Intensity coding by TMJ-responsive neurons in superficial laminae of caudal medullary dorsal horn of the rat

Olfactory stimulation Inhibits Nociceptive Signal Processing at the Input Stage of the Central Trigeminal System

The spinal trigeminal nucleus caudalis (SpVc) in the mammalian brainstem serves a pivotal function in pain processing. As the main relay center for nociceptive signals, SpVc conducts pain-related signals from various regions of the head toward higher levels of central processing such as the thalamus. SpVc also receives modulatory signals from other brain areas, which can alleviate the perception of headache. We studied the impact of olfactory co-stimulation on pain-related behavior and SpVc neural activity in mice. Using the TRPA1 agonist allyl isothiocyanate (AITC) as noxious stimulus, we quantified the aversive response and the perceived pain intensity by evaluating explorative running and the mouse grimace scale, respectively. We found that the floral odorants phenylethyl alcohol (PEA) and lavender oil mitigated the aversive response to AITC. Consistent with this finding, a newly developed, automated quantification of c-Fos expression in SpVc revealed that co-stimulation with PEA or lavender profoundly reduced network activity in the presence of AITC. These results demonstrated a substantial analgesic potential of odor stimulation in the trigeminal system and provide an explanation for the palliative effect of odors in the treatment of headache.

Experimental Methods to Inform Diagnostic Approaches for Painful TMJ Osteoarthritis

Temporomandibular joint (TMJ) osteoarthritis (OA) is a degenerative disease of the joint that can produce persistent orofacial pain as well as functional and structural changes to its bone, cartilage, and ligaments. Despite advances in the clinical utility and reliability of the Diagnostic Criteria for Temporomandibular Disorders, clinical tools inadequately predict which patients will develop chronic TMJ pain and degeneration, limiting clinical management. The challenges of managing and treating TMJ OA are due, in part, to a limited understanding of the mechanisms contributing to the development and maintenance of TMJ pain. OA is initiated by multiple factors, including injury, aging, abnormal joint mechanics, and atypical joint shape, which can produce microtrauma, remodeling of joint tissues, and synovial inflammation. TMJ microtrauma and remodeling can increase expression of cytokines, chemokines, and catabolic factors that damage synovial tissues and can activate free nerve endings in the joint. Although studies have separately investigated inflammation-driven orofacial pain, acute activity of the trigeminal nerve, or TMJ tissue degeneration and/or damage, the temporal mechanistic factors leading to chronic TMJ pain are undefined. Limited understanding of the interaction between degeneration, intra-articular chemical factors, and pain has further restricted the development of targeted, disease-modifying drugs to help patients avoid long-term pain and invasive procedures, like TMJ replacement. A range of animal models captures features of intra-articular inflammation, joint overloading, and tissue damage. Although those models traditionally measure peripheral sensitivity as a surrogate for pain, recent studies recognize the brain's role in integrating, modulating, and interpreting nociceptive inputs in the TMJ, particularly in light of psychosocial influences on TMJ pain. The articular and neural contributors to TMJ pain, imaging modalities with clinical potential to identify TMJ OA early, and future directions for clinical management of TMJ OA are reviewed in the context of evidence in the field.

Bilateral increases in ERK activation at the spinomedullary junction region by acute masseter muscle injury during temporomandibular joint inflammation in the rats

We determined the role of persistent monoarthritis of temporomandibular joint region (TMJ) on bilateral masseter muscle (MM) nociception in male rats using orofacial nocifensive behaviors, phosphorylated extracellular signal-regulated kinase and Fos induction at the trigeminal subnucleus caudalis/upper cervical spinal cord (Vc/C₂) region in response to formalin injection to the MM region. TMJ inflammation was induced by local injection of CFA into the left TMJ region. Orofacial nocifensive behaviors evoked by formalin injection ipsilateral or contralateral to the TMJ inflammation appeared to be increased at 1-14 days or at 1, 10 and 14 days after induction of TMJ inflammation, respectively, while increases in behavioral duration were seen mainly in the late phase rather than the early phase. The number of pERK positive cells was investigated in superficial laminae at the Vc/C₂ region at 3, 10, 20, 60 and 80 min after MM stimulation with formalin at 14 days after TMJ inflammation. TMJ-inflamed rats displayed greater responses of pERK expression by the ipsilateral MM stimulation at 3-60 min, while contralateral MM stimulation increased pERK expression at 3, 10 and 20 min compared to non-CFA rats. Fos expression by MM stimulation was increased at 14 days after induction of TMJ inflammation regardless of the affected side. These findings showed that persistent TMJ inflammation for 10 and 14 days is sufficient to enhance MM nociception indicated by behaviors and neural responses in superficial laminae at the Vc/C₂ region.

