GCG and diabetes mellitus: Novel dual-GLP-1/GIP receptor agonists that have been developed to treat diabetes (Finan et al., 2013) or AD and PD (Hölscher, 2018) can cross the BBB at various degrees (Kastin et al., 2002; Kastin and Akerstrom, 2003; Li et al., 2020; Salameh et al., 2020; Zhang et al., 2020).