Neuroanatomy of the pain system and of the pathways that modulate pain

Distribution of the EP3 prostaglandin E(2) receptor subtype in the rat brain: relationship to sites of interleukin-1-induced cellular responsiveness

The activation of neurosecretory neurons that express corticotropin-releasing hormone (CRH) in response to increased circulating levels of interleukin-1beta (IL-1beta) depends on prostaglandin E(2) (PGE(2)) acting locally within the brain parenchyma. To identify potential central targets for PGE(2) relevant to pituitary-adrenal control, the distribution of mRNA encoding the PGE(2) receptor subtype EP3 (EP3R) was analyzed in rat brain. Hybridization histochemistry revealed prominent labeling of cells in discrete portions of the olfactory system, iso- and hippocampal cortices, and subcortical telencephalic structures in the septal region and amygdala. Labeling over the midline, intralaminar, and anterior thalamic groups was particularly prominent. EP3R expression was enriched in the median preoptic nucleus and adjoining aspects of the medial preoptic area (MPO) implicated in thermoregulatory/febrile responses and sleep induction. EP3R-expressing cells were also prominent in brainstem cell groups involved in nociceptive information processing/modulation (periaqueductal gray, locus coeruleus (LC), parabrachial nucleus (PB), caudal raphé nuclei), arousal and wakefulness (LC, midbrain raphé and tuberomammillary nuclei); and in conveying interoceptive input, including systemic IL-1 signals, to the endocrine hypothalamus (nucleus of the solitary tract (NTS) and rostral ventrolateral medulla [VLM]). Combined hybridization histochemical detection of EP3R mRNA with immunolocalization of IL-1beta-induced Fos protein expression identified cytokine-sensitive, EP3R-positive cells in the medial NTS, rostral VLM, and, to a lesser extent, aspects of the MPO. These findings are consistent with the view that increased circulating IL-1 may stimulate central neural mechanisms, including hypothalamic CRH neurons, through an EP3R-dependent mechanism involving PGE(2)-mediated activation of cells in the caudal medulla and/or preoptic region.

Regional cerebral blood flow in fibromyalgia: single-photon-emission computed tomography evidence of reduction in the pontine tegmentum and thalami

OBJECTIVE

To determine whether regional cerebral blood flow (rCBF) is abnormal in any cerebral structure of women with fibromyalgia (FM), following a report that rCBF is reduced in the thalami and heads of caudate nuclei in FM.

METHODS

Seventeen women with FM and 22 healthy women had a resting single-photon-emission computed tomography (SPECT) brain scan to assess rCBF and a T1-weighted magnetic resonance imaging (MRI) scan to enable precise anatomic localization. Additionally, all participants underwent 2 manual tender point examinations and completed a set of questionnaires evaluating clinical features. SPECT scans were analyzed for differences in rCBF between groups using statistical parametric mapping (SPM) and regions of interest (ROIs) manually drawn on coregistered MRI.

RESULTS

Compared with control subjects, the rCBF in FM patients was significantly reduced in the right thalamus (P = 0.006), but not in the left thalamus or head of either caudate nucleus. SPM analysis indicated a statistically significant reduction in rCBF in the inferior pontine tegmentum (corrected P = 0.006 at the cluster level and corrected P = 0.023 for voxel of maximal significance), with consistent findings from ROI analysis (P = 0.003). SPM also detected a reduction in rCBF on the perimeter of the right lentiform nucleus. No correlations were found with clinical features or indices of pain threshold.

CONCLUSION

Our finding of a reduction in thalamic rCBF is consistent with findings of functional brain imaging studies of other chronic clinical pain syndromes, while our finding of reduced pontine tegmental rCBF is new. The pathophysiologic significance of these changes in FM remains to be elucidated.

Are 'neutral cells' in the rostral ventro-medial medulla subtypes of on- and off-cells?

The classification of cells in the rostral ventromedial medulla (RVM) is based on the response pattern to noxious tail heat: on-cell activity increased, off-cell activity decreased, and activity of neutral cells is unaffected by noxious heat tail stimulation. It is generally assumed that on-, off- and neutral cells respond equally to noxious stimulation applied anywhere on the body surface, but so far this assumption has not been systematically examined. In the present study the effects of thermal and mechanical stimuli applied to the tail, the extremities and the orofacial region on the extracellularly recorded activity of 14 neutral cells were investigated in lightly anesthetized rats. Although the neutral cells did not respond to noxious tail heat, all of them responded to most of the other stimuli in an on- or off-manner. Especially cell responses to pinch stimuli applied to the skin of the ear, the forehead and the nose differed from the neutral behavior. The fact that the neutral cells in the present study responded in an off- or on-manner by applying noxious stimuli different from noxious tail heat suggests that these cells are possibly subtypes of on- and off-cells in the RVM.

Expression of metabotropic glutamate receptors mRNA in the thalamus and brainstem of monoarthritic rats

Evidence for the involvement of metabotropic glutamate receptors (mGluR) in sensory processing has been emerging. Additionally, the differential distribution of distinct mGluR subtypes mRNA in particular thalamic nuclei of normal rats suggests that they could be involved in the processing of somatosensory information. In the present study, mGluR1, 3, 4 and 7 mRNAs expression was investigated by in situ hybridisation in selected brainstem and thalamic nuclei of adult monoarthritic rats at different time points of the disease (2, 4 and 14 days). Monoarthritic rats displayed behavioural and physical signs of painful arthritis at all time points. At 2 days of monoarthritis the mGluR1 mRNA expression was decreased mainly in the ventrobasal complex (VB) and in the posterior thalamic nuclei (Po) contralateral to the inflamed joint. The mGluR4 mRNA expression was also reduced, but minimum values were found at 4 days of monoarthritis, when no changes could be found in mGluR1 mRNA expression. At 14 days, mGluR4 mRNA expression was similar to controls, while mGluR1 mRNA was again reduced. Similar decreases of mGluR7 mRNA expression in the VB and Po were found at all time points, while mGluR3 mRNA expression was bilaterally increased in the reticular thalamic nucleus (Rt). In the brainstem no changes could be found in the expression of any mGluR subtype mRNA. The reduced expression of mGluR1, 4 and 7 transcripts in VB and Po, and the increases of mGluR3 mRNA in the Rt may contribute to counteract the increased noxious input arising from the periphery.

Neurophysiology of Cancer Pain: From the Laboratory to the Clinic

Pain is one of the most distressing symptoms associated with cancer. Basic science research has provided much insight into the mechanisms of peripheral and central pain and the actions of new drugs. Despite these advances, pain accompanying malignancy can be difficult to treat. Pain most commonly presents when the tumor has invaded somatic,visceral, or neural structures. An understanding of pain mechanisms is essential when deciding on the appropriate treatment. New therapeutic options have been developed and will hopefully provide clinicians with tools to successfully alleviate cancer pain.

Endogenous morphine

It is now well accepted that endogenous morphine is present in animals, both in invertebrates and vertebrates. It is a key signaling molecule that plays an important role in downregulating physiological responses, such as those in the immune system, including immune elements in the CNS. It has been demonstrated that a specific mu-opiate-receptor subtype, mu3, mediates these downregulatory effects through release of NO. This article examines morphine as an endogenous signaling molecule, in terms of its role in neural and immune regulation.

