Organization of cortical projections to the medullary subnucleus reticularis dorsalis: A retrograde and anterograde tracing study in the rat

Functional roles of descending projections from the cerebral cortex to the trigeminal spinal subnucleus caudalis in orofacial nociceptive information processing

BACKGROUND

The trigeminal spinal subnucleus caudalis (Sp5C), also known as the medullary dorsal horn, receives orofacial somatosensory inputs, particularly nociceptive inputs, from the trigeminal nerve. In the Sp5C, excitatory and inhibitory neurons, glutamatergic and GABAergic/glycinergic neurons, respectively, form the local circuits. The axons of the glutamatergic neurons in lamina I ascend toward the thalamic and parabrachial nuclei, and this projection is the main pathway of orofacial nociception. Additionally, the axons of the higher brain regions, including the locus coeruleus, dorsal raphe, and cerebral cortex, are sent to the Sp5C.

HIGHLIGHT

Among these descending projections, this review focuses on the functional profiles of the corticotrigeminal projections to the Sp5C, along with their anatomical aspects. The primary and secondary somatosensory and insular cortices are of particular interest.

CONCLUSION

Corticotrigeminal projections from the somatosensory cortex to the Sp5C play a suppressive role in nociceptive information processing, whereas recent studies have demonstrated a facilitative role of the insular cortex in nociceptive information processing at the Sp5C level.

Deciphering the functional role of insular cortex stratification in trigeminal neuropathic pain

Trigeminal neuropathic pain (TNP) is a major concern in both dentistry and medicine. The progression from normal to chronic TNP through activation of the insular cortex (IC) is thought to involve several neuroplastic changes in multiple brain regions, resulting in distorted pain perception and associated comorbidities. While the functional changes in the insula are recognized contributors to TNP, the intricate mechanisms underlying the involvement of the insula in TNP processing remain subjects of ongoing investigation. Here, we have overviewed the most recent advancements regarding the functional role of IC in regulating TNP alongside insights into the IC's connectivity with other brain regions implicated in trigeminal pain pathways. In addition, the review examines diverse modulation strategies that target the different parts of the IC, thereby suggesting novel diagnostic and therapeutic management of chronic TNP in the future.

Identification of brain areas in mice with peak neural activity across the acute and persistent phases of post-traumatic headache

BACKGROUND

Post-traumatic headache is very common after a mild traumatic brain injury. Post-traumatic headache may persist for months to years after an injury in a substantial proportion of people. The pathophysiology underlying post-traumatic headache remains unknown but is likely distinct from other headache disorders. Identification of brain areas activated in acute and persistent phases of post-traumatic headache can provide insights into the underlying circuits mediating headache pain. We used an animal model of mild traumatic brain injury-induced post-traumatic headache and c-fos immunohistochemistry to identify brain regions with peak activity levels across the acute and persistent phases of post-traumatic headache.

METHODS

Male and female C57BL/6 J mice were briefly anesthetized and subjected to a sham procedure or a weight drop closed-head mild traumatic brain injury . Cutaneous allodynia was assessed in the periorbital and hindpaw regions using von Frey filaments. Immunohistochemical c-fos based neural activity mapping was then performed on sections from whole brain across the development of post-traumatic headache (i.e. peak of the acute phase at 2 days post- mild traumatic brain injury), start of the persistent phase (i.e. >14 days post-mild traumatic brain injury) or after provocation with stress (bright light). Brain areas with consistent and peak levels of c-fos expression across mild traumatic brain injury induced post-traumatic headache were identified and included for further analysis.

RESULTS

Following mild traumatic brain injury, periorbital and hindpaw allodynia was observed in both male and female mice. This allodynia was transient and subsided within the first 14 days post-mild traumatic brain injury and is representative of acute post-traumatic headache. After this acute post-traumatic headache phase, exposure of mild traumatic brain injury mice to a bright light stress reinstated periorbital and hindpaw allodynia for several hours - indicative of the development of persistent post-traumatic headache. Acute post-traumatic headache was coincident with an increase in neuronal c-fos labeling in the spinal nucleus of the trigeminal caudalis, primary somatosensory cortex, and the nucleus accumbens. Neuronal activation returned to baseline levels by the persistent post-traumatic headache phase in the spinal nucleus of the trigeminal caudalis and primary somatosensory cortex but remained elevated in the nucleus accumbens. In the persistent post-traumatic headache phase, coincident with allodynia observed following bright light stress, we observed bright light stress-induced c-fos neural activation in the spinal nucleus of the trigeminal caudalis, primary somatosensory cortex, and nucleus accumbens.

CONCLUSION

Examination of mild traumatic brain injury-induced changes in peak c-fos expression revealed brain regions with significantly increased neural activity across the acute and persistent phases of post-traumatic headache. Our findings suggest mild traumatic brain injury-induced post-traumatic headache produces neural activation along pain relevant pathways at time-points matching post-traumatic headache-like pain behaviors. These observations suggest that the spinal nucleus of the trigeminal caudalis, primary somatosensory cortex, and nucleus accumbens may contribute to both the induction and maintenance of post-traumatic headache.

