Impaired inhibitory G-protein function contributes to increased calcium currents in rats with diabetic neuropathy

PIEZO1 as a new target for hyperglycemic stress-induced neuropathic injury: The potential therapeutic role of bezafibrate

Hyperglycemic stress can directly lead to neuronal damage. The mechanosensitive ion channel PIEZO1 can be activated in response to hyperglycemia, but its role in hyperglycemic neurotoxicity is unclear. The role of PIEZO1 in hyperglycemic neurotoxicity was explored by constructing a hyperglycemic mouse model and a high-glucose HT22 cell model. The results showed that PIEZO1 was significantly upregulated in response to high glucose stress. In vitro experiments have shown that high glucose stress induces changes in neuronal cell morphology and membrane tension, a key mechanism for PIEZO1 activation. In addition, high glucose stress upregulates serum/glucocorticoid-regulated kinase-1 (SGK1) and activates PIEZO1 through the Ca2+ pool and store-operated calcium entry (SOCE). PIEZO1-mediated Ca2+ influx further enhances SGK1 and SOCE, inducing intracellular Ca2+ peaks in neurons. PIEZO1 mediated intracellular Ca2+ elevation leads to calcium/calmodulin-dependent protein kinase 2α (CaMK2α) overactivation, which promotes oxidative stress and apoptosis signalling through p-CaMK2α/ERK/CREB and ox-CaMK2α/MAPK p38/NFκB p65 pathways, subsequently inducing synaptic damage and cognitive impairment in mice. The intron miR-107 of pantothenic kinase 1 (PANK1) is highly expressed in the brain and has been found to target PIEZO1 and SGK1. The PANK1 receptor is activated by peroxisome proliferator-activated receptor α (PPARα), an activator known to upregulate miR-107 levels in the brain. The clinically used lipid-lowering drug bezafibrate, a known PPARα activator, may upregulate miR-107 through the PPARɑ/PANK1 pathway, thereby inhibiting PIEZO1 and improving hyperglycemia-induced neuronal cell damage. This study provides a new idea for the pathogenesis and drug treatment of hyperglycemic neurotoxicity and diabetes-related cognitive dysfunction.

Diabetes-induced electrophysiological alterations on neurosomes in ganglia of peripheral nervous system

Diabetes mellitus (DM) leads to medical complications, the epidemiologically most important of which is diabetic peripheral neuropathy (DPN). Electrophysiology is a major component of neural functioning and several studies have been undertaken to elucidate the neural electrophysiological alterations caused by DM and their mechanisms of action. Due to the importance of electrophysiology for neuronal function, the review of the studies dealing predominantly with electrophysiological parameters and mechanisms in the neuronal somata of peripheral neural ganglia of diabetic animals during the last 45 years is here undertaken. These studies, using predominantly techniques of electrophysiology, most frequently patch clamp for voltage clamp studies of transmembrane currents through ionic channels, have investigated the experimental DPN. They also have demonstrated that various cellular and molecular mechanisms of action of diabetic physiopathology at the level of biophysical electrical parameters are affected in DPN. Thus, they have demonstrated that several passive and active transmembrane voltage parameters, related to neuronal excitability and neuronal functions, are altered in diabetes. The majority of the studies agreed that DM produces depolarization of the resting membrane potential; alters excitability, increasing and decreasing it in dorsal root ganglia (DRG) and in nodose ganglion, respectively. They have tried to relate these changes to sensorial alterations of DPN. Concerning ionic currents, predominantly studied in DRG, the most frequent finding was increases in Na+, Ca2+, and TRPV1 cation current, and decreases in K+ current. This review concluded that additional studies are needed before an understanding of the hierarchized, time-dependent, and integrated picture of the contribution of neural electrophysiological alterations to the DPN could be reached. DM-induced electrophysiological neuronal alterations that so far have been demonstrated, most of them likely important, are either consistent with the DPN symptomatology or suggest important directions for improvement of the elucidation of DPN physiopathology, which the continuation seems to us very relevant.

Inhibition of phosphorylated calcium/calmodulin-dependent protein kinase IIα relieves streptozotocin-induced diabetic neuropathic pain through regulation of P2X3 receptor in dorsal root ganglia

Diabetic neuropathic pain (DNP) is frequent among patients with diabetes. We previously showed that P2X3 upregulation in dorsal root ganglia (DRG) plays a role in streptozotocin (STZ)-induced DNP but the underlying mechanism is unclear. Here, a rat model of DNP was established by a single injection of STZ (65 mg/kg). Fasting blood glucose was significantly elevated from the 1^st to 3^rd week. Paw withdrawal thresholds (PWTs) and paw withdrawal latencies (PWLs) in diabetic rats significantly reduced from the 2^nd to 3^rd week. Western blot analysis revealed that elevated p-CaMKIIα levels in the DRG of DNP rats were accompanied by pain-associated behaviors while CaMKIIα levels were unchanged. Immunofluorescence revealed significant increase in the proportion of p-CaMKIIα immune positive DRG neurons (stained with NeuN) in the 2^nd and 3^rd week and p-CaMKIIα was co-expressed with P2X3 in DNP rats. KN93, a CaMKII antagonist, significantly reduce mechanical hyperalgesia and thermal hyperalgesia and these effects varied dose-dependently, and suppressed p-CaMKIIα and P2X3 upregulation in the DRGs of DNP rats. These results revealed that the p-CaMKIIα upregulation in DRG is involved in DNP, which possibly mediated P2X3 upregulation, indicating CaMKIIα may be an effective pharmacological target for DNP management.

Platelet-rich plasma application in diabetic ulcers: A review

There are 422 million diabetic people in the world. 25% of these individuals are diagnosed with diabetic foot ulcer (DFU). 20% of patients with DFU will suffer amputation of the lower limbs. Following amputation procedures, the mortality rate of patients is over 70% in 5 years. Diabetes has no cure and, therefore, treatment aims to prevent and treat its complications. Autologous platelet-rich plasma (PRP) has been shown to be a therapeutic tool for many types of disorders, including the treatment of DFU. This manuscript aims to carry out a review to provide more knowledge about the efficacy and safety of autologous PRP for wound closure in patients with DFU. The majority of studies included in this review state that PRP promotes improvement of DFU lesions by accelerating tissue healing processes. However, many studies have a small sample size and thus require larger sample range in order to improve robustness of data in the literature.

Application of Sygen® in Diabetic Peripheral Neuropathies—A Review of Biological Interactions

This study investigates the role of Sygen^® in diabetic peripheral neuropathy, a severe disease that affects the peripheral nervous system in diabetic individuals. This disorder often impacts the lower limbs, causing significant discomfort and, if left untreated, progresses into more serious conditions involving chronic ulcers and even amputation in many cases. Although there are management strategies available, peripheral neuropathies are difficult to treat as they often present multiple causes, especially due to metabolic dysfunction in diabetic individuals. Gangliosides, however, have long been studied and appreciated for their role in neurological diseases. The monosialotetrahexosylganglioside (GM1) ganglioside, popularly known as Sygen, provides beneficial effects such as enhanced neuritic sprouting, neurotrophism, neuroprotection, anti-apoptosis, and anti-excitotoxic activity, being particularly useful in the treatment of neurological complications that arise from diabetes. This product mimics the roles displayed by neurotrophins, improving neuronal function and immunomodulation by attenuating exacerbated inflammation in neurons. Furthermore, Sygen assists in axonal stabilization and keeps nodal and paranodal regions of myelin fibers organized. This maintains an adequate propagation of action potentials and restores standard peripheral nerve function. Given the multifactorial nature of this complicated disorder, medical practitioners must carefully screen the patient to avoid confusion and misdiagnosis. There are several studies analyzing the role of Sygen in neurological disorders. However, the medical literature still needs more robust investigations such as randomized clinical trials regarding the administration of this compound for diabetic peripheral neuropathies, specifically.

