The thermal grill illusion: unmasking the burn of cold pain

Investigation of the paradoxical painful sensation (‘illusion of pain’) produced by a thermal grill

A paradoxical painful sensation can be elicited by the simultaneous application of innocuous warm and cold stimuli to the skin. In the present study, we analyzed the conditions of production of this unique experimental illusion of pain in 52 healthy volunteers (27 men, 25 women). The stimuli were produced by a thermode composed of six bars whose temperature was controlled by Peltier elements. The temperature of alternate (even- and odd-numbered) bars could be controlled independently to produce various patterns of the 'thermal grill'. After measuring the cold and heat pain thresholds, a series of combinations of warm and cold stimuli, whose distance to the thermal pain threshold was at least 4 degrees C, were applied on the palmar surface of the right hand during 30s. After each stimulus, the subjects had to describe and rate their sensations on visual analog scales. Paradoxical painful sensations, mostly described as burning, were reported by all the subjects but three. However, the phenomenon was less frequent in approximately one third of ('low responder') volunteers. The frequency and intensity of such painful sensations were directly related to the magnitude (i.e. 5-25 degrees C) of the difference of the temperature between the warm and cold bars of the grill. The combination of increasingly colder temperature to a given warm temperature induces similar effects as combining increasingly warmer temperature to a given cold temperature. These results suggest that pain can be the result of a simple addition of non-noxious warm and cold signals.

Innocuous skin cooling modulates perception and neurophysiological correlates of brief CO2 laser stimuli in humans

The present study examined the influence of innocuous skin cooling on the perception and neurophysiological correlates of brief noxious CO2 laser stimuli. In nine normal subjects, brief CO2 laser pulses of four different intensities (duration 50 ms; diameter 5 mm; intensity range 5.8-10.6 mJ/mm2) were delivered at random every 5-10 s on the dorsum of the hand. Innocuous skin cooling was performed by a thermode (20 degrees C; 3x3 cm) with a central hole for the laser test stimuli. Quality and intensity (VAS) of perceptions, reaction times and laser evoked potentials (LEPs) were examined. Signal detection theory analysis was performed to evaluate discrimination performance and decision criterion. During innocuous skin cooling, detection threshold increased from 4.8+/-1.81 to 8.2+/-1.05 mJ/mm2 and pain threshold from 8.7+/-1.53 to 13.5+/-1.57 mJ/mm2. proportion of detected stimuli decreased from 87% to 48% and pain reports from 42% to 10%. The well localized 'pricking' sensation mediated by Adelta-nociceptors almost vanished. The intensity of sensations (VAS scores) was considerably reduced. Sensory discriminative performance was significantly depressed but decision criterion remained unchanged. Reaction times were delayed. The late-LEPs, correlates of Adelta-nociceptor activations, were also significantly depressed while the ultra-late LEPs, correlates of C-nociceptors, were not affected. Taken together, these results strongly suggest that innocuous skin cooling interfered with the sensory processing of laser heat stimuli and more prominently with those related to Adelta-nociceptive input.

Two populations of cold-sensitive neurons in rat dorsal root ganglia and their modulation by nerve growth factor

Cold sensing in mammals is not completely understood, although significant progress has been made recently with the cloning of two cold-activated ion channels, TRPM8 and TRPA1. We have used rat DRG neurons in primary culture and calcium fluorimetry to identify distinct populations of cold-sensitive neurons, which may underlie different functions. Menthol sensitivity clearly separated two classes of cold-responding neurons. One group was menthol-sensitive (MS), was activated at warmer temperatures and responded faster and with a larger increase in intracellular calcium concentration during cooling; the fraction of MS neurons in culture and their cold sensitivity were both increased in the presence of nerve growth factor. Neurons in the menthol-insensitive (MI) group required stronger cooling for activation than MS cells and neither their proportion nor their cold sensitivity were significantly altered by nerve growth factor. The two groups of cold-sensitive neurons also had different pharmacology. A larger fraction of MS cells were capsaicin-sensitive and coexpression of menthol and capsaicin sensitivity was observed in the absence of NGF. MI neurons were not stimulated by the super-cooling agent icilin or by the irritant mustard oil. Taken together these findings support a picture in which TRPM8 is the major player in detecting gentle cooling, while TRPA1 does not seem to be involved in cold sensing by MI neurons, at least in the temperature range between 32 and 12 degrees C.

Lamina I, but not Lamina V, Spinothalamic Neurons Exhibit Responses That Correspond With Burning Pain

Single-unit recordings from monkey spinothalamic tract (STT) neurons reveal that the responses of polymodal nociceptive lamina I STT neurons correspond with the profile of burning pain elicited in human subjects by repeated brief-contact heat. In contrast, lamina V wide-dynamic-range (WDR) neurons show a significantly different response pattern. This finding indicates that burning pain is signaled by modality-selective lamina I neurons and not convergent lamina V WDR neurons.

Allodynia in patients with post-stroke central pain (CPSP) studied by statistical quantitative sensory testing within individuals

The disinhibition hypothesis of post-stroke central pain (CPSP) suggests that 'the excessive response (dysesthesia/hyperalgesia/allodynia) is accompanied by a em leader loss of sensation' resulting from a lesion of a 'lateral nucleus' of thalamus or of 'cortico-thalamic paths' [Brain 34 (1911) 102]. One recent elaboration of this hypothesis proposes a submodality specific relationship, such that injury to a cool-signaling lateral thalamic pathway disinhibits a nociceptive medial thalamic pathway, thereby producing both burning, cold, ongoing pain and cold allodynia. The current study quantitatively evaluated the sensory loss and sensory abnormalities to discern submodality relationships between these sensory features of CPSP. The present results were statistically tested within individuals so that sensory loss and sensory abnormality are directly related by occurrence in the same individual. The results demonstrate that individuals with CPSP and normal tactile detection thresholds experience tactile allodynia significantly more often than those with tactile hypoesthesia. Most patients (11/13) exhibited hypoesthesia for the perception of cool stimuli, but few of these (2/11) showed cold allodynia. The most dramatic case of cold allodynia occurred in a patient who had a normal detection threshold for cold. Individuals with cold hypoesthesia, strictly contralateral to the cerebro-vascular accident (CVA or stroke), were often characterized by the presence of burning, cold, ongoing pain, and by the absence, not the presence, of cold allodynia. Overall, these results in CPSP suggest that tactile allodynia occurs in disturbances of thermal/pain pathways that spare the tactile-signaling pathways, and that cold hypoesthesia is neither necessary nor sufficient for cold allodynia.

Spinal cord injury pain--mechanisms and treatment

In spinal cord injury (SCI), pain is a major cause of disability. A review of experimental and human studies, which provide insight into the mechanisms and treatment of SCI neuropathic pain are presented. Each of a series of pathophysiologic changes after SCI may be relevant for the development of SCI neuropathic pain. These changes are discussed in relation to neuropathic pain at and below the level of SCI. SCI neuropathic pain is difficult to treat. Experimental and human randomized, double-blind, placebo-controlled, clinical trials on pharmacologic treatment of SCI pain are summarized.

Sensory perception in complete spinal cord injury

OBJECTIVES

To describe sensations evoked by painful or repetitive stimulation below injury level in patients with a clinically complete (American Spinal Injury Association, ASIA Grade A) spinal cord injury (SCI).

MATERIAL AND METHODS

Twenty-four patients (11 with central neuropathic pain and 13 without pain) with a traumatic SCI above the tenth thoracic vertebra were examined using quantitative sensory testing, MR imaging, and somatosensory evoked potentials (SEP).

