Forebrain mechanisms of nociception and pain: analysis through imaging

Alterations of regional spontaneous neuronal activity and corresponding brain circuit changes during resting state in migraine without aura

Although previous resting-state studies have reported abnormal functional cerebral changes in patients with migraine without aura (MwoA), few have focused on alterations in both regional spontaneous neuronal activity and corresponding brain circuits in MwoA patients during rest. Eighteen MwoA patients and 18 age- and gender-matched healthy controls (HC) were recruited in the current study. Baseline cerebral alterations were investigated using amplitude of low-frequency fluctuation (ALFF) and region of interest (ROI)-based functional connectivity (FC) analyses. Compared with HC, MwoA patients showed decreased ALFF values in the left rostral anterior cingulate cortex (rACC) and bilateral prefrontal cortex (PFC) as well as increased ALFF values in the right thalamus. FC analysis also revealed abnormal FCs associated with these ROIs. In addition, ALFF values of the left rACC correlated with duration of disease in MwoA. Our findings could lead to a better understanding of intrinsic functional architecture of baseline brain activity in MwoA, providing both regional and brain circuit spontaneous neuronal activity properties.

Astrocytes are involved in long-term facilitation of neuronal excitation in the anterior cingulate cortex of mice with inflammatory pain

Neuronal plasticity in the pain-processing pathway is thought to be a mechanism underlying pain hypersensitivity and negative emotions occurring during a pain state. Recent evidence suggests that the activation of astrocytes in the anterior cingulate cortex (ACC) contributes to the development of negative emotions during pain hypersensitivity after peripheral inflammation. However, it is unknown whether these activated astrocytes contribute to neuronal plasticity in the ACC. In this study, by using optical imaging with voltage- and Ca(2+)-sensitive dyes, we examined the long-term facilitation of neuronal excitation induced by high-frequency conditioning stimulation (HFS) in ACC slices of control mice and mice with peripheral inflammation induced by the injection of complete Freund adjuvant (CFA) to the hind paw. Immunoreactivity of glial fibrillary acidic protein in laminae II-III of the ACC in the CFA-injected mice was higher than in the control mice. Neuronal excitation in ACC slices from the CFA-injected mice was gradually increased by HFS, and the magnitude of this long-term facilitation was greater than in the control mice. The long-term facilitation in the CFA-injected mice was inhibited by the astroglial toxin, the N-methyl-d-aspartate (NMDA) receptor antagonist and NMDA receptor glycine binding site antagonist. The increase of intracellular Ca(2+) concentration in astrocytes during HFS was higher in the CFA-injected mice than in the control mice and was inhibited by l-α-aminoadipate (l-α-AA). These results suggest that the activation of astrocytes in the ACC plays a crucial role in the development of negative emotions and LTP during pain hypersensitivity after peripheral inflammation.

Stress-induced modulation of nociceptive responses in the rat anterior cingulate cortex

In the limbic system, the anterior cingulate cortex (ACCX) is one of the key areas involved in the close association between pain and emotion. However, neuronal changes in ACCX nociceptive responses after stress conditioning have not yet been quantitatively investigated. We investigated the modulation of nociceptive responses in the ACCX neurons following restraint stress in rats. The present study demonstrated that stress-conditioning enhanced excitatory nociceptive responses in the ACCX following tail stimuli in the mid-term (7 days). Short-term (3 days) and long-term (21 days) of stress conditioning did not affect these responses significantly. Nociceptive responses evoked by other sites of the body (nose, back and four paws) stimulation were not changed by stress-conditioning, indicating that neural information from the tail is important for emotional system modulation. It is suggested that the emotional/affective part of the pain sensation is strongly modified by stress through neuroplasticity in the ACCX.

Effects of painful stimulation and acupuncture on attention networks in healthy subjects

Pain is a subjective sensory and emotional experience, and it has been reported that many different brain regions are regulated by pain, and that pain can impact attention. Acupuncture is an important treatment component of Chinese traditional medicine, and has been used for thousands of years to treat a wide variety of conditions. Although several studies have shown that acupuncture improves consciousness, the precise impact of both acupuncture and painful stimulation on attention is unclear. Are all of the attention networks modulated, or do these stimuli act on a specific network? Is the effect of painful stimulation similar to that of acupuncture? We administered the attention network test to 30 participants (15 males) to investigate the relative efficiencies of three independent attention networks (alerting, orienting, and executive control networks) under three conditions: baseline, after painful stimulation, and after acupuncture. The degree of pain experienced was assessed on a horizontally oriented visual analogue scale. The results showed that painful stimulation and acupuncture had similar effects on the orienting and executive control networks; however, there was a significantly different effect between the three conditions on the alerting network. In conclusion, (1) painful stimulation can selectively impact attention; (2) acupuncture can also selectively impact attention; i.e., both have selective influences on the alerting and executive control networks, but not on the orienting network; (3) the effects of acupuncture and painful stimulation are not identical. The mechanisms by which painful stimulation and acupuncture influence attention warrant further research.

Pervasive competition between threat and reward in the brain

In the current functional MRI study, we investigated interactions between reward and threat processing. Visual cues at the start of each trial informed participants about the chance of winning monetary reward and/or receiving a mild aversive shock. We tested two competing hypothesis: according to the 'salience hypothesis', in the condition involving both reward and threat, enhanced activation would be observed because of increased salience; according to the 'competition hypothesis', the processing of reward and threat would trade-off against each other, leading to reduced activation. Analysis of skin conductance data during a delay phase revealed an interaction between reward and threat processing, such that the effect of reward was reduced during threat and the effect of threat was reduced during reward. Analysis of imaging data during the same task phase revealed interactions between reward and threat processing in several regions, including the midbrain/ventral tegmental area, caudate, putamen, bed nucleus of the stria terminalis, anterior insula, middle frontal gyrus and dorsal anterior cingulate cortex. Taken together, our findings reveal conditions during which reward and threat trade-off against each other across multiple sites. Such interactions are suggestive of competitive processes and may reflect the organization of opponent systems in the brain.

