Causalgia, pathological pain, and adrenergic receptors

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Modified Carpal Tunnel Release: A New Approach to Minimizing Pillar Pain

INTRODUCTION

Pillar pain is a frequent postoperative complication of carpal tunnel release (CTR). The precise definition of pillar pain is lacking, but most authors describe it as diffuse aching pain and tenderness in the thenar and hypothenar area. The etiology of pillar pain is unclear. However, the most prevalent theory is the neurogenic theory, which attributes the pain to the damage of small nerve branches of palmar cutaneous branches of median nerve after surgical incision, with resulting entrapment of the nerves in the scar tissue at the incision site. We postulated that a main source of pillar pain is sensory neuromas along the incision site.In this article, we describe a simple modification of the standard CTR technique with intent to decrease neuroma formation and thus minimizing pillar pain.

MATERIALS AND METHODS

This is a retrospective study comparing the incidence and duration of pillar pain between patients who underwent standard CTR (SCTR, n = 53) versus the minimizing pillar pain CTR technique (n = 55). Based on duration of pillar pain, the groups were placed into 3 subgroups (<3, 3-6, and >6 months). Presence and duration of pillar pain in each group were recorded along with return to work (RTW), complications, and patient satisfaction.

RESULTS

The SCTR group had a total of 17 patients with pillar pain (32.1%), 5 of which resolved within 3 months, 7 within 3 to 6 months, and 5 in more than 6 months. The group that underwent the minimizing pillar pain technique had a total of 4 patients with pillar pain (7.2%). Three resolved within 3 months, 1 resolved within 3 to 6 months, and there were no patients with pillar pain lasting more than 6 months. Average RTW time for minimization of pillar pain CTR (MPPCTR) was 34.9 days. Average RTW time for SCTR was 54.8 days. Satisfaction was higher among patients who underwent surgery with MPPCTR.

CONCLUSIONS

Based on these results, we concluded that MPPCTR compared with SCTR had equal complication rate, however, significantly lower incidence and duration of pillar pain, higher rate of satisfaction, and earlier RTW.

Neuroma Management: Capping Nerve Injuries With an Acellular Nerve Allograft Can Limit Axon Regeneration

Background: Management of painful neuromas continues to challenge clinicians. Controlling axon growth to prevent neuroma has gained considerable traction. A logical extension of this idea is to therefore develop an approach to control and arrest axon growth. Given the limits in axonal regeneration across acellular nerve allografts (ANAs), these constructs could provide a means to reliably terminate axon regeneration from an injured nerve. The purpose of this study was to determine if attaching an ANA to an injured nerve could provide a means to control and limit axon regeneration in a predictable manner. Methods: Twenty (20) adult rats received a sciatic nerve transection, where only the proximal nerve was repaired using an ANA of variable length (0.5, 2.5, and 5.0 cm) or left unrepaired (control). The nerves were harvested 5 weeks post-operatively for gross and histomorphometric analysis. The extent of myelinated axons in regenerated tissue was quantified. Results: At 5 weeks, limited axon regeneration within the ANAs was observed. All lengths of ANAs lead to reduced myelinated axon numbers in the most terminal tissue region compared to untreated injured nerve (P = .002). Additionally, ANA length 2.5 cm or greater did not contain any axons at the most terminal tissue region. Conclusions: This study demonstrates a proof of concept that ANAs attached to the proximal end of an injured nerve can limit axon growth in a controlled manner. Furthermore, the extent of axon growth from the injured nerve into the ANA is dependent on the ANA length.

Noradrenaline modulates mechanically evoked responses in the rat spinal dorsal horn: an in vivo patch-clamp study

Purpose: We investigated the effects of noradrenaline (NA) on physiologically evoked synaptic responses of substantia gelatinosa (SG) neurons using anesthetized animals.

Methods: Male Sprague–Dawley rats (6–8 weeks, 200–300 g, n=21) were anesthetized. The lumbar spinal cord was exposed from L3 to L5; subsequently, the rats were fixed to a stereotaxic apparatus. The electrode was advanced at an angle of 30–45 degrees into the SG using a micromanipulator. We recorded excitatory post-synaptic currents (EPSC). Under these conditions, innocuous or noxious mechanical stimuli were applied to the receptive field of the ipsilateral hindlimb with or without NA, respectively.

