Sensitization of pain pathways in the spinal cord: cellular mechanisms

Neurochemical and behavioral features in genetic absence epilepsy and in acutely induced absence seizures

The absence epilepsy typical electroencephalographic pattern of sharp spikes and slow waves (SWDs) is considered to be due to an interaction of an initiation site in the cortex and a resonant circuit in the thalamus. The hyperpolarization-activated cyclic nucleotide-gated cationic I h pacemaker channels (HCN) play an important role in the enhanced cortical excitability. The role of thalamic HCN in SWD occurrence is less clear. Absence epilepsy in the WAG/Rij strain is accompanied by deficiency of the activity of dopaminergic system, which weakens the formation of an emotional positive state, causes depression-like symptoms, and counteracts learning and memory processes. It also enhances GABAA receptor activity in the striatum, globus pallidus, and reticular thalamic nucleus, causing a rise of SWD activity in the cortico-thalamo-cortical networks. One of the reasons for the occurrence of absences is that several genes coding of GABAA receptors are mutated. The question arises: what the role of DA receptors is. Two mechanisms that cause an infringement of the function of DA receptors in this genetic absence epilepsy model are proposed.

Effect of high-frequency alternating current on spinal afferent nociceptive transmission

OBJECTIVE

The study was performed to test the hypothesis that high-frequency alternating current (HFAC) ranging from 2 to 100 kHz delivered to the spinal dorsal roots reduces activity of spinal wide dynamic range (WDR) dorsal horn neurons (DHNs) during noxious peripheral stimulation.

MATERIALS AND METHODS

This hypothesis was tested in both small and large animal in vivo preparations. Single-unit extracellular spinal DHN recordings were performed in seven adult rats and four adult goats while testing various parameters of HFAC delivered to the nerve roots or dorsal root entry zone using various electrode types. Frequencies tested ranged from 2 to 100 kHz but focused on the 3 to 50 kHz range. This study investigated the ability of HFAC to inhibit WDR neuronal activity evoked by noxious mechanical (pinch), and electrical stimuli was tested but was primarily focused on electrical stimulation.

RESULTS

Rat Study: Effects of HFAC were successfully tested on 11 WDR neurons. Suppression or complete blockade of evoked activity was observed in all 11 of these neurons. Complete data sets for neurons systematically tested with 15 baseline and post-HFAC stimulus sweeps were obtained in five neurons, the nociceptive activity of which was suppressed by an average of 69 ± 9.7% (p < 0.0001). Goat Study: HFAC was successfully tested on 15 WDR neurons. Conclusive suppression or complete nociceptive blockade was observed for 12/15 and complete data sets with at least 20 baseline and post-HFAC stimulus sweeps were obtained from eight DHNs. For these neurons the mean activity suppression was 70 ± 10% (p < 0.005).

CONCLUSIONS

Delivery of HFAC to the region of epidural nerve root or nerve root entry inhibited afferent nociceptive input and therefore may have potential to serve as an alternative to traditional spinal cord stimulation without sensory paresthesia as neuronal activation cannot occur at frequencies in this range.

Self-reported comorbid pains in severe headaches or migraines in a US national sample

AIMS

To compare prevalence of self-reported comorbid temporomandibular joint muscle disorder-type, neck, back, and joint pains in people with severe headache or migraine; and analyze these self-reported pains in the 2000-2005 US National Health Interview Survey by gender and age for non-Hispanic whites, Hispanics, and non-Hispanic blacks (African Americans).

METHODS

National Health Interview Survey data included information on gender, age, race, ethnicity, health status, and common pain types: severe headache or migraine, temporomandibular joint muscle disorder-type, neck, and low back in the last 3 months, as well as prior-month joint pains. Analyses included survey prevalence estimation and survey logistic regression to obtain odds ratios and 95% confidence intervals.

RESULTS

The study included 189,967 adults: 48% males, 52% females; 73% white, 12% Hispanic, and 11% black. Of the entire sample, 29,712 (15%) reported severe headache or migraine, and 19,228 (64%) had severe headache or migraine with at least 1 comorbid pain. Two or more comorbid pains were reported in 10,200 (33%), with no gender difference, and with Hispanics (n = 1847 or 32%) and blacks (n = 1301 or 30%) less likely to report 2 or more comorbid pains than whites (n = 6747 or 34%) (odds ratio = 0.91, P = .032; OR = 0.82, P < .001, respectively). This group also reported significantly lower ratings of self-rated health (P < .001). Differences in type of comorbid pain by age patterns were found.

CONCLUSIONS

Severe headache or migraine is often associated with other common pains, seldom existing alone. Two or more comorbid pains are common, similarly affecting gender and racial/ethnic groups.

Timing in the absence of supraspinal input III: regularly spaced cutaneous stimulation prevents and reverses the spinal learning deficit produced by peripheral inflammation

In the absence of brain input, spinal systems can adapt to new environmental relations. For example, spinally transected rats given a legshock each time the leg is extended exhibit a progressive increase in flexion duration that minimizes net shock exposure, a simple form of instrumental learning. This capacity for learning is modulated by prior stimulation; both variable shock and inflammation produce a lasting inhibition of learning. An extended exposure to fixed spaced shock has no adverse effect on learning and opposes the consequences of variable shock. The present studies expand on these findings and demonstrate that fixed stimulation ameliorates the impact of peripheral inflammation. Spinally transected rats were administered 900 fixed spaced legshocks before (Experiment 1) or 1,800 legshocks after (Experiment 2) a subcutaneous hindpaw injection of capsaicin. Learning was assessed 24 hr later. Treatment with fixed shock attenuated the capsaicin-induced inhibition of learning. These findings suggest that fixed stimulation promotes adaptive plasticity and may foster recovery after injury.

