Analgesia produced by injection of lidocaine into the anterior cingulum bundle of the rat

Investigating the brain functional abnormalities underlying pain hypervigilance in chronic neck and shoulder pain: a resting-state fMRI study

PURPOSE

To investigate pain hypervigilance in individuals suffering from chronic neck and shoulder pain (CNSP) and its underlying brain mechanism.

METHODS

The evaluation of pain vigilance was conducted through the utilization of pain vigilance and awareness questionnaires. Voxel-wise regional homogeneity (ReHo) from 60 CNSP patients and 60 healthy controls (HCs) using resting-state fMRI data. Voxel-wise two-sample T-test was conducted to reveal the ReHo variations between CNSP and HC. Correlation analyses were utilized to reveal the connection between brain abnormalities and medical measurements. Furthermore, a mediation analysis was conducted to elucidate the pathway-linking changes in brain function with medical measurements.

RESULTS

Our present study revealed three main findings. Firstly, patients with CSNP demonstrated a heightened vigilance of pain in comparison to healthy adults, a common occurrence among individuals with chronic pain conditions. Secondly, we observed brain abnormalities in various brain regions in CSNP patients, and these alterations were associated with the extent of pain vigilance. Lastly, the pain hypervigilance impact on the severity of pain was found to be controlled by regional neural activity in the anterior cingulate cortex (ACC) in subjects with CSNP.

CONCLUSION

Our findings suggested that long-term repetitive nociceptive input caused by chronic pain further aggravates the pain intensity by impairing the vigilance-related pain processing within the anterior cingulate cortex in CNSP patients.

Divergent brainstem opioidergic pathways that coordinate breathing with pain and emotions

Breathing can be heavily influenced by pain or internal emotional states, but the neural circuitry underlying this tight coordination is unknown. Here we report that Oprm1 (μ-opioid receptor)-expressing neurons in the lateral parabrachial nucleus (PBL) are crucial for coordinating breathing with affective pain in mice. Individual PBL^Oprm1 neuronal activity synchronizes with breathing rhythm and responds to noxious stimuli. Manipulating PBL^Oprm1 activity directly changes breathing rate, affective pain perception, and anxiety. Furthermore, PBL^Oprm1 neurons constitute two distinct subpopulations in a "core-shell" configuration that divergently projects to the forebrain and hindbrain. Through non-overlapping projections to the central amygdala and pre-Bötzinger complex, these two subpopulations differentially regulate breathing, affective pain, and negative emotions. Moreover, these subsets form recurrent excitatory networks through reciprocal glutamatergic projections. Together, our data define the divergent parabrachial opioidergic circuits as a common neural substrate that coordinates breathing with various sensations and behaviors such as pain and emotional processing.

Cerebral current-source distribution associated with pain improvement by non-invasive painless signaling therapy in patients with failed back surgery syndrome

BACKGROUND

Non-invasive painless signaling therapy (NPST) is an electro-cutaneous treatment that converts endogenous pain information into synthetic non-pain information. This study explored whether pain improvement by NPST in failed back surgery syndrome (FBSS) patients is related to cerebral modulation.

METHODS

Electroencephalography (EEG) analysis was performed in 11 patients with FBSS. Subjects received daily NPST for 5 days. Before the first treatment, patients completed the Brief Pain Inventory (BPI) and Beck Depression Inventory and underwent baseline EEG. After the final treatment, they responded again to the BPI, reported the percent pain improvement (PPI), and then underwent post-treatment EEG. If the PPI grade was zero, they were assigned to the ineffective group, while all others were assigned to the effective group. We used standardized low-resolution brain electromagnetic tomography (sLORETA) to explore the EEG current-source distribution (CSD) associated with pain improvement by NPST.

RESULTS

The 11 participants had a median age of 67.0 years, and 63.6% were female. The sLORETA images revealed a beta-2 CSD increment in 12 voxels of the right anterior cingulate gyrus (ACG) and the right medial frontal area. The point of maximal CSD changes was in the right ACG. The alpha band CSD increased in 2 voxels of the left transverse gyrus.

CONCLUSIONS

Pain improvement by NPST in FBSS patients was associated with increased cerebral activity, mainly in the right ACG. The change in afferent information induced by NPST seems to be associated with cerebral pain perception.

Role of the Anterior Cingulate Cortex in Translational Pain Research

As the most common symptomatic reason to seek medical consultation, pain is a complex experience that has been classified into different categories and stages. In pain processing, noxious stimuli may activate the anterior cingulate cortex (ACC). But the function of ACC in the different pain conditions is not well discussed. In this review, we elaborate the commonalities and differences from accumulated evidence by a variety of pain assays for physiological pain and pathological pain including inflammatory pain, neuropathic pain, and cancer pain in the ACC, and discuss the cellular receptors and signaling molecules from animal studies. We further summarize the ACC as a new central neuromodulation target for invasive and non-invasive stimulation techniques in clinical pain management. The comprehensive understanding of pain processing in the ACC may lead to bridging the gap in translational research between basic and clinical studies and to develop new therapies.

Resting-state white matter-cortical connectivity in non-human primate brain

Numerous studies have used functional magnetic resonance imaging (fMRI) to characterize functional connectivity between cortical regions by analyzing correlations in blood oxygenation level dependent (BOLD) signals in a resting state. However, to date, there have been only a handful of studies reporting resting state BOLD signals in white matter. Nonetheless, a growing number of reports has emerged in recent years suggesting white matter BOLD signals can be reliably detected, though their biophysical origins remain unclear. Moreover, recent studies have identified robust correlations in a resting state between signals from cortex and specific white matter tracts. In order to further validate and interpret these findings, we studied a non-human primate model to investigate resting-state connectivity patterns between parcellated cortical volumes and specific white matter bundles. Our results show that resting-state connectivity patterns between white and gray matter structures are not randomly distributed but share notable similarities with diffusion- and histology-derived anatomic connectivities. This suggests that resting-state BOLD correlations between white matter fiber tracts and the gray matter regions to which they connect are directly related to the anatomic arrangement and density of WM fibers. We also measured how different levels of baseline neural activity, induced by varying levels of anesthesia, modulate these patterns. As anesthesia levels were raised, we observed weakened correlation coefficients between specific white matter tracts and gray matter regions while key features of the connectivity pattern remained similar. Overall, results from this study provide further evidence that neural activity is detectable by BOLD fMRI in both gray and white matter throughout the resting brain. The combined use of gray and white matter functional connectivity could also offer refined full-scale functional parcellation of the entire brain to characterize its functional architecture.

The cingulum bundle: Anatomy, function, and dysfunction

The cingulum bundle is a prominent white matter tract that interconnects frontal, parietal, and medial temporal sites, while also linking subcortical nuclei to the cingulate gyrus. Despite its apparent continuity, the cingulum's composition continually changes as fibres join and leave the bundle. To help understand its complex structure, this review begins with detailed, comparative descriptions of the multiple connections comprising the cingulum bundle. Next, the impact of cingulum bundle damage in rats, monkeys, and humans is analysed. Despite causing extensive anatomical disconnections, cingulum bundle lesions typically produce only mild deficits, highlighting the importance of parallel pathways and the distributed nature of its various functions. Meanwhile, non-invasive imaging implicates the cingulum bundle in executive control, emotion, pain (dorsal cingulum), and episodic memory (parahippocampal cingulum), while clinical studies reveal cingulum abnormalities in numerous conditions, including schizophrenia, depression, post-traumatic stress disorder, obsessive compulsive disorder, autism spectrum disorder, Mild Cognitive Impairment, and Alzheimer's disease. Understanding the seemingly diverse contributions of the cingulum will require better ways of isolating pathways within this highly complex tract.

