Ketogenic diet effects on inflammatory allodynia and ongoing pain in rodents

The ketogenic diet mitigates opioid-induced hyperalgesia by restoring short-chain fatty acids-producing bacteria in the gut

ABSTRACT

Opioids are commonly prescribed to patients with chronic pain. Chronic opioid usage comes with a slew of serious side effects, including opioid-induced hyperalgesia (OIH). The patients with long-term opioid treatment experience paradoxical increases in nociceptive hypersensitivity, namely, OIH. Currently, treatment options for OIH are extremely lacking. In this study, we show that the ketogenic diet recovers the abnormal pain behavior caused by chronic morphine treatment in male mice, and we further show that the therapeutic effect of the ketogenic diet is mediated through gut microbiome. Our 16S rRNA sequencing demonstrates that chronic morphine treatment causes changes in mouse gut microbiota, specifically a decrease in short-chain fatty acids-producing bacteria, and the sequencing data also show that the ketogenic diet rescues those bacteria in the mouse gut. More importantly, we show that supplementation with short-chain fatty acids (butyrate, propionate, and acetate) can delay the onset of OIH, indicating that short-chain fatty acids play a direct role in the development of OIH. Our findings suggest that gut microbiome could be targeted to treat OIH, and the ketogenic diet can be used as a complementary approach for pain relief in patients with chronic opioid treatment. We only used male mice in this study, and thus, our findings cannot be generalized to both sexes.

Addressing Pain Using a Mediterranean Ketogenic Nutrition Program in Older Adults with Mild Cognitive Impairment

Chronic pain has negative physical and cognitive consequences in older adults and may lead to a poorer quality of life. Mediterranean ketogenic nutrition (MKN) is a promising nonpharmacological intervention for pain management, but long-term adherence is challenging due to the carbohydrate restrictive diet regimen. The main objective of this study was to evaluate the effects of the pilot MKN Adherence (MKNA) Program on pain in older adults with mild cognitive impairment and to assess whether improvements in self-reported pain were associated with adherence to MKN. Older adults (N = 58) aged 60-85 with possible mild cognitive impairment were randomized to a 6-week MKNA arm or an MKN Education (MKNE) program arm. Both arms received the same nutrition education and group format; however, the MKNA arm received additional motivational interviewing and cognitive behavioral skills to enhance adherence. Changes in self-reported pain (Brief Pain Inventory, Roland Morris, Patient's Global Impression of Change) and adherence to MKN (ketone levels, self-reported adherence) were assessed at baseline, 6-weeks, and 3-months post intervention. Both arms showed clinically significant reductions in pain. Greater adherence to MKN across the 6-week intervention was associated with higher ratings of pain-related changes on the Patient's Global Impression of Change scale. Based on these findings, adherence to MKN may promote improvements in self-reported pain in older adults with mild cognitive impairment and findings support the need for future full-scale randomized clinical trials evaluating MKN programs on pain. Trial Registration: Clinicaltrials.gov ID: NCT04817176.

Differential peripheral immune signatures elicited by vegan versus ketogenic diets in humans

Nutrition has broad impacts on all physiological processes. However, how nutrition affects human immunity remains largely unknown. Here we explored the impact of a dietary intervention on both immunity and the microbiota by performing a post hoc analysis of a clinical trial in which each of the 20 participants sequentially consumed vegan or ketogenic diets for 2 weeks ( NCT03878108 ). Using a multiomics approach including multidimensional flow cytometry, transcriptomic, proteomic, metabolomic and metagenomic datasets, we assessed the impact of each diet, and dietary switch, on host immunity and the microbiota. Our data revealed that overall, a ketogenic diet was associated with a significant upregulation of pathways and enrichment in cells associated with the adaptive immune system. In contrast, a vegan diet had a significant impact on the innate immune system, including upregulation of pathways associated with antiviral immunity. Both diets significantly and differentially impacted the microbiome and host-associated amino acid metabolism, with a strong downregulation of most microbial pathways following ketogenic diet compared with baseline and vegan diet. Despite the diversity of participants, we also observed a tightly connected network between datasets driven by compounds associated with amino acids, lipids and the immune system. Collectively, this work demonstrates that in diverse participants 2 weeks of controlled dietary intervention is sufficient to significantly and divergently impact host immunity, which could have implications for precision nutritional interventions. ClinicalTrials.gov registration: NCT03878108 .