Streptozotocin-Induced Diabetic Neuropathic Pain Is Associated with Potentiated Calcium-Permeable AMPA Receptor Activity in the Spinal Cord

Neuronal hyperactivity in the spinal dorsal horn can amplify nociceptive input in diabetic neuropathic pain. The glutamate N-methyl-d-aspartate and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (NMDA receptors and AMPA receptors, respectively) are involved in spinal nociceptive transmission. It is unclear, however, whether painful diabetic neuropathy is associated with changes in the activity of synaptic NMDA receptors and AMPA receptors in spinal dorsal horn neurons. AMPA receptors lacking GluA2 are Ca2+-permeable (CP-AMPA receptors), and their currents display characteristic inward rectification. In this study, we showed that evoked excitatory postsynaptic currents (EPSCs), induced by streptozotocin, exhibited inward rectification in spinal dorsal neurons in diabetic rats. Presynaptic and postsynaptic NMDA receptor activity in the spinal dorsal horn was similar in diabetic and control rats. In the dorsal spinal cord, the membrane GluA2 protein level was significantly lower in diabetic than in control rats, whereas the cytosolic GluA2 level was greater in diabetic than in control rats. In contrast, the GluA1 subunit levels in the plasma membrane and cytosol did not differ between the two groups. Blocking CP-AMPA receptors significantly reduced the amplitude of EPSCs of dorsal horn neurons in diabetic but not in control rats. Furthermore, blocking spinal CP-AMPA receptors reduced pain hypersensitivity in diabetic rats but had no effect on nociception in control rats. Our study suggests that diabetic neuropathy augments CP-AMPA receptor activity in the spinal dorsal horn by causing intracellular retention of GluA2 and impairing GluA2 membrane trafficking. Increased prevalence of spinal CP-AMPA receptors sustains diabetic neuropathic pain. SIGNIFICANCE STATEMENT: This study demonstrates that the prevalence of synaptic calcium-permeable AMPA receptors is increased in the spinal dorsal horn, which mediates pain hypersensitivity in diabetic neuropathy. Thus, calcium-permeable AMPA receptors play an important role in glutamatergic synaptic plasticity in the spinal cord in painful diabetic neuropathy. This new knowledge improves our understanding of the mechanisms involved in central sensitization associated with diabetic neuropathic pain and suggests that calcium-permeable AMPA receptors are an alternative therapeutic target for treating this chronic pain condition.

Methylglyoxal Disrupts Paranodal Axoglial Junctions via Calpain Activation

Nodes of Ranvier and associated paranodal and juxtaparanodal domains along myelinated axons are essential for normal function of the peripheral and central nervous systems. Disruption of these domains as well as increases in the reactive carbonyl species methylglyoxal are implicated as a pathophysiology common to a wide variety of neurological diseases. Here, using an ex vivo nerve exposure model, we show that increasing methylglyoxal produces paranodal disruption, evidenced by disorganized immunostaining of axoglial cell-adhesion proteins, in both sciatic and optic nerves from wild-type mice. Consistent with previous studies showing that increase of methylglyoxal can alter intracellular calcium homeostasis, we found upregulated activity of the calcium-activated protease calpain in sciatic nerves after methylglyoxal exposure. Methylglyoxal exposure altered clusters of proteins that are known as calpain substrates: ezrin in Schwann cell microvilli at the perinodal area and zonula occludens 1 in Schwann cell autotypic junctions at paranodes. Finally, treatment with the calpain inhibitor calpeptin ameliorated methylglyoxal-evoked ezrin loss and paranodal disruption in both sciatic and optic nerves. Our findings strongly suggest that elevated methylglyoxal levels and subsequent calpain activation contribute to the disruption of specialized axoglial domains along myelinated nerve fibers in neurological diseases.

Diabetic neuropathy research: from mouse models to targets for treatment

Diabetic neuropathy is one of the most serious complications of diabetes, and its increase shows no sign of stopping. Furthermore, current clinical treatments do not yet approach the best effectiveness. Thus, the development of better strategies for treating diabetic neuropathy is an urgent matter. In this review, we first discuss the advantages and disadvantages of some major mouse models of diabetic neuropathy and then address the targets for mechanism-based treatment that have been studied. We also introduce our studies on each part. Using stem cells as a source of neurotrophic factors to target extrinsic factors of diabetic neuropathy, we found that they present a promising treatment.

Comprehensive analysis of differentially expressed microRNAs and mRNAs in dorsal root ganglia from streptozotocin-induced diabetic rats

Diabetic peripheral neuropathy is a common complication associated with diabetes mellitus with a pathogenesis that is incompletely understood. By regulating RNA silencing and post-transcriptional gene expression, microRNAs participate in various biological processes and human diseases. However, the relationship between microRNAs and the progress of diabetic peripheral neuropathy still lacks a thorough exploration. Here we used microarray microRNA and mRNA expression profiling to analyze the microRNAs and mRNAs which are aberrantly expressed in dorsal root ganglia from streptozotocin-induced diabetic rats. We found that 37 microRNAs and 1357 mRNAs were differentially expressed in comparison to non-diabetic samples. Bioinformatics analysis indicated that 399 gene ontology terms and 29 Kyoto Encyclopedia of Genes and Genomes pathways were significantly enriched in diabetic rats. Additionally, a microRNA-gene network evaluation identified rno-miR-330-5p, rno-miR-17-1-3p and rno-miR-346 as important players for network regulation. Finally, quantitative real-time polymerase chain reaction analysis was used to confirm the microarray results. In conclusion, this study provides a systematic perspective of microRNA and mRNA expression in dorsal root ganglia from diabetic rats, and suggests that dysregulated microRNAs and mRNAs may be important promotors of peripheral neuropathy. Our results may be the underlying framework of future studies regarding the effect of the aberrantly expressed genes on the pathophysiology of diabetic peripheral neuropathy.

The Impact of Microbiota-Gut-Brain Axis on Diabetic Cognition Impairment

Progressive cognitive dysfunction is a central characteristic of diabetic encephalopathy (DE). With an aging population, the incidence of DE is rising and it has become a major threat that seriously affects public health. Studies within this decade have indicated the important role of risk factors such as oxidative stress and inflammation on the development of cognitive impairment. With the recognition of the two-way communication between gut and brain, recent investigation suggests that “microbiota-gut-brain axis” also plays a pivotal role in modulating both cognition function and endocrine stability. This review aims to systemically elucidate the underlying impact of diabetes on cognitive impairment.

