Functional plasticity in primate somatosensory thalamus following chronic lesion of the ventral lateral spinal cord

The role of spinal neurons targeted by corticospinal neurons in central poststroke neuropathic pain

BACKGROUND

Central poststroke pain (CPSP) is one of the primary sequelae following stroke, yet its underlying mechanisms are poorly understood.

METHODS

By lesioning the lateral thalamic nuclei, we first established a CPSP model that exhibits mechanical and thermal hypersensitivity. Innocuous mechanical stimuli following the thalamic lesion evoked robust neural activation in somatosensory corticospinal neurons (CSNs), as well as in the deep dorsal horn, where low threshold mechanosensory afferents terminate. In this study, we used viral-based mapping and intersectional functional manipulations to decipher the role of somatosensory CSNs and their spinal targets in the CPSP pathophysiology.

RESULTS

We first mapped the post-synaptic spinal targets of lumbar innervating CSNs using an anterograde trans-synaptic AAV1-based strategy and showed these spinal interneurons were activated by innocuous tactile stimuli post-thalamic lesion. Functionally, tetanus toxin-based chronic inactivation of spinal neurons targeted by CSNs prevented the development of CPSP. Consistently, transient chemogenetic silencing of these neurons alleviated established mechanical pain hypersensitivity and innocuous tactile stimuli evoked aversion linked to the CPSP. In contrast, chemogenetic activation of these neurons was insufficient to induce robust mechanical allodynia typically observed in the CPSP.

CONCLUSION

The CSNs and their spinal targets are required but insufficient for the establishment of CPSP hypersensitivity. Our study provided novel insights into the neural mechanisms underlying CPSP and potential therapeutic interventions to treat refractory central neuropathic pain conditions.

Role of auditory-somatosensory corticothalamic circuit integration in analgesia

Our sensory environment is permeated by a diverse array of auditory and somatosensory stimuli. The pairing of acoustic signals with concurrent or forthcoming tactile cues are abundant in everyday life and various survival contexts across species, thus deeming the ability to integrate sensory inputs arising from the combination of these stimuli as crucial. The corticothalamic system plays a critical role in orchestrating the construction, integration and distribution of the information extracted from these sensory modalities. In this mini-review, we provide a circuit-level description of the auditory corticothalamic pathway in conjunction with adjacent corticothalamic somatosensory projections. Although the extent of the functional interactions shared by these pathways is not entirely elucidated, activation of each of these systems appears to modulate sensory perception in the complementary domain. Several specific issues are reviewed. Under certain environmental noise conditions, the spectral information of a sound could induce modulations in nociception and even induce analgesia. We begin by discussing recent findings by Zhou et al. (2022) implicating the corticothalamic system in mediating sound-induced analgesia. Next, we describe relevant components of the corticothalamic pathway's functional organization. Additionally, we describe an emerging body of literature pointing to intrathalamic circuitry being optimal for controlling and selecting sensory signals across modalities, with the thalamic reticular nucleus being a candidate mechanism for directing cross-modal interactions. Finally, Ca2+ bursting in thalamic neurons evoked by the thalamic reticular nucleus is explored.

Thalamic anterior part of the ventral posterolateral nucleus and central lateral nucleus in the genesis of central post-stroke pain

BACKGROUND

The genesis of central post-stroke pain (CPSP) is important but difficult to understand. We evaluated the involvement of the thalamic anterior part of the ventral posterolateral nucleus (VPLa) and central lateral nucleus (CL) in the occurrence of CPSP.

METHOD

Stereotactic thalamotomy was performed on the posterior part of the ventral lateral nucleus (VLp)-VPLa and CL in 9 patients with CPSP caused by deep-seated intracerebral hemorrhage. Computed tomography (CT) did not reveal definite thalamic lesion in 5 patients but did in 4 patients. Electrophysiological studies of these thalamic nuclei were carried out during the surgery. Anatomical studies using CT were performed in another 20 patients with thalamic hemorrhage who had clear consciousness but had sensory disturbance at onset.

RESULTS

Neural activities were preserved and hyperactive and unstable discharges (HUDs) were often recognized along the trajectory in the thalamic VLp-VPLa in 5 patients without thalamic lesion. Surgical modification of this area ameliorated pain, particularly movement-related pain. Neural activities were hypoactive in the other 4 patients with thalamic lesion. However, neural activities were preserved and HUDs were sometimes recognized in the CL. Sensory responses were seen, but at low rate, in the sensory thalamus. Anatomical study showed that the thalamic lesion was obviously smaller in the patients with developing pain in the chronic stage.

CONCLUSIONS

Change in neural activities around the cerebrovascular disease lesion in the thalamic VPLa or CL might affect the perception of sensory impulses or sensory processing in those thalamic nuclei, resulting in the genesis of CPSP.

Changes in Prefrontal Cortex-Thalamic Circuitry after Acoustic Trauma

In the adult auditory system, loss of input resulting from peripheral deafferentation is well known to lead to plasticity in the central nervous system, manifested as reorganization of cortical maps and altered activity throughout the central auditory pathways. The auditory system also has strong afferent and efferent connections with cortico-limbic circuitry including the prefrontal cortex and the question arises whether this circuitry is also affected by loss of peripheral input. Recent studies in our laboratory showed that PFC activation can modulate activity of the auditory thalamus or medial geniculate nucleus (MGN) in normal hearing rats. In addition, we have shown in rats that cochlear trauma resulted in altered spontaneous burst firing in MGN. However, whether the PFC influence on MGN is changed after cochlear trauma is unknown. We investigated the effects of electrical stimulation of PFC on single neuron activity in the MGN in anaesthetized Wistar rats 2 weeks after acoustic trauma or sham surgery. Electrical stimulation of PFC showed a variety of effects in MGN neurons both in sham and acoustic trauma groups but inhibitory responses were significantly larger in the acoustic trauma animals. These results suggest an alteration in functional connectivity between PFC and MGN after cochlear trauma. This change may be a compensatory mechanism increasing sensory gating after the development of altered spontaneous activity in MGN, to prevent altered activity reaching the cortex and conscious perception.

