Elevated substance-P-like immunoreactivity levels in spinal dialysates during the formalin test in normal and diabetic rats

Neurokinin receptor mechanisms in forebrain medial septum modulate nociception in the formalin model of inflammatory pain

The present study has explored the hypothesis that neurokinin1 receptors (NK1Rs) in medial septum (MS) modulate nociception evoked on hind paw injection of formalin. Indeed, the NK1Rs in MS are localized on cholinergic neurons which have been implicated in nociception. In anaesthetized rat, microinjection of L-733,060, an antagonist at NK1Rs, into MS antagonized the suppression of CA1 population spike (PS) evoked on peripheral injection of formalin or on intraseptal microinjection of substance P (SP), an agonist at NK1Rs. The CA1 PS reflects the synaptic excitability of pyramidal cells in the region. Furthermore, microinjection of L-733,060 into MS, but not LS, attenuated formalin-induced theta activation in both anaesthetized and awake rat, where theta reflects an oscillatory information processing by hippocampal neurons. The effects of L-733,060 on microinjection into MS were nociceptive selective as the antagonist did not block septo-hippocampal response to direct MS stimulation by the cholinergic receptor agonist, carbachol, in anaesthetized animal or on exploration in awake animal. Interestingly, microinjection of L-733,060 into both MS and LS attenuated formalin-induced nociceptive flinches. Collectively, the foregoing novel findings highlight that transmission at NK1R provide an affective valence to septo-hippocampal information processing and that peptidergic transmission in the septum modulates nociceptive behaviours.

Diabetic neuropathy and neuropathic pain: a (con)fusion of pathogenic mechanisms?

Neuropathy is a common complication of long-term diabetes that impairs quality of life by producing pain, sensory loss and limb amputation. The presence of neuropathy in both insulin-deficient (type 1) and insulin resistant (type 2) diabetes along with the slowing of progression of neuropathy by improved glycemic control in type 1 diabetes has caused the majority of preclinical and clinical investigations to focus on hyperglycemia as the initiating pathogenic lesion. Studies in animal models of diabetes have identified multiple plausible mechanisms of glucotoxicity to the nervous system including post-translational modification of proteins by glucose and increased glucose metabolism by aldose reductase, glycolysis and other catabolic pathways. However, it is becoming increasingly apparent that factors not necessarily downstream of hyperglycemia can also contribute to the incidence, progression and severity of neuropathy and neuropathic pain. For example, peripheral nerve contains insulin receptors that transduce the neurotrophic and neurosupportive properties of insulin, independent of systemic glucose regulation, while the detection of neuropathy and neuropathic pain in patients with metabolic syndrome and failure of improved glycemic control to protect against neuropathy in cohorts of type 2 diabetic patients has placed a focus on the pathogenic role of dyslipidemia. This review provides an overview of current understanding of potential initiating lesions for diabetic neuropathy and the multiple downstream mechanisms identified in cell and animal models of diabetes that may contribute to the pathogenesis of diabetic neuropathy and neuropathic pain.

Lost in Translation? Measuring Diabetic Neuropathy in Humans and Animals

The worldwide diabetes epidemic is estimated to currently afflict almost 500 million persons. Long-term diabetes damages multiple organ systems with the blood vessels, eyes, kidneys and nervous systems being particularly vulnerable. These complications of diabetes reduce lifespan, impede quality of life and impose a huge social and economic burden on both the individual and society. Peripheral neuropathy is a debilitating complication that will impact over half of all persons with diabetes. There is no treatment for diabetic neuropathy and a disturbingly long history of therapeutic approaches showing promise in preclinical studies but failing to translate to the clinic. These failures have prompted re-examination of both the animal models and clinical trial design. This review focuses on the functional and structural parameters used as indices of peripheral neuropathy in preclinical and clinical studies and the extent to which they share a common pathogenesis and presentation. Nerve conduction studies in large myelinated fibers have long been the mainstay of preclinical efficacy screening programs and clinical trials, supplemented by quantitative sensory tests. However, a more refined approach is emerging that incorporates measures of small fiber density in the skin and cornea alongside these traditional assays at both preclinical and clinical phases.

Topical Delivery of Muscarinic Receptor Antagonists Prevents and Reverses Peripheral Neuropathy in Female Diabetic Mice

Muscarinic antagonists promote sensory neurite outgrowth in vitro and prevent and/or reverse multiple indices of peripheral neuropathy in rodent models of diabetes, chemotherapy-induced peripheral neuropathy, and HIV protein-induced neuropathy when delivered systemically. We measured plasma concentrations of the M₁ receptor-selective muscarinic antagonist pirenzepine when delivered by subcutaneous injection, oral gavage, or topical application to the skin and investigated efficacy of topically delivered pirenzepine against indices of peripheral neuropathy in diabetic mice. Topical application of 2% pirenzepine to the paw resulted in plasma concentrations 6 hours postdelivery that approximated those previously shown to promote neurite outgrowth in vitro. Topical delivery of pirenzepine to the paw of mice with streptozotocin-induced diabetes dose-dependently (0.1%-10.0%) prevented tactile allodynia, thermal hypoalgesia, and loss of epidermal nerve fibers in the treated paw and attenuated large fiber motor nerve conduction slowing in the ipsilateral limb. Efficacy against some indices of neuropathy was also noted in the contralateral limb, indicating systemic effects following local treatment. Topical pirenzepine also reversed established paw heat hypoalgesia, whereas withdrawal of treatment resulted in a gradual decline in efficacy over 2-4 weeks. Efficacy of topical pirenzepine was muted when treatment was reduced from 5 to 3 or 1 day/wk. Similar local effects were noted with the nonselective muscarinic receptor antagonist atropine when applied either to the paw or to the eye. Topical delivery of muscarinic antagonists may serve as a practical therapeutic approach to treating diabetic and other peripheral neuropathies. SIGNIFICANCE STATEMENT: Muscarinic antagonist pirenzepine alleviates diabetic peripheral neuropathy when applied topically in mice.

Insulin-like growth factor-1 activates AMPK to augment mitochondrial function and correct neuronal metabolism in sensory neurons in type 1 diabetes

Objective

Diabetic sensorimotor polyneuropathy (DSPN) affects approximately half of diabetic patients leading to significant morbidity. There is impaired neurotrophic growth factor signaling, AMP-activated protein kinase (AMPK) activity and mitochondrial function in dorsal root ganglia (DRG) of animal models of type 1 and type 2 diabetes. We hypothesized that sub-optimal insulin-like growth factor 1 (IGF-1) signaling in diabetes drives loss of AMPK activity and mitochondrial function, both contributing to development of DSPN.

Methods

Age-matched control Sprague-Dawley rats and streptozotocin (STZ)-induced type 1 diabetic rats with/without IGF-1 therapy were used for in vivo studies. For in vitro studies, DRG neurons from control and STZ-diabetic rats were cultured and treated with/without IGF-1 in the presence or absence of inhibitors or siRNAs.

