Perioperative administration of the alpha2-adrenoceptor agonist clonidine at the site of nerve injury reduces the development of mechanical hypersensitivity and modulates local cytokine expression

The development of chronic pain after surgery is not rare. Nerve injury from complete or partial nerve section during surgery leads to macrophage recruitment and release of pro-inflammatory cytokines, leading in turn to sensitization. Macrophages also express alpha2-adrenoceptors, and we previously demonstrated a prolonged reduction in hypersensitivity following peri-neural injection of the alpha2-adrenoceptor agonist, clonidine, in rats with chronic nerve injury. The current study tested whether peri-neural clonidine at the time of injury could also prevent development of hypersensitivity. Rats underwent partial ligation of one sciatic nerve, and peri-neural saline, clonidine or a combination of clonidine and the alpha2A-adrenceptor-preferring antagonist, BRL44408, were administered before wound closure and, in some animals, also 24 and 48 h later. The single clonidine injection reduced hypersensitivity for only 5 h, whereas repeated injection for three days reduced hypersensitivity for 28 days. Peri-neural clonidine reduced the increase in tissue content of the proinflammatory cytokines IL-1beta and particularly TNFalpha in sciatic nerve, DRG and spinal cord while increasing concentrations of the anti-inflammatory cytokine TGF-beta1. Clonidine's effects on behavior and TNFalpha content were blocked by BRL44408. We conclude that peri-neural administration of clonidine at the site and time of injury reduces the degree of hypersensitivity in part by altering the balance of pro- and anti-inflammatory cytokines through activation of alpha2A-adrenoceptors. These results support testing of whether clonidine, as an adjuvant in continuous peripheral nerve blocks in settings of known major nerve injury, such as limb amputation, might prevent the development of chronic pain.     

Overexpression of GDNF in the Uninjured DRG Exerts Analgesic Effects on Neuropathic Pain Following Segmental Spinal Nerve Ligation in Mice

Glial cell line-derived neurotrophic factor (GDNF), a survival-promoting factor for a subset of nociceptive small-diameter neurons, has been shown to exert analgesic effects on neuropathic pain. However, its detailed mechanisms of action are still unknown. In the present study, we investigated the site-specific analgesic effects of GDNF in the neuropathic pain state using lentiviral vector-mediated GDNF overexpression in mice with left fifth lumbar (L5) spinal nerve ligation (SNL) as a neuropathic pain model. A lentiviral vector expressing both GDNF and enhanced green fluorescent protein (EGFP) was constructed and injected into the left dorsal spinal cord, uninjured fourth lumbar (L4) dorsal root ganglion (DRG), injured L5 DRG, or plantar skin of mice. In SNL mice, injection of the GDNF-EGFP-expressing lentivirus into the dorsal spinal cord or uninjured L4 DRG partially but significantly reduced the mechanical allodynia in association with an increase in GDNF protein expression in each virus injection site, whereas injection into the injured L5 DRG or plantar skin had no effects. These results suggest that GDNF exerts its analgesic effects in the neuropathic pain state by acting on the central terminals of uninjured DRG neurons and/or on the spinal cells targeted by the uninjured DRG neurons.

Perspective

This article shows that GDNF exerts its analgesic effects on neuropathic pain by acting on the central terminals of uninjured DRG neurons and/or on the spinal cells targeted by these neurons. Therefore, research focusing on these GDNF-dependent neurons in the uninjured DRG would provide a new strategy for treating neuropathic pain.     

