Chronic widespread pain disorders, such as fibromyalgia (FM), are highly represented in our population but well understudied. Upwards of 2% of the U.S. population suffers from FM, with women being two-fold more likely to be diagnosed (1). The lack of clinical understandings of the disorder and largely ineffective treatment modalities on a multidimensional level has resulted in a large fiscal and societal burden (2). The current 2016 American College of Rheumatology diagnostic criteria for FM in adults includes: (1) generalized pain in at least 4 of 5 regions; (2) symptom presence for at least 3 months; (3) a Widespread Pain Index (WPI) score ≥7 and a Symptom Severity (SS) score ≥5, or a WPI score of 4–6 and a SS score ≥9; and (4) FM diagnosis does not exclude the presence of other clinical disorders (3). The current diagnostic criteria have allowed for FM to be assessed along a continuum of its unique symptoms, providing a more precise quantification of widespread pain while reducing misclassification or discrimination based on the presence of coexisting disorders (3). The inclusion of more clearly defined FM diagnostic criteria on a multidimensional level has proven vital in that FM is often both exacerbated and defined by its affective and cognitive dimensions—especially in the development of preclinical representations of the disorder—including depression and anxiety, difficulty sleeping, and various cognitive deficits (4–6). The close relationship between FM and its association with affective disturbances demonstrates the need for diagnostic methods to be inclusive of the negative emotionality associated with the disorder.

Without a definitive etiological understanding of FM, it has been difficult to develop a preclinical model of FM that allows for the comprehensive study of negative emotionality associated with this pain disorder (7). While there are a handful of models developed for preclinical FM research, we focus on Sluka et al.'s acidic saline model due to long-lasting incorporation of both central and peripheral pain factors with the absence of peripheral tissue damage which is commonly found in FM patients (8). The model utilizes repeated injections of 4.0 ± 0.1 pH saline into the left gastrocnemius muscle of the rodent to induce a bilateral hyperalgesia lasting up to 4 weeks (8). Previous research has focused on the ability of the acidic saline model to induce a comprehensive FM-like experience, including the decreased pain thresholds and anxio-depressive characteristics of the disorder. Some studies have reported that the acidic saline model induces both anxiety- and depression-like behaviors (9–15). However, Pratt et al. provide evidence that the acidic saline model may not be as efficacious in the replication of FM-related affective behaviors as previously thought (16). While it is sufficiently understood that acid-induced hyperalgesia adequately models the sensory and possible etiology of FM, the conflicting results on the model's ability to induce negative emotionality, such as anxio-depressive symptoms, calls for further investigation into potential alterations of emotionality associated with the model.

In the study of chronic pain, hyperbaric oxygen (HBO₂₎ has provided evidence of anti-nociceptive and anti-inflammatory mechanisms (17), with efficacy of treatment having been shown in animal models of injury-related pain (18), carrageenan-induced paw edema (19), arthritic pain (20), and neuropathic pain (21). Within the clinical literature, a randomized controlled trial conducted by Yildiz et al. indicates that patients diagnosed with FM experienced a significant decrease in FM-associated tender points and visual analog scale scores, alongside an increase in pain thresholds (22). Efrati et al. also investigated the potential role of HBO₂ in a clinical FM population and also found that treatment was effective in increasing physical pain thresholds and decreasing tender point count (23). Additionally, the same study found a significant increase in physical functionality as measured by the Fibromyalgia Impact Questionnaire (FIQ), a decrease in psychological distress as measured by the Symptom Check List (SCL-90), and an overall increase in quality of life (QoL) assessments (23). A more recent randomized controlled trial by Hadanny et al. showed improvement in women's scores on various measures of FM pain, such as the WPI, alongside improvement in various measures of PTSD (24). Further study, by Curtis et al. sought to validate the ability of HBO₂ to both improve and maintain physical and affective FM symptoms (25). Curtis et al. also reported similar results as seen in other randomized controlled trials, with functional impairment symptoms, anxio-depressive symptoms, and sleep disturbances all being alleviated for 3 months following treatment (25). The use of HBO₂ for the attenuation of both negative physicality and emotionality, such as depression- and anxiety-like behaviors, among various types of pain models and central nervous system disorders has persisted through both clinical (17, 26–28) and preclinical studies (17, 29). With FM being a disorder strongly associated with negative affective components, there is promise in the potential therapeutic benefits of hyperbaric treatment for both alleviation of FM-related hyperalgesia, as well as its related affective dimensions.

