Response characteristics of specific thermoreceptive afferents innervating monkey facial skin and their relationship to human thermal sensitivity

Cellular models of pain: New technologies and their potential to progress preclinical research

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Human sensory neurons can reduce the translational gap in analgesic development.

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Access to dorsal root ganglion (hDRG) neurons is increasing.

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Diverse sensory neuron subtypes can now be generated via stem cell technology.

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Advances of these technologies will improve our understanding of human nociception.

In vitro models fill a vital niche in preclinical pain research, allowing detailed study of molecular pathways, and in the case of humanised systems, providing a translational bridge between in vivo animal models and human patients. Significant advances in cellular technology available to basic pain researchers have occurred in the last decade, including developing protocols to differentiate sensory neuron-like cells from stem cells and greater access to human dorsal root ganglion tissue. In this review, we discuss the use of both models in preclinical pain research: What can a human sensory neuron in a dish tell us that rodent in vivo models cannot? How similar are these models to their endogenous counterparts, and how should we judge them? What limitations do we need to consider? How can we leverage cell models to improve translational success? In vitro human sensory neuron models equip pain researchers with a valuable tool to investigate human nociception. With continual development, consideration for their advantages and limitations, and effective integration with other experimental strategies, they could become a driving force for the pain field's advancement.

Transcriptional Programming of Human Mechanosensory Neuron Subtypes from Pluripotent Stem Cells

Efficient and homogeneous in vitro generation of peripheral sensory neurons may provide a framework for novel drug screening platforms and disease models of touch and pain. We discover that, by ovesssrexpressing NGN2 and BRN3A, human pluripotent stem cells can be transcriptionally programmed to differentiate into a surprisingly uniform culture of cold- and mechano-sensing neurons. Although such a neuronal subtype is not found in mice, we identify molecular evidence for its existence in human sensory ganglia. Combining NGN2 and BRN3A programming with neural crest patterning, we produce two additional populations of sensory neurons, including a specialized touch receptor neuron subtype. Finally, we apply this system to model a rare inherited sensory disorder of touch and proprioception caused by inactivating mutations in PIEZO2. Together, these findings establish an approach to specify distinct sensory neuron subtypes in vitro, underscoring the utility of stem cell technology to capture human-specific features of physiology and disease.

Nickolls et al. develop a method, using human stem cells, to generate specific types of sensory neurons that detect cold temperature and mechanical force. This approach uncovers a class of neuron found in humans, but not mice, and enables the modeling of a rare sensory disorder of touch and proprioception.

Lasting modulation of human cortical swallowing motor pathways following thermal tongue stimulation

BACKGROUND

Thermal tactile oropharyngeal stimulation has been clinically used to facilitate swallowing initiation in dysphagic patients. We previously demonstrated that thermal stimulation applied to the oral cavity provokes an immediate excitability in pharyngeal motor cortex. The aim of the current study was to investigate whether thermal stimulation can produce longer lasting effects on the corticopharyngeal neural pathway.

METHODS

Healthy volunteers (n = 8/12) underwent baseline pharyngeal motor evoked potential (PMEP) measurements evoked by transcranial magnetic stimulation. In the first experiment, subjects received thermal stimulation alternating 30 seconds of 15 and 36°C applied to the tongue surface for either 10 minutes, 5 minutes, or sham. In the second experiment, one of three intermittent thermal stimulus patterns was delivered: cold (alternating 30 seconds of 15 and 36°C), warm (continuous 36°C), or hot (alternating 30 seconds of 45 and 36°C) for 10 minutes. In both experiments, PMEP were remeasured every 15 minutes up to 60 minutes following thermal stimulation.

KEY RESULTS

Repeated measures ANOVA for each stimulus time in the first experiment showed a significant increased change in PMEP amplitude at 30 minutes following only 10-minute stimulation compared with sham (P < .05). In the second experiment, we found that cold stimulation was more effective than the other stimulation (P < .05) at increasing PMEP amplitudes.

CONCLUSIONS AND INFERENCES

Ten-minute cold stimulation on the tongue can induce a delayed (30 minutes) increase in pharyngeal cortical excitability, providing a clinically useful therapeutic window for its application in dysphagic patients.