Inhibition of Adenylyl Cyclase in the Spinal Cord Alleviates Painful Diabetic Neuropathy in Zucker Diabetic Fatty Rats

OBJECTIVES

Diabetic neuropathy is the most common complication of both type 1 and type 2 diabetes. In this study, we tested the hypotheses that impaired Gi protein expression/function in the spinal cord is associated with the development of painful neuropathy in people with type 2 diabetes and that reduction of cyclic adenosine monophosphate (cAMP) production by inhibiting adenylyl cyclase in the spinal cord can alleviate diabetic neuropathy.

METHODS

To this end, we examined the levels of cAMP, cAMP-dependent protein kinase (PKA) and cAMP response element-binding protein (CREB) in the spinal cord after the development of neuropathic pain in Zucker diabetic fatty (ZDF) rats with type 2 diabetes. We evaluated the effects of intrathecal injections of SQ22536, an adenylyl cyclase inhibitor, on mechanical allodynia and thermal hyperalgesia in rats with painful diabetic neuropathy.

RESULTS

We found that diabetic ZDF rats exhibited mechanical allodynia and thermal hyperalgesia, which are associated with enhanced cAMP production, increased PKA activation and elevated CREB phosphorylation in the spinal cord. Additionally, diabetic ZDF rats exhibited attenuated expression of Giα, but not Gsα, in the spinal cord. Furthermore, intrathecal administrations of SQ22536 dose-dependently alleviated mechanical allodynia and thermal hyperalgesia in diabetic ZDF rats and reduced cAMP production, PKA activation and p-CREB expression in the spinal cord.

CONCLUSIONS

Taken together, our study suggested that cAMP-mediated signalling in the spinal cord is likely critical for the development of painful neuropathy in people with type 2 diabetes.

Mechanisms of diabetic neuron damage: Molecular pathways

Diabetic polyneuropathy (DPN) is a common but intractable degenerative disorder of peripheral neurons. DPN first results in retraction and loss of sensory terminals in target organs such as the skin, whereas the perikarya (cell bodies) of neurons are relatively preserved. This is important because it implies that regrowth of distal terminals, rather than neuron replacement or rescue, may be useful clinically. Although a number of neuronal molecular abnormalities have been examined in experimental DPN, several are prominent: loss of structural proteins, neuropeptides, and neurotrophic receptors; upregulation of "stress" and "repair" proteins; elevated nitric oxide synthesis; increased AGE-RAGE signaling, NF-κB and PKC; altered neuron survival pathways; changes of pain-related ion channel investment. There is also a role for abnormalities of direct signaling of neurons by insulin, an important trophic factor for neurons that express its receptors. While evidence implicating each of these pathways has emerged, how they link together and result in neuronal degeneration remains unclear. However, several offer interesting new avenues for more definitive therapy of this condition.

Painful neuropathy: Mechanisms

Painful neuropathy, like the other complications of diabetes, is a growing healthcare concern. Unfortunately, current treatments are of variable efficacy and do not target underlying pathogenic mechanisms, in part because these mechanisms are not well defined. Rat and mouse models of type 1 diabetes are frequently used to study diabetic neuropathy, with rats in particular being consistently reported to show allodynia and hyperalgesia. Models of type 2 diabetes are being used with increasing frequency, but the current literature on the progression of indices of neuropathic pain is variable and relatively few therapeutics have yet been developed in these models. While evidence for spontaneous pain in rodent models is sparse, measures of evoked mechanical, thermal and chemical pain can provide insight into the pathogenesis of the condition. The stocking and glove distribution of pain tantalizingly suggests that the generator site of neuropathic pain is found within the peripheral nervous system. However, emerging evidence demonstrates that amplification in the spinal cord, via spinal disinhibition and neuroinflammation, and also in the brain, via enhanced thalamic activity or decreased cortical inhibition, likely contribute to the pathogenesis of painful diabetic neuropathy. Several potential therapeutic strategies have emerged from preclinical studies, including prophylactic treatments that intervene against underlying mechanisms of disease, treatments that prevent gains of nociceptive function, treatments that suppress enhancements of nociceptive function, and treatments that impede normal nociceptive mechanisms. Ongoing challenges include unraveling the complexity of underlying pathogenic mechanisms, addressing the potential disconnect between the perceived location of pain and the actual pain generator and amplifier sites, and finding ways to identify which mechanisms operate in specific patients to allow rational and individualized choice of targeted therapies.

Painful Diabetic Neuropathy: Prevention or Suppression?

Pain-sensing sensory neurons (nociceptors) of the dorsal root ganglia (DRG) and dorsal horn (DH) can become sensitized (hyperexcitable) in response to pathological conditions such as diabetes, which in turn may lead to the development of painful peripheral diabetic neuropathy (PDN). Because of incomplete knowledge about the mechanisms underlying painful PDN, current treatment for painful PDN has been limited to somewhat nonspecific systemic drugs that have significant side effects or potential for abuse. Recent studies have established that several ion channels in DRG and DH neurons are dysregulated and make a previously unrecognized contribution to sensitization of pain responses by enhancing excitability of nociceptors in animal models of type 1 and type 2 PDN. Furthermore, it has been reported that targeting posttranslational modification of nociceptive ion channels such as glycosylation and methylglyoxal metabolism can completely reverse mechanical and thermal hyperalgesia in diabetic animals with PDN in vivo. Understanding details of posttranslational regulation of nociceptive channel activity may facilitate development of novel therapies for treatment of painful PDN. We argue that pharmacological targeting of the specific pathogenic mechanism rather than of the channel per se may cause fewer side effects and reduce the potential for drug abuse in patients with diabetes.

Inside the Diabetic Brain: Role of Different Players Involved in Cognitive Decline

Diabetes mellitus is the most common metabolic disease, and its prevalence is increasing. A growing body of evidence, both in animal models and epidemiological studies, has demonstrated that metabolic diseases like obesity, insulin resistance, and diabetes are associated with alterations in the central nervous system (CNS), being linked with development of cognitive and memory impairments and presenting a higher risk for dementia and Alzheimer's disease. The rising prevalence of diabetes together with its increasing earlier onset suggests that diabetes-related cognitive dysfunction will increase in the near future, causing substantial socioeconomic impact. Decreased insulin secretion or action, dysregulation of glucose homeostasis, impairment in the hypothalamic-pituitary-adrenal axis, obesity, hyperleptinemia, and inflammation may act independently or synergistically to disrupt neuronal homeostasis and cause diabetes-associated cognitive decline. However, the crosstalk between those factors and the mechanisms underlying the diabetes-related CNS complications is still elusive. During the past few years, different strategies (neuroprotective and antioxidant drugs) have emerged as promising therapies for this complication, which still remains to be preventable or treatable. This Review summarizes fundamental past and ongoing research on diabetes-associated cognitive decline, highlighting potential contributors, mechanistic mediators, and new pharmacological approaches to prevent and/or delay this complication.

