Cerebral mechanisms operating in the presence and absence of inflammatory pain

Pharmacological FMRI in the development of new analgesic compounds

Chronic pain is a major problem for the individual and for society. Despite a range of drugs being available to treat chronic pain, only inadequate pain relief can be achieved for many patients. There is therefore a need for the development of new analgesic compounds. The assessment of pain depends to date entirely on the subjective report of the patient, in contrast to many other clinical conditions where biomarkers that help determine the severity and stage of the disease enable the physician to monitor the course of the disease and treatment effects longitudinally. In this article, we illustrate that magnetic resonance-based imaging techniques have the potential to provide sensitive and specific biomarkers of the pain experience, as well as clarifying disease mechanisms. Functional magnetic resonance imaging (FMRI) is particularly suited to investigating the effects of pharmacological agents on pain processing within the human central nervous system. Combination of FMRI and drug administration is termed pharmacological FMRI (phFMRI). In addition to outlining several methodological considerations that have to be taken into account when performing phFMRI, we discuss phFMRI studies that have already used this technique to study the effects of analgesic compounds. These studies provide promising data for the use of phFMRI as sensitive tool in assessing a potential drug effect. Such pharmacodynamic readouts obtained early in the process of drug development would not only save the pharmaceutical industry substantial amounts of money, but would also avoid the unnecessary exposure of patients to molecules with limited or no therapeutic value. We are therefore optimistic that phFMRI will be used as a tool with high sensitivity and specificity for evaluating analgesic agents in early drug development and clinical studies.

The physiology and processing of pain: a review

Despite the many advances in our understanding of the mechanisms underlying pain processing, pain continues to be a major healthcare problem in the United States. Each day, millions of Americans are affected by both acute and chronic pain conditions, costing in excess of $100 billion for treatment-related costs and lost work productivity. Thus, it is imperative that better treatment strategies be developed. One step toward improving pain management is through increased knowledge of pain physiology. Within the nervous system, there are several pathways that transmit information about pain from the periphery to the brain. There is also a network of pathways that carry modulatory signals from the brain and brainstem that alter the incoming flow of pain information. This article provides a review to the physiology and processing of pain.

Pain mechanisms and their disorders

The purpose of this article is to summarise how functional imaging techniques have changed our understanding of normal and abnormal pain mechanisms, how they inform a change in clinical practice and to speculate on possible future clinical uses.

Rational and targeted pharmacologic treatment of fibromyalgia

Despite disappointing results when subjected to randomized clinical trials, pharmacologic agents remain an important component of FM management. Addressing the main symptoms of pain, disturbed sleep, mood disturbances, fatigue, and associated conditions is essential to improve patient functioning and enhanced quality of life. However, much work remains to design clinical trials which address the complexity of FM, while satisfying evidence based medicine paradigms.

Central activation by histamine-induced itch: analogies to pain processing: a correlational analysis of O-15 H2O positron emission tomography studies

The aim of this study was to identify the functional cerebral network involved in the central processing of itch and to detect analogies and differences to previously identified cerebral activation patterns triggered by painful noxious stimuli. Repeated positron emission tomography regional cerebral blood flow (rCBF) measurements using O15-labeled water were performed in six healthy right-handed male subjects (mean age 32 +/- 2 years). Each subject underwent 12 sequential rCBF measurements. In all subjects a standardized skin prick test was performed on the right forearm 2 min before each rCBF measurement. For activation, histamine was applied in nine tests in logarithmically increasing concentrations from 0.03 to 8%. Three tests were performed with isotonic saline solution serving as a control condition. Itch intensity and unpleasantness were registered with a visual analogue scale during each test. Subtraction analysis between activation and control conditions as well as correlation analysis with covariates were performed. Itch induced a significant activation in the predominantly contralateral somatosensory cortex and in the ipsilateral and contralateral motor areas (supplementary motor area (SMA), premotor cortex, primary motor cortex). Additional significant activations were found in the prefrontal cortex and the cingulate gyrus, but not in subcortical structures nor in the secondary somatosensory cortex. In correlation analyses, several cortical areas showed a graded increase in rCBF with the logarithm of the histamine concentration (bilateral sensorimotor areas and cingulate cortex; contralateral insula, superior temporal cortex and prefrontal cortex) and with itch unpleasantness (contralateral sensorimotor cortex, prefrontal cortex and posterior insula; ipsilateral SMA). Induction of itch results in the activation of a distributed cerebral network. Itch and pain seem to share common pathways (a medial and a lateral processing pathway and a strong projection to the motor system). In contrast to pain activation studies, no subcortical (i.e. thalamic) activations were detected and correlation analyses suggest differences in subjective processing of the two sensations.