Activation of rostral ventromedial medulla neurons by noxious stimulation of cutaneous and deep craniofacial tissues

The rostral ventromedial medulla (RVM) projects to the medullary and spinal dorsal horns and is a major source of descending modulation of nociceptive transmission. Traditionally, neurons in the RVM are classified functionally as on, off, and neutral cells on the basis of responses to noxious cutaneous stimulation of the tail or hind paw. On cells facilitate nociceptive transmission, off cells are inhibitory, whereas neutral cells are unresponsive to noxious stimuli and their role in pain modulation is unclear. Classification of RVM neurons with respect to stimulation of craniofacial tissues is not well defined. In isoflurane-anesthetized male rats, RVM neurons first were classified as on (25.5%), off (25.5%), or neutral (49%) cells by noxious pinch applied to the hind paw. Pinching the skin overlying the temporomandibular joint (TMJ) altered the proportions of on (39.2%), off (42.2%), and neutral (19.6%) cells. To assess the response of RVM cells to specialized craniofacial inputs, adenosine triphosphate (ATP; 0.01-1 mM) was injected into the TMJ and capsaicin (0.1%) was applied to the ocular surface. TMJ and ocular surface stimulation also resulted in a reduced proportion of neutral cells compared with hind paw pinch. Dose-effect analyses revealed that on and off cells encoded the intra-TMJ concentration of ATP. These results suggest that somatotopy plays a significant role in the functional classification of RVM cells and support the notion that neutral cells likely are subgroups of on and off cells. It is suggested that a portion of RVM neurons serve different functions in modulating craniofacial and spinal pain conditions.

GABAergic influence on temporomandibular joint-responsive spinomedullary neurons depends on estrogen status

Sensory input from the temporomandibular joint (TMJ) to neurons in superficial laminae at the spinomedullary (Vc/C1-2) region is strongly influenced by estrogen status. This study determined if GABAergic mechanisms play a role in estrogen modulation of TMJ nociceptive processing in ovariectomized female rats treated with high- (HE) or low-dose (LE) estradiol (E2) for 2days. Superficial laminae neurons were activated by ATP (1mM) injections into the joint space. The selective GABAA receptor antagonist, bicuculline methiodide (BMI, 5 or 50μM, 30μl), applied at the site of recording greatly enhanced the magnitude and duration of ATP-evoked responses in LE rats, but not in units from HE rats. The convergent cutaneous receptive field (RF) area of TMJ neurons was enlarged after BMI in LE but not HE rats, while resting discharge rates were increased after BMI independent of estrogen status. By contrast, the selective GABAA receptor agonist, muscimol (50μM, 30μl), significantly reduced the magnitude and duration of ATP-evoked activity, resting discharge rate, and cutaneous RF area of TMJ neurons in LE and HE rats, whereas lower doses (5μM) affected only units from LE rats. Protein levels of GABAA receptor β3 isoform at the Vc/C1-2 region were similar for HE and LE rats. These results suggest that GABAergic mechanisms contribute significantly to background discharge rates and TMJ-evoked input to superficial laminae neurons at the Vc/C1-2 region. Estrogen status may gate the magnitude of GABAergic influence on TMJ neurons at the earliest stages of nociceptive processing at the spinomedullary region.

