Further characterization of the pituitary‐adrenocortical responses to stress in Chiroptera

Previous studies on bats from this laboratory have revealed the presence of exceptionally high circulating levels of glucocorticoids in two species of the sub‐order Megachiroptera. In the present study, the following questions were asked: (1) what effect does the routine handling and examination of captive bats have on the activity of their hypothalamic‐pituitary‐adrenocortical (HPA) axis?; (2) are the unusually high plasma levels of cortisol and corticosterone found in Pteropus hypomelanus associated with high levels of circulating adrenocorticotropic hormone (ACTH)?; (3) are there diurnal changes in stress responsivity in this species?; and (4) how do levels of glucocorticoids in P. hypomelanus compare with those found in other species of Chiroptera (both micro and megachiropteran species)? Of five species examined, P. hypomelanus had slightly higher total glucocorticoid levels than P. pumulis, but ∼‐fold higher levels than in three species of Microchiroptera (Artibeus jamaicensis, Carollia perspicillata, and Myotis lucifigus). There was a pronounced diurnal rhythm in glucocorticoid levels in one species (M. lucifigus) for which this was determined. A 1‐h period of restraint stress increased glucose and glucocorticoid levels in P. pumulis, and also increased ACTH and glucocorticoids in P. hypomelanus. Fifteen minutes of routine handling (weighing, measuring, etc.) elicited a significant rise in plasma glucocorticoids in P. hypomelanus to combined peak (cortisol plus corticosterone) levels of over 1,000 ng/ml (100 μg%). There was no significant difference in the response to handling in bats tested in the morning or evening. Basal ACTH levels as detected by radioimmunoassay were low in P. hypomelanus, in spite of high steroid levels. Displacement of labeled ACTH by plasma of P. hypomelanus was roughly parallel to that produced by synthetic human ACTH. The identity of immunoreactive ACTH was confirmed by subjecting separate aliquots of plasma to an immunoradiometric assay (IRMA), which operates on a different principle from the RIA and necessitates an intact molecule closely resembling human ACTH. We conclude that bats possess a “stress response” similar in some ways to that seen in other mammals. The extremely high constitutive and stress‐induced levels of glucocorticoids in the Megachiroptera are nearly unique among mammals. Although high circulating levels of ACTH were detected by IRMA, but not by RIA, it is possible that the Chiropteran adrenal gland is hypersensitive to ACTH or possesses exceptionally active intracellular steroidogenic mechanisms. Finally, the routine daily procedures associated with measuring, weighing, and monitoring bats in captivity constitutes a significant stress to the animal as revealed by pronounced activation of the HPA axis.