To understand how antidepressants work, you need to have a general understanding of how the cells communicate with each other in the brain. Within the brain, there are many cells called neurons. Messages pass from neuron to neuron using chemical messengers called neurotransmitters. The messages can pass on information about emotions, behavior, body temperature, appetite, or many other functions. The type of information sent depends on which neurons are activated and what part of the brain is stimulated.

A message passes from a sending neuron to a receiving neuron. The neurotransmitters leave the sending neuron and enter the space between the sending and receiving neurons. This space is called the synapse. The neurotransmitters then hook up to a receptor on the receiving neuron to deliver their message.

Once neurotransmitters have sent their message, they return and can be reabsorbed by the sending neuron in a process called reuptake. Reuptake allows the messengers to be reused. Two of these neurotransmitters are serotonin and norepinephrine. Low levels of serotonin and norepinephrine in the synapse are associated with depression and sadness. Some medications used to treat depression work by increasing the amount of certain neurotransmitters that are available to carry messages.

Each type of antidepressant works on brain chemistry a little differently. All antidepressant medications influence how certain neurotransmitters, especially serotonin and norepinephrine, work in the brain.

SSRIs and tricyclic antidepressants. Antidepressants, such as selective serotonin reuptake inhibitors, or SSRIs, and tricyclic antidepressants, work by slowing or blocking the sending neuron from taking back the released serotonin. In that way, more of this chemical is available in the synapse. The more of this neurotransmitter that is available, the more likely the message is received, and depression is reduced. To learn more about how these antidepressants work, see Tricyclic Antidepressants (TCAs) and Selective Serotonin Reuptake Inhibitors (SSRIs).

MAOIs. The antidepressants known as MAO inhibitors, or MAOIs, affect neurotransmitters differently. Monoamine oxidase (MAO) is a natural enzyme that breaks down neurotransmitters. The drug MAOI disrupts the action of the enzyme MAO. In that way, there is an increase in the amount of neurotransmitters in the synapse, making more messengers available to the receiving neuron, and thus reducing depression. To learn more about how these antidepressants work, see Monoamine Oxidase Inhibitors (MAOIs).