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.

Effects of Transcranial Direct Current Stimulation (t-DCS) of the Cerebellum on Pain Perception and Endogenous Pain Modulation: a Randomized, Monocentric, Double-Blind, Sham-Controlled Crossover Study

Accumulating evidence demonstrates a role of the cerebellum in nociception. Some studies suggest that this is mediated via endogenous pain modulation. Here, we used t-DCS to test the effects of modulation of cerebellar function on nociception and endogenous pain modulation. Anodal, cathodal, and sham cerebellar t-DCS were investigated in a cross-over design in 21 healthy subjects. The nociceptive flexor (RIII) reflex, conditioning pain modulation (CPM), and offset analgesia (OA) paradigms were used to assess endogenous pain modulation. Somatosensory evoked potentials (SEPs) and pain ratings were used to assess supraspinal nociception and pain perception, respectively. No significant t-DCS effects were detected when including all t-DCS types and time points (baseline, 0, 30, 60 min post t-DCS) in the analysis. Exploratory analysis revealed an increased RIII reflex size immediately after cathodal t-DCS (compared to sham, P = 0.046, η2p = 0.184), in parallel with a trend for a decrease in electrical pain thresholds (P = 0.094, η2p = 0.134), and increased N120 SEP amplitudes 30 min after cathodal compared to anodal t-DCS (P = 0.007, η2p = 0.374). OA was increased after anodal compared to sham stimulation (P = 0.023, η2p = 0.232). Exploratory results suggested that cathodal (inhibitory) cerebellar t-DCS increased pain perception and reduced endogenous pain inhibition while anodal (excitatory) t-DCS increased endogenous pain inhibition. Results are principally compatible with activation of endogenous pain inhibition by cerebellar excitation. However, maybe due to limited t-DCS skull penetration, effects were small and unlikely to be clinically significant.

Effects of stimulation area and temperature rates on offset analgesia

INTRODUCTION

Offset analgesia describes the effect of a slightly reduced nociceptive stimulus, resulting in a disproportionate large reduction in the pain perception. This effect may be associated with descending pain inhibition, but parameters influencing this phenomenon are poorly understood.

OBJECTIVES

In this study, 2 separate experiments were conducted to investigate both, the spatial aspects of offset analgesia and the influence of different rates of temperature rise.

METHODS

In both experiments, 29 healthy participants received individualized and heat-based offset analgesia paradigms applied to the forearm, with continuous assessment of pain intensity. In experiment 1, offset analgesia paradigms with 3 different rates of temperature rise were applied, whereas in experiment 2, offset analgesia paradigms with 2 different heat application areas were used.

RESULTS

The results of experiment 1 showed that different temperature rates had no effect on the offset analgesia response (P > 0.05). Experiment 2, however, showed the influence of the size of a stimulated area on offset analgesia (P = 0.009), which can be explained mainly by the influence of spatial summation of pain and habituation processes.

CONCLUSIONS

The study showed a lack of influence of different temperature rates on offset analgesia; however, spatial aspects of offset analgesia could be identified. These are most likely based on spatial summation of pain and altered adaptation to pain.

Offset analgesia is increased intra-orally

BACKGROUND

Offset analgesia (OA) is commonly used to quantify endogenous pain inhibition. However, the potential role of afferent inputs and the subsequent peripheral factors from different body areas on the underlying mechanisms are still unclear.

OBJECTIVES

The aim of this cross-sectional study was to compare the magnitude of OA in four different body areas representing a) glabrous and non-glabrous skin, b) trigeminal and extra-trigeminal areas, and c) intra- and extra-oral tissue.

METHODS

OA was assessed at the oral mucosa of the lower lip, at the skin of the cheek, the forearm and the palm of the hand in 32 healthy and pain-free participants. OA testing included two trials: (1) a constant trial (30 seconds of constant heat stimulation at an individualized temperature of Pain₅₀ (pain intensity of 50 out of 100)), and (2) an offset trial (10 seconds of individualized Pain₅₀ , followed by 5 seconds at Pain₅₀ +1°C and 15 seconds at Pain₅₀ ). Participants continuously rated their pain during each trial with a computerized visual analog scale.

RESULTS

A significant OA response was recorded at the oral mucosa (p<0.001, d=1.24), the cheek (p<0.001, d=0.84) and the forearm (p<0.001, d=1.04), but not at the palm (p=0.19, d=0.24). Significant differences were shown for OA recorded at the cheek versus the mucosa (p=0.02), and between palm and mucosa (p=0.007), but not between the remaining areas (p>0.05).

CONCLUSION

This study suggests that intra-oral endogenous pain inhibition assessed with OA is enhanced and supports the role of peripheral mechanisms contributing to the OA response.