Mechanisms of Below-Level Pain Following Spinal Cord Injury (SCI)

Mechanisms of below-level pain are discoverable as neural adaptations rostral to spinal injury. Accordingly, the strategy of investigations summarized here has been to characterize behavioral and neural responses to below-level stimulation over time following selective lesions of spinal gray and/or white matter. Assessments of human pain and the pain sensitivity of humans and laboratory animals following spinal injury have revealed common disruptions of pain processing. Interruption of the spinothalamic pathway partially deafferents nocireceptive cerebral neurons, rendering them spontaneously active and hypersensitive to remaining inputs. The spontaneous activity among these neurons is disorganized and unlikely to generate pain. However, activation of these neurons by their remaining inputs can result in pain. Also, injury to spinal gray matter results in a cascade of secondary events, including excitotoxicity, with rostral propagation of excitatory influences that contribute to chronic pain. Establishment and maintenance of below-level pain results from combined influences of injured and spared axons in the spinal white matter and injured neurons in spinal gray matter on processing of nociception by hyperexcitable cerebral neurons that are partially deafferented. A model of spinal stenosis suggests that ischemic injury to the core spinal region can generate below-level pain. Additional questions are raised about demyelination, epileptic discharge, autonomic activation, prolonged activity of C nocireceptive neurons, and thalamocortical plasticity in the generation of below-level pain. PERSPECTIVE: An understanding of mechanisms can direct therapeutic approaches to prevent development of below-level pain or arrest it following spinal cord injury. Among the possibilities covered here are surgical and other means of attenuating gray matter excitotoxicity and ascending propagation of excitatory influences from spinal lesions to thalamocortical systems involved in pain encoding and arousal.

Selective deficiencies in descending inhibitory modulation in neuropathic rats: implications for enhancing noradrenergic tone

Supplemental Digital Content is Available in the Text.

Descending noradrenergic pathways modulate spontaneous but not evoked thalamic neuronal hyperexcitability in neuropathic pain states. Spinal clonidine inhibits evoked and spontaneous firing, whereas reboxetine selectively inhibits evoked firing.

Pontine noradrenergic neurones form part of a descending inhibitory system that influences spinal nociceptive processing. Weak or absent descending inhibition is a common feature of chronic pain patients. We examined the extent to which the descending noradrenergic system is tonically active, how control of spinal neuronal excitability is integrated into thalamic relays within sensory-discriminative projection pathways, and how this inhibitory control is altered after nerve injury. In vivo electrophysiology was performed in anaesthetised spinal nerve–ligated (SNL) and sham-operated rats to record from wide dynamic range neurones in the ventral posterolateral thalamus (VPL). In sham rats, spinal block of α₂-adrenoceptors with atipamezole resulted in enhanced stimulus-evoked and spontaneous firing in the VPL, and produced conditioned place avoidance. However, in SNL rats, these conditioned avoidance behaviours were absent. Furthermore, inhibitory control of evoked neuronal responses was lost, but spinal atipamezole markedly increased spontaneous firing. Augmenting spinal noradrenergic tone in neuropathic rats with reboxetine, a selective noradrenergic reuptake inhibitor, modestly reinstated inhibitory control of evoked responses in the VPL but had no effect on spontaneous firing. By contrast, clonidine, an α₂ agonist, inhibited both evoked and spontaneous firing, and exhibited increased potency in SNL rats compared with sham controls. These data suggest descending noradrenergic inhibitory pathways are tonically active in sham rats. Moreover, in neuropathic states, descending inhibitory control is diminished, but not completely absent, and distinguishes between spontaneous and evoked neuronal activity. These observations may have implications for how analgesics targeting the noradrenergic system provide relief.

Pain After Spinal Cord Injury Is Associated With Abnormal Presynaptic Inhibition in the Posterior Nucleus of the Thalamus

Pain after spinal cord injury (SCI-Pain) is one of the most debilitating sequelae of spinal cord injury, characterized as relentless, excruciating pain that is largely refractory to treatments. Although it is generally agreed that SCI-Pain results from maladaptive plasticity in the pain processing pathway that includes the spinothalamic tract and somatosensory thalamus, the specific mechanisms underlying the development and maintenance of such pain are yet unclear. However, accumulating evidence suggests that SCI-Pain may be causally related to abnormal thalamic disinhibition, leading to hyperactivity in the posterior thalamic nucleus (PO), a higher-order nucleus involved in somatosensory and pain processing. We previously described several presynaptic mechanisms by which activity in PO is regulated, including the regulation of GABAergic as well as glutamatergic release by presynaptic metabotropic gamma-aminobutyric acid (GABA_B) receptors. Using acute slices from a mouse model of SCI-Pain, we tested whether such mechanisms are affected by SCI-Pain. We reveal 2 abnormal changes in presynaptic signaling in the SCI-Pain condition. The substantial tonic activation of presynaptic GABA_B receptors on GABAergic projections to PO-characteristic of normal animals-was absent in mice with SCI-Pain. Also absent in mice with SCI-Pain was the normal presynaptic regulation of glutamatergic projections to the PO by GABA_B receptors. The loss of these regulatory presynaptic mechanisms in SCI-Pain may be an element of maladaptive plasticity leading to PO hyperexcitability and behavioral pain, and may suggest targets for development of novel treatments.

PERSPECTIVE

This report presents synaptic mechanisms that may underlie the development and maintenance of SCI-Pain. Because of the difficulty in treating SCI-Pain, a better understanding of the underlying neurobiological mechanisms is critical, and may allow development of better treatment modalities.

Wo der Schmerz in das Bewusstsein tritt: das thalamo-kortikale System bei der Schmerzverarbeitung

Die Übersetzung von schmerzhaften Reizen in Schmerzempfindungen wird durch mehrere periphere und zentrale Signalwege des Nervensystems verwirklicht. Man nimmt heute an, dass die Organisation dieser Signalwege die beiden Hauptfunktionen der Schmerzwahrnehmung wiederspiegeln: die Bewertung von schmerzhaften Reizen (wo, was, wie stark) und die Generierung negativer Emotionen. Experimentelle Befunde deuten darauf hin, dass aufsteigende Schmerzsignale über zwei Hauptwege im thalamokortikalen (TK) System verlaufen, die diese beiden Funktionen erfüllen. Wir diskutieren daher hier die strukturellen und funktionellen Befunde, die zu der Auffassung führten, dass diskriminierende Schmerzbewertung im lateralen TK-Weg ausgeführt wird, während der mediale TK-Weg schmerzassoziierte aversive Emotionen generiert. Obwohl der Schwerpunkt dieses Übersichtsartikels auf akuter Schmerzverarbeitung liegt, gehen wir zum Schluss darauf ein, wie Veränderungen in diesen Signalwegen zu pathologischen Schmerzempfindungen bei Menschen und Tiermodellen führen können.

