alpha2-adrenoceptors inhibit the intracellular Ca2+ response to electrical stimulation in normal and injured sensory neurons, with increased inhibition of calcitonin gene-related peptide expressing neurons after injury

Tail nerve electrical stimulation promoted the efficiency of transplanted spinal cord-like tissue as a neuronal relay to repair the motor function of rats with transected spinal cord injury

Following transected spinal cord injury (SCI), there is a critical need to restore nerve conduction at the injury site and activate the silent neural circuits caudal to the injury to promote the recovery of voluntary movement. In this study, we generated a rat model of SCI, constructed neural stem cell (NSC)-derived spinal cord-like tissue (SCLT), and evaluated its ability to replace injured spinal cord and repair nerve conduction in the spinal cord as a neuronal relay. The lumbosacral spinal cord was further activated with tail nerve electrical stimulation (TNES) as a synergistic electrical stimulation to better receive the neural information transmitted by the SCLT. Next, we investigated the neuromodulatory mechanism underlying the action of TNES and its synergism with SCLT in SCI repair. TNES promoted the regeneration and remyelination of axons and increased the proportion of glutamatergic neurons in SCLT to transmit brain-derived neural information more efficiently to the caudal spinal cord. TNES also increased the innervation of motor neurons to hindlimb muscle and improved the microenvironment of muscle tissue, resulting in effective prevention of hindlimb muscle atrophy and enhanced muscle mitochondrial energy metabolism. Tracing of the neural circuits of the sciatic nerve and tail nerve identified the mechanisms responsible for the synergistic effects of SCLT transplantation and TNES in activating central pattern generator (CPG) neural circuits and promoting voluntary motor function recovery in rats. The combination of SCLT and TNES is expected to provide a new breakthrough for patients with SCI to restore voluntary movement and control their muscles.

Effect of sympathetic sprouting on the excitability of dorsal root ganglion neurons and afferents in a rat model of neuropathic pain

Increased sympathetic nerve excitability has been reported to aggravate a variety of chronic pain conditions, and an increase in the number of sympathetic nerve fibers in the dorsal root ganglion (DRG) has been found in neuropathic pain (NP) models. However, the mechanism of the neurotransmitter norepinephrine (NE) released by sympathetic nerve fiber endings on the excitability of DRG neurons is still controversial, and the adrenergic receptor subtypes involved in this biological process are also controversial. In our study, we have two objectives: (1) To determine the effect of the neurotransmitter NE on the excitability of different neurons in DRG; (2) To determine which adrenergic receptors are involved in the excitability of DRG neurons by NE released by sprouting sympathetic fibers. In this experiment, a unique field potential recording method of spinal cord dorsal horn was innovatively adopted, which can be used for electrophysiological study in vivo. The results showed that： Forty days after SNI, patch clamp and field potential recording methods confirmed that NE enhanced the excitability of ipsilateral DRG large neurons, and then our in vivo electrophysiological results showed that the α₂ receptor blocker Yohimbine could block the excitatory effect of NE on A-fiber and the inhibitory effect on C-fiber, while the α_2A-adrenergic receptor agonist guanfacine (100 μM) had the same biological effect as NE. Finally, we concluded that NE from sympathetic fiber endings is involved in the regulation of pain signaling by acting on α_2A-adrenergic receptors in DRG.

Implantable, Programmable, and Wireless Device for Electrical Stimulation of the Dorsal Root Ganglion in Freely-Moving Rats: A Proof of Concept Study

Objective

This was a proof of concept study, based on systematic reviews of the efficacy and safety of the dorsal root ganglion (DRG) stimulation. The main objective was to develop an implantable, programmable, and wireless device for electrical stimulation of DRG and a methodology that can be used in translational research, especially to understand the mechanism of neuromodulation and to test new treatment modalities in animal models of pain.

Methods

We developed and tested a stimulator that uses a battery-powered microelectronic circuit, to generate constant current square biphasic or monophasic pulsed waveform of variable amplitudes and duration. It is controlled by software and an external controller that allows radio frequency communication with the stimulator. The stimulator was implanted in Sprague–Dawley (SD) rats. The lead was positioned at the L5 DRG level, while the stimulator was placed in the skin pocket at the ipsilateral side. Forty-five animals were used and divided into six groups: spinal nerve ligation (SNL), chronic compression injury of the DRG (CCD), SNL + active DRG stimulation, intact control group, group with the implanted sham stimulator, and sham lead. Behavioral testing was performed on the day preceding surgery and three times postoperatively (1st, 3rd, and 7th day).