Bestrophin-1 Participates in Neuropathic Pain Induced by Spinal Nerve Transection but not Spinal Nerve Ligation

Previous studies have reported that L5/L6 spinal nerve ligation (SNL), but not L5 spinal nerve transection (SNT), enhances anoctamin-1 in injured and uninjured dorsal root ganglia (DRG) of rats suggesting some differences in function of the type of nerve injury. The role of bestrophin-1 in these conditions is unknown. The aim of this study was to investigate the role of bestrophin-1 in rats subjected to L5 SNT and L5/L6 SNL. SNT up-regulated bestrophin-1 protein expression in injured L5 and uninjured L4 DRG at day 7, whereas it enhanced GAP43 mainly in injured, but also in uninjured DRG. In contrast, SNL enhanced GAP43 at day 1 and 7, while bestrophin-1 expression increased only at day 1 after nerve injury. Accordingly, intrathecal injection of the bestrophin-1 blocker CaCC_inh-A01 (1-10 µg) reverted SNT- or SNL-induced tactile allodynia in a concentration-dependent manner. Intrathecal injection of CaCC_inh-A01 (10 µg) prevented SNT-induced upregulation of bestrophin-1 and GAP43 at day 7. In contrast, CaCC_inh-A01 did not affect SNL-induced up-regulation of GAP43 nor bestrophin-1. Bestrophin-1 was mainly expressed in small- and medium-size neurons in naïve rats, while SNT increased bestrophin-1 immunoreactivity in CGRP+, but not in IB4+ neuronal cells in DRG. Intrathecal injection of bestrophin-1 plasmid (pCMVBest) induced tactile allodynia and increased bestrophin-1 expression in DRG and spinal cord in naïve rats. CaCC_inh-A01 reversed bestrophin-1 overexpression-induced tactile allodynia and restored bestrophin-1 expression. Our data suggest that bestrophin-1 plays a relevant role in neuropathic pain induced by SNT, but not by SNL.PerspectiveSNT, but not SNL, up-regulates bestrophin-1 and GAP43 protein expression in injured L5 and uninjured L4 DRG. SNT increases bestrophin-1 immunoreactivity in CGRP+ neurons in DRG. Bestrophin-1 overexpression induces allodynia. CaCCinh-A01 reduces allodynia and restores bestrophin-1 expression. Our data suggest bestrophin-1 is differentially regulated depending on the neuropathic pain model.     

神经病理性痛大鼠背根神经节细胞电生理学特征的改变

目的:研究神经病理性痛大鼠背根神经节(dorsal root ganglion, DRG)细胞电生理特征的改变.方法:以单侧L5和L6脊神经结扎制备大鼠神经病理性痛模型,运用全细胞电流钳技术进行电生理记录.结果:引起损伤侧DGR中、小直径神经元兴奋的基强度降低,动作电位爆发阈值下降,小直径神经元的动作电位时程变宽,自发放电的细胞比率明显升高.结论:神经损伤诱导初级感觉神经元的兴奋性增强,这可能是引起动物神经病理性痛敏的原因之一.     

Alterations in neuropeptide Y, tyrosine hydroxylase, and Y-receptor subtype distribution following spinal nerve injury to rats