Human Thalamic Somatosensory Nucleus (Ventral Caudal, Vc) as a Locus for Stimulation by INPUTS from Tactile, Noxious and Thermal Sensors on an Active Prosthesis

The forebrain somatic sensory locus for input from sensors on the surface of an active prosthesis is an important component of the Brain Machine Interface. We now review the neuronal responses to controlled cutaneous stimuli and the sensations produced by Threshold Stimulation at Microampere current levels (TMIS) in such a locus, the human thalamic Ventral Caudal nucleus (Vc). The responses of these neurons to tactile stimuli mirror those for the corresponding class of tactile mechanoreceptor fiber in the peripheral nerve, and TMIS can evoke sensations like those produced by the stimuli that optimally activate each class. These neuronal responses show a somatotopic arrangement from lateral to medial in the sequence: leg, arm, face and intraoral structures. TMIS evoked sensations show a much more detailed organization into anterior posteriorly oriented rods, approximately 300 microns diameter, that represent smaller parts of the body, such as parts of individual digits. Neurons responding to painful and thermal stimuli are most dense around the posterior inferior border of Vc, and TMIS evoked pain sensations occur in one of two patterns: (i) pain evoked regardless of the frequency or number of spikes in a burst of TMIS; and (ii) the description and intensity of the sensation changes with increasing frequencies and numbers. In patients with major injuries leading to loss of somatic sensory input, TMIS often evokes sensations in the representation of parts of the body with loss of sensory input, e.g., the phantom after amputation. Some patients with these injuries have ongoing pain and pain evoked by TMIS of the representation in those parts of the body. Therefore, thalamic TMIS may produce useful patterned somatotopic feedback to the CNS from sensors on an active prosthesis that is sometimes complicated by TMIS evoked pain in the representation of those parts of the body.

Functional Characterization of At-Level Hypersensitivity in Patients With Spinal Cord Injury

At-level and above-level hypersensitivity was assessed in patients with chronic complete thoracic spinal cord injury (SCI). Patients were classified using somatosensory mapping (brush, cold, pinprick) and assigned into 2 groups (ie, patients with at-level hypersensitivity [SCIHs, n = 8] and without at-level hypersensitivity [SCINHs, n = 7]). Gender and age-matched healthy subjects served as controls. Quantitative sensory testing (QST), electrically- and histamine-induced pain and itch, laser Doppler imaging, and laser-evoked potentials (LEP) were recorded at-level and above-level in SCI-patients. Six of 8 SCIHs, but 0 of 7 SCINHs patients suffered from neuropathic below-level pain. Clinical sensory mapping revealed spreading of hypersensitivity to more cranial areas (above-level) in 3 SCIHs. Cold pain threshold measures confirmed clinical hypersensitivity at-level in SCIHs. At-level and above-level hypersensitivity to electrical stimulation did not differ significantly between SCIHs and SCINHs. Mechanical allodynia, cold, and pin-prick hypersensitivity did not relate to impaired sensory function (QST), axon reflex flare, or LEPs. Clinically assessed at-level hypersensitivity was linked to below-level neuropathic pain, suggesting neuronal hyperexcitability contributes to the development of neuropathic pain. However, electrically evoked pain was not significantly different between SCI patients. Thus, SCI-induced enhanced excitability of nociceptive processing does not necessarily lead to neuropathic pain. QST and LEP revealed no crucial role of deafferentation for hypersensitivity development after SCI.

PERSPECTIVE

At-level hypersensitivity after complete thoracic SCI is associated with neuropathic below-level pain if evoked by clinical sensory stimuli. QST, LEP, and electrically-induced axon reflex flare sizes did not indicate somatosensory deafferentation in SCIHs.

Neuronal responses to tactile stimuli and tactile sensations evoked by microstimulation in the human thalamic principal somatic sensory nucleus (ventral caudal)

The normal organization and plasticity of the cutaneous core of the thalamic principal somatosensory nucleus (ventral caudal, Vc) have been studied by single-neuron recordings and microstimulation in patients undergoing awake stereotactic operations for essential tremor (ET) without apparent somatic sensory abnormality and in patients with dystonia or chronic pain secondary to major nervous system injury. In patients with ET, most Vc neurons responded to one of the four stimuli, each of which optimally activates one mechanoreceptor type. Sensations evoked by microstimulation were similar to those evoked by the optimal stimulus only among rapidly adapting neurons. In patients with ET, Vc was highly segmented somatotopically, and vibration, movement, pressure, and sharp sensations were usually evoked by microstimulation at separate sites in Vc. In patients with conditions including spinal cord transection, amputation, or dystonia, RFs were mismatched with projected fields more commonly than in patients with ET. The representation of the border of the anesthetic area (e.g., stump) or of the dystonic limb was much larger than that of the same part of the body in patients with ET. This review describes the organization and reorganization of human Vc neuronal activity in nervous system injury and dystonia and then proposes basic mechanisms.

Neuronal hyperexcitability in the ventral posterior thalamus of neuropathic rats: modality selective effects of pregabalin

Studies on brain mechanisms of neuropathic pain are lacking. This study characterizes the properties of rat ventral posterior thalamic wide dynamic range (WDR) and nociceptive-specific (NS) neurons, the latter of which are uncharacterized in a neuropathic state. We provide evidence of phenotypic changes in neuronal sensitivity that may underlie cold and brush hypersensitivity, and that WDR neurons, and not NS neurons, encode hypersensitivity to low-intensity stimuli. Pregabalin reversed neuronal hyperexcitability in spinal nerve-ligated rats in a modality-selective manner.

