INTRODUCTION:

The major depressive disorder (MDD) arises from the interaction of environmental, genetic and epigenetic factors, producing a deficit in monoaminergic transmission within the brain. However, our understanding of its pathophysiology is quite limited.

OBJECTIVE:

To reach an integrative view of the MDD pathophysiology, as well as the mechanisms of action of antidepressant drugs.

METHOD:

We used the PubMed database to search for the documents by using the appropriate key words. Most of them are experimental research and molecular genetics and brain imaging studies in humans.

RESULTS:

The pathophysiology of MDD is characterized by: i) shrinkage of the cingulate anterior cortex; ii) hyper-metabolism of the Cg25 area; iii) lower expression of the 5-HT1A receptor; iv) enhanced expression of monoamine oxidase A. Besides, certain gene polymorphisms are strongly linked to the pathophysiology, and there is evidence that 5-HT1A receptor expression is reduced by psychological stress. Antidepressants reverse the hyper-metabolic state of Cg25, stimulate neurogenesis and the cAMP pathway. We found that imipramine increases and reduces the expression of $G_{\alpha s}$ and $G_{\alpha z}$, respectively (data no published).

DISCUSSION AND CONCLUSION:

The disruption in monoaminergic transmission could be mediated by: i) the G1463A hTPH2 polymorphism that reduces the serotonin synthesis; ii) the C(-1019)G 5-HT1A polymorphism that increases the receptor expression in the dorsal rafe, and reduces serotonin release; iii) an increase in monoamine degradation. The reduced 5-HT1A expression is discussed considering its inhibitory properties in the prefrontal cortex. The effects of imipramine on $G_{\alpha s}$ and $G_{\alpha z}$ are in agreement with the antidepressant-induced stimulation of the cAMP pathway.

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