Subcortical structures involved in pain processing: evidence from single-trial fMRI

Structural and Functional Brain Patterns of Non-Motor Syndromes in Parkinson's Disease

Parkinson’s disease (PD) is a common, progressive and multisystem neurodegenerative disorder characterized by motor and non-motor symptoms. Advanced magnetic resonance imaging, positron emission tomography, and functional magnetic resonance imaging can render the view toward understanding the neural basis of these non-motor syndromes, as they help to understand the underlying pathophysiological abnormalities. This review provides an up-to-date description of structural and functional brain alterations in patients with PD with cognitive deficits, visual hallucinations, fatigue, impulsive behavior disorders, sleep disorders, and pain.

Ictal headache: Insights from two cases

Background

We report the first literature description of ictal epileptic headaches closely mimicking glossopharyngeal neuralgia and short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing.

Case 1

A 37-year-old man complained of short-lasting, electric-shock like headache, confined to the pharynx. During the episodes, he could not speak because he felt “words blocked at the throat”. An EEG recorded epileptic discharges concomitant with headache; a brain MRI disclosed frontal polymicrogyria.

Case 2

A 66-year-old man complained of short-lasting, right periocular headache, associated with ipsilateral ptosis, conjunctival injection and lacrimation. Some episodes were followed by tonic contraction of the right facial and limb muscles; on one occasion, headache was followed by a generalized seizure. A brain MRI revealed hippocampal abnormalities.

Discussion

These cases highlight the complex relationship between headache and epilepsy, and suggest a possible contribution of cortical structures to the genesis of paroxysmal headaches such as glossopharyngeal neuralgia and short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing.

Differential efficiency of transcutaneous electrical nerve stimulation in dominant versus nondominant hands in fibromyalgia: placebo-controlled functional near-infrared spectroscopy study

Using functional near-infrared spectroscopy (fNIRS), modulation of hemodynamic responses by transcutaneous electrical nerve stimulation (TENS) during delivery of nociceptive stimulation was investigated in fibromyalgia (FM) patients and healthy controls for both hands. Two experiments were conducted: (1) median nerve stimulation with TENS and (2) painful stimulation using electronic von Frey filaments with TENS/placebo TENS. Mean [Formula: see text] brain activity was compared across groups and conditions using factorial ANOVA. Dominant (right) hand stimulation indicated significant interactions between group and condition in both hemispheres. Post hoc results revealed that FM patients showed an increased activation in "pain + TENS" condition compared to the "pain + placebo TENS" condition while the brain activity patterns for these conditions in controls were reversed. Left-hand stimulation resulted in similar TENS effects (reduced activation for "pain + TENS" than "pain + placebo TENS") in both groups. TENS effects in FM patients might be manipulated by the stimulation side. While the nondominant hand was responsive to TENS treatment, the dominant hand was not. These results indicate that stimulation side might be an effective factor in FM treatment by using TENS. Future studies are needed to clarify the underlying mechanism for these findings.

The impact of fibromyalgia symptoms on brain morphometry

Fibromyalgia (FMS) is a complex clinical syndrome that includes many symptoms beyond chronic pain. The studies that have addressed brain morphometry in FMS have had very heterogeneous results. Thus, the question of which specific FMS symptoms and clinical features-pain, but also psychological distress, sleep-related problems, health status, and medication intake-impact on brain morphometry remains open. Here, we wanted to determine if brain changes in FMS are "symptom-related" more than "diagnostic-related". We performed an observational study of 46 premenopausal women (23 FMS patients and 23 age-matched healthy participants). Magnetic resonance images were analyzed using voxel-based morphometry and subcortical segmentation. We used multiple regression models to assess the associations between total and local brain volumes and FMS clinical characteristics. Furthermore, we calculated associations between subcortical structures' shapes and volumes and FMS clinical characteristics. Larger psychological distress, anxiety, and sleepiness, and higher analgesic consumption accounted for 38 % of FMS patients' smaller total gray matter volume (GMV). For both groups, local decrements of GMV in the medial orbitofrontal cortex were associated to larger psychological distress. Local increases of GMV were positively related to pain scores (superior frontal gyrus), psychological distress (cerebellum), anxiety (medial orbitofrontal cortex), and sleepiness (frontal superior medial cortex). FMS clinical characteristics were also associated to deformations in subcortical structures and volumes changes. This study reveals that total and local GMV changes in FMS go beyond the traditional "pain matrix" alterations. We demonstrated that brain morphology is altered by pain, but also by clinical characteristics that define the FMS experience.

Attenuation of pCREB and Egr1 expression in the insular and anterior cingulate cortices associated with enhancement of CFA-evoked mechanical hypersensitivity after repeated forced swim stress

The perception and response to pain are severely impacted by exposure to stressors. In some animal models, stress increases pain sensitivity, which is termed stress-induced hyperalgesia (SIH). The insular cortex (IC) and anterior cingulate cortex (ACC), which are typically activated by noxious stimuli, affect pain perception through the descending pain modulatory system. In the present study, we examined the expression of phospho-cAMP response element-binding protein (pCREB) and early growth response 1 (Egr1) in the IC and ACC at 3h (the acute phase of peripheral tissue inflammation) after complete Freund's adjuvant (CFA) injection in naïve rats and rats preconditioned with forced swim stress (FS) to clarify the effect of FS, a stressor, on cortical cell activities in the rats showing SIH induced by FS. The CFA injection into the hindpaw induced mechanical hypersensitivity and increased the expression of the pCREB and Egr1 in the IC and ACC at 3h after the injection. FS (day 1, 10min; days 2-3, 20min) prior to the CFA injection enhanced the CFA-induced mechanical hypersensitivity and attenuated the increase in the expression of pCREB and Egr1 in the IC and ACC. These findings suggested that FS modulates the CFA injection-induced neuroplasticity in the IC and ACC to enhance the mechanical hypersensitivity. These findings are thought to signify stressor-induced dysfunction of the descending pain modulatory system.

Nondermatomal somatosensory deficits in chronic pain are associated with cerebral grey matter changes

OBJECTIVES

Widespread sensory deficits occur in 20-40% of chronic pain patients on the side of pain, independent of pain aetiology, and are known as nondermatomal sensory deficits (NDSDs). NDSDs can occur in absence of central or peripheral nervous system lesions. We hypothesised that NDSDs were associated with cerebral grey matter changes in the sensory system and in pain processing regions, detectable with voxel-based morphometry.

