Traditional antidepressants like Prozac work on a group of chemical messengers in the brain called the serotonin system. Researchers once thought that a lack of serotonin was the cause of depression, and that these drugs worked simply by boosting serotonin levels. Recent research suggests a more complicated explanation. Serotonin drugs work by stimulating the birth of new neurons, which eventually form new connections in the brain. Ketamine, in contrast, activates a different chemical system in the brain - the glutamate system. Researcher Ron Duman at Yale thinks ketamine rapidly increases the communication among existing neurons by creating new connections. This is a quicker process than waiting for new neurons to form and accomplishes the same goal of enhancing brain circuit activity. Ketamine has been used for decades as an anesthetic. It also has become a wildly popular but illegal club drug known as Special K." Mental health researchers got interested in ketamine because of reports that it could make depression vanish almost instantly. Carlos Zarate ... does ketamine research at the NIH. Zarate says patients typically say, "'I feel that something's lifted or feel that I've never been depressed in my life. I feel I can work. I feel I can contribute to society.' And it was a different experience from feeling high. This was feeling that something has been removed."