Central generators of migraine and autonomic cephalalgias as targets for personalized pain management: Translational links

BACKGROUND AND OBJECTIVE

Migraine oscillates between different states in association with internal homeostatic functions and biological rhythms that become more easily dysregulated in genetically susceptible individuals. Clinical and pre-clinical data on migraine pathophysiology support a primary role of the central nervous system (CNS) through 'dysexcitability' of certain brain networks, and a critical contribution of the peripheral sensory and autonomic signalling from the intracranial meningeal innervation. This review focuses on the most relevant back and forward translational studies devoted to the assessment of CNS dysfunctions involved in primary headaches and discusses the role they play in rendering the brain susceptible to headache states.

METHODS AND RESULTS

We collected a body of scientific literature from human and animal investigations that provide a compelling perspective on the anatomical and functional underpinnings of the CNS in migraine and trigeminal autonomic cephalalgias. We focus on medullary, hypothalamic and corticofugal modulation mechanisms that represent strategic neural substrates for elucidating the links between trigeminovascular maladaptive states, migraine triggering and the temporal phenotype of the disease.

CONCLUSION

It is argued that a better understanding of homeostatic dysfunctional states appears fundamental and may benefit the development of personalized therapeutic approaches for improving clinical outcomes in primary headache disorders.

SIGNIFICANCE

This review focuses on the most relevant back and forward translational studies showing the crucial role of top-down brain modulation in triggering and maintaining primary headache states and how these central dysfunctions may interact with personalized pain management strategies.

Descending projections from the insular cortex to the trigeminal spinal subnucleus caudalis facilitate excitatory outputs to the parabrachial nucleus in rats

ABSTRACT

Nociceptive information from the orofacial area projects to the trigeminal spinal subnucleus caudalis (Sp5C) and is then conveyed to several nuclei, including the parabrachial nucleus (PBN). The insular cortex (IC) receives orofacial nociceptive information and sends corticofugal projections to the Sp5C. The Sp5C consists of glutamatergic and GABAergic/glycinergic interneurons that induce excitatory postsynaptic currents and inhibitory postsynaptic currents, respectively, in projection neurons. Therefore, quantification of glutamatergic IC inputs in combination with identifying postsynaptic neuronal subtypes is critical to elucidate IC roles in the regulation of Sp5C activities. We investigated features of synaptic transmission from the IC to glutamatergic and GABAergic/glycinergic Sp5C neurons of laminae I/II using vesicular GABA transporter-Venus transgenic rats that received an injection of adeno-associated virus-channelrhodopsin-2-mCherry into the IC. Selective stimulation of IC axon terminals in Sp5C slice preparations induced monosynaptic excitatory postsynaptic currents in both excitatory glutamatergic and inhibitory GABAergic/glycinergic Sp5C neurons with a comparable amplitude. Paired whole-cell patch-clamp recordings showed that unitary inhibitory postsynaptic currents from inhibitory neurons influencing excitatory neurons, including neurons projecting to the PBN, exhibited a high failure rate and were suppressed by both bicuculline and strychnine, suggesting that excitatory neurons in the Sp5C receive both GABAergic and glycinergic inhibition with low impact. Moreover, selective stimulation of IC axons increased the firing rate at the threshold responses. Finally, we demonstrated that selective stimulation of IC axons in the Sp5C by a chemogenetic approach decreased the thresholds of both mechanical and thermal nociception. Thus, IC projection to the Sp5C is likely to facilitate rather than suppress excitatory outputs from the Sp5C.

Cerebro-Cerebellar Networks in Migraine Symptoms and Headache

The cerebellum is associated with the biology of migraine in a variety of ways. Clinically, symptoms such as fatigue, motor weakness, vertigo, dizziness, difficulty concentrating and finding words, nausea, and visual disturbances are common in different types of migraine. The neural basis of these symptoms is complex, not completely known, and likely involve activation of both specific and shared circuits throughout the brain. Posterior circulation stroke, or neurosurgical removal of posterior fossa tumors, as well as anatomical tract tracing in animals, provided the first insights to theorize about cerebellar functions. Nowadays, with the addition of functional imaging, much progress has been done on cerebellar structure and function in health and disease, and, as a consequence, the theories refined. Accordingly, the cerebellum may be useful but not necessary for the execution of motor, sensory or cognitive tasks, but, rather, would participate as an efficiency facilitator of neurologic functions by improving speed and skill in performance of tasks produced by the cerebral area to which it is reciprocally connected. At the subcortical level, critical regions in these processes are the basal ganglia and thalamic nuclei. Altogether, a modulatory role of the cerebellum over multiple brain regions appears compelling, mainly by considering the complexity of its reciprocal connections to common neural networks involved in motor, vestibular, cognitive, affective, sensory, and autonomic processing—all functions affected at different phases and degrees across the migraine spectrum. Despite the many associations between cerebellum and migraine, it is not known whether this structure contributes to migraine initiation, symptoms generation or headache. Specific cerebellar dysfunction via genetically driven excitatory/inhibitory imbalances, oligemia and/or increased risk to white matter lesions has been proposed as a critical contributor to migraine pathogenesis. Therefore, given that neural projections and functions of many brainstem, midbrain and forebrain areas are shared between the cerebellum and migraine trigeminovascular pathways, this review will provide a synopsis on cerebellar structure and function, its role in trigeminal pain, and an updated overview of relevant clinical and preclinical literature on the potential role of cerebellar networks in migraine pathophysiology.