The intranasal trigeminal system

Many odors activate the intranasal chemosensory trigeminal system where they produce cooling and other somatic sensations such as tingling, burning, or stinging. Specific trigeminal receptors are involved in the mediation of these sensations. Importantly, the trigeminal system also mediates sensitivity to airflow. The intranasal trigeminal and the olfactory system are closely connected. With regard to central nervous processing, it is most interesting that trigeminal stimuli can activate the piriform cortex, which is typically viewed as the primary olfactory cortex. This suggests that interactions between the two systems may form at a relatively early stage of processing. For example, there is evidence showing that acquired olfactory loss leads to reduced trigeminal sensitivity, probably on account of the lack of interaction in the central nervous system. Decreased trigeminal sensitivity may also be responsible for changes in airflow perception, leading to the impression of congested nasal airways.

Thermoreceptive neurons in the dorsal portion of the trigeminal principal nucleus in rats

The dorsal margin of the trigeminal principal nucleus (PV) contains neurons responsive to innocuous thermal stimulation of the tongue and maybe a thermal relay (Hayama and Hashimoto, 2011). The present electrophysiological study examined whether PV thermoreceptive neurons project to the thalamus and investigated response properties to cold (20°C) or warm (40°C) stimulation of the tongue. Twenty-three thermoreceptive neurons were identified in the dorsal portion of the PV. Twenty of the 23 neurons were examined but none projected to the thalamus. Impulse frequencies of 8 of the 11 thermoreceptive neurons examined rapidly increased with cold stimulation, then decreased and gradually increased to steady state level, and rapidly decreased with warm stimulation. Thermal receptive fields were examined for six PV thermoreceptive neurons; five had a large receptive field extending over the whole anterior tongue ipsilateral to the recording side. These findings suggest that the dorsal portion of the PV is not a thermal relay mediating thermal information from the tongue to the thalamus.

A possible new relay for tongue thermal sense in the dorsal margin of the trigeminal principal nucleus in rats

Neurons responding to innocuous thermal stimulation of the anterior tongue were newly identified in the dorsal margin of the trigeminal principal nucleus (PV). Connections of the dorsal margin of the PV were neuroanatomically identified using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) with electrophysiological techniques. Injection of WGA-HRP into the dorsal margin of the PV demonstrated anterograde labels distributed from the most dorsomedial portion of the posteromedial ventral nucleus (VPM) to the dorsolateral portion of the parvicellular part of the VPM and retrograde labels in the superficial layers of the caudal subnucleus of the spinal trigeminal nucleus (SpVc). Injection of WGA-HRP in the most dorsomedial portion of the VPM demonstrated retrogradely labeled cells in the dorsal margin of the PV as well as in the superficial layers of the SpVc. Tracer injection into the superficial layers of the SpVc resulted in anterograde labels in the dorsal margin of the PV as well as in the dorsomedial portion of the VPM. These findings show that neurons in the PV respond to innocuous thermal stimulation of the trigeminal field and suggest that the dorsal margin of the PV is a possible new relay for the tongue thermal sense, receiving thermal information from the superficial layers of the SpVc, the primary thermal relay for the trigeminal field, and passing the information to the thalamic relay.

Threshold and rate sensitivity of low-threshold thermal nociception

Previous studies have shown that sensations of burning, stinging or pricking can be evoked by warming or cooling the skin to innocuous temperatures [low-threshold thermal nociception (LTN)] below the thresholds of cold- and heat-sensitive nociceptors. LTN implies that some primary afferent fibers classically defined as warm and cold fibers relay stimulation to the nociceptive system. We addressed this question in humans by determining if different adaptation temperatures (ATs) and rates of temperature change would affect thermal sensation and LTN similarly. In Experiment 1 subjects rated the intensity of warmth, cold and nociceptive sensations produced by increasing steps in temperature (+/-0.5 degrees C increments) from ATs of 35, 33 and 31 degrees C for cooling, and 30, 32 and 34 degrees C for heating. Depending upon the AT, thresholds for nociceptive and thermal sensations estimated from the rating data differed by as little as -1.0 degrees C for cooling and +1.5 degrees C for heating. Thresholds of thermal and nociceptive sensations shifted by similar amounts across the three ATs during cooling, whereas during heating the nociceptive threshold was significantly affected only between ATs of 32 and 34 degrees C. In Experiment 2, increasing the rate of temperature change from 0.5 to 4.0 degrees C/s increased the intensity of thermal and nociceptive sensations significantly but the effect was greatest for nociceptive sensations during heating. The results of both experiments are consistent with the mediation of LTN by low-threshold thermoreceptors, although LTN caused by heating may depend on a subset of fibers that express less sensitive TRP channels than those that serve sensations of warmth at the mildest temperatures.