Results: NA (50 μM) pre-application induced three types of responses for pinch-evoked EPSCs. The number of neurons showing inhibition, facilitation, and no-effect was 15 (71.4%), 2 (9.5%), and 4 (19%), respectively (n=21). Pre-treatment with NA also induced three different types of responses for puff-evoked EPSC (n=21). The number of neurons showing inhibition, facilitation, and no-effect was 9 (42.9%), 9 (42.9%), and 3 (14.2%), respectively. Further, there was a significant difference in the rate distribution (inhibition, facilitation, and no change) between puff- and pinch-evoked responses.

Conclusion: Our present data indicate that NA acts on noxious and innocuous mechanical transmission in the SG. Considering the distinct sensory inputs to the SG, the different actions of NA on the transmission of sensory information imply that NA exerts its analgesic effects in a manner more complicated than previously believed.

Review of overlap between thermoregulation and pain modulation in fibromyalgia

Fibromyalgia (FM) syndrome is characterized by widespread pain that is exacerbated by cold and stress but relieved by warmth. We review the points along thermal and pain pathways where temperature may influence pain. We also present evidence addressing the possibility that brown adipose tissue activity is linked to the pain of FM given that cold initiates thermogenesis in brown adipose tissue through adrenergic activity, whereas warmth suspends thermogenesis. Although females have a higher incidence of FM and more resting thermogenesis, they are less able to recruit brown adipose tissue in response to chronic stress than males. In addition, conditions that are frequently comorbid with FM compromise brown adipose activity making it less responsive to sympathetic stimulation. This results in lower body temperatures, lower metabolic rates, and lower circulating cortisol/corticosterone in response to stress--characteristics of FM. In the periphery, sympathetic nerves to brown adipose also project to surrounding tissues, including tender points characterizing FM. As a result, the musculoskeletal hyperalgesia associated with conditions such as FM may result from referred pain in the adjacent muscle and skin.

Up-regulation of cutaneous α1 -adrenoceptors in complex regional pain syndrome type I

BACKGROUND

In a small radioligand-binding study of cutaneous α1 -adrenoceptors in complex regional pain syndrome (CRPS), signal intensity was greater in the CRPS-affected limb than in controls. However, it was not possible to localize heightened expression of α1 -adrenoceptors to nerves, sweat glands, blood vessels, or keratinocytes using this technique.

METHODS

To explore this in the present study, skin biopsies were obtained from 31 patients with CRPS type I and 23 healthy controls of similar age and sex distribution. Expression of α1 -adrenoceptors on keratinocytes and on dermal blood vessels, sweat glands, and nerves was assessed using immunohistochemistry.

RESULTS

α1 -Adrenoceptors were expressed more strongly in dermal nerve bundles and the epidermis both on the affected and contralateral unaffected side in patients than in controls (P<0.05). However, expression of α1 -adrenoceptors in sweat glands and blood vessels was similar in patients and controls. α1 -Adrenoceptor staining intensity in the CRPS-affected epidermis was associated with pain intensity (P < 0.05), but a similar trend for nerve bundles did not achieve statistical significance.

DISCUSSION

Epidermal cells influence nociception by releasing ligands that act on sensory nerve fibers. Moreover, an increased expression of α1 -adrenoceptors on nociceptive afferents has been shown to aggravate neuropathic pain. Thus, the heightened expression of α1 -adrenoceptors in dermal nerves and epidermal cells might augment pain and neuroinflammatory disturbances after tissue injury in patients with CRPS type I.

Sensory innervation of the dorsal longitudinal ligament and the meninges in the lumbar spine of the dog

Although intervertebral disc herniation is a well-known disease in dogs, pain management for this condition has remained a challenge. The goal of the present study is to address the lack of information regarding the innervation of anatomical structures within the canine vertebral canal. Immunolabeling was performed with antibodies against protein gene product 9.5, Tuj-1 (neuron-specific class III β-tubulin), calcitonin gene-related peptide, and neuropeptide Y in combination with the lectin from Lycopersicon esculentum as a marker for blood vessels. Staining was indicative of both sensory and sympathetic fibers. Innervation density was the highest in lateral areas, intermediate in dorsal areas, and the lowest in ventral areas. In the dorsal longitudinal ligament (DLL), the highest innervation density was observed in the lateral regions. Innervation was lower at mid-vertebral levels than at intervertebral levels. The presence of sensory and sympathetic fibers in the canine dura and DLL suggests that pain may originate from both these structures. Due to these regional differences in sensory innervation patterns, trauma to intervertebral DLL and lateral dura is expected to be particularly painful. The results ought to provide a better basis for the assessment of medicinal and surgical procedures.

