Levels of Neurohypophyseal Peptides in the Rat during the First Month of Life. II. Response to Physiological Stimuli*

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Abstract
Levels of vasopressin, oxytocin, and neurophysin were measured by RIA in the hypothalamus, pituitary, and plasma of infant rats (2–30 days old). At all ages, ip injection of a hypertonic solution of 5 g/100 ml NaCl produced a marked increase in levels of vasopressin and neurophysin in plasma, up to 21 μU/ml and 51 ng/ml, respectively. After dehydration, there was a decrease of 26–38% in the levels of neurohypophyseal peptides in the pituitary. Depletion of neurohypophyseal peptides from the pituitary was greater after 24 h of dehydration in younger rats (26%) than in older rats (7%). Levels of vasopressin in plasma were less than 1.7 μU/ml after dehydration in younger rats but were greater in older rats. Immaturity of the neurohypophysis may contribute to the inability of newborn rats to withstand prolonged dehydration.