Pain in the trigeminal system: irritation of the nasal mucosa using short- and long-lasting stimuli

The paper describes methods which allow intranasal irritation using short- and long-lasting painful stimuli in humans. Short-lasting pain is induced by gaseous CO(2), while long-lasting pain is induced by a stream of dry air. Both models have been explored regarding their major determinants, e.g. stimulus duration, stimulus intensity, or repeated stimulation. Short-lasting, non-inflammatory pain stimuli seem to provide specific indicators of A(delta)-fiber function, while responses to long-lasting, inflammatory pain appear to be indicative of C-fiber function. Responses to both types of painful stimuli are modulated by analgesic drugs. As these well-investigated models allow the detailed and precise analysis of modulatory effects on intranasal nociception, they appear to be suited for the investigation of subtle changes of intranasal irritation, e.g. induced by environmental agents.

Mechanosensory and thermosensory changes across the border of impaired sensitivity to pinprick after mandibular nerve injury

PURPOSE

The study goal was to determine how sensory function varies across the border of impaired sensitivity to pinprick in patients with mandibular nerve injuries.

PATIENTS AND METHODS

Borders of decreased sensitivity to pinprick were mapped in 15 patients who reported altered sensation. Four mechanoreceptive, 2 thermoreceptive, and 2 thermonociceptive functions were studied at 5 sites separated by 0.6 cm across the border. The tests were repeated to evaluate day-to-day consistency in the pattern of variation for each sensory measure.

RESULTS

The estimates of sensory function were not found to vary in a systematic manner from outside to inside the pinprick-impaired area for all patients for any of the 8 tests. However, for every test, some patients exhibited large variations. On average, the magnitudes of loss in contact detection, subjective intensity of light touch, and direction discrimination were greatest; the magnitudes of loss in 2-point perception and in heat and cold pain perception were least. Some patients provided no evidence of impairment on certain tests. For some patients, the estimates suggested increased sensitivity within the pinprick-impaired area (eg, to noxious cold stimuli).

CONCLUSIONS

Although certain patients exhibit impairment, there is no obligatory loss in light touch, 2-point perception, direction discrimination, or temperature perception across the border of decreased sensitivity to pinprick. The differences among patients suggest that the data from individual patients should be evaluated in clinical studies and in clinical practice. Researchers should not rely solely on average values and summary statistics.

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.

Assessment of intranasal trigeminal function

Intranasal trigeminal function is more and more understood as an integral part of human chemosensory perception. Sensations like burning, stinging, warmth, coolness, or itching are produced by almost all odorants so that they can be perceived by anosmics. Electrophysiological responses to trigeminal stimuli allow the specific assessment of trigeminally mediated information at different levels of processing including the periphery or the cortex. Information regarding the localization of these processes can be derived from magnetoencephalographic recordings or functional imaging data. When using these techniques in combination with psychophysical measures, it seems to be possible to specifically describe how and where the processing of irritation takes place, how it may interact with olfactory mediated sensations, and how it is modulated, e.g. by environmental influences or analgesic drugs.

Facilitation of a nociceptive flexion reflex in man by nonnoxious radiant heat produced by a laser

Electromyographic recordings were made in healthy volunteers from the knee-flexor biceps femoris muscle of the nociceptive RIII reflex elicited by electrical stimulation of the cutaneous sural nerve. The stimulus intensity was adjusted to produce a moderate pricking-pain sensation. The test responses were conditioned by a nonnoxious thermal (</=40 degrees C) stimulus applied to the receptive field of the sural nerve. This stimulus was delivered by a CO2 laser stimulator and consisted of a 100-ms pulse of heat with a beam diameter of 20 mm. Its power was 22.7 +/- 4.2 W (7.2 mJ/mm2), and it produced a sensation of warmth. The maximum surface temperature reached at the end of the period of stimulation was calculated to be 7 degrees C above the actual reference temperature of the skin (32 degrees C). The interval between the laser (conditioning) and electrical (test) stimuli was varied from 50 to 3, 000 ms in steps of 50 ms. It was found that the nociceptive flexion reflex was facilitated by the thermal stimulus; this modulation occurred with particular conditioning-test intervals, which peaked at 500 and 1,100 ms with an additional late, long-lasting phase between 1,600 and 2,300 ms. It was calculated that the conduction velocities of the cutaneous afferent fibers responsible for facilitating the RIII reflex, fell into three ranges: one corresponding to A delta fibers (3.2 m/s) and two in the C fiber range (1.3 and 0.7 m/s). It is concluded that information emanating from warm receptors and nociceptors converges. In this respect, the present data show, for the first time, that in man, conditioning nonnociceptive warm thermoreceptive A delta and C fibers results in an interaction at the spinal level with a nociceptive reflex. This interaction may constitute a useful means whereby signals add together to trigger flexion reflexes in defensive reactions and other basic motor behaviors. It also may contribute to hyperalgesia in inflammatory processes. The methodology used in this study appears to be a useful noninvasive tool for exploring the thermoalgesic mechanisms in both experimental and clinical situations.

