Mediation of spinal nerve injury induced tactile allodynia by descending facilitatory pathways in the dorsolateral funiculus in rats

Descending facilitatory control of mechanically evoked responses is enhanced in deep dorsal horn neurones following peripheral nerve injury

Pain resulting from peripheral nerve injury, characterised by ongoing pain, hyperalgesia and allodynia arises from peripheral and central processes. Here, we studied the potential role of central facilitations in nerve injury by investigating the effect of blocking the excitatory 5HT3 receptor with ondansetron. 5HT3 receptors play a pronociceptive role in the spinal cord and ondansetron has previously been shown to produce antinociception in behavioural studies. We investigated the effects of spinally administered ondansetron (10, 50 and 100 microg) on the responses of deep dorsal horn neurones, evoked by peripheral electrical stimuli and a range of natural (mechanical punctate and heat) stimuli, 2 weeks after nerve injury induced through tight ligation of L5/6 spinal nerves (SNL). Comparisons were made between SNL rats and a sham-operated group. Ondansetron produced little effect on the electrically evoked responses (Abeta-, Adelta- and C-fibre-evoked responses, postdischarge); however, responses to mechanical punctate stimuli (von Frey filaments 1-75 g) were markedly reduced in both SNL and control groups. Furthermore, the drug effect was significantly enhanced after SNL (p<0.05). In particular, the lowest dose (10 microg) now became effective after SNL. Ondansetron produced less marked effects on thermal responses. Our results demonstrate that neuropathic pain states are associated with an enhanced descending facilitatory control of mechanical responses of spinal neurones, mediated through the activation of spinal 5HT3 receptors. These excitatory influences are likely to contribute to the development and maintenance of central sensitisation in the spinal cord, and furthermore, to the behavioural manifestation of tactile allodynia.

Antinociceptive and nociceptive actions of opioids

Although the opioids are the principal treatment options for moderate to severe pain, their use is also associated with the development of tolerance, defined as the progressive need for higher doses to achieve a constant analgesic effect. The mechanisms which underlie this phenomenon remain unclear. Recent studies revealed that cholecystokinin (CCK) is upregulated in the rostral ventromedial medulla (RVM) during persistent opioid exposure. CCK is both antiopioid and pronociceptive, and activates descending pain facilitation mechanisms from the RVM enhancing nociceptive transmission at the spinal cord and promoting hyperalgesia. The neuroplastic changes elicited by opioid exposure reflect adaptive changes to promote increased pain transmission and consequent diminished antinociception (i.e., tolerance).

Enhanced evoked excitatory transmitter release in experimental neuropathy requires descending facilitation

Nerve injury-induced afferent discharge is thought to elicit spinal sensitization and consequent abnormal pain. Experimental neuropathic pain, however, also depends on central changes, including descending facilitation arising from the rostral ventromedial medulla (RVM) and upregulation of spinal dynorphin. A possible intersection of these influences at the spinal level was explored by measuring evoked, excitatory transmitter release in tissues taken from nerve-injured animals with or without previous manipulation of descending modulatory systems. Spinal nerve ligation (SNL) produced expected tactile and thermal hyperesthesias. Capsaicin-evoked calcitonin gene-related peptide (CGRP) release was markedly enhanced in lumbar spinal tissue from SNL rats when compared with sham-operated controls. Enhanced, evoked CGRP release from SNL rats was blocked by anti-dynorphin A(1-13) antiserum; this treatment did not alter evoked release in tissues from sham-operated rats. Dorsolateral funiculus lesion (DLF) or destruction of RVM neurons expressing mu-opioid receptors with dermorphin-saporin, blocked tactile and thermal hypersensitivity, as well as SNL-induced upregulation of spinal dynorphin. Spinal tissues from these DLF-lesioned or dermorphin-saporin-treated SNL rats did not exhibit enhanced capsaicin-evoked CGRP-IR release. These data demonstrate exaggerated release of excitatory transmitter from primary afferents after injury to peripheral nerves, supporting the likely importance of increased afferent input as a driving force of neuropathic pain. The data also show that modulatory influences of descending facilitation are required for enhanced evoked transmitter release after nerve injury. Thus, convergence of descending modulation, spinal plasticity, and afferent drive in the nerve-injured state reveals a mechanism by which some aspects of nerve injury-induced hyperesthesias may occur.