Recent animal models of experimental nerve injury have proven useful in evaluating potential sympathetic involvement in neuropathic pain syndromes. We have employed a widely adopted unilateral L5/L6 spinal nerve ligation model to compare the development of mechanical allodynia with neurochemical changes both at the site of peripheral nerve injury and in the dorsal root ganglia (DRG). We have focused on the expression of neuropeptide Y (NPY), a well-studied regulatory peptide and phenotypic marker of sympathetic neurons, and functionally related Y-receptor binding sites following nerve injury. In sympathetic neurons, NPY is colocalized and coreleased with norepinephrine (NE) at peripheral sites of action. Furthermore, NPY gene expression is induced within the population of medium- and large-diameter DRG neurons of the A beta-fiber class after experimental nerve injury. We therefore hypothesized that concurrent alterations in NPY and NE expression by sympathetic and sensory neurons may be a contributing factor to sympathetically-maintained neuropathic conditions. Animals with unilateral L5/L6 spinal nerve ligation developed mechanical allodynia of the hind paw ipsilateral to the site of injury that persisted until sacrifice at postoperative day 10. A significant induction of preproneuropeptide Y-encoding (PPNPY) mRNA, as detected by in situ hybridization histochemistry (ISHH), occurred in populations of medium- and large-diameter DRG neurons ipsilateral to the site of injury. Immunohistochemical analysis indicated a marked decline in the number of labeled sympathetic axons positive for tyrosine hydroxylase-like and NPY-like immunoreactivities (TH-LI and NPY-LI, respectively) proximal to the site of nerve injury and almost complete elimination of immunopositive fibers distal to the site of ligation. Whereas, the extent of colocalization of NPY-LI to TH-LI-positive sympathetic axons in unaffected L4 or L5 nerve segments exceeded 80%, this figure declined to approximately 50% in regenerating axons of ligated spinal nerve L5. The portion of NPY-LI that was not colocalized to sympathetic TH-LI-positive fibers was most likely contributed by regenerating sensory axons, consistent with marked de novo synthesis of NPY by DRG neurons. In end bulb axon terminals, i.e. morphological profiles characteristic of neuromas, NPY-LI-positive elements that were not colocalized to TH-LI-positive sympathetic elements appeared to be spatially segregated from those of sympathetic origin with colocalized TH-LI and NPY-LI. Receptor autoradiography indicated that small- and medium-diameter DRG somata of the C-fiber class normally express both Y1 and Y2 receptor subtypes. The pattern of the distribution of Y-receptor binding sites appeared to be relatively unaffected by spinal nerve ligation. In contrast, there was a marked increase in the density of Y2 receptor binding sites in the proximal segment of ligated spinal nerve L5, consistent with previously published data indicating differential transport of the Y2 autoregulatory receptor subtype to nerve terminals. Induction of NPY gene expression in injured DRG neurons is consistent with appearance of NPY-LI-positive end bulbs derived from regenerating sensory axons that are found in developing neuromas containing a relatively high density of transported prejunctional Y2 receptors. Newly established functional interactions of spatially segregated sensory- and sympathetically-derived end bulbs in developing neuromas may enhance neuronal hyperexcitability engendered by aberrant electrical activity at the site of injury. Injury-related alterations in the regulatory activities of NPY released within the DRG at somally-distributed Y-receptors may also contribute to the development and/or persistence of symptoms characteristic of sympathetically-maintained pain. Finally, at later times NPY-mediated modulation of NE release from invading sympathetic axon terminals within the DRG may affect the extent of alpha2 rece     

Induction of high mobility group box-1 in dorsal root ganglion contributes to pain hypersensitivity after peripheral nerve injury

Pro-inflammatory cytokine high mobility group box-1 (HMGB-1) is involved in inflammation in the central nervous system, but less is known about its biological effects in the peripheral nervous system. In the present study, the role of HMGB-1 in the primary afferent nerve was investigated in the context of the pathophysiology of peripheral nerve injury-induced pain hypersensitivity. Real-time PCR confirmed an increase in HMGB-1 mRNA expression in the dorsal root ganglion (DRG) and spinal nerve at 1 day after spinal nerve ligation (SNL). Induction of HMGB-1 mRNA was observed in both injured L5 and uninjured L4. Immunohistochemistry for HMGB-1 revealed that SNL-induced HMGB-1 expression in the primary afferent neurons and satellite glial cells (SGCs) in the DRG, and in Schwann cells in the spinal nerve. Up-regulation of HMGB-1 was associated with translocation of its signal from the nucleus to the cytoplasm. Injection of HMGB-1 into the sciatic nerve produces transient behavioural hyperalgesia. Neutralizing antibody against HMGB-1 successfully alleviated the mechanical allodynia observed after SNL treatment. Receptor for advanced glycation end products (RAGE), one of the major receptors for HMGB-1, was expressed in the primary afferent neurons and SGCs in the DRG, as well as in Schwann cells in the spinal nerve. These results indicate that HMGB-1 is synthesized and secreted into the DRG and spinal nerve, and contributes to the development of neuropathic pain after nerve injury. Blocking HMGB-1/RAGE signalling might thus be a promising therapeutic strategy for the management of neuropathic pain.     

