11 August 2011
Scared of the wrong things? Enzyme imbalance may be the cause
by Kate Melville
Do you run when you should stay? Are you afraid of certain things for no logical reason? An enzyme deficiency might be to blame, say researchers from the University of Southern California.
Writing in the International Journal of Neuropharmacology, the authors explain how mice lacking a certain enzyme due to genetic mutation are unable to properly assess threat. The affected mice exhibited defensive behaviors in the presence of neutral stimuli, such as plastic bottles. Conversely, in the presence of true danger cues such as a rat, the mice with the enzyme mutation were less cautious and defensive than their littermates. Mice without the enzyme also took longer to leave an open chamber, indicating reduction in exploratory tendencies.
"Taken together, our findings suggest that monoamine oxidase A deficiency leads to a general inability to appropriately assess contextual risk, as indicated by the inappropriateness of their defensive behaviors," said study author Jean C. Shih.
Monoamine oxidase A is the main enzyme in the brain that breaks down serotonin, norepinephrine and dopamine, which have been shown to contribute to the "fight or flight" impulse by raising heart rates and increasing blood and oxygen flow. Previous research by Shih showed that deficiency in monoamine oxidase A causes aggression in mice and humans, but this study strongly suggests what was perceived as aggressiveness may more accurately be described as an inability to properly respond to environmental cues.
"Mice without monoamine oxidase A exhibited a distinct inability to attune their response to the situation," said co-author Sean Godar. "The paradoxical responses to neutral and fear-inducing stimuli are markedly reminiscent of deficits in facial affect processing in schizophrenia and autism."
Interestingly, the researchers found no significant differences in sensory ability between the mice with a monoamine oxidase A deficiency and their littermates - both groups found buried treats at about the same rate and were similarly able to traverse a ledge and recognize objects.
The researchers suggest that the strange defensive behavior exhibited by the enzyme-deficient mice may actually reflect a limited range of adaptive responses and lack of emotional flexibility - the mice may only have one gear for fear.