Regional changes in forebrain activation during the early and late phase of formalin nociception: analysis using cerebral blood flow in the rat

Orexin receptors in the hippocampal dentate gyrus modulated the restraint stress-induced analgesia in the animal model of chronic pain

Previous studies have shown that stressful stimuli induced an adaptive response of reduced nociception, known as stress-induced analgesia (SIA). Since orexin neuropeptides are involved in pain modulation, and orexin neurons, primarily located in the lateral hypothalamus (LH), project to various hippocampal regions, such as the dentate gyrus (DG), the current study aimed to examine the role of orexin receptors within the DG region in the restraint SIA in the animal model of chronic pain. One hundred-thirty adult male Wistar rats (230-250 g) were unilaterally implanted with a cannula above the DG region. Animals were given SB334867 or TCS OX2 29 (1, 3, 10, and 30 nmol, 0.5 µl/rat) into the DG region as orexin-1 receptor (OX1r) and orexin-2 receptor (OX2r) antagonists, respectively, five min before exposure to a 3-hour restraint stress (RS) period. Animals were then undergone the formalin test to assess pain-related behaviors as the animal model of chronic pain. The results showed that RS produces an analgesic response during the early and late phases of the formalin test. However, intra-DG microinjection of OX1r and OX2r antagonists attenuated the restraint SIA. OX2r antagonist was more potent than OX1r antagonist in the early phase of the formalin test, while OX1r antagonist was little more effective in the late phase. Predominantly, it could be concluded that the orexinergic system in the DG region might act as a potential endogenous pain control system and a novel target for treating stress-related disorders.

Longitudinal FDG-PET scan study of brain changes in mice with cancer-induced bone pain and after morphine analgesia

Morphine is the most commonly used drug for treating physical and psychological suffering caused by advanced cancer. Although morphine is known to elicit multiple supraspinal analgesic effects, its behavioral correlates with respect to the whole-brain metabolic activity during cancer-induced bone pain have not been elucidated. We injected 4T1 mouse breast cancer cells into the left femur bone marrow cavity of BALB/c mice. All mice developed limb use deficits, mechanical allodynia, and hypersensitivity to cold, which were effectively suppressed with morphine. Serial 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) was performed for each mouse before cancer induction (0 day), after cancer-induced bone pain was established (14 days), and during effective morphine treatment (16 days). The longitudinal FDG-PET imaging analysis demonstrated that cancer-induced bone pain increased glucose uptake in the insular cortex and hypothalamus and decreased the activity of the retrosplenial cortex. Morphine reversed the activation of the insular cortex and hypothalamus. Furthermore, morphine activated the amygdala and rostral ventromedial medulla and suppressed the activity of anterior cingulate cortex. Our findings of hypothalamic and insular cortical activation support the hypothesis that cancer-induced bone pain has strong inflammatory and affective components in freely moving animals. Morphine may provide descending inhibitory and facilitatory actions in the treatment of cancer-induced bone pain in a clinical setting.

BOLD fMRI of C-Fiber Mediated Nociceptive Processing in Mouse Brain in Response to Thermal Stimulation of the Forepaws

Functional magnetic resonance imaging (fMRI) in rodents enables non-invasive studies of brain function in response to peripheral input or at rest. In this study we describe a thermal stimulation paradigm using infrared laser diodes to apply noxious heat to the forepaw of mice in order to study nociceptive processing. Stimulation at 45 and 46°C led to robust BOLD signal changes in various brain structures including the somatosensory cortices and the thalamus. The BOLD signal amplitude scaled with the temperature applied but not with the area irradiated by the laser beam. To demonstrate the specificity of the paradigm for assessing nociceptive signaling we administered the quaternary lidocaine derivative QX-314 to the forepaws, which due to its positive charge cannot readily cross biological membranes. However, upon activation of TRPV1 channels following the administration of capsaicin the BOLD signal was largely abolished, indicative of a selective block of the C-fiber nociceptors due to QX-314 having entered the cells via the now open TRPV1 channels. This demonstrates that the cerebral BOLD response to thermal noxious paw stimulation is specifically mediated by C-fibers.

Identifying the integrated neural networks involved in capsaicin-induced pain using fMRI in awake TRPV1 knockout and wild-type rats

In the present study, we used functional MRI in awake rats to investigate the pain response that accompanies intradermal injection of capsaicin into the hindpaw. To this end, we used BOLD imaging together with a 3D segmented, annotated rat atlas and computational analysis to identify the integrated neural circuits involved in capsaicin-induced pain. The specificity of the pain response to capsaicin was tested in a transgenic model that contains a biallelic deletion of the gene encoding for the transient receptor potential cation channel subfamily V member 1 (TRPV1). Capsaicin is an exogenous ligand for the TRPV1 receptor, and in wild-type rats, activated the putative pain neural circuit. In addition, capsaicin-treated wild-type rats exhibited activation in brain regions comprising the Papez circuit and habenular system, systems that play important roles in the integration of emotional information, and learning and memory of aversive information, respectively. As expected, capsaicin administration to TRPV1-KO rats failed to elicit the robust BOLD activation pattern observed in wild-type controls. However, the intradermal injection of formalin elicited a significant activation of the putative pain pathway as represented by such areas as the anterior cingulate, somatosensory cortex, parabrachial nucleus, and periaqueductal gray. Notably, comparison of neural responses to capsaicin in wild-type vs. knock-out rats uncovered evidence that capsaicin may function in an antinociceptive capacity independent of TRPV1 signaling. Our data suggest that neuroimaging of pain in awake, conscious animals has the potential to inform the neurobiological basis of full and integrated perceptions of pain.

Gabapentin reverses central hypersensitivity and suppresses medial prefrontal cortical glucose metabolism in rats with neuropathic pain

BACKGROUND

Gabapentin (GBP) is known to suppress neuropathic hypersensitivity of primary afferents and the spinal cord dorsal horn. However, its supra-spinal action sites are unclear. We identify the brain regions where GBP changes the brain glucose metabolic rate at the effective dose that alleviates mechanical allodynia using 18 F-fluorodeoxyglucose-positron emission tomography (FDG-PET) scanning.

RESULTS

Comparing the PET imaging data before and after the GBP treatment, the spared nerve injury-induced increases of glucose metabolism in the thalamus and cerebellar vermis were reversed, and a significant decrease occurred in glucose metabolism in the medial prefrontal cortex (mPFC), including the anterior cingulate cortex. GBP treatment also reversed post-SNI connectivity increases between limbic cortices and thalamus.