Results

In animals with SNL, neurostimulation reduced pain-related behavior, tested with pinprick hyperalgesia, pinprick withdrawal test, and cold test, while the leads per se did not cause DRG compression. The rats well tolerated the stimulator. It did not hinder animal movement, and it enabled the animals to be housed under regular conditions.

Conclusion

A proof-of-concept experiment with our stimulator verified the usability of the device. The stimulator enables a wide range of research applications from adjusting stimulation parameters for different pain conditions, studying new stimulation methods with different frequencies and waveforms to obtain knowledge about analgesic mechanisms of DRG stimulation.

Spinal α2 -adrenoceptors and neuropathic pain modulation; therapeutic target

Neuropathic pain can arise from disease or damage to the nervous system. The most common symptoms of neuropathic pain include spontaneous pain, allodynia, and hyperalgesia. There is still limited knowledge about the factors that initiate and maintain neuropathic pain. However, ample evidence has proved the antinociceptive role of spinal α-adrenoceptors following nerve injury. It is well-documented that noradrenergic descending pathways from supraspinal loci exert an inhibitory influence on the spinal cord nociceptive neurons, mostly through the activation of spinal α₂ -adrenoceptors. This, in turn, suppresses transmission of pain input and the hyperexcitability of spinal dorsal horn neurons. There is considerable evidence demonstrating that spinal application of α₂ -adrenoceptor agonists leads to analgesic effects in animal models of neuropathic pain. Today, despite the recent rapid development of neuroscience and drug discovery, effective drugs with clear basic mechanisms have remained a mystery. Here, we give an overview of the cellular mechanisms through which brainstem adrenergic descending inhibitory processing can alter spinal pain transmission to the higher centres, and how these pathways change in neuropathic pain conditions focusing on the role of spinal α₂ -adrenoceptors in the spinal dorsal horn. We then suggest that α₂ -adrenoceptor agonist may be useful to treat neuropathic pain. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Resveratrol-induced antinociception is involved in calcium channels and calcium/caffeine-sensitive pools

Resveratrol has been widely investigated for its potential health properties, although little is known about its mechanism in vivo. Previous studies have indicated that resveratrol produces antinociceptive effects in mice. Calcium channels and calcium/caffeine-sensitive pools are reported to be associated with analgesic effect. The present study was to explore the involvement of Ca²⁺ channel and calcium/caffeine-sensitive pools in the antinociceptive response of resveratrol. Tail-flick test was used to assess antinociception in mice treated with resveratrol or the combinations of resveratrol with MK 801, nimodipine, CaCl₂, ryanodine and ethylene glycol tetraacetic acid (EGTA), respectively. The Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and brain-derived neurotrophic factor (BDNF) levels in the spinal cord were also investigated when treated with the above drugs. The results showed that resveratrol increased the tail flick latency in the tail-flick test, in dose-dependent manner. N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK 801 potentiated the antinociceptive effects of sub-threshold dose of resveratrol at 10 mg/kg. Ca²⁺ channel blocker, however, abolished the antinociceptive effects of resveratrol. In contrast to these results, EGTA or ryanodine treatment (i.c.v.) potentiated resveratrol-induced antinociception. There was a significant decrease in p-CaMKII and an increase in BDNF expression in the spinal cord when combined with MK 801, nimodipine, ryanodine and EGTA. While an increase in p-CaMKII level and a decrease in BDNF expression were observed when high dose of resveratrol combined with CaCl₂. These findings suggest that resveratrol exhibits the antinociceptive effects by inhibition of calcium channels and calcium/caffeine-sensitive pools.