Opioid Receptors

Opioids are the oldest and most potent drugs for the treatment of severe pain. Their clinical application is undisputed in acute (e.g., postoperative) and cancer pain, but their long-term use in chronic pain has met increasing scrutiny. This article reviews mechanisms underlying opioid analgesia and other opioid actions. It discusses the structure, function, and plasticity of opioid receptors; the central and peripheral sites of analgesic actions and side effects; endogenous and exogenous opioid receptor ligands; and conventional and novel opioid compounds. Challenging clinical situations, such as the tension between chronic pain and addiction, are also illustrated.

CaV1.2 and CaV1.3 channel hyperactivation in mouse islet β cells exposed to type 1 diabetic serum

The voltage-gated Ca(2+) (CaV) channel acts as a key player in β cell physiology and pathophysiology. β cell CaV channels undergo hyperactivation subsequent to exposure to type 1 diabetic (T1D) serum resulting in increased cytosolic free Ca(2+) concentration and thereby Ca(2+)-triggered β cell apoptosis. The present study was aimed at revealing the subtypes of CaV1 channels hyperactivated by T1D serum as well as the biophysical mechanisms responsible for T1D serum-induced hyperactivation of β cell CaV1 channels. Patch-clamp recordings and single-cell RT-PCR analysis were performed in pancreatic β cells from CaV1 channel knockout and corresponding control mice. We now show that functional CaV1.3 channels are expressed in a subgroup of islet β cells from CaV1.2 knockout mice (CaV1.2(-/-)). T1D serum enhanced whole-cell CaV currents in islet β cells from CaV1.3 knockout mice (CaV1.3(-/-)). T1D serum increased the open probability and number of functional unitary CaV1 channels in CaV1.2(-/-) and CaV1.3(-/-) β cells. These data demonstrate that T1D serum hyperactivates both CaV1.2 and CaV1.3 channels by increasing their conductivity and number. These findings suggest CaV1.2 and CaV1.3 channels as potential targets for anti-diabetes therapy.

Alteration of the mu opioid receptor: Ca2+ channel signaling pathway in a subset of rat sensory neurons following chronic femoral artery occlusion

The exercise pressor reflex, a crucial component of the cardiovascular response under physiological and pathophysiological states, is activated via metabolic and mechanical mediators that originate from contracting muscles and stimulate group III and IV afferents. We reported previously that stimulation of mu opioid receptors (MOR), expressed in both afferents, led to a significant attenuation of the reflex in rats whose femoral arteries had been occluded for 72 h. The present study examined the effect of arterial occlusion on the signaling components involved in the opioid-mediated modulation of Ca(2+) channels in rat dorsal root ganglion neurons innervating the triceps surae muscles. We focused on neurons that were transfected with cDNA coding for enhanced green fluorescent protein whose expression is driven by the voltage-gated Na(+) channel 1.8 (Na(V)1.8) promoter region, a channel expressed primarily in nociceptive neurons. With the use of a small interference RNA approach, our results show that the pertussis toxin-sensitive Gα(i3) subunit couples MOR with Ca(2+) channels. We observed a significant leftward shift of the MOR agonist [D-Ala2-N-Me-Phe4-Glycol5]-enkephalin concentration-response relationship in neurons isolated from rats with occluded arteries compared with those that were perfused freely. Femoral occlusion did not affect Ca(2+) channel density or the fraction of the main Ca(2+) channel subtype. Furthermore, Western blotting analysis indicated that the leftward shift did not result from either increased Gα(i3) or MOR expression. Finally, all neurons from both groups exhibited an inward current following exposure of the transient potential receptor vanilloid 1 (TRPV1) agonist, 8-methyl-N-vanillyl-6-nonenamide. These findings suggest that sensory neurons mediating the exercise pressor reflex express Na(V)1.8 and TRPV1 channels, and femoral occlusion alters the MOR pharmacological profile.

Calcium signalling in sensory neurones and peripheral glia in the context of diabetic neuropathies

Peripheral sensory nervous system is comprised of neurones with their axons and neuroglia that includes satellite glial cells in sensory ganglia, myelinating, non-myelinating and perisynaptic Schwann cells. Pathogenesis of peripheral diabetic polyneuropathies is associated with aberrant function of both neurones and glia. Deregulated Ca(2+) homoeostasis and aberrant Ca(2+) signalling in neuronal and glial elements contributes to many forms of neuropathology and is fundamental to neurodegenerative diseases. In diabetes both neurones and glia experience metabolic stress and mitochondrial dysfunction which lead to deregulation of Ca(2+) homeostasis and Ca(2+) signalling, which in their turn lead to pathological cellular reactions contributing to development of diabetic neuropathies. Molecular cascades responsible for Ca(2+) homeostasis and signalling, therefore, can be regarded as potential therapeutic targets.

Nociceptive neurons differentially express fast and slow T-type Ca²⁺ currents in different types of diabetic neuropathy

T-type Ca²⁺ channels are known as important participants of nociception and their remodeling contributes to diabetes-induced alterations of pain sensation. In this work we have established that about 30% of rat nonpeptidergic thermal C-type nociceptive (NTCN) neurons of segments L4-L6 express a slow T-type Ca²⁺ current (T-current) while a fast T-current is expressed in the other 70% of these neurons. Streptozotocin-induced diabetes in young rats resulted in thermal hyperalgesia, hypoalgesia, or normalgesia 5-6 weeks after the induction. Our results show that NTCN neurons obtained from hyperalgesic animals do not express the slow T-current. Meanwhile, the fraction of neurons expressing the slow T-current did not significantly change in the hypo- and normalgesic diabetic groups. Moreover, the peak current density of fast T-current was significantly increased only in the neurons of hyperalgesic group. In contrast, the peak current density of slow T-current was significantly decreased in the hypo- and normalgesic groups. Experimental diabetes also resulted in a depolarizing shift of steady-state inactivation of fast T-current in the hyperalgesic group and slow T-current in the hypo- and normalgesic groups. We suggest that the observed changes may contribute to expression of different types of peripheral diabetic neuropathy occurring during the development of diabetes mellitus.

Reduced number, G protein coupling, and antinociceptive efficacy of spinal mu-opioid receptors in diabetic rats are reversed by nerve growth factor

This study investigated putative mechanisms of impaired spinal opioid antinociception such as a downregulation of mu-opioid receptor (MOR) number, coupling, and efficacy in rats with advanced (12 weeks) streptozotocin (STZ)-induced diabetes. Intravenous injection of STZ (45 mg/kg) in Wistar rats led to selective degeneration of insulin-producing pancreatic ß-cells, elevated blood glucose, and mechanical hyperalgesia. In these animals, dose-dependent and naloxone-reversible intrathecal fentanyl antinociception was significantly impaired and associated with a loss in MOR immunoreactivity of calcitonin gene-related peptide-immunoreactive (CGRP-IR) sensory nerve terminals, membrane-bound MOR binding sites, and MOR-stimulated G protein coupling within the dorsal horn of the spinal cord. Intrathecal delivery of nerve growth factor (NGF) in diabetic animals normalized spinal MOR number and G protein coupling and rescued spinal fentanyl-induced antinociception. These findings identify for the first time a loss in functional MOR on central terminals of sensory neurons as a contributing factor for the impaired spinal opioid responsiveness during advanced STZ-induced diabetes that can be reversed by NGF. Moreover, they support growing evidence of a distinct regulation of opioid responsiveness during various painful states of disease (eg, arthritis, cancer, neuropathy) and may give novel therapeutic incentives.