Microstructural White Matter Abnormalities in the Dorsal Cingulum of Adolescents with IBS

Alterations in fractional anisotropy (FA) have been considered to reflect microstructural white matter (WM) changes in disease conditions; however, no study to date has examined WM changes using diffusion tensor imaging (DTI) in adolescents with irritable bowel syndrome (IBS). The objective of the present study was two-fold: (1) to determine whether differences in FA, and other non-FA metrics, were present in adolescents with IBS compared to healthy controls using whole-brain, region of interest (ROI)-restricted tract-based spatial statistics (TBSS) and canonical ROI DTI analyses for the cingulum bundle, and (2) to determine whether these metrics were related to clinical measures of disease duration and pain intensity in the IBS group. A total of 16 adolescents with a Rome III diagnosis of IBS (females = 12; mean age = 16.29, age range: 11.96-18.5 years) and 16 age- and gender-matched healthy controls (females = 12; mean age = 16.24; age range: 11.71-20.32 years) participated in this study. Diffusion-weighted images were acquired using a Siemens 3-T Trio Tim Syngo MRI scanner with a 32-channel head coil. The ROI-restricted TBSS and canonical ROI-based DTI analyses revealed that adolescents with IBS showed decreased FA in the right dorsal cingulum bundle compared to controls. No relationship between FA and disease severity measures was found. Microstructural WM alterations in the right dorsal cingulum bundle in adolescents with IBS may reflect a premorbid brain state or the emergence of a disease-driven process that results from complex changes in pain- and affect-related processing via spinothalamic and corticolimbic pathways.

Antinociceptive tolerance to NSAIDs in the anterior cingulate cortex is mediated via endogenous opioid mechanism

BACKGROUND

In the past decade several studies have reported that in some brain areas, particularly, in the midbrain periaqueductal gray matter, rostral ventro-medial medulla, central nucleus of amygdala, nucleus raphe magnus, and dorsal hippocampus, microinjections of non-steroidal anti-inflammatory drugs (NSAIDs) induce antinociception with distinct development of tolerance. Given this evidence, in this study we investigated the development of tolerance to the analgesic effects of NSAIDs diclofenac, ketorolac and xefocam microinjected into the rostral part of anterior cingulate cortex (ACC) in rats.

METHODS

Male Wistar experimental and control (saline) rats were implanted with a guide cannula in the ACC and tested for antinociception following microinjection of NSAIDs into the ACC in the tail-flick (TF) and hot plate (HP) tests. Repeated measures of analysis of variance with post-hoc Tukey-Kramer multiple comparison tests were used for statistical evaluations.

RESULTS

Treatment with each NSAID significantly enhanced the TF and HP latencies on the first day, followed by a progressive decrease in the analgesic effect over a 4-day period, i.e., developed tolerance. Pretreatment with an opioid antagonist naloxone completely prevented the analgesic effects of the three NSAIDs in both behavioral assays.

CONCLUSIONS

These findings support the concept that the development of tolerance to the antinociceptive effects of NSAIDs is mediated via an endogenous opioid system possibly involving descending pain modulatory systems.

Large Scale In Vivo Recording of Sensory Neuron Activity with GCaMP6

Greater emphasis on the study of intact cellular networks in their physiological environment has led to rapid advances in intravital imaging of the central nervous system (CNS), while the peripheral system remains largely unexplored. To assess large networks of sensory neurons, we selectively label primary afferents with GCaMP6s in male and female C57bl/6 mice and visualize their functional responses to peripheral stimulation in vivo. We show that we are able to monitor the activity of hundreds of sensory neurons simultaneously, with sufficient sensitivity to detect, in most cases, single action potentials with a typical rise time of around 200 ms, and an exponential decay with a time constant of approximately 700 ms. With this technique we are able to characterize the responses of large populations of sensory neurons to innocuous and noxious mechanical and thermal stimuli under normal and inflammatory conditions. We demonstrate that the majority of primary afferents are polymodal with between 50-80% of thermally sensitive DRG neurons responding also to noxious mechanical stimulation. We also specifically assess the small population of peripheral cold neurons and demonstrate significant sensitization to cooling after a model of sterile and persistent inflammation, with significantly increased sensitivity already at decreases of 5°C when compared to uninflamed responses. This not only reveals interesting new insights into the (patho)physiology of the peripheral nervous system but also demonstrates the sensitivity of this imaging technique to physiological changes in primary afferents.

A nociceptive stress model of adolescent physical abuse induces contextual fear and cingulate nociceptive neuroplasticities

Adolescent physical abuse impairs emotional development and evokes cingulate pathologies, but its neuronal and circuit substrates are unknown. Conditioning adolescent rabbits with noxious colorectal distension for only 2 h over 3 weeks simulated the human child abuse in amplitude, frequency, and duration. Thermal withdrawal thresholds were unchanged suggesting that sensitized spinal mechanisms may not be operable. Unchanged weight, stools, colorectal histology, and no evidence of abdominal pain argue against tissue injury or irritable bowel syndrome. Contextual fear was amplified as they avoided the site of their abuse. Conditioning impacted anterior cingulate and anterior midcingulate (ACC, aMCC) neuron excitability: (1) more neurons responded to cutaneous and visceral (VNox) noxious stimuli than controls engaging latent nociception (present but not manifest in controls). (2) Rear paw stimulation increased responses over forepaws with shorter onsets and longer durations, while forepaw responses were of higher amplitude. (3) There were more VNox responses with two excitatory phases and longer durations. (4) Some had unique three-phase excitatory responses. (5) Long-duration VNox stimuli did not inhibit neurons as in controls, suggesting the release of an inhibitory circuit. (6) aMCC changes in cutaneous but not visceral nociception confirmed its role in cutaneous nociception. For the first time, we report neuroplasticities that may be evoked by adolescent physical abuse and reflect psychogenic pain: i.e., no ongoing peripheral pain and altered ACC nociception. These limbic responses may be a cognitive trace of abuse and may shed light on impaired human emotional development and sexual function.

Altered white matter microarchitecture in the cingulum bundle in women with primary dysmenorrhea: A tract-based analysis study

Primary dysmenorrhea (PD), as characterized by painful menstrual cramps without organic causes, is associated with central sensitization and brain function changes. Previous studies showed the integrated role of the default mode network (DMN) in the pain connectome and its key contribution on how an individual perceives and copes with pain disorders. Here, we aimed to investigate whether the cingulum bundle connecting hub regions of the DMN was disrupted in young women with PD. Diffusion tensor imaging was obtained in 41 PD patients and 41 matched healthy controls (HC) during their periovulatory phase. The production of prostaglandins (PGs) was obtained in PD patients during their pain-free and pain phases. As compared with HC, PD patients had similar scores of pain intensity, anxiety, and depression in their pain-free phase. However, altered white matter properties mainly located in the posterior section of the cingulum bundle were observed in PD. Besides PGs being related to menstrual pain, a close relationship was found between the white matter properties of the cingulum bundle during the pain-free phase and the severity of the menstrual pain in PD patients. Our study suggested that PD had trait changes of white matter integrities in the cingulum bundle that persisted beyond the time of menstruation. We inferred that altered anatomical connections may lead to less-flexible communication within the DMN, and/or between the DMN and other pain-related brain networks, which may result in the central susceptibility to develop chronic pain conditions in PD's later life. Hum Brain Mapp 38:4430-4443, 2017. © 2017 Wiley Periodicals, Inc.