Analgesic Mechanisms of Antidepressants for Neuropathic Pain

Tricyclic antidepressants and serotonin noradrenaline reuptake inhibitors are used to treat chronic pain, such as neuropathic pain. Why antidepressants are effective for treatment of neuropathic pain and the precise mechanisms underlying their effects, however, remain unclear. The inhibitory effects of these antidepressants for neuropathic pain manifest more quickly than their antidepressive effects, suggesting different modes of action. Recent studies of animal models of neuropathic pain revealed that noradrenaline is extremely important for the inhibition of neuropathic pain. First, increasing noradrenaline in the spinal cord by reuptake inhibition directly inhibits neuropathic pain through α₂-adrenergic receptors. Second, increasing noradrenaline acts on the locus coeruleus and improves the function of an impaired descending noradrenergic inhibitory system. Serotonin and dopamine may reinforce the noradrenergic effects to inhibit neuropathic pain. The mechanisms of neuropathic pain inhibition by antidepressants based mainly on experimental findings from animal models of neuropathic pain are discussed in this review.

Electrical Stimulation of Dorsal Root Ganglion in the Context of Pain: A Systematic Review of In Vitro and In Vivo Animal Model Studies

OBJECTIVE

Dorsal root ganglion (DRG) has recently emerged as an attractive target for neuromodulation therapy since primary sensory neurons and their soma in DRGs are important sites for pathophysiologic changes that lead to neuropathic pain. Our aim was to create evidence synthesis about the effects of electrical stimulation of DRG in the context of pain from in vitro and in vivo animal models, analyze methodology and quality of studies in the field.

METHODS

For conducting systematic review we searched three data bases: MEDLINE, Embase and Web of Science. The quality of included studies was assessed with the Systematic Review Centre for Laboratory Animal Experimentation risk of bias tool for animal studies. The study was registered in the Collaborative Approach to Meta-Analysis and Review of Animal Data from Experimental Studies data base.

RESULTS

We included six in vitro and eight in vivo animal studies. All included in vitro studies combined neurostimulation with substances or drugs and reported an improvement in pain-related parameters due to neurostimulation. Among in vivo studies, six used pulsed radiofrequency, while two used electrical field stimulation. All in vivo studies reported improvement in pain-related behavior following stimulation. Meta-analysis was not possible because of heterogeneity and missing data. The quality of included studies was suboptimal since all had an unclear risk of bias in multiple domains.

CONCLUSIONS

Limited data from in vitro and in vivo animal studies indicate that electrical stimulation of DRG has a positive therapeutic effect in the context of pain-related outcomes. Further studies with a standardized methodological approach and outcomes will provide useful information about electrical stimulation of DRG in animal models.

The analgesic effect of trans-resveratrol is regulated by calcium channels in the hippocampus of mice

Resveratrol has been widely studied in terms of it's potential to slow the progression of many diseases. But little is known about the mechanism of action in neuropathic pain. Neuropathic pain is the main type of chronic pain associated with tissue injury. Calcium channels and calcium/caffeine-sensitive pools are associated with analgesic pathway involving neuropathic pain. Our previous study suggested that the antinociceptive effect of resveratrol was involved in Ca²⁺/calmodulin-dependent signaling in the spinal cord of mice. The aim of this study was to explore the involvement of Ca²⁺ in analgesic effects of trans-resveratrol in neuropathic pain and signal pathway in hippocampus. Hot plate test was used to assess antinociceptive response when mice were treated with trans-resveratrol alone or in combination with Mk 801, nimodipine, CaCl₂, ryanodine or EGTA. The effects of trans-resveratrol and the combination on Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and BDNF (brain-derived neurotrophic factor) expression in hippocampus were also investigated. The results showed that trans-resveratrol increased paw withdraw latency in the hot plate test. The effect of resveratrol was enhanced by Mk 801 and nimodipine. Central administration of Ca²⁺, however, abolished the antinociceptive effects of resveratrol. In contrast, centrally administered EGTA or ryanodine improved trans-resveratrol induced antinociception. There was a significant increase in p-CaMKII and BDNF expression in the hippocampus when resveratrol were combined with Mk 801, nimodipine, ryanodine and EGTA. Administration of CaCl₂ blocked changes in p-CaMKII and BDNF levels in the hippocampus. These findings suggest that trans-resveratrol exerts the effects of antinociception through regulation of calcium channels and calcium/caffeine-sensitive pools.