Cannabinoid receptor CB2 localisation and agonist-mediated inhibition of capsaicin responses in human sensory neurons

Cannabinoid receptor 2 (CB2) agonists provide the potential for treating chronic pain states without CNS effects associated with CB1 receptor activation. Animal models suggest that they act mainly via non-neuronal cells, possibly inhibition of inflammatory cells in the periphery or CNS, or via release of beta-endorphin; however, the clinical relevance and mechanism of analgesic action is uncertain. Here, we demonstrate colocalisation of CB2 with CB1 and the capsaicin receptor TRPV1 in human dorsal root ganglion (DRG) sensory neurons and increased levels of CB2 receptors in human peripheral nerves after injury, particularly painful neuromas. In primary cultures of human DRG neurons, selective CB2 agonists blocked activation of inward cation currents and elevation of cytoplasmic Ca2+ in response to capsaicin. These inhibitory effects were reversed by GW818646X a CB2 antagonist, and 8-bromo cAMP, but not by SR141716 a CB1 antagonist, or naloxone. Thus CB2 receptor agonists functionally inhibited nociceptive signalling in human primary sensory neurons via a mechanism shared with opioids, of adenylyl cyclase inhibition, but not via mu-opioid receptors. We conclude that CB2 agonists deserve imminent clinical trials for nociceptive, inflammatory and neuropathic chronic pain, in which capsaicin or heat-activated responses via TRPV1 may provide a clinical marker.     

Spinal Nerve Ligation in Mouse Upregulates TRPV1 Heat Function in Injured IB4-Positive Nociceptors

Peripheral nerve injury leads to neuropathic pain, but the underlying mechanisms are not clear. The TRPV1 channel expressed by nociceptors is one receptor for noxious heat and inflammatory molecules. Lumbar 4 (L4) spinal nerve ligation (SNL) in mice induced persistent heat hyperalgesia 4 to 10 days after injury. The heat hypersensitivity was completely reversed by the TRPV1 antagonist A-425619. Furthermore, DRG neurons were isolated from the injured L4 ganglia or adjacent L3 ganglia 4 to 10 days after L4 SNL. Whole-cell patch-clamp recordings were performed and heat stimuli (22°C to 50°C/3 s) were applied to the soma. Neurons were classified by soma size and isolectin-B4 (IB4) binding. Among directly injured L4 neurons, SNL increased the percentage of small-diameter IB4-positive neurons that were heat-sensitive from 13% (naive controls) to 56% and conversely decreased the proportion of small IB4-negative neurons that were heat-sensitive from 66% (naive controls) to 34%. There was no change in IB4 binding in neurons from the injured ganglia. Surprisingly, in neurons from the adjacent L3 ganglia, SNL had no effect on the heat responsiveness of either IB4-positive or negative small neurons. Also, SNL had no effect on heat responses in medium-large–diameter neurons from either the injured or adjacent ganglia.PerspectiveTRPV1 function is upregulated in IB4-positive sensory neurons, and TRPV1 is responsible for the behavioral heat hypersensitivity in the spinal nerve ligation model. Because IB4-positive neurons may contribute to the emotional perception of pain, TRPV1 antagonists, targeting both sensory and affective pain components, could have broad analgesic effects.     