Alterations in pain processing circuitries in episodic migraine

Background

The precise underlying mechanisms of migraine remain unknown. Although we have previously shown acute orofacial pain evoked changes within the brainstem of individuals with migraine, we do not know if these brainstem alterations are driven by changes in higher cortical regions. The aim of this investigation is to extend our previous investigation to determine if higher brain centers display altered activation patterns and connectivity in migraineurs during acute orofacial noxious stimuli.

Methods

Functional magnetic resonance imaging was performed in 29 healthy controls and 25 migraineurs during the interictal and immediately (within 24-h) prior to migraine phases. We assessed activation of higher cortical areas during noxious orofacial heat stimulation using a thermode device and assessed whole scan and pain-related changes in connectivity.

Results

Despite similar overall pain intensity ratings between all three groups, migraineurs in the group immediately prior to migraine displayed greater activation of the ipsilateral nucleus accumbens, the contralateral ventrolateral prefrontal cortex and two clusters in the dorsolateral prefrontal cortex (dlPFC). Reduced whole scan dlPFC [Z + 44] connectivity with cortical/subcortical and brainstem regions involved in pain modulation such as the putamen and primary motor cortex was demonstrated in migraineurs. Pain-related changes in connectivity of the dlPFC and the hypothalamus immediately prior to migraine was also found to be reduced with brainstem pain modulatory areas such as the rostral ventromedial medulla and dorsolateral pons.

Conclusions

These data reveal that the modulation of brainstem pain modulatory areas by higher cortical regions may be aberrant during pain and these alterations in this descending pain modulatory pathway manifests exclusively prior to the development of a migraine attack.

Vortioxetine as an analgesic in preclinical inflammatory pain models: Mechanism of action

Vortioxetine is a novel atypical antidepressant with multimodal activity that has recently demonstrated efficacy against neuropathic pain. There is no published data about its analgesic properties in models characterized by peripheral inflammation and consequent pain pathway sensitization, nor data on its mechanism of antinociceptive action. This study aimed to investigate vortioxetine's antinociceptive/antihyperalgesic effects in trigeminal, visceral, and somatic inflammatory pain models, and provide evidence on its mechanism of action in the modulation of trigeminal nociception. Vortioxetine's effects on the nociceptive behavior in orofacial formalin test (OFT) and acetic acid-writhing test in mice and on mechanical hyperalgesia in carrageenan-induced paw inflammation in rats were examined following peroral single administration. The involvement of serotonergic/adrenergic/cholinergic/cannabinoid/adenosine receptors was evaluated in OFT by intraperitoneally treating mice with an appropriate antagonist immediately after vortioxetine application. We used antagonists of 5-HT_1B/1D serotonergic (GR 127935), α₁ -adrenergic (prazosin), α₂ -adrenergic (yohimbine), β₁ -adrenergic (metoprolol), muscarinic (atropine), α₇ nicotinic (methyllycaconitine), CB₁ /CB₂ cannabinoid (AM251 and AM630), and adenosine A₁ (DPCPX) receptors. Vortioxetine dose-dependently reduced pain behavior in OFT and acetic acid writhing test, as well as inflammatory hyperalgesia in paw pressure test. All examined antagonists except prazosin dose-dependently inhibited vortioxetine's antinociceptive effects. In conclusion, vortioxetine exerted analgesic efficacy in trigeminal, visceral, and somatic inflammatory pain. The effect is at least in part mediated by 5-HT_1B/1D serotonergic, α₂ /β₁ -adrenergic, muscarinic and nicotinic cholinergic, CB₁ /CB₂ cannabinoid, and adenosine A₁ receptors. These findings contribute to better understanding of the analgesic effect of vortioxetine and suggest its potential usefulness for inflammatory pain treatment.

Enhanced pain modulation capacity among individuals with borderline personality disorder: A possible mechanism underlying their hypoalgesia

BACKGROUND

Ample evidence suggests that individuals with borderline personality disorder (BPD) exhibit hyposensitivity to pain. Since the underlying mechanism of the pain hyposensitivity is unknown, we tested here for the first time whether this hyposensitivity is pain specific or exists also for innocuous sensation, and whether it is associated with enhanced descending pain modulation capabilities.

METHODS

Participants were 55 women; 22 patients with BPD and 33 matched healthy controls. Testing included the measurement of warmth sensation threshold (WST), heat-pain threshold (HPT), pain adaptation, conditioned pain modulation (CPM) and temporal summation of heat-pain (TSP). The level of dissociation was also evaluated.

RESULTS

Women with BPD had higher WST and HPT compared with healthy controls. Moreover, women with BPD had greater magnitude of pain adaptation and CPM as well as higher dissociation level compared to controls. In neither the BPD nor the control group did WST and HPT correlate with pain adaptation, CPM or dissociation. In the BPD group only, HPT inversely correlated with the magnitude of TSP.

CONCLUSIONS

Women with BPD present generalized hyposensitivity to both innocuous and noxious stimuli. Furthermore, women with BPD exhibit more efficient pain inhibition capabilities than healthy controls. While efficient pain modulation may underlie pain hyposensitivity in BPD, both traits may exist independently from each other, or may be moderated by another factor such as dissociation.

