RESPONSES OF SINGLE DORSAL CORD CELLS TO PERIPHERAL CUTANEOUS UNMYELINATED FIBRES

Bilateral thermal hyperalgesia in trigeminal and extra-trigeminal regions in patients with myofascial temporomandibular disorders

Our aim was to assess thermal sensitivity in both trigeminal and extra-trigeminal regions in patients with myofascial temporomandibular disorder (TMD) but without comorbid conditions as compared to age-matched controls. Twenty women (age 24 +/- 3 years) diagnosed with myofascial TMD according to the research diagnostic criteria for TMD and 20 healthy women (age 24 +/- 4 years) were included. Warm and cold detection thresholds (WDT and CDT, respectively) and heat and cold pain thresholds (HPT and CPT, respectively) were bilaterally assessed over the masseter and frontalis muscles (trigeminal regions) and the wrist (extra-trigeminal region). The mean of three determinations at each site was calculated and used for analysis. The order of the test sites was randomized. A two-way ANOVA was used to test for differences between groups and sides (most painful/contra-lateral side; dominant/non-dominant). No significant differences between groups for WDT or CDT in trigeminal and extra-trigeminal regions (ANOVA, P > 0.389) were found. There were significant differences between groups, but not between sides, for HPT and CPT in both trigeminal and extra-trigeminal areas (ANOVA, P < 0.001). CPT (P < 0.001) over the trigeminal area was positively correlated with both pain intensity and duration of pain symptoms: the longer the history of pain or the greater the pain intensity, the higher the CPT (i.e., the greater cold hyperalgesia) over the trigeminal region. Our findings revealed bilateral thermal hyperalgesia (lower HPT and higher CPT) but normal WDT and CDT in trigeminal and extra-trigeminal regions in women with myofascial TMD as compared to healthy controls. Bilateral heat/cold hyperalgesia in trigeminal and extra-trigeminal areas may reflect a dysfunction of thermal channels in myofascial TMD patients as result of some combination of peripheral sensitization, facilitation of central nociceptive processing and/or decreased descending inhibition.

Bilateral mechanical-pain sensitivity over the trigeminal region in patients with chronic mechanical neck pain

The aim of this study was to investigate bilateral pressure-pain sensitivity over the trigeminal region, the cervical spine, and the tibialis anterior muscle in patients with mechanical chronic neck pain. Twenty-three patients with neck pain (56% women), aged 20 to 37 years old, and 23 matched controls (aged 20 to 38 years) were included. Pressure pain thresholds (PPTs) were bilaterally assessed over masseter, temporalis, and upper trapezius muscles, the C5-C6 zygapophyseal joint, and the tibialis anterior muscle in a blinded design. The results showed that PPT levels were significantly decreased bilaterally over the masseter, temporalis, and upper trapezius muscles, and also the C5-C6 zygapophyseal joint (P < .001), but not over the tibialis anterior muscle (P = .4) in patients with mechanical chronic neck pain when compared to controls. The magnitude of PPT decreases was greater in the cervical region as compared to the trigeminal region (P < .01). PPTs over the masseter muscles were negatively correlated to both duration of pain symptoms and neck-pain intensity (P < .001). Our findings revealed pressure-pain hyperalgesia in the trigeminal region in patients with mechanical chronic neck pain, suggesting spreading of sensitization to the trigeminal region in this patient population.

PERSPECTIVE

This article reveals the presence of bilateral pressure-pain hypersensitivity in the trigeminal region in patients with idiopathic neck pain, suggesting a sensitization process of the trigemino-cervical nucleus caudalis in this population. This finding has implications for development of management strategies.

Central sensitization: a generator of pain hypersensitivity by central neural plasticity

Central sensitization represents an enhancement in the function of neurons and circuits in nociceptive pathways caused by increases in membrane excitability and synaptic efficacy as well as to reduced inhibition and is a manifestation of the remarkable plasticity of the somatosensory nervous system in response to activity, inflammation, and neural injury. The net effect of central sensitization is to recruit previously subthreshold synaptic inputs to nociceptive neurons, generating an increased or augmented action potential output: a state of facilitation, potentiation, augmentation, or amplification. Central sensitization is responsible for many of the temporal, spatial, and threshold changes in pain sensibility in acute and chronic clinical pain settings and exemplifies the fundamental contribution of the central nervous system to the generation of pain hypersensitivity. Because central sensitization results from changes in the properties of neurons in the central nervous system, the pain is no longer coupled, as acute nociceptive pain is, to the presence, intensity, or duration of noxious peripheral stimuli. Instead, central sensitization produces pain hypersensitivity by changing the sensory response elicited by normal inputs, including those that usually evoke innocuous sensations.

PERSPECTIVE

In this article, we review the major triggers that initiate and maintain central sensitization in healthy individuals in response to nociceptor input and in patients with inflammatory and neuropathic pain, emphasizing the fundamental contribution and multiple mechanisms of synaptic plasticity caused by changes in the density, nature, and properties of ionotropic and metabotropic glutamate receptors.

Short-term synaptic plasticity in the nociceptive thalamic-anterior cingulate pathway

Background

Although the mechanisms of short- and long-term potentiation of nociceptive-evoked responses are well known in the spinal cord, including central sensitization, there has been a growing body of information on such events in the cerebral cortex. In view of the importance of anterior cingulate cortex (ACC) in chronic pain conditions, this review considers neuronal plasticities in the thalamocingulate pathway that may be the earliest changes associated with such syndromes.

Results

A single nociceptive electrical stimulus to the sciatic nerve induced a prominent sink current in the layer II/III of the ACC in vivo, while high frequency stimulation potentiated the response of this current. Paired-pulse facilitation by electrical stimulation of midline, mediodorsal and intralaminar thalamic nuclei (MITN) suggesting that the MITN projection to ACC mediates the nociceptive short-term plasticity. The short-term synaptic plasticities were evaluated for different inputs in vitro where the medial thalamic and contralateral corpus callosum afferents were compared. Stimulation of the mediodorsal afferent evoked a stronger short-term synaptic plasticity and effectively transferred the bursting thalamic activity to cingulate cortex that was not true for contralateral stimulation. This short-term enhancement of synaptic transmission was mediated by polysynaptic pathways and NMDA receptors. Layer II/III neurons of the ACC express a short-term plasticity that involves glutamate and presynaptic calcium influx and is an important mechanism of the short-term plasticity.

