Changes in the response states of primate spinothalamic tract cells caused by mechanical damage of the skin or activation of descending controls

Wo der Schmerz in das Bewusstsein tritt: das thalamo-kortikale System bei der Schmerzverarbeitung

Die Übersetzung von schmerzhaften Reizen in Schmerzempfindungen wird durch mehrere periphere und zentrale Signalwege des Nervensystems verwirklicht. Man nimmt heute an, dass die Organisation dieser Signalwege die beiden Hauptfunktionen der Schmerzwahrnehmung wiederspiegeln: die Bewertung von schmerzhaften Reizen (wo, was, wie stark) und die Generierung negativer Emotionen. Experimentelle Befunde deuten darauf hin, dass aufsteigende Schmerzsignale über zwei Hauptwege im thalamokortikalen (TK) System verlaufen, die diese beiden Funktionen erfüllen. Wir diskutieren daher hier die strukturellen und funktionellen Befunde, die zu der Auffassung führten, dass diskriminierende Schmerzbewertung im lateralen TK-Weg ausgeführt wird, während der mediale TK-Weg schmerzassoziierte aversive Emotionen generiert. Obwohl der Schwerpunkt dieses Übersichtsartikels auf akuter Schmerzverarbeitung liegt, gehen wir zum Schluss darauf ein, wie Veränderungen in diesen Signalwegen zu pathologischen Schmerzempfindungen bei Menschen und Tiermodellen führen können.

Sustained neuronal hyperexcitability is evident in the thalamus after a transient cervical radicular injury

STUDY DESIGN

This study used extracellular electrophysiology to examine neuronal hyperexcitability in the ventroposterolateral nucleus (VPL) of the thalamus in a rat model of painful radiculopathy.

OBJECTIVE

The goal of this study was to quantify evoked neuronal excitability in the VPL at day 14 after a cervical nerve root compression to determine thalamic processing of persistent radicular pain.

SUMMARY OF BACKGROUND DATA

Nerve root compression often leads to radicular pain. Chronic pain is thought to induce structural and biochemical changes in the brain affecting supraspinal signaling. In particular, the VPL of the thalamus has been implicated in chronic pain states.

METHODS

Rats underwent a painful transient C7 nerve root compression or sham procedure. Ipsilateral forepaw mechanical allodynia was assessed on days 1, 3, 5, 7, 10, and 14 and evoked thalamic neuronal recordings were collected at day 14 from the contralateral VPL, whereas the injured forepaw was stimulated using a range of non-noxious and noxious mechanical stimuli. Neurons were classified on the basis of their response to stimulation.

RESULTS

Behavioral sensitivity was elevated after nerve root compression starting at day 3 and persisted until day 14 (P < 0.049). Thalamic recordings at day 14 demonstrated increased neuronal hyperexcitability after injury for all mechanical stimuli (P < 0.024). In particular, wide dynamic range neurons demonstrated significantly more firing after injury compared with sham in response to von Frey stimulation (P < 0.0001). Firing in low threshold mechanoreceptive neurons was not different between groups.

CONCLUSION

These data demonstrate that persistent radicular pain is associated with sustained neuronal hyperexcitability in the contralateral VPL of the thalamus. These findings suggest that thalamic processing is altered during radiculopathy and these changes in neuronal firing are associated with behavioral sensitivity.

LEVEL OF EVIDENCE

N/A.

Memantine attenuates responses of spinothalamic tract cells to cutaneous stimulation in neuropathic monkeys

Several lines of evidence indicate that N-methyl-D-aspartate (NMDA) receptors play an important role in nociception in general and in pathological pain in particular. It has been previously demonstrated in behavioral studies that NMDA receptor antagonists attenuate pathological pain in humans and nociceptive behaviors in animals. In the present study, we investigated the effect of the NMDA receptor antagonist memantine (MEM) on the responses of spinothalamic tract (STT) cells in normal and neuropathic monkeys. Memantine was delivered into the spinal cord through a microdialysis fiber acutely implanted into the dorsal horn. Responses of STT cells to peripheral stimulation within their receptive fields were recorded before and after MEM infusion. In normal animals (n = 7), 10 mm MEM did not affect STT cell (n = 7) baseline activity or responses to mechanical stimuli (brush, press or pinch). In neuropathic animals (n = 6), 1.0, 3.0, 10.0 and 100 mm MEM did not affect baseline activity of STT cells (n = 7); however, in a dose-dependent fashion, it significantly reduced responses of these cells to all cutaneous stimuli. The data suggest that MEM can have a direct effect on STT cells, blocking NMDA receptors known to be present on this cell population and, furthermore, may be a therapeutic agent for chronic pain.

Selective Nerve Root Injections Can Predict Surgical Outcome For Lumbar and Cervical Radiculopathy

OBJECTIVE

Diagnostic selective nerve root injection (SNI) results were analyzed in 101 patients who underwent lumbar or cervical decompression for radiculopathy and compared to surgical outcome 1 year postoperatively. A comparison of surgical outcomes was also examined between magnetic resonance imaging (MRI) and SNI results.

RESULTS

Of the 101 patients, 91 (90%) had positive and 10 had negative SNI results at the level operated. Ninety-one percent of the patients with a positive SNI had good surgical outcomes, whereas 60% of the patients with a negative SNI had good outcomes. Of the patients with a positive MRI result, 87% had good surgical outcomes, whereas a similar percentage of the patients with a negative MRI (85%) had good surgical outcomes. When findings between SNI and MRI differed (n = 20), surgery at a level consistent with the SNI was more strongly associated with a good surgical outcome. Of the patients with a poor surgical outcome, surgery was most often performed at a level inconsistent with the SNI finding.

