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Adaptation to Mild, Intermittent Stress Delays Development of Hyperglycemia in the Zucker Diabetic Fatty Rat Independent of Food Intake: Role of Habituation of the Hypothalamic-Pituitary-Adrenal Axis

Departments of Physiology (H.E.B., A.S.S., M.A.K., J.T.Y.Y., D.G.M., S.G.M., M.V.), Obstetrics and Gynecology (S.G.M.), and Medicine (S.G.M., M.V.), Medical Sciences Building, University of Toronto, Toronto, Ontario, Canada M5S 1A8

Address all correspondence to: Holly Bates, Room 3363 Medical Sciences Building, 1 King’s College Circle, Toronto, Ontario, Canada M5S 1A8. E-mail: holdoug{at}yahoo.ca.

Hypothalamic-pituitary-adrenal (HPA) axis hyperactivity occurs in type 2 diabetes, and stress is assumed to play a causal role. However, intermittent restraint stress, a model mimicking some mild stressors, delays development of hyperglycemia in Zucker diabetic fatty (ZDF) rats. We examine whether such stress delays hyperglycemia independent of stress-induced reductions in hyperphagia and is due to adaptations in gene expression of HPA-related peptides and receptors that ameliorate corticosteronemia and thus hyperglycemia. ZDF rats were intermittently restraint stressed (1 h/d, 5 d/wk) for 13 wk and compared with obese control, pair fed, and lean ZDF rats. After 13 wk, basal hormones were repeatedly measured over 24 h, and HPA-related gene expression was assessed by in situ hybridization. Although restraint initially induced hyperglycemia, this response habituated over time, and intermittent restraint delayed hyperglycemia. This delay was partly related to 5–15% decreased hyperphagia, which was not accompanied by decreased arcuate nucleus NPY or increased POMC mRNA expression, although expression was altered by obesity. Obese rats demonstrated basal hypercorticosteronemia and greater corticosterone responses to food/water removal. Basal hypercorticosteronemia was further exacerbated after 13 wk of pair feeding during the nadir. Importantly, intermittent restraint further delayed hyperglycemia independent of food intake, because glycemia was 30–40% lower than after 13 wk of pair feeding. This may be mediated by increased hippocampal MR mRNA, reduced anterior pituitary POMC mRNA levels, and lower adrenal sensitivity to ACTH, thus preventing basal and stress-induced hypercorticosteronemia. In contrast, 24-h catecholamines were unaltered. Thus, rather than playing a causal role, intermittent stress delayed deteriorations in glycemia and ameliorated HPA hyperactivity in the ZDF rat.