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Abstract
Chronic osmotic stress inhibits, while repeated physical stress can increase pituitary
ACTH responsiveness to a novel stress. The interaction between these effects was studied
in rats subjected to repeated i.p. injection of hypertonic saline, a strong aversive
stimulus with osmotic and painful and psychological stress components, for 14 days.
Hypertonic saline injection caused marked drinking responses, transient increases
in plasma vasopressin (VP), and marked increases in VP mRNA and irVP in magnocellular
cell bodies in the hypothalamus. Parvicellular activity was also enhanced as indicated
by increases in VP immunostaining in the external zone of the median eminence and
CRH mRNA and irCRH in the PVN. Plasma ACTH levels increased 10-fold after 30 min hypertonic
saline injection, returning to basal levels in 4 h, and there was no desensitization
of the ACTH responses after repeated injections (from basal values of 76 +/- 10 to
782 +/- 57, 788 +/- 83 and 779 +/- 31 pg/ml 30 min after the first, 4th and 14th injection,
respectively). Basal ACTH levels were normal 24 h after the last injection, but pituitary
POMC mRNA levels were increased by 95%, and ACTH responses to a novel stress (15 min
immobilization) were significantly larger than in controls (P < 0.01) despite increases
in morning plasma corticosterone levels (1.5 +/- 0.4 and 9.2 +/- 3.1 micrograms/dl
in controls and stressed rats, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)