CONCLUSIONS

Our study found that a diagnostic SNI can safely and accurately discern the presence or absence of cervical or lumbar radiculopathy. The diagnostic SNI can persuade surgeons from operating on an initially suspicious, but incorrect, level of radiculopathy. In cases where MRI findings are equivocal, multilevel, and/or do not agree with the patient's symptoms, the result of a negative diagnostic SNI (ie, lack of presence of radiculopathy) becomes superior in predicting the absence of an offending lesion.

Protein phosphatase modulates the phosphorylation of spinal cord NMDA receptors in rats following intradermal injection of capsaicin

The present study investigates the role of serine/threonine protein phosphatase 2A (PP2A) in the modulation of the phosphorylation of the NR1 and NR2B subunits of NMDA receptors in the spinal cord of rats following intradermal injection of capsaicin. The effects of a specific inhibitor of PP2A, fostriecin, on the expression of NR1, phospho-NR1, NR2B, and phospho-NR2B subunits of the NMDA receptor in the spinal cord of rats following noxious stimulation were examined. After continually perfusing with ACSF or fostriecin (3 microM) through a previously implanted microdialysis fiber for 30 min, central sensitization was initiated by injection of capsaicin into the plantar surface of the left paw of rats. The spinal cord was removed at different time points (30, 60, 90, 120, 180 min) after intradermal injection of capsaicin. Western blots were performed to examine the expression of NMDA subunits in spinal cord tissue by using specific antibodies. We found that the upregulated phosphorylation of both NR1 and NR2B subunits induced by capsaicin injection was significantly potentiated by the PP2A inhibitor without affecting the NR1 and NR2B protein itself. These results suggest that PP2A may have a regulatory effect on central sensitization induced by noxious stimuli in the periphery by regulating the phosphorylation state of NMDA receptors.

Response characterstics of spinothalamic tract neurons that project to the posterior thalamus in rats

A sizeable number of spinothalamic tract axons terminate in the posterior thalamus. The functional roles and precise areas of termination of these axons have been a subject of recent controversy. The goals of this study were to identify spinothalamic tract neurons (STT) within the cervical enlargement that project to this area, characterize their responses to mechanical and thermal stimulation of their receptive fields, and use microantidromic tracking methods to determine the nuclei in which their axons terminate. Forty-seven neurons were antidromically activated using low-amplitude (< or =30 microA) current pulses in the contralateral posterior thalamus. The 51 points at which antidromic activation thresholds were lowest were surrounded by ineffective tracks indicating that the surrounded axons terminated within the posterior thalamus. The areas of termination were located primarily in the posterior triangular, medial geniculate, posterior and posterior intralaminar, and suprageniculate nuclei. Recording points were located in the superficial and deep dorsal horn. The mean antidromic conduction velocity was 6.4 m/s, a conduction velocity slower than that of other projections to the thalamus or hypothalamus in rats. Cutaneous receptive fields appeared to be smaller than those of neurons projecting to other areas of the thalamus or to the hypothalamus. Each of the examined neurons responded exclusively or preferentially to noxious stimuli. These findings indicate that the STT carries nociceptive information to several target nuclei within the posterior thalamus. We discuss the evidence that this projection provides nociceptive information that plays an important role in fear conditioning.

Dorsal horn neuron response patterns to graded heat stimuli in the rat

Sensory input from various receptors in the periphery first becomes integrated in the spinal cord dorsal horn. The response of the spinal cord dorsal horn neurons to mechanical stimuli are classified as low threshold, high threshold, and wide dynamic range neurons. However, the response pattern of deep dorsal horn cells to heat has not been well described. In this study, the response of the spinal cord dorsal horn neurons to graded heat stimuli were characterized in 147 neurons in rats by extracellular single cell recording. After a differentiable cell was identified, the Peltier heat stimulator was applied to the receptive field and the base temperature was set at 30 degrees C. The heat stimulus was delivered for 10 s from 37-51 degrees C in 2 degrees C increments, with an inter-stimulus interval of 30 s. Out of the 147 neurons, five statistically distinguishable response patterns were identified by latent class cluster analysis. It is concluded that variation of temperature may account for the observed results and indicate functionally different subsets of heat-responsive cells in the deep dorsal horn.

The functional anatomy of neuropathic pain

The generation of neuropathic pain is a complex phenomenon involving a process of peripheral and central sensitization producing enhanced transmission of nociceptive inputs to the brain associated with the loss of discriminatory processing of noxious and innocuous stimuli. This increased flow of abnormally processed nociceptive inputs to the brain may overcome the ability of descending modulatory pathways to produce analgesia, causing further worsening of the pain. Several crucial locations involved in the physiologic generation of pain inputs (eg, peripheral nociceptors, dorsal horns, thalamus, cortex) show evidence of functional reorganization and altered nociceptive processing in association with chronic pain. These locations present the best targets for therapeutic intervention, including systemic administration of drugs able to counteract the chemical storm induced by neural injuries in the nociceptive afferents and dorsal horns, or for more focused intervention, such as neuroablative procedures; intrathecal drug delivery; and spinal cord, deep brain, or motor cortex stimulation.

A critical review of the role of the proposed VMpo nucleus in pain

The evidence presented by Craig and his colleagues for an important projection from lamina I spinothalamic tract neurons to a renamed thalamic nucleus (the posterior part of the ventral medial nucleus or VMpo), as well as to the ventrocaudal medial dorsal and the ventral posterior inferior thalamic nuclei, is critically reviewed. Of particular concern is the denial of an important nociceptive lamina I projection to the ventrobasal complex. Contrary evidence is reviewed that strongly favors a role of spinothalamic projections from both lamina I and deep layers of the dorsal horn to the ventrobasal complex and other thalamic nuclei and from there to the SI and SII somatosensory cortices in the sensory-discriminative processing of pain and temperature information.

