Plasticity of pain-related neuronal activity in the human thalamus

Exploring Functional Connectivity in Chronic Spinal Cord Injury Patients With Neuropathic Pain Versus Without Neuropathic Pain

The great majority of spinal cord injury (SCI) patients have debilitating chronic pain. Despite decades of research, these pain pathways of neuropathic pain (NP) are unknown. SCI patients have been shown to have abnormal brain pain pathways. We hypothesize that SCI NP patients' pain matrix is altered compared to SCI patients without NP. This study examines the functional connectivity (FC) in SCI patients with moderate-severe chronic NP compared to SCI patients with mild-no NP. These groups were compared to control subjects. The Neuropathic Pain Questionnaire and neurological evaluation based on the International Standard Neurological Classification of SCI were utilized to define the severity and level of injury. Of the 10 SCI patients, 7 (48.6 ± 17.02 years old, 6 male and 1 female) indicated that they had NP and 3 did not have NP (39.33 ± 8.08 years old, 2 male and 1 female). Ten uninjured neurologically intact participants were used as controls (24.8 ± 4.61 years old, 5 male and 5 female). FC metrics were obtained from the comparisons of resting-state functional magnetic resonance imaging among our various groups (controls, SCI with NP, and SCI without NP). For each comparison, a region-of-interest (ROI)-to-ROI connectivity analysis was pursued, encompassing a total of 175 ROIs based on a customized atlas derived from the AAL3 atlas. The analysis accounted for covariates such as age and sex. To correct for multiple comparisons, a strict Bonferroni correction was applied with a significance level of p < 0.05/NROIs. When comparing SCI patients with moderate-to-severe pain to those with mild-to-no pain, specific thalamic nuclei had altered connections. These nuclei included: medial pulvinar; lateral pulvinar; medial geniculate nucleus; lateral geniculate nucleus; and mediodorsal magnocellular nucleus. There was increased FC between the lateral geniculate nucleus and the anteroventral nucleus in NP post-SCI. Our analysis additionally highlights the relationships between the frontal lobe and temporal lobe with pain. This study successfully identifies thalamic neuroplastic changes that occur in patients with SCI who develop NP. It additionally underscores the pain matrix and involvement of the frontal and temporal lobes as well. Our findings complement that the development of NP post-SCI involves cognitive, emotional, and behavioral influences.

The Current View on the Paradox of Pain in Autism Spectrum Disorders

Autism spectrum disorder (ASD) is a neurodevelopmental disorder, which affects 1 in 44 children and may cause severe disabilities. Besides socio-communicational difficulties and repetitive behaviors, ASD also presents as atypical sensorimotor function and pain reactivity. While chronic pain is a frequent co-morbidity in autism, pain management in this population is often insufficient because of difficulties in pain evaluation, worsening their prognosis and perhaps driving higher mortality rates. Previous observations have tended to oversimplify the experience of pain in autism as being insensitive to painful stimuli. Various findings in the past 15 years have challenged and complicated this dogma. However, a relatively small number of studies investigates the physiological correlates of pain reactivity in ASD. We explore the possibility that atypical pain perception in people with ASD is mediated by alterations in pain perception, transmission, expression and modulation, and through interactions between these processes. These complex interactions may account for the great variability and sometimes contradictory findings from the studies. A growing body of evidence is challenging the idea of alterations in pain processing in ASD due to a single factor, and calls for an integrative view. We propose a model of the pain cycle that includes the interplay between the molecular and neurophysiological pathways of pain processing and it conscious appraisal that may interfere with pain reactivity and coping in autism. The role of social factors in pain-induced response is also discussed. Pain assessment in clinical care is mostly based on subjective rather than objective measures. This review clarifies the strong need for a consistent methodology, and describes innovative tools to cope with the heterogeneity of pain expression in ASD, enabling individualized assessment. Multiple measures, including self-reporting, informant reporting, clinician-assessed, and purely physiological metrics may provide more consistent results. An integrative view on the regulation of the pain cycle offers a more robust framework to characterize the experience of pain in autism.

