A recent study in Cincinnati examined how hypernatremia (increased sodium concentration in blood) affected the behavior of rats when they were exposed to psychological stress. Response to stress was assessed by measuring blood levels of substances such as renin, oxytocin and vasopressin. The researchers also measured the expression of the c-fos protein in brain neurons. Expression of this protein is high whenever an animal is exposed to stress. When hypernatremia was produced in rats, it was found that their response to stress was reduced.
Animals and human beings suffer from stress caused by two factors. The first is exposure to a real danger or some abnormality in the body. The second factor is perceived threat or a psychological disturbance. When stress is produced, signals from both these pathways reach the same portion of the brain – the hypothalamus. The present study investigated whether by altering the signals from one pathway, one can change the response to signals of the other pathway. In this experiment, researchers used hypernatremia, or increased blood sodium concentration as an actual danger signal and they isolated rats in a cage as a source of psychological stress. They also evaluated how the rats interacted with other rats to measure their social behavior.
- A solution of sodium chloride was injected into several rats to produce a state of hypernatremia. After 1 hour, a few rats were killed. Their blood was collected and levels of renin, oxytocin, and vasopressin were measured.
- The remaining rats were isolated in cages to induce psychological stress. At 15, 30, and 60 minutes, the rats were killed and levels of the same 3 hormones were assessed. Expression of the c-fos protein in the brain was measured by using a technique called immunohistochemistry.
- In the second part of the experiment, transmitters were fitted in the aorta, a major blood vessel, to record cardiovascular changes to stress. These transmitters recorded blood pressure and heart rate.
- Social interaction of rats was measured by counting the number of times a rat touched or sniffed the other rats.
- Hypernatremia, or increased blood sodium concentration, caused less elevation of levels of the hormones renin, oxytocin and vasopressin. Expression of c-fos was significantly elevated.
- Cardiovascular response was also eased when hypernatremia was induced in the rats.
- “Rats that were treated with sodium chloride before the initiation of social interaction test spent significantly more time engaging with unfamiliar rats than did control rats.” This suggests a decrease in anxious behaviour and an improvement in social behavior, as a result of acute osmotic dehydration (hypernatremia).
This study has proved that by inducing some physical changes, one can change the body’s response to psychological stress. This will also help in improving social interactions. To some extent, cardiovascular changes in response to psychological stress, such as elevation of blood pressure, can be minimized by simultaneous exposure to physical stress. In the present experiment, the physical stress was hypernatremia. It is important to understand whether similar responses will be observed to other physical stresses also, as it is practically impossible to induce hypernatremia in humans. The results of this experiment can be helpful in devising newer techniques to alleviate the problem of psychological stress and its harmful effects on the body.
For More Information:
Hydration State Controls Stress Responsiveness and Social Behavior
Publication Journal: The Journal of Neuroscience, April 2011
By Eric G Krause; Annette D de Kloet
From the University of Cincinnati, Cincinnati, Ohio
*FYI Living Lab Reports Are Summaries of the Original Research.