Polymorphism of the Glutamate Transporter Protein EAAT2 and Migraine Transformation into Chronic Daily Headache

Background and Purpose

The progression of migraine into chronic daily headache involves multiple risk factors, but the main contributor is not known. Glutamate is the major excitatory neurotransmitter in central sensitization, which is an important process in the pathogenesis of migraine transformation. The glutamate transporter protein excitatory amino acid transporter 2 (EAAT2) is the primary modulator of glutamatergic neurotransmission, and genetic polymorphisms of its gene, EEAT2, have been identified. The aim of this study was to determine the effect of EAAT2 polymorphisms on migraine transformation into chronic daily headache.

Methods

We included 74 migraine patients with episodic attack (M-E) and 59 migraine patients with chronic daily headache (M-CDH). After amplifying EAAT2 by polymerase chain reaction, we assessed its genotype frequencies based on restriction fragment length polymorphisms. We reclassified all migraine patients into two groups according to their EAAT2 genotype, either with the A allele (n=62) or without it (n=71), and compared the clinical variables between the two groups.

Results

The genotype frequencies of EAAT2 polymorphisms did not differ between the M-E and M-CDH groups. Comparison between EEAT2 genotypes revealed that the frequency of analgesic usage was significantly higher among migraine patients with the A allele (12.9±1.6 days/month) than in those without the A allele (8.1±1.2 days/month; p=0.019). The other clinical variables of migraine did not differ between the two groups.

Conclusions

The results suggest that EEAT2 polymorphism contributes to the tendency toward frequent analgesic usage in migraine patients. This implies a potential genetic influence on the progression of migraine into chronic daily headache through the development of medication-overuse headache.

Gene therapy for chronic neuropathic pain: how does it work and where do we stand today?

OBJECTIVES

Chronic neuropathic pain has been an enigma to physicians and researchers for decades. A better understanding of its pathophysiology has given us more insight into its various mechanisms and possible treatment options. We now have an understanding of the role of various ionic channels, biologically active molecules involved in pain, and also the intricate pain pathways where possible interventions might lead to substantial pain relief. The recent research on laboratory animals using virus-based vectors for gene transfer at targeted sites is very promising and may lead to additional human clinical trials. However, one needs to be aware that this "novel" approach is still in its infancy and that many of its details need to be further elucidated. The purpose of this article is to thoroughly review the current available literature and analyze the deficiencies in our current knowledge.

DESIGN

Literature review.

METHODS

After an extensive online literature search, a total of 133 articles were selected to synthesize a comprehensive review about chronic neuropathic pain and gene therapy in order to understand the concepts and mechanisms.

RESULTS

Most of the studies have shown benefits of gene therapy in animal models, and recently, phase 1 human trials using herpes simplex virus vector have started for intractable cancer pain.

CONCLUSION

Although animal data have shown safety and efficacy, and initial human trials have been promising, additional studies in humans are required to more completely understand the actual benefits and risks of using gene therapy for the treatment of chronic neuropathic pain.

Studies of locomotor network neuroprotection by the selective poly(ADP-ribose) polymerase-1 inhibitor PJ-34 against excitotoxic injury to the rat spinal cord in vitro

Delayed neuronal destruction after acute spinal injury is attributed to excitotoxicity mediated by hyperactivation of poly(ADP-ribose) polymerase-1 (PARP-1) that induces 'parthanatos', namely a non-apoptotic cell death mechanism. With an in vitro model of excitotoxicity, we have previously observed parthanatos of rat spinal cord locomotor networks to be decreased by a broad spectrum PARP-1 inhibitor. The present study investigated whether the selective PARP-1 inhibitor N-(6-oxo-5,6-dihydrophenanthridin-2-yl)-(N,N-dimethylamino)acetamide.HCl (PJ-34) not only protected networks from kainate-evoked excitotoxicity, but also prevented loss of locomotor patterns recorded as fictive locomotion from lumbar (L) ventral roots (VRs) 24 h later. PJ-34 (60 μm) blocked PARP-1 activation and preserved dorsal, central and ventral gray matter with maintained reflex activity even after a large dose of kainate. Fictive locomotion could not, however, be restored by either electrical stimulation or bath-applied neurochemicals (N-methyl-D-aspartate plus 5-hydroxytryptamine). A low kainate concentration induced less histological damage that was widely prevented by PJ-34. Nonetheless, fictive locomotion was observed in just over 50% of preparations whose histological profile did not differ (except for the dorsal horn) from those lacking such a rhythm. Our data show that inhibition of PARP-1 could amply preserve spinal network histology after excitotoxicity, with return of locomotor patterns only when the excitotoxic stimulus was moderate. These results demonstrated divergence between histological and functional outcome, implying a narrow borderline between loss of fictive locomotion and neuronal preservation. Our data suggest that either damage of a few unidentified neurons or functional network inhibition was critical for ensuring locomotor cycles.

Intrinsic membrane properties of spinal dorsal horn neurones modulate nociceptive information processing in vivo

The dorsal horn of the spinal cord is the first central relay where nociceptive inputs are processed. Based on the expression and modulation of intrinsic electrophysiological properties in in vitro slice preparations, dorsal horn neurones (DHNs) display different discharge patterns (tonic, plateau or rhythmic), which shape the neurone's response to sensory inputs. However, it is unclear whether intrinsic properties play any role in sensory processing in vivo. Using in vivo patch clamp recordings in the adult rat, we here examine whether these intrinsic properties are present, and to what extent they determine the DHN response to natural stimulation. We focused primarily on wide dynamic range neurones in deep laminae. These cells displayed a multicomponent peripheral receptive field, comprising an excitatory firing zone, a low-probability firing fringe, and adjacent inhibitory zones. Deep DHNs presented similar intrinsic properties to those observed in vitro, including plateau potentials. These plateaus, underlying high frequency accelerating discharges and after-discharges, were triggered by mechanical stimulation of the excitatory receptive field. Persistent activities induced by activation of plateau potentials were interrupted by stimulation of peripheral inhibitory zones. Moreover, we show that plateau activation is necessary for the expression of windup in response to repetitive, nociceptive stimulation. Finally, using the spinal nerve ligation model of neuropathy, we demonstrate a significant increase in the proportion of plateau neurones in deep dorsal laminae. Our data, therefore, establish that intrinsic amplification properties are expressed within intact spinal circuits and suggest their involvement in neuropathy-induced hyperexcitability of deep DHNs.

