CENTRAL AND PERIPHERAL SYMPATHETIC ACTIVITIES IN RATS DURING RECOVERYFROM SIMULATED WEIGHTLESSNESS

Rats were tail suspended, keeping their forelimbs weight bearing for 1 4 days, and then allowed to recover for a short (6-h) or a long (24-h) period to assess the behavior of the sympathetic nervous system after weightless simulation. Sympathetic activity was determined by measuri ng norepinephrine (NE) turnover in the brain stem cell groups involved in central blood pressure control and in organs playing a key role in the cardiovascular regulation (heart and kidneys). The NE turnover wa s greatly reduced in the rostral (-56%; P < 0.001) and caudal (-73%; P < 0.001) A2 nucleus of suspended rats but was unchanged in the Al, A5 , and A6 cell groups compared with attached rats. The NE turnover in t he cardiac atria (-34%; P < 0.001) and ventricles (-35%; P < 0.001) an d kidneys (-31%; P < 0.001) was decreased after suspension. The centra l and peripheral sympathetic activities returned to normal within 24 h of release from suspension, but there was hyperactivity after 6 h of recovery. This raises the problem of interpreting the results obtained in animals killed a few hours after return from spaceflight.