Alteration of neuronal activity after digit amputation in rat anterior cingulate cortex

Phantom limb pain is experienced by nearly 50 - 80% of the patients following limb amputation. The anterior cingulate cortex (ACC) is a part of the limbic system that is an essential component in mediating the affective and emotional component of pain responses. To explore the role of ACC in the phantom limb pain, we recorded evoked excitatory postsynaptic potentials (EPSPs), cortical network activity and electrophysiological properties of pyramid neurons in adult rat ACC before and after a third hind paw digit amputation using in vivo intracellular or extracellular recording and staining techniques. The recorded neurons were morphologically identified as pyramidal neurons in the ACC region. The spontaneous activity of ACC neurons significantly reduced with a more percentage of down state after amputation, this is correlated with a decrease in spontaneous spikes in medial thalamus. However, the amplitude of the evoked EPSPs was increased significantly shortly after amputation and lasted for up to 7 days. This potentiation is associated with an increase of paired-pulse facilitation (PPF), suggesting the involvement of presynaptic component in this process. No significant difference in membrane properties was observed after amputation. On the other hand, administration of Complete Freund's Adjuvant (CFA) into the hind paw, a model of inflammatory pain, induced the potentiation of EPSPs in ACC neurons at 7 days after injection. These results demonstrate that digit amputation induced a long-lasting potentiation of synaptic transmission and decrease of cortical network activity in ACC in rats, which might contribute to the phantom limb pain.

Enhanced pain expectation in migraine: EEG-based evidence for impaired prefrontal function

BACKGROUND

Dysexcitability characterizes the interictal migraineous brain. The main central expressions of this dysexcitability are decreased habituation and enhanced anticipation and attention to pain and other external sensory stimuli.

OBJECTIVE

This study evaluates the effects of anticipation on pain modulation and their neural correlates in migraine.

METHODS

In 39 migraineurs (20 migraine with aura [MWA] and 19 migraine without aura [MOA]) and 22 healthy controls, cortical responses to 2 successive trains of noxious contact-heat stimuli, presented in either predicted or unpredicted manner, were analyzed using standardized low-resolution electromagnetic tomography key.

RESULTS

A lack of habituation to repeated predicted pain was associated with significantly increased pain-evoked potential amplitudes in MWAs (increase of 3.9 μV) and unchanged ones in MOAs (1.1 μV) but not in controls (decrease of 5 μV). Repeated unpredicted pain resulted in enhanced pain-evoked potential amplitudes in both MWA and MOA groups (increase of 5.5 μV and 4.4 μV, respectively) compared with controls (decrease of 0.2 μV). Source localization revealed reduced activations in the anterior-medial prefrontal cortices and subsequent increased somatosensory activity in migraineurs (P < .05). The prefrontal-somatosensory dysfunction positively correlated with lifetime headache duration (P < .05) and concern of upcoming migraine attacks (P < .05) in MWAs, and with frequency of migraine attacks in MOAs (P < .05).

CONCLUSIONS

Our findings of impaired modulation of anticipated pain in migraine suggest a heightened state of anticipatory readiness combined with ineffective recruitment of prefrontal inhibitory pathways during experience of pain; the latter might account for the former, at least partially. In line, less efficient inhibitory capability is a plausible mechanistic explanation for patients' high concern about their upcoming migraine attacks.

Electroacupuncture alleviates affective pain in an inflammatory pain rat model

Pain has both sensory-discriminative and emotional-affective dimensions. Previous studies demonstrate that electroacupuncture (EA) alleviates the sensory dimension but do not address the affective. An inflammatory pain rat model, produced by a complete Freund adjuvant (CFA) injection into the hind paw, was combined with a conditioned place avoidance (CPA) test to determine whether EA inhibits spontaneous pain-induced affective response and, if so, to study the possibility that rostral anterior cingulate cortex (rACC) opioids underlie this effect. Male Sprague-Dawley rats (250-275 g, Harlan) were used. The rats showed place aversion (i.e. affective pain) by spending less time in a pain-paired compartment after conditioning than during a preconditioning test. Systemic non-analgesic morphine (0.5 and 1.0 mg/kg, i.p.) inhibited the affective reaction, suggesting that the affective dimension is underpinned by mechanisms different from those of the sensory dimension of pain. Morphine at 0.5 and at 1 mg/kg did not induce reward. Rats given EA treatment before pain-paired conditioning at GB 30 showed no aversion to the pain-paired compartment, indicating that EA inhibited the affective dimension. EA treatment did not produce reward or aversive effect. Intra-rACC administration of D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr amide (CTOP), a selective mu opioid receptor antagonist, but not norbinaltorphimine (nor-BNI), a selective kappa opioid receptor antagonist, blocked EA inhibition of the affective dimension. These data demonstrate that EA activates opioid receptors in the rACC to inhibit pain-induced affective responses and that EA may be an effective therapy for both the sensory-discriminative and the affective dimensions of pain.

An fMRI study on the acute effects of exercise on pain processing in trained athletes

Endurance exercise is known to promote sustained antinociceptive effects, and there is evidence that the reduction of pain perception mediated by exercise is driven by central opioidergic neurotransmission. To directly investigate the involved brain areas and the underlying neural mechanisms in humans, thermal heat-pain challenges were applied to 20 athletes during 4 separate functional magnetic resonance imaging (fMRI) scans, i.e., before and after 2 hours of running (exercise condition) and walking (control condition), respectively. Imaging revealed a reproducible pattern of distributed pain-related activation in all 4 conditions, including the mesial and lateral pain systems, and the periaqueductal gray (PAG) as a key region of the descending antinociceptive pathway. At the behavioral level, running as compared with walking decreased affective pain ratings. The influence of exercise on pain-related activation was reflected in a significant time × treatment interaction in the PAG, along with similar trends in the pregenual anterior cingulate cortex and the middle insular cortex, where pain-induced activation levels were elevated after walking, but decreased or unchanged after running. Our findings indicate that enhanced reactive recruitment of endogenous antinociceptive mechanisms after aversive repeated pain exposure is attenuated by exercise. The fact that running, but not walking, reproducibly elevated β-endorphin levels in plasma indicates involvement of the opioidergic system in exercise. This may argue for an elevated opioidergic tone in the brain of athletes, mediating antinociceptive mechanisms. Our findings provide the first evidence using functional imaging to support the role of endurance exercise in pain modulation.

