Plasticity of the nervous system at the systematic, cellular and molecular levels: a mechanism of chronic pain and hyperalgesia

Laminin polymer treatment accelerates repair of the crushed peripheral nerve in adult rats

Promoting axon growth after peripheral nerve injury may support recovery. Soluble laminin polymers formed at pH 4 (aLam) accelerate axon growth from adult dorsal root ganglion neurons in vitro. We used an adult rat model of a peripheral (peroneal) nerve crush to investigate whether an injection of aLam enhances axon growth and functional recovery in vivo. Rats that received an injection of aLam into the crush at 2 days post-injury show significant improvements in hind limb motor function at 2 and 5 weeks after injury compared with control rats that received phosphate-buffered saline. Functional improvement was not associated with changes in sensitivity to thermal or mechanical stimuli. Treatment with aLam decreased the occurrence of autophagia and abolished non-compliance with treadmill walking. Rats treated with aLam showed increased axon presence in the crush site at 2 weeks post-injury and larger axon diameter at 10 weeks post-injury compared with controls. Treatment with aLam did not affect Schwann cell presence or axon myelination. Our results demonstrated that aLam accelerates axon growth and maturity in a crushed peroneal nerve associated with expedited hind limb motor function recovery. Our data support the therapeutic potential of injectable aLam polymers for treatment of peripheral nerve crush injuries. STATEMENT OF SIGNIFICANCE: Incidence of peripheral nerve injury has been estimated to be as high as 5% of all cases entering a Level 1 trauma center and the majority of cases are young males. Peripheral nerves have some endogenous repair capabilities, but overall recovery of function remains limited, which typically has devastating effects on the individual, family, and society, as wages are lost and rehabilitation is extended until the nerves can repair. We report here that laminin polymers injected into a crush accelerated repair and recovery, had no adverse effects on sensory function, obliterated non-compliance for walking tests, and decreased the occurrence of autophagia. These data support the use of laminin polymers for safe and effective recovery after peripheral nerve injury.

Reversible inhibition of the glycine transporter GlyT2 circumvents acute toxicity while preserving efficacy in the treatment of pain

BACKGROUND AND PURPOSE

Available medications for chronic pain provide only partial relief and often cause unacceptable side effects. There is therefore a need for novel molecular targets to develop new therapeutics with improved efficacy and tolerability. Despite encouraging efficacy data in rodents with inhibitors of the neuronal glycine transporter-2 (GlyT2), there are also some reports of toxicity and their development was discontinued.

EXPERIMENTAL APPROACH

In order to clarify the possibility of targeting GlyT2 for the treatment of pain, we have used an integrated approach comprising in vitro pharmacology, selectivity, bioavailability, in vivo efficacy and safety assessment to analyse the properties and efficacy of ALX-1393 and Org-25543, the two published GlyT2 inhibitors from which in vivo data are available.

KEY RESULTS

We report that these compounds have a different set of undesirable properties that limit their usefulness as pharmacological tools. Importantly, we discover that inhibitors of GlyT2 can exert an apparent reversible or irreversible inhibition of the transporter and describe a new class of reversible GlyT2 inhibitors that preserves efficacy while avoiding acute toxicity.

CONCLUSIONS AND IMPLICATIONS

Our pharmacological comparison of two closely related GlyT2 inhibitors with different modes of inhibition provides important insights into their safety and efficacy profiles, uncovering that in the presence of a GlyT2 mechanism-based toxicity, reversible inhibitors might allow a tolerable balance between efficacy and toxicity. These findings shed light into the drawbacks associated with the early GlyT2 inhibitors and describe a new mechanism that might serve as the starting point for new drug development.

