Positron emission tomography reveals changes in global and regional cerebral blood flow during noxious stimulation of normal and inflamed elbow joints in anesthetized cats

[Hippocampus is involved in 17β-estradiol exacerbating experimental occlusal inter- ference-induced chronic masseter hyperalgesia in ovariectomized rats]

OBJECTIVE

To investigate the influence of chronic masseter hyperalgesia induced by 17β-estradiol (E2) and experimental occlusal interference (EOI) on underlying mechanism in hippocampus of ovariectomized (OVX) rats.

METHODS

In the study, 32 OVX rats were randomly divided into 4 groups (8 rats/group): The control group was OVX group, and 0 μg/d E2 (vehicle) injection was started 7 d after OVX without EOI; in the experimental group (1) OVX + E2 group, 80 μg/d E2 injection was started 7 d after OVX without EOI; in the experimental group (2) OVX + EOI group, vehicle injection was started 7 d after OVX and EOI was applied 17 d after OVX; in the experimental group (3) OVX + E2 + EOI group, 80 μg/d E2 injection was started 7 d after OVX and EOI was applied 17 d after OVX. Bilateral masseter muscle mechanical withdrawal thresholds were measured before OVX, 7 days after OVX (before E2 injection), 17 days after OVX (10 days after E2 injection and before EOI) and 24 days after OVX (7 days after EOI). Immunofluorescence staining was used to reveal phospho-extracellular signal regulated kinase 1/2 (p-ERK1/2)-positive neurons in CA3 of hippocampus. The protein expression of p-ERK1/2 in hippocampus was detected using Western Blot.

RESULTS

Compared with the control group [left side: (135.3±8.5) g, right side: (135.4±10.8) g], bilateral masseter muscle mechanical withdrawal thresholds of OVX+E2 group [left side: (113.3±5.6) g, right side: (112.5 ± 5.6) g] and OVX+EOI group [left side: (93.3±5.4) g, right side: 90.8±5.5) g] were decreased (P < 0.01). Bilateral masseter muscle mechanical withdrawal thresholds were significantly lower in OVX+E2+EOI group [left side: (81.2±6.2) g, right side: 79.8±7.7) g] than in the control, OVX+E2 and OVX+EOI groups (P < 0.05). The proportion of p-ERK1/2 positive neurons in the CA3 region of the hippocampus was increased in the control, OVX+E2, OVX+EOI and OVX+E2+EOI groups in turn, and the difference between the groups was statistically significant (P < 0.05). p-ERK1/2 protein expression was increased in the control, OVX+E2 and OVX+EOI groups in turn, but the difference was not statistically significant (P>0.05). p-ERK1/2 expression was significantly higher in OVX+E2+EOI group than in the other three groups (P < 0.05).

CONCLUSION

High concentration of E2 could exacerbated EOI-induced chronic masseter hyperalgesia in ovariectomized rats, and its central mechanism may be related to the upregulation of the phosphorylation of ERK1/2 in hippocampus.

Functional MRI-based identification of brain regions activated by mechanical noxious stimulation and modulatory effect of remifentanil in cats

This study was conducted to identify the brain regions corresponding to mechanical noxious stimulation in cats using functional magnetic resonance imaging (fMRI) and to investigate the modulatory effect of remifentanil on the activation of these regions. Six healthy cats were anesthetized using a constant-rate infusion of alfaxalone. Cats were allocated to one of three treatment groups: remifentanil 0 (saline), 0.25, and 0.5μg/kg/min. A 3.0-T MRI unit was used to collect fMRI data. During the fMRI scanning, mechanical noxious stimulation was applied by tail clamping. The brain regions activated by the stimulation were identified based on blood oxygenation level-dependent (BOLD) responses. The modulatory effects of remifentanil were evaluated using a region of interest (ROI) analysis comparing signal changes in each brain region. Increased activity from noxious stimulation was observed in the somatosensory area (the postcruciatus gyrus, the anterior part of the marginalis gyrus, and the anterior part of the ectomarginalis gyrus), the parietal association area (the middle part of the marginalis gyrus and the middle part of the ectomarginalis gyrus), the cingulate cortex, the hippocampus, and the cerebellum. The results of the ROI analysis indicated that activations in the somatosensory area, the cingulate cortex, the hippocampus, and the cerebellum were significantly modulated (P<0.05) by remifentanil. In cats, activation patterns evoked by mechanical noxious stimulation were observed in several brain regions thought to be involved in various aspects of pain processing, including sensory discrimination and integration, affect, and motor response. These brain responses were modulated by remifentanil.