RESULTS

Painful (pressure, pinch, heat or cold) or repetitive (pinprick) stimuli elicited vague localized sensations in 12 patients (50%). Pain, spasticity, and spasms were equally seen in SCI patients with or without localized sensations. SEP and MRI did not differentiate between these two groups.

CONCLUSION

The present study suggests retained sensory communication across the injury in complete SCI, i.e. 'sensory discomplete' SCI.

Temperature perception and nociception

The specificity theory of somesthesis holds that perceptions of warmth, cold, and pain are served by separate senses. Although no longer accepted in all its details, the theory's basic assumptions of anatomical and functional specificity have remained guiding principles in research on temperature perception and its relationship to pain. This article reviews the response characteristics of thermoreceptors, temperature-sensitive nociceptors, and their associated pathways in the context of old and new perceptual phenomena, most of which cannot be satisfactorily explained by the specificity theory. The evidence indicates that throughout most of the perceptual range, temperature sensitivity depends upon coactivation of, and interactions among, thermal and nociceptive pathways that are composed of both specific "labeled lines" and nonspecific, multimodal fibers. Adding to this complexity is evidence that tactile stimulation can influence the way in which thermal stimulation is perceived. It is argued that thermoreception is best defined as a functional subsystem of somesthesis that serves the very different and sometimes conflicting demands of thermoregulation, protection from thermal injury, and haptic perception.

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.

Postural stability is altered by the stimulation of pain but not warm receptors in humans

BACKGROUND

It is now recognized that large diameter myelinated afferents provide the primary source of lower limb proprioceptive information for maintaining an upright standing position. Small diameter afferents transmitting noxious stimuli, however, can also influence motor behaviors. Despite the possible influence of pain on motor behaviors, the effects of pain on the postural control system have not been well documented.

METHODS

Two cutaneous heat stimulations (experiment 1: non-noxious 40 degrees C; experiment 2: noxious 45 degrees C) were applied bilaterally on the calves of the subject with two thermal grills to stimulate A delta and C warm receptors and nociceptors in order to examine their effects on postural stability. The non-noxious stimulation induced a gentle sensation of warmth and the noxious stimulation induced a perception of heat pain (visual analogue scores of 0 and 46 mm, respectively). For both experiments, ten healthy young adults were tested with and without heat stimulations of the lower limbs while standing upright on a force platform with eyes open, eyes closed and eyes closed with tendon co-vibration of tibialis anterior and triceps surae muscles. The center of pressure displacements were analyzed to examine how both stimulations affected the regulation of quiet standing and if the effects were exacerbated when vision was removed or ankle proprioception perturbed.

RESULTS

The stimulation of the warm receptors (40 degrees C) did not induce any postural deterioration. With pain (45 degrees C), subjects showed a significant increase in standard deviation, range and mean velocity of postural oscillations as well as standard deviation of the center of pressure velocity. The effects of heat pain were exacerbated when subjects had both their eyes closed and ankle tendons vibrated (increased standard deviation of the center of pressure velocity and mean velocity of the center of pressure).

CONCLUSIONS

A non-noxious stimulation (40 degrees C) of the small diameter afferents is not a sufficiently intense sensory stimulation to alter the control of posture. A painful stimulation (45 degrees C) of the skin thermoreceptors, however, yielded a deterioration of the postural control system. The observed deteriorating effects of the combined stimulation of nociceptors and Ia afferents (when ankle tendons were vibrated) could result from the convergence of these afferents at the spinal level. This could certainly lead to the hypothesis that individuals suffering from lower limb pain present alterations of the postural control mechanisms; especially populations already at risk of falling (for example, frail elderly) or populations suffering from concomitant lower limb pain and sensory deficits (for example, diabetic polyneuropathy).

Relationships between skin temperature and temporal summation of heat and cold pain

Temporal summation of heat pain during repetitive stimulation is dependent on C nociceptor activation of central N-methyl-d-aspartate (NMDA) receptor mechanisms. Moderate temporal summation is produced by sequential triangular ramps of stimulation that control skin temperature between heat pulses but do not elicit distinct first and second pain sensations. Dramatic summation of second pain is produced by repeated contact of the skin with a preheated thermode, but skin temperature between taps is not controlled by this procedure. Therefore relationships between recordings of skin temperature and psychophysical ratings of heat pain were evaluated during series of repeated skin contacts. Surface and subcutaneous recordings of skin temperatures revealed efficient thermoregulatory compensation for heat stimulation at interstimulus intervals (ISIs) ranging from 2 to 8 s. Temporal summation of heat pain was strongly influenced by the ISIs and cannot be explained by small increases in skin temperature between taps or by heat storage throughout a stimulus series. Repetitive brief contact with a precooled thermode was utilized to evaluate whether temporal summation of cold pain occurs, and if so, whether it is influenced by skin temperature. Surface and subcutaneous recordings of skin temperature revealed a sluggish thermoregulatory compensation for repetitive cold stimulation. In contrast to heat stimulation, skin temperature did not recover between cold stimuli throughout ISIs of 3-8 s. Psychophysically, repetitive cold stimulation produced an aching pain sensation that progressed gradually and radiated beyond the site of stimulation. The magnitude of aching pain was well related to skin temperature and thus appeared to be established primarily by peripheral factors.

The significance of A-delta and C fibres for the perception of synthetic heat

Synthetic heat is a perception of strong, but not painful, heat arising when skin is stimulated by an alternating pattern of adjacent cold and warmth. This study examines the contribution of different classes of nerve fibres to this perception. In 40 subjects changes in synthetic heat and thermal perceptions were studied during a 30-min ischaemic nerve block in one reaction time, and one threshold determination task. Synthetic heat stimuli were described as hot or warm, but not as painful, and were preceded by a transient cold. Reaction times for synthetic heat stimuli did not differ from those for cold stimuli. Thresholds for synthetic heat and thermal stimuli were similar. During A fibre nerve block the perception of synthetic heat lost the cold component whereas the frequency of hot and warm descriptors did not change. The perception of cold stimuli changed, such that pure cold was replaced by dysaesthetic descriptors. Reaction times and thresholds for thermal and synthetic heat stimuli increased equally during the nerve block. It is concluded that the perception of synthetic heat most likely arises from the fusion of signals dependent on unmyelinated low threshold cold and warm receptors. It is not dependent on A-delta cold fibres, and a contribution of nociceptors is quite unlikely. The possibility of a psychological contribution at the perceptual level is discussed.

Clinical and electrophysiologic correlates of quantitative sensory testing in patients with incomplete spinal cord injury

OBJECTIVE

To determine the degree of association among indices of preserved sensation derived from quantitative sensory testing (QST), somatosensory evoked potentials (SEPs), and the clinical characteristics of patients with spinal cord injury (SCI).

DESIGN

A controlled correlational study of diverse measures of preserved sensory function.

SETTING

Regional SCI rehabilitation center in Ontario, Canada.

PARTICIPANTS

Thirty-three patients with incomplete SCI and 14 able-bodied controls.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

QST measures of perceptual threshold for temperature and vibration, American Spinal Injury Association sensory scores (light touch, pinprick), and tibial nerve SEPs.

RESULTS

There was a low degree of association (kappa) between QST results and sensory scores (|kappa|=.05-.44). QST measures yielded greater numbers of patients with SCI being classified as impaired, suggesting a greater sensitivity of QST to detect more subtle sensory deficits. QST measures of vibration threshold generally corresponded to the patients' SEP recordings. QST measures of modalities conveyed within the same tract were significantly (P<.05) correlated (|r|=.46-.84) in patients with SCI, but not in controls, whereas those modalities mediated by different pathways had lower and generally nonsignificant correlations (|r|=.05-.44) in both patients and controls.

