BDNF polymorphism and interhemispheric balance of motor cortex excitability: a preliminary study

Preclinical studies have demonstrated that brain-derived neurotrophic factor (BDNF) plays a crucial role in the homeostatic regulation of cortical excitability and excitation/inhibition balance. Using transcranial magnetic stimulation techniques, we investigated whether BDNF polymorphism could influence cortical excitability of the left and right primary motor cortex in healthy humans. Twenty-nine participants were recruited and genotyped for the presence of the BDNF Val66Met polymorphism, namely homozygous for the valine allele (Val/Val), heterozygotes (Val/Met), and homozygous for the methionine allele (Met/Met). Blinded to the latter, we evaluated inhibitory and facilitatory circuits of the left (LH) and right motor cortex (RH) by measuring resting (RMT) and active motor threshold (AMT), short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF). For each neurophysiological metric, we also considered the interhemispheric balance expressed by the laterality index (LI). Val/Val participants (n = 21) exhibited an overall higher excitability of the LH compared with the RH, as probed by lower motor thresholds, lower SICI, and higher ICF. Val/Val participants displayed positive LI, especially for AMT and ICF (all P < 0.05), indicating higher LH excitability and more pronounced interhemispheric excitability imbalance as compared with Met carriers. Our preliminary results suggest that BDNF Val66Met polymorphism might influence interhemispheric balance of motor cortex excitability. NEW & NOTEWORTHY BDNF Val66Met polymorphism might influence interhemispheric balance of motor cortex excitability. Specifically, Val/Val carriers display higher excitability of the left compared with the right primary motor cortex, whereas Met carriers do not show any significant corticomotor excitability imbalance. These preliminary results are relevant to understanding aberrant interhemispheric excitability and excitation/inhibition balance in neurological disorders.