Altered temporal pattern of mechanically evoked C-fiber activity in a model of diabetic neuropathy in the rat

Blocking peripheral drive from colorectal afferents by subkilohertz dorsal root ganglion stimulation

ABSTRACT

Clinical evidence indicates dorsal root ganglion (DRG) stimulation effectively reduces pain without the need to evoke paresthesia. This paresthesia-free anesthesia by DRG stimulation can be promising to treat pain from the viscera, where paresthesia usually cannot be produced. Here, we explored the mechanisms and parameters for DRG stimulation using an ex vivo preparation with mouse distal colon and rectum (colorectum), pelvic nerve, L6 DRG, and dorsal root in continuity. We conducted single-fiber recordings from split dorsal root filaments and assessed the effect of DRG stimulation on afferent neural transmission. We determined the optimal stimulus pulse width by measuring the chronaxies of DRG stimulation to be below 216 µs, indicating spike initiation likely at attached axons rather than somata. Subkilohertz DRG stimulation significantly attenuates colorectal afferent transmission (10, 50, 100, 500, and 1000 Hz), of which 50 and 100 Hz show superior blocking effects. Synchronized spinal nerve and DRG stimulation reveals a progressive increase in conduction delay by DRG stimulation, suggesting activity-dependent slowing in blocked fibers. Afferents blocked by DRG stimulation show a greater increase in conduction delay than the unblocked counterparts. Midrange frequencies (50-500 Hz) are more efficient at blocking transmission than lower or higher frequencies. In addition, DRG stimulation at 50 and 100 Hz significantly attenuates in vivo visceromotor responses to noxious colorectal balloon distension. This reversible conduction block in C-type and Aδ-type afferents by subkilohertz DRG stimulation likely underlies the paresthesia-free anesthesia by DRG stimulation, thereby offering a promising new approach for managing chronic visceral pain.

GPR40 Activation Abolishes Diabetes-Induced Painful Neuropathy by Suppressing VEGF-A Expression

G-protein-coupled receptor 40 (GPR40) is a promising target to support glucose-induced insulin release in patients with type 2 diabetes. We studied the role of GPR40 in the regulation of blood-nerve barrier integrity and its involvement in diabetes-induced neuropathies. Because GPR40 modulates insulin release, we used the streptozotocin model for type 1 diabetes, in which GPR40 functions can be investigated independently of its effects on insulin release. Diabetic wild-type mice exhibited increased vascular endothelial permeability and showed epineural microlesions in sciatic nerves, which were also observed in naïve GPR40-/- mice. Fittingly, expression of vascular endothelial growth factor-A (VEGF-A), an inducer of vascular permeability, was increased in diabetic wild-type and naïve GPR40-/- mice. GPR40 antagonists increased VEGF-A expression in murine and human endothelial cells as well as permeability of transendothelial barriers. In contrast, GPR40 agonists suppressed VEGF-A release and mRNA expression. The VEGF receptor inhibitor axitinib prevented diabetes-induced hypersensitivities and reduced endothelial and epineural permeability. Importantly, the GPR40 agonist GW9508 reverted established diabetes-induced hypersensitivity, an effect that was blocked by VEGF-A administration. Thus, GPR40 activation suppresses VEGF-A expression, thereby reducing diabetes-induced blood-nerve barrier permeability and reverting diabetes-induced hypersensitivities.

Spinal cord-wide structural disruption in type 2 diabetes rescued by exenatide "a glucagon-like peptide-1 analogue" via down-regulating inflammatory, oxidative stress and apoptotic signaling pathways

The mechanisms of spinal cord-wide structural and functional disruption in diabetic patients remain elusive. This study evaluated histopathological alterations of the spinal cord cytoarchitecture in T2DM model of rats and assessed the potential ameliorating effect of exenatide "a potent GLP-1 analogue". Thirty male rats were allocated into three groups; I (control), II (Diabetic): T2DM was induced by high fat diet for 8 weeks followed by a single I.P injection of STZ (25 mg/kg BW) and III (Diabetic/Exenatide): T2DM rats injected with exenatide (10 μg/Kg, S.C. twice daily for 2 weeks). Neurobehavioral sensory and motor tests were carried out and glycemic control biomarkers and indices of insulin resistance and sensitivity were measured. In addition, the spinal cord was processed for histological and immunohistochemical studies besides assessing its tissue homogenate levels of pro-inflammatory/anti-inflamatory cytokines and oxidant/antioxidant biomarkers. Moreover, RT-qPCR was performed to measure the expression of proapoptotic/antiapoptotic and neurotrophic genes. The diabetic rats exhibited thermal hyperalgesia, mechanical allodynia and decreased locomotor activity along with increased serum glucose, insulin, HbA1c, HOMA-IR while, quantitative insulin sensitivity check index (QUICKI) was decreased. Also, IL-1β NF-kB, MDA increased while IL-10, SOD activity and β-endorphin decreased in the spinal tissue. Up regulation of caspase-3 and down regulation of Bcl-2, nerve growth factor (NGF) and glial cell-derived neurotrophic (GDNF) in diabetic rats. Also, they exhibited histopathological changes and increased CD68 positive microglia and Bax immunoreactivity in the spinal cord. Subsequent to exenatide treatment, most biomolecular, structural and functional impairments of the spinal cord were restored in the diabetic rats. In conclusion, the neuro-modulating effect of exenatide against diabetic-induced spinal cord affection warrants the concern about its therapeutic relevance in confronting the devastating diabetic neuropathic complications.

The implication of transient receptor potential canonical 6 in BDNF-induced mechanical allodynia in rat model of diabetic neuropathic pain

AIMS

Brain-derived neurotrophic factor (BDNF) is vital in the pathogenesis of mechanical allodynia with a paucity of reports available regarding diabetic neuropathy pain (DNP). Herein we identified the involvement of BDNF in driving mechanical allodynia in DNP rats via the activation of transient receptor potential canonical 6 (TRPC6) channel.

MATERIALS AND METHODS

The DNP rat model was established via streptozotocin (STZ) injection, and allodynia was assessed by paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL). The expression profiles of BDNF and TRPC6 in dorsal root ganglia (DRG) and spinal cord were illustrated by immunofluorescence and Western blotting. Intrathecal administration of K252a or TrkB-Fc was performed to inhibit BNDF/TrkB expression, and respective injection of GsMTX-4, BTP2 and TRPC6 antisense oligodeoxynucleotides (TRPC6-AS) was likewise conducted to inhibit TRPC6 expression in DNP rats. Calcium influx in DRG was monitored by calcium imaging.

KEY FINDINGS

The time-dependent increase of BDNF and TRPC6 expression in DRG and spinal cord was observed since the 7th post-STZ day, correlated with the development of mechanical allodynia in DNP rats. Intrathecal administration of K252a, TrkB-Fc, GsMTX-4 and BTP2 prevented mechanical allodynia in DNP rats. Pre-treatment of TRPC6-AS reversed the BDNF-induced pain-like responses in DNP rats rather than the naïve rats. In addition, the TRPC6-AS reversed BDNF-induced increase of calcium influx in DRG neurons in DNP rats.

SIGNIFICANCE

The intrathecal inhibition of TRPC6 alleviated the BDNF-induced mechanical allodynia in DNP rat model. This finding may validate the application of TRPC6 antagonists as interesting strategy for DNP management.

