Melatonin prevents mitochondrial dysfunction and insulin resistance in rat skeletal muscle.

Om hvordan melatonin beskytter mitokondriene i muskler og insulin resistens.

Teodore et.al. 2014. http://www.ncbi.nlm.nih.gov/pubmed/24981026

Abstract

Melatonin has a number of beneficial metabolic actions and reduced levels of melatonin may contribute to type 2 diabetes. The present study investigated the metabolic pathways involved in the effects of melatonin on mitochondrial function and insulin resistance in rat skeletal muscle. The effect of melatonin was tested both in vitro in isolated rats skeletal muscle cells and in vivo using pinealectomized rats (PNX). Insulin resistance was induced in vitro by treating primary rat skeletal muscle cells with palmitic acid for 24 hr. Insulin-stimulated glucose uptake was reduced by palmitic acid followed by decreased phosphorylation of AKT which was prevented my melatonin. Palmitic acid reduced mitochondrial respiration, genes involved in mitochondrial biogenesis and the levels of tricarboxylic acid cycle intermediates whereas melatonin counteracted all these parameters in insulin-resistant cells. Melatonin treatment increases CAMKII and p-CREB but had no effect on p-AMPK. Silencing of CREB protein by siRNA reduced mitochondrial respiration mimicking the effect of palmitic acid and prevented melatonin-induced increase in p-AKT in palmitic acid-treated cells. PNX rats exhibited mild glucose intolerance, decreased energy expenditure and decreased p-AKT, mitochondrial respiration, and p-CREB and PGC-1 alpha levels in skeletal muscle which were restored by melatonin treatment in PNX rats. In summary, we showed that melatonin could prevent mitochondrial dysfunction and insulin resistance via activation of CREB-PGC-1 alpha pathway. Thus, the present work shows that melatonin play an important role in skeletal muscle mitochondrial function which could explain some of the beneficial effects of melatonin in insulin resistance states.

Inflammatory Cytokine Concentrations Are Acutely Increased by Hyperglycemia in Humans

Denne viser hvordan selv de uten diabetes får økt cytokinverdi (betennelse) i blodet i 1-2 timer etter blodsukkerstigning. I denne studien var det snakk om blodsukker over 15 mmol/L. De sier at blodsukker økninger påvirker cytokinnivået mer enn et stabilt høyt blodsukker.

http://circ.ahajournals.org/content/106/16/2067.full

Control Subjects:

Plasma IL-6 levels rose from a basal value of 2.0±0.7 pg/mL to a peak of 3.1±0.9 pg/mL at 1 hour (P<0.01) and returned to basal level at 3 hours (Figure 2).

Fasting plasma TNF-α levels were 3.3±1.2 pg/mL; they peaked at 1 hour (4.9±1.4 pg/mL, P<0.01), and returned to baseline at 3 hours.

Plasma IL-18 levels rose from a basal value of 116±28 pg/mL to a peak of 140±31 pg/mL at 2 hours (P<0.01) and returned to basal levels at 3 hours (110±26 pg/mL).

The novel findings of the present study were that (1) acute hyperglycemia in control and in IGT subjects induces an increase in plasma IL-6, TNF-α, and IL-18 concentrations; (2) the effect of sustained hyperglycemia is reproduced by transient oscillations in plasma glucose and is amplified by the IGT status; and (3) the antioxidant glutathione completely prevents the rise in plasma cytokines induced by hyperglycemia. These results indicate that hyperglycemic spikes affect cytokine concentrations more than continuous hyperglycemia, at least in the short term, and suggest that an oxidative mechanism mediates the effect of hyperglycemia.

Another finding of the present study was that glutathione, a powerful antioxidant, completely prevented cytokine increase induced by oscillatory hyperglycemia in healthy humans. Hyperglycemia-induced oxidative stress, 32 along with soluble advanced glycation end products and products of lipid peroxidation, possibly serves as a key activator of upstream kinases, leading to induction of inflammatory gene expression.33

Hyperglycemia enhances the cytokine production and oxidative responses to a low but not high dose of endotoxin in rats.

