Researchers at Hamburg-Eppendorf have found that natural opioids in the body reduce “conditioned fear” caused by fearful stimuli.

Volunteers inside an MRI scanner watched symbols on a screen. One symbol was sometimes followed by a painful application of heat, and the other symbol was innocuous.

Half the volunteers had naloxone, which blocks the effects of opioids. Their fear response–the amygdala’s activation, visible in the MRI scan–didn’t change over time when the “fearful” symbol appeared. The control group’s responses diminished over time.

The researchers speculate that opioid deficiency could be a contributing factor in anxiety disorders, noting that knock-out mice whose opioid systems have been genetically “switched off” appear to be prone to anxieties and exaggerated conditioned fear responses.