This possibility is supported by both animal and human studies. In rats, tiny injections of norepinephrine into the amygdala, a region of the brain that encodes fear, can enhance fear conditioning. And Marieke Soeter at the University of Amsterdam recently conducted an experiment in which college students were shown a picture paired with a small electric shock. Before viewing the pictures, subjects were randomly given yohimbine, a drug that releases norepinephrine in the brain, or a placebo. When students were tested 48 hours later, those who had received yohimbine had greater fear-associated learning and had a harder time “unlearning” the fear — when presented with the picture in the absence of a shock — than those students who had taken the placebo.
The study implies that soldiers exposed to elevated norepinephrine levels from taking stimulants are also at risk of relapse when re-exposed to the initial stressor. And because the treatment of PTSD involves unlearning fear responses, soldiers exposed to stimulants during trauma could well be more resistant to treatment.
And in fact, blocking the effects of norepinephrine with beta blockers can stop fear-conditioning and possibly even prevent post-traumatic stress disorder.
Roger Pittman, a psychiatrist at Harvard Medical School, led a small study in 2002 in which he randomly assigned emergency-room patients to either the beta blocker propranolol or a placebo within six hours of their experiencing a traumatic event. After one month, subjects who took the propranolol had significantly fewer symptoms of PTSD than subjects who took the placebo.
Does all of this prove that stimulants promote the development of post-traumatic stress disorder?
No. Because two things are correlated doesn’t mean there is a causal link. There are other factors that might play an important role, like incurring a traumatic brain injury, which is a known risk factor for the disorder, and growing steadily during these wars.
Still, it is an open question whether the use of stimulants in combat does more good than harm. The next step should be a rigorous epidemiologic study of a possible link between stimulants and PTSD in our troops.