'A new role for new neurons' contribution of adult-generated granule cells to dentate gyrus network physiology in vivo

Throughout the adult life of most mammals, new neurons are generated and integrate into the dentate gyrus of the hippocampus. Recent work has documented specific cognitive deficits after elimination of adult hippocampal neurogenesis in rodents, suggesting that these neurons may contribute to information processing in hippocampal circuits. Young adult-born neurons exhibit enhanced excitability and have altered capacity for synaptic plasticity. Still, little is known about the effect of adult-born granule cells on hippocampal activity in vivo. In order to assess the impact of these new neurons on neural circuits in the dentate, I recorded perforant-path evoked responses and spontaneous network activity from the dentate gyrus of urethane-anesthetized mice whose hippocampus had been focally X-irradiated to eliminate the population of young adult-born granule cells. After X-irradiation, perforant-path responses were reduced in magnitude. In contrast, there was a marked increase in the amplitude of spontaneous gamma-frequency bursts in the dentate gyrus and hilus, as well as increased synchronization of dentate neuron firing to these bursts. These data suggest that young neurons may have the effect of inhibiting or destabilizing recurrent network activity in the dentate and hilus. This unexpected result yields a new perspective on how a modest number of young adult-generated granule cells may function to modulate activity in the larger population of mature granule cells, rather than acting solely as independent encoding units.