Results

Dysregulation of mRNAs for IGF-1, AMPKα2, ATP5a1 (subunit of ATPase), and PGC-1β occurred in DRG of diabetic vs. control rats. IGF-1 up-regulated mRNA levels of these genes in cultured DRGs from control or diabetic rats. IGF-1 treatment of DRG cultures significantly (P < 0.05) increased phosphorylation of Akt, P70S6K, AMPK and acetyl-CoA carboxylase (ACC). Mitochondrial gene expression and oxygen consumption rate (spare respiratory capacity), ATP production, mtDNA/nDNA ratio and neurite outgrowth were augmented (P < 0.05). AMPK inhibitor, Compound C, or AMPKα1-specific siRNA suppressed IGF-1 elevation of mitochondrial function, mtDNA and neurite outgrowth. Diabetic rats treated with IGF-1 exhibited reversal of thermal hypoalgesia and, in a separate study, reversed the deficit in corneal nerve profiles. In diabetic rats, IGF-1 elevated the levels of AMPK and P70S6K phosphorylation, raised Complex IV-MTCO1 and Complex V-ATP5a protein expression, and restored the enzyme activities of Complex IV and I in the DRG. IGF-1 prevented TCA metabolite build-up in nerve.

Conclusions

In DRG neuron cultures IGF-1 signals via AMPK to elevate mitochondrial function and drive axonal outgrowth. We propose that this signaling axis mediates IGF-1-dependent protection from distal dying-back of fibers in diabetic neuropathy.

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IGF-1 activates AMPK to drive mitochondrial phenotype in sensory neurons.

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AMPK isoforms differentially regulate mitochondrial gene expression and respiratory activity.

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IGF-1 treatment of type 1 diabetic rodents protected from neuropathy.

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DRG of diabetic rats treated with IGF-1 exhibited raised activity and gene expression of AMPK and respiratory complexes.

Endomorphin-2 Inhibition of Substance P Signaling within Lamina I of the Spinal Cord Is Impaired in Diabetic Neuropathic Pain Rats

Opiate analgesia in the spinal cord is impaired in diabetic neuropathic pain (DNP), but until now the reason is unknown. We hypothesized that it resulted from a decreased inhibition of substance P (SP) signaling within the dorsal horn of the spinal cord. To investigate this possibility, we evaluated the effects of endomorphin-2 (EM2), an endogenous ligand of the μ-opioid receptor (MOR), on SP release within lamina I of the spinal dorsal horn (SDH) in rats with DNP. We established the DNP rat model and compared the analgesic efficacy of EM2 between inflammation pain and DNP rat models. Behavioral results suggested that the analgesic efficacy of EM2 was compromised in the condition of painful diabetic neuropathy. Then, we measured presynaptic SP release induced by different stimulating modalities via neurokinin-1 receptor (NK1R) internalization. Although there was no significant change in basal and evoked SP release between control and DNP rats, EM2 failed to inhibit SP release by noxious mechanical and thermal stimuli in DNP but not in control and inflammation pain model. We also observed that EM2 decreased the number of FOS-positive neurons within lamina I of the SDH but did not change the amount of FOS/NK1R double-labeled neurons. Finally, we identified a remarkable decrease in MORs within the primary afferent fibers and dorsal root ganglion (DRG) neurons by Western blot (WB) and immunohistochemistry (IHC). Taken together, these data suggest that reduced presynaptic MOR expression might account for the loss of the inhibitory effect of EM2 on SP signaling, which might be one of the neurobiological foundations for decreased opioid efficacy in the treatment of DNP.

Acute spinal cord injury (SCI) transforms how GABA affects nociceptive sensitization

Noxious input can sensitize pain (nociceptive) circuits within the spinal cord, inducing a lasting increase in spinal cord neural excitability (central sensitization) that is thought to contribute to chronic pain. The development of spinally-mediated central sensitization is regulated by descending fibers and GABAergic interneurons. The current study provides evidence that spinal cord injury (SCI) transforms how GABA affects nociceptive transmission within the spinal cord, recapitulating an earlier developmental state wherein GABA has an excitatory effect. In spinally transected rats, noxious electrical stimulation and inflammation induce enhanced mechanical reactivity (EMR), a behavioral index of nociceptive sensitization. Pretreatment with the GABAA receptor antagonist bicuculline blocked these effects. Peripheral application of an irritant (capsaicin) also induced EMR. Both the induction and maintenance of this effect were blocked by bicuculline. Cellular indices of central sensitization [c-fos expression and ERK phosphorylation (pERK)] were also attenuated. In intact (sham operated) rats, bicuculline had the opposite effect. Pretreatment with a GABA agonist (muscimol) attenuated nociceptive sensitization in intact, but not spinally injured, rats. The effect of SCI on GABA function was linked to a reduction in the Cl- transporter, KCC2, leading to a reduction in intracellular Cl- that would attenuate GABA-mediated inhibition. Pharmacologically blocking the KCC2 channel (with i.t. DIOA) in intact rats mimicked the effect of SCI. Conversely, a pharmacological treatment (bumetanide) that should increase intracellular Cl- levels blocked the effect of SCI. The results suggest that GABAergic neurons drive, rather than inhibit, the development of nociceptive sensitization after spinal injury.

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.

Role of neurokinin type 1 receptor in nociception at the periphery and the spinal level in the rat

OBJECTIVES

Noxious stimuli activate small to medium-sized dorsal root ganglion (DRG) neurons. Intense noxious stimuli result in the release of substance P (SP) from the central terminals of these neurons. It binds to the neurokinin type 1 receptor (NK1r) and sensitises the dorsal horn neurons. SP is also released from the peripheral terminals leading to neurogenic inflammation. However, their individual contribution at spinal and peripheral levels to postincisional nociception has not been delineated as yet.

METHODS

Sprague-Dawley rats were administered different doses (3-100 μg) of an NK1r antagonist (L760735) by intrathecal (i.t.) route before hind paw incision. On the basis of its antinociceptive effect on guarding behaviour, the 30 μg dose was selected for further study. In different sets of animals, this was administered i.t. (postemptive) and intrawound (i.w.). Finally, in another group, drug (30 μg) was administered through both i.t and i.w. routes. The antinociceptive effect was assessed and compared. Expression of SP was examined in the spinal cord. Intrawound concentration of SP and inflammatory mediators was also evaluated.

RESULTS

Postemptive i.t. administration significantly attenuated guarding and allodynia. Guarding was alone decreased after i.w. drug treatment. Combined drug administration further attenuated all nociceptive parameters, more so after postemptive treatment. Expression of SP in the spinal cord decreased post incision but increased in the paw tissue. Inflammatory mediators like the nerve growth factor also increased after incision.

CONCLUSION

In conclusion, SP acting through the NK1r appears to be an important mediator of nociception, more so at the spinal level. These findings could have clinical relevance.

