Fatigue and paradoxical enhancement of heat response in C-fiber nociceptors from cross-modal excitation

Assessing the influence of non-ischaemic A-fiber conduction blockade on offset analgesia: an experimental study

Offset analgesia (OA) is believed to reflect the efficiency of the endogenous pain modulatory system. However, the underlying mechanisms are still being debated. Previous research suggested both, central and peripheral mechanisms, with the latter involving the influence of specific A-delta-fibers. Therefore, this study aimed to investigate the influence of a non-ischaemic A-fiber conduction blockade on the OA response in healthy participants. A total of 52 participants were recruited for an A-fiber conduction blockade via compression of the superficial radial nerve. To monitor fiber-specific peripheral nerve conduction capacity, quantitative sensory testing was performed continuously. Before, during and after the A-fiber block, an individualized OA-paradigm was applied to the dorsum of both hands (blocked and control side were randomized). Pain intensity of each heat stimulus was evaluated by an electronic visual analogue scale. A successful A-fiber conduction blockade was achieved in thirty participants. Offset analgesia has been verified within time (before, during, after blockade), and condition (blocked and control side) (p < 0.01, d > 0.5). Repeated measurements ANOVA showed no significant interaction effects between OA within condition and time (p = 0.24, η²p = 0.05). Hence, no significant effect of A-fiber blockade was detected on OA during noxious heat stimulation. The results suggest that peripheral A-fiber afferents may play a minor role in OA compared to alternative central mechanisms or other fibers. However, further studies are needed to substantiate a central rather than peripheral influence on OA. PERSPECTIVE: This article presents the observation of offset analgesia before, during and after a successful A-fiber conduction blockade in healthy volunteers. A better understanding of the mechanisms of offset analgesia and endogenous pain modulation in general may help to explain the underlying aspects of pain disorders.

Neural Circuitry Polarization in the Spinal Dorsal Horn (SDH): A Novel Form of Dysregulated Circuitry Plasticity during Pain Pathogenesis

Pathological pain emerges from nociceptive system dysfunction, resulting in heightened pain circuit activity. Various forms of circuitry plasticity, such as central sensitization, synaptic plasticity, homeostatic plasticity, and excitation/inhibition balance, contribute to the malfunction of neural circuits during pain pathogenesis. Recently, a new form of plasticity in the spinal dorsal horn (SDH), named neural circuit polarization (NCP), was discovered in pain models induced by HIV-1 gp120 and chronic morphine administration. NCP manifests as an increase in excitatory postsynaptic currents (EPSCs) in excitatory neurons and a decrease in EPSCs in inhibitory neurons, presumably facilitating hyperactivation of pain circuits. The expression of NCP is associated with astrogliosis. Ablation of reactive astrocytes or suppression of astrogliosis blocks NCP and, concomitantly, the development of gp120- or morphine-induced pain. In this review, we aim to compare and integrate NCP with other forms of plasticity in pain circuits to improve the understanding of the pathogenic contribution of NCP and its cooperation with other forms of circuitry plasticity during the development of pathological pain.

Differential Effects of Thermal Stimuli in Eliciting Temporal Contrast Enhancement: A Psychophysical Study

Offset analgesia (OA) is observed when pain relief is disproportional to the reduction of noxious input and is based on temporal contrast enhancement (TCE). This phenomenon is believed to reflect the function of the inhibitory pain modulatory system. However, the mechanisms contributing to this phenomenon remain poorly understood, with previous research focusing primarily on painful stimuli and not generalizing to nonpainful stimuli. Therefore, the aim of this study was to investigate whether TCE can be induced by noxious as well as innocuous heat and cold stimuli. Asymptomatic subjects (n = 50) were recruited to participate in 2 consecutive experiments. In the first pilot study (n = 17), the parameters of noxious and innocuous heat and cold stimuli were investigated in order to implement them in the main study. In the second (main) experiment, subjects (n = 33) participated in TCE paradigms consisting of 4 different modalities, including noxious heat (NH), innocuous heat (IH), noxious cold (NC), and innocuous cold (IC). The intensity of the sensations of each thermal modality was assessed using an electronic visual analog scale. TCE was confirmed for NH (P < .001), NC (P = .034), and IC (P = .002). Conversely, TCE could not be shown for IH (P = 1.00). No significant correlation between TCE modalities was found (r < .3, P > .05). The results suggest that TCE can be induced by both painful and nonpainful thermal stimulation but not by innocuous warm temperature. The exact underlying mechanisms need to be clarified. However, among other potential mechanisms, this may be explained by a thermo-specific activation of C-fiber afferents by IH and of A-fiber afferents by IC, suggesting the involvement of A-fibers rather than C-fibers in TCE. More research is needed to confirm a peripheral influence. PERSPECTIVE: This psychophysical study presents the observation of temporal contrast enhancement during NH, NC, and innocuous cold stimuli but not during stimulation with innocuous warm temperatures in healthy volunteers. A better understanding of endogenous pain modulation mechanisms might be helpful in explaining the underlying aspects of pain disorders.

Exploring the Use of the Current Perception Threshold in Pharyngeal Paresthesia Patients

To explore pharyngeal sensory function by current perception threshold (CPT) measurement in paresthetic pharynx. In total, 58 healthy participants and 66 patients with pharyngeal paresthetic symptoms underwent CPT evaluation. Pharyngeal paresthesia (n = 66) was classified into three categories based on aetiologies: six cases with pain in pharynx; 34 neuropathic patients with glossopharyngeal nerve and/or vagus nerve or recurrent laryngeal nerve injury; and 26 patients with globus pharyngeus. CPT measurements were obtained from bilateral palatoglossal arch and tongue base at 2000, 250 and 5 Hz stimulation frequencies. Ranked from high to low, the CPT values for the bilateral palatoglossal arches and tongue bases were: lower cranial neuropathic patients, globus pharyngeus, healthy participants and patients with pain. The CPT values for neuropathic patients on the injured side were significantly higher than those on the healthy side (P < 0.05). The CPT values for patients with pain in pharynx were significantly lower than those of healthy participants (P < 0.05) when the bilateral tongue bases were stimulated. The CPT measurement is a reliable method for quantitatively assessing pharyngeal sensory function and able to differentiate pharyngeal paresthesia between lower cranial neuropathic and subjective discomfort. Pharyngeal sensory function is more sensitive in patients with pain in pharynx. Pharyngeal sensory function is significantly reduced in lower cranial neuropathic patients, especially on the injured side. Patients with globus pharyngeus have pharyngeal hyposensitivity.

