Pain Assessment in an NGF-Induced Animal Model of Low Back Pain

Research Objectives: To characterize spontaneous and mechanical pain in an NGF-induced feline model of low back pain. It is hypothesized that lumbar NGF injections will increase spontaneous and mechanical pain.

Design: Experimental Study

Setting: Animal Care Facility

Participants: Twenty-four adult cats (15 male, 9 female).

Interventions: The multifidus muscle was bilaterally injected at L6 & L7 with nerve growth factor (NGF; 0.8µM, 100µL; n=10) or phosphate buffer solution (PBS-vehicle; n=14) on Day 0 and Day 5 under isoflurane anesthesia. Animals were placed in a large plexiglass container with a mesh top and videotaped for 20 minutes on Days 0 (prior to injection) and on Day 11 to capture facial expressions. A screenshot was taken every two minutes and five action units (orbital tightening, ear position, whisker change, muzzle tension, head position) scored for the Feline Grimace Scale (FGS). The Pain Sensitivity Test (PST) and the Feline Musculoskeletal Pain Index (FMPI) assessments were also performed. The PST was assessed at the foretrunk, hindtrunk, and L6 spinous process.

Main Outcome Measures: FGS and FMPI assessments for spontaneous pain and PST for mechanical pain. The mean FGS scores of each group were compared using a repeated measures ANCOVA, while the PST and FMPI assessments were analyzed using the Wilcoxon Ranked Sum test

Results: Comparing mean FGS changes from Day 11 to Day 0, there was a slight increase in spontaneous facial pain expression among the NGF group, but these changes failed to reach significance (p=0.65). FMPI assessment at Day 11 indicated as significant decrease in FMPI scoring among NGF animals compared to PBS animals (p=0.02). The PST showed no significant difference between groups for the foretrunk, but significant NGF pain sensitivity changes at the hindtrunk and L6 sites (p=0.002).

Conclusions: FMPI and PST support the hypothesis that NGF increases localized musculoskeletal pain in felines, while the FGS indicated no significant change in feline facial expression due to NGF injections.

Author(s) Disclosures: Authors declare no conflicts of interests. This work was supported by NIH/NCCIH R21 AT010517 to WRR.