ATP-sensitive potassium channels in rat primary afferent neurons: the effect of neuropathic injury and gabapentin

Metformin inhibits paclitaxel-induced mechanical allodynia by activating opioidergic pathways and reducing cytokines production in the dorsal root ganglia and thalamus

It has been shown that AMP-activated protein kinase (AMPK) is involved in the nociceptive processing. This observation has prompted us to investigate the effects of the AMPK activator metformin on the paclitaxel-induced mechanical allodynia, a well-established model of neuropathic pain. Mechanical allodynia was induced by four intraperitoneal (i.p) injections of paclitaxel (2 mg/kg.day) in mice. Metformin was administered per os (p.o.). Naltrexoneandglibenclamide were used to investigate mechanisms mediating metformin activity. Concentrations of cytokines in the dorsal root ganglia (DRG) and thalamus were determined. After a single p.o. administration, the two highest doses of metformin (500 and 1000 mg/kg) attenuated the mechanical allodynia. This response was attenuated by all doses of metformin (250, 500 and 1000 mg/kg) when two administrations, 2 h apart, were carried out. Naltrexone (5 and 10 mg/kg, i.p.), but not glibenclamide (20 and 40 mg/kg, p.o.), attenuated metformin activity. Concentrations of tumor necrosis factor (TNF)-α, interleukin (IL)-1β and CXCL-1 in the DRG were increased after administration of paclitaxel. Metformin (1000 mg/kg) reduced concentrations of TNF-α, IL-1β and CXCL-1 in the DRG. Concentration of IL-6, but not TNF-α, in the thalamus was increased after administration of paclitaxel. Metformin (1000 mg/kg) reduced concentration of IL-6 in the thalamus. In summary, metformin exhibits activity in the model of neuropathic pain induced by paclitaxel. This activity may be mediated by activation of opioidergic pathways and reduced production of TNF-α, IL-1β and CXCL-1 in the DRG and IL-6 in the thalamus.

Emerging Nonpharmacologic Interventions to Treat Diabetic Peripheral Neuropathy

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

Poly (beta-amino ester) as an in vivo nanocarrier for therapeutic nucleic acids

Therapeutic nucleic acids are an emerging class of therapy for treating various diseases through immunomodulation, protein replacement, gene editing, and genetic engineering. However, they need a vector to effectively and safely reach the target cells. Most gene and cell therapies rely on ex vivo gene delivery, which is laborious, time-consuming, and costly; therefore, devising a systematic vector for effective and safe in vivo delivery of therapeutic nucleic acids is required to target the cells of interest in an efficient manner. Synthetic nanoparticle vector poly beta amino ester (PBAE), a class of degradable polymer, is a promising candidate for in vivo gene delivery. PBAE is considered the most potent in vivo vector due to its excellent transfection performance and biodegradability. PBAE nanoparticles showed tunable charge density, diverse structural characteristics, excellent encapsulation capacity, high stability, stimuli-responsive release, site-specific delivery, potent binding to nucleic acids, flexible binding ability to various conjugates, and effective endosomal escape. These unique properties of PBAE are an essential contribution to in vivo gene delivery. The current review discusses each of the components used for PBAE synthesis and the impact of various environmental and physicochemical factors of the body on PBAE nanocarrier.

ATP-Sensitive Potassium Channels in Migraine: Translational Findings and Therapeutic Potential

Globally, migraine is a leading cause of disability with a huge impact on both the work and private life of affected persons. To overcome the societal migraine burden, better treatment options are needed. Increasing evidence suggests that ATP-sensitive potassium (KATP) channels are involved in migraine pathophysiology. These channels are essential both in blood glucose regulation and cardiovascular homeostasis. Experimental infusion of the KATP channel opener levcromakalim to healthy volunteers and migraine patients induced headache and migraine attacks in 82-100% of participants. Thus, this is the most potent trigger of headache and migraine identified to date. Levcromakalim likely induces migraine via dilation of cranial arteries. However, other neuronal mechanisms are also proposed. Here, basic KATP channel distribution, physiology, and pharmacology are reviewed followed by thorough review of clinical and preclinical research on KATP channel involvement in migraine. KATP channel opening and blocking have been studied in a range of preclinical migraine models and, within recent years, strong evidence on the importance of their opening in migraine has been provided from human studies. Despite major advances, translational difficulties exist regarding the possible anti-migraine efficacy of KATP channel blockage. These are due to significant species differences in the potency and specificity of pharmacological tools targeting the various KATP channel subtypes.

Morphine Efficacy, Tolerance, and Hypersensitivity Are Altered After Modulation of SUR1 Subtype KATP Channel Activity in Mice

ATP-sensitive potassium (K_ATP) channels are found in the nervous system and are downstream targets of opioid receptors. K_ATP channel activity can effect morphine efficacy and may beneficial for relieving chronic pain in the peripheral and central nervous system. Unfortunately, the K_ATP channels exists as a heterooctomers, and the exact subtypes responsible for the contribution to chronic pain and opioid signaling in either dorsal root ganglia (DRG) or the spinal cord are yet unknown. Chronic opioid exposure (15 mg/kg morphine, s.c., twice daily) over 5 days produces significant downregulation of Kir6.2 and SUR1 in the spinal cord and DRG of mice. In vitro studies also conclude potassium flux after K_ATP channel agonist stimulation is decreased in neuroblastoma cells treated with morphine for several days. Mice lacking the K_ATP channel SUR1 subunit have reduced opioid efficacy in mechanical paw withdrawal behavioral responses compared to wild-type and heterozygous littermates (5 and 15 mg/kg, s.c., morphine). Using either short hairpin RNA (shRNA) or SUR1 cre-lox strategies, downregulation of SUR1 subtype K_ATP channels in the spinal cord and DRG of mice potentiated the development of morphine tolerance and withdrawal. Opioid tolerance was attenuated with intraplantar injection of SUR1 agonists, such as diazoxide and NN-414 (100 μM, 10 μL) compared to vehicle treated animals. These studies are an important first step in determining the role of K_ATP channel subunits in antinociception, opioid signaling, and the development of opioid tolerance, and shed light on the potential translational ability of K_ATP channel targeting pharmaceuticals and their possible future clinical utilization. These data suggest that increasing neuronal K_ATP channel activity in the peripheral nervous system may be a viable option to alleviate opioid tolerance and withdrawal.