Neuropathic pain represents a substantial clinical challenge; understanding the underlying neural mechanisms and back-translation of therapeutics could aid targeting of treatments more effectively. The ventral posterior thalamus (VP) is the major termination site for the spinothalamic tract and relays nociceptive activity to the somatosensory cortex; however, under neuropathic conditions, it is unclear how hyperexcitability of spinal neurons converges onto thalamic relays. This study aimed to identify neural substrates of hypersensitivity and the influence of pregabalin on central processing. In vivo electrophysiology was performed to record from VP wide dynamic range (WDR) and nociceptive-specific (NS) neurons in anesthetized spinal nerve-ligated (SNL), sham-operated, and naive rats. In neuropathic rats, WDR neurons had elevated evoked responses to low- and high-intensity punctate mechanical stimuli, dynamic brushing, and innocuous and noxious cooling, but less so to heat stimulation, of the receptive field. NS neurons in SNL rats also displayed increased responses to noxious punctate mechanical stimulation, dynamic brushing, noxious cooling, and noxious heat. Additionally, WDR, but not NS, neurons in SNL rats exhibited substantially higher rates of spontaneous firing, which may correlate with ongoing pain. The ratio of WDR-to-NS neurons was comparable between SNL and naive/sham groups, suggesting relatively few NS neurons gain sensitivity to low-intensity stimuli leading to a “WDR phenotype.” After neuropathy was induced, the proportion of cold-sensitive WDR and NS neurons increased, supporting the suggestion that changes in frequency-dependent firing and population coding underlie cold hypersensitivity. In SNL rats, pregabalin inhibited mechanical and heat responses but not cold-evoked or elevated spontaneous activity.

Functional characterization of a mouse model for central post-stroke pain

Background

Stroke patients often suffer from a central neuropathic pain syndrome called central post-stroke pain. This syndrome is characterized by evoked pain hypersensitivity as well as spontaneous, on-going pain in the body area affected by the stroke. Clinical evidence strongly suggests a dysfunction in central pain pathways as an important pathophysiological factor in the development of central post-stroke pain, but the exact underlying mechanisms remain poorly understood. To elucidate the underlying pathophysiology of central post-stroke pain, we generated a mouse model that is based on a unilateral stereotactic lesion of the thalamic ventral posterolateral nucleus, which typically causes central post-stroke pain in humans.

Results

Behavioral analysis showed that the sensory changes in our model are comparable to the sensory abnormalities observed in patients suffering from central post-stroke pain. Surprisingly, pharmacological inhibition of spinal and peripheral key components of the pain system had no effect on the induction or maintenance of the evoked hypersensitivity observed in our model. In contrast, microinjection of lidocaine into the thalamic lesion completely reversed injury-induced hypersensitivity.

Conclusions

These results suggest that the evoked hypersensitivity observed in central post-stroke pain is causally linked to on-going neuronal activity in the lateral thalamus.

Cortical Responsiveness to Nociceptive Stimuli in Patients with Chronic Disorders of Consciousness: Do C-Fiber Laser Evoked Potentials Have a Role?

It has been shown that the presence of Aδ-fiber laser evoked potentials (Aδ-LEP) in patients suffering from chronic disorders of consciousness (DOC), such as vegetative state (VS) and minimally conscious state (MCS), may be the expression of a residual cortical pain arousal. Interestingly, the study of C-fiber LEP (C-LEP) could be useful in the assessment of cortical pain arousal in the DOC individuals who lack of Aδ-LEP. To this end, we enrolled 38 DOC patients following post-anoxic or post-traumatic brain injury, who met the international criteria for VS and MCS diagnosis. Each subject was clinically evaluated, through the coma recovery scale-revised (CRS-R) and the nociceptive coma scale-revised (NCS-R), and electrophysiologically tested by means of a solid-state laser for Aδ-LEP and C-LEP. VS individuals showed increased latencies and reduced amplitudes of both the Aδ-LEP and C-LEP components in comparison to MCS patients. Although nearly all of the patients had both the LEP components, some VS individuals showed only the C-LEP ones. Notably, such patients had a similar NCS-R score to those having both the LEP components. Hence, we could hypothesize that C-LEP generators may be rearranged or partially spared in order to still guarantee cortical pain arousal when Aδ-LEP generators are damaged. Therefore, the residual presence of C-LEP should be assessed when Aδ-LEP are missing, since a potential pain experience should be still present in some patients, so to properly initiate, or adapt, the most appropriate pain treatment.

Tactile representation in somatosensory thalamus (VPL) and cortex (S1) of awake primate and the plasticity induced by VPL neuroprosthetic stimulation

To further understand how tactile information is carried in somatosensory cortex (S1) and the thalamus (VPL), and how neuronal plasticity after neuroprosthetic stimulation affects sensory encoding, we chronically implanted microelectrode arrays across hand areas in both S1 and VPL, where neuronal activities were simultaneously recorded during tactile stimulation on the finger pad of awake monkeys. Tactile information encoded in the firing rate of individual units (rate coding) or in the synchrony of unit pairs (synchrony coding) was quantitatively assessed within the information theoretic-framework. We found that tactile information encoded in VPL was higher than that encoded in S1 for both rate coding and synchrony coding; rate coding carried greater information than synchrony coding for the same recording area. With the aim for neuroprosthetic stimulation, plasticity of the circuit was tested after 30 min of VPL electrical stimulation, where stimuli were delivered either randomly or contingent on the spiking of an S1 unit. We showed that neural encoding in VPL was more stable than in S1, which depends not only on the thalamic input but also on recurrent feedback. The percent change of mutual-information after stimulation was increased with closed-loop stimulation, but decreased with random stimulation. The underlying mechanisms during closed-loop stimulation might be spike-timing-dependent plasticity, while frequency-dependent synaptic plasticity might play a role in random stimulation. Our results suggest that VPL could be a promising target region for somatosensory stimulation with closed-loop brain-machine-interface applications.

Comparison of operant escape and reflex tests of nociceptive sensitivity

Testing of reflexes such as flexion/withdrawal or licking/guarding is well established as the standard for evaluating nociceptive sensitivity and its modulation in preclinical investigations of laboratory animals. Concerns about this approach have been dismissed for practical reasons - reflex testing requires no training of the animals; it is simple to instrument; and responses are characterized by observers as latencies or thresholds for evocation. In order to evaluate this method, the present review summarizes a series of experiments in which reflex and operant escape responding are compared in normal animals and following surgical models of neuropathic pain or pharmacological intervention for pain. Particular attention is paid to relationships between reflex and escape responding and information on the pain sensitivity of normal human subjects or patients with pain. Numerous disparities between results for reflex and operant escape measures are described, but the results of operant testing are consistent with evidence from humans. Objective reasons are given for experimenters to choose between these and other methods of evaluating the nociceptive sensitivity of laboratory animals.