SIGNIFICANCE

On the basis of testing pronociceptive and antinociceptive components among individuals with BPD and healthy controls, this study reveals enhanced ability to inhibit pain among woman with borderline personality disorder (BPD) which may underlie hyposensitivity to both noxious and innocuous stimuli and perhaps also self-injurious behaviour among these individuals. The study contributes novel information on possible mechanisms involved in BPD manifestations.

Cortical integration of bilateral nociceptive signals: when more is less

Integration of nociceptive information is essential to produce adapted responses, to promote body integrity and survival. However, how the brain integrates nociceptive inputs from different body areas remains unknown. The aim of this study was to examine the cortical integration of bilateral nociceptive inputs evoked by laser heat stimuli. Sixteen healthy volunteers (8 F, 8 M; age: 25.5 ± 4.3) were recruited to participate in one session during which painful laser stimuli were applied to their hands with 2 Nd:YAP laser systems. Electroencephalographic activity was recorded to measure laser-evoked potentials and event-related spectral perturbations. Twenty nociceptive stimuli were applied in each of the 4 counterbalanced conditions: (1) right hand, (2) left hand, and both hands with (3) attention to the right or (4) attention to the left. Compared with unilateral conditions, N2 and P2 peak amplitude as well as gamma oscillation power were decreased in bilateral conditions (P < 0.05), but these effects were not affected by the direction of attention (P > 0.1). By contrast, pain was not significantly different in any condition (P > 0.05). These findings show that although more nociceptive inputs reach the brain with multiple nociceptive stimuli, their sensory representation is decreased while pain perception remains unchanged. These interactions between cerebral processing of nociceptive information from different body regions could support coordinated behavioral responses when pain origins from multiple sources.

Cortical influences on brainstem circuitry responsible for conditioned pain modulation in humans

Conditioned pain modulation (CPM) is a powerful endogenous analgesic mechanism which can completely inhibit incoming nociceptor signals at the primary synapse. The circuitry responsible for CPM lies within the brainstem and involves the subnucleus reticularis dorsalis (SRD). While the brainstem is critical for CPM, the cortex can significantly modulate its expression, likely via the brainstem circuitry critical for CPM. Since higher cortical regions such as the anterior, mid-cingulate, and dorsolateral prefrontal cortices are activated by noxious stimuli and show reduced activations during other analgesic responses, we hypothesized that these regions would display reduced responses during CPM analgesia. Furthermore, we hypothesized that functional connectivity strength between these cortical regions and the SRD would be stronger in those that express CPM analgesia compared with those that do not. We used functional magnetic resonance imaging to determine sites recruited during CPM expression and their influence on the SRD. A lack of CPM analgesia was associated with greater signal intensity increases during each test stimulus in the presence of the conditioning stimulus compared to test stimuli alone in the mid-cingulate and dorsolateral prefrontal cortices and increased functional connectivity with the SRD. In contrast, those subjects exhibiting CPM analgesia showed no change in the magnitude of signal intensity increases in these cortical regions or strength of functional connectivity with the SRD. These data suggest that during multiple or widespread painful stimuli, engagement of the prefrontal and cingulate cortices prevents the generation of CPM analgesia, raising the possibility altered responsiveness in these cortical regions underlie the reduced CPM observed in individuals with chronic pain. Hum Brain Mapp 37:2630-2644, 2016. © 2016 Wiley Periodicals, Inc.

A Functional Magnetic Resonance Imaging Study to Investigate the Utility of a Picture Imagination Task in Investigating Neural Responses in Patients with Chronic Musculoskeletal Pain to Daily Physical Activity Photographs

Pain-related anxiety and fear are associated with increased difficulties in attention, increased awareness of pain, impaired disengagement from pain, and can moderate the effects of attentional coping attempts. Accurately assessing the direct impact of pain-related anxiety and fear on pain behavior has proved difficult. Studies have demonstrated no or limited influence of pain-related fear and anxiety on behavior but this may be due to inherent problems with the scales used. Neuroimaging has improved the understanding of neural processes underlying the factors that influence pain perception. This study aimed to establish if a Picture and Imagination Task (PIT), largely developed from the Photographs of Daily Activity (PHODA) assessment tool, could help explore how people living with chronic pain process information about daily activities. Blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) was used to compare brain responses in patients with chronic musculoskeletal pain (CMSKP) (n = 15) and healthy controls (n = 15). Subjects were asked to imagine how they would feel mentally and physically if asked to perform daily activities illustrated in PIT. The results found that a number of regions involved in pain processing saw increased BOLD activation in patients compared with controls when undertaking the task and included the insula, anterior cingulate cortex, thalamus and inferior and superior parietal cortices. Similarly, increased BOLD responses in patients compared to controls in the frontal pole, paracingulate and the supplementary motor cortex may be suggestive of a memory component to the responses The amygdala, orbitofrontal cortex, substantia nigra/ventral tegmentum, putamen, thalamus, pallidum, inferior parietal (supramarginal and angular gyrus) and cingulate cortex were also seen to have greater differences in BOLD signal changes in patients compared with controls and many of these regions are also associated with general phobic responses. Therefore, we suggest that PIT is a useful task to explore pain- and movement-related anxiety and fear in fMRI studies. Regions in the Default Mode Network remained active or were less deactivated during the PIT task in patients with CMSKP compared to healthy controls supporting the contention that the DMN is abnormal in patients with CMSKP.

