Chronic BDNF deficiency leads to an age-dependent impairment in spatial learning

• Age-dependent deficit in Morris water maze learning in heterozygous BDNF-KO mice.
• Correlation of hippocampal BDNF content with learning impairment.
• No effects of reduced BDNF on weight gain, motor skills and short-term memory in BDNF+/−.

Brain-derived neurotrophic factor (BDNF) is a crucial mediator of neural plasticity and, consequently, of memory formation. In hippocampus-dependent learning tasks BDNF also seems to play an essential role. However, there are conflicting results concerning the spatial learning ability of aging BDNF+/− mice in the Morris water maze paradigm. To evaluate the effect of chronic BDNF deficiency in the hippocampus on spatial learning throughout life, we conducted a comprehensive study to test differently aged BDNF+/− mice and their wild type littermates in the Morris water maze and to subsequently quantify their hippocampal BDNF protein levels as well as expression levels of TrkB receptors. We observed an age-dependent learning deficit in BDNF+/− animals, starting at seven months of age, despite stable hippocampal BDNF protein expression and continual decline of TrkB receptor expression throughout aging. Furthermore, we detected a positive correlation between hippocampal BDNF protein levels and learning performance during the probe trial in animals that showed a good learning performance during the long-term memory test.