Hindlimb unloading in adult rats using an alternative tail harness design

Introduction: Hindlimb unloading has proven to be an effective model for studying the physiological effects of spaceflight. However, using current methodologies, maintenance of adult rats in hindlimb unloading for long periods is challenging. Therefore, our goal was to develop a technique allowing long-term hindlimb unloading in adult rats. Methods: Adult male Sprague Dawley rats were assigned to control (C), control pinned (CP), and hindlimb unloaded (HU) groups. All rats were anesthetized and a stainless steel needle was inserted through the skin in the ventral side of the tail of CP and HU groups. A cable was inserted through the needle, wrapped around to the dorsal side, secured, and stabilized with casting tape. HU rats were hindlimb suspended for 28 d by attaching the cable to an adjustable bar, resulting in a 30° head-down tilt, and were monitored for health status and bodyweight. Fecal corticosterone levels were used as a stress index. Hindlimb muscles and adrenals were weighed. Results: Attrition due to animals slipping from their tail harness was eliminated. HU animals lost 9.9% bodyweight within the first 6 d of hindlimb unloading, but maintained that bodyweight the remaining 22 d. Stress levels, as measured by fecal corticosterone and adrenal weights, were not elevated significantly during suspension and muscle weights were decreased significantly. Discussion: Results indicate that this method is suitable for long-term hindlimb unloading of adult rats, providing an alternative approach to study loss of musculoskeletal mass in simulated microgravity.