Mice shed light on fearless behaviour

US scientists have found that mice lacking one copy of a gene involved in brain development take more risks than normal mice, and are less able to form 'emotional' memories. The study, carried out by researchers at the Fred Hutchinson Cancer Research Center, shows that the neuroD2 gene is crucial for the growth of the amygdala - the brain region that controls the fear response. The findings, published online in the Proceedings of the National Academy of Sciences, may help explain why some people are more likely to be 'risk-takers' than others.

The amygdala is a small, almond-shaped area of the brain responsible for processing fear and emotion. 'Most of us are familiar with the fact that we can remember things better if those memories are formed at a time when there is a strong emotional impact - times when we are frightened, angry or falling in love', said team leader James Olson, adding 'that's emotional-memory formation'.

Olson and his colleagues firstly showed that in mice with no copies of the neuroD2 gene, the amygdala doesn't form properly, and the animals die shortly after birth. The team then studied mice with just one copy of the neuroD2 gene, and found that they had fewer neurons in their amygdala than normal mice. The scientists carried out experiments to see if the missing gene affected the fear-processing ability of the animals. They exposed mice to a noise followed by a mild foot shock, a procedure that that makes normal mice crouch down and not move next time they hear the noise. However, the mice missing a copy of neuroD2 didn't freeze in anticipation of the shock.

In another experiment, the scientists placed the mice in an elevated maze and gave them the choice of walking along either unprotected walkways, or ones with protective walls. The neuroD2 deficient mice chose the unprotected routes, while normal mice almost always stuck to the protected ones. Olson said that the neuroD2-deficient mice 'show a profound difference in unconditioned anxiety levels as well as their ability to form emotional memories'. He added that more research is needed to see if variation in the human version of neuroD2 affects human brain development and behaviour.