CONCLUSIONS

Our results indicate that GBP analgesic effect may be mediated by reversing central hypersensitivity, and suppressing mPFC, a crucial part of the cortical representation of pain, in the brain.

Inactivation of the prelimbic rather than infralimbic cortex impairs acquisition and expression of formalin-induced conditioned place avoidance

Conditioned place avoidance (CPA) paradigm has been used to investigate the affective component of pain. Although the anterior cingulate cortex (ACC) has been demonstrated to play an important role in the affective aspect of pain, whether the other prefrontal subdivisions are involved in pain-related aversion is unknown. The present study investigated the role of the prelimbic cortex (PL) and infralimbic cortex (IL) in the acquisition and expression of formalin-induced CPA (F-CPA) in rats. GABAA receptor agonist muscimol was bilaterally microinjected into PL/IL before or after the formalin-paired training, to explore the effect of temporary inactivation of PL/IL on the acquisition and expression of F-CPA, respectively. The results showed that inactivation of PL rather than IL impaired the acquisition and expression of F-CPA. Moreover, the PL inactivation did not block the acquisition of LiCl-induced CPA, suggesting that PL may be specifically implicated in the pain-emotion related encoding. These results indicate that PL but not IL is involved in the aversive dimension of pain.

Synchronized electrical stimulation of the rat medial forebrain bundle and perforant pathway generates an additive BOLD response in the nucleus accumbens and prefrontal cortex

To study how a synchronized activation of two independent pathways affects the fMRI response in a common targeted brain region, blood oxygen dependent (BOLD) signals were measured during electrical stimulation of the right medial forebrain bundle (MFB), the right perforant pathway (PP) and concurrent stimulation of the two fiber systems. Repetitive electrical stimulations of the MFB triggered significant positive BOLD responses in the nucleus accumbens (NAcc), septum, anterior cingulate cortex/medial prefrontal cortex (ACC/mPFC), ventral tegmental area/substantia nigra (VTA/SN), right entorhinal cortex (EC) and colliculus superior, which, in general, declined during later stimulation trains. At the same time, negative BOLD responses were observed in the striatum. Thus, the same stimulus caused region-specific hemodynamic responses. An identical electrical stimulation of the PP generated positive BOLD responses in the right dentate gyrus/hippocampus proper/subiculum (DG/HC), the right entorhinal cortex and the left entorhinal cortex, which remained almost stable during consecutive stimulation trains. Co-stimulation of the two fiber systems resulted in an additive activation pattern, i.e., the BOLD responses were stronger during the stimulation of the two pathways than during the stimulation of only one pathway. However, during the simultaneous stimulation of the two pathways, the development of the BOLD responses to consecutive trains changed. The BOLD responses in regions that were predominantly activated by MFB stimulation (i.e., NAcc, septum and ACC/mPFC) did not decline as fast as during pure MFB stimulation, thus an additive BOLD response was only observed during later trains. In contrast, in the brain regions that were predominantly activated by PP stimulation (i.e., right EC, DG/HC), co-stimulation of the MFB only resulted in an additive effect during early trains but not later trains. Consequently, the development of the BOLD responses during consecutive stimulations indicates the presence of an interaction between the two pathways in a target region, whereas the observed averaged BOLD responses do not.

Methylglyoxal modification of Nav1.8 facilitates nociceptive neuron firing and causes hyperalgesia in diabetic neuropathy

This study establishes a mechanism for metabolic hyperalgesia based on the glycolytic metabolite methylglyoxal. We found that concentrations of plasma methylglyoxal above 600 nM discriminate between diabetes-affected individuals with pain and those without pain. Methylglyoxal depolarizes sensory neurons and induces post-translational modifications of the voltage-gated sodium channel Na(v)1.8, which are associated with increased electrical excitability and facilitated firing of nociceptive neurons, whereas it promotes the slow inactivation of Na(v)1.7. In mice, treatment with methylglyoxal reduces nerve conduction velocity, facilitates neurosecretion of calcitonin gene-related peptide, increases cyclooxygenase-2 (COX-2) expression and evokes thermal and mechanical hyperalgesia. This hyperalgesia is reflected by increased blood flow in brain regions that are involved in pain processing. We also found similar changes in streptozotocin-induced and genetic mouse models of diabetes but not in Na(v)1.8 knockout (Scn10(-/-)) mice. Several strategies that include a methylglyoxal scavenger are effective in reducing methylglyoxal- and diabetes-induced hyperalgesia. This previously undescribed concept of metabolically driven hyperalgesia provides a new basis for the design of therapeutic interventions for painful diabetic neuropathy.

Cerebrovascular responses of the rat brain to noxious stimuli as examined by functional near-infrared whole brain imaging

While near-infrared (NIR) spectroscopy has been increasingly used to detect stimulated brain activities with an advantage of dissociating regional oxy- and deoxyhemoglobin concentrations simultaneously, it has not been utilized much in pain research. Here, we investigated and demonstrated the feasibility of using this technique to obtain whole brain hemodynamics in rats and speculated on the functional relevance of the NIR-based hemodynamic signals during pain processing. NIR signals were emitted and collected using a 26-optodes array on rat's dorsal skull surface after the removal of skin. Following the subcutaneous injection of formalin (50 μl, 3%) into a hindpaw, several isolable brain regions showed hemodynamic changes, including the anterior cingulate cortex, primary/secondary somatosensory cortexes, thalamus, and periaqueductal gray (n = 6). Time courses of hemodynamic changes in respective regions matched with the well-documented biphasic excitatory response. Surprisingly, an atypical pattern (i.e., a decrease in oxyhemoglobin concentration with a concomitant increase in deoxyhemoglobin concentration) was seen in phase II. In a separate group of rats with innocuous brush and noxious pinch of the same area (n = 11), results confirmed that the atypical pattern occurred more likely in the presence of nociception than nonpainful stimulation, suggesting it as a physiological substrate when the brain processes pain. In conclusion, the NIR whole brain imaging provides a useful alternative to study pain in vivo using small-animal models. Our results support the notion that neurovascular response patterns depend on stimuli, bringing attention to the interpretation of vascular-based neuroimaging data in studies of pain.