Denne beskriver hvordan hyperglycemi (regnes som blodsukker over 7 mmol/L i lengre perioder, eller fastende blodsukker over 7) gir økt cytokin-aktivitet i flere timer etter en stressende episode. Om man spiser en snickers går blodsukkeret opp til over 10, og om man kontinuerlig spiser mat som øker blodsukkeret er det en stor sjangse for at man har en kronisk betennelsesreaskjon med økt cytokin aktivitet.

Kobler vi det med denne, som nevner at cytokiner tilført fra utenfor muskelen kan gi hyperalgesi, så begynner bildet å bli klarere: «One mechanism of action, the immune-to-brain communication through activation of brain and spinal cord glial cells was reviewed by Wieseler-Frank et al. (2005). Activation of CNS glia and subsequent production of inflammatory cytokines can lead to hyperalgesia.» http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1552097/

Abstract

OBJECTIVE:

The aim of this study was to investigate whether hyperglycemia enhances the systemic inflammatory response and oxidative stress induced by endotoxin.

DESIGN:

Laboratory investigation.

SETTING:

University medical school.

SUBJECTS:

Forty-one male Sprague-Dawley rats.

INTERVENTIONS:

A hyperglycemic condition was produced in rats by glucose clamp for 3 hrs. Immediately on stopping the glucose infusion, animals received different doses of endotoxin injection (0, 0.2, or 1 mg/kg), and then blood glucose concentration was monitored over the ensuing 2 hrs. At the end of 2 hrs, levels of tumor necrosis factor-alpha, interleukin-1beta, interleukin-6, corticosterone, and alpha-1 acid glycoprotein were determined in serum, and malondialdehyde and total glutathione content were determined in the liver.

MEASUREMENTS AND MAIN RESULTS:

Over the 2-hr period, blood glucose concentrations returned to normal in initially hyperglycemic rats. However, the levels of cytokines, corticosterone, and alpha-1 acid glycoprotein were significantly higher in these animals compared with nonhyperglycemic controls, demonstrating an extended effect of prior hyperglycemia on markers of systemic inflammation. With low-dose (0.2 mg/kg) but not high-dose (1 mg/kg) endotoxin administration, hyperglycemic animals had significantly higher levels of cytokines compared with controls, indicating that prior hyperglycemia can enhance the systemic inflammatory response to a moderate endotoxin dose, but that the maximum effects of endotoxin on production of inflammatory cytokines are not altered by transient high glucose exposure.

CONCLUSIONS:

Systemic inflammation persists for a period following hyperglycemia, and this can enhance the systemic inflammatory response to a subsequent moderate stress.

Noen studier om hvordan Substans P forholder seg til mat

Denne nevner at en 10% reduksjon av anbefalt daglig magnesium inntakt øker sjangsen for osteoporose og Substans P

Bone Loss Induced by Dietary Magnesium Reduction to 10% of the Nutrient Requirement in Rats Is Associated with Increased Release of Substance P and Tumor Necrosis Factor-α1 

http://jn.nutrition.org/content/134/1/79.long

These data demonstrated that a Mg intake of 10% of NR in rats causes bone loss that may be secondary to the increased release of substance P and TNF-α.

Denne nevner hvordan tiltak som reduserer SP bidrar til å redusere de negative virkningene av magnesiummangel.

Neurogenic Inflammation and Cardiac Dysfunction due to Hypomagnesemia.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3753099/

Significant protection against most of these MgD-mediated events has been observed with interventions that modulate neuronal SP release or its bioactivity, and with several antioxidants (vitamin E, probucol, epicaptopril, d-propranolol). In view of the clinical prevalence of hypomagnesemia, new treatments, beyond magnesium repletion, may be needed to diminish deleterious neurogenic and prooxidative components described in this article.

Denne nevner hvordan SP er involvert i insulin regulering og diabetes.

Role of Substance P in the Regulation of Glucose Metabolism via Insulin Signaling-Associated Pathways

 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3230056/

Our results demonstrate an important role for SP in adipose tissue responses and obesity-associated pathologies. These novel SP effects on molecules that enhance insulin resistance at the adipocyte level may reflect an important role for this peptide in the pathophysiology of type 2 diabetes.