Inflammation in CRPS: role of the sympathetic supply

Acute Complex Regional Pain Syndrome (CRPS) is associated with signs of inflammation such as increased skin temperature, oedema, skin colour changes and pain. Pro-inflammatory cytokines (tumour necrosis factor-α (TNF-α), interleukin-2 (IL-2), IL-1beta, IL-6) are up-regulated, whereas anti-inflammatory cytokines (IL-4, IL-10) are diminished. Adaptive immunity seems to be involved in CRPS pathophysiology as many patients have autoantibodies directed against β2 adrenergic and muscarinic-2 receptors. In an animal tibial fracture model changes in the innate immune response such as up-regulation of keratinocytes are also found. Additionally, CRPS is accompanied by increased neurogenic inflammation which depends mainly on neuropeptides such as CGRP and Substance P. Besides inflammatory signs, sympathetic nervous system involvement in CRPS results in cool skin, increased sweating and sympathetically-maintained pain. The norepinephrine level is lower in the CRPS-affected than contralateral limb, but sympathetic sprouting and up-regulation of alpha-adrenoceptors may result in an adrenergic supersensitivity. The sympathetic nervous system and inflammation interact: norepinephrine influences the immune system and the production of cytokines. There is substantial evidence that this interaction contributes to the pathophysiology and clinical presentation of CRPS, but this interaction is not straightforward. How inflammation in CRPS might be exaggerated by sympathetic transmitters requires further elucidation.

Early demyelination of primary A-fibers induces a rapid-onset of neuropathic pain in rat

Some types of peripheral neuropathic pain are associated with damage to myelin rather than to axons of primary sensory neurons. It is extremely important to develop selective demyelination animal models for understanding neuropathic pain caused by demyelination. We induced a rapid-onset and reversible demyelination of peripheral A-fibers and neuropathic pain behaviors in adult rats by a single injection of cobra venom into the sciatic nerve. The relation between A-fiber demyelination and the abnormal pain behaviors was investigated using this model. Microfilament recordings revealed that cobra venom selectively blocked A-fibers, but not C-fibers. Selective blockade of A-fibers may result from A-fiber demyelination at the site of venom injection as demonstrated by microscope examination. The axons of the demyelinated A-fibers appeared to be otherwise normal. Neuropathic pain behaviors appeared almost immediately after venom injection and lasted about 3 weeks. Electrophysiological studies indicated that venom injection induced loss of conduction in A-fibers, increased sensitivity of C-polymodal nociceptors to innocuous stimuli, and triggered spontaneous activity from both peripheral and central terminals of C-fiber nociceptors. Neurogenic inflammatory responses were also observed in the affected skin via Evan's Blue extravasation experiments. Both antidromic C-fiber spontaneous activity and neurogenic inflammation were substantially decreased by continuous A-fiber threshold electric stimuli applied proximally to the venom injection site. The data suggest that normal activity of peripheral A-fibers may produce inhibitory modulation of C-fiber polymodal nociceptors. Removal of inhibition to C-fiber polymodal nociceptors following demyelination of A-fibers may result in pain and neurogenic inflammation in the affected receptive field.