METHODS

Twenty-five patients with NDSDs, 23 patients without NDSDs ("pain-only"), and 29 healthy controls were studied with high resolution structural MRI of the brain. A comprehensive clinical and psychiatric evaluation based on Diagnostic and Statistical Manual was performed in all patients.

RESULTS

Patients with NDSDs and "pain-only" did not differ concerning demographic data and psychiatric diagnoses, although anxiety scores (HADS-A) were higher in patients with NDSDs. In patients with NDSDs, grey matter increases were found in the right primary sensory cortex, thalamus, and bilaterally in lateral temporal regions and the hippocampus/fusiform gyrus. "Pain-only" patients showed a bilateral grey matter increase in the posterior insula and less pronounced changes in sensorimotor cortex.

CONCLUSIONS

Dysfunctional sensory processing in patients with NDSDs is associated with complex changes in grey matter volume, involving the somatosensory system and temporal regions.

Association of clinical headache features with stroke location: An MRI voxel-based symptom lesion mapping study

Background We have recently shown that the presence of headache in ischemic stroke is associated with lesions of the insular cortex. The aim of this post-hoc subgroup analysis was to investigate the association of specific headache features with stroke location in patients with acute ischemic stroke. Methods In this observational study, patients (mean age: 61.5, 58% males) with ischemic stroke and acute headache (n = 49) were investigated. Infarcts were manually outlined on 3D diffusion weighted magnetic resonance imaging (MRI) scans and transformed into standard stereotaxic space; lesions of the left hemisphere were mirrored in the x-axis to allow a voxel-wise group analysis of all patients. We analyzed the association of lesion location and the following phenotypical characteristics by voxel-based symptom lesion mapping: Headache intensity, different qualities of headache (pulsating, tension-type like and stabbing), and the presence of nausea, of cranial autonomic symptoms and of light or noise sensitivity. Results Headache intensity was associated with lesions of the posterior insula, the operculum and the cerebellum. "Pulsating" headache occurred with widespread cortical and subcortical strokes. The presence of "tension-like" and "stabbing" headache was not related to specific lesion patterns. Nausea was associated with lesions in the posterior circulation territory. Cranial-autonomic symptoms were related to lesions of the parietal lobe, the somatosensory cortex (SI) and the middle temporal cortex. The presence of noise sensitivity was associated with cerebellar lesions, whereas light sensitivity was not related to specific lesions in our sample. Conclusion Headache phenotype in ischemic stroke appears to be related to specific ischemic lesion patterns.

The Role of Stress Regulation on Neural Plasticity in Pain Chronification

Pain, especially chronic pain, is one of the most common clinical symptoms and has been considered as a worldwide healthcare problem. The transition from acute to chronic pain is accompanied by a chain of alterations in physiology, pathology, and psychology. Increasing clinical studies and complementary animal models have elucidated effects of stress regulation on the pain chronification via investigating activations of the hypothalamic-pituitary-adrenal (HPA) axis and changes in some crucial brain regions, including the amygdala, prefrontal cortex, and hippocampus. Although individuals suffer from acute pain benefit from such physiological alterations, chronic pain is commonly associated with maladaptive responses, like the HPA dysfunction and abnormal brain plasticity. However, the causal relationship among pain chronification, stress regulation, and brain alterations is rarely discussed. To call for more attention on this issue, we review recent findings obtained from clinical populations and animal models, propose an integrated stress model of pain chronification based on the existing models in perspectives of environmental influences and genetic predispositions, and discuss the significance of investigating the role of stress regulation on brain alteration in pain chronification for various clinical applications.

Altered functional connectivity of the marginal division in migraine: a resting-state fMRI study

BACKGROUND

The marginal division of neostriatum (MrD) is a flat, pan-shaped zone between the neostriatum and the globus pallidus, and previous documents demonstrated that it was involved in the modulation of pain. The aim of this study is to investigate the roles of the MrD of the human brain in the chronicization migraine using resting state functional magnetic resonance imaging (rs-fMRI).

METHODS

Conventional MRI, 3D structure images, and rs-fMRI were performed in 18 patients with episodic migraines (EM), 16 patients with chronic migraine (CM), 44 patients with medication overuse headache plus chronic migraine (MOH + CM), and 32 normal controls (NC). MrD was defined using manual delineation on structural images, and was selected as the seed to calculate the functional connectivity (FC).

RESULTS

Compared with the NC group, the decreased FC of MrD was observed in the EM and CM groups, and increased FC of MrD was demonstrated in all patient groups. Compared with the EM group, the decreased FC of MrD was revealed in the CM and MOH + CM groups, and the increased FC occurred only in the CM group. Increased FC of MrD alone was observed in the MOH + CM group compared with that in the CM group.

CONCLUSION

This study confirmed the double neuromodulation network of MrD in pain modulation and migraine chronicization; however, the mechanism requires further investigation.

Structural Imaging Changes and Behavioral Correlates in Patients with Crohn's Disease in Remission

Background: Crohn’s disease (CD) is a subtype of inflammatory bowel disease caused by immune-mediated inflammation in the gastrointestinal tract. The extent of morphologic brain alterations and their associated cognitive and affective impairments remain poorly characterized.

Aims: We used magnetic resonance imaging to identify structural brain differences between patients with Crohn’s disease in remission compared to age-matched healthy controls and evaluated for structural-behavioral correlates.

Methods: Nineteen patients and 20 healthy, age-matched controls were recruited in the study. Group differences in brain morphometric measures and correlations between brain measures and performance on a cognitive task, the verbal fluency (VF) task, were examined. Correlations between brain measures and cognitive measures as well as self-reported measures of depression, personality, and affective scales were examined.

Results: Patients showed significant cortical thickening in the left superior frontal region compared to controls. Significant group differences were observed in sub-cortical volume measures in both hemispheres. Investigation of brain-behavior correlations revealed significant group differences in the correlation between cortical surface area and VF performance, although behavioral performance was equivalent between the two groups. The left middle temporal surface area was a significant predictor of VF performance with controls showing a significant positive correlation between these measures, and patients showing the opposite effect.

Conclusion: Our results indicate key differences in structural brain measures in patients with CD compared to controls. Additionally, correlation between brain measures and behavioral responses suggest there may be a neural basis to the alterations in patients’ cognitive and affective responses.

Assessment and Management of Pain in Patients With Disorders of Consciousness

Pain is a first-person experience that must be reported, verbally or nonverbally, to be correctly assessed. How, then, is pain perception determined in persons who are noncommunicative? This determination is a major clinical challenge because patients with disorders of consciousness are unable to communicate their feelings and possible pain experiences. This review will describe the current knowledge of evaluating pain perception in a minimally conscious state compared with an unconscious state (also known as vegetative state/unresponsive wakefulness syndrome) and how to approach the management of pain in these 2 populations.

