Separable roles of hippocampal granule cells in forgetting and pyramidal cells in remembering spatial information.

To investigate the roles individual hippocampal cell groups play in processing of spatial information for memory, we administered low-intensity electrical stimulation to the granule cells, CA3 and CA1 pyramidal cells of the dorsal hippocampus at selected times before and after acquisition of the solution to a radial maze win-stay task. Stimulation of any of the 3 cells populations yielded a variable duration anterograde disruption of memory performance, while stimulation of dentate gyrus granule cells alone produced a declarative memory-specific retrograde amnesia. The amnestic effect of granule cell stimulation was not associated with after discharges in the hippocampus and was prevented by systemic administration of the opiate antagonist naloxone. Our results support the view that this electrical stimulus does not disrupt, but rather, activates the normal function of the granule cell system, resulting in erasure of information held in declarative memory. In contrast, similar activation of the pyramidal cell system does not yield retrograde amnesia, suggesting a normal role for these cells in promoting memory for spatial information.