Nerve growth factor and ATP excite different neck muscle nociceptors in anaesthetized mice

Research Progress on the Experimental Model and Underlying Mechanistic Studies of Tension-Type Headaches

PURPOSE OF REVIEW

Tension-type headaches (TTH) significantly diminish patients' quality of life and increase absenteeism, thereby imposing a substantial economic burden. Animal models are essential tools for studying disease mechanisms and drug development. However, until now, little focus has been placed on summarizing the animal models of TTH and associated mechanistic studies. This narrative review discusses the current animal models of TTH and related mechanistic studies to provide insights into the pathophysiological mechanisms of and treatments for TTH.

RECENT FINDINGS

The primary method for constructing an animal model of TTH involves injecting a solution of pain relievers, such as adenosine triphosphate, nerve growth factor, or a high concentration of salt solution, into the neck to initiate harmful cervical muscle responses. This model enables the examination of the interaction between peripheral muscles and central sensitization, which is crucial for understanding the pathophysiology of TTH. Mechanistic studies based on this model have investigated the effect of the P2X receptor antagonist, P2X7 receptor blockade, the P2Y1 receptor agonist 2-MESADP, P2Y1 receptor antagonist MRS2179, nitric oxide synthase inhibitors, and acetylsalicylic acid. Despite notable advancements, the current model of TTH has limitations, including surgical complexity and the inability to replicate chronic tension-type headache (CTTH). To gain a more comprehensive understanding and develop more effective treatment methods, future studies should focus on simplifying surgical procedures, examining other predisposing factors, and establishing a model for chronic TTH. This will offer a deeper insight into the pathophysiological mechanism of TTH and pave the way for improved treatment approaches.

Role of ATP in migraine mechanisms: focus on P2X3 receptors

Migraine is a major health burden worldwide with complex pathophysiology and multifarious underlying mechanisms. One poorly understood issue concerns the early steps in the generation of migraine pain. To elucidate the basic process of migraine pain further, it seems useful to consider key molecular players that may operate synergistically to evoke headache. While the neuropeptide CGRP is an important contributor, we propose that extracellular ATP (that generally plays a powerful nociceptive role) is also a major component of migraine headache, acting in concert with CGRP to stimulate trigeminal nociceptive neurons. The aim of the present focused review is to highlight the role of ATP activating its P2X3 membrane receptors selectively expressed by sensory neurons including their nerve fiber terminals in the meninges. Specifically, we present data on the homeostasis of ATP and related purines in the trigeminovascular system and in the CNS; the basic properties of ATP signalling at peripheral and central nerve terminals; the characteristics of P2X3 and related receptors in trigeminal neurons; the critical speed and persistence of P2X3 receptor activity; their cohabitation at the so-called meningeal neuro-immune synapse; the identity of certain endogenous agents cooperating with ATP to induce neuronal sensitization in the trigeminal sensory system; the role of P2X3 receptors in familial type migraine; the current state of P2X3 receptor antagonists and their pharmacological perspectives in migraine. It is proposed that the unique kinetic properties of P2X3 receptors activated by ATP offer an interesting translational value to stimulate future studies for innovative treatments of migraine pain.

Characterisation of One Class of Group III Sensory Neurons Innervating Abdominal Muscles of the Mouse

Group III/IV striated muscle afferents are small diameter sensory neurons that play important roles in reflexes and sensation. To date, the morphological features of physiologically characterised group III/IV muscular afferents have not been identified. Here, the electrophysiological and morphological characteristics of sensory neurons innervating striated muscles of the mouse abdominal wall were investigated, ex vivo. Extracellular recordings were made from subcostal nerve trunks innervating the muscles. A distinctive class of mechanosensitive afferents was identified by a combination of physiological features including sensitivity to local compression, saturating response to graded stretch and, in most cases, absence of spontaneous firing. Studies were restricted to these distinctive units. These units had conduction velocities averaging 14 ± 4 m/s (range: 8-20 m/s, n = 7); within the range of group III fibres in mice. Von Frey hairs were used to map receptive fields, which covered an area of 0.36 ± 0.18 mm² (n = 7). In 7 preparations, biotinamide filling of recorded nerve trunks revealed a single axon in the marked receptive field, with distinctive axonal branching and terminations meandering through the connective tissue sandwiched between two closely associated muscle layers. These axons were not immunoreactive for CGRP (n = 7) and were not activated by application of capsaicin (1 µM, n = 14). All of these afferents were strongly activated by a "metabolite mix" containing lactate, adenosine triphosphate and reduced pH. Responses to mechanical stimuli and to metabolites were additive. We have characterised a distinctive class of mechano- and chemo-sensitive group III afferent endings associated with connective tissue close to muscle fibres.