From Mechanism to Cure: Renewing the Goal to Eliminate the Disease of Pain

Objective

Persistent pain causes untold misery worldwide and is a leading cause of disability. Despite its astonishing prevalence, pain is undertreated, at least in part because existing therapeutics are ineffective or cause intolerable side effects. In this review, we cover new findings about the neurobiology of pain and argue that all but the most transient forms of pain needed to avoid tissue damage should be approached as a disease where a cure can be the goal of all treatment plans, even if attaining this goal is not yet always possible.

Design

We reviewed the literature to highlight recent advances in the area of the neurobiology of pain.

Results

We discuss barriers that are currently hindering the achievement of this goal, as well as the development of new therapeutic strategies. We also discuss innovations in the field that are creating new opportunities to treat and even reverse persistent pain, some of which are in late-phase clinical trials.

Conclusion

We conclude that the confluence of new basic science discoveries and development of new technologies are creating a path toward pain therapeutics that should offer significant hope of a cure for patients and practitioners alike. Classification of Evidence. Our review points to new areas of inquiry for the pain field to advance the goal of developing new therapeutics to treat chronic pain.

The Antiallodynic Effects of Nefopam Are Mediated by the Adenosine Triphosphate-Sensitive Potassium Channel in a Neuropathic Pain Model

BACKGROUND

Nefopam hydrochloride is a centrally acting compound that induces antinociceptive and antihyperalgesic properties in neuropathic pain models. Previous reports have shown that activation of adenosine triphosphate (ATP)-sensitive and calcium-activated potassium (KATP and KCa2+) channels has antiallodynic effects in neuropathic pain. In the present study, we evaluated the relationship between potassium channels and nefopam to determine whether the antiallodynic effects of nefopam are mediated by potassium channels in a neuropathic pain model.

METHODS

Mechanical allodynia was induced by spinal nerve ligation (SNL) in rats, and the paw withdrawal threshold (PWT) was evaluated by the use of von Frey filaments. Nefopam was administered intraperitoneally before or after SNL. We assessed the relationship between nefopam and intrathecal injection of the KCa2+ channel antagonists apamin and charybdotoxin, and the KATP channel blocker glibenclamide to assess their abilities to reverse the antiallodynic effects of nefopam. In addition, we evaluated whether the KATP channel opener pinacidil had antiallodynic effects and promoted the antiallodynic effects of nefopam.

RESULTS

Administration of nefopam before and after SNL induced significant antiallodynic effects (P < .01, respectively), which were significantly reduced by glibenclamide (P < .01). Pinacidil improved the antiallodynic effects of nefopam (P < .01); however, apamin and charybdotoxin had little effects on the antiallodynic properties of nefopam.

CONCLUSIONS

The antiallodynic effects of nefopam are increased by a KATP channel agonist and reversed by a KATP channel antagonist. These data suggest that the KATP channel is involved in the antiallodynic effects of nefopam in a neuropathic pain model.

Optimization of Polyplex Formation between DNA Oligonucleotide and Poly(ʟ-Lysine): Experimental Study and Modeling Approach

The polyplexes formed by nucleic acids and polycations have received a great attention owing to their potential application in gene therapy. In our study, we report experimental results and modeling outcomes regarding the optimization of polyplex formation between the double-stranded DNA (dsDNA) and poly(l-Lysine) (PLL). The quantification of the binding efficiency during polyplex formation was performed by processing of the images captured from the gel electrophoresis assays. The design of experiments (DoE) and response surface methodology (RSM) were employed to investigate the coupling effect of key factors (pH and N/P ratio) affecting the binding efficiency. According to the experimental observations and response surface analysis, the N/P ratio showed a major influence on binding efficiency compared to pH. Model-based optimization calculations along with the experimental confirmation runs unveiled the maximal binding efficiency (99.4%) achieved at pH 5.4 and N/P ratio 125. To support the experimental data and reveal insights of molecular mechanism responsible for the polyplex formation between dsDNA and PLL, molecular dynamics simulations were performed at pH 5.4 and 7.4.

Delta Opioid Receptor Expression and Function in Primary Afferent Somatosensory Neurons

The functional diversity of primary afferent neurons of the dorsal root ganglia (DRG) generates a variety of qualitatively and quantitatively distinct somatosensory experiences, from shooting pain to pleasant touch. In recent years, the identification of dozens of genetic markers specifically expressed by subpopulations of DRG neurons has dramatically improved our understanding of this diversity and provided the tools to manipulate their activity and uncover their molecular identity and function. Opioid receptors have long been known to be expressed by discrete populations of DRG neurons, in which they regulate cell excitability and neurotransmitter release. We review recent insights into the identity of the DRG neurons that express the delta opioid receptor (DOR) and the ion channel mechanisms that DOR engages in these cells to regulate sensory input. We highlight recent findings derived from DORGFP reporter mice and from in situ hybridization and RNA sequencing studies in wild-type mice that revealed DOR presence in cutaneous mechanosensory afferents eliciting touch and implicated in tactile allodynia. Mechanistically, we describe how DOR modulates opening of voltage-gated calcium channels (VGCCs) to control glutamatergic neurotransmission between somatosensory neurons and postsynaptic neurons in the spinal cord dorsal horn. We additionally discuss other potential signaling mechanisms, including those involving potassium channels, which DOR may engage to fine tune somatosensation. We conclude by discussing how this knowledge may explain the analgesic properties of DOR agonists against mechanical pain and uncovers an unanticipated specialized function for DOR in cutaneous mechanosensation.