Blockade of adrenomedullin receptors reverses morphine tolerance and its neurochemical mechanisms

Adrenomedullin (AM) has been demonstrated to be involved in the development of opioid tolerance. The present study further investigated the role of AM in the maintenance of morphine tolerance, morphine-associated hyperalgesia and its cellular mechanisms. Intrathecal (i.t.) injection of morphine for 6 days induced a decline of its analgesic effect and hyperalgesia. Acute administration of the AM receptor antagonist AM(22-52) resumed the potency of morphine in a dose-dependent manner (12, 35.8 and 71.5 μg, i.t.). The AM(22-52) treatment also suppressed morphine tolerance-associated hyperalgesia. Furthermore, i.t. administration of AM(22-52) at a dose of 35.8 μg reversed the morphine induced-enhancement of nNOS (neuronal nitric oxide synthase) and CGRP immunoreactivity in the spinal dorsal horn and/or dorsal root ganglia (DRG). Interestingly, chronic administration of morphine reduced the expression of the endogenous opioid peptide bovine adrenal medulla 22 (BAM22) in small- and medium-sized neurons in DRG and this reduction was partially reversed by the administration of AM(22-52) (35.8 μg). These results suggest that the activation of AM receptors was involved in the maintenance of morphine tolerance mediating by not only upregulation of the pronociceptive mediators, nNOS and CGRP but also the down-regulation of pain-inhibiting molecule BAM22. Our data support the hypothesis that the level of both pronociceptive mediators and endogenous pain-inhibiting molecules has an impact on the potency of morphine analgesia. Targeting AM receptors is a promising approach to maintain the potency of morphine analgesia during chronic use of this drug.

Modulation of sensory neuron-specific receptors in the development of morphine tolerance and its neurochemical mechanisms

Prevention of opiate tolerance is a critical issue in pain management. The present study was designed to characterize the pharmacological properties of sensory neuron-specific receptors (SNSR; also known as Mas-related gene receptors, or Mrg) for their modulation in the development of morphine tolerance and to investigate the underlying mechanism(s). Daily coadministration of the SNSR agonist BAM8-22 at a dose of 0.01 or 0.001, but not 1.0, nmol with morphine (intrathecally, or i.t., 20 microg/day) for 6 days significantly decreased the development of morphine tolerance. Coadministration of BAM8-22 (i.t., 1.0 nmol) on days 1, 3, and 5 completely blocked tolerance to morphine-induced analgesia. Intermittent coadministration of the structurally dissimilar SNSR agonist (Tyr(6))-2-MSH-6-12 (MSH; 5 nmol) also produced similar modulation. Chronic administration of morphine (20 microg, i.t.) increased expression of neuronal nitric oxide synthase (nNOS) and calcitonin gene-related peptide (CGRP) in superficial layers of the spinal cord and dorsal root ganglia. All these increases were abolished when BAM8-22 or MSH was intermittently coadministered. Furthermore, intermittent administration of BAM8-22 inhibited morphine-induced increase in protein kinase C gamma (PKC gamma) in both membrane and cytosol of spinal dorsal horn neurons. These results suggest that moderate activation of SNSR modulated morphine tolerance by inhibition of the PKC signaling pathway, leading to abolishment of enhancement of nNOS and CGRP. As SNSR are uniquely located ina subset of small-sized neurons in dorsal root and trigeminal ganglia, intermittent combination of SNSR agonist could be a promising adjunct for sustained use of opiates without central nervous system side effects.

A role for protein kinase C-dependent upregulation of adrenomedullin in the development of morphine tolerance in male rats

Adrenomedullin (AM) belongs to calcitonin gene-related peptide (CGRP) family and is a pronociceptive mediator. This study investigated whether AM plays a role in the development of tolerance to morphine-induced analgesia. Repetitive intrathecal injection of morphine increased the expression of AM-like immunoreactivity (AM-IR) in the spinal dorsal horn and dorsal root ganglion (DRG) neurons. Ganglion explant culture study showed that this upregulation of AM-IR was μ-opioid receptor dependent through the use of another agonist, fentanyl, and a selective antagonist, CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)). The coadministration of the selective AM receptor antagonist AM(22-52) markedly attenuated the development of morphine tolerance, associated thermal hyperalgesia, and increase in AM-IR. A likely autocrine mechanism is supported by the finding that AM-IR is colocalized with AM receptor components in DRG neurons. Furthermore, opiate-induced increase in AM content was blocked by protein kinase C (PKC) inhibitors, whereas a PKC activator increased AM synthesis and release. A treatment with AM(22-52) also inhibited increases in the expression of CGRP-IR in the spinal cord and DRGs as well as in culture ganglion explants, whereas exposure to CGRP failed to alter AM content. Together, these results reveal that a sustained opiate treatment induces an upregulation of AM through the activation of μ-opioid receptors and the PKC signaling pathway. This phenomenon contributes to the development of tolerance to the antinociceptive effects of opiates at least partially via the upregulation of CGRP. Targeting AM and its receptors should be considered as a novel approach to preserve the analgesic potency of opiates during their chronic use.