Endogenous opioid-dopamine neurotransmission underlie negative CBV fMRI signals

Previous studies showed noxious unilateral forepaw electrical stimulation surprisingly evoked negative blood-oxygenation-level-dependent (BOLD), cerebral blood flow (CBF), and cerebral blood volume (CBV) fMRI responses in the bilateral striatum whereas the local neuronal spike and c-Fos activities increased. These negative responses are associated with vasoconstriction and appeared to override the increased hemodynamic responses that typically accompanied with increased neural activity. The current study aimed to investigate the role of μ-opioid system in modulating vasoconstriction in the striatum associated with noxious stimulation on a 4.7-Tesla MRI scanner. Specifically, we investigated: i) how morphine (a μ-opioid receptor agonist) affects the vasoconstriction in the bilateral striatum associated with noxious electrical forepaw stimulation in rats, and ii) how naloxone (an opioid receptor antagonist) and eticlopride (a dopamine D(2)/D(3) receptor antagonist) modulates the morphine effects onwards. Injection of morphine enhanced the negative striatal CBV responses to noxious stimulation. Sequential injection of naloxone in the same animals abolished the stimulus-evoked vasoconstriction. In a separate group of animals, injection of eticlopride following morphine also reduced the vasoconstriction. Our findings suggested that noxious stimulation endogenously activated opioid and dopamine receptors in the striatum and thus leading to vasoconstriction.

A combination of atlas-based and voxel-wise approaches to analyze metabolic changes in autoradiographic data from Alzheimer's mice

Murine models are commonly used in neuroscience research to improve our knowledge of disease processes and to test drug effects. To accurately study brain glucose metabolism in these animals, ex vivo autoradiography remains the gold standard. The analysis of 3D-reconstructed autoradiographic volumes using a voxel-wise approach allows clusters of voxels representing metabolic differences between groups to be revealed. However, the spatial localization of these clusters requires careful visual identification by a neuroanatomist, a time-consuming task that is often subject to misinterpretation. Moreover, the large number of voxels to be computed in autoradiographic rodent images leads to many false positives. Here, we proposed an original automated indexation of the results of a voxel-wise approach using an MRI-based 3D digital atlas, followed by the restriction of the statistical analysis using atlas-based segmentation, thus taking advantage of the specific and complementary strengths of these two approaches. In a preliminary study of transgenic Alzheimer's mice (APP/PS1), and control littermates (PS1), we were able to achieve prompt and direct anatomical indexation of metabolic changes detected between the two groups, revealing both hypo- and hypermetabolism in the brain of APP/PS1 mice. Furthermore, statistical results were refined using atlas-based segmentation: most interesting results were obtained for the hippocampus. We thus confirmed and extended our previous results by identifying the brain structures affected in this pathological model and demonstrating modified glucose uptake in structures like the olfactory bulb. Our combined approach thus paves the way for a complete and accurate examination of functional data from cerebral structures involved in models of neurodegenerative diseases.

Striatal and cortical BOLD, blood flow, blood volume, oxygen consumption, and glucose consumption changes in noxious forepaw electrical stimulation

Recent reports showed noxious forepaw stimulation in rats evoked an unexpected sustained decrease in cerebral blood volume (CBV) in the bilateral striatum, whereas increases in spike activity and Fos-immunoreactive cells were observed. This study aimed to further evaluate the hemodynamic and metabolic needs in this model and the sources of negative functional magnetic resonance imaging (fMRI) signals by measuring blood oxygenation-level-dependent (BOLD), cerebral-blood-flow (CBF), CBV, and oxygen-consumption (i.e., cerebral metabolic rate of oxygen (CMRO(2))) changes using an 11.7-T MRI scanner, and glucose-consumption (i.e., cerebral metabolic rate of glucose (CMRglc)) changes using micro-positron emission tomography. In the contralateral somatosensory cortex, BOLD, CBF, CBV, CMRO(2) (n=7, P<0.05), and CMRglc (n=5, P<0.05) increased. In contrast, in the bilateral striatum, BOLD, CBF, and CBV decreased (P<0.05), CMRO(2) decreased slightly, although not significantly from baseline, and CMRglc was not statistically significant from baseline (P>0.05). These multimodal functional imaging findings corroborate the unexpected negative hemodynamic changes in the striatum during noxious forepaw stimulation, and support the hypothesis that striatal hemodynamic response is dominated by neurotransmitter-mediated vasoconstriction, overriding the stimulus-evoked fMRI signal increases commonly accompany elevated neuronal activity. Multimodal functional imaging approach offers a means to probe the unique attributes of the striatum, providing novel insights into the neurovascular coupling in the striatum. These findings may have strong implications in fMRI studies of pain.

MicroPET imaging of noxious thermal stimuli in the conscious rat brain

Small animal positron emission tomography (microPET) has been utilized in the investigation of nociception. However, a possible drawback from previous studies is the reduced activation pattern due to the application of anesthesia. The purpose of the present study was to demonstrate a potential means of avoiding anesthesia during stimulation, as well as minimizing the confounding anesthetic effect. Sodium pentobarbital and ketamine were first evaluated to determine their effect on microPET images in the current study. [(18)F]-Fluorodeoxyglucose ((18)F-FDG) was an appropriate radiotracer to reveal activated regions in rat brains. Pentobarbital anesthesia significantly reduced (18)F-FDG uptake in neural tissues, blurrier to lower contrast; therefore, ketamine was used to anesthetize animals during microPET. After the rats were anesthetized and secured in a laboratory-made stereotaxic frame, a simple, noninvasive stereotaxic technique was used to position their heads in the microPET scanner and to roughly conform the images in the stereotaxic atlas. For functional imaging, conscious rats were restrained in cages with minimal ambient noise; short repetitive thermal stimuli were applied to each rat's tail subsequently. The rats were adequately anesthetized with ketamine following 30 min of scanning without stimulation. An activation index (AI) was calculated from microPET data to quantify the local metabolic activity changes according to the normalized (18)F-FDG dosage. The average AI indicated a side-to-side difference for all innocuous stimulations in the thalamus. However, such side-to-side difference was only observed for noxious heat and cold stimulations in primary somatosensory cortex (SI), secondary somatosensory cortex (SII), and agranular insular cortex (AIC). The present study demonstrated the feasibility of the microPET technique to image metabolic functions of the conscious rat brain, offering better rationale and protocol designs for future pain studies.

