BDNF and Alzheimer disease: Apelin-13 (i.c.v.)alleviated memory performance deficits in a rat model of chronic water-immersion restraint stress (Shen et al., 2019) and cognitive deficits in a streptozotocin-induced rat model of Alzheimer’s disease (Luo et al., 2019), each of which was mediated by brain-derived neurotrophic factor (BDNF) signaling.