The antidepressants can be classified in two categories:

I. MAO inhibitors, mainly hydrazine derivatives. These substances inhibit the effects of the enzyme monoamine oxidase, which breaks down noradrenaline, adrenaline and serotonin, leading to an increase in the effects of these neurotransmitters. The first known MAO inhibitor was iproniazid, an INH derivative, which was synthesized in 1950 as tuberculostaticum. In 1952 it was established that this substance inhibits MAO, and in 1953 its positive effect among TB patients was discovered. Its positive effects on depressive patients were described in 1957. A series of similar substances were synthesized in the 1960s. The application of MAO inhibitors has remained limited because of the sideeffects. (note 80)

II. Reuptake inhibitors and release reinforcers, which thereby enhance the activity of dopamine, noradrenaline and serotonin. The best known antidepressants, the socalled tricyclic antidepressants, belong to this category: amitriptyline, nortriptyline, imipramine, etc. Their antidepressant capacity is primarily due to the fact that they increase the secretion of serotonin by serotonergic neurons. The tetracyclic antidepressants, such as mianserin, work on the same principle.

Recently substances have also been developed which inhibit the reuptake of serotonin, such as fluoxetine (Prozac). It is because of this effect that this substance has occasionally been marketed in Italy as an illegal drug under the name "bye bye blues". (note 81)

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