Role of soluble guanylate cyclase in the trigeminal subnucleus caudalis in capsaicin-induced muscle hypersensitivity

Peripheral glutamate receptor and transient receptor potential channel mechanisms of craniofacial muscle pain

Temporomandibular joint disorder is a common chronic craniofacial pain condition, often involving persistent, widespread craniofacial muscle pain. Although the etiology of chronic muscle pain is not well known, sufficient clinical and preclinical information supports a contribution of trigeminal nociceptors to craniofacial muscle pain processing under various experimental and pathological conditions. Here, we review cellular and molecular mechanisms underlying sensitization of muscle nociceptive afferents. In particular, we summarize findings on pronociceptive roles of peripheral glutamate in humans, and we discuss mechanistic contributions of glutamate receptors, including N-methyl-D-aspartate receptors and metabotropic glutamate receptors, which have considerably increased our understanding of peripheral mechanisms of craniofacial muscle pain. Several members of the transient receptor potential (TRP) family, such as transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential ankyrin 1, also play essential roles in the development of spontaneous pain and mechanical hypersensitivity in craniofacial muscles. Furthermore, glutamate receptors and TRP channels functionally and bi-directionally interact to modulate trigeminal nociceptors. Activation of glutamate receptors invokes protein kinase C, which leads to the phosphorylation of TRPV1. Sensitization of TRPV1 by inflammatory mediators and glutamate receptors in combination with endogenous ligands contributes to masseter hyperalgesia. The distinct intracellular signaling pathways through which both receptor systems engage and specific molecular regions of TRPV1 are offered as novel targets for the development of mechanism-based treatment strategies for myogenous craniofacial pain conditions.

Behavioral characteristics of capsaicin mediated cutaneous, myogenic, and arthrogenic orofacial nociception in rats

OBJECTIVE

To assess changes in orofacial tactile sensitivity and gnawing related to capsaicin-mediated cutaneous, myogenic, and arthrogenic nociception in the rat.

DESIGN

After recovery from anesthesia, orofacial tactile sensitivity and gnawing were assessed using operant testing methods following capsaicin application. Twenty female CD-Hairless rats were tested with bilateral capsaicin cream application to the cheek or with isoflurane anesthesia alone. Following several weeks of recovery, animals (n = 20) received either 10 μL unilateral masseter injections of vehicle, or phosphate buffered saline (PBS) to assess injection sensitization. After several weeks, masseter capsaicin (1.0%) injections (10 μL) were assessed compared to vehicle and PBS (n = 13). Weeks later capsaicin TMJ injections were evaluated. Animals (n = 11) received either 10 μL unilateral TMJ injections of capsaicin solution (1%) or vehicle.

RESULTS

Capsaicin cream to the skin significantly altered gnawing activity (increased puncture time by 248 s (p = 0.0002)) and tactile sensitivity (decreased tolerated bottle distance by 0.980 cm compared to isoflurane only (p = 0.0001)). Similarly, capsaicin masseter injection increased puncture time (339.6 s, p = 0.07) and decreased tolerated bottle distance (1.04 cm, p = 0.005) compared to vehicle. However, intra-articular capsaicin in the TMJ only modified gnawing (increased puncture time by 133 s), with no changes found in tactile sensitivity compared to vehicle.

CONCLUSION

Application of capsaicin to the skin and masseter had similar behavioral effects; however, intra-articular injections to the TMJ only affected gnawing. These data indicate the behavioral changes in rodent models of myogenic and cutaneous pain may be markedly different than models of arthrogenic pain originating from the TMJ.