Effects of type 1 diabetes on reflexive cardiovascular responses to intermittent muscle contraction

This is the first study to provide evidence that early-stage type 1 diabetes mellitus (T1DM) leads to an exaggerated exercise pressor reflex evoked by intermittent muscle contraction, resulting in substantially higher cardiovascular strain. These findings are significant as they indicate that interventions targeting the exercise pressor reflex may work to alleviate the increased cardiovascular strain and overall burden during exercise in T1DM.

Exaggerated mechanoreflex in early-stage type 1 diabetic rats: role of Piezo channels

Recent findings have shown that muscle contraction evokes an exaggerated pressor response in type 1 diabetes mellitus (T1DM) rats; however, it is not known whether the mechanoreflex, which is commonly stimulated by stretching the Achilles tendon, contributes to this abnormal response. Furthermore, the role of mechano-gated Piezo channels, found on thin-fiber afferent endings, in evoking the mechanoreflex in T1DM is also unknown. Therefore, in male and female streptozotocin (STZ, 50 mg/kg)-induced T1DM and healthy control (CTL) rats, we examined the pressor and cardioaccelerator responses to tendon stretch during the early stage of the disease. To determine the role of Piezo channels, GsMTx-4, a selective Piezo channel inhibitor, was injected into the arterial supply of the hindlimb. At 1 wk after STZ injection in unanesthetized, decerebrate rats, we stretched the Achilles tendon for 30 s and measured pressor and cardioaccelerator responses. We then compared pressor and cardioaccelerator responses to tendon stretch before and after GsMTx-4 injection (10 µg/100 ml). We found that the pressor (change in mean arterial pressure) response [41 ± 5 mmHg ( n = 15) for STZ and 18 ± 3 mmHg ( n = 11) for CTL ( P < 0.01)] and cardioaccelerator (change in heart rate) response [18 ± 4 beats/min for STZ ( n = 15) and 8 ± 2 beats/min ( n = 11) for CTL ( P < 0.05)] to tendon stretch were exaggerated in STZ rats. Local injection of GsMTx-4 attenuated the pressor [55 ± 7 mmHg ( n = 6) before and 27 ± 9 mmHg ( n = 6) after GsMTx-4 ( P < 0.01)], but not the cardioaccelerator, response to tendon stretch in STZ rats and had no effect on either response in CTL rats. These data suggest that T1DM exaggerates the mechanoreflex response to tendon stretch and that Piezo channels play a role in this exaggeration.

Neuroactive Steroids and Sex-Dimorphic Nervous Damage Induced by Diabetes Mellitus

Diabetes mellitus is a metabolic disease where improper glycaemic control may induce severe complications in different organs. In this review, we will discuss alterations occurring in peripheral and central nervous system of patients with type 1 (i.e., insulin dependent diabetes mellitus,) or type 2 diabetes (i.e., non-insulin dependent diabetes mellitus), as well as related experimental models. A particular focus will be on the role exerted by neuroactive steroids (i.e., important regulators of nervous functions) in the nervous damage induced by diabetes. Indeed, the nervous levels of these molecules are affected by the pathology and, in agreement, their neuroprotective effects have been reported. Interestingly, the sex is another important variable. As discussed, nervous diabetic complications show sex dimorphic features in term of incidence, functional outcomes and neuroactive steroid levels. Therefore, these features represent an interesting background for possible sex-oriented therapies with neuroactive steroids aimed to counteract nervous damage observed in diabetic pathology.

Lumbar Spinal Cord Activity and Blood Biochemical Changes in Individuals With Diabetic Peripheral Neuropathy During Electrical Stimulation

It is difficult to perform an in vivo evaluation of the nerve conduction mechanism in a patient with diabetic peripheral neuropathy (DPN). We aim to explore possible activation differences to enable a further understanding of the nerve conduction mechanisms of diabetic neuropathy and to present a novel clinical method to evaluate nerve injury and recovery. DPN patients (n = 20) and healthy volunteers (n = 20) were included in this study to detect the functional activation of the lumbar spinal cord via electric stimulation. Spinal fMRI data sets were acquired via a single-shot fast spin echo (SSFSE) sequence. A task-related fMRI was performed via low-frequency electrical stimulation. After post-processing, the active voxels and the percentage of signal changes were calculated for the DPN evaluation and the correlations between the blood biochemical indexes, such as glucose, total cholesterol, and hemoglobin A1c were explored. Activation in the DPN patients was primarily observed in the T12 (10/13) vertebral level. The percentage of signal changes in DPN patients was higher than that in the control group (Z = -2.757, P < 0.05). Positive correlation between the percentage of signal changes and the total cholesterol/glucose in the DNP group was found (P < 0.05). Lumbar spinal cord fMRI, based on the SEEP effect, was determined to be feasible. The repetitive activation distribution was primarily located at the T12 vertebral level. Lumbar spinal cord fMRI might be used as a potential tool to assess and reveal the nerve conduction mechanisms in DPN.

Diabetes-induced microvascular complications at the level of the spinal cord: a contributing factor in diabetic neuropathic pain

KEY POINTS

Diabetes is thought to induce neuropathic pain through activation of dorsal horn sensory neurons in the spinal cord. Here we explore the impact of hyperglycaemia on the blood supply supporting the spinal cord and chronic pain development. In streptozotocin-induced diabetic rats, neuropathic pain is accompanied by a decline in microvascular integrity in the dorsal horn. Hyperglycaemia-induced degeneration of the endothelium in the dorsal horn was associated with a loss in vascular endothelial growth factor (VEGF)-A₁₆₅ b expression. VEGF-A₁₆₅ b treatment prevented diabetic neuropathic pain and degeneration of the endothelium in the spinal cord. Using an endothelial-specific VEGFR2 knockout transgenic mouse model, the loss of endothelial VEGFR2 signalling led to a decline in vascular integrity in the dorsal horn and the development of hyperalgesia in VEGFR2 knockout mice. This highlights that vascular degeneration in the spinal cord could be a previously unidentified factor in the development of diabetic neuropathic pain.

ABSTRACT

Abnormalities of neurovascular interactions within the CNS of diabetic patients is associated with the onset of many neurological disease states. However, to date, the link between the neurovascular network within the spinal cord and regulation of nociception has not been investigated despite neuropathic pain being common in diabetes. We hypothesised that hyperglycaemia-induced endothelial degeneration in the spinal cord, due to suppression of vascular endothelial growth factor (VEGF)-A/VEGFR2 signalling, induces diabetic neuropathic pain. Nociceptive pain behaviour was investigated in a chemically induced model of type 1 diabetes (streptozotocin induced, insulin supplemented; either vehicle or VEGF-A₁₆₅ b treated) and an inducible endothelial knockdown of VEGFR2 (tamoxifen induced). Diabetic animals developed mechanical allodynia and heat hyperalgesia. This was associated with a reduction in the number of blood vessels and reduction in Evans blue extravasation in the lumbar spinal cord of diabetic animals versus age-matched controls. Endothelial markers occludin, CD31 and VE-cadherin were downregulated in the spinal cord of the diabetic group versus controls, and there was a concurrent reduction of VEGF-A₁₆₅ b expression. In diabetic animals, VEGF-A₁₆₅ b treatment (biweekly i.p., 20 ng g^-1 ) restored normal Evans blue extravasation and prevented vascular degeneration, diabetes-induced central neuron activation and neuropathic pain. Inducible knockdown of VEGFR2 (tamoxifen treated Tie2CreER^T2 -vegfr2^flfl mice) led to a reduction in blood vessel network volume in the lumbar spinal cord and development of heat hyperalgesia. These findings indicate that hyperglycaemia leads to a reduction in the VEGF-A/VEGFR2 signalling cascade, resulting in endothelial dysfunction in the spinal cord, which could be an undiscovered contributing factor to diabetic neuropathic pain.

