BDNF and atrial fibrillation: The neurotrophic factors and/or neurotransmitters can also be upregulated by TNF‐α.41 To investigate this, human AF cells were exposed to Link N for 48 hours in the presence of TNF‐α, and relative gene expression was evaluated for NGF, BDNF, SP, and their receptors (Figures 6 and 7).