ACUTE STRESS ATTENUATES BUT DOES NOT ABOLISH CIRCADIAN RHYTHMICITY OFSERUM THYROTROPIN AND GROWTH-HORMONE IN THE RAT

The effects of acute immobilization (IMO) on daily rhythms of corticos terone, thyroid-stimulating hormone (TSH) and growth hormone (GH) were studied in adult male rats. Two hours of IMO increased serum corticos terone, this increase still being observed 3 h after finishing stress exposure. In the dark period corticosterone levels did not differ in c ontrol and IMO rats, but higher levels were observed again in the morn ing of the day after. Immobilization lowered serum GH and TSH levels t hroughout the 24-h period that followed exposure to the stressor. Such an effect was more marked in GH than in TSH, In addition, GH, but not TSH, levels were found to be reduced significantly by IMO at 08.30 h of the next day. None the less, daily rhythms of GH and TSH were still persistent and roughly similar to those of control rats. The daily rh ythm of food intake was measured in a separate experiment and it was o bserved, as expected, that IMO reduced food intake only in the dark pe riod of the lighting cycle. It appears therefore unlikely that IMO-ind uced anorexia was the major factor responsible for the inhibition of G H and TSH caused by IMO at 11.00 and 19.00 h, considering that the amo unt of food intake was very low and similar in control and IMO rats du ring this period, However, anorexia might have contributed to inhibiti on of GH and TSH secretion afterwards, Thus, in a third experiment we studied the contribution of IMO-induced anorexia to the changes in hor mone levels observed 24 h after stress by introducing a group of pair- fed rats. It was found that IMO, but not pair-feeding, reduced TSH lev els, whereas a similar reduction of GH was found in the two conditions . It might be concluded that acute stress transiently altered corticos terone secretion, the only long-lasting effect being a slight increase in its morning levels on the following stress. Immobilization also ca uses an inhibition of GH and TSH secretion in the rat that persists fo r several hours after finalization of exposure to the stressor, but da ily rhythms were still apparent. It appears that the contribution of s tress-induced anorexia is different in GH than in TSH. In conclusion, an acute severe stressor such as IMO, although modifying circulating l evels of some hormones, particularly in the hours following exposure t o the stressor, did not appear to interfere greatly with the expressio n of circadian rhythms of anterior pituitary hormones.