Conclusion

The potentiation of ACC neuronal activity induced by thalamic bursting suggest that short-term synaptic plasticities enable the processing of nociceptive information from the medial thalamus and this temporal response variability is particularly important in pain because temporal maintenance of the response supports cortical integration and memory formation related to noxious events. Moreover, these modifications of cingulate synapses appear to regulate afferent signals that may be important to the transition from acute to chronic pain conditions associated with persistent peripheral noxious stimulation. Enhanced and maintained nociceptive activities in cingulate cortex, therefore, can become adverse and it will be important to learn how to regulate such changes in thalamic firing patterns that transmit nociceptive information to ACC in early stages of chronic pain.

Translational pain research: achievements and challenges

The achievements in both preclinical and clinical pain research over the past 4 decades have led to significant progress in clinical pain management. However, pain research still faces enormous challenges and there remain many obstacles in the treatment of clinical pain, particularly chronic pain. Translational pain research needs to involve a number of important areas including: 1) bridging the gap between pain research and clinical pain management; 2) developing objective pain-assessment tools; 3) analyzing current theories of pain mechanisms and their relevance to clinical pain; 4) exploring new tools for both preclinical and clinical pain research; and 5) coordinating research efforts among basic scientists, clinical investigators, and pain-medicine practitioners. These issues are discussed in this article in light of the achievements and challenges of translational pain research.

PERSPECTIVE

The subjective nature of clinical pain calls for innovative research approaches. As translational pain research emerges as an important field in pain medicine, it will play a unique role in improving clinical pain management through coordinated bidirectional research approaches between bedside and bench.

Chronic tension-type headache: what is new?

PURPOSE OF REVIEW

This review discusses current data on nosological boundaries related to diagnosis, pathophysiology and therapeutic strategies in chronic tension-type headache (CTTH).

RECENT FINDINGS

Diagnostic criteria of CTTH should be adapted to improve its sensitivity against migraine. It seems that mechanical pain sensitivity is a consequence and not a causative factor of CTTH. Recent evidence is modifying previous knowledge about relationships between muscle tissues and CTTH, suggesting a potential role of muscle trigger points in the genesis of pain. An updated pain model suggests that headache perception can be explained by referred pain from trigger points in the craniocervical muscles, mediated through the spinal cord and the trigeminal nucleus caudalis rather than only tenderness of the muscles themselves. Different therapeutic strategies, pharmacological, physical therapy, psychological and acupuncture, are generally used. The therapeutic efficacy of nonsteroidal anti-inflammatory drugs remains incomplete. The tricyclic antidepressants are the most used first-line therapeutic agents for CTTH. Surprisingly, few controlled studies have been performed and not all of them have found an efficacy superior to placebo. Further, there is insufficient evidence to support/refute the efficacy of physical therapy in CTTH.

SUMMARY

Although there is an increasing scientific interest in CTTH, future studies incorporating subgroups of patients who will likely to benefit from a specific treatment (clinical prediction rules) should be conducted.

Synaptically evoked glutamate transporter currents in Spinal Dorsal Horn Astrocytes

Background

Removing and sequestering synaptically released glutamate from the extracellular space is carried out by specific plasma membrane transporters that are primarily located in astrocytes. Glial glutamate transporter function can be monitored by recording the currents that are produced by co-transportation of Na⁺ ions with the uptake of glutamate. The goal of this study was to characterize glutamate transporter function in astrocytes of the spinal cord dorsal horn in real time by recording synaptically evoked glutamate transporter currents.

Results

Whole-cell patch clamp recordings were obtained from astrocytes in the spinal substantia gelatinosa (SG) area in spinal slices of young adult rats. Glutamate transporter currents were evoked in these cells by electrical stimulation at the spinal dorsal root entry zone in the presence of bicuculline, strychnine, DNQX and D-AP5. Transporter currents were abolished when synaptic transmission was blocked by TTX or Cd²⁺. Pharmacological studies identified two subtypes of glutamate transporters in spinal astrocytes, GLAST and GLT-1. Glutamate transporter currents were graded with stimulus intensity, reaching peak responses at 4 to 5 times activation threshold, but were reduced following low-frequency (0.1 – 1 Hz) repetitive stimulation.

Conclusion

These results suggest that glutamate transporters of spinal astrocytes could be activated by synaptic activation, and recording glutamate transporter currents may provide a means of examining the real time physiological responses of glial cells in spinal sensory processing, sensitization, hyperalgesia and chronic pain.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Activation of ERK1/2 in the primary injury site is required to maintain melittin-enhanced wind-up of rat spinal wide-dynamic-range neurons

Peripheral modulation of wind-up enhancement induced by peripheral tissue injury is investigated in rat spinal wide-dynamic-range (WDR) neurons. After subcutaneous (s.c.) injection of melittin, a pain-related peptidergic component separated from bee venom, the responsiveness of spinal cord WDR neuron to repeated suprathreshold (1.5T, the intensity threshold) electrical stimuli is enhanced. Comparing with the less effects on early response (0-100 ms), melittin significantly increases late response (100 ms to the next stimulus artifact) and after-discharge (starting from 2s after the last stimulus artifact) with 189% and 546%, respectively. Peripheral administration of a specific MEK inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis-[o-aminophenylmercapto] butadiene (U0126, 1 microg) gradually suppresses, but not completely blocks melittin-enhanced wind-up to the similar level of baseline. The inhibitions of U0126 are mainly on late response and after-discharge with 49% and 65%, respectively. Peripheral administration of three doses of U0126 (0.1, 1, 10 microg) has no effects on melittin-induced local paw edema regardless of either pre- or post-treatment of the drug. We conclude that peripheral ERKs pathway in the primary injury site is required to maintain melittin-enhanced wind-up of rat spinal cord wide-dynamic-range neurons.