PERSPECTIVE

In diabetic neuropathy a loss in sensory neuron mu-opioid receptor number and coupling contributes to impaired spinal opioid antinociception that can be reversed by NGF. These findings support growing evidence of a distinct regulation of opioid responsiveness during various painful diseases and may give novel therapeutic incentives.

Specific functioning of Cav3.2 T-type calcium and TRPV1 channels under different types of STZ-diabetic neuropathy

Streptozotocin (STZ)-induced type 1 diabetes in rats leads to the development of peripheral diabetic neuropathy (PDN) manifested as thermal hyperalgesia at early stages (4th week) followed by hypoalgesia after 8weeks of diabetes development. Here we found that 6-7 week STZ-diabetic rats developed either thermal hyper- (18%), hypo- (25%) or normalgesic (57%) types of PDN. These developmentally similar diabetic rats were studied in order to analyze mechanisms potentially underlying different thermal nociception. The proportion of IB4-positive capsaicin-sensitive small DRG neurons, strongly involved in thermal nociception, was not altered under different types of PDN implying differential changes at cellular and molecular level. We further focused on properties of T-type calcium and TRPV1 channels, which are known to be involved in Ca(2+) signaling and pathological nociception. Indeed, TRPV1-mediated signaling in these neurons was downregulated under hypo- and normalgesia and upregulated under hyperalgesia. A complex interplay between diabetes-induced changes in functional expression of Cav3.2 T-type calcium channels and depolarizing shift of their steady-state inactivation resulted in upregulation of these channels under hyper- and normalgesia and their downregulation under hypoalgesia. As a result, T-type window current was increased by several times under hyperalgesia partially underlying the increased resting [Ca(2+)]i observed in the hyperalgesic rats. At the same time Cav3.2-dependent Ca(2+) signaling was upregulated in all types of PDN. These findings indicate that alterations in functioning of Cav3.2 T-type and TRPV1 channels, specific for each type of PDN, may underlie the variety of pain syndromes induced by type 1 diabetes.

Calcium activity of upper thoracic dorsal root ganglion neurons in zucker diabetic Fatty rats

The aim of the present study was to examine the calcium activity of C8-T5 dorsal root ganglion (DRG) neurons from Zucker diabetic fatty rats. In total, 8 diabetic ZDF fatty animals and 8 age-matched control ZDF lean rats were employed in the study. C8-T5 dorsal root ganglia were isolated bilaterally from 14 to 18 weeks old rats, and a primary culture was prepared. Calcium activity was measured ratiometrically using the fluorescent Ca(2+)-indicator Fura-2 acetoxymethyl ester. All neurons were stimulated twice with 20 mM K(+), followed by stimulation with either 0.3 or 0.5  μ M Capsaicin, alone or in combination with algogenic chemicals (bradykinin, serotonin, prostaglandin E2 (all 10(-5) M), and adenosine (10(-3) M)) at pH 7.4 and 6.0. Neurons from diabetic animals exhibited an overall increased response to stimulation with 20 mM K(+) compared to neurons from control. Stimulation with Capsaicin alone caused an augmented response in neurons from diabetic animals compared to control animals. When stimulated with a combination of Capsaicin and algogenic chemicals, no differences between the two groups of neurons were measured, neither at pH 7.4 nor 6.0. In conclusion, diabetes-induced alterations in calcium activity of the DRG neurons were found, potentially indicating altered neuronal responses during myocardial ischemia.

Contemporary insights into painful diabetic neuropathy and treatment with spinal cord stimulation

A substantial body of literature is available on the natural history of diabetes, but much less is understood of the natural history of painful diabetic peripheral neuropathy (PDPN), a pervasive and costly complication of diabetes mellitus. Multiple mechanisms have been proposed, including polyol pathway activation, advanced glycosylation end-product formation, and vasculopathic changes. Nevertheless, specific treatment modalities addressing these basic issues are still lacking. The mainstay of treatment includes pharmacological management with antidepressants, anticonvulsants, and opioids, but these drugs are often limited by unfavorable side-effect profiles. For over 30 years, spinal cord stimulation (SCS) has been used extensively for the management of various chronic neuropathic pain states. In the past decade, interest in the use of SCS for treatment of PDPN has increased. This article reviews pathophysiological mechanisms of PDPN, proposed mechanisms of SCS, and the role of SCS for the treatment of PDPN.

Sensory neurons derived from diabetic rats have diminished internal Ca2+ stores linked to impaired re-uptake by the endoplasmic reticulum

Distal symmetrical sensory neuropathy in diabetes involves the dying back of axons, and the pathology equates with axonal dystrophy generated under conditions of aberrant Ca2+ signalling. Previous work has described abnormalities in Ca2+ homoeostasis in sensory and dorsal horn neurons acutely isolated from diabetic rodents. We extended this work by testing the hypothesis that sensory neurons exposed to long-term Type 1 diabetes in vivo would exhibit abnormal axonal Ca2+ homoeostasis and focused on the role of SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase). DRG (dorsal root ganglia) sensory neurons from age-matched normal and 3-5-month-old STZ (streptozotocin)-diabetic rats (an experimental model of Type 1 diabetes) were cultured. At 1-2 days in vitro an array of parameters were measured to investigate Ca2+ homoeostasis including (i) axonal levels of intracellular Ca2+, (ii) Ca2+ uptake by the ER (endoplasmic reticulum), (iii) assessment of Ca2+ signalling following a long-term thapsigargin-induced blockade of SERCA and (iv) determination of expression of ER mass and stress markers using immunocytochemistry and Western blotting. KCl- and caffeine-induced Ca2+ transients in axons were 2-fold lower in cultures of diabetic neurons compared with normal neurons indicative of reduced ER calcium loading. The rate of uptake of Ca2+ into the ER was reduced by 2-fold (P<0.05) in diabetic neurons, while markers for ER mass and ER stress were unchanged. Abnormalities in Ca2+ homoeostasis in diabetic neurons could be mimicked via long-term inhibition of SERCA in normal neurons. In summary, axons of neurons from diabetic rats exhibited aberrant Ca2+ homoeostasis possibly triggered by sub-optimal SERCA activity that could contribute to the distal axonopathy observed in diabetes.

Diabetes and alpha lipoic Acid

Diabetes mellitus is a multi-faceted metabolic disorder where there is increased oxidative stress that contributes to the pathogenesis of this debilitating disease. This has prompted several investigations into the use of antioxidants as a complementary therapeutic approach. Alpha lipoic acid, a naturally occurring dithiol compound which plays an essential role in mitochondrial bioenergetic reactions, has gained considerable attention as an antioxidant for use in managing diabetic complications. Lipoic acid quenches reactive oxygen species, chelates metal ions, and reduces the oxidized forms of other antioxidants such as vitamin C, vitamin E, and glutathione. It also boosts antioxidant defense system through Nrf-2-mediated antioxidant gene expression and by modulation of peroxisome proliferator activated receptors-regulated genes. ALA inhibits nuclear factor kappa B and activates AMPK in skeletal muscles, which in turn have a plethora of metabolic consequences. These diverse actions suggest that lipoic acid acts by multiple mechanisms, many of which have only been uncovered recently. In this review we briefly summarize the known biochemical properties of lipoic acid and then discussed the oxidative mechanisms implicated in diabetic complications and the mechanisms by which lipoic acid may ameliorate these reactions. The findings of some of the clinical trials in which lipoic acid administration has been tested in diabetic patients during the last 10 years are summarized. It appears that the clearest benefit of lipoic acid supplementation is in patients with diabetic neuropathy.