Thalamus: The 'promoter' of endogenous modulation of pain and potential therapeutic target in pathological pain

More recently, the thalamic mediodorsal (MD) and ventromedial (VM) nuclei have been revealed to be functioned as 'nociceptive discriminator' in discriminating noxious and innocuous peripheral afferents, and exhibits distinct different descending controls of nociception. Of particularly importance, the function of thalamic nuclei in engaging descending modulation of nociception is 'silent' or inactive during the physiological state as well as in condition exposed to insufficient noxious stimulation. Once initiation by sufficient noxious or innocuous C-afferents associated with temporal and spatial summation, the thalamic MD and VM nuclei exhibit salient, different effects: facilitation and inhibition, on noxious mechanically and heat evoked nociception, respectively. Based on series of experimental evidence, we here summarize a novel hypothesis involving thalamic MD and VM nuclei functioned as 'promoter' in initiating descending facilitation and inhibition of pain with specific spatiotemporal characteristics. We further hypothesize that clinical remedy in targeting thalamic VM nucleus by enhancing its activities in recruiting inhibition alone or decreasing thalamic MD nucleus induced facilitation may provide promising way in effectively control of pathological pain.

Aberrant Thalamic-Centered Functional Connectivity in Patients with Persistent Somatoform Pain Disorder

PURPOSE

Recent task-based fMRI studies have shown that Persistent Somatoform Pain Disorder (PSPD) patients demonstrated aberrant activity in a wide range of brain regions associated with sensation, cognition and emotion. However, these specific task-based studies could not clearly uncover the alterations in the spontaneous brain networks that were associated with the general pain-related symptoms in PSPD.

PATIENTS AND METHODS

In the present study, 13 PSPD patients and 23 matched healthy controls (HCs) were enrolled. Resting state and 3D structural imaging data were collected during magnetic resonance imaging (MRI) scans. Ninety regions of interest (ROIs) were selected from the automated anatomical labeling (AAL) template. The functional connectivity toolbox "CONN" was used to calculate the functional connectivity (FC) coefficients.

RESULTS

Our results showed that PSPD patients exhibited increased FCs between the left thalamus and the right amygdala, the right hippocampus, and multiple sub-regions of the occipital lobe when compared to HCs. Correlation analysis revealed a negative correlation between the left thalamus-right amygdala FC and the level of anxiety in PSPD patients.

CONCLUSION

These findings suggest that the altered FC between thalamus and amygdala may be the neural mechanisms underlying the pain-related anxiety in PSPD.

The Use of Neuromodulation for Symptom Management

Pain and other symptoms of autonomic dysregulation such as hypertension, dyspnoea and bladder instability can lead to intractable suffering. Incorporation of neuromodulation into symptom management, including palliative care treatment protocols, is becoming a viable option scientifically, ethically, and economically in order to relieve suffering. It provides further opportunity for symptom control that cannot otherwise be provided by pharmacology and other conventional methods.

Differential mRNA expression of neuroinflammatory modulators in the spinal cord and thalamus of type 2 diabetic monkeys

BACKGROUND

Given that diabetes-associated complications are closely associated with neuroinflammation, it is imperative to study potential changes in neuroinflammatory modulators in the central nervous system of diabetic primates.

METHODS

The mRNA levels of pro- and anti-inflammatory cytokines, toll-like receptors (TLRs), growth factors, and cannabinoid receptors were compared in the spinal dorsal horn (SDH) and thalamus of naturally occurring type 2 diabetic monkeys and an age-matched control group using reverse transcription and quantitative real-time polymerase chain reaction.

RESULTS

In the SDH of diabetic monkeys, mRNA levels of proinflammatory cytokines (i.e. interleukin [IL]-1β and tumor necrosis factor [TNF] α), TLR1, and TLR2 were increased, whereas mRNA levels of IL-10, an anti-inflammatory cytokine, were decreased. No changes were observed in the mRNA levels of growth factors and cannabinoid receptors. In line with the mRNA data, TNFα immunoreactivity was significantly increased in diabetic monkeys. Moreover, mRNA expression levels of IL-1β, TNFα, TLR1, and TLR2 in the SDH were positively correlated with plasma glucose concentrations in all monkeys.

CONCLUSIONS

Several ligands and receptors involved in neuroinflammation are simultaneously dysregulated in the spinal cord of diabetic monkeys. This primate disease model will facilitate the design of novel treatment approaches to ameliorate neuroinflammation-driven adverse effects in diabetic patients.