Brief, low frequency stimulation of rat peripheral C-fibres evokes prolonged microglial-induced central sensitization in adults but not in neonates

The sensitization of spinal dorsal horn neurones leads to prolonged enhancement of pain behaviour and can be evoked by intense C-fibre stimulation, tissue inflammation and peripheral nerve injury. Activation of central immune cells plays a key role in establishing pain hypersensitivity but the exact nature of the afferent input that triggers the activation of microglia and other glial cells within the CNS, remains unclear. Here intense but non-damaging, electrical stimulation of intact adult rat C-fibres for 5 min at 10 Hz induced central sensitization characterized by significant decreases in mechanical withdrawal thresholds 3, 24 and 48 h later. This maintained (>3 h) hypersensitivity was not observed following topical skin application of capsaicin. C-fibre evoked sensitization was accompanied by significant microglial activation, shown by increased Iba-1 immunoreactivity throughout the dorsal horn at 24 and 48 h and significant upregulation of markers of microglial activation: IL-6 and Mcp-1 at 3 h and Mmp3, CSF-1 and CD163 at 24 and 48 h. C-fibre stimulation caused no nerve damage at ultrastructural and molecular levels. Lower intensity stimulation that did not activate C-fibres or sham stimulation did not increase Iba-1 immunoreactivity or induce behavioural sensitivity. Pre-treatment with minocycline (40 mg/kg, i.p.) prevented the C-fibre evoked sensitization and microglial activation. Identical C-fibre stimulation in 10-day old rat pups failed to activate microglia or change behaviour. These results demonstrate that a brief period of low frequency C-fibre stimulation, in the absence of nerve damage, is sufficient to activate microglia resulting in behavioural hyperalgesia.

Spatial and temporal summation of pain evoked by mechanical pressure stimulation

Chronic pain patients often suffer from widespread and long lasting pain. The integrative effect of combined spatial and temporal summation on pain intensity has not been quantitatively tested. The present study was designed to investigate: (1) if the size of the stimulation area would facilitate the temporal summation of pain to repetitive pressure stimulation, and (2) if temporal summation is effective when stimulating separated sites, repetitively. Twenty healthy male subjects participated in this study. The test sites were located on the bilateral upper trapezius and tibialis anterior muscles. The ten stimuli (each with a duration of 1s) were applied to a single site at three inter-stimulus intervals (ISI: 1, 5, 30s) using five different probe sizes (0.5, 1, 2, 4 and 8cm(2)). The stimulation intensity was equal to the pressure pain threshold (PPT) determined for each probe size. Similar repetitive stimulations at two inter-stimulus intervals (5s and 30s) using two sizes of probes (0.5cm(2) and 2cm(2)) were applied to ten separate sites. The PPT at the trapezius muscle decreased significantly with the increase in stimulus area from 0.5cm(2) to 8cm(2) (P<0.001) due to spatial summation. Temporal summation of pain was evoked by repetitive pressure stimuli on the same site for all ISI and was more pronounced at 5s and 30s ISI with larger probe areas (2, 4, and 8cm(2)) compared to smaller probe areas. There was no temporal summation of pain to stimuli with ISI 5 and 30s when stimulating the separated sites. The current study indicated that spatial summation facilitated the temporal summation of pain for stimuli given at 5s and 30s ISI. The combination of temporal and spatial integration of nociceptive input facilitates the pain intensity, suggesting that temporal summation is clinically relevant in conditions with widespread pain.

Effect of Static and Dynamic Heat Pain Stimulus Profiles on the Temporal Dynamics and Interdependence of Pain Qualities, Intensity, and Affect

Acute and chronic pains are characterized by a particular constellation of pain qualities, such as burning, aching, stinging, or sharp feelings. However, the temporal pattern of specific pain qualities and their relationship with pain and affect is not well understood. In addition, little is known about how the temperature time course of the stimulus impacts the temporal dynamics of pain qualities and the relationship between pain qualities. Therefore we applied two types of stimuli to the feet of 16 healthy subjects, each calibrated to evoke a similar pain magnitude (50/100): static stimulus held at constant intensity and dynamic stimulus increased in intensity in small steps. Stimulus runs consisted of three 30-s stimuli (either static or dynamic) with an interstimulus interval of 60 s. Continuous on-line ratings of pain, burning, sharp, stinging, cutting, and annoyance were obtained in separate runs, and the evoked responses were characterized by within-stimulus adaptation (early: 0- to 15-s peak vs. late: 25- to 40-s peak) and by their temporal properties (time to onset, peak, and end). The temporal profile of the burning sensation was similar to the pain and annoyance evoked by the static and dynamic stimuli. However, the sharp, stinging and cutting sensations attenuated in response to the static stimuli ( P < 0.01) but intensified along with pain and affect in response to the dynamic stimuli ( P < 0.05), whereas there was no attenuation in the evoked profiles of pain ( P = 0.61), annoyance ( P = 0.27), or burning quality ( P = 0.27). These data demonstrate that specific pain qualities with known differences in underlying mechanisms have distinct temporal dynamics that depend on the stimulus intensity dynamics.

Anatomical and functional characterization of neuropil in the gracile fasciculus

A fundamental organizational principle of the central nervous system is that gray matter is the province of neuronal somata, white matter their processes. However, the rat and primate dorsal columns (archetypal spinal "white matter" tracts) are actually of intermediate character, insofar as they contain a surprisingly prominent neuropil of unknown function. Here I report on the morphology, inputs, projections, and functional properties of these neurons. Small fusiform and larger lentiform neurons are most abundant in the gracile fasciculus of the cervical and lumbar enlargements and are absent from the cuneate fasciculus and corticospinal tract. Many have dendrites that run along the dorsal pia, and, although in transverse sections these neurons appear isolated from the gray matter, they are also connected to area X by varicose and sometimes loosely fasciculated dendrites. These neurons receive neurochemically diverse, compartmentalized synaptic inputs (primary afferent, intrinsic and descending), half express the substance P receptor, and some project supraspinally. Unlike substantia gelatinosa neurons, they do not express protein kinase C gamma. Functionally, they have small receptive fields, which are somatotopically appropriate with respect to their anterior-posterior position along the neuraxis. They respond to innocuous and/or noxious mechanical stimulation of the distal extremities, and some are prone to central sensitization or "windup." Morphologically, neurochemically, and functionally, therefore, these cells most closely resemble neurons in laminae III-VI in the dorsal horn. The proximity of their dorsal dendrites to the pia mater may also reflect an ability to integrate internal (e.g., changes in cerebrospinal fluid compostition) and external (e.g., somatic) stimuli.

