Depression is believed to stem from a chemical imbalance in the brain, which can be addressed by medication. Alterations are made by changing the levels of different neurotransmitters – chemicals which conduct electrical impulses between neurons – in the brain. It is these chemicals that regulate or control a number of your bodily functions, including your mood. Experts believe that depression is caused by a lack of the neurotransmitters serotonin and noradrenaline.