Oligodendrocyte ablation triggers central pain independently of innate or adaptive immune responses in mice

Mechanisms underlying central neuropathic pain are poorly understood. Although glial dysfunction has been functionally linked with neuropathic pain, very little is known about modulation of pain by oligodendrocytes. Here we report that genetic ablation of oligodendrocytes rapidly triggers a pattern of sensory changes that closely resemble central neuropathic pain, which are manifest before overt demyelination. Primary oligodendrocyte loss is not associated with autoreactive T- and B-cell infiltration in the spinal cord and neither activation of microglia nor reactive astrogliosis contribute functionally to central pain evoked by ablation of oligodendrocytes. Instead, light and electron microscopic analyses reveal axonal pathology in the spinal dorsal horn and spinothalamic tract concurrent with the induction and maintenance of nociceptive hypersensitivity. These data reveal a role for oligodendrocytes in modulating pain and suggest that perturbation of oligodendrocyte functions that maintain axonal integrity can lead to central neuropathic pain independent of immune contributions.

Whether oligodendrocytes have a role in the development of chronic pain is not clear. Here the authors show that oligodendrocyte depletion causes a neuropathic pain that sets in before demyelination and is independent of immune cell activation and infiltration.

Therapeutic time window of noninvasive brain stimulation for pain treatment: inhibition of maladaptive plasticity with early intervention

Neuromodulatory effects of noninvasive brain stimulation (NIBS) have been extensively studied in chronic disorders such as major depression, chronic pain and stroke. However, few studies have explored the use of these techniques in acute conditions. A possible use of NIBS in acute disorders is to prevent or reverse ongoing maladaptive plastic alterations, seemingly responsible for treatment refractoriness and detrimental behavioral changes. In this review, the authors discuss the potential role of NIBS in blocking maladaptive plasticity using the transition of acute to chronic pain in conditions such as postsurgical pain, central poststroke pain, pain after spinal cord injury and pain after traumatic brain injury as a model. The authors also present suggestions for clinical trial design using NIBS in the acute stage of illnesses.

Excitotoxic injury to thoracolumbar gray matter alters sympathetic activation and thermal pain sensitivity

Studies of humans, monkeys and rodents have implicated combined gray and white matter damage as important for development of chronic pain following spinal cord injury (SCI). Below-level chronic pain and hyperalgesia following injury to the spinal white matter, including the spinothalamic tract (STT), can be enhanced by excitotoxic influences within the gray matter at the site of SCI. Also, excitotoxic injury of thoracic gray matter without interruption of the STT results in below-level heat hyperalgesia. The present study evaluates the possibility that thoracolumbar gray matter injury increases sensitivity to nociceptive heat stimulation by altering spinal sympathetic outflow. Thermal preferences of rats for heat (45 °C) versus cold (15 °C) were evaluated before and after thoracolumbar injections of quisqualic acid (QUIS). A pre-injury preference for heat changed to a post-injury preference for cold. Systemic activation of the sympathetic nervous system by restraint stress decreased the heat preference pre-injury and increased the cold preference post-injury. The heat aversive effect of stress was magnified and prolonged post-injury, compared to pre-injury. Also, peripheral sympathetic activation by nociceptive stimulation was evaluated pre- and post-injury by measuring thermal transfer through a hindpaw during stimulation with 44.5 °C. Skin temperature recordings revealed enhanced sympathetic activation by nociceptive heat stimulation following spinal QUIS injury. However, increased sympathetic activation with peripheral vasoconstriction should enhance cold aversion, in contrast to the observed increase in heat aversion. Thus, peripheral sympathetic vasoconstriction can be ruled out as a mechanism for heat hyperalgesia following excitotoxic gray matter injury.

Evaluation of lateral spinal hemisection as a preclinical model of spinal cord injury pain

Operant escape from nociceptive thermal stimulation of 13 Long-Evans rats was compared before and after lateral spinal hemisection, to determine whether this lesion configuration provides an appropriate preclinical model of the hyperalgesia that can be associated with human spinal cord injury. Escape from 44 °C and from 47 °C stimulation was not affected following sham spinal surgery but was significantly reduced over 20 weeks of postoperative testing following lateral spinal hemisection. This result is opposite to previous reports of enhanced reflex withdrawal in response to thermal stimulation of rats following lateral spinal hemisection. In addition, the latency of reflexive lick/guard responses to 44 °C was increased and the duration of lick/guard responding was decreased in the present study (hyporeflexia). Thus, previous assessments of simple withdrawal reflexes have described a hyperreflexia following lateral spinal hemisection that was not replicated by lick/guard testing, and postoperative escape responding revealed hypoalgesia rather than the increased pain sensitivity expected in a model of chronic pain.

Health survey of numbness/pain and its associated factors in Kotohira, Japan

We conducted a survey of adults in Kotohira, a town of about 10,000 people located in the Nakatado District of Kagawa Prefecture, Japan. The survey was distributed to 8184 individuals, and effective responses were received from 3863 persons (response rate, 47.2%) during the survey period. Results regarding numbness and pain showed numbness alone in 7.7%, pain alone in 7.2%, both numbness and pain in 6.0%, and neither numbness nor pain in 79.6%. Spine and spinal cord damage was reported present by 5.4%, and absent by 94.6%. Analysis using the Short-Form Health Survey questionnaire, with comparison between subjects reporting both numbness and pain in the extremities and subjects with either numbness or pain alone, showed lower scores for in Short-Form Health Survey subscales (physical functioning, role [physical, emotional], bodily pain, vitality, and mental health). Subjects with numbness alone generally reported no disability in daily life. In a secondary survey, analysis of neurological findings by specialists identified 6 cases of "pain following spinal cord damage" in which spinal cord-related pain developed in the hands or feet. This represented 0.15% of the survey population starting from the primary survey.

Role of primary somatosensory cortex in the coding of pain

The intensity and submodality of pain are widely attributed to stimulus encoding by peripheral and subcortical spinal/trigeminal portions of the somatosensory nervous system. Consistent with this interpretation are studies of surgically anesthetized animals, demonstrating that relationships between nociceptive stimulation and activation of neurons are similar at subcortical levels of somatosensory projection and within the primary somatosensory cortex (in cytoarchitectural areas 3b and 1 of somatosensory cortex, SI). Such findings have led to characterizations of SI as a network that preserves, rather than transforms, the excitatory drive it receives from subcortical levels. Inconsistent with this perspective are images and neurophysiological recordings of SI neurons in lightly anesthetized primates. These studies demonstrate that an extreme anterior position within SI (area 3a) receives input originating predominantly from unmyelinated nociceptors, distinguishing it from posterior SI (areas 3b and 1), long recognized as receiving input predominantly from myelinated afferents, including nociceptors. Of particular importance, interactions between these subregions during maintained nociceptive stimulation are accompanied by an altered SI response to myelinated and unmyelinated nociceptors. A revised view of pain coding within SI cortex is discussed, and potentially significant clinical implications are emphasized.