Systematic comparison of adeno-associated virus and biotinylated dextran amine reveals equivalent sensitivity between tracers and novel projection targets in the mouse brain

As an anterograde neuronal tracer, recombinant adeno-associated virus (AAV) has distinct advantages over the widely used biotinylated dextran amine (BDA). However, the sensitivity and selectivity of AAV remain uncharacterized for many brain regions and species. To validate this tracing method further, AAV (serotype 1) was systematically compared with BDA as an anterograde tracer by injecting both tracers into three cortical and 15 subcortical regions in C57BL/6J mice. Identical parameters were used for our sequential iontophoretic injections, producing injections of AAV that were more robust in size and in density of neurons infected compared with those of BDA. However, these differences did not preclude further comparison between the tracers, because the pairs of injections were suitably colocalized and contained some percentage of double-labeled neurons. A qualitative analysis of projection patterns showed that the two tracers behave very similarly when injection sites are well matched. Additionally, a quantitative analysis of relative projection intensity for cases targeting primary motor cortex (MOp), primary somatosensory cortex (SSp), and caudoputamen (CP) showed strong agreement in the ranked order of projection intensities between the two tracers. A detailed analysis of the projections of two brain regions (SSp and MOp) revealed many targets that have not previously been described in the mouse or rat. Minor retrograde labeling of neurons was observed in all cases examined, for both AAV and BDA. Our results show that AAV has actions equivalent to those of BDA as an anterograde tracer and is suitable for analysis of neural circuitry throughout the mouse brain.

The pronociceptive dorsal reticular nucleus contains mostly tonic neurons and shows a high prevalence of spontaneous activity in block preparation

Despite the importance and significant clinical impact of understanding information processing in the nociceptive system, the functional properties of neurons in many parts of this system are still unknown. In this work we performed whole cell patch-clamp recording in rat brain stem blocks to characterize the electrophysiological properties of neurons in the dorsal reticular nucleus (DRt), a region known to be involved in pronociceptive modulation. We also compared properties of DRt neurons with those in the adjacent parvicellular reticular nucleus and in neighboring regions outside the reticular formation. We found that neurons in the DRt and parvicellular reticular nucleus had similar electrophysiological properties and exhibited mostly toniclike firing patterns, whereas neurons outside the reticular formation showed a larger diversity of firing patterns. Interestingly, more than one-half of the neurons also showed spontaneous activity. While the general view of the reticular formation, being a loosely associated mesh of groups of neurons with diverse function, and earlier reports suggests more electrophysiological heterogeneity, we showed that this is indeed not the case. Our results indicate that functional difference of neurons in the reticular formation may mostly be determined by their connectivity profiles and not by their intrinsic electrophysiological properties. The dominance of tonic neurons in the DRt supports previous conclusions that these neurons encode stimulus intensity through their firing frequency, while the high prevalence of spontaneous activity most likely shapes nociceptive modulation by this brain stem region.

Migraine pathophysiology: anatomy of the trigeminovascular pathway and associated neurological symptoms, CSD, sensitization and modulation of pain

Scientific evidence support the notion that migraine pathophysiology involves inherited alteration of brain excitability, intracranial arterial dilatation, recurrent activation and sensitization of the trigeminovascular pathway, and consequential structural and functional changes in genetically susceptible individuals. Evidence of altered brain excitability emerged from clinical and preclinical investigation of sensory auras, ictal and interictal hypersensitivity to visual, auditory and olfactory stimulation, and reduced activation of descending inhibitory pain pathways. Data supporting the activation and sensitization of the trigeminovascular system include the progressive development of cephalic and whole-body cutaneous allodynia during a migraine attack. Also, structural and functional alterations include the presence of subcortical white mater lesions, thickening of cortical areas involved in processing sensory information, and cortical neuroplastic changes induced by cortical spreading depression. Here, we review recent anatomical data on the trigeminovascular pathway and its activation by cortical spreading depression, a novel understanding of the neural substrate of migraine-type photophobia, and modulation of the trigeminovascular pathway by the brainstem, hypothalamus and cortex.

Abnormal endogenous pain modulation is a shared characteristic of many chronic pain conditions