Pain in people with chronic obstructive pulmonary disease (COPD)

INTRODUCTION

The prevalence and characteristics of pain are not known in COPD patients. The purposes of this study were to determine if pain is more common in COPD patients than in healthy people and if it was related to self-reported physical activity, health related quality of life (HRQoL) and comorbidities.

METHODS

Participants returned a mailed survey package that contained: 1) McGill Pain Questionnaire (MPQ) and Brief Pain Inventory (BPI) to evaluate pain severity and how pain interferes with activities; 2) Tampa Scale for Kinesiophobia (TSK) to evaluate fear of movement related to pain; 3) Short Form-36 (SF-36), to measure HRQoL; 4) Community Health Activities Model Program for Seniors (CHAMPS) to evaluate physical activity; 5) a form to list medications and comorbidities.

RESULTS

Forty-seven COPD patients and 47 age- and gender-matched healthy people responded. People with COPD demonstrated more pain (MPQ and BPI, p = 0.000), a greater pain-related interference in their lives (BPI, p = 0.000), a higher pain-related fear of movement, and lower frequency and energy expenditure of physical activities (CHAMPS, p = 0.000) than healthy people (TSK, p < 0.001). Pain severity (MPQ and BPI) was indirectly correlated to the Physical Component Score of the SF-36. COPD patients identified pain in the neck and trunk 3.1 times more often than healthy people. The number of comorbidities was the most consistent independent correlate of pain in COPD patients.

CONCLUSIONS

COPD patients demonstrate more pain which interferes with activities more so than healthy people of similar age. Pain is also negatively associated with HRQoL in COPD.

Psychophysical and cerebral responses to heat stimulation in patients with central pain, painless central sensory loss, and in healthy persons

Patients with central pain (CP) typically have chronic pain within an area of reduced pain and temperature sensation, suggesting an impairment of endogenous pain modulation mechanisms. We tested the hypothesis that some brain structures normally activated by cutaneous heat stimulation would be hyperresponsive among patients with CP but not among patients with a central nervous system lesion causing a loss of heat or nociceptive sensation with no pain (NP). We used H(2)(15)O positron emission tomography to measure, in 15 healthy control participants, 10 NP patients, and 10 CP patients, increases in regional cerebral blood flow among volumes of interest (VOI) from the resting (no stimulus) condition during bilateral contact heat stimulation at heat detection, heat pain threshold, and heat pain tolerance levels. Both patient groups had a reduced perception of heat intensity and unpleasantness on the clinically affected side and a bilateral impairment of heat detection. Compared with the HC group, both NP and CP patients had more hyperactive and hypoactive VOI in the resting state and more hyperresponsive and hyporesponsive VOI during heat stimulation. Compared with NP patients, CP patients had more hyperresponsive VOI in the intralaminar thalamus and sensory-motor cortex during heat stimulation. Our results show that focal CNS lesions produce bilateral sensory deficits and widespread changes in the nociceptive excitability of the brain. The increased nociceptive excitability within the intralaminar thalamus and sensory-motor cortex of our sample of CP patients suggests an underlying pathophysiology for the pain in some central pain syndromes.

Unmasking the mysteries of the habenula in pain and analgesia

The habenula is a small bilateral structure in the posterior-medial aspect of the dorsal thalamus that has been implicated in a remarkably wide range of behaviors including olfaction, ingestion, mating, endocrine and reward function, pain and analgesia. Afferent connections from forebrain structures send inputs to the lateral and medial habenula where efferents are mainly projected to brainstem regions that include well-known pain modulatory regions such as the periaqueductal gray and raphe nuclei. A convergence of preclinical data implicates the region in multiple behaviors that may be considered part of the pain experience including a putative role in pain modulation, affective, and motivational processes. The habenula seems to play a role as an evaluator, acting as a major point of convergence where external stimuli is received, evaluated, and redirected for motivation of appropriate behavioral response. Here, we review the role of the habenula in pain and analgesia, consider its potential role in chronic pain, and review more recent clinical and functional imaging data of the habenula from animals and humans. Even through the habenula is a small brain structure, advances in structural and functional imaging in humans should allow for further advancement of our understanding of its role in pain and analgesia.

Imaging of acute versus pathological pain in humans

Pain subserves different functions. Acute pain from the intact body alerts the victim to immediately react and withdraw from the bodily threat, ideally before an injury happens. However, during manifest injury and tissue inflammation, withdrawal and flight are no longer adaptive. Instead, sparing the affected body part to promote healing requires heightened awareness and avoidance behaviour over longer periods of time. Quality and time scales of behavioural adaptations are therefore substantially different between pain during normal compared to abnormal tissue states. Given these functional differences we postulated that the two phenomena also recruit different forebrain systems. We used positron emission tomography (PET) and subtracted scans obtained during painful heating of normal skin from scans during equally intense but normally non-painful heating of capsaicin-treated skin. This comparison reveals the specific activation of a medial thalamic pathway to limbic forebrain structures such as anterior insula, perigenual anterior cingulate, ventral striatum, and prefrontal cortex during pain originating in the chemically sensitised skin. It is possible that the unique forebrain recruitment by pain under a patho-physiological tissue status is caused by a significantly greater facilitation of the multi-synaptic projections from the spino-parabrachial tract of the superficial dorsal horn to the medial thalamus compared to deeper and direct lateral thalamic projections from the spino-thalamic tract.

