Effects of Chronic Immobilization Stress on Pituitary Hormone Secretion, on Hypothalamic Factor Levels, and on Pituitary Responsiveness to LHRH and TRH in Female Rats

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Abstract
The effects of 8 h of daily immobilization applied for 10 consecutive days on pituitary and plasma levels of anterior pituitary (AP) hormones as well as on TRH, LHRH and somatostatin (SRIF) content in the hypothalamus, amygdala and forebrain were studied in adult female rats. The in vivo pituitary responsiveness to intraventricular injections of LHRH combined with TRH was also tested in groups of rats submitted to the same stress procedure. Long-term intermittent immobilization stress markedly reduced GH, LH, PRL and TSH plasma levell, while FSH titers were not modified. Parallel to these variations, FSH and LH pituitary levels increased, while those of GH, PRL and TSH remained unchanged. No modification of hypothalamic or amygdala TRH, LHRH or SRIF content have been observed, whereas forebrain TRH levels decreased significantly. In nonstressed animals, LHRH + TRH significantly increased LH and TSH plasma levels and decreased those of PRL, FSH and GH. In immobilized rats, the hypothalamic factors further stimulated the secretion of LH and TSH, released FSH and PRL, and had a greater inhibiting effect on plasma GH levels. These results indicate that chronic stress-induced decreases in plasma AP hormone levels are not associated with a diminished pituitary responsiveness to releasing factors, nor with modification of hypothalamic factor content. Furthermore, the enhanced liberation of LH, and the release of FSH after LHRH + TRH administration during stress could be secondary to the increased pituitary content of these hormones.

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