TRPV1 channel-mediated bilateral allodynia induced by unilateral masseter muscle inflammation in rats

Pain in masticatory muscles is among the most prominent symptoms of temperomandibular disorders (TMDs) that have diverse and complex etiology. A common complaint of TMD is that unilateral pain of craniofacial muscle can cause a widespread of bilateral pain sensation, although the underlying mechanism remains unknown. To investigate whether unilateral inflammation of masseter muscle can cause a bilateral allodynia, we generated masseter muscle inflammation induced by unilateral injection of complete Freund’s adjuvant (CFA) in rats, and measured the bilateral head withdrawal threshold at different time points using a von Frey anesthesiometer. After behavioral assessment, both right and left trigeminal ganglia (TRG) were dissected and examined for histopathology and transient receptor potential vanilloid 1 (TRPV1) mRNA expression using quantitative real-time PCR analysis. A significant increase in TRPV1 mRNA expression occurred in TRG ipsilateral to CFA injected masseter muscle, whereas no significant alteration in TRPV1 occurred in the contralateral TRG. Interestingly, central injection of TRPV1 antagonist 5-iodoresiniferatoxin into the hippocampus significantly attenuated the head withdrawal response of both CFA injected and non-CFA injected contralateral masseter muscle. Our findings show that unilateral inflammation of masseter muscle is capable of inducing bilateral allodynia in rats. Upregulation of TRPV1 at the TRG level is due to nociception caused by inflammation, whereas contralateral nocifensive behavior in masticatory muscle nociception is likely mediated by central TRPV1, pointing to the involvement of altered information processing in higher centers.

The effect of the cold pressor test on a visually evoked cerebral blood flow velocity response

We investigated the hypothesis that during tonic pain stimulus, neurovascular coupling (NVC) decreases, measuring visually evoked cerebral blood flow velocity response (VEFR) during cold pressor test (CPT) in healthy human subjects as a test. VEFR was calculated as a relative increase in blood flow velocity in the posterior cerebral artery from average values during the last 5 s of the stimulus-OFF period to average values during the last 10 s of the stimulus-ON period. Three consecutive experimental phases were compared: basal, CPT and recovery. During CPT, end-diastolic and mean VEFR increased from 20.2 to 23.6% (p < 0.05) and from 17.5 to 20.0% (p < 0.05), respectively. In recovery phase, end-diastolic and mean VEFR decreased to 17.7% and 15.5%, respectively. Both values were statistically significantly different from CPT phase (p < 0.05). Compared with the basal phase, only end-diastolic VEFR was statistically significantly different in the recovery phase (p < 0.05). Our results are consistent with the assumption that there is a change in the activity of NVC during CPT because of the modulatory influence of subcortical structures activated during tonic pain. Contrary to our expectations, the combined effect of such influences increases rather than decreases NVC.

17-Beta-estradiol enhanced allodynia of inflammatory temporomandibular joint through upregulation of hippocampal TRPV1 in ovariectomized rats

