Genetic increase in brain-derived neurotrophic factor levels enhances learning and memory.

Brain-derived neurotrophic factor (BDNF), a neurotrophin, is known to promote neuronal differentiation stimulating neurite outgrowth in the developing CNS, and is also known to modulate synaptic plasticity, thereby contributing to learning and memory in the mature brain. Here, we investigated the role of increased levels of intracerebral BDNF in learning and memory function. Using genetically engineered transgenic BDNF overexpressing mice (RTG-BDNF), young adult, homozygous (+/+), heterozygous (+/-), or wild-type (-/-) littermates, we analyzed escape latency to a hidden-platform and swimming velocity in the Morris Water Maze test (MWM) with modifications for the mice. The MWM comprised 4 trials per day over 5 consecutive days (sessions) without prior or subsequent training. In a separate set of animals, BDNF protein levels in the cortex, thalamostriatum and the hippocampus were measured quantitatively using ELISA. In the BDNF (+/-) mice, the BDNF levels in the cortex, the thalamostriatum and the hippocampus were significantly high, compared to the wild-type littermates; 238%, 158%, and 171%, respectively (P<0.01, one-way ANOVA and a post-hoc test in each region). The BDNF levels in the BDNF (+/+) mice were not elevated. The BDNF (+/-), but not the (+/+) mice, demonstrated significantly shorter escape latency, shorter total path length in the MWM, and more frequent arrivals at the location where the platform had been placed previously in the probe trial, compared with the wild-type littermates (P<0.05, at each time pint). Because the maximum swimming velocity was not affected in the BDNF-transgenic mice, increased BDNF levels in the brain were found to enhance spatial learning and memory function. Although it has been postulated that excessive BDNF is deteriorating for neuronal survival or neurite outgrowth, further investigations are needed to clarify the mechanism of paradoxical lack of increase in BDNF levels in the (+/+) mouse brain.