The antinociceptive effects of magnesium sulfate and MK-801 in visceral inflammatory pain model: The role of NO/cGMP/K(+)ATP pathway

CONTEXT

Magnesium and MK-801 (dizocilpine), antagonists of N-methyl-d-aspartate receptors, are involved in the processing of pain.

OBJECTIVE

This study determines whether magnesium sulfate (MS) and MK-801 affects visceral inflammatory pain and determines a possible mechanism of action.

MATERIALS AND METHODS

Analgesic activity was assessed using the acetic acid-induced writhing test in rats. MS (1-45 mg/kg) or MK-801 (0.005-0.03 mg/kg) was administrated subcutaneously (s.c.). To assess possible mechanisms of action, we examined the effects of l-NAME (10 mg/kg, intraperitoneal), methylene blue (0.5 mg/kg, s.c.), and glibenclamide (3 mg/kg, s.c.) on the effect of MS or MK-801.

RESULTS

MS and MK-801 showed biphasic and linear dose-response pattern, respectively. MS reduces the number of writhing on the dose of 1, 5, and 15 mg/kg by 60, 50, and 78%, respectively, while it has no effects on the doses of 30 and 45 mg/kg. MK-801 (0.005-0.03 mg/kg) showed decrease in the number of writhing by 33-79%. The mean effective doses of MS and MK-801 were 6.6 (first phase) and 0.009 mg/kg, respectively. Both drugs did not impair the rotarod performance. l-NAME, methylene blue, and glybenclamide reduced the effect of MK-801 by 100, 43, and 64%, respectively, but not the effect of MS.

CONCLUSIONS

The results suggest that MS and MK-801 may be useful analgesics in the management of visceral inflammatory pain, at doses that do not induce motor impairment. The modulation of NO/cGMP/K+ATP pathway plays an important role in the antinociceptive mechanism of MK-801, but does not contribute to the antinociceptive effect of MS.

Pregabalin attenuates excitotoxicity in diabetes

Diabetes can exacerbate seizures and worsen seizure-related brain damage. In the present study, we aimed to determine whether the standard antiepileptic drug pregabalin (PGB) protects against pilocarpine-induced seizures and excitotoxicity in diabetes. Adult male Sprague-Dawley rats were divided into either a streptozotocin (STZ)-induced diabetes group or a normal saline (NS) group. Both groups were further divided into subgroups that were treated intravenously with either PGB (15 mg/kg) or a vehicle; all groups were treated with subcutaneous pilocarpine (60 mg/kg) to induce seizures. To evaluate spontaneous recurrent seizures (SRS), PGB-pretreated rats were fed rat chow containing oral PGB (450 mg) for 28 consecutive days; vehicle-pretreated rats were fed regular chow. SRS frequency was monitored for 2 weeks from post-status epilepticus day 15. We evaluated both acute neuronal loss and chronic mossy fiber sprouting in the CA3 area. In addition, we performed patch clamp recordings to study evoked excitatory postsynaptic currents (eEPSCs) in hippocampal CA1 neurons for both vehicle-treated rats with SRS. Finally, we used an RNA interference knockdown method for Kir6.2 in a hippocampal cell line to evaluate PGB's effects in the presence of high-dose ATP. We found that compared to vehicle-treated rats, PGB-treated rats showed less severe acute seizure activity, reduced acute neuronal loss, and chronic mossy fiber sprouting. In the vehicle-treated STZ rats, eEPSC amplitude was significantly lower after PGB administration, but glibenclamide reversed this effect. The RNA interference study confirmed that PGB could counteract the ATP-sensitive potassium channel (K_ATP)-closing effect of high-dose ATP. By opening K_ATP, PGB protects against neuronal excitotoxicity, and is therefore a potential antiepileptogenic in diabetes. These findings might help develop a clinical algorithm for treating patients with epilepsy and comorbid metabolic disorders.

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.

Polymeric nucleic acid carriers: current issues and novel design approaches

To deliver nucleic acids including plasmid DNA (pDNA) and short interfering RNA (siRNA), polymeric gene carriers equipped with various functionalities have been extensively investigated. The functionalities of these polymeric vectors have been designed to overcome various extracellular and intracellular hurdles that nucleic acids and their carriers encounter during their journey from injection site to intracellular target site. This review briefly introduces known extracellular and intracellular issues of nucleic acid delivery and their solution strategies. We examine significant yet overlooked factors affecting nucleic acid delivery (e.g., microenvironmental pH, polymer/siRNA complexation, and pharmaceutical formulation) and highlight our reported approaches to solve these problems.

Blockade of ATP-sensitive potassium channels prevents the attenuation of the exercise pressor reflex by tempol in rats with ligated femoral arteries

We reported previously that tempol attenuated the exercise pressor and muscle mechanoreceptor reflexes in rats whose femoral arteries were ligated, whereas tempol did not attenuate these reflexes in rats whose femoral arteries were freely perfused. Although the mechanism whereby tempol attenuated these reflexes in rats whose femoral artery was ligated was independent of its ability to scavenge reactive oxygen species, its nature remains unclear. An alternative explanation for the tempol-induced attenuation of these reflexes involves ATP-sensitive potassium channels (K(ATP)) and calcium-activated potassium channels (BK(Ca)), both of which are opened by tempol. We tested the likelihood of this explanation by measuring the effects of either glibenclamide (0.1 mg/kg), which blocks K(ATP) channels, or iberiotoxin (20 or 40 μg/kg), which blocks BK(Ca) channels, on the tempol-induced attenuation of the exercise pressor and muscle mechanoreceptor reflexes in decerebrated rats whose femoral arteries were ligated. We found that glibenclamide prevented the tempol-induced attenuation of both reflexes, whereas iberiotoxin did not. We also found that the amount of protein comprising the pore of the K(ATP) channel in the dorsal root ganglia innervating hindlimbs whose femoral artery was ligated was significantly greater than that in the dorsal root ganglia innervating hindlimbs whose femoral arteries were freely perfused. In contrast, the amounts of protein comprising the BK(Ca) channel in the dorsal root ganglia innervating the ligated and freely perfused hindlimbs were not different. We conclude that tempol attenuated both reflexes by opening K(ATP) channels, an effect that hyperpolarized muscle afferents stimulated by static contraction or tendon stretch.

