http://discovermagazine.com/2010/jun/15-brain-switches-that-can-turn-mental-illness-on-off
To figure out how licking had altered the rats, Meaney and his colleagues looked closely at the animals’ brains. They discovered major differences in the rats’ hippocampus, a part of the brain that helps organize memories. Neurons in the hippocampus regulate the response to stress hormones by making special receptors. When the receptors grab a hormone, the neurons respond by pumping out proteins that trigger a cascade of reactions. These reactions ripple through the brain and reach the adrenal glands, putting a brake on the production of stress hormones.
In order to make the hormone receptors, though, the hippocampus must first receive signals. Those signals switch on a series of genes, which finally cause neurons in the hippocampus to build the receptors. Meaney and his colleagues discovered something unusual in one of these genes, known as the glucocorticoid receptor gene: The stretch of DNA that serves as the switch for this gene was different in the rats that got a lot of licks, compared with the ones that did not. In the rats without much licking, the switch for the glucocorticoid receptor gene was capped by methyl groups, and the neurons in the underlicked rats did not produce as many receptors. The hippocampus neurons therefore were less sensitive to stress hormones and were less able to tamp down the animal’s stress response. As a result, the underlicked rats were permanently stressed out.
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