Assessment of brain responses to innocuous and noxious electrical forepaw stimulation in mice using BOLD fMRI

Functional magnetic resonance imaging (fMRI) using the blood oxygen level-dependent (BOLD) contrast was used to study sensory processing in the brain of isoflurane-anesthetized mice. The use of a cryogenic surface coil in a small animal 9.4T system provided the sensitivity required for detection and quantitative analysis of hemodynamic changes caused by neural activity in the mouse brain in response to electrical forepaw stimulation at different amplitudes. A gradient echo-echo planar imaging (GE-EPI) sequence was used to acquire five coronal brain slices of 0.5mm thickness. BOLD signal changes were observed in primary and secondary somatosensory cortices, the thalamus and the insular cortex, important regions involved in sensory and nociceptive processing. Activation was observed consistently bilateral despite unilateral stimulation of the forepaw. The temporal BOLD profile was segregated into two signal components with different temporal characteristics. The maximum BOLD amplitude of both signal components correlated strongly with the stimulation amplitude. Analysis of the dynamic behavior of the somatosensory 'fast' BOLD component revealed a decreasing signal decay rate constant k(off) with increasing maximum BOLD amplitude (and stimulation amplitude). This study demonstrates the feasibility of a robust BOLD fMRI protocol to study nociceptive processing in isoflurane-anesthetized mice. The reliability of the method allows for detailed analysis of the temporal BOLD profile and for investigation of somatosensory and noxious signal processing in the brain, which is attractive for characterizing genetically engineered mouse models.

Formalin pain increases the concentration of serotonin and its 5-hydroxyindoleacetic acid metabolite in the CA1 region of hippocampus

BACKGROUND AND THE PURPOSE OF THE STUDY

The hippocampal formation is involved in nociception. Prenatal serotonin depletion results in a significant decrease in the concentration of nociceptive sensitivity during the second phase of behavioral response in the formalin test.

METHODS

A microdialysis probe was inserted via a guide cannula into the right CA1 region of the hippocampus. Extracellular serotonin (5HT) and its 5- hydroxyindoleacetic acid (5HIAA) metabolite overflow were collected every 10 min during the formalin test and measured by HPLC with electrochemichal detector.

RESULTS

Compared to the sham group, formalin injection in the hind paw of the rat significantly increased 5HT after 10, 30, 40, and 50 min and increased 5HIAA after 10, 30, 40, 50, and 60 min collection time periods in hippocampal dialysate. (n=6 for each group at each sampling time). In the formalin treated rats serotonin and 5HIAA concentrations increased in the biphasic pattern in concert with the first and second phases of formalin pain.

CONCLUSION

The hippocampal formation might be involved in the processing of nociceptive information and serotonin-related mechanisms in the hippocampus may play a role in the biphasic behavioral responses to formalin noxious stimulation.

A new scenario for negative functional magnetic resonance imaging signals: endogenous neurotransmission

Functional magnetic resonance imaging (fMRI) has revolutionized investigations of brain functions. Increases in fMRI signals are usually correlated with neuronal activation, but diverse explanations have been proposed for negative fMRI responses, including decreases in neuronal activity, the vascular-steal effect, and large increases in oxygen consumption. These possible scenarios, although encompassing a wide range of potential neurovascular responses, cannot yet be used to interpret certain types of negative fMRI signals. Recent studies have found that intravenous injection of dopamine D(2) receptor (D2DR) agonist reduced the hemodynamic responses in the caudate-putamen (CPu); however, whether endogenous dopaminergic neurotransmission contributes to fMRI signals remains obscure. Since it has been suggested that the D2DR is involved in pain modulation, and the CPu shows equivocal fMRI signals during noxious stimulation, the present study established an animal model based on graded electrical stimulation to elicit different levels of nociception, and aimed to determine whether nociception-induced endogenous dopaminergic neurotransmission is sufficient to generate negative fMRI responses. Our results from cerebral blood volume (CBV)-weighted fMRI, Fos immunohistochemistry, and electrophysiological recording demonstrated a salient bilateral CBV decreases associated with heightened neuronal activity in the CPu induced by unilateral noxious electrical stimulation. In addition, preinjection of D2DR antagonist reduced the observed CBV decreases. Our findings reveal the role of the D2DR in regulating striatal vascular responses and suggest that endogenous neurotransmission-induced CBV decreases underlie negative fMRI signals. Hence, the influence of endogenous neurotransmission should be considered when interpreting fMRI data, especially in an area involved in strong vasoactive neurotransmission.

Effects of maternal corticosterone and stress on behavioral and hormonal indices of formalin pain in male and female offspring of different ages

In previous studies, we showed for the first time that prenatal stress in rats produces long-term alterations of formalin-induced pain behavior that are dependent on age and sex, and we demonstrated an important role of the serotonergic system in mechanisms of prenatal stress (Butkevich, I.P. and Vershinina, E.A., 2001; Butkevich, I.P. and Vershinina, E.A., 2003; Butkevich, I.P., Mikhailenko, V.A., Vershinina, E.A., Khozhai, L.I., Grigorev, I.P., Otellin, V.A., 2005; Butkevich, I.P., Mikhailenko, V.A., Khozhai, L.I., Otellin, V.A., 2006). In the present study, we focus on the influence of the maternal corticosterone milieu and its role in the effects of stress during pregnancy on formalin-induced pain and the corticosterone response to it in male and female offspring of different ages. For this purpose, we used adrenalectomy (AD) in female rats 3-4 weeks before mating (as distinct from AD typically performed at the beginning of pregnancy). Since AD is considered a reliable method to treat hypercortisolism, researches on the effects of long-term AD in dams on the systems responsible for adaptive behavior in offspring are important (such studies are not described in the literature). The results demonstrate that the differences in the corticosterone response to injection of formalin and saline are obvious in 90-day-old (adult) female offspring but masked in 25-day-old ones. AD promoted the corticosterone response to formalin-induced pain but not to injection of saline in prenatally non-stressed female offspring of both ages. Prenatal stress canceled the differences in corticosterone response to injection of formalin and saline in 25-day-old offspring of AD dams and in adult offspring of sham-operated (SH) dams but caused similar differences in adult offspring of AD dams. Sex differences were found in basal corticosterone levels in AD prenatally stressed rats of both age groups, with a higher level in females, and in the corticosterone response to formalin-induced pain in the adult rats of all groups investigated, with higher corticosterone levels in females. In regard to pain behavior, AD induced significant changes in flexing+shaking in prenatally non-stressed adult offspring and canceled the differences in this behavior between non-stressed and stressed 25-day-old offspring. There were sex differences in pain behavior of the adult rats: greater flexing+shaking in AD non-stressed males but in SH non-stressed females; greater licking in prenatally-stressed AD and SH females. These results indicate that the long-term influences of maternal corticosterone on formalin-induced pain and the corticosterone response to it are determined by the sex and age of the offspring and suggest that other mechanisms, including serotonergic ones revealed in our previous studies, are involved in the effects of prenatal stress on inflammatory pain behavior.