Neuroactive steroids and diabetic complications in the nervous system

Important complications of diabetes mellitus in the nervous system are represented by diabetic peripheral neuropathy and diabetic encephalopathy. In this context, an important link is represented by neuroactive steroids (i.e., steroids coming from peripheral glands and affecting nervous functionality as well as directly synthesized in the nervous system). Indeed, diabetes does not only affect the reproductive axis and consequently the levels of sex steroid hormones, but also those of neuroactive steroids. Indeed, as will be here summarized, the levels of these neuromodulators present in the central and peripheral nervous system are affected by the pathology in a sex-dimorphic way. In addition, some of these neuroactive steroids, such as the metabolites of progesterone or testosterone, as well as pharmacological tools able to increase their levels have been demonstrated, in experimental models, to be promising protective agents against diabetic peripheral neuropathy and diabetic encephalopathy.

Action Potential Broadening in Capsaicin-Sensitive DRG Neurons from Frequency-Dependent Reduction of Kv3 Current

Action potential (AP) shape is a key determinant of cellular electrophysiological behavior. We found that in small-diameter, capsaicin-sensitive dorsal root ganglia neurons corresponding to nociceptors (from rats of either sex), stimulation at frequencies as low as 1 Hz produced progressive broadening of the APs. Stimulation at 10 Hz for 3 s resulted in an increase in AP width by an average of 76 ± 7% at 22°C and by 38 ± 3% at 35°C. AP clamp experiments showed that spike broadening results from frequency-dependent reduction of potassium current during spike repolarization. The major current responsible for frequency-dependent reduction of overall spike-repolarizing potassium current was identified as Kv3 current by its sensitivity to low concentrations of 4-aminopyridine (IC₅₀ <100 μm) and block by the peptide inhibitor blood depressing substance I (BDS-I). There was a small component of Kv1-mediated current during AP repolarization, but this current did not show frequency-dependent reduction. In a small fraction of cells, there was a component of calcium-dependent potassium current that showed frequency-dependent reduction, but the contribution to overall potassium current reduction was almost always much smaller than that of Kv3-mediated current. These results show that Kv3 channels make a major contribution to spike repolarization in small-diameter DRG neurons and undergo frequency-dependent reduction, leading to spike broadening at moderate firing frequencies. Spike broadening from frequency-dependent reduction in Kv3 current could mitigate the frequency-dependent decreases in conduction velocity typical of C-fiber axons.SIGNIFICANCE STATEMENT Small-diameter dorsal root ganglia (DRG) neurons mediating nociception and other sensory modalities express many types of potassium channels, but how they combine to control firing patterns and conduction is not well understood. We found that action potentials of small-diameter rat DRG neurons showed spike broadening at frequencies as low as 1 Hz and that spike broadening resulted predominantly from frequency-dependent inactivation of Kv3 channels. Spike width helps to control transmitter release, conduction velocity, and firing patterns and understanding the role of particular potassium channels can help to guide new pharmacological strategies for targeting pain-sensing neurons selectively.

Involvement of spinal glutamate transporter-1 in the development of mechanical allodynia and hyperalgesia associated with type 2 diabetes

Little is known about the effects of the development of type 2 diabetes on glutamate homeostasis in the spinal cord. Therefore, we quantified the extracellular levels of glutamate in the spinal cord of Zucker diabetic fatty (ZDF) rats using in vivo microdialysis. In addition, protein levels of glutamate transporter-1 (GLT-1) in the spinal cord of ZDF rats were measured using Western blot. Finally, the effects of repeated intrathecal injections of ceftriaxone, which was previously shown to enhance GLT-1 expression, on the development of mechanical allodynia and hyperalgesia as well as on basal extracellular level of glutamate and the expression of GLT-1 in the spinal cord of ZDF rats were evaluated. It was found that ZDF rats developed mechanical hyperalgesia and allodynia, which were associated with increased basal extracellular levels of glutamate and attenuated levels of GLT-1 expression in the spinal cord, particularly in the dorsal horn. Furthermore, repeated intrathecal administrations of ceftriaxone dose-dependently prevented the development of mechanical hyperalgesia and allodynia in ZDF rats, which were correlated with enhanced GLT-1 expression without altering the basal glutamate levels in the spinal cord of ZDF rats. Overall, the results suggested that impaired glutamate reuptake in the spinal cord may contribute to the development of neuropathic pains in type 2 diabetes.

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.

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.

Nociceptor Sensitization Depends on Age and Pain Chronicity(1,2,3)

Peripheral inflammation causes mechanical pain behavior and increased action potential firing. However, most studies examine inflammatory pain at acute, rather than chronic time points, despite the greater burden of chronic pain on patient populations, especially aged individuals. Furthermore, there is disagreement in the field about whether primary afferents contribute to chronic pain. Therefore, we sought to evaluate the contribution of nociceptor activity to the generation of pain behaviors during the acute and chronic phases of inflammation in both young and aged mice. We found that both young (2 months old) and aged (>18 months old) mice exhibited prominent pain behaviors during both acute (2 day) and chronic (8 week) inflammation. However, young mice exhibited greater behavioral sensitization to mechanical stimuli than their aged counterparts. Teased fiber recordings in young animals revealed a twofold mechanical sensitization in C fibers during acute inflammation, but an unexpected twofold reduction in firing during chronic inflammation. Responsiveness to capsaicin and mechanical responsiveness of A-mechanonociceptor (AM) fibers were also reduced chronically. Importantly, this lack of sensitization in afferent firing during chronic inflammation occurred even as these inflamed mice exhibited continued behavioral sensitization. Interestingly, C fibers from inflamed aged animals showed no change in mechanical firing compared with controls during either the acute or chronic inflammatory phases, despite strong behavioral sensitization to mechanical stimuli at these time points. These results reveal the following two important findings: (1) nociceptor sensitization to mechanical stimulation depends on age and the chronicity of injury; and (2) maintenance of chronic inflammatory pain does not rely on enhanced peripheral drive.