The Role of Regulatory Transporters in Neuropathic Pain

Neuropathic pain arises from an injury or disease of the somatosensory nervous system rather than stimulation of pain receptors. As a result, the fine balance between excitation and inhibition is perturbed leading to hyperalgesia and allodynia. Various neuropathic pain models provide considerable evidence that changes in the glutamatergic, GABAergic, and monoaminergic systems. Neurotransmitter reuptake transporter proteins have the potential to change the temporal and spatial profile of various neurotransmitters throughout the nervous system. This, in turn, can affect the downstream effects of these neurotransmitters and hence modulate pain. This chapter explores various reuptake transporter systems and implicates their role in pain processing. Understanding the transporter systems will enhance drug discovery targeting different facets of neuropathic pain.

The antihyperalgesic effects of intrathecal bupropion, a dopamine and noradrenaline reuptake inhibitor, in a rat model of neuropathic pain

BACKGROUND

Antidepressants are often used for the treatment of neuropathic pain, and their analgesic effects rely on increased noradrenaline and serotonin levels in the spinal cord. Clinical studies have also shown that bupropion, a dopamine and noradrenaline reuptake inhibitor, has strong efficacy in neuropathic pain; however, the role of spinal cord dopamine in neuropathic pain is unknown. We hypothesized that bupropion inhibits neuropathic pain by increasing noradrenaline and dopamine in the spinal cord. In the present study, we determined the efficacy and underlying mechanisms of intrathecal administration of bupropion in a rat model of neuropathic pain.

METHODS

Male Sprague-Dawley rats were anesthetized, and right L5 spinal nerve ligation (SNL) was performed to produce mechanical hyperalgesia of the hindpaw. Withdrawal threshold to a paw pressure test was measured before and after intrathecal administration of bupropion, without or with intrathecal antagonists for α2-adrenoceptors and dopamine D2 receptors. In vivo microdialysis was performed in the dorsal horn of the lumbar spinal cord to measure noradrenaline and dopamine concentrations after intrathecal injection of bupropion. We also measured the noradrenaline and dopamine contents in the ipsilateral dorsal lumbar spinal cord in normal rats and in rats 2, 3, and 4 weeks after SNL.

RESULTS

Intrathecal injection of bupropion produced a dose-dependent antihyperalgesic effect (3, 10, 30, and 100 μg, P < 0.001). The effect (30 μg) was dose-dependently reversed by intrathecal pretreatment (15 minutes before bupropion injection) with the α2-adrenoceptor antagonist idazoxan (3, 10, and 30 μg, P < 0.001) and D2 receptor antagonist sulpiride (3, 10, and 30 μg, P < 0.001). Microdialysis revealed that noradrenaline and dopamine concentrations in the spinal dorsal horn were increased after intrathecal injection of bupropion (30 μg, P < 0.001 and P = 0.001, respectively). Furthermore, the noradrenaline and dopamine contents in the spinal dorsal horn were increased 2 weeks after SNL (P < 0.001 and P = 0.044, respectively) and then decreased gradually.

CONCLUSIONS

These findings suggest that plasticity of descending inhibitory pathways such as the noradrenaline and dopamine systems contributes to the maintenance of neuropathic pain and that spinal cord noradrenaline and dopamine both play an inhibitory role in neuropathic pain.

Alpha-adrenoceptor modulation in central nervous system trauma: pain, spasms, and paralysis--an unlucky triad

Many researchers have attempted to pharmacologically modulate the adrenergic system to control locomotion, pain, and spasms after central nervous system (CNS) trauma, although such efforts have led to conflicting results. Despite this, multiple studies highlight that α-adrenoceptors (α-ARs) are promising therapeutic targets because in the CNS, they are involved in reactivity to stressors and regulation of locomotion, pain, and spasms. These functions can be activated by direct modulation of these receptors on neuronal networks in the brain and the spinal cord. In addition, these multifunctional receptors are also broadly expressed on immune cells. This suggests that they might play a key role in modulating immunological responses, which may be crucial in treating spinal cord injury and traumatic brain injury as both diseases are characterized by a strong inflammatory component. Reducing the proinflammatory response will create a more permissive environment for axon regeneration and may support neuromodulation in combination therapies. However, pharmacological interventions are hindered by adrenergic system complexity and the even more complicated anatomical and physiological changes in the CNS after trauma. This review is the first concise overview of the pros and cons of α-AR modulation in the context of CNS trauma.

