NEONATAL HIPPOCAMPAL DAMAGE IN RATS - LONG-TERM SPATIAL MEMORY DEFICITS AND ASSOCIATIONS WITH MAGNITUDE OF HIPPOCAMPAL DAMAGE

This study investigated the effects of neonatal hippocampal ablation o n the development of spatial learning and memory abilities in rats. Ne wborn rats sustained bilateral electrolytic lesions of the hippocampus or were sham-operated on postnatal day 1 (PN1). At PN20-25, PN50-55, or PN90-95, separate groups of rats were tested in a Morris water maze on a visible ''cue'' condition (visible platform in a fixed location of the maze), a spatial ''place'' condition (submerged platform in a f ixed location), or a no-contingency ''random'' condition (submerged pl atform in a random location). Rats were tested for 6 consecutive days, with 12 acquisition trials and 1 retention (probe) trial per day. Dur ing acquisition trials, the rat's latency to escape the maze was recor ded. During retention trials (last trial for each day, no escape platf orm available), the total time the rat spent in the probe quadrant was recorded. Data from rats with hippocampal lesions tested as infants ( PN20-25) or as adults (PN50-55 and PN90-95) converged across measures to reveal that 1) spatial (place) memory deficits were evident through out developmental testing, suggesting that the deficits in spatial mem ory were long-lasting, if not permanent, and 2) behavioral performance measures under the spatial (place) condition were significantly corre lated with total volume of hippocampal tissue damage, and with volume of damage to the right and anterior hippocampal regions. These results support the hypothesis that hippocampal integrity is important for th e normal development of spatial learning and memory functions, and sho w that other brain structures do not assume hippocampal-spatial memory functions when the hippocampus is damaged during the neonatal period (even when testing is not begun until adulthood). Thus, neonatal hippo campal damage in rats may serve as a rodent model for assessing treatm ent strategies (e.g., pharmacological) relevant to human perinatal bra in injury and developmental disabilities within the learning and memor y realm. (C) 1997 Wiley-Liss, Inc.