Sex differences in primary muscle afferent sensitization following ischemia and reperfusion injury

BACKGROUND

Chronic pain conditions are more prevalent in women, but most preclinical studies into mechanisms of pain generation are performed using male animals. Furthermore, whereas group III and IV nociceptive muscle afferents provoke central sensitization more effectively than their cutaneous counterparts, less is known about this critical population of muscle nociceptors. Here, we compare the physiology of individual muscle afferents in uninjured males and females. We then characterize the molecular, physiological, and behavioral effects of transient ischemia and reperfusion injury (I/R), a model we have extensively studied in males and in females.

METHODS

Response properties and phenotypes to mechanical, thermal, and chemical stimulation were compared using an ex vivo muscle/nerve/dorsal root ganglia (DRG)/spinal cord recording preparation. Analyses of injury-related changes were also performed by assaying evoked and spontaneous pain-related behaviors, as well as mRNA expression of the affected muscle and DRGs. The appropriate analyses of variance and post hoc tests (with false discovery rate corrections when needed) were performed for each measure.

RESULTS

Females have more mechanically sensitive muscle afferents and show greater mechanical and thermal responsiveness than what is found in males. With I/R, both sexes show fewer cells responsive to an innocuous metabolite solution (ATP, lactic acid, and protons), and lower mechanical thresholds in individual afferents; however, females also possess altered thermal responsiveness, which may be related to sex-dependent changes in gene expression within the affected DRGs. Regardless, both sexes show similar increases in I/R-induced pain-like behaviors.

CONCLUSIONS

Here, we illustrate a unique phenomenon wherein discrete, sex-dependent mechanisms of primary muscle afferent sensitization after ischemic injury to the periphery may underlie similar behavioral changes between the sexes. Furthermore, although the group III and IV muscle afferents are fully developed functionally, the differential mechanisms of sensitization manifest prior to sexual maturity. Hence, this study illustrates the pressing need for further exploration of sex differences in afferent function throughout the lifespan for use in developing appropriately targeted pain therapies.

Peripheral Mechanisms of Ischemic Myalgia

Musculoskeletal pain due to ischemia is present in a variety of clinical conditions including peripheral vascular disease (PVD), sickle cell disease (SCD), complex regional pain syndrome (CRPS), and even fibromyalgia (FM). The clinical features associated with deep tissue ischemia are unique because although the subjective description of pain is common to other forms of myalgia, patients with ischemic muscle pain often respond poorly to conventional analgesic therapies. Moreover, these patients also display increased cardiovascular responses to muscle contraction, which often leads to exercise intolerance or exacerbation of underlying cardiovascular conditions. This suggests that the mechanisms of myalgia development and the role of altered cardiovascular function under conditions of ischemia may be distinct compared to other injuries/diseases of the muscles. It is widely accepted that group III and IV muscle afferents play an important role in the development of pain due to ischemia. These same muscle afferents also form the sensory component of the exercise pressor reflex (EPR), which is the increase in heart rate and blood pressure (BP) experienced after muscle contraction. Studies suggest that afferent sensitization after ischemia depends on interactions between purinergic (P2X and P2Y) receptors, transient receptor potential (TRP) channels, and acid sensing ion channels (ASICs) in individual populations of peripheral sensory neurons. Specific alterations in primary afferent function through these receptor mechanisms correlate with increased pain related behaviors and altered EPRs. Recent evidence suggests that factors within the muscles during ischemic conditions including upregulation of growth factors and cytokines, and microvascular changes may be linked to the overexpression of these different receptor molecules in the dorsal root ganglia (DRG) that in turn modulate pain and sympathetic reflexes. In this review article, we will discuss the peripheral mechanisms involved in the development of ischemic myalgia and the role that primary sensory neurons play in EPR modulation.

