Abstract
Seasonal variations in day length trigger clear changes in the behavior, growth, food intake, and reproductive status of photoperiod-sensitive animals, such as Fischer 344 rats. However, there is little information about the effects of seasonal fluctuations in day length on glucose and lipid metabolisms and their underlying mechanisms in this model. To gain knowledge on these issues, three groups of male Fischer 344 rats were fed with a standard diet and exposed to different photoperiods for 14 weeks: normal photoperiod (L12, 12 h light/day), long photoperiod (L18, 18 h light/day), and short photoperiod (L6, 6 h light/day). A multivariate analysis carried out with 239 biometric, serum, hepatic and skeletal muscle parameters revealed a clear separation among the three groups. Compared with L12 rats, L6 animals displayed a marked alteration of glucose homeostasis and fatty acid uptake and oxidation, which were evidenced by the following observations: (1) increased circulating levels of glucose and non-esterified fatty acids; (2) a sharp down-regulation of the phosphorylated Akt2 levels, a downstream post-receptor target of insulin, in both the soleus and gastrocnemius muscles; (3) decreased expression in the soleus muscle of the glucose metabolism-related microRNA-194 and lower mRNA levels of the genes involved in glucose metabolism (Irs1, soleus, and Glut2, liver), β-oxidation (Had and Cpt1β, soleus) and fatty acid transport (Cd36, soleus, and liver). L18 animals also displayed higher blood glucose levels than L12 rats and profound changes in other glucose and lipid metabolism-related parameters in the blood, liver, and skeletal muscles. However, the mechanisms that account for the observed effects were less evident than those reported in L6 animals. In conclusion, exposure to different photoperiods strongly modulated glucose and lipid metabolisms in normoweight rats. These findings emphasize the relevance of circannual rhythms in metabolic homeostasis regulation and suggest that Fischer 344 rats are a promising animal model with which to study glucose- and lipid-related pathologies that are influenced by seasonal variations, such as obesity, cardiovascular disease and seasonal affective disorder.
Highlights
It has generally been established that many mammals are season- or photoperiod-sensitive and are able to change their behavior, morphology and physiology to anticipate climate and food availability changes among the seasons (Prendergast, 2005; Morgan et al, 2006)
Food intake, growth, energy balance and reproduction have been observed to vary during the year in some species to ensure their survival (Ebling, 2015). This seasonal responsiveness can be regulated by two mechanisms: the first is promoted by environmental cues that indicate the time of year, such as the day length, in which melatonin plays a key role (Nelson and Demas, 1997); the second corresponds to the endogenous circannual rhythms, which are adjusted by environmental cues and can adapt to seasonal processes as a response to variations in the photoperiod or other external signals (Paul et al, 2008)
We demonstrated that male Fischer 344 rats exhibited profound changes in parameters related with glucose, lipid and energy metabolisms in the serum, liver and skeletal muscle when chronically exposed to different photoperiods
Summary
It has generally been established that many mammals are season- or photoperiod-sensitive and are able to change their behavior, morphology and physiology to anticipate climate and food availability changes among the seasons (Prendergast, 2005; Morgan et al, 2006). Since fat accretion and decreased physical activity can increase the risk of insulin resistance and cardiovascular disease (CVD), the seasonal variations in these and other parameters can place the human health at more risk in winter than in summer (Reilly and Peiser, 2006) This fact is illustrated by the increased number of cardiac events observed in winter both north and south of the equator, by the additional 20,000 deaths per year caused by coronary and cerebrovascular events that were reported in England and Wales during this season, and by the strong negative correlation found between CVD mortality and the hours of sunshine (Pell and Cobbe, 1999). The use of animal models, which can be maintained under constant temperature and social input, have emerged as a useful strategy to shed more light on how exposure to different photoperiods impacts physiology and health
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