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2018, Physiology & behavior
Glucocorticoids, a class of metabolic hormones, impact a wide range of traits (e.g., behavior, skeletal growth, muscle maintenance, glucose metabolism), and variation in concentrations of circulating glucocorticoids (such as corticosterone), at the level of natural individual variation, in relation to endocrine disorders, or from exogenous supplementation, have manifold effects. Changes in circulating corticosterone concentrations can also impact multiple aspects of locomotor behavior, including both motivation and physical ability for exercise. To examine further the role of corticosterone in locomotor behavior and associated traits, we utilized laboratory house mice from a long-term experiment that selectively breeds for high levels of voluntary exercise. As compared with four non-selected control (C) lines, mice from the four replicate High Runner (HR) lines have ~2-fold higher baseline circulating corticosterone concentrations as well as ~3-fold higher voluntary wheel running on a daily basis, higher home-cage activity when deprived of wheels, higher maximal aerobic capacity, and smaller body size; potentially, all of these differences could be modulated by circulating corticosterone. We administered 50 μg/mL corticosterone-21-hemisuccinate in the drinking water of both HR and C male mice from weaning through ~8 weeks of age. As compared with mice from C lines, HR mice had higher endogenous corticosterone levels; higher daily wheel-running distance, duration, and speed; higher maximal oxygen consumption during forced exercise (VO2max); spent more time in the closed arms of an elevated plus maze; and had larger reproductive fat pads. For both HR and C mice, corticosterone treatment strongly suppressed endogenous circulating corticosterone levels, decreased growth rate and adult body mass, increased food and water consumption (both adjusted for body mass), increased entries into closed arms of an elevated plus maze, decreased home-cage activity (total and average intensity), decreased wheel-running distance and maximum speed, and decreased VO2max. At the suborganismal level, corticosterone treatment decreased relative adrenal, liver, and triceps surae muscle mass, as well as tail length, but increased both subdermal and reproductive fat pad masses, as well as hematocrit. Overall, the responses of both HR and C mice to corticosterone supplementation were "negative" from a health perspective. These results have significant implications for understanding both the evolution of baseline corticosterone levels and stress-related effects on activity levels. They also suggest that patients experiencing extended periods of glucocorticoid treatment might benefit from attempts to increase their physical activity as an adjuvant.