London, May 21 : A new study has revealed that body's natural pain killing system may aid in blocking phobias.
It has long been known that combining an innocuous stimulus (such as a tone) with something aversive (such as a shock to the feet), animals, including humans, learn to show a "conditioned fear" response. And both the learning and the instigation of the 'conditioned fear' response occur in a part of the brain known as the amygdala. Moreover, people showing this response actually tend to experience less pain the more they are exposed to the dreading stimulus.
A study conducted in using rodents showed that this was because of opioids, a chemical substance that has a morphine-like action in the body. The main use is for pain relief.
The researchers found with the increased exposure to the painful stimulus opioids came into action during the conditioning, thus alleviating the pain. When these chemicals were blocked they not only stopped lessening pain but also intensifies the learning process.
The team led by Falk Eippert at the University Medical Centre of Hamburg-Eppendorf, Germany wanted to know if something similar happened in humans.
In the study involving 30 male volunteers, the participants were asked to watch green triangles and blue pentagons on a screen inside an MRI scanner.
One of the symbols was followed half the time by a moderately painful application of heat to the forearm; the other was never followed by pain.alf the participants were given naloxone, a drug that blocks the effects of opioids, while others were given saline solution as a control.
The brain scans showed that in people whose opioid systems had been blocked, the amygdala showed a fear response that did not reduce with exposure. Every time they saw the symbol associated with pain, their amygdalas reacted strongly. In the control group, however, the activation decreased over the course of the experiment.
The participants who were given naloxone reacted more fearfully which the team believes suggested that they were learning the association more intensively.
In the beginning of each trial, volunteers had to perform a reaction time task pressing a button to indicate on which half of the screen the symbol had appeared.
The subjects reacted more quickly to the cue signalling pain than the cue signalling nothing, however, opioid-free subjects reacted significantly faster.
"This natural response advantage to something dangerous was much stronger in the people who received naloxone," New Scientist quoted Eippert, as saying.
The scientists believe that opioid deficiency could be a contributing factor to anxiety disorders and amplified fear responses.
"It potentially has far reaching implications," said Jon-Kar Zubieta at the University of Michigan in Ann Arbor, US.
"There is tremendous variability in how individuals respond to threats and stress, he says, which is thought to relate to the risk developing anxiety disorders.
"This study examines the circuits and neurochemical systems that are likely to underlie that response heterogeneity," Kar Zubieta added.