Differential expression of alpha-adrenoceptor subtypes in rat dorsal root ganglion after chronic constriction injury

mRNAs of alpha-adrenoceptor (α-AR) subtypes are found in neurons in dorsal root ganglion (DRG) and change after peripheral nerve injury. In this study, the distribution of α-AR subtype proteins was studied in L5 DRG of normal rats and rats with chronic constriction injury of sciatic nerve (CCI). Using immunofluorescence technique, it was found that α1A-, α1B-, and α2A-AR proteins were expressed in large, medium, and small size neurons in normal DRG, and significantly increased in all size neurons 14 days after CCI. α1D- and α2C-AR was also expressed in all size neurons in normal DRG. However, α1D-AR was significantly increased and α2C-AR was decreased in small size neurons 14 days post CCI. α2B-AR neurons were not detectable in normal and CCI DRG. Co-expression of α1A- and α2A-AR in the same neuron was observed in normal DRG and increased post CCI. Collectively, these results indicated that there is distinct distribution of α-AR subtypes in DRG neurons, and the distribution and levels of expression of α-AR subtypes change differently after CCI. The up-regulation of α-AR subtypes in DRG neurons may play an important role in the process of generating and transmitting neuropathic pain.

Highlights in opioid agonists and antagonists

This review highlights new insights in to opioid agonists and antagonists, focusing on their mechanism of action with spinal and systemic administration, chronic use and main adverse effects. Short-cuts on some opioid agonists and antagonists of clinical interest are also presented, revealing potential clinical implications and future clinical directions as part of multimodal analgesia.

The role of alpha-2 adrenoceptor subtype in the antiallodynic effect of intraplantar dexmedetomidine in a rat spinal nerve ligation model

The purpose of this study was to examine the effects of intraplantar dexmedetomidine to relieve neuropathic pain and determine the role of peripheral α2-adrenoceptors. Neuropathic pain was induced by ligating the L5 and L6 spinal nerves in male Sprague-Dawley rats, and mechanical allodynia was assessed using von Frey filaments. Several antagonists were injected into the hindpaws to evaluate the mechanisms of action of dexmedetomidine, a nonselective α2-adrenoceptor antagonist yohimbine, an α2A-adrenoceptor antagonist BRL 44408, an α2B-adrenoceptor antagonist ARC 239, and a α2C-adrenoceptor antagonist JP 1302. The expression of α2A-adrenoceptor, α2B-adrenoceptor, and α2C-adrenoceptor genes in the lumbar segment of the spinal cord and the plantar skin of the nerve-injured leg was detected by reverse transcription-polymerase chain reaction. Ipsilateral intraplantar injection of dexmedetomidine produced dose-dependent antiallodynia. Ipsilateral, but not contraleral, intraplantar injection of yohimbine reversed the antinociception of dexmedetomidine. Intraplantar BRL 44408, ARC 239, and JP 1302 reversed the antinociception of dexmedetomidine. The expression levels of α2-adrenoceptor genes in the lumbar spinal cord did not differ between rats with neuropathic pain and naïve rats. The expression levels of α2B-adrenoceptor and α2C-adrenoceptor genes of plantar skin were upregulated significantly in the model group, whereas α2A-adrenoceptor expression was unchanged. These results suggest that intraplantar injection of dexmedetomidine produced an antiallodynic effect in spinal nerve ligation-induced neuropathic pain. All three types of peripheral α2A, α2B, and α2C-adrenoceptors were involved in the antiallodynic mechanism of dexmedetomidine.

Monoaminergic modulation of spinal viscero-sympathetic function in the neonatal mouse thoracic spinal cord