Long-lasting effect evoked by tonic muscle pain on parietal EEG activity in humans

OBJECTIVE

To explore EEG changes evoked by tonic experimental muscle pain compared to a non-painful vibratory stimulus.

METHODS

Thirty-one EEG channels were recorded before, during and after painful and non-painful stimulation. Pain was induced in the left brachioradialis muscle by injection of hypertonic (5%) saline. The vibratory stimulus was applied to the skin area overlying the brachioradialis muscle. The power of the major frequency components of the EEG activity (FFT, fast Fourier transform) was quantified and t-maps between the different experimental conditions were evaluated in frequency domain.

RESULTS

The main effect of muscle pain, compared to non-painful stimulation, was a significant and long-lasting increase of delta (1-3 Hz) power and an alpha-1 (9-11 Hz) power increase over the contralateral parietal locus. This finding could suggest a decreased excitability of the primary somatosensory cortex during muscle pain. The main effect of vibration, compared to its unstimulated baseline, consisted in an increase of beta-1 (14-20 Hz) power in the right frontal region.

CONCLUSIONS

Our data demonstrate significant and specific topographic EEG changes during tonic muscle pain. Since these modifications differ from those produced by an unstimulated baseline and during non-painful tonic stimulation, they might reflect mechanisms involved in the processing of nociceptive and adverse tonic stimuli.

Meta-Analysis of Thirty-Four Independent Samples Studied Using PET Reveals a Significantly Attenuated Central Response to Noxious Stimulation in Clinical Pain Patients

Chronic pain disorder is widely understood as a "biopsychosocial" phenomenon, meaning that it is influenced by psychology and certain life events. This broad understanding of chronic pain suggests that central responses during pain experience should be altered in patients compared with pain-free volunteers. A total of 34 studies are reviewed, revealing a widespread "neuromatrix" of activated regions. These regions include the brain stem, thalamus, and lentiform nucleus, and the insula, prefrontal, parietal, and anterior cingulate cortices. Meta-analysis of these studies does not reveal any single region or pattern of activity to be of particular influence during chronic pain but does reveal a generally reduced response to noxious stimulation in patients with concomitant clinical pain. The relevance of this finding remains unclear with the most parsimonious explanation being increased response variability in patients. More specific findings can be revealed when using a hypothesis-generated approach; further investigation of genetic and developmental predisposition is suggested.

Newer Concepts in Pain Mechanisms

This review addresses investigations of how nociceptive information is processed in the brain, and the role of nerve growth factor in the neurobiology of nociception. The brain studies probe the neuronal matrix concept of how pain is perceived, and are based on brain imaging techniques, positron emission tomography (PET), and functional magnetic resonance imaging (fMRI). The critical role of nerve growth factor in the embryogenesis of primary afferent cell bodies (nociceptors, small diameter axons, dorsal root ganglia and their terminals in the spinal cord) and in inflammatory processes in adults are reviewed. Neuroimaging data support the neuronal matrix concept, and the potential therapeutic opportunities offered by pharmacologically targeting various aspects of nerve growth factor and its interaction with its receptor are clear.

Functional magnetic resonance imaging in rats subjected to intense electrical and noxious chemical stimulation of the forepaw