The Current State of Deep Brain Stimulation for Chronic Pain and Its Context in Other Forms of Neuromodulation

Chronic intractable pain is debilitating for those touched, affecting 5% of the population. Deep brain stimulation (DBS) has fallen out of favour as the centrally implantable neurostimulation of choice for chronic pain since the 1970–1980s, with some neurosurgeons favouring motor cortex stimulation as the ‘last chance saloon’. This article reviews the available data and professional opinion of the current state of DBS as a treatment for chronic pain, placing it in the context of other neuromodulation therapies. We suggest DBS, with its newer target, namely anterior cingulate cortex (ACC), should not be blacklisted on the basis of a lack of good quality study data, which often fails to capture the merits of the treatment.

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.

Rehabilitation strategies for wrist sensorimotor control impairment: From theory to practice

This clinical review discusses the organization, neuroanatomy, assessment, clinical relevance, and rehabilitation of sensorimotor (SM) control impairment after wrist trauma. The wrist SM control system encompasses complex SM pathways that control normal wrist active range of motion and mediate wrist joint neuromuscular stability for maintaining joint function. Among various known assessment methods of wrist SM control impairment, the active wrist joint position sense test is determined to be a clinically meaningful and responsive measure for wrist SM control impairment after wrist fracture. Wrist trauma may involve significant soft tissue injury (ie, skin, ligament, muscle), which could disrupt the generation and transmission of adequate proprioceptive input from wrist mechanoreceptors, thus leading to significant joint SM impairment. Various clinical examples of wrist trauma (eg, distal radius fracture, scapholunate joint injury) along with known prognostic factors (eg, pain) that may influence wrist SM control impairment recovery are discussed to illustrate this point. This article proposes promising rehabilitation strategies toward restoring wrist joint conscious and unconscious SM control impairments, integrating current research evidence with clinical practice. These strategies require more rigorous evaluation in clinical trials.

LEVEL OF EVIDENCE

5.

Gain control mechanisms in the nociceptive system

The "gate control theory of pain" of 1965 became famous for integrating clinical observations and the understanding of spinal dorsal horn circuitry at that time into a testable model. Although it became rapidly clear that spinal circuitry is much more complex than that proposed by Melzack and Wall, their prediction of the clinical efficacy of transcutaneous electrical nerve stimulation and spinal cord stimulation has left an important clinical legacy also 50 years later. In the meantime, it has been recognized that the sensitivity of the nociceptive system can be decreased or increased and that this "gain control" can occur at peripheral, spinal, and supraspinal levels. The resulting changes in pain sensitivity can be rapidly reversible or persistent, highly localized or widespread. Profiling of spatio-temporal characteristics of altered pain sensitivity (evoked pain to mechanical and/or heat stimuli) allows implications on the mechanisms likely active in a given patient, including peripheral or central sensitization, intraspinal or descending inhibition. This hypothesis generation in the diagnostic process is an essential step towards a mechanism-based treatment of pain. The challenge now is to generate the rational basis of multimodal pain therapy algorithms by including profile-based stratification of patients into studies on efficacy of pharmacological and nonpharmacological treatment modalities. This review outlines the current evidence base for this approach.

Nonpharmacological Interventions in Targeting Pain-Related Brain Plasticity

Chronic pain is a highly prevalent and debilitating condition that is frequently associated with multiple comorbid psychiatric conditions and functional, biochemical, and anatomical alterations in various brain centers. Due to its widespread and diverse manifestations, chronic pain is often resistant to classical pharmacological treatment paradigms, prompting the search for alternative treatment approaches that are safe and efficacious. The current review will focus on the following themes: attentional and cognitive interventions, the role of global environmental factors, and the effects of exercise and physical rehabilitation in both chronic pain patients and preclinical pain models. The manuscript will discuss not only the analgesic efficacy of these therapies, but also their ability to reverse pain-related brain neuroplasticity. Finally, we will discuss the potential mechanisms of action for each of the interventions.

Post-stroke hip fracture in older people: A narrative review

Background/Aims:

A hip fracture is a serious and common complication after stroke. Individuals often experience particular gait problems post-stroke and may have difficulty with everyday activities such as walking. Therefore, the interventions for falls prevention in older patients should be re-examined when applied to the post-stroke population. This study reviews the risk factors for falls and hip fractures that are specific to the post-stroke population and the measures that may be taken to prevent hip fractures and/or falls in this patient group.

