TCAs increase neurotransmitter levels by preventing nerve endings — called synapses — from drawing these chemicals back into their tissues, which is normally how the body reduces their concentrations. They increase levels of norepinephrine and serotonin, two neurotransmitters, and block the action of acetylcholine, another neurotransmitter, this helps nerve signalling in the brain, which can help relieve depression. Tricyclic antidepressants are most effective for people with severe depression compared with those with mild to moderate depression, but they are also given as alternatives to SSRIs (selective serotonin re-uptake inhibitors) and are sometimes used in panic disorder, obsessive compulsive disorder and the treatment of chronic pain.