these substances work by stimulating the brain's natural activity. There are three main classes of anti-depressant drug. Monoamine oxidase inhibitors (MAOIs) which block the breakdown of the neurotransmitter noradrenaline, increasing its level in the brain. Tricyclic antidepressants (TCAs). These acted the same way as MAOIs, to increase levels of noradrenaline, but don't have the same dangerous side effects. The newest type of anti-depressant drug includes the drug Prozac It works by correcting the balance of a neurotransmitter called serotonin.