Electrophysiological changes in adult rat dorsal horn neurons after neonatal peripheral inflammation

Neonatal peripheral inflammation has been shown to produce profound anatomical changes in the dorsal horn of adult rats. In this study, we explored whether parallel physiological changes exist. Neonatal rats were injected with complete Freund's adjuvant (CFA) into the left hind paw. At 8-10 wk of age, single dorsal horn neurons were recorded in response to graded intensities of mechanical stimuli delivered to the receptive field. In addition, cord dorsum potentials, produced by electrical stimuli delivered to the left sciatic nerve at 2.5x threshold, were recorded bilaterally from L2 to S3. There were significant increases in background activity and responses to brush and pinch in neonatal rats that were treated with CFA, as compared with control rats. Further analysis showed similar significant changes when dorsal horn neurons were categorized into wide dynamic range (WDR), high-threshold (HT), and low-threshold (LT) groups. The receptive field was significantly larger in neonatally treated rats as compared with control rats. Additionally, there was a significant increase in the response to a 49 degrees C heat stimulus in neonatally treated rats as compared with control rats. There was also a trend for the amplitudes of N1, N2, and P waves of the cord dorsum potential to increase and latencies to decrease in neonatally treated rats, but no significant differences were detected between different levels of the spinal cord (L2 to S3). These data further support the notion that anatomical and physiological plasticity changes occurred in the spinal cord following early neonatal CFA treatment.

Serotonergic neural precursor cell grafts attenuate bilateral hyperexcitability of dorsal horn neurons after spinal hemisection in rat

Hemisection of the rat spinal cord at thoracic level 13 provides a model of spinal cord injury that is characterized by chronic pain attributable to hyperexcitability of dorsal horn neurons. Presuming that this hyperexcitability can be explained in part by interruption of descending inhibitory modulation by serotonin, we hypothesized that intrathecal transplantation of RN46A-B14 serotonergic precursor cells, which secrete serotonin and brain-derived neurotrophic factor, would reduce this hyperexcitability by normalizing the responses of low-threshold mechanoreceptive, nociceptive-specific, and multireceptive dorsal horn neurons. Three groups (n=45 total) of 30-day-old male Sprague-Dawley rats underwent thoracic level 13 spinal hemisection, after which four weeks were allowed for development of allodynia and hyperalgesia. The three groups of animals received transplants of no cells, 10(6) RN46A-V1 (vector-only) or 10(6) RN46A-B14 cells at lumbar segments 2-3. Electrophysiological experiments were done two weeks later. Low-threshold mechanoreceptive, nociceptive-specific, and multireceptive cells (n=394 total) were isolated at depths of 1-300 and 301-1000 micro in the lumbar enlargement. Responses to innocuous and noxious peripheral stimuli were characterized, and analyses of population responses were performed. Compared with normal animals, dorsal horn neurons of all types in hemisected animals showed increased responsiveness to peripheral stimuli. This was true for neurons on both sides of the spinal cord. After hemisection, the proportion of neurons classified as multireceptive cells increased, and interspike intervals of spontaneous discharges became less uniform after hemisection. Transplantation of RN46A-B14 cells restored evoked responses to near-control levels, normalized background activity, and returned the proportion of multireceptive cells to the control level. Restoration of normal activity was reversed with methysergide.These electrophysiological results corroborate anatomical and behavioral studies showing the effectiveness of serotonergic neural precursors in correcting phenomena associated with chronic central pain following spinal cord injury, and provide mechanistic insights regarding mode of action.

Projections from the marginal zone and deep dorsal horn to the ventrobasal nuclei of the primate thalamus

It has been concluded recently that if a projection from the marginal zone to the ventral posterior lateral (VPL) nucleus exists, it is sparse. Given the importance of the marginal zone in nociception, this conclusion has raised doubts about the significance of the role of the ventrobasal complex in nociception. We have reexamined this projection using injections of the retrograde tracer, cholera toxin subunit B, into one side of the lateral thalamus in macaque monkeys. The injections were confined to the ventrobasal complex (with minimal spread to adjacent nuclei that do not receive spinal projections) in two animals. Many retrogradely labeled neurons were found in lamina I (as well as in lamina V) of the contralateral spinal and medullary dorsal horn. The results are consistent with the view that neurons in the marginal zone contribute prominently to the spinothalamic and trigeminothalamic projections to the VPL and ventral posterior medial (VPM) nuclei. This pathway is likely to be important for the sensory-discriminative processing of nociceptive information with respect to the location and intensity of painful stimuli.

Locations of spinothalamic tract axons in cervical and thoracic spinal cord white matter in monkeys

The spinothalamic tract (STT) is the primary pathway carrying nociceptive information from the spinal cord to the brain in humans. The aim of this study was to understand better the organization of STT axons within the spinal cord white matter of monkeys. The location of STT axons was determined using method of antidromic activation. Twenty-six lumbar STT cells were isolated. Nineteen were classified as wide dynamic range neurons and seven as high-threshold cells. Fifteen STT neurons were recorded in the deep dorsal horn (DDH) and 11 in superficial dorsal horn (SDH). The axons of 26 STT neurons were located at 73 low-threshold points (<30 microA) within the lateral funiculus from T(9) to C(6). STT neurons in the SDH were activated from 33 low-threshold points, neurons in the DDH from 40 low-threshold points. In lower thoracic segments, SDH neurons were antidromically activated from low-threshold points at the dorsal-ventral level of the denticulate ligament. Neurons in the DDH were activated from points located slightly ventral, within the ventral lateral funiculus. At higher segmental levels, axons from SDH neurons continued in a position dorsal to those of neurons in the DDH. However, axons from neurons in both areas of the gray matter were activated from points located in more ventral positions within the lateral funiculus. Unlike the suggestions in several previous reports, the present findings indicate that STT axons originating in the lumbar cord shift into increasingly ventral positions as they ascend the length of the spinal cord.

