances, depression, steatohepatitis and cognitive dysfunction. Based on this, we now propose that circadian disruption may be an important underlying aetiological factor for the Metabolic Syndrome and we suggest that it be renamed the 'Circadian Syndrome'. With the increased recognition of the 'Circadian Syndrome', circadian medicine, through the timing of exercise, light exposure, food consumption, dispensing of medications and sleep, is likely to play a much greater role in the maintenance of both individual and population health in the future.

Red white and blue - bright light effects in a diurnal rodent model for seasonal affective disorder

Despite the common use of bright light exposure for treatment of seasonal affective disorder (SAD), the underlying biology of the therapeutic effect is not clear. Moreover, there is a debate regarding the most efficacious wavelength of light for treatment. Whereas according to the traditional approach full-spectrum light is used, recent studies suggest that the critical wavelengths are within the range of blue light (460 and 484 nm). Our previous work shows that when diurnal rodents are maintained under short photoperiod they develop depression- and anxiety-like behavioral phenotype that is ameliorated by treatment with wide-spectrum bright light exposure (2500 lux at the cage, 5000 K). Our current study compares the effect of bright wide-spectrum (3,000 lux, wavelength 420- 780 nm, 5487 K), blue (1,300 lux, wavelength 420-530 nm) and red light (1,300 lux, wavelength range 600-780 nm) exposure in the fat sand rat (Psammomys Obesus) model of SAD. We report results of experiments with six groups of sand rats that were kept under various photoperiods and light treatments, and subjected to behavioral tests related to emotions: forced swim test, elevated plus maze and social interactions. Exposure to either intense wide-spectrum white light or to blue light equally ameliorated depression-like behavior whereas red light had no effect. Bright wide-spectrum white light treatment had no effect on animals maintained under neutral photoperiod, meaning that light exposure was only effective in the pathological-like state. The resemblance between the effects of bright white light and blue light suggests that intrinsically photosensitive retinal ganglion cells (ipRGCs) are involved in the underlying biology of SAD and light therapy.

Diurnality, Type 2 Diabetes, and Depressive-Like Behavior

Although type 2 diabetes (T2DM) and depression are associated with disturbances in circadian rhythms, most studies of these diseases use nocturnal mice and rats while modeling diurnal humans. We suggest that the development of T2DM and depression are related to changes that accompany the switch from the mammalian ancestral nocturnal activity to the current diurnal one. We show that diurnal sand rats ( Psammomys obesus) held outdoors in laboratory cages (where they are exposed to natural environmental conditions) and fed a standard rodent diet do not develop T2DM in contrast to animals held indoors (where the only cycling environmental condition is light) fed the same diet. Moreover, keeping sand rats under a short photoperiod dampened behavioral and molecular daily rhythms, resulted in anxiety- and depressive-like behavior, and accelerated the development of T2DM. We suggest that the disturbed rhythms disrupt the internal temporal order and metabolic pathways controlled by feeding and the circadian system, resulting in the development of T2DM and depressive-like behavior. We further suggest that using nocturnal mice and rats as sole model animals may limit research, especially when studying circadian rhythm-related diseases.

The Antidepressant Effect of Light Therapy from Retinal Projections

Observations from clinical trials have frequently demonstrated that light therapy can be an effective therapy for seasonal and non-seasonal major depression. Despite the fact that light therapy is known to have several advantages over antidepressant drugs like a low cost, minimal side-effects, and fast onset of therapeutic effect, the mechanism underlying light therapy remains unclear. So far, it is known that light therapy modulates mood states and cognitive functions, involving circadian and non-circadian pathways from retinas into brain. In this review, we discuss the therapeutic effect of light on major depression and its relationship to direct retinal projections in the brain. We finally emphasize the function of the retino-raphe projection in modulating serotonin activity, which probably underlies the antidepressant effect of light therapy for depression.

Revisiting the validity of the mouse forced swim test: Systematic review and meta-analysis of the effects of prototypic antidepressants

One problem area regarding animal models for affective disorders is unclear reproducibility, including external validity or generalizability. One way to evaluate external validity is with systematic reviews and meta-analyses. The current study presents a meta-analysis of the effects of prototypic antidepressants in the mouse forced swim test (FST). We identified studies that examined effects of antidepressants in the FST in mice and used standard protocol, male mice and acute drug administration. We calculated Effect sizes using Cohen's d, homogeneity using Q statistic and correlations using Pearson's correlation. Results indicate that all drugs reduce immobility in the FST. However, effect sizes for most drugs are heterogeneous and do not show a consistent dose/response relationship across variability factors. Reducing variability by examining only one strain or data from individual laboratories partially increases dose response relationship. These findings suggest that whereas the FST is a valid tool to qualitatively screen antidepressant effects its validity in the context of hierarchical comparison between doses or compounds might be relevant only to single experiments.