Endogenous opioid-dopamine neurotransmission underlie negative CBV fMRI signals

Previous studies showed noxious unilateral forepaw electrical stimulation surprisingly evoked negative blood-oxygenation-level-dependent (BOLD), cerebral blood flow (CBF), and cerebral blood volume (CBV) fMRI responses in the bilateral striatum whereas the local neuronal spike and c-Fos activities increased. These negative responses are associated with vasoconstriction and appeared to override the increased hemodynamic responses that typically accompanied with increased neural activity. The current study aimed to investigate the role of μ-opioid system in modulating vasoconstriction in the striatum associated with noxious stimulation on a 4.7-Tesla MRI scanner. Specifically, we investigated: i) how morphine (a μ-opioid receptor agonist) affects the vasoconstriction in the bilateral striatum associated with noxious electrical forepaw stimulation in rats, and ii) how naloxone (an opioid receptor antagonist) and eticlopride (a dopamine D(2)/D(3) receptor antagonist) modulates the morphine effects onwards. Injection of morphine enhanced the negative striatal CBV responses to noxious stimulation. Sequential injection of naloxone in the same animals abolished the stimulus-evoked vasoconstriction. In a separate group of animals, injection of eticlopride following morphine also reduced the vasoconstriction. Our findings suggested that noxious stimulation endogenously activated opioid and dopamine receptors in the striatum and thus leading to vasoconstriction.

Fatigue-induced increase in intracortical communication between mid/anterior insular and motor cortex during cycling exercise

In the present study, intracortical communication between mid/anterior insular and motor cortex was investigated during a fatiguing cycling exercise. From 16 healthy male subjects performing a constant-load test at 60% peak oxygen consumption (VO(2peak)) until volitional exhaustion, electroencephalography data were analysed during repetitive, artefact-free periods of 1-min duration. To quantify fatigue-induced intracortical communication, mean intra-hemispheric lagged phase synchronization between mid/anterior insular and motor cortex was calculated: (i) at the beginning of cycling; (ii) at the end of cycling; and (iii) during recovery cycling. Results revealed significantly increased lagged phase synchronization at the end of cycling, which returned to baseline during recovery cycling after subjects' cessation of exercise. Following previous imaging studies reporting the mid/anterior insular cortex as an essential instance processing a variety of sensory stimuli and signalling forthcoming physiological threat, our results provide further evidence that during a fatiguing exercise this structure might not only integrate and evaluate sensory information from the periphery, but also act in communication with the motor cortex. To the best of our knowledge, this is the first study to empirically demonstrate that muscle fatigue leads to changes in interaction between structures of a brain's neural network.

Double-blind, placebo-controlled, randomized pilot study of cerebral blood flow patterns employing SPECT imaging in dental postsurgical pain patients with and without pain relief

BACKGROUND

Single-photon emission computed tomography (SPECT) has been employed in the study of altered regional cerebral blood flow (CBF) in experimental and chronic pain. CBF patterns have not been evaluated in patients with acute postoperative pain.

OBJECTIVE

The purpose of this pilot study was to employ SPECT to measure CBF distribution associated with postoperative dental pain and to compare these CBF patterns to subsequent images in the same patients who were experiencing pain relief versus continued or worsening pain who had received active or placebo analgesic interventions. The primary outcome measure was the percentage change in blood flow in various regions of interest.

METHODS

Twenty-two healthy individuals (10 males and 12 females, age range 20-29 years) who underwent the removal of ≥1 partial or full bony impacted mandibular third molars were evaluated for pain intensity as the local anesthesia dissipated, employing a 0 to10 numeric rating scale (0 = no pain; 10 = worst imaginable). When the subjects' pain level reached ≥4/10, they were injected intravenously with 260 MBq of technetium Tc 99m bicisate (ethyl cysteinate dimer). Under double-blind conditions and 10 minutes before being placed in the SPECT scanner, the first 10 subjects were randomized to receive intravenous ketorolac 15 mg or saline while the remaining 12 subjects were randomized to receive by mouth either ibuprofen 400 mg, ibuprofen 200 mg, acetaminophen 1000 mg, or placebo. One hour after drug administration, subjects were reevaluated for pain, injected with 925 MBq of technetium Tc 99m bicisate, given rescue medication if required, and then rescanned. CBF ratios were obtained for regions of interest and by normalizing to average whole brain activity.

RESULTS

Subjects generally had a moderate degree (mean [SD], 7.3% [4.0%]) of thalamic asymmetry on initial scans with pain; after treatment, subjects reporting worsening pain regardless of the intervention had higher thalamic asymmetry (8.1% vs 2.8%) than those reporting relief of pain. Subjects who reported reduced pain after the intervention had significantly different (P < 0.05) mean CBF changes compared with those reporting worsening pain in the left prefrontal cortex, left sensorimotor area, right anterior cingulate, and right caudate.

CONCLUSIONS

Acute postoperative dental pain was associated with moderate thalamic asymmetry that improved following successful pain management. Sustained or worsening pain was associated with increased CBF in brain regions associated with pain pathways, whereas pain relief was associated with decreased activity in the same areas.

Cerebral blood flow alterations in pain-processing regions of patients with fibromyalgia using perfusion MR imaging

BACKGROUND AND PURPOSE

Widespread pain sensitivity in patients with FM suggests a CNS processing problem. The purpose of this study was to assess alterations in perfusion as measured by DSC in a number of brain regions implicated in pain processing between patients with FM and healthy controls.