With much of our knowledge of the clinical management and underlying etiology of FM stemming from information obtained from preclinical models, it is crucial to understand to what extent these animal models fully mirror the disorder and to what extent these models can be utilized for further experimental treatment studies. Therefore, we examined these goals in relation to the acidic saline model's ability to replicate alterations of emotionality by measuring anxio-depressive like behaviors, while further investigating the efficacy of a known FM therapeutic agent, pregabalin, in alleviating sensory and affective FM symptoms. In addition, we further investigated the validity of the acidic saline model in producing anxio-depressive FM symptoms, as well as explored the role of HBO₂ as a therapeutic approach for treating alterations of the sensory and affective dimensions of symptoms associated with FM.

The purpose of the current studies was to further investigate the efficacy of the acidic saline model of FM in replicating negative emotionality as reported in clinical manifestations of FM. Further, we sought to utilize a back-translational approach to gauge the potential efficacy of HBO₂ as a therapeutic mechanism for the alleviation of acidic saline induced pain and its associated negative affectivity. Animals were induced into the acidic saline model and measures of anxiety and place escape/avoidance were recorded, with replication of the pharmacological profile being confirmed through the administration of pregabalin. In addition, some animals were induced into the acidic saline model, where measures of anxiety and anhedonia were recorded, with HBO₂ being investigated as a potential therapeutic agent for alleviation of both reduced thresholds and potentially negative affect.

The results showed a reduction in mechanical thresholds among animals randomized to the FM condition, such that there were indications of hyperalgesia associated with the injections of hypertonic saline. Administration of pregabalin significantly reduced mechanical hyperalgesia. Animals randomized to the FM condition who received vehicle injections showed the most avoidance behavior during the PEAP test, implicative of negative affect associated with the FM-like pain. Of interest is that pregabalin treatment reduced the avoidance behavior to a level comparable to that of non-FM control animals. These results imply that negative affect associated with the acidic saline model can be measured using PEAP methodology (16) and is sensitive to therapeutic interventions such as pregabalin.

We had expected to see similar outcomes of anxiety-like behavior and locomotion, but there was no evidence for animals in the FM condition showing differences in anxiety-like behaviors compared to controls. Interestingly, saline control animals randomized to receive a pregabalin treatment showed an increase in locomotion at post-treatment compared to pre-treatment. This increase in locomotion provides evidence that pregabalin dosages at 30 mg/kg likely were not involved in the absence of anxiety-like behavior through the induction of lethargy or ataxia, as has been previously observed (33). However, control animals who received sham treatment showed less anxiety over each testing session, as displayed by an increase in distance traveled in the center of the apparatus. The gradual reduction of anxiety like behavior is likely due to a test–retest error and should be examined in future experiments.

Prior to HBO₂ treatment, there was a significant difference of mechanical withdrawal thresholds between the FM condition and the saline control condition following induction of the FM model, replicating the pregabalin results and providing further validity for the acidic saline model in its ability to produce FM-like mechanical evoked pain. It was also anticipated that after each treatment session, specifically among FM animals randomized to the HBO₂ condition, there would be an increase in paw withdrawal thresholds following each treatment session. Data indicated no significant differences in thresholds between FM animals that received HBO₂ compared to those who received a control treatment, until 72 h following the second treatment. Then, animals in the FM condition who received HBO₂ showed significantly lower thresholds than FM animals that received a control treatment. Though this study is the first to investigate the potential effect HBO₂ has on the acidic saline model of FM, these results were not in line with results of the few clinical studies that exhibited HBO₂'s efficacy in FM pain management (22–25). In fact, the results of these studies provided strong evidence within clinical populations for the use of HBO₂ as a potential FM treatment method. Additionally, applying a back-translational approach within the current study indicated that the efficacy of HBO₂ seen in clinical populations did not render increased thresholds among rats induced into a prominent animal model of FM pain. However, it is plausible that the treatment regimen investigated by this study did not reach the therapeutic threshold needed to produce pain alleviation. Therefore, these results from novel exploration of the back-translational application of HBO₂ within Sluka et al.'s acidic saline model of FM (8) suggests future studies should seek to analyze variations in oxygenation and pressurization dosages, as well as the impact of more treatment schedules to potentially alleviate FM-like pain.

