Effect of neonatal dentate gyrus lesion on allothetic and idiothetic navigation in rats

Goal-directed navigation is believed to be the combined product of idiothet ic and allothetic orientation. Although both navigation systems require the hippocampal formation, it is probable that different circuits implement th em. Examination of Long-Evans rats with dentate gyrus lesions induced by ne onatal X-ray irradiation may show the dissociation of these two components of navigation. Two recently developed place avoidance tasks on a rotating c ircular arena were used to test this hypothesis. in the first test, the pos ition of the punished area is stable in the room frame but is permanently c hanging on the surface of the arena. This task requires the rat to use allo thetic orientation and to disregard idiothetic orientation. In the second t est, the prohibited area is fixed in the coordinate system of the arena and the experiment is conducted in complete darkness, forcing the rat to rely exclusively on idiothesis supported by substratal cues. The results suggest that the dentate gyrus lesion interferes less with idiothetic orientation than with allothetic orientation. Ln addition, an attempt was made to contr ol the number of developing granule cells by exact timing of a single high dose of perinatal irradiation, and to measure the ensuing behavioral defici ts. Rats irradiated at 6, 18, or 24 h after birth were tested as adults in the Morris water maze. Irradiated animals showed significant, but highly va riable, learning deficit, but histological examination indicated that the g ranule cell loss did not correlate with the degree of behavioral impairment . (C) 2001 Academic Press.