Motor Skills Assessment

Motor function evaluation is critical and often defines certain phenotypes. Mutant mouse models are often used to study ataxias and other neuromuscular disease. Thus, there is a need for tests that will enable investigators to examine and evaluate motor function. These tests are important for the evaluation of abnormalities in aging mice as well as specific muscular disease studies such as muscular dystrophy.

The first step in evaluating neuromuscular function is a complete neurologic exam including neurologic reflex tests. Easy tests for normal reflexes include the righting reflex, postural reflex, eye blink reflex, ear twitch reflex, and whisker-orienting reflex. In the righting reflex, the mouse is turned over onto its back; normal mice will immediately turn themselves over onto all four feet. The postural reflex involves placing the mouse in an empty cage and shaking the cage back and forth and up and down; normal mice will extend all four limbs to maintain the proper upright position. The eye blink and ear twitch reflexes are based on the normal avoidance action in response to a cotton swab nearing the eye or touching the ear. The whisker-orienting response is tested by lightly brushing the whiskers of a freely moving animal with a small brush; normal mice will stop moving their whiskers when touched and may turn their head to the side being brushed.22

The open field locomotion test can be used to evaluate motor function. The testing apparatus consists of a digiscan open field plexiglass box with a series of photocell receptors and emitters. When a mouse moves through a beam, the beam path is broken, and the photocell analyzer records the beam break. A computerassisted analyzer tallies the number of beam breaks for each set of photocells, with an ability to distinguish between vertical and horizontal activity. Computer software can calculate a large number of relevant variables over a preset time period, including vertical and horizontal activities, total distance traversed, total number of movements, and time spent in the center of the open field versus time spent in the perimeter of the open field. A 5-min test session is sufficient to evaluate gross abnormalities in locomotion.22

Performance on the rotarod can be used as a way to detect cerebellar defects as well as other neuromotor deficits in mice. The rotarod is a machine with revolving wheels upon which the mice walk at constantly accelerating speeds. During experimentation, mice are placed on the wheels and the latency period for mice to fall off is measured. Mice with deficits in motor coordination or balance will fall off the rotarod well before the end of a 5-min test session.

In addition to the rotarod, the balance-beam test and the vertical pole test can be used to assess motor coordination and balance in transgenic mice. In the balance-beam exam, mice are trained to traverse a series of graded beams in order to reach an enclosed safety platform. A bright light illuminates the beginning of the beams to further encourage the mouse to walk across the beams toward the enclosed box at the other side. Common beam sizes are 28-mm,2 12-mm,2 5-mm,2 28-mm round, 17-mm round, and 11-mm round. Evaluations are based on latency to cross the beams and the number of times the hind feet slip off the beams. The vertical pole test also uses minimal amounts of equipment and is easy to perform. In this exam, a metal or plastic pole, 2 cm in diameter and 40 cm long, wrapped with cloth tape to increase traction, is held in a horizontal position with the mouse placed in the center. Gradually, the pole is lifted into a vertical position with latency to fall off the measured variable. Normal mice will stay on the pole even in the complete vertical position and may even walk up and down. Mice with deficits in motor coordination or balance will fall off before the pole hits 45°.22

Walking patterns or gait abnormalities can be evaluated with footprint analysis. The hindpaws of the mice are dipped in a nontoxic ink or paint, and the mice are allowed to walk on white paper within a dark tunnel. The mice walk down the tunnel and leave their footprints on the paper. The footprints can then be examined for any signs of ataxia or gait abnormalities. The ability of the mouse to walk a straight line is measured by variability in stride length, variability around a linear axis, hindbase width and frontbase width between left and right paws, overlap of paws, and the distance between each stride.22 Motor strength can be evaluated using the hanging wire exam. Mice use balance and grip strength to hang upside down on a wire. A standard wire cage lid can be used for this assessment, with duck tape around the edge so the animal will not walk off the edge. Simply place the mouse on the cage, shake the lid several times to cause the mouse to grip the wires, and turn the lid over so that the mouse is hanging by its feet to the underside of the lid. There should be enough of a drop from the lid that the mouse does not simply drop off, but not a distance so large that it will be injured should it fall during the exam; 20 cm from cage lid to ground is an adequate height. Latency to fall off is measured, as normal mice can hang upside down by their grip alone for several minutes. 22

Some genetically engineered mouse models show neuromuscular dysfunction in more specific areas than these general neurologic exams can identify. For example, some transgenic mice will exhibit stereotypic behaviors, or repetitive, invariant, perseverative motor patterns that do not appear to be directed toward a specific goal. There are scoring systems to characterize such behaviors as chronic grooming, sniffing, or head dipping. Lesions on one side of the brain can cause circling, with the direction of the motion correlative with the side of the lesion. Circadian rhythms and sleep patterns can be disrupted in transgenic mice. Some transgenic mice will show extreme signs of neurologic dysfunction such as seizures, ataxia, immobility, or twitching that are indicative of loss of vestibular function.22

Diagnostic exams should be tailored toward the neurologic area most likely to be the leading cause of the visualized problems.

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