Val66Met in brain-derived neurotrophic factor affects stimulus-induced plasticity in the human pharyngeal motor cortex

Background & aims: Polymorphisms in brain-derived neurotrophic factor (BDNF) can affect brain and behavioral responses. However, little is known about the effects of a single nucleotide polymorphism (SNP) in BDNF, at codon 66 (the Val-Met substitution, detected in approximately 33% of the Caucasian population) on stimulation-induced plasticity in the cortico-bulbar system. We examined whether this SNP influenced outcomes of different forms of neurostimulation applied to the pharyngeal motor cortex.

Methods: Thirty-eight healthy volunteers were assessed for corticobulbar excitability after single-pulse, transcranial magnetic stimulation of induced pharyngeal electromyographic responses, recorded from a swallowed intraluminal catheter. Thereafter, volunteers were conditioned with pharyngeal electrical stimulation, or 2 forms of repetitive (1 and 5 Hz) transcranial magnetic stimulation (rTMS). Repeated measurements of pharyngeal motor-evoked potentials were assessed with transcranial magnetic stimulation for as long as 1 hour after the 3 forms of neurostimulation and correlated with SNPs at codon 66 of BDNF (encoding Val or Met).

Results: Pharyngeal electrical stimulation significantly increased the amplitude of motor-evoked potentials in individuals with the SNP that encoded Val66, compared to those that encoded Met66, with a strong GENOTYPE*TIME interaction (F₈,₁₁₂ = 2.4; P = .018). By contrast, there was a significant reduction in latencies of subjects with the SNP that encoded Met66 after 5-Hz rTMS (F₃,₆₀ = 4.9; P = .04). In addition, the expected inhibitory effect of 1-Hz rTMS on amplitude was not observed in subjects with the SNP that encoded Met66 in BDNF (F₇,₁₄₀ = 2.23; P = .035).

Conclusions: An SNP in human BDNF at codon 66 affects plasticity of the pharyngeal cortex to different forms of neurostimulation. Genetic analysis might help select specific forms of neurostimulation as therapeutics for patients with disorders such as dysphagic stroke.