Effect of an Anterior Cingulotomy on Pain, Cognition, and Sensory Pathways

BACKGROUND

Anterior cingulotomy (AC) was originally used to treat patients with a psychiatric disorder, but it is also useful for treating patients with chronic intractable pain. We reviewed 24 patients at our hospital who underwent AC for chronic intractable pain to determine whether surgery influenced patient cognition and the pain circuit.

METHODS

A visual analog scale (VAS) was used to evaluate patients' pain scale preoperatively, at 1 month and 3-6 months postoperatively, and at the final follow-up. Mini-Mental State Examination (MMSE) and Cognitive Abilities Screening Instrument (CASI) were used to evaluate postoperative cognitive function. The latencies of peaks P20 and P37 of the somatosensory evoked potential (SSEP) conductive time were used to evaluate the intactness of the thalamocortical tract after AC.

RESULTS

The median preoperative VAS score was 8, MMSE score was 27, and CASI score was 86.8. Six patients underwent a reoperation because of recurrent pain. Pain was significantly reduced after AC, and the median VAS score at the last follow-up was 5. There was no significant pain improvement in patients who underwent a reoperation. There were no significant changes in MMSE and CASI scores or SSEP after cingulotomy. There were no operation-related complications in the patients.

CONCLUSIONS

A stereotactic AC was safe and effective in resolving chronic refractory pain. It did not affect patient cognition or the sensory conductive pathway. However, patients who had recurrent intractable pain after a cingulotomy did not respond well to the reoperation.

Spatial and temporal plasticity of synaptic organization in anterior cingulate cortex following peripheral inflammatory pain: multi-electrode array recordings in rats

To explore whether experiencing inflammatory pain has an impact upon intracortical synaptic organization, the planar multi-electrode array (MEA) technique and 2-dimensional current source density (2D-CSD) imaging were used in slice preparations of the anterior cingulate cortex (ACC) from rats. Synaptic activity across different layers of the ACC was evoked by deep layer stimulation through one electrode. The layer-localization of both local field potentials (LFPs) and the spread of current sink calculated by 2D-CSD analysis was characterized pharmacologically. Moreover, the induction of long-term potentiation (LTP) and changes in LTP magnitude were also evaluated. We found that under naïve conditions, the current sink was initially generated in layer VI, then spread to layer V and finally confined to layers II-III. This spatial pattern of current sink movement typically reflected changes in depolarized sites from deep layers (V-VI) to superficial layers (II-III) where intra- and extracortical inputs terminate. In the ACC slices from rats in an inflamed state (for 2 h) caused by intraplantar bee-venom injection, the spatial profile of intra-ACC synaptic organization was significantly changed, showing an enlarged current sink distribution and a leftward shift of the stimulus-response curves relative to the naïve and saline controls. The change was more distinct in the superficial layers (II-III) than in the deep site. In terms of temporal properties, the rate of LTP induction was significantly increased in layers II-III by inflammatory pain. However, the magnitude of LTP was not significantly enhanced by this treatment. Taken together, these results show that inflammatory pain results in distinct spatial and temporal plasticity of synaptic organization in the ACC, which may lead to altered synaptic transmission and modulation.

Depressive-like history alters persistent pain behavior in rats: Opposite contribution of frontal cortex and amygdala implied

Numerous studies have shown that pain perception is strongly influenced by depression. However, very few studies have examined whether pain perception is altered in the remission period of depression, and what role the fronto-limbic circuits may play in the behavioral changes associated with remission. Using an unpredictable chronic mild stress (UCMS) animal model of depression, the present study investigated pain-related behaviors in rats with prior exposure to a UCMS stimulus. The γ-aminobutyric acid (GABA)A receptor agonist muscimol was microinjected bilaterally into the basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC) to examine the modulation of pain by these brain regions in the recovery state. Rats with a depression-like history displayed increased ongoing pain behavior in the formalin test, although their thermal pain thresholds were unchanged. Intra-BLA muscimol during the recovery phase dramatically decreased formalin-induced pain behavior and also significantly increased rats' sucrose preference. By contrast, in the mPFC, muscimol produced the opposite effect, suggesting different, perhaps opposing, roles of the BLA and mPFC in mediating the influence of prior UCMS exposure on pain perception. Taken together, these results demonstrated that a depressive experience may cause long-term alterations in limbic circuit excitability and thus lead to long-lasting changes in pain perception.

Adult-age inflammatory pain experience enhances long-term pain vigilance in rats

BACKGROUND

Previous animal studies have illustrated a modulatory effect of neonatal pain experience on subsequent pain-related behaviors. However, the relationship between chronic pain status in adulthood and future pain perception remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

In the current study, we investigated the effects of inflammatory pain experience on subsequent formalin-evoked pain behaviors and fear conditioning induced by noxious stimulation in adult rats. Our results demonstrated an increase of the second but not the first phase of formalin-induced pain behaviors in animals with a history of inflammatory pain that have recovered. Similarly, rats with persistent pain experience displayed facilitated acquisition and prolonged retention of pain-related conditioning. These effects of prior pain experience on subsequent behavior were prevented by repeated morphine administration at an early stage of inflammatory pain.

CONCLUSIONS/SIGNIFICANCE

These results suggest that chronic pain diseases, if not properly and promptly treated, may have a long-lasting impact on processing and perception of environmental threats. This may increase the susceptibility of patients to subsequent pain-related disorders, even when chronic pain develops in adulthood. These data highlight the importance of treatment of chronic pain at an early stage.

Roles of the anterior cingulate cortex and medial thalamus in short-term and long-term aversive information processing

Background

The anterior cingulate cortex (ACC) and medial thalamus (MT) are two of the main components of the medial pain pathway that subserve the affective aspect of pain. The hypothesis of the present study was that the ACC is involved in short-term aversive information processing and that the MT is critical for encoding unconditioned nociceptive information. The roles of these two components in short-term and long-term aversive information processing was investigated using a step-through inhibitory avoidance task.

Results

Behavioral training began 1 week after surgery, in which radiofrequency lesions of the ACC or MT were performed. The retention tests were conducted 30 s (short-term) or 24 h (long-term) after training. Pretraining radiofrequency lesions of the ACC impaired performance in the 30 s, but not 24 h, retention test. Microinfusions of lidocaine into the ACC immediately after training impaired performance in the retention test conducted 10 min later. Pretraining radiofrequency lesions of the MT impaired performance in both the 30 s and 24 h retention tests. However, posttraining, but not pretest, microinfusions of lidocaine into the MT impaired performance in the 24 h retention test.

Conclusions

These results suggest that the ACC may play an important role in short-term, but not long-term, nociceptive information processing. In contrast, the MT may be important for the consolidation of nociceptive information storage.

Chronic stress induces transient spinal neuroinflammation, triggering sensory hypersensitivity and long-lasting anxiety-induced hyperalgesia

Chronic stressful events induce biochemical, physiological and psychological changes, resulting in stress-related neuropsychiatric disorders, such as anxiety or depression. Using repeated social defeat as a stressful event model, we show that this preclinical paradigm induces a transient increase in the expression of the genes encoding the pro-inflammatory molecules iNOS and COX-2. We provide the first demonstration that chronic stress affects spinal plasticity through a mechanism involving local neuroinflammation. The functional consequences of such neuroinflammation are associated with a transient decrease in the mechanical nociceptive threshold. Administration of the cholecystokinin(CCK)-2 receptor antagonist, CI-988, directly into the Rostral Ventromedial Medulla reverses the chronic stress-induced decrease in the nociceptive threshold. These data strongly suggest that chronic stress induces a spinal neuroinflammation associated with transient sensory hypersensitivity involving the activation of CCK-dependent nociceptive descending facilitatory pathways. Pharmacological data show that chronic social stress-induced long-lasting state of anxiety is not responsible for maintaining the spinal neuroinflammation and, therefore, for the associated sensory hypersensitivity. Conversely, an evaluation of pain-related behavior in the formalin model indicates that anxiety is directly related to prolonged hyperalgesia prevented by systemic benzodiazepine or CCK-2 receptor antagonist treatments. The present study highlights the adverse effects of chronic stress on spinal neuroinflammation triggering sensory hypersensitivity. Exploration of this phenomenon points out the divergence between pain sensitivity and anxiety-induced hyperalgesia, which is in agreement with clinical observations. Altogether, these data open up new perspectives for clinical research devoted to the evaluation and treatment of pain in anxio-depressive patients.

