The darkness and the light: diurnal rodent models for seasonal affective disorder

A Light-Responsive Neural Circuit Suppresses Feeding

Light plays an essential role in a variety of physiological processes, including vision, mood, and glucose homeostasis. However, the intricate relationship between light and an animal's feeding behavior has remained elusive. Here, we found that light exposure suppresses food intake, whereas darkness amplifies it in male mice. Interestingly, this phenomenon extends its reach to diurnal male Nile grass rats and healthy humans. We further show that lateral habenula (LHb) neurons in mice respond to light exposure, which in turn activates 5-HT neurons in the dorsal Raphe nucleus (DRN). Activation of the LHb→5-HTDRN circuit in mice blunts darkness-induced hyperphagia, while inhibition of the circuit prevents light-induced anorexia. Together, we discovered a light-responsive neural circuit that relays the environmental light signals to regulate feeding behavior in mice.

SAD rats: Effects of short photoperiod and carbohydrate consumption on sleep, liver steatosis, and the gut microbiome in diurnal grass rats

Seasonal affective disorder (SAD) is a recurrent depression triggered by exposure to short photoperiods, with a subset of patients reporting hypersomnia, increased appetite, and carbohydrate craving. Dysfunction of the microbiota - gut - brain axis is frequently associated with depressive disorders, but its role in SAD is unknown. Nile grass rats (Arvicanthis niloticus) are potentially useful for exploring the pathophysiology of SAD, as they are diurnal and have been found to exhibit anhedonia and affective-like behavior in response to short photoperiods. Further, given grass rats have been found to spontaneously develop metabolic syndrome, they may be particularly susceptible to environmental triggers of metabolic dysbiosis. We conducted a 2 × 2 factorial design experiment to test the effects of short photoperiod (4 h:20 h Light:Dark (LD) vs. neutral 12:12 LD), access to a high concentration (8%) sucrose solution, and the interaction between the two, on activity, sleep, liver steatosis, and the gut microbiome of grass rats. We found that animals on short photoperiods maintained robust diel rhythms and similar subjective day lengths as controls in neutral photoperiods but showed disrupted activity and sleep patterns (i.e. a return to sleep after an initial bout of activity that occurs ~ 13 h before lights off). We found no evidence that photoperiod influenced sucrose consumption. By the end of the experiment, some grass rats were overweight and exhibited signs of non-alcoholic fatty liver disease (NAFLD) with micro- and macro-steatosis. However, neither photoperiod nor access to sucrose solution significantly affected the degree of liver steatosis. The gut microbiome of grass rats varied substantially among individuals, but most variation was attributable to parental effects and the microbiome was unaffected by photoperiod or access to sucrose. Our study indicates short photoperiod leads to disrupted activity and sleep in grass rats but does not impact sucrose consumption or exacerbate metabolic dysbiosis and NAFLD.

The Physiological and Pharmacological Significance of the Circadian Timing of the HPA Axis: A Mathematical Modeling Approach

As a potent endogenous regulator of homeostasis, the circadian time-keeping system synchronizes internal physiology to periodic changes in the external environment to enhance survival. Adapting endogenous rhythms to the external time is accomplished hierarchically with the central pacemaker located in the suprachiasmatic nucleus (SCN) signaling the hypothalamus-pituitary-adrenal (HPA) axis to release hormones, notably cortisol, which help maintain the body's circadian rhythm. Given the essential role of HPA-releasing hormones in regulating physiological functions, including immune response, cell cycle, and energy metabolism, their daily variation is critical for the proper function of the circadian timing system. In this review, we focus on cortisol and key fundamental properties of the HPA axis and highlight their importance in controlling circadian dynamics. We demonstrate how systems-driven, mathematical modeling of the HPA axis complements experimental findings, enhances our understanding of complex physiological systems, helps predict potential mechanisms of action, and elucidates the consequences of circadian disruption. Finally, we outline the implications of circadian regulation in the context of personalized chronotherapy. Focusing on the chrono-pharmacology of synthetic glucocorticoids, we review the challenges and opportunities associated with moving toward personalized therapies that capitalize on circadian rhythms.

Adolescent nonpharmacological interventions for early-life stress and their mechanisms

Those with a negative experience of psychosocial stress during the early stage of life not only have a high susceptibility of the psychiatric disorder in all phases of their life span, but they also demonstrate more severe symptoms and poorer response to treatment compared to those without a history of early-life stress. The interventions targeted to early-life stress may improve the effectiveness of treating and preventing psychiatric disorders. Brain regions associated with mood and cognition develop rapidly and own heightened plasticity during adolescence. So, manipulating nonpharmacological interventions in fewer side effects and higher acceptance during adolescence, which is a probable window of opportunity, may ameliorate or even reverse the constantly deteriorating impact of early-life stress. The present article reviews animal and people studies about adolescent nonpharmacological interventions for early-life stress. We aim to discuss whether those adolescent nonpharmacological interventions can promote individuals' psychological health who expose to early-life stress.