Effects of repeated exposures to experimental cold pain stimulus on pain perception in healthy young Indian men

Background

The influence of repeated exposures to cold pain stimulus, a surrogate of clinical pain, has remained largely unexplored. The study was planned to test the effect of repeated exposures to cold pain through cold pressor task on pain sensitivity and vascular reactivity.

Methods

Single-group experimental study. Thirty-seven healthy male volunteers (18-25 years) were exposed to cold pressor tasks for seven consecutive days and on the 14th day on the nondominant hand. The same was repeated on dominant hand on first and seventh days; 31 completed the protocol.

Results

Pain threshold and tolerance in the nondominant hand increased from day 1 to day 7 (p < 0.001) and were positively correlated on day 1 ( ρ =0.45, p = 0.011) and day 7 ( ρ =0.38, p = 0.036). Diastolic blood pressure response was found to increase by day 7 (p < 0.0024) and positively correlated with tolerance. On the dominant hand, the threshold reduced from day 1 to day 7, while tolerance increased. Both threshold and tolerance remained lower than that of nondominant hand. Day 14 values of threshold and tolerance were in between day 1 and day 7 values but not significantly different from both.

Conclusion

Habituation in pain threshold and tolerance was observed on repeated exposure to cold pain, which was not significantly retained till the 14th day. The same was not observed with subjective feeling of pain perception. The increased diastolic blood pressure response is suggestive of peripheral vasoconstriction. Increased tolerance in the dominant hand by day 7 demonstrates a systemic effect in habituation.

The capsaicin receptor TRPV1 is the first line defense protecting from acute non damaging heat: a translational approach

Background

Pain is the vital sense preventing tissue damage by harmful noxious stimuli. The capsaicin receptor TRPV1 is activated by noxious temperatures, however, acute heat pain is only marginally affected in mice after TRPV1 knockout but completely eliminated in mice lacking TRPV1 positive fibers. Exploring contribution of candidate signal transduction mechanisms to heat pain in humans needs translational models.

Methods

We used focused, non-damaging, short near-infrared laser heat stimuli (wavelength 1470/1475 nm) to study the involvement of TRPV1-expressing nerve fibers in the encoding of heat pain intensity. Human psychophysics (both sexes) were compared to calcium transients in native rat DRG neurons and heterologously expressing HEK293 cells.

Results

Heating of dermal and epidermal nerve fibers in humans with laser stimuli of ≥ 2.5 mJ (≥ 25 ms, 100 mW) induced pain that increased linearly as a function of stimulus intensity in double logarithmic space across two orders of magnitude and was completely abolished by desensitization using topical capsaicin. In DRG neurons and TRPV1-expressing HEK cells, heat sensitivity was restricted to capsaicin sensitive cells. Strength duration curves (2–10 ms range) and thresholds (DRGs 0.56 mJ, HEK cells 0.52 mJ) were nearly identical. Tachyphylaxis upon repetitive stimulation occurred in HEK cells (54%), DRGs (59%), and humans (25%).

Conclusion

TRPV1-expressing nociceptors encode transient non-damaging heat pain in humans, thermal gating of TRPV1 is similar in HEK cells and DRG neurons, and TRPV1 tachyphylaxis is an important modulator of heat pain sensitivity. These findings suggest that TRPV1 expressed in dermal and epidermal populations of nociceptors serves as first line defense against heat injury.

Thermal block of action potentials is primarily due to voltage-dependent potassium currents: a modeling study

OBJECTIVE

Thermal block of action potential conduction using infrared lasers is a new modality for manipulating neural activity. It could be used for analysis of the nervous system and for therapeutic applications. We sought to understand the mechanisms of thermal block.

APPROACH

To analyze the mechanisms of thermal block, we studied both the original Hodgkin/Huxley model, and a version modified to more accurately match experimental data on thermal responses in the squid giant axon.

MAIN RESULTS

Both the original and modified models suggested that thermal block, especially at higher temperatures, is primarily due to a depolarization-activated hyperpolarization as increased temperature leads to faster activation of voltage-gated potassium ion channels. The minimum length needed to block an axon scaled with the square root of the axon's diameter.

SIGNIFICANCE

The results suggest that voltage-dependent potassium ion channels play a major role in thermal block, and that relatively short lengths of axon could be thermally manipulated to selectively block fine, unmyelinated axons, such as C fibers, that carry pain and other sensory information.

The Effect of Moxibustion Stimulation on Local and Distal Skin Temperature in Healthy Subjects

The aim of this study is to investigate the response of local and distal skin temperature to moxibustion stimulation (MS) and explore the effects of MS on sympathetic nerve activity. The distal skin temperatures of fingertips, as an indicator for sympathetic reflex response, were recorded using infrared camera during resting period (10 min), MS period (10 min), and natural cooling period (15 min), respectively. The MS without ash cleaning (AC) was applied to acupoints Quze (PC3) (Group I) and Lao Gong (PC8) (Group II), respectively. In Group III, the MS with the operation of AC was performed on PC8. The temperature responses of the local stimulation points and corresponding control points were also investigated. At the beginning of MS period, a significant increase of temperature on the stimulation point accompanied by a simultaneous reduction of temperature on fingertips was observed. A marked negative correlation was also obtained between temperature changes in the stimulation point and in the fingertips. At the end of natural cooling period (t = 34 min), the temperature of stimulation point was obviously higher than baseline values. In contrast, the temperatures of fingertips increased and then returned to the baseline levels during the second minute of MS period. In Group III, the temperature of stimulation point increased every time with the operation of AC, accompanied by the temperature decrease of middle fingertip. The findings suggest that moxibustion may trigger the sympathetic nervous system and induce the reduction of microcirculation, accompanied by a reduction of fingertip temperature. In addition, the operation of AC caused repeated cycles of thermal stimulation on the stimulation point, which may repetitively activate cutaneous sympathetic nerve fibres and evoke the temperature reduction of fingertips.