The effect of environmental pH on polymeric transfection efficiency

Although polymers, polyplexes, and cells are exposed to various extracellular and intracellular pH environments during polyplex preparation and polymeric transfection, the impact of environmental pH on polymeric transfection has not yet been investigated. This study aims to understand the influence of environmental pH on polymeric transfection by modulating the pH of the transfection medium or the culture medium. Changes in the extracellular pH affected polymeric transfection by way of complex factors such as pH-induced changes in polymer characteristics (e.g., proton buffering capacity and ionization), polyplex characteristics (e.g., size, surface charge, and decomplexation), and cellular characteristics (e.g., cellular uptake, cell cycle phases, and intracellular pH environment). Notably, acidic medium delayed endocytosis, endosomal acidification, cytosolic release, and decomplexation of polyplexes, thereby negatively affecting gene expression. However, acidic medium inhibited mitosis and reduced dilution of gene expression, resulting in increased transfection efficiency. Compared to pH 7.4 medium, acidic transfection medium reduced gene expression 1.6-7.7-fold whereas acidic culture medium enhanced transfection efficiency 2.1-2.6-fold. Polymeric transfection was affected more by the culture medium than by the transfection medium. Understanding the effects of extracellular pH during polymeric transfection may stimulate new strategies for determining effective and safe polymeric gene carriers.

Activation of ATP-sensitive potassium channels antagonize nociceptive behavior and hyperexcitability of DRG neurons from rats

Background

Nociceptive responses to noxious stimuli are initiated at peripheral nociceptor terminals. Ion channels play a vital role in pain signal initiation and conduction. Activation of K_ATP channels has been implicated in mediating the analgesic effects of agents such as morphine. However, systematic studies regarding the effects of K_ATP activators on nociception and neuronal excitability are scarce.

Results

In this study, we describe the antagonistic effects of K_ATP activators pinacidil and diazoxide on nocifensive behavior induced by bradykinin (BK), thermo and mechanical stimuli, and the bradykinin-induced hyperexcitability of DRG neurons. We also found that K_ATP activators can moderately activate K_ATP in DRG neurons. Because the effects of K_ATP activators can be reversed by the K_ATP blocker glyburide, direct activation of K_ATP is most likely the underlying mechanism.

Conclusion

This systematic study clearly demonstrates that activation of K_ATP could have significant modulatory effects on the excitability of sensory neurons and thus on sensory behaviors, such as nociception. K_ATP activators can be evaluated clinically for the treatment of pain symptoms.

Adenosine triphosphate-sensitive potassium channel blockers attenuate the antiallodynic effect of R-PIA in neuropathic rats

BACKGROUND

Nerve injury can generate neuropathic pain. The accompanying mechanical allodynia may be reduced by the intrathecal administration of adenosine. The neuroprotective effects of adenosine are mediated by the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel. We assessed the relationship between the adenosine A1 receptor agonist, N⁶-(R)-phenylisopropyl adenosine (R-PIA), and K(ATP) channels to determine whether the antiallodynic effects of R-PIA are also mediated through K(ATP) channels in a rat nerve ligation injury model of neuropathic pain.

METHODS

Mechanical allodynia was induced by tight ligation of the left lumbar fifth and sixth spinal nerves. Mechanical allodynia in the left hindpaw was evaluated using von Frey filaments to measure withdrawal thresholds. R-PIA (0.5, 1, or 2 μg) was administered intrathecally to induce antiallodynia. We assessed whether pretreatment with the K(ATP) channel blockers glibenclamide or 5-hydroxydecanoate reversed the antiallodynic effect of R-PIA. Also, we evaluated whether diazoxide, a K(ATP) channel opener, had an antiallodynic effect and promoted the antiallodynic effect of R-PIA. Lastly, we investigated whether the voltage-activated K channel blocker 4-aminopyridine attenuated the effect of R-PIA.

RESULTS

Intrathecal R-PIA produced maximal antiallodynia at 2 μg (P < 0.05). Intrathecal pretreatment with glibenclamide and intraperitoneal pretreatment 5-hydroxydecanoate significantly reduced the antiallodynic effect of R-PIA. Diazoxide produced an antiallodynic effect and also enhanced the antiallodynic action of R-PIA. 4-Aminopyridine had no effect on the antiallodynic action of R-PIA.

CONCLUSIONS

The antiallodynic effects of adenosine A1 receptor stimulation may be related to K(ATP) channel activity in a rat model of nerve ligation injury.

Sex differences in the contribution of ATP-sensitive K+ channels in trigeminal ganglia under an acute muscle pain condition

In this study, we examined whether functional subunits of the ATP-dependent K+ channel (KATP) are expressed in trigeminal ganglia (TG), which contains sensory neurons that innervate oral and facial structures. We also investigated whether direct activation of the KATP effectively attenuates mechanical hypersensitivity in the context of an acute orofacial muscle pain condition. The KATP expression in TG and behavioral studies were conducted in age matched male and female Sprague-Dawley rats. RT-PCR experiments showed that the mRNAs for the inwardly rectifying pore-forming subunits, Kir6.1 and Kir6.2, as well as the regulatory sulfonylurea subunits, SUR1 and SUR2, were reliably detected in TG. Subsequent western blot analysis confirmed that proteins for all four subunits are expressed in TG, and showed that Kir6.2 is expressed at a significantly higher level in male TG compared to that of female rats. This observation was confirmed by the immunohistochemical demonstration of higher percentages of Kir6 positive masseter afferents in female rats. Masseteric injection of capsaicin evokes a time dependent increase in masseter sensitivity to noxious mechanical stimulation. A specific KATP agonist, pinacidil, dose-dependently attenuated the capsaicin-induced mechanical hypersensitivity in male rats. The dose of pinacidil (20 μg) that completely blocked the capsaicin responses in male rats was ineffective in female rats regardless of their estrus phases. Only at the highest dose (300 μg) we used, pinacidil was partially effective in female rats. Similarly, another KATP agonist, diazoxide which targets different KATP subunits also showed sex specific responses in attenuating capsaicin-induced masseter hypersensitivity. These data suggested that sex differences in functional KATP expression in TG may underlie sex specific responses to KATP agonists. The present study provided novel information on sex differences in KATP expression in TG and its contribution under an orofacial muscle pain condition.