The forebrain medial septal region and nociception

The forebrain medial septum, which is an integral part of the septo-hippocampal network, is implicated in sensorimotor integration, fear and anxiety, and spatial learning and memory. A body of evidence also suggests that the septal region affects experimental pain. Indeed, some explorations in humans have raised the possibility that the region may modulate clinical pain as well. This review explores the evidence that implicates the medial septum in nociception and suggests that non-overlapping circuits in the region facilitate acute nociceptive behaviors and defensive behaviors that reflect affect and cognitive appraisal, especially in relation to persistent nociception. In line with a role in nociception, the region modulates nociceptive responses in the neuraxis, including the hippocampus and the anterior cingulate cortex. The aforementioned forebrain regions have also been implicated in persistent/long-lasting nociception. The review also weighs the effects of the medial septum on nociception vis-à-vis the known roles of the region and emphasizes the fact that the region is a part of network of forebrain structures which have been long associated with reward, cognition and affect-motivation and are now implicated in persistent/long-lasting nociception.

Cerebellar direct current stimulation modulates pain perception in humans

PURPOSE

The cerebellum is involved in a wide number of integrative functions, but its role in pain experience and in the nociceptive information processing is poorly understood. In healthy volunteers we evaluated the effects of transcranial cerebellar direct current stimulation (tcDCS) by studying the changes in the perceptive threshold, pain intensity at given stimulation intensities (VAS:0-10) and laser evoked potentials (LEPs) variables (N1 and N2/P2 amplitudes and latencies).

METHODS

Fifteen subjects were studied before and after anodal, cathodal and sham tcDCS. LEPs were obtained using a neodymium:yttrium-aluminium-perovskite (Nd:YAP) laser and recorded from the dorsum of the left hand. VAS was evaluated by delivering laser pulses at two different intensities, respectively two and three times the perceptive threshold.

RESULTS

Cathodal polarization dampened significantly the perceptive threshold and increased the VAS score, while the anodal one had opposite effects. Cathodal tcDCS increased significantly the N1 and N2/P2 amplitudes and decreased their latencies, whereas anodal tcDCS elicited opposite effects. Motor thresholds assessed through transcranial magnetic stimulation were not affected by cerebellar stimulation.

CONCLUSIONS

tcDCS modulates pain perception and its cortical correlates. Since it is effective on both N1 and N2/P2 components, we speculate that the cerebellum engagement in pain processing modulates the activity of both somatosensory and cingulate cortices. Present findings prompt investigation of the cerebellar direct current polarization as a possible novel and safe therapeutic tool in chronic pain patients.

Deep Brain Stimulation for Intractable Chronic Cluster Headache: Proposals for Patient Selection

Cluster headache is the most severe of the primary headaches. Positron emission tomography and functional MRI studies have shown that the ipsilateral posterior hypothalamus is activated during cluster headache attacks and is structurally asymmetric in these patients. These changes are highly specific for the condition and suggest that the cluster headache generator may be located in that brain area; they further suggest that electrical stimulation of that region might produce clinical improvement in chronic cluster headache sufferers refractory to medical therapy. In five patients with severe intractable chronic cluster headache, hypothalamic electrical stimulation produced complete and long-term pain relief with no relevant side-effects. We therefore consider it essential to propose criteria for selecting chronic cluster headache patients for hypothalamic deep brain stimulation before this procedure is undertaken at other academic medical centres.

New Insights in Trigeminal Anatomy: A Double Orofacial Tract for Nociceptive Input

Orofacial pain in patients relies on the anatomical pathways that conduct nociceptive information, originating from the periphery towards the trigeminal sensory nucleus complex (TSNC) and finally, to the thalami and the somatosensorical cortical regions. The anatomy and function of the so-called trigeminothalamic tracts have been investigated before. In these animal-based studies from the previous century, the intracerebral pathways were mapped using different retro- and anterograde tracing methods. We review the literature on the trigeminothalamic tracts focusing on these animal tracer studies. Subsequently, we related the observations of these studies to clinical findings using fMRI trials. The intracerebral trigeminal pathways can be subdivided into three pathways: a ventral (contralateral) and dorsal (mainly ipsilateral) trigeminothalamic tract and the intranuclear pathway. Based on the reviewed evidence we hypothesize the co-existence of an ipsilateral nociceptive conduction tract to the cerebral cortex and we translate evidence from animal-based research to the human anatomy. Our hypothesis differs from the classical idea that orofacial pain arises only from nociceptive information via the contralateral, ventral trigeminothalamic pathway. Better understanding of the histology, anatomy and connectivity of the trigeminal fibers could contribute to the discovery of a more effective pain treatment in patients suffering from various orofacial pain syndromes.

Pain issues in disorders of consciousness

BACKGROUND

The assessment of pain and nociception in non-communicative patients with disorders of consciousness (DOC) is a real challenge for clinicians. It is, therefore, important to develop sensitive standardized tools usable at the bedside.

OBJECTIVES

This review aims to provide an overview of the current knowledge about pain processing and assessment in patients with DOC.

METHODS

A search was performed on PubMed using MeSH terms including vegetative state, unresponsive wakefulness syndrome, minimally conscious state, consciousness disorders, pain, nociception, neuroimaging and pain assessment.

RESULTS

Neuroimaging studies investigating pain processing in patients with DOC and their implication for clinicians are reviewed. Current works on the development of standardized and sensitive tools for assessing nociception are described.

CONCLUSION

The suggested pain perception capacity highlighted by neuroimaging studies in patients in a MCS and in some patients in a VS/UWS supports the idea that these patients need analgesic treatment and monitoring. The first tool which has been developed to assess nociception and pain in patients with DOC is the NCS. Its revised version represents a rapid, standardized and sensitive scale which can be easily implemented in a clinical setting. Complementary pain assessments are also under validation in order to offer more options to clinicians.