On the relation between sensory deafferentation, pain and thalamic activity in Wallenberg's syndrome: A PET-scan study before and after motor cortex stimulation

Decrease of thalamic blood flow contralateral to neuropathic pain has been described by several groups, but its relation with sensory deafferentation remains unclear. Here we report one instance where the thalamic effects of sensory deafferentation could be dissociated from those of neuropathic pain. A 50-year-old patient underwent a left medullary infarct leading to right-sided thermal and pain hypaesthesia up to the third right trigeminal division, as well as in the left face. During the following months the patient developed neuropathic pain limited to the left side of the face. Although the territory with sensory loss was much wider in the right (non painful) than in the left (painful) side of the body, PET-scan demonstrated significant reduction of blood flow in the right thalamus (contralateral to the small painful area) relative to its homologous region. After 3 months of right motor cortex stimulation the patient reported 60% relief of his left facial pain, and a new PET-scan showed correction of the thalamic asymmetry. We conclude that thalamic PET-scan hypoactivity contralateral to neuropathic pain does not merely reflect deafferentation, but appears related to the pain pathophysiology, and may be normalized in parallel with pain relief. The possible mechanisms linking thalamic hypoactivity and pain are discussed in relation with findings in epileptic patients, possible compensation phenomena and bursting thalamic discharges described in animals and humans. Restoration of thalamic activity in neuropathic pain might represent one important condition to obtain successful relief by analgesic procedures, including cortical neurostimulation.

Chronic pain following spinal cord injury

Most patients with insults to the spinal cord or central nervous system suffer from excruciating, unrelenting, chronic pain that is largely resistant to treatment. This condition affects a large percentage of spinal cord injury patients, and numerous patients with multiple sclerosis, stroke and other conditions. Despite the recent advances in basic science and clinical research the pathophysiological mechanisms of pain following spinal cord injury remain unknown. Here we describe a novel mechanism of loss of inhibition within the thalamus that may predispose for the development of this chronic pain and discuss a potential treatment that may restore inhibition and ameliorate pain.

Sex differences in effects of excitotoxic spinal injury on below-level pain sensitivity

Effects of excitotoxic injury to the thoracic gray matter on sensitivity to below-level nociceptive stimulation were evaluated for female and male Long-Evans rats. Operant escape and lick/guard (L/G) reflex responses to thermal stimulation were evaluated before and for 13-15 weeks after: 1) injections of quisqualic acid (QUIS) into the thoracic gray matter (T8-9), 2) laminectomy and spinal exposure and penetration without injection (sham) or 3) no surgical procedure (control). L/G responding to heat stimulation (44 °C) was unaffected for females and males following thoracic QUIS injections. Similarly, male escape performance was not significantly altered for 44 °C or 10 °C stimulation after QUIS injections or sham surgery. However, escape testing following QUIS and sham injections revealed increased heat sensitivity (44 °C) and decreased cold sensitivity (10 °C) for females. This selective effect is indicative of altered sympathetic activation by the thoracic injections. The effect of sham surgery suggests that female rats are vulnerable to ischemic injury during exposure and manipulation of the spinal cord. Escape from nociceptive heat and cold sensitivity of control males and females was unchanged over 13-15 weeks of testing.

Maladaptive homeostatic plasticity in a rodent model of central pain syndrome: thalamic hyperexcitability after spinothalamic tract lesions

Central pain syndrome (CPS) is defined as pain associated with a lesion of the CNS and is a common consequence of spinal cord injuries. We generated a rodent model of CPS by making unilateral electrolytic or demyelinating lesions centered on the spinothalamic tract in rats. Thermal hyperalgesia and mechanical allodynia occurred in both hind paws and forepaws by 7 d postlesion and were maintained >31 d. Field potentials in the ventral posterior lateral nucleus (VPL) in thalamic brain slices from lesioned animals displayed an increased probability of burst responses. Ethosuximide, a T-type calcium channel blocker, eliminated busting in lesioned thalamic slices and attenuated lesion-induced hyperalgesia and allodynia. We conclude that CPS in this model results from an increase in the excitability of thalamic nuclei that have lost normal ascending inputs as the result of a spinal cord injury and suggest that ethosuximide will relieve human CPS by restoring normal thalamic excitability.

Large-scale reorganization in the somatosensory cortex and thalamus after sensory loss in macaque monkeys

Adult brains undergo large-scale plastic changes after peripheral and central injuries. Although it has been shown that both the cortical and thalamic representations can reorganize, uncertainties exist regarding the extent, nature, and time course of changes at each level. We have determined how cortical representations in the somatosensory area 3b and the ventroposterior (VP) nucleus of thalamus are affected by long standing unilateral dorsal column lesions at cervical levels in macaque monkeys. In monkeys with recovery periods of 22-23 months, the intact face inputs expanded into the deafferented hand region of area 3b after complete or partial lesions of the dorsal columns. The expansion of the face region could extend all the way medially into the leg and foot representations. In the same monkeys, similar expansions of the face representation take place in the VP nucleus of the thalamus, indicating that both these processing levels undergo similar reorganizations. The receptive fields of the expanded representations were similar in somatosensory cortex and thalamus. In two monkeys, we determined the extent of the brain reorganization immediately after dorsal column lesions. In these monkeys, the deafferented regions of area 3b and the VP nucleus became unresponsive to the peripheral touch immediately after the lesion. No reorganization was seen in the cortex or the VP nucleus. A comparison of the extents of deafferentation across the monkeys shows that even if the dorsal column lesion is partial, preserving most of the hand representation, it is sufficient to induce an expansion of the face representation.