Descending serotonergic, noradrenergic, and dopaminergic systems project diffusely to sensory, motor and autonomic spinal cord regions. Using neonatal mice, this study examined monoaminergic modulation of visceral sensory input and sympathetic preganglionic output. Whole-cell recordings from sympathetic preganglionic neurons (SPNs) in spinal cord slice demonstrated that serotonin, noradrenaline, and dopamine modulated SPN excitability. Serotonin depolarized all, while noradrenaline and dopamine depolarized most SPNs. Serotonin and noradrenaline also increased SPN current-evoked firing frequency, while both increases and decreases were seen with dopamine. In an in vitro thoracolumbar spinal cord/sympathetic chain preparation, stimulation of splanchnic nerve visceral afferents evoked reflexes and subthreshold population synaptic potentials in thoracic ventral roots that were dose-dependently depressed by the monoamines. Visceral afferent stimulation also evoked bicuculline-sensitive dorsal root potentials thought to reflect presynaptic inhibition via primary afferent depolarization. These dorsal root potentials were likewise dose-dependently depressed by the monoamines. Concomitant monoaminergic depression of population afferent synaptic transmission recorded as dorsal horn field potentials was also seen. Collectively, serotonin, norepinephrine and dopamine were shown to exert broad and comparable modulatory regulation of viscero-sympathetic function. The general facilitation of SPN efferent excitability with simultaneous depression of visceral afferent-evoked motor output suggests that descending monoaminergic systems reconfigure spinal cord autonomic function away from visceral sensory influence. Coincident monoaminergic reductions in dorsal horn responses support a multifaceted modulatory shift in the encoding of spinal visceral afferent activity. Similar monoamine-induced changes have been observed for somatic sensorimotor function, suggesting an integrative modulatory response on spinal autonomic and somatic function.

An increase in spinal cord noradrenaline is a major contributor to the antihyperalgesic effect of antidepressants after peripheral nerve injury in the rat

Antidepressants are often used for the treatment of neuropathic pain. Clinical studies suggest that the efficacy of serotonin (5-HT) and noradrenaline (NA) reuptake inhibitors (SNRIs) for neuropathic pain is greater than that of selective 5-HT reuptake inhibitors (SSRIs). In the present study, we determined the efficacy and mechanisms involved in the antihyperalgesic effects of milnacipran, an SNRI, compared with paroxetine, an SSRI, and maprotiline, a selective NA reuptake inhibitor, using a rat model of neuropathic pain. Male Sprague-Dawley rats underwent spinal nerve ligation (SNL), and the withdrawal threshold to paw pressure was measured. Intraperitoneal injection of milnacipran (3-30mg/kg) produced a dose-dependent antihyperalgesic effect. The effect was reversed by intrathecal injection of the α(2)-adrenoceptor antagonist idazoxan (30μg), but not by various 5-HT receptor antagonists. Paroxetine produced an antihyperalgesic effect only at the highest dose tested (10mg/kg). This effect was reversed by intrathecal injection of both idazoxan and ondansetron (30μg), a 5-HT3 receptor antagonist. Maprotiline produced an antihyperalgesic effect (10 and 30mg/kg), and the effect was reversed by intrathecal idazoxan. In microdialysis studies, NA and 5-HT concentrations in the spinal dorsal horn were increased after injection of either milnacipran or paroxetine, and only NA was increased after maprotiline. Furthermore, the NA content in the spinal cord of SNL rats was greater than that in normal animals. These findings suggest that an increase in NA in the spinal cord plays an important role in the antihyperalgesic effects of not only NA reuptake inhibitors but also SSRIs.

Oxytocin inhibits the membrane depolarization-induced increase in intracellular calcium in capsaicin sensitive sensory neurons: a peripheral mechanism of analgesic action

BACKGROUND

Lumbar intrathecal injection of oxytocin produces antinociception in rats and analgesia in humans. Classically, oxytocin receptors couple to stimulatory G proteins, increase inositol-3-phosphate production, and result in neuronal excitation. Most work to date has focused on a spinal site of oxytocin to excite γ-aminobutyric acid interneurons to produce analgesia. Here we ask whether oxytocin might also affect primary sensory afferents by modulating high voltage-gated calcium channels, such as it does in the brain.

METHODS

Dorsal root ganglion cells from adult rats were acutely dissociated and cultured, and changes in intracellular calcium determined by fluorescent microscopy using an indicator dye. The effects of oxytocin alone and in the presence of transient depolarization from increased extracellular KCl concentration were determined, and the pharmacology of these effects were studied. Cells from injured dorsal root ganglion cells after spinal nerve ligation were also studied.

RESULTS

Oxytocin produced a concentration-dependent inhibition of the increase in intracellular calcium from membrane depolarization, an effect blocked more efficiently by oxytocin- than vasopressin-receptor selective antagonists. Oxytocin-induced inhibition was present in cells responding to capsaicin, and when internal stores of calcium were depleted with thapsigargin. Oxytocin produced similar inhibition in cells from animals with spinal nerve ligation.

