Neurons and Neurotransmitters
The brain consists of a series of interconnected cells (called neurons) as well as the cells that protect and support the neurons. Neurons are responsible for the activity in the nervous system, which means that in the brain they control cognition, movement and emotions. Neurons communicate with each other via small spaces between neurons called synapses. Neurons use chemical signals, called neurotransmitters, to communicate across synapses. When a neuron gets stimulated and wants to pass on a signal to another neuron, it releases neurotransmitters into the synapse. Other neurons in the synapse then get stimulated or inhibited by these neurotransmitters.
Depression and Neurotransmitters
Scientists are still studying what exactly causes depression. MedicineNet notes that our understanding of how the human brain works focuses in large part on neurotransmitter levels. It is well established that certain disorders are associated with abnormal levels of certain neurotransmitters. Depression is often linked to abnormally low levels of the the neurotransmitters serotonin, dopamine and norepinephrine, which all primarily work to stimulate neurons in the brain. Although the causal relationship between depression and these neurotransmitters has not been established (meaning that researchers can't prove that abnormal neurotransmitter levels directly cause depression, only that the two are associated), most antidepressants work by increasing the levels of serotonin, norepinephrine and dopamine in the brain.
Antidepressant Drugs
As the Virtual Chembook at Elmhurst College explains, antidepressant drugs work by either slowing the rate at which certain neurotransmitters are pumped out of the synapses or by reducing the speed with which they are broken down. One class of drug, called selective serotonin reuptake inhibitors, keeps serotonin from being pumped out of the synapse, allowing it to stimulate neurons for longer. Tricyclic antidepressants work in a similar fashion but primarily block the pumping out of norepinephrine, though many of them also increase how long serotonin and dopamine are in the synapse as well. Finally, monoamine oxidase inhibitors work by blocking the enzymes that break down serotonin, dopamine, and norepinephrine, which keeps these neurotransmitters in the synapse for longer.