For measures of anhedonia, we anticipated that animals in the FM condition that received a control treatment would exhibit the greatest levels of anhedonia, as indicated by the lowest magnitude of sucrose preference. In addition, FM animals that were treated with HBO₂ were anticipated to show sucrose preference similar to controls. Analysis of the sucrose preference data did not support these hypotheses, as no significant differences were found between groups at 24 h, 48 h, or 72 h following the second hyperbaric treatment. These results countered previous work (9, 10) indicating animals within an acidic saline model displayed significant differences in anhedonic behaviors. It should be noted, however, that the current study utilized a different, and more continuous, methodological approach compared to that of Liu and colleagues (9, 10). As a result, we believe these unique results have two potential implications: (1) the acidic saline model was not effective in producing anhedonia, or (2) the pleasure associated with sucrose consumption among rodents is more salient than the pain associated with repeated insults of acidic saline. The consistent and voluminous intake of sucrose by rats has been well-documented (34–36) and has even served to contribute to the development of rodent models of binge eating disorders and the display of addiction-like behaviors (34, 37, 38). Therefore, the natural behavioral repertoire of rats displays strong reward processing associated with sweetened solutions. It is possible that the acidic saline model was not efficacious in inducing a state of anhedonia. However, anhedonia has been documented among both clinical FM patients (39–42) and among acidic saline preclinical representations (9, 10). It is possible that the magnitude of pain produced by the model in this study was not aversive enough to compete with the associated sucrose reward and, thus, presented as an absence of anhedonic behaviors.

It was further hypothesized that animals in the FM condition that received HBO₂ would show significantly less anxiety-like behavior within the open field paradigm indicated by spending less time along the perimeter of the apparatus. Comparably, we anticipated that animals in the FM condition that received a control treatment would show significantly more anxiety-like behavior. Contrary to our hypotheses, analyses of the distance traveled in both the center and the perimeter of the open field apparatus did not support this relationship, as there were no significant between-groups differences. Previous studies produced results indicating that FM is associated with increased anxiety-like behavior in the open field paradigm (9, 11, 12), which calls into question the complexity of the relationship between the acidic saline model and the circumstances under which the model is capable of replicating negative emotionality associated with FM.

The absence of anxiety-like behavior observed in the current studies contradicts previous studies that also utilized the open field paradigm to investigate the negative emotionality associated with the acidic saline model (9, 11, 12). Failure to show anxiolytic behavior might be attributed to errors associated with repeated measures of open field activity. However, a single between-subjects measure of open field activity produced a similar, unexpected absence of anxiety-like behavior (14, 15). The absence of a collective negative affect and sensory experience characteristic of FM likely produced a much less aversive experience and, thus, failed to produce changes in the behavioral repertoire of the animals that might have been observed in measures of sucrose preference or open field activity. Further investigations are necessary to identify the underlying variables that may be contributing to the variation in development of negative affect that has been previously observed.

The expected back-translational efficacy of HBO₂ as a therapeutic agent for treatment of FM pain was derived from clinical manifestations of improved symptomology and pain thresholds associated with the treatment modality (22–25). Additionally, evidence for the use of HBO₂ in treatment of various other presentations of preclinical pain has provided even further promise (17–21). However, the inability of this study to bridge the gap between preclinical pain research and clinical FM research might conceivably be attributed to the treatment regimen employed by this study. This study utilized two treatments of 100% oxygen at 2.0 ATA which, compared to other preclinical studies, was a rather low pressurization and number of treatment sessions (17–21). However, this treatment dosage of 2.0 ATA did prove to be beneficial for previous clinical FM samples (23–25). Overall, there is potential that variations in treatment dosages—as well as variations in the roles that pressurization and oxygenation play as independent factors in HBO₂—may elicit more beneficial therapeutic outcomes than observed in the current study.

The information obtained from preclinical investigations of various pathologies and their affective dimensions serves as a crucial foundation for approaching clinical disease manifestations. In order to successfully utilize these preclinical models, understanding the full translational efficacy, including replication of pathological characteristics and cognition/affectivity, is vital. In the study of FM, a disorder with no single identified etiology or treatment, it is essential we have a complete understanding of the preclinical models used to investigate disease state variability and potential therapeutic approaches. The findings from the current study have provided controversial evidence for the ability of the acidic saline model to fully replicate the complex experience of FM by encompassing its sensory and affective elements. The degree to which we understand the relationship between commonly-observed affectivity in clinical populations versus in preclinical models directly influences further research into underlying biological and psychological mechanisms associated with less-understood pain states, such as FM. Exploring the ability of potential therapeutic modalities to alleviate negative affect associated with chronic pain serves to steer clinical treatment approaches towards a focus not just on the sensory alleviation of pain but, rather, treatment of pain as a sensory, affective, and cognitive experience. While the current study provided conflicting data on the ability of a prominent FM model to replicate its characteristic multidimensional pain experience, as well as alleviate the pain through HBO₂ treatment, future investigations should focus on experimental factors that contribute to development of the previously observed negative affect within the acidic saline model. Additionally, further research should appraise potential alterations in HBO₂ regimens to probe the effects on reduced thresholds produced by the acidic saline model.