Homotopic and heterotopic effects of endogenous analgesia in healthy volunteers

'Pain inhibits pain' mechanisms: Is pain modulation simply due to distraction?

'Diffuse noxious inhibitory controls' (DNIC), a form of supraspinal descending endogenous analgesia, requires a noxious conditioning stimulus for pain attenuation. This may be partly dependent on a distraction effect. The term "conditioned pain modulation" (CPM) has recently been introduced to describe the psychophysical paradigm to test DNIC. The present study aimed to determine whether distraction and tonic heat stimulation inhibit pain through the same or different mechanisms by looking at whether there is a similar or even an additive effect on pain attenuation. Test pain was brief heat stimulation applied to the left volar of 34 healthy volunteers. For conditioning, the right hand was immersed in 46.5 degrees C water. Distraction was provided by three different difficulty levels of continuous cognitive visual tasks. Experimental blocks consisted of test pain: (1) alone; 'baseline', (2) with conditioning pain; 'CPM', (3) with distraction; 'distraction' and (4) with conditioning pain and distraction; 'combined'. They were randomized and repeated three times and pain intensity and unpleasantness rated. Results showed an overall effect of experimental block on test pain intensity (P=0.0125). Post-hoc tests revealed a significant reduction in pain intensity ratings under Combined (21.2+/-2.3; mean+/-SEM) compared to CPM alone (16.0+/-2.3) (P<0.05). Furthermore, at all levels of distraction there were always a few subjects who were not distracted despite expressing CPM. Based on the additive effect of CPM and distraction on pain inhibition, and the cases of no distraction despite CPM, we suggest that CPM acts independently from distraction.

Heterotopic pruritic conditioning and itch--analogous to DNIC in pain?

Pain can be endogenously modulated by heterotopic noxious conditioning stimulation (HNCS) through a mechanism which is known as diffuse noxious inhibitory control (DNIC). Since DNIC can be impaired in patients suffering from chronic pain, a comparable impaired itch inhibition may exist in patients suffering from chronic itch. The aim of the present study was to investigate whether heterotopic pruritic conditioning stimulation (HPCS) would display an impaired modulation of itch in patients suffering from chronic itch compared with healthy subjects. To this end, electrical stimuli were applied before and after histamine application (HPCS) to female patients with psoriasis and healthy female control subjects. Subjects reported the intensity of electrically evoked itch before and after HPCS. In order to replicate earlier findings for DNIC, electrically evoked pain was additionally investigated before and after cold stimulation (HNCS). As expected, the intensity of itch evoked by the electrical stimulus was significantly less after than before HPCS in healthy subjects, and the same was found for the intensity of electrically evoked pain after compared to before HNCS. Contrarily, in the patients levels of electrically evoked itch were significantly higher after than before HPCS, and no significant difference in pain intensity before and after HNCS was observed. In line with pain modulation, results suggest that there is a DNIC analogous mechanism for itch, i.e., diffuse pruritic inhibitory control (DPIC), which is impaired in patients with chronic itch, possibly due to a dysregulation of descending itch modulatory systems.

Sex, gender, and pain: a review of recent clinical and experimental findings

Sex-related influences on pain and analgesia have become a topic of tremendous scientific and clinical interest, especially in the last 10 to 15 years. Members of our research group published reviews of this literature more than a decade ago, and the intervening time period has witnessed robust growth in research regarding sex, gender, and pain. Therefore, it seems timely to revisit this literature. Abundant evidence from recent epidemiologic studies clearly demonstrates that women are at substantially greater risk for many clinical pain conditions, and there is some suggestion that postoperative and procedural pain may be more severe among women than men. Consistent with our previous reviews, current human findings regarding sex differences in experimental pain indicate greater pain sensitivity among females compared with males for most pain modalities, including more recently implemented clinically relevant pain models such as temporal summation of pain and intramuscular injection of algesic substances. The evidence regarding sex differences in laboratory measures of endogenous pain modulation is mixed, as are findings from studies using functional brain imaging to ascertain sex differences in pain-related cerebral activation. Also inconsistent are findings regarding sex differences in responses to pharmacologic and non-pharmacologic pain treatments. The article concludes with a discussion of potential biopsychosocial mechanisms that may underlie sex differences in pain, and considerations for future research are discussed.

PERSPECTIVE

This article reviews the recent literature regarding sex, gender, and pain. The growing body of evidence that has accumulated in the past 10 to 15 years continues to indicate substantial sex differences in clinical and experimental pain responses, and some evidence suggests that pain treatment responses may differ for women versus men.