Exploring nociceptive response by BOLD fMRI in alpha-chloralose anesthetized rats

The technique of blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) was used to provide a spatial-temporal mapping of nociceptive activation in brain. We contrived to obtain an illustration of the pain related regions by injecting formalin at the hindpaw using a 4.7 T MR system in alpha-chloralose anesthetized rats. In order to obtain the pain response, we avoided any invasive surgery on animals to purify the signal of nociception. The dynamic data were analyzed by mapping correlation coefficient and the time activity curves were calculated by atlas-based region of interest selection. The BOLD signals showed obvious difference in anterior cingulated cortex, somatosensory cortex, medial thalamus, and striatum after stimulation. The results not only show the global somatotopic organization of noxious stimulation on hindpaw in rats, but also provided invaluable information for neuroscience research.

Dynamic evaluation of [18F]-FDG uptake in the rat brain by microPET imaging

This study aims to acquire the functional image of the rat brain, small animal positron emission tomography (microPET) with high resolution and sensitivity is adopted to assess the metabolic activity corresponding to the neuronal activity induced by the electrical stimulation of the rat tail using [18F] fluorodeoxyglucose (FDG) as the radiotracer. The microPET imaging technology can provide anatomical and functional information on neuronal activity used to analyze responses in pathway sequence relationships between the thalamus and the cerebral cortex.

Longitudinal evaluation of the Hdh(CAG)150 knock-in murine model of Huntington's disease

Several murine genetic models of Huntington's disease (HD) have been developed. Murine genetic models are crucial for identifying mechanisms of neurodegeneration in HD and for preclinical evaluation of possible therapies for HD. Longitudinal analysis of mutant phenotypes is necessary to validate models and to identify appropriate periods for analysis of early events in the pathogenesis of neurodegeneration. Here we report longitudinal characterization of the murine Hdh(CAG)150 knock-in model of HD. A series of behavioral tests at five different time points (20, 40, 50, 70, and 100 weeks) demonstrates an age-dependent, late-onset behavioral phenotype with significant motor abnormalities at 70 and 100 weeks of age. Pathological analysis demonstrated loss of striatal dopamine D1 and D2 receptor binding sites at 70 and 100 weeks of age, and stereological analysis showed significant loss of striatal neuron number at 100 weeks. Late-onset behavioral abnormalities, decrease in striatal dopamine receptors, and diminished striatal neuron number observed in this mouse model recapitulate key features of HD. The Hdh(CAG)150 knock-in mouse is a valid model to evaluate early events in the pathogenesis of neurodegeneration in HD.

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.

Differences in forebrain activation in two strains of rat at rest and after spinal cord injury

Forebrain activation patterns in normal and spinal-injured Sprague-Dawley (SD) rats were determined by measuring regional cerebral blood flow as an indicator of neuronal activity. Data are compared to our previously published findings from normal and spinal-injured Long-Evans (LE) rats and reveal a striking degree of overlap, as well as differences, between strains in the basal (unstimulated) forebrain activation in normal animals. Specifically, 81% of the structures sampled showed similar activation in both strains, suggesting a consistent and identifiable pattern of basal cerebral activation in the rat. LE controls showed significantly greater basal activation in the remaining structures compared to SD control group, including the anterior dorsal thalamus, basolateral amygdala, SII cortex, and the hypothalamic paraventricular nucleus. In contrast, spinal cord injury (SCI) resulted in strain-specific changes in forebrain activation categorized by structures that showed significant increases in: (1) only LE SCI rats (posterior, ventrolateral, and ventroposterolateral thalamic nuclei); (2) only SD SCI rats (anterior-dorsal and medial thalamus, basolateral amygdala, cingulate and retrosplenial cortex, habenula, interpeduncular nucleus, hypothalamic paraventricular nucleus, periaqueductal gray); or (3) both strains (arcuate nucleus, ventroposteromedial thalamus, SI and SII somatosensory cortex). These results provide information related to the remote, i.e. supraspinal, effects of spinal cord injury and suggest that genetic differences play an important part in the forebrain response to such injury. Brain activation studies therefore provide a useful tool in understanding the full extent of secondary consequences following spinal injury and for identifying potential central mechanism responsible for the development of pain.

Representations of pain in the brain

In this paper, the relationships between neural mechanisms of persistent pain and the neural representations of these conditions in the human and animal brain will be reviewed. Animal models of chronic pain, such as the sciatic nerve constrictive injuries, are accompanied by somatotopically organized increases in several pain-related areas of the brain. Recent human brain imaging studies utilizing functional magnetic resonance imaging and positron emission tomography have elucidated the cerebral representations of visceral and somatic hypersensitivity. Both forms of hypersensitivity are represented in similar brain regions that are activated during acute pain, yet have a more extensive or intense cerebral representation. This suggests that these somatic and visceral hyperalgesic states may be represented by increased activity in the same cerebral pathways and centers that are involved in nociceptive stimuli in normal individuals. Hyperalgesic states during clinically relevant pain are especially reflected in brain areas such as the anterior cingulate and prefrontal cortical regions.

Functional magnetic resonance imaging studies of opioid receptor-mediated modulation of noxious-evoked BOLD contrast in rats

RATIONALE

Functional magnetic resonance imaging (fMRI) in rats can non-invasively identify brain regions activated by physiological stimuli and the effects of pharmacological intervention on these responses.

OBJECTIVES

This study was conducted to investigate the effects of systemic administration of the mu-opioid receptor agonist morphine on whole brain functional signal intensity in anaesthetised rats; to investigate whether pre-treatment with the opioid receptor antagonist naloxone blocks the effects of morphine; to determine whether pre-treatment with morphine attenuates noxious-evoked changes in whole brain functional signal intensity.

METHODS

Continuous whole brain fMRI scanning was used to study brain signal intensity prior to, and following, systemic administration of morphine (5 mg/kg, n=7), systemic administration of naloxone (1 mg/kg) and morphine (n=8). Effects of pre-treatment with saline (n=5) or morphine (5 mg/kg, n=5) on formalin (5%, intraplantar)-evoked changes in signal intensity were determined. Data were processed using SMP99 with fixed-effects analysis (p<0.05).

RESULTS

Morphine produced significant positive bilateral increases in signal intensity in the cingulate cortex, amygdala, thalamus, hypothalamus and PAG (p<0.05), and these effects were blocked by naloxone. Intraplantar injection of formalin produced a significant positive increase in signal intensity in the cingulate cortex, somatosensory cortex, amygdala, thalamus, hypothalamus and PAG (p<0.05). Morphine attenuated formalin-evoked increases in signal intensity in the PAG, amygdala, hypothalamus and cingulate cortex.

