Biochemical factorsDepression is a type of mood disorder that some believe is triggered when neurotransmitters in the brain are out of balance. Neurotransmitters are chemical messengers that help the brain communicate with other parts of the body. These chemicals help regulate many physiological functions.Low levels of neurotransmitters may play a role in why some people are more susceptible to depression, including the neurotransmitters:serotoninnorepinephrinedopamine