Mediation of spinal nerve injury induced tactile allodynia by descending facilitatory pathways in the dorsolateral funiculus in rats

Evidence exists to indicate that tactile allodynia arising from peripheral nerve injury is integrated predominately at supraspinal, rather than spinal, sites. In the present experiments, the possibility that disruption of descending pathways through the dorsolateral funiculus (DLF) might alter expression of nerve-injury induced tactile allodynia was explored. Male, Sprague-Dawley rats received L(5)/L(6) spinal nerve ligation (SNL). Lesions to the DLF were made ipsilateral or contralateral to SNL. Tactile allodynia was determined by measuring withdrawal thresholds to probing with von Frey filaments. Rats with DLF lesions presented no apparent motor deficits and did not alter sensory threshold in sham-SNL operated rats. DLF lesions made ipsilateral to SNL completely blocked tactile allodynia in SNL rats. Contralateral DLF lesions and sham surgery did not have any effect on SNL-induced allodynia. These results indicate that tactile allodynia after peripheral nerve injury is dependent upon tonic activation of net descending facilitation from supraspinal sites and support the hypothesis of tonic activation of descending facilitation as a basis for chronic pain.

Antinociceptive effects of morphine injected into the nucleus parafascicularis thalami of the rat

The antinociceptive action of morphine microinjected into the nucleus parafascicularis thalami (nPf) on pain behaviors organized at different levels of the neuraxis was examined in the rat. Behaviors organized at spinal (spinal motor reflexes, SMRs), medullary (vocalizations during shock, VDSs), and forebrain (vocalization afterdischarges, VADs) levels were elicited by noxious tailshock. Morphine administered into nPf generated dose-dependent increases in thresholds of VDS and VAD, but failed to elevate SMR thresholds. Increases in vocalization thresholds were reversed in a dose-dependent manner by the microinjection of the mu-opiate receptor antagonist, methylnaloxonium, into nPf. Results are discussed in terms of the relative influence of nPf-administered morphine on nociceptive processing at spinal versus supraspinal levels of the neuraxis.

Periaqueductal gray matter projections to midline and intralaminar thalamic nuclei of the rat

The periaqueductal gray matter (PAG) projections to the intralaminar and midline thalamic nuclei were examined in rats. Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected in discrete regions of the PAG, and axonal labeling was examined in the thalamus. PHA-L was also placed into the dorsal raphe nuclei or nucleus of Darkschewitsch and interstitial nucleus of Cajal as controls. In a separate group of rats, the retrograde tracer cholera toxin beta-subunit (CTb) was injected into one of the intralaminar thalamic nuclei-lateral parafascicular, medial parafascicular, central lateral (CL), paracentral (PC), or central medial nucleus-or one of the midline thalamic nuclei-paraventricular (PVT), intermediodorsal (IMD), mediodorsal, paratenial, rhomboid (Rh), reuniens (Re), or caudal ventral medial (VMc) nucleus. The distribution of CTb labeled neurons in the PAG was then mapped. All PAG regions (the four columns of the caudal two-thirds of the PAG plus rostral PAG) and the precommissural nucleus projected to the rostral PVT, IMD, and CL. The ventrolateral, lateral, and rostral PAG provided additional inputs to most of the other intralaminar and midline thalamic nuclei. PAG inputs to the VMc originated from the rostral and ventrolateral PAG areas. In addition, the lateral and rostral PAG projected to the zona incerta. No evidence was found for a PAG input to the ventroposterior lateral parvicellular, ventroposterior medial parvicellular, caudal PC, oval paracentral, and reticular thalamic nuclei. PAG --> thalamic circuits may modulate autonomic-, nociceptive-, and behavior-related forebrain circuits associated with defense and emotional responses.

Spinal and supraspinal mechanisms of neuropathic pain

Neuropathic pain is associated with abnormal tactile and thermal responses that may be extraterritorial to the injured nerve. Importantly, tactile allodynia and thermal hyperalgesia may involve separate pathways, since complete and partial spinal cord lesions have blocked allodynia, but not hyperalgesia, after spinal nerve ligation (SNL). Furthermore, lesions of the dorsal column, and lidocaine microinjected into dorsal column nuclei block only tactile allodynia. Conversely, thermal hyperalgesia, but not tactile allodynia was blocked by desensitization of C-fibers with resiniferotoxin. Therefore, it seems that tactile allodynia is likely to be mediated by large diameter A beta fibers, and not susceptible to modulation by spinal opioids, whereas hyperalgesia is mediated by unmyelinated C-fibers, and is sensitive to blockade by spinal opioids. Additionally, abnormal, spontaneous afferent drive in neuropathic pain may contribute to NMDA-mediated central sensitization by glutamate and by non-opioid actions of spinal dynorphin. Correspondingly, SNL elicited elevation in spinal dynorphin content in spinal segments at and adjacent to the zone of entry of the injured nerve along with signs of neuropathic pain. Antiserum to dynorphin A(1-17) or MK-801 given spinally blocked thermal hyperalgesia, but not tactile allodynia, after SNL, and also restored diminished morphine antinociception. Finally, afferent drive may induce descending facilitation from the rostroventromedial medulla (RVM). Blocking afferent drive with bupivicaine also restored lost potency of PAG morphine, as did CCK antagonists in the RVM. This observation is consistent with afferent drive activating descending facilitation from the RVM, and thus diminishing opioid activity, and may underlie the clinical observation of limited responsiveness of neuropathic pain to opioids.

Involvement of mGluR(5) on acute nociceptive transmission

The effect of the mGluR(5) antagonist, MPEP (2-Methyl-6-(phenylethynyl)-pyridine), and of the mGluR(1) antagonist, AIDA((RS)-1-Aminoindan-1,5-dicarboxylic acid), were examined on nociceptive neurons in the ventroposterolateral (VPL) nucleus of the thalamus in response to pressure stimuli to the contralateral hindpaw of rats under urethane anesthesia. Intravenous (i.v.) injection of MPEP (0.1, 1, and 10 mg/kg) blocked responses to noxious stimulation in a dose-dependent and reversible manner. AIDA (3 and 15 mg/kg, i.v.), in contrast, had no effect on these cells. MPEP action was selective to noxious stimulation because even when tested at the highest dose (10 mg/kg, i.v.) it did not alter the responses of non-nociceptive neurons to brush stimulation. To investigate the site of action of MPEP, intra-thalamic injections were made during electrophysiological recordings. Using this method, the mGluR(5) antagonist did not affect nociceptive responses, suggesting that thalamic receptors were not involved in this action. On the other hand, the NMDA thalamic receptors seem to be involved because the NMDA receptor antagonist, MK801, successfully blocked responses to noxious pressure stimulation following intra-thalamic injections. In the spinal cord in vitro model, MPEP (30 microM, 60 min) was also able to attenuate ventral root potentials after single shock electrical stimulation of the dorsal root and inhibit wind-up response evoked by repetitive stimulation. Taken together, these findings suggest that blockade of the mGluR(5), but not mGluR(1) decreases nociceptive transmission in the thalamus and that these effects may be mediated by spinal cord receptors.

Descending facilitatory modulation of a behavioral nociceptive response by stimulation in the adult rat anterior cingulate cortex

It is well documented that the descending endogenous analgesia system, including the periaqueductal gray (PAG) and the rostral ventral medulla (RVM), play an important role in modulation of nociceptive transmission and morphine- and cannabinoid-produced analgesia. Neurons in the PAG receive inputs from different nuclei of higher structures, including the anterior cingulate cortex (ACC). However, it is unclear if stimulation of neurons in the ACC modulates spinal nociceptive transmission. The present study has examined the effects of electrical stimulation and chemical activation of metabotropic glutamate receptors (mGluRs) in the ACC on a spinal nociceptive tail-flick (TF) reflex induced by noxious heating. Activation of the ACC at high intensities (up to 500 microA) of electrical stimulation did not produce any antinociceptive effect. Instead, at most sites within the ACC (n = 36 of 41 sites), electrical stimulation produced significant facilitation of the TF reflex (i.e. decreases in TF latency). Chemical activation of mGluRs within the ACC also produced a facilitatory effect. Descending facilitation from the ACC apparently relays at the RVM. Electrical stimulation in the RVM produces a biphasic modulatory effect, showing facilitation at low intensities and inhibition at higher intensities. The present study provides evidence that activation of mGluRs within the ACC can facilitate spinal nociception.