The role of TRPA1 in muscle pain and mechanical hypersensitivity under inflammatory conditions in rats

Transient receptor potential cation channel, subfamily A, member 1 (TRPA1) is expressed in muscle afferents and direct activation of these receptors induces acute mechanical hypersensitivity. However, the functional role of TRPA1 under pathological muscle pain conditions and mechanisms by which TRPA1 mediate muscle pain and hyperalgesia are not clearly understood. Two rodent behavioral models validated to assess craniofacial muscle pain conditions were used to study ATP- and N-Methyl-D-aspartate (NMDA)-induced acute mechanical hypersensitivity and complete Freund's adjuvant (CFA)-induced persistent mechanical hypersensitivity. The rat grimace scale (RGS) was utilized to assess inflammation-induced spontaneous muscle pain. Behavioral pharmacology experiments were performed to assess the effects of AP18, a selective TRPA1 antagonist under these conditions. TRPA1 expression levels in trigeminal ganglia (TG) were examined before and after CFA treatment in the rat masseter muscle. Pre-treatment of the muscle with AP18 dose-dependently blocked the development of acute mechanical hypersensitivity induced by NMDA and α,β-methylene adenosine triphosphate (αβmeATP), a specific agonist for NMDA and P2X3 receptor, respectively. CFA-induced mechanical hypersensitivity and spontaneous muscle pain responses were significantly reversed by post-treatment of the muscle with AP18 when CFA effects were most prominent. CFA-induced myositis was accompanied by significant up-regulation of TRPA1 expression in TG. Our findings showed that TRPA1 in muscle afferents plays an important role in the development of acute mechanical hypersensitivity and in the maintenance of persistent muscle pain and hypersensitivity. Our data suggested that TRPA1 may serve as a downstream target of pro-nociceptive ion channels, such as P2X3 and NMDA receptors in masseter afferents, and that increased TRPA1 expression under inflammatory conditions may contribute to the maintenance of persistent muscle pain and mechanical hyperalgesia. Mechanistic studies elucidating transcriptional or post-translational regulation of TRPA1 expression under pathological pain conditions should provide important basic information to further advance the treatment of craniofacial muscle pain conditions.

A novel operant-based behavioral assay of mechanical allodynia in the orofacial region of rats

BACKGROUND

Detecting behaviors related to orofacial pain in rodent models often relies on subjective investigator grades or methods that place the animal in a stressful environment. In this study, an operant-based behavioral assay is presented for the assessment of orofacial tactile sensitivity in the rat.

NEW METHODS

In the testing chamber, rats are provided access to a sweetened condensed milk bottle; however, a 360° array of stainless steel wire loops impedes access. To receive the reward, an animal must engage the wires across the orofacial region. Contact with the bottle triggers a motor, requiring the animal to accept increasing pressure on the face during the test. To evaluate this approach, tolerated bottle distance was measured for 10 hairless Sprague Dawley rats at baseline and 30 min after application of capsaicin cream (0.1%) to the face. The experiment was repeated to evaluate the ability of morphine to reverse this effect.

RESULTS

The application of capsaicin cream reduced tolerated bottle distance measures relative to baseline (p<0.05). As long as morphine did not cause reduced participation due to sedation, subcutaneous morphine dosing reduced the effects of capsaicin (p<0.001). Comparison with existing method: For behavioral tests, experimenters often make subjective decisions of an animal's response. Operant methods can reduce these effects by measuring an animal's selection in a reward-conflict decision. Herein, a method to measure orofacial sensitivity is presented using an operant system.

CONCLUSIONS

This operant device allows for consistent measurement of heightened tactile sensitivity in the orofacial regions of the rat.

P2X₃ and TRPV1 functionally interact and mediate sensitization of trigeminal sensory neurons

