Summary
D-serine, which acts on a type of N-methyl-D-aspartate receptors (NMDARs) is thought to be important for neuronal plasticity. As people age they produce less D-serine, and it is hypothesized that this is one of the reasons why it becomes more difficult to learn as we age. D-serine can be absorbed in the digestive tract and it can cross the blood-brain barrier. It is hypothesized that the D-serine levels of the aged brain can be restored via oral supplementation, reversing the cognitive decline to a degree.
The experiment used to test age-related learning deficit is the following: a rat is trained to press one of two levers to receive food. Then, the experiments switch the correct lever, such that now the rat needs to press the other lever to get the reward. The cognitive flexibility is tested by measuring how long it changes for the rat to begin pressing the other lever. Young rats learn to press the other lever quickly, while older rats continue pressing the old lever for longer. When supplemented with D-serine, the ability of the old rats to re-learn which lever to press improves dramatically. They show that D-serine supplementation has no effect on the young rats' performance, and they show that this effect is dose dependent, supporting their original hypothesis that D-serine supplementation can restore some of the brain's plasticity that is lost as the rat ages.