The challenge of chronic pain

Chronic pain is a complex problem with staggering negative health and economic consequences. The complexity of chronic pain is presented within Cervero and Laird's model that describes three phases of pain, including pain without tissue damage, pain with tissue damage and inflammation, and neuropathic pain. The increased afferent input in phases 2 and 3 of chronic pain produces marked changes in primary afferents, dorsal root ganglia, and spinal cord dorsal horn. These changes promote the symptoms of chronic pain, including spontaneous pain, hyperalgesia, and allodynia. Increased afferent input also evokes supraspinal input to the dorsal horn, including biphasic innervation from the ventromedial medulla and A7 catecholamine cell group, that promotes hyperalgesia and allodynia. More rostral brain structures, such as the lateral hypothalamus, amygdala, and hippocampus, may also play a role in chronic pain. Although much has been discovered about the multiple pathological mechanisms involved in chronic pain, further research is needed to fully comprehend these mechanisms.

Neurotrophic factors as novel therapeutics for neuropathic pain

Neuropathic pain is a chronic condition that is caused by injury to the nervous system. Unlike acute pain, which is protective, neuropathic pain persists and serves no useful purpose, and severely affects quality of life. However, present therapies have modest efficacy in most patients, are palliative rather than curative, and their side effects represent significant limitations. Tremendous progress has been made over the past decade in our understanding of the biology of pain sensory neurons. The recent discovery that neurotrophic factors play an important role in neuropathic pain indicates that these pathways could serve as novel intervention points for therapy. Moreover, neurotrophic factors have the potential to address the underlying pathophysiology of neuropathic pain, thereby halting or reversing the disease process.

Peripheral and central sites of action for A-85380 in the spinal nerve ligation model of neuropathic pain

Neuronal nicotinic receptor (NNR) agonists such as ABT 594 have been shown to be effective in a wide range of preclinical models of acute and neuropathic pain. The present study, using the NNR agonist A-85380, sought to determine if NNR agonists are acting via similar or differing mechanisms to induce anti-nociception and anti-allodynia. A systemic administration of the quaternary NNR antagonist chlorisondamine (0.4 micromol/kg, intraperitoneal (i.p.)) did not alter A-85380-induced (0.75 micromol/kg, i.p.) anti-nociception in the rat paw withdrawal model of acute thermal pain. In contrast, previous studies have demonstrated that blockade of central NNRs by prior administration of chlorisondamine (10 microg i.c.v.) prevents A-85380 induced anti-nociception indicating a predominantly central site of action of NNR agonists in relieving acute pain. In the rat spinal nerve ligation model of neuropathic pain, A-85380 induced a dose-dependent anti-allodynia (0.5-1.0 micromol/kg) that was blocked by pretreatment with mecamylamine (1 micromol/kg). Interestingly, unlike acute pain, both systemic and central administration of chlorisondamine blocked A-85380-induced anti-allodynia, an effect that was determined not to be due to a non-specific effect of chlorisondamine or to chlorisondamine crossing the blood-brain barrier. The peripheral site of action was shown not to be the primary receptive field, since A-85380 had equally potent anti-allodynic effects when it was injected into either the affected or unaffected paw. In contrast, infusion of A-85380 directly onto the L5 dorsal root ganglion on the affected side resulted in a dose-dependent and marked anti-allodynia (10-20 microg) at doses that had no effect when injected systemically. This effect was blocked by pretreatment with chlorisondamine. Together these data further support the idea that different mechanisms underlie different pain states and suggest that the effects of NNR agonists in neuropathic pain may be due in part to peripheral actions of the compounds.

Physiology of chronic spinal pain syndromes: from animal models to biomechanics

STUDY DESIGN

The literature and current research related to spinal pain mechanisms were reviewed, as well as animal models related to its study.

OBJECTIVES

To provide a pragmatic discussion of spinal pain that both reviews relevant research and coherently synthesizes the existing body of literature related to pain, nociception, animal modeling, and injury biomechanics.