Treating Diabetes with Exercise – Focus on the Microvasculature

Veldig viktig studie som nevner at det ikke finnes glatt muskulatur i kapillærene, så det er arteriolene som avgjør blodsirkulasjonen i kapillærene. His blodsirkulasjonen i en arteriole blir dårlig stopper sirkulasjonen opp i et område av muskelen som serveres av kapillærene.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4000229

Abstract

The rising incidence of diabetes and the associated metabolic diseases including obesity, cardiovascular disease and hypertension have led to investigation of a number of drugs to treat these diseases. However, lifestyle interventions including diet and exercise remain the first line of defense. The benefits of exercise are typically presented in terms of weight loss, improved body composition and reduced fat mass, but exercise can have many other beneficial effects. Acute effects of exercise include major changes in blood flow through active muscle, an active hyperemia that increases the delivery of oxygen to the working muscle fibers. Longer term exercise training can affect the vasculature, improving endothelial health and possibly basal metabolic rates. Further, insulin sensitivity is improved both acutely after a single bout of exercise and shows chronic effects with exercise training, effectively reducing diabetes risk. Exercise-mediated improvements in endothelial function may also reduce complications associated with both diabetes and other metabolic disease. Thus, while drugs to improve microvascular function in diabetes continue to be investigated, exercise can also provide many similar benefits on endothelial function and should remain the first prescription when treating insulin resistance and diabetes. This review will investigate the effects of exercise on the blood vessel and the potential benefits of exercise on cardiovascular disease and diabetes.

At rest, a low proportion of capillaries are exposed to blood flow at one time, with a rapid increase in the number of perfused capillaries after exercise [31], thus increasing functional capillary density.

Vascular smooth muscle cells are located around the arterioles and some venules, and can constrict to change blood flow patterns, while capillaries do not typically contribute to blood flow changes [30] (Figure 1). Blood flow through capillaries is controlled upstream by small arterioles at rest, and the rapid recruitment of unperfused capillaries by exercise could suggest that nerves are responsible for this action [34]. The sympathetic nervous system is mainly responsible for the vasoconstrictor responses, and as the arterioles and larger vessels are innervated [38] the majority of sympathetic nervous system activity is localized to that area of the vascular tree. Physical exercise can enhance sympathetic nerve activity [39] to maintain arterial pressure, and may be involved in maintaining exercise tolerance, as reviewed by Thomas and Segal [38].

Structural differences between artery, arteriole and capillary. No vascular smooth muscle is located on the capillary; therefore flow through capillaires is modified by pre-capillary arterioles. Cessation of flow through arterioles will prevent flow through a portion of the muscle.

Insulin relies on endothelium-dependent vasodilation to enhance perfusion, thus endothelial dysfunction reduces insulin-mediated increases in muscle perfusion, which can contribute to the metabolic deficit in diabetes. As exercise-mediated changes in perfusion are typically endothelium-independent, exercise is still able to recruit capillaries and thus increase muscle perfusion in obesity and type 2 diabetes, even in the face of endothelial dysfunction. Numerous studies have now shown that while insulin’s vascular effects may be blocked in diabetes, exercise still maintains its ability to increase the distribution of blood flow through muscle [42].

Nitric oxide (NO) is the main vasodilator from the endothelium specifically involved in blood flow and blood distribution, and while reduction in nitric oxide synthesis lowered total blood flow, exercise-mediated capillary recruitment was not affected [46]. In fact, inhibition of NO formation enhances both resting and exercise-mediated muscle oxygen uptake [47]; despite a reduction in total flow, microvascular flow was not affected, suggesting that NO is not involved in the vascular response to exercise.