Temporal properties of pain contrast enhancement using repetitive stimulation

BACKGROUND

Offset analgesia (OA) is characterized by a disproportionately large reduction in pain following a small decrease in noxious stimulation and is based on temporal pain contrast enhancement (TPCE). The underlying mechanisms of this phenomenon are still poorly understood. This study is aiming to investigate whether TPCE can also be induced by repetitive stimulation, i.e., by stimuli clearly separated in time.

METHODS

A repetitive TPCE paradigm was induced in healthy, pain-free subjects (n = 33) using heat stimuli. Three different interstimulus intervals (ISIs) were used: 5, 15, and 25 seconds. All paradigms were contrasted with a control paradigm without temperature change. Participants continuously rated perceived pain intensity. In addition, electrodermal activity (EDA) was recorded as a surrogate measure of autonomic arousal.

RESULTS

Temporal pain contrast enhancement was confirmed for ISI 5 seconds (p < 0.001) and ISI 15 seconds (p = 0.005) but not for ISI 25 seconds (p = 0.07), however, the magnitude of TPCE did not differ between ISIs (p = 0.11). A TPCE-like effect was also detected with increased EDA values.

CONCLUSIONS

TPCE can be induced by repetitive stimulation. This finding may be explained by a combination of the mechanisms underlying the OA and a facilitated pain habituation.

Functional connectivity modulations during offset analgesia in chronic pain patients: an fMRI study

Patients with neuropathic pain and fibromyalgia showed reduced or absent offset analgesia (OA) response and attenuated cerebral activity in descending pain modulatory and reward systems in patients. However, neural network modifications of OA in chronic pain have not been determined. We enrolled 23 patients with various chronic pain and 17 age- and gender- matched healthy controls. All participants were given OA-related noxious thermal stimuli, including 3 repeats of offset analgesia paradigm at 46-47-46 °C and constant paradigm at 46 °C on the left volar forearm under whole-brain functional magnitude resonance imaging (fMRI). We evaluated magnitude of OA, examined OA modulated functional connectivity using psychophysiological interaction analysis and resting-state functional connectivity analysis and explored their behavioral correlations in patients compared with controls.Compared to controls, chronic pain patients showed smaller magnitude of OA (P = 0.047). OA modulated connectivity decreased between posterior cingulate cortex (PCC) and right medial prefrontal cortex (MPFC) in proportion to current chronic pain (P = 0.018); decreased between right pallidum and right thalamus, and increased between right caudate nucleus and left primary somatosensory cortex (P _FDR < 0.05).The impaired PCC-MPFC connectivity might play an important role in dysfunction of OA and contribute to pain chronification.

Offset analgesia is associated with opposing modulation of medial versus dorsolateral prefrontal cortex activations: A functional near-infrared spectroscopy study

Offset analgesia is defined by a dramatic drop in perceived pain intensity with a relatively small decrease in noxious input. Although functional magnetic resonance imaging studies implicate subcortical descending inhibitory circuits during offset analgesia, the role of cortical areas remains unclear. The current study identifies cortical correlates of offset analgesia using functional near infrared spectroscopy (fNIRS). Twenty-four healthy volunteers underwent fNIRS scanning during offset (OS) and control (Con) heat stimuli applied to the forearm. After controlling for non-neural hemodynamic responses in superficial tissues, widespread increases in cortical oxygenated hemoglobin concentration were observed, reflecting cortical activation during heat pain. OS-Con contrasts revealed deactivations in bilateral medial prefrontal cortex (mPFC) and bilateral somatosensory cortex (SSC) associated with offset analgesia. Right dorsolateral prefrontal cortex (dlPFC) showed activation only during OS. These data demonstrate opposing cortical activation patterns during offset analgesia and support a model in which right dlPFC underlies ongoing evaluation of pain intensity change. With predictions of decreasing pain intensity, right dlPFC activation likely inhibits ascending noxious input via subcortical pathways resulting in SSC and mPFC deactivation. This study identifies cortical circuitry underlying offset analgesia and introduces the use of fNIRS to study pain modulation in an outpatient clinical environment.

Offset analgesia and onset hyperalgesia with different stimulus ranges

A comparison between the effects of offset analgesia and onset hyperalgesia and how these effects relate to the stimulus range of thermal stimulation.

Introduction:

Offset analgesia (OA), a large reduction in pain after a brief increase in intensity of an otherwise stable painful stimulus, has been established by a large body of research. But the opposite effect, onset hyperalgesia (OH), a disproportional hyperalgesic response after a briefly decreased intensity of a painful stimulus, has only been investigated in one previous study.

Objectives:

The aim of this study was to induce OA and OH in healthy participants and explore the effects of different stimulus ranges (increase/decrease of temperature) on OA and OH.

