Davis, M., Myers, K.M., Ressler, K.J. Rothbaum, B.O. (2005). Facilitation of Extinction of Conditioned Fear by D-Cycloserine. American Psychological Association, Volume 14, Number 4, 214-219.
Conditioned fear can be suppressed through a process known as extinction, in which repeated exposure to a fearful stimuli minus any aversive effects leads to a gradual reduction in the fear response. Extinction is not thought to be an "unlearning" process, but rather the addition of a supplementary learning process which actively inhibits or suppresses fear responses that are no longer adaptive. Although not much is known about the neural underpinnings of extinction, it is known that the process depends on NMDA receptors within the amygdala which help to consolidate extinction. The compound D-cycloserine binds to the NMDA receptor and improves its efficiency, and administration of D-cycloserine in rats was shown to dose-dependently enhance extinction.
Taking this laboratory research to the clinic, researchers were curious to see if similar effects would be witnessed in human subjects. This article showed that D-cycloserine used alongside exposure therapy for acrophobics (people suffering from a fear of heights) resulted in significantly larger reductions in phobic symptoms and faster improvement as compared with placebo-controlled subjects. While this paints an optimistic picture for the use of D-cycloserine with phobias, it remains to be seen whether the compound will show equally impressive results in improving cognitive behavioral therapies for more complex anxiety disorders such as PTSD.
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