Task difficulty in the Morris water task influences the survival of new neurons in the dentate gyrus

Adult neurogenesis continues throughout life in the mammalian hippocampus. The precise function of the adult generated neurons remains uncertain although there is growing evidence that they are involved in hippocampus-dependent learning and memory. Training rats on a hidden platform version of the Morris water task has been shown to increase or decrease the survival of newly produced cells in the dentate gyrus (DG) compared to training on a visible platform version. Here we investigated whether the difficulty of the task is related to the degree or direction of the change in neurogenesis. We trained rats on either a visible platform version of the Morris water task or one of three different hidden platform paradigms: four training trials per session version, two training trials per session, and reduced-cue (a version in which the majority of the distal cues were removed from the room). BrdU was administered 6 days prior to training and rats were perfused 24 h after the last training session. As expected, training on the four trial hidden platform version increased cell survival compared to training on the visible platform version. However, training on the more difficult reduced-cue hidden platform version resulted in a decrease in cell survival. Rats that received fewer trials per session did not differ in terms of cell survival in comparison to rats trained on the visible platform version. These findings demonstrate that altering the difficulty of the spatial task has an impact on the corresponding change in cell survival. The lack of obvious distal cues likely changed the strategy used by the rats to determine the location of the platform and resulted in a decrease, instead of an increase in cell survival in the hippocampus. In conclusion, different types of hippocampus-dependent learning can differentially impact cell survival.