Nerve growth factor augments neuronal responsiveness to noradrenaline in cultured dorsal root ganglion neurons of rats

Increasing evidence indicates that both the nerve growth factor (NGF) and adrenergic systems play a very important role in the development of nociception. However, there is little information concerning the functional interactions between these two systems in the dorsal root ganglion (DRG). The present study tested the hypothesis that NGF could affect neuronal responsiveness to noradrenaline (NA) on the nociceptive DRG neurons, thus enhancing the nociceptive signals. To investigate this issue, spontaneous action potentials were recorded in cultured DRG neurons using current-clamp recording. When NGF (50 ng/ml, 24 h) was administered in the neuronal cultures, the neuronal firing response to NA (10 μM) was augmented in TrkA-positive neurons (3.02±0.28 Hz with NGF treatment vs. 1.36±0.14 Hz in control, P<0.05), indicating that chronic NGF treatment significantly enhanced the neuronal response to NA. Pretreatment of neurons with either the α-adrenergic receptor (AR) antagonist phentolamine (100 μM) or α1-AR antagonist prazosin (50 μM) significantly inhibited the enhanced firings of DRG neurons induced by NA. In addition, treatment of neuronal cultures with NGF (50 ng/ml, 24 h) induced a two-fold increase in α1b-AR expression, as detected with real-time reverse transcription PCR (RT-PCR) and Western blots, but had no effect on α2-AR expression. These observations indicate that NGF augmented neuronal responsiveness to NA in DRG neurons via increasing α1b-AR expression, and this could contribute to the development of pain sensitization.

Tapentadol in pain management: a μ-opioid receptor agonist and noradrenaline reuptake inhibitor

Several mechanisms can be proposed to explain an apparent synergistic analgesic action between μ-opioid and α(2)-adrenergic receptor agonists. Combining both effects in a single molecule eliminates the potential for drug-drug interactions inherent in multiple drug therapy. Tapentadol is the first US FDA-approved centrally acting analgesic having both μ-opioid receptor agonist and noradrenaline (norepinephrine) reuptake inhibition activity with minimal serotonin reuptake inhibition. This dual mode of action may make tapentadol particularly useful in the treatment of neuropathic pain. Having limited protein binding, no active metabolites and no significant microsomal enzyme induction or inhibition, tapentadol has a limited potential for drug-drug interactions. Clinical trial evidence in acute and chronic non-cancer pain and neuropathic pain supports an opioid-sparing effect that reduces some of the typical opioid-related adverse effects. Specifically, the reduction in treatment-emergent gastrointestinal adverse effects for tapentadol compared with equianalgesic pure μ-opioid receptor agonists results in improved tolerability and adherence to therapy for both the immediate- and extended-release formulations of tapentadol.

A standardized surgical technique for rat superior cervical ganglionectomy

Superior cervical ganglionectomy (SCGx) is a valuable microsurgical model to study the role of the sympathetic nervous system in a vast array of physiological and pathological processes, including homeostatic regulation, circadian biology and the dynamics of neuronal dysfunction and recovery after injury. Despite having several experimental applications in the rat, a thorough description of a standardized procedure has never been published. Here, we provide a brief review of the principal features and experimental uses of the SCGx, the surgical anatomy of the neck and sympathetic cervical chain, and a step-by-step description of how to consistently remove the superior cervical ganglia through the omohyoid muscle or the carotid triangle. Furthermore, we suggest procedures and precautions to be taken during and after surgery to optimize results and describe tools to validate surgical success. We expect that the following standardized and optimized protocol will allow researchers to organize knowledge into a cohesive framework in those areas where the SCGx is applied.

Blockade of intra-articular adrenergic receptors increases analgesic demands for pain relief after knee surgery

Activation of opioid receptors on peripheral sensory nerve terminals by opioid peptides that are produced and released from immune cells can result in inhibition of inflammatory pain. This study tests the hypothesis that postoperative pain is attenuated endogenously through a local sympathetic neurotransmitter-activated release of opioids in patients undergoing knee surgery. We examined the expression of opioid peptides and adrenergic receptors in cells infiltrating inflamed synovial tissue and we hypothesized that intra-articular (i.a.) administration of the adrenergic receptor antagonist labetalol will increase postoperative analgesic consumption and/or pain intensity in these patients. In a double-blind, randomized manner, 75 patients undergoing therapeutic knee arthroscopy received i.a. placebo (20 ml saline) or labetalol (2.5 or 5 mg in 20 ml saline) at the end of surgery. Postoperative pain intensity was assessed by visual analog and verbal rating scales at rest and on exertion, and by the consumption of morphine via patient-controlled analgesia. Synovial biopsies were taken during the operation for double-immunofluorescence confocal microscopy studies. Alpha(1)- and beta(2)-adrenergic receptors were co-expressed in opioid peptide-containing cells. No significant difference was seen in pain scores, but patients receiving 2.5 mg labetalol requested significantly higher amounts of morphine. These findings are consistent with the notion that surgical stress induces sympathetically activated release of endogenous opioids from inflammatory cells and subsequent analgesia via activation of peripheral opioid receptors.