MATERIALS AND METHODS

Twenty-one patients with FM and 27 healthy controls underwent conventional MR imaging and DSC. For DSC, 12 regions of interest were placed in brain regions previously implicated in pain processing. rCBF values were calculated for each region of interest. Subjects answered mood/pain coping questionnaires and underwent clinical/experimental pain assessment.

RESULTS

There were significant correlations between the thalamic rCBF values and the pain-control beliefs of FM subjects. The strength of the relationship between clinical pain measures and thalamic rCBF values increased after adjusting for pain-control beliefs. There was a significantly different distribution pattern of rCBF values across various brain regions between the FM group and the healthy controls. There was a lower degree of correlation in the FM group between the thalamic rCBF values and the other brain regions relative to the healthy controls.

CONCLUSIONS

Significant correlations were found between thalamic rCBF values and pain belief values. These data suggest that there are baseline alterations of brain perfusion in patients with FM. rCBF values of the thalami exhibited lower correlations with respect to other brain regions thought to be involved in pain processing compared with those in healthy controls.

Neuronal and microglial mechanisms of neuropathic pain

Neuropathic pain is generally defined as a chronic pain state resulting from peripheral and/or central nerve injury. Effective treatment for neuropathic pain is still lacking, due in part to poor understanding of pathological mechanisms at the molecular level. Neuronal mechanisms of neuropathic pain, especially synaptic plasticity, are the major focus of many investigators. N-methyl-D-aspartate (NMDA) receptor dependent synaptic plasticity at the spinal and cortical levels is believed to contribute to enhanced sensory responses after injury. Glial cells, including astrocytes and microglia, have recently been implicated in neuropathic pain. These glial cells form close interactions with neurons and thus may modulate nociceptive transmission under pathological conditions. In this review, we present recent progress in the study of neuronal and microglial mechanisms underlying neuropathic pain. We propose that activity-dependent neuronal plasticity is a key target for treatment in neuropathic pain.

Coordinate-based meta-analysis of experimentally induced and chronic persistent neuropathic pain

Differences in brain activation in experimentally induced and chronic neuropathic pain conditions are useful for understanding central mechanisms leading to chronic neuropathic pain. Many mapping studies investigating both pain conditions are now available, and the latest tools for coordinate-based meta-analysis offer the possibility of random effects statistics. We performed a meta-analysis based on a literature search of published functional magnetic resonance imaging group studies to compare patterns of activity during experimentally induced and chronic neuropathic pain, for the later including four fibromyalgia studies. Stimulus-dependent activation in experimental pain was further divided into "thermal" and "non thermal" stimuli. A conjunction of experimentally induced and chronic neuropathic pain revealed activation of the bilateral secondary somatosensory cortex, right middle cingulate cortex, right inferior parietal lobe, supplementary motor area, right caudal anterior insula, and bilateral thalamus. Primary somatosensory activation was only observed during experimental non-thermal stimulation. Chronic neuropathic pain studies showed increased activation in the left secondary somatosensory cortex, anterior cingulate cortex, and right caudal anterior insula when compared to experimentally induced pain. Activation clusters in the anterior cingulate cortex and caudal anterior insula suggest a strong emotional contribution to the processing of chronic neuropathic pain.

Synaptic plasticity and pain aversion

Negative affective emotions are defined as the conceptual feature of pain. A number of clinical and animal studies have indicated that the limbic system including the anterior cingulate cortex (ACC) and amygdala plays a critical role in the processing of affective components of pain. Glutamatergic transmission plays an important role in the processing of affective aspects of pain. Long-term changes on glutamatergic synapses contribute to the expression of aversion behavior induced by pain. In this article, the neurocircuits involved in the processing of affective aspects of pain, the glutamatergic synaptic plasticity in these brain regions, and the epigenetic mechanisms underlying pain-related synaptic plasticity will be reviewed and discussed. New discoveries regarding the interaction between the synaptic plasticity and affective components of pain may advance our understanding on the pain mechanism, and lead to new strategies for pain treatment.

Neural specificity of acupuncture stimulation from support vector machine classification analysis

Acupoint specificity, as a crucial issue in acupuncture neuroimaging studies, is still a controversial topic. Previous studies have generally adopted a block-based general linear model (GLM) approach, which predicts the temporal changes in the blood oxygenation level-dependent signal conforming to the "on-off" specifications. However, this method might become impractical since the precise timing and duration of acupuncture actions cannot be specified a priori. In the current study, we applied a data-driven multivariate classification approach, namely, support vector machine (SVM), to explore the neural specificity of acupuncture at gall bladder 40 (GB40) using kidney 3 (KI3) as a control condition (belonging to different meridians but the same nerve segment). In addition, to verify whether the typical GLM approach is sensitive enough in exploring the neural response patterns evoked by acupuncture, we also employed the GLM method to the same data sets. The SVM analysis detected distinct neural response patterns between GB40 and KI3--positive predominantly for the GB40, while negative following the KI3. By contrast, group analysis from the GLM showed that acupuncture at these different acupoints can both evoke similar widespread signal decreases in multiple brain regions, and most of these regions were spatially overlapped, mainly distributing in the limbic and subcortical structures. Our findings may provide additional evidence to support the specificity of acupuncture, relevant to its clinical efficacy. Moreover, we also proved that GLM analysis is prone to be susceptible to errors and is not appropriate for detecting neural response patterns evoked by acupuncture stimulation.

Divergent neural processes specific to the acute and sustained phases of verum and SHAM acupuncture

PURPOSE

To discuss which brain region potentially functioned and switched between the immediate and delayed response of acupuncture.

MATERIALS AND METHODS

A nonrepeated event-related functional MRI (fMRI) design was used to investigate the spatial and temporal patterns of acupuncture effects induced by needling an acupoint ST36 (ACU) and a nonmeridian point (SHAM). The standard general linear model was used to detect the immediate neural responses of acupuncture. Graph theory analysis was used to characterize the functional integrated network of the acupuncture delayed effect.