T-type voltage-gated calcium channels as targets for the development of novel pain therapies

It is well recognized that voltage-gated calcium (Ca(2+)) channels modulate the function of peripheral and central pain pathways by influencing fast synaptic transmission and neuronal excitability. In the past, attention focused on the modulation of different subtypes of high-voltage-activated-type Ca(2+) channels; more recently, the function of low-voltage-activated or transient (T)-type Ca(2+) channels (T-channels) in nociception has been well documented. Currently, available pain therapies remain insufficient for certain forms of pain associated with chronic disorders (e.g. neuropathic pain) and often have serious side effects. Hence, the identification of selective and potent inhibitors and modulators of neuronal T-channels may help greatly in the development of safer, more effective pain therapies. Here, we summarize the available information implicating peripheral and central T-channels in nociception. We also discuss possible future developments aimed at selective modulation of function of these channels, which are highly expressed in nociceptors.

Forced-exercise delays neuropathic pain in experimental diabetes: effects on voltage-activated calcium channels

Physical exercise produces a variety of psychophysical effects, including altered pain perception. Elevated levels of centrally produced endorphins or endocannabinoids are implicated as mediators of exercise-induced analgesia. The effect of exercise on the development and persistence of disease-associated acute/chronic pain remains unclear. In this study, we quantified the physiological consequence of forced-exercise on the development of diabetes-associated neuropathic pain. Euglycemic control or streptozotocin (STZ)-induced diabetic adult male rats were subdivided into sedentary or forced-exercised (2-10 weeks, treadmill) subgroups and assessed for changes in tactile responsiveness. Two weeks following STZ-treatment, sedentary rats developed a marked and sustained hypersensitivity to von Frey tactile stimulation. By comparison, STZ-treated diabetic rats undergoing forced-exercise exhibited a 4-week delay in the onset of tactile hypersensitivity that was independent of glucose control. Exercise-facilitated analgesia in diabetic rats was reversed, in a dose-dependent manner, by naloxone. Small-diameter (< 30 μm) DRG neurons harvested from STZ-treated tactile hypersensitive diabetic rats exhibited an enhanced (2.5-fold) rightward (depolarizing) shift in peak high-voltage activated (HVA) Ca(2+) current density with a concomitant appearance of a low-voltage activated (LVA) Ca(2+) current component. LVA Ca(2+) currents present in DRG neurons from hypersensitive diabetic rats exhibited a marked depolarizing shift in steady-state inactivation. Forced-exercise attenuated diabetes-associated changes in HVA Ca(2+) current density while preventing the depolarizing shift in steady-state inactivation of LVA Ca(2+) currents. Forced-exercise markedly delays the onset of diabetes-associated neuropathic pain, in part, by attenuating associated changes in HVA and LVA Ca(2+) channel function within small-diameter DRG neurons possibly by altering opioidergic tone.

Diabetic visceral hypersensitivity is associated with activation of mitogen-activated kinase in rat dorsal root ganglia

OBJECTIVE

Diabetic patients often experience visceral hypersensitivity and anorectal dysfunction. We hypothesize that the enhanced excitability of colon projecting dorsal root ganglia (DRG) neurons observed in diabetes is caused by a decrease in the amplitude of the transient A-type K(+) (I(A)) currents resulting from increased phosphorylation of mitogen-activated protein kinases (MAPK) and reduced opening of K(v)4.2 channels.

RESEARCH DESIGN AND METHODS

We performed patch-clamp recordings of colon projecting DRG neurons from control and streptozotocin-induced diabetic (STZ-D) rats. Western blot analyses and immunocytochemistry studies were used to elucidate the intracellular signaling pathways that modulate the I(A) current. In vivo studies were performed to demonstrate that abnormal MAPK signaling is responsible for the enhanced visceromotor response to colorectal distention in STZ-D rats.

RESULTS

Patch-clamp studies demonstrated that I(A) current was diminished in the colon projecting DRG neurons of STZ-D rats. Western blot analysis of STZ-D DRG neurons revealed increases in phosphorylated MAPK and K(V)4.2. In diabetic DRG neurons, increased intracellular Ca(2+) ([Ca(2+)](i)), protein kinase C (PKC), and MAPK were involved in the regulation of I(A) current through modulation of K(v)4.2. Hypersensitive visceromotor responses to colorectal distention in STZ-D rats were normalized by administration of MAPK inhibitor U0126.

CONCLUSIONS

We demonstrated that reduction of the I(A) current in STZ-D DRG neurons is triggered by impaired [Ca(2+)](i) ion homeostasis, and this in turn activates the PKC-MAPK pathways, resulting in decreased opening of the K(v)4.2 channels. Hence, the PKC-MAPK-K(v)4.2 pathways represent a potential therapeutic target for treating visceral hypersensitivity in diabetes.

Evaluation of the aldose reductase inhibitor fidarestat on ischemia-reperfusion injury in rat retina

This study evaluated the effects of retinal ischemia-reperfusion (IR) injury and pre-treatment with the potent and specific aldose reductase inhibitor fidarestat on apoptosis, aldose reductase and sorbitol dehydrogenase expression, sorbitol pathway intermediate concentrations, and oxidative-nitrosative stress. Female Wistar rats were pre-treated with either vehicle (N-methyl-D-glucamine) or fidarestat, 32 mg kg(-1) d(-1) for both, in the right jugular vein, for 3 consecutive days. A group of vehicle- and fidarestat-treated rats were subjected to 45-min retinal ischemia followed by 24-h reperfusion. Ischemia was induced 30 min after the last vehicle or fidarestat administration. Retinal IR resulted in a remarkable increase in retinal cell death. The number of TUNEL-positive nuclei increased 48-fold in the IR group compared with non-ischemic controls (p<0.01), and this increase was partially prevented by fidarestat. AR expression (Western blot analysis) increased by 19% in the IR group (p<0.05), and this increase was prevented by fidarestat. Sorbitol dehydrogenase and nitrated protein expressions were similar among all experimental groups. Retinal sorbitol concentrations tended to increase in the IR group but the difference with non-ischemic controls did not achieve statistical significance (p=0.08). Retinal fructose concentrations were 2.2-fold greater in the IR group than in the non-ischemic controls (p<0.05). Fidarestat pre-treatment of rats subjected to IR reduced retinal sorbitol concentration to the levels in non-ischemic controls. Retinal fructose concentrations were reduced by 41% in fidarestat-pre-treated IR group vs. untreated ischemic controls (p=0.0517), but remained 30% higher than in the non-ischemic control group. In conclusion, IR injury to rat retina is associated with a dramatic increase in cell death, elevated AR expression and sorbitol pathway intermediate accumulation. These changes were prevented or alleviated by the AR inhibitor fidarestat. The results identify AR as an important therapeutic target for diseases involving IR injury, and provide the rationale for development of fidarestat and other AR inhibitors.