Quantitative analysis of orofacial thermoreceptive neurons in the superficial medullary dorsal horn of the rat

Surprisingly little is known concerning the central processing of innocuous thermal somatosensory information. The aim of the present study was to obtain quantitative data on the characteristics of neurons in the rat superficial medullary dorsal horn (sMDH) that responded to innocuous thermal stimulation of the rat's face and tongue. Single-unit extracellular recordings were obtained in chloralose-urethane anesthetized rats. A total of 153 thermoreceptive neurons was studied. Of these, 146 were excited by cooling and inhibited by warming and were classified as COLD cells. The remaining seven cells were excited by innocuous warming of the skin or tongue. Of 123 COLD cells tested, 33% were excited by touch and 22% by pinch stimuli delivered to the thermoreceptive field. Of the 50 COLD cells tested, 46% were excited also by noxious heating (> or = 50 degrees C for 5 s). Most (82/121) of the receptive fields were located on the upper lip, 25 on the tongue, and most of the remaining on the lower lip. Receptive fields were generally small (1-5 mm2). In some experiments, electrical stimulation in the thalamus was performed, and nine COLD cells could be activated antidromically. The responses of 38 COLD cells to incremental 5 degrees C cooling steps were examined quantitatively. Thermal stimuli were applied to facial or lingual receptive fields of sMDH neurons with a computer-controlled Peltier thermode starting from 33 degrees C, decreasing to 8 or 3 degrees C, and returning to 33 degrees C. Most COLD cells (26/38) had both static and dynamic responses; 7 had mainly dynamic and 5 mainly static responses to step decreases in temperature. Rat sMDH COLD cells could be classified into three groups depending on their stimulus-response functions. The first group (Type 1, n = 19) had a bell-shaped static stimulus response function. The second group (Type 2) had a high maintained or increasing static firing rate as the temperature decreased < 18 degrees C (n = 10). Type 3 COLD cells had mainly dynamic properties (n = 7). Many of the cells in all groups were excited by noxious mechanical stimulation. Type 2 cells differed from the other two groups in that most did not respond to noxious thermal stimuli (hot) and many responded to innocuous tactile stimuli. Neurons from each of the three groups of COLD cells could be activated antidromically from contralateral thalamus. These data suggest that there is little central processing of thermal information at the first central synapse for Type 1 neurons, however, the responses of the other two types may be due to central processing and convergence. The demonstration of rat sMDH COLD cells with distinctive stimulus-response functions to thermal shifts suggests separate functional roles of these neurons in the ascending thermal sensory pathway.

Noxious and innocuous cold discrimination in humans: evidence for separate afferent channels

The present study evaluated the ability of humans to discriminate temperature decreases in the noxious and innocuous cold range. Two groups of five subjects detected changes in cold stimuli applied to the maxillary face. For five subjects, adapting temperatures of 22 degrees, 16 degrees, 6 degrees and 0 degrees C were used, and thresholds for detecting temperature decreases were determined using an adaptive psychophysical paradigm. Visual analogue scale (VAS) ratings of cold and pain sensation were also recorded at 5-min intervals throughout each session. A second group of five subjects performed a similar detection task, but in this case using classical psychophysical techniques (method of constant stimuli) and adapting temperatures of 22 degrees, 16 degrees, 10 degrees and 6 degrees C. These subjects described the quality of the detected change in sensation, in addition to rating overall cold and pain sensation throughout the session. Detection thresholds were 0.27 degrees, 0.48 degrees, 4.8 degrees, 8.0 degrees and >10.0 degrees C for baselines of 22 degrees, 16 degrees, 10 degrees, 6 degrees and 0 degrees C, respectively, indicating that discrimination was better in the innocuous cool (22 degrees and 16 degrees C) than in the noxious and near-noxious cold (10-0 degrees C) range (P < 0.05). Tonic adapting temperatures of 22 degrees and 16 degrees C were always rated as cool but not painful, whereas adapting temperatures of 10 degrees and 6 degrees were sometimes and 0 degrees C usually rated as painful. Phasic temperature decreases from 22 degrees and 16 degrees C always produced cooling sensations, whereas decreases from baselines of 10 degrees and 6 degrees C produced primarily sensations of painful and non-painful prickle. These data suggest that different afferent channels mediate cool and noxious cold perception and add support to the hypothesis that noxious cold sensation is mediated by subdermal nociceptors.

