Neural mechanisms underlying placebo and nocebo effects in tonic muscle pain.

Pain is a highly subjective and multidimensional experience, significantly influenced by various psychological factors. Placebo analgesia and nocebo hyperalgesia exemplify this influence, where inert treatments result in pain relief or exacerbation, respectively. While extensive research has elucidated the psychological and neural mechanisms behind these effects, most studies have focused on transient pain stimuli. To explore these mechanisms in the context of tonic pain, we conducted a study using a 15-minute tonic muscle pain induction procedure, where hypertonic saline was infused into the left masseter of healthy participants. We collected real-time Visual Analogue Scale (VAS) scores and functional magnetic resonance imaging (fMRI) data during the induction of placebo analgesia and nocebo hyperalgesia via conditioned learning. Our findings revealed that placebo analgesia was more pronounced and lasted longer than nocebo hyperalgesia. Real-time pain ratings correlated significantly with neural activity in several brain regions. Notably, the putamen was implicated in both effects, while the caudate and other regions were differentially involved in placebo and nocebo effects. These findings confirm that the tonic muscle pain paradigm can be used to investigate the mechanisms of placebo and nocebo effects and indicate that placebo analgesia and nocebo hyperalgesia may have more distinct than common neural bases.

Direct Inhibition of Microglia Activation by Pretreatment With Botulinum Neurotoxin A for the Prevention of Neuropathic Pain.

Peripheral injection of botulinum neurotoxin A (BoNT/A) has been demonstrated to have a long-term analgesic effect in treating neuropathic pain. Around peripheral nerves, BoNT/A is taken up by primary afferent neurons and inhibits neuropeptide release. Moreover, BoNT/A could also be retrogradely transported to the spinal cord. Recent studies have suggested that BoNT/A could attenuates neuropathic pain by inhibiting the activation of spinal glial cells. However, it remains unclear whether BoNT/A directly interacts with these glial cells or via their interaction with neurons. Our aim here is to determine the direct effect of BoNT/A on primary microglia and astrocytes. We show that BoNT/A pretreatment significantly inhibits lipopolysaccharide (LPS) -induced activation and pro-inflammatory cytokine release in primary microglia (1 U/mL BoNT/A in medium), while it has no effect on the activation of astrocytes (2 U/mL BoNT/A in medium). Moreover, a single intrathecal pre-administration of a low dose of BoNT/A (1 U/kg) significantly prohibited the partial sciatic nerve ligation (PSNL)- induced upregulation of pro-inflammatory cytokines in both the spinal cord dorsal horn and dorsal root ganglions (DRGs), which in turn prevented the PSNL-induced mechanical allodynia and thermal hyperalgesia. In conclusion, our results indicate that BoNT/A pretreatment prevents PSNL-induced neuropathic pain by direct inhibition of spinal microglia activation.

Abnormal brain activation during emotion processing of euthymic bipolar patients taking different mood stabilizers.

Numerous functional magnetic resonance imaging studies have been conducted to elucidate emotion processing of patients with bipolar disorder (BD), but due to different inclusion criteria used, especially for the history of medication use, the results for euthymic BD patients are inconsistent. For this reason, brain functional effects of psychopharmacological treatments on BD patients have been investigated by numerous fMRI studies, but there is no existing report for brain functional effects of different mood stabilizers. In this study, we compared the emotion processing in BD patients treated by two popularly used mood stabilizer, lithium (N = 13; 30 ± 9 years) and valproate (N = 16; 33 ± 8 years), as well as healthy controls (HC; N = 16; 29 ± 7 years). Two emotional tasks were applied in this study: one used emotional pictures of everyday objects and scenes, and another used emotional facial expression pictures. The main findings were that BD on lithium showed increased fMRI activation in the right dorsal anterior cingulate cortex and bilateral lingual gyrus in response to the positive pictures relative to neutral pictures compared with BD on valproate and HC. Besides, no abnormal activation was observed in the amygdala. Limitations of this study comprise the small sample size and the cross-sectional design. Therefore, the results were suggestive of a different effect of lithium and valproate on brain activities during emotion processing but no causal role can be proposed. The enduring impairments in euthymic state could provide clues to the brain regions involved in the primary pathology of BD.

Neural Activation During Tonic Pain and Interaction Between Pain and Emotion in Bipolar Disorder: An fMRI Study.

