The oldest types of antidepressants (MAOIs and tricyclics) were developed in the 1950s and 1960s. The next types of antidepressants (SSRIs, SNRIs and NASSAs) were developed in the 1980s and 1990s.
The first antidepressant was isoniazid, an MAOI, which was actually designed as a treatment for tuberculosis. However it was accidentally found to have euphoric effects on patients, some of whom showed increased vigour and appetite, weight gain, improved sleep, and sociability. This discovery led to the development of a range of other MAOI antidepressants including isocarboxazid, phenelzine and tranylcypramine.
The next group of antidepressants to be discovered accidentally were tricyclics, drugs that were actually designed to work as antihistamines. The first tricyclic antidepressant was imipramine which was followed by a range of other tricyclics including amitriptyline, desipramine and trimipramine.
The comparable effects of MAOIs and tricyclics led researchers to suggest that these drugs shared a common mode of action, that is, they both led to an increased availability of serotonin in the brain.
The first SSRI was zimeldine, a drug specifically designed to increase the production of serotonin within the brain. It was soon followed by other SSRIs including citalopram, fluoxetine and sertraline.
The first SNRIs, were venlafaxine and duloxetine, drugs specifically designed to block the absorption of serotonin within the brain. The first NASSA was mirtazapine, a drug specifically designed to block the absorption of noradrenaline and serotonin in the brain.