Musculoskeletal pain conditions, particularly those associated with temporomandibular disorders (TMD) affect a large percentage of the population. Identifying mechanisms underlying hyperalgesia could contribute to the development of new treatment strategies for the management of TMD and other muscle pain conditions. In this study, we provide evidence of functional interactions between two ligand-gated channels, P2X₃ and transient receptor potential V1 (TRPV1), in trigeminal sensory neurons, and propose that the interactions serve as an underlying mechanism for the development of mechanical hyperalgesia. Mechanical sensitivity of the masseter muscle was assessed in lightly anesthetized rats via an electronic anesthesiometer (Ro et al., 2009). Direct intramuscular injection of a selective P2X₃ agonist, alpha,beta-methylene adenosine triphosphate (αβmeATP), induced a dose- and time-dependent hyperalgesia. Mechanical sensitivity in the contralateral muscle was unaffected suggesting local P2X₃ mediate hyperalgesia. Anesthetizing the overlying skin had no effect on αβmeATP-induced hyperalgesia confirming the contribution of P2X₃ from the muscle. Importantly, the αβmeATP-induced hyperalgesia was prevented by pretreatment of the muscle with a TRPV1 antagonist, AMG9810. P2X₃ was co-expressed with TRPV1 in the masseter muscle afferents confirming the possibility for intracellular interactions. Additionally, in a subpopulation of P2Xv/TRPV1 positive neurons, capsaicin-induced Ca(2+) transients were significantly amplified following P2X₃ activation. Finally, activation of P2X₃ induced phosphorylation of serine, but not threonine, residues in TRPV1 in trigeminal ganglia cultures. Significant phosphorylation was observed at 15 min, the time point at which behavioral hyperalgesia was prominent. Previously, activation of either P2X₃ or TRPV1 had been independently implicated in the development of mechanical hyperalgesia. Our data propose P2X₃ and TRPV1 interact in a facilitatory manner, which could contribute to the peripheral sensitization known to underlie masseter hyperalgesia.

Functional interactions between NMDA receptors and TRPV1 in trigeminal sensory neurons mediate mechanical hyperalgesia in the rat masseter muscle

The NMDA and TRPV1 receptors that are expressed in sensory neurons have been independently demonstrated to play important roles in peripheral pain mechanisms. In the present study, we investigated whether the 2 receptor-channel systems form a functional complex that provides the basis for the development of mechanical hyperalgesia. In the masseter muscle, direct application of NMDA induced a time-dependent increase in mechanical sensitivity, which was significantly blocked when the muscle was pretreated with a specific TRPV1 antagonist, AMG9810. The NR1 subunit of the NMDA receptor and TRPV1 were coexpressed in 32% of masseter afferents in trigeminal ganglia (TG). Furthermore, NR1 and NR2B formed protein-protein complexes with TRPV1 in TG as demonstrated by coimmunoprecipitation experiments. Calcium imaging analyses further corroborated that NMDA and TRPV1 receptors functionally interact. In TG culture, application of NMDA resulted in phosphorylation of serine, but not threonine or tyrosine, residues of TRPV1 in a time course similar to that of the development of NMDA-induced mechanical hyperalgesia. The NMDA-induced phosphorylation was significantly attenuated by CaMKII and PKC inhibitors, but not by a PKA inhibitor. Consistent with the biochemical data, the NMDA-induced mechanical hyperalgesia was also effectively blocked when the muscle was pretreated with a CaMKII or PKC inhibitor. Thus, NMDA receptors and TRPV1 functionally interact via CaMKII and PKC signaling cascades and contribute to mechanical hyperalgesia. These data offer novel mechanisms by which 2 ligand-gated channels in sensory neurons interact and reinforce the notion that TRPV1 functions as a signal integrator under pathological conditions.

Masseter inflammation differentially regulates three nitric oxide synthases in the rat trigeminal subnucleus caudalis

OBJECTIVE

The aim of the present study was to evaluate changes in expression levels of three nitric oxide synthases (NOSs), namely inducible NOS (iNOS), neuronal NOS (nNOS) and endothelial NOS (eNOS), in the subnucleus caudalis of the trigeminal sensory nuclear complex (Vc) under experimental myositis conditions.

DESIGN

Male Sprague Dawley rats were injected with an inflammatory agent, complete Freund's adjuvant (CFA), or capsaicin in the masseter muscle. The brainstem region containing the Vc was extracted at both immediate (30 and 60 min) and longer (1, 3, 7 days) time points to examine the changes in the three NOS protein levels via the Western blot technique. Subsequently, the RT-PCR experiments were carried out to verify the changes in iNOS mRNA.

RESULTS

Following the injections of CFA, there were no significant changes in the level of the three NOS proteins at the immediate time points. However, there was a significant upregulation of iNOS mRNA and protein 3 days after CFA-induced inflammation. Neither nNOS nor eNOS showed significant changes in the protein level at any of the longer time points. Capsaicin injection in the masseter, which we recently reported to upregulate all three NOS at the immediate time points, did not result in significant changes at longer time points.

