Background/Purpose:

Destabilization of the medial meniscus (DMM) in mice results in experimental osteoarthritis, characterized by progressive joint damage (severe damage by week 12) and associated pain-related behaviors. The purpose of this study was to determine if mice also develop changes in motor coordination and balance by using two assays designed to test these parameters: the balance beam and horizontal ladder.

Methods:

A 60-cm long, 6-mm wide square wooden balance beam was elevated 35 cm. Mice were filmed from one end as they crossed the beam. Videos were analyzed for (1) Time (in seconds) the mouse took to cross the beam; (2) Whether or not the mouse used its tail.

We used a 60-cm long horizontal ladder with 4-cm long metal rungs spaced 2 cm apart in a straight line. Mice were filmed from the side as they crossed the ladder. Videos were analyzed for (1) Time (in seconds) the mouse needed to cross the ladder; (2) The number of errors made in placing the right hind paw on a rung.

DMM or sham surgery was performed in the right knee of 10-week old male C57BL/6 mice. Age-matched naïve mice were also used. Twelve or fourteen weeks after surgery, mice were tested on either the beam or ladder.

In addition, two strains of naïve DREADD (Designer Receptor Activated by a Designer Drug) male mice were used, in order to selectively silence either nociceptors (Na_V1.8-Pdi mice) or proprioceptors (PV-Pdi mice) (Ray et al, Science, 333:637, 2011). Mice were injected with either clozapine-N-oxide (CNO) (10 mg/kg, i.p.) or vehicle (PBS) (n=6-10 mice/strain/treatment). One hour post injection, mice were tested by crossing the ladder or beam. The experimenter was blinded to the groups.

Results:

Balance beam – Twelve weeks after surgery, DMM mice (n=13) took longer to cross the beam than shams (n=11) (p=0.04), and DMM mice used their tail more often than sham mice while crossing the beam (p=0.03).

Ladder – By week 14 after surgery, DMM mice took longer to cross the ladder compared to naïve mice (n=5/group) (p=0.001), while sham mice were not different from either naïve (p=0.16) or DMM (p=0.1). The number of errors made was not different among the groups at this time point.

On the beam, CNO caused both naive PV-Pdi (p=0.05) and naïve Na_V1.8-Pdi (p=0.04) mice to take longer to cross the beam compared to PBS, but only PV-Pdi mice showed a difference in tail use when given CNO (p=0.008). Together, this suggests that a mixture of nociceptors and proprioceptors are needed to accomplish the beam task.

On the ladder, naïve PV-Pdi mice given CNO took longer to cross the ladder (p=0.01) and made more errors while crossing (p=0.09) compared to PBS. There was no difference between CNO and PBS in Na_V1.8-Pdi mice, providing evidence that this assay assesses proprioceptive function.

Conclusion:

Using two different balance assays, we determined that mice developed impairments following DMM surgery, particularly during the late stage of the model, when joint damage and pain-related behaviors are most apparent. Chemogenetic silencing of proprioceptors but not nociceptors resulted in deficits in the horizontal ladder assay, while the beam test appeared to require both kinds of neurons to accomplish the task depending on the parameter measured.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/measuring-balance-in-an-experimental-mouse-model-of-osteoarthritis/