Cholinergic mechanism involved in the nociceptive modulation of dentate gyrus

Acetylcholine (ACh) causes a wide variety of anti-nociceptive effects. The dentate gyrus (DG) region of the hippocampal formation (HF) has been demonstrated to be involved in nociceptive perception. However, the mechanisms underlying this anti-nociceptive role have not yet been elucidated in the cholinergic pain-related neurons of DG. The electrical activities of pain-related neurons of DG were recorded by a glass microelectrode. Two kinds of pain-related neurons were found: pain-excited neurons (PEN) and pain-inhibited neurons (PIN). The experimental protocol involved intra-DG administration of muscarinic cholinergic receptor (mAChR) agonist or antagonist. Intra-DG microinjection of 1 microl of ACh (0.2 microg/microl) or 1 microl of pilocarpine (0.4 microg/microl) decreased the discharge frequency of PEN and prolonged firing latency, but increased the discharge frequency of PIN and shortened PIN inhibitory duration (ID). Intra-DG administration of 1 microl of atropine (1.0 microg/microl) showed exactly the opposite effects. According to the above experimental results, we can presume that cholinergic pain-related neurons in DG are involved in the modulation of the nociceptive response by affecting the discharge of PEN and PIN.

Differential involvement of the anterior cingulate and primary sensorimotor cortices in sensory and affective functions of pain

The present study examined the role of neurons in different pain-related functions of the anterior cingulate cortex (ACC) and primary sensorimotor cortex (SmI) by assessing their abilities to code different levels of noxious heat and activity changes evoked by classical fear conditioning involving electric shocks. Multiple single-unit activity was recorded with microwires implanted in the SmI and ACC of each rat. In the first set of experiments, the middle segment of the tail in each rat was irradiated with laser-heat pulses of various intensities. Neuronal responses in both the SmI and ACC increased with the intensity of the laser heat, although there was a significantly higher percentage of intensity-related units in the SmI. Furthermore, the stimulus-response curve of SmI ensemble activity had a steeper slope than that of the ACC. In the second set of experiments, rats were trained and tested on a conditioned fear-potentiated startle task in which a light was paired with an electric shock and, later, the startle response was elicited by a burst of noise in the presence or absence of light. A higher percentage of ACC units changed their neuronal responses to the conditioned stimulus after the light-shock pairing and the average activity change was also significantly stronger. Our results suggest that SmI neurons are better at coding laser-heat intensity than ACC neurons, whereas more ACC neurons are involved in conditioned fear associated with an electric shock than SmI neurons. These data provide evidence for differential contributions of the SmI and ACC to sensory and affective dimensions of pain.

Distinct roles of the anterior cingulate cortex in spinal and supraspinal bee venom-induced pain behaviors

A wide variety of human and animal experiments suggest that the anterior cingulate cortex (ACC) is one of the key brain substrates subserving higher order processing of noxious information. However, no sufficient data are now available regarding the mediation by ACC of different levels of pain processing as well as its potential descending modulation of spinal nociception. Using the well-developed rat bee venom (BV) model, the present study evaluated the effect of lesions of bilateral ACC on two levels of spontaneous nociceptive behaviors (spinally-processed persistent paw flinching reflex and supraspinally-processed paw lifting/licking) and heat or mechanical hypersensitivity under the inflammatory pain state. In contrast to the sham lesion group (saline microinjection into the ACC), bilateral complete ACC chemical lesions (kainic acid microinjection into the ACC) significantly decreased the BV-induced paw lifting and licking behavior (less time spent by the animal in paw lifting/licking) but produced no influence upon spinally-processed spontaneous paw flinching reflex (no change in number of paw flinches following subcutaneous BV injection). Moreover, the bilateral ACC lesions relieved the BV-evoked primary thermal or mechanical hypersensitivity compared with the sham control group. However, incomplete lesions of bilateral ACC failed to affect the abovementioned pain-related behaviors. No effects were seen on basal pain sensitivity in either group of rats. Motor coordination, as measured by Rota-Rod treadmill test, was not impaired by bilateral ACC lesions. These results implicate that the ACC area of the brain plays differential roles in the mediation of different levels of spontaneous pain-related behaviors. The present study also provides additional evidence for the ACC-mediated descending facilitation of primary hyperalgesia (pain hypersensitivity) identified in the injured area under inflammatory pain state.

Decreased pain response in mice following cortex-specific knockout of the N-methyl-D-aspartate NR1 subunit

Studies have shown that N-methyl-D-aspartate (NMDA) receptors play a critical role in pain processing at different levels of the central nervous system. In this study, we used cortex-specific NR1 knockout mice (C57BL/6 strain) to elucidate the role of cortical NMDA receptors in pain processes. On post-natal day 20, paw withdrawal latency (PWL) to a noxious thermal stimulus was measured in male and female knockout (KO), control (Ctrl), and C57BL/6 (C57) mice. Twenty-four hours later, the same mice were tested in the formalin-pain assay (20 microl of 5% formalin injected into one hind-paw). The results show that KO mice (both male and female) have significantly reduced pain responses during both early and late phases of formalin test, as compared with Ctrl and C57 mice (p<0.01). By contrast, no differences among groups were found in PWL to a noxious thermal stimulus. Taken together, these results demonstrate dissociation in the role of cortical NMDA receptors in mediating different types of pain.

GABAA but not GABAB receptors in the rostral anterior cingulate cortex selectively modulate pain-induced escape/avoidance behavior

The rostral anterior cingulate cortex (rACC) is involved in supraspinal nociceptive processing. ACC lesions relieve persistent pain, but do not affect the patient's ability to localize a noxious stimulus. Since the rACC has a high density of GABA receptors, it is possible that pain processing is influenced by these receptors in the rACC. The present experiments examined the involvement of rat rACC GABA(A) and GABA(B) receptors in regard to sensitivity to mechanical stimulation and escape/avoidance behavior in response to a noxious stimulus following L5 spinal nerve ligation. Rats were or were not afflicted with a neuropathic pain condition by an L5 spinal nerve ligation. rACC microinjection of 10 microg/microl GABA, a GABA(A) agonist (0.001 microg/microl, 0.1 microg/microl, or 0.5 microg/microl muscimol), a GABA(B) agonist (0.1 microg/microl, 1 microg/microl, or 5 microg/microl baclofen), or saline, did not alter mechanical withdrawal thresholds. Importantly, following 10 microg/microl GABA, 0.1 microg/microl, or 0.5 microg/microl muscimol microinjected into the rACC, place escape/avoidance behavior to a noxious mechanical stimulus was attenuated in injured animals. The attenuation was specific to the rACC and was blocked by a preadministered microinjection of the appropriate antagonist(s) into the rACC. In conclusion, microinjection of GABA and higher doses of muscimol did not decrease mechanical hyperalgesia but did attenuate place escape/avoidance behavior that is associated with mechanical stimulation of the ligated paw. These results provide additional support for the role of the rACC in higher order supraspinal processing of noxious events and suggest that rACC GABA(A) receptors significantly contribute to this processing.

