PAW PREFERENCE AND INTRA- INFRAPYRAMIDAL MOSSY FIBERS IN THE HIPPOCAMPUS OF THE MOUSE/

The size of the intra-/infrapyramidal messy fiber projections (IIP-MF) and their left/right asymmetry were assessed in 86 mice of either sex , including 26 animals from two mouse lines bred for strong or weak pa w preference, 38 mice of a randomly bred F-3 generation derived from a n eight-way cross, and 22 mice with variably sized corpora callosa in which only the left hippocampus was measured. Prior to morphometry, al l mice were tested for paw preference. In addition, we compared the st rain means in paw preference as observed in nine inbred mouse strains with known differences in their IIP-MF distribution. Mice bred for str ong paw preference had a 70% larger IIP-MF projection than weakly late ralized and dyscallosal mice; random-bred mice fell in-between the ext remes. The individual scores of the strength of paw preference were po sitively correlated with the extent of the IIP-MF Among the inbred str ains, the extent of the IIP-MF was similarly correlated with the stren gth of paw preference. The acallosal mice showed a significant negativ e correlation between extent of the IIP-MF projection and test-retest reliability of paw use. The left-right asymmetry of the IIP-MF was sig nificantly and positively correlated with the direction of paw prefere nce in the entire sample. We conclude that size and asymmetry of the I IP-MF projection are some of the many factors influencing the directio n of paw preference and its strength, albeit moderately. We hypothesiz e that mice with larger IIP-MF projections use a given paw more consis tently, being perhaps more resistant to interferences, and that left-r ight asymmetries of the IIP-MF may bias and/or reinforce an initial ch oice of a paw. In addition, the data provide another example of correl ations between IIP-MF variations and nonspatial behavior.