Processing noxious information at the subnucleus reticularis dorsalis (SRD) of anesthetized cats: wind-up mechanisms

With the exception of one monkey's study, where wind-up was not reported, electrophysiological data from SRD neurons were obtained in rodents where they show wind-up. This work was designed to examine the response properties of SRD neurons in anesthetized cats to study how general the data from rats may be. Since cat's SRD cells showed wind-up, its underlying mechanisms were approached, an issue not previously addressed at supraspinal level. Electrical stimulation, extracellular (combined with microiontophoresis) and intracellular techniques revealed that A delta information reaches the SRD via the ventrolateral cord, whereas C information preferentially follows a dorsal route. Wind-up was usually generated by spinal and peripheral stimulation, but it was also evoked either by stimulating the nucleus reticularis gigantocellularis (NRGc), even after spinal cord section and bilateral full thickness removal of the cerebral cortex, or by applying microiontophoretic pulses of l-glutamate at 0.3-1 Hz. Wind-up relied on afferent repetitive activity gradually depolarizing the SRD neurons leading 3-4.5 Hz subthreshold membrane rhythmic activity to threshold. Riluzole retarded wind-up generation and decreased the number of spikes per stimulus during wind-up. GABA or glycine abolished spontaneous and sensory-evoked activity and bicuculline, but not strychnine, increased spontaneous and stimulus-evoked activity. These results demonstrate that wind-up at the SRD is not merely the reflection of spinal wind-up, but (i) can be locally generated, (ii) is partially dependent upon persistent sodium currents, and (iii) is under the modulation of a tonic GABAa-dependent inhibition decreasing SRD excitability. Injury and/or inflammation producing tonic C-fiber activation will surpass tonic inhibition generating wind-up.

Quantitative testing of pain perception in subjects with PTSD--implications for the mechanism of the coexistence between PTSD and chronic pain

Post-traumatic stress disorder (PTSD) often co-occurs with chronic pain. Neither the underlying mechanism of this comorbidity nor the nature of pain perception among subjects with PTSD is well defined. This study is the first systematic and quantitative evaluation of pain perception and chronic pain in subjects with PTSD. The study group consisted of 32 outpatients with combat- and terror-related PTSD, 29 outpatients with anxiety disorder and 20 healthy controls. Quantitative somatosensory testing included the measurement of warm, cold, light touch and heat-pain thresholds and responses to acute suprathreshold heat and mechanical stimuli. Chronic pain was characterized, and levels of PTSD and anxiety symptomatology were assessed by self-report questionnaires. Subjects with PTSD exhibited higher rates of chronic pain, more intense chronic pain and more painful body regions compared with the other two groups. PTSD severity correlated with chronic pain severity. Thresholds of subjects with PTSD were significantly higher than those of subjects with anxiety and healthy controls, but they perceived suprathreshold stimuli as being much more intense than the other two groups. These results suggest that subjects with PTSD exhibit an intense and widespread chronic pain and a unique sensory profile of hyposensitivity to pain accompanied by hyper-reactivity to suprathreshold noxious stimuli. These features may be attributed to the manner with which PTSD subjects emotionally interpret and respond to painful stimuli. Alternatively, but not mutually exclusive, the findings may reflect altered sensory processing among these subjects.

From acute musculoskeletal pain to chronic widespread pain and fibromyalgia: application of pain neurophysiology in manual therapy practice

During the past decade, scientific research has provided new insight into the development from an acute, localised musculoskeletal disorder towards chronic widespread pain/fibromyalgia (FM). Chronic widespread pain/FM is characterised by sensitisation of central pain pathways. An in-depth review of basic and clinical research was performed to design a theoretical framework for manual therapy in these patients. It is explained that manual therapy might be able to influence the process of chronicity in three different ways. (I) In order to prevent chronicity in (sub)acute musculoskeletal disorders, it seems crucial to limit the time course of afferent stimulation of peripheral nociceptors. (II) In the case of chronic widespread pain and established sensitisation of central pain pathways, relatively minor injuries/trauma at any locations are likely to sustain the process of central sensitisation and should be treated appropriately with manual therapy accounting for the decreased sensory threshold. Inappropriate pain beliefs should be addressed and exercise interventions should account for the process of central sensitisation. (III) However, manual therapists ignoring the processes involved in the development and maintenance of chronic widespread pain/FM may cause more harm then benefit to the patient by triggering or sustaining central sensitisation.

A Role For Wind-Up in Trigeminal Sensory Processing: Intensity Coding of Nociceptive Stimuli in the Rat

Wind-up is a progressive, frequency-dependent increase in the excitability of trigeminal and spinal dorsal horn wide dynamic range (WDR) nociceptive neurons evoked by repetitive stimulation of primary afferent nociceptive C-fibres. The correlate of wind-up in humans is temporal summation, which is an increase in pain perception to repetitive constant nociceptive stimulation. Although wind-up is widely used as a tool for studying the processing of nociceptive information, including central sensitization, its actual role is still unknown. Here, we recorded from trigeminal WDR neurons using in vivo electrophysiological techniques in rats and assessed the wind-up phenomenon in response to stimuli of different intensities and frequencies. First, we found that the amplitude of C-evoked responses of WDR neurons to repetitive stimulation increased progressively to reach a peak, then consistently showed a stable or slightly decreasing plateau phase. Only the first phase of this time course fitted in with the wind-up description. Therefore, to assess wind-up, we measured a limited number of initial responses. Second, we showed that wind-up, i.e. the slope of the frequency-dependent increase in the response to C-fibre stimulation, was linearly correlated to the stimulus intensity. Intensities of brief C-fibre inputs were thus coded into frequencies of action potentials by second-order neurons through frequency-dependent potentiation of the evoked responses. Third, wind-up also occurred at stimulation intensities below the threshold for C-evoked responses in WDR neurons, suggesting that wind-up can amplify subthreshold C-fibre inputs to WDR neurons. This might account for the observation that sparse, subliminal, neuronal activity in nociceptors can become painful via central integration of neural responses. Altogether, the present results show that wind-up can provide trigeminal WDR neurons with the capability to encode the intensity of short-duration orofacial nociceptive stimuli and to detect subthreshold nociceptive input. Thus, not only may wind-up play a physiological role in trigeminal sensory processing, but its enhancement may also underlie the pathophysiology of chronic orofacial pain conditions.