Common and divergent molecular mechanisms of fasting and ketogenic diets

Intermittent short-term fasting (ISTF) and ketogenic diets (KDs) exert overlapping but not identical effects on cell metabolism, function, and resilience. Whereas health benefits of KD are largely mediated by the ketone bodies (KBs), ISTF engages additional adaptive physiological responses. KDs act mainly through inhibition of histone deacetylases (HDACs), reduction of oxidative stress, improvement of mitochondria efficiency, and control of inflammation. Mechanisms of action of ISTF include stimulation of autophagy, increased insulin and leptin sensitivity, activation of AMP-activated protein kinase (AMPK), inhibition of the mechanistic target of rapamycin (mTOR) pathway, bolstering mitochondrial resilience, and suppression of oxidative stress and inflammation. Frequent switching between ketogenic and nonketogenic states may optimize health by increasing stress resistance, while also enhancing cell plasticity and functionality.

Ketogenic Diet Alleviates Mechanical Allodynia in the Models of Inflammatory and Neuropathic Pain in Male Mice

Inflammation is involved in the induction of chronic inflammatory and neuropathic pain. Moreover, the ketogenic diet, a high-fat, low-carbohydrate, and adequate protein diet, has an anti-inflammatory effect. Thus, we hypothesized that a ketogenic diet has a therapeutic effect on both types of chronic pain. In the present study, we investigated the effect of a ketogenic diet on mechanical allodynia, a chronic pain symptom, in formalin-induced chronic inflammatory pain and nerve injury-induced neuropathic pain models using adult male mice. Formalin injection into the hind paw induced mechanical allodynia in both the injected and intact hind paws, and the ketogenic diet alleviated mechanical allodynia in both hind paws. In addition, the ketogenic diet prevented formalin-induced edema. Furthermore, the diet alleviated mechanical allodynia induced by peripheral nerve injury. Thus, these findings indicate that a ketogenic diet has a therapeutic effect on chronic pain induced by inflammation and nerve injury.

A Potential Role for the Ketogenic Diet in Alzheimer's Disease Treatment: Exploring Pre-Clinical and Clinical Evidence

Given the remarkable progress in global health and overall quality of life, the significant rise in life expectancy has become intertwined with the surging occurrence of neurodegenerative disorders (NDs). This emerging trend is poised to pose a substantial challenge to the fields of medicine and public health in the years ahead. In this context, Alzheimer's disease (AD) is regarded as an ND that causes recent memory loss, motor impairment and cognitive deficits. AD is the most common cause of dementia in the elderly and its development is linked to multifactorial interactions between the environment, genetics, aging and lifestyle. The pathological hallmarks in AD are the accumulation of β-amyloid peptide (Aβ), the hyperphosphorylation of tau protein, neurotoxic events and impaired glucose metabolism. Due to pharmacological limitations and in view of the prevailing glycemic hypometabolism, the ketogenic diet (KD) emerges as a promising non-pharmacological possibility for managing AD, an approach that has already demonstrated efficacy in addressing other disorders, notably epilepsy. The KD consists of a food regimen in which carbohydrate intake is discouraged at the expense of increased lipid consumption, inducing metabolic ketosis whereby the main source of energy becomes ketone bodies instead of glucose. Thus, under these dietary conditions, neuronal death via lack of energy would be decreased, inasmuch as the metabolism of lipids is not impaired in AD. In this way, the clinical picture of patients with AD would potentially improve via the slowing down of symptoms and delaying of the progression of the disease. Hence, this review aims to explore the rationale behind utilizing the KD in AD treatment while emphasizing the metabolic interplay between the KD and the improvement of AD indicators, drawing insights from both preclinical and clinical investigations. Via a comprehensive examination of the studies detailed in this review, it is evident that the KD emerges as a promising alternative for managing AD. Moreover, its efficacy is notably enhanced when dietary composition is modified, thereby opening up innovative avenues for decreasing the progression of AD.