Efficacy of a long-acting C-peptide analogue against peripheral neuropathy in streptozotocin-diabetic mice

AIMS

To investigate the efficacy of a pegylated C-peptide (Peg-C-peptide) against indices of peripheral neuropathy in a mouse model of type 1 diabetes and to compare efficacy of this C-peptide analogue against that of the native molecule.

METHODS

C57Bl/6 mice were injected with two consecutive doses of streptozotocin (STZ) to induce type 1 diabetes. Mice were treated twice daily with native C-peptide [0.4-1.3 mg/kg subcutaneously (s.c.)] or twice weekly with Peg-C-peptide (0.1-1.3 mg/kg s.c.) for 20 weeks. Motor and sensory nerve conduction velocities, thermal and tactile responses and rate dependent H-wave depression were assessed after 20 weeks of diabetes. Foot skin intraepidermal fibres and corneal nerves were counted, and sciatic nerve substance P and plasma C-peptide levels were also determined.

RESULTS

After 5 months of STZ-induced diabetes, mice exhibited significant motor and sensory nerve conduction slowing, thermal hypoalgesia, tactile allodynia and attenuation of rate-dependent depression of the H reflex. These functional disorders were accompanied by nerve substance P depletion but not loss of small sensory fibres in the hind paw epidermis or the cornea. The efficacy of twice-daily treatment with native C-peptide in preventing these disorders was matched or exceeded by twice-weekly treatment with Peg-C-peptide. Both native and Peg-C-peptide also increased corneal nerve occupancy in the sub-basal nerve plexus of control rats.

CONCLUSIONS

These data identify actions of C-peptide against novel and clinically pertinent aspects of diabetic neuropathy in mice and also establish Peg-C-peptide as a long-acting therapeutic method of potential clinical value.

Antinociceptive effects of endomorphin-2: suppression of substance P release in the inflammatory pain model rat

Endomorphin-2 (EM2) and Substance P (SP) exert suppressive and facilitative influences upon nociception, respectively. Although EM2 and SP were often co-expressed in single neurons in dorsal root ganglion (DRG), it is still unknown if and how the nociception-suppressive influences of EM2 might be exerted upon nociception-facilitative effects of SP in the DRG neurons. We examined these issues in the inflammatory pain model rats produced by subcutaneous injection of the complete Freund's adjuvant into the hind paw. The paw withdrawal threshold for mechanical allodynia was measured. Changes of EM2 and SP release were estimated by measuring intrathecal levels of EM2 and SP through in vivo microdialysis analysis of cerebrospinal fluid. The mechanical allodynia was dose-dependently attenuated by intrathecal injection of EM2 or a neurokinin-1 receptor antagonist, and facilitated by intrathecal injection of SP or a mu-opioid receptor (MOR) antagonist. Importantly, intrathecal level of SP was found to be lowered by intrathecal injection of EM2. Morphologically, colocalization of EM2-, MOR- and SP-immunoreactivity in single DRG neurons was observed by immunofluorescent histochemistry, and co-expression of EM2 and SP in large, dense-cored presynaptic vesicles in primary afferents, as well as localization of MOR on pre- and postsynaptic membrane in spinal dorsal horn, was also confirmed electron miscroscopically. Thus, the results indicated that analgesic influences of EM2 upon inflammatory pain might be exerted through suppression of SP release, supporting the assumptions that binding of EM2 to presynaptic MOR might induce such effects.

Activated microglia in the spinal cord underlies diabetic neuropathic pain

Diabetes mellitus is an increasingly common chronic medical condition. Approximately 30% of diabetic patients develop neuropathic pain, manifested as spontaneous pain, hyperalgesia and allodynia. Hyperglycemia induces metabolic changes in peripheral tissues and enhances oxidative stress in nerve fibers. The damages and subsequent reactive inflammation affect structural properties of Schwann cells and axons leading to the release of neuropoietic mediators, such as pro-inflammatory cytokines and pro-nociceptive mediators. Therefore, diabetic neuropathic pain (DNP) shares some histological features and underlying mechanisms with traumatic neuropathy. DNP displays, however, other distinct features; for instance, sensory input to the spinal cord decreases rather than increasing in diabetic patients. Consequently, development of central sensitization in DNP involves mechanisms that are distinct from traumatic neuropathic pain. In DNP, the contribution of spinal cord microglia activation to central sensitization and pain processes is emerging as a new concept. Besides inflammation in the periphery, hyperglycemia and the resulting production of reactive oxygen species affect the local microenvironment in the spinal cord. All these alterations could trigger resting and sessile microglia to the activated phenotype. In turn, microglia synthesize and release pro-inflammatory cytokines and neuroactive molecules capable of inducing hyperactivity of spinal nociceptive neurons. Hence, it is imperative to elucidate glial mechanisms underlying DNP for the development of effective therapeutic agents. The present review highlights the recent developments regarding the contribution of spinal microglia as compelling target for the treatment of DNP.

Animal models of diabetes-induced neuropathic pain

Neuropathy will afflict over half of the approximately 350 million people worldwide who currently suffer from diabetes and around one-third of diabetic patients with neuropathy will suffer from painful symptoms that may be spontaneous or stimulus evoked. Diabetes can be induced in rats or mice by genetic, dietary, or chemical means, and there are a variety of well-characterized models of diabetic neuropathy that replicate either type 1 or type 2 diabetes. Diabetic rodents display aspects of sensorimotor dysfunction such as stimulus-evoked allodynia and hyperalgesia that are widely used to model painful neuropathy. This allows investigation of pathogenic mechanisms and development of potential therapeutic interventions that may alleviate established pain or prevent onset of pain.

Extracellular signal-regulated protein kinase activation in spinal cord contributes to pain hypersensitivity in a mouse model of type 2 diabetes

Painful peripheral neuropathy is a common complication of diabetes mellitus. The symptom of pain can become a major factor that decreases the quality of life of patients with diabetes, while effective treatment is lacking. In the present study, we aimed to investigate the changes of pain threshold in the early stage of diabetes in db/db mice, an animal model of type 2 diabetes mellitus, and the underlying molecular mechanisms. We found that (1) db/db mice (with a leptin receptor-null mutation and characterized by obesity and hyperglycemia) showed hypersensitivity to mechanical and thermal stimuli at the early stage of diabetes; (2) phosphorylated extracellular signal-regulated kinase (pERK), but not total ERK in the spinal cord and dorsal root ganglia in db/db mice significantly increased compared with wild-type mice. The increased pERK immunoreactivity occurred in both NeuN-expressing neurons and GFAP-expressing astrocytes, but not in Iba-1-expressing microglia; (3) both single and consecutive (for 5 days) intrathecal injections of U0126 (2 nmol per day), a selective MEK (an ERK kinase) inhibitor beginning at 8 weeks of age, attenuated the bilateral mechanical allodynia in the von-Frey test and heat hyperalgesia in Hargreave's test; and (4) db/db mice also displayed increased nocifensive behavior during the formalin test, and this was blocked by intrathecal injection of U0126. Also, the expression of pERK1 and pERK2 was upregulated following the formalin injection. Our results suggested that the activation of ERK in spinal neurons and astrocytes is correlated with pain hypersensitivity of the type 2 diabetes animal model. Inhibiting the ERK pathway may provide a new therapy for pain control in type 2 diabetes.