CONCLUSION

Our data demonstrate that morphine modulates noxious-evoked changes in signal intensity in discrete brain regions. fMRI studies in rats are able to identify specific brain regions involved in the pharmacological modification of physiologically evoked changes in regional brain activation.

Reduced serotonin synthesis during early embryogeny changes effect of subsequent prenatal stress on persistent pain in the formalin test in adult male and female rats

The considerable evidence supporting a role for serotonin (5-HT) in the embryonic formation of CNS, mediation of prenatal stress, and pain processing is reviewed. Long-term influences of prenatal 5-HT depletion as well as its combination with prenatal stress effects on tonic nociceptive system in 90-day-old Wistar rats were studied in the formalin test. Pregnant dams were injected with para-chlorophenylalanine (pCPA, 400 mg/kg/2 ml, ip), producing 5-HT depletion during the early period of fetal serotonergic system development. The adult offspring from pCPA-treated dams revealed changes in behavioral indices of persistent pain (flexing + shaking and licking) in the formalin test (2.5%, 50 microl) that were accompanied by irreversible morphological alterations in the dorsal raphe nuclei. In the other series of experiments, the role of 5-HT in the mediation of prenatal stress on the behavioral indices of persistent pain was investigated in the adult offspring from dams with 5-HT depletion followed by restraint stress. Stress during the last embryonic week caused much more increase in flexing + shaking and licking in the second tonic phase of the response to formalin in offspring from pCPA- than saline-treated (control) dams. The former was characterized by alterations in the durations of the interphase, the second phase, and the whole behavioral response too. In offspring from pCPA-treated dams, sex dimorphism was revealed in tonic pain evaluated by licking. Together with our previous results in juvenile rats demonstrating the necessity of definite level of prenatal 5-HT for normal development of tonic nociceptive system, the present pioneering findings obtained in adult rats indicate that prenatal 5-HT depletion causes long-term morphological abnormalities in the dorsal raphe nuclei accompanied by alterations in behavioral indices of tonic pain. Early prenatal 5-HT depletion increases vulnerability of tonic nociceptive circuits to the following prenatal stress.

Effects of ketamine anesthesia on central nociceptive processing in the rat: a 2-deoxyglucose study

Ketamine is a dissociative anesthetic with complex actions on the CNS. We investigated here the effects of ketamine anesthesia on somatosensory processing in the rat spinal cord, thalamus, and cerebral cortex, using the quantitative 2-deoxyglucose mapping technique. Unanesthetized or ketamine-anesthetized male Sprague-Dawley rats received a s.c. injection of a dilute formaldehyde solution (5%, 0.08 ml) into a forepaw, inducing prolonged noxious afferent input, or an equal volume of isotonic saline as a control stimulus. The 2-deoxyglucose experiments started 30 min after the injection. In the cervical enlargement of the spinal cord, ketamine had no significant effect on glucose metabolic rates in saline-injected animals, whereas it prevented the metabolic increases elicited by prolonged noxious stimulation in unanesthetized animals. At the thalamic level, ketamine increased glucose uptake in both saline- and formalin-injected rats in the lateral posterior, lateral dorsal, medial dorsal, gelatinosus, antero-ventral and antero-medial thalamic nuclei, whereas it decreased metabolic activity in the ventro-basal complex. At the cortical level, the drug increased metabolic activity in both control and formalin groups in the lacunosus-molecularis layer of the dorsal hippocampus, posterior parietal, retrosplenial, cingulate and frontal cortex; significant metabolic decreases were found in the CA1 region of the dorsal hippocampus and in the parietal 1 and 2 cortical areas. In the investigated brain regions, ketamine did not abolish noxious-evoked increases in glucose uptake, which were in fact enhanced in the forelimb cortex and in the lacunosus-molecularis layer of the hippocampus. The dissociation between the spinal and supraspinal effects of ketamine suggests a specific antinociceptive action on spinal circuits, in parallel with complex changes of the activity of brain circuits involved in somatosensory processing. More generally, this study shows that functional imaging techniques are able to quantitatively assess the effects of anesthetic drugs on nociceptive processing at different levels of the neuraxis.

Hypothalamo-pituitary-adrenal axis dysfunction as a contributory factor to chronic pain and depression

Chronic pain and depressive illness are variably resistant to treatment with current pharmacologic therapies. Pain as a reflex sensory response is accompanied by a fast autonomic and delayed neuroendocrine response mediated by the sympathoadrenal and hypothalamo-pituitary-adrenal (HPA) axis, respectively. The emotional aspect of the pain response is encoded by corticolimbic systems (including the HPA axis) to encapsulate the relationship between pain, memory, and mood. These same systems contribute to the symptomatology of depression, a common symptom of which is pain. Conversely, many chronic pain patients may suffer from depressive illness, which appears to develop as a consequence of chronic pain. A comparison of key changes in HPA function after chronic stress in animals with clinical depression in humans, reveals some striking similarities. In this article, the role of the HPA axis in the etiology of chronic pain and depression is discussed.

Decreased serotonin level during pregnancy alters morphological and functional characteristics of tonic nociceptive system in juvenile offspring of the rat

Serotonin (5-HT) contributes to the prenatal development of the central nervous system, acting as a morphogen in the young embryo and later as a neurotransmitter. This biologically active agent influences both morphological and biochemical differentiation of raphe neurons, which give rise to the descending serotonergic paths that regulate the processing of acutely evoked nociceptive inputs. The involvement of 5-HT in the prenatal development of tonic nociceptive system has not been studied. In the present study we evaluated the effects of a single injection (400 mg/kg, 2 ml, i.p.) of the 5-HT synthesis inhibitor, para-chlorophenylalanine (pCPA), given to pregnant rats during the critical period fetal serotonin development. The functional integrity of the tonic nociceptive response was investigated in 25 day old rats using the classic formalin test. Morphological analysis of brain structures involved in formalin-induced pain and 5-HT levels in the heads of 12-day embryos were also evaluated. Embryonic levels of 5-HT were significantly lowered by the treatment. The juvenile rats from pCPA-treated females showed altered brain morphology and cell differentiation in the developing cortex, hippocampus, raphe nuclei, and substantia nigra. In the formalin test, there were significant decreases in the intensity and duration of the second phase of the formalin-induced response, characterizing persistent, tonic pain. The extent of impairments in the brain structures correlated positively with the level of decrease in the behavioral responses. The data demonstrate the involvement of 5-HT in the prenatal development of the tonic nociceptive system. The decreased tonic component of the behavioral response can be explained by lower activity of the descending excitatory serotonergic system originating in the raphe nuclei, resulting in decreased tonic pain processing organized at the level of the dorsal horn of the spinal cord.