Feasibility and impact of ketogenic dietary interventions in polycystic kidney disease: KETO-ADPKD-a randomized controlled trial

Ketogenic dietary interventions (KDIs) are beneficial in animal models of autosomal-dominant polycystic kidney disease (ADPKD). KETO-ADPKD, an exploratory, randomized, controlled trial, is intended to provide clinical translation of these findings (NCT04680780). Sixty-six patients were randomized to a KDI arm (ketogenic diet [KD] or water fasting [WF]) or the control group. Both interventions induce significant ketogenesis on the basis of blood and breath acetone measurements. Ninety-five percent (KD) and 85% (WF) report the diet as feasible. KD leads to significant reductions in body fat and liver volume. Additionally, KD is associated with reduced kidney volume (not reaching statistical significance). Interestingly, the KD group exhibits improved kidney function at the end of treatment, while the control and WF groups show a progressive decline, as is typical in ADPKD. Safety-relevant events are largely mild, expected (initial flu-like symptoms associated with KD), and transient. Safety assessment is complemented by nuclear magnetic resonance (NMR) lipid profile analyses.

The Ketogenic Effect of SGLT-2 Inhibitors-Beneficial or Harmful?

Sodium-glucose cotransporter-2 (SGLT-2) inhibitors, also called gliflozins or flozins, are a class of drugs that have been increasingly used in the management of type 2 diabetes mellitus (T2DM) due to their glucose-lowering, cardiovascular (CV), and renal positive effects. However, recent studies suggest that SGLT-2 inhibitors might also have a ketogenic effect, increasing ketone body production. While this can be beneficial for some patients, it may also result in several potential unfavorable effects, such as decreased bone mineral density, infections, and ketoacidosis, among others. Due to the intricate and multifaceted impact caused by SGLT-2 inhibitors, this initially anti-diabetic class of medications has been effectively used to treat both patients with chronic kidney disease (CKD) and those with heart failure (HF). Additionally, their therapeutic potential appears to extend beyond the currently investigated conditions. The objective of this review article is to present a thorough summary of the latest research on the mechanism of action of SGLT-2 inhibitors, their ketogenesis, and their potential synergy with the ketogenic diet for managing diabetes. The article particularly discusses the benefits and risks of combining SGLT-2 inhibitors with the ketogenic diet and their clinical applications and compares them with other anti-diabetic agents in terms of ketogenic effects. It also explores future directions regarding the ketogenic effects of SGLT-2 inhibitors.

ATP-gated potassium channels contribute to ketogenic diet-mediated analgesia in mice

Chronic pain is a substantial health burden and options for treating chronic pain remain minimally effective. Ketogenic diets are emerging as well-tolerated, effective therapeutic strategies in preclinical models of chronic pain, especially diabetic neuropathy. We tested whether a ketogenic diet is antinociceptive through ketone oxidation and related activation of ATP-gated potassium (KATP) channels in mice. We demonstrate that consumption of a ketogenic diet for one week reduced evoked nocifensive behaviors (licking, biting, lifting) following intraplantar injection of different noxious stimuli (methylglyoxal, cinnamaldehyde, capsaicin, or Yoda1) in mice. A ketogenic diet also decreased the expression of p-ERK, an indicator of neuronal activation in the spinal cord, following peripheral administration of these stimuli. Using a genetic mouse model with deficient ketone oxidation in peripheral sensory neurons, we demonstrate that protection against methylglyoxal-induced nociception by a ketogenic diet partially depends on ketone oxidation by peripheral neurons. Injection of tolbutamide, a KATP channel antagonist, prevented ketogenic diet-mediated antinociception following intraplantar capsaicin injection. Tolbutamide also restored the expression of spinal activation markers in ketogenic diet-fed, capsaicin-injected mice. Moreover, activation of KATP channels with the KATP channel agonist diazoxide reduced pain-like behaviors in capsaicin-injected, chow-fed mice, similar to the effects observed with a ketogenic diet. Diazoxide also reduced the number of p-ERK+ cells in capsaicin-injected mice. These data support a mechanism that includes neuronal ketone oxidation and activation of KATP channels to provide ketogenic diet-related analgesia. This study also identifies KATP channels as a new target to mimic the antinociceptive effects of a ketogenic diet.