Comparative study of viscerosomatic input onto postsynaptic dorsal column and spinothalamic tract neurons in the primate

The purpose of the present investigation was to examine, in the primate, the role of the postsynaptic dorsal column (PSDC) system and that of the spinothalamic tract (STT) in viscerosensory processing by comparing the responses of neurons in these pathways to colorectal distension (CRD). Experiments were done on four anesthetized male monkeys (Macaca fascicularis). Extracellular recordings were made from a total of 100 neurons randomly located in the L(6)-S(1) segments of the spinal cord. Most of these neurons had cutaneous receptive fields in the perineal area, on the hind limbs or on the rump. Forty-eight percent were PSDC neurons activated antidromically from the upper cervical dorsal column or the nucleus gracilis, 17% were STT neurons activated antidromically from the thalamus, and 35% were unidentified. Twenty-one PSDC neurons, located mostly near the central canal, were excited by CRD and three were inhibited. Twenty-four PSDC neurons, mostly located in the nucleus proprius, did not respond to CRD. Of the 17 STT neurons, 7 neurons were excited by CRD, 4 neurons were inhibited, and 6 neurons did not respond to CRD. Of the unidentified neurons, 23 were excited by CRD, 7 were inhibited, and 5 did not respond. The average responses of STT and PSDC neurons excited by CRD were comparable in magnitude and duration. These results suggest that the major role of the PSDC pathway in viscerosensory processing may be due to a quantitative rather than a qualitative neuronal dominance over the STT.

Pathophysiological mechanisms of tension-type headache: a review of epidemiological and experimental studies

In this present thesis I have discussed the epidemiology and possible pathophysiological mechanisms of tension-type headache. A population-based study of 1000 subjects randomly selected from a general population, two clinical studies, and a method study of EMG recordings, were conducted. Tension-type headache was the most prevalent form of headache, with a life-time prevalence of 78% in a general adult population. Thirty percent were affected more than 14 days per year and 3% were chronically affected, i.e. had headache at least every other day. Females were more frequently affected than males, and young subjects more frequently affected than older subjects. Females were more sensitive to mechanical pressure pain and revealed more tenderness from pericranial muscles and tendon insertions than males, and young subjects were more pain-sensitive than older subjects. Significantly higher tenderness in pericranial muscles was found in subjects with tension-type headache compared to migraineurs and to subjects without any experience of headache. Tenderness increased significantly with increasing frequency of tension-type headache in both males and females, whereas no such relation was found for mechanical pain thresholds. The applied EMG methodology was fairly reliable and nonpainful, but due to intersubject variability paired studies should be preferred. Subjects with chronic tension-type headache had slightly increased EMG levels during resting conditions and decreased levels during maximal voluntary contraction compared with headache-free subjects, indicating insufficient relaxation at rest and impaired recruitment at maximal activity. In a subsequent clinical, controlled study, the effect of 30 min of sustained tooth clenching was studied. Within 24 h, 69% of patients and 17% of controls developed a tension-type headache. Shortly after clenching, tenderness was increased in the group who subsequently developed headache, whereas tenderness was stable in the group of patients who remained headache-free, indicating that tenderness might be a causative factor of the headache. Likewise, psychophysical and EMG parameters were studied in 28 patients with tension-type headache, both during and outside of a spontaneous episode of tension-type headache. It was concluded that a peripheral mechanism of tension-type headache is most likely in the episodic subform, whereas a secondary, segmental central sensitization and/or an impaired supraspinal modulation of incoming stimuli seems to be involved in subjects with chronic tension-type headache. Prolonged nociceptive stimuli from myofascial tissue may be of importance for the conversion of episodic into chronic tension-type headache. The author emphasizes that tension-type headache is a multifactorial disorder with several concurrent pathophysiological mechanisms, and that extracranial myofascial nociception may constitute only one of them. The present thesis supplements the understanding of the balance between peripheral and central components in tension-type headache, and thereby, hopefully, leads us to a better prevention and treatment of the most prevalent type of headache.

Acute pain mechanisms

The systems activated by tissue-injuring stimuli are complex. The nociceptive primary afferents have little spontaneous activity under normal conditions; however, after tissue injury, they display longlasting, ongoing activity. This results, in part, because the injury elicits the release of active factors that sensitize or excite the peripheral nerve terminal. A threshold that is lowered to the extent that body temperature and the pressure of edema are adequate stimuli results in spontaneous pain. This phenomenon is mediated by a variety of blood-borne active factors released during plasma extravasation, by agents released from local inflammatory cells, and by neurotransmitters released from the peripheral terminals of the primary afferent fibers themselves. Well-defined projections into the dorsal horn convey the "pain message" to at least two well-defined populations of neurons: those that are nociceptive specific and those that display an intensity-linked discharge over a range of stimuli from innocuous to noxious. Convergence from various fiber types, modalities, and end organs permits the encoding of afferent traffic with respect to intensity and location. The convergence of axons from somatic and visceral structures reflects the mechanism for the so-called "referred pain state." Most importantly, these dorsal horn systems have a dynamic component in addition to the hard-wiring; their output can be regulated both up and down. The up-regulation provides the basis for much of the facilitated processing that is believed to account for a significant percentage of the postinjury pain state. The facilitated state has a unique pharmacology, with the underlying mechanisms reflecting a cascade of actions that starts with the NMDA receptor and proceeds through the spinal release of intermediaries, such as prostaglandins and nitric oxide. Conversely, the ability to down-regulate the dorsal horn stimulus response function accounts for the powerful control exerted by a wide variety of diverse factors, including the spinal delivery of opioid and nonopioid analgesics and the "endogenous analgesia system." These linkages reflect the complexity of the encoding mechanisms that transduce the tissue injury into the behavioral sequela known as pain. This article also emphasizes that, although considerable progress has been made in the past decade, the current pace of research promises greater insights.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Complex effects of general anesthesia on sensory processing in the spinal dorsal horn

A chronic animal preparation allowed us to compare activity of the same single, spinal dorsal horn neurons in the physiologically intact, awake, drug-free state and in the anesthetized state. The inhalation anesthetic enflurane produced profound, and at times, opposite effects on spinal dorsal horn neuron responses to non-noxious and noxious receptive field stimulation. Some effects would not have been predicted, based upon current understanding of anesthetics.

