BDNF and peripheral nerve injury: Studies in animal models have demonstrated that in a relatively early stage of neuropathic pain following peripheral nerve injury, descending noradrenergic pathways exert an effective inhibition against mechanical and thermal hypersensitivity by increasing brain-derived neurotrophic factor (BDNF) levels (Hayashida et al., 2008; Hayashida and Eisenach, 2010).