Effects of relaxation and stress on the capsaicin-induced local inflammatory response

OBJECTIVE

Although stress is known to modulate the inflammatory response, there has been little experimental examination of the effects of stress and stress reduction on inflammation in humans. In particular, the effects of stress and relaxation on neurogenic inflammation have been minimally studied. This study examines the effects of three experimental manipulations: mental stress, relaxation, and control on the local inflammatory response evoked by the intradermal injection of capsaicin, the active ingredient in chili peppers.

METHODS

Fifty subjects (28 men and 22 women) were pretrained in relaxation using an imagery-based relaxation tape and then randomized to experimental condition. Subjects participated in an evening reactivity session including 20 minutes of a stress (Stroop test), relaxation (tape), or control (video) manipulation, followed by a capsaicin injection in the forearm. Digitized flare measurements were taken for 1 hour postcapsaicin, and measurements of cardiovascular variables, cortisol, adrenocorticotrophic hormone, and norepinephrine were taken at regular intervals.

RESULTS

The size of the maximum capsaicin-induced flare was significantly smaller in the relaxation condition than in the stress or control conditions, which did not differ from each other. Increases in norepinephrine, heart rate, and systolic blood pressure during the experimental task, but not after capsaicin, significantly predicted size of maximum flare and total area under the curve of flare measurements.

CONCLUSIONS

These findings suggest that stress reduction may affect local inflammatory processes. Results are consistent with sympathetic modulation of the effects of relaxation on the flare response.

Evidence for ascending visceral nociceptive information in the dorsal midline and lateral spinal cord

The effect of acute, mid-cervical spinal cord lesions on neuronal and reflex activity evoked by the noxious visceral stimulus, colorectal distension (CRD; 80 mmHg, 20 s), was determined in halothane-anesthetized rats. Extracellular recordings were performed of neurons stereotaxically located within the ventrobasal group of the thalamus and in the region of the medullary lateral reticular nucleus. CRD-evoked activity of thalamic neurons was attenuated by lesions of the dorsal midline, but minimally affected by lateral lesions of the spinal cord. In contrast, CRD-evoked activity of medullary neurons was attenuated by lateral lesions ipsilateral to the recording site, but minimally affected by contralateral lateral lesions or dorsal midline lesions. Pseudo-affective visceromotor/cardiovascular responses were vigorous in rats with dorsal midline lesions and absent/attenuated in rats with bilateral lateral spinal lesions. This study presents evidence that visceral nociceptive information ascends in the spinal cord by both dorsal midline and lateral spinal pathways.

Prepulse inhibition of the blink reflex by laser stimuli in normal humans

The subcortical integrative effects of laser-induced activation of pain ascending tracts were examined in 11 healthy volunteers, aged 22-52 years. Subjects underwent either CO2 laser stimulation at the dorsum of the hand, electrical stimulation of digital nerves at the 3rd finger, or mechanical taps to the first dorsal interosseous space, preceding a blink reflex elicited by a supraorbital nerve electrical stimulus. The percentage inhibition induced in the R2 response of the blink reflex was similar for the three different stimulus modalities, but occurred at a different time interval. Compared to control trials, the R2 response of the test trials was a mean of 23.1% at the interval of 250 ms with laser stimuli, 17.4% at the interval of 100 ms with electrical stimuli to the 3rd finger, and 20.6% at the interval of 90 ms with a mechanical tap to the 1st interosseous space. Activation of pain receptors induces prepulse inhibition of the blink reflex at a delay corresponding to a slowly conducting pathway. The percentage inhibition is similar to that observed with other somatosensory inputs.

Interferon-gamma-responsive neuronal sites in the normal rat brain: receptor protein distribution and cell activation revealed by Fos induction

Constitutive expression of the interferon-gamma receptor protein (IFN-gammaR), and the distribution of cells in which Fos, a marker of cell activation, is induced by intracerebroventricular administration of IFN-gamma, were studied in the rat brain by immunohistochemistry. IFN-gammaR immunopositivity was found in neuronal elements, which exhibited a selective distribution being concentrated in the piriform and entorhinal cortex, midline thalamus and medial hypothalamic structures, brainstem nociceptive relays (including the periaqueductal gray, the parabrachial nuclei and the caudal part of the spinal trigeminal nuclei), and circumventricular organs such as the median eminence and area postrema. IFN-gamma-induced Fos expression mostly corresponded to neuronal sites of receptor distribution. Because of its topographical distribution, it is suggested that activation of the IFN-gammaR in neurons may play a role to limit spread of infections in the brain and, in concert with other proinflammatory cytokines, to modulate adaptive responses to an antigen challenge mediated by the central nervous system.

Punctate midline myelotomy for the relief of visceral cancer pain

OBJECT

This study offers clinical support for the concept that neurosurgical interruption of a midline posterior column pathway by performing a punctate midline myelotomy (PMM) provides significant pain relief without causing adverse neurological sequelae in cancer patients with visceral pain refractory to other therapies.

METHODS

A PMM of the posterior columns was performed in six cancer patients in whom visceral pain had been refractory to other therapies. The cause of the visceral pain was related to residual, progressive, or recurrent local cancer or postirradiation effects. Clinical efficacy of the procedure was examined by comparing patient pain ratings and narcotic usage pre- and post-PMM. Follow-up periods ranged from 3 to 31 months. Examination of the results indicates a significant reduction in pain ratings as well as a significant reduction in daily narcotic use. No adverse neurological effects were observed. One spinal cord has been recovered for postmortem examination.

CONCLUSIONS

These findings provide corroborating clinical evidence for the existence of a newly recognized midline posterior column pathway that mediates the perception of visceral pelvic and abdominal pain. Preliminary data indicate that significant pain relief can be obtained following PMM with minimal neurological morbidity and suggest that the procedure may provide an alternative treatment modality for cancer-related pain in patients in whom adequate pain control with narcotics cannot be achieved or narcotic side effects cannot be tolerated.

Correlations between serotonin level and single-cell firing in the rat's nucleus raphe magnus

