[Effects of cold acclimation and repetitive stress on stress-induced neuroendocrine response].

In order to elucidate the mechanism (S) involved in improved cold tolerance by means of enhanced non-shivering thermogenesis of the repetitively stressed rats, noradrenaline (NA) turnover of brown adipose tissue (BAT) and adrenocortical responses were investigated. (1) NA turnover of BAT in the cold-acclimated rats was greater than that in the resting controls. NA level of BAT in the cold-acclimated rats decreased to about 40% of the control level. It was thus inferred that this lower NA levels was induced by accelerated NA turnover. (2) NA turnover of BAT in the stressed rats after repetitive immobilization stress was higher than that of the non-stressed controls. Therefore, increased sympathetic activity of BAT would be one of the mechanisms of cross adaptation between cold and stress. (3) NA turnovers of BAT in the controls, the cold-acclimated rats and stressed ones were increased by acute cold exposure (-5 degrees C). NA turnovers of BAT in the controls and the stressed rats were increased by acute immobilization stress. (4) Plasma corticoids (corticosterone and deoxycorticosterone) in the cold-acclimated rats were higher than that of controls. Plasma corticoids in the controls, the cold-acclimated rats and stressed ones were increased by acute cold exposure (-5 degrees C, 15 min) and acute immobilization stress (30 min). The extents of increases in plasma corticoids in the stressed rats, but not in the cold-acclimated ones, were greater than those in the controls. It was suggested that repetitive immobilization stress could enhance nonshivering thermogenesis via an enhanced responsiveness of adrenocortical secretion to acute stress and cold. It would be concluded from these results that enhanced responses of corticoid secretion and accelerated sympathetic activity were associated with the establishment of cross adaptation between cold and stress. It was suggested that the extent of participation of these factors was not necessarily the same between the cold-acclimated and the stressed organisms.