LTP of spinal A beta and C-fibre evoked responses after electrical sciatic nerve stimulation

Plasticity in the central nervous system may play an important role in clinical pain. The present study shows that long-term potentiation (LTP) may be induced in single wide dynamic range (WDR) neurons in the dorsal horn after high-frequency stimulation of the sciatic nerve in intact urethane anaesthetized rats. Extracellular recordings of firing responses after single pulse stimuli were made. The high-frequency conditioning stimulus increased the A beta- and C-fibre-mediated firing responses to single pulse stimuli by 60 and 130%, respectively, for more than 6 h. This finding supports a role for WDR neurons in 'nociceptive memory' in the dorsal horn. The model presented here may be an important tool for further investigations of mechanisms of plasticity within the dorsal horn.

Sensitization of postsynaptic dorsal column neuronal responses by colon inflammation

The role of a newly identified component of the postsynaptic dorsal column (PSDC) system in viscerosensory processing has been recently described. The purpose of this study was to examine the effect of colon inflammation on the responses of single PSDC cells, located in the vicinity of the central canal at L6-S1 spinal segments, to graded colorectal distension (CRD) and to cutaneous stimulation. Experiments were conducted on seven male Sprague-Dawley rats anesthetized with pentobarbital. Recordings were made from seven PSDC cells located around the central canal at L6-S1 in response to CRD and cutaneous stimulation before and after colon inflammation. Inflammation of the colon with mustard oil (MO) induced an increase in the background activity of these cells. Colon inflammation also potentiated the responses of the PSDC cells to graded CRD but not to cutaneous stimulation. This is consistent with previously observed effects of colon inflammation on the responses of viscerosensitive cells in the ventral posterolateral (VPL) nucleus of the thalamus and in the nucleus gracilis (NG). These observations support a role of the PSDC system in viscerosensory processing and primary visceral hyperalgesia.

Dextrorphan attenuates responses of spinothalamic tract cells in normal and nerve-injured monkeys

Spinal cord N-methyl-D-aspartate (NMDA) receptors play an important role in the transmission of acute and chronic pain. The present study investigated the ability of dextrorphan (DEX), a metabolite of dextromethorphan and a clinically safe NMDA antagonist, to attenuate the responses of nociceptive spinothalamic tract (STT) neurons in anesthetized monkeys. The STT cells were recorded extracellularly in the lumbosacral enlargement and were identified by antidromic activation from the ventral posterior lateral thalamic nucleus. DEX administered through a microdialysis fiber inserted into the dorsal horn inhibited the responses of STT cells in normal animals to noxious pinch and heat stimuli. In monkeys made neuropathic by tight ligation of the L7 or S1 spinal nerve, DEX significantly attenuated the responses of STT cells to noxious pinch and heat, as well as to innocuous brushing, pressure and von Frey filament stimuli. These findings strongly suggest that DEX should be considered a potentially useful therapeutic agent for the treatment of neuropathic pain in humans.

Comparison of three rodent neuropathic pain models

To characterize various animal models of neuropathic pain, we compared three previously developed rat models using the same behavioral testing methods. These models involve: (1) chronic constriction injury by loose ligation of the sciatic nerve (CCI); (2) tight ligation of the partial sciatic nerve (PSL); and (3) tight ligation of spinal nerves (SNL). Comparisons were made for the time course of behavioral signs representing various components of neuropathic pain as well as for the effects of surgical sympathectomy. In general, all three methods of peripheral nerve injury produced behavioral signs of both ongoing and evoked pain with similar time courses. However, there was a considerable difference in the magnitude of each pain component between models. Signs of mechanical allodynia were largest in the SNL injury and smallest in the CCI model. On the other hand, behavioral signs representing ongoing pain were much more prominent in the CCI model than in the other two. Although the behavioral signs of neuropathic pain tended to decrease after sympathectomy in all three models, the change was most evident in the SNL model. The results of the present study suggest that the three rat models tested have contrasting features, yet all are useful neuropathic pain models, possibly representing different populations of human neuropathic pain patients.

Neuroanatomy of the pain system and of the pathways that modulate pain

We review many of the recent findings concerning mechanisms and pathways for pain and its modulation, emphasizing sensitization and the modulation of nociceptors and of dorsal horn nociceptive neurons. We describe the organization of several ascending nociceptive pathways, including the spinothalamic, spinomesencephalic, spinoreticular, spinolimbic, spinocervical, and postsynaptic dorsal column pathways in some detail and discuss nociceptive processing in the thalamus and cerebral cortex. Structures involved in the descending analgesia systems, including the periaqueductal gray, locus ceruleus, and parabrachial area, nucleus raphe magnus, reticular formation, anterior pretectal nucleus, thalamus and cerebral cortex, and several components of the limbic system are described and the pathways and neurotransmitters utilized are mentioned. Finally, we speculate on possible fruitful lines of research that might lead to improvements in therapy for pain.