The relation between serotonin release and electrical activity was examined in the nucleus raphe magnus of rats anesthetized with pentobarbital. Serotonin levels were monitored through a carbon-fiber microelectrode by fast cyclic voltammetry (usually at 1 Hz). Single-cell firing was recorded through the same microelectrode, except during the voltammetry waveform and associated electrical artifact (totaling about 30 ms). Multi-barrel micropipettes incorporating the voltammetry electrode were used for iontophoresis of drugs. Cells were inhibited, excited or unaffected by noxious mechanical skin stimulation. These were respectively designated as off(M) cells, on(M) cells and neutral(M) cells, M denoting mechanical. During 3 min of pinching, serotonin slowly rose near seven of 14 on(M) cells and 26 of 46 off(M) cells; it fell near two off(M) cells; it was unchanged near all other cells, including six neutral(M) cells. On a finer spatiotemporal scale, near four of seven on(M) cells, 10 of 14 off(M) cells and 0 of four neutral(M) cells, average serotonin levels fell significantly within +/- 100 ms of spontaneous spikes. Lower serotonin may have caused the higher spike probability; the converse is theoretically unlikely, since delays between release and detection are estimated to exceed 100 ms. Increased serotonin and decreased firing were always seen following iontophoresis or intravenous injection (1 mg/kg) of the serotonin re-uptake inhibitor clomipramine (n = 7). Iontophoresis of +/- propranolol, whose serotonergic actions include antagonism and partial agonism at 5-HT1 receptors, also increased serotonin and decreased firing (n=4). Methiothepin (intravenous, 1 mg/kg), whose serotonergic actions include 5-HT1 and 5-HT2 antagonism, typically raised serotonin levels (four of five cells) and always blocked inhibition by clomipramine (n = 3). Iontophoresis of glutamate always lowered serotonin and increased firing (n = 4). Since serotonin levels and firing were usually inversely correlated, except near on(M) cells during pinch, we propose that serotonin is released from terminals of incoming nociceptive afferents. Prior neuroanatomical knowledge favors a midbrain origin for these afferents, while some of the drug findings suggest that their terminals possess inhibitory serotonergic autoreceptors, possibly of 5-HT1b subtype. The released serotonin could contribute to the inhibition of off(M) cells and excitation of on(M) cells by noxious stimulation, since inhibitory 5-HT1a receptors and excitatory 5-HT2 receptors, respectively, have previously been shown to dominate their serotonergic responses.

GM1 ganglioside restores abnormal responses to acute thermal and mechanical stimuli in aged rats

We investigated the effect of aging on the responses to thermal and mechanical stimuli in rats. Young (3-5 months old) and aged (22-24 months old) male Sprague-Dawley rats were tested in the hot plate, high- and low-intensity radiant heat tail flick, and von Frey hair assays. Compared to young rats, aged rats displayed longer latencies in the hot plate and the high-intensity tail flick assays (hypoalgesia), but there was no difference in the low-intensity tail flick assay. In addition, aged rats had decreased thresholds to mechanical stimuli produced by von Frey hairs compared with young rats (mechanical allodynia). Administration of GM1 ganglioside, 30 mg/kg, i.p., once daily for 30 days, to aged rats partially restored the responses in the hot plate and von Frey hair assays. GM1 had no effect on the altered responses in the tail flick test in aged rats, and in general, had no effect on any sensory modality tested in young rats.

Neurophysiology of cancer pain

BACKGROUND

Recent basic science research has greatly added to our knowledge of pain mechanisms. Application of this knowledge to cancer pain syndromes has led to new and innovative approaches to cancer pain management.

METHODS

The mechanisms involved in the three main cancer pain syndromes (somatic, visceral, and neuropathic) are reviewed, and various therapeutic options are discussed.

RESULTS

Advances in knowledge in neurophysiology, neuroanatomy, and pharmacology have allowed a greater understanding of the peripheral and central mechanisms of pain. New drugs and interventional techniques based on this knowledge have improved the control of cancer pain.

CONCLUSIONS

Understanding the neurophysiology of cancer pain promotes use of the most appropriate palliative measures for pain control.

Propofol directly depresses lumbar dorsal horn neuronal responses to noxious stimulation in goats

PURPOSE

We tested the hypothesis that propofol, acting in the brain, would either enhance, or have no effect, on lumbar dorsal horn neuronal responses to a noxious mechanical stimulus applied to the hindlimb. We recorded the response of lumbar dorsal horn neurons during differential delivery of propofol to the brain and torso of goats.

METHODS

Goats were anesthetized with isoflurane and neck dissections performed which permitted cranial bypass. A laminectomy was made to allow microelectrode recording of lumbar dorsal horn neuronal activity. Isoflurane was maintained at 0.8+/-0.1% to both head and torso throughout the study. During cranial bypass propofol was separately administered to the torso (1 mg x kg(-1), n = 7; 3.75 mg x kg(-1), n = 8) or cranial (0.04 mg x kg(-1), n = 7; 0.14 mg kg(-1), n = 8) circulations.

RESULTS

Propofol administered to the torso depressed dorsal horn neuronal responses to noxious stimulation: low dose: 500+/-243 to 174+/-240 impulses x min(-1) at one minute post-injection, P<0.001; high dose: 478+/-204 to 91+/-138 impulses x min(-1) at one minute post-injection, P<0.05). Propofol administered to the cranial circulation had no effect: low dose: 315+/-150 to 410+/-272 impulses x min(-1), P>0.05; high dose: 462+/-261 to 371+/-196 impulses x min(-1), P>0.05.

CONCLUSIONS

These data indicate that propofol has a direct depressant effect on dorsal horn neuronal responses to noxious stimulation, with little or no indirect supraspinal effect.

Limbically Augmented Pain Syndrome (LAPS): Kindling, Corticolimbic Sensitization, and the Convergence of Affective and Sensory Symptoms in Chronic Pain Disorders

There is abundant clinical evidence that depression occurs with high frequency among chronic pain patients. When compared with other serious medical disorders, the prevalence of depression in chronic pain appears high. The fundamental reason for this association is unknown. Theories have attempted to explain the link between pain and depression in terms of psychologic mechanisms. Other theories highlight shared neurobiologic substrates. However, a comprehensive theory integrating biologic and psychologic viewpoints remains elusive. In this article, we draw on research on neuroplastic processes in corticolimbic structures to model the linkage between the sensory and affective domains of pain. Our hypothesis is based on kindling experiments in animals that elucidate the complex neurobiologic mechanisms that transduce exteroceptive and interoceptive stimuli into "memory" at the cellular/synaptic level. This experimental model has found application in the affective disorders to explain how a person's history of exposure to psychologic trauma configures the neurobiologic substrate for later-amplified pathologic response. In applying kindling research to pain, we begin by reviewing the literature on nociception-induced neuroplasticity at the corticolimbic level. We suggest that kindling and related models of neuroplasticity can be used to describe ways in which exposure to a noxious stimulus may, under certain conditions, lead to a sensitized corticolimbic state. This sensitized state can be described in terms of the kindling properties of amplification, spontaneity, neuroanatomic spreading, and cross-sensitization. A case example illustrates how these properties offer a neurobiologic framework for understanding the sensory/affective/behavioral symptom complex seen in a subset of chronic pain patients. These patients are characterized by atypical and treatment-refractory pain complaints, in association with disturbances of mood, sleep, energy, libido, memory/concentration, behavior, and stress intolerance. We introduce the term "limbically augmented pain syndrome" to describe this symptom complex.

Excitatory amino acid profiles of synovial fluid from patients with arthritis

OBJECTIVE

Previous studies in an experimental synovitis model in rats determined that administration of glutamate and aspartate into the joint produces hyperalgesic responses, while their receptor antagonists provide protection against the development of a hyperalgesic state. We examined concentrations of amino acids in synovial fluid (SF) to determine if increases might be relevant to human joint pathology.

METHODS

One hundred forty-four repository SF samples from patients undergoing diagnostic or therapeutic arthrocentesis and 14 SF samples from 7 cadavers were analyzed by high pressure liquid chromatography and compared as arthritic and control cohorts.

RESULTS

Compared to the average concentrations from the autopsy cases, the excitatory amino acids (EAA) glutamate and aspartate in SF from patients with synovitis were 54 and 28 times higher, respectively. Increases for all other amino acids ranged from 3 to 18-fold. The values for glutamate and aspartate were significantly higher than the mean increase for other amino acids compared using unpaired t tests (p < 0.0001). The mean ratio of glutamate and aspartate elevations over the mean increase for other amino acids was 4-fold and 2-fold, respectively. The EAA were highest in Reiter's, infectious arthropathies, and systemic lupus erythematosus, but did not appreciably segregate to diagnosis or SF white blood cell count.