RESULTS

Acupuncture induced significant signal changes in the limbic/paralimbic areas, neocortical regions, brainstem, and cerebellum for immediate effect both in ACU and SHAM. Some of these brain regions showed strong functional connectivity for a delayed effect in ACU. Conjunction analysis showed that the insula played a critical role during the overall process of ACU. No overlapping brain regions were found in SHAM.

CONCLUSION

The findings of this study suggested that the delayed effects may reflect a more significant characteristic underlying acupuncture. Given that the insula as a relay station switched between the immediate and delayed response, it suggested that divergent functional connectivity patterns may mediate the acupuncture-related effects for ACU and SHAM.

Lesion of the rostral anterior cingulate cortex eliminates the aversiveness of spontaneous neuropathic pain following partial or complete axotomy

Neuropathic pain is often "spontaneous" or "stimulus-independent." Such pain may result from spontaneous discharge in primary afferent nociceptors in injured peripheral nerves. However, whether axotomized primary afferent nociceptors give rise to pain is unclear. The rostral anterior cingulate cortex (rACC) mediates the negative affective component of inflammatory pain. Whether the rACC integrates the aversive component of chronic spontaneous pain arising from nerve injury is not known. Here, we used the principle of negative reinforcement to show that axotomy produces an aversive state reflecting spontaneous pain driven from injured nerves. Additionally, we investigated whether the rACC contributes to the aversiveness of nerve injury-induced spontaneous pain. Partial or complete hind paw denervation was produced by sciatic or sciatic/saphenous axotomy, respectively. Conditioned place preference resulting from presumed pain relief was observed following spinal clonidine in animals with sciatic axotomy but not in sham-operated controls. Similarly, lidocaine administration into the rostral ventromedial medulla (RVM) produced place preference selectively in animals with sciatic/saphenous axotomy. In rats with spinal nerve ligation (SNL) injury, lesion of the rACC blocked the reward elicited by RVM lidocaine but did not alter acute stimulus-evoked hypersensitivity. Lesion of the rACC did not block cocaine-induced reward, indicating that rACC blockade did not impair memory encoding or retrieval but did impair spontaneous aversiveness. These data indicate that spontaneous pain arising from injured nerve fibers produces a tonic aversive state that is mediated by the rACC. Identification of the circuits mediating aversiveness of chronic pain should facilitate the development of improved therapies.

Involvement of the heme oxygenase-carbon monoxide-cGMP pathway in the nociception induced by acute painful stimulus in rats

Heme oxygenase-carbon monoxide-cGMP (HO-CO-cGMP) pathway has been reported to be involved in peripheral and spinal modulation of inflammatory pain. However, the involvement of this pathway in the modulation of acute painful stimulus in the absence of inflammation remains unknown. Thus, we evaluated the involvement of the HO-CO-cGMP pathway in nociception by means the of analgesia index (AI) in the tail flick test. Rats underwent surgery for implantation of unilateral guide cannula directed toward the lateral ventricle and after the recovery period (5-7 days) were subjected to the measures of baseline tail flick test. Animals were divided into groups to assess the effect of intracerebroventricular administration (i.c.v.) of the following compounds: ZnDPBG (HO inhibitor) or vehicle (Na(2)CO(3)), heme-lysinate (substrate overload) or vehicle (l-lysine), or the selective inhibitor of soluble guanilate cyclase ODQ or vehicle (DMSO 1%) following the administration of heme-lysinate or vehicle. Heme overload increased AI, indicating an antinociceptive role of the pathway. This response was attenuated by i.c.v. pretreatment with the HO inhibitor ZnDPBG. In addition, this effect was dependent on cGMP activity, since the pretreatment with ODQ blocked the increase in the AI. Because CO produces most of its actions via cGMP, these data strongly imply that CO is the HO product involved in the antinociceptive response. This modulation seems to be phasic rather than tonic, since i.c.v. treatment with ZnDPBG or ODQ did not alter the AI. Therefore, we provide evidence consistent with the notion that HO-CO-cGMP pathway plays a key phasic antinociceptive role modulating noninflammatory acute pain.

Dyspnoea and the brain

Chronic dyspnoea is a devastating symptom that debilitates millions of people worldwide. It causes a large burden on both patient and carer, and significant costs to society and health services. Treatment options are limited. Much effort has been directed at optimising lung function and improving exercise capacity, however, the brain mechanisms underlying dyspnoea perception have received less attention. In this review, we focus on cognitive and affective aspects of dyspnoea and discuss how novel neuroimaging methods can provide quantitative measures of these subjective sensations. We draw parallels with the more advanced field of chronic pain, and explain some of the challenges faced when imaging dyspnoea. To date, brain mechanisms of dyspnoea have been investigated in a handful of studies by a limited number of authors. These have found consistent activation in the insular cortex, the anterior cingulate cortex and the amygdala. Novel neuroimaging methods and an improved understanding of perceptual mechanisms underlying dyspnoea now position us to transform dyspnoea research. Future research should investigate how brain regions associated with dyspnoea interact, as well as accurately correlate this neuronal activation with reliable behavioural measures. A better understanding of the brain processes underlying dyspnoea perception will lead to new therapies that will improve quality of life for a very large group of patients.

Limitation of physical performance in a muscle fatiguing handgrip exercise is mediated by thalamo-insular activity

In this study, we investigated central/supraspinal processes mediating cessation of a muscle fatiguing exercise. Fifteen male subjects performed 39 intermittent, isometric handgrip contractions (13 s on, 5-6 s off) with the dominant right hand while brain activation was assessed by means of functional magnetic resonance imaging (fMRI). An adaptive, partly stochastic protocol was designed such that in approximately 50% of the contraction trials the required force could not be held until the end of the trial (task failure trial). Trials performed in compliance with the force requirements (succeeded trial) were compared with task failure trials concerning neural activity during a small time window before task failure occurred. The data revealed significantly increased activation contralaterally in both the mid/anterior insular cortex and the thalamus during the investigated time window in the case of subsequent task failure. In accordance with other studies investigating sensations that alert the organism to urgent homeostatic imbalance such as air hunger, hunger for food, and pain, we assume that an increased thalamo-insular activation in the context of a fatigue-induced handgrip exercise could reflect increased homeostatic disturbance in the exercising muscle and may be of essential importance by mediating task failure to maintain the integrity of the organism.