Brain Mapping-Based Model of Δ(9)-Tetrahydrocannabinol Effects on Connectivity in the Pain Matrix

Cannabinoids receive increasing interest as analgesic treatments. However, the clinical use of Δ(9)-tetrahydrocannabinol (Δ(9)-THC) has progressed with justified caution, which also owes to the incomplete mechanistic understanding of its analgesic effects, in particular its interference with the processing of sensory or affective components of pain. The present placebo-controlled crossover study therefore focused on the effects of 20 mg oral THC on the connectivity between brain areas of the pain matrix following experimental stimulation of trigeminal nocisensors in 15 non-addicted healthy volunteers. A general linear model (GLM) analysis identified reduced activations in the hippocampus and the anterior insula following THC administration. However, assessment of psychophysiological interaction (PPI) revealed that the effects of THC first consisted in a weakening of the interaction between the thalamus and the secondary somatosensory cortex (S2). From there, dynamic causal modeling (DCM) was employed to infer that THC attenuated the connections to the hippocampus and to the anterior insula, suggesting that the reduced activations in these regions are secondary to a reduction of the connectivity from somatosensory regions by THC. These findings may have consequences for the way THC effects are currently interpreted: as cannabinoids are increasingly considered in pain treatment, present results provide relevant information about how THC interferes with the affective component of pain. Specifically, the present experiment suggests that THC does not selectively affect limbic regions, but rather interferes with sensory processing which in turn reduces sensory-limbic connectivity, leading to deactivation of affective regions.

Intrinsic functional connectivity of periaqueductal gray subregions in humans

The periaqueductal gray matter (PAG) is a key brain region of the descending pain modulation pathway. It is also involved in cardiovascular functions, anxiety, and fear; however, little is known about PAG subdivisions in humans. The aims of this study were to use resting-state fMRI-based functional connectivity (FC) to parcellate the human PAG and to determine FC of its subregions. To do this, we acquired resting-state fMRI scans from 79 healthy subjects and (1) used a data-driven method to parcellate the PAG, (2) used predefined seeds in PAG subregions to evaluate PAG FC to the whole brain, and (3) examined sex differences in PAG FC. We found that clustering of the left and right PAG yielded similar patterns of caudal, middle, and rostral subdivisions in the coronal plane, and dorsal and ventral subdivisions in the sagittal plane. FC analysis of predefined subregions revealed that the ventolateral(VL)-PAG was supfunctionally connected to brain regions associated with descending pain modulation (anterior cingulate cortex (ACC), upper pons/medulla), whereas the lateral (L) and dorsolateral (DL) subregions were connected with brain regions implicated in executive functions (prefrontal cortex, striatum, hippocampus). We also found sex differences in FC including areas implicated in pain, salience, and analgesia including the ACC and the insula in women, and the MCC, parahippocampal gyrus, and the temporal pole in men. The organization of the human PAG thus provides a framework to understand the circuitry underlying the broad range of responses to pain and its modulation in men and women.

[Imaging techniques and pain]

Over the last 15 years, functional brain imaging techniques have provided critical insights into cortical, subcortical and even spinal mechanisms involved in pain perception and pain modulation in humans. The pivotal contribution of brain imaging studies conducted in Germany have thereby been internationally acknowledged. One of the key challenges for the next decade is to shift the focus from studies in healthy volunteers to different clinical populations suffering from chronic pain to characterize CNS mechanisms, as well as neurobiological predictors and resilience factors of pain chronification. Ultimately, the knowledge gained by this work may help identify individual or syndrome-specific CNS changes as biomarkers to make therapeutic decisions.

Effect of Repeated Electroacupuncture Intervention on Hippocampal ERK and p38MAPK Signaling in Neuropathic Pain Rats

Results of our past studies showed that hippocampal muscarinic acetylcholine receptor (mAChR)-1 mRNA and differentially expressed proteins participating in MAPK signaling were involved in electroacupuncture (EA) induced cumulative analgesia in neuropathic pain rats, but the underlying intracellular mechanism remains unknown. The present study was designed to observe the effect of EA stimulation (EAS) on hippocampal extracellular signal-regulated kinases (ERK) and p38 MAPK signaling in rats with chronic constrictive injury (CCI) of the sciatic nerve, so as to reveal its related intracellular targets in pain relief. After CCI, the thermal pain thresholds of the affected hind were significantly decreased compared with the control group (P < 0.05). Following one and two weeks' EAS of ST 36-GB34, the pain thresholds were significantly upregulated (P < 0.05), and the effect of EA2W was remarkably superior to that of EA2D and EA1W (P < 0.05). Correspondingly, CCI-induced decreased expression levels of Ras, c-Raf, ERK1 and p-ERK1/2 proteins, and p38 MAPK mRNA and p-p38MAPK protein in the hippocampus tissues were reversed by EA2W (P < 0.05). The above mentioned results indicated that EA2W induced cumulative analgesic effect may be closely associated with its function in removing neuropathic pain induced suppression of intracellular ERK and p38MAPK signaling in the hippocampus.

Longitudinal FDG microPET imaging of neuropathic pain: does cerebellar activity correlate with neuropathic pain development in a rat model?

BACKGROUND

We used [F-18] FDG microPET imaging as part of a longitudinal study to investigate changes in the brain.

METHODS

Glucose metabolism during the development of neuropathic pain after tibial and sural nerve transection (TST) model rats. MicroPET images were obtained 1 week before operation and then weekly for 8 weeks post-operation.

RESULTS

The behavioral test was performed immediately after the every FDG administration. After TST modeling, neuropathic pain rats showed increased mechanical sensitivity of the injured hind paw. The withdrawal response to mechanical pain stimulation by von Frey filaments was observed within the first week (3.8 ± 0.73), and it rapidly increased in the third week (7.13 ± 0.82). This response reached a peak in the fourth week after surgery (9.0 ± 0.53), which persisted until the eighth week. In microPET scan imaging, cerebellum, which initially started from the ansiform lobule, was activated gradually to all part from the third week in all image acquisitions through the eighth week.

CONCLUSIONS

The longitudinal microPET scan study of brains from neuropathic pain rat models showed sequential cerebellar activity that was in accordance with results from behavioral test responses, thus supporting a role for the cerebellum in the development of neuropathic pain.

Cortical plasticity in episodic and chronic cluster headache

Cluster headache (CH) is characterized by recurrent episodes of excruciatingly painful, unilateral headache attacks typically accompanied by trigeminal autonomic symptoms. Due to its rhythm with alternating episodes of pain and no-pain, it is an excellent model to investigate whether structural brain changes detected by magnetic resonance based voxel-based-morphometry (VBM) reflect the cause of the disease, may be a consequence of the underlying disease other than pain, or may simply be caused by the sensation of pain itself. We investigated 91 patients with CH in different stages of their disease using VBM and compared them to 78 age- and gender-matched healthy controls. We detected distinct regional gray matter (GM) changes in different brain regions including the temporal lobe, the hippocampus, the insular cortex and the cerebellum. The extent, location and direction of observed GM alterations depended on the state of disease and appeared dynamic in relation to pain state (i.e., pain vs. no-pain). No hypothalamic changes were detected in CH patients compared to healthy controls. The GM changes observed in this study are highly dynamic and thereby reflect the cortical plasticity of the brain in regard to pain. This observed dynamic may provide an explanation of the diverse results of previous VBM studies in pain. Regarding CH the results suggest that the disease is more likely to be caused by a network dysfunction rather than by a single malfunctioning structure.