The distribution of blood through muscle increases the capacity for nutrient exchange. In exercise the primary purpose of functional hyperemiais for oxygen delivery, as the oxygen required by exercising muscle is much higher than resting muscle (reviewed in [37]). Recruitment of capillaries can decrease the velocity of blood flow by increasing the cross-sectional area of the capillary bed and the time available for exchange. Recruitment also increases surface area for exchange and decreases perfusion distances to promote oxygen delivery to tissues with exercise [34] (Figure 2). While in exercise the main metabolite required at the working muscle is oxygen, distribution of other nutrients can also be affected, including glucose, fats, other hormones and cytokines. Muscle metabolism can therefore be altered by perfusion of the tissue [48,49]. While there can be regulated transport of certain larger hormones across the vasculature [50,51], smaller molecules can diffuse across the endothelium easily, possibly making muscle perfusion a more important player in the delivery of glucose and oxygen to the tissue.

Vasodilation affects delivery, and thus metabolism. The rate of transfer across the endothelium is dependent on surface area, permeability of the endothelium, diffusion distance, and concentration difference (Fick’s first law of diffusion). Vasodilation increases surface area in arterioles for exchange, but will also recruit downstream capillaries, which will reduce diffusion distance and increase surface area for exchange. Working muscle increases oxygen utilization, increasing the concentration difference from the blood vessel to the tissue.

Mitochondrial dysfunction has been proposed to be both a cause [72] and a consequence [73] of insulin resistance, and may contribute to endothelial dysfunction [74]. If oxygen delivery is a component of mitochondrial health and biogenesis, it is possible that impaired perfusion may contribute to fiber type switching, where an oxidative fiber, which is typically highly vascularized and contains mitochondria, switches to a glycolytic fiber with less vascularity and mitochondria. As exercise can improve oxidative capacity, increase mitochondria content [75], and also increase muscle perfusion [31,32,34,45,76], the relationship between muscle perfusion, fiber type and mitochondrial function needs to be clarified.

The vascular component of exercise may well be linked to the reduction of diabetic complication such as retinopathy, peripheral neuropathy and nephropathy, as there is a vascular basis to many of these complications. The endothelium has been implicated in diabetic nephropathy [88], and the blood vessels formed in response to reduced perfusion in retinopathy show abnormal structure and function [89].

Care and Feeding of the Endocannabinoid System: A Systematic Review of Potential Clinical Interventions that Upregulate the Endocannabinoid System

Denne beskriver endocannabinoider(eCB) og hvordan man kan øke produksjonen av dem og reseptorene for dem. eCB er et kroppens viktigste naturlige smertstillende stoffer som kan produseres og påvirker alle nerver i kroppen. Spesielt viktig i hjernen, men også i det perifere nervesystem.

Massasje, kiropraktikk og hard trening (f.eks. runners high) utløser eCB i kroppen. Det gjør også omegabalanse (mer n-3), probiotica, NSAIDs, m.m. Også yoga, meditasjon, pust og andre stressreduserende påvirker eCB. Og trening, men kun om man gjør det jevnlig over tid.

Den nevner at langvarig stress reduserer eCB i kroppen siden det er koblet til kortisol. Men den nevner også at noen tilstander kan ha forhøyet eCB i kroppen, f.eks. overvekt.

Med høyt nivå av n-6 relativt til n-3 blir det en overvekt av AA (arakidonsyre) som produserer en overvekt av eCB, som dermed fører til en reduksjon av eCB reseptorer. Dette gjør at smertestillende medikamenter fungerer dårligere, og at det blir lettere kronisk smerte. Tilskudd av n-3 gjør at eCB reseptorene øker. Studiene er gjort på mus og innebærer 17 g/kg.

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0089566

The endocannabinoid (eCB) system consists of receptors, endogenous ligands, and ligand metabolic enzymes. Metaphorically the eCB system represents a microcosm of psychoneuroimmunology or mind-body medicine. Cannabinoid receptor 1 (CB1) is the most abundant G protein-coupled receptor expressed in the brain, with particularly dense expression in (rank order): the substantia nigra, globus pallidus, hippocampus, cerebral cortex, putamen, caudate, cerebellum, and amygdala [1]. CB1 is also expressed in non-neuronal cells, such as adipocytes and hepatocytes, and in musculoskeletal tissues. Cannabinoid receptor 2 (CB2) is principally associated with cells governing immune function, although it may also be expressed in the central nervous [2][3].

