According to MedlinePlus, approximately 19 million adults in the United States are affected by depression each year. Depression can cause an extremely low mood, as well as feelings of worthlessness and a loss of pleasure in everyday activities. Depression is considered to be a chronic medical condition because it can affect the brain.
Neurotransmitters
Depression's effects on the brain are best understood in the context of neurotransmitters. Neurotransmitters are chemicals that neurons make and secrete in order to communicate with other neurons throughout the brain. Neurotransmitters are secreted by one neuron into a inter-neuron space called a synapse. Once there the neurotransmitters can either activate or inhibit the other neuron in the synapse. According to MedlinePlus, depression is thought to be caused by abnormal levels of neurotransmitters which disrupt the balance of activity in the brain. In particular, depression is linked with abnormal neurotransmitter levels in the limbic system (controls emotions) and parts of the cortex (controls thoughts and consciousness). Although for many people depression can be linked to a single event that throws the brain's chemistry out of whack, in some people these chemical changes occur with no identifiable trigger.
Glutamate
According to the Mayo Clinic, one of the important brain chemicals involved in depression is glutamate. Glutamate is actually an amino acid, a substance whose major role is to make proteins. Glutamate turns off activity in some parts of the brain and activates other areas. In particular, glutamate is important in neural pathways that lead to depression and its symptoms. There is a "chicken and egg" type of debate as to whether depression causes abnormal activation of these neurons (in the cortex and limbic system) or if the activation of these pathways causes depression, but it is clear that glutamate-mediated signals are part of depression.
Monoamines
Another important set of neurotransmitters involved in depression is the monoamine class, which includes norepinephrine, dopamine and serotonin. These neurotransmitters in essence work to counteract the effects of glutamate. They are thought to work by binding to neurons and making them less sensitive to the effects of glutamate. This means that it is harder for the depression nerve signals to get sent out. It is also thought that depression causes patients to make less of these neurotransmitters, which is why most kinds of antidepressants work at increasing the levels of norepinephrine and/or serotonin to help counteract these changes. Most antidepressant pharmaceuticals focus on elevating levels of serotonin, norepinephrine or dopamine, or some combination of the three. One type of medication works by slowing the rate at which these neurotransmitters are pumped out of the synapse. These drugs are called selective serotonin reuptake inhibitors (SSRIs) or norepinephrine and dopamine reuptake inhibitors (NDRIs), depending on what neurotransmitters are affected. Other antidepressants, such as tricyclic antidepressants, decrease the pumping out of all three neurotransmitters, though these are less commonly used because they lead to more side effects. Regardless, most pharmaceutical treatments for depression target these monoamines in order to take advantage of their effects on glutamate signaling and its effects on mood and thoughts.