Utilization of Diurnal Rodents in the Research of Depression

Preclinical Research Most neuropsychiatric research, including that related to the circadian system, is performed using nocturnal animals, mainly laboratory mice and rats. Mood disorders are known to be associated with circadian rhythm abnormalities, but the mechanisms by which circadian rhythm disruptions interact with depression remain unclear. As the circadian system of diurnal and nocturnal mammals differs, we previously suggested that the utilization of diurnal animal models may be advantageous for understanding these relations. During the last 10 years, we and others established the validity of several diurnal rodent species as a model for the interactions between circadian rhythms and depression. Diurnal rodents respond to photoperiod manipulation in a similar way to humans, the behavioral outcome is directly related to the circadian system, and treatment that is effective in patients is also effective in the model. Moreover, less effective treatments in patients are also less effective in the model. We, therefore, suggest that using diurnal animal models to study circadian rhythms-related affective disorders, such as depression, will provide new insights that will hopefully lead to the development of more effective treatments. Drug Dev Res 77 : 347-356, 2016. © 2016 Wiley Periodicals, Inc.

Biological adaptability under seasonal variation of light/dark cycles

3A substantial amount of experimental models designed to understand rhythms entrainment and the effects of different regimens of light exposure on health have been proposed. However, many of them do not relate to what occurs in real life. Our objective was to evaluate the influence of "seasonal-like" variation in light/dark cycles on biological rhythms. Twenty adult male Wistar rats were assigned to three groups: control (CT), kept in 12:12 light/dark (LD) cycle; long photoperiod/short photoperiod (LP/SP), kept in 16.5:7.5 LD cycle for 18 days (phase A), then 17 days of gradual reductions in light time (phase B), then 18 days of shorter exposure (7.5:16.5 LD cycle, phase C); short photoperiod/long photoperiod (SP/LP) group, with same modifications as the LP/SP group, but in reverse order, starting phase A in 7.5:16.5 LD cycle. Activity and temperature were recorded constantly, and melatonin and cortisol concentrations were measured twice. Activity and temperature acrophases of all groups changed according to light. The correlation between activity and temperature was, overall, significantly lower for SP/LP group compared with LP/SP and CT groups. Regarding melatonin concentration, LP/SP group showed significant positive correlation between phase A and C (p = 0.018). Animals changed temperature and activity according to photoperiod and demonstrated better adaptability in transitioning from long to short photoperiod. Since this model imitates seasonal variation in light in a species that is largely used in behavioral experiments, it reveals promising methods to improve the reliability of experimental models and of further environmental health research.

Chronotype and seasonality: morningness is associated with lower seasonal mood and behavior changes in the Old Order Amish

BACKGROUND

Several studies documented that lower scores on the Morningness-Eveningness Questionnaire (MEQ) are associated with a higher global seasonality of mood (GSS). As for the Modern Man artificial lighting predominantly extends evening activity and exposure to light, and as evening bright light phase is known to delay circadian rhythms, this chronic exposure could potentially lead to both lower Morningness as well as higher GSS. The aim of the study was to investigate if the MEQ-GSS relationship holds in the Old Order Amish of Lancaster County, PA, a population that does not use network electrical light.

METHODS

489 Old Order Amish adults (47.6% women), with average (SD) age of 49.7 (14.2) years, completed both the Seasonal Pattern Assessment Questionnaire (SPAQ) for the assessment of GSS, and MEQ. Associations between GSS scores and MEQ scores were analyzed using linear models, accounting for age, gender and relatedness by including the relationship matrix in the model as a random effect.

RESULTS

GSS was inversely associated with MEQ scores (p=0.006, adjusted).

LIMITATIONS

include a potential recall bias associated with self-report questionnaires and no actual light exposure measurements.

CONCLUSION

We confirmed the previously reported inverse association between MEQ scores and lower seasonality of mood, for the first time in a population that does not use home network electrical lighting. This result suggests that the association is not a byproduct of exposure to network electric light, and calls for additional research to investigate mechanisms by which Morningness is negatively associated with seasonality.

The catecholaminergic-cholinergic balance hypothesis of bipolar disorder revisited

Bipolar disorder is a unique illness characterized by fluctuations between mood states of depression and mania. Originally, an adrenergic-cholinergic balance hypothesis was postulated to underlie these different affective states. In this review, we update this hypothesis with recent findings from human and animal studies, suggesting that a catecholaminergic-cholinergic hypothesis may be more relevant. Evidence from neuroimaging studies, neuropharmacological interventions, and genetic associations support the notion that increased cholinergic functioning underlies depression, whereas increased activations of the catecholamines (dopamine and norepinephrine) underlie mania. Elevated functional acetylcholine during depression may affect both muscarinic and nicotinic acetylcholine receptors in a compensatory fashion. Increased functional dopamine and norepinephrine during mania on the other hand may affect receptor expression and functioning of dopamine reuptake transporters. Despite increasing evidence supporting this hypothesis, a relationship between these two neurotransmitter systems that could explain cycling between states of depression and mania is missing. Future studies should focus on the influence of environmental stimuli and genetic susceptibilities that may affect the catecholaminergic-cholinergic balance underlying cycling between the affective states. Overall, observations from recent studies add important data to this revised balance theory of bipolar disorder, renewing interest in this field of research.

