Spinal Contusion Injury Induces Long‐Lasting Changes in Home Cage Activity and Respiration which Correlate with Spontaneous and Evoked Indices of Neuropathic Pain

Abstract

Spinal cord injury (SCI) often results in chronic neuropathy and dysregulation of motor and respiratory behavior. Quantifying these changes in rodents is difficult, often invasive, and can alter the target measure. Despite its importance, few studies have undertaken the attempt [Urban et al 2011; 10.1016/j.pain.2010.12.003]. The objective of this work is to correlate measures of pain after SCI to changes in physiological events of naturally‐behaving animals using non‐invasive home‐cage monitoring.All animal work conformed to Emory University Department of Animal Resources and FASEB guidelines. Animal home cages were instrumented with non‐contact electric potential integrated circuit sensors (Plessey Inc.) able to non‐invasively detect animal activity and respiratory variables from outside the home cage [Noble et al 2017; doi.org/10.1016/j.jneumeth.2016.12.007]. After three weeks of acclimation and baseline recordings, seven female adult c57/bl6 mice were recorded for 15 hours overnight for six weeks subsequent to a thoracic T10 spinal cord contusion injury (SCI; 50 kDyne impact). Hindpaw mechanical sensitivity was assessed with von Frey hairs using Chaplan's up‐down protocol. At 6 weeks post‐injury, SCI animals (n=7) and age‐matched naïve c57/bl6 mice (n=3) were euthanized, and in vitro electrophysiologic recordings of spontaneous afferent population activity were obtained from isolated lumbar dorsal root ganglia (DRG). Spinal cords and DRG were immersion fixed for subsequent histological assessment. Spontaneous activity was calculated as average firing rates over 5 trials (5 sec per trial). Data were analyzed using 1‐way ANOVAs and Tukey's post‐hoc test for significance. Linear regressions were calculated to assess correlation between data.Relative to baseline, SCI animals exhibited heightened mechanical sensitivity consistent with evoked allodynia at all time points except week 1 (p<0.001, Fig 1A). SCI animals were less active after SCI (p<0.034, Fig 1B). While there was no change in resting respiratory rate (RR, 3.6±1.4 Hz), there was an increase in RR variability at weeks 5 and 6 relative to baseline (p<0.045, Fig 1C). The response for mechanical sensitivity and change in active time significantly correlated (R2=0.11, p=0.005) with a biphasic shape, exhibiting an acute response right after surgery then recovering to baseline levels before declining into a chronic response. Spontaneous DRG firing rates were higher in SCI animals than naïve animals (p=0.045, Fig 1D). With the removal of one outlying animal, spontaneous DRG activity strongly correlated with respiration rate variability (R2=0.87, p=0.0002) and with near significance to mechanical sensitivity (R2=0.42, p=0.055).In summary, contusion SCI leads to increased RR variability, which correlated to increased afferent spontaneous activity. SCI also reduced motor activity, which correlated to mechanical allodynia; These correlations may be causally linked. We hypothesize that ongoing spontaneous pain (i.e. increased spontaneous nociceptor activity) increases RR variability, and ongoing hindpaw mechanical allodynia reduces motor activity. While further studies are planned to more definitively link pain to alterations in physiological events in naturally‐behaving animals, it is clear that non‐invasive home‐cage monitoring can detect important consequences of SCI.Support or Funding InformationThis work was supported by the Craig H. Neilsen Foundation and NIH/NIGMS Institutional Research and Academic Career Development Award 5k12‐GM000680‐17This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.