Characterization of the Dahl salt-sensitive rat as a rodent model of inherited, widespread, persistent pain

A multivariate statistical index, the fibromyalgia analog model, identifies Dahl S rats as a model of fibromyalgia with improved face validity

ABSTRACT

Fibromyalgia syndrome (FMS) is clinically diagnosed using a widespread pain index and symptom severity scores. Despite this, multivariate statistical models using clinical data consistently suggest the presence of single overarching factor that can capture much of the variation across a spectrum of biomarkers for FMS, potentially aiding diagnosis. We hypothesized that a comparable single index could be developed for preclinical FMS rat models to quantitatively compare and rigorously test new candidates for improved translational potential and therapeutic screening. We used male and female reserpine-treated Sprague Dawley (SD) rats to produce the fibromyalgia analog model (FAM) index, a tool to systematically determine the robustness of potential models of FMS. Features associated with FMS were assessed by behavior tests, including widespread nociceptive sensitivity evaluated on the hind paws ("paw") and gastrocnemius muscles ("mus") using a Randall-Selitto device, and face ("VF") using von Frey filaments; depression (by the forced swim test [FST]); anxiety (using the Elevated Zero Maze [EZM]); and fatigue (using a Home Cage Monitoring system [HCM]). A multivariate statistical analysis was used to determine the FAM index and document internal and external model validity. The index equation was calculated to be F = 0.410 × rzpaw + 0.618 × rzmus + 0.429 × rzVF + 0.455 × zFST + 0.183 × RZEZM + 0.160 × rzHCM, where each z is a standardized score. Our data showed the ability of the FAM index to differentiate saline-treated SD rats as a control, "normal" group, vs either acidic saline-injected SD or Dahl S rats as models displaying fibromyalgia-like behaviors.

Impact of homeostatic body hydration status, evaluated by hemodynamic measures, on different pain sensitization paths to a chronic pain syndrome

Contrasting findings on the mechanisms of chronic pain and hypertension development render the current conventional evidence of a negative relationship between blood pressure (BP) and pain severity insufficient for developing personalized treatments. In this interdisciplinary study, patients with fibromyalgia (FM) exhibiting clinically normal or elevated BP, alongside healthy participants were assessed. Different pain sensitization responses were evaluated using a dynamic 'slowly repeated evoked pain' (SREP) measure, as well as static pain pressure threshold and tolerance measures. Cardiovascular responses to clino-orthostatic (lying-standing) challenges were also examined as acute re- and de-hydration events, challenging cardiovascular and cerebrovascular homeostasis. These challenges involve compensating effects from various cardiac preload or afterload mechanisms associated with different homeostatic body hydration statuses. Additionally, hair cortisol concentration was considered as a factor with an impact on chronic hydration statuses. Pain windup (SREP) and lower pain threshold in FM patients were found to be related to BP rise during clinostatic (lying) rehydration or orthostatic (standing) dehydration events, respectively. These events were determined by acute systemic vasoconstriction (i.e., cardiac afterload response) overcompensating for clinostatic or orthostatic cardiac preload under-responses (low cardiac output or stroke volume). Lower pain tolerance was associated with tonic blood pressure reduction, determined by permanent hypovolemia (low stroke volume) decompensated by permanent systemic vasodilation. In conclusion, the body hydration status profiles assessed by (re)activity of systemic vascular resistance and effective blood volume-related measures can help predict the risk and intensity of different pain sensitization components in chronic pain syndrome, facilitating a more personalized management approach.

Descending Control of Nociception Poorly Predicts the Development of Persistent Postsurgical Pain-like Behavior in Consomic Dahl S Rat Strains

BACKGROUND

Chronic postsurgical pain is a poorly recognized outcome of surgery where patients experience pain long after healing from the surgical insult. Descending control of nociception, a phenomenon whereby application of a strong nociceptive stimulus to one part of the body of animals inhibits pain in remote body regions, offers one strategy to identify a propensity to develop chronic postsurgical pain-like behavior. Here, consomic rat panel was used to test the hypothesis that pain persistence is mechanistically linked to ineffective descending control of nociception.