CONCLUSION

Our data provide evidence of increased glutamate and aspartate in the SF of humans with active arthritis, suggesting that glutamate mediated events may contribute to the pathogenesis of human arthritic conditions.

Activation of the anterior cingulate cortex by thalamic stimulation in patients with chronic pain: a positron emission tomography study

Object

Deep brain stimulation (DBS) of the sensory thalamus has been used to treat chronic, intractable pain. The goal of this study was to investigate the thalamocortical pathways activated during thalamic DBS.

Methods

The authors compared positron emission tomography (PET) images obtained before, during, and after DBS in five patients with chronic pain. Two of the five patients reported significant DBS-induced pain relief during PET scanning, and the remaining three patients did not report any analgesic effect of DBS during scanning. The most robust effect associated with DBS was activation of the anterior cingulate cortex (ACC). An anterior ACC activation was sustained throughout the 40 minutes of DBS, whereas a more posteriorly located ACC activation occurred at a delay after onset of DBS, although these activations were not dependent on the degree of pain relief reported during DBS. However, implications specific to the analgesic effect of DBS require further study of a larger, more homogeneous patient population. Additional effects of thalamic DBS were detected in motor-related regions (the globus pallidus, cortical area 4, and the cerebellum) and visual and association cortical areas.

Conclusions

The authors demonstrate that the ACC is activated during thalamic DBS in patients with chronic pain.

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.

Modulation of nociceptive transmission by NMDA/glycine site receptor in the ventroposterolateral nucleus of the thalamus

NMDA-type glutamate receptors are involved in the generation and maintenance of altered pain states. In the present study, we examined the effect of an NMDA-glycine site antagonist, GV196771A [E-4, 6-dichloro-3-(2-oxo-1-phenyl-pyrrolidin-3-ylidenemethyl)-1H- indole-2- carboxylic acid sodium salt], on responses to noxious stimuli both in normal rats and during peripheral mononeuropathy induced by chronic constriction injury (CCI) of the sciatic nerve. In one series of experiments, activity of nociceptive neurons in the ventroposterolateral (VPL) nucleus of the thalamus was recorded in response to pressure stimuli to the contralateral hindpaw. Intravenous injection (iv) of the glycine antagonist had no effect on these cells in normal rats. When tested in rats with CCI induced 2-3 weeks previously, however, GV196771A (0.125, 0.5 and 2.0mg/kg) blocked responses to noxious stimulation in a dose-dependent and reversible manner. Morphine (0.5mg/kg, iv) and the NMDA channel blocker MK801 (0.1mg/kg, iv) suppressed noxious stimulus-evoked activity of VPL neurons in both normal and CCI-treated rats. MK801 also decreased the responses of non-nociceptive neurons to brush stimulation in both sets of animals, in contrast to the glycine antagonist which did not alter the responses of these cells. Similar results were obtained from a series of behavior experiments in which the latency for paw withdrawal from heat stimulation was measured in normal and CCI-treated rats. GV196771A (3 and 10mg/kg) injected orally, reduced the hyperalgesic response in the treated rats but did not change the withdrawal latency in normal rats. Taken together, these findings suggest that block of the NMDA receptor decreases nociceptive transmission in the thalamus and can modulate hyperalgesic states. GV196771A and glycine antagonists in general may represent innovative and safe agents for the treatment of neuropathic pain.

Differential distribution of metabotropic glutamate receptor subtype mRNAs in the thalamus of the rat

L-Glutamate (L-Glu) is present in most excitatory synapses of the mammalian brain, acting on several receptor subtypes. Height different genes encoding metabotropic glutamate receptors (mGluRs) subtypes have been described (mGluR1-8), having a distinct distribution in the brain. In the present study, the distribution of mGluR1, 3, 4, 5 and 7 mRNAs was determined in 20 thalamic nuclei of adult rats by performing in situ hybridisation with subtype-specific 35S-labelled oligonucleotide probes. High expression of mGluR1 mRNA mainly occurred in midline nuclei such as the centromedial/centrolateral (CM/CL) nuclei, parafascicular and submedius nuclei, and in the ventroposteromedial (VPM) and posterior (Po) nuclei. In contrast, mGluR5 mRNA was more uniformly distributed at weak to moderate levels, except in the reuniens nucleus where a strong signal was detected. The mGluR3 mRNA was highly expressed in the reticular thalamic nucleus and almost not detectable in any other thalamic region. Additionally, mGluR3 mRNA was found not only in neurones but also in putative glial cells. The mGluR4 mRNA was abundant in most thalamic nuclei, with prominent expression in the CM/CL, Po and ventrobasal complex (VPM and ventroposterolateral, VPL). Finally, mGluR7 transcripts were found evenly distributed throughout the thalamus at moderate levels, the highest signal being detected in the paraventricular thalamic nucleus, VPM, VPL and Po. This differential distribution of mGluR subtypes in the rat thalamus may contribute to the heterogeneity of glutamate effects on thalamic neurones. The mGluR1, mGluR4 and mGluR7 receptors may be involved in the processing of somatosensory information because they are expressed in nuclei that receive direct sensory input.

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.

Acupuncture modulates the limbic system and subcortical gray structures of the human brain: evidence from fMRI studies in normal subjects

Acupuncture, an ancient therapeutic technique, is emerging as an important modality of complementary medicine in the United States. The use and efficacy of acupuncture treatment are not yet widely accepted in Western scientific and medical communities. Demonstration of regionally specific, quantifiable acupuncture effects on relevant structures of the human brain would facilitate acceptance and integration of this therapeutic modality into the practice of modern medicine. Research with animal models of acupuncture indicates that many of the beneficial effects may be mediated at the subcortical level in the brain. We used functional magnetic resonance imaging (fMRI) to investigate the effects of acupuncture in normal subjects and to provide a foundation for future studies on mechanisms of acupuncture action in therapeutic interventions. Acupuncture needle manipulation was performed at Large Intestine 4 (LI 4, Hegu) on the hand in 13 subjects [Stux, 1997]. Needle manipulation on either hand produced prominent decreases of fMRI signals in the nucleus accumbens, amygdala, hippocampus, parahippocampus, hypothalamus, ventral tegmental area, anterior cingulate gyrus (BA 24), caudate, putamen, temporal pole, and insula in all 11 subjects who experienced acupuncture sensation. In marked contrast, signal increases were observed primarily in the somatosensory cortex. The two subjects who experienced pain instead of acupuncture sensation exhibited signal increases instead of decreases in the anterior cingulate gyrus (BA 24), caudate, putamen, anterior thalamus, and posterior insula. Superficial tactile stimulation to the same area elicited signal increases in the somatosensory cortex as expected, but no signal decreases in the deep structures. These preliminary results suggest that acupuncture needle manipulation modulates the activity of the limbic system and subcortical structures. We hypothesize that modulation of subcortical structures may be an important mechanism by which acupuncture exerts its complex multisystem effects.

Activation of the anterior cingulate cortex by thalamic stimulation in patients with chronic pain: a positron emission tomography study

OBJECT

Deep brain stimulation (DBS) of the sensory thalamus has been used to treat chronic, intractable pain. The goal of this study was to investigate the thalamocortical pathways activated during thalamic DBS.

METHODS

The authors compared positron emission tomography (PET) images obtained before, during, and after DBS in five patients with chronic pain. Two of the five patients reported significant DBS-induced pain relief during PET scanning, and the remaining three patients did not report any analgesic effect of DBS during scanning. The most robust effect associated with DBS was activation of the anterior cingulate cortex (ACC). An anterior ACC activation was sustained throughout the 40 minutes of DBS, whereas a more posteriorly located ACC activation occurred at a delay after onset of DBS, although these activations were not dependent on the degree of pain relief reported during DBS. However, implications specific to the analgesic effect of DBS require further study of a larger, more homogeneous patient population. Additional effects of thalamic DBS were detected in motor-related regions (the globus pallidus, cortical area 4, and the cerebellum) and visual and association cortical areas.

