Conditional catecholamine changes in rhesus monkeys

Magnitude of changes in levels of plasma norepinephrine and epinephrine from basal levels was compared in four rhesus monkeys upon presentations of positive and of negative conditional stimuli (CS + and CS -) during sessions of Pavlovian fear conditioning. The two monkeys providing the least information as to when shock would be delivered both showed significantly increased levels of epinephrine during CS + sessions. Following a course of extinction trials, these differences disappeared. Changes in levels of plasma norepinephrine were not as consistent, possibly because of the fact that only limited amounts of this neurohumor find their way into the circulation. The data demonstrate that neurohumors with a very brief half-life can be conditioned. Moreover, since the amount of shock used was the same for the two subgroups studied, the data on epinephrine levels are consistent with our hypothesis that environmental factors other than the physical stimulus itself may play a role in habituation of a visceral response. Data from behavioral studies would suggest that this occurs because the animal learns when it is safe or in danger.