Neuronal model of tactile allodynia produced by spinal strychnine: effects of excitatory amino acid receptor antagonists and a mu-opiate receptor agonist

Touch evoked agitation (allodynia) can be induced by spinal delivery of strychnine and this effect is antagonized by intrathecal NMDA and non-NMDA receptor antagonists, but not by mu-opiate receptor agonists. In this study, we sought to characterize the effect of focal glycine-receptor inhibition on spontaneous and evoked activity in dorsal horn neurons of the chloralose-anesthetized cat. Strychnine (1 mM) applied near the neurons through a dialysis fiber caused an enhanced response to hair deflection, enlargement of the low threshold receptive fields and in some cells, an increase in afterdischarge. These changes were observed only in cells that were activated by both hair deflection and high intensity mechanical stimulation. Subsequent co-administration of an NMDA receptor antagonist (AP-7, 2.0 mM) preferentially blocked strychnine-associated effects without changing the original receptive field characteristics. Co-administration of a non-NMDA excitatory amino acid receptor antagonist (CNQX, 1 mM) with the strychnine served to block low (brush) and high intensity (pinch) afferent input. In contrast, addition of a mu-opiate receptor agonist (alfentanil 2.4 mM) to the strychnine perfusate selectively reduced responsiveness to high intensity stimulation, while having no effect on the exaggerated response to hair deflection. Given the functional and pharmacological similarity of the effects of spinal strychnine to post-nerve injury states in man, disinhibition due to a loss of glycinergic input may be associated with large myelinated fiber-mediated nociceptive states. Consistent with these data is the contention that under normal circumstances, afferent hair follicle input onto convergent neurons is regulated by a tonic glycinergic circuit. Removal of this regulatory influence leads to a magnification of low threshold tactile throughput in dorsal horn. This model may help to provide pharmacological insights into more efficacious treatments for such pain states that are relatively refractory to opioid therapies.

Basal and touch-evoked fos-like immunoreactivity during experimental inflammation in the rat

Fos-immunoreactivity can readily be induced in spinal cord neurones by noxious, but to a much more limited extent, by innocuous peripheral stimuli. The present study has investigated whether low intensity stimuli and electrical stimulation of A beta afferents elicit greater c-fos expression during the behavioural sensory hypersensitivity generated by experimental peripheral inflammation. We have examined the time-course of c-fos expression after inflammation produced by either an intra-plantar injection of the irritant turpentine oil or of complete Freund's adjuvant (CFA). In the former case, a significant initial expression in all dorsal horn laminae was followed by a gradual decrease, whereas after CFA injection, an initial expression limited to the superficial laminae subsequently extended into the deep laminae, with a decrease at 24 h and an increase in labelling at later times. Low intensity touch stimuli repeated for 10 min, when applied at 24 h and 48 h after CFA injection, elicited a significant increase in the number of Fos-immunoreactive neurons in both the superficial and deep laminae of the dorsal horn compared to non-inflamed animals. Electrical stimulation of the sciatic nerve 24 h post-CFA injection, at a strength sufficient only to activate A beta-afferents fibres (100 microA, 50 microseconds, 10 min), also elicited a significant increase in labelling relative to the same stimuli applied in control animals, especially in laminae V-VI. The present results demonstrate that low intensity cutaneous stimuli elicit a significantly greater increase in c-fos expression in dorsal horn neurons during peripheral inflammation and that A beta-afferent input contributes to this, a finding that may relate to the allodynia experienced during inflammation.

Is there a pathway in the posterior funiculus that signals visceral pain?

The present report provides evidence that axons in the medial part of the posterior column at T10 convey ascending nociceptive signals from pelvic visceral organs. This evidence was obtained from human surgical case studies and histological verification of the lesion in one of these cases, along with neuroanatomical and neurophysiological findings in animal experiments. A restricted lesion in this area can virtually eliminate pelvic pain due to cancer. The results remain excellent even in cases in which somatic structures of the pelvic body wall are involved. Following this procedure, neurological testing reveals no additional neurological deficit. There is no analgesia to pinprick stimuli applied to the body surface, despite the relief of the visceral pain. Since it is reasonable to attribute the favorable results of limited midline myelotomies to the interruption of axons of visceral nociceptive projection neurons in the posterior column, we have performed experiments in rats to test this hypothesis. The results in rats indicate that the dorsal column does indeed include a nociceptive component that signals pelvic visceral pain. The pathway includes neurons of the postsynaptic dorsal column pathway at the L6-S1 segmental level, axons of these neurons in the fasciculus gracilis, and neurons of the nucleus gracilis and the ventral posterolateral nucleus of the thalamus.

Three electrophysiological classes of guinea pig sympathetic postganglionic neurone have distinct morphologies

Sympathetic postganglionic neurones can be differentiated electrophysiologically into three classes (phasic, Ph; tonic, T; and long-afterhyperpolarising, LAH) based on their potassium channel expression and consequent differences in excitability. We tested whether neuronal morphology differs between these classes. Neurones in coeliac, inferior mesenteric, and lower lumbar paravertebral ganglia of guinea pigs were filled with biocytin during in vitro experiments in which electrical properties were recorded. The dimensions of somata and dendrites were measured in approximately equal numbers of stained neurones of each class. The three electrophysiological classes were distinct in terms of soma shape, soma size (Ph < T = LAH), total dendritic length (LAH < Ph < T) and average length of dendrites (LAH < Ph < T) (P < 0.0001, multivariate analysis of variance). The mean number of primary dendrites also differed (LAH 13, Ph 16, T 20). The majority of dendrites did not branch, the ratios of terminations to primary dendrites being 1.36 (LAH), 1.63 (Ph) and 1.81 (T). Overall, LAH neurones, with medium-sized somata but the smallest dendritic trees, were more distinct morphologically than Ph and T neurones. The morphological differences between classes were not dependent on differences in location. Further, there was no apparent relation between morphology and the pattern of synaptic input each class receives. The results indicate that three distinct groups of sympathetic postganglionic neurone exist in adult guinea pigs, although more than three functions are subserved by these neurones.