CONCLUSIONS

The authors demonstrate that the ACC is activated during thalamic DBS in patients with chronic pain.

Visceral nociception

Visceral pain is of great concern to the medical community because it remains particularly resistant to current clinical treatments. A serendipitous and initially unexplainable clinical finding that a punctate midline dorsal column lesion is effective in eliminating visceral pain, however, has initiated a resurgence of interest in the study of the basic mechanisms of visceral nociception. Clinical and anatomic findings have determined that visceral pain either of thoracic or pelvic origin can be relieved by carefully placed lesions directed at the lateral edge or the medial edge of the gracile fasciculus, respectively. Studies are demonstrating that visceral pain is quite unique from cutaneous pain.

Ascending projections from the area around the spinal cord central canal: A Phaseolus vulgaris leucoagglutinin study in rats

A single small iontophoretic injection of Phaseolus vulgaris leucoagglutinin labels projections from the area surrounding the spinal cord central canal at midthoracic (T6-T9) or lumbosacral (L6-S1) segments of the spinal cord. The projections from the midthoracic or lumbosacral level of the medial spinal cord are found: 1) ascending ipsilaterally in the dorsal column near the dorsal intermediate septum or the midline of the gracile fasciculus, respectively; 2) terminating primarily in the dorsal, lateral rim of the gracile nucleus and the medial rim of the cuneate nucleus or the dorsomedial rim of the gracile nucleus, respectively; and 3) ascending bilaterally with slight contralateral predominance in the ventrolateral quadrant of the spinal cord and terminating in the ventral and medial medullary reticular formation. Other less dense projections are to the pons, midbrain, thalamus, hypothalamus, and other forebrain structures. Projections arising from the lumbosacral level are also found in Barrington's nucleus. The results of the present study support previous retrograde tract tracing and physiological studies from our group demonstrating that the neurons in the area adjacent to the central canal of the midthoracic or lumbosacral level of the spinal cord send long ascending projections to the dorsal column nucleus that are important in the transmission of second-order afferent information for visceral nociception. Thus, the axonal projections through both the dorsal and the ventrolateral white matter from the CC region terminate in many regions of the brain providing spinal input for sensory integration, autonomic regulation, motor and emotional responses, and limbic activation.

Ascending projections from the area around the spinal cord central canal: A Phaseolus vulgaris leucoagglutinin study in rats

A single small iontophoretic injection of Phaseolus vulgaris leucoagglutinin labels projections from the area surrounding the spinal cord central canal at midthoracic (T6-T9) or lumbosacral (L6-S1) segments of the spinal cord. The projections from the midthoracic or lumbosacral level of the medial spinal cord are found: 1) ascending ipsilaterally in the dorsal column near the dorsal intermediate septum or the midline of the gracile fasciculus, respectively; 2) terminating primarily in the dorsal, lateral rim of the gracile nucleus and the medial rim of the cuneate nucleus or the dorsomedial rim of the gracile nucleus, respectively; and 3) ascending bilaterally with slight contralateral predominance in the ventrolateral quadrant of the spinal cord and terminating in the ventral and medial medullary reticular formation. Other less dense projections are to the pons, midbrain, thalamus, hypothalamus, and other forebrain structures. Projections arising from the lumbosacral level are also found in Barrington's nucleus. The results of the present study support previous retrograde tract tracing and physiological studies from our group demonstrating that the neurons in the area adjacent to the central canal of the midthoracic or lumbosacral level of the spinal cord send long ascending projections to the dorsal column nucleus that are important in the transmission of second-order afferent information for visceral nociception. Thus, the axonal projections through both the dorsal and the ventrolateral white matter from the CC region terminate in many regions of the brain providing spinal input for sensory integration, autonomic regulation, motor and emotional responses, and limbic activation.

Developmental tuning in a spinal nociceptive system: effects of neonatal spinalization

Recent studies indicate a modular organization of the nociceptive withdrawal reflex system. Each module has a characteristic receptive field, closely matching the withdrawal movement caused by its effector muscle. In the rat, the strength of the sensory input to each module is tuned during the first postnatal weeks, i.e., erroneous spinal connections are depressed, and adequate connections are strengthened. To clarify if this tuning is dependent on supraspinal structures, the effect of a complete neonatal spinal cord transection on the postnatal tuning of withdrawal reflexes was studied. The nociceptive receptive fields of single hindlimb muscles and compound withdrawal reflexes were examined in decerebrate unanesthetized and awake rats, respectively. Noxious thermal CO(2) laser stimulation was used to evoke reflex responses. Neonatal spinal cord transection resulted in a disrupted reflex organization in the adult rat, resembling that previously found in neonatal rats. The receptive fields of single hindlimb muscles exhibited abnormal distribution of sensitivity not matching the withdrawal action of the effector muscles. Likewise, the composite nocifensive movements, as documented in the awake rat, often resulted in erroneous movements toward the stimulus. It is concluded that withdrawal reflexes do not become functionally adapted in rats spinalized at birth. These findings suggest a critical role for supraspinal systems in the postnatal tuning of spinal nociceptive systems.

A passion of the soul: an introduction to pain for consciousness researchers

Pain is an important focus for consciousness research because it is an avenue for exploring somatic awareness, emotion, and the genesis of subjectivity. In principle, pain is awareness of tissue trauma, but pain can occur in the absence of identifiable injury, and sometimes substantive tissue injury produces no pain. The purpose of this paper is to help bridge pain research and consciousness studies. It reviews the basic sensory neurophysiology associated with tissue injury, including transduction, transmission, modulation, and central representation. In addition, it highlights the central mechanisms for the emotional aspects of pain, demonstrating the physiological link between tissue trauma and mechanisms of emotional arousal. Finally, we discuss several current issues in the field of pain research that bear on central issues in consciousness studies, such as sickness and sense of self.

Developmental tuning in a spinal nociceptive system: effects of neonatal spinalization

Recent studies indicate a modular organization of the nociceptive withdrawal reflex system. Each module has a characteristic receptive field, closely matching the withdrawal movement caused by its effector muscle. In the rat, the strength of the sensory input to each module is tuned during the first postnatal weeks, i.e., erroneous spinal connections are depressed, and adequate connections are strengthened. To clarify if this tuning is dependent on supraspinal structures, the effect of a complete neonatal spinal cord transection on the postnatal tuning of withdrawal reflexes was studied. The nociceptive receptive fields of single hindlimb muscles and compound withdrawal reflexes were examined in decerebrate unanesthetized and awake rats, respectively. Noxious thermal CO(2) laser stimulation was used to evoke reflex responses. Neonatal spinal cord transection resulted in a disrupted reflex organization in the adult rat, resembling that previously found in neonatal rats. The receptive fields of single hindlimb muscles exhibited abnormal distribution of sensitivity not matching the withdrawal action of the effector muscles. Likewise, the composite nocifensive movements, as documented in the awake rat, often resulted in erroneous movements toward the stimulus. It is concluded that withdrawal reflexes do not become functionally adapted in rats spinalized at birth. These findings suggest a critical role for supraspinal systems in the postnatal tuning of spinal nociceptive systems.

