The response pattern of noradrenaline release to repeated stress in the hypothalamic paraventricular nucleus differs according to the form of stress in rats.

The effect of two repeated forms of stress, manual restraint and tail-pinch, on noradrenaline (NA) release in the hypothalamic paraventricular nucleus of the rat was examined by intracerebral microdialysis. Manual restraint significantly increased NA release, but the stimulatory effect gradually declined when the stress was repeated at intervals of 120 min. High K+ induced a great increase in NA release even when manual restraint produced no significant effect on NA release. In contrast, tail-pinch significantly increased NA release to a greater extent than manual restraint, and the increase in NA release did not change when the stress was repeated three times at intervals of 120 min. These results suggest that desensitization of NA neurons ending in the hypothalamic paraventricular nucleus occurs in response to painless stress, such as manual restraint, whereas no attenuation of NA release is caused by repeated stress accompanied by pain, such as tail-pinch.