CONCLUSIONS

These data suggest that oxytocin produces antinociception after intrathecal delivery in part by reducing excitatory neurotransmitter release from the central terminals of nociceptors.

Acute inhibition of Ca2+/calmodulin-dependent protein kinase II reverses experimental neuropathic pain in mice

The limited data that currently exist for the role of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in neuropathic pain are conflicting. In the present study, we tested the hypothesis that CaMKII is required for the maintenance of neuropathic pain in a rodent model of experimental mononeuropathy. Spinal nerve L(5)/L(6) ligation (SNL) was found to increase the spinal activity of CaMKII (pCaMKII) on the ipsilateral (but not contralateral) side. This effect was blocked by 2-[N-(2-hydroxyethyl)-N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine) (KN93) (intrathecal injection), a CaMKII inhibitor. Acute treatment with KN93 dose-dependently reversed SNL-induced thermal hyperalgesia and mechanical allodynia. The action of KN93 lasted for at least 2 to 4 h. 2-[N-(4-Methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine (KN92) (45 nmol i.t.), an inactive analog of KN93, showed no effect on SNL-induced CaMKII activation, allodynia, or hyperalgesia. We further examined the pharmacologic action of trifluoperazine, a clinically used antipsychotic drug that we found to be a potent CaMKII inhibitor in these assays. Trifluoperazine (administered intraperitoneally or by mouth) dose-dependently reversed SNL-induced mechanical allodynia, thermal hyperalgesia, and CaMKII activation without causing locomotor impairment in mice at the highest doses used. In conclusion, our findings support a critical role of CaMKII in neuropathic pain. Blocking CaMKII or CaMKII-mediated signaling may offer a novel therapeutic target for the treatment of neuropathic pain.

Gabapentin acts within the locus coeruleus to alleviate neuropathic pain

BACKGROUND

Gabapentin recruits descending inhibition to produce analgesia after nerve injury, but whether this is a local action in the brainstem is unknown. The authors hypothesized that gabapentin activates noradrenergic neurons in the locus coeruleus (LC) by a local action.

METHODS

Male rats underwent L5-L6 spinal nerve ligation (SNL) and received drugs by intra-LC or systemic routes for behavior testing, immunohistochemistry in the LC, and microdialysis in the spinal dorsal horn. In other studies, brainstem slices from normal and SNL animals were used for immunohistochemistry.

RESULTS

SNL increased phosphorylated cyclic adenosine monophosphate response element binding protein (pCREB)-expressing nuclei bilaterally in the LC, and increased noradrenaline release in the spinal dorsal horn. Gabapentin, whether in isolated brainstem slices or in conscious or anesthetized animals, increased pCREB-expressing nuclei in the LC. The net increase in pCREB expression by gabapentin did not differ between normal and SNL conditions. This gabapentin-induced pCREB activation in LC neurons was abolished by an AMPA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Intra-LC-injected gabapentin reduced hypersensitivity in SNL rats in a dose-dependent manner. Both intra-LC coadministration of CNQX and intrathecal administration of the alpha2-adrenoceptor antagonist idazoxan blocked antihypersensitivity by intra-LC gabapentin. Intravenous gabapentin induced noradrenaline release in the spinal dorsal horn. The net amount of noradrenaline release by gabapentin is larger in SNL rats compared with the normal condition, although the percentage increases from the baseline were the same.

CONCLUSIONS

These results suggest that gabapentin acts directly in the brainstem via a glutamate-dependent mechanism to stimulate descending inhibition to produce antihypersensitivity after peripheral nerve injury.

Effects of clonidine on lidocaine-induced inhibition of axonal transport in cultured mouse dorsal root ganglion neurones

BACKGROUND

The alpha(2)-adrenoceptor agonist clonidine is used in combination with lidocaine for anaesthesia. Lidocaine inhibits axonal transport and neurite growth, whereas alpha(2)-adrenoceptor agonists have neurotrophic effects. Here we have investigated whether clonidine reduces lidocaine-induced inhibition of axonal transport in cultured mouse dorsal root ganglion neurones.

METHODS

Axonal transport of organelles and neurite growth were assessed by video microscopy in cells treated with clonidine and lidocaine for 1 h. Stable responses were achieved within this period.

