BDNF and fragile X syndrome: On the same way and consistent with previous results linking cAMP with BDNF transcription [143], inhibition of PDEs has been reported to increase BDNF levels [52, 53, 85, 92, 103], whose deregulation is involved in the pathophysiology of depression [144], SCZ [133, 145], RTT [126, 146], and FXS [147].