Transient vasopressin release and thirst in response to prolonged intracerebroventricular infusions of hypertonic mannitol in saline

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Abstract
Infusions of 0.4 M mannitol in 0.15 M NaCl into the lateral cerebral ventricle (40 or 100 min, 0.02 ml/min) caused slight, transient vasopressin release and temporary thirst in the conscious goat, whereas infusions of pure, hypertonic (0.7M) mannitol did not elicit thirst and inhibited the basic vasopressin release in the nonhydrated animal. Infusions of equiosmolal (0.35M) NaCl induced persistent thirst and pronounced elevation of the plasma vasopressin concentration throughout the infusion period. The CSF osmolality was raised by the same order of magnitude (13%) after the mannitol/NaCl and the hypertonic NaCl infusions. The CSF Na+ concentration was elevated by > 10% at 5 min after the hypertonic NaCl infusions, but it was reduced by approximately 10% at 5 min after the mannitol/NaCl infusions. There was no appreciable difference in the CSF K+ concentration after the infusions. The results are discussed with regard to the possible importance of CSF Na+ concentration as opposed to strict osmotic factors for the excitation of receptors involved in the control of water balance.