Objective: Pain and affective disorders have clear clinical relevance; however, very few studies have investigated the association between pain and bipolar disorder. This study investigated the brain activity of patients with bipolar disorder (BPs) undergoing tonic pain and assessed the interaction between pain and emotion. Methods: Ten BPs and ten healthy controls (HCs) were exposed to emotional pictures (positive, neutral, or negative), tonic pain only (pain session), and emotional pictures along with tonic pain (combined session). A moderate tonic pain was induced by the infusion of hypertonic saline (5% NaCl) into the right masseter muscle with a computer-controlled system. Whole-brain blood oxygenation level dependent (BOLD) signals were acquired using 3T functional resonance imaging (fMRI). Results: Ten BPs and ten healthy participants were included in the final analysis. During the pain session, BPs accepted more saline, but showed lower pain rating scores than HCs. When experiencing pain, BPs showed a significant decrease in the BOLD signal in the bilateral insula, left inferior frontal gyrus (IFG), and left cerebellum as compared with HCs. In the combined session, the activated regions for positive mood (pain with positive mood > baseline) in BPs were the left cerebellum, right temporal gyrus, and left occipital gyrus; the activated regions for negative mood (pain with negative mood > baseline) were the right occipital gyrus, left insula, left IFG, and bilateral precentral gyrus. Conclusions: This study presents the preliminary finding of the interaction between pain and emotion in BPs. BPs exhibited lower sensitivity to pain, and the activation of insula and IFG may reflect the interaction between emotion and pain stimulus.

Electroacupuncture Stimulation of Language-Implicated Acupoint Tongli (HT 5) in Healthy Subjects: An fMRI Evaluation Study.

OBJECTIVE: To explore brain activations associated with electroacupuncture simulation at Tongli (HT 5) and its comparison with brain activations during picture-naming task. METHODS: Twenty healthy subjects were enrolled in this study. Half of them received electroacupuncture stimulation at HT 5 (ACUP group) and the other half of them received stimulation at a nonmeridian sham acupoint (SHAM group). All subjects performed picture-naming task. Each subject finished two runs of functional magnetic resonance imaging examinations in one session and picture-naming task was performed before electroacupuncture stimulation. Subjective brain activations were obtained using generalized linear model and inter-group analyses were performed after that. RESULTS: The electroacupuncture stimulation at HT 5 induced significant brain activations in both the anterior and posterior language regions, including the left inferior frontal gyrus, which was in consistent with activations induced during picture-naming task. Group analysis showed a tendency of increased activation of ACUP group in left inferior frontal gyrus compared with SHAM group (P<0.05 FDR corrected). CONCLUSIONS: Electroacupuncture treatment at the acupoint HT 5 has modulation effect on typical language-implicated brain regions in healthy subjects, which provides supporting evidence for beneficial effects of needling at HT 5 for recovery of language function in aphasia.

Increased cerebellar activation after repetitive transcranial magnetic stimulation over the primary motor cortex in patients with multiple system atrophy.

BACKGROUND: Previous review reported that the high-frequency repetitive transcranial magnetic stimulation (rTMS) over the primary motor area (M1) of Parkinson's disease (PD) patients could alleviate their symptoms. This study aimed to investigate the effect of rTMS over the left M1 of patients with multiple system atrophy (MSA). METHODS: Fifteen MSA patients were randomly assigned to receive a 10-session real (EP: group of experimental patients; n=7) or sham (CP: group of control patients; n=8) rTMS stimulation over two weeks. The overall experimental procedure consisted of two functional magnetic resonance imaging (fMRI) sessions, before and after a 10-session rTMS treatment. A complex self-paced sequential tapping task was performed during fMRI scanning. In addition, 18 age and gender matched healthy controls (HC) were enrolled. Subjects from the HC group did not receive any rTMS treatment and they underwent fMRI examination only once. The primary end point was the motor score change of the Unified Multiple System Atrophy Rating Scale (UMSARS-II) measured before and after the 5th and 10th session. Task-related activation was also compared among groups. RESULTS: After active rTMS treatment, only patients of EP group significant improvement in UMSARS-II score. Compared to HC, MSA patients showed significant activation over similar brain areas except for the cerebellum. Increased activation was obtained in the bilateral cerebellum after rTMS treatment in the EP group. On the contrary, no increased activation was identified in the CP group. CONCLUSIONS: Our results highlight rTMS over M1 induced motor improvement in MSA patients that may be associated with increased activation in the cerebellum.

