Inhibition of rat corticotropin-releasing factor and adrenocorticotropin secretion by an osmotic stimulus.

This study demonstrates that a chronic osmotic stimulus can influence the hypothalamo-pituitary axis by inhibiting the secretion of basal and adrenalectomy-elevated adrenocorticotropin (ACTH) from the anterior pituitary. In rats given 340 mM NaCl instead of tap water to drink for 12 days, plasma concentrations of ACTH decreased to 105 +/- 27 pM (mean +/- S.E.M., n = 6) compared with control animals (182 +/- 13 pM). In adrenalectomised (ADX) rats given 150 mM NaCl to drink for 12 days, plasma ACTH concentrations were greatly increased (783 +/- 141 pM) but in ADX rats treated with 340 mM NaCl for the same period, plasma ACTH was similar to controls (237 +/- 59 pM). The corticotropin-releasing factor (CRF-41) content of the median eminence (ME) was reduced in ADX rats on 150 mM NaCl (854 +/- 78 fmol) and further reduced in ADX rats given 340 mM NaCl (510 +/- 56 fmol) compared with control animals (1239 +/- 114 fmol), suggesting that the decrease in plasma ACTH concentrations in saline-treated animals is secondary to a decrease in the secretion of CRF-41 from the ME. These data are the first evidence for a central mechanism, independent of glucocorticoid feedback, through which a chronic osmotic stimulus can inhibit the activity of CRF-41 neurons.