Meeting Abstract

32.3  Jan. 5  Circadian Pattern of Plasma Corticosterone and Binding Globulins in Mice Selectively Bred for High Activity Levels MALISCH, J.L.**; GOMES, F.; BREUNER, C.W.; GARLAND, JR., T.; Univ. of California, Riverside; Univ. of California, Riverside; Univ. of Montana; Univ. of California, Riverside jbunk001@ucr.edu

In vertebrates, baseline glucocorticoids vary predictably on a diel basis, typically peaking shortly before the onset of activity. Presumably, circadian patterns in glucocorticoid secretion have evolved to match predictable rises in energetic need. In mice from lines selectively bred for high voluntary wheel running (HR lines), total baseline levels of corticosterone (CORT) are significantly elevated (2-fold) above that of non-selected control (C) lines (Malisch et al., in press, Physiological and Biochemical Zoology). We propose that increased baseline CORT is a necessary adaptation to permit the high levels (nearly 3-fold higher than C mice) of wheel running exhibited by HR mice. Here we examine baseline CORT levels, corticosterone-binding globulin (CBG) capacity, and free CORT levels (CORT not bound to CBG and hence biologically active) at six points during the 24-hour cycle. Plasma samples were obtained from 4 individuals from each of the 4 HR lines and 4 C lines at 6 equally spaced timepoints. Preliminary analyses indicate that CORT is elevated at all timepoints, significantly so at three of the six. CBG binding capacity does not differ between HR and C lines; therefore, free CORT is also higher at all timepoints in the HR lines. Additionally, the circadian pattern does not appear to differ between HR and C mice. These findings support the hypothesis that elevated CORT is an adaptation to support increased activity levels in HR mice. Supported by NSF IOB-0543429 to T.G and IBN-0202676 to C.W.B.