Vascular endothelial growth factor-A165b prevents diabetic neuropathic pain and sensory neuronal degeneration

Diabetic peripheral neuropathy affects up to half of diabetic patients. This neuronal damage leads to sensory disturbances, including allodynia and hyperalgesia. Many growth factors have been suggested as useful treatments for prevention of neurodegeneration, including the vascular endothelial growth factor (VEGF) family. VEGF-A is generated as two alternative splice variant families. The most widely studied isoform, VEGF-A165a is both pro-angiogenic and neuroprotective, but pro-nociceptive and increases vascular permeability in animal models. Streptozotocin (STZ)-induced diabetic rats develop both hyperglycaemia and many of the resulting diabetic complications seen in patients, including peripheral neuropathy. In the present study, we show that the anti-angiogenic VEGF-A splice variant, VEGF-A165b, is also a potential therapeutic for diabetic neuropathy. Seven weeks of VEGF-A165b treatment in diabetic rats reversed enhanced pain behaviour in multiple behavioural paradigms and was neuroprotective, reducing hyperglycaemia-induced activated caspase 3 (AC3) levels in sensory neuronal subsets, epidermal sensory nerve fibre loss and aberrant sciatic nerve morphology. Furthermore, VEGF-A165b inhibited a STZ-induced increase in Evans Blue extravasation in dorsal root ganglia (DRG), saphenous nerve and plantar skin of the hind paw. Increased transient receptor potential ankyrin 1 (TRPA1) channel activity is associated with the onset of diabetic neuropathy. VEGF-A165b also prevented hyperglycaemia-enhanced TRPA1 activity in an in vitro sensory neuronal cell line indicating a novel direct neuronal mechanism that could underlie the anti-nociceptive effect observed in vivo. These results demonstrate that in a model of Type I diabetes VEGF-A165b attenuates altered pain behaviour and prevents neuronal stress, possibly through an effect on TRPA1 activity.

Spike propagation through the dorsal root ganglia in an unmyelinated sensory neuron: a modeling study

Unmyelinated C-fibers are a major type of sensory neurons conveying pain information. Action potential conduction is regulated by the bifurcation (T-junction) of sensory neuron axons within the dorsal root ganglia (DRG). Understanding how C-fiber signaling is influenced by the morphology of the T-junction and the local expression of ion channels is important for understanding pain signaling. In this study we used biophysical computer modeling to investigate the influence of axon morphology within the DRG and various membrane conductances on the reliability of spike propagation. As expected, calculated input impedance and the amplitude of propagating action potentials were both lowest at the T-junction. Propagation reliability for single spikes was highly sensitive to the diameter of the stem axon and the density of voltage-gated Na⁺ channels. A model containing only fast voltage-gated Na⁺ and delayed-rectifier K⁺ channels conducted trains of spikes up to frequencies of 110 Hz. The addition of slowly activating KCNQ channels (i.e., K_V7 or M-channels) to the model reduced the following frequency to 30 Hz. Hyperpolarization produced by addition of a much slower conductance, such as a Ca²⁺-dependent K⁺ current, was needed to reduce the following frequency to 6 Hz. Attenuation of driving force due to ion accumulation or hyperpolarization produced by a Na⁺-K⁺ pump had no effect on following frequency but could influence the reliability of spike propagation mutually with the voltage shift generated by a Ca²⁺-dependent K⁺ current. These simulations suggest how specific ion channels within the DRG may contribute toward therapeutic treatments for chronic pain.

Docosahexaenoic acid inhibits mechanical allodynia and thermal hyperalgesia in diabetic rats by decreasing the excitability of DRG neurons

Diabetes mellitus is a common metabolic disease in human beings with characteristic symptoms of hyperglycemia, chronic inflammation and insulin resistance. One of the most common complications of early-onset diabetes mellitus is peripheral diabetic neuropathy, which is manifested either by loss of nociception or by allodynia and hyperalgesia. Dietary fatty acids, especially polyunsaturated fatty acids, have been shown the potential of anti-inflammation and modulating neuron excitability. The present study investigated the effects of docosahexaenoic acid (DHA) on the excitability of dorsal root ganglion (DRG) neurons in streptozotocin (STZ)-induced diabetes rats. The effects of DHA on the allodynia and hyperalgesia of diabetic rats were also evaluated. Dietary DHA supplementation effectively attenuated both allodynia and hyperalgesia induced by STZ injection. DHA supplementation decreased the excitability of DRG neurons by decreasing the sodium currents and increasing potassium currents, which may contribute to the effect of alleviating allodynia and hyperalgesia in diabetic rats. The results suggested that DHA might be useful as an adjuvant therapy for the prevention and treatment of painful diabetic neuropathy.

Control of somatic membrane potential in nociceptive neurons and its implications for peripheral nociceptive transmission

Peripheral sensory ganglia contain somata of afferent fibres conveying somatosensory inputs to the central nervous system. Growing evidence suggests that the somatic/perisomatic region of sensory neurons can influence peripheral sensory transmission. Control of resting membrane potential (E_rest) is an important mechanism regulating excitability, but surprisingly little is known about how E_rest is regulated in sensory neuron somata or how changes in somatic/perisomatic E_rest affect peripheral sensory transmission. We first evaluated the influence of several major ion channels on E_rest in cultured small-diameter, mostly capsaicin-sensitive (presumed nociceptive) dorsal root ganglion (DRG) neurons. The strongest and most prevalent effect on E_rest was achieved by modulating M channels, K2P and 4-aminopiridine-sensitive K_V channels, while hyperpolarization-activated cyclic nucleotide-gated, voltage-gated Na⁺, and T-type Ca²⁺ channels to a lesser extent also contributed to E_rest. Second, we investigated how varying somatic/perisomatic membrane potential, by manipulating ion channels of sensory neurons within the DRG, affected peripheral nociceptive transmission in vivo. Acute focal application of M or K_ATP channel enhancers or a hyperpolarization-activated cyclic nucleotide-gated channel blocker to L5 DRG in vivo significantly alleviated pain induced by hind paw injection of bradykinin. Finally, we show with computational modelling how somatic/perisomatic hyperpolarization, in concert with the low-pass filtering properties of the t-junction within the DRG, can interfere with action potential propagation. Our study deciphers a complement of ion channels that sets the somatic E_rest of nociceptive neurons and provides strong evidence for a robust filtering role of the somatic and perisomatic compartments of peripheral nociceptive neuron.

Exercise-induced modulation of pain in adults with and without painful diabetic neuropathy

The purpose of this study was to examine exercise-induced pain modulation in diabetic adults with painful diabetic neuropathy (PDN) compared to diabetic adults without PDN. Eighteen adults diagnosed with type 2 diabetes with and without PDN (mean age of 49 years) completed 2 sessions. During the familiarization session, participants completed questionnaires, were familiarized with the pain testing protocols, and completed maximal isometric contractions. During the exercise session, experimental pain testing was completed before and following exercise consisting of 3 minutes of isometric exercise performed at 25% maximal voluntary contraction. Ratings of perceived exertion and muscle pain were assessed every 30 seconds during exercise. Results indicated that ratings of perceived exertion and muscle pain during exercise were significantly higher (P < .05) for diabetic adults with PDN versus diabetic adults without PDN. Diabetic adults with PDN did not experience changes in thermal pain ratings following exercise, whereas diabetic adults without PDN reported significantly lower pain ratings following exercise. It is concluded that diabetic adults with PDN experienced high levels of muscle pain during exercise and a lack of exercise-induced hypoalgesia following exercise, in comparison to diabetic adults without PDN, who experienced lower levels of muscle pain during exercise and a hypoalgesic response following exercise.

PERSPECTIVE

Very little research has been conducted examining the impact of exercise on pain modulation in diabetic adults with PDN. This study provides support that adults with PDN exhibit exercise-induced endogenous pain modulatory system dysfunction.

