Age-related naturally occurring depression of hippocampal neurogenesis does not affect trace fear conditioning

New neuron production throughout adulthood in granule cell layer (GCL) of rat hippocampus is a well‐known phenomenon. A role of new neurons in hippocampal learning has been proposed, but the question is still open. A reduction of neural precursor proliferation in GCL of 2‐month‐old rats to about 20%, induced by the cytostatic agent methylazoxymethanol, was found to cause impairment in trace conditioning, suggesting a role of immature neurons in this kind of hippocampus‐dependent learning (Shors et al., Hippocampus 2002;12:578–584). Neurogenesis decreases with increasing age. In this study, neural precursor proliferation and newborn cell survival were evaluated in GCL of adult rats within a range of ages following development and preceding old age. In 5‐month‐old rats, neural precursor proliferation was reduced to 57% and newborn cell survival was reduced to 40% in comparison to rats of 2 months of age; in 12‐month‐old rats, the decrease was to 5 and 4%, respectively. Consistently, the density of immature neurons decreased to 41 and 13% in 5‐ and 12‐month‐old rats, respectively. The role of neurogenesis in trace fear conditioning was studied in this natural model of neurogenesis depression. No impairment in trace fear conditioning was found both in 5‐ and 12‐month‐old rats in comparison to 2‐month‐old rats, notwithstanding the decrease of neurogenesis that is marked in 12‐month‐old rats. This finding shows that a lower rate of neurogenesis is sufficient for learning in 12‐month‐old rats in comparison to young rats. © 2005 Wiley‐Liss, Inc.