Morphine exposure in early life increases nociceptive behavior in a rat formalin tonic pain model in adult life

Considering the importance of a deeper understanding of the effect throughout life of opioid analgesia at birth, our objective was to determine whether morphine administration in early life, once a day for 7 days in 8-day-old rats, alters the nociceptive response over the short (P16), medium (P30), and long term (P60) and to evaluate which system is involved in the altered nociceptive response. The nociceptive responses were assessed by the formalin test, and the behavior analyzed was the total time spent in biting and flicking of the formalin-injected hindpaw, recorded during the first 5 min (phase I) and from 15-30 min (phase II). The morphine group showed no change in nociceptive response at P16, but at P30 and P60, the nociceptive response was increased in phase I, and in both phases, respectively. At P30 and P60, the animals received a non-steroidal anti-inflammatory drug (indomethacin) or NMDA receptor antagonist (ketamine) 30 min before the formalin test. The increase in the nociceptive response was completely reversed by ketamine, and partially by indomethacin. These results indicate that early morphine exposure causes an increase in the nociceptive response in adult life. It is possible that this lower nociception threshold is due to neuroadaptations in nociceptive circuits, such as the glutamatergic system. Thus, this work demonstrates the importance of evaluating clinical consequences related to early opioid administration and suggests a need for a novel design of agents that may counteract opiate-induced neuroplastic changes.

Stimulation of dorsal root afferents increases the excitability of ascending sensory axons in the isolated spinal cord of mature mice

The phenomenon of windup has often been used to assess excitability increases of spinal neurons induced by repetitive stimulation of nociceptive afferents. Windup has been studied in individual spinal cord neurons and in spinal motor reflexes neither of which accurately reflect the forward transmission of nociceptive signals to the brain. In addition, most in vitro studies of spinal windup have been conducted on immature or juvenile animals and it is challenging to extrapolate these results to the adult spinal cord. In the present study, we have used an in vitro whole spinal cord preparation from functionally mature mice (up to 8 weeks old) to record windup activity in ascending axons in the mid-thoracic region evoked by electrical stimulation of a lumbar or sacral dorsal root. Windup responses were observed in axons in the ipsi- and contralateral dorsolateral funiculus (iDLF and cDLF) and in the contralateral ventrolateral funiculus (cVLF). No windup responses were evoked in postsynaptic axons of the ipsilateral dorsal columns (iDC) and no postsynaptic responses were elicited in the ipsilateral ventrolateral funiculus (iVLF) or contralateral dorsal columns (cDC). Between 40% and 45% of all axons in the DLF and cVLF that responded to a single dorsal root stimulus also showed windup. The NMDA receptor antagonist MK-801 reversibly blocked such windup responses. These results illustrate that windup can be consistently recorded from ascending pathways in the mature spinal cord in vitro but also show that windup can only be elicited in a proportion of sensory axons projecting through some, but not all, ascending spinal cord pathways.

NMDA Channels Together With L-Type Calcium Currents and Calcium-Activated Nonspecific Cationic Currents Are Sufficient to Generate Windup in WDR Neurons

Windup is characterized as a frequency-dependent increase in the number of evoked action potentials in dorsal horn neurons in response to electrical stimulation of afferent C-fibers. This phenomenon was first described in the mid-60s, but the core mechanisms behind it still remain elusive. Several factors affecting its dynamics have been identified, but the distinction between modulating mechanisms from generating mechanisms is not always clear. Several mechanisms contribute to the excitation of dorsal horn neurons exhibiting windup, and one of our main aims was to help making this distinction. The approach presented here relies on mathematical and computational analysis to study the mechanism(s) underlying windup. From experimentally obtained windup profiles, we extract the time scale of the facilitation mechanisms that may support the characteristics of windup. Guided by these values and using simulations of a biologically realistic compartmental model of a wide dynamic range (WDR) neuron, we are able to assess the contribution of each mechanism for the generation of action potentials windup. We show that the key mechanisms giving rise to windup is the temporal summation of N-methyl-d-aspartate (NMDA) long-lasting postsynaptic responses taking place on top of a membrane potential cumulative depolarization. Calcium-activated nonspecific cationic currents driven by calcium influx from L-type calcium channels and synaptic currents support this cumulative depolarization and plateau formation in WDR neuron membrane potential. The effects of different nonhomogeneous stimulation protocols are explored, and their important role in clarifying many aspects of the windup generation is shown. The models are used to produce several predictions that can be tested experimentally.

Multiple firing patterns in deep dorsal horn neurons of the spinal cord: computational analysis of mechanisms and functional implications

Deep dorsal horn relay neurons (dDHNs) of the spinal cord are known to exhibit multiple firing patterns under the control of local metabotropic neuromodulation: tonic firing, plateau potential, and spontaneous oscillations. This work investigates the role of interactions between voltage-gated channels and the occurrence of different firing patterns and then correlates these two phenomena with their functional role in sensory information processing. We designed a conductance-based model using the NEURON software package, which successfully reproduced the classical features of plateau in dDHNs, including a wind-up of the neuronal response after repetitive stimulation. This modeling approach allowed us to systematically test the impact of conductance interactions on the firing patterns. We found that the expression of multiple firing patterns can be reproduced by changes in the balance between two currents (L-type calcium and potassium inward rectifier conductances). By investigating a possible generalization of the firing state switch, we found that the switch can also occur by varying the balance of any hyperpolarizing and depolarizing conductances. This result extends the control of the firing switch to neuromodulators or to network effects such as synaptic inhibition. We observed that the switch between the different firing patterns occurs as a continuous function in the model, revealing a particular intermediate state called the accelerating mode. To characterize the functional effect of a firing switch on information transfer, we used correlation analysis between a model of peripheral nociceptive afference and the dDHN model. The simulation results indicate that the accelerating mode was the optimal firing state for information transfer.