CONCLUSIONS

The low degree of association between QST measures and sensory scores is likely attributable to measurement limitations of both assessments, as well as various neuroanatomic and neuropathologic factors. QST provides more sensitive detection of preserved sensory function than does standard clinical examination in patients with incomplete SCI.

Venous malformations associated with central pain: report of a case

IMPLICATIONS

The authors describe an unusual case of central pain (CP) that resulted from giant venous hemangiomas. The patient was treated with a variety of medications, including the N-methyl-D-aspartate antagonist dextromethorphan. We report the first known association between venous malformations and CP and briefly describe why the use of dextromethorphan in this disorder requires further evaluation.

Central pain

PURPOSE OF REVIEW

As a result of its accompanying co-morbidity, our lack of understanding regarding its mechanisms, and its resistance to conventional treatment, central pain is one of the most formidable challenges pain physicians are faced with. The objective of this review is to summarize recent advances in our understanding of the etiology, clinical presentation, and treatment of central pain, with special emphasis being placed on studies published within the past year.

RECENT FINDINGS

Recent evidence suggests that not only injuries commonly associated with central pain, such as strokes and spinal cord lesions, but also disorders such as fibromyalgia, phantom limb pain and tension-type headaches may involve central phenomena. Perhaps because of the lack of clinical trials, treatment is still largely based on traditional prescribing methods and anecdotal evidence. Recent studies have indicated possible roles for tricyclic antidepressants, anti-seizure medications, and motor cortex stimulation in the treatment of central pain.

SUMMARY

Injury to the spinothalamocortical pathways is a necessary, but not sufficient factor in the pathogenesis of central pain. Perhaps because of the similarities in mechanisms, there is considerable overlap between effective treatments for central pain and those for peripheral neuropathic pain. Our poor understanding of the etiology of central pain and the relative lack of effective treatments emphasize the need for further research into this elusive disorder.

How do you feel? Interoception: the sense of the physiological condition of the body

As humans, we perceive feelings from our bodies that relate our state of well-being, our energy and stress levels, our mood and disposition. How do we have these feelings? What neural processes do they represent? Recent functional anatomical work has detailed an afferent neural system in primates and in humans that represents all aspects of the physiological condition of the physical body. This system constitutes a representation of 'the material me', and might provide a foundation for subjective feelings, emotion and self-awareness.

Noxious cold evokes multiple sensations with distinct time courses

A noxious cold stimulus can evoke multiple sensations each occurring with a different time course. We have performed psychophysical studies to identify the time course of five sensations evoked by a noxious cold stimulus applied to the hand. Subjects continuously rated either pain, ache, cold, heat or prickle sensations throughout repeated presentations of a noxious cold stimulus (3 degrees C) from a neutral (32 degrees C) baseline. Separate runs were used to assess each of the five types of sensation. Cold was reported throughout the period of cooling. The time course of pain and ache sensations were similar. However, prickle and heat sensations had time courses that could be distinguished from each other, and from ache and pain. Identification of these temporal profiles could provide clues to their underlying mechanisms. The temporal dissociation of these sensations will also enable neuroimaging studies of the cortical mechanisms associated with these sensations. Thus our results constitute a first step toward identifying the distinct modes of neural activity associated with different types of pain sensation.

Tuning of membrane properties regulates subliminal synapses in dorsal horn neurons of intact rats

Functional plasticity in receptive field properties underlies the mechanism whereby spinal dorsal horn neurons encode changes in pain sensitivity following peripheral injury. Activation of "silent" or subliminal excitatory synapses was hypothesized to account for this injury-induced neural plasticity. To better characterize the mechanisms governing subliminal inputs, we adapted whole-cell patch clamp to the study of dorsal horn neurons in intact, anesthetized rats. In this report we show that the membrane properties of spinal cells correlate to functional class defined by action potential responses to cutaneous stimuli. In addition, we report the discovery of a novel "silent" population of neurons with solely subliminal excitatory inputs at rest that can be activated by membrane depolarization. Finally, an induced change in baseline membrane potential to a level nearer that of a different functional class results in a corresponding change in the responses to cutaneous stimuli of a given cell to that of the new functional class. In summary our findings suggest that biophysical membrane properties are key factors determining the functional profile of spinal neurons. The rapid change of such properties may regulate the function of silent synapses in spinal neurons and underlie rapid development of neural plasticity.

Disruption of thermal perception in a multiple sclerosis patient with central pain

OBJECTIVE

To investigate integrative thermal perception in a patient with multiple sclerosis.

DESIGN

Quantitative thermosensory testing was used to evaluate pain and other sensations produced by heat, cold, and the thermal grill pain illusion.

PATIENT

The authors report on a 43-year-old patient with central pain manifest most strongly in her left arm and hand, contralateral to an upper cervical spinothalamic lesion due to multiple sclerosis.

OUTCOME MEASURES AND RESULTS

Quantitative thermosensory testing showed that the patient had heat hypalgesia (no pain with stimuli of 45-50 degrees C) and cold allodynia (pain with innocuous cool temperatures, 25-10 degrees C). Whereas healthy subjects rated 20 degrees and 40 degrees C as nonpainful, but the thermal grill (intermixed 20 and 40 degrees C stimuli) as painful, the patient rated the thermal grill as less painful than 20 degrees C.

CONCLUSIONS

The absence of thermal grill-evoked pain is consistent with the hypothesis that in some cases of central pain the loss of the thermosensory pathway results in disruption of the normal cold inhibition of burning pain.

Responses of spinothalamic lamina I neurons to repeated brief contact heat stimulation in the cat

It was recently shown that repeated heat stimulation, using brief contacts (<1 s) with a preheated thermode at sufficiently short interstimulus intervals (ISIs <5 s) and high temperatures (> or =51 degrees C), will elicit in humans a sensation of rapidly augmenting "second" (burning) pain with only a weak "first" (sharp) pain sensation. Most strikingly, at short intertrial intervals (ITIs >5 s) such summation will reset, or begin again at baseline. In the present experiments, the responses of nociceptive lamina I spinothalamic (STT) neurons in the lumbosacral dorsal horn of barbiturate-anesthetized cats were examined using this repeated brief contact heat paradigm. The neurons were classified as nociceptive-specific (NS, n = 8) or polymodal nociceptive (HPC, n = 8) based on their responses to quantitative thermal stimuli; all had receptive fields on the glabrous ventral hindpaw. A pneumatic piston was used to apply a thermode preheated to 34, 46, 49, 53, or 58 degrees C with a contact dwell time of approximately 0.7 s to the ventral hindpaw repeatedly (15 times) at ISIs of 2, 3, and 5 s, with 3-5 min between trials. The mean responses of the 16 nociceptive lamina I STT cells showed rapid temporal summation that was directly dependent on temperature and inversely dependent on ISI, with the greatest increases occurring between the 3rd and 10th contacts. The temporal profiles of this family of curves correspond with the psychophysical data on human sensation. Further analysis showed that this summation was due to the HPC cells, which all showed strong summation; in contrast, the NS cells showed little, if any. The HPC responses to the repeated heat stimuli lagged each contact by approximately 1 s, consistent with the strong, monosynaptic C-fiber input that is characteristic of HPC cells and also with the dependence of second pain on C-fiber nociceptors. HPC cells also displayed the reset phenomenon at short ITIs, again in correspondence with the psychophysical data. The summation and the reset displayed by HPC cells were not related to skin temperature. Thus the results presented in this study, together with those in the preceding article, demonstrate a double dissociation indicating that NS and HPC lamina I STT cells can subserve the qualitatively distinct sensations of first (sharp) and second (burning) pain, respectively. These findings support the concept that the lamina I STT projection comprises several discrete sensory channels that are integrated in the forebrain to generate distinct sensations.