Estrogen status and psychophysical stress modify temporomandibular joint input to medullary dorsal horn neurons in a lamina-specific manner in female rats

Estrogen status and psychological stress contribute to the expression of several chronic pain conditions including temporomandibular muscle and joint disorders (TMJD). Sensory neurons that supply the temporomandibular joint (TMJ) region terminate in laminae I and V of the spinal trigeminal nucleus (Vc/C1-2 region); however, little is known about lamina-specificity and environmental influences on the encoding properties of TMJ brainstem neurons. To test the hypothesis that Vc/C1-2 neurons integrate both interoceptive and exteroceptive signals relevant for TMJ nociception, we recorded TMJ-evoked activity in superficial and deep laminae of ovariectomized rats under high and low estradiol (E2) and stress conditions. Rats received daily injections of low (LE) or high (HE) dose E2 and were subjected to forced swim (FS) or sham swim conditioning for 3days. The results revealed marked lamina-specificity in that HE rats displayed enhanced TMJ-evoked activity in superficial, but not deep, laminae independent of stress conditioning. By contrast, FS conditioned rats displayed increased background firing and TMJ-evoked activity of neurons in deep, but not superficial, laminae independent of E2 status. FS also enhanced TMJ-evoked masseter muscle activity and suggested the importance of deep dorsal horn neurons in mediating evoked jaw muscle activity. In conclusion, E2 status and psychophysical stress play a significant role in modifying the encoding properties of TMJ-responsive medullary dorsal horn neurons with a marked lamina-specificity.

Suppression of neurokinin-1 receptor in trigeminal ganglia attenuates central sensitization following inflammation

This study examined whether local application of a neurokinin-1 (NK1) receptor antagonist into the trigeminal ganglia (TRGs) modulates hyperexcitability of trigeminal spinal nucleus caudalis (SpVc) wide-dynamic range (WDR) neuron activity innervating both the temporomandibular joint (TMJ) region and facial skin following TMJ inflammation. Extracellular single unit recording combined with multibarrel electrodes was used. TMJ inflammation was induced by the injection of complete Freund's adjuvant (CFA). WDR neurons responding to electrical stimuli of the TMJ region and facial skin were recorded from the SpVc in anesthetized rats. The spontaneous and mechanical stimulation-induced discharge frequencies of WDR neurons were significantly larger in inflamed rats than in control rats. The spontaneous WDR activities were current-dependently decreased by local iontophoretic application of an NK1 receptor antagonist into the TRGs after 1 and 2 days of inflammation. The firing frequency of WDR neurons and threshold evoked by mechanical stimulation of facial skin returned to control levels by application of the NK1 receptor antagonist into TRGs after 1 day, but not 2 days, of inflammation. These results suggest that in the early stages of inflammation suppression of the NK1 receptor mechanism in TRGs may prevent central sensitization of SpVc nociceptive neurons.

Temporomandibular joint-evoked responses by spinomedullary neurons and masseter muscle are enhanced after repeated psychophysical stress

Psychological stress is a risk factor for the development of musculoskeletal pain of the head and neck; however, the basis for this relationship remains uncertain. This study tested the hypothesis that psychophysical stress alone was sufficient to alter the encoding properties of spinomedullary dorsal horn neurons and masseter muscle activity in male rats. Repeated forced swim conditioning increased markedly both the background firing rate and temporomandibular joint (TMJ)-evoked activity of neurons in deep dorsal horn, while neurons in superficial laminae were less affected. Stress also increased the responses to stimulation of facial skin overlying the TMJ of neurons in deep and superficial dorsal horn. TMJ-evoked masseter muscle activity was enhanced significantly in stressed rats, an effect that was reduced by prior blockade of the spinomedullary junction region. These data indicated that repeated psychophysical stress induced widespread effects on the properties of medullary dorsal horn neurons and masseter muscle activity. The effects of stress were seen preferentially on neurons in deep dorsal horn and included enhanced responses to chemosensory input from the TMJ and mechanical input from overlying facial skin. The stress-induced elevation in TMJ-evoked masseter muscle activity matched well with the changes seen in dorsal horn neurons. It is concluded that the spinomedullary junction region plays a critical role in the integration of psychophysical stress and sensory information relevant for nociception involving deep craniofacial tissues.