Antinozizeptive Wirkungen von α2-Adrenozeptoragonisten („analgetische“ Wirkungen im Tierversuch)(“analgetische” Wirkungen im Tierversuch)

alpha(2)-Adrenoceptor agonists like clonidine, dexmedetomidine, and ST-91, inhibit nociceptive reflex activity predominantly by a spinal mode of action. They mimic the action of the inhibitory transmitter noradrenaline, which is released from the terminals of bulbospinal monoaminergic pathways. The inhibition by noradrenaline is due partly to hyperpolarization of the postsynaptic neuronal membrane; however, the selective antinociceptive effect of the alpha(2)-adrenoceptor agonists results from reduction of the release of the excitatory transmitters such as glutamate and substance P, blockade of the binding of substance P to spinal neurones, and enhancement of the action of the inhibitory transmitter, 5-hydroxytryptamine. Clonidine and dexmedetomidine stimulate adrenoceptors of the alpha(2A) subtype, while ST-91 stimulates alpha(2B) adrenoceptors. Antinociception is manifested not only by depression of nociceptive reflexes and behaviour, but also by inhibition of the expression of immediate early genes in dorsal horn neurones following noxious stimulation. The inhibitory control from the brain stem of spinal nociceptive activity can be triggered by alpha(2)-adrenoceptor agonists. Moreover, impulse conduction in C fibres of peripheral nerves is far more reduced by these compounds than that in A fibres. Antinociceptive effects are reported to occur in various models of clinical pain, e.g. the formalin test, adjuvans-induced arthritis, autotomy following deafferentation, and "hyperalgesia" after nerve ligation. Therefore, the mechanisms involved in antinociception may also be responsible for the analgesia produced by alpha(2)-adrenoceptor agonists.     

Role of the endothelium on the response to adrenoceptor agonists of rabbit aorta during cooling

The role of the endothelium in the effects of cooling on the response to alpha1- and alpha2-adrenoceptor agonists of rabbit aorta was studied. The contractions induced by clonidine (10(-9)-3 x 10(-4) M) and xylazine (10(-7)-10(-3) M) but not phenylephrine (10(-9)-3 x 10(-4) M) and methoxamine (10(-9)-3 x 10(-4) M) were enhanced in endothelium-denuded or N(G)-nitro-L arginine methyl- ester (L-NAME) (10(-5) M) pretreated rabbit aorta. The sensitivity, but not the maximal response, of both alpha1- and alpha2-adrenoceptor agonists was significantly lower at 28 degrees C (cooling) than at 37 degrees C. Endothelium removal did not affect the action of cooling. These results were taken as evidence for the specificity of alpha2-adrenoceptor agonists on the production and release of nitric oxide from vascular endothelium. The results suggest that the endothelium seems to have no role in the cooling-induced responses of both alpha1- and alpha2-adrenoceptors.     

Dynamic temporal and spatial regulation of mu opioid receptor expression in primary afferent neurons following spinal nerve injury

Despite using prescribed pain medications, patients with neuropathic pain continue to experience moderate to severe pain. There is a growing recognition of a potent peripheral opioid analgesia in models of inflammatory and neuropathic pain. The goal of this study was to characterize the temporal and spatial expression of mu opioid receptor (mOR) mRNA and protein in primary afferent neurons in a rat L5 spinal nerve ligation model of persistent neuropathic pain. Bilateral L4 and L5 dorsal root ganglia (DRGs), L4 and L5 spinal cord segments, and hind paw plantar skins were collected on days 0 (naïve), 3, 7, 14, and 35 post‐spinal nerve ligation or post‐sham surgery. We found that expression of mOR mRNA and protein in primary afferent neurons changed dynamically and site‐specifically following L5 spinal nerve ligation. Real‐time RT—PCR, immunohistochemistry, and Western blot analysis demonstrated a down‐regulation of mOR mRNA and protein in the injured L5 DRG. In contrast, in the uninjured L4 DRG, mOR mRNA transiently decreased on day 7 and then increased significantly on day 14. Western blot analysis revealed a persistent increase in mOR protein expression, although immunohistochemistry showed no change in number of mOR‐positive neurons in the uninjured L4 DRG. Interestingly, mOR protein expression was reduced in the skin on days 14 and 35 post‐nerve injury and in the L4 and L5 spinal cord on day 35 post‐nerve injury. These temporal and anatomically specific changes in mOR expression following nerve injury are likely to have functional consequences on pain‐associated behaviors and opioid analgesia.     