SUMMARY OF BACKGROUND DATA

A detailed body of literature suggests that spinal pain mechanisms are quite complicated and involve a host of different processes (e.g., genetics, gender, neurophysiology, and biomechanics) that may contribute to clinical manifestations and symptoms.

METHODS

Both a review of the literature and a presentation of current and ongoing laboratory research are presented. Specific findings from the authors' laboratory using a rodent model of lumbar radiculopathy are presented to elucidate the role of local nerve root biomechanics in initiating and maintaining behavioral symptoms of nociception and pain.

RESULTS

For an understanding of chronic pain, a bidirectional-translational approach that incorporates cross-disciplinary methods such as in vivo biomechanical techniques is required. A conceptual model of chronic spine pain is proposed that details the dynamic and integrated roles of injury, biomechanics, and nociceptive physiology.

CONCLUSIONS

Areas of continued research are highlighted that may help guide the management of painful spine symptoms and syndromes.

Abnormal brain processing of cutaneous pain in migraine patients during the attack

We examined cutaneous pain thresholds using CO(2) laser stimuli during migraine attacks, and defined the evoked cortical potential characteristics. Ten patients without aura were studied during attacks and for at least 72 h subsequently. Pain stimuli were generated on the dorsum of both hands and the right and left supraorbital zones, using pulses from a CO(2) laser. Absolute latencies of scalp potentials were measured at the highest peak of each response component, and the peak-to-peak amplitudes of N2a-P2 components were recorded. Cutaneous pain thresholds were significantly reduced on both the symptomatic and non-symptomatic sides during the attack, in comparison with the headache-free phase. The N2a-P2 complexes also increased in amplitude during attacks in comparison with the pain-free side. Thus, cutaneous hyperalgesia occurs during migraine attack, and is subtended by central sensitization phenomena, probably involving the cortex.

Time-dependent descending facilitation from the rostral ventromedial medulla maintains, but does not initiate, neuropathic pain

Although injury-induced afferent discharge declines significantly over time, experimental neuropathic pain persists unchanged for long periods. These observations suggest that processes that initiate experimental neuropathic pain may differ from those that maintain such pain. Here, the role of descending facilitation arising from developing plasticity in the rostral ventromedial medulla (RVM) in the initiation and maintenance of experimental neuropathic pain was explored. Tactile and thermal hypersensitivity were induced in rats by spinal nerve ligation (SNL). RVM lidocaine blocked SNL-induced tactile and thermal hypersensitivity on post-SNL days 6-12 but not on post-SNL day 3. Lesion of RVM cells expressing mu-opioid receptors with dermorphin-saporin did not prevent the onset of SNL-induced tactile and thermal hypersensitivity, but these signs reversed to baseline levels beginning on post-SNL day 4. Similarly, lesions of the dorsolateral funiculus (DLF) did not prevent the onset of SNL-induced tactile and thermal hypersensitivity, but these signs reversed to baseline levels beginning on post-SNL day 4. Lesions of the DLF also blocked the SNL-induced increase in spinal dynorphin content, which has been suggested to promote neuropathic pain. These data distinguish mechanisms that initiate the neuropathic state as independent of descending supraspinal influences and additional mechanism(s) that require supraspinal facilitation to maintain such pain. In addition, the data indicate that these time-dependent descending influences can underlie some of the SNL-induced plasticity at the spinal level. Such time-dependent descending influences driving associated spinal changes, such as the upregulation of dynorphin, are key elements in the maintenance, but not initiation, of neuropathic states.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Inhibition of neuropathic pain by selective ablation of brainstem medullary cells expressing the mu-opioid receptor