Platelet-rich plasma and the elimination of neuropathic pain

Peripheral neuropathic pain typically results from trauma-induced nociceptive neuron hyperexcitability and their spontaneous ectopic activity. This pain persists until the trauma-induced cascade of events runs its full course, which results in complete tissue repair, including the nociceptive neurons recovering their normal biophysical properties, ceasing to be hyperexcitable, and stopping having spontaneous electrical activity. However, if a wound undergoes no, insufficient, or too much inflammation, or if a wound becomes stuck in an inflammatory state, chronic neuropathic pain persists. Although various drugs and techniques provide temporary relief from chronic neuropathic pain, many have serious side effects, are not effective, none promotes the completion of the wound healing process, and none provides permanent pain relief. This paper examines the hypothesis that chronic neuropathic pain can be permanently eliminated by applying platelet-rich plasma to the site at which the pain originates, thereby triggering the complete cascade of events involved in normal wound repair. Many published papers claim that the clinical application of platelet-rich plasma to painful sites, such as muscle injuries and joints, or to the ends of nerves evoking chronic neuropathic pain, a process often referred to as prolotherapy, eliminates pain initiated at such sites. However, there is no published explanation of a possible mechanism/s by which platelet-rich plasma may accomplish this effect. This paper discusses the normal physiological cascade of trauma-induced events that lead to chronic neuropathic pain and its eventual elimination, techniques being studied to reduce or eliminate neuropathic pain, and how the application of platelet-rich plasma may lead to the permanent elimination of neuropathic pain. It concludes that platelet-rich plasma eliminates neuropathic pain primarily by platelet- and stem cell-released factors initiating the complex cascade of wound healing events, starting with the induction of enhanced inflammation and its complete resolution, followed by all the subsequent steps of tissue remodeling, wound repair and axon regeneration that result in the elimination of neuropathic pain, and also by some of these same factors acting directly on neurons to promote axon regeneration thereby eliminating neuropathic pain.

Angiotensin II receptor type 2 activation is required for cutaneous sensory hyperinnervation and hypersensitivity in a rat hind paw model of inflammatory pain

Many pain syndromes are associated with abnormal proliferation of peripheral sensory fibers. We showed previously that angiotensin II, acting through its type 2 receptor (AT2), stimulates axon outgrowth by cultured dorsal root ganglion neurons. In this study, we assessed whether AT2 mediates nociceptor hyperinnervation in the rodent hind paw model of inflammatory pain. Plantar injection of complete Freund's adjuvant (CFA), but not saline, produced marked thermal and mechanical hypersensitivity through 7 days. This was accompanied by proliferation of dermal and epidermal PGP9.5-immunoreactive (ir) and calcitonin gene-related peptide-immunoreactive (CGRP-ir) axons, and dermal axons immunoreactive for GFRα2 but not tyrosine hydroxylase or neurofilament H. Continuous infusion of the AT2 antagonist PD123319 beginning with CFA injection completely prevented hyperinnervation as well as hypersensitivity over a 7-day period. A single PD123319 injection 7 days after CFA also reversed thermal hypersensitivity and partially reversed mechanical hypersensitivity 3 hours later, without affecting cutaneous innervation. Angiotensin II-synthesizing proteins renin and angiotensinogen were largely absent after saline but abundant in T cells and macrophages in CFA-injected paws with or without PD123319. Thus, emigrant cells at the site of inflammation apparently establish a renin-angiotensin system, and AT2 activation elicits nociceptor sprouting and heightened thermal and mechanical sensitivity.

PERSPECTIVE

Short-term AT2 activation is a potent contributor to thermal hypersensitivity, whereas long-term effects (such as hyperinnervation) also contribute to mechanical hypersensitivity. Pharmacologic blockade of AT2 signaling represents a potential therapeutic strategy aimed at biologic mechanisms underlying chronic inflammatory pain.

