Functional testing in animal models of spinal cord injury: not as straight forward as one would think

Abstract

When exploring potential treatments for spinal cord injury (SCI), functional recovery is deemed the most relevant outcome measure when it comes to translational considerations. Yet, assessing such recovery and potential treatment effects is challenging and the pitfalls are frequently underestimated. The consequences are that in many cases positive results cannot be reliably replicated, and likely treatments that appear to lack effects have been dismissed prematurely. In this article we review the relationships between lesion location/severity and functional outcomes with specific consideration given to floor and ceiling effects. The roles of compensatory strategies, the challenges of distinguishing them from bona fide recovery, and of comparing function to pre-injury levels given the variability inherent in animal testing are discussed. Ultimately, we offer a series of considerations to enhance the power of functional analysis in animal models of SCI.

Keywords: compensation; grasping; locomotion; recovery; spinal cord injury.

Figure 1

Nonlinearity of lesion severity and functional recovery. (A) Clustering of animals in the BBB locomotor score. Although T8 lesion severity and location (dorsal versus ventral) were applied randomly, two scoring points were assigned most frequently, indicating thresholds in the BBB scale. Adapted from Schucht et al. (2002). (B) Correlations of lesion and reaching success. Total lesioned area at C4 was not correlated with final single pellet reaching success. Success is consistently high with small lesions and low with extensive lesions, however, when approximately 14–38% of the dorsolateral spinal cross section is lesioned, there is substantial variability in reaching recovery (grey area). Adapted from Hurd et al. (2013).

Figure 2

Decreased performance in behavioral tests by rats in the vicinity of stress. Reaching performance dropped substantially when a second (independent) group of rats housed in the same room underwent surgery (as indicated by the arrow), an intus cella effect. Reaching performance remained low for over a week. Data is shown as mean ± SEM, n = 12.

Figure 3

Abnormal light: dark cycle impairs locomotor recovery. Graph shows BBB scores over time for a group of eight female SD rats housed in pairs. Due to a malfunction, we believe that the animals were in constant light (24:0, light:dark) starting 1 day post-injury until the 12:12, light:dark cycle was restored at 5 weeks post-injury. Data is shown as mean ± SD.

Figure 4

Rats traverse a significantly greater distance when housed in pairs as compared to single housed. Graph shows total distance traveled for female SD rats with 12.5 g-cm NYU contusive injuries at T10 in standard 10″ × 18″ cages. Digital video was made of 1 min for every 10 min throughout the dark (12 h) period. Video was made a 4 Hz using a Basler 602f camera and custom-built IR lighting. Distance was determined using MaxTraq by Innovision Systems for three nights from each 10 day bin. Data is shown as mean ± SD.

Figure 5

Variability in reaching success throughout the week. (A) Reaching recovery was assessed following a C4 dorsolateral quadrant spinal cord lesion. Success during rehabilitative training was found to be lowest on Mondays. Data is shown as mean ± SEM, n = 10. (B) Reaching success varied throughout the week among individual rats. Graph shows the pre-injury reaching performance for two different rats over the course of one week.