Vagal damage enhances polyneuropathy pain: additive effect of two algogenic mechanisms

While the major pain generation in polyneuropathy is in the somatic peripheral nerves, pathologies at visceral nerves might be involved as well. Decreased vagal afferent activity is known to disinhibit pain perception, and therefore might contribute to pain in polyneuropathy. In this study we explored this potential contribution by employing a rat model of vincristine (VCR)-induced pain after sub-diaphragmatic vagotomy (SDV). Forty rats were divided into 4 groups: VCR, SDV, VCR+SDV and controls. Each rat underwent a variety of pain-related behavioral tests including assessment of spontaneous pain, allodynia and hyperalgesia to thermal and mechanical stimuli. We found that VCR+SDV rats had enhanced painful neuropathy compared to VCR alone, expressed as: (1) earlier development of central sensitization: at the first week in rats that underwent SDV+VCR (p<0.0001) and only at the second week in rats injected with VCR alone (p<0.0001), (2) increased incidence of spontaneous pain behavior (p=0.0036), (3) spreading of the spontaneous pain behavior to the forelimbs, (4) higher mechanical dynamic allodynia (tendency, p=0.08) and (5) augmentation of the response to repetitive painful and non-painful mechanical stimuli (p<0.001). Thus, decreased vagal activity aggravates both the severity and the time course of painful polyneuropathy. Therefore, the two mechanisms add to each other in generating the pain picture.

The role of myofascial trigger points in musculoskeletal pain syndromes of the head and neck

Neck and head pain syndromes are common problems seen in clinical practice. Pain features of commonly designated idiopathic neck pain and some primary headaches (ie, tension-type headache or migraine) fit the descriptions of referred pain originating in muscle trigger points (TrPs). This article discusses the scientific evidence supporting the role of muscle TrPs in chronic musculo-skeletal disorders of the neck and head. The relevance of referred pain elicited by muscle TrPs in patients with neck pain has been investigated in few studies. Some authors found that both muscle TrPs in neck-shoulder muscles and cervical joint dysfunctions contribute at the same time to neck pain perception. Furthermore, it seems that referred pain originated in muscle TrPs could also contribute to neck symptoms perceived by subjects after a rear-end crash. In addition, several recent studies reported that both TTH and migraine are associated with referred pain from TrPs in the suboccipital, upper trapezius, sternocleidomastoid, temporalis, or superior oblique muscles. Referred pain elicited by active TrPs mimics the pain areas observed during head pain attacks in these primary headaches. Based on available data, it seems that the pain profile of neck and head syndromes may be provoked referred pain from TrPs in the posterior cervical, head, and shoulder muscles. Additional studies are needed to delineate more information on the relation between muscle TrPs and musculoskeletal pain syndromes of the head and neck.

Age Interacts with Stimulus Frequency in the Temporal Summation of Pain

OBJECTIVE

There is growing interest in the impact of aging on the plasticity of pain responses. Up-regulation characterizes pain responses in clinical situations, and consequently aging effects on the development and resolution of increased sensitivity have important implications for the experience of pain in those older age groups who are more likely to suffer from chronic conditions. This study examined temporal summation of pain at different stimulus frequencies to gain further insights into the effect of age on pain plasticity.

DESIGN

In a group of younger and a group of older subjects, trains of five brief electrical stimuli were applied to the skin over the sural nerve at frequencies ranging between 0.2 and 2.0 Hz. Nociceptive reflexes were recorded throughout the application of stimuli. Single pulses and the fifth pulse of each series were rated for pain intensity with a visual analog scale.

RESULTS

The younger subjects demonstrated temporal summation at frequencies of stimulation that were consistent with previous reports, namely 0.33 to 2.0 Hz. The older group had a greater mean rating of the fifth pulse relative to a single pulse at all frequencies of stimulation. The behavior of the nociceptive reflex to repeated stimuli was equivalent for the two age groups, only summating at a frequency of 2.0 Hz.

CONCLUSIONS

The temporal summation of low-frequency stimuli in the older subjects suggests that aging impacts on the capacity of the nociceptive system to down-regulate subsequent to sensitization.

Noninvasive assessment of the facilitation of the nociceptive withdrawal reflex by repeated electrical stimulations in conscious dogs

OBJECTIVE

To investigate the facilitation of the nociceptive withdrawal reflex (NWR) by repeated electrical stimuli and the associated behavioral response scores in conscious, nonmedicated dogs as a measure of temporal summation and analyze the influence of stimulus intensity and frequency on temporal summation responses.

ANIMALS

8 adult Beagles.

PROCEDURES

Surface electromyographic responses evoked by transcutaneous constant-current electrical stimulation of ulnaris and digital plantar nerves were recorded from the deltoideus, cleidobrachialis, biceps femoris, and cranial tibial muscles. A repeated stimulus was given at 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, and 1.1 x I(t) (the individual NWR threshold intensity) at 2, 5, and 20 Hz. Threshold intensity and relative amplitude and latency of the reflex were analyzed for each stimulus configuration. Behavioral reactions were subjectively scored.

RESULTS

Repeated sub-I(t) stimuli summated and facilitated the NWR. To elicit temporal summation, significantly lower intensities were needed for the hind limb, compared with the forelimb. Stimulus frequency did not influence temporal summation, whereas increasing intensity resulted in significantly stronger electromyographic responses and nociception (determined via behavioral response scoring) among the dogs.

CONCLUSIONS AND CLINICAL RELEVANCE

In dogs, it is possible to elicit nociceptive temporal summation that correlates with behavioral reactions. These data suggest that this experimental technique can be used to evaluate nociceptive system excitability and efficacy of analgesics in canids.