Hypersensitivity and hyperinnervation of the rat hind paw following carrageenan-induced inflammation

Studies of human tissue show that many chronic pain syndromes are accompanied by abnormal increases in numbers of peripheral sensory nerve fibers. It is not known if sensory nerve sprouting occurs as a result of inflammation present in these conditions, or other factors such as infection or extensive tissue damage. In the present study, we used a well established model of inflammation to examine cutaneous innervation density in relation to mechanical and thermal hypersensitivity. Adult female rats were ovariectomized to eliminate fluctuations in female reproductive hormones and one week later, a hind paw was injected with carrageenan or saline vehicle. Behavioral testing showed that saline vehicle injection did not alter thermal or mechanical thresholds compared to pre-injection baselines. Carrageenan injections resulted in markedly reduced paw withdrawal thresholds at 24 and 72 h after injection; this was accompanied by increased mechanical sensitivity of the contralateral paw at 72 h. Analysis of innervation density using PGP9.5 as a pan-neuronal marker at 72 h showed that inflammation resulted in a 2-fold increase in cutaneous innervation density. We conclude that inflammation alone is sufficient to induce sprouting of sensory cutaneous axon endings leading local tissue hyperinnervation, which may contribute to hypersensitivity that occurs in painful inflammatory conditions.

Role of Helicobacter pylori infection on neuronal expression in the stomach and spinal cord of a murine model

OBJECTIVE

To investigate the effect of Helicobacter pylori (H. pylori) infection on neuronal expressions in the stomach and spinal cord of mice so as to explain dyspepsia symptoms in H. pylori infected patients.

METHODS

C57BL/6 female mice were studied at 2 weeks (acute infection group) and 12 weeks (chronic infection group) after H. pylori inoculation. Histological analyses for gastric inflammation and bacterial colonization were assessed by HE staining and Warthin-Starry staining. Fos, vasoactive intestinal polypeptide (VIP) and calcitonin gene-related peptide expressions (CGRP) were studied by immunohistochemistry.

RESULTS

H. pylori colonization was present mainly in pyloric region, but bacterial density was similar in both infected groups. The intensity of mucosal inflammation and activity was significantly higher in two infected groups than in those in the control group. The degree of mononuclear and polymorphonuclear cell infiltration in proventricular-glandular region and gastric corpus at 12 weeks after H. pylori inoculation was higher than that at 2 weeks after inoculation. The neuronal expressions of fos, VIP, and CGRP in the stomach and spinal cord were significantly more marked in the infected groups than in the control group, but there was no significant difference between two infected groups.

CONCLUSION

H. pylori infection induced different degrees of gastric mucosal inflammation in the murine model. Both early and chronic infection groups of mice showed enhanced neuronal expressions of fos, VIP and CGRP of stomach and spinal cord and these could form a basis for appearance of functional dyspeptic symptoms in patients with H. pylori infection.

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.

Changes in expression of nociceptin/orphanin FQ and its receptor in spinal dorsal horn during electroacupuncture treatment for peripheral inflammatory pain in rats

The neuropeptide nociceptin/orphanin FQ (N/OFQ), the endogenous agonist of the N/OFQ peptide receptor (NOP receptor), has been demonstrated to be involved in many physiological and pathological functions including pain modulation. It was reported that electroacupuncture (EA) had a potent analgesic effect on inflammatory pain by activating various endogenous transmitters such as the opioid peptides. In the present study, we investigated the effect of EA on peripheral inflammatory pain and the expression of N/OFQ and the NOP receptor in the spinal dorsal horn of rats, using a behavioral test, RT-PCR, immunohistochemistry and Western blot analysis techniques. The results showed: (1) EA had an accumulative analgesic effect on chronic inflammatory pain; (2) in the superficial layers of the spinal dorsal horn, the level of mRNA of the precursor protein for N/OFQ (preproN/OFQ, ppN/OFQ) was increased and the N/OFQ immunoreactivity was decreased after peripheral inflammation, and could be significantly increased by EA treatment; (3) both mRNA and protein levels of the NOP receptor in the spinal dorsal horn were significantly increased after chronic inflammatory pain and could be further enhanced by EA treatment. The present data demonstrated that EA could activate the endogenous N/OFQ-NOP receptor system, and this might underlie the effectiveness of EA in the treatment of inflammatory pain.

