Genotyping
Peripheral whole blood samples were collected in BD Vacutainer lavender top blood collection tubes at baseline and were stored at − 80 °C. DNA was extracted from whole blood using an automated DNA extraction machine (AutoGen FLEX STAR, Hollisten, MA). Genotyping of the BDNFval66met polymorphism (GenBank dbSNP: rs6265) was performed with TaqMan SNP assay (C-11592758-10) in an optimized ABI 7300 (Applied Biosystems). Due to the low frequency of the Met/Met genotype in the population (less than 5%), the established convention in the field was followed and Met/Met and Val/Met participants were combined into a single group.