Differential ascending projections of temporomandibular joint-responsive brainstem neurons to periaqueductal gray and posterior thalamus of male and female rats

Several craniofacial pain conditions, including temporomandibular joint disorders (TMJDs), are more prevalent in women than men. The basis for sex differences in deep craniofacial pain is not known. The present study compared the magnitude of ascending projections from temporomandibular joint (TMJ)-responsive neurons in trigeminal brainstem with the ventrolateral periaqueductal gray (vlPAG) or posterior nucleus of the thalamus (Po) in males and female rats. Fluorogold (FG) was injected into vlPAG or Po, and TMJ-responsive neurons were identified by Fos-like immunoreactivity (Fos-LI) after mustard oil injection. TMJ-evoked Fos-LI was similar in males and females; however, significant differences in cell counts were seen for FG single-labeled and Fos/FG double-labeled neurons in trigeminal brainstem. After vlPAG injections, the number of FG-labeled neurons in trigeminal subnucleus interpolaris (Vi), ventral interpolaris/caudalis transition (vl-Vi/Vc), and dorsal paratrigeminal region (dPa5) was greater in females than males. The percentage of Fos/FG double-labeled neurons in vl-Vi/Vc and dPa5 after vlPAG injection also was greater in females than males. In contrast, after Po injections, males displayed a greater number of FG-labeled neurons in superficial laminae (Lam I/II) of trigeminal subnucleus caudalis (Vc) and upper cervical spinal cord (C(1-2)) and deeper laminae (Lam III/V) at C(1-2) than females. The percentage of Fos/FG double-labeled neurons in Lam I/II of Vc after Po injection also was greater in males than females. These data revealed significant sex differences in ascending projections from TMJ-responsive neurons in trigeminal brainstem. Such differences may influence the ability of males and females to recruit autonomic reflexes and endogenous pain control circuits relevant for TMJ nociception.

Cortical NR2B NMDA subunit antagonism reduces inflammatory pain in male and female rats

Background

Studies have shown that N-methyl-D-aspartate (NMDA) receptors play a critical role in pain processing at different levels of the central nervous system.

Methods

In this study, we used adult Wistar rats to examine gender differences in the effects of NR2B NMDA antagonism at the level of the anterior cingulate cortex in phasic pain, and in the first and second phases of a formalin test. Rats underwent stereotactic surgery for cannula implantation in the anterior cingulate cortex. After recovery, paw withdrawal latency to a noxious thermal stimulus was assessed. Rats were also subjected to a formalin pain test whereby 60 μL of 5% formalin was injected into the right hind paw.

Results

Female and male rats that received Ro 25-6981, an NR2B antagonist, before formalin injection showed significantly reduced pain responses to the formalin test compared with saline-injected control rats (P < 0.05). No gender differences in phasic pain responses were found in rats treated with Ro 25-6981.

Conclusion

These results suggest that cortical antagonism of the NR2B subunit reduces inflammatory pain levels in both genders of rat.

Psychophysical stress increases the expression of phospho-CREB, Fos protein and neurokinin-1 receptors in superficial laminae of trigeminal subnucleus caudalis in female rats

Psychological stress and estrogen status are risk factors to develop painful temporomandibular joint disorders (TMJD); however, the neural basis for this relationship is not known. This study tested the hypothesis that repeated forced swim stress and estradiol treatment alter the phosphorylation of cAMP responsive element-binding protein (pCREB) in trigeminal subnucleus caudalis (Vc), the initial site of sensory input from the TMJ. Ovariectomized female rats were given low or high dose estradiol and subjected to repeated forced swim stress for 3 days and on day 4 an intra-TMJ injection of mustard oil or vehicle was given. Forced swim alone increased the number of pCREB-positive neurons, independent of estradiol treatment or TMJ stimulation, in superficial and deep laminae of Vc. Forced swim also increased the number of Fos-positive neurons in superficial laminae and neurokinin-1 receptor mRNA in whole dorsal Vc, independent of estradiol treatment. These results indicated that persistent psychophysical stress alone was sufficient to increase the expression of pCREB and downstream regulated genes associated with enhanced excitability in the caudal medullary dorsal horn, a brainstem region thought to be critical for TMJD pain.