Diabetic neuropathic pain: a role for testosterone metabolites

Diabetic neuropathy is associated with neuropathic pain in about 50% of diabetic subjects. Clinical management of neuropathic pain is complex and so far unsatisfactory. In this study, we analyzed the effects of the testosterone metabolites, dihydrotestosterone (DHT), and 3α-diol, on nociceptive and allodynia thresholds and on molecular and functional parameters related to pain modulation in the dorsal horns of the spinal cord and in the dorsal root ganglia of rats rendered diabetic by streptozotocin injection. Furthermore, the levels of DHT and 3α-diol were analyzed in the spinal cord. Diabetes resulted in a significant decrease in DHT levels in the spinal cord that was reverted by DHT or 3α-diol treatments. In addition, 3α-diol treatment resulted in a significant increase in 3α-diol in the spinal cord compared with control values. Both steroids showed analgesic properties on diabetic neuropathic pain, affecting different pain parameters and possibly by different mechanisms of action. Indeed, DHT counteracted the effect of diabetes on the mechanical nociceptive threshold, pre- and post-synaptic components, glutamate release, astrocyte immunoreactivity, and expression of interleukin-1β (IL1β), while 3α-diol was effective on tactile allodynia threshold, glutamate release, astrocyte immunoreactivity and the expression of substance P, toll-like receptor 4, tumor necrosis factor-α, transforming growth factor β-1, IL1β, and translocator protein. These results indicate that testosterone metabolites are potential agents for the treatment of diabetic neuropathic pain.

Failure of action potential propagation in sensory neurons: mechanisms and loss of afferent filtering in C-type units after painful nerve injury

The T-junction of sensory neurons in the dorsal root ganglion (DRG) is a potential impediment to action potential (AP) propagation towards the CNS. Using intracellular recordings from rat DRG neuronal somata during stimulation of the dorsal root, we determined that the maximal rate at which all of 20 APs in a train could successfully transit the T-junction (following frequency) was lowest in C-type units, followed by A-type units with inflected descending limbs of the AP, and highest in A-type units without inflections. In C-type units, following frequency was slower than the rate at which AP trains could be produced in either dorsal root axonal segments or in the soma alone, indicating that the T-junction is a site that acts as a low-pass filter for AP propagation. Following frequency was slower for a train of 20 APs than for two, indicating that a cumulative process leads to propagation failure. Propagation failure was accompanied by diminished somatic membrane input resistance, and was enhanced when Ca(2+)-sensitive K(+) currents were augmented or when Ca(2+)-sensitive Cl(-) currents were blocked. After peripheral nerve injury, following frequencies were increased in axotomized C-type neurons and decreased in axotomized non-inflected A-type neurons. These findings reveal that the T-junction in sensory neurons is a regulator of afferent impulse traffic. Diminished filtering of AP trains at the T-junction of C-type neurons with axotomized peripheral processes could enhance the transmission of activity that is ectopically triggered in a neuroma or the neuronal soma, possibly contributing to pain generation.

NON-INVASIVE EVALUATION OF NERVE CONDUCTION IN SMALL DIAMETER FIBERS IN THE RAT

A novel non-invasive technique was applied to measure velocity within slow conducting axons in the distal extreme of the sciatic nerve (i.e., digital nerve) in a rat model. The technique is based on the extraction of rectified multiple unit activity (MUA) from in vivo whole nerve compound responses. This method reliably identifies compound action potentials in thinly myelinated fibers conducting at a range of 9-18 m/s (Aδ axons), as well as in a subgroup of unmylinated C fibers conducting at approximately 1-2 m/s. The sensitivity of the method to C-fiber conduction was confirmed by the progressive decrement of the responses in the 1-2 m/s range over a 20-day period following the topical application of capsaicin (ANOVA p<0.03). Increasing the frequency of applied repetitive stimulation over a range of 0.75 Hz to 6.0 Hz produced slowing of conduction and a significant decrease in the magnitude of the compound C-fiber response (ANOVA p<0.01). This technique offers a unique opportunity for the non-invasive, repeatable, and quantitative assessment of velocity in the subsets of Aδ and C fibers in parallel with evaluation of fast nerve conduction.

Gastrodin inhibits allodynia and hyperalgesia in painful diabetic neuropathy rats by decreasing excitability of nociceptive primary sensory neurons

Painful diabetic neuropathy (PDN) is a common complication of diabetes mellitus and adversely affects the patients' quality of life. Evidence has accumulated that PDN is associated with hyperexcitability of peripheral nociceptive primary sensory neurons. However, the precise cellular mechanism underlying PDN remains elusive. This may result in the lacking of effective therapies for the treatment of PDN. The phenolic glucoside, gastrodin, which is a main constituent of the Chinese herbal medicine Gastrodia elata Blume, has been widely used as an anticonvulsant, sedative, and analgesic since ancient times. However, the cellular mechanisms underlying its analgesic actions are not well understood. By utilizing a combination of behavioral surveys and electrophysiological recordings, the present study investigated the role of gastrodin in an experimental rat model of STZ-induced PDN and to further explore the underlying cellular mechanisms. Intraperitoneal administration of gastrodin effectively attenuated both the mechanical allodynia and thermal hyperalgesia induced by STZ injection. Whole-cell patch clamp recordings were obtained from nociceptive, capsaicin-sensitive small diameter neurons of the intact dorsal root ganglion (DRG). Recordings from diabetic rats revealed that the abnormal hyperexcitability of neurons was greatly abolished by application of GAS. To determine which currents were involved in the antinociceptive action of gastrodin, we examined the effects of gastrodin on transient sodium currents (I(NaT)) and potassium currents in diabetic small DRG neurons. Diabetes caused a prominent enhancement of I(NaT) and a decrease of potassium currents, especially slowly inactivating potassium currents (I(AS)); these effects were completely reversed by GAS in a dose-dependent manner. Furthermore, changes in activation and inactivation kinetics of I(NaT) and total potassium current as well as I(AS) currents induced by STZ were normalized by GAS. This study provides a clear cellular basis for the peripheral analgesic action of gastrodin for the treatment of chronic pain, including PDN.

5-HT7 receptor activation attenuates thermal hyperalgesia in streptozocin-induced diabetic mice

The role of 5-HT7 receptors in the nociceptive processing received most attention during the last few years. The involvement of 5-HT₇ receptors in nerve injury-induced neuropathic pain states have been reported only recently; however, there are no reports on its contribution in diabetic neuropathic pain. We therefore planned to investigate the effect of 5-HT₇ receptor activation on the changes of nociceptive threshold in diabetic mice. Diabetes was induced by a single intraperitoneal injection of streptozocin (150 mg/kg, i.p.). The nociceptive responses in normal and diabetic animals were tested in the hot-plate and tail-flick assays. Both hot-plate and tail-flick latencies significantly shortened at 1-3/4 weeks (thermal hyperalgesia) and prolonged at 6-7 weeks (thermal hypoalgesia) after streptozocin administration. At the dose of 10 mg/kg, systemic injections of AS-19, a selective 5-HT₇ receptor agonist, reduced thermal hyperalgesia at early stage of diabetes, but did not influence thermal hypoalgesia at late stage. Co-administration of SB-258719, a selective 5-HT₇ receptor antagonist, at a dose that had no effect on its own (10 mg/kg), reversed the anti-hyperalgesic effect of AS-19. Our results indicate that systemic administration of 5-HT₇ receptor agonists may have clinical utility in treating diabetic neuropathic pain.

