Some people are naturally more anxious than others.
How anxiously we react to threat or adversity is part of our personality. This characteristic is called trait anxiety, and those with high trait anxiety are more prone to mental disorders such as depression, substance abuse and psychosis. Trait anxiety is heritable, with genes explaining much of the variability between individuals.
Oler et al have investigated genetic effects on the activity of brain regions that mediate trait anxiety.
The subjects of this study were rhesus monkeys, from several generations of a single-family pedigree – at an average age corresponding to that of humans just before puberty. The authors exposed the animals to a human intruder, a social-threat procedure that reveals an anxiety trait. They found, by examining both blood levels of the stress hormone cortisol and behaviours such as ‘freezing: that some monkeys reacted with high anxiety, and others with less.
Oler et al. also measured metabolic activity in the brain by injecting the monkeys with 18FDG, a radioactive analogue of glucose that is taken up and trapped in nerve cells according to their activity at the time of exposure to the social threat. The authors then anaesthetized the monkeys in order to image, using positron emission tomography, a ‘snapshot’ of regional brain metabolism during the preceding stress procedure.
The results indicate that, in anxious monkeys, brain activity is higher in a variety of areas, but most prominently in two key signalling structures for negative emotion, the amygdala and the anterior hippocampus. Activity in these two structures explained a sizeable proportion of the variance in anxiety behaviour from monkey to monkey
Much research in anxiety has focused on the amygdala , which signals environmental danger and triggers ‘fight-or-flight’ responses. But extensive evidence also links the anterior hippo¬campus, an essential structure for ‘declarative’ memory – to anxious behaviour and trait anxiety. Furthermore, there are strong interactions between the amygdala and hippocampus, which mediate emotional memory.
Activity in the anterior hippocampus is under greater genetic influence but found no significant heritability in the amygdala.

Oler et al 2010 Amygdala and hippocampal substrates of anxious temperament differ in their heritability vol 466 pp 864-868.
Meyer-Lindenberg 2010 Genes and the anxious brain Nature vol 466 pp827-8

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