Method:

A narrative review of articles written in English in the scientific literature on the risk factors, natural history and prevention of hip fractures in post-stroke patients.

Results:

The interventions that were reviewed showed some benefit to post-stroke patients. These included interventions designed to increase physical activity, water-based exercise, general falls prevention programmes, and the use of force platforms and body weight supported treadmills. More solid evidence was found to support strategies such as the symmetrical use of limbs, which provides appropriate support and an environment where an individual can regain more normal movement patterns. This can be achieved by training in a low-gravity environment using water-based exercises or a body-weight supported treadmill. To perform more symmetrical movements, biofeedback (visual feedback) or a force platform can be used in addition to symmetrical exercises.

Conclusions:

In a post-stroke population, it is better to pay special attention to the prevention of falls and hip fracture than to rehabilitate after a fracture has occurred. Evaluation and management of the risk factors that contribute to falls should be a priority in post-stroke rehabilitation, focusing on as many risk factors as possible in hip fracture prevention programmes. A variety of techniques should be used to facilitate functional recovery, emphasising symmetry and optimal movement patterns.

Basic anatomy and physiology of pain pathways

This article provides an integrated review of the basic anatomy and physiology of the pain processing pathways. The transmission and parcellation of noxious stimuli from the peripheral nervous system to the central nervous system is discussed. In addition, the inhibitory and excitatory systems that regulate pain along with the consequences of dysfunction are considered.

Neuropathic pain and deep brain stimulation

Deep brain stimulation (DBS) is a neurosurgical intervention the efficacy, safety, and utility of which are established in the treatment of Parkinson's disease. For the treatment of chronic, neuropathic pain refractory to medical therapies, many prospective case series have been reported, but few have published findings from patients treated with current standards of neuroimaging and stimulator technology over the last decade . We summarize the history, science, selection, assessment, surgery, programming, and personal clinical experience of DBS of the ventral posterior thalamus, periventricular/periaqueductal gray matter, and latterly rostral anterior cingulate cortex (Cg24) in 113 patients treated at 2 centers (John Radcliffe, Oxford, UK, and Hospital de São João, Porto, Portugal) over 13 years. Several experienced centers continue DBS for chronic pain, with success in selected patients, in particular those with pain after amputation, brachial plexus injury, stroke, and cephalalgias including anesthesia dolorosa. Other successes include pain after multiple sclerosis and spine injury. Somatotopic coverage during awake surgery is important in our technique, with cingulate DBS under general anesthesia considered for whole or hemibody pain, or after unsuccessful DBS of other targets. Findings discussed from neuroimaging modalities, invasive neurophysiological insights from local field potential recording, and autonomic assessments may translate into improved patient selection and enhanced efficacy, encouraging larger clinical trials.

Estradiol attenuates spinal cord injury-induced pain by suppressing microglial activation in thalamic VPL nuclei of rats

In our previous study we showed that central pain syndrome (CPS) induced by electrolytic injury caused in the unilateral spinothalamic tract (STT) is a concomitant of glial alteration at the site of injury. Here, we investigated the activity of glial cells in thalamic ventral posterolateral nuclei (VPL) and their contribution to CPS. We also examined whether post-injury administration of a pharmacological dose of estradiol can attenuate CPS and associated molecular changes. Based on the results,in the ipsilateral VPL the microglial phenotype switched o hyperactive mode and Iba1 expression was increased significantly on days 21 and 28 post-injury. The same feature was observed in contralateral VPL on day 28 (P<.05). These changes were strongly correlated with the onset of CPS (r(2)=0.670). STT injury did not induce significant astroglial response in both ipsilateral and contralateral VPL. Estradiol attenuated bilateral mechanical hypersensitivity 14 days after STT lesion (P<.05). Estradiol also suppressed microglial activation in the VPL. Taken together, these findings indicate that selective STT lesion induces bilateral microglia activation in VPL which might contribute to mechanical hypersensitivity. Furthermore, a pharmacological dose of estradiol reduces central pain possibly via suppression of glial activity in VPL region.