Ketolysis is required for the proper development and function of the somatosensory nervous system

Ketogenic diets are emerging as protective interventions in preclinical and clinical models of somatosensory nervous system disorders. Additionally, dysregulation of succinyl-CoA 3-oxoacid CoA-transferase 1 (SCOT, encoded by Oxct1), the fate-committing enzyme in mitochondrial ketolysis, has recently been described in Friedreich's ataxia and amyotrophic lateral sclerosis. However, the contribution of ketone metabolism in the normal development and function of the somatosensory nervous system remains poorly characterized. We generated sensory neuron-specific, Advillin-Cre knockout of SCOT (Adv-KO-SCOT) mice and characterized the structure and function of their somatosensory system. We used histological techniques to assess sensory neuronal populations, myelination, and skin and spinal dorsal horn innervation. We also examined cutaneous and proprioceptive sensory behaviors with the von Frey test, radiant heat assay, rotarod, and grid-walk tests. Adv-KO-SCOT mice exhibited myelination deficits, altered morphology of putative Aδ soma from the dorsal root ganglion, reduced cutaneous innervation, and abnormal innervation of the spinal dorsal horn compared to wildtype mice. Synapsin 1-Cre-driven knockout of Oxct1 confirmed deficits in epidermal innervation following a loss of ketone oxidation. Loss of peripheral axonal ketolysis was further associated with proprioceptive deficits, yet Adv-KO-SCOT mice did not exhibit drastically altered cutaneous mechanical and thermal thresholds. Knockout of Oxct1 in peripheral sensory neurons resulted in histological abnormalities and severe proprioceptive deficits in mice. We conclude that ketone metabolism is essential for the development of the somatosensory nervous system. These findings also suggest that decreased ketone oxidation in the somatosensory nervous system may explain the neurological symptoms of Friedreich's ataxia.

Emerging Nonpharmacologic Interventions to Treat Diabetic Peripheral Neuropathy

Significance: Diabetic peripheral neuropathy (DPN), a complication of metabolic syndrome, type I and type II diabetes, leads to sensory changes that include slow nerve conduction, nerve degeneration, loss of sensation, pain, and gate disturbances. These complications remain largely untreatable, although tight glycemic control can prevent neuropathy progression. Nonpharmacologic approaches remain the most impactful to date, but additional advances in treatment approaches are needed. Recent Advances: This review highlights several emerging interventions, including a focus on dietary interventions and physical activity, that continue to show promise for treating DPN. We provide an overview of our current understanding of how exercise can improve aspects of DPN. We also highlight new studies in which a ketogenic diet has been used as an intervention to prevent and reverse DPN. Critical Issues: Both exercise and consuming a ketogenic diet induce systemic and cellular changes that collectively improve complications associated with DPN. Both interventions may involve similar signaling pathways and benefits but also impact DPN through unique mechanisms. Future Directions: These lifestyle interventions are critically important as personalized medicine approaches will likely be needed to identify specific subsets of neuropathy symptoms and deficits in patients, and determine the most impactful treatment. Overall, these two interventions have the potential to provide meaningful relief for patients with DPN and provide new avenues to identify new therapeutic targets.

