Chronic restraint stress affected fuel utilization by altering the respiratory exchange ratio in adult male rats

INTRODUCTION Brain metabolism is a fundamental process involved in the maintenance of the main higher functions in humans and an imbalance of bioenergetics has been observed in patients with major depressive disorder [1]. Indeed, among the several systems affected by stress exposure, metabolic perturbations may predispose to the development of psychiatric disorders. Recently, by employing a chronic stress protocol in rats, we demonstrated that metabolic mechanisms involved in the stress response are differentially modulated in the ventral hippocampus of resilient compared with vulnerable rats [2], with the latter preferring fatty acid utilization over glycolysis to produce ketone bodies and ultimately Acetyl-CoA as a main energy source [3]. AIMS The aim of this study is to investigate whether chronic stress exposure influences the hedonic phenotype and the whole-body metabolic profiling of adult male rats through indirect calorimetry by the measurement of oxygen consumption and carbon dioxide production, food and water intake and locomotor activity. METHODS Adult male Wistar Han rats were exposed to two weeks of chronic restraint stress (CRS) (1hr/twice a day). After one week of CRS, half of the animals were allocated in the metabolic cages for one week, while continuing the stress procedure. Metabolic analyses were performed by indirect calorimetry using a computer-controlled system (Promethion Metabolic Screening, Sable Systems International, Las Vegas, NV, USA). At weekly intervals, rats were tested with the sucrose consumption test to monitor their hedonic phenotype. Metabolic data were analyzed using the web tool Indirect Calorimetry Experiments CalR17 and presented as hourly averages for the light-dark periods considered. The analyses of the data behavioral and metabolic were performed with the Unpaired t test. SUMMARY OF RESULTS Rats exposed to two weeks of CRS showed a statistically significant reduction of the sucrose preference (p<0.05 vs No stress, Unpaired t test) in comparison to the no stress group. Moreover, CRS induced a significant decrease of the body weight (p<0.05 vs No stress, Unpaired t test). In vivo metabolic profiling through indirect calorimetry showed that after one week of CRS rats did not show alteration in the metabolic parameters whereas after two weeks of CRS, animals presented a statistically significant decrease in the respiratory exchange ratio (RER) both in the dark (p<0.001 vs No stress, Unpaired t test) and in the light phases (p<0.001 vs No stress, Unpaired t test), changes not due to the food intake or to the locomotor activity. CONCLUSIONS These results suggest that different substrates are utilized for the energy production after two weeks of chronic restraint stress. Indeed, the reduction of the sucrose preference observed in chronically stressed rats is associated with an alteration of the RER, indicating that prolonged stress exposure causes a shift in the energetic metabolism by inducing a preferential use of fatty acid over carbohydrates as energy source.