BDNF and major depressive disorder: Given the evident interplay between the BDNF–TrkB and TGF-β1 systems, we suggest that the BDNF–TrkB system plays a role in the rapid-acting antidepressant-like actions of TGF-β1 (Fig. 2), and although further study of this interaction is needed, intranasal TGF-β1 administration could be a novel therapeutic approach for depression.