Supraspinal metabolic activity changes in the rat during adjuvant monoarthritis

Pain is a multi-dimensional experience including sensory-discriminative and affective-motivational components. The attribution of such components to a corresponding cerebral neuronal substrate in the brain refers to conclusions drawn from electrical brain stimulation, lesion studies, topographic mappings and metabolic imaging. Increases in neuronal metabolic activity in supraspinal brain regions, suggested to be involved in the central processing of pain, have previously been shown in various animal studies. The present investigation is the first to describe supraspinal structures which show increased metabolic activity during ongoing monoarthritic pain at multiple time-points. Experimental chronic monoarthritis of a hindlimb induced by complete Freund's adjuvant is one of the most used models in studies of neuronal plasticity associated with chronic pain. Such animals show typical symptoms of hyperalgesia and allodynia for a prolonged period. Metabolic activity changes in supraspinal brain regions during monoarthritis were assessed using the quantitative [14C]-2deoxyglucose technique at two, four, 14 days of the disease and, furthermore, in a group of 14-day monoarthritic rats which were mechanically stimulated by repeated extensions of the inflamed joint. Local glucose utilization was determined ipsi- and contralateral to the arthritic hindpaw in more than 50 brain regions at various supraspinal levels, and compared with saline-injected controls. At two and 14 days of monoarthritis significant bilateral increases in glucose utilization were seen in many brain structures, including brainstem, thalamic, limbic and cortical regions. Within the brainstem, animals with 14-day monoarthritis showed a higher number of regions with increased metabolic activity compared with two days. No differences between ipsi- and contralateral sides were detected in any of the experimental groups. Average increases ranged from 20 to 40% compared with controls and maximum values were detected in specific brain regions, such as the anterior pretectal nucleus, the anterior cingulate cortex and the nucleus accumbens. Interestingly, at four days of monoarthritis, the glucose utilization values were in the control range in almost all regions studied. Moreover, in monoarthritic rats receiving an additional noxious mechanical stimulation, the rates of glucose utilization were also comparable to controls in all brain areas investigated. Such patterns of brain metabolic activity agreed with concomitant changes in the lumbar spinal cord, described in the accompanying report. The present data show that a large array of supraspinal structures displays elevated metabolic activity during painful monoarthritis, with a non-linear profile for the time-points investigated. This observation most probably reflects mechanisms of transmission and modulation of nociceptive input arising from the monoarthritis and accompanying its development.

Analysis of the perception of and reactivity to pain and heat in patients with wallenberg syndrome and severe spinothalamic tract dysfunction

BACKGROUND

The aim of the study was to assess the consequences of severe spinothalamic tract lesions resulting from lateral medullary infarct and to show that a specific pain perception can be elicited by strong thermal stimulation.

CASE DESCRIPTIONS

Both patients examined presented with severe thermoalgic dissociation of the limbs contralateral to the lesion, with normal discriminative somatosensory perception and motor strength. They reported pain perception when touching very warm (>50 degrees C to 60 degrees C) objects and a brisk, occasionally uncontrolled withdrawal reaction of the arm and hand under the same conditions, without any perception of the heat nature of the stimulus. Warm stimulation, <45 degrees C, elicited no thermal perception or discrimination. Pain perception could be elicited in both patients by increasing the temperature, with a reproducible threshold of 47 degrees C to 49 degrees C. Pain always occurred after a prolonged delay of 8 to 10 seconds in response to threshold heat, and was described as deep and osseous, and clearly different from that perceived on the nonaffected side. The delay was much shorter when the temperature was increased by 4 degrees C to 5 degrees C. Cold stimulation elicited similar pain perception in one patient. Analysis of subjective perception of laser stimulation showed a much higher pain threshold on the affected hand. There were no laser-evoked potentials on this side, which suggested major spinothalamic injury. Assessment of the RIII noxious reflex revealed persistent response withdrawal reactions, with an increased threshold on the affected side, and partial consciousness of the noxious nature of the stimulus.

CONCLUSIONS

To our knowledge, this is the first description of the appearance of pain perception of high temperatures in patients with severe spinothalamic injury who are suffering from a complete loss of temperature perception. This implies that noxious thermal stimulation can still be perceived via extra spinothalamic pathways (which are slow and multisynaptic), such as the spinoreticulothalamic tract. Patients with Wallenberg syndrome should be informed and made aware of their residual perception of and reactions to noxious stimulation.

Group I metabotropic glutamate receptors: implications for brain diseases

Glutamate is the major excitatory neurotransmitter in the brain and plays a unique role in a variety of central nervous system (CNS) functions. The discovery of the metabotropic receptors (mGluRs), a family of G-protein coupled receptors than can be activated by glutamate, has led to an impressive number of studies in recent years aimed at understanding their biochemical, physiological and pharmacological characteristics. The eight mGluRs now known are divided into three groups according to their sequence homology, signal transduction mechanisms, and agonist selectivity. Group I mGluRs include mGluR1 and mGluR5, which are linked to the activation of phospholipase C; Groups II and III include all others and are negatively coupled to adenylyl cyclases. The availability in recent years of agents selective for Group I mGluRs has made possible the study of the physiological roles of these receptors in the CNS. In addition to mediating glutamatergic neurotransmission, Group I mGluRs can modulate other neurotransmitter receptors, including GABA and the ionotropic glutamate receptors. Group I mGluRs are involved in many CNS functions and may participate in a variety of disorders such as pain, epilepsy, ischemia, and chronic neurodegenerative diseases. This class of receptor may provide important pharmacological therapeutic targets and elucidating its functions will be relevant to develop new treatments for neurological and psychiatric disorders in which glutamatergic neurotransmission is abnormally regulated. In this review anatomical, physiological and pharmacological results are presented with a special emphasis on the role of Group I mGluRs in functional and pathological processes.

The effects of cannabinoids on the brain

Cannabinoids have a long history of consumption for recreational and medical reasons. The primary active constituent of the hemp plant Cannabis sativa is delta9-tetrahydrocannabinol (delta9-THC). In humans, psychoactive cannabinoids produce euphoria, enhancement of sensory perception, tachycardia, antinociception, difficulties in concentration and impairment of memory. The cognitive deficiencies seem to persist after withdrawal. The toxicity of marijuana has been underestimated for a long time, since recent findings revealed delta9-THC-induced cell death with shrinkage of neurons and DNA fragmentation in the hippocampus. The acute effects of cannabinoids as well as the development of tolerance are mediated by G protein-coupled cannabinoid receptors. The CB1 receptor and its splice variant CB1A, are found predominantly in the brain with highest densities in the hippocampus, cerebellum and striatum. The CB2 receptor is found predominantly in the spleen and in haemopoietic cells and has only 44% overall nucleotide sequence identity with the CB1 receptor. The existence of this receptor provided the molecular basis for the immunosuppressive actions of marijuana. The CB1 receptor mediates inhibition of adenylate cyclase, inhibition of N- and P/Q-type calcium channels, stimulation of potassium channels, and activation of mitogen-activated protein kinase. The CB2 receptor mediates inhibition of adenylate cyclase and activation of mitogen-activated protein kinase. The discovery of endogenous cannabinoid receptor ligands, anandamide (N-arachidonylethanolamine) and 2-arachidonylglycerol made the notion of a central cannabinoid neuromodulatory system plausible. Anandamide is released from neurons upon depolarization through a mechanism that requires calcium-dependent cleavage from a phospholipid precursor in neuronal membranes. The release of anandamide is followed by rapid uptake into the plasma and hydrolysis by fatty-acid amidohydrolase. The psychoactive cannabinoids increase the activity of dopaminergic neurons in the ventral tegmental area-mesolimbic pathway. Since these dopaminergic circuits are known to play a pivotal role in mediating the reinforcing (rewarding) effects of the most drugs of abuse, the enhanced dopaminergic drive elicited by the cannabinoids is thought to underlie the reinforcing and abuse properties of marijuana. Thus, cannabinoids share a final common neuronal action with other major drugs of abuse such as morphine, ethanol and nicotine in producing facilitation of the mesolimbic dopamine system.