Reduced conduction failure of the main axon of polymodal nociceptive C-fibres contributes to painful diabetic neuropathy in rats

Painful diabetic neuropathy is a common complication of diabetes mellitus and can affect many aspects of life and severely limit patients' daily functions. Signals of painful diabetic neuropathy are believed to originate in the peripheral nervous system. However, its peripheral mechanism of hyperalgesia has remained elusive. Numerous studies have accumulated that polymodal nociceptive C-fibres play a crucial role in the generation and conduction of pain signals and sensitization of which following injury or inflammation leads to marked hyperalgesia. Traditionally, the number of nociceptive primary afferent firings is believed to be determined at the free nerve endings, while the extended main axon of unmyelinated C-fibres only involves the reliable and faithful propagation of firing series to the central terminals. We challenged this classic view by showing that conduction of action potential can fail to occur in response to repetitive activity when they travel down the main axon of polymodal nociceptive C-fibres. Quantitative analysis of conduction failure revealed that the degree of conduction failure displays a frequency-dependent manner. Local administration of low threshold, rapidly activating potassium current blocker, α-dendrotoxin (0.5 nM) and persistent sodium current blocker, low doses of tetrodotoxin (<100 nM) on the main axon of C-fibres can reciprocally regulate the degree of conduction failure, confirming that conduction failure did occur along the main axon of polymodal nociceptive C-fibres. Following streptozotocin-induced diabetes, a subset of polymodal nociceptive C-fibres exhibited high-firing-frequency to suprathreshold mechanical stimulation, which account for about one-third of the whole population of polymodal nociceptive C-fibres tested. These high-firing-frequency polymodal nociceptive C-fibres in rats with diabetes displayed a marked reduction of conduction failure. Delivery of low concentrations of tetrodotoxin and Nav1.8 selective blocker, A-803467 on the main axon of C-fibres was found to markedly enhance the conduction failure in a dose-dependent manner in diabetic rats. Upregulated expression of sodium channel subunits Nav1.7 and Nav1.8 in both small dorsal root ganglion neurons and peripheral C-fibres as well as enhanced transient and persistent sodium current and increased excitability in small dorsal root ganglion neurons from diabetic rats might underlie the reduced conduction failure in the diabetic high-firing-frequency polymodal nociceptive C-fibres. This study shed new light on the functional capability in the pain signals processing for the main axon of polymodal nociceptive C-fibres and revealed a novel mechanism underlying diabetic hyperalgesia.

Characterization of upper thoracic spinal neurons receiving noxious cardiac and/or somatic inputs in diabetic rats

The aim of the present study was to examine spinal processing of cardiac and somatic nociceptive input in rats with STZ-induced diabetes. Type 1 diabetes was induced with streptozotocin (50mg/kg) in 14 male Sprague-Dawley rats and citrate buffer was injected in 14 control rats. After 4-11 weeks, the rats were anesthetized with pentobarbital, ventilated and paralyzed. A laminectomy enabled extracellular recording of T(3) spinal cord neuronal activity. Intrapericardial administration of a mixture of algogenic chemicals (bradykinin, serotonin, prostaglandin E(2) (all at 10(-5)M), and adenosine (10(-3)M)) was applied to activate nociceptors of cardiac afferent nerve endings. Furthermore, somatic receptive properties were examined by applying innocuous (brush and light pressure) and noxious (pinch) cutaneous mechanical stimuli. Diabetes-induced increases in spontaneous activity were observed in subsets of neurons exhibiting long-lasting excitatory responses to administration of the algogenic mixture. Algogenic chemicals altered activity of a larger proportion of neurons from diabetic animals (73/111) than control animals (55/115, P<0.05). Some subtypes of neurons exhibiting long-lasting excitatory responses, elicited prolonged duration and others, had a shortened latency. Some neurons exhibiting short-lasting excitatory responses in diabetic animals elicited a shorter latency and some a decreased excitatory change. The size of the somatic receptive field was increased for cardiosomatic neurons from diabetic animals. Cutaneous somatic mechanical stimulation caused spinal neurons to respond with a mixture of hyper- and hypoexcitability. In conclusion, diabetes induced changes in the spinal processing of cardiac input and these might contribute to cardiovascular autonomic neuropathy in patients with diabetes.

Abnormal muscle afferent function in a model of Taxol chemotherapy-induced painful neuropathy

Despite muscle pain being a well-described symptom in patients with diverse forms of peripheral neuropathy, the role of neuropathic mechanisms in muscle pain have received remarkably little attention. We have recently demonstrated in a well-established model of chemotherapy-induced painful neuropathy (CIPN) that the anti-tumor drug paclitaxel (Taxol) produces mechanical hyperalgesia in skeletal muscle, of similar time course to and with shared mechanism with cutaneous symptoms. In the present study, we evaluated muscle afferent neuron function in this rat model of CIPN. The mechanical threshold of muscle afferents in rats exposed to paclitaxel was not significantly different from the mechanical threshold of muscle afferents in control animals (P = 0.07). However, paclitaxel did produce a marked increase in the number of action potentials elicited by prolonged suprathreshold fixed intensity mechanical stimulation and a marked increase in the conduction velocity. In addition, the interspike interval (ISI) analysis (to evaluate the temporal characteristics of the response of afferents to sustained mechanical stimulation) showed a significant difference in rats treated with paclitaxel; there was a significantly greater ISI percentage of paclitaxel-treated muscle afferents with 0.01- and 0.02-s ISI. In contrast, an analysis of variability of neuronal firing over time (CV2 analysis) showed no effect of paclitaxel administration. These effects of paclitaxel on muscle afferent function contrast with the previously reported effects of paclitaxel on the function of cutaneous nociceptors.

Olesoxime (TRO19622): A Novel Mitochondrial-Targeted Neuroprotective Compound

Olesoxime (TRO19622) is a novel mitochondrial-targeted neuroprotective compound undergoing a pivotal clinical efficacy study in Amyotrophic Lateral Sclerosis (ALS) and also in development for Spinal Muscular Atrophy (SMA). It belongs to a new family of cholesterol-oximes identified for its survival-promoting activity on purified motor neurons deprived of neurotrophic factors. Olesoxime targets proteins of the outer mitochondrial membrane, concentrates at the mitochondria and prevents permeability transition pore opening mediated by, among other things, oxidative stress. Olesoxime has been shown to exert a potent neuroprotective effect in various in vitro and in vivo models. In particular olesoxime provided significant protection in experimental animal models of motor neuron disorders and more particularly ALS. Olesoxime is orally active, crosses the blood brain barrier, and is well tolerated. Collectively, its pharmacological properties designate olesoxime as a promising drug candidate for motor neuron diseases.

Functional magnetic resonance imaging of the spinal cord during sensory stimulation in diabetic rats

PURPOSE

To determine if differences exist between control and diabetic rats in functional magnetic resonance imaging (fMRI) activity of the spinal cord and if fMRI can provide a means of early detection of diabetic neuropathy.