Peripheral electrophysiological responses decrease in response to repetitive painful stimulation of the human nasal mucosa

The study aimed to investigate the relation of both negative mucosal potentials (NMP) obtained from respiratory epithelium, and pain ratings to repetitive stimulation with CO2 (21 subjects). Trains of four stimuli of identical intensity (70% v/v) were applied at a constant interseries interval (approximately 60 s) but different interstimulus intervals (ISI) of 2 s or 6 s. At an ISI of 6 s, ratings decreased while they increased at an interval of 2 s (P < 0.01). This change was accompanied by the buildup of burning pain probably relating to the 'wind-up' of spinal neurons. In contrast, the decrease of NMP amplitudes was stronger the shorter the ISI (P < 0.01). These findings are in line with the view that the NMP reflects activation of epithelial nociceptors (C-fibers and/or A delta-fibers).

Chemo-somatosensory event-related potentials in response to repetitive painful chemical stimulation of the nasal mucosa

The aim of the study was to investigate how chemo-somatosensory event-related potentials (CSSERPs) and pain ratings are modified by repetitive painful stimulation of the nasal mucosa (58% v/v CO2, 200 msec duration). Twenty-two subjects performed 3 experiments during which trains of stimuli were applied. The interstimulus interval (ISI) between stimuli was constant for each experiment, but varied between experiments (8, 4, and 2 sec). CSSERPs were obtained from 5 positions (Fz, C3, Cz, C4, and Pz). The subjects not only rated the overall perceived intensities but also reported the quality of the stimuli. At an ISI of 8 sec estimates decreased and only stinging sensations were reported. In contrast, at an interval of 2 sec estimates increased being accompanied by the buildup of burning pain. This phenomenon was interpreted in terms of the superposition of first (sharp and stinging pain: A delta fibers) and second pain (dull and burning pain: C fibers), respectively. However, given the special circumstances of short ISIs CSSERP amplitudes decreased the more the shorter the ISI was. In line with previous investigations it is hypothesised that CSSERPs predominantly reflect nociceptive information transmitted via A delta fibers.

Response properties of primary somatosensory cortical neurons responsive to cold stimulation of the facial skin and oral mucous membrane

The distribution and response characteristics of the primary somatosensory cortical (SI) neurons activated by cold stimulation of the facial skin and oral mucous membrane were studied in cats. The discharge activities of 53 cold-sensitive SI neurons that responded to a decrease in temperature of the facial skin and/or oral mucous membrane were recorded. Each of these neurons was classified according to its responsiveness to mechanical stimulation as follows: LTM (low-threshold mechanoreceptive, 14/53), WDR (wide dynamic range, 39/53) and NS (nociceptive-specific, none identified). Encoding and non-encoding SI cold-sensitive neurons were identified, according to their responsiveness to decremental thermal stimulation. Only 14 cold-sensitive SI neurons demonstrated increased firing frequencies when subjected to incremental stimulus intensity increases and were classified as the encoding-type, whereas 39 non-encoding-type neurons did not.

Toward a computational model of constraint-driven exploration and haptic object identification

A conceptual model of the human haptic system in relation to object identification is presented. The model encompasses major architectural elements including representations of haptically accessible object properties and exploratory procedures (EPs)--dedicated movement patterns that are specialized to extract particular properties. These architectural units are related in processing-specific ways. Properties are associated with exploratory procedures in keeping with the extent to which a given procedure delivers information about a given property. The EPs are associated with one another in keeping with their compatibility, as determined by parameters of motor execution and interactions with the object and the workspace. The resulting architecture is treated as a system of constraints which guide the exploration of an object during the course of identification. The selection of the next step in a sequence of exploration requires that constraints be optimally satisfied. A network approach to constraint satisfaction is implemented and shown to account for a number of previous empirical results concerning the time course of exploration, object classification speed, and incidental learning about object properties. This system has potential applications for robotic haptic exploration.