RESULTS

Clonidine (10 and 100 microM) increased and lidocaine (10, 100 microM, and 1 mM) decreased axonal transport. The inhibitory effects of lidocaine were reduced by simultaneous treatment with clonidine. The actions of clonidine were antagonized by the alpha(2)-adrenoceptor antagonist yohimbine. Since clonidine was reported to block N-type channels, we further investigated the role of ion channels in the antagonistic action of clonidine on the lidocaine response. The action of lidocaine on axonal transport was not mimicked by the Na+ channel blocker tetrodotoxin and not blocked by the Na+ channel activator veratridine. The action of lidocaine was not blocked by the L-type Ca2+ channel blocker nifedipine, but was blocked by the N-type channel blocker omega-conotoxin MVIIA. These effects on axonal transport correlated with the effects on neurite growth.

CONCLUSIONS

Inhibition of axonal transport induced by lidocaine, which may be mediated by N-type channel activation, can be blocked by clonidine. Clonidine may alleviate the effects of lidocaine on neuronal dysfunction.

Brain derived nerve growth factor induces spinal noradrenergic fiber sprouting and enhances clonidine analgesia following nerve injury in rats

Many treatments for neuropathic pain activate or augment norepinephrine release in the spinal cord, yet these treatments are less effective against acute nociceptive stimuli. We previously showed in mice that peripheral nerve injury results in sprouting of spinal noradrenergic fibers, possibly reflecting the substrate for this shift in drug efficacy. Here, we tested whether such sprouting also occurs in rats after nerve injury and examined one signal for such sprouting. Ligation of L5 and L6 spinal nerves unilaterally in rats resulted in hypersensitivity to tactile stimulation of the ipsilateral paw, and sprouting of noradrenergic fibers in the dorsal horn of the lumbar spinal cord. Brain derived nerve growth factor (BDNF) content increased in L4-L6 dorsal root ganglia ipsilateral to injury and in lumbar spinal cord following nerve injury, and intrathecal infusion of BDNF antiserum prevented spinal noradrenergic sprouting. This treatment also prevented the increased analgesic efficacy of intrathecal clonidine observed after nerve injury. Intraspinal injection of BDNF in non-injured rats mimicked the sprouting of spinal noradrenergic fibers seen after nerve injury. These results suggest that increased BDNF synthesis and release drives spinal noradrenergic sprouting following nerve injury, and that this sprouting may paradoxically increase the capacity for analgesia in the setting of neuropathic pain from drugs which utilize or mimic the noradrenergic pathway.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Inhibitory M2 muscarinic receptors are upregulated in both axotomized and intact small diameter dorsal root ganglion cells after peripheral nerve injury

Acetylcholine reduces nociceptive input in part by activating inhibitory M2 muscarinic receptors on primary sensory neurons, and acetylcholinesterase inhibitors and muscarinic agonists produce analgesia in humans and animals. M2 muscarinic receptors are upregulated in animals with diabetic neuropathy, but their level of expression and function after peripheral nerve injury has not been previously examined. This study tested, using intracellular Ca(2+) response to membrane depolarization, the effect of the M2 muscarinic receptor agonist bethanechol on individual dorsal root ganglion cells from normal and L5-6 spinal nerve-ligated rats, followed by M2 muscarinic receptor immunostaining. We also examined functional transient receptor potential for vanilloids-1 activity by determining intracellular Ca(2+) response evoked by capsaicin in M2 muscarinic receptor immunoreactive cells. In normal dorsal root ganglion cells, bethanechol inhibited the Ca(2+) response in a concentration-related fashion, and this inhibition was blocked by the M2 muscarinic receptor antagonist gallamine. Cells expressing M2 muscarinic receptors by immunostaining were significantly inhibited by bethanechol, whereas those lacking positive staining were not. The proportion of studied dorsal root ganglion neurons with positive M2 muscarinic receptor staining increased significantly in the injured ipsilateral L5-6 and the uninjured ipsilateral L4 ganglia, but not in the contralateral dorsal root ganglion neurons compared with normals. In contrast, the proportion of neurons responding to capsaicin significantly decreased in the injured ipsilateral L5-6 dorsal root ganglion cells. These results suggest that inhibitory M2 muscarinic receptors are upregulated in small- and medium-sized axotomized dorsal root ganglion neurons and their uninjured neighbors following nerve injury, and may represent an appropriate target for analgesia in this setting.