Pregabalin for the management of fibromyalgia syndrome

This last article in a three-part series on approved medications for managing fibromyalgia syndrome (FMS) reviews pregabalin (Lyrica^®). Pregabalin was the first drug approved for FMS management and, as an anticonvulsant, differs from the other approved agents that are antidepressants. Pregabalin inhibits presynaptic excitatory neurotransmitter release by blocking α₂δ calcium channels. Five randomized, placebo-controlled trials have demonstrated pregabalin reduces pain and improves sleep and health-related quality of life in FMS patients. While indicated dosing is 300–450 mg divided twice daily, initial dosing of 25–50 mg at night is recommended owing to side effects including somnolence, dizziness, and cognitive dysfunction. Since side effects such as weight gain and peripheral edema are dose-related, uptitration in weekly increments based on tolerability and therapeutic response is recommended. Due to its lack of protein binding and negligible hepatic metabolism, pregabalin can be safely combined with other medications and used in patients with renal failure when the dose is appropriate. Pregabalin may worsen sedation when combined with central nervous system depressants. Pregabalin should be discontinued gradually. Pregabalin-treated patients should be monitored for the emergence or worsening of depression, suicidal thoughts or behavior. Pregabalin in combination with the other approved medications may be synergistic in treating FMS.

The pathophysiology, diagnosis and treatment of fibromyalgia

Research in fibromyalgia has increased understanding of the possible genetic and environmental factors that could be involved in the etiology of fibromyalgia. There is now substantial evidence for augmentation of central pain processing in fibromyalgia. Because the clinical presentation of fibromyalgia is heterogeneous, treatment recommendations must be individualized for each patient. The rapid growth of trials in fibromyalgia in recent years has resulted in new evidence-based approaches to pharmacological and nonpharmacological treatment.

Allodynia in migraine: association with comorbid pain conditions

BACKGROUND

Cutaneous allodynia (CA) in migraine is a clinical manifestation of central nervous system sensitization. Several chronic pain syndromes and mood disorders are comorbid with migraine. In this study we examine the relationship of migraine-associated CA with these comorbid conditions. We also evaluate the association of CA with factors such as demographic profiles, migraine characteristics, and smoking status that may have an influence on the relationships of CA to pain and mood.

METHODS

Data are from a cross-sectional multicenter study of comorbid conditions in persons seeking treatment in headache clinics. Diagnosis of migraine was determined by a physician based on the International Classification of Headache Disorders-II criteria. Participants completed a self-administered questionnaire ascertaining sociodemographics, migraine-associated allodynia, physician-diagnosed comorbid medical and psychiatric disorders, headache-related disability, current depression, and anxiety.

RESULTS

A total of 1413 migraineurs (mean age = 42 years, 89% women) from 11 different headache treatment centers completed a survey on the prevalence of comorbid conditions. Aura was reported by 38% and chronic headache by 35% of the participants. Sixty percent of the study population reported at least one migraine-related allodynic symptom, 10% reported > or =4 symptoms. Symptoms of CA were associated with female gender, body mass index, current smoking, presence of aura, chronic headaches, transformed headaches, severe headache-related disability, and duration of migraine illness from onset. The prevalence of self-reported physician diagnosis of comorbid pain conditions (irritable bowel syndrome, chronic fatigue syndrome, fibromyalgia) and psychiatric conditions (current depression and anxiety) was also associated with symptoms of CA. Adjusted ordinal regression indicated a significant association between number of pain conditions and severity of CA (based on symptom count). Adjusting for sociodemographics, migraine characteristics, and current depression and anxiety, the likelihood of reporting symptoms of severe allodynia was much higher in those with 3 or more pain conditions (odds ratio = 3.03, 95% confidence interval: 1.78-5.17), and 2 pain conditions (odds ratio = 2.67, 95% confidence interval: 1.78-4.01) when compared with those with no comorbid pain condition.

CONCLUSION

Symptoms of CA in migraine were associated with current anxiety, depression, and several chronic pain conditions. A graded relationship was observed between number of allodynic symptoms and the number of pain conditions, even after adjusting for confounding factors. This study also presents the novel association of CA symptoms with younger age of migraine onset, and with cigarette smoking, in addition to confirming several previously reported findings.

Effects of neurokinin-1 receptor agonism and antagonism in the rostral ventromedial medulla of rats with acute or persistent inflammatory nociception

The rostral ventromedial medulla (RVM), a central relay in the bulbospinal pathways that modulate nociception, contains high concentrations of substance P (Sub P) and neurokinin-1 (NK1) receptors. However, the function of Sub P in the RVM is poorly understood. This study characterized the actions of Sub P in the RVM in the absence of injury and then used two NK1 receptor antagonists, L-733,060 and L-703, 606, to probe the role of endogenously released Sub P in the development and maintenance of persistent inflammatory nociception of immune or neurogenic origin. In uninjured rats, microinjection of Sub P in the RVM produced a transient thermal antinociception that was attenuated by pretreatment with L-733,060 or L-703,606. It did not alter threshold to withdrawal from tactile stimulation with von Frey filaments. Microinjection of the antagonists alone did not alter paw withdrawal latency (PWL) or threshold suggesting that Sub P is not tonically released in the RVM in the absence of injury. However, microinjection of either antagonist in the RVM was sufficient to reverse heat hyperalgesia 4 h, 4 days or 2 weeks after intraplantar (ipl) injection of complete Freund's adjuvant (CFA). Antagonism of NK1 receptors in the RVM did not prevent or reverse tactile hypersensitivity induced by CFA, but did attenuate that produced by capsaicin. NK1 receptor antagonism did not prevent the development of thermal hyperalgesia, tactile hypersensitivity or spontaneous pain behaviors induced by mustard oil (MO). The results suggest that Sub P has bimodal actions in the RVM and that following inflammatory injury, it can play a critical role as a pronociceptive agent in the development and maintenance of hyperalgesia and tactile hypersensitivity. However, its actions are highly dependent on the stimulus modality and the type of injury, and this may be an additional basis for the poor efficacy of NK1 receptor antagonists in clinical trials.