ATP-sensitive muscle afferents activate spinal trigeminal neurons with meningeal afferent input in rat - pathophysiological implications for tension-type headache

BACKGROUND

Tension-type headache and other primary headaches may be triggered or aggravated by disorders of pericranial muscles, which is possibly due to convergent or collateral afferent input from meningeal and muscular receptive areas. In rodent models high extracellular concentrations of ATP caused muscle nociception and central sensitization of second order neurons. In a rat model of meningeal nociception we asked if spinal trigeminal activity induced by ATP can be modulated by local anaesthesia of distinct muscles.

METHODS

Ongoing activity was recorded from spinal trigeminal neurons with afferent input from the cranial dura mater, the temporal muscle and neck muscles. The stable ATP analogue α,β-methylene adenosine 5'-triphosphate (α,β-meATP, 10 mM) was injected into the ipsilateral temporal muscle, 30 min later followed by injection of local anaesthetics (lidocaine, 2 %) into the ipsilateral neck muscles and/or the temporal muscle.

RESULTS

Injection of α,β-meATP into the temporal muscle caused progressive increase in ongoing activity of most of the spinal trigeminal neurons within 30 min. Injection of lidocaine into the neck muscles and/or the temporal muscle reduced this activation to previous levels within 10 min.

CONCLUSIONS

Distinct spinal trigeminal neurons processing meningeal nociceptive information are under the control of convergent afferent input from several pericranial muscles. Blockade of at least one of these inputs can normalize central trigeminal activity. This may explain why therapeutic manipulations of head muscles can be beneficial in primary headaches.

Sensitization of group III and IV muscle afferents in the mouse after ischemia and reperfusion injury

Ischemic myalgia is a unique type of muscle pain in the patient population. The role that discrete muscle afferent subpopulations play in the generation of pain during ischemic events, however, has yet to be determined. Using 2 brachial artery occlusion models to compare prolonged ischemia or transient ischemia with reperfusion of the muscles, we found that both injuries caused behavioral decrements in grip strength, as well as increased spontaneous pain behaviors. Using our ex vivo forepaw muscles, median and ulnar nerves, dorsal root ganglion, and spinal cord recording preparation, we found after both prolonged and transient ischemia that there was a significant increase in the number of afferents that responded to both noxious and non-noxious chemical (lactate, adenosine triphosphate, varying pH) stimulation of the muscles compared to uninjured controls. However, we found an increase in firing to heat stimuli specifically in muscle afferents during prolonged ischemia, but a distinct increase in afferent firing to non-noxious chemicals and decreased mechanical thresholds after transient ischemia. The unique changes in afferent function observed also corresponded with distinct patterns of gene expression in the dorsal root ganglia. Thus, the development of ischemic myalgia may be generated by unique afferent-based mechanisms during prolonged and transient ischemia.

PERSPECTIVE

This study analyzed the response properties of thinly myelinated group III and unmyelinated group IV muscle afferents during prolonged and transient ischemia in addition to pain behaviors and alterations in DRG gene expression in the mouse. Results suggest that mechanisms of pain generation during prolonged ischemia may be different from ischemia/reperfusion.

Comprehensive phenotyping of group III and IV muscle afferents in mouse

While much is known about the functional properties of cutaneous nociceptors, relatively little is known about the comprehensive functional properties of group III and IV muscle afferents. We have developed a mouse ex vivo forepaw muscle, median and ulnar nerve, dorsal root ganglion (DRG), spinal cord recording preparation to examine the functional response properties, neurochemical phenotypes, and spinal projections of individual muscle afferents. We found that the majority of group III and IV muscle afferents were chemosensitive (52%) while only 34% responded to mechanical stimulation and fewer (32%) responded to thermal stimuli. The chemosensitive afferents could be grouped into those that responded to a "low"-metabolite mixture containing amounts of lactate and ATP at pH 7.0 simulating levels observed in muscle during exercise (metaboreceptors) and a "high"-metabolite mixture containing lactic acid concentrations and ATP at pH 6.6 mimicking levels observed during ischemic contractions (metabo-nociceptors). While the majority of the metabo-nociceptive fibers responding to the higher concentration levels were found to contain acid-sensing ion channel 3 (ASIC3) and/or transient receptor potential vanilloid type 1 (TRPV1), metaboreceptors responding to the lower concentration levels lacked these receptors. Anatomically, group III muscle afferents were found to have projections into laminae I and IIo, and deeper laminae in the spinal cord, while all functional types of group IV muscle afferents projected primarily into both laminae I and II. These results provide novel information about the variety of sensory afferents innervating the muscle and provide insight into the types of fibers that may exhibit plasticity after injuries.