Specificity of diagnostic nerve blocks: a prospective, randomized study of sciatica due to lumbosacral spine disease

Temporary nerve blocks using local anesthetic are employed extensively in the evaluation of pain problems, particularly lumbosacral spine disease. Their specificity and sensitivity in localizing anatomic sources of pain have never been studied formally, however, and so their diagnostic and prognostic value is questionable. There have been anecdotal reports of relief of pain by temporary blocks directed to areas of pain referral, as opposed to areas of documented underlying pathology; but there has been no study to define the frequency or magnitude of this effect. We have examined the specificity and sensitivity of a battery of local anesthetic blocks in a series of 33 patients with a chief complaint of sciatica, attributable in all cases to spinal disease (radiculopathy, with some clinical features of arthropathy). As determined by blinded patient analog ratings in randomized sequence, three different nerve blocks were significantly more effective than control lumbar subcutaneous injection of an identical volume of 3 ml of 0.5% bupivacaine (P < 0.05). Not only paraspinal lumbosacral root blocks and medial branch posterior primary ramus blocks (at or proximal to the pathology), but also sciatic nerve blocks (distal or collateral to the pathology) produced temporary relief in a majority of patients. This confirmed the study hypothesis that false positive results are common, and specificity is low. For sciatic nerve blocks, specificity was between 24% and 36%. Patterns of responses specific to the established diagnosis of radiculopathy (i.e., root block most effective) had sensitivities between 9% and 42%. Statistical analysis of clinical and technical prognostic factors revealed that the only association with pain relief by any block were the effects of other blocks. The strongest association was between relief by sciatic nerve block and relief by medial branch posterior primary ramus (facet) block (P = 0.001, odds ratio 16.0). There were no associations between the results of blocks and clinical findings (history, physical examination, diagnostic imaging) in these patients, chosen for their homogeneous clinical presentation and absence of functional signs. Our findings indicate a limited role for uncontrolled local anesthetic blocks in the diagnostic evaluation of sciatica and referred pain syndromes in general. Negative blocks or a pattern of responses may have some predictive value, but isolated, positive blocks are non-specific. This lack of specificity may, however, be advantageous in therapeutic applications.

Contributions of injured and intact afferents to neuropathic pain in an experimental rat model

This study was conducted to determine the contribution of peripheral inputs from injured and intact afferent fibers to behavioral signs of neuropathic pain, using a previously developed neuropathic rat model. Neuropathic injury was produced by tightly ligating the left L5 and L6 spinal nerves; this procedure induced rats to display neuropathic pain behaviors in the ipsilateral hindlimb. The behaviors included signs of mechanical and cold allodynia, as well as ongoing pain. Five days after neuropathic injury, peripheral inputs from injured segments (L5 and L6) or intact segments (L3 and L4) were blocked by either transection of the dorsal roots or application of a local anesthetic (bupivacaine) to the roots. Dorsal rhizotomy of the injured segments reduced all components of neuropathic pain behaviors. In contrast, dorsal rhizotomy of the uninjured segments abolished behavioral signs of mechanical and cold allodynia, but signs of ongoing pain were preserved. Blocking afferent inputs by application of bupivacaine mimicked the results of dorsal rhizotomy, in a reversible manner. These results suggest that afferent signals from injured and intact fibers play distinctively different roles in neuropathic pain: inputs from injured afferents maintain all components of neuropathic pain, while those from intact afferents mediate evoked pain such as mechanical and cold allodynia. An hypothesis is proposed to explain the results of the present as well as other published studies.

Role of WDR neurons in a hind limb noxious heat evoked flexion withdrawal reflex

Behavioral experiments and neurophysiological experiments, the two major types of preclinical studies which have paved the way for the development of spinal analgesia were compared under identical conditions utilizing the same animals. The results demonstrate that the activation of the wide dynamic range (WDR) neurons preceded the behavioral withdrawal reflexes, and that the activation of the WDR neurons occurred at lower stimulus temperature than that for the withdrawal reflex. The results suggest that the neuronal activation began before the behavioral reflex but also that the firing frequency of the WDR neurons at the time of the withdrawal reflex could not distinguish between non-noxious and noxious stimuli. Further study is needed to elucidate the neuronal mechanisms of the activation of the behavioral reflex.

The pathophysiology of chronic pain--increased sensitivity to low threshold A beta-fibre inputs

Chronic pain is characterized by abnormal sensitivity, which is due to the generation of pain in response to the activation of the low-threshold mechanoreceptive A beta fibres that normally generate innocuous sensations. Three different processes in the spinal cord can account for this dramatic alteration in sensory processing in the somatosensory system: increased excitability, decreased inhibition and structural reorganization. All have been shown to occur and each may contribute separately or together to the wide range of chronic inflammatory and neuropathic pain disorders. The unravelling of the cellular mechanisms involved both offers the potential for developing novel therapeutic strategies, which reduce functional synaptic plasticity and prevent central atrophic and regenerative responses in injured neurones, and illustrates the capacity of the adult nervous system for maladaptive modification.

Sensitization of high mechanothreshold superficial dorsal horn and flexor motor neurones following chemosensitive primary afferent activation

Nociceptive primary afferents have the capacity to induce a state of increased excitability or central sensitization in dorsal horn neurones. This contributes to the mechanical hypersensitivity (allodynia) which occurs after peripheral tissue injury where low-mechanothreshold primary afferent activation begins to elicit pain. The relative susceptibility of dorsal horn cells with an apparent exclusive nociceptive input (nociceptive-specific (NS) or high-threshold (HT) cells) and those with a convergent input from low- and high-threshold mechanoreceptors (wide-dynamic-range (WDR) or multireceptive neurones) to sensitivity changes has been disputed. We have examined whether high-mechanothreshold neurones in the superficial dorsal and the ventral horn can modify their sensitivity following cutaneous application of the chemical irritant mustard oil. This produced both a prolonged reduction in the mechanical threshold of the cutaneous flexion withdrawal reflex, recorded from semitendinosus alpha-motor neurones, and an increase in the activity evoked in these neurones by low-intensity touch stimuli to the glabrous skin. Eight NS or HT only cells, defined in terms of their cutaneous mechanoreceptive field properties, were recorded in the superficial dorsal horn before and after cutaneous application of mustard oil. Mustard oil was applied outside of the mechanical receptive field of the cells and produced a transient increase in action potential discharge in 4 cells but increased the mechanoreceptive field size in all cells for 30-60 min. Mechanical thresholds declined in 6 cells to levels associated with low-threshold (LT) and WDR cells, and this was accompanied by recruitment of a novel brush/touch response in 5 cells. The responses evoked by graded electrical stimulation of the sural nerve were tested in 5 cells. Only 1 cell failed to show any change after mustard oil. In 3 cells, an increase in the response to A-fibre afferents occurred, a novel A-fibre response was recruited in 2 cells and the C-fibre response increased in 2 cells. Cells in the superficial dorsal horn of the rat spinal cord that are normally NS can begin, therefore, to respond to LT primary afferent mechanoreceptors after an increase in central excitability produced by activation of peripheral chemoreceptors. Sensitization of these, as well as of WDR cells, may contribute to the generation of post-injury mechanical pain and reflex hypersensitivity.