Multiple representations of pain in human cerebral cortex

The representation of pain in the cerebral cortex is less well understood than that of any other sensory system. However, with the use of magnetic resonance imaging and positron emission tomography in humans, it has now been demonstrated that painful heat causes significant activation of the contralateral anterior cingulate, secondary somatosensory, and primary somatosensory cortices. This contrasts with the predominant activation of primary somatosensory cortex caused by vibrotactile stimuli in similar experiments. Furthermore, the unilateral cingulate activation indicates that this forebrain area, thought to regulate emotions, contains an unexpectedly specific representation of pain.

Studies of heat pain sensation in man: perception thresholds, rate of stimulus rise and reaction time

Afferent impulse frequency, one of the determinants of subjective magnitude of sensation, varies with the rate of rise of stimulus intensity: the faster the increase in stimulus energy, the higher the frequency of firing for a given amount of energy. This predicts that the steeper the stimulus ramp the lower will be the threshold for perception. While such inverse relation holds for myelinated fibre mediated cold sensation and mechanical pressure sensation, the opposite has been reported for unmyelinated fibre mediated heat pain and cold pain sensations. These paradoxical results intuitively suggest possible reaction time artefact. Indeed, a fixed time interval that includes conduction of the impulses to the brain, central processing and efferent conduction, intervenes between sufficient peripheral stimulus and the voluntary signal in reaction to subjective experience. As stimulus temperature continues to rise along this time, an artefactually high threshold reading results: the steeper the temperature rise, the larger will be the artefact, particularly for submodalities with longer reaction time. The present study compared heat pain threshold, obtained through a method that involves reaction time participation, with heat pain thresholds obtained bypassing reaction time. It was found in 16 volunteers that: (a) Heat pain thresholds decreased as the rate of temperature rise increased when reaction time was not a factor (P less than 0.001). (b) Heat pain thresholds determined through the method involving reaction time participation were significantly higher than those obtained bypassing reaction time (P less than 0.01). Such difference increased with increasing rates of temperature rise. (c) Peripheral conduction velocity calculated from average reaction time was found to be approximately 0.6 m/sec.(ABSTRACT TRUNCATED AT 250 WORDS)

Cutaneous pain and detection thresholds to short CO2 laser pulses in humans: evidence on afferent mechanisms and the influence of varying stimulus conditions

Pain and detection thresholds to short CO2 laser pulses were studied in healthy human subjects. Pain thresholds were significantly higher than detection thresholds in both hairy and glabrous skin; in the glabrous skin both thresholds were higher in the hairy skin. The range from detection threshold to pain threshold was larger in the glabrous skin. The minimal energy per surface area needed to produce any sensation (detection) or pain sensation decreased with increasing stimulus surface, and this spatial summation effect was to equal magnitude in the hairy and the glabrous skin. With decreasing stimulus pulse duration (from 45 to 15 msec) the detection and pain thresholds were elevated: this effect was stronger on pain thresholds. With increasing adapting skin temperature, less energy was needed to produce any sensation (detection) or pain sensation. The effect of adapting skin temperature was equal on pain and detection thresholds. The conduction velocity of fibers mediating laser evoked first sensations was in the thin fiber range (less than 10 msec), according to a reaction time study. The results suggest that short CO2 laser pulses produce both non-pain and pain sensations, but that both these sensations are based on the activation of the same primary afferent fiber population of slowly conducting nociceptive fibers. Central summation of primary afferent impulses is needed to elicit a liminal non-painful sensation, and an increased number of impulses in the same fibers produces pain.

Does acute intraoral pain alter cutaneous sensibility?

Cutaneous sensibility was tested in eight patients suffering from acute postoperative intraoral pain. Tactile-, cold-, warm-, and heat-pain thresholds as well as reaction time to cold pulses were unaffected by the presence of pain. However, reaction time to warm pulses was increased in the painful area on the day of pain compared to a non-painful state. The findings are discussed in relation to (1) functional convergence of different sensory fibres on central neurons (2) the phenomenon of diffuse noxious inhibitory controls and (3) secondary hyperalgesia. The observed effect of clinical pain on the warm pathway could be explained as an intrasegmental noxious inhibitory effect.

Analysis of cold and warm receptor activity in vampire bats and mice

The response characteristics of facial thermoreceptors of the common vampire bat and of the mouse have been quantitatively studied. Cold receptors were identified in bat and mouse; warm receptors were only established in the bat. Cold and warm receptor populations of the two species share most of their properties with facial thermoreceptor populations of various mammalian species investigated so far. The temporal pattern of activity of cold receptors of the mouse corresponded to that observed in cats, dogs and monkeys: impulse groups at lower, and beating activity at higher temperatures. At maintained temperature, no impulse groups were initiated in cold receptors of the bat. However, cooling steps from various initial temperatures induced a transient grouped discharge in both cold receptor populations. A discharge in regular groups of impulses was occasionally generated in warm receptors of the bat at maintained temperatures and following warming steps. The data indicate that the temperature dependence of periodic activity in warm receptors is not as uniform as it is in cold receptors. It is concluded that cyclic processes are involved in sensory transduction of both warm and cold receptors, and that this cyclic behavior seems to be a general property of thermoreceptors of presumably all vertebrate species.