Epinephrine, phenylephrine, and methoxamine induce infiltrative anesthesia via alpha1-adrenoceptors in rats

AIM

To assess whether epinephrine, phenylephrine, and methoxamine act via certain subtypes of adrenoceptors to exert their local anesthetic activity.

METHODS

We investigated cutaneous anesthesia from adrenoceptor agonists and/or antagonists in conscious, unanesthetized Sprague-Dawley male rats (weight 200-250 g). Cutaneous anesthesia was evidenced by a block of the cutaneous trunci muscle reflex, which is characterized by reflex movement of the skin over the back produced by twitches of lateral thoracispinal muscles in response to local dorsal cutaneous noxious pinprick.

RESULTS

Local infiltration of epinephrine, L-phenylephrine, or methoxamine alone induces cutaneous anesthesia in rats in a dose-dependent way. Epinephrine is found to be 19 and 29 times more potent than those of methoxamine and L-phenylephrine, respectively. The cutaneous anesthesia induced by epinephrine, phenylephrine, or methoxamine can be significantly reduced by alpha(1)-adrenoceptor antagonists (eg, prazosin), alpha1, alpha2-adrenoceptor antagonist, alpha(1A)-adrenoceptor antagonist (eg, 5-methylurapdil), alpha(1B)-adrenoceptor antagonist (eg, chloroethylclonidine), or alpha(1D)-adrenoceptor antagonist (eg, BMY7873).

CONCLUSION

Our results indicate that epinephrine, phenylephrine and methoxamine all act mainly via mixed subtypes of alpha(1)-adrenoceptors to induce cutaneous anesthesia in the rat.

Translational pain research: achievements and challenges

The achievements in both preclinical and clinical pain research over the past 4 decades have led to significant progress in clinical pain management. However, pain research still faces enormous challenges and there remain many obstacles in the treatment of clinical pain, particularly chronic pain. Translational pain research needs to involve a number of important areas including: 1) bridging the gap between pain research and clinical pain management; 2) developing objective pain-assessment tools; 3) analyzing current theories of pain mechanisms and their relevance to clinical pain; 4) exploring new tools for both preclinical and clinical pain research; and 5) coordinating research efforts among basic scientists, clinical investigators, and pain-medicine practitioners. These issues are discussed in this article in light of the achievements and challenges of translational pain research.

PERSPECTIVE

The subjective nature of clinical pain calls for innovative research approaches. As translational pain research emerges as an important field in pain medicine, it will play a unique role in improving clinical pain management through coordinated bidirectional research approaches between bedside and bench.

Antinociception occurs with a reversal in alpha 2-adrenoceptor regulation of TNF production by peripheral monocytes/macrophages from pro- to anti-inflammatory