Characterisation of the alpha-adrenoceptors of the rat renal vascular bed

Postjunctional renal alpha-adrenoceptors were studied (1) in vivo, on the renal vasculature of the anaesthesized rat and compared with those in the femoral vasculature, and (2) in vitro, on the renal vascular bed of isolated perfused rat kidney. In vivo, renal and iliac blood flows were measured with an electromagnetic flow meter. The i.v. injection of (-)-phenylephrine (1-16 micrograms/kg) and B-HT 920 (0.6-600 micrograms/kg) induced an increase in both renal and iliac vascular resistance, inhibited respectively with prazosin (300 micrograms/kg) or yohimbine (300 micrograms/kg). In the kidney, maximum response to B-HT 920 was equivalent to 64% of that to (-)-phenylephrine; on the iliac vasculature, vasoconstrictor responses to both drugs were identical, but only corresponded to 50% of the maximum renal response to (-)-phenylephrine. This indicates the predominance of alpha 1- over alpha 2-adrenoceptors in the renal vascular bed. In vitro, on the isolated perfused rat kidney, vasoconstriction was induced by the preferential alpha 1-adrenoceptor agonists [(-)-phenylephrine, cirazoline and methoxamine] and the preferential alpha 2-adrenoceptor agonists (alpha-methylnoradrenaline, dopamine and clonidine) at concentrations at which they lose their selectivity for the alpha 2-adrenoceptors; all responses were antagonised by prazosin but not by yohimbine. B-HT 920, the selective alpha 2-adrenoceptor agonist, only induced renal vasoconstriction in vitro under concomitant infusion of rabbit plasma.     

Different effects of opioid and cannabinoid receptor agonists on C-fiber-induced extracellular signal-regulated kinase activation in dorsal horn neurons in normal and spinal nerve-ligated rats

Nerve injury results in neuropathic pain, a debilitating pain condition. Whereas cannabinoids are consistently shown to attenuate neuropathic pain, the efficacy of opioids is highly controversial. Molecular mechanisms underlying analgesic effects of opioids and cannabinoids are not fully understood. We have shown that the signaling molecule ERK (extracellular signal-regulated kinase) is activated by C-fiber stimulation in dorsal horn neurons and contributes to pain sensitization. In this study, we examined whether opioids and cannabinoids can affect C-fiber-induced ERK phosphorylation (pERK) in dorsal horn neurons in spinal cord slices from normal and spinal nerve-ligated rats. In normal control spinal slices, capsaicin induced a drastic pERK expression in superficial dorsal horn neurons, which was suppressed by morphine (10 microM), the selective mu-opioid receptor agonist DAMGO [[d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (1 microM)], and the selective CB1 receptor ACEA agonist [arachidonyl-2'-chloroethylamide (5 microM)]. One week after spinal nerve ligation when neuropathic pain is fully developed, capsaicin induced less pERK expression in the injured L(5)-spinal segment. This pERK induction was not suppressed by morphine (10 microM) and DAMGO (1 microM) but was enhanced by high concentration of DAMGO (5 microM). In contrast, ACEA (10 microM) was still very effective in inhibiting capsaicin-induced pERK expression. In the adjacent L(4) spinal segment, both DAMGO and ACEA significantly suppressed pERK induction by capsaicin. These results indicate that, after nerve injury, opioids lose their capability to suppress C-fiber-induced spinal neuron activation in the injured L(5) but not in the intact L(4) spinal segment, whereas cannabinoids still maintain their efficacy.     