Up-Regulation of ProBDNF/p75NTR Signaling in Spinal Cord Drives Inflammatory Pain in Male Rats

Background

The spinal cord expresses brain-derived neurotrophic factor precursor (proBDNF) and its receptor pan neurotrophin receptor 75 (p75^NTR). However, the role of spinal proBDNF signaling in the pathogenesis of inflammatory pain remains unknown.

Methods

Rats were locally injected with complete Freund's adjuvant (CFA) to induce inflammatory pain. The proBDNF signal expression was detected by double-labeled immunofluorescence. ProBDNF protein, p75^NTR extracellular domain (p75^NTR-ECD), or monoclonal anti-proBDNF (McAb-proB) were administrated by intrathecal injection to investigate their effects on pain behavior. Paw withdrawal thermal latency (PWL) and paw withdrawal mechanical threshold (PWT) were performed to evaluate pain behavior. Immunoblotting, immunohistochemistry, and immunofluorescence were used to assess inflammation-induced biochemical changes.

Results

CFA induced a rapid increase in proBDNF in the ipsilateral spinal cord, and immunofluorescence revealed that CFA-enhanced proBDNF was expressed in NeuN positive neurons and GFAP positive astrocytes. The administration of furin cleavage-resistant proBDNF via intrathecal injection (I.t.) significantly decreased the PWT and PWL, whereas McAb-proB by I.t. alleviated CFA-induced pain-like hypersensitivity in rats. Meanwhile, CFA administration triggered the activation of p75^NTR and its downstream signaling extracellular signal-regulated kinase 1/2 (ERK1/2) and nuclear factor (NF)-kappaB p65 in the spinal cord. I.t. administration of p75^NTR-ECD suppressed CFA-induced pain and neuroinflammation, including the expression of p-ERK1/2, p-p65, and the gene expression of tumor necrosis factor-α (TNF-α), and interleukin 6 (IL-6).

Conclusion

Our study reveals that the activated proBDNF/p75^NTRsignaling in the spinal cord contributes to the development of CFA-induced inflammatory pain. McAb-proB and p75^NTR-ECD appear to be promising therapeutic agents for inflammatory pain.

Ketogenic Diet and Inflammation: Implications for Mood and Anxiety Disorders

The ketogenic diet, known as a low-carbohydrate, high-protein, and high-fat diet, drastically restrains the major source of energy for the body, forcing it to burn all excess fat through a process called ketosis-the breaking down of fat into ketone bodies. First suggested as a medical treatment for children suffering from epilepsy, this diet has gained increased popularity as a rapid weight loss strategy. Over the past few years, there have been numerous studies suggesting that the ketogenic diet may provide therapeutic effects for several psychiatric conditions such as mood- and anxiety-related disorders. However, despite significant progress in research, the mechanisms underlying its therapeutic effects remain largely unexplored and are yet to be fully elucidated. This chapter provides an in-depth overview of preclinical and clinical evidence supporting the use of a ketogenic diet in the management of mood and anxiety disorders and discusses its relationship with inflammatory processes and potential mechanisms of actions for its therapeutic effects.

Ketogenic diet prevents methylglyoxal-evoked nociception by scavenging methylglyoxal

