Paw preference and intra-/infrapyramidal mossy fibers in the hippocampus of the mouse.

The size of the intra-/infrapyramidal mossy 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 paw preference, 38 mice of a randomly bred F3 generation derived from an eight-way cross, and 22 mice with variably sized corpora callosa in which only the left hippocampus was measured. Prior to morphometry, all mice were tested for paw preference. In addition, we compared the strain means in paw preference as observed in nine inbred mouse strains with known differences in their IIP-MF distribution.

Selective breeding for extremes in open-field activity of mice entails a differentiation of hippocampal mossy fibers.

The brains of mice selectively bred for differential locomotor activity in an open field (DeFries et al., Behav. Genet.

Swimming navigation and structural variations of the infrapyramidal mossy fibers in the hippocampus of the mouse.

The extent of the infrapyramidal mossy fiber projection in CA3 (IIP-MF) at the midseptotemporal level correlates negatively with two-way avoidance learning and positively with performance in the radial maze, both tasks known to be sensitive to hippocampal lesions. If hippocampal structural variations are causing behavioral variations, one must predict positive correlations between the extent of the IIP-MF and performance in swimming navigation. Thus, the authors studied learning and reversal learning of swimming navigation in mice in which the size of the IIP-MF had been randomized by means of systematic crosses and in 2 mouse strains known for differential infrapyramidal projections (C57BL/6 and DBA/2).

Using genetically-defined rodent strains for the identification of hippocampal traits relevant for two-way avoidance behavior: a non-invasive approach.

Genetically-defined rodent strains permit the identification of hippocampal traits which are of functional relevance for the performance of two-way avoidance behavior. This is exemplified here by analyzing the relationship between infrapyramidal mossy fibers (a tiny projection terminating upon the basal dendrites of hippocampal pyramidal neurons) and two-way avoidance learning in about 800 animals. The necessary steps include 1) identification of structural traits sensitive to selective breeding for extremes in two-way avoidance, 2) testing the robustness of the associations found by studying individual and genetical correlations between hippocampal traits and behavior, 3) establishing causal relationships by Mendelian crossing of strains with extreme structural traits and studying the behavioral consequences of such structural 'randomization', 4) confirming causal relationships by manipulating the structural variable in inbred (isogenic) strains, thereby eliminating the possibility of genetic linkage, and 5) ruling out the possibility of spurious associations by studying the correlations between the hippocampal trait and other behaviors known to depend on hippocampal functioning.