Suppression of Vasopressin Secretion by Classically Conditioned Stimuli in Rats

The possibility that emotional stress suppresses footshock-induced, secretion of antidiuretic hormone (VP) was tested in rats by a classical conditioning paradigm. As a training trial, rats received a flash and a brief sound (2kHz, 0.5s) as conditioned stimuli (CS) followed by footshocks (FS, 5ms pulses of 3mA intensity, 50Hz) for 1s period as unconditioned stimuli. Rats were trained by 100 trials repeated at an interval of 6s. A various length of time after the training, rats were tested by CS repeated at an interval of 15s for the period of 120s and FS of 60s period, which started 60s after the CS onset. Testing CS further augmented FS-induced increase in plasma adrenocorticotrophic hormone (ACTH) but suppressed FS-induced increase in plasma VP in a time-dependent manner. The CS also increased the degree of an inhibition of motor behavior known as "freezing" behavior. Augmentation of ACTH response and suppression of VP response to testing FS were dependent on the training shock intensity. These data are consistent with the hypothesis that VP secretion is potentiated by physical but suppressed by emotional stress.