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We identified gray matter (GM) changes in pain processing areas and beyond in patients suffering cluster headache (CH) compared to healthy controls

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GM changes significantly differed for different courses of the disease (chronic, episodic inside bout, episodic outside bout).

•

GM decrease was predominantly observed in chronic CH, while episodic CH showed a more complex and partly opposite behavior.

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This dynamic probably reflects the brain's adaptation capacity to changing stimuli in regard to cortical plasticity and may provide an explanation of the diverse results of previous VBM studies in pain

The effect of treatment history on therapeutic outcome: psychological and neurobiological underpinnings

It is increasingly recognized that the efficacy of medical treatments is determined in critical part by the therapeutic context in which it is delivered. An important characteristic of that context is treatment history. We recently reported first evidence for a carry-over of treatment experience to subsequent treatment response across different treatment approaches. Here we expand on these findings by exploring the psychological and neurobiological underpinnings of the effect of treatment experience on future treatment response in an experimental model of placebo analgesia with a conditioning procedure. In a combined behavioral and neuroimaging study we experimentally induced positive or negative experiences with an analgesic treatment in two groups of healthy human subjects. Subsequently we compared responses to a second, different analgesic treatment between both groups. We found that participants with an experimentally induced negative experience with the first treatment showed a substantially reduced response to a second analgesic treatment. Intriguingly, several psychological trait variables including anxiety, depression and locus of control modulate the susceptibility for the effects of prior treatment experiences on future treatment outcome. These behavioral effects were supported by neuroimaging data which showed significant differences in brain regions encoding pain and analgesia between groups. These differences in activation patterns were present not only during the pain phase, but also already prior to painful stimulation and scaled with the individual treatment response. Our data provide behavioral and neurobiological evidence showing that the influence of treatment history transfers over time and over therapeutic approaches. Our experimental findings emphasize the careful consideration of treatment history and a strictly systematic treatment approach to avoid negative carry-over effects.

Illness behavior in patients with chronic low back pain and activation of the affective circuitry of the brain

OBJECTIVE

Patients with chronic low back pain (cLBP) show a range of behavioral patterns that do not correlate with degree of spinal abnormality found in clinical, radiological, neurophysiological, or laboratory investigations. This may indicate an augmented central pain response, consistent with factors that mediate and maintain psychological distress in this group.

METHODS

Twenty-four cLBP patients were scanned with functional magnetic resonance imaging while receiving noxious thermal stimulation to the right hand. Patients were clinically assessed into those with significant pain-related illness behavior (Waddell signs [WS]-H) or without (WS-L) based on WS.

RESULTS

Our findings revealed a significant increase in brain activity in WS-H versus WS-L patients in response to noxious heat in the right amygdala/parahippocampal gyrus and ventrolateral prefrontal and insular cortex (at a VoxelPThreshold = 0.01). We found no difference between groups for heat pain thresholds (t(22) = -1.17, p = .28) or sensory-discriminative pain regions.

CONCLUSIONS

Patients with cLBP displaying major pain behavior have increased activity in the emotional circuitry of the brain. This study is the first to suggest an association between a specific clinical test in cLBP and neurobiology of the brain. Functional magnetic resonance imaging may provide a tool capable of enhancing diagnostic accuracy and affecting treatment decisions in cases where no structural cause can be identified.

Hippocampal subfields differentially correlate with chronic pain in older adults

Although previous studies have demonstrated that the hippocampus plays a role in pain processing, the role of hippocampal subfields is uncertain. The goal of this study was to examine the relationship between hippocampal subfield volumes and chronic pain in nondemented older adults. The study sample included 86 community-residing adults age 70 or older who were free of dementia and recruited from the Einstein Aging Study. Chronic pain was defined as pain over the last 3 months, that was moderate or severe (minimum rating of 4 out of 10) most, or all of the time. Hippocampal subfield volumes were estimated using FreeSurfer software. We modeled the association between chronic pain and hippocampal and subfield volume using linear regression. The sample had a mean age of 80 and was 58% female. Chronic pain, present in 55% of the sample, was associated with smaller right and total hippocampal volumes, particularly in women, after adjusting for age, education, and intracranial volume (eTICV). In addition, in women, volume was significantly reduced in participants with chronic pain in right CA2-3 (β=-0.35, p=0.010), right CA4-DG (β=-0.35, p=0.011), left presubiculum (β=-0.29, p=0.030), and left fimbria (β=-0.30, p=0.023). In men, chronic pain was not associated with the volume of any of the hippocampal subfield volumes. Chronic pain in women is associated with a reduction in the volume of right hippocampus and also selected hippocampal subfields. Future studies should clarify the mechanisms underlying the association between regional hippocampal volumes and chronic pain, particularly in women.

Brain mediators of the effects of noxious heat on pain

Recent human neuroimaging studies have investigated the neural correlates of either noxious stimulus intensity or reported pain. Although useful, analyzing brain relationships with stimulus intensity and behavior separately does not address how sensation and pain are linked in the central nervous system. In this study, we used multi-level mediation analysis to identify brain mediators of pain--regions in which trial-by-trial responses to heat explained variability in the relationship between noxious stimulus intensity (across 4 levels) and pain. This approach has the potential to identify multiple circuits with complementary roles in pain genesis. Brain mediators of noxious heat effects on pain included targets of ascending nociceptive pathways (anterior cingulate, insula, SII, and medial thalamus) and also prefrontal and subcortical regions not associated with nociceptive pathways per se. Cluster analysis revealed that mediators were grouped into several distinct functional networks, including the following: somatosensory, paralimbic, and striatal-cerebellar networks that increased with stimulus intensity; and 2 networks co-localized with "default mode" regions in which stimulus intensity-related decreases mediated increased pain. We also identified "thermosensory" regions that responded to increasing noxious heat but did not predict pain reports. Finally, several regions did not respond to noxious input, but their activity predicted pain; these included ventromedial prefrontal cortex, dorsolateral prefrontal cortex, cerebellar regions, and supplementary motor cortices. These regions likely underlie both nociceptive and non-nociceptive processes that contribute to pain, such as attention and decision-making processes. Overall, these results elucidate how multiple distinct brain systems jointly contribute to the central generation of pain.