On the enigma of pain and hyperalgesia: A molecular perspective

Pain is a common symptom of injuries and inflammatory-related conditions. The perception of pain, commonly known as nociception, depends on integrated receptors and molecular pathways. Inflammatory mediators are involved in the genesis, persistence, and severity of pain. Noxious stimuli can trigger a cascade of inflammatory loops that feedback onto sensory modalities and domains of the CNS, in an attempt to alert the brain of deregulated homeostasis. Understanding the mechanisms of pain continue to make nociception and hyperalgesia a burgeoning field of research.

GFAP and Fos immunoreactivity in lumbo-sacral spinal cord and medulla oblongata after chronic colonic inflammation in rats

AIM: To investigate the response of astrocytes and neurons in rat lumbo-sacral spinal cord and medulla oblongata induced by chronic colonic inflammation, and the relationship between them.

METHODS: Thirty-three male Sprague-Dawley rats were randomly divided into two groups: experimental group (n = 17), colonic inflammation was induced by intra-luminal administration of trinitrobenzenesulfonic acid (TNBS); control group (n = 16), saline was administered intra-luminally. After 3, 7, 14, and 28 d of administration, the lumbo-sacral spinal cord and medulla oblongata were removed and processed for anti-glial fibrillary acidic protein (GFAP), Fos and GFAP/Fos immunohistochemistry.

RESULTS: Activated astrocytes positive for GFAP were mainly distributed in the superficial laminae (laminae I-II) of dorsal horn, intermediolateral nucleus (laminae V), posterior commissural nucleus (laminae X) and anterolateral nucleus (laminae IX). Fos-IR (Fos-immunoreactive) neurons were mainly distributed in the deeper laminae of the spinal cord (laminae III-IV, V-VI). In the medulla oblongata, both GFAP-IR astrocytes and Fos-IR neurons were mainly distributed in the medullary visceral zone (MVZ). The density of GFAP in the spinal cord of experimental rats was significantly higher after 3, 7, and 14 d of TNBS administration compared with the controls (50.4±16.8, 29.2±6.5, 24.1±5.6, P<0.05). The density of GFAP in MVZ was significantly higher after 3 d of TNBS administration (34.3±2.5, P<0.05). After 28 d of TNBS administration, the density of GFAP in the spinal cord and MVZ decreased and became comparable to that of the controls (18.0±4.9, 14.6±6.4, P>0.05).

CONCLUSION: Astrocytes in spinal cord and medulla oblongata can be activated by colonic inflammation. The activated astrocytes are closely related to Fos-IR neurons. With the recovery of colonic inflammation, the activity of astrocytes in the spinal cord and medulla oblongata is reduced.

Foot temperature in diabetic polyneuropathy: innocent bystander or unrecognized accomplice?

AIM

To explore mechanisms by which temperature could influence the pathogenesis and symptoms of diabetic polyneuropathy.

METHODS

We conducted a literature review attempting to identify mechanisms by which diabetic polyneuropathy could be affected by temperature.

RESULTS

Cooling can theoretically hasten the progression of diabetic polyneuropathy through several different mechanisms. Specifically, cooling can enhance neuronal ischaemia, increase formation of reactive oxygen species, slow axonal transport, increase protein kinase C activity, and interfere with immune function. Short-term temperature fluctuations (both warming and cooling) can initiate and exacerbate neuropathic pain by causing neuronal hyperexcitability and functional deafferentation. Although normal fluctuations of distal extremity temperature may be sufficient for these effects, impaired thermoregulation may make the distal extremities more susceptible to temperature extremes. Eventually, a 'vicious cycle' may ensue, resulting in neuronal deterioration with further disruption of temperature regulation. Limited epidemiological data suggest a higher prevalence of diabetic polyneuropathy in populations living in colder locations, supporting our hypothesis.