CONCLUSION

Acute and chronic muscle inflammation differentially modulates the expression of the three NOS in the Vc. These data suggest that the contribution of each NOS in craniofacial muscle pain processing under inflammatory conditions may be anticipated with distinct temporal profiles.

The role of nitric oxide in orofacial pain

Nitric oxide (NO) is a free radical gas that has been shown to be produced by nitric oxide synthase (NOS) in different cell types and recognized to act as a neurotransmitter or neuromodulator in the nervous system. NOS isoforms are expressed and/or can be induced in the related structures of trigeminal nerve system, in which the regulation of NOS biosynthesis at different levels of gene expression may allow for a fine control of NO production. Several lines of evidence suggest that NO may play a role through multiple mechanisms in orofacial pain processing. This report will review the latest evidence for the role of NO involved in orofacial pain and the potential cellular mechanisms are also discussed.

Organization of pERK-immunoreactive cells in trigeminal spinal nucleus caudalis, upper cervical cord, NTS and Pa5 following capsaicin injection into masticatory and swallowing-related muscles in rats

Many phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive (IR) cells are expressed in the trigeminal spinal subnucleus caudalis (Vc), upper cervical spinal cord (C1-C2), nucleus tractus solitarii (NTS) and paratrigeminal nucleus (Pa5) after capsaicin injection into the whisker pad (WP), masseter muscle (MM), digastric muscle (DM) or sternohyoideus muscle (SM). The pERK-IR cells also showed NeuN immunoreactivity, indicating that ERK phosphorylation occurs in neurons. The pERK-IR cells were significantly reduced after intrathecal injection of MEK 1/2 inhibitor PD98059. The pERK-IR cells expressed bilaterally in the Vc and C1-C2 after capsaicin injection into the unilateral DM or SM, whereas unilaterally in the Vc and C1-C2 after unilateral WP or MM injection. After capsaicin injection into the WP or MM, the pERK-IR cell expression in the Vc was restricted rostrocaudally within a narrow area. However, the distribution of pERK-IR cells was more wide spread without a clear peak in the Vc and C1-C2 after capsaicin injection into the DM or SM. In the NTS, the unimodal pERK-IR cell expression peaked at 0-720μm rostral from the obex following capsaicin injection into WP, MM, DM or SM. In the ipsilateral Pa5, many pERK-IR cells were observed following capsaicin injection into the SM. The number of swallows elicited by distilled water administration was significantly smaller after capsaicin injection into the WP, MM or DM but not SM compared to that of vehicle-injected rats. Various noxious inputs due to the masticatory or swallowing-related muscle inflammation may be differentially involved in muscle pain and swallowing reflex activity.

Activation of peripheral delta-opioid receptors leads to anti-hyperalgesic responses in the masseter muscle of male and female rats

In this project, we examined peripheral δ-opioid receptor (DOR)-mediated anti-hyperalgesic responses in the context of an acute orofacial muscle pain condition in both male and female rats. We also investigated whether the ATP-sensitive K+ channel (KATP), a downstream target of OR signaling, contributes to DOR-mediated anti-hyperalgesic responses. Local pretreatment of the masseter with a DOR agonist, [D-Pen², D-Pen⁶]-enkephalin (DPDPE), dose-dependently attenuated capsaicin-induced mechanical hypersensitivity in both male and female rats. However, there were sex differences in the potency of local DPDPE in that a 10-fold higher dose of DPDPE was required in female rats to produce the level of anti-hyperalgesia achieved in male rats. The sex differences in the DPDPE effect may not be fully explained by DOR expression level since there was no significant sex difference in DOR mRNA levels in trigeminal ganglia (TG). Finally, pretreatment of the masseter with the KATP antagonist, glibenclamide, significantly blocked the effects of DPDPE in male rats suggesting that the peripheral DOR effect is mediated by the KATP. These studies revealed novel information about sex differences with regards to peripherally localized DOR-mediated anti-hyperalgesia under an orofacial muscle pain condition.