ABSTRACT

Methylglyoxal (MGO) is a reactive dicarbonyl byproduct of glycolysis implicated in a growing number of neuropathic pain conditions, including chemotherapy-induced peripheral neuropathy, diabetic peripheral neuropathy, and radiculopathy with lumbar disk herniation. Recent studies show success in preclinical models treating these disorders with an interventional ketogenic diet. Here, we tested the hypothesis that a ketogenic diet modifies pathological MGO signaling as a mechanism underlying neuropathy improvement. We found that mice injected with MGO displayed nocifensive behaviors, whereas mice prefed a ketogenic diet were resistant to mechanical allodynia elicited by MGO. In addition, levels of circulating MGO were reduced in ketogenic diet-fed mice and negatively correlated with levels of the ketone body β-hydroxybutyrate (β-HB). Methylglyoxal is normally scavenged by the glyoxalase system, and ketogenic diet-fed mice displayed increased glyoxalase 1 activity compared with chow-fed control mice. Recent studies also suggest that ketone bodies contribute to MGO detoxification, consistent with a negative correlation between β-HB and MGO. To assess whether ketone bodies modified MGO-evoked nociception through direct MGO detoxification, we coincubated either acetoacetate or β-HB with MGO before injection. Mice receiving intraplantar MGO injection exhibit increased nociceptive behavior (lifting, licking, biting, and scratching), which was significantly reduced by coincubation with either acetoacetate or β-HB. Methylglyoxal increased phospho-extracellular signal-regulated kinase-positive cells in the spinal dorsal horn, and this evoked spinal activation was ameliorated by preincubation with acetoacetate or β-HB. These results suggest that a ketogenic diet and ketone bodies ameliorate MGO-evoked nociception, partially through detoxification of MGO, and provide rationale for therapeutic intervention with a ketogenic diet in MGO-driven pathologies.

Low-Carbohydrate High-Fat Diet: A SWOC Analysis

Insulin resistance (IR) plays a role in the pathogenesis of many diseases, such as type 2 diabetes mellitus, cardiovascular disease, non-alcoholic fatty liver disease, obesity, and neurodegenerative diseases, including Alzheimer's disease. The ketogenic diet (KD) is a low-carbohydrate/high-fat diet that arose in the 1920s as an effective treatment for seizure control. Since then, the KD has been studied as a therapeutic approach for various IR-related disorders with successful results. To date, the use of the KD is still debatable regarding its safety. Some studies have acknowledged its usefulness, while others do not recommend its long-term implementation. In this review, we applied a SWOC (Strengths, Weaknesses, Opportunities, and Challenges) analysis that revealed the positive, constructive strengths of the KD, its potential complications, different conditions that can make used for it, and the challenges faced by both physicians and subjects throughout a KD. This SWOC analysis showed that the KD works on the pathophysiological mechanism of IR-related disorders such as chronic inflammation, oxidative stress and mitochondrial stress. Furthermore, the implementation of the KD as a potential adjuvant therapy for many diseases, including cancer, neurodegenerative disorders, polycystic ovary syndrome, and pain management was proven. On the other hand, the short and long-term possible undesirable KD-related effects, including nutritional deficiencies, growth retardation and nephrolithiasis, should be considered and strictly monitored. Conclusively, this review provides a context for decision-makers, physicians, researchers, and the general population to focus on this dietary intervention in preventing and treating diseases. Moreover, it draws the attention of scientists and physicians towards the opportunities and challenges associated with the KD that requires attention before KD initiation.

Pharmacological and non-pharmacological therapeutic interventions for the treatment of spinal cord injury-induced pain

