Symptoms of depression
Depression, also known as clinical depression, major depression or major depressive disorder, is characterised by symptoms including anxiety, loss of interest in activities that are usually pleasurable, disturbed sleep, change in appetite, fatigue, feelings of worthlessness or guilt, difficulty thinking or concentrating, and recurrent thoughts of suicide. If these symptoms continue for more than two weeks this suggests that the depression is a clinical condition and not just feeling low. Once an episode of major depression has occurred there is a high risk of recurrence of another depressive episode. Mood enhancing medications are used to treat depressive episodes and are also used as maintenance against relapse or recurrence of depressive episode.
Mood enhancing neurotransmitters
Serotonin and noradrenaline are the two most important neurotransmitters in the area of the brain that regulates mood. When serotonin or noradrenaline is released from one nerve cell it passes across the gap between nerve cells (synapse) and fires up the next nerve cell. This sets up a communication signal that is transmitted from one nerve cell to another. If there is not sufficient serotonin or noradrenaline released to trigger this nerve signal, then this communication pathway does not work properly and can lead to symptoms of depression.
Inhibition of neurotransmitter reuptake
Both serotonin and noradrenaline are recycled after use by being taken back up into the presynaptic neurone or sending cell. Serotonin noradrenaline reuptake inhibitors (SNRIs) work by inhibiting the reuptake of both serotonin and noradrenaline, whereas the selective serotonin reuptake inhibitors (SSRI) are very selective for serotonin and do not affect other neurotransmitters. SNRIs also have a small effect on other neurotransmitters like dopamine. If neurotransmitter reuptake is inhibited, there is more serotonin or noradrenaline remaining in the synapse to trigger the next cell and continue communication between mood-enhancing neurones.
Tricyclic antidepressants also work by inhibiting neurotransmitter reuptake, however, they non-specific in their action as they also affect several other brain chemicals like histamine and acetylcholine, which accounts for their sedative properties and other side effects.
Reversible inhibitors of monoamine oxidase
Reversible inhibitors of monoamine oxidase (RIMA) antidepressants work by a different mechanism from other classes of antidepressant to boost the amount of mood enhancing neurotransmitters. Once they have done their job by stimulating a nerve impulse, neurotransmitters are removed and one way this is done is by being inactivated by the enzyme monoamine oxidase. Inhibition of monoamine oxidase results in more serotonin and noradrenaline remaining to trigger the next cell and continue communication between mood-enhancing neurones. RIMAs are short lived and their effects reversible so they do not have such serious side effects as earlier drugs.