PLASMA OSMOTIC PRESSURE AND VOLUME CHANGES AS DETERMINANTS OF DRINKING THRESHOLDS

Experiment I determined drinking thresholds of salt-injected rats given differential exposure on a deprivation schedule. Results showed that rats given a subcutaneous 16% NaCl injection drank reliably after 5-7 min postinjection. Plasma osmotic pressure increased 2-3% at the initiation of drinking, this 2-3% increase representing a drinking threshold. Experiment II determined similar thresholds in rats allowed to alter their osmotic pressure by eating dry food. The mean time lapse from the onset of eating to the reinitiation of drinking was 51 min. Although the temporal characteristics of the animals which ate were different from the injected rats, the osmotic pressure changes were equal (2-3% increase) to those of Experiment I. Two more experiments tested two hypotheses generated by Experiment I. Results are discussed in terms of mechanism involved in the onset of drinking behavior. © 1969.