KATP channel subunits in rat dorsal root ganglia: alterations by painful axotomy

Background

ATP-sensitive potassium (K_ATP) channels in neurons mediate neuroprotection, they regulate membrane excitability, and they control neurotransmitter release. Because loss of DRG neuronal K_ATP currents is involved in the pathophysiology of pain after peripheral nerve injury, we characterized the distribution of the K_ATP channel subunits in rat DRG, and determined their alterations by painful axotomy using RT-PCR, immunohistochemistry and electron microscopy.

Results

PCR demonstrated Kir6.1, Kir6.2, SUR1 and SUR2 transcripts in control DRG neurons. Protein expression for all but Kir6.1 was confirmed by Western blots and immunohistochemistry. Immunostaining of these subunits was identified by fluorescent and confocal microscopy in plasmalemmal and nuclear membranes, in the cytosol, along the peripheral fibers, and in satellite glial cells. Kir6.2 co-localized with SUR1 subunits. Kir6.2, SUR1, and SUR2 subunits were identified in neuronal subpopulations, categorized by positive or negative NF200 or CGRP staining. K_ATP current recorded in excised patches was blocked by glybenclamide, but preincubation with antibody against SUR1 abolished this blocking effect of glybenclamide, confirming that the antibody targets the SUR1 protein in the neuronal plasmalemmal membrane.

In the myelinated nerve fibers we observed anti-SUR1 immunostaining in regularly spaced funneled-shaped structures. These structures were identified by electron microscopy as Schmidt-Lanterman incisures (SLI) formed by the Schwann cells. Immunostaining against SUR1 and Kir6.2 colocalized with anti-Caspr at paranodal sites.

DRG excised from rats made hyperalgesic by spinal nerve ligation exhibited similar staining against Kir6.2, SUR1 or SUR2 as DRG from controls, but showed decreased prevalence of SUR1 immunofluorescent NF200 positive neurons. In DRG and dorsal roots proximal to axotomy SLI were smaller and showed decreased SUR1 immunofluorescence.

Conclusions

We identified Kir6.2/SUR1 and Kir6.2/SUR2 K_ATP channels in rat DRG neuronal somata, peripheral nerve fibers, and glial satellite and Schwann cells, in both normal state and after painful nerve injury. This is the first report of K_ATP channels in paranodal sites adjacent to nodes of Ranvier and in the SLI of the Schwann cells. After painful axotomy K_ATP channels are downregulated in large, myelinated somata and also in SLI, which are also of smaller size compared to controls.

Because K_ATP channels may have diverse functional roles in neurons and glia, further studies are needed to explore the potential of K_ATP channels as targets of therapies against neuropathic pain and neurodegeneration.

Suppressed Ca2+/CaM/CaMKII-dependent K(ATP) channel activity in primary afferent neurons mediates hyperalgesia after axotomy

Painful axotomy decreases K(ATP) channel current (IK(ATP)) in primary afferent neurons. Because cytosolic Ca(2+) signaling is depressed in injured dorsal root ganglia (DRG) neurons, we investigated whether Ca(2+)-calmodulin (CaM)-Ca(2+)/CaM-dependent kinase II (CaMKII) regulates IK(ATP) in large DRG neurons. Immunohistochemistry identified the presence of K(ATP) channel subunits SUR1, SUR2, and Kir6.2 but not Kir6.1, and pCaMKII in neurofilament 200-positive DRG somata. Single-channel recordings from cell-attached patches revealed that basal and evoked IK(ATP) by ionomycin, a Ca(2+) ionophore, is activated by CaMKII. In axotomized neurons from rats made hyperalgesic by spinal nerve ligation (SNL), basal K(ATP) channel activity was decreased, and sensitivity to ionomycin was abolished. Basal and Ca(2+)-evoked K(ATP) channel activity correlated inversely with the degree of hyperalgesia induced by SNL in the rats from which the neurons were isolated. Inhibition of IK(ATP) by glybenclamide, a selective K(ATP) channel inhibitor, depolarized resting membrane potential (RMP) recorded in perforated whole-cell patches and enhanced neurotransmitter release measured by amperometry. The selective K(ATP) channel opener diazoxide hyperpolarized the RMP and attenuated neurotransmitter release. Axotomized neurons from rats made hyperalgesic by SNL lost sensitivity to the myristoylated form of autocamtide-2-related inhibitory peptide (AIPm), a pseudosubstrate blocker of CaMKII, whereas axotomized neurons from SNL animals that failed to develop hyperalgesia showed normal IK(ATP) inhibition by AIPm. AIPm also depolarized RMP in control neurons via K(ATP) channel inhibition. Unitary current conductance and sensitivity of K(ATP) channels to cytosolic ATP and ligands were preserved even after painful nerve injury, thus providing opportunities for selective therapeutic targeting against neuropathic pain.

