Motor skills and motor learning in Lurcher mutant mice during aging
Résumé
Motor learning abilities on the rotorod and motor skills (muscular strength, motor coordination, static and dynamic equilibrium) were investigated in three-, nine-, 15-and 21-month-old Lurcher and control mice. Animals were subjected to motor training on the rotorod before being subjected to motor skills tests. The results showed that control mice exhibited decrease of muscular strength and speci®c equilibrium impairments in static conditions with age, but were still able to learn the motor task on the rotorod even in old age. These results suggest that, in control mice, ef®ciency of the reactive mechanisms, which are sustained by the lower transcerebellar loop (cerebello-rubro-olivo-cerebellar loop), decreased with age, while the ef®ciency of the proactive adjustments, which are sustained by the upper transcerebellar loop (cerebello-thalamo-cortico-ponto-cerebellar loop), did not. In spite of their motor de®cits, Lurcher mutants were able to learn the motor task at three months, but exhibited severe motor learning de®cits as soon as nine months. Such a de®cit seems to be associated with dynamic equilibrium impairments, which also appeared at nine months in these mutants. By two months of age, degeneration of the cerebellar cortex and the olivocerebellar pathway in Lurcher mice has disrupted both lower and upper transcerebellar loops. Disruption of the lower loop could well explain precocious static equilibrium de®cits. However, in spite of disruption of the upper loop, motor learning and dynamic equilibrium were preserved in young mutant mice, suggesting that either deep cerebellar nuclei and/or other motor structures involved in proactive mechanisms needed to maintain dynamic equilibrium and to learn motor tasks, such as the striatopallidal system, are suf®cient. The fact that, in Lurcher mutant mice, motor learning decreased by the age of nine months suggests that the above-mentioned structures are less ef®cient, likely due to degeneration resulting from precocious and focused neurodegeneration of the cerebellar cortex. From this behavioral approach of motor skills and motor learning during aging in Lurcher mutant mice, we postulated the differential involvement of two transcerebellar systems in equilibrium maintenance and motor learning. Moreover, in these mutants, we showed that motor learning abilities decreased with age, suggesting that the precocious degeneration of the cerebellar Purkinje cells had long-term effects on motor structures which are not primarily affected. Thus, from these results, Lurcher mutant mice therefore appear to be a good model to study the pathological evolution of progressive neurodegenera-tion in the central nervous system during aging.