P2X7 receptor blockade reverses purinergic facilitation of neck muscle nociception in mice

INTRODUCTION

Facilitation of neck muscle nociception mediated via purinergic signalling may play a role in the pathophysiology of tension-type headache (TTH). The present study addressed reversal of purinergic facilitation of brainstem nociception via P2X7 antagonist action in anaesthetized mice.

METHODS

Following administration of α,β-meATP (i.m. 20 µL/min, 20 µL each) into semispinal neck muscles, the impact of neck muscle nociceptive input on brainstem processing was monitored by the jaw-opening reflex in anaesthetized mice (n = 20). The hypothesized involvement of the P2X7 receptor in the α,β-meATP effect was addressed with i.p. (systemic) and i.m. (semispinalis, 20 µL/min, 20 µL each) administration of P2X7 inhibitor A438079 during established facilitation; i.p. saline served as control.

RESULTS

α,β-meATP reliably induced jaw-opening reflex facilitation (256 ± 48% (mean ± SEM), n = 20). I.p. A438079 (150, 300 µmol/kg) completely reversed this α,β-meATP effect dose-dependently. Neither saline nor intramuscular A438079 (100 µM) altered facilitated brainstem nociceptive processing.

DISCUSSION

These data suggest that muscular structures are not directly involved in the P2X7 antagonist-mediated reversal of purinergic facilitation. Instead, involvement of neuronal structures, particularly of the central nervous system, seems more probable. The results from this animal experimental model may point to involvement of purinergic P2X7 receptors in TTH pathophysiology and may suggest potential future targets for its pharmacological treatment.

Acetylsalicylic acid inhibits α,β-meATP-induced facilitation of neck muscle nociception in mice--implications for acute treatment of tension-type headache

Infusion of α,β-methylene ATP (α,β-meATP) into murine neck muscle facilitates brainstem nociception. This animal experimental model is suggested to be appropriate for investigating pathophysiological mechanisms in tension-type headache. It was hypothesized that d-lysine acetylsalicylic acid (ASA, aspirin®) reverses this α,β-meATP effect. Facilitation of neck muscle nociceptive processing was induced via bilateral infusion of α,β-meATP into semispinal neck muscles (100 nM, 20 μl each) in 42 anesthetized mice. Brainstem nociception was monitored by the jaw-opening reflex elicited via electrical tongue stimulation. The hypothesis was addressed by subsequent (15, 30, 60 mg/kg) and preceding (60 mg/kg) intraperitoneal ASA injection. Saline served as control to ASA solution. Subsequent ASA dose-dependently reversed α,β-meATP-induced reflex facilitation and was the most prominent with 60 mg/kg. Preceding 60 mg/kg ASA prevented reflex facilitation. Cyclooxygenases are involved in nociceptive transmission. Former experiments showed that unspecific inhibition of cyclooxygenases does not alter the α,β-meATP effect. This suggests a specific mode of action of ASA. The concept is accepted that neck muscle nociception is involved in the pathophysiology of tension-type headache. Thus, objective proof of ASA effects in this experimental model may emphasize its major role in pharmacological treatment of tension-type headache attacks.

Neuronal nitric oxide synthase is involved in the induction of nerve growth factor-induced neck muscle nociception

BACKGROUND

Neck muscle nociception mediated by nitric oxide may play a role in the pathophysiology of tension-type headache.

OBJECTIVE

The present study addresses the involvement of neuronal nitric oxide synthase (nNOS) in the facilitation of neck muscle nociception after local application of nerve growth factor (NGF).

METHODS

After administration of NGF into semispinal neck muscles, the impact of neck muscle noxious input on brainstem processing was monitored by the jaw-opening reflex in anesthetized mice. The modulatory effect of preceding and subsequent administration of an inhibitor of neuronal nitric oxide synthase on central facilitation was addressed in a controlled study.

RESULTS

With preceding i.p. application of saline or 0.096 mg/kg of the specific nNOS inhibitor Nω-propyl-L-arginine (NPLA), NGF induced a sustained reflex facilitation within 60 minutes. Preceding injection of 0.96 mg/kg or 1.92 mg/kg NPLA completely prevented the potentially facilitatory effect of NGF. Subsequent administration of 0.96 mg/kg NPLA did not affect established NGF-evoked reflex facilitation. Thus, NPLA prevents facilitation of brainstem processing by noxious myofascial input from neck muscles in a dose-dependent manner.

