Hippocampal mossy fibers and swimming navigation learning in two vole species occupying different habitats

We showed previously for mice that size differences of the infrapyramdal: h ippocampal mossy fiber projection (IIP-MF) correlate with spatial learning abilities. In order to clarify the role of the IIP-MF in:a natural environm ent, we studied the bank vole (Clethrionomys glareolus), adapted to a wide range of different habitats, and the root vole (Microtus oeconomos), living in homogenous grassland habitats with small home ranges. Morphometry on Timm-stained horizontal brain sections of six C. glareolus a nd six M. oeconomus revealed that the size of the entire mossy fiber projec tion was 42 % larger in C. glareolus than M. oeconomus. C. glareolus had al so an IIP-MF projection about 230% larger than that of the root vole. A sam ple of captured animals was then transferred to the laboratory (C. glareolu s, n = 23; M. oeconomus, n = 15) and underwent testing for swimming navigat ion according to a standardized protocol used to assess water maze learning in about 2,000 normal acid transgenic mice. Both species learned faster th an laboratory mice. Overall escape times showed no differences, but path le ngth was significantly reduced in C. glareolus, which also showed superior performance in a variety of scores assessing spatial search patterns. On th e other hand, M. oeconomus showed faster swimming speed, and strong thigmot axis combined with circular swimming. M. oeconomos also scored at chance le vels during the probe trial, about as poorly as mutant knockout mice consid ered to be deficient in spatial memory. These differences probably reflect differential styles of water maze learni ng rather than spatial memory deficits: C. glareolus appears to:be superior in inhibiting behavior interfering with proper spatial search behavior, wh ile M. oeconomus succeeds in escaping by using rapid circular swimming. We assume that size variations of the IIP-MF correspend to a mechanism stabili zing hippocampal processing during spatial learning or complex activities. This corresponds to the ecological lifestyle of the two species and is in l ine with previous observations on the role of the IIP-MF. Hippocampus 2000; 10:17-30. (C) 2000 Wiley-Liss, Inc.