Placebo Analgesia Changes Alpha Oscillations Induced by Tonic Muscle Pain: EEG Frequency Analysis Including Data during Pain Evaluation.

Placebo exhibits beneficial effects on pain perception in human experimental studies. Most of these studies demonstrate that placebo significantly decreased neural activities in pain modulatory brain regions and pain-evoked potentials. This study examined placebo analgesia-related effects on spontaneous brain oscillations. We examined placebo effects on four order-fixed 20-min conditions in two sessions: isotonic saline-induced control conditions (with/without placebo) followed by hypertonic saline-induced tonic muscle pain conditions (with/without placebo) in 19 subjects using continuous electroencephalography (EEG) recording. Placebo treatment exerted significant analgesic effects in 14 placebo responders, as subjective intensity of pain perception decreased. Frequency analyses were performed on whole continuous EEG data, data during pain perception rating and data after rating. The results in the first two cases revealed that placebo induced significant increases and a trend toward significant increases in the amplitude of alpha oscillation during tonic muscle pain compared to control conditions in frontal-central regions of the brain, respectively. Placebo-induced decreases in the subjective intensity of pain perception significantly and positively correlated with the increases in the amplitude of alpha oscillations during pain conditions. In conclusion, the modulation effect of placebo treatment was captured when the pain perception evaluating period was included. The strong correlation between the placebo effect on reported pain perception and alpha amplitude suggest that alpha oscillations in frontal-central regions serve as a cortical oscillatory basis of the placebo effect on tonic muscle pain. These results provide important evidence for the investigation of objective indicators of the placebo effect.

Corrigendum: Placebo Analgesia Changes Alpha Oscillations Induced by Tonic Muscle Pain: EEG Frequency Analysis Including Data during Pain Evaluation.

[This corrects the article on p. 45 in vol. 10, PMID: 27242501.].

Changes of gamma-band oscillatory activity to tonic muscle pain.

It is well know that phasic pain could induce suppression of alpha oscillations and enhancement of gamma oscillations. However, the cortical responses to tonic pain, especially tonic pain originating from deep tissue, which was proposed to better resemble the clinical pain, are not well understood. Here we aimed to investigate electroencephalographic (EEG) responses to tonic muscle pain. EEG signals and pain perceptions of three order-counterbalanced conditions: innocuous condition (A, infusion of isotonic saline), noxious conditions with low (B) and medium (C) intensities (infusion of hypertonic saline) were recorded from 43 subjects. We observed the enhancement of gamma oscillations in frontal-central region in condition C, as compared to either condition A or B. Positive relationship between the amplitude of gamma oscillations and pain intensity was also observed in frontal-central region. Therefore, we provide novel evidence for the encoding of frontal-central gamma oscillations in tonic pain processing.

Alterations in regional homogeneity of resting-state brain activity in patients with major depressive disorder screening positive on the 32-item hypomania checklist (HCL-32).

BACKGROUND: Bipolar disorder (BD) is difficult to diagnose in the early stages of the illness, with the most frequent misdiagnosis being major depressive disorder (MDD). We aimed to use a regional homogeneity (ReHo) approach with resting-state functional magnetic resonance imaging (rs-fMRI) to investigate the features of spontaneous brain activity in MDD patients screening positive on the 32-item Hypomania Checklist (HCL-32). METHODS: Nineteen MDD patients screening positive (HCL-32(+); 9 males; 24.9±5.7 years) and 18 patients screening negative (HCL-32(-); 9 males; 27.1±6.7 years), together with 24 healthy controls (HC; 11 males; 26.4±3.9 years) were studied. ReHo maps were compared and an receiver operating characteristic (ROC) analysis was conducted to confirm the utility of the identified ReHo differences in classifying the patients. RESULTS: The MDD versus HC showed different ReHo in many brain areas, especially in the frontal and parietal cortex. The HCL-32(+) versus HCL-32(-) showed significant increase of ReHo in the right medial superior frontal cortex, left inferior parietal cortex and middle/inferior temporal cortex, and decrease of ReHo in the left postcentral cortex and cerebellum. ROC analysis showed good sensitivity and specificity for distinguishing these two subgroups of MDD. LIMITATIONS: Recruited patients were all on antidepressants and standard mania rating scales were not performed to assess their hypomanic symptoms. CONCLUSIONS: The rs-fMRI measurement of ReHo in distributed brain regions may be putative biomarkers which could differentiate subthreshold BD from MDD.