Spinal administration of alpha 2-adrenoceptor agonists and opioids or local anaesthetic agents

The effects of combined spinal administration of alpha(2)-adrenoceptor agonists, local anaesthetics, and opioids have been extensively studied. The motor and the sensory block of spinal and epidural anaesthesia is enhanced and prolonged by the combination of clonidine with the local anaesthetics lidocaine, tetracaine and bupivacaine. Because higher plasma levels of local anaesthetics were measured when clonidine was injected epidurally, the enhancement of the local anaesthetic's effect by clonidine is not due to slowed resorption, but rather to direct spinal and supraspinal effects of clonidine. Furthermore, direct local anaesthetic properties of clonidine on nerve fibres are discussed. In addition, in children the combination of clonidine and bupivacaine for caudal anaesthesia resulted in a marked prolongation of postoperative pain relief. Circulatory effects of combined clonidine and local anaesthetics are the result of the specific spinal blockade and the central and peripheral effects of clonidine. The combined epidural and intrathecal administration of opioids and alpha(2)-adrenoceptor agonists provides better postoperative pain relief than the administration of either substance alone. In humans, the interaction seems to be additive rather than supra-additive. Because of its limited duration of action, continuous administration of clonidine is recommended, especially when it is used in combination with opioids. Neither the incidence nor the severity of side effects is increased by a combined therapy with opioids. Despite the sedative properties of clonidine, there is no increased risk of respiratory depression when clonidine is given in combination with opioids. The inhibiting effect on the sympathetic nervous system activity regularly observed during spinal administration of clonidine supports the value of this therapy and will support its use in the future. Therefore, the combination of alpha(2)-adrenoceptor agonists with local anaesthetics or opioids is reasonable and may improve anaesthetic practice.     

Distribution and regulation of alpha(2)-adrenoceptors in rat dorsal root ganglia

Using in situ hybridization with riboprobes the distribution of alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptor mRNAs were studied in normal rat dorsal root ganglia and after unilateral peripheral nerve injury (total nerve transection) or inflammation. The most common adrenoceptor mRNA was of the alpha(2C) subtype (almost 80% of all neuron profiles) followed by the alpha(2A) subtype (almost 20%), whereas alpha(2B)-adrenoceptor mRNA was only found in small numbers of neuron profiles. The most dramatic effect of peripheral nerve injury was observed for the alpha(2A)-adrenoceptor mRNA, which increased to 45% of all neuron profiles. In contrast, alpha(2C) adrenoceptor mRNA showed a small decrease in this situation. Carrageenan-induced peripheral inflammation did not affect the percentage of alpha(2A)- or alpha(2C)-adrenoceptor mRNA-positive profiles. These findings suggest that, if any of the alpha(2) adrenoceptor, the alpha(2A) subtype represents the most likely candidate in DRG neurons to be involved in sympathetically maintained pain.     

Switching of bradykinin-mediated nociception following partial sciatic nerve injury in mice

Bradykinin (BK) is well known as a potent mediator of pain and hyperalgesia. Using a highly sensitive nociception test, we found that intraplantar (i.pl.) injection of BK produced nociceptive hyper-responses in partial sciatic nerve-injured mice, compared with the control sham-operated animals. By use of selective agonists and antagonists, we revealed that BK nociception in sham-operated mice was mediated through B2 receptor, whereas that in injured mice was mediated through B1 receptor. When we examined the activation of extracellular signal-regulated protein kinase (ERK) in dorsal root ganglion (DRG) neurons upon i.pl. injection of BK, phosphorylated ERK was mainly observed in unmyelinated neurons in sham-operated mice, and in case of nerve-injured mice, ERK was mainly activated in myelinated neurons and satellite cells. The B1 receptor agonist, [Lys-des-Arg(9)]-BK also produced nociceptive response and activated ERK only in nerve-injured mice. BK or B1 agonist-induced activation of ERK in DRG neurons of nerve-injured mice was completely blocked by pretreatment with antisense oligodeoxynucleotide (AS-ODN) for B1 receptor. We found that in sham-operated mice mainly B2 receptors were expressed in unmyelinated DRG neurons with a very little presence of B1 receptor. After nerve injury, B2 receptor expression drastically decreased, whereas B1 receptors were newly expressed mainly in myelinated DRG neurons and satellite cells. Finally, BK nociception in sham-operated mice was blocked by AS-ODN for B2 receptors and that in injured mice by AS-ODN for B1 receptors. Altogether, these findings confirm a switching of receptor and fiber subtype for BK nociception after peripheral nerve injury, which might contribute to the pathobiology of neuropathic pain.     

