Abstracts

The hippocampal formation is important for spatial learning and memory. Many individual pyramidal cells of CA1 and CA3 in normal rats act as place cells. Each place cell discharges rapidly only when the rat[rsquo]s head is in a cell-specific part of the environment called the firing field. Firing fields are stable over long times (weeks or months) in a constant environment, implying that the across-cell representation is remembered and not created de novo each time the rat enters the environment. Previous studies have shown that immature rats are capable of learning visual-spatial tasks, such as the hidden platform in the water maze, as early as postnatal day (P) 20. However, the performance of young rats in the water maze is impaired compared to adult rats. Previous reports have found that reliable place cells do not occur prior to P50 raising the possibility that the poor specificity and stability of hippocampal location firing may account for the poor visual-spatial skills seen in these animals. The goal of this study was to determine whether young animals have reproducible place cells. Rats had electrode placement at P25 (n = 5) or P70 (n = 7). A movable array of sixteen 25 micron electrodes was used. Place cells were recorded at P31-P38, P48, or P80. Well-delineated place cells were recorded as early as P31. Place cells were recorded at P31-P38 (n = 25 cells), P48 (n = 9 cells), and P80 (n =21 cells). There were no differences in center rate, field rate or field area in the three groups. However, the coherences were lower in the immature rats than the adults demonstrating the precision of the firing field was higher in adults compared to the immature rats. Stability of the cells was lower in the immature rats with significant differences in angular displacement among the three groups. While similarity scores increased with age, the differences were not statistically different. When angular displacement and similarity scores were compared after 6 hour in the P31-38 and P80 recordings the angular displacement and similarity scores were significantly longer in the in younger rats (Angular displacement P31-P38: 61.57[plusmn]10.66, P80 8.40[plusmn]2.30, p[lt]0.001; Similarity score P31-P38: 0.494[plusmn]0.052, P80 0.620[plusmn]0.030, p = 0.036). In this study we demonstrated that well-formed place cells can be recorded in very young rats. When we utilized a small chamber we found pace cells were easy to record and were quite robust in their firing patterns. However, we found place cells in young rats had reduced coherences, a measure of the local smoothness of spatial firing patterns. In addition, stability of place cells as measured by angular displacement and similarity score, was reduced in the younger animals compared to older animals. (Supported by National Institutes of Health (NINDS)(NS044296).)