We examined whether cerebral activation to two different intense and painful stimuli could be detected using functional magnetic resonance imaging (fMRI) in alpha-chloralose anesthetized rats. Experiments were performed using a 9.4 T magnet and a surface coil centered over the forebrain. A set of gradient echo images were acquired and analyzed using our software based on fuzzy cluster analysis (EvIdent). Following the injection of 50 microl of formalin (5%) into the forepaw we observed a regional increase in signal intensity in the MR images in all animals. Anterior cingulate cortex, frontal cortex and sensory-motor cortex were some of the regions that activated frequently and often bilaterally. Surprisingly, activation appeared sequentially, often occurring first in either the right or the left hemisphere with a separation of seconds to minutes between peak activations. Morphine pre-treatment (1 mg/kg, i. v.) delayed and/or reduced the intensity of the activation resulting in a decrease in the overall response. Following episodes of intense electrical stimulation, produced by two brief stimulations (15 V, 0. 3 ms, 3 Hz) of the forepaw, activation was observed consistently in the sensory-motor cortex contralateral to the stimulation. Activation also occurred frequently in the anterior cingulate cortex, ipsilateral sensory-motor cortex and frontal cortical regions. All these regions of activation were markedly reduced during nitrous oxide inhalation. Treatment with morphine resulted in an inhibition of the activation response to electrical stimulation in most regions except for sensory-motor cortex. Thus, electrical and chemical noxious stimuli activated regions that are known to be involved in the central processing of pain and morphine modified the activation observed. fMRI combined with appropriate exploratory data analysis tools could provide an effective new tool with which to study novel analgesics and their effects on the CNS processing of pain in animal models.

Pain perception: is there a role for primary somatosensory cortex?

Anatomical, physiological, and lesion data implicate multiple cortical regions in the complex experience of pain. These regions include primary and secondary somatosensory cortices, anterior cingulate cortex, insular cortex, and regions of the frontal cortex. Nevertheless, the role of different cortical areas in pain processing is controversial, particularly that of primary somatosensory cortex (S1). Human brain-imaging studies do not consistently reveal pain-related activation of S1, and older studies of cortical lesions and cortical stimulation in humans did not uncover a clear role of S1 in the pain experience. Whereas studies from a number of laboratories show that S1 is activated during the presentation of noxious stimuli as well as in association with some pathological pain states, others do not report such activation. Several factors may contribute to the different results among studies. First, we have evidence demonstrating that S1 activation is highly modulated by cognitive factors that alter pain perception, including attention and previous experience. Second, the precise somatotopic organization of S1 may lead to small focal activations, which are degraded by sulcal anatomical variability when averaging data across subjects. Third, the probable mixed excitatory and inhibitory effects of nociceptive input to S1 could be disparately represented in different experimental paradigms. Finally, statistical considerations are important in interpreting negative findings in S1. We conclude that, when these factors are taken into account, the bulk of the evidence now strongly supports a prominent and highly modulated role for S1 cortex in the sensory aspects of pain, including localization and discrimination of pain intensity.

Cerebral responses to pain in patients suffering acute post-dental extraction pain measured by positron emission tomography (PET)

Previous studies with normal volunteers have demonstrated distributed cortical responses to experimental heat pain within a network of structures. The network includes the insula, anterior cingulate, prefrontal, inferior parietal and somatosensory cortices. Patients suffering from chronic nociceptive pain following rheumatoid arthritis (RA) have shown damped central responses to experimental heat pain applied to the back of the right hand. In this study of patients with acute, left-sided, post-molar-extraction (surgical) pain, we assessed the cortical responses to experimental heat pain, applied to the back of the right hand, using positron emission tomography (PET), and compared the responses with a previously reported control group and the RA group. In response to the experimental heat pain, the surgical group indicated significantly increased regional cerebral blood flow in the prefrontal cortex [Brodman's area (BA) 44] ipsilateral to the heat stimulus. Contralateral increases were detected in the putamen and transverse temporal gyrus (BA 40/41/42) with bilateral increases in the insular cortex. Compared to the control and RA group, there were significantly reduced responses in the anterior cingulate (BA 24), pre-frontal medial, and orbito-frontal (BA 9/10/32/47) cortices. These results suggest that relatively discrete regions of the cerebral cortex are responsible for acute nociceptive processing during an acute inflammatory episode. The reduced frontal and anterior cingulate responses to the experimental heat pain (applied to the right hand) during acute inflammatory pain (left jaw) illustrates cortical modulation of nociceptive processing that may be related to non-somatotopic, bilateral, nociceptive inputs to these areas. Copyright 1999 European Federation of Chapters of the International Association for the Study of Pain.