Methods:

A total of 62 participants were tested in 2 identical experiments. Offset analgesia and OH conditions included 2 different temperature deviations (±1°C/±2°C) from initial temperature and were compared with a constant temperature (control).

Results:

Offset analgesia was successfully elicited in OA_1°C in experiment 1, and in OA_1°C and OA_2°C in experiment 2. Results indicate a continuous stimulus–response relationship between the stimulus range and the resulting hypoalgesic response. Onset hyperalgesia was only elicited in OH_2°C in experiment 1. Exploratory analysis showed that the lack of OH response in experiment 2 could be explained by sex differences, and that OA and OH responses were only weakly correlated.

Conclusions:

The asymmetry between pain responses after a brief temperature increase and decrease suggests that different mechanisms are involved in the pain responses to increasing and decreasing temperature. This asymmetry may also be explained by high temperatures in OA condition (+1°C/+2°C above baseline) that could be seen as salient “learning signals,” which augment the response to following changes in temperature.

Tapentadol treatment results in long-term pain relief in patients with chronic low back pain and associates with reduced segmental sensitization

The endogenous pain system may be used as a biomarker in the pharmacological treatment of patients with CLBP, enabling an individualized, mechanism-based treatment approach.

Introduction:

Chronic low back pain (CLBP) is one of the most common chronic pain conditions in pain practice.

Objectives:

In the current study, we describe phenotypes of patients with CLBP based on the status of their endogenous pain modulatory system.

Methods:

Conditioned pain modulation (a measure of central pain inhibition), temporal summation (TS, a measure of pain facilitation), and offset analgesia (a measure of temporal filtering of nociception) were evaluated in 53 patients with CLBP at painful and nonpainful sites. Next, in a double-blind, randomized, placebo-controlled trial, 40 patients with defective conditioned pain modulation responses received treatment with tapentadol prolonged-release or placebo for 3 months.

Results:

The majority of patients (87%) demonstrated loss of central pain inhibition combined with segmentally increased TS and reduced offset analgesia at the lower back region. During treatment, tapentadol reduced pain intensity more than placebo (tapentadol −19.5 ± 2.1 mm versus placebo −7.1 ± 1.8 mm, P = 0.025). Furthermore, tapentadol significantly decreased pain facilitation by reduction of TS responses at the lower back (tapentadol −0.94 ± 1.9 versus placebo 0.01 ± 1.5, P = 0.020), which correlated with pain reduction (P < 0.001).

Conclusion:

Patients with CLBP demonstrated different phenotypes of endogenous pain modulation. In patients with reduced conditioned pain modulation, tapentadol produced long-term pain relief that coincided with reduction of signs of pain facilitation. These data indicate that the endogenous pain system may be used as a biomarker in the pharmacological treatment of CLBP, enabling an individualized, mechanism-based treatment approach.

Pain inhibition is not affected by exercise-induced pain

Introduction:

Offset analgesia (OA) and conditioned pain modulation (CPM) are frequently used paradigms to assess the descending pain modulation system. Recently, it was shown that both paradigms are reduced in chronic pain, but the influence of acute pain has not yet been adequately examined.

Objectives:

The aim of this study is to investigate OA and CPM after exercise-induced pain to evaluate whether these tests can be influenced by delayed-onset muscle soreness (DOMS) at a local or remote body site.

Methods:

Forty-two healthy adults were invited to 3 separate examination days: a baseline appointment, the consecutive day, and 7 days later. Participants were randomly divided into a rest (n = 21) and an exercise group (n = 21). The latter performed a single intensive exercise for the lower back. Before, immediately after, and on the following examination days, OA and CPM were measured at the forearm and the lower back by blinded assessor.

Results:

The exercise provoked a moderate pain perception and a mild delayed-onset muscle soreness on the following day. Repeated-measurements analysis of variance showed no statistically significant main effect for either OA or CPM at the forearm or lower back (P > 0.05).

Conclusion:

Delayed-onset muscle soreness was shown to have no effect on the inhibitory pain modulation system neither locally (at the painful body part), nor remotely. Thus, OA and CPM are robust test paradigms that probably require more intense, different, or prolonged pain to be modulated.

Neural Mechanisms of Offset Analgesia

One feels a disproportionately large decrease of pain sensation on a slight decrease of thermal pain stimulus. Such phenomenon is termed offset analgesia and considered mediated by endogenous analgesic mechanisms. Offset analgesia was found attenuated in patients with neuropathic pain. We further found that such attenuation occurred in a more heterogeneous population of patients with chronic pain. By functional magnetic resonance imaging, we also found negative blood oxygenation level-dependent signals at those areas concerned with descending pain modulatory and reward systems during offset analgesia in the same cohort of patients. We propose that dysfunction of those systems, as revealed by attenuation of offset analgesia, might well be part of neural mechanisms of pain chronification.

