[The biology of depression].

There are several arguments in favor of a biological cause of endogenous depressions coming from clinical experience, genetic studies and the pharmacology of antidepressants. Impaired impulse transmission across the synapses of noradrenergic and serotoninergic systems has been for 20 years considered to be the underlying defect. In contrast to earlier theories this defect consists more probably in an altered sensitivity of post-synaptic receptors than in reduced concentration of the amines. As neurotransmission is regulated in a complex way the interpretation of experimental results is often ambiguous. The noradrenergic medial forebrain bundle, part of the reward system, is of special importance for a biological theory of depression.