CNS pattern of metabolic activity during tonic pain: evidence for modulation by beta-endorphin

CNS correlates of acute prolonged pain, and the effects of partial blockade of the central beta-endorphin system, were investigated by the quantitative 2-deoxyglucose technique in unanaesthetized, freely moving rats. Experiments were performed during the second, tonic phase of the behavioural response to a prolonged chemical noxious stimulus (s.c. injection of dilute formalin into a forepaw), or after minor tissue injury (s.c. saline injection). During formalin-induced pain, local glucose utilization rates in the CNS were bilaterally increased in the grey matter of the cervical spinal cord, in spinal white matter tracts and in several supraspinal structures, including portions of the medullary reticular formation, locus coeruleus, lateral parabrachial region, anterior pretectal nucleus, the medial, lateral and posterior thalamic regions, basal ganglia, and the parietal, cingulate, frontal, insular and orbital cortical areas. Pretreatment with anti-beta-endorphin antibodies, injected i.c.v., led to increased metabolism in the tegmental nuclei, locus coeruleus, hypothalamic and thalamic structures, putamen, nucleus accumbens, diagonal band nuclei and dentate gyrus, and in portions of the parietal, cingulate, insular, frontal and orbital cortex. In formalin-injected rats, pretreated with anti-beta-endorphin, behavioural changes indicative of hyperalgesia (increased licking response) were found, which were paralleled by a significant enhancement of functional activity in the anterior pretectal nucleus and in thalamo-cortical systems. A positive correlation was found between the duration of the licking response and metabolic activity of several forebrain regions. These results provide a map of the CNS pattern of metabolic activity during tonic somatic pain, and demonstrate a modulatory role for beta-endorphin in central networks that process somatosensory inputs.

Pain in fibromyalgia

Just as our caveman forebears were frail in the face of predatory animals, we are frail in today's society of childhood neglect or abuse, bumper-to-bumper traffic, frustration at work, and multiple daily hassles. The same neuroendocrine systems and pain regulatory mechanisms that protected early man during acute stress are still encoded in our genome, but may be maladaptive in psychologically and physiologically vulnerable people faced with chronic stress. Many patients with fibromyalgia become vulnerable because of the long-lasting psychological and neurophysiological effects of negative experiences in childhood. Ill-equipped with positive cognitive, emotional, and behavioral skills as adults, they display maladaptive coping strategies, low self-efficacy, and negative mood when confronted with the inevitable stressors of life. Psychological distress ensues, which reduces thresholds for pain perception and tolerance (already relatively low in women) even further. Converging lines of psychological and neurobiological evidence strongly suggest that chronic stress-related blunting of the HPA, sympathetic, and other axes of the stress response together with associated alterations in pain regulatory mechanisms may finally explain the pain and fatigue of fibromyalgia. Vulnerable people who can be classified by the ACR criteria as having fibromyalgia do not have a discrete disease. They are simply the most ill in a continuum of distress, chronic pain, and painful tender points in the general population.

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.

Region-specific encoding of sensory and affective components of pain in the human brain: a positron emission tomography correlation analysis

Brain imaging with positron emission tomography has identified some of the principal cerebral structures of a central network activated by pain. To discover whether the different cortical and subcortical areas process different components of the multidimensional nature of pain, we performed a regression analysis between noxious heat-related regional blood flow increases and experimental pain parameters reflecting detection of pain, encoding of pain intensity, as well as pain unpleasantness. The results of our activation study indicate that different functions in pain processing can be attributed to different brain regions; ie, the gating function reflected by the pain threshold appeared to be related to anterior cingulate cortex, the frontal inferior cortex, and the thalamus, the coding of pain intensity to the periventricular gray as well as to the posterior cingulate cortex, and the encoding of pain unpleasantness to the posterior sector of the anterior cingulate cortex.

