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Val homozygote genotype may serve as a vulnerability factor in MDD regarding hippocampal volume loss. This finding can be considered as a supportive evidence for the neurotrophic factor hypothesis of depression.

Aim: Serotonin is known for its importance in the pathophysiology of major depressive disorder. Although the hippocampus is one of the key regions in which neurogenesis occurs, and serotonin plays an important role in neurogenesis, results of studies that investigate effect of the 5-HTTLPR polymorphism on hippocampal volumes in major depressive disorder are inconclusive. Method: We looked for a relationship between the 5-HTTLPR polymorphism and hippocampal volumes in 44 depressed patients (mean age ± SD 33.6 ± 9.5 years) and 43 healthy controls (30.4 ± 6.7 years). Region of interest analysis was conducted on the images acquired via MRI. Results: Although hippocampal volumes were similar in healthy and patient groups, there was a significant interaction between genotype and diagnosis on hippocampus volumes. Post-hoc ANCOVA showed that hippocampal volumes of S/S homozygous depressed patients were smaller compared to healthy controls in both hemispheres. Conclusion: The 5-HTTLPR polymorphism has an effect on hippocampal volumes of depressed patients, which is apparent only in S/S genotype. It seems that decreased neurogenesis by effects of reduced serotoninergic transmission may be responsible for smaller hippocampal volumes observed in S/S homozygous depressed patients.