CONCLUSIONS

Variations in foot temperature may play an important but as yet unrecognized role in the development and symptoms of diabetic polyneuropathy. Further basic and clinical research exploring this concept could help elucidate the natural history of diabetic polyneuropathy and lead to novel therapeutic strategies.

The neuromatrix theory of pain: implications for selected nonpharmacologic methods of pain relief for labor

Women experience the pain of labor differently, with many factors contributing to their overall perception of pain. The neuromatrix theory of pain provides a framework that may explain why selected nonpharmacologic methods of pain relief can be quite effective for the relief of pain for the laboring woman. The concept of a pain "neuromatrix" suggests that perception of pain is simultaneously modulated by multiple influences. The theory was developed by Ronald Melzack and represents an expansion beyond his original "gate theory" of pain, first proposed in 1965 with P. D. Wall. This article reviews several nonpharmacologic methods of pain relief with implications for the practicing clinician. Providing adequate pain relief during labor and birth is an important component of caring for women during labor and birth.

A parametric study of electroacupuncture on persistent hyperalgesia and Fos protein expression in rats

We previously reported the anti-hyperalgesia of electroacupuncture (EA) on persistent inflammatory pain in an unrestrained, unsedated, and conscious rat model. Using this model, induced by injecting complete Freund's adjuvant (CFA) into one hind paw, we systematically evaluated the anti-hyperalgesia of EA stimulation parameters (frequency, intensity, treatment duration, and pulse width). We assessed hyperalgesia by paw withdrawal latency (PWL) to a noxious thermal stimulus and found that 10- and 100-Hz EA frequencies at a current intensity of 3 mA produced the greatest anti-hyperalgesia, when compared to other parameters. Both frequencies significantly increased PWL in the early phases of hyperalgesia (2.5 and 24 h; p < 0.05), and 10 Hz EA also significantly increased PWL in later phases (5 to 7 days; p < 0.05). A sufficient but tolerable intensity of 3 mA was more effective than lower intensities (1-2 mA). A 20-min treatment produced better anti-hyperalgesia than longer and shorter (10 and 30 min) treatments. Acupoint specificity study demonstrated that GB30 produced significant EA anti-hyperalgesia, while Waiguan (TE5) and sham points, an abdominal point and a point at the opposite aspect of GB30, did not. The spinal Fos protein expression study demonstrated that the optimal EA selectively suppressed Fos expression in superficial laminae (I/II) and activated it in deeper laminae (III/IV) of the spinal dorsal horn. The results suggest that the EA anti-hyperalgesia is parameter-dependent and point-specific, and they provide important information for designing further clinical acupuncture research on persistent inflammatory pain.

Distinctive pattern of c-fos expression in the feline cervico-lumbar spinal cord after stimulation of vanilloid receptors in dorsal neck muscles

In the present study, c-fos expression in the spinal cord has been used as a marker of neuronal activation induced by capsaicin-sensitive sensory afferents from the dorsal neck muscles in cats (n = 6). The number of Fos-immunoreactive neurons, which were revealed using the avidin-biotin-peroxidase method, was significantly increased in the cervical and lumbar spinal cord. In contrast to the control group (n = 3), 2 h after intramuscular capsaicin injection, c-fos expression was more extensive ipsilaterally to the injected side in the C3-C6 segments, and bilaterally in the L4-L6 segments. Most labeled neurons in the cervical spinal cord were small and giant cells, predominantly located in the middle and lateral parts of lamina I and, additionally, at the neck of the dorsal horn (lamina V), i.e., within the zones of termination of high-threshold muscle afferents. The widespread distribution of labeled cells throughout the cervical cord within the intermediate zone (lamina VII) coincided with the sites of last-order premotor interneurons and cells of origin of long crossed and uncrossed descending propriospinal pathways to the lumbar spinal cord. These findings suggest possible mechanisms for spreading of nociceptive signals between cervical and lumbar regions.

