BDNF and Alzheimer disease: Mairin, also named as betulinic acid, that is, a pentacyclic triterpenoid, could prevent Aβ/streptozotocin-induced spatial and passive avoidance memory deficits and reduce Aβ fibril plaques in the hippocampus region of the AD rat model through protecting microcirculation, alleviating inflammation, and upregulating BDNF expression [26, 27].