MATERIALS AND METHODS

fMRI of the spinal cord, using noxious electrical stimulation (15 V ( approximately 8 mA), 0.3 msec, 3 Hz) of the hind paw, was performed in groups of control and streptozotocin (STZ)-induced type 1 diabetic rats.

RESULTS

Diabetic rats were lighter, hyperglycemic, and had lower blood pH than controls. fMRI activity at the lumbar enlargement of the spinal cord was identified in the dorsal horn ipsilateral to stimulus of all animals. Signal intensity changes across the lumbar spinal cord during periods of activity were not significantly different between control and diabetic rats, with a trend toward greater signal changes in controls. When specific regions of the spinal cord were analyzed, control rats exhibited significantly increased blood-oxygen level-dependent (BOLD) fMRI activity in both ipsilateral and contralateral dorsal horn compared to diabetic rats.

CONCLUSION

The results of this study are consistent with reports that primary afferent input to the spinal cord is diminished by diabetes, and suggest that BOLD fMRI may be useful in early detection of diabetic neuropathy.

Characterization of upper thoracic spinal neurons responding to esophageal distension in diabetic rats

The aim of this study was to examine spinal neuronal processing of innocuous and noxious mechanical inputs from the esophagus in diabetic rats. Streptozotocin (50 mg/kg, ip) was used to induce diabetes in 15 male Sprague-Dawley rats, and vehicle (10 mM citrate buffer) was injected into 15 rats as control. Four to eleven weeks after injections, extracellular potentials of single thoracic (T3) spinal neurons were recorded in pentobarbital anesthetized, paralyzed, and ventilated rats. Esophageal distensions (ED, 0.2, 0.4 ml, 20 s) were produced by water inflation of a latex balloon in the thoracic esophagus. Noxious ED (0.4 ml, 20 s) altered activity of 44% (55/126) and 38% (50/132) of spinal neurons in diabetic and control rats, respectively. The short-lasting excitatory responses to ED were encountered more frequently in diabetic rats (27/42 vs 15/41, P<0.05). Spinal neurons with low threshold for excitatory responses to ED were more frequently encountered in diabetic rats (33/42 vs 23/41, P<0.05). However, mean excitatory responses and duration of responses to noxious ED were significantly reduced for high-threshold neurons in diabetic rats (7.4+/-1.1 vs 13.9+/-3.3 imp/s; 19.0+/-2.3 vs 31.2+/-5.5 s; P<0.05). In addition, more large size somatic receptive fields were found for spinal neurons with esophageal input in diabetic rats than in control rats (28/42 vs 19/45, P<0.05). These results suggested that diabetes influenced response characteristics of thoracic spinal neurons receiving mechanical esophageal input, which might indicate an altered spinal visceroceptive processing underlying diabetic esophageal neuropathy.

Diabetes affects the expression of GABA and potassium chloride cotransporter in the spinal cord: a study in streptozotocin diabetic rats

Painful diabetic neuropathy is associated to hyperexcitability and spontaneous hyperactivity of spinal cord neurons. The underlying pathophysiological mechanisms are not clear. Increases in excitatory neurotransmission at the spinal cord, involving glutamate and SP, seem to account for the abnormal neuronal activity, but inhibitory influences were never evaluated. This study aims to analyse the expression of GABA, its synthesizing enzyme glutamic acid decarboxylase (GAD) and the potassium chloride cotransporter (KCC2), in the spinal dorsal horn of streptozotocin (STZ)-induced diabetic rats. Four weeks after saline or STZ (60mg/kg) injection, animals were sacrificed and the spinal segments L2-L3 were removed and immunoreacted for GABA, GAD and KCC2, or processed for western blotting for KCC2. Densitometric quantification was performed in the superficial dorsal horn (laminae I, II and III) of immunoreacted sections and in the immunoblots. STZ rats presented a significant increase of GABA expression in laminae II and III when compared with control animals, while no differences were detected in GAD expression. A significant decrease in KCC2 expression was detected by immunohistochemistry in laminae I and II, which was confirmed by immunoblotting. Increased GABA levels, along with decrease in KCC2 expression, may underlie the abnormal neuronal activity detected in the spinal cord of diabetic rats. Reduction in KCC2 expression was shown to lead to increases in intracellular chloride concentration and, in such condition, GABA binding to GABA(A) receptor induces membrane depolarization, provoking neuronal excitation rather than inhibition. Based on these findings, we propose that a loss of GABA-mediated inhibitory tone at the spinal cord may result in neuronal hyperexcitability and spontaneous hyperactivity during diabetes.

Concurrent activation of the somatosensory forebrain and deactivation of periaqueductal gray associated with diabetes-induced neuropathic pain

We combined behavioral testing with brain imaging using (99m)Tc-HMPAO (Amersham Health) to identify CNS structures reflecting alterations in pain perception in the streptozotocin (STZ) model of type I diabetes. We induced diabetic hyperglycemia (blood glucose >300 mg/dl) by injecting male Sprague-Dawley rats with STZ (45 mg/kg i.p.). Four weeks after STZ-diabetic rats exhibited behaviors indicative of neuropathic pain (hypersensitivity thermal stimuli) and this hypersensitivity persisted for up to 6 weeks. Imaging data in STZ-diabetic rats revealed significant increases in the activation of brain regions involved in pain processing after 6 weeks duration of diabetes. These regions included secondary somatosensory cortex, ventrobasal thalamic nuclei and the basolateral amygdala. In contrast, the activation in habenular nuclei and the midbrain periaqueductal gray were markedly decreased in STZ rats. These data suggest that pain in diabetic neuropathy may be due in part to hyperactivity in somatosensory structures coupled with a concurrent deactivation of structures mediating antinociception.

Mechanically-evoked C-fiber activity in painful alcohol and AIDS therapy neuropathy in the rat

While altered activities in sensory neurons were noticed in neuropathic pain, caused by highly diverse insults to the peripheral nervous system, such as diabetes, alcohol ingestion, cancer chemotherapy and drugs used to treat AIDS, other infections and autoimmune diseases, as well as trauma, our understanding of how these various peripheral neuropathies manifest as altered neuronal activity is still rudimentary. The recent development of models of several of those neuropathies has, however, now made it possible to address their impact on primary afferent nociceptor function. We compared changes in mechanically-evoked C-fiber activity, in models of painful peripheral neuropathy induced by drinking ethanol (alcohol) or administering 2',3'-dideoxycytidine (ddC), a nucleoside reverse transcriptase inhibitor for AIDS therapy, two co-morbid conditions in which pain is thought to be mediated by different second messenger signaling pathways. In C-fiber afferents, ddC decreased conduction velocity. In contrast, alcohol but not ddC caused enhanced response to mechanical stimulation (i.e., decrease in threshold and increase in response to sustained threshold and supra-threshold stimulation) and changes in pattern of evoked activity (interspike interval and action potential variability analyses). These marked differences in primary afferent nociceptor function, in two different forms of neuropathy that produce mechanical hyperalgesia of similar magnitude, suggest that optimal treatment of neuropathic pain may differ depending on the nature of the causative insult to the peripheral nervous system, and emphasize the value of studying co-morbid conditions that produce painful peripheral neuropathy by different mechanisms.