Autonomic nervous system changes in men with chronic pelvic pain syndrome

PURPOSE

We investigated the role of the autonomic nervous system in the chronic pelvic pain syndrome by studying blood pressure changes, power spectral analysis of heart rate variability, and hand and foot sympathetic skin responses in men with the chronic pelvic pain syndrome and healthy controls.

MATERIALS AND METHODS

We recruited 22 men with the chronic pelvic pain syndrome and 20 controls. The subjects underwent 5-minute resting and standing blood pressure readings as well as electrocardiogram recordings. Heart rate variability was evaluated using the low frequency (sympathetic) and high frequency (parasympathetic) components of power spectral analysis. We recorded sympathetic skin responses with surface electrodes placed on the dorsal and ventral sides of both hands and feet, measuring amplitude and latency.

RESULTS

Mean blood pressure readings for supine and standing were higher in the chronic pelvic pain syndrome group. Between group comparisons revealed no differences in low or high power spectra for supine or standing positions. When subjects were taken to standing from the supine position, mean low frequency power increased in the controls and decreased in the chronic pelvic pain syndrome group. High frequency power, a reflection of parasympathetic activity, decreased significantly in the control group while the decrease was not statistically significant in the chronic pelvic pain syndrome group. There was no difference between the 2 groups for hand and foot sympathetic skin response latency and amplitudes.

CONCLUSIONS

The heart rate variability and blood pressure changes suggest altered autonomic nervous system responses in men with the chronic pelvic pain syndrome. Sympathetic skin response findings were not conclusive. Heart rate variability and blood pressure findings provide a basis for therapeutic manipulation of the autonomic nervous system for management of the chronic pelvic pain syndrome.

Myofascial pain syndromes and their evaluation

Myofascial pain refers to a specific form of soft-tissue rheumatism that results from irritable foci (trigger points) within skeletal muscles and their ligamentous junctions. It must be distinguished from bursitis, tendonitis, hypermobility syndromes, fibromyalgia and fasciitis. On the other hand it often exists as part of a clinical complex that includes these other soft-tissue conditions, i.e., it is not a diagnosis of exclusion. The clinical science of trigger points can be traced to the pioneering work of Kellgren in the 1930s, with his mapping of myotomal referral patterns of pain resulting from the injection of hypertonic saline into muscle and ligaments. Most muscles have characteristic myotomal patterns of referred pain; this feature forms the basis of the clinical recognition of myofascial trigger points in the form of a tender locus within a taut band of muscle which restricts the full range of motion and refers pain centrifugally when stimulated. Although myofascial pain syndromes have been described in the medical literature for about the last 100 years, it is only recently that scientific studies have revealed objective abnormalities.

C. Offset analgesia: a temporal contrast mechanism for nociceptive information

Temporal filtering of afferent information is an intrinsic component of the processing of numerous types of sensory information. To date, no temporal filtering mechanism has been identified for nociceptive information. The phenomenon of offset analgesia, the disproportionately large decrease in perceived pain following slight decreases in noxious thermal intensity, however, suggests the existence of such a mechanism. To test the hypothesis that a temporal filtering mechanism is engaged during noxious stimulus offset, subjects rated heat pain intensity while stimulus fall rates were varied from -0.5 to -5.0 degrees C/s. In the absence of a temporal filtering mechanism, pain intensity would be expected to decrease in direct proportion to the stimulus fall rate. However, psychophysical fall rates were considerably faster than stimulus fall rates, such that subjects reported no pain while stimulus temperatures were clearly within the noxious range (47.2 degrees C). In addition, paired noxious stimuli were presented simultaneously to determine if offset analgesia evoked by one stimulus could inhibit pain arising from a separate population of primary afferent neurons. Pain ratings were significantly lower than those reported from two constant 49 degrees C stimuli when offset analgesia was induced proximal to, but not distal to, a second noxious stimulus. These asymmetric spatial interactions are not readily explained by peripheral mechanisms. Taken together, these findings indicate that offset analgesia is mediated in part by central mechanisms and reflect a temporal filtering of the sensory information that enhances the contrast of dynamic decreases in noxious stimulus intensity.

Dissociation of nociceptive modulation of a human jaw reflex from the influence of stress

In human beings, inhibitory jaw reflexes can be depressed by painful stimulation of remote parts of the body. Since similar effects can be produced by the stress of anticipating pain, we wished to investigate whether the effects of remote painful stimuli are dependent on stress. EMG recordings were made from a masseter muscle while subjects maintained activity in the muscle at approximately 12.5% of maximum using visual feedback. The protocols involved three sequences: (1) "standard controls" in which reflexes were evoked by electrical test stimuli applied to the upper lip; (2) "standard conditioning" in which painful electrical conditioning stimuli were applied over the sural nerve 100 ms before the test stimuli; (3) "random sequences" in which test-only and conditioning-test combinations were employed in a double-blind, random, order. Data are presented as means +/- SEMs. In the standard controls, the stimuli evoked clear inhibitory reflexes (latency 37 +/- 1.3 ms, duration 62 +/- 5.6 ms; n = 10) in all the subjects. During standard conditioning, the reflex magnitude was reduced significantly (by 50.0 +/- 8.5%, P = 0.0002, one-sample t-test). When the test-only and conditioning-test responses were extracted from the random sequences, there was also a significant reduction in the reflex magnitude following conditioning (by 34.6 +/- 5.5%, P = 0.0002, one-sample t-test) albeit less so than between the standard sequences (P = 0.03, paired t-test). A second series of experiments suggested that these lesser effects during the random sequences were not substantially due to any loss of temporal summation of the conditioning mechanisms. The evidence for this was that application of pairs of conditioning stimuli did not produce a significantly greater effect than single conditioning stimuli within a random sequence (39.9 +/- 9.6% as opposed to 32.7 +/- 9.1% reductions in the reflex, P = 0.117, paired t-test). Therefore since any stress in the random sequences would not have been "tied" to the conditioned responses alone, the effects of remote painful stimuli on this inhibitory jaw reflex cannot be entirely secondary to stress.