Repetitive transcranial magnetic stimulation of dorsolateral prefrontal cortex increases tolerance to human experimental pain

Dorsolateral prefrontal cortex (DLPFCx) has been implicated in pain perception and in a pain modulation pathway. However, the precise participation of this region is not completely understood. The aim of this study was to evaluate whether 1 Hz rTMS of DLPFCx modifies threshold and tolerance in experimental pain. The effect of 1 Hz rTMS during 15 min at 100% motor threshold was tested in one hundred and eighty right-handed healthy volunteers, using a parallel-group stimulation design. The stimulation sites were right or left DLPFCx, right or left motor cortex, vertex or sham. rTMS was applied in two experimental contexts: (1) To evaluate its transitory effect (interference or facilitation) during cold pressor threshold (CPTh) and tolerance (CPTt) and (2) to evaluate its long-term effect by stimulating before CPTh, CPTt, pain heat thermal threshold, pain pressure threshold and tolerance. During rTMS of right DLPFCx, an increase in left hand CPTt (mean +/- SD; 17.63 s +/- 5.58 to 30.94 s +/- 14.84, P < 0.001) and in right hand CPTt (18.65 s +/- 6.47 to 26.74 s +/- 11.85, P < 0.001) were shown. No other stimulation site modified any of the pain measures during or after rTMS. These results show that 1 Hz rTMS of right DLPFCx has a selective effect by increasing pain tolerance and also sustains a right hemisphere preference in pain processing.

Selective regulation of pain affect following activation of the opioid anterior cingulate cortex system

Morphine and surgical cingulotomy, or transection of the anterior cingulate cortex (ACC), provides relief of chronic pain by selectively decreasing the affective dimension of the condition without altering sensory processing. Clinical reports suggest that morphine might be acting at the level of the ACC to alter the complex experience of pain. Therefore, the purpose of this experiment was to directly examine the functional role of the ACC in processing the aversive nature of pain induced by ligation of the L5 spinal nerve. Systemic administration of low dose morphine produced a selective attenuation of pain affect, as indicated by a decrease in the aversiveness of noxious cutaneous stimulation in nerve-damaged animals, with no alteration of mechanical paw withdrawal threshold. Supraspinally, microinjection of morphine into the ACC produced a selective naloxone reversible reduction in pain affect, as indicated by a decrease in the aversiveness of noxious cutaneous stimulation in nerve-damaged animals, with no alteration of response to mechanical stimulation. These data demonstrate the central role of the ACC opioid system in selectively processing the aversive quality of noxious mechanical stimulation in animals with a persistent pain condition.

Contributions of the anterior cingulate cortex and amygdala to pain- and fear-conditioned place avoidance in rats

The pain experience includes a sensory-discriminative and an affective-emotional component. The sensory component of pain has been extensively studied, while data about the negative affective component of pain are quite limited. The anterior cingulate cortex (ACC), and amygdala are thought to be key neural substrates underlying emotional responses. Using formalin-induced conditioned place avoidance (F-CPA) and electric foot-shock conditioned place avoidance (S-CPA) models, the present study observed the effects of bilateral excitotoxic (quinolinic acid 200 nmol/microl) lesions of the ACC and amygdala on pain and fear induced negative emotion, as well as on sensory component of pain. In the place-conditioning paradigm, both intraplantar (i.pl.) injection of formalin and electric foot-shock produced conditioned place avoidance. Excitotoxin-induced lesion of either the ACC or amygdala significantly reduced the magnitude of F-CPA. However, the decrease in the magnitude of S-CPA occurred only in the amygdala, but not ACC lesioned animals. Neither ACC nor amygdala lesion significantly changed formalin-induced acute nociceptive behaviors. These results suggest that the amygdala is involved in both pain- and fear-related negative emotion, and the ACC might play a critical role in the expression of pain-related negative emotion.

Kortikale Repr�sentation von Schmerz

Contrary to the traditional view that the cerebral cortex is not involved in pain perception an extensive cortical network associated with pain processing has been revealed during the past decades. This network consistently includes the primary (S1) and secondary somatosensory cortices (S2), the insular cortex, and the anterior cingulate cortex (ACC). These cortical areas are organized in parallel and contribute to different dimensions of pain experience. The S1 cortex is mainly involved in discriminative aspects of pain, while the S2 cortex seems to have an important role in cognitive aspects of pain perception. The insula has been proposed to be involved in autonomic reactions to noxious stimuli and in pain-related learning and memory. The ACC is closely related to pain affect and may subserve the integration of general affect, cognition, and response selection. Furthermore, first pain appears to be particularly related to activation of S1 whereas second pain is closely related to ACC activation.

Differential effect of anterior cingulate cortex lesion on mechanical hypersensitivity and escape/avoidance behavior in an animal model of neuropathic pain

Various limbic system structures have been implicated in processing noxious information. One such structure is the anterior cingulate cortex (ACC), a region that is thought to modulate higher order processing of noxious input related to the affective/motivational component of pain. The present experiment examined the involvement of the ACC in higher order pain processing by measuring paw withdrawal threshold and escape/avoidance responses in the L5 spinal nerve ligation model of neuropathic pain before and following electrolytic lesion of the ACC. In the place/escape avoidance paradigm, the afflicted paw is mechanically stimulated when the animal is in the preferred dark area of the chamber and the contralateral paw is stimulated when the animal is in the light area. Escape/avoidance was defined as a shift from the preferred dark area to an increase of time spent in the light area of the chamber. Animals with L5 ligation had significantly lower mechanical paw withdrawal threshold (hypersensitivity) and enhanced escape/avoidance behavior. ACC lesion in animals with L5 ligation did not alter mechanical hypersensitivity, but did significantly decrease escape/avoidance behavior. Anxiety, as measured using the light-enhanced startle paradigm, was not altered by ACC lesion. These results highlight the utility of novel behavioral test paradigms and provide additional support for the role of the ACC in higher order processing of noxious information, suggesting that ACC lesions selectively decrease negative affect associated with neuropathy-induced hypersensitivity.

Glutamatergic activation of anterior cingulate cortex produces an aversive teaching signal

Noxious stimuli have motivational power and can support associative learning, but the neural circuitry mediating such avoidance learning is poorly understood. The anterior cingulate cortex (ACC) is implicated in the affective response to noxious stimuli and the motivational properties of conditioned stimuli that predict noxious stimulation. Using conditioned place aversion (CPA) in rats, we found that excitatory amino acid microinjection into the ACC during conditioning produces avoidance learning in the absence of a peripheral noxious stimulus. Furthermore, microinjection of an excitatory amino acid antagonist into the ACC during conditioning blocked learning elicited by a noxious stimulus. ACC lesions made after conditioning did not impair expression of CPA. Thus, ACC neuronal activity is necessary and sufficient for noxious stimuli to produce an aversive teaching signal. Our results support the idea that a shared ACC pathway mediates both pain-induced negative affect and a nociceptor-driven aversive teaching signal.

