Beneficial effects of voluntary wheel running on activity rhythms, metabolic state, and affect in a diurnal model of circadian disruption

Circadian desynchrony and glucose metabolism

The circadian timing system controls glucose metabolism in a time-of-day dependent manner. In mammals, the circadian timing system consists of the main central clock in the bilateral suprachiasmatic nucleus (SCN) of the anterior hypothalamus and subordinate clocks in peripheral tissues. The oscillations produced by these different clocks with a period of approximately 24-h are generated by the transcriptional-translational feedback loops of a set of core clock genes. Glucose homeostasis is one of the daily rhythms controlled by this circadian timing system. The central pacemaker in the SCN controls glucose homeostasis through its neural projections to hypothalamic hubs that are in control of feeding behavior and energy metabolism. Using hormones such as adrenal glucocorticoids and melatonin and the autonomic nervous system, the SCN modulates critical processes such as glucose production and insulin sensitivity. Peripheral clocks in tissues, such as the liver, muscle, and adipose tissue serve to enhance and sustain these SCN signals. In the optimal situation all these clocks are synchronized and aligned with behavior and the environmental light/dark cycle. A negative impact on glucose metabolism becomes apparent when the internal timing system becomes disturbed, also known as circadian desynchrony or circadian misalignment. Circadian desynchrony may occur at several levels, as the mistiming of light exposure or sleep will especially affect the central clock, whereas mistiming of food intake or physical activity will especially involve the peripheral clocks. In this review, we will summarize the literature investigating the impact of circadian desynchrony on glucose metabolism and how it may result in the development of insulin resistance. In addition, we will discuss potential strategies aimed at reinstating circadian synchrony to improve insulin sensitivity and contribute to the prevention of type 2 diabetes.

Effects of photoperiod and food on glucose intolerance and subsequent ocular pathology in the fat sand rat

Type 2 diabetes mellitus (T2DM) and its ocular complications, such as cataract and diabetic retinopathy (DR) have been linked to circadian rhythm-disturbances. Using a unique diurnal animal model, the sand rat (Psammomys obesus) we examined the effect of circadian disruption by short photoperiod acclimation on the development of T2DM and related ocular pathologies. We experimented with 48 male sand rats. Variables were day length (short photoperiod, SP, vs. neutral photoperiod NP) and diet (standard rodent diet vs. low-energy diet). Blood glucose, the presence of cataract and retinal pathology were monitored. Histological slides were examined for lens opacity, retinal cell count and thickness. Animals under SP and fed standard rodent diet (SPSR) for 20 weeks had higher baseline blood glucose levels and lower glucose tolerance compared with animals kept under NP regardless of diet, and under SP with low energy diet (SPLE). Animals under SPSR had less cells in the outer nuclear layer, a lower total number of cells in the retina, and a thickened retina. Higher blood glucose levels correlated with lower number of cells in all cellular layers of the retina and thicker retina. Animals under SPSR had higher occurrence of cataract, and a higher degree of cataract, which correlated with higher blood glucose levels. Sand rats kept under SPSR develop cataract and retinal abnormalities indicative of DR, whereas sand rats kept under NP regardless of diet, or under SPLE, do not. These ocular abnormalities significantly correlate with hyperglycemia.

Comparison of intermittent fasting and voluntary wheel running on physical and cognitive abilities in high-fat diet-induced obese rats

Regular physical activity is a proven routine for weight management in addressing obesity. Another method that has gained attention for its health benefits is intermittent fasting (IF). Physical and cognitive abilities while on these routines are poorly understood in the obese population. Sixty-five male Sprague Dawley rats at 7 weeks of age were subjected to diet-induced obesity by feeding a high-fat diet (HFD) or a standard diet (SD) for 8 weeks, after which behavioral testing was performed to detect any changes in physical and cognitive abilities. Rats from the HFD-fed (now considered obese) and SD-fed groups were then subjected to IF (18-hour fast and 6-hour feeding daily), voluntary wheel running (VWR), or control conditions for 3 weeks before repeating the same behavioral testing protocol. IF resulted in less weight gain (p<0.05) and elevated ketone levels (p<0.05) in both SD and HFD-fed groups. IF improved physical activity when compared to VWR and control animals in both SD and HFD-fed groups (p<0.05) while the VWR group in the SD-fed rats exhibited less physical fatigue compared to IF and controls (p<0.05). Additionally, elevated ketone levels were weakly correlated with decreased physical (p<0.0001) and exploratory behavior (p<0.01). These results suggest that IF is more effective than VWR in HFD and SD-fed rats in minimizing weight gain and retaining physical activity, and ketones may play a part in establishing the reported physical benefits. Exploration of physiological mechanisms between ketones, diet, and exercise will help fight obesity and many associated diseases.

The effect of voluntary exercise on light cycle stress-induced metabolic resistance

PURPOSE

Disruption of circadian genes affects metabolic homeostasis. Regular exercise programs prevent metabolic dysfunction and alter circadian gene expression In this study, we investigated whether exercise affects light stress-induced circadian rhythm derangement and metabolic resistance.

METHODS

A circadian rhythm derangement mouse model was designed by extending the light exposure by two hours (14 L/10 D) for three weeks. Nine-weekold male mice were single-caged and divided into four groups: sedentary groups with or without light stress, and voluntary wheel-trained groups with or without light stress. In addition, differentiated myotubes were cultured in the presence of dexamethasone with or without 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR). The comprehensive laboratory animal monitoring system was used to analyze the metabolic changes in mice. Moreover, reverse transcription-polymerase chain reaction (RT-PCR) was used to quantify the mRNA expression levels of circadian genes in animal and cell culture models.

RESULTS

Three weeks of light stress reduced the running distance and increased the weight of mice. In addition, VO2 consumption and heat production were increased during the night cycle under non-stress conditions but not under stress conditions. PCR analysis revealed that exercise and stress altered the expression levels of circadian genes in the hypothalamus and quadriceps muscles. mRNA expression levels of period circadian regulator 1 were downregulated in the quadriceps muscles of the stressed sedentary group compared to that in muscles of the non-stressed sedentary group. Furthermore, differentiated myotube cells cultured in the presence of dexamethasone, with or without AICAR, showed distinct oscillation patterns at various time points.

CONCLUSION

Our study demonstrates that exercise partially prevents metabolic disruption by regulating the circadian gene expression in skeletal muscles.

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.