Hypovolemia stimulates intraoral intake of water and NaCl solution in intact rats but not in chronic decerebrate rats

This experiment tested the hypothesis that afferent signals from cardiac baroreceptors to the caudal brain stem are integrated by hindbrain systems to control ingestive behavior in response to plasma volume deficits in rats. A supracollicular transection was made which should not interfere with the neural signal of volume depletion to the hindbrain. Decerebrate (n = 5) and control rats (n = 7) were given subcutaneous injections of 30% polyethylene glycol (PEG) to induce hypovolemia or of isotonic saline as a control. Four hours after the injection, either water or 0.1 M NaCl was administered through an intraoral cannula, and intakes were measured. Decerebrate rats did not ingest significantly more water or saline after PEG treatment than after the control treatment, whereas control rats ingested both fluids in significantly larger volumes after PEG treatment. In another test using the same animals, heart rate was monitored after intravenous injections of phenylephrine (to raise blood pressure) and nitroprusside (to lower it). Similar reflexive changes in heart rate were observed in control and decerebrate rats, showing that baroreflex function was not impaired by decerebration. These results indicate that baroafferent signals are processed at multiple levels of the neuraxis, with hindbrain systems mediating autonomic cardiovascular reflexes in response to changes in blood pressure, and midbrain or forebrain systems mediating behavioral responses associated with thirst. (C) 2003 Elsevier Inc. All rights reserved.