Peripheral neuropathy in adult type 1 Gaucher disease: a 2-year prospective observational study

Type 1 Gaucher disease is currently categorized as non-neuronopathic, although recent studies suggest peripheral neurological manifestations. We report prevalence and incidence data for peripheral neuropathy and associated conditions from a multinational, prospective, longitudinal, observational cohort study in patients with type 1 Gaucher disease, either untreated or receiving enzyme replacement therapy. The primary outcome parameters were the prevalence and incidence of polyneuropathy, evaluated by standardized assessments of neurological symptoms and signs, and electrophysiological studies. All diagnoses of polyneuropathy were adjudicated centrally. Secondary outcome parameters included the prevalence and incidence of mononeuropathy, other neurological or electrophysiological abnormalities not fulfilling the criteria for a mono- or polyneuropathy and general type 1 Gaucher disease symptoms. Furthermore, a literature search was performed to identify all studies reporting on prevalence and incidence of polyneuropathy in the general population. One hundred and three patients were enrolled [median (range) age: 42 (18-75) years; disease duration: 15 (0-56) years; 52% female]; 14 (13.6%) were untreated and 89 (86.4%) were on enzyme replacement therapy. At baseline, 11 patients [10.7%; 95% confidence interval (CI): 5.9-18.3] were diagnosed with sensory motor axonal polyneuropathy. Two (1.9%; 95% CI: 0.1-7.2) had a mononeuropathy of the ulnar nerve. The 2-year follow-up period revealed another six cases of polyneuropathy (2.9 per 100 person-years; 95% CI: 1.2-6.3). Patients with polyneuropathy were older than those without (P<0.001). Conditions possibly associated with polyneuropathy were identified in four patients only, being monoclonal gammopathy, vitamin B(1) deficiency, folic acid deficiency, type 2 diabetes mellitus, renal insufficiency, alcohol abuse and exposure to toxins related to profession. The 11 cases of polyneuropathy found at baseline were confirmed during follow-up. According to the literature, the prevalence of polyneuropathy in the general population was estimated between 0.09 and 1.3% and the incidence was estimated between 0.0046 and 0.015 per 100 person-years. Thus, we conclude that the prevalence and incidence of polyneuropathy in patients with type 1 Gaucher disease is increased compared with the general population.

Changes in the gene expression of specific G-protein subunits correlate with morphine insensitivity in streptozotocin-induced diabetic rats

Several animal and human studies have shown a decreased analgesic potency of morphine in diabetic subjects. Since G-protein subunits have an important role in morphine effects at the cellular level and the exact mechanism(s) of diabetes-induced morphine insensitivity has not been fully clarified yet, the present study was designed to determine the changes in the levels of G(alphai), G(alphas), G(beta) mRNAs and proteins involved in this phenomenon. All experiments were carried out on male Wistar rats. The tail-flick test was used to assess the nociceptive threshold. Diabetes was induced by injection of 50 mg/kg (i.p.) streptozotocin. Four weeks after diabetes induction, the dorsal half of the lumbar spinal cord was assayed for the expression of G-protein subunits using semiquantitative RT-PCR and immunoblotting. The antinociceptive effect of intrathecal morphine (5, 10 and 15 microg i.t.) was significantly reduced in diabetic rats and these effects were reversed with insulin replacement. In diabetic animals, a significant increase in the mRNA levels of G(alphai) (23.5%) was observed in the dorsal portion of the lumbar spinal cord. The mRNA level of G(alphas) and G(beta) did not change. Following diabetes a significant decrease in the protein levels of G(alphai) was induced. In contrast, no significant changes were observed in the protein level of G(alphas) and G(beta). In diabetic animals that received insulin, levels of G(alphai) mRNA and protein were close to those in control rats. In conclusion, our results demonstrate that the expression pattern of the cellular components involved in morphine analgesia changes in diabetic animals. This may be, at least partly, responsible for diabetes-induced morphine insensitivity.

Mitochondrial stress and the pathogenesis of diabetic neuropathy

Diabetic neuropathy is a major complication of diabetes that affects the sensory and autonomic nervous systems and leads to significant morbidity and impact on quality of life of patients. Mitochondrial stress has been proposed as a major mediator of neurodegeneration in diabetes. This review briefly summarizes the nature of sensory and autonomic nerve dysfunction and presents these findings in the context of diabetes-induced nerve degeneration mediated by alterations in mitochondrial ultrastructure, physiology and trafficking. Diabetes-induced dysfunction in calcium homeostasis is discussed at length and causative associations with sub-optimal mitochondrial physiology are developed. It is clear that across a range of complications of diabetes that mitochondrial physiology is impaired, in general a reduction in electron transport chain capability is apparent. This abnormal activity may predispose mitochondria to generate elevated reactive oxygen species (ROS), although experimental proof remains lacking, but more importantly will deleteriously alter the bioenergetic status of neurons. It is proposed that the next five years of research should focus on identifying changes in mitochondrial phenotype and associated cellular impact, identifying sources of ROS in neurons and analyzing mitochondrial trafficking under diabetic conditions.

Alpha-lipoic Acid and diabetic neuropathy

Diabetic neuropathy presents a major public health problem. It is defined by the symptoms and signs of peripheral nerve dysfunction in diabetic patients, in whom other causes of neuropathy have been excluded. Pathogenetic mechanisms that have been implicated in diabetic neuropathy are: a) increased flux through the polyol pathway, leading to accumulation of sorbitol, a reduction in myo-inositol, and an associated reduced Na+-K+-ATPase activity, and b) endoneurial microvascular damage and hypoxia due to nitric oxide inactivation by increased oxygen free radical activity. Alpha-lipoic acid seems to delay or reverse peripheral diabetic neuropathy through its multiple antioxidant properties. Treatment with alpha-lipoic acid increases reduced glutathione, an important endogenous antioxidant. In clinical trials, 600 mg alpha-lipoic acid has been shown to improve neuropathic deficits. This review focuses on the relationship of alpha-lipoic acid and auto-oxidative glycosylation. It discusses the impact of alpha-lipoic acid on hyperglycemia-induced oxidative stress, and examines the role of alpha-lipoic acid in preventing glycation process and nerve hypoxia.

Abnormal calcium homeostasis in peripheral neuropathies

Abnormal neuronal calcium (Ca2+) homeostasis has been implicated in numerous diseases of the nervous system. The pathogenesis of two increasingly common disorders of the peripheral nervous system, namely neuropathic pain and diabetic polyneuropathy, has been associated with aberrant Ca2+ channel expression and function. Here we review the current state of knowledge regarding the role of Ca2+ dyshomeostasis and associated mitochondrial dysfunction in painful and diabetic neuropathies. The central impact of both alterations of Ca2+ signalling at the plasma membrane and also intracellular Ca2+ handling on sensory neurone function is discussed and related to abnormal endoplasmic reticulum performance. We also present new data highlighting sub-optimal axonal Ca2+ signalling in diabetic neuropathy and discuss the putative role for this abnormality in the induction of axonal degeneration in peripheral neuropathies. The accumulating evidence implicating Ca2+ dysregulation in both painful and degenerative neuropathies, along with recent advances in understanding of regional variations in Ca2+ channel and pump structures, makes modulation of neuronal Ca2+ handling an increasingly viable approach for therapeutic interventions against the painful and degenerative aspects of many peripheral neuropathies.