Suffering: the contributions of persistent pain

Pain is a perceived threat or damage to one's biological integrity. Suffering is the perception of serious threat or damage to the self, and it emerges when a discrepancy develops between what one expected of one's self and what one does or is. Some patients who experience sustained unrelieved pain suffer because pain changes who they are. At a physiological level, chronic pain promotes an extended and destructive stress response characterised by neuroendocrine dysregulation, fatigue, dysphoria, myalgia, and impaired mental and physical performance. This constellation of discomforts and functional limitations can foster negative thinking and create a vicious cycle of stress and disability. The idea that one's pain is uncontrollable in itself leads to stress. Patients suffer when this cycle renders them incapable of sustaining productive work, a normal family life, and supportive social interactions. Although patients suffer for many reasons, the physician can contribute substantially to the prevention or relief of suffering by controlling pain. Suffering is a nebulous concept for most physicians, and its relation to pain is unclear. This review offers a medically useful concept of suffering that distinguishes it from pain, accounts for the contributory relation of pain to suffering by describing pain as a stressor, and explores the implications of these ideas for the care of patients.

The neurobiology of pain

Understanding the plasticity of pain and analgesia exhibited in different pain states may improve therapies for the two major types of pain, neuropathic and inflammatory pain, in which nerve and tissue damage leads to alterations at both peripheral and central levels. At the level of the peripheral nerve, drugs that act on particular sodium channels may target only pain-related activity. Agents that act on some of the peripheral mediators of pain may control peripheral nerve activity. A new generation of non-steroidal anti-inflammatory drugs, cyclo-oxygenase 2 inhibitors, that lack gastric actions are becoming available. In the spinal cord, the release of peptides and glutamate causes activation of multiple receptors, particularly, the N-methyl-D-aspartate receptor for glutamate, which, in concert with other spinal systems, generates spinal hypersensitivity. Blocking the generation of excitability is one approach, but increasing inhibitions may also provide analgesia. Opioid actions are via presynaptic and post-synaptic inhibitory effects on central and peripheral C fibre terminals, spinal neurones, and supraspinal mechanisms. Our knowledge of brain mechanisms of pain is still, however, limited. Other new targets have been revealed by molecular biology and animal models of clinical pain, but the possibility of a "magic bullet" is doubtful. Thus, another approach could be single molecules with dual drug actions, that encompass targets where additive or synergistic effects of different mechanisms may enable pain relief without major adverse effects.

Intracerebroventricular administration of nocistatin reduces inflammatory hyperalgesia in rats

Nocistatin is a biologically active peptide derived from prepronociceptin, and its intrathecal administration has been reported to reduce nociceptin- or prostaglandin E2-induced hyperalgesia and allodynia in mice. In this study, we investigated the effects of intracerebroventricular (i.c.v.) administration of nocistatin on the inflammatory hyperalgesia induced by hindlimb intraplantar injection of carrageenan/kaolin in the rat paw-pressure test. Intracerebroventricular administration of nocistatin (0.5-50 pmol/rat) dose-dependently reduced carrageenan/kaolin-induced hyperalgesia, which peaked at 15-30 m. However, i.c.v. administration of nocistatin (50 pmol/rat) had no effect on the nociceptive threshold of non-inflamed rats. These results indicate that nocistatin has anti-hyperalgesic effects on the inflammatory hyperalgesia induced by carrageenan/kaolin at the supraspinal level.

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.

Alzheimer patients report less pain intensity and pain affect than non-demented elderly

Pain assessment for patients with Alzheimer's disease (AD) is generally aimed at quantifying pain, i.e., the intensity and locations of pain. Based on the extensive neuropathology in limbic brain areas with this disorder, we hypothesized that, compared to control patients, AD patients would report an additional loss of qualitative aspects of pain, i.e., pain affect. This hypothesis was tested by administering specific parts of three pain questionnaires and comparing the use of analgesics in 19 AD patients with that of 18 elderly patients without dementia who were matched for the presence of painful conditions. Results reveal that AD patients, compared to controls, experience less intense pain and less pain affect. In contrast, the number of AD patients using analgesics did not differ from the number of controls. These findings suggest that pain assessment for patients with AD should be focused on both quantitative and qualitative aspects of pain.

Branching and/or collateral projections of spinal dorsal horn neurons

Branching and/or collateral projections of spinal dorsal horn neurons is a common phenomenon. Evidence is presented for the existence of STTm/STTl, STTc/STTi, STT/SMT, STT/SRT, SCT/DCPS, SST/DCPS, SCT/SST, STT/SHT, STeT/SHT, STeTs and other doubly or multiply projecting spinal neurons that have been anatomically and physiologically identified and named based on the locations of the cells of origin and their terminations in the brain. These newly discovered spinal projection neurons are characterized by a single cell body and branched axons and/or collaterals that project to two or more target areas in the brain. These novel populations of neurons seem to be a fuzzy set of spinal projection neurons that function as an intersection set of the corresponding single projection spinal neurons and to be at an intermediate stage phylogenetically. Identification strategies are discussed, and general concluding remarks are made in this review.

Central pain and dysesthesia syndrome

This article presents recent observations about different recognized central pain syndromes (CPS) and discusses them in light of contemporary microelectrode and imaging findings. Different theories regarding the generation of CPS are reviewed, with an emphasis on difficulties in diagnosis and treatment. The author discourages destructive procedures for treatment of CPS, favoring, instead, reversible procedures such as stimulation techniques and drug delivery systems.

Midbrain periaqueductal gray (PAG) inhibits nociceptive inputs to sacral dorsal horn nociceptive neurons through alpha2-adrenergic receptors

Modulation of sacral spinal dorsal horn neurons by the ventrolateral PAG was studied by extracellular recording combined with microiontophoretic applications of alpha-adrenergic agonists or antagonists. Bicuculline (BIC, 15 ng) microinjected into the ventrolateral PAG produced a consistent inhibition of the responses of nociceptive dorsal horn neurons. After PAG-BIC applications, the total number of spikes per heat stimulation period was significantly decreased to a mean of 37 +/- 19% (n = 8) of the pre-BIC control. Local iontophoresis of the selective alpha2-adrenoceptor antagonists idazoxan or yohimbine but not the selective alpha1 antagonist benoxathian significantly reversed PAG-BIC-evoked inhibition. At low ejection currents, clonidine, an alpha2-adrenoceptor agonist, markedly reduced noxious heat-evoked responses but had no consistent action on the responses to iontophoresed excitatory amino acids [EAA; N-methyl--aspartate (NMDA) or kainic acid]. At ejection currents higher than required to block descending inhibition, idazoxan potentiated responses to both heat and EAA iontophoresis. At higher ejection currents, EAA responses were inhibited by clonidine. This indicates that both presynaptic and postsynaptic alpha2 receptors are capable of inhibiting the recorded neurons. Activation of the alpha1 adrenoceptors by iontophoresis of methoxamine often led to a marked increase in the responses to kainic acid and, to a lesser extent, to NMDA iontophoresis or noxious heat. Together with previously reported work, the current experiments demonstrate that PAG neurons inhibit nociceptive dorsal horn neurons primarily through an indirect alpha2 adrenoceptor mechanism. In this same population of dorsal horn neurons, norepinephrine has a direct alpha1-mediated excitatory effect.