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 (2 kHz, 0.5 s) as conditioned stimuli (CS) followed by footshocks (FS, 5 ms pulses of 3 mA intensity, 50 Hz) for 1 s period as unconditioned stimuli. Rats were trained by 100 trials repeated at an interval of 6 s. A various length of time after the training, rats were tested by CS repeated at an interval of 15 s for the period of 120 s and FS of 60 s period, which started 60 s 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.