Deep brain stimulation for pain

Deep brain stimulation (DBS) is a neurosurgical intervention whose efficacy, safety, and utility have been shown in the treatment of movement disorders. For the treatment of chronic pain refractory to medical therapies, many prospective case series have been reported, but few have published findings from patients treated during the past decade using current standards of neuroimaging and stimulator technology. We summarize the history, science, selection, assessment, surgery, and personal clinical experience of DBS of the ventral posterior thalamus, periventricular/periaqueductal gray matter, and, latterly, the rostral anterior cingulate cortex (Cg24) in 100 patients treated now at two centers (John Radcliffe Hospital, Oxford, UK, and Hospital de São João, Porto, Portugal) over 12 years. Several experienced centers continue DBS for chronic pain with success in selected patients, in particular those with pain after amputation, brachial plexus injury, stroke, and cephalalgias including anesthesia dolorosa. Other successes include pain after multiple sclerosis and spine injury. Somatotopic coverage during awake surgery is important in our technique, with cingulate DBS considered for whole-body pain or after unsuccessful DBS of other targets. Findings discussed from neuroimaging modalities, invasive neurophysiological insights from local field potential recording, and autonomic assessments may translate into improved patient selection and enhanced efficacy, encouraging larger clinical trials.

Visualizing the complex brain dynamics of chronic pain

Chronic pain is now recognized as a disease state that involves changes in brain function. This concept is reinforced by data that document structural and morphological remapping of brain circuitry under conditions of chronic pain. Evidence for aberrant neurophysiology in the brain further confirms neuroplasticity at cellular and molecular levels. Proper detection of pain-induced changes using emerging non-invasive and cost-effective technologies, such as analytical electroencephalography methods, could yield objective diagnostic measures and may guide therapeutic interventions targeting the brain for effective management of chronic pain.

Combination Drug Therapy for Pain following Chronic Spinal Cord Injury

A number of mechanisms have been elucidated that maintain neuropathic pain due to spinal cord injury (SCI). While target-based therapeutics are being developed based on elucidation of these mechanisms, treatment for neuropathic SCI pain has not been entirely satisfactory due in part to the significant convergence of neurological and inflammatory processes that maintain the neuropathic pain state. Thus, a combination drug treatment strategy, wherein several pain-related mechanism are simultaneously engaged, could be more efficacious than treatment against individual mechanisms alone. Also, by engaging several targets at once, it may be possible to reduce the doses of the individual drugs, thereby minimizing the potential for adverse side effects. Positive preclinical and clinical studies have demonstrated improved efficacy of combination drug treatment over single drug treatment in neuropathic pain of peripheral origin, and perhaps such combinations could be utilized for neuropathic SCI pain. At the same time, there are mechanisms that distinguish SCI from peripheral neuropathic pain, so novel combination therapies will be needed.

Underlying pathology and associated factors of hemiplegic shoulder pain

The prevalence of hemiplegic shoulder pain is approximately 22%-23% in the general population of stroke survivors and approximately 54%-55% among stroke patients in rehabilitation settings. Hemiplegic shoulder pain causes a reduced quality-of-life, poor functional recovery, depression, disturbed sleep, and prolonged hospitalization. Herein, we attempted to understand, based on a literature review and experts' opinion, the pathologic processes underlying hemiplegic shoulder pain and the major associated factors contributing to its development. The systematization of underlying pathologies was proposed, which might eventually enable a more constructive clinical approach in evaluating and treating patients with hemiplegic shoulder pain.

Interactions between Pain and the Motor Cortex: Insights from Research on Phantom Limb Pain and Complex Regional Pain Syndrome

PURPOSE

Pain is a significantly disabling problem that often interacts with other deficits during the rehabilitation process. The aim of this paper is to review evidence of interactions between pain and the motor cortex in order to attempt to answer the following questions: (1) Does acute pain interfere with motor-cortex activity? (2) Does chronic pain interfere with motor-cortex activity, and, conversely, does motor-cortex plasticity contribute to chronic pain? (3) Can the induction of motor plasticity by means of motor-cortex stimulation decrease pain? (4) Can motor training result in both motor-cortex reorganization and pain relief?

SUMMARY OF KEY POINTS

Acute experimental pain has been clearly shown to exert an inhibitory influence over the motor cortex, which can interfere with motor learning capacities. Current evidence also suggests a relationship between chronic pain and motor-cortex reorganization, but it is still unclear whether one causes the other. However, there is growing evidence that interventions aimed at normalizing motor-cortex organization can lead to pain relief.

