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

Abstract

We showed previously for mice that size differences of the infrapyramidal hippocampal mossy fiber projection (IIP-MF) correlate with spatial learning abilities. In order to clarify the role of the IIP-MF in a natural environment, we studied the bank vole (Clethrionomys glareolus), adapted to a wide range of different habitats, and the root vole (Microtus oeconomus), living in homogenous grassland habitats with small home ranges. Morphometry on Timm-stained horizontal brain sections of six C. glareolus and six M. oeconomus revealed that the size of the entire mossy fiber projection was 42% larger in C. glareolus than M. oeconomus. C. glareolus had also an IIP-MF projection about 230% larger than that of the root vole. A sample of captured animals was then transferred to the laboratory (C. glareolus, n = 23; M. oeconomus, n = 15) and underwent testing for swimming navigation according to a standardized protocol used to assess water maze learning in about 2,000 normal and transgenic mice. Both species learned faster than laboratory mice. Overall escape times showed no differences, but path length was significantly reduced in C. glareolus, which also showed superior performance in a variety of scores assessing spatial search patterns. On the other hand, M. oeconomus showed faster swimming speed, and strong thigmotaxis combined with circular swimming. M. oeconomus also scored at chance levels during the probe trial, about as poorly as mutant knockout mice considered to be deficient in spatial memory. These differences probably reflect differential styles of water maze learning rather than spatial memory deficits: C. glareolus appears to be superior in inhibiting behavior interfering with proper spatial search behavior, while M. oeconomus succeeds in escaping by using rapid circular swimming. We assume that size variations of the IIP-MF correspond to a mechanism stabilizing hippocampal processing during spatial learning or complex activities. This corresponds to the ecological lifestyle of the two species and is in line with previous observations on the role of the IIP-MF.

Role of a neuronal small non-messenger RNA: behavioural alterations in BC1 RNA-deleted mice

“…Vole species with different home ranges and habitats vary with respect to hippocampal size and microstructure. This correlates with spatial behaviour in the field and in the laboratory [19,36]. In natural mouse populations similar aptations occur [5].…”

Long-term monitoring of hippocampus-dependent behavior in naturalistic settings: Mutant mice lacking neurotrophin receptor TrkB in the forebrain show spatial learning but impaired behavioral flexibility

“…In those reports, they are interpreted as an inability of spatial learning, but see above. Likewise, variations in hippocampal mossy fiber patterns known to be associated with hippocampal functions correlate moderately with thigmotaxis (Pleskacheva et al, 2000) and place reversal learning in the Morris maze (Schöpke et al, 1991;Bernasconi-Guastalla et al, 1994); further, mice with hippocampal lesions show excessive wall swimming (Mojaheri et al, unpublished observations). Finally, lesions of the mouse hippocampus do not only impair the acquisition of finding an invisible platform, but retard, unlike in rats, learning of cued escape learning (Logue et al, 1997).…”

Dissecting the behaviour of transgenic mice: is it the mutation, the genetic background, or the environment?

“…85.6 631 individually. In addition, data from two wild vole species are shown (Pleskacheva et al 2000): Microtus agrestis (9) and Clethrionomys glareolus (7). The second subset consisted of 933 mice in which the mutations under study affected at least one parameter of swimming navigation learning.…”