Effects of varying doses of estrogen and caudal pressure on wheel running in orchidectomized male mice.

Physical inactivity is a leading cause of hypokinetic diseases - obesity, heart disease, diabetes, and certain types of cancers. Increased city walkability, better access to fitness facilities, and remediation of socioeconomic barriers prove successful for limited populations within the confines of stringently controlled environments; however, these strategies fail to reverse the ever-increasing physical inactivity epidemic on a global scale indicating the existence of other unidentified factors. These purported biological factors remain critical targets to understand the regulation of this complex phenotype. An estrogenic mechanism that incompletely or slowly adjusts physical activity levels following reintroduction of estrogenic compounds to surgically gonadectomized mice has been postulated to exist. Currently, this mechanism remains scrutinized due to concerns that elevated estrogen levels induce urinary bladder distension. The distension of the urinary bladder may mechanically disrupt physical activity, masking any physiological effects estrogen has on physical activity. The purpose of this study was to evaluate the effects of estrogen on physical activity levels while employing dose-related strategies to alleviate distension in mice. Wheel running data were collected under normal physiological conditions, following removal of endogenous sex steroids via orchidectomy, and during estrogen replacement at various doses (0%, 10%, 50% or 100% estrogen-containing implants) to induce varying degrees of urinary bladder distension. Wheel running distance (P = 0.005) and duration (P = 0.006) decreased after orchidectomy, but slowly increased following estrogen replacement. During the study, wheel running did not return to the levels observed in physiologically intact mice. Significant distension was not observed between estrogen treatment groups indicating that a slow-responding estrogen effect exists in male mice that prevents wheel running from returning to normal levels immediately following steroid reintroduction. The limited increase in wheel running during estrogen treatment following orchidectomy is not an artifact of induced urinary bladder distension.