Sex differences in the contribution of ATP-sensitive K+ channels in trigeminal ganglia under an acute muscle pain condition

In this study, we examined whether functional subunits of the ATP-dependent K+ channel (KATP) are expressed in trigeminal ganglia (TG), which contains sensory neurons that innervate oral and facial structures. We also investigated whether direct activation of the KATP effectively attenuates mechanical hypersensitivity in the context of an acute orofacial muscle pain condition. The KATP expression in TG and behavioral studies were conducted in age matched male and female Sprague-Dawley rats. RT-PCR experiments showed that the mRNAs for the inwardly rectifying pore-forming subunits, Kir6.1 and Kir6.2, as well as the regulatory sulfonylurea subunits, SUR1 and SUR2, were reliably detected in TG. Subsequent western blot analysis confirmed that proteins for all four subunits are expressed in TG, and showed that Kir6.2 is expressed at a significantly higher level in male TG compared to that of female rats. This observation was confirmed by the immunohistochemical demonstration of higher percentages of Kir6 positive masseter afferents in female rats. Masseteric injection of capsaicin evokes a time dependent increase in masseter sensitivity to noxious mechanical stimulation. A specific KATP agonist, pinacidil, dose-dependently attenuated the capsaicin-induced mechanical hypersensitivity in male rats. The dose of pinacidil (20 μg) that completely blocked the capsaicin responses in male rats was ineffective in female rats regardless of their estrus phases. Only at the highest dose (300 μg) we used, pinacidil was partially effective in female rats. Similarly, another KATP agonist, diazoxide which targets different KATP subunits also showed sex specific responses in attenuating capsaicin-induced masseter hypersensitivity. These data suggested that sex differences in functional KATP expression in TG may underlie sex specific responses to KATP agonists. The present study provided novel information on sex differences in KATP expression in TG and its contribution under an orofacial muscle pain condition.

Expression changes of K+-Cl- co-transporter 2 and Na+-K+-Cl- co-transporter1 in mouse trigeminal subnucleus caudalis following pulpal inflammation

Cation chloride co-transporters, including K(+)-Cl(-) co-transporter 2 (KCC2) and Na(+)-K(+)-Cl(-) co-transporter 1 (NKCC1), are of particular importance to GABAergic transmission and thus involved in the development of hyperalgesia at the spinal level. However, it is largely unknown whether these co-transporters in the trigeminal system contribute to dental pain. In this study, we investigated the expression of KCC2 and NKCC1 mRNAs in mouse trigeminal subnucleus caudalis (Vc) after lipopolysaccharide (LPS) application to the tooth pulp by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) method. KCC2 mRNA was found to be down-regulated at 1d after pulpal inflammation, while NKCC1 was up-regulated. Blockade of endogenous brain-derived neurotrophic factor-tyrosine receptor kinase B pathway with K252a produced pronounced antinociception as evidenced by decreased tongue protrusion behavior in LPS-treated mice. These data suggest that KCC2 and NKCC1 in Vc may play a critical role in the nociception and transmission of dental pain during pulpal inflammation.

Electrophysiological characterization of the rat trigeminal caudalis (Vc) neurons following intramuscular injection of capsaicin

Extracellular single unit recording experiments were performed to examine response characteristics of wide dynamic range neurons in the Vc that receive masseter afferent input in Sprague-Dawley rats. Capsaicin, or its vehicle, was directly administered into the masseter muscle and changes in resting discharge, responses to mechanical stimulation on the cutaneous receptive field and the electrical threshold for masseter nerve stimulation were assessed. Intramuscular capsaicin induced significant increase in the background discharge and mechanical hypersensitivity to the cutaneous stimulation and lowered the threshold for masseter nerve stimulation-evoked responses in the majority of neurons. The capsaicin-induced increase in evoked responses, but not the resting discharge, was partially attenuated when the muscle was pretreated with a mGluR antagonist. The present study suggests that injury or inflammation in the masseter muscle induce generalized hyperexcitability of central trigeminal neurons and that the blockade of peripherally localized mGluR5 can effectively attenuate muscular hypersensitivity.

