I just read the excellent Not Rocket Science for the first time. He has a nice writeup of the propranolol story that is making the rounds. Some researchers conditioned some subjects to get stressed when they saw a picture of a spider by shocking them while viewing spider pictures. Then, everyone got more spider pictures (with no shocks), this time with loud noises! The fear response to these picture/noise combos was measured by observing the subjects’ blinking.
On the second day of the experiment, everyone got a pill with their spider picture and loud noise. Some of them got a placebo, and some got propranolol.
Propranolol is a “beta-adrenergic antagonist” – that is, it blocks (antagonizes) a certain subset (beta) of adrenergic receptors (which respond to small molecules like adrenalin). Because the response to stimuli like loud noises, shocks, and spiders is often adrenalin-mediated, it’s reasonable (in fact, well-known) that propranolol will have an effect on one’s response to this sort of stressor. Of course, placebos work, too, and both groups actually mellowed out a little when subjected to spider+noise.
The next (third) day, the shock-propranolol group had an attenuated fear response relative to the shock-placebo group. The authors go on to speculate that neurotransmitters like adrenalin play a role in the maintenance of these frightening memories, and blocking the molecule’s action at the time the memory is recalled attenuates the associated emotions.