Movements modulate cortical activities evoked by noxious stimulation.

To evaluate the effects of movement on cortical activities evoked by noxious stimulation, we recorded magnetoencephalography following noxious YAG laser stimulation applied to the dorsum of the left hand in normal volunteers. Results of the present study can be summarized as follows: (1) active movement of the hand ipsilateral to the side of noxious stimulation resulted in significant attenuation of both primary and secondary somatosensory cortices (SI and SII) in the hemisphere contralateral to the stimulated hand (cSI and cSII). Activity in the hemisphere ipsilateral to the side of stimulation (iSII) was not affected. (2) Active movement of the hand contralateral to the side of noxious stimulation resulted in significant attenuation of cSII. Activity in cSI and iSII was not affected. (3) Passive movement of the hand ipsilateral to the side of noxious stimulation resulted in significant attenuation of cSI. Activity in cSII and iSII was not affected. (4) Visual analogue scale (VAS) changes showed a similar pattern to the amplitude changes of cSII. These results suggest that activities in three regions are modulated by movements differently. Inhibition in cSI was considered to be mainly due to an interaction in SI by the signals ascending from the stimulated and movement hand. Inhibition in cSII was considered to be mainly due to particular brain activities relating to motor execution and/or movement execution associated with a specific attention effect. In addition, since VAS changes showed a similar relationship with the amplitude changes of cSII, cSII may play a role in pain perception.

Effects of sleep on pain-related somatosensory evoked magnetic fields in humans.

We investigated the effects of sleep on pain-related somatosensory evoked magnetic fields (SEFs) following painful electrical stimulation to identify the mechanisms generating them in both fast A-beta fibers relating to touch and slow A-delta fibers relating to pain. While the subjects were awake, non-painful and painful electrical stimulations were applied, and while asleep, painful stimulation was applied to the left index finger. During awake, five components (1M-5M) were identified following both non-painful and painful stimulation, but the 4M and 5M at around 70-100 ms and 140-180 ms, respectively, were significantly enhanced following painful stimulation. During sleep, 1M and 2M generated in the primary somatosensory cortex (SI) did not show a significant change, 3M in SI showed a slight but significant amplitude reduction, and 4M and 5M generated in both SI and the secondary somatosensory cortex (SII) were significantly decreased in amplitude or disappeared. The 4M and 5M are complicated components generated in SI and SII ascending through both A-beta fibers and A-delta fibers. They are specifically enhanced by painful stimulation due to an increase of signals ascending through A-delta fibers, and are markedly decreased during sleep, because they much involve cognitive function.

Effects of sleep on pain-related somatosensory evoked potentials in humans.

We investigated effects of sleep on pain-related somatosensory evoked potentials (SEP) following painful electrical stimulation of the left index finger. The biggest advantage of this method is that signals ascending through both A-beta fibers relating to touch and A-delta fibers relating to pain can be recorded simultaneously. While the subject was awake, non-painful stimulation evoked early- and middle latency components, N20, P30 and N60, at the C4 electrode, and painful stimulation evoked not only early- and middle latency components at the C4 but also later pain-specific components, N130 and P240, at the Cz electrode. During sleep, N20 and P30 did not show a significant change in amplitude, N60 showed a slight but significant amplitude reduction, and N130 and P240 significantly decreased in amplitude or disappeared, as compared with those while awake. Therefore, we speculate on the mechanisms generating each component as follows; (1) N20 and P30 are the primary components generated in SI ascending through A-beta fibers. (2) N60 is the secondary component generated in SI involving cognitive function to some degree. (3) N130-P240 are the pain-specific components ascending through A-delta fibers, and closely related to cognitive function, because they were much affected by consciousness, different from the components ascending through A-beta fibers.

Cerebral responses following stimulation of unmyelinated C-fibers in humans: electro- and magneto-encephalographic study.