Temporal and intensity coding of pain in human cortex

Temporal and intensity coding of pain in human cortex. J. Neurophysiol. 80:3312-3320, 1998. We used a high-resolution functional magnetic resonance imaging (fMRI) technique in healthy right-handed volunteers to demonstrate cortical areas displaying changes of activity significantly related to the time profile of the perceived intensity of experimental somatic pain over the course of several minutes. Twenty-four subjects (ascorbic acid group) received a subcutaneous injection of a dilute ascorbic acid solution into the dorsum of one foot, inducing prolonged burning pain (peak pain intensity on a 0-100 scale: 48 +/- 3, mean +/- SE; duration: 11.9 +/- 0.8 min). fMRI data sets were continuously acquired for approximately 20 min, beginning 5 min before and lasting 15 min after the onset of stimulation, from two sagittal planes on the medial hemispheric wall contralateral to the stimulated site, including the cingulate cortex and the putative foot representation area of the primary somatosensory cortex (SI). Neural clusters whose fMRI signal time courses were positively or negatively correlated (P < 0.0005) with the individual pain intensity curve were identified by cross-correlation statistics in all 24 volunteers. The spatial extent of the identified clusters was linearly related (P < 0.0001) to peak pain intensity. Regional analyses showed that positively correlated clusters were present in the majority of subjects in SI, cingulate, motor, and premotor cortex. Negative correlations were found predominantly in medial parietal, perigenual cingulate, and medial prefrontal regions. To test whether these neural changes were due to aspecific arousal or emotional reactions, related either to anticipation or presence of pain, fMRI experiments were performed with the same protocol in two additional groups of volunteers, subjected either to subcutaneous saline injection (saline: n = 16), inducing mild short-lasting pain (peak pain intensity 23 +/- 4; duration 2.8 +/- 0.6 min) or to nonnoxious mechanical stimulation of the skin (controls: n = 16) at the same body site. Subjects did not know in advance which stimulus would occur. The spatial extent of neural clusters whose signal time courses were positively or negatively correlated with the mean pain intensity curve of subjects injected with ascorbic acid was significantly larger (P < 0.001) in the ascorbic acid group than both saline and controls, suggesting that the observed responses were specifically related to pain intensity and duration. These findings reveal distributed cortical systems, including parietal areas as well as cingulate and frontal regions, involved in dynamic encoding of pain intensity over time, a process of great biological and clinical relevance.

Neurophysiological evaluation of pain

Neurophysiological techniques for the evaluation of pain in humans have made important advances in the last decade. A number of features of neuroanatomy and physiology of nociception qualifies pain as a multidimensional phenomenon which is rather unique among the sensory systems and which poses a number of technical and procedural requirements for its appropriate diagnostic assessment. Various stimulation techniques to induce defined pain in humans and used in combination with the methodology of evoked electrical brain potentials and magnetic fields are presented. Most recent knowledge gathered from scalp topography and dipole source analysis of pain-relevant evoked potentials and fields is discussed. Particular emphasis is put upon laser-evoked potentials and their application for diagnosis, pathophysiological description and monitoring of patients with neurological disorders and abnormal pain states. Future perspectives in this growing field of research are discussed briefly.

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.

Reduced cortical responses to noxious heat in patients with rheumatoid arthritis

OBJECTIVES

To test the hypothesis that patients with chronic inflammatory pain develop adaptive cortical responses to noxious stimulation characterised by reduced anterior cingulate responses.

METHODS

Positron emission tomography was used to measure changes in regional cerebral blood flow (rCBF) in response to an acute experimental pain stimulus in six patients with rheumatoid arthritis (RA) in comparison to six age and sex matched controls. A standardised and reproducible non-painful and painful phasic heat stimulus was delivered by a thermal probe to the back of the right hand during six two minute periods during which time rCBF measurements were made. The effects of non-painful heat were subtracted from those of painful heat to weight the analysis towards the non-discriminatory or 'suffering' components of pain processing. Significance maps of pain processing were generated and compared in each group and contrasted with results obtained in a group of patients with atypical facial pain (AFP) that have been previously published.

RESULTS

The RA patients showed remarkably damped cortical and subcortical responses to pain compared with the control group. Significant differences between the two groups were observed in the prefrontal (BA 10) and anterior cingulate (BA 24) and cingulofrontal transition cortical (BA 32) areas. The reduced anterior cingulate responses to standardised heat pain were compared with the increased cingulate responses seen in patients with psychogenically maintained pain (AFP) who had both lower pain tolerance and mood than the RA group.

CONCLUSIONS

Major cortical adaptive responses to standardised noxious heat can be measured and contrasted in patients with different types of chronic pain. The different pattern of cingulate and frontal cortical responses in the patients with inflammatory and non-nociceptive pain suggest that different mechanisms are operating, possibly at a thalamocortical level. Implications for treatment strategies for chronic pain are discussed.