Temporomandibular disorders (TMDs) predominantly affect reproductive female patients, with pain the most frequent complaint. Although estrogens are believed to play important roles in TMD pain, the mechanism underlying modulation of TMD pain by estrogens remains largely unknown. Accumulating evidence implies that the hippocampus is involved in sexual dimorphism of pain sensitivity. In this study, we investigated the hippocampal TRPV1 (transient receptor potential vanilloid 1) expression in ovariectomized rats that received 17-beta-estradiol substitution and found that 17-beta-estradiol enhanced the mechanical allodynia of inflamed temporomandibular joint (TMJ) induced by complete Freund's adjuvant. Real-time PCR and immunoblotting demonstrated that TMJ inflammation significantly induced hippocampal TRPV1 expression compared with the control group but failed to induce it in the ovariectomized rats that received no estradiol replacement. In addition, estradiol potentiated TMJ inflammation-induced hippocampal TRPV1 expression in a dose-dependent manner in the ovariectomized rats. In contrast, TRPV1 transcription in amygdala, prefrontal cortex, and thalamus was not affected by TMJ inflammation and estradiol. Immunostaining showed TRPV1 localized in the processes and cytoplasm of pyramidal neurons in CA1-CA3 regions of the hippocampus. Moreover, intrahippocampal injection of TRPV1 antagonists capsazepine and 5'-iodo-resiniferatoxin into the CA1 region of the hippocampus significantly attenuated allodynia of inflamed TMJ in both nonovariectomized and ovariectomized rats that received estradiol replacement. Our results suggested that hippocampal TRPV1 can modulate central pain processing and estradiol may contribute to the sexual dimorphism of TMD pain sensitivity through upregulation of TRPV1 expression in the hippocampus.

Roles of the hippocampal formation in pain information processing

Pain is a complex experience consisting of sensory-discriminative, affective-motivational, and cognitive-evaluative dimensions. Now it has been gradually known that noxious information is processed by a widely-distributed, hierarchically- interconnected neural network, referred to as neuromatrix, in the brain. Thus, identifying the multiple neural networks subserving these functional aspects and harnessing this knowledge to manipulate the pain response in new and beneficial ways are challenging tasks. Albeit with elaborate research efforts on the cortical responses to painful stimuli or clinical pain, involvement of the hippocampal formation (HF) in pain is still a matter of controversy. Here, we integrate previous animal and human studies from the viewpoint of HF and pain, sequentially representing anatomical, behavioral, electrophysiological, molecular/biochemical and functional imaging evidence supporting the role of HF in pain processing. At last, we further expound on the relationship between pain and memory and present some unresolved issues.

Imaging of pain: recent developments

Brain imaging of pain has made remarkable strides in the past year and a half. The basic regional activation pattern after acute nociceptive stimulation is now fairly well clarified. The extension of imaging studies from normal subjects to include cohorts of pathological pain patients is occurring. The techniques of positron emission tomography, functional magnetic resonance imaging and single photon emission computed tomography have all been applied to the study of human pain processing and the assessment of physiological interventions or psychological manipulations. Studies using labelled ligands to trace receptor alterations have also been conducted. Although more work could be done on the pharmacology and physiology of anesthesiology, the resulting set of observations provides a deeper understanding of the basic human neurophysiology of pain and a potential neural framework for better pain management.

The articulo-cardiac sympathetic reflex in spinalized, anesthetized rats

Somatic afferent regulation of heart rate by noxious knee joint stimulation has been proven in anesthetized cats to be a reflex response whose reflex center is in the brain and whose efferent arc is a cardiac sympathetic nerve. In the present study we examined whether articular stimulation could influence heart rate by this efferent sympathetic pathway in spinalized rats. In central nervous system (CNS)-intact rats, noxious articular movement of either the knee or elbow joint resulted in an increase in cardiac sympathetic nerve activity and heart rate. However, although in acutely spinalized rats a noxious movement of the elbow joint resulted in a significant increase in cardiac sympathetic nerve activity and heart rate, a noxious movement of the knee joint had no such effect and resulted in only a marginal increase in heart rate. Because this marginal increase was abolished by adrenalectomy suggests that it was due to the release of adrenal catecholamines. In conclusion, the spinal cord appears to be capable of mediating, by way of cardiac sympathetic nerves, the propriospinally induced reflex increase in heart rate that follows noxious stimulation of the elbow joint, but not the knee joint.

