An automated task for the training and assessment of distal forelimb function in a mouse model of ischemic stroke

Abstract

Background: Behavioral models relevant to stroke research seek to capture important aspects of motor skills typically impaired in human patients, such as coordination of distal musculature. Such models may focus on mice since many genetic tools are available for use only in that species and since the training and behavioral demands of mice can differ from rats even for superficially similar behavioral readouts. However, current mouse assays are time consuming to train and score, especially in a manner producing continuous quantification. An automated assay of mouse forelimb function may provide advantages for quantification and speed, and may be useful for many applications including stroke research.

New method: We present an automated assay of distal forelimb function. In this task, mice reach forward, grip and pull an isometric handle with a prescribed force. The apparatus partially automates the training process so that mice can be trained quickly and simultaneously.

Results: Using this apparatus, it is possible to measure long-lasting impairment in success rate, force pulled, latency to pull, and latency to success up to 22 weeks following photothrombotic cortical strokes in mice.

Comparison with existing method(s): This assessment measures forelimb function as do pellet reach tasks, but it utilizes a different motion and provides automatic measures that can ease and augment the research process.

Conclusions: This high-throughput behavioral assay can detect long-lasting motor impairments, eliminates the need for subjective scoring, and produces a rich, continuous data set from which many aspects of the reach and grasp motion can be automatically extracted.

Keywords: Behavior; Forelimb; Ischemic stroke; Mouse; Stroke.

Figure 1 – 1 or 1.5 column

A pparatus a. Schematic representation without peanut oiIdispenser. b. Wide-angle pictures of complete apparatus. c. Close up of handle and positioning relative to apparatus.

Figure 2 – 1 or 1.5 column

a. Example Trials. Data traces for sample trials showing raw measurements (paw extension and force) and derived measurements (latency to success, highest force in trial, latency to pull). Both signals are stable while the mouse is not interacting with the slot or handle, and both signals change upon behavioral performance with a clear signal relative to noise. The left trace shows 3 attempts before success. b. Program Interface. An experimenter can control and monitor a session via this GUI. Individual trails can be seen in a list on the lower right, and a maximum trial force through time for both hits and misses can be seen in a graph at the middle right. Raw data for the past several seconds can be seen in the middle left, and settings for the session at the top. Sessions are controlled by buttons on the lower left. The GUI is displayed in color on computer screens. c. Photos of a mouse reaching for the handle. The mouse first brings its nose close to the opening then extends the right forepaw with the long axis of the wrist close to vertical. The first digits then wrap around the handle and exert force in the direction of the mouse’s body.

Figure 3 – 2 column

a. Cresyl Violet stains of 6 serial sections 720μm apart from one mouse who received photothrombotic stroke. Stroke included the forelimb representation of the motor cortex and disrupted subcortical white matter tracts in all but two animals. b. Infarct volumes between 1.7 mm anterior to bregma to 2.6 mm posterior to bregma. c. Baseline and post-stroke performance in each of 5 measures, p values from paired two-tailed t-tests. After stroke, the average success rate and maximum force were nearly two standard deviations below baseline, while latency measures were one standard deviation below baseline. d. Decrease from baseline average in success and force on day 7 following stroke is correlated with infarct volume.

Figure 4 – 2 column

Recovery data (n=13) collected at days 3, 7, and weekly until day 154 after stroke: error bars represent SEM. Stars above individual time points represent Fishers LSD individual comparisons between that time point and the last baseline time point: p .05 (*), p .05 (**), p .005 (***),and p .0005 (****).