This group is one of three main types of drugs used to treat DEPRESSION, and was the first to be introduced (in the 1950s). Tricyclic drugs work by blocking the re-uptake of the neurotransmitters SEROTONIN and NORADRENALINE (see NEUROTRANSMITTER), thus increasing the amount of the neurotransmitters at the nerve cell's receptors. In people with depression, fewer neurotransmitters than normal are released, resulting in a slowing of neural activities. The drugs have a sedative effect, which can be useful for depressives with sleep problems, and an ANTIMUSCARINE action which can cause dry mouth and constipation. Overdosage can produce COMA, fits (see SEIZURE) and irregular heart rhythm (ARRHYTHMIA). (See also ANTIDEPRESSANT DRUGS.)