The eCB system’s salient homeostatic roles have been summarized as, “relax, eat, sleep, forget, and protect” [5]. It modulates embryological development, neural plasticity, neuroprotection, immunity and inflammation, apoptosis and carcinogenesis, pain and emotional memory, and most importantly from the viewpoint of recent drug development: hunger, feeding, and metabolism. Obese individuals seem to display an increased eCB tone, driving CB1activation in a chronic, feed-forward dysfunction (reviewed by [6]).

Other diseases are associated with suboptimal functioning of the eCB system. Russo [8]proposed that migraine, fibromyalgia, irritable bowel syndrome, and related conditions represent CEDS, “clinical endocannabinoid deficiency syndromes.” Fride [9] speculated that a dysfunctional eCB system in infants contributes to “failure to thrive” syndrome. Hill and Gorzalka [10] hypothesized that deficient eCB signaling could be involved in the pathogenesis of depressive illnesses. In human studies, eCB system deficiencies have been implicated in uncompensated schizophrenia [11], migraine [12], multiple sclerosis [13], Huntington’s [14],[15], uncompensated Parkinson’s [16], irritable bowel syndrome [17], uncompensated anorexia[18], and chronic motion sickness [19].

NSAIDs inhibit two cyclooxygenase (COX) enzymes, COX1 and COX2, and thereby block the conversion of arachidonic acid (AA) into inflammatory prostaglandins. Ibuprofen, ketorolac, and flurbiprofen also block the hydrolysis of AEA into arachidonic acid and ethanolamine [27]. SeeFigure 2. A binding site for some NSAIDs on FAAH has also been identified [28]. NSAID inhibition of COX2 blocks the metabolism of AEA and 2-AG into prostaglandin ethanolamides (PG-EAs) and prostaglandin glycerol esters (PG-GEs), respectively [29].

Combining NSAIDs with cannabinoids (either eCBs or exogenous cannabinoids) produces additive or synergistic effects. A sub-effective dose of WIN55,212-2 became fully antinociceptive following administration of indomethacin in rats [36].

In summary, preclinical studies indicate that some NSAIDs inhibit FAAH and enhance the activity of eCBs, phytocannabinoids, and synthetic cannabinoids. Combinational effects may be particularly relevant at peripheral sites, such as the peripheral terminals of nociceptors.

The distribution of glucocorticoid receptors (GRs) and CB1 overlap substantially in the central nervous system and other tissues, as do GRs and CB2 in immune cells. Dual activation of GRs and CBs may participate in glucocorticoid-mediated anti-inflammatory activity, immune suppression, insulin resistance, and acute psychoactive effects.

The acute administration of glucocorticoids may shift AA metabolism toward eCB synthesis in parts of the brain.

Chronic exposure to glucocorticoids downregulates the eCB system. Chronic corticosterone administration decreased CB1 densities in rat hippocampus [59] and mouse hippocampus and amygdala [61]. Chronic corticosterone administration in male rats led to visceral hyperalgesia in response to colorectal distension, accompanied by increased AEA, decreased CB1 expression, and increased TRPV1 expression in dorsal root ganglia. Co-treatment with the corticoid receptor antagonist RU-486 prevented these changes [62].

Polyunsaturated fatty acids (PUFAs) play fundamental roles in many cellular and multicellular processes, including inflammation, immunity, and neurotransmission. They must be obtained through diet, and a proper balance between omega-6 (ω-6) PUFAs and ω-3 PUFAs is essential. The typical Western diet contains a surfeit of ω-6s and a deficiency of ω-3s [130].

The inflammatory metabolites of AA are countered by dietary ω-3s. The two best-known ω-3s are eicosapentaenoic acid (EPA, 20:5ω-3) and docosahexaenoic acid (DHA, 22:6ω-3).

eCBs are derived from AA (see Figure 2). Several preclinical studies showed that dietary supplementation with AA increased serum levels of AEA and 2-AG, summarized in Table 1. Although we clearly need AA to biosynthesize eCBs, excessive levels of AA, administered chronically, may lead to excessive levels of eCBs. This in turn may lead to desensitized and downregulated CB1 and CB2 receptors.