Distinct brain mechanisms support spatial vs temporal filtering of nociceptive information

The role of endogenous analgesic mechanisms has largely been viewed in the context of gain modulation during nociceptive processing. However, these analgesic mechanisms may play critical roles in the extraction and subsequent utilization of information related to spatial and temporal features of nociceptive input. To date, it remains unknown if spatial and temporal filtering of nociceptive information is supported by similar analgesic mechanisms. To address this question, human volunteers were recruited to assess brain activation with functional magnetic resonance imaging during conditioned pain modulation (CPM) and offset analgesia (OA). CPM provides one paradigm for assessing spatial filtering of nociceptive information while OA provides a paradigm for assessing temporal filtering of nociceptive information. CPM and OA both produced statistically significant reductions in pain intensity. However, the magnitude of pain reduction elicited by CPM was not correlated with that elicited by OA across different individuals. Different patterns of brain activation were consistent with the psychophysical findings. CPM elicited widespread reductions in regions engaged in nociceptive processing such as the thalamus, insula, and secondary somatosensory cortex. OA produced reduced activity in the primary somatosensory cortex but was associated with greater activation in the anterior insula, dorsolateral prefrontal cortex, intraparietal sulcus, and inferior parietal lobule relative to CPM. In the brain stem, CPM consistently produced reductions in activity, while OA produced increases in activity. Conjunction analysis confirmed that CPM-related activity did not overlap with that of OA. Thus, dissociable mechanisms support inhibitory processes engaged during spatial vs temporal filtering of nociceptive information.

Altered experimental pain perception after cerebellar infarction

Animal studies have suggested that the cerebellum, in addition to its motor functions, also has a role in pain processing and modulation, possibly because of its extensive connections with the prefrontal cortex and with brainstem regions involved in descending pain control. Consistently, human imaging studies have shown cerebellar activation in response to painful stimulation. However, it is presently not clear whether cerebellar lesions affect pain perception in humans. In the present study, we used experimental pain testing to compare acute pain perception and endogenous pain inhibition in 30 patients 1 to 11 years after cerebellar infarction and in 30 sex- and age-matched healthy control subjects. Compared to controls, patients exhibited a significantly increased pain perception in response to acute heat stimuli (44 °C-48 °C, average pain intensity rating for patients 3.4±2.8 and for controls 1.5±1.7 [on a numeric rating scale of 0-10], P<.01) and to repeated 256 mN pinprick stimuli (1.3±1.9 vs. 0.6±1.0 [0-10], P<.05). Heat hyperalgesia in patients was more pronounced on the body side ipsilateral to the infarction. In addition, patients showed reduced offset analgesia (change in pain intensity rating: 0.0%±15.8% vs. -16.9%±36.3%, P<.05) and reduced placebo analgesia (change in pain intensity rating: -1.0±1.1 vs. -1.8±1.3 [0-10], P<.05) compared to controls. In contrast, heat and pressure pain thresholds were not significantly different between groups. These results show that, after cerebellar infarction, patients perceive heat and repeated mechanical stimuli as more painful than do healthy control subjects and have deficient activation of endogenous pain inhibitory mechanisms (offset and placebo analgesia). This suggests that the cerebellum has a previously underestimated role in human pain perception and modulation.

Offset analgesia is reduced in older adults

Recent studies indicate that aging is associated with dysfunctional changes in pain modulatory capacity, potentially contributing to increased incidence of pain in older adults. However, age-related changes in offset analgesia (offset), a form of temporal pain inhibition, remain poorly characterized. The purpose of this study was to investigate age differences in offset analgesia of heat pain in healthy younger and older adults. To explore the peripheral mechanisms underlying offset, an additional aim of the study was to test offset at 2 anatomical sites with known differences in nociceptor innervation. A total of 25 younger adults and 20 older adults completed 6 offset trials in which the experimental heat stimulus was presented to the volar forearm and glabrous skin of the palm. Each trial consisted of 3 continuous phases: an initial 15-second painful stimulus (T1), a slight increase in temperature from T1 for 5 seconds (T2), and a slight decrease back to the initial testing temperature for 10 seconds (T3). During each trial, subjects rated pain intensity continuously using an electronic visual analogue scale (0-100). Older adults demonstrated reduced offset compared to younger adults when tested on the volar forearm. Interestingly, offset analgesia was nonexistent on the palm for all subjects. The reduced offset found in older adults may reflect an age-related decline in endogenous inhibitory systems. However, although the exact mechanisms underlying offset remain unknown, the absence of offset at the palm suggests that peripheral mechanisms may be involved in initiating this phenomenon.