Cortical responses to noxious stimuli during sleep

We used magnetoencephalography to study effects of sleep on cortical responses to noxious stimuli and to clarify the mechanisms underlying pain perception. For a noxious stimulus, painful intra-epidermal electrical stimulation, which selectively activates A-delta fibers, was applied to the dorsum of the left hand. While awake, subjects were asked to count the number of stimuli silently (Attention) or ignore the stimuli (Control). During sleep, magnetic fields recorded in stage 1 sleep and stage 2 sleep were analyzed. One main component at a latency around 140-160 ms was identified in the awake condition. Multiple source analysis indicated that this main component was generated by activities in the contralateral primary somatosensory cortex (SI), bilateral secondary somatosensory cortex (SII) and insular cortex. The medial temporal area (MT) and cingulate cortex were activated later than the main component. Cortical responses in the contralateral SI, ipsilateral SII and MT, bilateral insula and cingulate cortex were significantly enhanced in Attention as compared with Control. The main component 1 M as well as later magnetic fields were markedly attenuated during sleep, suggesting that all these cortical areas are involved in pain cognition.

Imaging of central itch modulation in the human brain using positron emission tomography

The unpleasantness of itching is reduced by cooling. Although previous research suggests the presence of a central itch modulation system, there is little documentation about the modulation system in the brain. In the present study, we investigated the modulating system of the itching sensation in human brains using positron emission tomography and H(2) (15)O. The significant increases of regional cerebral blood flow caused by histamine stimuli using iontophoresis were observed in the anterior cingulate cortex (BA24), the thalamus, the parietal cortex (BA40 and BA7), the dorsolateral prefrontal cortex (BA46) and the premotor cortex (BA6). We did not observe any changes in the secondary somatosensory cortex (S2) during the itching stimulus, corresponding to the previous imaging studies concerning itching. Activation in these areas related to itching stimuli was decreased by a simultaneous stimulation of itching and cold pain (the dual stimuli), as compared to itching alone. Interestingly, the midbrain, including periaqueductal gray matter (PAG), was only activated during the dual stimuli. PAG is well known to be a modulating noxious stimulus. Here we hypothesize that the activation of PAG may also be related to the itch modulation. These findings indicate that the modified brain activities in the PAG, the cingulate, the frontal and the parietal cortex might be associated with the itch modulation in the central nervous system and that the S2 might not be primarily involved in processing the itching perception in the brain since the activity of S2 was not observed in any concentration of itching stimuli.

Persistent pain and depression: a biopsychosocial perspective

This review highlights recent research findings on the relationship between persistent pain and depression and discusses the implications of these findings for future research in persons who suffer from both pain and depression. First, we briefly discuss advances in theories of pain that underscore the important role that depression can play in the chronic pain experience. Second, we discuss depression in persons suffering from chronic pain from a biopsychosocial perspective that takes into account both biological and psychosocial mechanisms linking pain and depression. Third, we address biomedical, psychosocial, and combined medical-psychosocial approaches to treatment in persons with persistent pain and depression. We conclude by highlighting future directions for research related to screening and diagnosis of depression in persons having persistent pain, treatment of comorbid pain and depression, and individual and subgroup differences in the experience of persistent pain and depression.

Effects of naloxone on rewarding and aversive brain sites

The effects of naloxone on the rewarding and aversive properties of brain stimulation derived from the ventral tegmental area and the nucleus reticularis gigantocellularis, respectively, were assessed in rats, based on the following measures-the current threshold for latency to escape aversive nucleus reticularis gigantocellularis stimulation, the frequency threshold for rewarding ventral tegmental area stimulation, and the frequency threshold for self-stimulation obtained from delivery of concurrent ventral tegmental area and nucleus reticularis gigantocellularis stimulation, before and after three systemic doses of naloxone (0, 10, and 20mg/kg); in the latter case, the stimulation trains were interdigitated with an interpulse interval of 2 ms. Initially, thresholds for concurrent stimulation were elevated relative to the values obtained for ventral tegmental area stimulation alone, returning to baseline values only when the nucleus reticularis gigantocellularis stimulation no longer induced escape. After each pairing of the two sites, the current threshold for escape gradually increased until the maximum value administered, 700 microA, at which point aversive responses were no longer observed. This required very few pairings, between one and five trials across animals. Drug tests were then begun and produced a significant dose-response threshold increase across animals, without reinstating the latency to escape nucleus reticularis gigantocellularis stimulation. These findings are discussed in terms of a dissociation between the analgesic and rewarding properties of ventral tegmental area stimulation.

Contribution of the anterior cingulate cortex to laser-pain conditioning in rats

The emotional component of nociception is seldom distinguished from pain behavioral testing. The aim of the present study was to develop a behavioral test that indicates the emotional pain responses using the classical conditioning paradigm. The role of the anterior cingulate cortex (ACC) in the process of this pain conditioning response was also evaluated. In laser-pain conditioning, free moving rats were trained to associate a tone (conditioned stimulus, CS) and short CO(2) laser pulsation (unconditioned stimulus, US). Monotonous tone (800 Hz, 0.6 s) was delivered through a loud-speaker as CS. CO(2) laser pulses (5 W at 50 or 100 ms in duration) applied to the hind paw was adopted as US. The CS-US interval was 0.5 s. Laser-pain conditioning was developed during 40 CS-US pairings. CS and US pairing with 100-ms laser pulse stimuli was more effective in establishing conditioning responses than that of 50-ms stimuli. The conditioning responses remained, tested by presenting CS alone, immediate to and 24 h subsequent to training. The performance of laser-pain conditioning was significantly reduced after bilateral lesioning of the ACC. Similar results were also obtained by bilateral lesions of the amygdala. The conditioning responses were also diminished following morphine treatment. The association between a neutral stimulus and a noxious stimulus could be demonstrated in a Pavlovian conditioning test in free moving rats. Thus, the conditioned response may be employed as a measure of the emotional component of the nociception. It is also suggested that the ACC may play an important role in mediating this conditioning effect.

Potentiation of local field potentials in the anterior cingulate cortex evoked by the stimulation of the medial thalamic nuclei in rats

The limbic thalamus and cingulate cortex are essential components in mediating the affective component of pain responses. In the present study, we examined the excitatory properties of medial thalamus (MT)-evoked field potentials in the anterior cingulated cortex (ACC). We also examined the effects of paired pulses and brief tetanic stimuli of the MT. The aim of this study was to determine whether nociceptive inputs to medial thalamic afferents cause plastic changes in the ACC. In alpha-chloralose (50 mg/kg, i.v.) anaesthetized rats, tungsten microelectrodes were used to stimulate the MT and to record field potentials in the ACC. The locations of MT were identified by searching and examining their responses to peripheral noxious stimuli. Early negative (about 4.7 ms latency) and late positive (about 11.7 ms) potentials could be evoked in the ACC by MT stimuli. The evoked field potentials were potentiated by prepulse stimulation. Maximal paired pulse facilitation (509+/-51%) was produced in 80-150 ms interpulse intervals. Evoked field potentials were also potentiated (28.8+/-6.3% and 29.6+/-5.9%, respectively) by low (10 Hz/10 s) and high (100 Hz/2s/2x) frequency tetanic stimulation of the MT, with a duration maintained for about 90 s and 120 min, respectively. The potentiation of MT-evoked ACC potentials provides a neural basis for synaptic plasticity, which may be essential for the establishment of pain-initiated conditioning behavior and affective responses to noxious stimuli.

Sensory and affective components of orofacial pain: is it all in your brain?

In this paper, we shall review several chronic orofacial pain conditions with emphasis on those that are essentially refractory to treatment. We shall present a review of current and past literature that describes the various pain phenomena as well as their underlying central mechanisms. New data concerning refractory pain will be used to underscore the importance of central processing of pain, with particular emphasis on neuropsychological and cognitive function and capacity that may play important roles in pain processing and maintenance of the pain state. Further, neurophysiological data showing that the anterior cingulate cortex (ACC) and other structures in the brain may play key roles in modulation of chronic pain will also be discussed. Although peripheral triggering events surely play an important role in initiating pain, the development of chronic and, in particular, refractory pain may depend on changes or malfunctions in the central nervous system. These changes may be quite subtle and require sophisticated approaches, such as functional MRI, to study them, as is now being done. New findings obtained therefore may lead to more rational and reliable treatment for orofacial pain.

