High-fat diet causes mechanical allodynia in the absence of injury or diabetic pathology.

Understanding the interactions between diet, obesity, and diabetes is important to tease out mechanisms in painful pathology. Western diet is rich in fats, producing high amounts of circulating bioactive metabolites. However, no research has assessed how a high-fat diet (HFD) alone may sensitize an individual to non-painful stimuli in the absence of obesity or diabetic pathology. To investigate this, we tested the ability of a HFD to stimulate diet-induced hyperalgesic priming, or diet sensitization in male and female mice. Our results revealed that 8 weeks of HFD did not alter baseline pain sensitivity, but both male and female HFD-fed animals exhibited robust mechanical allodynia when exposed to a subthreshold dose of intraplantar Prostaglandin E2 (PGE2) compared to mice on chow diet. Furthermore, calcium imaging in isolated primary sensory neurons of both sexes revealed HFD induced an increased percentage of capsaicin-responsive neurons compared to their chow counterparts. Immunohistochemistry (IHC) showed a HFD-induced upregulation of ATF3, a neuronal marker of injury, in lumbar dorsal root ganglia (DRG). This suggests that a HFD induces allodynia in the absence of a pre-existing condition or injury via dietary components. With this new understanding of how a HFD can contribute to the onset of pain, we can understand the dissociation behind the comorbidities associated with obesity and diabetes to develop pharmacological interventions to treat them more efficiently.

Alleviation of paclitaxel-induced mechanical hypersensitivity and hyperalgesic priming with AMPK activators in male and female mice.

AMP-activated protein kinase (AMPK) is an energy-sensing kinase that has emerged as a novel therapeutic target for pain due to its ability to inhibit mechanistic target of rapamycin (mTOR) and mitogen activated protein kinase (MAPK) signaling, two signaling pathways that are linked to pain promotion after injury as well as the development of hyperalgesic priming. MAPK and mTOR signaling are also implicated in chemotherapy induced peripheral neuropathy (CIPN). We conducted a series of experiments to gain further insight into how AMPK activators might best be used to treat pain in both sexes in the setting of CIPN from paclitaxel. We also assessed whether hyperalgesic priming emerges from paclitaxel treatment and if this can be prevented by AMPK targeting. AMPK can be pharmacologically activated indirectly through regulation of upstream kinases like liver kinase B1 (LKB1) or directly using positive allosteric modulators. We used the indirect AMPK activators metformin and narciclasine, both of which have been shown to reduce pain in preclinical models but with much different potencies and different efficacies depending on the sex of the animal. We used the direct AMPK activator MK8722 because it is the most potent and specific such activator described to date. Here, the AMPK activators were used in 2 different treatment paradigms. First the drugs were given concurrently with paclitaxel to test whether they prevent mechanical hypersensitivity. Second the AMPK activators were given after the completion of paclitaxel treatment to test whether they reverse established mechanical hypersensitivity. Consistent with our previously published findings with metformin, narciclasine (1 mg/kg) produced an anti-hyperalgesic effect, preventing paclitaxel-induced neuropathy in outbred mice of both sexes. In contrast to metformin, narciclasine also reversed mechanical hypersensitivity in established CIPN. Both metformin (200 mg/kg) and narciclasine prevented the development of hyperalgesic priming induced by paclitaxel treatment. MK8722 (30 mg/kg) had no effect on mechanical hypersensitivity caused by paclitaxel in either the prevention or reversal treatment paradigms. However, MK8722 did attenuate hyperalgesic priming in male and female mice. We conclude that paclitaxel induces robust hyperalgesic priming that is prevented by AMPK targeting and that narciclasine is a particularly attractive candidate for further development as a CIPN treatment.

Hyperosmolar potassium inhibits myofibroblast conversion and reduces scar tissue formation.

Scar formation is a natural result of almost all wound healing in adult mammals. Unfortunately, scarring disrupts normal tissue function and can cause significant physical and psychological distress. In addition to improving surgical techniques to limit scar formation, several therapies are under development towards the same goal. Many of these treatments aim to disrupt transforming growth factor ß1 (TGFß1) signaling, as this is a critical control point for fibroblast differentiation into myofibroblasts; a contractile cell that organizes synthesized collagen fibrils into scar tissue. The present study aimed to examine the role of hyperosmolar potassium gluconate (KGluc) on fibroblast function in skin repair. KGluc was first determined to negatively regulate fibroblast proliferation, metabolism, and migration in a dose-dependent manner in vitro. Increasing concentrations of KGluc also inhibited differentiation into myofibroblasts, suggesting that local KGluc treatment might reduce fibrosis. KGluc delivery was confirmed via loading into collagen hydrogels and used to treat a full thickness skin wound in mice. KGluc qualitatively slowed initial closure of the wounds and resulted in tissue that more closely resembled mature, healthy skin (epidermal thickness and dermal-epidermal morphology) when compared to unloaded collagen hydrogels. KGluc treatment significantly reduced the number of myofibroblasts within the dermis while upregulated blood vessel density with respect to unloaded hydrogels, likely a result of disruption of TGFß1 signaling. Taken together, these data demonstrate the effectiveness of KGluc treatment on skin wound healing and suggest that this may be an efficient treatment to limit scar formation.