Identification of a cold receptor reveals a general role for TRP channels in thermosensation

The cellular and molecular mechanisms that enable us to sense cold are not well understood. Insights into this process have come from the use of pharmacological agents, such as menthol, that elicit a cooling sensation. Here we have characterized and cloned a menthol receptor from trigeminal sensory neurons that is also activated by thermal stimuli in the cool to cold range. This cold- and menthol-sensitive receptor, CMR1, is a member of the TRP family of excitatory ion channels, and we propose that it functions as a transducer of cold stimuli in the somatosensory system. These findings, together with our previous identification of the heat-sensitive channels VR1 and VRL-1, demonstrate that TRP channels detect temperatures over a wide range and are the principal sensors of thermal stimuli in the mammalian peripheral nervous system.

Sensory determinants of thermal pain

It is still unclear whether the quality of painful thermal sensation is determined only by conduction in specific, dedicated nociceptive channels (i.e. C or Adelta nociceptors) or whether it is a result of integrated activity in both nociceptive and non-nociceptive systems. To evaluate this question, we conducted quantitative and qualitative somatosensory testing in spinal cord injury subjects who suffered from partial or complete loss of thermal sensibility. Testing was performed in skin areas, below the level of the lesion, which were either lacking any thermal sensibility, lacking only one thermal sensation (either heat or cold) or having normal thermal sensations. We found that, in areas lacking any thermal sensibility, warm and cold stimuli produced a sensation of pricking pain, which had no thermal quality and was detected at significantly higher thresholds than in normal controls (48.5 +/- 1.8 and 9.7 +/- 5.1 degrees C for noxious heat- and noxious cold-induced pricking pain, respectively). Normal thermal pain sensations, consisting of normal perception of thermal quality and normal mean pain thresholds, were present both in normal skin areas (42.1 +/- 1.9 and 27.6 +/- 2.25 degrees C for heat and cold pain, respectively) and in areas in which only one thermal modality remained intact, when tested for that modality. Thus, testing for heat pain in areas in which only warm sensation was intact, or cold pain when only cold was intact produced normal qualities and thresholds of pain (42.8 +/- 3.4 and 24.4 +/- 6.2 degrees C for heat and cold pain, respectively). No spatial summation of pricking pain was observed, in contrast to the marked summation of heat pain in normal areas. In areas with only a single intact thermal modality, the quality of the perceived non-painful sensation was not determined by the thermal stimulus but by the intact modality (paradoxical sensation). Cold stimuli were perceived as warm in areas in which only warm sensation was preserved, and vice versa. A similar pattern was also seen for pain perception in areas with intact warm sensation. In these areas, both noxious heat and cold elicited a sensation of heat pain. No consistent pattern of heat-elicited pain was observed in areas in which only cold sensation was intact. These data suggest that the integrity of non-noxious thermal systems is essential for the normal perception of thermal pain, and that the subjective sensation of pain depends on the integration of information from nociceptive and non-nociceptive channels.

Quantitative and qualitative perceptual analysis of cold dysesthesia and hyperalgesia in fibromyalgia

Somatosensory perception thresholds, perceived intensity, and quality of perceptions were assessed in 20 women with fibromyalgia syndrome (FMS) and in 20 healthy age-matched female controls. All patients and controls scaled perceived intensity and described perceived quality of randomized thermal (Thermotest) and tactile (von Frey filaments) stimulation. Perceived intensity was scaled by free-number magnitude estimation and interindividual comparability was accomplished by Master Scaling. Perceived quality was assessed by choosing verbal descriptors from a list. Thenar was used as a reference for each modality tested. All patients were able to reliably scale perceived intensity at thenar, as well as in pain-affected body areas. Perception thresholds for cold pain, heat pain, cold-pain tolerance and heat-pain tolerance were significantly lower in patients than controls. For cold and tactile stimulation, the master scaled perceived intensities were significantly higher in patients' pain-affected areas, whereas for warmth/heat stimulation, the intensities were significantly lower. In the qualitative perceptual analysis the most striking and significant finding was the aberration of cold-evoked perceptions in all patients: most stimuli in the range of 30-10 degrees C were reported as heat or other paresthetic or dysesthetic perceptions. The perceptual quality of warmth, and of touch, did not differ from the controls. Another aberration was observed in the nociceptive range of thermal and of tactile stimulation as significantly more frequent pain-related descriptors than in controls. This indicates a general nociceptive facilitation in addition to the lower thermal pain thresholds. The combination of cold hyperesthesia, cold dysesthesia, and multimodal hyperalgesia suggests a selective pathophysiology at a particular level of integration, possibly in the insular cortex. It is suggested that the aberrations revealed by the supraliminal sensory evaluation may be generic for FMS. Particularly, the aberrations established in all patients for perceived quality and intensity in the cold sensory channel may be an additional diagnostic criterion.

Neurogenic hyperalgesia versus painful hypoalgesia: two distinct mechanisms of neuropathic pain

Patients with sensory disturbances of painful and non-painful character show distinct changes in touch and/or pain sensitivity. The patterns of sensory changes were compared to those of human surrogate models of neuropathic pain to assess the underlying mechanisms. We investigated 30 consecutive in-patients with dysaesthesia of various origins (peripheral, spinal, and brainstem lesions) and 15 healthy subjects. Tactile thresholds were determined with calibrated von Frey hairs (1.1mm). Thresholds and stimulus-response functions for pricking pain were determined with a series of calibrated punctate mechanical stimulators (0.2mm). Allodynia was tested by light stroking with a brush, Q-tip, and cotton wisp. Perceptual wind-up was tested by trains of punctate stimuli at 0.2 or 1Hz. Intradermal injection of capsaicin (n=7) and A-fiber conduction blockade (n=8) served as human surrogate models for neurogenic hyperalgesia and partial nociceptive deafferentation, respectively. Patients without pain (18/30) showed a continuous distribution of threshold shifts in the dysaesthetic skin area with a low to moderate increase in pain threshold (by 1.52+/-0.45 log2 units). Patients with painful dysaesthesia presented as two separate groups (six patients each): one showing lowered pain thresholds (by -1.94+/-0.46 log2 units, hyperalgesia) and the other elevated pain thresholds (by 3.02+/-0.48 log2 units, hypoalgesia). The human surrogate model of neurogenic hyperalgesia revealed nearly identical leftward shifts in stimulus-response function for pricking pain as patients with spontaneous pain and hyperalgesia (by a factor of about 5 each). The sensory changes in the human surrogate model of deafferentation were similar to patients with hypoalgesia and spontaneous pain (rightward shift of the stimulus-response function with a decrease in slope). Perceptual wind-up did not differ between symptomatic and control areas. There was no exclusive association of any parameter obtained by quantitative sensory testing with a particular disease (of either peripheral or central origin). Our findings suggest that neuropathic pain is based on two distinct mechanisms: (I) central sensitization (neurogenic hyperalgesia; in patients with minor sensory impairment) and (II) partial nociceptive deafferentation (painful hypoalgesia; in patients with major sensory deficit). This distinction as previously postulated for postherpetic neuralgia, is obviously valid also for other conditions. Our findings emphasize the significance of a mechanism-based classification of neuropathic pain.