Demonstration of cutaneous allodynia in association with chronic pelvic pain

Pelvic pain is a common condition that is associated with dysmenorrhea and endometriosis. In some women the severe episodes of cyclic pain change and the resultant pain becomes continuous and this condition becomes known as Chronic Pelvic Pain. This state can be present even after the appropriate medical or surgical therapy has been instituted. It can be associated with pain and tenderness in the muscles of the abdomen wall and intra-pelvic muscles leading to severe dyspareunia. Additional symptoms of irritable bowel and interstitial cystitis are common. A common sign of the development of this state is the emergence of cutaneous allodynia which emerges from the so-called viscero-somatic reflex. A simple bedside test for the presence of cutaneous allodynia is presented that does not require excessive time or special equipment. This test builds on previous work associated with changes in sensation related to gall bladder function and the viscera-somatic reflex(1;2). The test is undertaken with the subject s permission after an explanation of how the test will be performed. Allodynia refers to a condition in which a stimulus that is not normally painful is interpreted by the subject as painful. In this instance the light touch associated with a cotton-tipped applicator would not be expected to be painful. A positive test is however noted by the woman as suddenly painful or suddenly sharp. The patterns of this sensation are usually in a discrete pattern of a dermatome of the nerves that innervate the pelvis. The underlying pathology is now interpreted as evidence of neuroplasticity as a consequence of severe and repeating pain with changes in the functions of the dorsal horns of the spinal cord that results in altered function of visceral tissues and resultant somatic symptoms(3). The importance of recognizing the condition lies in an awareness that this process may present coincidentally with the initiating condition or after it has been treated. It also permits the clinician to evaluate the situation from the perspective that alternative explanations for the pain may be present that may not require additional surgery.

Spinal NMDA NR1 subunit expression following transient TNBS colitis

BACKGROUND

N-methyl-D-aspartic acid (NMDA) receptors play an important role in the development of hypersensitivity to visceral and somatic stimuli following inflammation or tissue injury. Our objective was to investigate the role of NMDA NR1 receptors in the spinal cord (T10-L1; L4-S1) of a subset of rats that remain hypersensitive following the histological resolution of TNBS-induced colitis compared to saline treated rats and rats that had recovered both behaviorally and histologically. We hypothesized that NMDA NR1 subunit expression mediates hypersensitivity following transient TNBS colitis.

METHODS

Male Sprague-Dawley rats (150 g-250 g) received 20 mg/rat intracolonic trinitrobenzene sulfonic acid (TNBS) in 50% ethanol or saline. Animals underwent nociceptive visceral/somatic pain testing 16 weeks after resolution of TNBS colitis. Animals were sacrificed and their spinal cords (T10-L1; L4-S1) were retrieved and 2-dimensional polyacrylamide gel electrophoresis and immunohistocytochemistry techniques were used to investigate spinal-NMDA receptor expression.

RESULTS

NR1(001) was the only NMDA NR1 receptor subunit that was expressed in recovered and control rats, whereas hypersensitive animals expressed NR1(011) and NR1(111) as well as NR1(001) subunits. Immunohistochemistry analysis demonstrated increased expression of NMDA NR1-N1, C1, and C2-plus expression in laminae I and II of the spinal cord (T10-L1; L4-S1) in hypersensitive rats but not in recovered/control rats.

CONCLUSIONS

Selective increases in the expression of the NMDA NR1 splice variants occur in hypersensitive rats following resolution of TNBS colitis. This suggests that the NMDA NR1 receptor plays an important role in the development of neuronal plasticity and central sensitization. The recombination of NR1 splice variants may serve as a key functional protein that maintains hypersensitivity following resolution of TNBS colitis.

Recent findings on the pathogenesis of bronchial asthma. Part I. Asthma as a neurohumoral disorder, a pathological vago-vagal axon reflex

The novel data on the pathogenesis of asthma are summarized in this three-part review. Its immunological background is well established but it is more than an immunological disorder. Multiple lines indicate that both peripheral and central neural mechanisms are also involved in the pathogenesis of asthma. In the present first part of the review asthma is described as vago-vagal axon reflex brought about by multiple positive feed-back mechanisms, receptor upregulation, wind-up, phenotypic switch and formation of a pathological conditioned reflex. In the coming second part the main dispositional (mostly hormonal) and external contributing factors are reviewed, while the third part deals with the role of inheritance, i.e., with gene alleles leading to enhanced production of mediators of asthma.