Cold-induced limb pain decreases sensitivity to pressure-pain sensations in the ipsilateral forehead

The aim of this study was to investigate the effect of unilateral limb pain on sensitivity to pain on each side of the forehead. In the first experiment, pressure-pain thresholds and sharpness sensations were assessed on each side of the forehead in 45 healthy volunteers before and after a 10 degrees C cold pressor of the hand and in 18 controls who were not subjected to the cold pressor. In a second experiment, forehead sensitivity was assessed in 32 healthy volunteers before and after a 2 degrees C cold pressor. The assessments were repeated without the cold pressor, and before and after six successive 4 degrees C cold pressor tests. The 10 degrees C cold pressor did not influence forehead sensitivity, whereas the 2 degrees C cold pressor and the 4 degrees C cold pressor tests resulted in bilateral analgesia to sharpness and pressure. The analgesia to pressure was greater in the ipsilateral forehead. Stress-induced analgesia and diffuse noxious inhibitory controls may have contributed to the analgesia to pressure-pain and sharpness sensations bilaterally after the most painful cold pressor tests. The locus coeruleus inhibits ipsilateral nociceptive activity in dorsal horn neurons during limb inflammation, and thus may have mediated the ipsilateral component of analgesia. Pain-evoked changes in forehead sensitivity differed for sharpness and pressure, possibly due to separate thalamic or cortical representations of cutaneous and deep tissue sensibility. These findings suggest that several mechanisms act concurrently to influence pain sensitivity at sites distant from a primary site of painful stimulation.

Experimental muscle pain impairs descending inhibition

In chronic musculoskeletal pain conditions, the balance between supraspinal facilitation and inhibition of pain shifts towards an overall decrease in inhibition. Application of a tonic painful stimulus results in activation of diffuse noxious inhibitory controls (DNIC). The aims of the present experimental human study were (1) to compare DNIC, evoked separately, by hypertonic saline (6%)-induced muscle pain (tibialis anterior) or cold pressor pain; (2) to investigate DNIC evoked by concomitant experimental muscle pain and cold pressor pain, and (3) to analyze for gender differences. Ten males and 10 age matched females participated in two sessions. In the first session unilateral muscle pain or unilateral cold pressor pain were induced separately; in the second session unilateral muscle pain and unilateral cold pressor pain were induced concomitantly. Pressure pain thresholds (PPT) were measured around the knee joint before, during, and after DNIC induction. Cold pressor pain increased PPT in both males and females with greater increases in males. Hypertonic saline-evoked muscle pain significantly increased PPT in males but not in females. When cold pressor and muscle pain were applied concomitantly the PPT increases were smaller when compared to the individual sessions. This study showed for the first time that two concurrent conditioning tonic pain stimuli (muscle pain and cold pressor pain) cause less DNIC compared with either of the conditioning stimuli given alone; and males showed greater DNIC than females. This may explain why patients with chronic musculoskeletal pain have impaired DNIC.

Acidic buffer induced muscle pain evokes referred pain and mechanical hyperalgesia in humans

While tissue acidosis causes local deep-tissue pain, its effect on referred pain and mechanical muscle hyperalgesia is unknown. The aim of this study was to investigate a human experimental acidic muscle pain model using a randomized, controlled, single-blinded study design. Seventy-two subjects (36 female) participated in three visits, each involving one 15 min intramuscular infusion into the anterior tibialis muscle: acidic phosphate buffer (pH 5.2) at 40 ml/h (N=69) or 20 ml/h (N=54), normal phosphate buffer (pH 7.3) at 40 ml/h (N=70), or isotonic saline at 40 ml/h (N=19). Pain ratings and pressure sensitivity of superficial and deep tissues were assessed before, during, and 20 min after infusion. Acidic buffer produced light to moderate, rate-dependent, muscle pain (not sex-dependent) compared to the control infusions, that referred pain to the ankle in 80% of women and 40% of men. Pain did not vary across self-reported menstrual phases. Pressure pain thresholds (PPTs) were reduced over the infused muscle with acidic infusion, defined as primary mechanical hyperalgesia. PPTs decreased at the ankle in those with referred pain in response to acidic buffer, i.e. referred mechanical hyperalgesia, but not at the foot. No pain or changes in PPTs occurred in the contralateral leg. These results demonstrate muscle acidosis can lead to local and referred pain and hyperalgesia, with significant sex differences in development of referred pain.

