BDNF and depressive symptom measurement: Upregulation of PDE4 induced by CACS may inhibit CREB-BDNF-mediated neurogenesis in the hippocampus, which is associated with the pathogenesis of depression and anxiety (O'Donnell and Zhang, 2004; Xu et al., 2011; Wang et al., 2014; Zhang et al., 2017).