Territorial and extra-territorial distribution of Fos protein in the lumbar spinal dorsal horn neurons in rats with chronic constriction nerve injuries

This study aimed to examine the relationship between temporal and spatial expression patterns of Fos protein in the spinal dorsal horn neurons and thermal hyperalgesia behaviors in rats with chronic constriction injury (CCI) to the sciatic nerve. Our results demonstrated that Fos protein expression in the spinal dorsal horn neurons at L5 segment ipsilateral and contralateral to CCI of the sciatic nerve was significantly greater than in sham rats from days 10 to 30 postoperatively (PO 10d to 30d), and was concentrated on the injury (ipsilateral) side. Unlike the short-lived expression after tissue inflammation, laminae I to VI (especially laminae III/IV) displayed a persistent greater number of Fos-like immunoreactive (Fos-LI) neurons for at least 30 days after CCI of the sciatic nerve. After the increase in laminae III/IV, Fos-LI neurons tended to gradually increase in laminae I/II and V/VI at L5 segment from PO 2d to 30d, which were correlated with the heat hyperalgesia (48 degrees C) behaviors measured by paw withdrawal latency in CCI rats but not in sham rats. Interestingly, a persistent increase of Fos-LI neurons in laminae I to VI at L5 segment of the ipsilateral and contralateral sides and at the L1 segment that was out of the normal central terminations of the sciatic nerve suggested the probable presence of territorial and extra-territorial central sensitization or inadequate central nervous system (CNS) adaptive mechanisms. These findings may partly explain why abnormal pain sensations are sometimes distributed in a pattern that does not coincide with the territories of nerves or with the posterior roots of the peripheral nerve after injury.

Ensuring competency in end-of-life care: controlling symptoms

BACKGROUND: Palliative medicine is assuming an increasingly important role in patient care. The Education for Physicians in End-of-life Care (EPEC) Project is an ambitious program to increase core palliative care skills for all physicians. It is not intended to transmit specialty level competencies in palliative care. METHOD: The EPEC Curriculum was developed to be a comprehensive syllabus including trainer notes, multiple approaches to teaching the material, slides, and videos of clinical encounters to trigger discussion are provided. The content was developed through a combination of expert opinion, participant feedback and selected literature review. Content development was guided by the goal of teaching core competencies not included in the training of generalist and non-palliative medicine specialist physicians. RESULTS: Whole patient assessment forms the basis for good symptom control. Approaches to the medical management of pain, depression, anxiety, breathlessness (dyspnea), nausea/vomiting, constipation, fatigue/weakness and the symptoms common during the last hours of life are described. CONCLUSION: While some physicians will have specialist palliative care services upon which to call, most in the world will need to provide the initial approaches to symptom control at the end-of-life.

Transitions in the concept of chronic pain

Chronic pain is a worldwide problem with significant physical, psychological, and social impacts. Despite its prevalence and cost, the phenomenon is not well understood. An inductive method of concept analysis was used to study the concept of chronic, non-cancerous pain in adults. A random sample of nursing, psychology, and neurophysiology literature published over a 30-year period (1969 through 1999) was used to generate a consensual definition of chronic pain. The transition in the attributes, antecedents, consequences, related concepts, and surrogate terms of chronic pain is described, and the implications of the findings for practice and research are discussed.

Pathophysiological tissue changes associated with repetitive movement: a review of the evidence

Work-related musculoskeletal disorders (WMSDs) represent approximately one third of workers' compensation costs in US private industry, yet estimates of acceptable exposure levels for forceful and repetitive tasks are imprecise, in part, due to lack of measures of tissue injury in humans. In this review, the authors discuss the scope of upper-extremity WMSDs, the relationship between repetition rate and forcefulness of reaching tasks and WMSDs, cellular responses to injury in vivo and in vitro, and animal injury models of repetitive, forceful tasks. The authors describe a model using albino rats and present evidence related to tissue injury and inflammation due to a highly repetitive reaching task. A conceptual schematic for WMSD development and suggestions for further research are presented. Animal models can enhance our ability to predict risk and to manage WMSDs in humans because such models permit the direct observation of exposed tissues as well as motor behavior.