Tumor necrosis factor-alpha (TNF) plays a role in neuropathic pain. During neuropathic pain development in the chronic constriction injury model, elevated TNF levels in the brain occur in association with enhanced alpha 2-adrenoceptor inhibition of norepinephrine release. alpha 2-Adrenoceptors are also located on peripheral macrophage where they normally function as pro-inflammatory, since they increase the production of the cytokine TNF, a proximal mediator of inflammation. How the central increase in TNF affects peripheral alpha 2-adrenoceptor function was investigated. Male, Sprague-Dawley rats had four loose ligatures placed around the right sciatic nerve. Thermal hyperalgesia was determined by comparing hind paw withdrawal latencies between chronic constriction injury and sham-operated rats. Chronic constriction injury increased TNF immunoreactivity at the lesion and the hippocampus. Amitriptyline, an antidepressant that is used as an analgesic, was intraperitoneally administered (10 mg/kg) starting simultaneous with ligature placement (day-0) or at days-4 or -6 post-surgery. Amitriptyline treatment initiated at day-0 or day-4 post-ligature placement alleviated hyperalgesia. When initiated at day-0, amitriptyline prevented increased TNF immunoreactivity in the hippocampus and at the lesion. A peripheral inflammatory response, macrophage production of TNF, was also assessed in the current study. Lipopolysaccharide (LPS)-stimulated production of TNF by whole blood cells and peritoneal macrophages was determined following activation of the alpha 2-adrenoceptor in vitro. alpha 2-Adrenoceptor regulation of TNF production from peripheral immune-effector cells reversed from potentiation in controls to inhibition in chronic constriction injured rats. This effect is accelerated with amitriptyline treatment initiated at day-0 or day-4 post-ligature placement. Amitriptyline treatment initiated day-6 post-ligature placement did not alleviate hyperalgesia and prevented the switch from potentiation to inhibition in alpha 2-adrenoceptor regulation of TNF production. Recombinant rat TNF i.c.v. microinfusion reproduces the response of peripheral macrophages from rats with chronic constriction injury. A reversal in peripheral alpha 2-adrenoceptor regulation of TNF production from pro- to anti-inflammatory is associated with effective alleviation of thermal hyperalgesia. Thus, alpha 2-adrenoceptor regulation of peripheral TNF production may serve as a potential biomarker to evaluate therapeutic regimens.

The sympathetic nervous system and pain

The sympathetic nervous system (SNS) and pain interact on many levels of the neuraxis. In healthy subjects, activation of the SNS in the brain usually suppresses pain mainly by descending inhibition of nociceptive transmission in the spinal cord. Furthermore, some experimental data even suggest that the SNS might control peripheral inflammation and nociceptive activation. However, even subtle changes in pathophysiology can dramatically change the effect of SNS on pain, and vice versa. In the periphery, inflammation or nociceptive activation is enhanced, spinal descending inhibition is reversed to spinal facilitation, and finally the awareness of all these changes will induce anxiety, which furthermore amplifies pain perception, affects pain behavior, and depresses mood. Unraveling the detailed molecular mechanisms of how this interaction of SNS and pain is established in health and disease will help us to treat pain more successfully in the future.

Mechanisms underlying development of spatially distributed chronic pain (fibromyalgia)

Chronic fibromyalgia (FM) pain is prevalent (estimated as high as 13%), predominantly affects women, and is associated with a variety of focal pain conditions. Ongoing FM pain is referred to deep tissues and is described as widespread but usually is maximally located within a restricted region such as the shoulders. Palpation of deep tissues reveals an enhanced nociceptive sensitivity that is not restricted to regions of clinical pain. Similarly, psychophysical testing reveals allodynia and hyperalgesia for cutaneous stimulation at locations beyond regions of clinical pain referral. The combination of widely distributed clinical pain and generalized hypersensitivity is highly disabling, but no satisfactory treatment is regularly prescribed. A thorough understanding of mechanisms will likely be required to develop and document adequate therapies. The generalized hypersensitivity associated with FM has focused considerable interest on central (CNS) mechanisms for the disorder. These include central sensitization, central disinhibition and a dysfunctional hypothalamic-pituitary-adrenal (HPA) axis. However, the central effects associated with FM can be produced by a peripheral source of pain. Chronic nociceptive input induces central sensitization, magnifying pain, and it activates the HPA and the sympathetic nervous system. Chronic sympathetic activation indirectly sensitizes peripheral nociceptors and sets up a vicious cycle. Thus, it appears that central mechanisms of FM pain are dependent on abnormal peripheral input(s) for development and maintenance of this condition. A substantial literature defines peripheral-CNS-peripheral interactions that are integral to FM pain. These reciprocal actions and related phenomena of relevance to FM pain are reviewed here, leading to suggestions for testing of therapeutic approaches.

Plasticity of pelvic autonomic ganglia and urogenital innervation

Pelvic ganglia contain a mixture of sympathetic and parasympathetic neurons and provide most of the motor innervation of the urogenital organs. They show a remarkable sensitivity to androgens and estrogens, which impacts on their development into sexually dimorphic structures and provide an array of mechanisms by which plasticity of these neurons can occur during puberty and adulthood. The structure of pelvic ganglia varies widely among species, ranging from rodents, which have a pair of large ganglia, to humans, in whom pelvic ganglion neurons are distributed in a large, complex plexus. This plexus is frequently injured during pelvic surgical procedures, yet strategies for its repair have yet to be developed. Advances in this area will come from a better understanding of the effects of injury on the cellular signaling process in pelvic neurons and also the role of neurotrophic factors during development, maintenance, and repair of these axons.