Cerebral regional cortical blood flow response during joint stimulation in cats

Noxious stimulation of an elbow joint in the anesthetized cat increases cerebral blood flow over broad, bilateral areas of the cerebral cortex and increases systemic blood pressure. In order to eliminate the confounding effects of elevated blood pressure on cerebral blood flow, we re-examined this phenomenon in cats with a transected spinal cord at the T1 level. Noxious stimulation of an elbow joint resulted in a significant increase in blood flow in the forelimb area of the contralateral primary somatosensory cortex; the blood pressure remained unchanged. These data in cats suggest that the previously described bilateral increase in cerebral blood flow following noxious joint stimulation was due, in part, to the increased blood pressure.

Quantitative in vivo measurement of central benzodiazepine receptors in the brain of cats by use of positron-emission tomography and [11C]flumazenil

OBJECTIVE

To map central benzodiazepine receptors (BZRs) in the brain of cats by use of positron-emission tomography (PET) and [11C]flumazenil.

ANIMALS

6 male cats that weighed between 2.0 and 3.6 kg.

PROCEDURE

Brain images obtained by PET evaluation of [11C]flumazenil were superimposed on T2-weighted magnetic-resonance imaging (MRI) scans of the same cats. Detailed anatomic regions, such as the cerebral cortex, striatum, thalamus, midbrain, and cerebellum, on the PET images were evident by PET-MRI registration. Regional binding of [11C]flumazenil to BZRs was quantitatively measured by use of a model with 2 tissue compartments and 4 variables.

RESULTS

The highest value for distribution volume was observed in the cerebral cortex, and the lowest value was found in the midbrain of cats.

CONCLUSIONS AND CLINICAL RELEVANCE

Binding of [11C]flumazenil to BZRs in the brain of cats can be quantitatively measured by use of PET with the aid of PET-MRI registration. It is difficult to diagnose changes in these neuroreceptors within the field of current veterinary science. In the future, PET should prove useful for investigating and diagnosing brain disorders in animals in clinical settings.

Evidence of involvement of central neural mechanisms in generating fibromyalgia pain

Fibromyalgia syndrome (FMS) is characterized by widespread pain, fatigue, sleep abnormalities, and distress. Because FMS lacks consistent evidence of tissue abnormalities, recent investigations have focused on central nervous system mechanisms of pain. Abnormal temporal summation of second pain (wind-up) and central sensitization have been described recently in patients with FMS. Wind-up and central sensitization, which rely on central pain mechanisms, occur after prolonged C-nociceptor input and depend on activation of nociceptor-specific neurons and wide dynamic range neurons in the dorsal horn of the spinal cord. Other abnormal central pain mechanisms recently detected in patients with FMS include diffuse noxious inhibitory controls. These pain inhibitory mechanisms rely on spinal cord and supraspinal systems involving pain facilitatory and pain inhibitory pathways. Brain-imaging techniques that can detect neuronal activation after nociceptive stimuli have provided additional evidence for abnormal central pain mechanisms in FMS. Brain images have corroborated the augmented reported pain experience of patients with fibromyalgia during experimental pain stimuli. In addition, thalamic activity, which contributes significantly to pain processing, was decreased in fibromyalgia. However, central pain mechanisms of fibromyalgia may not depend exclusively on neuronal activation. Neuroglial activation has been found to play an important role in the induction and maintenance of chronic pain. These findings may have important implications for future research and the treatment of fibromyalgia pain.

Peripheral and central sensitization in fibromyalgia: pathogenetic role

Characteristic symptoms of fibromyalgia syndrome include widespread pain, fatigue, sleep abnormalities, and distress. Patients with fibromyalgia show psychophysical evidence of mechanical, thermal, and electrical hyperalgesia. Peripheral and central abnormalities of nociception have been described in fibromyalgia. Important nociceptor systems in the skin and muscles seem to undergo profound changes in patients with fibromyalgia through unknown mechanisms. They include sensitization of vanilloid receptor, acid-sensing ion channel receptors, and purino-receptors. Tissue mediators of inflammation and nerve growth factors can excite these receptors and cause extensive changes in pain sensitivity, but patients with fibromyalgia lack consistent evidence for inflammatory soft tissue abnormalities. Therefore, recent investigations have focused on central nervous system mechanisms of pain in fibromyalgia.