Evidence for a spinal involvement in temporal pain contrast enhancement

Spatiotemporal filtering and amplification of sensory information at multiple levels during the generation of perceptual representations is a fundamental processing principle of the nervous system. While for the visual and auditory system temporal filtering of sensory signals has been noticed for a long time, respective contrast mechanisms within the nociceptive system became only recently subject of investigations, mainly in the context of offset analgesia (OA) subsequent to noxious stimulus decreases. In the present study we corroborate in a first experiment the assumption that offset analgesia involves a central component by showing that an OA-like effect accounting for 74% of a corresponding OA reference can be evoked by decomposing the stimulus offset into two separate box-car stimuli applied within the same dermatome but to separate populations of primary afferent neurons. In order to draw conclusions about the levels of the CNS at which temporal filtering of nociceptive information takes place during OA we investigate in a second experiment neuronal activity in the spinal cord during a painful thermal stimulus offset employing high-resolution fMRI in healthy volunteers. Pain-related BOLD responses in the spinal cord were significantly reduced during OA and their time course followed widely behavioral hypoalgesia, but not the thermal stimulation profile. In summary, the results suggest that temporal pain contrast enhancement during OA comprises a central mechanism and this mechanism becomes already effective at the level of the spinal cord.

Mechanically sensitive Aδ nociceptors that innervate bone marrow respond to changes in intra-osseous pressure

KEY POINTS

Sensory neurons that innervate the bone marrow provide the CNS with information about pain associated with bone disease and pathology, but little is known of their function. Here we use a novel in vivo bone-nerve electrophysiological preparation to study how they respond to noxious mechanical stimulation delivered by increasing intra-osseous pressure. We provide evidence that sensory neurons that innervate the bone marrow respond to high threshold noxious mechanical stimulation, have response properties consistent with a role in nociception, provide information about different features of an intra-osseous pressure stimulus and express the Piezo2 mechano-transducer molecule. Our findings show how some bone marrow nociceptors signal pain in bony diseases and pathologies that involve a mechanical disturbance or increased intra-osseous pressure, and that the Piezo2 mechano-transducer may be involved.

ABSTRACT

Whilst the sensory neurons and nerve terminals that innervate bone marrow have a morphology and molecular phenotype consistent with a role in nociception, little is known about their physiology or the mechanisms that generate and maintain bone pain. In the present study, we provide evidence that Aδ nociceptors that innervate the bone marrow respond to high threshold noxious mechanical stimulation, exhibit fatigue in response to prior stimulation and in some cases can be sensitized by capsaicin. They can be classified on the basis of their response properties as either phasic-tonic units that appear to code for different intensities of intra-osseous pressure, or phasic units that code for the rate of change in intra-osseous pressure. Three different subclasses of mechanically sensitive Aδ units were observed: phasic units that were sensitized by capsaicin, phasic units that were not sensitized by capsaicin and phasic-tonic units (that were not sensitized by capsaicin). These could also, in part, be distinguished by differences in their thresholds for activation, mean discharge frequency, latency to peak activation and peak-to-peak action potential amplitude. The majority of small-diameter myelinated sensory neurons projecting to the bone marrow expressed Piezo2. Our findings indicate that Aδ mechano-nociceptors are likely to play an important role in generating and maintaining pain in response to bony pathologies that involve a mechanical disturbance or increased intra-osseous pressure, and imply that Piezo2 signalling may be involved in mechano-transduction in these receptors.

Sleep Fragmentation Hypersensitizes Healthy Young Women to Deep and Superficial Experimental Pain

The effect of sleep deprivation on pain sensitivity has typically been studied using total and partial sleep deprivation protocols. These protocols do not mimic the fragmented pattern of sleep disruption usually observed in individuals with clinical pain conditions. Therefore, we conducted a controlled experiment to investigate the effect of sleep fragmentation on pain perception (deep pain: forearm muscle ischemia, and superficial pain: graded pin pricks applied to the skin) in 11 healthy young women after 2 consecutive nights of sleep fragmentation, compared with a normal night of sleep. Compared with normal sleep, sleep fragmentation resulted in significantly poorer sleep quality, morning vigilance, and global mood. Pin prick threshold decreased significantly (increased sensitivity), as did habituation to ischemic muscle pain (increased sensitivity), over the course of the 2 nights of sleep fragmentation compared with the night of normal sleep. Sleep fragmentation did not increase the maximum pain intensity reported during muscle ischemia (no increase in gain), and nor did it increase the number of spontaneous pains reported by participants. Our data show that sleep fragmentation in healthy, young, pain-free women increases pain sensitivity in superficial and deep tissues, indicating a role for sleep disruption, through sleep fragmentation, in modulating pain perception.

PERSPECTIVE

Our findings that pain-free, young women develop hyperalgesia to superficial and deep muscle pain after short-term sleep disruption highlight the need for effective sleep management strategies in patients with pain. Findings also suggest the possibility that short-term sleep disruption associated with recurrent acute pain could contribute to increased risk for future chronic pain conditions.