Differentially expressed genes in rat dorsal root ganglia following peripheral nerve injury

Ordered differential display PCR was used to identify differentially expressed genes in rat dorsal root ganglia at 7 days following chronic constriction injury (CCI) of the sciatic nerve. Fourteen differentially displayed cDNA bands were isolated, cloned and verified by RT-PCR. The four mRNAs were increased, which included mRNAs encoding heat shock protein 27, fatty acid binding protein, apolipoprotein D and one novel gene. Six down-regulated clones were microtubule-associated protein 1B, protein tyrosine phosphatase alpha, Kv1.2 channel, myelin protein SR13, medium-sized neurofilament protein, and one novel gene. Our results show that many differentially regulated genes after CCI may play a role in nerve degeneration and/or regeneration and provide a molecular framework for understanding the peripheral mechanism underlying neuropathic pain.

The pain of being sick: implications of immune-to-brain communication for understanding pain

This review focuses on the powerful pain facilitatory effects produced by the immune system. Immune cells, activated in response to infection, inflammation, or trauma, release proteins called proinflammatory cytokines. These proinflammatory cytokines signal the central nervous system, thereby creating exaggerated pain as well as an entire constellation of physiological, behavioral, and hormonal changes. These changes are collectively referred to as the sickness response. Release of proinflammatory cytokines by immune cells in the body leads, in turn, to release of proinflammatory cytokines by glia within the brain and spinal cord. Evidence is reviewed supporting the idea that proinflammatory cytokines exert powerful pain facilitatory effects following their release in the body, in the brain, and in the spinal cord. Such exaggerated pain states naturally occur in situations involving infection, inflammation, or trauma of the skin, of peripheral nerves, and of the central nervous system itself. Implications for human pain conditions are discussed.

Painful peripheral neuropathies

This article reviews the current pathophysiology of painful peripheral neuropathies, their differential diagnosis, and management.

Inflammation Causes a Long-Term Hyperexcitability in the Nociceptive Sensory Neurons of Aplysia

Nerve injury, tissue damage, and inflammation all cause hyperalgesia. A factor contributing to this increased sensitivity is a long-term (>24 hr) hyperexcitability (LTH) in the sensory neurons that mediate the responses. Using the cluster of nociceptive sensory neurons in Aplysia californica as a model, we are examining how inflammation induces LTH. A general inflammatory response was induced by inserting a gauze pad into the animal. Within 4 days, the gauze is enmeshed in an amorphous material that contains hemocytes, which comprise a cellular immune system. Concurrently, LTH appears in both ipsilateral and contralateral sensory neurons. The LTH is manifest as increased action potential discharge to a normalized stimulus. Immunocytochemistry revealed that hemocytes have antigens recognized by antibodies to TGFβ1, IL-6, and 5HT. When a localized inflammation was elicited on a nerve, hemocytes containing the TGFβ1 antigen were present near axons within the nerve and those containing the IL-6 were on the surface. Western blots of hemocytes, or of gauze that had induced a foreign body response, contained a 28-kD polypeptide recognized by the anti-TGFβ1 antibody. Exposure of the nervous system to recombinant human TGFβ1 elicited increased firing of the nociceptive neurons and a decrease in threshold. The TGFβ1 also caused an activation of protein kinase C (PKC) in axons but did not affect a kinase that is activated in axons after injury. Our findings, in conjunction with previous results, indicate that a TGFβ1-homolog can modulate the activity of neurons that respond to noxious stimuli. This system could also contribute to interactions between the immune and nervous systems via regulation of PKC.