NMDA receptor blockade reduces temporomandibular joint-evoked activity of trigeminal subnucleus caudalis neurons in an estrogen-dependent manner

Estrogen status is a risk factor in painful temporomandibular disorders (TMJD). Previously we reported that estradiol (E2) enhanced nociceptive processing of TMJ input by neurons in superficial laminae at the spinomedullary (Vc/C(1-2)) region; however, the mechanisms for this enhancement are not known. The present study determined if ionotropic glutamate receptors contribute to TMJ nociceptive processing in an E2-dependent manner. Ovariectomized (OvX) female rats were treated with high E2 (HE2) or low dose E2 (LE2) for 2 days and neural activity was recorded in laminae I-II at the Vc/C(1-2) region. TMJ-responsive units were activated by ATP injections into the joint space. ATP-evoked unit responses in HE2 rats were reduced significantly by topical application of the N-methyl-D-aspartate receptor antagonist, D(-)-2-amino-5-phosphonopentanoic acid (AP5) in a dose-related manner, while units from LE2 were not affected. Application of the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), inhibited the ATP-evoked responses in both groups. Spontaneous activity of TMJ units was not influenced by AP5, whereas it was reduced by DNQX similarly in both groups. The high threshold convergent cutaneous receptive field area of TMJ units was not changed by AP5, whereas DNQX caused a significant reduction in both groups. These results suggest that NMDA-dependent mechanisms contribute to the enhanced ATP-evoked responses of TMJ units in superficial laminae at the Vc/C(1-2) region under high E2 conditions, while non-NMDA-dependent mechanisms modify the encoding properties of TMJ units independent of E2 status.

Chronic inflammation and estradiol interact through MAPK activation to affect TMJ nociceptive processing by trigeminal caudalis neurons

The mitogen-activated protein kinase/extracellular regulated kinase (MAPK/ERK) pathway plays a key role in mediating estrogen actions in the brain and neuronal sensitization during inflammation. Estrogen status is a risk factor in chronic temporomandibular muscle/joint (TMJ) disorders; however, the basis for this relationship is not known. The present study tested the hypothesis that estrogen status acts through the MAPK/ERK signaling pathway to alter TMJ nociceptive processing. Single TMJ-responsive neurons were recorded in laminae I-II at the spinomedullary (Vc/C(1-2)) junction in naïve ovariectomized (OvX) female rats treated for 2 days with high-dose (20 microg/day; HE2) or low-dose estradiol (2 microg/day; LE2) and after chronic inflammation of the TMJ region by complete Freund's adjuvant for 12-14 days. Intra-TMJ injection of ATP (1 mM) was used to activate Vc/C(1-2) neurons. The MAPK/ERK inhibitor (PD98059, 0.01-1 mM) was applied topically to the dorsal Vc/C(1-2) surface at the site of recording 10 min prior to each ATP stimulus. In naïve HE2 rats, low-dose PD98059 caused a maximal inhibition of ATP-evoked activity, whereas even high doses had only minor effects on units in LE2 rats. By contrast, after chronic TMJ inflammation, PD98059 produced a marked and similar dose-related inhibition of ATP-evoked activity in HE2 and LE2 rats. These results suggested that E2 status and chronic inflammation acted, at least in part, through a common MAPK/ERK-dependent signaling pathway to enhance TMJ nociceptive processing by laminae I-II neurons at the spinomedullary junction region.

Differential effects of estradiol on encoding properties of TMJ units in laminae I and V at the spinomedullary junction in female rats

To determine whether estrogen status modulated dorsal horn neural activity relevant to temporomandibular joint (TMJ) processing single units were recorded in superficial and deep laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C1-2) junction of ovariectomized (OvX) female rats under barbiturate anesthesia after 17beta-estradiol (E2) treatment for 2 days. E2 dose-dependently enhanced the response to intra-TMJ stimulation by adenosine triphosphate (ATP) of neurons classified as nociceptive specific (NS), but not wide dynamic range (WDR), in superficial laminae. ATP caused similar responses among NS and WDR neurons from deep laminae in all groups. By contrast, the cutaneous receptive field areas of WDR, but not NS, units in superficial and deep laminae were enlarged in high E2-treated (HE2) compared with low E2-treated (LE2) females. Units from untreated or vehicle-treated male rats displayed responses similar to those of LE2 females. TMJ units in superficial laminae from females were more likely to receive convergent cutaneous input and respond to jaw movement than males, independent of E2 treatment. Western blot analysis revealed similar levels of P2X2 and P2X3 receptor protein in Vc/C1-2 or trigeminal ganglion samples in all groups. Immunohistochemistry revealed dense terminal labeling for P2X3 receptors in superficial laminae and moderate labeling in deep laminae at the Vc/C1-2 junction. These data indicated a significant linkage between estrogen status and the magnitude of articular input evoked by ATP from TMJ neurons in the superficial laminae at the Vc/C1-2 junction, whereas estrogenic modulation of TMJ neurons in deep laminae affected only the convergent input from overlying facial skin.