Use of Capsaicin to Treat Pain: Mechanistic and Therapeutic Considerations

Capsaicin is the pungent ingredient of chili peppers and is approved as a topical treatment of neuropathic pain. The analgesia lasts for several months after a single treatment. Capsaicin selectively activates TRPV1, a Ca²⁺-permeable cationic ion channel that is enriched in the terminals of certain nociceptors. Activation is followed by a prolonged decreased response to noxious stimuli. Interest also exists in the use of injectable capsaicin as a treatment for focal pain conditions, such as arthritis and other musculoskeletal conditions. Recently injection of capsaicin showed therapeutic efficacy in patients with Morton’s neuroma, a painful foot condition associated with compression of one of the digital nerves. The relief of pain was associated with no change in tactile sensibility. Though injection evokes short term pain, the brief systemic exposure and potential to establish long term analgesia without other sensory changes creates an attractive clinical profile. Short-term and long-term effects arise from both functional and structural changes in nociceptive terminals. In this review, we discuss how local administration of capsaicin may induce ablation of nociceptive terminals and the clinical implications.

Spatial and Temporal Brain Responses to Noxious Heat Thermal Stimuli in Burning Mouth Syndrome

Burning mouth syndrome (BMS) is an idiopathic orofacial pain condition. Although the pathophysiology of BMS is not clearly understood, central and peripheral neuropathic mechanisms are thought to be involved. The authors compared brain response to noxious heat stimuli in 16 right-handed women with primary BMS and 15 sex- and age-matched right-handed healthy female controls. A thermal stimulus sequence of 32 °C to 40 °C to 32 °C to 49 °C was repeated 4 times in a cycle. Warm and noxious heat stimuli were delivered with a Peltier thermode placed on the right palm or right lower lip for 32 s each in a session. Functional magnetic resonance imaging data were obtained by recording echoplanar images with a block design. Statistical Parametric Mapping 8 software was used to analyze the data. Patients and controls both reported feeling more pain during palm stimulation than during lip stimulation. Repetition of noxious heat stimulus on the lower lip but not on the palm induced habituation in brain activity in the cingulate cortex without reduction in pain perception. Multiple regression analysis revealed a correlation between perceived pain intensity and suppression of brain activity in the anterior cingulate cortex when the repeated thermal sequence was applied at the lower lip. Furthermore, the response of the parahippocampal area differed in BMS patients and controls when the same repeated thermal sequence was applied at the palm. The authors’ findings indicate that BMS patients show specific brain responses due to impaired function of the central and peripheral nervous systems (clinical trial registration: UMIN000015002).

Normothermic Mouse Functional MRI of Acute Focal Thermostimulation for Probing Nociception

Combining mouse genomics and functional magnetic resonance imaging (fMRI) provides a promising tool to unravel the molecular mechanisms of chronic pain. Probing murine nociception via the blood oxygenation level-dependent (BOLD) effect is still challenging due to methodological constraints. Here we report on the reproducible application of acute noxious heat stimuli to examine the feasibility and limitations of functional brain mapping for central pain processing in mice. Recent technical and procedural advances were applied for enhanced BOLD signal detection and a tight control of physiological parameters. The latter includes the development of a novel mouse cradle designed to maintain whole-body normothermia in anesthetized mice during fMRI in a way that reflects the thermal status of awake, resting mice. Applying mild noxious heat stimuli to wildtype mice resulted in highly significant BOLD patterns in anatomical brain structures forming the pain matrix, which comprise temporal signal intensity changes of up to 6% magnitude. We also observed sub-threshold correlation patterns in large areas of the brain, as well as alterations in mean arterial blood pressure (MABP) in response to the applied stimulus.

TNAP and Pain Control

Chronic pain is one of the most debilitating and expensive diseases, yet current therapies are often insufficient in bringing about long-term relief. Further, many treatments for chronic pain also carry significant side effects. The molecule adenosine has long been identified as a potent inhibitor of nociceptive circuits in the spinal cord; however, the widespread expression of adenosine receptors in many organ systems has limited its use as an analgesic. Recently several 5' ectonucleotidases, including tissue non-specific alkaline phosphatase (TNAP), have been characterized for their ability to generate endogenous adenosine in nociceptive circuitry of the dorsal spinal cord. These ectonucleotidases have the ability to hydrolyze the endogenous pronociceptive nucleotides like adenosine triphosphate (ATP) into the antinociceptive nucleoside adenosine. This chapter discusses the role of TNAP and other ectonucleotidases in nociceptive circuits, and their potential as future targets of new therapeutics to treat chronic pain.

Relationship between adaptation and cardiovascular response to tonic cold and heat pain Adaptability to tonic pain and cardiovascular responses

BACKGROUND

The mechanisms of adaptation to tonic pain are not elucidated. We hypothesized that the adaptability to tonic pain is related to the cardiovascular system.

METHODS

Twenty-six subjects received over two sessions in a random order: tonic cold (7 ± 0.2 °C) and heat pain (47.5 ± 0.5 °C) on the hand for 5 min. Pain intensity, blood pressure (BP), and heart rate (HR) were continuously monitored.

RESULTS

Pain experience during the heat (HIT) and cold (CIT) immersion tests exhibited different average time courses, being approximated with a linear and cubic function, respectively. In each test, two groups of participants could be identified based on the time course of their tonic thermal pain: one-third of participants were pain adaptive and two-thirds non adaptive. The adaptive group exhibited higher initial pain, lower last pain, and shorter latency to peak pain than the non-adaptive one. Interestingly, some participants were adaptive to both pain stimuli, most were not. HIT as well as CIT produced a stable elevation of BP. However, BP was higher during CIT than HIT (p = 0.034). HR was also increased during CIT and HIT, but the two tests differed with respect to the time course of responses. Finally, the intensity and time course of pain rating to both HIT and CIT correlated with neither BP nor HR responses.

CONCLUSIONS

These results suggest that individual sensitivity and adaptability to tonic thermal pain is related to the intensity of initial pain rating and the latency to peak pain but not to cardiovascular responses.

