BDNF and Cognitive impairment: Previous studies have found significant correlations between hyperglycemia and DPP4 activity, BDNF and cognitive impairment (Krabbe et al., 2007; Luchsinger et al., 2007; Zheng et al., 2014), to exclude the possible influences of hyperglycemia on DPP4 activity, BDNF and cognitive function.