Emotional regulation of pain: the role of noradrenaline in the amygdala

The perception of pain involves the activation of the spinal pathway as well as the supra-spinal pathway, which targets brain regions involved in affective and cognitive processes. Pain and emotions have the capacity to influence each other reciprocally; negative emotions, such as depression and anxiety, increase the risk for chronic pain, which may lead to anxiety and depression. The amygdala is a key-player in the expression of emotions, receives direct nociceptive information from the parabrachial nucleus, and is densely innervated by noradrenergic brain centers. In recent years, the amygdala has attracted increasing interest for its role in pain perception and modulation. In this review, we will give a short overview of structures involved in the pain pathway, zoom in to afferent and efferent connections to and from the amygdala, with emphasis on the direct parabrachio-amygdaloid pathway and discuss the evidence for amygdala's role in pain processing and modulation. In addition to the involvement of the amygdala in negative emotions during the perception of pain, this brain structure is also a target site for many neuromodulators to regulate the perception of pain. We will end this article with a short review on the effects of noradrenaline and its role in hypoalgesia and analgesia.

Posttraumatic stress disorder: a theoretical model of the hyperarousal subtype

Posttraumatic stress disorder (PTSD) is a frequent and distressing mental disorder, about which much remains to be learned. It is a heterogeneous disorder; the hyperarousal subtype (about 70% of occurrences and simply termed PTSD in this paper) is the topic of this article, but the dissociative subtype (about 30% of occurrences and likely involving quite different brain mechanisms) is outside its scope. A theoretical model is presented that integrates neuroscience data on diverse brain regions known to be involved in PTSD, and extensive psychiatric findings on the disorder. Specifically, the amygdala is a multifunctional brain region that is crucial to PTSD, and processes peritraumatic hyperarousal on grounded cognition principles to produce hyperarousal symptoms. Amygdala activity also modulates hippocampal function, which is supported by a large body of evidence, and likewise amygdala activity modulates several brainstem regions, visual cortex, rostral anterior cingulate cortex (rACC), and medial orbitofrontal cortex (mOFC), to produce diverse startle, visual, memory, numbing, anger, and recklessness symptoms. Additional brain regions process other aspects of peritraumatic responses to produce further symptoms. These contentions are supported by neuroimaging, neuropsychological, neuroanatomical, physiological, cognitive, and behavioral evidence. Collectively, the model offers an account of how responses at the time of trauma are transformed into an extensive array of the 20 PTSD symptoms that are specified in the Diagnostic and Statistical Manual of Mental Disorders, Fifth edition. It elucidates the neural mechanisms of a specific form of psychopathology, and accords with the Research Domain Criteria framework.

Imaging pain: a potent means for investigating pain mechanisms in patients

Chronic pain is a state of physical suffering strongly associated with feelings of anxiety, depression and despair. Disease pathophysiology, psychological state, and social milieu can influence chronic pain, but can be difficult to diagnose based solely on clinical presentation. Here, we review brain neuroimaging research that is shaping our understanding of pain mechanisms, and consider how such knowledge might lead to useful diagnostic tools for the management of persistent pain in individual patients.

The voice of conscience: neural bases of interpersonal guilt and compensation

People feel bad for inflicting harms upon others; this emotional state is termed interpersonal guilt. In this study, the participant played multiple rounds of a dot-estimation task with anonymous partners while undergoing fMRI. The partner would receive pain stimulation if the partner or the participant or both responded incorrectly; the participant was then given the option to intervene and bear a proportion of pain for the partner. The level of pain voluntarily taken and the activations in anterior middle cingulate cortex (aMCC) and bilateral anterior insula (AI) were higher when the participant was solely responsible for the stimulation (Self_Incorrect) than when both committed an error (Both_Incorrect). Moreover, the gray matter volume in the aMCC predicted the individual's compensation behavior, measured as the difference between the level of pain taken in the Self_Incorrect and Both_Incorrect conditions. Furthermore, a mediation pathway analysis revealed that activation in a midbrain region mediated the relationship between aMCC activation and the individual's tendency to compensate. These results demonstrate that the aMCC and the midbrain nucleus not only play an important role in experiencing interpersonal guilt, but also contribute to compensation behavior.

Pearls and pitfalls: Neuroimaging in headache

Premise

One of the most exciting developments in modern neuroscience was the development of imaging techniques providing a non-invasive technique for detection of structure-function relationships characteristic of pain and headache. There is no question that neuroimaging has provided us with a better understanding of how the aura in migraine develops, and it has served as a bridge between neurophysiological studies and clinical findings, although doubtless several questions remain.

Pearls

Neuroimaging drew attention toward central mechanisms in idiopathic headache syndromes. Outstanding functional studies have reinforced the crucial role of the brainstem in acute and chronic migraine and the hypothalamic area in trigemino-autonomic headaches. Several morphometric studies suggest a decreased gray matter in pain-transmitting areas in headache patients; however, those have to be seen in the light of a wealth of pain studies and studies on exercise-dependent plasticity.

Goal

This review focuses on neuroimaging as a scientific tool and highlights the recent advances made in studying primary headache syndromes using functional and structural neuroimaging techniques. It will also point toward open questions and gives recommendations for future studies.

Brainstem activation in cluster headache: an adaptive behavioural response?

INTRODUCTION

The functional neuroimaging of headache patients has revolutionized our understanding of the pathophysiology of primary headaches, providing unique insights into these syndromes. Indeed, functional neuroimaging studies have shown the activation of specific brain structures, the brainstem in migraine and posterior hypothalamus in cluster headache (CH), as well as in other trigeminal autonomic cephalalgias. We describe the functional neuroimaging findings in a patient suffering from CH headache, investigated with functional magnetic resonance imaging (fMRI) during typical pain attacks.

MATERIAL AND METHODS

Two typical, consecutive CH attacks were investigated by two fMRI imaging sessions on the same day. Both fMRI scans were performed at rest, during the CH attacks and the pain-free state induced by subcutaneous administration of sumatriptan.

RESULTS

Significant activation of the bilateral red nucleus, ventral pons and trigeminal root entry zone ipsilaterally to the pain side was detected during the pain state, in addition to the hypothalamic region ipsilaterally to the pain side.

CONCLUSION

Being that such structures are mainly involved in motor function and reactive behaviour, their activation, in our hypothesis, may be linked to pain avoidance and may well represent a defence reaction in cluster headache, which is characterised by a "fight-or-flight" type behavioural pattern during pain attacks.