The intensity of acute and chronic pain depends on interactions between peripheral impulse input and CNS pain mechanisms, including facilitation and inhibition. Whereas tonic pain inhibition is a characteristic of most pain-free individuals, pain facilitation can be detected in many chronic pain patients. The capability to inhibit pain is normally distributed along a wide continuum in the general population and can be used to predict chronic pain. Accumulating evidence suggests that endogenous pain inhibition depends on activation of the prefrontal cortex, periaqueductal gray and rostral ventral medulla. Quantitative sensory test paradigms have been designed to acquire detailed information regarding each individual's endogenous pain inhibition and facilitation. Such tests include: temporal summation of pain, which is mostly used to assess facilitatory pain modulation by measuring the change in pain perception during a series of identical nociceptive stimuli; and conditioned pain modulation, which tests pain inhibition by utilizing two simultaneously applied painful stimuli (the 'pain inhibits pain' paradigm). Considerable indirect evidence seems to indicate that not only increased pain facilitation but also ineffective pain inhibition represents a predisposition for chronic pain. This view is supported by the fact that many chronic pain syndromes (e.g., fibromyalgia, temporomandibular joint disorder, irritable bowel syndrome, headache and chronic fatigue syndrome) are associated with hypersensitivity to painful stimuli and reduced endogenous pain inhibition. However, future prospective studies will be necessary to provide definitive evidence for this relationship. Such research would not only provide important information about mechanisms relevant to chronic pain but would also permit identification of individuals at high risk for future chronic pain.

Cortical influences on rapid brainstem plasticity

Cortical contributions to brainstem plasticity in the somatosensory system are poorly understood. Tactile receptive fields (RFs) of brainstem dorsal column nuclei (DCN) neurons rapidly enlarge when peripheral inputs are disrupted by local anesthetic blocks with lidocaine (LID). Cortical inputs appear to influence this plasticity because enlargements have been shown to be greater when cortical inputs are disrupted. Like disruptions of peripheral inputs, disruptions of DCN inhibition by DCN administration of the GABAA receptor antagonist bicuculline methiodide (BMI) also cause rapid enlargements of DCN RFs when cortical inputs are intact. These findings leave questions about interactions between cortical inputs, DCN inhibition, and DCN RF plasticity. To study potential interactions, the present experiments evaluated RF sizes of DCN tactilely responsive neurons in anesthetized rats following DCN microinjection of BMI when cortical inputs were acutely disrupted or intact. These tests were also supplemented by subsequent LID tests to directly compare post-BMI and post-LID effects on the same RF. BMI caused DCN RF enlargements when cortical inputs were disrupted or intact; however, enlargements after cortical input disruption were greater than when cortical inputs were intact. Following RF enlargement and retraction after BMI, LID often caused a second enlargement of the same RF, across skin that partially matched skin involved in the enlargement after BMI. This occurred when cortical inputs were disrupted or intact. We hypothesize that cortical inputs are not required for BMI and LID to initiate partially matching enlargements in individual DCN tactile RFs, however, cortical inputs constrain magnitudes of these enlargements.

Differential responsiveness of c-Fos expression in the rat medulla oblongata to different treadmill running speeds

Expression of the inducible transcription factor c-Fos was mapped in the rat medulla oblongata to identify the brain areas respond to different running speeds. Rats were subjected to 30 min of running, either at high speed, low speed or just sitting on a treadmill (control). Blood lactate levels were measured to confirm the physiological impact of different exercise intensities. The number of c-Fos-ir cells was counted and their spatial distributions were mapped through the rostral to the caudal level in the medulla. A statistically significant exercise intensity-dependent induction of c-Fos was observed in the nucleus of the solitary tract (NTS) and caudal ventrolateral medulla (CVL) in the medulla. Further, c-Fos induction was more predominant in the caudal part of each nucleus. The present data clearly show that different running speeds cause differential activation of each nucleus in the medulla, and in particular, the caudal parts of the NTS and the CVL are the most responsive to speed changes. The present study identifies brain areas newly found to be responsive to changes in running speed. These findings are likely to be particularly helpful in studies of specific neural circuits and their functions in response to different running speeds.

Anterior cingulate cortex contributes to the descending facilitatory modulation of pain via dorsal reticular nucleus

Supraspinal centres biphasically modulate spinal nociceptive transmission, including descending inhibition and facilitation. Recent studies have revealed that descending facilitatory modulation is a key mechanism underlying induction and maintenance of neuropathic and inflammatory pain. The anterior cingulate cortex (ACC) is not only involved in the transmission of pain sensation but also plays a role in processing pain-related emotion. The ACC also widely connects with relevant regions of the descending modulation system. Here we used electrophysiological and behavioural techniques to study the possible pathways behind the modulation of spinal nociceptive transmission from the ACC. C-fibre-evoked field potentials in the spinal dorsal horn were produced by electrical stimulation of the sciatic nerve at an intensity high enough to excite C fibres, and paw withdrawal latencies (PWLs) to noxious heating were recorded. The results showed that high-frequency tetanic electrical stimulation of the ACC both unilaterally enhanced the C-fibre-evoked field potentials in the spinal dorsal horn and bilaterally shortened PWLs, indicating a facilitation of spinal nociception. A similar effect was observed after microinjection of N-methyl-d-aspartic acid (NMDA; 10 nm, 1 microL) or homocysteic acid (HCA; 0.1 m, 1 microL) into the ACC. When the dorsal reticular nucleus (DRt) was electrolytically lesioned, ACC-induced facilitation of spinal nociception was blocked. These results imply that: (i) activation of the ACC may facilitate spinal nociception; (ii) NMDA receptors in the ACC may be involved in descending facilitation; and (iii) the DRt plays a crucial role in mediating ACC-induced facilitation of spinal nociception.

