Only half of depressed individuals who take antidepressants actually get a mood lift. And now scientists think they know why: A study in mice found receptors on certain brain cells essentially block the effects of these medicines. If the same holds true in humans, the other 50 percent of depressed patients may have more effective treatment options. ‘The mouse model explains why someone may not respond to antidepressants,’ says Rene Hen, professor of pharmacology in the Departments of Psychiatry and Neuroscience at Columbia University. Antidepressants are designed to increase the levels of serotonin, so that when more of the serotonin neurotransmitter is sent to other parts of the brain, the person feels relief from depression. Hen identified a receptor, so he could replicate in mice what happens when antidepressants fail. Some of the genetically engineered mice were designed to have high levels of the receptor 1A, a type of receptor on nerve cells that produces serotonin. By watching their behavior, Hen determined how the mice responded to the drugs. Usually when mice take antidepressants, they act more daring. However, mice with high levels of certain serotonin receptors, did not act like they were on antidepressants. […]