Cooling inhibits capsaicin-induced currents in cultured rat dorsal root ganglion neurones

Whole-cell and single-channel recordings from rat dorsal root ganglion neurones were used to investigate the temperature dependence of currents through the capsaicin receptor (vanilloid receptor 1, VR1). Reducing the temperature from 31 to 14 degrees C inhibited the current induced by 0.5 microM capsaicin by 80%. The Q(10) (temperature coefficient over a 10 degrees C range) of the whole-cell capsaicin-induced current was 2.3 between 10 and 30 degrees C. Single-channel recordings showed that this inhibition by cooling was due to a marked reduction in the open probability (Q(10)=8.2 between 10 and 30 degrees C). This effect can explain the pain relief and reduction in inflammation caused by strong cooling of the skin.

Serotonin alters multi-segmental convergence patterns in spinal cord deep dorsal horn and intermediate laminae neurons in an in vitro young rat preparation

Each spinal neuron has a receptive field that corresponds to stimulation of a specific area of skin or subcutaneous tissue. Receptive fields are plastic and can be altered during development and injury but the actions of neuromodulators, such as serotonin (5-hydroxytryptamine, 5-HT) on receptive field properties are not well known. We used stimulation of multiple adjacent dorsal root spinal segments as a measure of "receptive field size" to determine the effects of 5-HT on multi-segmental convergent input onto neurons in laminae IV-VII. Whole-cell patch-clamp recordings were undertaken in the in vitro hemisected thoracolumbar spinal cord of rats aged 8-10 days old. Based on synaptic responses, neurons could be divided into two predominant groups and 5-HT exerted different effects on these groups. The first group received excitatory post-synaptic potentials (EPSPs) from the homonymous dorsal root but inhibitory post-synaptic potentials (IPSPs) with increasing amplitude from more distant dorsal roots. In this group, 5-HT preferentially depressed the IPSPs from adjacent nerve roots while leaving the EPSP intact. The second group received short-latency EPSPs from all segments stimulated and 5-HT potently depressed all synaptic input. In both populations the depressant actions of 5-HT increased with dose (0.1-10.0 microM). Bicuculline and strychnine did not affect the 5-HT induced short-latency synaptic depression. These results suggest that descending serotonergic systems depress spinal sensory convergence in a graded and differentiated manner. The findings are discussed in relation to the modulation of nociceptive signaling.

Quantitative assessment of cutaneous thermal and vibration sensation and thermal pain detection thresholds in healthy children and adolescents

Quantitative sensory testing (QST) is a noninvasive, computer-assisted method for assessing function in peripheral small and large sensory fibers. In order to use QST for clinical neurological assessment in children, it is necessary: (1) to determine whether children can reliably perform these tests and (2) to characterize normal ranges in healthy children. Values of cold sensation, warm sensation, cold pain, heat pain, and vibration sensation detection thresholds were determined in the hand and foot with the method of limits (MLI) and method of levels (MLE) in 101 healthy children aged 6-17 years using a commercially available device. Both MLI and MLE were well-accepted by children, and there was good reproducibility between two sessions. The MLE takes longer to perform but produces lower thermal detection thresholds than the MLI. In the MLI, vibration and warm sensation showed higher thresholds in the foot than hand, whereas cold pain showed lower thresholds in the foot than hand. Based on these results, QST may be used to document and monitor the clinical course of sensory abnormalities in children with neurological disorders or neuropathic pain.

Quantitative response characteristics of thermoreceptive and nociceptive lamina I spinothalamic neurons in the cat

The physiological characteristics of antidromically identified lamina I spinothalamic (STT) neurons in the lumbosacral spinal cord were examined using quantitative thermal and mechanical stimuli in barbiturate-anesthetized cats. Cells belonging to the three main recognized classes were included based on categorization with natural cutaneous stimulation of the hindpaw: nociceptive-specific (NS), polymodal nociceptive (HPC), or thermoreceptive-specific (COOL) cells. The mean central conduction latencies of these classes differed significantly; NS = 130.8 +/- 55.5 (SD) ms (n = 100), HPC = 72.1 +/- 28.0 ms (n = 128), and COOL = 58.6 +/- 25.3 ms (n = 136), which correspond to conduction velocities of 2.5, 4.6, and 5.6 m/s. Based on recordings made prior to any noxious stimulation, the mean spontaneous discharge rates of these classes also differed: NS = 0.5 +/- 0.7 imp/s (n = 47), HPC = 0.9 +/- 0.7 imp/s (n = 59), and COOL = 3.3 +/- 2.6 imp/s (n = 107). Standard, quantitative, thermal stimulus sequences applied with a Peltier thermode were used to characterize the stimulus-response functions of 76 COOL cells, 47 HPC cells, and 37 NS cells. The COOL cells showed a very linear output from 34 degrees C down to approximately 15 degrees C and a maintained plateau thereafter. The HPC cells showed a fairly linear but accelerating response to cold below a median threshold of approximately 24 degrees C and down to 9 degrees C (measured at the skin-thermode interface with a thermode temperature of 2 degrees C). The HPC cells and the NS cells both showed rapidly increasing, sigmoidal response functions to noxious heat with a fairly linear response between 45 and 53 degrees C, but they had significantly different thresholds; half of the HPC cells were activated at ~45.5 degrees C and half of the NS cells at approximately 43 degrees C. The 20 HPC lamina I STT cells and 10 NS cells tested with quantitative pinch stimuli showed fairly linear responses above a threshold of approximately 130 g/mm(2) for HPC cells and a threshold of approximately 100 g/mm(2) for NS cells. All of these response functions compare well (across species) with the available data on the characteristics of thermoreceptive and nociceptive primary afferent fibers and the appropriate psychophysics in humans. Together these results support the concept that these classes of lamina I STT cells provide discrete sensory channels for the sensations of temperature and pain.

Differential projections of thermoreceptive and nociceptive lamina I trigeminothalamic and spinothalamic neurons in the cat

The projections of 40 trigeminothalamic or spinothalamic (TSTT) lamina I neurons were mapped using antidromic activation from a mobile electrode array in barbiturate anesthetized cats. Single units were identified as projection cells from the initial array position and characterized with natural cutaneous stimuli as nociceptive-specific (NS, n = 9), polymodal nociceptive (HPC, n = 8), or thermoreceptive-specific (COOL, n = 22; WARM, n = 1) cells. Thresholds for antidromic activation were measured from each electrode in the mediolateral array at vertical steps of 250 microm over a 7-mm dorsoventral extent in two to eight (median = 6.0) anteroposterior planes. Histological reconstructions showed that the maps encompassed all three of the main lamina I projection targets observed in prior anatomical work, i.e., the ventral aspect of the ventroposterior complex (vVP), the dorsomedial aspect of the ventroposterior medial nucleus (dmVPM), and the submedial nucleus (Sm). The antidromic activation foci were localized to these sites (and occasional projections to other sites were also observed, such as the parafascicular nucleus and zona incerta). The projections of thermoreceptive and nociceptive cells differed. The projections of the thermoreceptive-specific cells were 20/23 to dmVPM, 21/23 to vVP, and 17/23 to Sm, whereas the projections of the NS cells were 1/9 to dmVPM, 9/9 to vVP, and 9/9 to Sm and the projections of the HPC cells were 0/8 to dmVPM, 7/8 to vVP, and 6/8 to Sm. Thus nearly all thermoreceptive cells projected to dmVPM, but almost no nociceptive cells did. Further, thermoreceptive cells projected medially within vVP (including the basal ventral medial nucleus), while nociceptive cells projected both medially and more laterally, and the ascending axons of thermoreceptive cells were concentrated in the medial mesencephalon, while the axons of nociceptive cells ascended in the lateral mesencephalon. These findings provide evidence for anatomical differences between these physiological classes of lamina I cells, and they corroborate prior anatomical localization of the lamina I TSTT projection targets in the cat. These results support evidence indicating that the ventral aspect of the basal ventral medial nucleus is important for thermosensory behavior in cats, consistent with the view that this region is a primordial homologue of the posterior ventral medial nucleus in primates.