Brain activity related to temporal summation of C-fiber evoked pain

Temporal summation of "second pain" (TSSP) is considered to be the result of C-fiber-evoked responses of dorsal horn neurons, termed 'windup'. This phenomenon is dependent on stimulus frequency (0.33 Hz) and relevant for central sensitization and chronic pain. Previous brain imaging studies have only been used to characterize neural correlates of second pain but not its temporal summation. We utilized functional magnetic resonance imaging (fMRI) in healthy volunteers to measure brain responses associated with TSSP. Region of interest analysis was used to assess TSSP related brain activation. Eleven pain-free normal subjects underwent fMRI scanning during repetitive heat pulses to the right foot at 0.33 and 0.17 Hz. Stimulus intensities were adjusted to each individual's heat sensitivity to achieve comparable TSSP ratings of moderate pain in all subjects. As predicted, experimental pain ratings showed robust TSSP during 0.33 Hz but not 0.17 Hz stimuli. fMRI statistical maps identified several brain regions with stimulus and frequency dependent activation consistent with TSSP, including contralateral thalamus (THAL), S1, bilateral S2, anterior and posterior insula (INS), mid-anterior cingulate cortex (ACC), and supplemental motor areas (SMA). TSSP ratings were significantly correlated with brain activation in somatosensory areas (THAL, S1, left S2), anterior INS, and ACC. These results show that neural responses related to TSSP are evoked in somatosensory processing areas (THAL, S2), as well as in multiple areas that serve other functions related to pain, such as cognition (ACC, PFC), affect (INS, ACC, PAG), pre-motor activity (SMA, cerebellum), and pain modulation (rostral ACC).

Myofascial trigger points and sensitization: an updated pain model for tension-type headache

Present pain models for tension-type headache suggest that nociceptive inputs from peripheral tender muscles can lead to central sensitization and chronic tension-type headache (CTTH) conditions. Such models support that possible peripheral mechanisms leading to pericranial tenderness include activation or sensitization of nociceptive nerve endings by liberation of chemical mediators (bradikinin, serotonin, substance P). However, a study has found that non-specific tender points in CTTH subjects were not responsible for liberation of algogenic substances in the periphery. Assuming that liberation of algogenic substances is important, the question arising is: if tender muscle points are not the primary sites of on-going neurogenic inflammation, which structure can be responsible for liberation of chemical mediators in the periphery? A recent study has found higher levels of algogenic substances, and lower pH levels, in active myofascial trigger point (TrPs) compared with control tender points. Clinical studies have demonstrated that referred pain elicited by head and neck muscles contribute to head pain patterns in CTTH. Based on available data, an updated pain model for CTTH is proposed in which headache can at least partly be explained by referred pain from TrPs in the posterior cervical, head and shoulder muscles. In this updated pain model, TrPs would be the primary hyperalgesic zones responsible for the development of central sensitization in CTTH.

Blocking sodium channels to treat neuropathic pain

Neuropathic pain remains a large unmet medical need. A number of therapeutic options exist, but efficacy and tolerability are less than satisfactory. Based on animal models and limited data from human patients, the pain and hypersensitivity that characterize neuropathic pain are associated with spontaneous discharges of normally quiescent nociceptors. Sodium channel blockers inhibit this spontaneous activity, reverse nerve injury-induced pain behavior in animals and alleviate neuropathic pain in humans. Several sodium channel subtypes are expressed primarily in sensory neurons and may contribute to the efficacy of sodium channel blockers. In this report, the authors review the current understanding of the role of sodium channels and of specific sodium channel subtypes in neuropathic pain signaling.

Peripheral and central mechanisms of pain generation

Pain research has uncovered important neuronal mechanisms that underlie clinically relevant pain states such as inflammatory and neuropathic pain. Importantly, both the peripheral and the central nociceptive system contribute significantly to the generation of pain upon inflammation and nerve injury. Peripheral nociceptors are sensitized during inflammation, and peripheral nerve fibres develop ectopic discharges upon nerve injury or disease. As a consequence a complex neuronal response is evoked in the spinal cord where neurons become hyperexcitable, and a new balance is set between excitation and inhibition. The spinal processes are significantly influenced by brain stem circuits that inhibit or facilitate spinal nociceptive processing. Numerous mechanisms are involved in peripheral and central nociceptive processes including rapid functional changes of signalling and long-term regulatory changes such as up-regulation of mediator/receptor systems. Conscious pain is generated by thalamocortical networks that produce both sensory discriminative and affective components of the pain response.

Spinal use-dependent plasticity of synaptic transmission in humans after a single cycling session

The spinal cord is able to express use-dependent plasticity, as demonstrated in spinalized cats following treadmill training. In humans, spinal use-dependent plasticity is inferred from modifications in the size of H reflex, which are often more prominent after skilled motor training. Plasticity can develop at synaptic connections between afferent fibres and/or descending tracts and motoneurones or interneurones interposed in the spinal pathways. Here we explore whether skilled training induces a change in synaptic efficacy at the synapse between Ia afferents and soleus (Sol) motoneurones. Synaptic efficacy can be modulated presynaptically through changes of the probability of transmitter release (homosynaptic depression, HD). The frequency-related depression of the Sol H reflex, thought to reflect HD, was tested at rest, before and after one single skilled (14 subjects) or non-skilled (9 subjects) cycling training session. Performance improved in both groups but to a larger extent with skilled training, while HD increased immediately after and the day following skilled training in the absence of changes with non-skilled training. These results support the view that spinal cord function is able to encode a local motor memory.

Central sensitization: a biopsychosocial explanation for chronic widespread pain in patients with fibromyalgia and chronic fatigue syndrome

In addition to the debilitating fatigue, the majority of patients with chronic fatigue syndrome (CFS) experience chronic widespread pain. These pain complaints show the greatest overlap between CFS and fibromyalgia (FM). Although the literature provides evidence for central sensitization as cause for the musculoskeletal pain in FM, in CFS this evidence is currently lacking, despite the observed similarities in both diseases. The knowledge concerning the physiological mechanism of central sensitization, the pathophysiology and the pain processing in FM, and the knowledge on the pathophysiology of CFS lead to the hypothesis that central sensitization is also responsible for the sustaining pain complaints in CFS. This hypothesis is based on the hyperalgesia and allodynia reported in CFS, on the elevated concentrations of nitric oxide presented in the blood of CFS patients, on the typical personality styles seen in CFS and on the brain abnormalities shown on brain images. To examine the present hypothesis more research is required. Further investigations could use similar protocols to those already used in studies on pain in FM like, for example, studies on temporal summation, spatial summation, the role of psychosocial aspects in chronic pain, etc.

