SPATIAL CORRELATES OF FIRING PATTERNS OF SINGLE CELLS IN THE SUBICULUM OF THE FREELY MOVING RAT

Hippocampal lesions cause spatial learning deficits, and single hippoc ampal cells show location-specific firing patterns, known as place fie lds. This suggests the hippocampus plays a critical role in navigation by providing an ongoing indication of the animal's momentary spatial location. One question that has received little attention is how this locational signal is used by downstream brain regions to orchestrate a ctual navigational behavior. As a first step, we have examined the spa tial firing correlates of cells in the dorsal subiculum as rats naviga te in an open-field, pellet-searching task. The subiculum is one of th e few major output zones for the hippocampus, and it, in turn, project s to numerous other brain areas, each thought to be involved in variou s learning and memory functions. Most subicular cells showed a robust locational signal. The patterns observed were different from those in the hippocampus, however, in that cells tended to fire throughout much of the environment, but showed graded, location-related rate modulati on, such that there were some localized regions of high firing and oth er regions with relatively low firing. There were slight quantitative differences between the proximal (adjacent to the hippocampus) and dis tal (farther from the hippocampus) subicular regions, with distal cell s showing slightly higher average firing rates, spatial signaling, and firing field size. This was of interest since these two regions have different efferent connections. Examination of spike trains allowed cl assification of cells into bursting, nonbursting, and theta (putative interneuron) categories, and this is similar to subicular cell types i dentified in vitro. Interestingly, the bursting and nonbursting types did not differ detectably in spatial firing properties, suggesting tha t differences in intrinsic membrane properties do not necessitate diff erences in coding of environmental inputs. The results suggest that th e subiculum transmits a robust, highly distributed spatial signal to e ach of its projection areas, and that this signal is transmitted in bo th a bursting and nonbursting mode.