BDNF and Mental deterioration: It is noteworthy that abnormally high levels of adenosine in the brain are also associated with cognitive decline.[63, 64] This is probably because adenosine at a high level acts primarily on the A2R (rather than A1R) receptor to activate the PKC signaling and to suppress BDNF expression and synaptogenesis, thereby impairing hippocampal LTP and memory.[65] Therefore, we propose that hippocampal adenosine should be controlled to a fine range of ≈15.9–39.0 ng mL−1 for mental health.