Alpha 2-adrenoceptor agonists for the treatment of chronic pain

The antinociceptive effect of alpha(2)-adrenoceptor agonists is mediated by activation of descending inhibiting noradrenergic systems, which modulates 'wide-dynamic-range' neurones. Furthermore, they inhibit the liberation of substance P and endorphines and activate serotoninergic neurones. Despite this variety of antinociceptive actions, there is still little experience with alpha(2)-adrenoceptor agonists as therapeutic agents for use in chronic pain syndromes. Studies in animals and patients have shown that the transdermal, epidural and intravenous administration of the alpha(2)-adrenoceptor agonist clonidine reduces pain intensity in neuropathic pain syndromes for periods varying from some hours up to 1 month. Patients suffering from lancinating or sharp pain respond best to this therapy. Topically applied clonidine (200-300 microg) relieves hyperalgesia in sympathetically maintained pain. Epidural administration of 300 microg clonidine dissolved in 5 ml NaCl 0.9 % has also been shown to be effective. In patients suffering from cancer pain tolerant to opioids, pain control has proved possible again with combinations of opioids and clonidine. In isolated cases clonidine has been administered epidurally at a dose of 1500 microg/day for almost 5 months without evidence for any histotoxic property of clonidine. Side effects often observed during administration of alpha(2)-adrenoceptor agonists are dry mouth, sedation, hypotension and bradycardia. Therapeutic interventions are usually not required.     

VR1, but not P2X(3), increases in the spared L4 DRG in rats with L5 spinal nerve ligation

We investigated the expression of two candidate transducers of noxious stimuli in peripheral tissues, the vanilloid receptor subtype 1 (VR1) and the P2X(3), a subunit of the ionotropic P2X receptor for ATP, in spared L4 DRG neurons following L5 spinal nerve ligation, a neuropathic pain model. VR1 mRNA expression increased in the small- and medium-sized DRG neurons from the first to 28th day after injury, and this up-regulation corresponded well with the development and maintenance of thermal hyperalgesia of the hind paw. The increase in VR1-immunoreactive (ir) neurons was confirmed at the third day after surgery. In contrast, there was no change in expression of P2X(3) mRNA over 4 weeks after ligation, or in the percentage of P2X(3)-ir neurons observed 3 days after surgery. Our data suggests that increased VR1 in the spared L4 DRG may contribute to the exaggerated heat response observed in this neuropathic pain model. Taken together with the previous reports that P2X(3) expression increases in the spared DRG neurons in other neuropathic pain models, there appears to be differences in the phenotypic changes and pathomechanisms of the various neuropathic pain models.     

Systemische Gabe von α2-Adrenozeptoragonisten zur postoperativen Analgesiezur postoperativen Analgesie

The alpha(2)-adrenoceptor agonist clonidine has analgesic properties comparable to those of opioids after systemic administration. It also has antihypertensive, antiemetic, anxiolytic, sedative and antisialogogue effects and reduces the incidence of shivering. Thus, the pharmacodynamic profile of clonidine seems to suit it quite well for the special problems related to recovery from anaesthesia. The respiratory depression is weak, particularly in comparison with opioids. This paper reviews previous experience with systemic administration of alpha(2)-adrenoceptor agonists for postoperative pain relief. Especially in combination with low-dose opioids, clonidine leads to a similar or even better level of pain relief with significantly reduced adverse side effects compared with opioid mono-therapy, perhaps because different sites of action are addressed and influence nociception. Therefore, balanced postoperative analgesia including alpha(2)-adrenoceptor agonists, therefore seems to be a beneficial addition to differentiated postoperative pain relief.