The beneficial effects of C-Peptide on diabetic polyneuropathy

Diabetic polyneuropathy (DPN) is a common complication in diabetes. At present, there is no adequate treatment, and DPN is often debilitating for patients. It is a heterogeneous disorder and differs in type 1 and type 2 diabetes. An important underlying factor in type 1 DPN is insulin deficiency. Proinsulin C-peptide is a critical element in the cascade of events. In this review, we describe the physiological role of C-peptide and how it provides an insulin-like signaling function. Such effects translate into beneficial outcomes in early metabolic perturbations of neural Na+/K+-ATPase and nitric oxide (NO) with subsequent preventive effects on early nerve dysfunction. Further corrective consequences resulting from this signaling cascade have beneficial effects on gene regulation of early gene responses, neurotrophic factors, their receptors, and the insulin receptor itself. This may lead to preventive and corrective results to nerve fiber degeneration and loss, as well as, promotion of nerve fiber regeneration with respect to sensory somatic fibers and small nociceptive nerve fibers. A characteristic abnormality of type 1 DPN is nodal and paranodal degeneration with severe consequences for myelinated fiber function. This review deals in detail with the underlying insulin-deficiency-related molecular changes and their correction by C-peptide. Based on these observations, it is evident that continuous maintenance of insulin-like actions by C-peptide is needed in peripheral nerve to minimize the sequences of metabolic and molecular abnormalities, thereby ameliorating neuropathic complications. There is now ample evidence demonstrating that C-peptide replacement in type 1 diabetes promotes insulin action and signaling activities in a more enhanced, prolonged, and continuous fashion than does insulin alone. It is therefore necessary to replace C-peptide to physiological levels in diabetic patients. This will have substantial beneficial effects on type 1 DPN.

In vivo silencing of the Ca(V)3.2 T-type calcium channels in sensory neurons alleviates hyperalgesia in rats with streptozocin-induced diabetic neuropathy

Earlier, we showed that streptozocin (STZ)-induced type 1 diabetes in rats leads to the development of painful peripheral diabetic neuropathy (PDN) manifested as thermal hyperalgesia and mechanical allodynia accompanied by significant enhancement of T-type calcium currents (T-currents) and cellular excitability in medium-sized dorsal root ganglion (DRG) neurons. Here, we studied the in vivo and in vitro effects of gene-silencing therapy specific for the Ca(V)3.2 isoform of T-channels, on thermal and mechanical hypersensitivities, and T-current expression in small- and medium-sized DRG neurons of STZ-treated rats. We found that silencing of the T-channel Ca(V)3.2 isoform using antisense oligonucleotides, had a profound and selective anti-hyperalgesic effect in diabetic rats and is accompanied by significant down-regulation of T-currents in DRG neurons. Anti-hyperalgesic effects of Ca(V)3.2 antisense oligonucleotides in diabetic rats were similar in models of rapid and slow onset of hyperglycemia following intravenous and intraperitoneal injections of STZ, respectively. Furthermore, treatments of diabetic rats with daily insulin injections reversed T-current alterations in DRG neurons in parallel with reversal of thermal and mechanical hypersensitivities in vivo. This confirms that Ca(V)3.2 T-channels, important signal amplifiers in peripheral sensory neurons, may contribute to the cellular hyperexcitability that ultimately leads to the development of painful PDN.

Endocannabinoid-mediated long-term depression in the avian midbrain expressed presynaptically and postsynaptically

Here, we examined long-term synaptic plasticity in the avian auditory midbrain, a region involved in experience-dependent learning. We found that coactivation of N-methyl-D-aspartate receptors (NMDAR) and type 1 cannabinoid receptors (CB1R) induces long-term depression (LTD) at the synapse between the central shell and the external portion of the inferior colliculus of the chicken. Although endocannabinoids are commonly thought of as presynaptic modulators, recent reports have suggested that they can also modulate the postsynaptic site. In the avian midbrain, we found that LTD is mediated by both presynaptic and postsynaptic changes. The presynaptic mechanism consists of a decrease in neurotransmitter release, whereas a depression of NMDAR-mediated current takes place on the postsynaptic side. Both the presynaptic and the postsynaptic effects depend on CB1R activation. The reduction of postsynaptic NMDAR currents represents a novel role of endocannabinoids in synaptic modulation.

Diabetes and the peripheral nerve

Diabetes-induced damage to peripheral nerve culminates in development of peripheral diabetic neuropathy (PDN), one of the most devastating complications of diabetes mellitus and a leading cause of foot amputation. The pathogenesis of PDN occurs as a consequence of complex interactions among multiple hyperglycemia-initiated mechanisms, impaired insulin signaling, inflammation, hypertension, and disturbances of fatty acid and lipid metabolism. This review describes experimental new findings in animal and cell culture models as well as clinical data suggesting the importance of 1) previously established hyperglycemia-initiated mechanisms such as increased aldose reductase activity, non-enzymatic glycation/glycooxidation, activation of protein kinase C, 2) oxidative-nitrosative stress and poly(ADP-ribose) polymerase activation; 3) mitogen-activated protein kinase and cyclooxygenase-2 activation, impaired Ca(++) homeostasis and signaling, and several other mechanisms, in PDN.

Experimental diabetes enhances Ca2+ mobilization and glutamate exocytosis in cerebral synaptosomes from mice

The present study was conducted to investigate the effects of the diabetic condition on the Ca(2+) mobilization and glutamate release in cerebral nerve terminals (synaptosomes). Diabetes was induced in male mice by intraperitoneal injection of streptozotocin. Cytosolic free Ca(2+) concentration ([Ca(2+)](i)) and glutamate release in synaptosomes were determined using fura-2 and enzyme-linked fluorometric assay, respectively. Diabetes significantly enhanced the ability of the depolarizing agents K(+) and 4-aminopyridine (4-AP) to increase [Ca(2+)](i). In addition, diabetes significantly enhanced K(+)- and 4-AP-evoked Ca(2+)-dependent glutamate release. The pretreatment of synaptosomes with a combination of omega-agatoxin IVA (a P-type Ca(2+) channel blocker) and omega-conotoxin GVIA (an N-type Ca(2+) channel blocker) inhibited K(+)- or 4-AP-induced increases in [Ca(2+)](i) and Ca(2+)-dependent glutamate release in synaptosomes from the control and diabetic mice to a similar extent, respectively. These results indicate that diabetes enhances a K(+)- or 4-AP-evoked Ca(2+)-dependent glutamate release by increasing [Ca(2+)](i) via stimulation of Ca(2+) entry through both P- and N-type Ca(2+) channels.

Diabetic neuropathy

In this review, an in-depth anatomic and molecular pathogenesis of diabetic neuropathy is provided. Classifications and clinical manifestations of diabetic neuropathy are discussed. The current modalities of treatment and clinical research on this disorder are summarized.