CONCLUSION

These findings suggest that nNOS is involved in the induction but not the maintenance of NGF-evoked facilitation of nociception in the brainstem. These results from an experimental animal model may support the idea of NOS and nNOS as potential targets for pharmacological treatment of tension-type headache.

Inhibition of nNOS prevents and inhibition of iNOS reverses α,β-meATP-induced facilitation of neck muscle nociception in mice

Infusion of α,β-methylene ATP (α,β-meATP) into murine neck muscle facilitates brainstem nociception. Unspecific nitric oxide synthase (NOS) inhibition prevents and reverses this sensitization. It is unclear whether neuronal (nNOS), inducible (iNOS) or endothelial NOS isoenzymes are involved in this α,β-meATP effect. Hypothesized involvement of nNOS isoenzyme was addressed by preceding (0.5, 1, and 2 mg/kg) and subsequent (2 mg/kg) intraperitoneal injection of the nNOS-inhibitor NPLA. iNOS involvement was addressed by subsequent, intraperitoneal administration of the iNOS-inhibitor 1400 W (2 mg/kg). Brainstem nociception was monitored by the jaw-opening reflex elicited via electrical tongue stimulation in 45 anesthetized mice. Preceding NPLA dose-dependently prevented α,β-meATP-induced reflex facilitation. Whereas subsequent inhibition of nNOS showed no effect, iNOS inhibition by 1400 W significantly reversed reflex facilitation. Data provide evidence that nNOS plays a major role in induction and iNOS in maintenance of facilitation in neck muscle nociception. Divergent roles of NOS isoenzymes may promote research on target specific treatment for headache and neck muscle pain.

Inhibition of nitric oxide synthases prevents and reverses alpha,beta-meATP-induced neck muscle nociception in mice

INTRODUCTION

Tension-type headache (TTH) is associated with noxious input from neck muscles. Intravenous administration of the unspecific nitric oxide synthase inhibitor L-NMMA in chronic TTH patients caused analgesia and reduction of neck muscle tenderness.

METHODS

The unspecific nitric oxide synthase inhibitor L-NMMA was applied in an experimental model for neck muscle nociception in anesthetized mice (N = 25).

RESULTS

Local injection of α,β-meATP into semispinal neck muscles induced sustained facilitation of brainstem nociception as monitored by the jaw-opening reflex. Preceding intraperitoneal administration of L-NMMA (0.05, 0.1, 1 mg/kg) prevented reflex facilitation evoked by α,β-meATP in a dose-dependent manner. Intraperitoneal injection of L-NMMA subsequent to intramuscular α,β-meATP application reversed established brainstem reflex facilitation back to baseline values.

DISCUSSION

Both experiments with preceding and subsequent L-NMMA indicate the involvement of nitric oxide synthases in the induction and maintenance of facilitation. However, future experiments will have to address the involvement of various isoenzymes in order to provide for new therapeutic concepts in TTH.

Excitatory and inhibitory purinergic control of neck muscle nociception in anaesthetized mice

Tension-type headache is associated with noxious input from neck muscles. Due to the importance of purinergic mechanisms in muscle nociception, experimental studies typically inject alpha,beta-methyleneadenosine 5'-triphosphate (alpha,beta-meATP). In contrast to native adenosine 5'-triphosphate (ATP), alpha,beta-meATP has a narrow receptor profile and remains stable in tissue. The present study administered alpha,beta-meATP or ATP in semi-spinal neck muscles in anaesthetized mice (n = 65) in order to address different effects in neck muscle nociception. The jaw-opening reflex monitored the impact of neck muscle noxious input on brainstem processing. Injection of alpha,beta-meATP induced reflex facilitation in a dose-dependent manner. In contrast, only the lowest ATP dosage evoked facilitation. Preceding P2Y(1) receptor blockade revealed facilitation even under high-dosage ATP. Ongoing facilitation after alpha,beta-meATP injection neutralized under subsequent activation of P2Y(1) receptors. Results demonstrate opposing excitatory P2X and inhibitory P2Y effects of ATP in neck muscle nociception. These mechanisms may be involved in the pathophysiology of neck muscle pain in man.