Cortical influences on sizes and rapid plasticity of tactile receptive fields in the dorsal column nuclei

The cerebral cortex influences subcortical processing. In the somatosensory system, descending cortical inputs contribute in specific ways to the sizes and plasticity of tactile receptive fields (RFs) in the thalamus, but less is known about cortical influences on these aspects of brainstem RFs. The present studies evaluated how loss of cortical inputs affects sizes and plasticity of RFs in the brainstem dorsal column nuclei (DCN) when peripheral inputs were normal and when peripheral inputs were acutely disrupted. Loss of cortical inputs was produced by acute lesion of somatosensory, motor, and adjacent cortex, whereas disruption of peripheral inputs was produced by cutaneous microinjection of lidocaine (LID). Modest or no changes in sizes of DCN RFs, comparable to changes during control periods of no treatment, were seen in response to cortical lesion. LID caused rapid enlargements in RFs when cortex was intact. LID also caused rapid RF enlargements after cortical lesion, and these enlargements were greater than post-LID enlargements when cortex was intact. These results indicate that normally sized RFs continue to be produced in the DCN after loss of cortical input. Cortex is also not required for RF enlargements after LID; however, cortical inputs have a constraining effect on these enlargements. Considered with findings from previous thalamic studies, these results suggest that cortical influences on RF size and plasticity in the DCN and thalamus differ in some respects.

Transcranial magnetic stimulation in the management of pain

Drug-resistant, neurogenic pain can be treated by chronic motor cortex stimulation using surgically-implanted epidural electrodes. High-frequency, subthreshold repetitive transcranial magnetic stimulation (rTMS) of the motor cortex was shown to be able to produce antalgic effects, at least transiently, in patients with chronic pain. Nevertheless, other cortical targets than the primary motor cortex are tempting (parietal or prefrontal areas for instance) for the management of pain and need to be studied. Motor cortex TMS was also found to modulate non-nociceptive sensory perception as well as acutely provoked pain in healthy subjects by means of a single conditioning pulse or repeated trains. On the contrary, spontaneous or provoked pain was shown to modify motor cortex excitability, as assessed by TMS technique. Taking into account all these observations, it appears that motor cortex function and pain process are closely related and that TMS is a potent tool to explore and to understand this relationship. Beyond this physiological purpose, rTMS could be useful to control episodes of neurogenic pain of limited duration or to select patients for the surgical implantation of a cortical stimulator.

Brain afferents to the lateral caudal ventrolateral medulla: a retrograde and anterograde tracing study in the rat

The ventrolateral medulla (VLM) modulates autonomic functions, motor reactions and pain responses. The lateralmost part of the caudal VLM (VLMlat) was recently shown to be the VLM area responsible for pain modulation. In the present study, the brain sources of VLMlat afferent fibers were determined by tract-tracing techniques. Following injection of cholera toxin subunit B into the VLMlat, retrogradely labeled neurons in the forebrain occurred at the somatosensory, insular, motor, limbic and infralimbic cortices, and at the central amygdaloid nucleus. Retrogradely labeled neurons in diencephalic regions were observed in the lateral hypothalamus, posterior hypothalamus and paraventricular nucleus. In the brainstem, retrograde labeling occurred at the periaqueductal gray, red nucleus, parabrachial area, nucleus raphe magnus, nucleus tractus solitarii, lateral reticular nucleus and dorsal and ventral medullary reticular formation. In the cerebellum, retrogradely labeled neurons occurred at the lateral nucleus. Following injections of the anterograde tracer biotinylated dextran amine (BDA) into the lateral hypothalamus or paraventricular nucleus, anterogradely labeled fibers were mainly observed in the VLMlat. Injections of BDA into the periaqueductal gray, red nucleus or lateral nucleus of the cerebellum resulted in anterograde labeling in the VLMlat and lateral reticular nucleus. The present study gives an account of the brain regions putatively involved in triggering the modulatory actions elicited from the VLMlat. These include areas committed to somatosensory processing, autonomic control, somatic and visceral motor activity and affective reactions. The findings suggest that the VLMlat may play a major homeostatic role in the integration of nociception with other brain functions.

[Neurobiology of trigeminal pain]

The brainstem trigeminal complex integrates somatosensory inputs from orofacial areas and meninges. Recent studies have shown the existence of a double representation of pain within the brainstem, at the level of both caudalis and oralis subnuclei. Noxious messages are mainly conveyed by C-fibers that activate the subnucleus caudalis neurons. These neurons in turn activate the subnucleus oralis whose neurons share similar features with the deep spinal dorsal horn neurons. In contrast with the nearness of the laminar organization of the dorsal horn, the vertical organization of the trigeminal complex offers an easier access for the study of segmental mechanisms of nociceptive processing. This model allowed us to show the existence of subtle NMDA-related mechanisms of segmental nocious processing. The trigeminal complex conveys nociceptive messages to several brainstem and thalamic relays that activate a number of cortical areas responsible for pain sensations and reactions. Cortical processing is sustained by reciprocal interactions with thalamic areas and also by a direct modulation of their pre-thalamic relays. The dysfunction of these multiple modulatory mechanisms probably plays a key role in the pathophysiology of chronic trigeminal pain.