Nociceptive behavior in animal models for peripheral neuropathy: spinal and supraspinal mechanisms

Since the initial description by Wall [Wall, P.D., 1967. The laminar organization of dorsal horn and effects of descending impulses. J. Neurophysiol. 188, 403-423] of tonic descending inhibitory control of dorsal horn neurons, several studies have aimed to characterize the role of various brain centers in the control of nociceptive input to the spinal cord. The role of brainstem centers in pain inhibition has been well documented over the past four decades. Lesion to peripheral nerves results in hypersensitivity to mild tactile or cold stimuli (allodynia) and exaggerated response to nociceptive stimuli (hyperalgesia), both considered as cardinal signs of neuropathic pain. The increased interest in animal models for peripheral neuropathy has raised several questions concerning the rostral conduction of the neuropathic manifestations and the role of supraspinal centers, especially brainstem, in the inhibitory control or in the abnormal contribution to the maintenance and facilitation of neuropathic-like behavior. This review aims to summarize the data on the ascending and descending modulation of neuropathic manifestations and discusses the recent experimental data on the role of supraspinal centers in the control of neuropathic pain. In particular, the review emphasizes the importance of the reciprocal interconnections between the analgesic areas of the brainstem and the pain-related areas of the forebrain. The latter includes the cerebral limbic areas, the prefrontal cortex, the intralaminar thalamus and the hypothalamus and play a critical role in the control of pain considered as part of an integrated behavior related to emotions and various homeostatic regulations. We finally speculate that neuropathic pain, like extrapyramidal motor syndromes, reflects a disorder in the processing of somatosensory information.

Modulation of thalamic nociceptive processing after spinal cord injury through remote activation of thalamic microglia by cysteine cysteine chemokine ligand 21

Spinal cord injury (SCI) results in the generation and amplification of pain caused in part by injury-induced changes in neuronal excitability at multiple levels along the sensory neuraxis. We have previously shown that activated microglia, through an ERK (extracellular signal-regulated kinase)-regulated PGE(2) (prostaglandin E(2)) signaling mechanism, maintain neuronal hyperexcitability in the lumbar dorsal horn. Here, we examined whether microglial cells in the thalamus contribute to the modulation of chronic pain after SCI, and whether microglial activation is governed by spinally mediated increases in the microglial activator cysteine-cysteine chemokine ligand 21 (CCL21). We report that CCL21 is upregulated in dorsal horn neurons, that tissue levels are increased in the dorsal horn and ventral posterolateral (VPL) nucleus of the thalamus 4 weeks after SCI, and that the increase can be differentially reduced by spinal blockade at T1 or L1. In intact animals, electrical stimulation of the spinothalamic tract induces increases in thalamic CCL21 levels. Recombinant CCL21 injected into the VPL of intact animals transiently activates microglia and induces pain-related behaviors, effects that could be blocked with minocycline. After SCI, intra-VPL antibody-mediated neutralization of CCL21 decreases microglial activation and evoked hyperexcitability of VPL neurons, and restores nociceptive thresholds to near-normal levels. These data identify a novel pathway by which SCI triggers upregulation of the neuroimmune modulator CCL21 in the thalamus, which induces microglial activation in association with pain phenomena.

Sodium channel expression and the molecular pathophysiology of pain after SCI

The chronic pain that develops as a result of spinal cord injury (SCI) is extremely debilitating and remains largely unmanageable by current therapeutic strategies. Voltage-gated sodium channels regulate the biophysical properties, and thus firing characteristics, of neurons. After SCI the repertoire of sodium channels produced by dorsal horn nociceptive neurons is altered, enabling neurons to fire at higher than normal rates in response to unchanged peripheral stimuli as well as to generate spontaneous discharges in the absence of stimuli, resulting in the genesis of neuropathic pain. Our results have shown increased expression of the Nav1.3 sodium channel in the spinal cord and thalamus. Nav1.3 upregulation allows dorsal horn neurons to generate ramp currents, enhanced persistent currents, and shifts in steady-state activation and inactivation. Further downstream, Nav1.3 causes increased spontaneous and evoked firing of neurons in the ventroposterior lateral (VPL) nucleus of the thalamus. Nav1.3 also underlies changes in burst firing properties of VPL neurons. The combination of spinal and thalamic generation and amplification of pain by Nav1.3 dysregulation contributes to post-SCI chronic pain. If proven to be similar in humans, targeting of this system after SCI may offer hope for treatment of clinical pain.

Central sensitization in thalamic nociceptive neurons induced by mustard oil application to rat molar tooth pulp

We have recently demonstrated that application of mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces central sensitization that is reflected in changes in spontaneous activity, mechanoreceptive field (RF) size, mechanical activation threshold, and responses to graded mechanical stimuli applied to the neuronal RF in trigeminal brainstem subnucleus caudalis and subnucleus oralis. The aim of this study was to test whether central sensitization can be induced in nociceptive neurons of the posterior thalamus by MO application to the pulp. Single unit neuronal activity was recorded in the ventroposterior medial nucleus (VPM) or posterior nuclear group (PO) of the thalamus in anesthetized rats, and nociceptive neurons were classified as wide dynamic range (WDR) or nociceptive-specific (NS). MO application to the pulp was studied in 47 thalamic nociceptive neurons and found to excite over 50% of the 35 VPM neurons tested and to produce significant long-lasting (over 40 min) increases in spontaneous activity, cutaneous pinch RF size and responses to graded mechanical stimuli, and a decrease in threshold in the 29 NS neurons tested; a smaller but statistically significant increase in mean spontaneous firing rate and decrease in activation threshold occurred following MO in the six WDR neurons tested. Vehicle application to the pulp did not produce any significant changes in six VPM NS neurons tested. MO application to the pulp produced pronounced increases in spontaneous activity, pinch RF size, and responses to mechanical stimuli, and a decrease in threshold in three of the six PO neurons. In conclusion, application of the inflammatory irritant MO to the tooth pulp results in central sensitization of thalamic nociceptive neurons and this neuronal hyperexcitability likely contributes to the behavioral consequences of peripheral inflammation manifesting as pain referral, hyperalgesia and allodynia.

