HIPPOCAMPAL MESSY FIBERS AND SWIMMING NAVIGATION IN MICE - CORRELATIONS WITH SIZE AND LEFT-RIGHT ASYMMETRIES

Individual differences in the extent of the infrapyramidal messy fiber projection (IIP-MF) correlate with performance in tasks sensitive to hippocampal lesions, notably two-way avoidance, radial maze learning, and swimming navigation. Previous studies of swimming navigation sugge sted that the capacity of reversal learning and measures of directiona lity might also be related to asymmetries in the distribution of the I IP-MF. In order to verify these findings, the authors crossed the Coll ins High- and Low-lateralized mice (known to differ in messy fiber mor phology and brain asymmetries) and obtained a F-2-generation character ized by strong individual differences in these traits. Twenty-three (1 3 females, 10 males) mice were tested during 3 days for acquisition of swimming navigation (16 trials) toward a central platform, and during two days (12 trials) for their capacity of reversal learning toward a shifted platform. Morphometry of Timm-stained hippocampi revealed sev eral, partially independent correlations: Larger IIP-MF projections we re associated with prolonged crossing over the former platform positio n during the entire reversal learning; larger IIP-MF projections on th e left were correlated with more precise crossing of the former platfo rm position during the first 45 seconds of reversal learning; both ext ent and asymmetry of IIP-MF correlated positively with overnight impro vement of reversal learning; the size of the entire messy fiber projec tion (CA4, suprapyramidal and IIP-MF) correlated positively with the t ime spent in the platform quadrant and measures of initial orientation during acquisition of the task; and the mice showed an ipsilateral tu rning bias (spin) toward the side with the larger messy fiber projecti on. The authors conclude that an intact hippocampus mediates different ial processes underlying swimming navigation, and that left and right subfields may have differential functions.