Increased systemic catecholamines in complex regional pain syndrome and relationship to psychological factors: a pilot study

We have demonstrated that subjects with complex regional pain syndrome (CRPS) have asymmetric venous pool plasma concentrations of norepinephrine (NE) when affected and unaffected limbs are compared, with most demonstrating decreased NE levels in the affected limb. This pilot study explored whether systemic venous plasma catecholamine levels in CRPS subjects with sympathetically maintained pain (SMP) differ from those found in healthy volunteers. We also explored whether catecholamine levels were correlated with scores on psychometric measures of depression, anxiety, and personality. Venous blood samples from 33 CRPS/SMP patients (from unaffected limbs) and 30 healthy control subjects were assayed for plasma NE and epinephrine (E) concentrations. Plasma NE levels were significantly higher in the CRPS group (P < 0.001). Statistical comparisons of E levels across groups did not achieve significance (P < 0.06), although 52% of CRPS/SMP patients had E levels exceeding the 95% confidence interval based on control data. Significant positive correlations were found between E levels and scores on the Beck Depression Inventory and Scales 1, 3, and 6 on the Minnesota Multiphasic Personality Inventory-2 (all P < 0.05). This preliminary work suggests that increased NE and E levels in CRPS/SMP patients may result from the pain of CRPS, consequent affective distress, or both. Alternatively, our findings could reflect premorbid adrenergic hyperactivity caused by affective, endocrine, or other pathology, which might predispose these individuals to develop the syndrome. Definitive studies are needed to examine these hypotheses in detail.

Actions of noradrenaline on substantia gelatinosa neurones in the rat spinal cord revealed by in vivo patch recording

To elucidate the mechanisms of antinociception mediated by the descending noradrenergic pathway in the spinal cord, the effects of noradrenaline (NA) on noxious synaptic responses of substantia gelatinosa (SG) neurones, and postsynaptic actions of NA were investigated in rats using an in vivo whole-cell patch-clamp technique. Under urethane anaesthesia, the rat was fixed in a stereotaxic apparatus after the lumbar spinal cord was exposed. In the current-clamp mode, pinch stimuli applied to the ipsilateral hindlimb elicited a barrage of EPSPs, some of which initiated an action potential. Perfusion with NA onto the surface of the spinal cord hyperpolarized the membrane (5.0-9.5 mV) and suppressed the action potentials. In the voltage-clamp mode (V(H), -70 mV), the application of NA produced an outward current that was blocked by Cs(+) and GDP-beta-S added to the pipette solution and reduced the amplitude of EPSCs evoked by noxious stimuli. Under the blockade of postsynaptic actions of NA, a reduction of the evoked and spontaneous EPSCs of SG neurones was still observed, thus suggesting both pre- and postsynaptic actions of NA. The NA-induced outward currents showed a clear dose dependency (EC(50), 20 microM), and the reversal potential was -88 mV. The outward current was mimicked by an alpha(2)-adrenoceptor agonist, clonidine, and suppressed by an alpha(2)-adrenoceptor antagonist, yohimbine, but not by alpha(1)- and beta-antagonists. These findings suggest that NA acts on presynaptic sites to reduce noxious stimuli-induced EPSCs, and on postsynaptic SG neurones to induce an outward current by G-protein-mediated activation of K(+) channels through alpha(2)-adrenoceptors, thereby producing an antinociceptive effect.