Diurnal rodents as an advantageous model for affective disorders: novel data from diurnal degu (Octodon degus)

Circadian rhythms are strongly associated with affective disorders and recent studies have suggested utilization of diurnal rodents as model animal for circadian rhythms-related domains of these disorders. Previous work with the diurnal fat sand rat and Nile grass rat demonstrated that short photoperiod conditions result in behavioral changes including anxiety- and depression-like behavior. The present study examined the effect of manipulating day length on activity rhythms and behavior of the diurnal degu. Animals were housed for 3 weeks under either a short photoperiod (5-h:19-h LD) or a neutral photoperiod (12-h:12-h LD) and then evaluated by sweet solution test and the forced swim test for depression-like behavior, and in the light/dark box and open field for anxiety-like behavior. Results indicate that short photoperiod induced depression-like behavior in the forced swim test and the sweet solution preference test and anxiety-like behavior in the open field compared with animals maintained in a neutral photoperiod. No effects were shown in the light/dark box. Short photoperiod-acclimated degu showed reduced total activity duration and activity was not restricted to the light phase. The present study further supports the utilization of diurnal rodents to model circadian rhythms-related affective change. Beyond the possible diversity in the mechanisms underlying diurnality in different animals, there are now evidences that in three different diurnal species, the fat sand rat, the grass Nile rat and the degu, shortening of photoperiod results in the appearance of anxiety- and depression-like behaviors.

Rodent models for mania: practical approaches

The scarcity of good animal models for bipolar disorder (BPD) and especially for mania is repeatedly mentioned as one of the rate-limiting factors in the process of gaining a better understanding into its pathophysiology and of developing better treatments. Standard models of BPD have some value but usually represent only one facet of the disease and have partial validity. A number of new approaches for modeling BPD and specifically mania have been suggested in the last few years and can be combined to improve models. These approaches include targeted mutation models representing reverse translation, the identification of advantageous strains for components of the disorder, a search for the most homologous species to address specific human pathology, and the exploration of individual differences of response including the separation between susceptible and resilient animals. Additionally, recent efforts have identified and developed new tests to distinguish between "normal" and "BPD-like" animals including the different utilization of known tests and novel tests such as the female-urine-sniffing test and behavior pattern monitor analysis. Additional tests relating to further domains of BPD are still needed. An ideal model for BPD that will encompass the entire disease and be useful for every demand will probably not become available until we have a full understanding of the pathophysiology of the disorder. However, the current advances in modeling should lead to better comprehension of the disorder and therefore to the gradual development of increasingly improved models.

Depression-like responses induced by daytime light deficiency in the diurnal grass rat (Arvicanthis niloticus)

Seasonal Affective Disorder (SAD) is one of the most common mood disorders with depressive symptoms recurring in winter when there is less sunlight. The fact that light is the most salient factor entraining circadian rhythms leads to the phase-shifting hypothesis, which suggests that the depressive episodes of SAD are caused by misalignments between the circadian rhythms and the habitual sleep times. However, how changes in environmental lighting conditions lead to the fluctuations in mood is largely unknown. The objective of this study is to develop an animal model for some of the features/symptoms of SAD using the diurnal grass rats Arvichantis niloticus and to explore the neural mechanisms underlying the light associated mood changes. Animals were housed in either a 12∶12 hr bright light∶dark (1000lux, BLD) or dim light∶dark (50lux, DLD) condition. The depression-like behaviors were assessed by sweet-taste Saccharin solution preference (SSP) and forced swimming test (FST). Animals in the DLD group showed higher levels of depression-like behaviors compared to those in BLD. The anxiety-like behaviors were assessed in open field and light/dark box test, however no significant differences were observed between the two groups. The involvement of the circadian system on depression-like behaviors was investigated as well. Analysis of locomotor activity revealed no major differences in daily rhythms that could possibly contribute to the depression-like behaviors. To explore the neural substrates associated with the depression-like behaviors, the brain tissues from these animals were analyzed using immunocytochemistry. Attenuated indices of 5-HT signaling were observed in DLD compared to the BLD group. The results lay the groundwork for establishing a novel animal model and a novel experimental paradigm for SAD. The results also provide insights into the neural mechanisms underlying light-dependent mood changes.