Orthostatic response in rats after hindlimb unloading: effect of transcranial electrical stimulation

Orthostatic hypotension is a commonly observed phenomenon after exposure to microgravity and in various forms of autonomic failure. It has been suggested that insufficient activation of supraspinal structures responsible for descending sympathetic drive could play a significant role in this disorder. We examined the effect of transcranial electrical stimulation (TES) of autonomic nuclei within the brain on the orthostatic hypotension induced by exposure to simulated microgravity using a hindlimb unloading model.

There were 20 male Wistar rats that were suspended by their tail with the angle of elevation between the cage floor and the rat's body approximately 40 degrees. There were 11 age-matched Wistar rats used as cage controls. Orthostatic stability was examined by using an orthostatic challenge test (450 head-up test for a period of 3 min). In 10 rats from the tail-suspended group, the orthostatic challenge test was applied during TES.

In the rats exposed to simulated microgravity (tail suspension), the orthostatic challenge test caused a significant decrease in mean arterial blood pressure by 18.4 +/- 2.2%. TES attenuated this microgravity-induced orthostatic hypotension to 9.5 +/- 1.8% (P < 0.05), which was similar to the observed response to an orthostatic challenge in the control group (6.9 +/- 1.1%).

Results of this study suggest that TES significantly reduces the changes in blood pressure during an orthostatic challenge test in animals exposed to simulated microgravity. Our observations support the notion that a reduction in descending sympathoexcitatory input from supraspinal structures could contribute to orthostatic hypotension and intolerance observed in astronauts following their return from spaceflight.