Men and women are different: diffusion tensor imaging reveals sexual dimorphism in the microstructure of the thalamus, corpus callosum and cingulum

INTRODUCTION

Numerous magnetic resonance imaging (MRI) studies have addressed the question of morphological differences of the brain of men and women, reporting conflicting results regarding brain size and the ratio of gray and white matter. In the present study, we used diffusion tensor imaging (DTI) to delineate sex differences of brain white matter.

METHODS

We investigated brain microstructure in 25 male and 25 female healthy subjects using a 3T MRI scanner. Whole-head DTI scans were analyzed without a-priori hypothesis using Tract-Based Spatial Statistics (TBSS) calculating maps of fractional anisotropy (FA), radial diffusivity (RD, a potential marker of glial alteration and changes in myelination) and axial diffusivity (AD, a potential marker of axonal changes).

RESULTS

DTI revealed regional microstructural differences between the brains of male and female subjects. Those were prominent in the thalamus, corpus callosum and cingulum. Men showed significantly (p<0.0001) higher values of fractional anisotropy and lower radial diffusivity in these areas, suggesting that the observed differences are mainly due to differences in myelination.

DISCUSSION

As a novel finding we showed widespread differences in thalamic microstructure that have not been described previously. Additionally, the present study confirmed earlier DTI studies focusing on sexual dimorphism in the corpus callosum and cingulum. All changes appear to be based on differences in myelination. The sex differences in thalamic microstructure call for further studies on the underlying cause and the behavioral correlates of this sexual dimorphism. Future DTI group studies may carefully control for gender to avoid confounding.

Acupuncture modulates temporal neural responses in wide brain networks: evidence from fMRI study

BACKGROUND

Accumulating neuroimaging studies in humans have shown that acupuncture can modulate a widely distributed brain network, large portions of which are overlapped with the pain-related areas. Recently, a striking feature of acupuncture-induced analgesia is found to be associated with its long-last effect, which has a delayed onset and gradually reaches a peak even after acupuncture needling being terminated. Identifying temporal neural responses in these areas that occur at particular time--both acute and sustained effects during acupuncture processes--may therefore shed lights on how such peripheral inputs are conducted and mediated through the CNS. In the present study, we adopted a non-repeated event-related (NRER) fMRI paradigm and control theory based approach namely change-point analysis in order to capture the detailed temporal profile of neural responses induced by acupuncture.

RESULTS

Our findings demonstrated that neural activities at the different stages of acupuncture presented distinct temporal patterns, in which consistently positive neural responses were found during the period of acupuncture needling while much more complex and dynamic activities found during a post-acupuncture period. These brain responses had a significant time-dependent effect which showed different onset time and duration of neural activities. The amygdala and perigenual anterior cingulate cortex (pACC), exhibited increased activities during the needling phase while decreased gradually to reach a peak below the baseline. The periaqueductal gray (PAG) and hypothalamus presented saliently intermittent activations across the whole fMRI session. Apart from the time-dependent responses, relatively persistent activities were also identified in the anterior insula and prefrontal cortices. The overall findings indicate that acupuncture may engage differential temporal neural responses as a function of time in a wide range of brain networks.

CONCLUSIONS

Our study has provided evidence supporting a view that acupuncture intervention involves complex modulations of temporal neural response, and its effect can gradually resolve as a function of time. The functional specificity of acupuncture at ST36 may involve multiple levels of differential activities of a wide range of brain networks, which are gradually enhanced even after acupuncture needle being terminated.

Assessment of brain responses to innocuous and noxious electrical forepaw stimulation in mice using BOLD fMRI

Functional magnetic resonance imaging (fMRI) using the blood oxygen level-dependent (BOLD) contrast was used to study sensory processing in the brain of isoflurane-anesthetized mice. The use of a cryogenic surface coil in a small animal 9.4T system provided the sensitivity required for detection and quantitative analysis of hemodynamic changes caused by neural activity in the mouse brain in response to electrical forepaw stimulation at different amplitudes. A gradient echo-echo planar imaging (GE-EPI) sequence was used to acquire five coronal brain slices of 0.5mm thickness. BOLD signal changes were observed in primary and secondary somatosensory cortices, the thalamus and the insular cortex, important regions involved in sensory and nociceptive processing. Activation was observed consistently bilateral despite unilateral stimulation of the forepaw. The temporal BOLD profile was segregated into two signal components with different temporal characteristics. The maximum BOLD amplitude of both signal components correlated strongly with the stimulation amplitude. Analysis of the dynamic behavior of the somatosensory 'fast' BOLD component revealed a decreasing signal decay rate constant k(off) with increasing maximum BOLD amplitude (and stimulation amplitude). This study demonstrates the feasibility of a robust BOLD fMRI protocol to study nociceptive processing in isoflurane-anesthetized mice. The reliability of the method allows for detailed analysis of the temporal BOLD profile and for investigation of somatosensory and noxious signal processing in the brain, which is attractive for characterizing genetically engineered mouse models.

