Working memory task decreases the survival of newly born neurons in hippocampus

Throughout life new neurons are generated in dentate gyrus of hippocampus. Previous studies have found that spatial tasks can rescue newly born neurons from death. However, it is still unknown whether new neurons are similarly affected by all types of hippocampal-dependent tasks. Here we investigated the possible effects of working memory task (WMT) on immature neurons. Mice were trained in reference memory task and WMT respectively. The reference memory task used the classical hidden platform (HP) water maze task, while WMT used a delayed matching-to-place (DMTP) water maze task. Bromodeoxyuridine (BrdU) was administrated during the early or late phase of training, or 1week prior to training, in order to label dividing proliferating cells. After water maze training, the number of BrdU-labeled cells in dentate gyrus of hippocampus was compared. In addition, hippocampal brain-derived neurotrophic factor (BDNF) and Notch 1 receptor were characterized using Western blot. Serum corticosterone levels were also measured using enzyme immunoassay. Results showed that HP task and DMTP task did not change the number of BrdU-labeled cells produced during the early or late phase of training. As expected, the HP task increased the number of BrdU-labeled cells produced 1 week prior to training. However, DMTP task decreased the number of BrdU-labeled cells produced 1 week prior to training. Both tasks lead to a significant increase in serum corticosterone levels and did not change the expression of BDNF and Notch 1 receptor in hippocampus. Taken together, these results demonstrate that WMT has different effects on survival of immature neurons, and therefore suggests immature neurons may have more than one role depending on the demands of the tasks.