Inflammation enhances Y1 receptor signaling, neuropeptide Y-mediated inhibition of hyperalgesia, and substance P release from primary afferent neurons

Neuropeptide Y (NPY) is present in the superficial laminae of the dorsal horn and inhibits spinal nociceptive processing, but the mechanisms underlying its anti-hyperalgesic actions are unclear. We hypothesized that NPY acts at neuropeptide Y1 receptors in the dorsal horn to decrease nociception by inhibiting substance P (SP) release, and that these effects are enhanced by inflammation. To evaluate SP release, we used microdialysis and neurokinin 1 receptor (NK1R) internalization in rat. NPY decreased capsaicin-evoked SP-like immunoreactivity in the microdialysate of the dorsal horn. NPY also decreased non-noxious stimulus (paw brush)-evoked NK1R internalization (as well as mechanical hyperalgesia and mechanical and cold allodynia) after intraplantar injection of carrageenan. Similarly, in rat spinal cord slices with dorsal root attached, [Leu(31), Pro(34)]-NPY inhibited dorsal root stimulus-evoked NK1R internalization. In rat dorsal root ganglion neurons, Y1 receptors colocalized extensively with calcitonin gene-related peptide (CGRP). In dorsal horn neurons, Y1 receptors were extensively expressed and this may have masked the detection of terminal co-localization with CGRP or SP. To determine whether the pain inhibitory actions of Y1 receptors are enhanced by inflammation, we administered [Leu(31), Pro(34)]-NPY after intraplantar injection of complete Freund's adjuvant (CFA) in rat. We found that [Leu(31), Pro(34)]-NPY reduced paw clamp-induced NK1R internalization in CFA rats but not uninjured controls. To determine the contribution of increased Y1 receptor-G protein coupling, we measured [(35)S]GTPγS binding simulated by [Leu(31), Pro(34)]-NPY in mouse dorsal horn. CFA inflammation increased the affinity of Y1 receptor G-protein coupling. We conclude that Y1 receptors contribute to the anti-hyperalgesic effects of NPY by mediating the inhibition of SP release, and that Y1 receptor signaling in the dorsal horn is enhanced during inflammatory nociception.

Assessment and treatment of pain in people with dementia

Many elderly people experience pain and regularly take analgesic medication. Pain is also frequent in people with dementia, particularly those with severe disease. As no robust clinical guidelines are available for the treatment of pain in the context of dementia, the risk of inadequate treatment in individuals with this condition is high. Furthermore, our understanding of the aetiology of pain and the potential role of dementia-associated neuropathology in pain is limited. These issues are important in the clinical management of individuals with dementia, as untreated pain is a major contributor to reduced quality of life and disability, and can lead to increased behavioural and psychological symptoms. Assessment scales to identify pain in people with dementia have been highlighted in recent studies, but there is little evidence for consistency between these tools. Numerous studies have evaluated various approaches for the treatment of pain, including stepped-care protocols and/or administration of paracetamol and opioid medications. In this Review, we summarize the best-available evidence regarding the aetiology, assessment and treatment of pain in people with dementia. Further validation of assessment tools and large-scale trials of treatment approaches in people with dementia are needed to improve clinical guidance for the treatment of pain in these individuals.

Botulinum toxin type A reduces pain supersensitivity in experimental diabetic neuropathy: bilateral effect after unilateral injection

We investigated antinociceptive activity of botulinum toxin type A (BTX-A) in a model of diabetic neuropathic pain in rats. Male Wistar rats were made diabetic by a single intraperitoneal injection of streptozotocin (80mg/kg). Sensitivity to mechanical and thermal stimuli was measured with the paw-pressure and hot-plate test, respectively. The formalin test was used to measure sensitivity to chemical stimuli. Diabetic animals with pain thresholds lower for at least 25% compared to the non-diabetic group were considered neuropathic and were injected with BTX-A either subcutaneously (3, 5 and 7U/kg) or intrathecally (1U/kg). Mechanical and thermal sensitivity was measured at several time-points. After peripheral application, BTX-A (5 and 7U/kg) reduced mechanical and thermal hypersensitivity not only on ipsilateral, but on contralateral side, too. The antinociceptive effect started 5days following BTX-A injection and lasted at least 15days. Formalin-induced hypersensitivity in diabetic animals was abolished as well. When applied intrathecally, BTX-A (1U/kg) reduced diabetic hyperalgesia within 24h supporting the assumption of retrograde axonal transport of BTX-A from the peripheral site of injection to central nervous system. The results presented here demonstrate the long-lasting pain reduction after single BTX-A injection in the animals with diabetic neuropathy. The bilateral pain reduction after unilateral toxin application and the effectiveness of lower dose with the faster onset after the intrathecal injection suggest the involvement of the central nervous system in the antinociceptive action of BTX-A in painful diabetic neuropathy.

Nerve growth factor mediates mechanical allodynia in a mouse model of type 2 diabetes

C57BLKS db/db (db/db) mice develop a neuropathy with features of human type 2 diabetic neuropathy. Here, we demonstrate that these mice develop transient mechanical allodynia at the early stage of diabetes. We hypothesized that nerve growth factor (NGF), which enhances the expression of key mediators of nociception (i.e. substance P [SP] and calcitonin gene-related peptide), contributes to the development of mechanical allodynia in these mice. We found that NGF, SP, and calcitonin gene-related peptide gene expression is upregulated in the dorsal root ganglion (DRG) of db/db mice before or during the period that they develop mechanical allodynia. There were more small- to medium-sized NGF-immunopositive DRG neurons in db/db mice than in control db+ mice; these neurons also expressed SP, consistent with its role in nociception. Nerve growth factor expression in the hind paw skin was also increased in a variety of dermal cell types and nerve fibers, suggesting the contribution of a peripheral source of NGF to mechanical allodynia. The upregulation of NGF coincided with enhanced tropomyosin-related kinase A receptor phosphorylation in the DRG. Finally, an antibody against NGF inhibited mechanical allodynia and decreased the numbers of SP-positive DRG neurons in db/db mice. These results suggest that inhibition of NGF action is a potential strategy for treating painful diabetic neuropathy.

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.

