The adrenal cortex during activity and hibernation in the male little brown bat, Myotis lucifugus lucifugus: Annual rhythm of plasma cortisol levels

Abstract

Plasma cortisol concentrations were measured by radioimmunoassay in male little brown bats, Myotis lucifugus lucifugus (Chiroptera: Vespertilionidae), collected from a wild population throughout the entire annual cycle of activity (May–September) and hibernation (October–April). Two populations of corticosteroid levels were observed which corresponded to the physiological states of activity and hibernation, respectively: lower levels ( X = 0.46 ± 0.04 μ g/ml ) were measured during the months of activity whereas higher levels ( X = 0.81 ± 0.05 μ g/ml ) prevailed during the period of hibernation. The hibernating levels of cortical hormone for this bat are higher than those that have been measured in the plasma of other mammalian hibernators. These data suggest that the adrenal cortex in this species may be highly active during hibernation, a result that is at variance with the longstanding concept of “polyglandular involution” of the endocrine organs as a prerequisite for preparation and maintenance of hibernation in mammals. The findings also imply a seasonal “resetting” of feedback control in the brain-pituitary-adrenal axis with respect to ACTH in this species. Finally, it is suggested that elevated glucocorticoids would be necessary for the energetic requirements of hibernation and that periodic spontaneous arousals during hibernation may be important in permitting or accelerating hormonal action.