Alterations in Burst Firing of Thalamic VPL Neurons and Reversal by Nav1.3 Antisense After Spinal Cord Injury

We recently showed that spinal cord contusion injury (SCI) at the thoracic level induces pain-related behaviors and increased spontaneous discharges, hyperresponsiveness to innocuous and noxious peripheral stimuli, and enlarged receptive fields in neurons in the ventral posterolateral (VPL) nucleus of the thalamus. These changes are linked to the abnormal expression of Nav1.3, a rapidly repriming voltage-gated sodium channel. In this study, we examined the burst firing properties of VPL neurons after SCI. Adult male Sprague–Dawley rats underwent contusion SCI at the T9 level. Four weeks later, when Nav1.3 protein was upregulated within VPL neurons, extracellular unit recordings were made from VPL neurons in intact animals, those with SCI, and in SCI animals after receiving lumbar intrathecal injections of Nav1.3 antisense or mismatch oligodeoxynucleotides for 4 days. After SCI, VPL neurons with identifiable peripheral receptive fields showed rhythmic oscillatory burst firing with changes in discrete burst properties, and alternated among single-spike, burst, silent, and spindle wave firing modes. Nav1.3 antisense, but not mismatch, partially reversed alterations in burst firing after SCI. These results demonstrate several newly characterized changes in spontaneous burst firing properties of VPL neurons after SCI and suggest that abnormal expression of Nav1.3 contributes to these phenomena.

Fire and phantoms after spinal cord injury: Na+ channels and central pain

Neuropathic pain and phantom phenomena occur commonly after spinal cord injury (SCI) but their molecular basis is not yet fully understood. Recent findings demonstrate abnormal expression of the Nav1.3 Na(+) channel within second-order spinal cord dorsal horn neurons and third-order thalamic neurons along the pain pathway after SCI, and suggest that this change makes these neurons hyperexcitable so that they act as pain amplifiers and generators. Delineation of molecular changes that contribute to hyperexcitability of pain-signaling neurons might lead to identification of molecular targets that will be useful in the treatment of neuropathic pain after SCI and related nervous system injuries.

Plasticity of pain-related neuronal activity in the human thalamus

Strokes and other forms of injury to the central nervous system cause changes in function because of the injuries themselves and indirectly because injuries cause expression of neural plasticity. Studies in humans undergoing neurosurgical procedures for implantation of electrodes for deep brain stimulation and for making lesions in the brain have contributed understanding of both normal and abnormal functions of the somatic sensory system. This chapter will specifically discuss the reorganization of the ventral caudal (Vc) sensory nucleus of the thalamus that occurs in connection with pain conditions after strokes and spinal cord injuries. It is shown that pain is associated with expression of neural plasticity that alters maps of noxious and innocuous stimulation in the thalamus and affect processing of sensory information. Results from studies of neural activity in the thalamus in humans will be compared with results from animal studies.

Psychophysical elements of place and modality specificity in the thalamic somatic sensory nucleus (ventral caudal, vc) of awake humans

Discrete anatomic structures in the monkey somatic sensory thalamus may segregate input arising from different peripheral receptors and from different parts of the body. It has been proposed that these structures serve as components of modality- and place-specific pathways from the periphery to the cortex. We now test this hypothesis by examining the modality- and place-specific segregation of sensations at sites where microstimulation (microA currents) within the region of ventral caudal (Vc; human principal somatic sensory nucleus) evokes somatic sensations. Microstimulation was delivered in an ascending staircase protocol consisting of different numbers of pulses (4-100) presented at different frequencies (10-200 Hz) during awake thalamic surgery for movement disorders. The results demonstrate that the part of the body where microstimulation evoked sensation (projected field) and the descriptors of nonpainful sensations were usually uniform across the staircase. These results strongly support the existence of psychophysical elements of place and modality specificity in the Vc thalamus. The proportion of sites at which the sensation included more than one part of the body almost always stayed constant over current intervals (plateaus) of 10 microA. Similar plateaus were not found for sites with more than one descriptor, suggesting that elements of modality-specificity are smaller than and located within those for place-specificity. The intensity of sensations varied with the number of stimulation pulses for mechanical/tingle and cool sensations. The results provide strong evidence for psychophysically defined elements that are responsible for modality specificity of nonpainful sensations, place specificity, and intensity coding of somatic sensation in the human thalamus.

Changes in electrophysiological properties and sodium channel Nav1.3 expression in thalamic neurons after spinal cord injury

Spinal cord contusion injury (SCI) is known to induce pain-related behaviour, as well as hyperresponsiveness in lumbar dorsal horn nociceptive neurons associated with the aberrant expression of Na(v)1.3, a rapidly repriming voltage-gated sodium channel. Many of these second-order dorsal horn neurons project to third-order neurons in the ventrobasal complex of the thalamus. In this study we hypothesized that, following SCI, neurons in the thalamus undergo electrophysiological changes linked to aberrant expression of Na(v)1.3. Adult male Sprague-Dawley rats underwent contusion SCI at the T9 thoracic level. Four weeks post-SCI, Na(v)1.3 protein was upregulated within thalamic neurons in ventroposterior lateral (VPL) and ventroposterior medial nuclei, where extracellular unit recordings revealed increased spontaneous discharge, afterdischarge, hyperresponsiveness to innocuous and noxious peripheral stimuli, and expansion of peripheral receptive fields. Altered electrophysiological properties of VPL neurons persisted after interruption of ascending spinal barrage by spinal cord transection above the level of the injury. Lumbar intrathecal administration of specific antisense oligodeoxynucleotides generated against Na(v)1.3 caused a significant reduction in Na(v)1.3 expression in thalamic neurons and reversed electrophysiological alterations. These results show, for the first time, a change in sodium channel expression within neurons in the thalamus after injury to the spinal cord, and suggest that these changes contribute to altered processing of somatosensory information after SCI.

Afferent pain pathways: a neuroanatomical review

Painful experience is a complex entity made up of sensory, affective, motivational and cognitive dimensions. The neural mechanisms involved in pain perception acts in a serial and a parallel way, discriminating and locating the original stimulus and also integrating the affective feeling, involved in a special situation, with previous memories. This review examines the concepts of nociception, acute and chronic pain, and also describes the afferent pathways involved in reception, segmental processing and encephalic projection of pain stimulus. The interaction model of the cerebral cortex areas and their functional characteristics are also discussed.

Cortical changes in cholecystokinin mRNA are related to spontaneous pain behaviors following excitotoxic spinal cord injury in the rat

Cholecystokinin (CCK) in the CNS antagonizes the opioid system and has been implicated post-spinal cord injury (SCI) pain. The current study found that excitotoxic SCI alters levels of CCK mRNA levels in the cortex, diencepahlon, and mesencephalon of rats. Animals that developed pain post-SCI had significantly higher levels than animals that did not develop pain. Upregulation of CCK mRNA in the cortex may be related to post-SCI pain in rats.