Allodynia and hyperalgesia in diabetic rats are mediated by GABA and depletion of spinal potassium-chloride co-transporters

Diabetic rats show behavioral indices of painful neuropathy that may model the human condition. Hyperalgesia during the formalin test in diabetic rats is accompanied by the apparently paradoxical decrease in spinal release of excitatory neurotransmitters and increase in the inhibitory neurotransmitter GABA. Decreased expression of the potassium-chloride co-transporter, KCC2, in the spinal cord promotes excitatory properties of GABA. We therefore measured spinal KCC2 expression and explored the role of the GABA(A) receptor in rats with painful diabetic neuropathy. KCC2 protein levels were significantly reduced in the spinal cord of diabetic rats, while levels of NKCC1 and the GABA(A) receptor were unchanged. Spinal delivery of the GABA(A) receptor antagonist bicuculline reduced formalin-evoked flinching in diabetic rats and also dose-dependently alleviated tactile allodynia. GABA(A) receptor-mediated rate-dependent depression of the spinal H reflex was absent in the spinal cord of diabetic rats. Control rats treated with the KCC2 blocker DIOA, mimicked diabetes by showing increased formalin-evoked flinching and diminished rate- dependent depression. The ability of bicuculline to alleviate allodynia and formalin-evoked hyperalgesia in diabetic rats is consistent with a reversal of the properties of GABA predicted by reduced spinal KCC2 and suggests that reduced KCC2 expression and increased GABA release contribute to spinally mediated hyperalgesia in diabetes.

Dissociation of thermal hypoalgesia and epidermal denervation in streptozotocin-diabetic mice

The quantification of epidermal innervation, which consists primarily of heat-sensitive C-fibers, is emerging as a tool for diagnosing and staging diabetic neuropathy. However, the relationship between changes in heat sensitivity and changes in epidermal innervation has not yet been adequately explored. Therefore, we assessed epidermal nerve fiber density and thermal withdrawal latency in the hind paw of Swiss Webster mice after 2 and 4 weeks of streptozotocin-induced diabetes. Thermal hypoalgesia developed after only 2 weeks of diabetes, but a measurable reduction in PGP9.5-immunoreactive epidermal nerve fiber density did not appear until 4 weeks. These data suggest that impaired epidermal nociceptor function contributes to early diabetes-induced thermal hypoalgesia prior to the loss of peripheral terminals.

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.

Glucose neurotoxicity

Neurons have a constantly high glucose demand, and unlike muscle cells they cannot accommodate episodic glucose uptake under the influence of insulin. Neuronal glucose uptake depends on the extracellular concentration of glucose, and cellular damage can ensue after persistent episodes of hyperglycaemia--a phenomenon referred to as glucose neurotoxicity. This article reviews the pathophysiological manifestation of raised glucose in neurons and how this can explain the major components of diabetic neuropathy.

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.

Kainate receptors are primarily postsynaptic to SP-containing axon terminals in the trigeminal dorsal horn

Kainate receptors (KARs) are involved in the modulation and transmission of nociceptive information from peripheral afferents to neurons in the spinal cord and trigeminal dorsal horns. KARs are found at both pre- and postsynaptic sites in the dorsal horn. We hypothesized that KARs and Substance P (SP), a modulatory neuropeptide that is used as a marker of nociceptive afferents, have a complex interactive relationship. To determine the cellular relationship and connectivity between KARs and SP afferents, we used electron microscopic dual immunocytochemical analysis to examine the ultrastructural localization of KAR subunits GluR5, 6 and 7 (GluR5,6,7) in relation to SP within laminae I and II in the rat trigeminal dorsal horn. KARs were distributed both postsynaptically in dendrites and somata (51% of GluR5,6,7 immunoreactive (-ir) profiles) and presynaptically in axons and axon terminals (45%). We also found GluR5,6,7-ir glial profiles (5%). The majority of SP-ir profiles were presynaptic axons and axon terminals. SP-ir dendritic profiles were rare, yet 23% contained GluR5,6,7 immunoreactivity. GluR5,6,7 and SP were also colocalized at presynaptic sites (18% of GluR5,6,7-ir axons and axon terminals contained SP; while 11% of SP-ir axons and axon terminals contained GluR5,6,7). The most common interaction between KARs and SP we observed was GluR5,6,7-ir dendrites contacted by SP-ir axon terminals; 54% of the dendritic targets of SP-ir axon terminals were GluR5,6,7-ir. These results provide anatomical evidence that KARs primarily mediate nociceptive transmission postsynaptic to SP-containing afferents and may also modulate the presynaptic release of SP and glutamate in trigeminal dorsal horn.

Sensory and autonomic nerve changes in the monosodium glutamate-treated rat: a model of type II diabetes

Rats that had been injected with monosodium glutamate (MSG) neonatally were studied for up to 70 weeks and compared with age-matched control rats to study changes in glucose tolerance and in sympathetic and sensory nerves. At 61 and 65 weeks of age, there were significant differences in glucose tolerance between the MSG and control groups, and the MSG group had raised fasting blood glucose. These changes were not associated with changes in the number of beta-cells in the islets of Langerhans. In addition, the diabetic MSG-treated rats had central obesity and cataracts. Hypoalgesia to thermal stimuli was present in MSG-treated rats as early as 6 weeks and persisted at 70 weeks. However, no differences were observed in the distribution of substance P, the neurokinin-1 receptor or calcitonin gene-related peptide in the dorsal horn of L3-L5 at this age (70 weeks). Diabetic MSG-treated animals at 65 and 70 weeks of age had significantly reduced noradrenaline concentrations in the heart, tail artery and ileum, while concentrations in the adrenal gland and corpus cavernosum were significantly increased. There was also a significant increase in adrenal adrenaline, dopamine and serotonin, largely attributable to changes in weight of the adrenal gland in the MSG-treated animals. The results indicate that MSG-treated animals develop a form of type II diabetes by about 60 weeks of age, and that there are significant changes in amine levels in various tissues associated with these developments.

Effects of honokiol and magnolol on acute and inflammatory pain models in mice

The antinociceptive actions of honokiol and magnolol, two major bioactive constituents of the bark of Magnolia officinalis, were evaluated using tail-flick, hot-plate and formalin tests in mice. The effects of honokiol and magnolol on the formalin-induced c-Fos expression in the spinal cord dorsal horn as well as motor coordination and cognitive function were examined. Data showed that honokiol and magnolol did not produce analgesia in tail-flick, hot-plate paw-shaking and neurogenic phase of the overt nociception induced by intraplantar injection of formalin. However, honokiol and magnolol reduced the inflammatory phase of formalin-induced licking response. Consistently, honokiol and magnolol significantly decreased formalin-induced c-Fos protein expression in superficial (I-II) laminae of the L4-L5 lumbar dorsal horn. However, honokiol and magnolol did not elicit motor incoordination and memory dysfunction at doses higher than the analgesic dose. These results demonstrate that honokiol and magnolol effectively alleviate the formalin-induced inflammatory pain without motor and cognitive side effects, suggesting their therapeutic potential in the treatment of inflammatory pain.

