Imaging activation of adult-generated granule cells in spatial memory

New neurons are continuously generated in the subgranular zone of the hippocampus throughout adulthood, and there is increasing interest as to whether these new neurons become functionally integrated into memory circuits. This protocol describes the immunohistochemical procedures to visualize the recruitment of new neurons into circuits supporting spatial memory in intact mice. To label adult-generated granule cells, mice are injected with the proliferation marker 5-bromo-2'-deoxyuridine (BrdU). At different delays after BrdU treatment, mice are trained to locate a hidden platform in the Morris water maze, and spatial memory can then be tested in a probe test with the platform removed from the pool. Ninety minutes after this probe test, mice are perfused and tissue is sectioned. Immunohistochemical procedures are used to quantify BrdU-labeled cells and expression of the immediate early gene, Fos. Because Fos expression is regulated by neuronal activity, the degree of overlap between BrdU-labeled and Fos-labeled neurons provides an indication of whether adult-generated granule neurons have been incorporated into spatial memory circuits.