Unilateral intra-perilymphatic infusion of substance P enhances ipsilateral vestibulo-ocular reflex gains in the sinusoidal rotation test

Previous studies have reported localization of substance P (SP) within the inner ear and that SP exists abundantly within vestibular endorgans. While SP's functional role in the inner ear remains unclear, SP can act as a neuromodulator in the CNS and directly influences neuronal excitability. We hypothesized that SP might influence neuronal excitability within the vestibular periphery. The present study used the sinusoidal rotation test to investigate the influence of SP after its local application in the guinea pig unilateral inner ear. A tiny hole was made adjacent to the round window in the right ears of Hartley white guinea pigs that had normal tympanic membranes and Preyer reflexes. An osmotic pump infused SP (10(-4)M, 10(-3)M, and 10(-2)M), neurokinin-1 (NK-1) receptor antagonist (10(-3)M) alone, or SP (10(-3)M)+NK-1 receptor antagonist (10(-3)M) through this hole, with rotation tests performed before, and 12h and 24h after the treatment. Results were used to calculate the vestibulo-ocular reflex (VOR) gains. After administration of 10(-3)M and 10(-2)M SP, significant increases in the VOR gains were noted at 12h after treatment, with these gains disappearing by 24h after treatment. This increase was not observed when there was simultaneous NK-1 receptor antagonist administration. There were also no changes in the VOR gains noted after administration of 10(-4)M SP or the NK-1 receptor antagonist alone. These results indicate the possibility that SP may act on vestibular endorgans as an excitatory factor via the NK-1 receptors.

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.

Age-related difference in nociceptive behavior between SAMP6 and SAMR1 strains

Senescence accelerated prone mouse 6 (SAMP6) mice have been known to be a model for accelerated aging. Compared with the normal control senescence accelerated resistant mouse 1 (SAMR1) mice, although the SAMP6 mice have normal bone mass at 4 months, they exhibit a significantly lower bone mass at 8 months. It was recently reported that SAMP6 has memory deficit at 4 months of age, indicating that the change of nervous function might be already detected at 4 months of age. To assess whether SAMP6 mice exhibit an age-related abnormality of nociceptive transmission, we examined a battery of tests using the von Frey test for mechanically induced response, the hot plate test for thermally induced response, and the formalin paw test for chemically induced response. SAMP6 and SAMR1 showed similar response patterns in the von Frey test and the hot plate test. In the formalin paw test, 1-month-old SAMP6 and SAMR1 had similar responses, while 4-month-old SAMP6 exhibited attenuated phase 2 response, but normal phase 1 response. These findings indicate that onset of age-related phenotypes in SAMP6 differs in different tissues. SAMP6 could be useful to delineate the involvement of age-related nociceptive mechanisms.

Classification, causation and treatment of masticatory myogenous pain and dysfunction

This article focuses on chronic myogenous pains affecting the masticatory muscles, including focal myalgia, regional myalgia, myofascial pain, and fibromyalgia. The probable mechanisms are discussed and treatment options, including self-directed treatment, physical medicine modalities, and several types of pharmacologic agents, are presented.

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.

Involvement of post-translational modification of neuronal plasticity-related proteins in hyperalgesia revealed by a proteomic analysis

To clarify roles of an endogenous pain modulatory system of the central nervous system (CNS) in hyperalgesia, we tried to identify qualitative and quantitative protein changes by a proteomic analysis using an animal model of hyperalgesia. Specifically, we first induced functional hyperalgesia on male Wistar rats by repeated cold stress (specific alternation of rhythm in temperature, SART). We then compared proteomes of multiple regions of CNS and the dorsal root ganglion between the hyperalgetic rats and non-treated ones by 2-D PAGE in the pI range of 4.0-7.0. We found that SART changed the proteomes prominently in the mesencephalon and cerebellum. We thus analyzed the two brain regions in more detail using gels with narrower pI ranges. As a result, 29 and 23 protein spots were significantly changed in the mesencephalon and the cerebellum, respectively. We successfully identified 12 protein spots by a MALDI-TOF/TOF MS and subsequent protein database searching. They included unc-18 protein homolog 67K, collapsin response mediator protein (CRMP)-2 and CRMP-4, which were reported to be involved in neurotransmitter release or axon elongation. Interestingly, mRNA expression levels of these three proteins were not changed significantly by the induction of hyperalgesia. Instead, we found that the detected changes in the protein spots are caused by the post-translational modification (PTM) of proteolysis or phosphorylation. Taken together, development of the hyperalgesia would be linked to PTM of these three CNS proteins. PTM regulation may be one of the useful ways to treat hyperalgesia.

Decreased nociceptive sensitization in mice lacking the fragile X mental retardation protein: role of mGluR1/5 and mTOR

Fragile X mental retardation is caused by silencing of the gene (FMR1) that encodes the RNA-binding protein (FMRP) that influences translation in neurons. A prominent feature of the human disorder is self-injurious behavior, suggesting an abnormality in pain processing. Moreover, FMRP regulates group I metabotropic glutamate receptor (mGluR1/5)-dependent plasticity, which is known to contribute to nociceptive sensitization. We demonstrate here, using the Fmr1 knock-out (KO) mouse, that FMRP plays an important role in pain processing because Fmr1 KO mice showed (1) decreased (approximately 50%) responses to ongoing nociception (phase 2, formalin test), (2) a 3 week delay in the development of peripheral nerve injury-induced allodynia, and (3) a near absence of wind-up responses in ascending sensory fibers after repetitive C-fiber stimulation. We provide evidence that the behavioral deficits are related to a mGluR1/5- and mammalian target of rapamycin (mTOR)-mediated mechanism because (1) spinal mGluR5 antagonism failed to inhibit the second phase of the formalin test, and we observed a marked reduction in nociceptive response to an intrathecal injection of an mGluR1/5 agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) in Fmr1 KO mice; (2) peripheral DHPG injection had no effect in KO mice yet evoked thermal hyperalgesia in wild types; and (3) the mTOR inhibitor rapamycin inhibited formalin- and DHPG-induced nociception in wild-type but not Fmr1 KO mice. These experiments show that translation regulation via FMRP and mTOR is an important feature of nociceptive plasticity. These observations also support the hypothesis that the persistence of self-injurious behavior observed in fragile X mental retardation patients could be related to deficits in nociceptive sensitization.