Non-invasive diode laser activation of transient receptor potential proteins in nociceptors

We investigated diode laser (980 nm) evoked activation of transient receptor potential proteins (TRPV1 and TRPV2). C and A-delta (Aδ) nociceptor families are primarily responsible for pain mediation in the peripheral nervous system. TRPV1 proteins have been associated with heat evoked pain in C fibers while Aδ fibers have been associated with TRPV2. Diode laser stimulation allows a margin of safety between non-invasive activation and damage (19, 22, 34). Laser pulses (20-50 ms, 0.1-10 W, 980 nm) were used to stimulate: A) in vitro: excised patches from HEK293 cells expressing TRPV1; B) in vitro: rat DRG nociceptors expressing either TRPV1 or TRPV2; and C) in vivo: C-fibers of the rat saphenous nerve (SN) trunk. Cell currents were recorded using standard patch clamp methods. The SN was also stimulated electrically with bipolar electrodes. Stimulation (20-50 ms) of HEK and DRG cells expressing TRPV1 was highly reproducible. Activation and peak currents were achieved at estimated peak temperatures of 55°C and 70°C. Threshold activation was also observed in DRG neurons expressing TRPV2. The conduction velocity for laser-activated saphenous nerve afferents was in the C fiber range (0.5-1 m/s). Electrically stimulated nerve contained stimulation artifacts and complex neural components with conduction velocities ranging from 0.3-30 m/s. Diode laser activation of TRPV1 protein is a reproducible and effective means to probe TRP activity in both in vivo and in vitro preparations.

The preventive and therapeutic effects of GCPII (NAALADase) inhibition on painful and sensory diabetic neuropathy

Excitotoxic glutamate release occurs in several neurological disorders. One source is derived from the hydrolysis of the neuropeptide N-acetyl aspartyl glutamate (NAAG) by glutamate carboxypeptidase II (GCPII, also known as NAALADase). Drugs that attenuate glutamate transmission have been shown to relieve neuropathic pain, however side effects have limited their clinical use. It appears that GCPII is exclusively recruited to provide a glutamate source in hyperglutamatergic, excitotoxic conditions and therefore would be devoid of such side effects. Here we report on the therapeutic effects of an orally bio-available GCP II inhibitor on established painful and sensory neuropathy in the spontaneously diabetic BB/Wor rat. It significantly improved hyperalgesia, nerve conduction velocity and underlying myelinated fiber atrophy. The data suggest that GCP II inhibition may provide a meaningful and effective approach to the treatment of painful diabetic neuropathy.

Aldose reductase pathway inhibition improved vascular and C-fiber functions, allowing for pressure-induced vasodilation restoration during severe diabetic neuropathy

Pressure-induced vasodilation, a neurovascular mechanism relying on the interaction between mechanosensitive C-fibers and vessels, allows skin blood flow to increase in response to locally nonnociceptive applied pressure that in turn may protect against pressure ulcers. We expected that severe neuropathy would dramatically affect pressure-induced vasodilation in diabetic mice, and we aimed to determine whether pressure-induced vasodilation alteration could be reversed in 8-week diabetic mice. Control and diabetic mice received no treatment or sorbinil, an aldose reductase inhibitor, or alagebrium, an advanced glycation end product breaker, the last 2 weeks of diabetes. Laser Doppler flowmetry was used to evaluate pressure-induced vasodilation and endothelium-dependent vasodilation after iontophoretic delivery of acetylcholine (ACh). We assessed the nervous function with measurements of motor nerve conduction velocity (MNCV) as well as the C-fiber-mediated nociception threshold. Pressure-induced vasodilation, endothelial response, C-fiber threshold, and MNCV were all altered in 8-week diabetic mice. None of the treatments had a significant effect on MNCV. Although sorbinil and alagebrium both restored ACh-dependent vasodilation, sorbinil was the sole treatment to restore the C-fiber threshold as well as pressure-induced vasodilation development. Therefore, the inhibition of aldose reductase pathway by sorbinil improved vascular and C-fiber functions that allow pressure-induced vasodilation restoration that could limit neuropathic diabetic cutaneous pressure ulcers.

Unmyelinated fiber sensory neuropathy differs in type 1 and type 2 diabetes

BACKGROUND

Neuropathic pain is common in diabetic patients. Degeneration of sensory C-fibers in peripheral nerve plays a prominent role in the generation of neuropathic pain. We examined degenerative changes of C-fibers in two rat models with type 1 and type 2 diabetes.

METHODS

Type 1 insulinopenic BB/Wor and type 2 hyperinsulinemic diabetic BBZDR/Wor-rats of 8 months duration with equal exposure to hyperglycemia were examined. Thermal hyperalgesia was monitored using an infrared thermal probe. C-fiber size, number, frequencies of denervated Schwann cells, regenerating C-fibers, type 2 axon/Schwann cell relationship and collagen pockets in the sural nerve were examined morphometrically. Neurotrophic receptor expression was examined by Western blotting. Neurotrophins and neuropeptides were examined by ELISA.

RESULTS

Type 1 rats showed increased thermal hyperalgesia followed by a decrease. Hyperalgesia in type 2 rats showed a slower progression. These findings were associated with a 50% (p < 0.001) loss of C-fibers, increased frequencies of denervated Schwann cells (p < 0.001), regenerating fibers (p < 0.001), collagen pockets (p < 0.001) and type 2 axon/Schwann cell relationship (p < 0.001) in type 1, but not in type 2 rats. Expression of insulin receptor, IGF-1R, TrkA and C was decreased in BB/Wor rats, whereas BBZDR/Wor rats showed milder or no deficits. NGF and NT-3 in sciatic nerve and substance P and calcitonin gene-related peptide in dorsal root ganglia were decreased in type 1, but not in type 2 rats.

CONCLUSION

The more severe molecular, functional and morphometric abnormalities of nociceptive C-fibers in type 1 insulinopenic rats compared to type 2 hyperinsulinemic rats suggest that impaired insulin action may play a more important pathogenetic role than hyperglycemia per se.

C-peptide prevents nociceptive sensory neuropathy in type 1 diabetes

We examined the effects of C-peptide replacement on unmyelinated fiber function in the hind paw, sural nerve C-fiber morphometry, sciatic nerve neurotrophins, and the expression of neurotrophic receptors and content of neuropeptides in dorsal root ganglia in type 1 diabetic BB/Wor-rats. C-peptide replacement from onset of diabetes had no effect on hyperglycemia, but it significantly prevented progressive thermal hyperalgesia and prevented C-fiber atrophy, degeneration, and loss. These findings were associated with preventive effects on impaired availability of nerve growth factor and neurotrophin 3 in the sciatic nerve and significant prevention of perturbed expression of insulin, insulin growth factor-1, nerve growth factor, and neurotrophin 3 receptors in dorsal root ganglion cells. These beneficial effects translated into prevention of the decreased content of dorsal root ganglia nociceptive peptides such as substance P and calcitonin gene-related peptide. From these findings we conclude that replacement of insulinomimetic C-peptide prevents abnormalities of neurotrophins, their receptors, and nociceptive neuropeptides in type 1 BB/Wor-rats, resulting in the prevention of C-fiber pathology and nociceptive sensory nerve dysfunction. The data indicate that perturbed insulin/C-peptide action plays an important pathogenetic role in nociceptive sensory neuropathy and that C-peptide replacement may be of benefit in treating painful diabetic neuropathy in insulin-deficient diabetic conditions.