In the 1950s, the first antidepressants were discovered. Iproniazid is a monoamine oxidase inhibitor (MAOI), and imipramine is a tricyclic antidepressant (TCA). Their positive effects on depression were actually discovered by accident. Iproniazid was initially developed to treat tuberculosis, and imipramine was meant to be an antihistamine. MAOIs and TCAs work on the brain differently, but both were found to increase the brain’s serotonin levels. Perhaps, scientists thought, low levels of serotonin were what caused depression. This hypothesis and the fact that MAOIs and TCAs both had serious side effects led to the development of selective serotonin reuptake inhibitors (SSRIs).