Independent time courses of supraspinal nociceptive activity and spinally mediated behavior during tonic pain

The behavioral response to acute tissue injury is usually characterized by different phases, but the brain mechanisms underlying changes in pain-related behavior over time are still poorly understood. We aimed to analyze time-dependent changes in metabolic activity levels of 49 forebrain structures in the formalin pain model, using the autoradiographic 2-deoxyglucose method in unanesthetized, freely moving rats. We examined rats during the first phase of pain-related reactions ('early' groups), or during the third recovery phase, 60 min later, when the supraspinally mediated behavioral responses were reduced ('late' group). In the early groups, metabolic rates were bilaterally increased over control values in the periaqueductal gray, zona incerta and in several thalamic nuclei (anteroventral, centrolateral, lateral dorsal, parafascicular, posteromedial, submedius, ventromedial, and ventrobasal complex), as well as in the habenulae and in the parietal, cingulate, antero-dorsal insular, and anterior piriform cortex. A contralateral, somatotopically specific activation was found in the putative hindlimb representation area of the somatosensory cortex. In the late group, noxious-induced activation declined in most structures. However, metabolic rates were higher than controls in the periaqueductal gray and zona incerta and in two other structures not previously active: the prerubral area/field of Forel and the arcuate hypothalamic nucleus. These findings provide a time-dependent functional map of nociceptive and anti-nociceptive forebrain circuits during tonic pain. The parallel decrease in licking behavior and forebrain activity, at times when spinally mediated limb flexion responses were still present, suggests that endogenous antinociceptive systems may differently modulate spinal and supraspinal nociceptive networks following acute tissue injury.

Formalin-induced pain and mu-opioid receptor density in brain and spinal cord are modulated by A1 and A2a adenosine agonists in mice

The effects of adenosine analogues on pain have been shown to depend on the subtype receptor involved as well as on the nociceptive stimuli and on the route of administration. In the first experiment of the present study intraperitoneal administration of the A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) (0.015, 0.03, 0.09, 0.15, 0.21, 0.3 mg/kg) induced dose-dependent analgesia to formalin pain in both phases characterizing the test. The A(2a) receptor agonist 2-[p-2-(carbonyl-ethyl)-phenyethylamino]-5'-N-ethylcarboxaminoadenosine (CGS21680) (0.025, 0.05, 0.1, 0.15 mg/kg) significantly affected behavioral responses to formalin only during the early phase. In the second experiment the interaction between adenosine and the opioid system was investigated through both behavioral and neurochemical studies. The opioid antagonist naltrexone (0.1 mg/kg) did not affect the antinociception induced by CPA (0.21 mg/kg) and CGS21680 (0.05 mg/kg). Autoradiographic studies showed that formalin administration significantly modified mu-opioid receptor density in the superficial laminae of the spinal cord and in the paracentral thalamic nucleus, contralateral to the side of formalin injection. CPA and CGS21680 counteracted these effects induced by formalin. In conclusion the present study confirms and extends the role of A(1) and A(2a) adenosine receptors in the modulation of inflammatory pain and their interaction with the mu-opioid system, and suggests further investigation of these purinergic receptors from a therapeutic perspective.

Irritable bowel syndrome as a common precipitant of central sensitization

Animal models of neuropathic pain have significantly advanced our knowledge of abnormalities in central pain processing mechanisms in chronic pain disorders. New neuroimaging techniques using functional magnetic resonance imaging and positron emission tomography scanning are beginning to provide insight into cortical participation in the processing of pain. Irritable bowel syndrome (IBS) is one of the most common gastrointestinal disorders seen by physicians. Visceral hypersensitivity or decreased pain thresholds to distension of the gut is considered to be a biologic marker for IBS and is present in most patients with this gastrointestinal disorder. Patients with IBS also have many extraintestinal symptoms consistent with a central hyperalgesic state. Recent studies suggest that patients with IBS may also have cutaneous hyperalgesia similar to that seen in other chronic pain disorders such as fibromyalgia. This suggests that abnormalities of central nociceptive processing are present in IBS.

Long-term changes in behavior and regional cerebral blood flow associated with painful peripheral mononeuropathy in the rat

We identified long-term (up to 12 weeks), bilateral changes in spontaneous and evoked pain behavior and baseline forebrain activity following a chronic constriction injury (CCI) of the sciatic nerve. The long-term changes in basal forebrain activation following CCI were region-specific and can be divided into forebrain structures that showed either: (1) no change, (2) an increase, or (3) a decrease in activity with regard to the short-term (2 weeks) changes we previously reported. All the rats showed spontaneous pain behaviors that persisted throughout the 12-week observation period, resembling the pattern of change found in four limbic system structures: the anterior dorsal thalamus, habenular complex, and the cingulate and retrosplenial cortices. In contrast, heat hyperalgesia was delayed in onset until 4 weeks following CCI, but then persisted, showing a nearly constant level of increased responsiveness. The forebrain activation that resembles this behavioral pattern of change is found in somatosensory cortex, and in the hypothalamic paraventricular nucleus and the basolateral amygdala. Finally, mechanical allodynia, which was maximal during the first 2 weeks following nerve injury and gradually recovered by the seventh post-operative week uniquely matches the time course of changes in ventrolateral and ventroposterolateral thalamic activity. Our results indicate that peripheral nerve damage results in persistent changes in behavior and resting forebrain systems that modulate pain perception. The persistent abnormalities in the somatosensory cortex and thalamus suggest that the sensory thalamocortical axis is functionally deranged in certain chronic pain states.

Neuropeptide expression in rat paraventricular hypothalamic neurons that project to the spinal cord

The paraventricular hypothalamic nucleus (PVH) exerts many of its regulatory functions through projections to spinal cord neurons that control autonomic and sensory functions. By using in situ hybridization histochemistry in combination with retrograde tract tracing, we analyzed the peptide expression among neurons in the rat PVH that send axons to the spinal cord. Projection neurons were labeled by immunohistochemical detection of retrogradely transported cholera toxin subunit B, and radiolabeled long riboprobes were used to identify neurons containing dynorphin, enkephalin, or oxytocin mRNA. Of the spinally projecting neurons in the PVH, approximately 40% expressed dynorphin mRNA, 40% expressed oxytocin mRNA, and 20% expressed enkephalin mRNA. Taken together with our previous findings on the distribution of vasopressin-expressing neurons in the PVH (Hallbeck and Blomqvist [1999] J. Comp. Neurol. 411:201-211), the results demonstrated that the different PVH subdivisions display distinct peptide expression patterns among the spinal cord-projecting neurons. Thus, the lateral parvocellular subdivision contained large numbers of spinal cord-projecting neurons that express any of the four investigated peptides, whereas the ventral part of the medial parvocellular subdivision displayed a strong preponderance for dynorphin- and vasopressin-expressing cells. The dorsal parvocellular subdivision almost exclusively contained dynorphin- and oxytocin-expressing spinal cord-projecting neurons. This parcellation of the peptide-expressing neurons suggested a functional diversity among the spinal cord-projecting subdivisions of the PVH that provide an anatomic basis for its various and distinct influences on autonomic and sensory processing at the spinal level.