Cortical mapping of visceral pain in patients with GI disorders using functional magnetic resonance imaging

OBJECTIVE

We sought to identify central loci that activate in response to visceral stimuli (stool and pain). We had a particular interest in observing the anterior cingulate gyrus and frontal cortex in normals and in patients with intestinal disease, including inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS).

METHODS

Subjects underwent rectal balloon distention to a sensation of stool and to a sensation of pain while undergoing blood oxygenation level-dependent functional magnetic resonance imaging. Experiments were conducted in a Magnex 3.0-T whole body magnet with a Bruker Biospec console and a quadrature head coil. Four contiguous 5.0-mm oblique axial slices designed to optimize coverage of areas believed to be responsive to noxious stimulation were acquired. Activations were detected by using cross-correlation maps (p < 0.001) for individual subjects. The experimental groups were compared using both an analysis of variance and profile analysis.

RESULTS

A significantly higher percentage of pixels activated in the anterior cingulate gyrus over both pain and stool conditions for the control group than for the IBS group and for the IBS group than for the IBD group (p < 0.035). Deactivation of left somatosensory cortex was greater for the IBS group than for the IBD group and greater for the IBD group than for the controls (p < 0.0065) in the boxcar condition. Frontal deactivation in controls compared with disease groups bordered on statistical significance. Profile analysis of the three groups across six regions of interest revealed that the control and IBD groups were distinguished by different profiles of response (p < 0.005). Nonparametric evaluation of the data suggests that, among the pixels in the anterior cingulate activating to pain, there are two patterns of response to pain-on/off and graded. This was true for both controls and disease groups.

CONCLUSIONS

Normal controls and subjects with IBD and IBS share similar loci of activations to visceral sensations of stool and pain. Both activation and deactivation of particular regions of interest differentiate the three groups, as do profiles of patterned response across six of the regions of interest for the control and IBD groups.

Lesion and electrical stimulation of the ventral tegmental area modify persistent nociceptive behavior in the rat

The ventral tegmental area (VTA) has been traditionally related with the control of motor responses. However, some studies show that this area is also involved in the processing of nociceptive information. It has been reported that this nucleus participates in the dissociative analgesia phenomenon. In the few works where electrical stimulation and lesion of the VTA have been performed, evaluated with persistent or chronic pain related behaviors, contradictory results have been obtained. Thus, a more detailed analysis of the role of the VTA in persistent pain is needed. Two series of experiments were performed: lesions of this nucleus were done with radiofrequency, (bilaterally at two points per side using a temperature range from 50 to 80 degrees C), and the VTA was electrically stimulated (10 min daily over 5 days, 2 ms rectangular pulses at 100 Hz during 1 s every 5 s) using two different schemes:10 min before the induction of the nociceptive stimulus and 90 min after the induction of the nociceptive stimulus. The latter allowed us to distinguish if the VTA electrical stimulation had a distinctive antinociceptive effect when applied before or after the induction of the nociceptive stimulus on a persistent pain related behavioral response in the rat, the self injury behavior (SIB). Our results showed that VTA lesions enhanced the occurrence of SIB; while activation of this same nucleus by electrical stimulation after the nociceptive stimulus, but not before, facilitates the analgesic process, expressed as a 1 day delay in SIB onset. These results indicate that the VTA is a brain structure that plays a key role in the processing and modulation of persistent pain information. Data are discussed in terms of the relationship of the VTA with the affective component of pain.

Neurophysiology and functional neuroanatomy of pain perception

The traditional view that the cerebral cortex is not involved in pain processing has been abandoned during the past decades based on anatomic and physiologic investigations in animals, and lesion, functional neuroimaging, and neurophysiologic studies in humans. These studies have revealed an extensive central network associated with nociception that consistently includes the thalamus, the primary (SI) and secondary (SII) somatosensory cortices, the insula, and the anterior cingulate cortex (ACC). Anatomic and electrophysiologic data show that these cortical regions receive direct nociceptive thalamic input. From the results of human studies there is growing evidence that these different cortical structures contribute to different dimensions of pain experience. The SI cortex appears to be mainly involved in sensory-discriminative aspects of pain. The SII cortex seems to have an important role in recognition, learning, and memory of painful events. The insula has been proposed to be involved in autonomic reactions to noxious stimuli and in affective aspects of pain-related learning and memory. The ACC is closely related to pain unpleasantness and may subserve the integration of general affect, cognition, and response selection. The authors review the evidence on which the proposed relationship between cortical areas, pain-related neural activations, and components of pain perception is based.

Functional imaging of brain responses to pain. A review and meta-analysis (2000)

Brain responses to pain, assessed through positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) are reviewed. Functional activation of brain regions are thought to be reflected by increases in the regional cerebral blood flow (rCBF) in PET studies, and in the blood oxygen level dependent (BOLD) signal in fMRI. rCBF increases to noxious stimuli are almost constantly observed in second somatic (SII) and insular regions, and in the anterior cingulate cortex (ACC), and with slightly less consistency in the contralateral thalamus and the primary somatic area (SI). Activation of the lateral thalamus, SI, SII and insula are thought to be related to the sensory-discriminative aspects of pain processing. SI is activated in roughly half of the studies, and the probability of obtaining SI activation appears related to the total amount of body surface stimulated (spatial summation) and probably also by temporal summation and attention to the stimulus. In a number of studies, the thalamic response was bilateral, probably reflecting generalised arousal in reaction to pain. ACC does not seem to be involved in coding stimulus intensity or location but appears to participate in both the affective and attentional concomitants of pain sensation, as well as in response selection. ACC subdivisions activated by painful stimuli partially overlap those activated in orienting and target detection tasks, but are distinct from those activated in tests involving sustained attention (Stroop, etc.). In addition to ACC, increased blood flow in the posterior parietal and prefrontal cortices is thought to reflect attentional and memory networks activated by noxious stimulation. Less noted but frequent activation concerns motor-related areas such as the striatum, cerebellum and supplementary motor area, as well as regions involved in pain control such as the periaqueductal grey. In patients, chronic spontaneous pain is associated with decreased resting rCBF in contralateral thalamus, which may be reverted by analgesic procedures. Abnormal pain evoked by innocuous stimuli (allodynia) has been associated with amplification of the thalamic, insular and SII responses, concomitant to a paradoxical CBF decrease in ACC. It is argued that imaging studies of allodynia should be encouraged in order to understand central reorganisations leading to abnormal cortical pain processing. A number of brain areas activated by acute pain, particularly the thalamus and anterior cingulate, also show increases in rCBF during analgesic procedures. Taken together, these data suggest that hemodynamic responses to pain reflect simultaneously the sensory, cognitive and affective dimensions of pain, and that the same structure may both respond to pain and participate in pain control. The precise biochemical nature of these mechanisms remains to be investigated.

