Two of the chemical messengers in the brain are called serotonin (sometimes called 5-HT) and noradrenaline. Both are important in the areas of the brain that control or regulate mood and thinking. It is known that these two chemical messengers are not as effective or active as normal in the brain of someone who is depressed.
Antidepressants increase the amount of these chemical messengers, helping to correct the lack of action of the messengers and improving mood.
Source: www.rethink.org