There are two kinds of pain, a sharp pain ascending through Adelta fibers (first pain) and a second burning pain ascending though C fibers (second pain). By using a novel method, the application of a low intensity CO(2) laser beam to a tiny area of skin using a very thin aluminum plate with numerous tiny holes as a spatial filter, we succeeded in selectively stimulating unmyelinated C fibers of the skin in humans, and could record consistent and clear brain responses using electroencephalography (EEG) and magnetoencephalography (MEG). The conduction velocity (CV) of the C fibers of the peripheral nerve and spinal cord, probably spinothalamic tract (STT), is approximately 1-4 m/s, which is significantly slower than that of Adelta (approximately 10-15 m/s) and Abeta fibers (approximately 50-70 m/s). This method should be very useful for clinical application. Following C fiber stimulation, primary and secondary somatosensory cortices (SI and SII) are simultaneously activated in the cerebral hemisphere contralateral to the stimulation, and then, SII in the hemisphere ipsilateral to the stimulation is activated. These early responses are easily detected by MEG. Then, probably limbic systems such as insula and cingulate cortex are activated, and those activities reflected in EEG components. Investigations of the cortical processing in pain perception including both first and second pain should provide a better understanding of pain perception and, therefore, contribute to pain relief in clinical medicine.

Sensory perception during sleep in humans: a magnetoencephalograhic study.

We reported the changes of brain responses during sleep following auditory, visual, somatosensory and painful somatosensory stimulation by using magnetoencephalography (MEG). Surprisingly, very large changes were found under all conditions, although the changes in each were not the same. However, there are some common findings. Short-latency components, reflecting the primary cortical activities generated in the primary sensory cortex for each stimulus kind, show no significant change, or are slightly prolonged in latency and decreased in amplitude. These findings indicate that the neuronal activities in the primary sensory cortex are not affected or are only slightly inhibited during sleep. By contrast, middle- and long-latency components, probably reflecting secondary activities, are much affected during sleep. Since the dipole location is changed (auditory stimulation), unchanged (somatosensory stimulation) or vague (visual stimulation) between the state of being awake and asleep, different regions responsible for such changes of activity may be one explanation, although the activated regions are very close to each other. The enhancement of activities probably indicates two possibilities, an increase in the activity of excitatory systems during sleep, or a decrease in the activity of some inhibitory systems, which are active in the awake state. We have no evidence to support either, but we prefer the latter, since it is difficult to consider why neuronal activities would be increased during sleep.

Effects of attention, distraction and sleep on CO(2) laser evoked potentials related to C-fibers in humans.

OBJECTIVES: The objective of this study is to evaluate the effects of attention, distraction and sleep on CO(2) laser-evoked potentials (LEP) relating to C-fibers (ultra-late LEP). METHODS: Non-painful CO(2) laser pulses were delivered to a tiny skin area of the dorsum of the right hand. Ultra-late LEP were recorded from 10 normal subjects in 5 different conditions: control (wakefulness), attention, distraction, drowsiness and sleep (stage 2). RESULTS: The amplitude of ultra-late LEP was slightly increased during attention and significantly decreased during distraction, relative to the control. The ultra-late LEP decreased much in amplitude or almost disappeared during sleep. However, significant differences in latency among the conditions were not found. CONCLUSIONS: We confirmed that the brain responses relating to signals ascending through C-fibers were much affected by the level of consciousness, being consistent with the findings of late LEP relating to Adelta-fibers. This is the first study to indicate the important characteristics of ultra-late LEP relating to consciousness, suggesting that they include cognitive function and also that one has to be careful of the change in alertness when recording.

Pain-related magnetic fields evoked by intra-epidermal electrical stimulation in humans.

OBJECTIVES: We recently developed a new method for the preferential stimulation of Adelta fibers in humans. The aim of the present study was to examine whether this method can serve as an appropriate stimulus in a magnetoencephalographic study. METHODS: We recorded somatosensory-evoked magnetic fields (SEFs) following intra-epidermal electrical stimulation applied to the hand and elbow. Superficial parts of the skin were electrically stimulated through a needle electrode whose tip was inserted in the epidermis. RESULTS: In all 13 subjects, the equivalent current dipole was estimated in the secondary somatosensory cortices (SII). In 5 out of 13 subjects, simultaneous activation of the primary somatosensory cortex (SI) in the hemisphere contralateral to the stimulation was identified. The mean peak latencies of magnetic fields corresponding to contralateral SI, SII and ipsilateral SII activation following hand stimulation were 162, 158 and 171 ms, respectively. The respective latency following elbow stimulation was 137, 139 and 157 ms, respectively. Estimated peripheral conduction velocity was 15.6m/s. CONCLUSIONS: All the results were consistent with previous findings in pain SEF studies. We concluded that our novel intra-epidermal electrical stimulation is useful for pain SEF studies since it does not need special equipment and is easy to control.