Thermal sensitivity is not changed by acute pain or afferent stimulation

The effect of conditioning stimulation on thermal sensitivity and clinical pain was studied in 40 patients and six healthy subjects. Thresholds regarding cold, warm and heat pain perception did not differ significantly between the painful and non-painful skin areas in patients or between patients and healthy subjects before stimulation. The patients received either 100 Hz TENS, 2 Hz TENS, 100 Hz vibration, or placebo. No significant changes in thermal sensitivity were observed during and after conditioning stimulation in any of the test groups, although 24/40 (60%) of the patients reported reduction of their clinical pain intensity. The results indicate that (a) thermal sensitivity is not influenced by the presence of clinical pain, (b) the effects of stimulation on thermal sensitivity (thresholds) and clinical pain are not closely related, (c) central inhibitory effects of TENS and vibration are crucial for their pain relieving capacity.

Psychophysical assessment of tactile, pain and thermal sensory functions in burning mouth syndrome

Tactile, two-point discrimination, thermal change detection and heat pain thresholds as well as oral stereognostic ability, warmth scaling and heat pain tolerance were compared in a group of 72 subjects with burning mouth syndrome (BMS) and 43 age- and sex-matched control subjects. No differences were found between the BMS and control subjects for any of the sensory modalities tested except for heat pain tolerance. Pain tolerance was significantly decreased for the BMS subjects at the tongue tip, a site of clinical pain in approximately 85% of the subjects tested in this study, but not at the cutaneous lower lip which was a site of pain only in approximately 17% of the subjects tested in this study. In addition, no differences in heat pain tolerance were found at the cutaneous lower lip between the control subjects and the BMS subjects who reported pain on the mucosal lower lip (approximately 49% of subjects), but heat pain tolerance was significantly decreased at this site for those BMS subjects tested without pain on the mucosal lower lip (approximately 51% of subjects). These findings do not suggest a psychogenic origin for the alteration of heat pain tolerance in the BMS subjects, but suggest instead specific changes in their peripheral or central sensory functions.

Effect of tourniquet-induced ischemia on cutaneous thermal thresholds

The effect of tourniquet-induced ischemia on human thermal thresholds was studied. After the development of the A-fibre block (= a sharp elevation of cool threshold) the heat-pain threshold was still uninfluenced. This result supports previous evidence indicating that C-fibres mediate the liminal heat pain sensation. Thus, the quantitative determination of cutaneous heat pain thresholds provides a rather selective method for testing C-fibre mediated pain sensitivity, at least when a contact thermostimulator with a slow or moderate rise of stimulus temperature is used. The second aim of this study was to examine whether ischemia or mechanical pressure is the cause of the tourniquet-induced block of A-fibres. This was studied by varying the mechanical pressure and the amount of ischemia. With increased ischemia (with muscle work) the A-fibre block (increased cool threshold) came earlier, but this finding was not significant.

Influence of the rate of temperature change on thermal thresholds in man

Thermal thresholds (cool, warm, heat, heat pain) were determined in four skin regions (cheek, glabrous skin of the hand, hairy forearm, leg) of eight healthy human subjects. The thermostimulator was composed of Peltier elements and three rates of continuous stimulation were used: 1.4, 2.4, and 3.9 degrees C/s. Warm, heat, and heat pain thresholds increased with increasing rate of temperature change, and the increase was of equal magnitude with these three thresholds. However, the effect of increasing stimulus rate on cool thresholds was nonsignificant. Similar results were obtained in all skin regions studied. It is suggested that liminal warm, heat, and heat pain sensations are mediated by afferent fibers with conduction velocities of the same range (C-fibers) whereas liminal cool sensations are signaled by faster conducting afferent fibers.