Enhanced responses of spinal dorsal horn neurons to heat and cold stimuli following mild freeze injury to the skin

The effects of a mild freeze injury to the skin on responses of nociceptive dorsal horn neurons to cold and heat stimuli were examined in anesthetized rats. Electrophysiological recordings were obtained from 72 nociceptive spinal neurons located in the superficial and deep dorsal horn. All neurons had receptive fields (RFs) on the glabrous skin of the hindpaw, and neurons were functionally divided into wide dynamic range (WDR) and high-threshold (HT) neurons. Forty-four neurons (61%) were classified as WDR and responded to both innocuous and noxious mechanical stimuli (mean mechanical threshold of 12.8 +/- 1.6 mN). Twenty-eight neurons (39%) were classified as HT and were excited only by noxious mechanical stimuli (mean mechanical threshold of 154.2 +/- 18.3 mN). Neurons were characterized for their sensitivity heat (35 to 51 degrees C) and cold (28 to -12 degrees C) stimuli applied to their RF. Among WDR neurons, 86% were excited by both noxious heat and cold stimuli, while 14% responded only to heat. For HT neurons, 61% responded to heat and cold stimuli, 32% responded only to noxious heat, and 7% responded only to noxious cold. Effects of a mild freeze injury (-15 degrees C applied to the RF for 20 s) on responses to heat and cold stimuli were examined in 30 WDR and 22 HT neurons. Skin freezing was verified as an abrupt increase in skin temperature at the site of injury due to the exothermic reaction associated with crystallization. Freezing produced a decrease in response thresholds to heat and cold stimuli in most WDR and HT neurons. WDR and HT neurons exhibited a mean decrease in response threshold for cold of 9.0 +/- 1.3 degrees C and 10.0 +/- 1.6 degrees C, respectively. Mean response thresholds for heat decreased 4.0 +/- 0.4 degrees C and 4.3 +/- 1.3 degrees C in WDR and HT neurons, respectively. In addition, responses to suprathreshold cold and heat stimuli increased. WDR and HT neurons exhibited an 89% and a 192% increase in response across all cold stimuli, and a 93 and 92% increase in responses evoked across all heat stimuli, respectively. Our results demonstrate that many spinal neurons encode intensity of noxious cold as well as noxious heat over a broad range of stimulus temperatures. Enhanced responses of WDR and HT neurons to cold and heat stimuli after a mild freeze injury is likely to contribute to thermal hyperalgesia following a similar freeze injury in humans.

Tactile illusion perception in patients with central pain

OBJECTIVE

To evaluate lemniscal pathways in patients with central pain (CP) by use of a novel tactile illusion (TI)-producing technique.

PATIENTS AND METHODS

Somatosensory (SS) and TI testing was performed on the unaffected and affected wrists of 27 patients with unilateral central nervous system injury and unilateral CP. Data on TI testing-naive control subjects were used to adjust for a learning effect.

RESULTS

Testing in CP patients with cervical and brainstem injuries revealed predominantly unchanged or decreased SS perception on the affected side compared with the unaffected side. The majority of cervical- and brainstem-injured subjects with CP had improved TI performance on the affected side compared with the unaffected side. All the thalamic-injured and most of the suprathalamic-injured CP patients had altered SS testing with either decreased or increased SS perception noted. In these same CP patients the majority had reduced TI perception on the affected side compared with the unaffected side. Unexpectedly, perseveration of numbers was noted in patients with Dejerine-Roussy syndrome, and expanded TI testing was performed to better define this observation.

CONCLUSION

Cervical spinal cord-injured CP patients may have improved TI perception, whereas the majority of patients with supraspinal injuries and CP have reduced SS and TI perception. A unique sensory phenomenon of TI perseveration was observed in some patients with Dejerine-Roussy syndrome.

Expectation of pain enhances responses to nonpainful somatosensory stimulation in the anterior cingulate cortex and parietal operculum/posterior insula: an event-related functional magnetic resonance imaging study

Although behavioral studies suggest that pain distress may alter the perception of somatic stimulation, neural correlates underlying such alteration remain to be clarified. The present study was aimed to test the hypothesis that expectation of pain might amplify brain responses to somatosensory stimulation in the anterior cingulate cortex (ACC) and the region including parietal operculum and posterior insula (PO/PI), both of which may play roles in regulating pain-dependent behavior. We compared brain responses with and subjective evaluation of physically identical nonpainful warm stimuli between two psychologically different contexts: one linked with pain expectation by presenting the nonpainful stimuli randomly intermixed with painful stimuli and the other without. By applying the event-related functional magnetic resonance imaging technique, brain responses to the stimuli were assessed with respect to signal changes and activated volume, setting regions of interest on activated clusters in ACC and bilateral PO/PI defined by painful stimuli. As a result, the uncertain expectation of painful stimulus enhanced transient brain responses to nonpainful stimulus in ACC and PO/PI. The enhanced responses were revealed as a higher intensity of signal change in ACC and larger volume of activated voxels in PO/PI. Behavioral measurements demonstrated that expectation of painful stimulus amplified perceived unpleasantness of innocuous stimulus. From these findings, it is suggested that ACC and PO/PI are involved in modulation of affective aspect of sensory perception by the uncertain expectation of painful stimulus.

Spinothalamic lamina I neurons selectively sensitive to histamine: a central neural pathway for itch

We found a class of lamina I spinothalamic tract (STT) neurons selectively excited by iontophoretic histamine. The responses of this class of neurons parallel the pure itching sensation this stimulus elicits in humans, and match the responses of peripheral C-fibers that have similar selectivity. These neurons have distinct central conduction velocities and thalamic projections, indicating that they constitute a unique subset of STT neurons. These findings can explain why a lesion of the lateral STT disrupts itch along with pain and temperature sensations. Our findings provide strong evidence that itch is subserved by specific neural elements both peripherally and centrally.

Quantitative sensory studies in complex regional pain syndrome type 1/RSD

OBJECTIVE

Patients with complex regional pain syndrome type I (CRPSD1) may have thermal allodynia after application of a non-noxious thermal stimulus to the affected limb. We measured the warm, cold, heat-evoked pain threshold and the cold-evoked pain threshold in the affected area of 16 control patients and patients with complex regional pain syndrome type 1/RSD to test the hypothesis that allodynia results from an abnormality in sensory physiology.

SETTING

A contact thermode was used to apply a constant 1 degrees C/second increasing (warm and heat-evoked pain) or decreasing (cold and cold-evoked pain) thermal stimulus until the patient pressed the response button to show that a temperature change was felt by the patient. Student t test was used to compare thresholds in patients and control patients.

RESULTS

The cold-evoked pain threshold in patients with CRPSD1/RSD (p <0.001) was significantly decreased when compared with the thresholds in control patients (i.e., a smaller decrease in temperature was necessary to elicit cold-pain in patients with CRPSD1/RSD than in control patients). The heat-evoked pain threshold in patients with CRPS1/RSD was (p <0.05) decreased significantly when compared with thresholds in control patients. The warm- and cold-detection thresholds in patients with CRPS1/RSD were similar to the thresholds in control patients.

CONCLUSIONS

This study suggests that thermal allodynia in patients with CRPS1/RSD results from decreased cold-evoked and heat-evoked pain thresholds. The thermal pain thresholds are reset (decreased) so that non-noxious thermal stimuli are perceived to be pain (allodynia).