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.

Bilateral behavioral and regional cerebral blood flow changes during painful peripheral mononeuropathy in the rat

A unilateral chronic constriction injury (CCI) of the sciatic nerve produced bilateral effects in both pain related behaviors and in the pattern of forebrain activation. All CCI animals exhibited spontaneous pain-related behaviors as well as bilateral hyperalgesia and allodynia after CCI. Further, we identified changes in baseline (unstimulated) forebrain activation patterns 2 weeks following CCI by measuring regional cerebral blood flow (rCBF). Compared to controls, CCI consistently produced detectable, well-localized and typically bilateral increases in rCBF within multiple forebrain structures in unstimulated animals. For example, the hindlimb region of somatosensory cortex was significantly activated (22%) as well as multiple thalamc nuclei, including the ventral medial (8%), ventral posterior lateral (10%) and the posterior (9%) nuclear groups. In addition, several forebrain regions considered to be part of the limbic system showed pain-induced changes in rCBF, including the anterior dorsal nucleus of the thalamus (23%), cingulate cortex (18%), retrosplenial cortex (30%), habenular complex (53%), interpeduncular nucleus (45%) and the paraventricular nucleus of the hypothalamus (30%). Our results suggest that bilateral somatosensory and limbic forebrain structures participate in the neural mechanisms of prolonged persistent pain produced by a unilateral injury.

Chronic, selective forebrain responses to excitotoxic dorsal horn injury

Intraspinal injection of the AMPA/metabotropic receptor agonist quisqualic acid (QUIS) results in excitotoxic injury which develops pathological characteristics similar to those associated with ischemic and traumatic spinal cord injury (SCI) (R. P. Yezierski et al., 1998, Pain 75: 141-155; R. P. Yezierski et al., 1993, J. Neurotrauma 10: 445-456). Since spinal injury can lead to partial or complete deafferentation of ascending supraspinal structures, it is likely that secondary to the disruption of spinal pathways these regions could undergo significant reorganization. Recently, T. J. Morrow et al. (Pain 75: 355-365) showed that autoradiographic estimates of regional cerebral blood flow (rCBF) can be used to simultaneously identify alterations in the activation of multiple forebrain structures responsive to noxious formalin stimulation. Accordingly, we examined whether excitotoxic SCI produced alterations in the activation of supraspinal structures using rCBF as a marker of neuronal activity. Twenty-four to 41 days after unilateral injection of QUIS into the T12 to L3 spinal segments, we found significant increases in the activation of 7 of 22 supraspinal structures examined. As compared to controls, unstimulated SCI rats exhibited a significant bilateral increase in rCBF within the arcuate nucleus (ARC), the hindlimb region of S1 cortex (HL), parietal cortex (PAR), and the thalamic posterior (PO), ventral lateral (VL), ventral posterior lateral (VPL), and ventral posterior medial (VPM) nuclei. All structures showing significantly altered rCBF are associated with the processing of somatosensory information. These changes constitute remote responses to injury and suggest that widespread functional changes occur within cortical and subcortical regions following injury to the spinal cord.

Monkey insular cortex neurons respond to baroreceptive and somatosensory convergent inputs

To investigate possible convergence of autonomic and somatosensory input in the insula of the non-human primate, extracellular single-unit recordings were obtained from 81 neurons (43 insular and 38 in surrounding cortex) during application of cutaneous nociceptive stimuli (pinch) and baroreceptor challenge in six anesthetized monkeys (Macaca fascicularis). All cells were also tested with light touch (brush) stimulation. Twenty-six units responded to blood pressure changes; 20 (80%) were identified within the insula (P < 0.001). The majority of these insular units (16/20) also responded to nociceptive pinch (convergent units). More units responsive to changes in blood pressure (unimodal and convergent) were found in the right (18/29, 62%) than in the left insular cortex (2/14, 14%)(P = 0.004). Twenty-nine insular neurons responded to nociceptive stimuli; 16 of these were convergent units and 13 showed unimodal responses to somatosensory stimuli alone. These cells had wide bilateral receptive fields including face, hand, foot and tail. Ten insular neurons were unresponsive to both sets of stimuli (non-responsive cells); significantly more of these cells (28/38) were identified in extrainsular locations (P < 0.01). We suggest that the primate insular cortex may be involved in the integration of cardiovascular function with somatosensory (principally nociceptive) input. This view supports the emerging role of the insular cortex as an important forebrain site of viscerosomatosensory regulation with clinical implications for cardiovascular regulation under conditions of stress and arousal.

Forebrain mechanisms of nociception and pain: analysis through imaging

Pain is a unified experience composed of interacting discriminative, affective-motivational, and cognitive components, each of which is mediated and modulated through forebrain mechanisms acting at spinal, brainstem, and cerebral levels. The size of the human forebrain in relation to the spinal cord gives anatomical emphasis to forebrain control over nociceptive processing. Human forebrain pathology can cause pain without the activation of nociceptors. Functional imaging of the normal human brain with positron emission tomography (PET) shows synaptically induced increases in regional cerebral blood flow (rCBF) in several regions specifically during pain. We have examined the variables of gender, type of noxious stimulus, and the origin of nociceptive input as potential determinants of the pattern and intensity of rCBF responses. The structures most consistently activated across genders and during contact heat pain, cold pain, cutaneous laser pain or intramuscular pain were the contralateral insula and anterior cingulate cortex, the bilateral thalamus and premotor cortex, and the cerebellar vermis. These regions are commonly activated in PET studies of pain conducted by other investigators, and the intensity of the brain rCBF response correlates parametrically with perceived pain intensity. To complement the human studies, we developed an animal model for investigating stimulus-induced rCBF responses in the rat. In accord with behavioral measures and the results of human PET, there is a progressive and selective activation of somatosensory and limbic system structures in the brain and brainstem following the subcutaneous injection of formalin. The animal model and human PET studies should be mutually reinforcing and thus facilitate progress in understanding forebrain mechanisms of normal and pathological 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.