The dynamics of pain: evidence for simultaneous site-specific habituation and site-nonspecific sensitization in thermal pain

Repeated exposure to noxious stimuli changes their painfulness, due to multiple adaptive processes in the peripheral and central nervous systems. Somewhat paradoxically, repeated stimulation can produce an increase (sensitization) or a decrease (habituation) in pain. Adaptation processes may also be body-site-specific or operate across body sites, and considering this distinction may help explain the conditions under which habituation versus sensitization occurs. To dissociate the effects of site-specific and site-nonspecific adaptation processes, we examined reported pain in 100 participants during counterbalanced sequences of noxious thermal stimulation on multiple skin sites. Analysis of pain ratings revealed 2 opposing sequential effects: repeated stimulations of the same skin site produced temperature-dependent habituation, whereas repeated stimulations across different sites produced sensitization. Stimulation trials were separated by ∼20 seconds, and sensitization was unrelated to the distance between successively stimulated sites, suggesting that neither temporal nor spatial summation occurred. To explain these effects, we propose a dynamic model with 2 adaptation processes, one site-specific and the other site-nonspecific. The model explains 93% of the variance in the group-mean pain ratings after controlling for current stimulation temperature, with its estimated parameters showing evidence for habituation for the site-specific process and sensitization for the site-nonspecific process. The 2 pain adaptation processes revealed in this study, and the ability to disentangle them, may hold keys to understanding multiple pain-regulatory mechanisms and their disturbance in chronic pain syndromes.

PERSPECTIVE

This article presents novel evidence for simultaneous site-specific habituation and site-nonspecific sensitization in thermal pain, which can be disentangled (and the direction and strength of each process estimated) by a dynamic model. The dissociation of site-specific and site-nonspecific adaptation processes may hold keys to understanding multiple pain-regulatory mechanisms in both healthy and patient populations.

Eugenol and carvacrol excite first- and second-order trigeminal neurons and enhance their heat-evoked responses

Eugenol and carvacrol from clove and oregano, respectively, are agonists of the warmth-sensitive transient receptor potential channel TRPV3 and the irritant-sensitive transient receptor potential ankyrin (TRPA)-1. Eugenol and carvacrol induce oral irritation that rapidly desensitizes, accompanied by brief enhancement of innocuous warmth and heat pain in humans. We presently investigated if eugenol and carvacrol activate nociceptive primary afferent and higher order trigeminal neurons and enhance their heat-evoked responses, using calcium imaging of cultured trigeminal ganglion (TG) and dorsal root ganglion (DRG) neurons, and in vivo single-unit recordings in trigeminal subnucleus caudalis (Vc) of rats. Eugenol and carvacrol activated 20-30% of TG and 7-20% of DRG cells, the majority of which additionally responded to menthol, mustard oil and/or capsaicin. TG cell responses to innocuous (39°) and noxious (42 °C) heating were enhanced by eugenol and carvacrol. We identified dorsomedial Vc neurons responsive to noxious heating of the tongue in pentobarbital-anesthetized rats. Eugenol and carvacrol dose-dependently elicited desensitizing responses in 55% and 73% of heat-sensitive units, respectively. Responses to noxious heat were briefly enhanced by eugenol and carvacrol. Many eugenol- and carvacrol-responsive units also responded to menthol, cinnamaldehyde and capsaicin. These data support a peripheral site for eugenol and carvacrol to enhance warmth- and noxious heat-evoked responses of trigeminal neurons, and are consistent with the observation that these agonists briefly enhance warmth and heat pain on the human tongue.

NON-INVASIVE EVALUATION OF NERVE CONDUCTION IN SMALL DIAMETER FIBERS IN THE RAT

A novel non-invasive technique was applied to measure velocity within slow conducting axons in the distal extreme of the sciatic nerve (i.e., digital nerve) in a rat model. The technique is based on the extraction of rectified multiple unit activity (MUA) from in vivo whole nerve compound responses. This method reliably identifies compound action potentials in thinly myelinated fibers conducting at a range of 9-18 m/s (Aδ axons), as well as in a subgroup of unmylinated C fibers conducting at approximately 1-2 m/s. The sensitivity of the method to C-fiber conduction was confirmed by the progressive decrement of the responses in the 1-2 m/s range over a 20-day period following the topical application of capsaicin (ANOVA p<0.03). Increasing the frequency of applied repetitive stimulation over a range of 0.75 Hz to 6.0 Hz produced slowing of conduction and a significant decrease in the magnitude of the compound C-fiber response (ANOVA p<0.01). This technique offers a unique opportunity for the non-invasive, repeatable, and quantitative assessment of velocity in the subsets of Aδ and C fibers in parallel with evaluation of fast nerve conduction.

Gastric electrical stimulation decreases gastric distension-induced central nociception response through direct action on primary afferents

Background & Aims

Gastric electrical stimulation (GES) is an effective therapy to treat patients with chronic dyspepsia refractory to medical management. However, its mechanisms of action remain poorly understood.

Methods

Gastric pain was induced by performing gastric distension (GD) in anesthetized rats. Pain response was monitored by measuring the pseudo-affective reflex (e.g., blood pressure variation), while neuronal activation was determined using c-fos immunochemistry in the central nervous system. Involvement of primary afferents was assessed by measuring phosphorylation of ERK1/2 in dorsal root ganglia.

Results

GES decreased blood pressure variation induced by GD, and prevented GD-induced neuronal activation in the dorsal horn of the spinal cord (T9–T10), the nucleus of the solitary tract and in CRF neurons of the hypothalamic paraventricular nucleus. This effect remained unaltered within the spinal cord when sectioning the medulla at the T5 level. Furthermore, GES prevented GD-induced phosphorylation of ERK1/2 in dorsal root ganglia.

Conclusions

GES decreases GD-induced pain and/or discomfort likely through a direct modulation of gastric spinal afferents reducing central processing of visceral nociception.