CONCLUSIONS

Interactions between pain and the motor cortex are complex, and more studies are needed to understand these interactions in our patients, as well as to develop optimal rehabilitative strategies.

Cross-modal plasticity in the human thalamus: evidence from intraoperative macrostimulations

During stereotactic functional neurosurgery, stimulation procedure to control for proper target localization provides a unique opportunity to investigate pathophysiological phenomena that cannot be addressed in experimental setups. Here we report on the distribution of response modalities to 487 intraoperative thalamic stimulations performed in 24 neurogenic pain (NP), 17 parkinsonian (PD) and 10 neuropsychiatric (Npsy) patients. Threshold responses were subdivided into somatosensory, motor and affective, and compared between medial (central lateral nucleus) and lateral (ventral anterior, ventral lateral and ventral medial) thalamic nuclei and between patients groups. Major findings were as follows: in the medial thalamus, evoked responses were for a large majority (95%) somatosensory in NP patients, 47% were motor in PD patients, and 54% affective in Npsy patients. In the lateral thalamus, a much higher proportion of somatosensory (83%) than motor responses (5%) was evoked in NP patients, while the proportion was reversed in PD patients (69% motor vs. 21% somatosensory). These results provide the first evidence for functional cross-modal changes in lateral and medial thalamic nuclei in response to intraoperative stimulations in different functional disorders. This extensive functional reorganization sheds new light on wide-range plasticity in the adult human thalamocortical system.

Thalamic neuron hyperexcitability and enlarged receptive fields in the STZ model of diabetic pain

Distal limb pain in diabetes mellitus is frequently attributed to hyperexcitability of primary afferents associated with peripheral neuropathy. However, prior studies have demonstrated that, after traumatic nerve injury, hyperexcitability develops not only within primary afferents but also within pain-signalling neurons of the spinal cord dorsal horn and thalamic ventral posterolateral (VPL) nucleus, establishing a basis for tiered central pain generators or amplifiers. In this study we asked whether hyperexcitability develops within thalamic neurons in experimental painful diabetes. Diabetes was induced in adult male Sprague-Dawley rats with streptozotocin (STZ). Behavioral testing for tactile allodynia, performed one week prior to STZ injection and weekly thereafter, indicated that, by six weeks after STZ injection, mechanical allodynia had developed (mechanical withdrawal threshold <4 g, STZ; 21.75 g, control). Thalamic unit recordings were obtained from the VPL nucleus at seven weeks after STZ injection, in rats that met a criterion withdrawal threshold of <4 g, at a time when mean glucose level for control rats was 104.8+/-2.9, and for diabetic rats was 420.1+/-42.0. Our analysis shows that, in this model of diabetic neuropathic pain, thalamic VPL neurons develop hyperexcitability, with increased responses to phasic brush, press, and pinch stimuli applied to identified peripheral receptive fields. VPL neurons from diabetic rats also display enhanced spontaneous activity, independent of ascending afferent barrage, and enlarged receptive fields. These results suggest that aberrant levels of spontaneous activity and hyper-responsiveness of VPL thalamic neurons may contribute to diabetic neuropathic pain.

Tinnitus and pain

Tinnitus has many similarities with the symptoms of neurological disorders such as paresthesia and central neuropathic pain. There is considerable evidence that the symptoms and signs of some forms of tinnitus and central neuropathic pain are caused by functional changes in specific parts of the central nervous system and that these changes are caused by expression of neural plasticity. The changes in the auditory nervous system that cause tinnitus and the changes in the somatosensory systems that cause central neuropathic pain may have been initiated from the periphery, i.e. the ear or the auditory nerve for tinnitus and receptors and peripheral nerves in the body for pain. In the chronic condition of tinnitus and pain, abnormalities in the periphery may no longer play a role in the pathology, but the tinnitus is still referred to the ear and central neuropathic pain is still referred to the location on the body of the original pathology. In this chapter we will discuss specific similarities between tinnitus and pain, and compare tinnitus with other phantom disorders. Since much more is known about pain than about tinnitus, it is valuable to take advantage of the knowledge about pain in efforts to understand the pathophysiology of tinnitus and find treatments for tinnitus.