Diffuse noxious inhibitory controls triggered by heterotopic CO2 laser conditioning stimulation decreased the SEP amplitudes induced by electrical tooth stimulation with different intensity at an equally inhibitory rate

The purpose of this study was to investigate (1) whether selective Adelta-fiber stimulation with CO(2) laser produces a diffuse noxious inhibitory controls (DNIC) effect in the trigeminal nerve territory; and (2) whether the DNIC effect differs depending on test stimulus intensities under constant conditioning stimuli. To examine whether the CO(2) laser radiation on the dorsum of the hand selectively stimulates Adelta-fibers, laser evoked potentials (LEP) were recorded. The mean peak latency of LEP was 381.4 ms. The findings revealed that the CO(2) laser selectively stimulated Adelta-fibers. Electrical tooth stimuli with 3 levels of intensities (1.2, 1.4, 1.6 times the pain threshold) were applied to subjects as test stimulation in randomized order, with a CO(2) laser stimulus of 18 mJ/mm(2) applied to the dorsum of the hand for 4 min as the noxious conditioning stimulus. Somatosensory evoked potentials (SEP) induced by electrical tooth stimulation were recorded and tooth pain intensity was evaluated using a visual analogue scale (VAS). The amplitudes of the SEP late component and VAS values were significantly decreased only during the conditioning stimuli without aftereffect. The inhibitory rates of the amplitudes ranged from 31.3% to 34.6% and the VAS values from 29.0% to 31.2%. There were no significant differences in their inhibitory rates between the 3 test stimulus intensities. The result indicated that selective Adelta-fiber stimulation with the CO(2) laser produces a DNIC effect in the trigeminal nerve territory and suggested that the DNIC effect does not depend on the intensity of the test stimuli.

Determinants of endogenous analgesia magnitude in a diffuse noxious inhibitory control (DNIC) paradigm: Do conditioning stimulus painfulness, gender and personality variables matter?

Descending modulation of pain can be demonstrated psychophysically by dual pain stimulation. This study evaluates in 31 healthy subjects the association between parameters of the conditioning stimulus, gender and personality, and the endogenous analgesia (EA) extent assessed by diffuse noxious inhibitory control (DNIC) paradigm. Contact heat pain was applied as the test stimulus to the non-dominant forearm, with stimulation temperature at a psychophysical intensity score of 60 on a 0-100 numerical pain scale. The conditioning stimulus was a 60s immersion of the dominant hand in cold (12, 15, 18 degrees C), hot (44 and 46.5 degrees C), or skin temperature (33 degrees C) water. The test stimulus was repeated on the non-dominant hand during the last 30s of the conditioning immersion. EA extent was calculated as the difference between pain scores of the two test stimuli. State and trait anxiety and pain catastrophizing scores were assessed prior to stimulation. EA was induced only for the pain-generating conditioning stimuli at 46.5 degrees C (p=0.011) and 12 degrees C (p=0.003). EA was independent of conditioning pain modality, or personality, but a significant gender effect was found, with greater EA response in males. Importantly, pain scores of the conditioning stimuli were not correlated with EA extent. The latter is based on both our study population, and on additional 82 patients, who participated in another study, in which EA was induced by immersion at 46.5 degrees C. DNIC testing, thus, seems to be relatively independent of the stimulation conditions, making it an easy to apply tool, suitable for wide range applications in pain psychophysics.

Pain additivity, diffuse noxious inhibitory controls, and attention: a functional measurement analysis

This study utilized the methodology of Functional Measurement theory to investigate the additivity of painful and non-painful thermally induced experiences at one body site with those produced by brief noxious and innocuous electrical stimuli at another. Forty healthy young subjects were tested, using a Peltier thermode to induce tonic pain and an electrocutaneous stimulator for presenting phasic pain, under conditions of either full attention or visual/cognitive distraction (counting numerous light signals) in order to evaluate whether the summed effects are attributable to refocused attention. Six levels of intensity were combined in a factorial design for both tonic and phasic pain. Subjects indicated the overall strength of their dual perception on a visual analog scale. Stimuli showed complex patterns of interaction. Two stimuli were generally rated as greater than one, but the summation was far from additive and greatly influenced by the intensity of the stronger stimulus, suggesting inhibitory action. In general, tonic heat pain strongly affected the perception of phasic electrocutaneous pain whereas the reverse was only partly true. Distraction had a very small effect, suggesting that the "pain inhibits pain" phenomenon attributable to diffuse noxious inhibitory controls (DNIC) is not due to attentional processes. Our data also relate to issues regarding spatial summation across dermatomes and to adaptation level effects in pain, in which a strong painful experience serves as an anchor or comparison point by which others are judged. The psychophysical findings provide a perceptual foundation for clinical phenomena in which patients face with comorbid pain disorders.