Pre-existing brain function predicts subsequent practice of mindfulness and compassion meditation

While a variety of meditation techniques are increasingly employed as health interventions, the fact that meditation requires a significant commitment of time and effort may limit its potential widespread utility. In the current study, we ask whether baseline subjective reports or brain activity in response to a "Pain for Self and Others" paradigm predicts subsequent engagement in mindfulness and compassion meditation. The study also investigated whether compassion training would impact neural responses when compared to an active health education control group. Prior to training, activation of the left and right anterior insula, an area thought to be important for empathy, in response to the Other pain task was positively related to engagement with compassion meditation as measured by practice time (n=13). On the other hand, activity in the left amygdala during the Self pain task was negatively correlated with mindfulness practice time. Following the study intervention, there was no difference between the compassion group (n=13), and the control group (n=8), in brain responses to either the Self or Other task. These results are the first to indicate that baseline neural responses may predict engagement with meditation training and suggest that pre-existing neurobiological profiles differentially predispose individuals to engage with disparate meditation techniques.

Differential proteomics analysis of the analgesic effect of electroacupuncture intervention in the hippocampus following neuropathic pain in rats

Background

Evidence is building steadily on the effectiveness of acupuncture therapy in pain relief and repeated acupuncture-induced pain relief is accompanied by improvement of hippocampal neural synaptic plasticity. To further test the cellular and molecular changes underlying analgesic effect of acupuncture, the global change of acupuncture associated protein profiles in the hippocampus under neuropathic pain condition was profiled.

Methods

The chronic constrictive injury (CCI) model was established by ligature of the unilateral sciatic nerve in adult Wistar rats. Rats were randomized into normal control (NC) group, CCI group, and CCI with electroacupuncture (EA) stimulation group. EA was applied to bilateral Zusanli (ST36) and Yanglingquan (GB34) in the EA group. Differentially expressed proteins in the hippocampus in the three groups were identified by two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization time of flight mass spectrometry. The functional clustering of the identified proteins was analyzed by Mascot software.

Results

After CCI, the thermal pain threshold of the affected hind footpad was decreased and was reversed gradually by 12 sessions of acupuncture treatment. Following EA, there were 19 hippocampal proteins identified with significant changes in expression (>2-fold), which are involved in metabolic, physiological, and cellular processes. The top three canonical pathways identified were “cysteine metabolism”, “valine, leucine, and isoleucine degradation” and “mitogen-activated protein kinase (MAPK) signaling”.

Conclusions

These data suggest that the analgesic effect of EA is mediated by regulation of hippocampal proteins related to amino acid metabolism and activation of the MAPK signaling pathway.

Pain in people with Alzheimer disease: potential applications for psychophysical and neurophysiological research

Pain management in people with dementia is a critical problem. Recently, psychophysical and neuroimaging techniques have been used to extend our understanding of pain processing in the brain as well as to identify structural and functional changes in Alzheimer disease (AD). But interpreting the complex relationship between AD pathology, brain activation, and pain reports is challenging. This review proposes a conceptual framework for designing and interpreting psychophysical and neuroimaging studies of pain processing in people with AD. Previous human studies describe the lateral (sensory) and medial (affective) pain networks. Although the majority of the literature on pain supports the lateral and medial networks, some evidence supports an additional rostral pain network, which is believed to function in the production of pain behaviors. The sensory perception of pain as assessed through verbal report and behavioral display may be altered in AD. In addition, neural circuits mediating pain perception and behavioral expression may be hyperactive or underactive, depending on the brain region involved, stage of the disease, and type of pain (acute experimental stimuli or chronic medical conditions). People with worsening AD may therefore experience pain but be unable to indicate pain through verbal or behavioral reports, leaving them at great risk of experiencing untreated pain. Psychophysical (verbal or behavioral) and neurophysiological (brain activation) approaches can potentially address gaps in our knowledge of pain processing in AD by revealing the relationship between neural processes and verbal and behavioral outcomes in the presence of acute or chronic pain.

Grey matter changes associated with medication-overuse headache: correlations with disease related disability and anxiety

OBJECTIVES

Medication-overuse headache (MOH) is associated with psychiatric comorbidities. Neurobiological similarities to substance dependence have been suggested. This study investigated grey matter changes, focussing on pain and reward systems.

METHODS

Using voxel-based morphometry, structural MRIs were compared between 29 patients with both, MOH and migraine, according to International Headache Society criteria, and healthy controls. The Migraine Disability Assessment (MIDAS) score was used. Anxiety and depression were screened for with the Hospital Anxiety and Depression Scale (HADS) and confirmed by a psychiatrist, using the Mini International Neuropsychiatric Interview.

RESULTS

Nineteen patients (66%) had a present or past psychiatric disorder, mainly affective (N = 11) and anxiety disorders (N = 8). In all patients a significant increase of grey matter volume (GMV) was found in the periaqueductal grey matter of the midbrain, which correlated positively with the MIDAS and the HADS-anxiety subscale. A GMV increase was found bilaterally in the thalamus, and the ventral striatum. A significant GMV decrease was detected in frontal regions including orbitofrontal cortex, anterior cingulate cortex, the left and right insula, and the precuneus.

CONCLUSION

These findings are consistent with dysfunction of antinociceptive systems in MOH, which is influenced by anxiety. Dysfunction of the reward system may be a neurobiological basis for dependence in a subgroup of MOH patients.

Modulation of pain by emotional sounds: a laser-evoked potential study

BACKGROUND

Previous studies have shown increases in experimental pain during induction of a negative emotion with visual stimuli, verbal statements or unpleasant odours. The goal of the present study was to analyse the spatio-temporal activation patterns underlying pain augmentation during negative emotional sounds.

METHODS

Negative (e.g., crying), positive (e.g., laughter) and neutral (e.g., distant traffic) sound samples of 4 s duration were presented while noxious laser stimuli were administered to the dorsum of the right hand in 16 healthy participants. The electroencephalographic laser-evoked potentials (LEPs) were modelled using six equivalent source dipoles located in the left and right fronto-opercular/anterior-insular cortex, left parietal operculum, left primary somatosensory cortex, medial parietal cortex and left medial temporal cortex.

RESULTS

Negative emotional sounds were associated with stronger pain than neutral or positive sounds. The source activity in the left medial temporal cortex, purportedly involving hippocampal formation, in the epoch 294-330 ms was greater during negative than neutral or positive sounds. In the left fronto-opercular/anterior-insular cortex, negative sounds failed to show the positive potential component at around 260 ms that was observed during positive and especially during neutral sounds.

