Knowledge Base

Locomotor Adaptation in Mice

Overview

The cerebellum, or "little brain," plays a crucial role in balance and locomotion. Disorder of the cerebellum can lead to ataxia, which is characterized by increased variability and poor accuracy of movements. Even though gait ataxia and limb coordination have been investigated thoroughly in human subjects, their cellular underpinnings have been relatively neglected due to technical deficiencies in measuring all aspects of locomotion in mice.

Furthermore, not much is known about the plasticity mechanisms that drive the timing and motor learning in locomotion. To study the role of specific mutations of plasticity in the cerebellar microcircuit, several mutant mice were tested in a task that assesses locomotion and locomotion adaptation.

Experiment

Apparatus

To study locomotion, the fully automated ErasmusLadder was used. The ErasmusLadder consists of a horizontal ladder between two shelter boxes, each equipped with an LED spotlight in the roof and two pressurized air outlets in the back. Sensory stimuli (light and air) serve to control the moment of departure of the mice. The ladder itself has 37 rungs on each side, and each rung can be displaced vertically. Even rungs on one side and odd rungs on the other were elevated by 6 mm, thereby creating a left/right alternating pattern. All rungs are equipped with custom-made pressure sensors that are continuously monitored.

Protocol

Each mouse performed one daily session of 72 trials during 8 days, structured as follows:

• During the first four sessions, naive locomotion was assessed in which none of the rungs moved.
• During the last four sessions, locomotion adaptation was tested by challenging the mouse to deal with the appearance of an obstacle, which was preceded by a tone 200 ms prior to its occurrence.

The obstacle was induced by elevating one of the lower rungs, creating an obstacle of 12 mm just in front of the mouse. The location of the obstacle on the ladder varied randomly between trials. The exact timing of the obstacle appearance depended on the walking pattern and the predicted trajectory of the mouse.

Mouse Lines

Four different types of wild-type controls and mutants were used. These mouse lines are characterized by the following:

• Pcd mice: Degeneration of Purkinje cells.
• Pp2b and Gamma-Delta-2 mice: Specific aberrations in the cellular mechanisms underlying motor learning in the cerebellar cortex.
• a6-Cacna1a mice: Reduced input to the principal cell of the cerebellar cortex, the Purkinje cell.

Results

Naïve locomotion during unperturbed sessions and locomotion adaptation during perturbed sessions were studied. With the ErasmusLadder, the number, length, and accuracy of steps could be reliably tested. The rich dataset makes it possible to study advanced parameters such as:

• Walking pattern consistency
• Efficiency
• Temporal aspects of locomotion
• Inter-limb coordination

Figure 1: Basic Walking Pattern Comparison Between Wild-Type and Pcd Mice

Each daily session consisted of 72 trials, during which the mice had to walk back and forth from one shelter box to the other. The upper rungs, indicated by closed yellow symbols, are positioned in a left–right alternating pattern. The blue footprints represent the typical touches of the front paws of a control mouse and a Pcd mouse during a representative trial on the ladder. A single step corresponds to a front paw step. Steps are classified according to their length and direction and are represented as colored rectangles located below the ladders. Consecutive single steps of the same length merge to build blocks. The time course of the trials is also depicted, with symbols representing single touches.

Conclusions

With the ErasmusLadder it was possible to successfully characterize and objectify the locomotion pattern of different mouse lines with specific aberrations in the cerebellar cortex. Key findings include:

• The ErasmusLadder enables precise temporal analysis of locomotion, allowing the study of interlimb coordination.
• Mice can be challenged during perturbed sessions in which they must cross a suddenly appearing obstacle.
• The ErasmusLadder yields a more complete and quantitative analysis of locomotion than other systems currently available.

Source: Case study locomotor, Noldus Information Technology