This defence reaction is vital for survival, but if it continues for prolonged periods it can be just as dangerous. Xin-Ming Ma and Stafford L. Lightman of the University of Bristol, UK, have now found how rats seem to cope with a repeated stressful stimulus, gradually reducing their stress response each time the same stimulus is encountered, while maintaining the ability to produce the full stress response to a novel threat. They describe their findings in the latest issue of the Journal of Physiology . Stressful stimuli trigger hormone release from specialized nerve cells deep in the brain.
Glucocorticoids prepare the body's energy stores and immune responses for defence.
Novel stimuli often trigger a stress response, but it would be dangerous for the body to react repeatedly as if there is a full-scale emergency. Long-term rises in blood pressure, abnormal immune function and increases in circulating fat and glucose, among other responses to glucocorticoids, can lead to diabetes and heart problems. But normally, the more often we experience something, the less stressful it becomes. Ma and Lightman have found an analogous hormonal response in the rat. Restraining the animals leads to a stress response, including raised glucocorticoid hormone levels.