Dietary supplementation with ω-3s predictably increased the concentration of EPA and/or DHA in tissues, cells, and plasma, and decreased the relative concentration of AA in tissues, cells, and plasma [132][133]. ω-3 supplementation also decreased AEA and 2-AG in tissues, cells, and plasma (Table 1).

Adequate levels of dietary ω-3s are required for proper eCB signaling. Mice supplemented with ω-3s, compared to mice on a control diet, expressed greater levels of CB1 and CB2 mRNA.

n summary, dietary ω-3s seem to act as homeostatic regulators of the eCB system. In obese rodents fed a high-AA diet, ω-3s significantly decrease eCBs, especially 2-AG, particularly in tissues that become dysregulated, such as adipose and liver tissues. Plasma eCB levels are reduced by krill oil also in obese humans. Little change in eCB levels are seen in normo-weight individuals not fed a high ω-6 diet, and dietary ω-3s are required for proper eCB signaling.

Human intestinal epithelial cells incubated with L. acidophilus produced more CB2 mRNA [145]. Feeding L. acidophilus to mice and rats increased the expression of CB2 mRNA in colonic epithelial cells. Lastly, mice fed L. acidophilus showed less pain behavior following colonic distension with butyrate than control mice, an effect reversed by the CB2 antagonist AM630[145].

Chronic or repeated stress results in a chronic elevation of endogenous corticosterone via the hypothalamic-pituitary-adrenocortical (HPA) axis. Chronic stress (repeated restraint) reduced AEA levels throughout the corticolimbic stress circuit in rodents [99][196][197].

In summary, chronic stress impairs the eCB system, via decreased levels of AEA and 2-AG. Changes in CB1 expression are more labile. Stress management may reverse the effects of chronic stress on eCB signaling, although few studies exploring this possibility have been performed to date. Clinical anecdotes suggests that stress-reduction techniques, such as meditation, yoga, and deep breathing exercises impart mild cannabimimetic effects [218].

Massage and osteopathic manipulation of asymptomatic participants increased serum AEA 168% over pretreatment levels; mean OEA levels decreased 27%, and no changes occurred in 2-AG. Participants receiving sham manipulation showed no changes [218].

Upregulation of the eCB system in obese humans seems to be driven by excessive production of eCBs in several peripheral tissues such as visceral adipose tissue, liver, pancreas, and skeletal muscle.

In summary, increased food intake, adiposity, and elevated levels of AEA and 2-AG apparently spiral in a feed-forward mechanism. Weight loss from caloric restriction breaks the cycle, possibly by reducing CB1 expression and reducing eCB levels.

Although both types of exercise regimens increased eCB ligand concentrations, only long-term-forced exercise led to sustained elevations of eCBs, and predictable CB1 downregulation.

In whole animals, however, caffeine’s effects are biphasic and vary by dosage and acute versus chronic administration. In humans, the acute administration of caffeine decreases headache pain, but exposure to chronic high doses, ≥300 mg/day, may exacerbate chronic pain [275].

Skin Biopsy as a Diagnostic Tool in Peripheral Neuropathy: Correlates of Intraepidermal Nerve Fiber Density

Denne nevner mye om nevropati og sammenhengen mellom small fiber density og smerte, pluss at den nevner hvordan trening og steroidbehandling øker tettheten igjen. Viktigst prinsipp å hente fra denne artikkelen er at c-fiber tettheten sier noe om intensiteten på smerten, men ikke noe om smertetilstanden. Man kan ha lav tetthet og lite smerte, men om man får smerte er intensiteten desto høyere. Man må desverre logge inn for å få opp linkene.

http://www.medscape.org/viewarticle/563262_6

In diabetic neuropathy, patients with pain had lower IENF densities than did asymptomatic patients, but IENF density did not correlate with pain intensity within the group of symptomatic patients.[82]

In patients with impaired glucose tolerance, diet and exercise induced a slight recovery of IENF density that was associated with a reduction in pain symptoms.[83] Similarly, epidermal reinnervation coincided with pain reduction after steroid treatment.[71]

