Plasticity in the Cerebellum and Primary Somatosensory Cortex Relating to Habitual and Continuous Slender Branch Climbing in Laboratory Mice (Mus musculus)

Abstract

The origin of the mammalian order Primates is nested within a Euarchontan ancestry that was probably exploiting the fine branch arboreal niche in a facultative way. A putative transition into this habitat may have begun with a more generalized small-bodied mammal that lacked climbing specializations for grasping hands and feet. Here, we investigate whether mice exhibit central nervous system (CNS) plasticity associated with learning to grasp/climb proficiently. House mice were used to study phenotypic plasticity within the cerebellum and primary somatosensory cortex associated with the fine branch niche. This experimental treatment has previously been shown to influence skeletal plasticity in part because climb-training encourages tail use and facultative pedal grasping. The CNS necessary to coordinate and control these locomotor behaviors was investigated in a standard mouse model (N = 10 male CD-1/ICR mice), and plasticity was detected by histomorphometric and immunohistologic changes within the cerebellum and cerebrum. The climbing group had a significantly smaller relative granule cell layer in cerebellar lobule 1-3 than the control group (P < 0.10), but increased nerve growth factor immunoreactivity in white matter tracts of these lobules (P < 0.05). Qualitative observations in the primary somatosensory cortex revealed greater pyramidal/stellate cell counts in climbers. We suggest that coordinated tail and hindlimb learning within the arboreal milieu is facilitated by increased growth factor expression and neuronal alterations in the CNS. These findings suggest that mammals with a generalized Euarchontogliran body plan were capable of facultative pedal grasping and tail use so as to exploit the terminal branch niche.