Responses of diencephalic nociceptive neurones to orofacial stimuli and effects of internal capsule stimulation in the rat

The effect of conditioning stimulation of the internal capsule on nociceptive neurones in the rat diencephalon was investigated. The animals were anaesthetised with N(2)O/O(2) (2:1) and 0.5% halothane, and immobilised with pancuronium bromide. Nociceptive neurones responding to noxious stimulation of the face and oral structures were recorded in the ventral posteromedial nucleus, posterior group and zona incerta. These neurones were classified into wide dynamic range and nociceptive-specific types. Functional segregation of these nociceptive neurones was not apparent within the nucleus or between nuclei. A test stimulus with a single rectangular pulse (5-70 V) was applied to the centre of the receptive field; the nociceptive neurones exhibited short- and/or long-latency responses. Both responses in about 45% of the nociceptive neurones were inhibited by conditioning stimuli to the contralateral internal capsule with trains of 33 pulses (300 microA) at 330 Hz. The percent inhibitory effects on the nociceptive neurones of each area were 68.0+/-14.8% (n = 6) in the ventral posteromedial nucleus, 72.8+/-12.4% (n = 4) in the posterior group and 61.5+/-7.5% (n = 4) in the zona incerta. Effective sites for conditioning stimulation were concentrated in the lateral side of the internal capsule, through which the corticofugal fibres from the somatosensory cortex pass. These findings suggest that the transmission of nociceptive information to the diencephalon is modulated by stimulation of the internal capsule at the level of the trigeminal sensory complex in the brainstem. They might provide a novel way to elucidate the neurophysiological basis for antinociception by stimulation of the internal capsule observed in clinical studies.

The integrative role of the rat medullary subnucleus reticularis dorsalis in nociception

Neurons within the medullary subnucleus reticularis dorsalis (SRD) of the rat convey selectively nociceptive information from all parts of the body. We have sought to define the neuronal networks that convey information from widespread noxious stimuli to the diffuse thalamocortical system and also modulate spinal outflow. The experiments, which were performed in rats, were designed to determine whether efferents from the SRD issue collaterals to the thalamus and spinal cord. Injections of the tracers fluorogold and tetramethylrhodamine-labelled dextran were centred stereotaxically in two areas that receive dense projections from the SRD: the cervical spinal cord and the lateral ventromedial thalamus (VMl), respectively. In other experimental series, SRD neurons were characterized electrophysiologically and individually labelled in a Golgi-like manner following juxtacellular iontophoresis of biotin-dextran. More than half reticulothalamic neurons within the SRD provided monosynaptic connections to the spinal cord. SRD neurons that responded to Adelta- or Adelta- and C-fibre activation from any area of the body had axons that gave both ascending and descending collaterals. Because the SRD innervates several areas involved in motor processing and receives strong, direct influences from several cortical regions, it could provide a structural basis for the processing of nociceptive and motor activities.

Brain afferents to the medullary dorsal reticular nucleus: a retrograde and anterograde tracing study in the rat

The medullary dorsal reticular nucleus (DRt) was recently shown to belong to the supraspinal pain control system; neurons within this nucleus give origin to a descending projection that increases spinal nociceptive transmission and facilitates pain perception [Almeida et al. (1999), Eur. J. Neurosci., 11, 110-122]. In the present study, the areas of the brain that may modulate the activity of DRt neurons were investigated by using of tract-tracing techniques. Injection of a retrograde tracer into the DRt resulted in labelling in multiple areas of the brain. In the contralateral orbital, prelimbic, infralimbic, insular, motor and somatosensory cortices labelling was prominent, but a smaller ipsilateral projection from these same areas was also detected. Strong labelling was also noted in the central amygdaloid nucleus, bed nucleus of stria terminalis and substantia innominata. Labelled diencephalic areas were mainly confined to the hypothalamus, namely its lateral and posterior areas as well as the paraventricular nucleus. In the mesencephalon, the periaqueductal grey, red nucleus and deep mesencephalic nucleus were strongly labelled, whereas, in the brainstem, the parabrachial nuclei, rostroventromedial medulla, nucleus tractus solitarius, spinal trigeminal nucleus, and the parvocellular, dorsal, lateral and ventral reticular nuclei were the most densely labelled regions. All deep cerebellar nuclei were labelled bilaterally. These data suggest that the DRt integrates information from the somatosensory, antinociceptive, autonomic, limbic, pyramidal and extrapyramidal systems while triggering its descending facilitating action upon the spinal nociceptive transmission.

Definite segregation of cortical neurons projecting to the dorsal column nuclei in the rat

The dorsal column nuclei (DCN) receive a substantial contingent of projections that arise from a number of somatosensory and motor cortical areas. We investigated the spatial organization of these projections in the rat by placing small deposits of two retrograde fluorescent tracers in adjacent foci within the DCN. Single-labeled neurons were abundant in layer V of the primary somatosensory (SI) and motor areas, and in the posterior parietal cortex. More sparse labeling was found in the medial agranular cortex (or MII), and the second somatosensory area. A somatotopic-like arrangement of these neurons was more clearly noticed in the granular zones of SI. Double-labeled neurons were very uncommon, and appeared at border regions where single-labeled cells intermingled. The segregation of these projections supports some differential modulatory effects that the cortex exerts on the somatosensory processing that takes place in the DCN.