Topical and peripherally acting analgesics

Acute nociceptive, inflammatory, and neuropathic pain all depend to some degree on the peripheral activation of primary sensory afferent neurons. The localized peripheral administration of drugs, such as by topical application, can potentially optimize drug concentrations at the site of origin of the pain, while leading to lower systemic levels and fewer adverse systemic effects, fewer drug interactions, and no need to titrate doses into a therapeutic range compared with systemic administration. Primary sensory afferent neurons can be activated by a range of inflammatory mediators such as prostanoids, bradykinin, ATP, histamine, and serotonin, and inhibiting their actions represents a strategy for the development of analgesics. Peripheral nerve endings also express a variety of inhibitory neuroreceptors such as opioid, alpha-adrenergic, cholinergic, adenosine and cannabinoid receptors, and agonists for these receptors also represent viable targets for drug development. At present, topical and other forms of peripheral administration of nonsteroidal anti-inflammatory drugs, opioids, capsaicin, local anesthetics, and alpha-adrenoceptor agonists are being used in a variety of clinical states. There also are some clinical data on the use of topical antidepressants and glutamate receptor antagonists. There are preclinical data supporting the potential for development of local formulations of adenosine agonists, cannabinoid agonists, cholinergic ligands, cytokine antagonists, bradykinin antagonists, ATP antagonists, biogenic amine antagonists, neuropeptide antagonists, and agents that alter the availability of nerve growth factor. Given that activation of sensory neurons involves multiple mediators, combinations of agents targeting different mechanisms may be particularly useful. Topical analgesics represent a promising area for future drug development.

Modulation of CGRP and PGE2 release from isolated rat skin by alpha-adrenoceptors and kappa-opioid-receptors

Norepinephrine (NE) reduces the release of neuropeptides from central terminals of primary afferent neurones by presynaptic inhibition. We investigated whether NE also affects stimulus-induced intracutaneous calcitonin gene-related peptide (CGRP) and secondary prostaglandin E2 (PGE2) release. For comparison, kappa-opioid effects were examined. Antidromic electrical nerve stimulation resulted in significant increases in the release of CGRP and PGE2. The PGE2 release was prevented by selective activation of alpha2-adrenoceptors whereas the CGRP release was not changed. In contrast, selective kappa-opioid receptor activation diminished electrically evoked release of both CGRP and PGE2. We conclude that NE affected stimulated PGE2 release via alpha2-adrenoceptors on cells other than cutaneous afferents while kappa-opioid receptors are expressed in peripheral terminals of cutaneous afferents and their activation reduced CGRP release and secondary PGE2 formation.

Vascular and nociceptive effects of localized prolonged sympathetic blockade in human skin

Supersensitivity to noradrenaline contributes to certain vascular disorders (e.g., hypertension) and chronic neuropathic pain conditions (e.g., complex regional pain syndrome). We aimed to develop a procedure for inducing adrenergic supersensitivity that could be used to investigate the role of catecholamines in these clinical conditions. In the first study, three doses of guanethidine were administered by iontophoresis to separate small patches of skin in the forearm of healthy human volunteers. Four to five hours later. the vasoconstrictor response to the adrenergic releasing agent tyramine was inhibited in a dose-dependent manner by iontophoretic pretreatment with guanethidine, indicating that guanethidine had depleted endogenous adrenergic stores. In a second study, guanethidine and saline were administered by iontophoresis four times over approximately 2 weeks at separate sites in the forearm. One to two days after the final pretreatment, vasoconstriction to the iontophoresis of a weak dose of noradrenaline was enhanced at sites pretreated with guanethidine. To investigate the effect of guanethidine pretreatment on thermal hyperalgesia. the experimental sites were sensitized to heat by the topical application of 0.6% capsaicin. Both before and after the application of capsaicin, the heat-pain threshold and heat-pain ratings to suprathreshold stimulation were similar at sites pretreated for 2 weeks with guanethidine or saline. However, after the iontophoresis of noradrenaline, thermal hyperalgesia was greater at the guanethidine-pretreated site than the saline pretreated site. These observations indicate that prolonged depletion of adrenergic stores by guanethidine induces adrenergic supersensitivity in cutaneous vessels, and that adrenergic supersensitivity enhances thermal hyperalgesia in the presence of noradrenaline.

Pain and syringomyelia: a review

The pathophysiological basis of chronic pain syndromes remains poorly defined. Central and dysesthetic pain are probably the most disabling of sensory disturbances associated with syringomyelia, and, unfortunately, effective treatment remains elusive. In this paper, the authors review their institutional experience with both clinical and laboratory studies of patients with syringomyelia, and they review the relevant literature. To date, there is no consensus as to the best treatment for central cord pain syndromes, although there are many promising areas of current research involving the use of neurochemicals in the spinal cord.