Pathogenesis of spinally mediated hyperalgesia in diabetes

Hyperalgesia to noxious stimuli is accompanied by increased spinal cyclooxygenase (COX)-2 protein in diabetic rats. The present studies were initiated to establish causality between increased spinal COX-2 activity and hyperalgesia during diabetes and to assess the potential involvement of polyol pathway activity in the pathogenesis of spinally mediated hyperalgesia. Rats with 1, 2, or 4 weeks of streptozotocin-induced diabetes exhibited significantly increased levels of spinal COX-2 protein and activity, along with exaggerated paw flinching in response to 0.5% paw formalin injection. Increased flinching of diabetic rats was attenuated by intrathecal pretreatment with a selective COX-2 inhibitor immediately before formalin injection, confirming the involvement of COX-2 activity in diabetic hyperalgesia. Chronic treatment with insulin or ICI222155, an aldose reductase inhibitor (ARI) previously shown to prevent spinal polyol accumulation and formalin-evoked hyperalgesia in diabetic rats, prevented elevated spinal COX-2 protein and activity in diabetic rats. In contrast, the ARI IDD676 had no effect on spinal polyol accumulation, elevated spinal COX-2, or hyperalgesia to paw formalin injection. In the spinal cord, aldose reductase immunoreactivity was present solely in oligodendrocytes, which also contained COX-2 immunoreactivity. Polyol pathway flux in spinal oligodendrocytes provides a pathogenic mechanism linking hyperglycemia to hyperalgesia in diabetic rats.

Diabetes-induced chemogenic hypoalgesia is paralleled by attenuated stimulus-induced fos expression in the spinal cord of diabetic mice

Chronic hyperglycemia in diabetes induces abnormal nerve pathologies, resulting in diabetic neuropathy (DN). Sensory symptoms of DN can manifest as positive (painful), negative (insensate), or both. Streptozotocin (STZ)-induced diabetic C57Bl/6 mice have reduced cutaneous innervation and display reduced behavioral responses to noxious stimuli, reflecting the insensate aspect of the human syndrome. Current studies were undertaken to determine whether the diabetes-induced deficits in pain responses are reflected by changes in spinal activation in this model of DN. Nocifensive responses of nondiabetic and diabetic mice to formalin injection were measured 1, 3, 5, and 7 weeks after STZ, and at each time point formalin-induced spinal Fos expression was quantified. Responses of diabetic mice were significantly reduced during the second phase of the formalin test beginning 3 weeks after STZ and during Phase 1 beginning 5 weeks after STZ. Consistent with the behavioral responses, the number of Fos-positive cells in the dorsal horn of diabetic animals was significantly reduced beginning 3 weeks after STZ and continuing 5 and 7 weeks after STZ. The deficits at 5 weeks after STZ were restored by 2-week treatments with insulin or neurotrophins. These results demonstrate that the reduced sensation occurring from progressive peripheral axon loss results in functional deficits in spinal cord activation.

PERSPECTIVE

The reduced expression of the immediate early gene Fos as an indicator of pain transmission supports the diabetes-induced loss of sensation in this Type 1 model of diabetes. This murine model may be better suited to understanding the insensate symptoms of diabetic patients in the absence of chronic pain.

Impaired formalin-evoked changes of spinal amino acid levels in diabetic rats

To investigate mechanisms by which diabetes alters sensory processing, we measured levels of amino acid neurotransmitters in spinal dialysates from awake, unrestrained control and diabetic rats under resting conditions and following hind paw formalin injection. Under resting conditions, glutamate concentrations in spinal dialysates were significantly (P<0.05) decreased in diabetic rats compared to those of control rats whereas aspartate, taurine, glycine and citrulline remained unchanged and GABA was significantly (P<0.05) increased. Noxious stimulation of the hind paw by subcutaneous injection of 0.5% formalin into the dorsum caused a defined flinching behavior in the afflicted paw, and the amount of flinching was significantly (P<0.05) greater in diabetic rats than in controls. Paw formalin injection significantly (P<0.05) increased dialysate levels of glutamate, aspartate, taurine, glycine and citrulline by 3- to 4-fold above basal in both control and diabetic rats. The concentration of glutamate in dialysate samples collected immediately after paw formalin injection remained significantly (P<0.05) lower in diabetic rats compared to those in controls. Formalin injection did not alter dialysate GABA concentrations in control rats, whereas in diabetic rats there was an increase of 151+/-15% above basal levels. These findings indicate that the selective depression of basal and stimulus-evoked glutamate levels in the spinal cord of diabetic rats occurs in parallel with elevated spinal GABA levels. Because increased pain-associated behavior is accompanied by an attenuated spinal glutamate spike following paw formalin injection, hyperalgesia in diabetic rats does not appear to be secondary to enhanced glutamatergic input to the spinal cord.

Involvement of cholecystokinin in peripheral nociceptive sensitization during diabetes in rats as revealed by the formalin response

The possible pronociceptive role of peripheral cholecystokinin (CCK-8) as well as CCK(A) and CCK(B) receptors in diabetic rats was assessed. Subcutaneous injection of 0.5% formalin induced a greater nociceptive behavior in diabetic than in non-diabetic rats. Moreover, local peripheral injection of CCK-8 (0.1-100 microg) significantly increased 0.5% formalin-induced nociceptive activity in diabetic, but not in non-diabetic, rats. This effect was restricted to the formalin-injected paw as administration of CCK-8 into the contralateral paw was ineffective. Local peripheral administration of CCK-8, in the absence of formalin injection, produced a low level of, but significant increase in, flinching behavior in diabetic compared to non-diabetic rats. In addition, local peripheral administration of the non-selective CCK receptor antagonist proglumide (1-100 microg), CCK(A) receptor antagonist lorglumide (0.1-100 microg) or CCK(B) receptor antagonist CR-2945 (0.1-100 microg), but not vehicle or contralateral administration of CCK receptor antagonists, significantly reduced 0.5% formalin-induced flinching in diabetic rats. CR-2945 was the most effective drug in this condition. These effects were not observed in non-diabetic rats. The local peripheral pronociceptive effect of CCK-8 (100 microg) was significantly reduced by proglumide (100 microg), lorglumide (100 microg), and CR-2945 (100 microg). Results suggest that diabetes-induced peripheral sensitization could be due to a local peripheral release of CCK-8, which in turn would act on CCK(B), mainly but also in CCK(A), receptors located on the primary afferent neurons.

Protection of sensory function in diabetic rats by Neotrofin

We investigated the ability of Neotrofin, an agent that enhances endogenous nerve growth factor (NGF) levels, to prevent phenotypic, functional and structural changes that occur in the peripheral nerve of streptozotocin-diabetic rats. Eight weeks of Neotrofin treatment prevented depletion of NGF protein in plantar foot skin and sciatic nerve of diabetic rats and increased NGF protein in associated skeletal muscles. These effects were accompanied by maintenance of normal nerve levels of the neuropeptides substance P and calcitonin gene related peptide. Thermal hypoalgesia and conduction slowing of large sensory fibres in diabetic rats were ameliorated by Neotrofin treatment, whereas there was no effect on conduction slowing in large motor fibres or on reduced myelinated fibre axonal calibre. Enhancing endogenous production of neurotrophic factors using small molecules may be an alternative to either exogenous treatment with neurotrophic factors or gene therapy as a therapeutic approach to treating diabetic neuropathy.