Spinal cord injury (SCI) is a complex neurophysiological disorder, which can result in many long-term complications including changes in mobility, bowel and bladder function, cardiovascular function, and metabolism. In addition, most individuals with SCI experience some form of chronic pain, with one-third of these individuals rating their pain as severe and unrelenting. SCI-induced chronic pain is considered to be "high impact" and broadly affects a number of outcome measures, including daily activity, physical and cognitive function, mood, sleep, and overall quality of life. The majority of SCI pain patients suffer from pain that emanates from regions located below the level of injury. This pain is often rated as the most severe and the underlying mechanisms involve injury-induced plasticity along the entire neuraxis and within the peripheral nervous system. Unfortunately, current therapies for SCI-induced chronic pain lack universal efficacy. Pharmacological treatments, such as opioids, anticonvulsants, and antidepressants, have been shown to have limited success in promoting pain relief. In addition, these treatments are accompanied by many adverse events and safety issues that compound existing functional deficits in the spinally injured, such as gastrointestinal motility and respiration. Non-pharmacological treatments are safer alternatives that can be specifically tailored to the individual and used in tandem with pharmacological therapies if needed. This review describes existing non-pharmacological therapies that have been used to treat SCI-induced pain in both preclinical models and clinical populations. These include physical (i.e., exercise, acupuncture, and hyper- or hypothermia treatments), psychological (i.e., meditation and cognitive behavioral therapy), and dietary interventions (i.e., ketogenic and anti-inflammatory diet). Findings on the effectiveness of these interventions in reducing SCI-induced pain and improving quality of life are discussed. Overall, although studies suggest non-pharmacological treatments could be beneficial in reducing SCI-induced chronic pain, further research is needed. Additionally, because chronic pain, including SCI pain, is complex and has both emotional and physiological components, treatment should be multidisciplinary in nature and ideally tailored specifically to the patient.

A ketogenic diet reduces mechanical allodynia and improves epidermal innervation in diabetic mice

ABSTRACT

Dietary interventions are promising approaches to treat pain associated with metabolic changes because they impact both metabolic and neural components contributing to painful neuropathy. Here, we tested whether consumption of a ketogenic diet could affect sensation, pain, and epidermal innervation loss in type 1 diabetic mice. C57Bl/6 mice were rendered diabetic using streptozotocin and administered a ketogenic diet at either 3 weeks (prevention) or 9 weeks (reversal) of uncontrolled diabetes. We quantified changes in metabolic biomarkers, sensory thresholds, and epidermal innervation to assess impact on neuropathy parameters. Diabetic mice consuming a ketogenic diet had normalized weight gain, reduced blood glucose, elevated blood ketones, and reduced hemoglobin-A1C levels. These metabolic biomarkers were also improved after 9 weeks of diabetes followed by 4 weeks of a ketogenic diet. Diabetic mice fed a control chow diet developed rapid mechanical allodynia of the hind paw that was reversed within a week of consumption of a ketogenic diet in both prevention and reversal studies. Loss of thermal sensation was also improved by consumption of a ketogenic diet through normalized thermal thresholds. Finally, diabetic mice consuming a ketogenic diet had normalized epidermal innervation, including after 9 weeks of uncontrolled diabetes and 4 weeks of consumption of the ketogenic diet. These results suggest that, in mice, a ketogenic diet can prevent and reverse changes in key metabolic biomarkers, altered sensation, pain, and axon innervation of the skin. These results identify a ketogenic diet as a potential therapeutic intervention for patients with painful diabetic neuropathy and/or epidermal axon loss.

How Is Peripheral Injury Signaled to Satellite Glial Cells in Sensory Ganglia?

Injury or inflammation in the peripheral branches of neurons of sensory ganglia causes changes in neuronal properties, including excessive firing, which may underlie chronic pain. The main types of glial cell in these ganglia are satellite glial cells (SGCs), which completely surround neuronal somata. SGCs undergo activation following peripheral lesions, which can enhance neuronal firing. How neuronal injury induces SGC activation has been an open question. Moreover, the mechanisms by which the injury is signaled from the periphery to the ganglia are obscure and may include electrical conduction, axonal and humoral transport, and transmission at the spinal level. We found that peripheral inflammation induced SGC activation and that the messenger between injured neurons and SGCs was nitric oxide (NO), acting by elevating cyclic guanosine monophosphate (cGMP) in SGCs. These results, together with work from other laboratories, indicate that a plausible (but not exclusive) mechanism for neuron-SGCs interactions can be formulated as follows: Firing due to peripheral injury induces NO formation in neuronal somata, which diffuses to SGCs. This stimulates cGMP synthesis in SGCs, leading to their activation and to other changes, which contribute to neuronal hyperexcitability and pain. Other mediators such as proinflammatory cytokines probably also contribute to neuron-SGC communications.