Sex differences in endogenous pain modulation by distracting and painful conditioning stimulation

Sex differences in endogenous pain modulation were tested in healthy volunteers (32 men, 30 women). Painful contact heat stimuli were delivered to the right leg alone, and then in combination with various electrical conditioning stimuli delivered to the left forearm. Four conditioning protocols were applied to each subject in separate sessions: mild, non-painful (control); distracting; stressful-yet-non-painful; strongly painful. Thermal stimuli were rated on visual analog scales for pain intensity (INT) and unpleasantness (UNP). Distracting and painful conditioning stimuli significantly reduced heat pain INT and UNP ratings for both sexes, with significantly larger distraction effects on INT ratings for men than women (p=0.004). No sex differences in pain-evoked hypoalgesia were detected (p>0.05). The stress protocol did not consistently reduce heat pain ratings, possibly because the protocol was not sufficiently stressful to activate endogenous modulatory systems. Regression analysis revealed that the magnitude of pain-evoked hypoalgesia was predicted by the perceived distraction (p=0.003) and stress (p=0.04) produced by the painful conditioning stimulation, providing evidence that distraction and stress contribute to pain-evoked hypoalgesia. However, the contribution of stress to pain-evoked hypoalgesia differed by sex (p=0.02), with greater perceived stress associated with greater hypoalgesia in men and the opposite trend in women, suggesting sex differences in the mechanisms underlying pain-evoked hypoalgesia. This study provides indirect evidence that multiple neural mechanisms are involved in endogenous pain modulation and suggests that sex-specific aspects of these systems may contribute to greater pain sensitivity and higher prevalence of many chronic pain conditions among women.

Abnormal endogenous pain modulation and somatic and visceral hypersensitivity in female patients with irritable bowel syndrome

AIM: To investigate the role of endogenous pain modulatory mechanisms in the central sensitization implicated by the visceral hypersensitivity demonstrated in patients with irritable bowel syndrome (IBS). Dysfunction of modulatory mechanisms would be expected to also result in changes of somatic sensory function.

METHODS: Endogenous pain modulatory mechanisms were assessed using heterotopic stimulation and somatic and visceral sensory testing in IBS. Pain intensities (visual analogue scale, VAS 0-100) during suprathreshold rectal distension with a barostat, cold pressor stimulation of the foot and during both stimuli simultaneously (heterotopic stimulation) were recorded in 40 female patients with IBS and 20 female healthy controls.

RESULTS: Rectal hypersensitivity (defined by 95% CI of controls) was seen in 21 (53%), somatic hypersensitivity in 22 (55%) and both rectal and somatic hypersensitivity in 14 of these IBS patients. Heterotopic stimulation decreased rectal pain intensity by 6 (-11 to -1) in controls, but increased rectal pain by 2 (-3 to +6) in all IBS patients (P < 0.05) and by 8 (-2 to +19) in IBS patients with somatic and visceral hypersensitivity (P < 0.02).

CONCLUSION: A majority of IBS patients had abnormal endogenous pain modulation and somatic hypersensitivity as evidence of central sensitization.

Effects of cold stimulation on secondary hyperalgesia (HA) induced by capsaicin in healthy volunteers

Little is known about the sensory characteristics and underlying mechanisms behind secondary hyperalgesia (HA) (2 degrees HA). The aim of the present study was to investigate the relationships between two different noxious stimuli, mechanical and cold on capsaicin-induced 2 degrees HA. Fourteen healthy volunteers were exposed to three different cold stimuli (20, 10, 0 degrees C) 30 s each, on both forearms. The cold stimuli were applied before (baseline) and 8 min after intradermal administration of 50-microg capsaicin to the forearm, distally to the injection site in the inspected area of 2 degrees HA. Pain intensities were assessed immediately after each cold stimulus by means of a visual analogue scale (cold-VAS). Additionally, areas of mechanical HA (cm(2)) were assessed distally and proximally to the injection site at three different time points: 5, 8 (right after the second series of cold stimuli), and 30 min after the injection. No significant differences in cold-VAS were found between pre- and post-capsaicin injection at the tested forearm (P= 0.334), whereas significant reduction from pre- to post-injection was found in cold-VAS in the control forearm (P= 0.024). Further, 8 min after the injection, the cold stimulation led to an expansion of 2 degrees HA area (from 5.1+/-1.38 to 11.4+/-1.72 cm(2)) to punctuate stimuli distally but not proximally to the injection site (P<0.05). It is concluded that there is no HA to cold stimuli within the area of mechanical 2 degrees HA. However, cooling acts as a conditioning stimulus and expands the area of capsaicin-induced punctuate HA.