Anticonvulsants for neuropathic pain syndromes: mechanisms of action and place in therapy

Neuropathic pain, a form of chronic pain caused by injury to or disease of the peripheral or central nervous system, is a formidable therapeutic challenge to clinicians because it does not respond well to traditional pain therapies. Our knowledge about the pathogenesis of neuropathic pain has grown significantly over last 2 decades. Basic research with animal and human models of neuropathic pain has shown that a number of pathophysiological and biochemical changes take place in the nervous system as a result of an insult. This property of the nervous system to adapt morphologically and functionally to external stimuli is known as neuroplasticity and plays a crucial role in the onset and maintenance of pain symptoms. Many similarities between the pathophysiological phenomena observed in some epilepsy models and in neuropathic pain models justify the rational for use of anticonvulsant drugs in the symptomatic management of neuropathic pain disorders. Carbamazepine, the first anticonvulsant studied in clinical trials, probably alleviates pain by decreasing conductance in Na+ channels and inhibiting ectopic discharges. Results from clinical trials have been positive in the treatment of trigeminal neuralgia, painful diabetic neuropathy and postherpetic neuralgia. The availability of newer anticonvulsants tested in higher quality clinical trials has marked a new era in the treatment of neuropathic pain. Gabapentin has the most clearly demonstrated analgesic effect for the treatment of neuropathic pain, specifically for treatment of painful diabetic neuropathy and postherpetic neuralgia. Based on the positive results of these studies and its favourable adverse effect profile, gabapentin should be considered the first choice of therapy for neuropathic pain. Evidence for the efficacy of phenytoin as an antinociceptive agent is, at best, weak to modest. Lamotrigine has good potential to modulate and control neuropathic pain, as shown in 2 controlled clinical trials, although another randomised trial showed no effect. There is potential for phenobarbital, clonazepam, valproic acid, topiramate, pregabalin and tiagabine to have antihyperalgesic and antinociceptive activities based on result in animal models of neuropathic pain, but the efficacy of these drugs in the treatment of human neuropathic pain has not yet been fully determined in clinical trials. The role of anticonvulsant drugs in the treatment of neuropathic pain is evolving and has been clearly demonstrated with gabapentin and carbamazepine. Further advances in our understanding of the mechanisms underlying neuropathic pain syndromes and well-designed clinical trials should further the opportunities to establish the role of anticonvulsants in the treatment of neuropathic pain.

Menthol desensitization of capsaicin irritation. Evidence of a short-term anti-nociceptive effect

Evidence is presented of a short-term antinociceptive effect of menthol that was discovered in the course of investigating menthol's potential to sensitize the mouth to capsaicin. Previous research had shown that treating the tongue with menthol 15 min before exposure to capsaicin could enhance the irritancy of capsaicin, and we wished to learn if this effect would increase as the time between exposure to menthol and capsaicin decreased. We found instead that when capsaicin followed menthol by only 3.5 min, or when it was presented in mixture with menthol for 2-3 min, sensory irritation was reduced rather than enhanced. We examined the duration of this apparent crossdesensitization in a second experiment by varying the delay between exposure to menthol and a block of three consecutive capsaicin stimuli. Cross-desensitization tended to decline as the interstimulus interval (ISI) increased to 5 min, and even when desensitization was maximal, it was significant only for the first of the three capsaicin stimuli. In the final experiment we investigated how menthol self- and cross-desensitization can influence the perception of menthol-capsaicin mixtures. During a series of five, 90-s stimulations, self- and cross-desensitization became evident at the beginning of the second exposure, but the effect on mixture intensity again diminished rapidly as stimulation continued. We infer from these results that method can transiently desensitize capsaicin-sensitive fibers, but that exposure to capsaicin rapidly overrides the effect. The implications these findings have for menthol's potential as a topical analgesic are discussed.

Thermosensory activation of insular cortex

Temperature sensation is regarded as a submodality of touch, but evidence suggests involvement of insular cortex rather than parietal somatosensory cortices. Using positron emission tomography (PET), we found contralateral activity correlated with graded cooling stimuli only in the dorsal margin of the middle/posterior insula in humans. This corresponds to the thermoreceptive- and nociceptive-specific lamina I spinothalamocortical pathway in monkeys, and can be considered an enteroceptive area within limbic sensory cortex. Because lesions at this site can produce the post-stroke central pain syndrome, this finding supports the proposal that central pain results from loss of the normal inhibition of pain by cold. Notably, perceived thermal intensity was well correlated with activation in the right (ipsilateral) anterior insular and orbitofrontal cortices.

Cold-evoked pain varies with skin type and cooling rate: a psychophysical study in humans

The psychophysical responses to noxious cold stimulation of the skin in normal human subjects are not well understood. Continuous pain ratings with the visual analogue scale is an important method to assess these responses. In this study, we addressed several important issues about the parameters with which stimuli are delivered: the type of skin stimulated, the rate with which the stimulus temperature decreases, and the dimension of the pain rated by subjects. Cold stimuli were delivered to the thenar eminence (glabrous skin) and the dorso-lateral hand (hairy skin) via a 4 cm(2) Peltier-type stimulator. Cold and pain thresholds were determined by the method of limits (MOL). A computerized visual analogue scale (VAS) was used to obtain continuous ratings of pain intensity and affect. The McGill Pain Questionnaire (MPQ) was used to assess the quality of cold-evoked pain. Supra-threshold stimuli (34 degrees C base) were delivered at 0.5, 1 or 2 degrees C/s to 2 degrees C, held for 20s and returned to baseline at 9 degrees C/s. These studies revealed: (1) Cold thresholds, measured with MOL, were lower (i.e. occurred at higher absolute temperatures) for the hairy skin of the dorso-lateral hand compared to the glabrous skin of the thenar eminence. (2) A similar pattern was evident for cold induced pain thresholds with MOL at 1.5 degrees C/s and with intensity and affect VAS scales at 0.5 and 1 degrees C/s. (3) Exponents for supra-threshold ratings fit to power functions were larger for the glabrous skin site than the hairy skin site regardless of cooling rate or dimension of pain measured. (4) All pain indices were higher for slower cooling rates. (5) No significant differences were found in the pain indices for pain ratings of intensity and affect. (6) A substantial proportion of subjects chose words representing paradoxical heat with the MPQ. (7) Painful paradoxical heat sensations occurred most often during cooling, while innocuous warm sensations mainly occurred during the rewarming phase.

Alkylamides that produce tingling paresthesia activate tactile and thermal trigeminal neurons

Alkylamides isolated from the fruit of Xanthoxylum, Szechuan pepper, produce a strong tingling sensation in the mouth. In order to determine the peripheral basis of this sensation, extracellular nerve recordings were obtained from the lingual nerve of rats. The primary pungent compound, hydroxy-alpha-sanshool (HO-alpha-S), altered the levels of spontaneous activity in cool-sensitive fibers as well as inducing activity in tactile fibers, cold nociceptors and silent fibers that were insensitive to innocuous thermal or tactile stimuli. Moreover, tactile or thermal sensitivity was induced in fibers that were initially insensitive to touch or cooling. The neuronal distribution of sensitivities to capsaicin and to HO-alpha-S indicate that this compound affects neurons mediating innocuous sensations. HO-alpha-S may be useful as a model stimulus for studies of paresthesia.