CORRELATES OF INTRAEPIDERMAL NERVE FIBER DENSITY

Clinical Picture, Etiology and Neuropathic Pain

The clinical picture of small-fiber neuropathy is dominated by spontaneous and stimulus-evoked positive sensory symptoms—namely thermal and pinprick hypoesthesia—that can mask the signs of small-fiber loss. Only a few studies have attempted to correlate IENF density with validated clinical scales. In patients with diabetic neuropathy, a negative correlation between IENF density and neuropathy symptom score was reported.[53,56]These studies also showed that the extent of epidermal denervation correlated with the duration of diabetes but not with hemoglobin A1C levels, suggesting that IENF density might be useful as a marker of neuropathy progression. A recent study found a high concordance between reduced IENF density and loss of pinprick sensation in the foot.[61]

Skin biopsy has allowed small-fiber neuropathy to be demonstrated in restless legs syndrome[75] and erythromelalgia.[76] In systemic diseases, such as systemic lupus erythematosus, sarcoidosis, Sjögren’s syndrome, celiac disease and hypothyroidism, skin biopsy has enabled correlations to be found between neuropathic symptoms and small-fiber degeneration.[52,65,77–79]Although IENF density is a general marker of axonal integrity in peripheral neuropathies, it cannot be used to directly address the question of etiology. Skin biopsy findings can, however, indirectly contribute to the assessment of etiology. For example, in 40% of patients with small-fiber neuropathy diagnosed only after skin biopsy, oral glucose tolerance testing revealed a previously undetected impaired glucose tolerance.[49] Similarly, the distribution of IENF loss can help to differentiate between a non-length-dependent sensory neuronopathy and a length-dependent axonal neuropathy,[78,80] thereby leading to focused screening for associated diseases.

The relationship between IENF density and neuropathic pain remains uncertain. In HIV neuropathy, IENF density correlated inversely with pain severity when assessed by the patient, but not when the Gracely Pain Scale was used.[66] Another study found only a trend towards an inverse correlation between IENF density and pain intensity in this setting.[81] In diabetic neuropathy, patients with pain had lower IENF densities than did asymptomatic patients, but IENF density did not correlate with pain intensity within the group of symptomatic patients.[82] In patients with impaired glucose tolerance, diet and exercise induced a slight recovery of IENF density that was associated with a reduction in pain symptoms.[83] Similarly, epidermal reinnervation coincided with pain reduction after steroid treatment.[71]In length-dependent neuropathies, therefore, more-severe IENF loss seems to increase the risk of developing pain, the intensity of which might decrease in parallel with recovery of IENF density.

In postherpetic neuralgia, on the basis of evidence of relatively preserved skin innervation in the area of severe allodynia, normal thermal sensory function, pain relief in response to topical lidocaine, and worsening of pain with application of capsaicin, surgical removal of painful skin has been attempted.[84] After initial relief, pain increased, became intractable, and spread to previously unaffected dermatomes, suggesting the involvement of central mechanisms in the pathogenesis of neuropathic pain.

Sensory Nerve Conduction Studies

Sural sensory nerve action potential (SNAP) amplitude, which reflects the integrity of largediameter fibers, showed concordance with IENF density in the distal part of the leg in patients with large-fiber or mixed small-fiber and largefiber neuropathy. Not surprisingly, skin biopsy analysis seemed to be more sensitive than sural nerve conduction studies for diagnosing smallfiber neuropathy.[62] One study,[85] however, showed that in patients with symptoms of small-fiber neuropathy and normal sural nerve conduction, reduced IENF density correlated with a decrease in SNAP amplitude in the medial plantar nerve. This finding suggests subclinical involvement of the most-distal large fibers in small-fiber neuropathy.