ATP-sensitive potassium currents in rat primary afferent neurons: biophysical, pharmacological properties, and alterations by painful nerve injury

ATP-sensitive potassium (K(ATP)) channels may be linked to mechanisms of pain after nerve injury, but remain under-investigated in primary afferents so far. We therefore characterized these channels in dorsal root ganglion (DRG) neurons, and tested whether they contribute to hyperalgesia after spinal nerve ligation (SNL). We compared K(ATP) channel properties between DRG somata classified by diameter into small or large, and by injury status into neurons from rats that either did or did not become hyperalgesic after SNL, or neurons from control animals. In cell-attached patches, we recorded basal K(ATP) channel opening in all neuronal subpopulations. However, higher open probabilities and longer open times were observed in large compared to small neurons. Following SNL, this channel activity was suppressed only in large neurons from hyperalgesic rats, but not from animals that did not develop hyperalgesia. In contrast, no alterations of channel activity developed in small neurons after axotomy. On the other hand, cell-free recordings showed similar ATP sensitivity, inward rectification and unitary conductance (70-80 pS) between neurons classified by size or injury status. Likewise, pharmacological sensitivity to the K(ATP) channel opener diazoxide, and to the selective blockers glibenclamide and tolbutamide, did not differ between groups. In large neurons, selective inhibition of whole-cell ATP-sensitive potassium channel current (I(K(ATP))) by glibenclamide depolarized resting membrane potential (RMP). The contribution of this current to RMP was also attenuated after painful axotomy. Using specific antibodies, we identified SUR1, SUR2, and Kir6.2 but not Kir6.1 subunits in DRGs. These findings indicate that functional K(ATP) channels are present in normal DRG neurons, wherein they regulate RMP. Alterations of these channels may be involved in the pathogenesis of neuropathic pain following peripheral nerve injury. Their biophysical and pharmacological properties are preserved even after axotomy, suggesting that K(ATP) channels in primary afferents remain available for therapeutic targeting against established neuropathic pain.

Nitric oxide activates ATP-sensitive potassium channels in mammalian sensory neurons: action by direct S-nitrosylation

Background

ATP-sensitive potassium (K_ATP) channels in neurons regulate excitability, neurotransmitter release and mediate protection from cell-death. Furthermore, activation of K_ATP channels is suppressed in DRG neurons after painful-like nerve injury. NO-dependent mechanisms modulate both K_ATP channels and participate in the pathophysiology and pharmacology of neuropathic pain. Therefore, we investigated NO modulation of K_ATP channels in control and axotomized DRG neurons.

Results

Cell-attached and cell-free recordings of K_ATP currents in large DRG neurons from control rats (sham surgery, SS) revealed activation of K_ATP channels by NO exogenously released by the NO donor SNAP, through decreased sensitivity to [ATP]i.

This NO-induced K_ATP channel activation was not altered in ganglia from animals that demonstrated sustained hyperalgesia-type response to nociceptive stimulation following spinal nerve ligation. However, baseline opening of K_ATP channels and their activation induced by metabolic inhibition was suppressed by axotomy. Failure to block the NO-mediated amplification of K_ATP currents with specific inhibitors of sGC and PKG indicated that the classical sGC/cGMP/PKG signaling pathway was not involved in the activation by SNAP. NO-induced activation of K_ATP channels remained intact in cell-free patches, was reversed by DTT, a thiol-reducing agent, and prevented by NEM, a thiol-alkylating agent. Other findings indicated that the mechanisms by which NO activates K_ATP channels involve direct S-nitrosylation of cysteine residues in the SUR1 subunit. Specifically, current through recombinant wild-type SUR1/Kir6.2 channels expressed in COS7 cells was activated by NO, but channels formed only from truncated isoform Kir6.2 subunits without SUR1 subunits were insensitive to NO. Further, mutagenesis of SUR1 indicated that NO-induced K_ATP channel activation involves interaction of NO with residues in the NBD1 of the SUR1 subunit.

Conclusion

NO activates K_ATP channels in large DRG neurons via direct S-nitrosylation of cysteine residues in the SUR1 subunit. The capacity of NO to activate K_ATP channels via this mechanism remains intact even after spinal nerve ligation, thus providing opportunities for selective pharmacological enhancement of K_ATP current even after decrease of this current by painful-like nerve injury.

Opposing effects of spinal nerve ligation on calcium-activated potassium currents in axotomized and adjacent mammalian primary afferent neurons

Calcium-activated potassium channels regulate AHP and excitability in neurons. Since we have previously shown that axotomy decreases I(Ca) in DRG neurons, we investigated the association between I(Ca) and K((Ca)) currents in control medium-sized (30-39 microM) neurons, as well as axotomized L5 or adjacent L4 DRG neurons from hyperalgesic rats following L5 SNL. Currents in response to AP waveform voltage commands were recorded first in Tyrode's solution and sequentially after: 1) blocking Na(+) current with NMDG and TTX; 2) addition of K((Ca)) blockers with a combination of apamin 1 microM, iberiotoxin 200 nM, and clotrimazole 500 nM; 3) blocking remaining K(+) current with the addition of 4-AP, TEA-Cl, and glibenclamide; and 4) blocking I(Ca) with cadmium. In separate experiments, currents were evoked (HP -60 mV, 200 ms square command pulses from -100 to +50 mV) while ensuring high levels of activation of I(K(Ca)) by clamping cytosolic Ca(2+) concentration with pipette solution in which Ca(2+) was buffered to 1 microM. This revealed I(K(Ca)) with components sensitive to apamin, clotrimazole and iberiotoxin. SNL decreases total I(K(Ca)) in axotomized (L5) neurons, but increases total I(K(Ca)) in adjacent (L4) DRG neurons. All I(K(Ca)) subtypes are decreased by axotomy, but iberiotoxin-sensitive and clotrimazole-sensitive current densities are increased in adjacent L4 neurons after SNL. In an additional set of experiments we found that small-sized control DRG neurons also expressed iberiotoxin-sensitive currents, which are reduced in both axotomized (L5) and adjacent (L4) neurons.

CONCLUSIONS

Axotomy decreases I(K(Ca)) due to a direct effect on K((Ca)) channels. Axotomy-induced loss of I(Ca) may further potentiate current reduction. This reduction in I(K(Ca)) may contribute to elevated excitability after axotomy. Adjacent neurons (L4 after SNL) exhibit increased I(K(Ca)) current.

