Regulation of the hypothalamic pituitary adrenal axis during chronic stress: responses to repeated intraperitoneal hypertonic saline injection

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 ± 10to782 ± 57, 788 ± 83and779 ±31pg/ml30 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 ± 0.4and9.2 ± 3.1μg/dl) in controls and stressed rats, respectively). Enhancing the osmotic component of the stress by daily withdrawal of the drinking water for 8 h following the injection, or by administration of 0.9% saline as drinking fluid, did not prevent the increases in CRH mRNA and hypersensitivity of the ACTH response to the novel stress. The data show that physical and psychological components of the stress overcome the inhibitory effect of chronic osmotic stimulation on ACTH secretion, and emphasizes the relationship between parvicellular activation and HPA hypersensitivity during chronic stress.