Effects of distraction on magnetoencephalographic responses ascending through C-fibers in humans.

OBJECTIVE: Using magnetoencephalography (MEG), we evaluated the cerebral regions relating to second pain perception ascending through C-fibers and investigated the effect of distraction on each region. METHODS: Thirteen normal subjects participated in this study. CO2 laser pulses were delivered to the dorsum of the left hand to selectively activate C-fibers. The MEG responses were analyzed using a multi-dipole model. RESULTS: (1) primary somatosensory cortex (SI), and (2) secondary somatosensory cortex (SII)--insula were the main generators for the primary component, 1M, whose mean peak latency was 744 ms. In addition to (1) and (2), (3) cingulate cortex and (4) medial temporal area (MT) were also activated for the subsequent component, 2M, whose mean peak latency was 947 ms. During a mental calculation task (Distraction), all 6 sources were significantly reduced in amplitude, but the SII-insula (P < 0.01) and cingulate cortex (P < 0.001) were more sensitive than the SI (P < 0.05) and MT (P < 0.05). CONCLUSIONS: We confirmed that SI in the contralateral hemisphere and SII-insula, cingulate cortex and MT in bilateral hemispheres play a major role in second pain perception, and all sites were much affected by a change of attention, indicating that these regions are related to the cognitive aspect of second pain perception. SIGNIFICANCE: The SI, SII, cingulate and MT were activated during the C-fiber-related MEG response, and responses in these regions were significantly diminished during mental distraction.

Microneurographic study of C fiber discharges induced by CO2 laser stimulation in humans.

We investigated C-fiber discharges and cerebral potentials evoked by weak CO(2) laser beams applied to a tiny skin area in five healthy subjects. Microneurography was performed from the peroneal nerve in the right popliteal area. Cerebral potentials were recorded from the Cz electrode referred to linked earlobes. The mean conduction velocity of five stable single units was 1.1+/-0.3 m/s. The mean latency of the positive peak of cerebral potentials was 1327.4+/-46.2 ms. These findings indicated that this new stimulation method selectively activated C-fiber nociceptors of the skin.

Human brain processing and central mechanisms of pain as observed by electro- and magneto-encephalography.

We review the recent progress of electroencephalography (EEG) and magnetoencephalography (MEG) to elucidate pain perception mechanisms in humans, since EEG and MEG have an excellent temporal resolution in order of msec. MEG is more useful to detect activated areas following painful stimulation, because the spatial resolution of EEG is not very high. For recording activities following Adelta fiber stimulation relating to the first pain, painful CO2 laser stimulation is now widely used, but our new method, epidermal stimulation (ES), is also very useful. The primary small activity was recorded from the primary somatosensory cortex (SI), probably in area 1, in the hemisphere contralateral to the stimulation. Then, secondary somatosensory cortex (SII) and insula were activated with the second activity in SI. These 3 regions were activated in parallel with almost the same time period. This is a very characteristic finding in pain perception. Then, the cingulate cortex and medial temporal area (MT) around the amygdala and hippocampus were activated. In the hemisphere ipsilateral to the stimulation as well, the above regions were activated, except for SI. Therefore, we speculated that SI plays a main role in localization of the stimulus point, the SII and insula are important sites for pain perception, and the cingulate and MT are mainly responsible for cognitive or emotional aspects of pain perception. For recording activities following C fiber stimulation relating to the second pain, we recently developed a new method, that is, applying weaker CO2 laser stimuli to tiny areas of the skin. MEG findings following C fiber stimulation were also similar to those following Adelta fiber stimulation. However, the effects of sleep and attention on MEG following C fiber stimulation was much larger than that following Adelta fiber stimulation. This finding may suggest greater effects of cognitive or emotional functions on second pain than the first pain.