ATP-sensitive potassium currents reduce the PGE2-mediated enhancement of excitability in adult rat sensory neurons

Behavioral studies have shown that the hyperalgesia arising from inflammatory agents, such as prostaglandin E(2) (PGE(2)), can be antagonized by activators of the ATP-sensitive potassium current (K(ATP)). This observation raises questions as to whether this suppression results from a direct action on sensory neurons and what are the cellular mechanisms giving rise to this inhibition. We found that small to medium diameter sensory neurons isolated from the L4-6 DRGs expressed the mRNAs for Kir6.1, Kir6.2, and SUR1. In perforated-patch clamp recordings from acutely dissociated sensory neurons from the young adult rat, exposure to 300 microM diazoxide, a K(ATP) channel agonist, significantly hyperpolarized the resting membrane potential, reduced the number of action potentials evoked by a ramp of depolarizing current, and increased the amplitude of inward K(ATP) currents evoked by the voltage ramp. Similar results were obtained with the protonophore FCCP, which is known to reduce the levels of intracellular ATP and lead to the activation of K(ATP). Only a subpopulation of sensory neurons was sensitive to diazoxide whereas other neurons were unaffected. Treatment with 1 microM PGE(2) significantly enhanced the excitability of these small to medium diameter capsaicin-sensitive sensory neurons; this enhancement was reversed by subsequent exposure to diazoxide in a subpopulation of neurons. Similar to diazoxide, exposure to 8-Br-cyclic GMP antagonized the PGE(2)-induced increase in excitability. The effects of 8-Br-cyclic GMP could be reversed by exposure to glibenclamide, an antagonist of K(ATP) channels. As with diazoxide, only a subpopulation of sensory neurons were affected by 8-Br-cyclic GMP. These results demonstrate that activation of K(ATP) can reverse the sensitization produced by PGE(2) and may be an important means to modulate the enhanced excitability that results from inflammatory or injury conditions.

The effect of combined administration of ethanol and gabapentin on rabbit electroencephalographic activity

The central effect of ethanol is mainly connected with the effect on GABAergic, glutamatergic, serotonergic and opioid transmission. The mechanism of gabapentin effect suggests that it may alleviate the rewarding effect of ethanol, which may be used in the treatment of addiction. We decided to examine the interaction of ethanol with gabapentin by a pharmaco-electroencephalographic (EEG) method. The influence of gabapentin on the effect of ethanol on EEG of rabbits (midbrain reticular formation, hippocampus, frontal cortex) was tested. Gabapentin was administered at a single dose (25 and 100 mg/kg orally) or repeatedly twice a day at a total dose of 25 mg/kg for 14 days. Ethanol was injected at a dose of 0.8 g/kg 60 min. after gabapentin treatment. Ethanol caused an increase in the slow frequencies (0.5-4 Hz) in the recording, as well as a marked decrease of the fastest frequencies (13-30 and 30-45 Hz). Gabapentin lead to changes in rabbit EEG recording suggesting an depressant effect on the CNS (increase of slow and decrease of fast frequencies). The effects were less pronounced after repeated doses, which may indicate adaptative changes in the receptors. Gabapentin administered both in a single dose and for 7 days markedly enhanced the effect of ethanol on EEG recordings in rabbits. Repeated doses of gabapentin decrease the sensitivity of the hippocampus to the effect of ethanol.

Antiallodynic effect of etidronate, a bisphosphonate, in rats with adjuvant-induced arthritis: involvement of ATP-sensitive K+ channels

Bisphosphonates, pyrophosphate analogues, known as inhibitors of bone resorption, appear to cause analgesia in certain clinical painful situations. To detect clinically relevant analgesic property of etidronate, a non-aminobisphosphonate, we examined and characterized its antiallodynic effect in the rat with adjuvant-induced arthritis, in comparison with alendronate, an aminobisphosphonate, as determined by the von Frey test. Repeated systemic administration of etidronate at 10-40 mg/kg/day suppressed the adjuvant-induced mechanical allodynia in rat hindpaw, an effect reaching a plateau in approximately 10 days. Systemic or intraplantar (i.pl.) administration of ATP-sensitive K+ (K+ ATP) channel inhibitors, glibenclamide and/or tolbutamide, completely reversed the antiallodynic effect of etidronate within 1h in the arthritic rats, without affecting the nociceptive scores in naïve or arthritic animals that had not received etidronate. Alendronate, administered repeatedly, also revealed similar glibenclamide-reversible antiallodynic effect. In contrast, the antiallodynic effect of repeated systemic indomethacin was resistant to i.pl. glibenclamide in the arthritic rats. Repeated administration of etidronate or alendronate only slightly attenuated the adjuvant-evoked hindpaw edema. Among K+ ATP channel subunits, mRNAs for Kir6.1, SUR1, SUR2A and SUR2B were abundant in rat dorsal root ganglia, while Kir6.2 mRNA was poor. Our data demonstrate that repeated etidronate as well as alendronate exhibits antiallodynic activity in arthritic rats, which might be clinically relevant, and suggest involvement of K+ ATP channels in the underlying mechanisms.

Mechanisms of the antinociceptive action of gabapentin

Gabapentin, a gamma-aminobutyric acid (GABA) analogue anticonvulsant, is also an effective analgesic agent in neuropathic and inflammatory, but not acute, pain systemically and intrathecally. Other clinical indications such as anxiety, bipolar disorder, and hot flashes have also been proposed. Since gabapentin was developed, several hypotheses had been proposed for its action mechanisms. They include selectively activating the heterodimeric GABA(B) receptors consisting of GABA(B1a) and GABA(B2) subunits, selectively enhancing the NMDA current at GABAergic interneurons, or blocking AMPA-receptor-mediated transmission in the spinal cord, binding to the L-alpha-amino acid transporter, activating ATP-sensitive K(+) channels, activating hyperpolarization-activated cation channels, and modulating Ca(2+) current by selectively binding to the specific binding site of [(3)H]gabapentin, the alpha(2)delta subunit of voltage-dependent Ca(2+) channels. Different mechanisms might be involved in different therapeutic actions of gabapentin. In this review, we summarized the recent progress in the findings proposed for the antinociceptive action mechanisms of gabapentin and suggest that the alpha(2)delta subunit of spinal N-type Ca(2+) channels is very likely the analgesic action target of gabapentin.