Activation of protein kinase A contributes to the expression but not the induction of long-term hyperexcitability caused by axotomy of Aplysia sensory neurons

Nociceptive sensory neurons (SNs) in Aplysia provide useful models to study both memory and adaptive responses to nerve injury. Induction of long-term memory in many species, including Aplysia, is thought to depend on activation of cAMP-dependent protein kinase (PKA). Because Aplysia SNs display similar alterations in models of memory and after nerve injury, a plausible hypothesis is that axotomy triggers memory-like modifications by activating PKA in damaged axons. The present study disproves this hypothesis. SN axotomy was produced by (1) dissociation of somata from the ganglion [which is shown to induce long-term hyperexcitability (LTH)], (2) transection of neurites of dissociated SNs growing in vitro, or (3) peripheral nerve crush. Application of the competitive PKA inhibitor Rp-8-CPT-cAMPS at the time of axotomy failed to alter the induction of LTH by each form of axotomy, although the inhibitor antagonized hyperexcitability produced by 5-HT application. Strong activation of PKA in the nerve by coapplication of a membrane-permeant analog of cAMP and a phosphodiesterase inhibitor was not sufficient to induce LTH of either the SN somata or axons. Furthermore, nerve crush failed to activate axonal PKA or stimulate its retrograde transport. Therefore, PKA activation plays little if any role in the induction of LTH by axotomy. However, the expression of LTH was reduced by intracellular injection of the highly specific PKA inhibitor PKI several days after nerve crush. This suggests that long-lasting activation of PKA in or near the soma contributes to the maintenance of long-term modifications produced by nerve injury.

Effects of neurotropin on hyperalgesia and allodynia in mononeuropathic rats

Neurotropin is commonly used in Japan for the treatment of chronic pain. Using a rat model, we evaluated the effect of neurotropin on a unilateral peripheral mononeuropathy produced by placing loose ligatures around the sciatic nerve. The effect of neurotropin upon the resultant hyperalgesia and mechanical allodynia was assessed using the Ugo Basile Plantar test and von Frey hairs test, respectively. Neurotropin reduced thermal hyperalgesia and produced an early recovery from hyperalgesia in a dose-dependent manner. No significant reduction in mechanical allodynia, however, was noted under the tested condition. A possibility of differential drug sensitivity for thermal hyperalgesia and mechanical allodynia was indicated in this model, although the reason still remain elusive.

Central and peripheral analgesia mediated by the acetylcholinesterase-inhibitor neostigmine in the rat inflamed knee joint model

Intrinsic cholinergic inhibitory pathways present a key modulating system in pain perception. The use of intrathecal (i.t.) acetylcholinesterase-inhibitors, such as neostigmine, result in analgesia in both preclinical and clinical models. However, whether i.t. neostigmine suppresses tonic persistent pain or has peripheral sites of antinociceptive action has not been determined. Thus, we studied central (i.t.) and peripheral (intraarticular; i.a.) neostigmine in a rat inflamed knee joint model. Inhibition of thermal and mechanical hyperalgesia was assessed over 28 h using a modified Hargreaves box and von Frey hairs, respectively. I.t. neostigmine resulted in a dose-dependent thermal analgesia (50% of maximal effective dose [ED50] 0-4 h: 6.6 microg, 24-28 h: 9.4 microg) and mechanical analgesia (ED50 0-4 h: 3.5 microg, 24-28 h: 4.3 microg). I.t. atropine reversed analgesia by i.t. neostigmine. I.a. neostigmine also resulted in an i.a. atropine reversible dose-dependent increase of thermal analgesia, although it did not exceed 60% of a maximal possible analgesic effect with the largest applied dose (ED50 0-4 h: 76.2 microg, 24-28 h: 140.1 microg). Partial suppression of mechanical hyperalgesia was observed after i.a. neostigmine. We conclude that centrally administered neostigmine modulates thermal and mechanical antinociception in this animal model of inflammatory pain. These data suggest a peripheral site of muscarinic antinociception.

IMPLICATIONS

This animal study shows that administration of the acetylcholinesterase-inhibitor neostigmine results in enhanced levels of the endogenous neurotransmitter acetylcholine, which seems to act as one of a group of analgesia-modulating compounds at central and peripheral sites in inflammatory pain.