Basic and clinical aspects of visceral hyperalgesia

Although physiological stimuli in the healthy gastrointestinal tract are generally not associated with conscious perception, chronic abdominal discomfort and pain are the most common symptoms resulting in patient visits with gastroenterologists. Symptoms may be associated with inflammatory conditions of the gut or occur in the form of so-called functional disorders. The majority of patients with functional disorders appear to primarily have inappropriate perception of physiological events and altered reflex responses in different gut regions. Recent breakthroughs in the neurophysiology of somatic and visceral sensation are providing a series of plausible mechanisms to explain the development of chronic hyperalgesia within the human gastrointestinal tract. A central concept to all these mechanisms is the development of hyperexcitability of neurons in the dorsal horn, which can develop either in response to peripheral tissue irritation or in response to descending influences originating in the brainstem. Taking clinical characteristics and the concept of central hyperexcitability into account, a model is proposed by which abdominal pain from chronic inflammatory conditions of the gut and functional bowel disorders such as noncardiac chest pain, nonulcer dyspepsia, and irritable bowel syndrome could develop by multiple mechanisms either alone or in combination.

The effect of trans-ACPD, a metabotropic excitatory amino acid receptor agonist, on the responses of primate spinothalamic tract neurons

The responses of primate spinothalamic tract (STT) neurons to innocuous and noxious mechanical stimuli applied to the skin can be enhanced for more than an hour following prolonged noxious stimulation. This increased responsiveness is thought to reflect sensitization of dorsal horn neurons and may help account for secondary hyperalgesia and mechanical allodynia. The proposal that central sensitization is due to the activation of second messenger system was tested in this study by examining the effect of trans-ACPD (trans-D,L-1-amino-1,3-cyclopentanedicarboxylic acid), an agonist of metabotropic excitatory amino acid (EAA) receptors, introduced into the dorsal horn by microdialysis. A low dose of trans-ACPD resulted in an increase in the responses of STT cells to an innocuous mechanical stimulus (BRUSH), but no increase in the responses to noxious mechanical and thermal stimuli or in the excitation produced by iontophoretically applied EAAs. A high dose of trans-ACPD caused a transient increase in background activity, but no change in the responsiveness of spinothalamic cells to any of the test stimuli. It is concluded that low doses of trans-ACPD can selectively enhance transmission through interneuronal pathways mediating tactile inputs to spinothalamic cells.

A new strategy for the treatment of inflammatory pain. Prevention or elimination of central sensitization

The optimal treatment of pain requires an understanding of the mechanisms involved. Pain is a sensory end-point that can be generated by a number of dissimilar processes. Consequently, the concept of treating pain as a unitary symptom is obsolete. The mechanisms responsible for specific types of pain need to be understood, and particular treatments should be aimed selectively at the various subtypes of pain. A major breakthrough in our understanding of pain has come from the appreciation that clinical pain is qualitatively quite different from physiological or nociceptive pain and is characterised by the appearance of abnormal hypersensitivity. Clinical pain is more than a reflection of sustained peripheral input and it is, to a large extent, the expression of changes produced in the CNS, including the phenomenon of central sensitization. We need to treat both the disease/injury process in the periphery and the changes it induces or triggers in the CNS. Prevention of central sensitization will substantially eliminate the hyperalgesia and allodynia that patients find so distressing, and it offers new possibilities for the development of novel analgesics or antihypersensitivity drugs.

Chronic effects of topical application of capsaicin to the sciatic nerve on responses of primate spinothalamic neurons

The responses of 144 spinothalamic tract (STT) cells were recorded in 15 anesthetized macaque monkeys (Macaca fascicularis). Three to 4 weeks prior to the acute experiment, the sciatic nerve was surgically exposed on one or both sides so that capsaicin or vehicle could be applied. Responses of STT cells recorded in 3 experimental groups were compared: untreated (21 cells), vehicle-treated (40 cells), and capsaicin-treated (83 cells). The background activity of cells in the vehicle- and capsaicin-treated groups was the same as in the untreated group (that is, cells on the side contralateral to surgery). Responses to innocuous (BRUSH) and noxious (PINCH) mechanical stimuli were unchanged by vehicle or by capsaicin treatment. However, responses to other noxious (PRESSURE and SQUEEZE) mechanical stimuli were significantly increased in the vehicle-treated group. Compared with a large reference population, all experimental groups showed a significant increase in overall responsiveness to mechanical stimuli (as determined by cluster analysis), greatest in the vehicle-treated group. Responses to noxious heat stimuli were significantly reduced in the capsaicin-treated group for 45 degrees C and 47 degrees C stimuli. Volleys in A fibers, probably A delta fibers, evoked prolonged responses in many STT cells of all treatment groups. Electron microscopic counts of axons in the sciatic nerves of animals treated with capsaicin showed a reduced number of C fibers but no appreciable loss of myelinated axons. This loss of unmyelinated sensory fibers was presumably responsible for the reduction in the responses of the STT cells to noxious heat stimuli. Increased responses to some noxious mechanical stimuli and to A fiber volleys may have been the consequence of several factors, including surgical manipulation, a chemical action of vehicle and a contralateral action of capsaicin treatment.