BUNKERS, J L; GOMES, F; REZENDE, E L; SALTZMAN, W; GARLAND, T Jr; Univ. of California, Riverside: Plasma Corticosterone of Mice Selectively Bred for High Voluntary Wheel Running: Levels at Rest and Following Restraint Stress

The hypothalamic-pituitary-adrenal (HPA) axis is important in regulating energy metabolism and mediates responses to a broad range of stressors. Glucocorticoids also affect the central nervous system and locomotor behavior. Therefore, to explore potential adaptive changes in the HPA axis as animals evolve higher activity levels, we studied plasma corticosterone (B) and adrenal mass in 4 replicate lines of house mice that had been selected for high voluntary wheel running (S lines) for 34 generations and in 4 non-selected control (C) lines. Previous work showed that S females (males were not studied) have higher B levels while running on wheels at night, and suggested they might also have elevated daytime resting levels (J. Appl. Physiol. 92:1553-1561). We tested whether S mice of both sexes have higher B levels at rest during the day and whether they have an elevated B response to a novel (non-exercise) stressor. Resting daytime B levels in S were approximately twice as high as in C for both sexes. Following 40 min of physical restraint, B levels were similar in S and C; thus, B increased less in S. Females had B levels twice as high as those of males under both conditions and in both S and C. Adrenal mass was similar in S and C mice, but S were significantly smaller in total body mass (both sexes); thus, S mice tended to have larger adrenals relative to body mass. Hematocrit did not differ between S and C. Future experiments will be designed to address the functional significance of elevated B levels in the S lines. For example, elevated basal levels may enhance glycogenolysis and gluconeogenesis, which could facilitate aerobically supported sustained wheel running at relatively high speeds. Supported by NSF IBN-0212567 to T.G.