Psychophysical Tests

The detection of thermal and pain thresholds using quantitative sensory testing has been widely used to assess the function of small nerve fibers. Although this approach is useful in population studies, it is an unreliable tool for diagnosing small-fiber neuropathy in clinical practice.[86] Moreover, the size of the probe used for the test can affect the results.[87]

In view of the fact that unmyelinated fibers and thinly myelinated fibers convey warm and cold sensation, respectively, thermal thresholds would be expected to correlate with IENF density. In diabetic neuropathy, IENF density was found to be inversely correlated with thermal and pain thresholds, showing the highest correlation with warm threshold.[53,56,82]Similarly, in Guillain–Barré syndrome lower IENF density was associated with increased warm threshold.[67]One study reported a significant correlation between cold pain threshold and signs of large-fiber impairment.[59]By contrast, others studies did not find any correlation between quantitative sensory testing results and IENF density.[45,51,88]

Autonomic Tests

As IENFs are somatic unmyelinated fibers, their density would not be expected to correlate with autonomic fiber function. Intriguingly, however, in patients with Guillain–Barré syndrome and chronic inflammatory demyelinating polyradiculoneuropathy, lower IENF density was associated with a higher risk of developing dysautonomia.[64,67]These findings suggest that the integrity of IENFs might reflect the integrity of the whole class of small nerve fibers, including autonomic fibers. A few studies have investigated the correlation between IENF density and the results of a quantitative sudomotor axonal reflex test in patients with painful neuropathy and autonomic symptoms in order to test the hypothesis that IENF density and sweat output might decrease concomitantly. IENF density correlated with test results in one study,[63] but not in another.[51] In leprosy neuropathy, reduced nicotine-induced axon-reflex sweating correlated with decreased innervation of sweat glands.[88]

Nonconventional Neurophysiological Tests

Laser-evoked potentials (LEPs) have been used to investigate peripheral and central nociceptive pathways in trigeminal neuralgia and peripheral neuropathies. Late LEPs, reflecting Aδ-fiber activation, are delayed in patients with neuropathic pain, but can be enhanced when the pain has a psychogenic origin.[89] Recording of ultralate LEPs, reflecting activation of unmyelinated C-fibers, is less reliable than recording of late LEPs, thereby limiting the overall usefulness of LEPs in clinical practice. LEPs and skin biopsy findings have been examined in single case reports.[90]In two patients with Ross syndrome, abnormal LEPs correlated with decreased IENF density and increased thermal thresholds.[91] No study has yet looked for a correlation between results of skin biopsy analysis and recording of contact heat-evoked potentials, a technique that was recently proposed for investigating smallfiber function, but which cannot be used to assess C-fiber-related responses.[92]

Microneurography allows single-fiber recordings from nerves in awake patients. This technique demonstrated loss of nociceptive and skin sympathetic C-fiber activity that correlated with IENF and sweat gland denervation in a patient with hereditary sensory and autonomic neuropathy type IV.[20]In two patients with generalized anhidrosis, C-fiber recording and sweat gland innervation analysis distinguished postganglionic autonomic nerve fiber impairment from eccrine gland dysfunction.[34]

Sural Nerve Biopsy

The diagnosis of small-fiber neuropathy is better assessed by skin biopsy than by sural nerve biopsy.[57]IENF density can be reduced despite normal morphometry of unmyelinated and thinly myelinated fibers in sural nerve biopsy.[58] In a large comparative study,[62] skin and sural nerve biopsy findings were concordant in 73% of patients, but in 23% of patients IENF density was the only indicator of small-fiber neuropathy. Skin biopsy offers the opportunity to differentiate small nerve fibers with somatic function from those with autonomic function, thereby giving it a further advantage over nerve biopsy. In Charcot–Marie–Tooth disease and related hereditary neuropathies, a biopsy sample of the glabrous skin demonstrated the typical neuropathological abnormalities known from sural nerve studies.[5,6]

Immunohistochemical studies demonstrated IgM deposited specifically in the myelinated fibers of hairy and glabrous skin in patients with anti-myelin-associated-glycoprotein neuropathy.[93] Although skin biopsy can be contemplated in genetic and immune-mediated neuropathies, sural nerve biopsy should always be considered to confirm the diagnosis in inflammatory polyradiculoneuropathy with atypical presentation, or when vasculitic or amyloid neuropathy is suspected.