An fMRI study of the anterior cingulate cortex and surrounding medial wall activations evoked by noxious cutaneous heat and cold stimuli

The anterior cingulate cortex (ACC) and adjacent regions in the medial wall have been implicated in sensory, motor and cognitive processes, including pain. Our previous functional magnetic resonance imaging (fMRI) studies have demonstrated pain-related activation of the posterior portion of the ACC during transcutaneous electrical nerve stimulation (TENS) and variable patterns of cortical activation with innocuous and noxious thermal stimuli in individual subjects. The present study represents the companion paper to our recent study of pain- and thermal-related cortical activations with the aim to use fMRI to delineate the activations in the ACC and surrounding regions of the medial wall during application of innocuous and noxious thermal stimuli as well as during performance of a motor task in individual subjects. Ten normal subjects were imaged on a conventional 1.5 T GE 'echospeed' system. Functional images were obtained from sagittal sections through each hemisphere centered at approximately 3-5 and 7-9 mm from midline. Each subject was imaged during innocuous (cool, warm) and noxious thermal (cold, hot) stimulation of the thenar eminence, and execution of a motor (sequential finger-thumb opposition) task. Task-related activations were mostly confined to contralateral and medial ipsilateral images. Although the present results demonstrate intersubject variability in the task-related activations, some general modality-specific patterns were apparent: (i) innocuous thermal-related activations were located mainly in the anterior ACC; (ii) noxious thermal-related activations were primarily located in the anterior ACC, the ventral portion of the posterior ACC, and the supplementary motor area (SMA); (iii) motor-related activations were primarily located in the SMA and dorsal portion of the posterior ACC. These results indicate that specific spatial patterns of activation exist within the ACC and surrounding regions of the medial wall for innocuous and noxious thermal stimuli, and that noxious thermal- and motor-related activations appear to be segregated within the ACC. Therefore, we propose a segregation of the ACC into an anterior non-specific attention/arousal system and a posterior pain system.

Modulation of memory processing in the cingulate cortex of mice

To evaluate the possible role of the cingulate cortex in memory processing for training using a noxious stimulus, we trained mice on foot shock avoidance in a T-maze. Cholinergic, GABAergic, serotonergic, and glutamatergic agonists and antagonists were administered into the cingulate cortex immediately after training. Retention for the foot shock avoidance training was tested 1 week later. The results indicate that muscarinic and nicotinic agonists improved retention, while antagonists impaired it. GABA and serotonin agonists impaired retention, while antagonists improved it. Drugs acting on GABA(A) and GABA(B) receptors had similar effects on retention, as did drugs acting on serotonin 1 and 2 receptor subtypes. Glutamate improved retention, and AP5, an antagonist of the excitatory amino acid site of the NMDA receptor, impaired retention. The cingulate cortex, like other parts of the limbic system, is involved in memory processing that occurs shortly after training.

Activation of the anterior cingulate cortex by thalamic stimulation in patients with chronic pain: a positron emission tomography study

Object

Deep brain stimulation (DBS) of the sensory thalamus has been used to treat chronic, intractable pain. The goal of this study was to investigate the thalamocortical pathways activated during thalamic DBS.

Methods

The authors compared positron emission tomography (PET) images obtained before, during, and after DBS in five patients with chronic pain. Two of the five patients reported significant DBS-induced pain relief during PET scanning, and the remaining three patients did not report any analgesic effect of DBS during scanning. The most robust effect associated with DBS was activation of the anterior cingulate cortex (ACC). An anterior ACC activation was sustained throughout the 40 minutes of DBS, whereas a more posteriorly located ACC activation occurred at a delay after onset of DBS, although these activations were not dependent on the degree of pain relief reported during DBS. However, implications specific to the analgesic effect of DBS require further study of a larger, more homogeneous patient population. Additional effects of thalamic DBS were detected in motor-related regions (the globus pallidus, cortical area 4, and the cerebellum) and visual and association cortical areas.

Conclusions

The authors demonstrate that the ACC is activated during thalamic DBS in patients with chronic pain.

Activation of the anterior cingulate cortex by thalamic stimulation in patients with chronic pain: a positron emission tomography study

OBJECT

Deep brain stimulation (DBS) of the sensory thalamus has been used to treat chronic, intractable pain. The goal of this study was to investigate the thalamocortical pathways activated during thalamic DBS.

METHODS

The authors compared positron emission tomography (PET) images obtained before, during, and after DBS in five patients with chronic pain. Two of the five patients reported significant DBS-induced pain relief during PET scanning, and the remaining three patients did not report any analgesic effect of DBS during scanning. The most robust effect associated with DBS was activation of the anterior cingulate cortex (ACC). An anterior ACC activation was sustained throughout the 40 minutes of DBS, whereas a more posteriorly located ACC activation occurred at a delay after onset of DBS, although these activations were not dependent on the degree of pain relief reported during DBS. However, implications specific to the analgesic effect of DBS require further study of a larger, more homogeneous patient population. Additional effects of thalamic DBS were detected in motor-related regions (the globus pallidus, cortical area 4, and the cerebellum) and visual and association cortical areas.

CONCLUSIONS

The authors demonstrate that the ACC is activated during thalamic DBS in patients with chronic pain.

Pain and functional imaging

Functional neuroimaging has fundamentally changed our knowledge about the cerebral representation of pain. For the first time it has been possible to delineate the functional anatomy of different aspects of pain in the medial and lateral pain systems in the brain. The rapid developments in imaging methods over the past years have led to a consensus in the description of the central pain responses between different studies and also to a definition of a central pain matrix with specialized subfunctions in man. In the near future we will see studies where a systems perspective allows for a better understanding of the regulatory mechanisms in the higher-order frontal and parietal cortices. Also, pending the development of experimental paradigms, the functional anatomy of the emotional aspects of pain will become better known.

The contribution of functional imaging techniques to our understanding of rheumatic pain

The main cerebral components of the human pain matrix have been defined using functional imaging techniques. The experience of pain is likely to be elaborated as a result of parallel processing within this matrix. There is not, therefore, a single pain center. The determinants of pain are as likely to be determined by top-down as by bottom-up processes. The precise function of the different components of the matrix are just beginning to be defined. There appear to be important adaptive responses in the forebrain components of the matrix during arthritic pain. Endogenous opioid peptides are strong candidates for the modulation of some of these responses. More extensive and sequential behavioral and functional imaging studies are required to establish the contribution these adaptive responses make to the perception of pain.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Outward current produced by somatostatin (SRIF) in rat anterior cingulate pyramidal cells in vitro

1. A high density of receptors for somatostatin (SRIF) exists in the anterior cingulate cortex but their function is unknown. Whole-cell patch clamp recordings were made from visualized deep layer pyramidal cells of the rat anterior cingulate cortex contained in isolated brain slices to investigate the putative effects of SRIF and to identify the receptor subtype(s) involved. 2. SRIF (1-1000 nM) produced a concentration-dependent outward current which was associated with an increased membrane conductance, was sensitive to Ba2+ (300 microM - 1 mM), and was absent in the presence of a maximal concentration of the GABA(B) receptor agonist, baclofen (100 microM). These observations suggest the outward current was carried by K+ ions. 3. SRIF analogues also elicited outward currents with a rank potency order of (EC50, nM): octreotide (1.8)>BIM-23027 (3.7)>SRIF (20)=L-362,855 (20). BIM-23056 was without agonist or antagonist activity. Responses to L-362,855 were unlike those to the other agonists since they were sustained for the duration of the application. 4. The sst2 receptor antagonist, L-Tyr8Cyanamid 154806 (1 microM), had no effect alone but partially reversed responses to submaximal concentrations of SRIF (100 nM, 44+/-6% reversal) and L-362,855 (100 nM, 70+/-6% reversal) and fully reversed the response to BIM-23027 (10 nM). In contrast, L-Tyr8Cyanamid 154806 did not antagonize the response to baclofen (10 microM). 5. We conclude that SRIF activates a K+ conductance in anterior cingulate pyramidal neurones via an action predominantly at sst2 receptors.