Behavioral thermosensitivity after lesions of thalamic target areas of a thermosensory spinothalamic pathway in the cat

The ability of 17 cats to discriminate floor temperatures 2-4 degrees C below the ambient temperature was tested before and after unilateral electrolytic thalamic lesions. The lesions were made contralateral to the paws showing better performance in the temperature discrimination task. They were aimed at one or more of the three main target areas of thermoreceptive-specific lamina I spinothalamic neurons [i.e., the nucleus submedius, the dorsomedial aspect of the ventral posterior medial nucleus, and the ventral aspect of the basal ventral medial nucleus (vVMb)], following microelectrode mapping of somatosensory thalamus. The thermosensory consequences of each lesion were measured in postoperative testing, beginning 6-8 days after the final preoperative test session. A mild but definite thermosensory deficiency was found in five cats, in which the response behavior on the contralateral side was reduced below the 69% criterion level for several sessions. Histological analysis indicated that these cats differed only by the inclusion in the lesion of all or part of vVMb. Consequently this area appears to be important for cats' thermosensory behavior. Nevertheless even large lesions of virtually all of the thermoreceptive lamina I spinothalamic projection areas produced only this mild thermosensory deficit in stark contrast with the massive defect observed previously after spinal lesions of the middle of the lateral funiculus, where lamina I axons ascend. Accordingly such spinal lesions were added at the C(4) level, on the same side as the thalamic lesions, in six cats 3 mo after the thalamic surgery. These lesions caused severe contralateral defects (i.e., chance level performance). Thus the present findings are taken to indicate that contralateral ascending projections to vVMb in the thalamus participate in cats' thermosensory discrimination but that ascending projections to the brain stem must play an important role in their behavioral thermosensitivity.

Forebrain mechanisms of nociception and pain: analysis through imaging

Pain is a unified experience composed of interacting discriminative, affective-motivational, and cognitive components, each of which is mediated and modulated through forebrain mechanisms acting at spinal, brainstem, and cerebral levels. The size of the human forebrain in relation to the spinal cord gives anatomical emphasis to forebrain control over nociceptive processing. Human forebrain pathology can cause pain without the activation of nociceptors. Functional imaging of the normal human brain with positron emission tomography (PET) shows synaptically induced increases in regional cerebral blood flow (rCBF) in several regions specifically during pain. We have examined the variables of gender, type of noxious stimulus, and the origin of nociceptive input as potential determinants of the pattern and intensity of rCBF responses. The structures most consistently activated across genders and during contact heat pain, cold pain, cutaneous laser pain or intramuscular pain were the contralateral insula and anterior cingulate cortex, the bilateral thalamus and premotor cortex, and the cerebellar vermis. These regions are commonly activated in PET studies of pain conducted by other investigators, and the intensity of the brain rCBF response correlates parametrically with perceived pain intensity. To complement the human studies, we developed an animal model for investigating stimulus-induced rCBF responses in the rat. In accord with behavioral measures and the results of human PET, there is a progressive and selective activation of somatosensory and limbic system structures in the brain and brainstem following the subcutaneous injection of formalin. The animal model and human PET studies should be mutually reinforcing and thus facilitate progress in understanding forebrain mechanisms of normal and pathological pain.

NK-1 receptor immunoreactivity in distinct morphological types of lamina I neurons of the primate spinal cord

In cat and monkey, lamina I cells can be classified into three basic morphological types (fusiform, pyramidal, and multipolar), and recent intracellular labeling evidence in the cat indicates that fusiform and multipolar lamina I cells are two different types of nociceptive cells, whereas pyramidal cells are innocuous thermoreceptive-specific. Because earlier observations indicated that only nociceptive dorsal horn neurons respond to substance P (SP), we examined which morphological types of lamina I neurons express receptors for SP (NK-1r). We categorized NK-1r-immunoreactive (IR) lamina I neurons in serial horizontal sections from the cervical and lumbar enlargements of four monkeys. Consistent results were obtained by two independent teams of observers. Nearly all NK-1r-IR cells were fusiform (42%) or multipolar (43%), but only 6% were pyramidal (with 9% unclassified). We obtained similar findings in three monkeys in which we used double-labeling immunocytochemistry to identify NK-1r-IR and spinothalamic lamina I neurons retrogradely labeled with cholera toxin subunit b from the thalamus; most NK-1r-IR lamina I spinothalamic neurons were fusiform (48%) or multipolar (33%), and only 10% were pyramidal. In contrast, most (approximately 75%) pyramidal and some (approximately 25%) fusiform and multipolar lamina I spinothalamic neurons did not display NK-1r immunoreactivity. These data indicate that most fusiform and multipolar lamina I neurons in the monkey can express NK-1r, consistent with the idea that both types are nociceptive, whereas only a small proportion of lamina I pyramidal cells express this receptor, consistent with the previous finding that they are non-nociceptive. However, these findings also indicate that not all nociceptive lamina I neurons express receptors for SP.

Paradoxical heat sensation in healthy subjects: peripherally conducted by A delta or C fibres?

Paradoxical heat sensation upon cooling of the skin has been reported in central as well as in peripheral neurological conditions. In our study, we examined this phenomenon in 35 naive healthy test subjects, of whom 23 experienced paradoxical heat sensation under test conditions. We measured the peripheral conduction velocities of cold sensation, warm sensation and of paradoxical heat sensation by using a quantitative sensory testing model of indirect peripheral conduction velocity measurement. This was based on comparison of measurements at a proximal and a distal site using two measurement methods, one inclusive and the other exclusive of reaction time. We found that the conduction velocity of paradoxical heat sensation (0.70 m/s) was similar to that of warm sensation (0.68 m/s), and that the conduction velocity of cold sensation (7.74-8.01 m/s) was considerably faster. Thus, we conclude that paradoxical heat sensation in healthy subjects is conducted peripherally via slow unmyelinated C fibres and not via the faster A delta fibres. Consequently, we propose that paradoxical heat sensation is encoded via the heat sensing pathway, in accordance with the labelled-line code theory. The mechanisms proposed suggest a malfunctioning cold-sensing pathway disinhibiting the heat-sensing pathway, at peripheral, central or both levels, thus facilitating a paradoxical heat sensation.

SOMESTHESIS

In this review we focus on the perceptual and psychophysical aspects of somesthesis, although some information on neurophysiological aspects will be included as well; we look primarily at studies that have appeared since 1988. In the section on touch, we cover peripheral sensory mechanisms and several topics related to spatial and temporal pattern perception, specifically measures of spatial sensitivity, texture perception with particular emphasis on perceived roughness, complex spatial-temporal patterns, and the use of touch as a possible channel of communication. Other topics under this section include the effects of attention on processing tactile stimuli, cortical mechanisms, and the effects of aging on sensitivity. We also deal with thermal sensitivity and some aspects of haptics and kinesthesis. In the section on pain, we review work on the gate-control theory, sensory fibers, and higher neural organization. In addition, studies on central neurochemical effects and psychophysics of pain are examined.

Stimulation of human thalamus for pain relief: possible modulatory circuits revealed by positron emission tomography

Stimulation of human thalamus for pain relief: possible modulatory circuits revealed by positron emission tomography. J. Neurophysiol. 80: 3326-3330, 1998. Stimulation of the somatosensory thalamus was used for more than 2 decades to treat chronic pain in the human. However, despite clinical reports of successful results, little is known about the actual mechanisms mediating this form of stimulation-produced analgesia. To reveal possible neuronal pathways evoked by thalamic stimulation, we measured regional changes in cerebral blood flow (rCBF) in five patients who received successful long-term relief of chronic pain with somatosensory thalamic stimulation. Positron emission tomography during thalamic stimulation revealed significant activation of the thalamus in the region of the stimulating electrodes as well as activation of the insular cortex ipsilateral to the thalamic electrodes (contralateral to the patients' clinical pain). For these patients, thalamic stimulation also evoked paresthesiae that included thermal sensations in addition to tingling sensations. Results of this study indicate that in some cases somatosensory thalamic stimulation may activate a thalamocortical pain modulation circuit that involves thermal pathways. These results are consistent with other recent reports suggesting that activation of thermal pathways may contribute to modulation of nociceptive information.