Down-regulation of K+ -Cl- co-transporter 2 in mouse medullary dorsal horn contributes to the formalin-induced inflammatory orofacial pain

Cation chloride co-transporters, including K(+)-Cl(-) co-transporter 2 (KCC2) and Na(+)-K(+)-Cl(-) co-transporter 1 (NKCC1), are of particular importance to GABAergic transmission and thus involved in the development of hyperalgesia at the spinal level. However, there are fewer relevant reports in the trigeminal system. In this study, we investigated the behavioral changes and the accompanying change in the expressions of KCC2 and NKCC1 mRNAs in mouse medullary dorsal horn (MDH) after subcutaneous injection of formalin into the left vibrissa pad. Furthermore, we observed the behavioral changes following intracisternal injection of KCC2 antisense oligodeoxynucleotides (ASO) into naïve mice. Subcutaneous injection of formalin-induced a significant increase in the number of face rubbing events which are the indicators of spontaneous pain. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) results indicated that, compared to the saline control or the contralateral side, the ipsilateral mRNA level of KCC2 but not NKCC1, was significantly reduced in formalin-injected mice during phase 1 observation, followed by gradual recovery. Intracisternal injection of KCC2 ASO into naïve mice led to behavioral hypersensitivity similar to the hyperalgesia observed in formalin experiments. These findings indicate that peripheral inflammation induces down-regulation of KCC2 in the MDH, which may in turn facilitate the development of acute inflammatory pain. These results also suggest that preventing the down-regulation of KCC2 is a possible way to combat orofacial pain.

Activation of TRPV1 and TRPA1 leads to muscle nociception and mechanical hyperalgesia

The involvement of TRPV1 and TRPA1 in mediating craniofacial muscle nociception and mechanical hyperalgesia was investigated in male Sprague-Dawley rats. First, we confirmed the expression of TRPV1 in masseter afferents in rat trigeminal ganglia (TG), and provided new data that TRPA1 is also expressed in primary afferents innervating masticatory muscles in double-labeling immunohistochemistry experiments. We then examined whether the activation of each TRP channel in the masseter muscle evokes acute nocifensive responses and leads to the development of masseter hypersensitivity to mechanical stimulation using the behavioral models that have been specifically designed and validated for the craniofacial system. Intramuscular injections with specific agonists for TRPV1 and TRPA1, capsaicin and mustard oil (MO), respectively, produced immediate nocifensive hindpaw responses followed by prolonged mechanical hyperalgesia in a concentration-dependent manner. Pretreatment of the muscle with a TRPV1 antagonist, capsazepine, effectively attenuated the capsaicin-induced muscle nociception and mechanical hyperalgesia. Similarly, pretreatment of the muscle with a selective TRPA1 antagonist, AP18, significantly blocked the MO-induced muscle nociception and mechanical hyperalgesia. We confirmed these data with another set of selective antagonist for TRPV1 and TRPA1, AMG9810 and HC030031, respectively. Collectively, these results provide compelling evidence that TRPV1 and TRPA1 can functionally contribute to muscle nociception and hyperalgesia, and suggest that TRP channels expressed in muscle afferents can engage in the development of pathologic muscle pain conditions.

Involvement of neuronal, inducible and endothelial nitric oxide synthases in capsaicin-induced muscle hypersensitivity

Nitric oxide, which has been implicated in the development of hyperalgesia in the spinal system, has not been systematically studied in the trigeminal system, especially in the context of inflammatory muscle pain condition. In this study, we investigated the functional role of centrally released nitric oxide in the pathogenesis of orofacial muscle pain. Specifically, we examined the contribution of neuronal, inducible and endothelial nitric oxide synthases, nNOS, iNOS and eNOS, respectively, in mediating masseter hypersensitivity under acute inflammatory condition. Time-dependent changes in nNOS, iNOS and eNOS protein expression in the subnucleus caudalis (Vc) were assessed following capsaicin injection in the masseter muscle of male Sprague Dawley rats. The expression of all three nitric oxide synthases was significantly up-regulated 30-60 min following capsaicin stimulation, which paralleled the time course of the development of capsaicin-induced masseter hypersensitivity. Pretreatment with each NOS inhibitor significantly attenuated the masseter hypersensitivity. These data showed that all three NOS in the Vc are functionally important for the development of craniofacial muscle hyperalgesia and suggest that the three NOS are closely orchestrated to regulate the level of nitric oxide under normal and pathologic conditions.