Neurons in the rostroventromedial medulla (RVM) project to spinal loci where the neurons inhibit or facilitate pain transmission. Abnormal activity of facilitatory processes may thus represent a mechanism of chronic pain. This possibility and the phenotype of RVM cells that might underlie experimental neuropathic pain were investigated. Cells expressing mu-opioid receptors were targeted with a single microinjection of saporin conjugated to the mu-opioid agonist dermorphin; unconjugated saporin and dermorphin were used as controls. RVM dermorphin-saporin, but not dermorphin or saporin, significantly decreased cells expressing mu-opioid receptor transcript. RVM dermorphin, saporin, or dermorphin-saporin did not change baseline hindpaw sensitivity to non-noxious or noxious stimuli. Spinal nerve ligation (SNL) injury in rats pretreated with RVM dermorphin-saporin failed to elicit the expected increase in sensitivity to non-noxious mechanical or noxious thermal stimuli applied to the paw. RVM dermorphin or saporin did not alter SNL-induced experimental pain, and no pretreatment affected the responses of sham-operated groups. This protective effect of dermorphin-saporin against SNL-induced pain was blocked by beta-funaltrexamine, a selective mu-opioid receptor antagonist, indicating specific interaction of dermorphin-saporin with the mu-opioid receptor. RVM microinjection of dermorphin-saporin, but not of dermorphin or saporin, in animals previously undergoing SNL showed a time-related reversal of the SNL-induced experimental pain to preinjury baseline levels. Thus, loss of RVM mu receptor-expressing cells both prevents and reverses experimental neuropathic pain. The data support the hypothesis that inappropriate tonic-descending facilitation may underlie some chronic pain states and offer new possibilities for the design of therapeutic strategies.

Pain Processing: Paradoxes and Predictions

During the last 25 years, there have been substantial advances in our understanding of the physiology and pathophysiology of pain. The development of animal models that more closely mimic clinical pain in humans has helped elucidate the putative mechanisms by which chronic pain develops and is maintained. However, our increased understanding of the neurobiology of pain has not translated into breakthrough treatments for pain management. As such, chronic pain is still primarily managed by drugs whose primary indication does not include pain (eg, antidepressants, anticonvulsants, antiarrhythmics, local anesthetics). These adjuvant analgesics have come into favor despite the fact that the mechanisms through which these drugs provide pain relief remain either largely unknown or are not selective for a single target. Moreover, the efficacy of adjuvant analgesics in animal models of pain is often validated only after case studies or clinical trials have been reported. This retrospective validation of "novel" analgesics in animal models of pain raises a question of the predictive validity of these models. This article reviews the use of several adjuvant and standard analgesics currently used to treat difficult-to-manage pain. What can these drugs teach us about the development of novel pain medicines? Within this context, the use of animal models of pain to predict analgesic efficacy in clinical pain conditions is considered.

Tonic descending facilitation from the rostral ventromedial medulla mediates opioid-induced abnormal pain and antinociceptive tolerance

Many clinical case reports have suggested that sustained opioid exposure can elicit unexpected, paradoxical pain. Here, we explore the possibility that (1) opioid-induced pain results from tonic activation of descending pain facilitation arising in the rostral ventromedial medulla (RVM) and (2) the presence of such pain manifests behaviorally as antinociceptive tolerance. Rats implanted subcutaneously with pellets or osmotic minipumps delivering morphine displayed time-related tactile allodynia and thermal hyperalgesia (i. e., opioid-induced "pain"); placebo pellets or saline minipumps did not change thresholds. Opioid-induced pain was observed while morphine delivery continued and while the rats were not in withdrawal. RVM lidocaine, or bilateral lesions of the dorsolateral funiculus (DLF), did not change response thresholds in placebo-pelleted rats but blocked opioid-induced pain. The intrathecal morphine antinociceptive dose-response curve (DRC) in morphine-pelleted rats was displaced to the right of that in placebo-pelleted rats, indicating antinociceptive "tolerance." RVM lidocaine or bilateral DLF lesion did not alter the intrathecal morphine DRC in placebo-pelleted rats but blocked the rightward displacement seen in morphine-pelleted animals. The subcutaneous morphine antinociceptive DRC in morphine-pelleted rats was displaced to the right of that in placebo-pelleted rats; this right shift was blocked by RVM lidocaine. The data show that (1) opioids elicit pain through tonic activation of bulbospinal facilitation from the RVM, (2) increased pain decreases spinal opioid antinociceptive potency, and (3) blockade of pain restores antinociceptive potency, revealing no change in antinociceptive signal transduction. These studies offer a mechanism for paradoxical opioid-induced pain and allow the development of approaches by which the loss of analgesic activity of opioids might be inhibited.