Prevalence of depression and depressive symptoms at high altitudes: A systematic review and meta-analysis

BACKGROUND

The aim of the present study was to determine the prevalence of depression or depressive symptoms in people residing in high-altitude regions.

METHODS

Eleven databases were searched for studies on depression and depressive symptoms: PubMed, ISI Web of Science, CINAHL, Embase, Scopus, PsycINFO, Psychology and Behavioral Sciences Collection, Psychology Database, Academic Search Ultimate, SciELO and LILACS. Systematic review and meta-analysis were performed based on the inclusion of these articles measuring the prevalence of depressive symptoms in people living at high altitude (≥1500 m above sea level [masl]). The protocol was registered in PROSPERO (CRD42021271069).

RESULTS

Eight articles with >40,000 participants from 4 different countries were included. Among the samples treated, the combined prevalence of depressive symptoms was 17.9 % (I²: 99 %) and the only estimate by subpopulation at the country level was possible for China, with >36,000 participants, being 28.7 % (I²: 4 %).

LIMITATIONS

Considerable heterogeneity was reported in the estimation of overall prevalence due to the quality of the studies and the instruments used to screen for depressive symptoms.

CONCLUSION

Almost two out of every 10 people living at high-altitude regions suffer from depressive symptoms. Therefore, it is necessary to adapt interventions to this condition and further research in the field is required.

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.

Association between altitude and depression in Peru: An 8-year pooled analysis of population-based surveys

BACKGROUND

In high altitude regions, people experience biological, inflammatory and brain structure changes that increase the risk of depressive symptoms. The aim of this study was to determine the association between altitude and depressive symptoms in the Peruvian population, adjusting by demographic, socioeconomic and exposure to health risk factors.

METHODS

We performed a cross-sectional analytical study of data collected annually by the Demographic and Family Health Survey during the period 2013-2020. The presence of depressive symptoms during the last 14 days prior to the survey were measured using scores obtained from the Patient Health Questionnaire-9 (PHQ-9). A generalized linear model (GLM) of gamma family and log link function was used to report the crude and adjusted β coefficients. A quantile regression model was performed as a sensitivity analysis.

RESULTS

Data from a total of 215,409 participants were included. After adjusting for demographic, socioeconomic and health risk exposures, the GLM showed that an increase in every 100 m of altitude of residence was positively and significantly associated with the depressive symptoms score (β=0•01 [95% confidence interval: 0•01-0•01]).

LIMITATIONS

The length of residence in high altitude areas of the population included cannot be established, requiring future research to determine if the results of the present study are similar in native people or permanent residents of high altitude regions.

CONCLUSIONS

Altitude was positively associated with depressive symptom scores. Our results will allow the development of mental health interventions based on factors that increase the likelihood of depressive symptoms in high-altitudes.

Light-induced synchronization of the SCN coupled oscillators and implications for entraining the HPA axis

The suprachiasmatic nucleus (SCN) synchronizes the physiological rhythms to the external light-dark cycle and tunes the dynamics of circadian rhythms to photoperiod fluctuations. Changes in the neuronal network topologies are suggested to cause adaptation of the SCN in different photoperiods, resulting in the broader phase distribution of neuron activities in long photoperiods (LP) compared to short photoperiods (SP). Regulated by the SCN output, the level of glucocorticoids is elevated in short photoperiod, which is associated with peak disease incidence. The underlying coupling mechanisms of the SCN and the interplay between the SCN and the HPA axis have yet to be fully elucidated. In this work, we propose a mathematical model including a multiple-cellular SCN compartment and the HPA axis to investigate the properties of the circadian timing system under photoperiod changes. Our model predicts that the probability-dependent network is more energy-efficient than the distance-dependent network. Coupling the SCN network by intra-subpopulation and inter-subpopulation forces, we identified the negative correlation between robustness and plasticity of the oscillatory network. The HPA rhythms were predicted to be strongly entrained to the SCN rhythms with a pro-inflammatory high-amplitude glucocorticoid profile under SP. The fast temporal topology switch of the SCN network was predicted to enhance synchronization when the synchronization is not complete. These synchronization and circadian dynamics alterations might govern the seasonal variation of disease incidence and its symptom severity.