Pigment epithelium-derived factor is differentially expressed in peripheral neuropathies

Peripheral neuropathies are characterized by asymmetrical slowly progressive weakness with no upper motor neuron signs, and can occur either with or without pain. Due to poor knowledge of the disease mechanisms, available pain treatment is very limited. Because of the difficulties and invasiveness involved when performing direct analysis on peripheral and CNS, pathological markers can be searched for in the cerebrospinal fluid (CSF) as an alternative. To investigate pain mechanisms in peripheral neuropathy and find diagnostic markers, CSF samples were analyzed by a differential expression proteomic approach. We studied CSF from: neuropathic patients with pain (PN), without pain (NPN) and healthy controls (CN). 2-DE analysis showed ten protein spots differentially expressed, and six of these were identified by MS. In NPN patients we found an expression level decrease of three pigment epithelium-derived factor (PEDF) protein isoforms. Immunoblot with a specific antibody revealed the presence of additional PEDF isoforms not highlighted by differential expression analysis. Fucose residues on the oligosaccharide chain were found only in the isoforms down regulated in NPN patients. Considered as PEDF has important neurobiological effects, it might be considered an interesting pathology marker.

Capsaicin-evoked substance P release in rat dorsal horn increases after peripheral inflammation: a microdialysis study

Numerous in vitro studies suggest that inflammation is associated with enhanced release of substance P (SP) in the dorsal horn. To test the hypothesis that inflammation increases the evoked concentration of SP in the intact animal, we used in vivo microdialysis with a highly sensitive radioimmunoassay to monitor SP-like immunoreactivity (SP-LI) in the dorsal horn. Seven days after the induction of persistent unilateral inflammation with hindpaw injection of complete Freund's adjuvant, perfusion of the microdialysis probe with 10 microM capsaicin (a concentration which failed to induce SP-LI release in rats without inflammation) induced a significant increase of microdialysate SP-LI. Inclusion of an NMDA antagonist in the perfusion fluid completely blocked this capsaicin-evoked SP release. Administration of a five-fold higher dose of capsaicin did not further increase SP release. These results in a rat model of chronic arthritis suggest that persistent inflammatory signaling facilitates capsaicin-evoked SP release in the dorsal horn in vivo.

Prevention of sensory disorders in diabetic Sprague-Dawley rats by aldose reductase inhibition or treatment with ciliary neurotrophic factor

AIMS/HYPOTHESIS

Sensory neuropathy in diabetic patients frequently presents itself as progressive loss of thermal perception, while some patients describe concurrent spontaneous pain, allodynia or hyperalgesia. Diabetic rats develop thermal hypoalgesia and tactile allodynia by unknown mechanisms. We investigated whether sensory disorders in rats were related to glucose metabolism by aldose reductase. We also explored the therapeutic potential of exogenous neurotrophic factors.

METHODS

Behavioural assessments of thermal and tactile sensitivity were performed in normal rats and in rats with streptozotocin-induced diabetes. Some of the rats were treated with insulin, aldose reductase inhibitors, ciliary neurotrophic factor or brain-derived neurotrophic factor.

RESULTS

Thermal hypoalgesia was present after 8 weeks of diabetes and was prevented by insulin treatment, which maintained normoglycaemia, by the aldose reductase inhibitor Statil or by ciliary neurotrophic factor. Brain-derived neurotrophic factor did not have an effect. When diabetic rats were tested after shorter durations of diabetes, they showed transient thermal hyperalgesia after 4 weeks which progressed to thermal hypoalgesia after 8 weeks. The aldose reductase inhibitor IDD 676 (Lidorestat), given from the onset of diabetes, prevented the development of thermal hyperalgesia and also stopped progression to thermal hypoalgesia when delivered in the last 4 weeks of an 8-week period of diabetes. Tactile allodynia was not prevented by neurotrophic factor or aldose reductase inhibitor treatment.

CONCLUSIONS/INTERPRETATION

Transient thermal hyperalgesia and subsequent progressive thermal hypoalgesia occur in diabetic rats secondary to exaggerated flux through the polyol pathway. A depletion of ciliary neurotrophic factor mediated by the polyol pathway may be involved in the aetiology of thermal hypoalgesia.

Stimulation of spinal 5-HT(2A/2C) receptors potentiates the capsaicin-induced in vivo release of substance P-like immunoreactivity in the rat dorsal horn

Stimulation of spinal serotonin (5-HT)(2A/2C) receptors has previously been reported to lead to either a pro-nociceptive or an anti-nociceptive response. Behavioral data have indicated that the pro-nociceptive effect is related to the release of substance P (SP). The aim of this in vivo microdialysis study was to investigate if stimulation of spinal 5-HT(2A/2C) receptors by the selective agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) induces spontaneous or capsaicin-evoked increase in the release of SP-like immunoreactivity (SP-LI) in the rat dorsal horn. A dose of capsaicin (25 microM in the perfusion medium administered for 30 min), which did not lead to a significant release of SP-LI on its own, induced a significant increase of greater than 4-fold of the SP-LI level following spinal application of 50 nmol DOI. Higher (500 nmol) or lower (5 nmol) doses of DOI failed to induce a similar effect. In rats with a peripheral inflammation, induced by carrageenan, capsaicin (25 microM) induced a non-significant increase of SP-LI. A significant 8-fold increase of the SP-LI level was detected following administration of 50 nmol DOI in combination with capsaicin. The effect of DOI, which was completely prevented by co-administration of the 5-HT(2A) receptor antagonist ketanserin in control animals without peripheral inflammation, was only partly blocked in animals with carrageenan induced peripheral inflammation. In conclusion, stimulation of 5-HT(2A/2C) receptors facilitates the capsaicin-evoked release of SP-LI in the dorsal horn in both animals with and without carrageenan-induced unilateral inflammation. The observation that the highest dose of DOI failed to induce SP-LI release may be due to an inhibitory postsynaptic action at this dose.