CONCLUSIONS

Results suggest increased input of pain-related information into the hippocampal formation when listening to negative emotional sounds, which may in turn facilitate temporal binding between representations of noxious and other behaviourally relevant stimuli, and perhaps associative learning. Absence of the positive potential component fronto-opercular/anterior-insular cortex during negative sounds points to a slow attentional disengagement from pain and increased awareness of the painful stimulus.

Dissociable influences of opiates and expectations on pain

Placebo treatments and opiate drugs are thought to have common effects on the opioid system and pain-related brain processes. This has created excitement about the potential for expectations to modulate drug effects themselves. If drug effects differ as a function of belief, this would challenge the assumptions underlying the standard clinical trial. We conducted two studies to directly examine the relationship between expectations and opioid analgesia. We administered the opioid agonist remifentanil to human subjects during experimental thermal pain and manipulated participants' knowledge of drug delivery using an open-hidden design. This allowed us to test drug effects, expectancy (knowledge) effects, and their interactions on pain reports and pain-related responses in the brain. Remifentanil and expectancy both reduced pain, but drug effects on pain reports and fMRI activity did not interact with expectancy. Regions associated with pain processing showed drug-induced modulation during both Open and Hidden conditions, with no differences in drug effects as a function of expectation. Instead, expectancy modulated activity in frontal cortex, with a separable time course from drug effects. These findings reveal that opiates and placebo treatments both influence clinically relevant outcomes and operate without mutual interference.

fMRI evidence of degeneration-induced neuropathic pain in diabetes: enhanced limbic and striatal activations

Persistent neuropathic pain due to peripheral nerve degeneration in diabetes is a stressful symptom; however, the underlying neural substrates remain elusive. This study attempted to explore neuroanatomical substrates of thermal hyperalgesia and burning pain in a diabetic cohort due to pathologically proven cutaneous nerve degeneration (the painful group). By applying noxious 44°C heat stimuli to the right foot to provoke neuropathic pain symptoms, brain activation patterns were compared with those of healthy control subjects and patients with a similar degree of cutaneous nerve degeneration but without pain (the painless group). Psychophysical results showed enhanced affective pain ratings in the painful group. After eliminating the influence of different pain intensity ratings on cerebral responses, the painful group displayed augmented responses in the limbic and striatal structures, including the perigenual anterior cingulate cortex (ACC), superior frontal gyrus, medial thalamus, anterior insular cortex, lentiform nucleus (LN), and premotor area. Among these regions, blood oxygen level-dependent (BOLD) signals in the ACC and LN were correlated with pain ratings to thermal stimulations in the painful group. Furthermore, activation maps of a simple regression analysis as well as a region of interest analysis revealed that responses in these limbic and striatal circuits paralleled the duration of neuropathic pain. However, in the painless group, BOLD signals in the primary somatosensory cortex and ACC were reduced. These results suggest that enhanced limbic and striatal activations underlie maladaptive responses after cutaneous nerve degeneration, which contributed to the development and maintenance of burning pain and thermal hyperalgesia in diabetes.

Resting-state functional connectivity of the vermal and hemispheric subregions of the cerebellum with both the cerebral cortical networks and subcortical structures

The human cerebellum is a heterogeneous structure, and the pattern of resting-state functional connectivity (rsFC) of each subregion has not yet been fully characterized. We aimed to systematically investigate rsFC pattern of each cerebellar subregion in 228 healthy young adults. Voxel-based analysis revealed that several subregions showed similar rsFC patterns, reflecting functional integration; however, different subregions displayed distinct rsFC patterns, representing functional segregation. The same vermal and hemispheric subregions showed either different patterns or different strengths of rsFCs with the cerebrum, and different subregions of lobules VII and VIII displayed different rsFC patterns. Region of interest (ROI)-based analyses also confirmed these findings. Specifically, strong rsFCs were found: between lobules I-VI and vermal VIIb-IX and the visual network; between hemispheric VI, VIIb, VIIIa and the auditory network; between lobules I-VI, VIII and the sensorimotor network; between lobule IX, vermal VIIIb and the default-mode network; between lobule Crus I, hemispheric Crus II and the fronto-parietal network; between hemispheric VIIb, VIII and the task-positive network; between hemispheric VI, VIIb, VIII and the salience network; between most cerebellar subregions and the thalamus; between lobules V, VIIb and the midbrain red nucleus; between hemispheric Crus I, Crus II, vermal VIIIb, IX and the caudate nucleus; between lobules V, VI, VIIb, VIIIa and the pallidum and putamen; and between lobules I-V, hemispheric VIII, IX and the hippocampus and amygdala. These results confirm the existence of both functional integration and segregation among cerebellar subregions and largely improve our understanding of the functional organization of the human cerebellum.

NMDA or non-NMDA receptor antagonism within the amygdaloid central nucleus suppresses the affective dimension of pain in rats: evidence for hemispheric synergy

The amygdala contributes to generation of affective behaviors to threats. The prototypical threat to an individual is exposure to a noxious stimulus and the amygdaloid central nucleus (CeA) receives nociceptive input that is mediated by glutamatergic neurotransmission. The present study evaluated the contribution of glutamate receptors in CeA to generation of the affective response to acute pain in rats. Vocalizations that occur following a brief noxious tail shock (vocalization afterdischarges) are a validated rodent model of pain affect, and were preferentially suppressed by bilateral injection into CeA of the NMDA receptor antagonist D-2-amino-5-phosphonovalerate (AP5, 1 μg, 2 μg, or 4 μg) or the non-NMDA receptor antagonist 6-Cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX, .25 μg, .5 μg, 1 μg, or 2 μg). Vocalizations that occur during tail shock were suppressed to a lesser degree, whereas spinal motor reflexes (tail flick and hind limb movements) were unaffected by injection of AP5 or CNQX into CeA. Unilateral administration of AP5 or CNQX into CeA of either hemisphere also selectively elevated vocalization thresholds. Bilateral administration of AP5 or CNQX produced greater increases in vocalization thresholds than the same doses of antagonists administered unilaterality into either hemisphere indicating synergistic hemispheric interactions.

PERSPECTIVE

The amygdala contributes to production of emotional responses to environmental threats. Blocking glutamate neurotransmission within the central nucleus of the amygdala suppressed rats' emotional response to acute painful stimulation. Understanding the neurobiology underlying emotional responses to pain will provide insights into new treatments for pain and its associated affective disorders.