Activation of central 5HT2A receptors reduces the craniofacial nociception of rats

We assessed the contribution of central 5HT2A receptors to the craniofacial tissue nociception in naïve male rats. First, we tested whether activation of central 5HT2A receptors affected nociceptive neural activities recorded from superficial laminae of the trigeminal subnucleus caudalis (Vc)/upper cervical spinal cord junction (Vc/C2) region. Two types of units, such as deep-nociceptive or skin-wide dynamic range (WDR) units were identified from extracellular recordings. Topical administration of 5HT2A receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) onto the Vc/C2 region significantly reduced deep-nociceptive unit discharges evoked by formalin injection into the masseter muscle. Noxious pinch stimulation to the facial skin-evoked skin-WDR unit discharges was significantly reduced by topical administration of 0.1 mg/rat DOI onto the Vc/C2 region. Second, we tested whether i.c.v. administration of DOI affected Fos-like immunoreactivity (-LI) evoked by formalin injection into the masseter muscle. Fos-LI was significantly induced mainly at the ventrolateral (vl) area of trigeminal subnucleus interpolaris (Vi)/Vc junction (vl-Vi/Vc) region and Vc/C2 region in vehicle-treated rats. Formalin-evoked Fos-LI was significantly reduced in laminae I-II of the Vc/C2, but not vl-Vi/Vc region after i.c.v. administration of DOI. Finally, orofacial nocifensive behavioral activities evoked by formalin injection into the masseter muscle were significantly reduced by intracisternal administration of DOI. These results suggest that 5HT2A receptors in the Vc/C2 region mediate antinociceptive effects in the craniofacial nociception.

The influence of gender and sex steroids on craniofacial nociception

Several pain conditions localized to the craniofacial region show a remarkable sex-related difference in their prevalence. These conditions include temporomandibular disorders and burning mouth syndrome as well as tension-type, migraine, and cluster headaches. The mechanisms that underlie sex-related differences in the prevalence of these craniofacial pain conditions remain obscure and likely involve both physiological and psychosocial factors. In terms of physiological factors relevant to the development of headache, direct evidence of sex-related differences in the properties of dural afferent fibers or durally activated second-order trigeminal sensory neurons has yet to be provided. There is, however, evidence for sex-related differences in the response properties of afferent fibers and second-order trigeminal sensory neurons that convey nociceptive input from other craniofacial tissues associated with sex-related differences in chronic pain conditions, such as those that innervate the masseter muscle and temporomandibular joint. Further, modulation of craniofacial nociceptive input by opioidergic receptor mechanisms appears to be dependent on biological sex. Research into mechanisms that may contribute to sex-related differences in trigeminal nociceptive processing has primarily focused on effect of the female sex hormone estrogen, which appears to alter the excitability of trigeminal afferent fibers and sensory neurons to noxious stimulation of craniofacial tissues. This article discusses current knowledge of potential physiological mechanisms that could contribute to sex-related differences in certain craniofacial pain conditions.

Contribution of peripheral 5-HT2A or 5-HT3 receptors to Fos expression in the trigeminal spinal nucleus produced by acute injury to the masseter muscle during persistent temporomandibular joint inflammation in rats