The antiallodynic action target of intrathecal gabapentin: Ca2+ channels, KATP channels or N-methyl-d-aspartic acid receptors?

Gabapentin is a novel analgesic whose mechanism of action is not known. We investigated in a postoperative pain model whether adenosine triphosphate (ATP)-sensitive K+ (K(ATP)) channels, N-methyl-d-aspartic acid (NMDA) receptors, and Ca2+ channels are involved in the antiallodynic effect of intrathecal gabapentin. Mechanical allodynia was induced by a paw incision in isoflurane-anesthetized rats. Withdrawal thresholds to von Frey filament stimulation near the incision site were measured before and after incision and after intrathecal drug administration. The antiallodynic effect of gabapentin (100 mug) was not affected by intrathecal pretreatment with antagonists of K(ATP) channels, NMDA receptors or gamma-aminobutyric acid (GABA)(A) receptors. K(ATP) channel openers and GABA(A) receptor agonist, per se, had little effect on the postincision allodynic response. The Ca2+ channel blocker of N-type (omega-conotoxin GVIA, 0.1-3 microg), but not of P/Q-type (omega-agatoxin IVA), L-type (verapamil, diltiazem or nimodipine), or T-type (mibefradil), attenuated the incision-induced allodynia, as did gabapentin. Both the antiallodynic effects of gabapentin and omega-conotoxin GVIA were attenuated by Bay K 8644, an L-type Ca2+ channel activator. These results provide correlative evidence to support the contention that N-type Ca2+ channels, but not K(ATP) channels or NMDA or GABA(A) receptors, might be involved in the antiallodynic effect of intrathecal gabapentin.

Local application of volatile anesthetics attenuates the response to a mechanical stimulus in humans

PURPOSE

Volatile anesthetics may cause local hyperalgesia and/or analgesia. This double-blind randomized study investigated the effect of these medications when applied locally on the response to a mechanical stimulus.

METHODS

In experiment 1, standard commercial preparations of halothane 1 mL, isoflurane 1.5 mL and sevoflurane 2.7 mL were randomly applied on the forearm of 30 volunteers for 30 min, after which the response to a mechanical stimulus was recorded. The other forearm received water as control. The next day, the experiment for each anesthetic was repeated in a reverse fashion. Thirty minutes after the application, the response to a standardized mechanical stimulus was recorded. In experiments 2 and 3, the response to the same mechanical stimulus was tested after local applications of 2, 4, and 6 mL of halothane or after a local application of 5 mL sevoflurane respectively.

RESULTS

Low doses of the three anesthetics did not alter the response to the mechanical stimulus (F = 3.055, df = 1,174, P = 0.082). Two, 4, and 6 mL of halothane attenuated the response to the mechanical stimulus by 36%, 27% and 29% respectively (F = 9.586, df = 1,114, P = 0.002). Five millilitres of sevoflurane attenuated the response to the mechanical stimulus by 36% (F = 5.111, df = 19, P < 0.001).

CONCLUSION

Low liquid volumes of volatile anesthetics, when applied locally to the skin, did not influence the response to a mechanical stimulus, but higher volumes attenuated this response.

Non-stimulated Ca2+ leak pathway in cerebellar granule neurones

The aim of this study was to investigate the pathways of calcium influx routes in non-stimulated cerebellar granule neurones by use of standard microspectrofluorimetric techniques. Repetitive application of Ca2+-free solutions for various time intervals induced decreases of resting cytosolic free Ca2+ concentration ([Ca2+]i) which were followed, on Ca2+ readmission, by a full recovery, always to the initial resting [Ca2+]i levels. Use of drugs to deplete calcium stores (thapsigargin, alone or combined with low levels of ionomycin) did not cause release of Ca2+ from the intracellular stores nor enhanced the activity of the Ca2+ entry pathway. This influx was mainly independent of voltage operated calcium channels, since both L-type channel blockers (nitrendipine) and the hyperpolarizing agent pinacidil (a K+-channel opener) were without effect. Contribution from glutamate receptors to this influx was eliminated since a combination of blockers of NMDA and AMPA glutamate receptors (NBQX and D-AP5) did not affect the properties of the Ca2+ response. The Ca2+ leak pathway was sensitive to micromolar levels of lanthanum and gadolinium, and to the compound 2-APB, features shared by several channels of the TRP superfamily. In summary, our results show the presence of a Ca2+ permeable pathway, active and patent in resting conditions in cerebellar granule neurones, and which is different from the voltage-operated calcium channels and not operated by depletion of the stores.

Modulation of Dural Nociceptor Mechanosensitivity by the Nitric Oxide-Cyclic GMP Signaling Cascade

The role of the nitric oxide (NO)-cGMP signaling cascade in modulation of peripheral nociception is controversial. Although behavioral studies have suggested both pro- and anti-nociceptive effects, little is known about the direct action of this signaling cascade on primary afferent nociceptive neurons that mediate these behaviors. Here, using single-unit recordings, we examined the direct effect of NO-cGMP signaling on spontaneous activity and mechanical responses of nociceptive afferents that innervate the dura mater. We found that the NO donor sodium nitroprusside (SNP), when applied topically to the neuronal receptive field, induced both sensitization and inhibition of the mechanical responses, albeit in different populations of neurons, which could be distinguished based on their baseline mechanical thresholds. SNP, however, did not change the level of spontaneous activity. Administration of the cGMP analogue 8-pCPT-cGMP mimicked only the inhibitory effect. When SNP was co-applied with either an inhibitor of guanylyl cyclase or a cGMP blocker, sensitization never occurred, and the inhibitory effect of SNP could also be blocked. Our findings suggest that NO can either increase or decrease the mechanical responsiveness of nociceptors and that its action might depend, in part, on the baseline level of neuronal excitability. Our results also implicate cGMP in mediating the inhibitory effect of NO.