Global trends and performances of acupuncture research

This study was designed to evaluate the global scientific output of acupuncture research in the Science Citation Index-Expanded and to assess the tendencies and research performances of leading countries/territories and institutes. Articles referring to acupuncture were assessed by distribution of document types, languages, journals, subject categories, source countries, and source institutes. Results showed that 15 languages were represented in articles from 65 countries/territories. Journal of Alternative and Complementary Medicine published the most articles, followed by American Journal of Chinese Medicine and Acupuncture & Electro-Therapeutics Research which were listed in category of integrative & complementary medicine. In the study period of 1991-2009, USA was the top producing country, followed by China (mainland) with a sharply growth trend. In 2009, publication of China (mainland) ranked top one in the world. In addition, an acupuncture research trend was found in two phases in terms of the increase of number of SCI-expanded journals' articles. Among the acupuncture research, pain control has been the most prevalent direction of study, and brain imaging is attracting the most recent attention.

Functional magnetic resonance imaging activation of the brain in children: real acupoint versus sham acupoint

The purpose was to examine the brain activation patterns with acupuncture using real acupoint (Liv3) versus sham acupoint in healthy, sedated children using functional magnetic resonance imaging (fMRI). Functional magnetic resonance imaging scans of the brain for 10 healthy, sedated children were taken during stimulation of real acupoint (Liv3 [Taichong]) and a nearby sham acupoint in a randomized order, employing twisting and nontwisting methods using a blocked paradigm using a 2.0-T scanner. The functional data were analyzed by using SPM 99. Various regions of the brain were activated in 2 acupoints. However, the pattern was different for the 2 acupoints. We suggest specific cerebral activation patterns with acupuncture might explain some of its therapeutic effect.

[Pharmacological fMRI : new possibilities for assessing the efficacy of analgesic agents]

Pain is a complex subjective phenomenon that so far cannot be objectively quantified by any standardized procedure. This fact renders it also difficult to measure the efficacy of analgesic drugs. In recent years the application of functional magnetic resonance imaging (fMRI) has significantly increased our current knowledge about the brain physiological correlates of pain in humans. The technique is non-invasive and detects the increased blood flow into neuronally active brain regions based on the so-called BOLD (blood oxygenation level dependent) effect of T2-weighted MRI. This paper gives an overview of the application of pharmacological fMRI (phfMRI) as an approach to evaluate the efficacy of analgesics. In contrast to EEG- and MEG-based methods phfMRI allows more flexibility in the design of experimental paradigms and stimulus protocols to account for the diversity of clinical pain types (inflammatory pain, tactile allodynia etc.) or their dependence upon psychological circumstances (anxiety, depression, stress) in which pain occurs. However, in order to specifically refer results from phfMRI to the neuronal processes underlying pain, future research needs to increase the understanding of the mechanisms underlying the neurovascular coupling reaction represented by the BOLD technique. The same applies for the influence of cerebrovascular diseases on the BOLD response.

Positron emission tomography analysis of the analgesic effects of acupuncture in rhesus monkeys

The purpose of this study was to examine whether pain-induced brain activation was suppressed by acupuncture analgesia. We investigated the suppression of the pain-induced neuronal activation in specific brain areas of three male rhesus monkeys (aged four years old) using positron emission tomography (PET), in which changes in the regional cerebral blood flow (rCBF) were examined as an index of the neuronal activation. The brain areas such as the thalamus, insula and anterior cingulate cortex were activated by heating the tail of monkeys in 47 degrees C water compared to the heating at 37 degrees C. Those activations were suppressed by electroacupuncture (EA) with a 2 sec alteration of the frequency of 4 Hz/60 Hz at the right ST36 (the upper anterior tibial muscle) and the right LI4 (the back palm between the first and second metacarpal) acupoints. Meanwhile, this EA analgesic effect was confirmed by prolonging the tail withdrawal latencies from hot water in the temperature range from 45 to 50 degrees C.These brain areas were corresponded to the pain-related areas as reported in previous studies. In conclusion, we were able to visualize the acupuncture analgesia in the CNS. We also detected the brain areas activated or inactivated by acupuncture. The areas that responded to acupuncture stimulation at 47 degrees C water were different from the regions at 37 degrees C. We consider that this difference in the response to acupuncture may support the variation of the clinical efficacy of acupuncture in patients bearing pain or other disorders.

Orbitofrontal disinhibition of pain in migraine with aura: an interictal EEG-mapping study

This study aimed to identify the cortical mechanisms underlying the processes of interictal dishabituation to experimental pain in subjects suffering from migraine with aura (MWA). In 21 subjects with MWA and 22 healthy controls, cortical responses to two successive trials of noxious contact-heat stimuli were analyzed using EEG-tomography software. When compared with controls, MWA patients showed significantly increased pain-evoked potential amplitudes accompanied by reduced activity in the orbitofrontal cortex (OFC) and increased activity in the pain matrix regions, including the primary somatosensory cortex (SI) (p < .05). Similarly to controls, MWA subjects displayed an inverse correlation between the OFC and SI activities, and positive interrelations between other pain-specific regions. The activity changes in the OFC negatively correlated with lifetime headache duration and longevity (p < .05). Reduced inhibitory functioning of the prefrontal cortex is a possible cause for disinhibition of the pain-related sensory cortices in migraine. The finding of OFC hypofunction over the disease course is in keeping with current concepts of migraine as a progressive brain disorder.

Additional evidence for the affective dimension of dyspnea in patients with COPD

The primary purpose of this secondary analysis was to determine whether 103 participants with chronic obstructive pulmonary disease rated the affective dimension of dyspnea (dyspnea-related anxiety and dyspnea-related distress) separately from the sensory dimension (intensity) during baseline exercise testing conducted as part of a randomized clinical trial. A secondary purpose was to determine if dyspnea-related anxiety and distress were rated distinctly different from other measurements of anxiety. At the end of a 6-minute walk and an incremental treadmill test, participant ratings of the magnitude of dyspnea-related anxiety and distress on the Modified Borg Scale were significantly different from their ratings of the intensity of dyspnea. Dyspnea-related anxiety and distress also appeared to be concepts independent from measures of state anxiety, negative affect, and anxiety before a treadmill test.