Differential effects of experimental central sensitization on the time-course and magnitude of offset analgesia

Pain perception is temporally altered during states of chronic pain and acute central sensitization; however, the mechanisms contributing to temporal processing of nociceptive information remain poorly understood. Offset analgesia is a phenomenon that reflects the presence of temporal contrast mechanisms for nociceptive information and can provide an end point to study temporal aspects of pain processing. In order to investigate whether offset analgesia is disrupted during sensitized states, 23 healthy volunteers provided real-time continuous visual analogue scale responses to noxious heat stimuli that evoke offset analgesia. Responses to these stimuli were evaluated during capsaicin-heat sensitization (45°C stimulus, capsaicin cream 0.1%) and heat-only sensitization (40°C stimulus, placebo cream). Capsaicin-heat sensitization produced significantly larger regions of secondary mechanical allodynia compared to heat-only sensitization. Although areas of mechanical allodynia were positively related to individual differences in heat pain sensitivity, this relationship was altered at later time points after capsaicin-heat sensitization. Heat hyperalgesia was observed in the secondary region following both capsaicin-heat and heat-only sensitization. Increased latencies to maximal offset analgesia and prolonged aftersensations were observed only in the primary regions directly treated by capsaicin-heat or heat alone. However, contrary to the hypothesis that offset analgesia would be reduced following capsaicin-heat sensitization, the magnitude of offset analgesia remained remarkably intact after both capsaicin-heat and heat-only sensitization in zones of both primary and secondary mechanical allodynia. These data indicate that offset analgesia is a robust phenomenon and engages mechanisms that interact minimally with those supporting acute central sensitization.

Thermoreception and nociception of the skin: a classic paper of Bessou and Perl and analyses of thermal sensitivity during a student laboratory exercise

About four decades ago, Perl and collaborators were the first ones who unambiguously identified specifically nociceptive neurons in the periphery. In their classic work, they recorded action potentials from single C-fibers of a cutaneous nerve in cats while applying carefully graded stimuli to the skin (Bessou P, Perl ER. Response of cutaneous sensory units with unmyelinated fibers to noxious stimuli. J Neurophysiol 32: 1025-1043, 1969). They discovered polymodal nociceptors, which responded to mechanical, thermal, and chemical stimuli in the noxious range, and differentiated them from low-threshold thermoreceptors. Their classic findings form the basis of the present method that undergraduate medical students experience during laboratory exercises of sensory physiology, namely, quantitative testing of the thermal detection and pain thresholds. This diagnostic method examines the function of thin afferent nerve fibers. We collected data from nearly 300 students that showed that 1) women are more sensitive to thermal detection and thermal pain at the thenar than men, 2) habituation shifts thermal pain thresholds during repetititve testing, 3) the cold pain threshold is rather variable and lower when tested after heat pain than in the reverse case (order effect), and 4) ratings of pain intensity on a visual analog scale are correlated with the threshold temperature for heat pain but not for cold pain. Median group results could be reproduced in a retest. Quantitative sensory testing of thermal thresholds is feasible and instructive in the setting of a laboratory exercise and is appreciated by the students as a relevant and interesting technique.

Thermoreceptors and thermosensitive afferents

Cutaneous thermosensation plays an important role in thermal regulation and detection of potentially harmful thermal stimuli. Multiple classes of primary afferents are responsive to thermal stimuli. Afferent nerve fibers mediating the sensation of non-painful warmth or cold seem adapted to convey thermal information over a particular temperature range. In contrast, nociceptive afferents are often activated by both, painful cold and heat stimuli. The transduction mechanisms engaged by thermal stimuli have only recently been discovered. Transient receptor potential (TRP) ion channels that can be activated by temperatures over specific ranges potentially provide the molecular basis for thermosensation. However, non-TRP mechanisms are also likely to contribute to the transduction of thermal stimuli. This review summarizes findings regarding the transduction proteins and the primary afferents activated by innocuous and noxious cold and heat.

Habituation to pain: further support for a central component

Habituation to repetitive painful stimulation may represent an important protection mechanism against the development of chronic pain states. However, the exact neurobiological mechanisms of this phenomenon remain unclear. In this study we (i) explore the somatotopic specificity of pain attenuation over time and (ii) investigate the role of the endogenous opioid system in its development. We investigated 24 healthy volunteers with a paradigm of daily painful stimulation of the left volar forearm for 1 week. Habituation was assessed by comparing pain-related responses (ratings and thresholds) between days 1 and 8. To test whether a repetition-dependent attenuation of pain is restricted to the site of stimulus application or induces additional systemic effects indicative of a central mechanism, we also measured pain-related responses at the contralateral arm and the left leg. To assess the role of the endogenous opioid system in this mechanism, we used the opioid-receptor antagonist naloxone in a double-blind design. Repetitive painful stimulation over several days resulted in a significant habituation to pain at the site of daily stimulation. In addition, we also observed significant pain attenuation at the non-stimulated limbs. This effect was less pronounced at the untreated arm compared to the treated arm and even weaker in the leg, displaying a significant Stimulation-Site x Time interaction. The development of pain habituation was unaffected by the opioid antagonist naloxone. Taken together, these results strongly support the role of central components in the mechanism of pain habituation that do not directly involve the endogenous opioid system.

Short-term synaptic plasticity in the nociceptive thalamic-anterior cingulate pathway

Background

Although the mechanisms of short- and long-term potentiation of nociceptive-evoked responses are well known in the spinal cord, including central sensitization, there has been a growing body of information on such events in the cerebral cortex. In view of the importance of anterior cingulate cortex (ACC) in chronic pain conditions, this review considers neuronal plasticities in the thalamocingulate pathway that may be the earliest changes associated with such syndromes.