Beneficial actions of neurotrophin treatment on diabetes-induced hypoalgesia in mice

Studies were carried out in streptozotocin-treated diabetic mice to evaluate their behavioral responses to different noxious stimuli. In opposition to rats, streptozotocin-injected diabetic mice display a persistent hypoalgesia to non-noxious mechanical stimulation (von Frey monofilament). Similarly, nocifensive responses of diabetic mice to formalin injection were significantly reduced in both acute and inflammatory phases. However, no overt differences were detected between nondiabetic and diabetic mice in their sensitivity to noxious heat (radiant heat), cold (acetone), or noxious mechanical (pinprick) stimuli applied to the hind paw. To evaluate whether neurotrophin treatment could normalize the sensory deficits, nerve growth factor (NGF) or glial cell line-derived neurotrophic factor (GDNF) was administered intrathecally to diabetic mice for 3 weeks. Neurotrophin-treated mice were also compared to mice that received insulin for 3 weeks. Both NGF and insulin treatment significantly restored mechanical and chemogenic behavioral responses of diabetic mice. In contrast, GDNF treatment only reversed behavioral responses to chemogenic stimuli during the acute phase of the formalin test. These results demonstrate that diabetic mice develop reduced sensitivity to mechanical and chemical stimuli. Furthermore, these studies show that dorsal root ganglion neurons in diabetic mice are responsive to treatment with either NGF or GDNF; however, these 2 neurotrophins differ in their ability to affect distinct somatosensations.

Microdialysis in pain research

In vivo microdialysis has been used in preclinical pain research for more than a decade. This valuable tool allows correlations between nociceptive behavior and neurotransmitter release in pain-related CNS sites. However, several methodological issues must be considered to adequately interpret microdialysis data. Thus, the aim of this review is to describe key considerations, potential pitfalls, and important control experiments. We focus on animal experiments which evaluate the effects of noxious stimulation on CNS neurotransmitter release, particularly those that address clinically relevant problems in patients with long-lasting painful conditions.

Cystatin C as a cerebrospinal fluid biomarker for pain in humans

Through a process of subtraction cloning and differential hybridization, we previously identified several new genes whose expression was induced by peripheral inflammation. One of these coded for cystatin C, a secreted cysteine protease inhibitor in the cystatin superfamily. We hypothesized that, concurrent with increased expression in dorsal horn, increased secretion would elevate the cystatin C content in cerebrospinal fluid (CSF) during active pain states. Alterations were assessed by immunoassay and by surface enhanced laser desorption ionization (SELDI) mass spectrometry with either reverse phase or immobilized anti-cystatin C antibody surfaces using CSF from ten age-matched obstetrical patients at term. Five control subjects were scheduled for an elective caesarian section and were not in pain. Another five subjects were in labor for 8.9+/-1h and were in severe pain as assessed with a visual analog scale and the McGill short form questionnaire. The level of cystatin C as measured by immunoassay in the non-pain patients was 2.77+/-0.75 microg/ml and in the pain patients 5.36+/-0.92 microg/ml (P<0.02). The elevation occurred without significant change in total CSF protein or beta-endorphin content. The cystatin C increase also was detectable by SELDI with either raw CSF or after antibody capture. These data are consistent with our previous animal study and the idea that persistent pain induces the synthesis and release of cystatin C in dorsal spinal cord, the surplus of which overflows into the CSF.

Potential mechanisms of neuropathic pain in diabetes

Abnormal sensations and pain are features of approximately 10% of all cases of diabvetic neuropathy and can cause marked diminution in the quality of life for these patients. The quality and distribution of pain are variable, although descriptions of burning pain in the hands and feet are commonly reported. Like other neuropathic pain states, painful diabetic neuropathy has an unknown pathogenesis and, in many cases, is not alleviated by nonsteriodal anti-inflammatory drugs or opiates. In the last decase, a number of behavioral and physiologic studies have revealed indices of sensory dysfunction in animal models of diabetes. These include hyperalgesia to mechanical and noxious chemical stimuli and allodynia to light touch. Animal models of painful diabetic neuropathy have been used to investigate the therapeutic potential of a range of experimental agents and also to explore potential etiologic mechanisms. There is relatively little evidence to suggest that the peripheral sensory nerves of diabetic rodents exhibit spontaneous activity or increased responsiveness to peripheral stimuli. Indeed, the weight of eveidence suggests that sensory input to the spinal cord is decreased rather than increased in diabetic rodents. Aberrant spinal or supraspinal modulation of sensory processing may therefore be involved in generating allodynia and hyperalgesia in these models. Studies have supported a role for spinally mediated hyeralgesia in diabetic rats that may reflect either a response to diminished peripheral input or a consequence of hyperglycemia on local or descending modulatory systems. Elucidating the affects of diabetes on spinal sensory processing may assist development of novel therapeutic strategies for preventing and alleviating painful diabetic neuropathy.

Elevated spinal cyclooxygenase and prostaglandin release during hyperalgesia in diabetic rats

Diabetic rats display exaggerated hyperalgesic behavior in response to noxious stimuli that may model aspects of painful diabetic neuropathy. This study examined the contribution of spinal prostaglandin production to this exaggerated hyperalgesic behavior. Rats were implanted with spinal dialysis probes and received noxious stimulation to the hind paw by subcutaneous injection of 0.5% formalin solution. Prostaglandin E(2) (PGE(2)) was measured in dialysates of lumbar spinal cerebrospinal fluid concurrent with behavioral responses to formalin injection. In separate experiments, formalin-evoked behavioral responses were measured after intrathecal delivery of either a cyclooxygenase inhibitor or an EP(1) receptor antagonist, and cyclooxygenase protein was measured in spinal cord homogenates. Diabetic rats exhibited exaggerated behavioral responses to paw formalin injection and a concurrent prolongation of formalin-evoked PGE(2) release. Formalin-evoked behavioral responses were dose-dependently reduced in diabetic rats by spinal delivery of a cyclooxygenase inhibitor or an EP(1) receptor antagonist. Protein levels of cyclooxygenase-2 were elevated in the spinal cord of diabetic rats, whereas cyclooxygenase-1 protein was reduced. Hyperalgesic behavior in diabetic rats is associated with both increased cyclooxygenase-2 protein and cyclooxygenase-mediated PGE(2) release. Spinal delivery of selective inhibitors of cyclooxygenase-2 or antagonists of prostaglandin receptors may have therapeutic potential for treating painful diabetic neuropathy.

Spinal substance P release in vivo during the induction of long-term potentiation in dorsal horn neurons

Long-term potentiation (LTP) in wide dynamic range (WDR) neurons in the dorsal horn has been suggested to contribute to central sensitization and the development of chronic pain. Indirect experimental evidence indicates an involvement of substance P (SP), in this respect. The aim of the present study was to monitor the extracellular level of substance P-like immunoreactivity (SP-LI) in the dorsal horn of the rat during and after induction of LTP in WDR neurons in vivo. Electrophysiological recordings of single (WDR) neurons were performed in parallel with microdialysis in the dorsal horn under urethane-anaesthesia. The amount of SP-LI in the microdialysate was determined by radioimmunoassay. As previously shown, high frequency conditioning stimulation of the sciatic nerve induced an increased firing response of WDR neurons. An increased response to C-fibre stimulation, but not A-fibre stimulation, could be determined. A significant increase of the extracellular level of SP-LI in the dorsal horn was detected during, but not after, induction of LTP. These data suggest that SP may be involved in the induction of LTP by high frequency stimulation. However, the maintenance of spinal LTP following high frequency peripheral nerve stimulation does not seem to depend on an increased release of SP.