BDNF and Cognitive impairment: Of importance in this review is a report that some of the steroids, such as the enantiomer of 17β-estradiol, may modulate prosurvival signaling pathways that, for example, induce an increase in brain-derived neurotrophic factor (BDNF) levels, which not only improves learning and memory, but importantly, reverses cognitive impairments caused by general anesthetics [13,51,52,53,54].