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.