METHODS

Male and female Brown Norway, Dahl S, and eight consomic strains (SS-xBN) were used to determine the presence of chronic postsurgical pain-like behaviors by using paw-withdrawal threshold evaluation (von Frey method) in the area adjacent to a hind paw plantar incision. Descending control of nociception was assessed by measuring hind paw-withdrawal thresholds (Randall-Selitto method) after capsaicin (125 µg) injection into a forepaw. Consomic rats were developed by introgressing individual Brown Norway chromosomes on the Dahl S rat genetic background, as Dahl S rats lack preoperative descending control of nociception.

RESULTS

Substitution of several chromosomes from the "pain-resistant" Brown Norway to the "pain-prone" Dahl S/Medical College of Wisconsin reduced mechanical nociceptive sensitivity and increased endogenous pain modulation capacity by differing degrees. Statistical modeling of these data revealed that descending control of nociception is a poor general predictor of the propensity to develop chronic postsurgical pain-like behavior (poor fit for model 1). However, a significant strain-by-descending control of nociception interaction was revealed (model 3, -2*log likelihood; 550.668, -2ll change; 18.093, P = 0.034) with SS-13BN and SS-15BN strains showing a negative descending control of nociception relationship with chronic postsurgical pain-like behavior.

CONCLUSIONS

Descending control of nociception poorly predicted which rat strains developed chronic postsurgical pain-like behavior despite controlling for genetic, environmental, and sex differences. Two consomic strains that mimic clinical chronic postsurgical pain criteria and display a strong negative correlation with descending control of nociception were identified, offering novel candidates for future experiments exploring mechanisms that lead to chronic postsurgical pain.

EDITOR’S PERSPECTIVE

Red Quinoa Hydrolysates with Antioxidant Properties Improve Cardiovascular Health in Spontaneously Hypertensive Rats

In recent years, quinoa has been postulated as an emerging crop for the production of functional foods. Quinoa has been used to obtain plant protein hydrolysates with in vitro biological activity. The aim of the present study was to evaluate the beneficial effect of red quinoa hydrolysate (QrH) on oxidative stress and cardiovascular health in an in vivo experimental model of hypertension (HTN) in the spontaneously hypertensive rat (SHR). The oral administration of QrH at 1000 mg/kg/day (QrHH) showed a significant reduction in SBP from baseline (-9.8 ± 4.5 mm Hg; p < 0.05) in SHR. The mechanical stimulation thresholds did not change during the study QrH groups, whereas in the case of SHR control and SHR vitamin C, a significant reduction was observed (p < 0.05). The SHR QrHH exhibited higher antioxidant capacity in the kidney than the other experimental groups (p < 0.05). The SHR QrHH group showed an increase in reduced glutathione levels in the liver compared to the SHR control group (p < 0.05). In relation to lipid peroxidation, SHR QrHH exhibited a significant decrease in plasma, kidney and heart malondialdehyde (MDA) values compared to the SHR control group (p < 0.05). The results obtained revealed the in vivo antioxidant effect of QrH and its ability to ameliorate HTN and its associated complications.

Discovering chronic pain treatments: better animal models might help us get there

Only three classes of pain medications have made it into clinical use in the past 60 years despite intensive efforts and the need for nonaddictive pain treatments. One reason for the failure involves the use of animal models that lack mechanistic similarity to human pain conditions, with endpoint measurements that may not reflect the human pain experience. In this issue of the JCI, Ding, Fischer, and co-authors developed the foramen lacerum impingement of trigeminal nerve root (FLIT) model of human trigeminal neuralgia that has improved face, construct, and predictive validities over those of current models. They used the FLIT model to investigate the role that abnormal, hypersynchronous cortical activity contributed to a neuropathic pain state. Unrestrained, synchronous glutamatergic activity in the primary somatosensory cortex upper lip and jaw (S1ULp-S1J) region of the somatosensory cortex drove pain phenotypes. The model establishes a powerful tool to continue investigating the interaction between the peripheral and central nervous systems that leads to chronic pain.