The tightly regulated balance between secretion and removal of neurotransmitters is not functioning properly in certain mental conditions like bipolar disorder, obsessive-compulsive disorder, anxiety, and depression. Neurotransmitters are signaling molecules used to transmit messages between neurons (nerve cells) in the brain. Serotonin is one of the neurotransmitters affected in depression and similar disorders. The most common class of drugs for the treatment of these conditions is called selective serotonin reuptake inhibitors (SSRI). The well-known Fluoxetine (Prozac) is a member of this class.
SSRIs work on the serotonin balance by inhibiting a transporter called SERT that selectively pumps serotonin back into the neurons. The action of the SERT molecule shortens the time that serotonin has to deliver its signal, inhibiting SERT therefore increases serotonin function. How exactly SSRIs inhibits SERT has not been clear until now, even though researchers knew that the mechanism cannot just be a simple binding to SERT and thus preventing SERT from binding to serotonin. If this were the case, the action of the drugs should kick in fully at latest a few days after the patient started taking them. One of the most perplexing properties about SSRIs, however, is that it can take weeks to months before they are fully active. Researchers therefore believed since a long time that SSRIs inhibit the SERT in a different way. Still, the exact mechanism was not known until now.