NMDA Receptors and Colitis: Basic Science and Clinical Implications

During the last decade, research focusing primarily on alterations in the peripheral and central nervous system has improved our understanding of the pathophysiological mechanisms of chronic visceral pain. These studies have demonstrated significant physiological changes following injury to the viscera in the firing patterns of both primary afferent neurons that transmit nociceptive information from the viscera and in central neurons that process the nociceptive information. A number of receptors, neurotransmitters, cytokines, and second messenger systems in these neurons have been implicated in the enhancement of visceral nociception. N-methyl-d-aspartic acid (NMDA) receptors play an important role in chronic visceral pain and hypersensitivity that is present in the setting of colonic inflammation. NMDA receptors are found in the peripheral nervous system as well as the central terminal of primary afferent neurons and have been shown to play an important role in regulating the release of nociceptive neurotransmitters. Recent work has demonstrated the presence of NMDA receptors in the enteric nervous system. In this article, we will discuss more recent evidence of the role of NMDA receptors in visceral pain associated with colitis.

Dorsal horn NK1-expressing neurons control windup of downstream trigeminal nociceptive neurons

Windup is a progressive, frequency-dependent increase in the excitability of trigeminal and spinal dorsal horn wide dynamic range (WDR) nociceptive neurons to repetitive stimulation of primary afferent nociceptive C-fibers. Superficial dorsal horn neurokinin 1 receptor (NK1R)-expressing neurons were recently shown to regulate sensitization of WDR nociceptive neurons through activation of a defined spino-bulbo-spinal loop. However, the windup of WDR nociceptive neurons was not regulated through this loop. In the present study, we sought to identify the alternative circuit activated by dorsal horn NK1Rs that mediates WDR neuron windup. As a model we used the rat spinal trigeminal nucleus, in which the subnucleus oralis (Sp5O) contains a pool of WDR neurons that receive their nociceptive C-input indirectly via interneurons located in the medullary dorsal horn (MDH). First, we found that intravenous injection of NK1R antagonists (SR140333 and RP67580) produced a reversible inhibition of Sp5O WDR neuron windup. Second, we anatomically identified in the MDH lamina III a subpopulation of NK1R-expressing local interneurons that relay nociceptive information from the MDH to downstream Sp5O neurons. Third, using microinjections of NK1R antagonists during in vivo electrophysiological recordings from Sp5O WDR neurons, we showed that WDR neuron windup depends on activation of NK1Rs located in the MDH laminae I-III. We conclude that, in contrast to central sensitization that is controlled by a spino-bulbo-spinal loop, Sp5O WDR neuron windup is regulated through a local circuit activated by MDH lamina III NK1Rs.

Regulation of Na(v)1.2 channels by brain-derived neurotrophic factor, TrkB, and associated Fyn kinase

Voltage-gated sodium channels are responsible for action potential initiation and propagation in neurons, and modulation of their function has an important impact on neuronal excitability. Sodium channels are regulated by a Src-family tyrosine kinase pathway, and this modulation can be reversed by specifically bound receptor phosphoprotein tyrosine phosphatase-beta. However, the specific tyrosine kinase and signaling pathway are unknown. We found that the sodium channels in rat brain interact with Fyn, one of four Src-family tyrosine kinases expressed in the brain. Na(V)1.2 channels and Fyn are localized together in the axons of cultured hippocampal neurons, the mossy fibers of the hippocampus, and cell bodies, dendrites, and axons of neurons in many other brain areas, and they coimmunoprecipitate with Fyn from cotransfected tsA-201 cells. Coexpression of Fyn with Na(V)1.2 channels decreases sodium currents by increasing the rate of inactivation and causing a negative shift in the voltage dependence of inactivation. Reconstitution of a signaling pathway from brain-derived neurotrophic factor (BDNF) to sodium channels via the tyrosine receptor kinase B (TrkB)/p75 neurotrophin receptor and Fyn kinase in transfected cells resulted in an increased rate of inactivation of sodium channels and a negative shift in the voltage dependence of inactivation after treatment with BDNF. These results indicate that Fyn kinase is associated with sodium channels in brain neurons and can modulate Na(V)1.2 channels by tyrosine phosphorylation after activation of TrkB/p75 signaling by BDNF.

Increased Levels of Neurotrophins Are Not Specific for Chronic Migraine: Evidence From Primary Fibromyalgia Syndrome

All data obtained in experimental animal pain models support the role of nerve growth factor (NGF) as a putative candidate intervening in the pathogenesis of chronic pain, including chronic daily headache (CDH). Few studies have been carried out to establish its role in maintaining pain states in humans. The present study was aimed at investigating cerebrospinal fluid (CSF) levels of NGF and brain-derived neurotrophic factor (BDNF), both measured by sensitive immunoassay, in 20 chronic migraine (CM) patients and 20 patients affected by primary fibromyalgia syndrome (PFMS), compared with those of 20 age-matched control subjects. Significantly higher levels of both neurotrophins and glutamate were found. A significantly positive correlation emerged between CSF values of BDNF and those of NGF (r = .61, P < .001; r = .53, P < .01) and glutamate (r = .44, P < .02; r = .51, P < .01) in CM and PFMS patients, respectively. These findings suggest the possibility of a NGF-mediated up-regulation of BDNF involved in the pathophysiological events underlying long-term neuroplastic changes in persistent chronic painful conditions, such as CM and fibromyalgia. NGF might indirectly exert its effect through enhancing glutamatergic transmission via BDNF. The above mechanisms could account for sustained central sensitization in both chronic pain states.

PERSPECTIVE

This article presents findings of higher NGF and BDNF levels correlated to increased glutamate levels in the CSF of both chronic migraine and fibromyalgia patients. This opens new insights into the pathogenic mechanisms of chronic pain and offers clinicians new therapeutic perspectives targeting the above mechanisms in both painful disorders.