Visualizing the complex brain dynamics of chronic pain. J Neuroimmune Pharmacol. 2013;8:510-517. [PMID: 22684310 DOI: 10.1007/s11481-012-9378-8]

The effects of tapentadol and oxycodone on central processing of tonic pain

OBJECTIVE

The present study investigated differences between opioids to experimental tonic pain in healthy men.

METHODS

Twenty-one males participated in this cross-over-trial. Interventions twice daily were oxycodone (10 mg), tapentadol (50 mg) and placebo for 14 days. Tonic pain was induced on day 1, 4 and 14 by immersing the hand in 2 °C water for 120 s. Electroencephalography was recorded during test pain at baseline and after 14 days. Spectral analysis and source localization were investigated in predefined frequency bands.

RESULTS

A decreased perception of pain on day 4 persisted throughout the 14 days compared to baseline (p < 0.006). Oxycodone decreased the electroencephalography spectral power in the delta and theta bands and increased power in the alpha1, alpha2 and beta1 bands (p < 0.03). Tapentadol increased spectral power in the alpha1 band (p < 0.001). Source localization revealed that oxycodone decreased activity of the temporal and limbic region in the delta band, and frontal lobe in the alpha2 and beta1 bands, whereas tapentadol decreased alpha1 band activity in the temporal lobe compared to placebo.

CONCLUSION

Oxycodone and tapentadol reduced pain perception and changed the central processing of tonic pain.

SIGNIFICANCE

Different mechanisms of action were involved, where oxycodone affected cortical structures more than tapentadol.

What does a pain 'biomarker' mean and can a machine be taught to measure pain?

Artificial intelligence allows machines to predict human faculties such as image and voice recognition. Can machines be taught to measure pain? We argue that the two fundamental requirements for a device with 'pain biomarker' capabilities are hardware and software. We discuss the merits and limitations of electroencephalography (EEG) as the hardware component of a putative embodiment of the device, and advances in the application of machine learning approaches to EEG for predicting pain.

Altered fMRI resting-state connectivity in individuals with fibromyalgia on acute pain stimulation

BACKGROUND

Fibromyalgia is a chronic widespread pain condition, with patients commonly reporting other symptoms such as sleep difficulties, memory complaints and fatigue. The use of magnetic resonance imaging (MRI) in fibromyalgia has allowed for the detection of neural abnormalities, with alterations in brain activation elicited by experimental pain and alterations in resting state connectivity related to clinical pain.

METHODS

In this study, we sought to monitor state changes in resting brain connectivity following experimental pressure pain in fibromyalgia patients and healthy controls. Twelve fibromyalgia patients and 15 healthy controls were studied by applying discrete pressure stimuli to the thumbnail bed during MRI. Resting-state functional MRI scanning was performed before and immediately following experimental pressure pain. We investigated changes in functional connectivity to the thalamus and the insular cortex.

RESULTS

Acute pressure pain increased insula connectivity to the anterior cingulate and the hippocampus. Additionally, we observed increased thalamic connectivity to the precuneus/posterior cingulate cortex, a known part of the default mode network, in patients but not in controls. This connectivity was correlated with changes in clinical pain.

CONCLUSIONS

These data reporting changes in resting-state brain activity following a noxious stimulus suggest that the acute painful stimuli may contribute to the alteration of the neural signature of chronic pain. WHAT DOES THIS STUDY/ADD?: In this study acute pain application shows an echo in functional connectivity and clinical pain changes in chronic pain.

Neuroimaging the brain-gut axis in patients with irritable bowel syndrome

AIM: To summarize and synthesize current literature on neuroimaging the brain-gut axis in patients with irritable bowel syndrome (IBS).

METHODS: A database search for relevant literature was conducted using PubMed, Scopus and Embase in February 2015. Date filters were applied from the year 2009 and onward, and studies were limited to those written in the English language and those performed upon human subjects. The initial search yielded 797 articles, out of which 38 were pulled for full text review and 27 were included for study analysis. Investigations were reviewed to determine study design, methodology and results, and data points were placed in tabular format to facilitate analysis of study findings across disparate investigations.

RESULTS: Analysis of study data resulted in the abstraction of four key themes: Neurohormonal differences, anatomic measurements of brain structure and connectivity, differences in functional responsiveness of the brain during rectal distention, and confounding/correlating patient factors. Studies in this review noted alterations of glutamate in the left hippocampus (HIPP), commonalities across IBS subjects in terms of brain oscillation patterns, cortical thickness/gray matter volume differences, and neuroanatomical regions with increased activation in patients with IBS: Anterior cingulate cortex, mid cingulate cortex, amygdala, anterior insula, posterior insula and prefrontal cortex. A striking finding among interventions was the substantial influence that patient variables (e.g., sex, psychological and disease related factors) had upon the identification of neuroanatomical differences in structure and connectivity.

CONCLUSION: The field of neuroimaging can provide insight into underlying physiological differences that distinguish patients with IBS from a healthy population.