Mapping adenosine A(1) receptors in the cat brain by positron emission tomography with [(11)C]MPDX

We evaluated the potential of [(11)C]MPDX as a radioligand for mapping adenosine A(1) receptors in comparison with previously proposed [(11)C]KF15372 in cat brain by PET. Two tracers showed the same brain distribution. Brain uptake of [(11)C]MPDX (Ki = 4.2 nM) was much higher and washed out faster than that of [(11)C]KF15372 (Ki = 3.0 nM), and was blocked by carrier-loading or displaced with an A(1) antagonist. The regional A(1) receptor distribution evaluated with kinetic analysis is consistent with that previously measured in vitro. [(11)C]MPDX PET has a potential for mapping adenosine A(1) receptors in brain.

Jun, Fos and Krox in the hippocampus after noxious stimulation: simultaneous-input-dependent expression and nuclear speckling

Stimulation of sensory C-fibres produces extensive expression of the Fos, Jun and Krox families of inducible transcription factors (ITFs) in many nociceptive CNS areas [28]. In the hippocampus, however, c-Fos is only weakly induced by such stimulation, and expression of the other ITFs has not been studied. Here we examine the effects of single, repeated and simultaneous C-fibre inputs on ITF expressions in the rat hippocampus. A brief, strong electrical stimulation of sciatic nerve C-fibres induced little or no expression of c-Fos or Krox-20. In contrast, FosB was induced and continued to rise in all areas, whereas the basal expressions of c-Jun and Krox-24 were initially reduced but then returned during the subsequent 36 h. A weak noxious cutaneous stimulus applied to one hindpaw induced only weak expressions of the ITFs. However, if the sciatic stimulation was applied contralaterally and 6 h beforehand, this weak stimulus strongly induced Krox-24, but not other ITFs, i.e. there was a potentiation of Krox-24 expression. When these two stimuli were applied simultaneously a few c-Fos labelled cells did appear, and there was and an increased Krox-24 expression. There was also a strong potentiation of FosB and a strong reduction in c-Jun expression. This simultaneous stimulation was the only type of stimulation to induce expression of Krox-20. Also after simultaneous stimulation the majority of the nuclear labelling for FosB, but not of the other ITFs, had a speckled appearance. MK-801 blocked these changes in ITF expressions, but it could also cause the C-fibre stimulations to induce c-Fos and c-Jun in specific areas of the hippocampus. Thus C-fibre stimulation does affect transcription factor activity in the hippocampus; and the strong responses of some ITFs to simultaneous inputs points to their having a role as 'genetic coincidence detectors' in the hippocampus.

Exploring the pain "neuromatrix"

A considerable number of functional imaging studies have demonstrated the involvement of multiple central regions during the experience of pain. These regions process information in circuits that can broadly be assumed to process the affective, sensory, cognitive, motor, inhibitory, and autonomic responses stimulated by a noxious event. The concept of a "neuromatrix" for pain processing is, therefore, well supported. There is, however, scant evidence for any particular regional or circuit dysfunction during clinical pain. To be clinically useful, functional imaging may have to step beyond the generalities of the neuromatrix.

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.

Emerging concepts in the neurobiology of chronic pain: evidence of abnormal sensory processing in fibromyalgia

Chronic pain often differs from acute pain. The correlation between tissue pathology and the perceived severity of the chronic pain experience is poor or even absent. Furthermore, the sharp spatial localization of acute pain is not a feature of chronic pain; chronic pain is more diffuse and often spreads to areas beyond the original site. Of importance, chronic pain seldom responds to the therapeutic measures that are successful in treating acute pain. Physicians who are unaware of these differences may label the patient with chronic pain as being neurotic or even a malingerer. During the past decade, an exponential growth has occurred in the scientific underpinnings of chronic pain states. In particular, the concept of nonnociceptive pain has been refined at a physiologic, structural, and molecular level. This review focuses on this new body of knowledge, with particular reference to the chronic pain state termed "fibromyalgia."