Background & Aims
Fibromyalgia syndrome (FMS) is clinically diagnosed using a widespread pain index and symptom severity scores(1). No equivalent index is currently used to assess animal models of FMS. Despite the lack of consensus, there is an assumption that the different symptoms of FMS captured in the 2 diagnostic indices share a common underlying mechanism(2). In fact, multivariate statistical models using clinical data consistently suggest the presence of a single coordinating FMS factor that can explain much of the variation among the distinct aspects of FMS. We hypothesized that a multivariate statistical model could be developed for preclinical FMS rat models to quantitatively compare and rigorously test new candidates for improved translational potential and therapeutic screening. We therefore used reserpine-treated rats(3) to produce the Fibromyalgia Analog Model (FAM) index, a tool to systematically determine the robustness of potential models of FMS.
Methods
Male and female Sprague-Dawley (SD) rats, divided in 2 groups per sex (n=30), received subcutaneous injection of saline (control) or reserpine (1mg/kg) once daily for three consecutive days. Features associated with FMS were assessed by behavior tests, including: a) widespread nociceptive sensitivity, evaluated on the hind paws (pawpain), gastrocnemius muscle (legpain) and face (VPpain), using a Randall-Selitto device or von Frey filaments; b) depression (FST, using Forced Swim test); c) anxiety (EZM, determined by Elevated Zero Maze or Open Field Test); c) d) dyscognition (assessed by the Novel Object Recognition test); e) fatigue, using the Treadmill test; f) sleep quality (RHpostD), using a Home Cage Monitoring system. A multivariate statistical analysis was used to establish the FAM index and determine internal and external model validity. Male and female SS rats(4) and SD rats injected with acidic saline(5) (n=20) were then similarly phenotyped and their FAM scores calculated.
Results
The FAM index was created with a multivariate regression modelling framework. The index equation was calculated to be F = 0.663*zpawpain +0.998*zlegpain+0.693*zVPpain+0.736*zFST +0.295*zEZM+0.258*zRHpostD, where each z is a standard score: z(y) = (y-mean(y))/SD(y). Five measures were used to determine the statistical quality of the calculated FAM: I) model fit – Root Mean Squared Error of Approximation (RMSEA) target <=0.06, Actual =0.039; II) results variability - R2 coefficient of determination target >0.5, Actual R2 =0.564; III) reliability, with the reserpine model scoring rjj?0.7 for all 6 tests with a factor score determinacy of 0.98, and IV) robustness, with the estimated mean latent factor difference between reserpine and saline-treated rats =2.40 standard deviation units, exceeding Cohen’s definition of a moderate effect size (d>0.5). FAM scores from SS and acidic saline-injected SD rats were compared to the scores determined from the reserpine-treated group.
Conclusions
The FAM index allowed the “quantification of FMS level” in each animal across the behavior tests, based on a hypothetical, “ideal” FMS model, considering as indicators pain thresholds (paw, leg, facial), anxiety and depression levels, and distance travelled, associated to a value that determined its contribution to the index. Our data showed the FAM index ability to identify a FMS model, by correctly differentiating the saline-treated SD rats as a control, “normal” group. Using 6 endpoints (behavior measures) provided significant precision to the index. We expect that our FAM index will be useful in the determination of the face and predictive validities of animal FMS models, specifically due to its close match to the clinical diagnostic criteria of FMS. We consider this method/formula reliable for researchers using similar behavior tests to compute the FAM-like levels for both individual rats and groups of rats, without ever having to conduct or deal with an actual factor analysis.
References
1. Boomershine CS. A comprehensive evaluation of standardized assessment tools in the diagnosis of fibromyalgia and in the assessment of fibromyalgia severity. Pain Res Treat. 2012;2012:653714.
2. Dadabhoy D, et al. Biology and therapy of fibromyalgia. Evidence-based biomarkers for fibromyalgia syndrome. Arthritis Res Ther. 2008;10(4):211.
3. Nagakura Y, Oe T, et al. Biogenic amine depletion causes chronic muscular pain and tactile allodynia accompanied by depression: A putative animal model of fibromyalgia. Pain. 2009 Nov;146(1-2):26-33.
4. Ferrari LF, et al. Characterization of the Dahl salt-sensitive rat as a rodent model of inherited, widespread, persistent pain. Sci Rep. 2022 Nov 11;12(1):19348.
5. Sluka KA, et al. Unilateral intramuscular injections of acidic saline produce a bilateral, long-lasting hyperalgesia. Muscle Nerve. 2001 Jan;24(1):37-46.