Results

A single nociceptive electrical stimulus to the sciatic nerve induced a prominent sink current in the layer II/III of the ACC in vivo, while high frequency stimulation potentiated the response of this current. Paired-pulse facilitation by electrical stimulation of midline, mediodorsal and intralaminar thalamic nuclei (MITN) suggesting that the MITN projection to ACC mediates the nociceptive short-term plasticity. The short-term synaptic plasticities were evaluated for different inputs in vitro where the medial thalamic and contralateral corpus callosum afferents were compared. Stimulation of the mediodorsal afferent evoked a stronger short-term synaptic plasticity and effectively transferred the bursting thalamic activity to cingulate cortex that was not true for contralateral stimulation. This short-term enhancement of synaptic transmission was mediated by polysynaptic pathways and NMDA receptors. Layer II/III neurons of the ACC express a short-term plasticity that involves glutamate and presynaptic calcium influx and is an important mechanism of the short-term plasticity.

Conclusion

The potentiation of ACC neuronal activity induced by thalamic bursting suggest that short-term synaptic plasticities enable the processing of nociceptive information from the medial thalamus and this temporal response variability is particularly important in pain because temporal maintenance of the response supports cortical integration and memory formation related to noxious events. Moreover, these modifications of cingulate synapses appear to regulate afferent signals that may be important to the transition from acute to chronic pain conditions associated with persistent peripheral noxious stimulation. Enhanced and maintained nociceptive activities in cingulate cortex, therefore, can become adverse and it will be important to learn how to regulate such changes in thalamic firing patterns that transmit nociceptive information to ACC in early stages of chronic pain.

Heat-induced action potential discharges in nociceptive primary sensory neurons of rats

Although several transducer molecules for noxious stimuli have been identified, little is known about the transformation of the resulting generator currents into action potentials (APs). Therefore we investigated the transformation process for stepped noxious heat stimuli (42-47 degrees C, 3-s duration) into membrane potential changes and subsequent AP discharges using the somata of acutely dissociated small dorsal root ganglion (DRG) neurons (diameter<or=32.5 microm) of adult rats as a model for their own peripheral terminals. Three types of heat-induced membrane potential changes were differentiated: type 1, heat-induced AP discharges (approximately 37% of the neurons); type 2, heat-induced membrane depolarization (40%); and type 3, responses not exceeding those of switching the superfusion (23%). Warming neurons from room temperature to 35 degrees C increased their background conductance, nearly doubled the AP threshold current, and led to smaller and narrower APs. Adaptation of heat-induced AP discharges was seen in about half of the type 1 neurons. The remaining half displayed accelerating discharges to both heat stimuli and depolarizing current injection. Repeated heat stimulation induced marked suppression of AP discharges. Under rapid calcium buffering using BAPTA, repolarization of heat-induced APs stopped at a plateau potential slowly decreasing from +16.5+/-2.9 to -2.2+/-5.5 mV, resulting in no further AP discharges. This study demonstrates that heat-induced AP discharges can be elicited in the soma of a subgroup of DRG neurons. These discharges display suppression on repetitive stimulation, but either adaptation or sensitization during prolonged stimuli. AP threshold and AP shape during these discharges suggest temperature dependence of background conductance and repolarizing currents.

Pruritic and nociceptive sensations and dysesthesias from a spicule of cowhage

Although the trichomes (spicules) of a pod of cowhage (Mucuna pruriens) are known to evoke a histamine-independent itch that is mediated by a cysteine protease, little is known of the itch and accompanying nociceptive sensations evoked by a single spicule and the enhanced itch and pain that can occur in the surrounding skin. The tip of a single spicule applied to the forearm of 45 subjects typically evoked 1) itch accompanied by nociceptive sensations (NS) of pricking/stinging and, to a lesser extent, burning, and 2) one or more areas of cutaneous dysesthesia characterized by hyperknesis (enhanced itch to pricking) with or without alloknesis (itch to stroking) and/or hyperalgesia (enhanced pricking pain). Itch could occur in the absence of NS or one or more dysesthesias but very rarely the reverse. The peak magnitude of sensation was positively correlated for itch and NS and increased (exhibited spatial summation) as the number of spicules was increased within a spatial extent of 6 cm but not 1 cm. The areas of dysesthesia did not exhibit spatial summation. We conclude that itch evoked by a punctate chemical stimulus can co-exist with NS and cutaneous dysesthesias as may occur in clinical pruritus. However, cowhage itch was not always accompanied by NS or dysesthesia nor was a momentary change in itch necessarily accompanied by a similar change in NS or vice versa. Thus there may be separate neural coding mechanisms for itch, nociceptive sensations, and each type of dysesthesia.

Modulation of meningeal nociceptors mechanosensitivity by peripheral proteinase-activated receptor-2: the role of mast cells

Inflammatory-related activation and sensitization of meningeal nociceptors is believed to play a key role in promoting the intracranial throbbing pain of migraine. We have shown recently that mast cell activation and various mast cell-derived inflammatory mediators can promote activation and sensitization of meningeal nociceptors. Mast cell tryptase has also been proposed to promote pain hypersensitivity by activating the proteinase-activated receptor 2 (PAR2) that is expressed on nociceptive neurons. In this study using in vivo single-unit recording in the trigeminal ganglion of anaesthetized rats, we found that local meningeal activation of PAR2 using the specific agonist SLIGRL-NH2 promoted sensitization of the threshold response while provoking desensitization of the suprathreshold responses. SLIGRL-NH2 also excited a subpopulation of meningeal nociceptors. Chronic mast cell depletion enhanced the sensitizing effects of PAR2 activation while curbing its desensitizing effects. Mast cell depletion did not change the PAR2-mediated excitatory effect. We propose that by enhancing the mechanical sensitivity of meningeal nociceptors local PAR2 activation could play a role in promoting the throbbing pain of migraine and that local mast cell degranulation may modulate such an effect.