Evidence for peripheral, but not central modulation of trigeminal cold receptive cells in the rat

The effects of locus coeruleus (LC), periaqueductal grey (PAG) and segmental stimulation (all of which are known to inhibit convergent nociceptive cells), were tested on the activity of cold receptive cells in the trigeminal system of the rat. LC and PAG stimulation from sites which inhibited convergent nociceptive cells had no effect on cells with cold receptive input in the trigeminal nucleus caudalis. Electrical or mechanical segmental stimulation caused suppression of activity in cold receptive trigeminal nucleus neurons. Recording from the trigeminal ganglion showed this suppression to be a property of the primary afferent cold receptors themselves and therefore it is not analogous to the proposed mechanism for the segmental inhibition of convergent nociceptive neurons.

Psychophysical detection and pain ratings of incremental thermal stimuli: a comparison with nociceptor responses in humans

The capacity of humans to detect and scale the magnitude of pain elicited by small increments in temperature, delivered by a contact thermal stimulator to localized areas of the arm or leg, was measured on non-painful and painful adaptation temperatures. Subjects continuously rated the magnitude of any pain sensation elicited by heat increments superimposed on base temperatures of 38, 44, 47 or 48 degrees C. Detection threshold was also measured using a two-alternative forced choice method. The increment detection thresholds were lower for a continuously painful base of 47 degrees C than for a non-painful base of 38 degrees C in normal skin, and likewise were lower for a base of 38 degrees C following hyperalgesia induced by a mild burn. Incremental pain thresholds were nearly equal to detection thresholds on the base of 47 degrees C. The sensitivity with which subjects could scale the magnitude of pain was 2-7 times better for increments delivered on a 48 degrees C as opposed to a 38 degrees C base. Evoked responses in 6 single C-fiber mechanoheat nociceptive afferents (CMHs) were recorded percutaneously from the peroneal nerves of 3 humans, who were simultaneously judging pain magnitude. For a base of 38 degrees C, both the pain and the neural response thresholds were an order of magnitude higher than corresponding thresholds on a base of 48 degrees C. For a base of 47 degrees C, response thresholds of the CMHs ranged from 0.1 to 0.5 degrees C and were comparable to detection thresholds of 0.1 to 0.3 degrees C. The sensitivity with which most nociceptors could signal increment size was 3-4 times better on a 48 degrees C than a 38 degrees C base. Incremental pain sensitivity was not altered by a compression block of activity in myelinated afferents that eliminated the sense of cool and touch. Thus, activity in unmyelinated fibers alone could account for the sensitivity to incremental thermal stimuli that were superimposed on a painful base temperature. Further, it is likely that CMH nociceptors alone could provide the peripheral information necessary to detect and to make magnitude judgments of pain elicited by these stimuli.

Discrimination of innocuous and noxious thermal stimuli applied to the face in human and monkey

Four humans and one monkey performed two-choice discriminations between simultaneous 3-sec heat stimuli applied to the face. All subjects produced more accurate discriminations in the noxious thermal range (approximately 47 degrees C) than in the innocuous thermal range (approximately 39 degrees C). The difference threshold, defined as the smallest temperature difference detected on 75% of the trials, was smaller for every subject at 47 degrees C than at 39 degrees C. The monkey's discriminative performance was comparable to that of humans in the noxious range, but inferior to that of humans in the innocuous range. Subjects' superior discrimination at noxious temperatures cannot be easily accounted for by differences in primary afferent activity of warm fibers and heat-nociceptive fibers. However, differences in central processing or attentional modulation could contribute to superior discrimination in the noxious range. These findings indicate that heat-sensitive nociceptors are capable of transmitting precise information about noxious thermal input to the skin.

Ischemic pain nonsegmentally produces a predominant reduction of pain and thermal sensitivity in man: a selective role for endogenous opioids

Ischemic pain was produced by a blood pressure cuff placed to the arm of healthy human subjects for 15 min which produced a mean pain score of 59% (visual analogue scale). Ischemia induced a significant dental pain threshold elevation (mean 67%) and 2 mg of naloxone did not reduce it. Thermal sensitivity of the upper lip had a tendency to reduction during ischemia and 2 mg of naloxone reduced this effect. Tactile thresholds in the forehead or in the contralateral arm were not markedly elevated. Neither ACTH nor prolactin level in the plasma was related to the dental pain threshold elevation during ischemia. The findings of the present study suggest that ischemic pain nonsegmentally produces a predominant inhibition of responses to thin afferents. Endogenous opioids may markedly contribute to the reduction of thermal sensitivity induced by ischemia, but their contribution to dental pain threshold elevations seems to be less important. Stress or other adenohypophyseal mechanisms involving the release of ACTH or prolactin do not explain the effects of ischemia found in the present study.