Electroencephalography and analgesics

To assess centrally mediated analgesic mechanisms in clinical trials with pain patients, objective standardized methods such as electroencephalography (EEG) has many advantages. The aim of this review is to provide the reader with an overview of present findings in analgesics assessed with spontaneous EEG and evoked brain potentials (EPs) in humans. Furthermore, EEG methodologies will be discussed with respect to translation from animals to humans and future perspectives in predicting analgesic efficacy. We searched PubMed with MeSH terms 'analgesics', 'electroencephalography' and 'evoked potentials' for relevant articles. Combined with a search in their reference lists 15 articles on spontaneous EEG and 55 papers on EPs were identified. Overall, opioids produced increased activity in the delta band in the spontaneous EEG, but increases in higher frequency bands were also seen. The EP amplitudes decreased in the majority of studies. Anticonvulsants used as analgesics showed inconsistent results. The N-methyl-D-aspartate receptor antagonist ketamine showed an increase in the theta band in spontaneous EEG and decreases in EP amplitudes. Tricyclic antidepressants increased the activity in the delta, theta and beta bands in the spontaneous EEG while EPs were inconsistently affected. Weak analgesics were mainly investigated with EPs and a decrease in amplitudes was generally observed. This review reveals that both spontaneous EEG and EPs are widely used as biomarkers for analgesic drug effects. Methodological differences are common and a more uniform approach will further enhance the value of such biomarkers for drug development and prediction of treatment response in individual patients.

Cortical theta is increased while thalamocortical coherence is decreased in rat models of acute and chronic pain

Thalamocortical oscillations are critical for sensory perception. Although pain is known to disrupt synchrony in thalamocortical oscillations, evidence in the literature is controversial. Thalamocortical coherence has been reported to be increased in patients with neurogenic pain but decreased in a rat model of central pain. Moreover, theta (4 to 8 Hz) oscillations in primary somatosensory (S1) cortex are speculated to predict pain in humans. To date, the link between pain and network oscillations in animal models has been understudied. Thus, we tested the hypothesis that pain disrupts thalamocortical coherence and S1 theta power in two rat models of pain. We recorded electrocorticography (ECoG) waveforms over S1 and local field potentials (LFP) within ventral posterolateral thalamus in freely behaving rats under spontaneous (stimulus-independent) pain conditions. Rats received intradermal capsaicin injection (Cap) in the hindpaw, followed hours later by chronic constriction injury (CCI) of the sciatic nerve lasting several days. Our results show that pain decreases coherence between LFP and ECoG waveforms in the 2- to 30-Hz range, and increases ECoG power in the theta range. These changes are short-lasting after Cap and longer-lasting after CCI. These data might be particularly relevant to preclinical correlates of spontaneous pain-like behavior, with potential implications to clinical biomarkers of ongoing pain.

Functional connectivity of the default mode network and its association with pain networks in irritable bowel patients assessed via lidocaine treatment

The default mode network (DMN), a group of brain regions implicated in passive thought processes, has been proposed as a potentially informative neural marker to aid in novel treatment development. However, the DMN's internal connectivity and its temporal relationship (ie, functional network connectivity) with pain-related neural networks in chronic pain conditions is poorly understood, as is the DMN's sensitivity to analgesic effects. The current study assessed how DMN functional connectivity and its temporal association with 3 pain-related networks changed after rectal lidocaine treatment in irritable bowel syndrome patients. Eleven females with irritable bowel syndrome underwent a rectal balloon distension paradigm during functional magnetic resonance imaging in 2 conditions: natural history (ie, baseline) and lidocaine. Results showed increased DMN connectivity with pain-related regions during natural history and increased within-network connectivity of DMN structures under lidocaine. Further, there was a significantly greater lag time between 2 of the pain networks, those involved in cognitive and in affective pain processes, comparing lidocaine to natural history. These findings suggest that 1) DMN plasticity is sensitive to analgesic effects, and 2) reduced pain ratings via analgesia reflect DMN connectivity more similar to pain-free individuals. Findings show potential implications of this network as an approach for understanding clinical pain management techniques.

PERSPECTIVE

This study shows that lidocaine, a peripheral analgesic, significantly altered DMN connectivity and affected its relationship with pain-related networks. These findings suggest that the DMN, which is hypothesized to represent non-goal-oriented activity, is sensitive to analgesic effects and could be useful to understand pain treatment mechanisms.

Neuroinflammation, neuroautoimmunity, and the co-morbidities of complex regional pain syndrome

Complex Regional Pain Syndrome (CRPS) is associated with non-dermatomal patterns of pain, unusual movement disorders, and somatovisceral dysfunctions. These symptoms are viewed by some neurologists and psychiatrists as being psychogenic in origin. Recent evidence, however, suggests that an autoimmune attack on self-antigens found in the peripheral and central nervous system may underlie a number of CRPS symptoms. From both animal and human studies, evidence is accumulating that neuroinflammation can spread, either anterograde or retrograde, via axonal projections in the CNS, thereby establishing neuroinflammatory tracks and secondary neuroinflammatory foci within the neuraxis. These findings suggest that neuroinflammatory lesions, as well as their associated functional consequences, should be evaluated during the differential diagnosis of non-dermatomal pain presentations, atypical movement disorders, as well as other "medically unexplained symptoms", which are often attributed to psychogenic illness.

Imaging biomarkers and the role of neuroinflammation in neuropathic pain

The papers from this thematic issue followed a translational research workshop, Imaging Neuroinflammation and Neuropathic Pain, that focused on the search for neuroimaging biomarkers to assess neuroinflammation associated with neuropathic pain. The topics covered in this issue include overviews of the historical and current knowledge regarding neuropathic pain, the potential mechanisms involved, the often under-recognized clinical presentations that can delay diagnosis, the various neuroimaging techniques that have been applied to evaluate neuropathic pain and neuroinflammation, to case series illustrating novel treatments of neuropathic pain. Furthermore, the use of telemedicine to disseminate knowledge and improve the diagnosis and treatment of pain syndromes is also discussed.