The seasonal pattern of cell proliferation and neuron number in the dentate gyrus of wild adult eastern grey squirrels

The dentate gyrus is one of two areas in the mammalian brain that produces neurons in adulthood. Neurogenesis (proliferation, survival, and differenti ation of new neurons) is regulated by experience, and increased neurogenesi s appears to be correlated with improved spatial learning in mammals and bi rds. We tested the hypothesis that in long-lived mammals that scatter-hoard food, seasonal variations in spatial memory processing (i.e. increased pro cessing during caching season in the autumn) might correlate with changes i n neurogenesis and neuron number in the granule cell layer of the dentate g yrus (gcl DG). We investigated the rate of cell proliferation and the total number of neurons in the granule cell layer of wild adult eastern grey squ irrels (Sciurus carolinensis) at three different times of the year (October , January and June). We found no seasonal differences in cell proliferation rate or in total neuron number in the granule cell layer. Our findings are in agreement with those of previous studies in laboratory mice and rats, a nd in free-ranging, food-caching, black-capped chickadees, as well as with current hypotheses regarding the relationship between neurogenesis and lear ning. Our results, however, are also in agreement with the hypothesis that neurogenesis in the dentate gyrus represents a maintenance system that may be regulated by environmental factors, and that changes in total neuron num ber previously reported in rodents represent developmental changes rather t han adult plasticity. The patterns observed in mature wild rodents, such as free-ranging squirrels, may represent more accurately the extent of hippoc ampal plasticity in adult mammals.