Peripheral and central components of habituation of heat pain perception and evoked potentials in humans

For the neurophysiological examination of nociceptive pathways, contact-heat evoked potentials (contact-heat EPs) are elicited by repetitive brief noxious heat stimuli. Suppression of heat responses in primary nociceptive neurons during repetitive stimulation has been shown in animal models in vivo and in vitro. We now investigated whether heat pain and contact-heat EPs in humans display equivalent signs of habituation. Heat pain and EPs were elicited in 16 volunteers with a contact thermode (30 degrees Cs(-1)). Heat pulses at three intensities (pain threshold, moderate noxious and maximum available) were applied to the right forearm either by moving the thermode after each pulse to variable locations or when fixed to one location (inter-stimulus intervals 8-10s). Contact-heat EPs consisted of an early negativity in temporal leads (N1), followed by a biphasic response at the vertex (N2-P2). Pain ratings and contact-heat EPs (N1 and N2-P2 components) displayed significant temperature dependence. N2-P2 correlated positively with ratings. With stimulation at variable locations, both measures slowly decreased with time constants tau of 2 min (ratings) and 12 min (EPs). With stimulation at a fixed location, habituation was much faster for both, ratings (tau=10s) and EPs (tau=33 s). As a consequence, both measures were significantly reduced (p<0.005) leading to a rightward shift of the stimulus-response function by 5 degrees C. In conclusion, human heat pain perception and contact-heat EPs display signs of rapid habituation when stimulation is restricted to a fixed location and thus, reflect fatigue of peripheral nociceptive neurons. Habituation within the central nervous system is slower and less pronounced.

Cortical oscillatory changes during warming and heating in humans

Warmth and heat are registered by different types of cutaneous receptors. To disentangle the cortical activation patterns of warming and heating, we analyzed the temporal evolution of the electroencephalographic 10 and 20 Hz oscillations with the time resolution of hundreds of milliseconds. Sixty heat (from 32 to 50.5 degrees C, rate of change 6 degrees C/s) and warm (from 32 to 42 degrees C, 6 degrees C/s) stimuli were applied on the right thenar using contact thermode. EEG was recorded from 111 scalp electrodes in 12 healthy subjects, and analyzed using event-related desynchronization and low-resolution electromagnetic tomography methods. During warming, the amplitudes of 10 and 20 Hz oscillations over the contralateral primary sensorimotor (SI/MI) and premotor cortices decreased, and the amplitude of 20 Hz oscillations in the anterior cingulate and ipsilateral premotor cortex increased. Heating was associated with additional profound amplitude decreases of 10 and 20 Hz oscillations over SI/MI and premotor cortex, and by amplitude increase of 20 Hz oscillations originating in the posterior cingulate cortex. Results suggest biphasic amplitude changes of the cortical oscillations during ramp increase of temperature attributable to the periods of warming and heating. The amplitude decreases of 10 and 20 Hz oscillations in SI/MI and premotor cortex possibly aid in preparation of motor withdrawal reaction in an event that temperature should reach intolerable pain. Synchronization of the 20 Hz oscillations in the anterior and especially in the posterior cingulate cortex may aid suppression of unwanted movements.

Modulation of Dural Nociceptor Mechanosensitivity by the Nitric Oxide-Cyclic GMP Signaling Cascade

The role of the nitric oxide (NO)-cGMP signaling cascade in modulation of peripheral nociception is controversial. Although behavioral studies have suggested both pro- and anti-nociceptive effects, little is known about the direct action of this signaling cascade on primary afferent nociceptive neurons that mediate these behaviors. Here, using single-unit recordings, we examined the direct effect of NO-cGMP signaling on spontaneous activity and mechanical responses of nociceptive afferents that innervate the dura mater. We found that the NO donor sodium nitroprusside (SNP), when applied topically to the neuronal receptive field, induced both sensitization and inhibition of the mechanical responses, albeit in different populations of neurons, which could be distinguished based on their baseline mechanical thresholds. SNP, however, did not change the level of spontaneous activity. Administration of the cGMP analogue 8-pCPT-cGMP mimicked only the inhibitory effect. When SNP was co-applied with either an inhibitor of guanylyl cyclase or a cGMP blocker, sensitization never occurred, and the inhibitory effect of SNP could also be blocked. Our findings suggest that NO can either increase or decrease the mechanical responsiveness of nociceptors and that its action might depend, in part, on the baseline level of neuronal excitability. Our results also implicate cGMP in mediating the inhibitory effect of NO.

Role of tetrodotoxin-resistant Na+ current slow inactivation in adaptation of action potential firing in small-diameter dorsal root ganglion neurons

When acutely dissociated small-diameter dorsal root ganglion (DRG) neurons were stimulated with repeated current injections or prolonged application of capsaicin, their action potential firing quickly adapted. Because TTX-resistant (TTX-R) sodium current in these presumptive nociceptors generates a large fraction of depolarizing current during the action potential, we examined the possible role of inactivation of TTX-R sodium channels in producing adaptation. Under voltage clamp, TTX-R current elicited by short depolarizations showed strong use dependence at frequencies as low as 1 Hz, although recovery from fast inactivation was complete in approximately 10-30 msec. This use-dependent reduction was the result of the entry of TTX-R sodium channels into slow inactivated states. Slow inactivation was more effectively produced by steady depolarization than by cycling channels through open states. Slow inactivation was steeply voltage dependent, with a Boltzmann slope factor of 5 mV, a midpoint near -45 mV (5 sec conditioning pulses), and completeness of approximately 93% positive to -20 mV. The time constant for entry (approximately 200 msec) was independent of voltage from -20 mV to +60 mV, whereas recovery kinetics were moderately voltage dependent (time constant, approximately 1.5 sec at -60 mV and approximately 0.5 sec at -100 mV). Using a prerecorded current-clamp response to capsaicin as a voltage-clamp command waveform, we found that adaptation of firing occurred with a time course similar to that of development of slow inactivation. Thus, slow inactivation of the TTX-R sodium current limits the duration of small DRG cell firing in response to maintained stimuli and may contribute to cross desensitization between chemical and electrical stimuli.