Antinociceptive effects, metabolism and disposition of ketamine in ponies under target-controlled drug infusion

Ketamine is widely used as an anesthetic in a variety of drug combinations in human and veterinary medicine. Recently, it gained new interest for use in long-term pain therapy administered in sub-anesthetic doses in humans and animals. The purpose of this study was to develop a physiologically based pharmacokinetic (PBPk) model for ketamine in ponies and to investigate the effect of low-dose ketamine infusion on the amplitude and the duration of the nociceptive withdrawal reflex (NWR). A target-controlled infusion (TCI) of ketamine with a target plasma level of 1 microg/ml S-ketamine over 120 min under isoflurane anesthesia was performed in Shetland ponies. A quantitative electromyographic assessment of the NWR was done before, during and after the TCI. Plasma levels of R-/S-ketamine and R-/S-norketamine were determined by enantioselective capillary electrophoresis. These data and two additional data sets from bolus studies were used to build a PBPk model for ketamine in ponies. The peak-to-peak amplitude and the duration of the NWR decreased significantly during TCI and returned slowly toward baseline values after the end of TCI. The PBPk model provides reliable prediction of plasma and tissue levels of R- and S-ketamine and R- and S-norketamine. Furthermore, biotransformation of ketamine takes place in the liver and in the lung via first-pass metabolism. Plasma concentrations of S-norketamine were higher compared to R-norketamine during TCI at all time points. Analysis of the data suggested identical biotransformation rates from the parent compounds to the principle metabolites (R- and S-norketamine) but different downstream metabolism to further metabolites. The PBPk model can provide predictions of R- and S-ketamine and norketamine concentrations in other clinical settings (e.g. horses).

Are patients with systemic lupus erythematosus at increased risk for Fibromyalgia?

Widespread chronic pain, fatigue, and distress do not represent risk factors for future systemic lupus erythematosus (SLE) or other autoimmune syndromes. On the other hand, SLE seems to be a significant risk factor for fibromyalgia (FM). Up to 47% of SLE patients fulfill FM criteria. SLE patients with concomitant FM are often highly symptomatic and dysfunctional. The presence of FM symptoms in SLE patients, however, does not predict more extensive organ involvement or lupus activity. The high concordance of SLE with FM suggests common mechanisms related to pain and distress in both patient groups. Recent research suggests involvement of N-methyl-D-aspartate (NMDA) and neurokinin receptor systems. Thus, autoimmune activity against these receptor systems in SLE patients could result in pain, cognitive defects, and chronic pain states including FM. Conversely, treatment of SLE-FM patients with inhibitors of NMDA or neurokinin receptors may prevent or alleviate cognitive abnormalities and chronic pain, as well as FM.

The Effect of Intravenous Ketorolac on Capsaicin-Induced Deep Tissue Hyperalgesia

Preclinical and clinical studies have emphasized that persistent small afferent input will induce a state of central facilitation that can be attenuated by systemically administered nonsteroidal antiinflammatory drugs. However, these studies have been performed using cutaneous models of hyperalgesia. In this study we evaluated the effects of IV ketorolac on an experimental model of deep tissue hyperalgesia using IM capsaicin. We used a double-blind, placebo-controlled, crossover design. Ten subjects received 60 mg of ketorolac or placebo in 2 sessions separated by 1 wk. Capsaicin (100 microg in 10 microL) was then injected into the flexor carpi ulnaris muscle of the left forearm. After injection, spontaneous pain scores, pressure pain scores, gripping pain, pain distribution, and pain quality were recorded at 0, 5, 10, 15, 20, and 25 min. Cutaneous allodynia and dysesthesia were then mapped and thermal and mechanical thresholds were measured. The IM injection of capsaicin resulted in a reliable report of pain, hyperalgesia, and referred pain. Ketorolac had no effect on spontaneous pain, elicited pain, pain distribution, or secondary hyperalgesia induced by capsaicin. The findings of this study support the feasibility of further pharmacological studies using the IM capsaicin pain model.

Windup in dorsal horn neurons is modulated by endogenous spinal mu-opioid mechanisms

The mu-opioid receptor (MOR) plays a critical role in morphine analgesia and nociceptive transmission. However, the physiological roles for endogenous MOR mechanisms in modulating spinal nociceptive transmission, and particularly in the enhanced excitability of spinal nociceptive neurons after repeated noxious inputs, are less well understood. Using a MOR gene knock-out (-/-) approach and an MOR-preferring antagonist, we investigated the roles of endogenous MOR mechanisms in processing of acute noxious input and in neuronal sensitization during windup-inducing stimuli in wide dynamic range (WDR) neurons. Extracellular single-unit activity of WDR neurons was recorded in isoflurane-anesthetized MOR(-/-) and wild-type C57BL/6 mice. There were no significant differences between the genotypes in the responses of deep WDR cells to acute mechanical stimuli, graded electrical stimuli, and noxious chemical stimuli applied to the receptive field. Intracutaneous electrical stimulation at 1.0 Hz produced similar levels of windup in both genotypes. In contrast, 0.2 Hz stimulation induced significantly higher levels of windup in MOR(-/-) mice compared with the wild-type group. In wild-type mice, spinal superfusion with naloxone hydrochloride (10 mM, 30 microl) significantly enhanced windup to 0.2 Hz stimulation in both deep and superficial WDR cells. A trend toward facilitation of windup was also observed during 1.0 Hz stimulation after naloxone treatment. These results suggest that endogenous MOR mechanisms are not essential in the processing of acute noxious mechanical and electrical stimuli by WDR neurons. However, MORs may play an important role in endogenous inhibitory mechanisms that regulate the development of spinal neuronal sensitization.