We investigated the contribution of peripheral 5-HT2A or 5-HT3 receptors to Fos expression in the trigeminal spinal nucleus (VSP) following acute masseter muscle injury in male rats with or without temporomandibular joint (TMJ) inflammation persisting for 7 days. TMJ inflammation was evoked by an injection of complete Freund's adjuvant (CFA). Two hours after formalin injection into the masseter muscle produced Fos-like immunoreactivity (Fos-LI) in several regions of the VSP and upper cervical spinal cord (C2), such as ventrolateral (vl) area of the trigeminal subnucleus caudalis (Vc)/subnucleus interpolaris (Vi) transition (vl-Vi/Vc), paratrigeminal nucleus (dPa5), middle portion of the Vc (mid-Vc) and Vc/C2 transition (Vc/C2) regions in both groups. Significant increases in the number of Fos-LI were observed in these areas in CFA group compared with non-CFA group. TMJ inflammation alone did not induce a significant level of Fos-LI in the VSP. In order to assess the effect of antagonizing 5-HT2A or 5-HT3 receptors on formalin-induced Fos-LI, rats were pre-treated with local (masseter muscle) administration of ketanserin or tropisetron (0.01, 0.1 mg/rat) 20 min prior to formalin injection. In CFA group, these antagonists given locally reduced the Fos-LI response in the laminae I-II at the mid-Vc and Vc/C2 regions. These antagonists reduced the Fos-LI response in the dPa5, but not in the vl-Vi/Vc region. The Fos-LI response was not affected by i.v. administration of ketanserin (0.01, 0.1 mg/rat) or tropisetron (0.01 mg/rat). In non-CFA group, these antagonists given locally did not reduce the Fos-LI response. These results suggest that peripheral 5-HT2A and 5-HT3 receptors contribute to nociceptive processing in the masseter muscle in TMJ inflammatory conditions.

Local blockade of integrins in the temporomandibular joint region reduces Fos-positive neurons in trigeminal subnucleus caudalis of female rats produced by jaw movement

This study assessed the influence of integrins on trigeminal brainstem neural activity evoked during jaw movement (JM). Limited range of motion and pain during jaw opening are common complaints of patients with temporomandibular joint (TMJ) disorders. JM (0.5 Hz, 30 min) was presented to ovariectomized (OvX) female rats given estrogen replacement and males under barbiturate anesthesia. Quantification of Fos-like immunoreactivity (Fos-LI) after JM served as an index of evoked neural activity. Rats were injected locally in the TMJ with either an active (GRGDS, 300 microM, 25 microl) or an inactive integrin antagonist (SDGRG) prior to JM. The effect of prior inflammation of the TMJ region was assessed in separate groups of rats by injecting bradykinin (10 microM, 25 microl) with or without integrin drugs prior to JM. Active integrin antagonist significantly reduced JM-evoked Fos-LI in superficial laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C2) junction in OvX compared to male rats independent of bradykinin pretreatment. Fos-LI produced in the dorsal paratrigeminal and trigeminal subnucleus interpolaris/caudalis (Vi/Vc) transition regions was not reduced by active integrin antagonist in males or OvX females. Active integrin antagonist did not affect Fos-LI produced after injection of bradykinin alone into the TMJ. These results suggest that RGD binding integrins contribute to JM-evoked neural activity at the Vc/C2 junction under naive and inflamed conditions in a sex-dependent manner.

Hypertonic saline-induced muscle nociception and c-fos activation are partially mediated by peripheral NMDA receptors

In this study, the animal model of hypertonic saline (HS) infusion protocol was developed and utilized to test the hypothesis that HS causes peripheral release of glutamate, and that blockade of peripheral NMDA receptors significantly reduces HS-induced nocifensive behavior and central neuronal activation. Nocifensive behavior and c-fos immunoreactivity, as a marker of central neuronal activation, were assessed from the animals that received intramuscular HS infusion with and without the NMDA receptor antagonist, MK-801. HS infusion (20 microl/min for 10 min) in the rat masseter produced prolonged nocifensive hindpaw shaking responses that peaked in the first minute and gradually diminished over the infusion period. The HS induced nocifensive behavior was dose-dependently attenuated by MK-801 pretreatments (0.3 mg/kg and 0.1 mg/kg), but not by vehicle pretreatment (isotonic saline; ISO), in the masseter muscle. HS infusion produced a significant number of Fos positive neurons in the ispsilateral subnucleus caudalis (Vc). Subsequent immunohistochemical studies showed that peripheral MK-801 pretreatment effectively reduced the HS induced neuronal activation in the Vc. These results provide compelling evidence that HS-induced muscle nociception is mediated, in part, by peripheral release of glutamate, and that blockade of peripheral glutamate receptors may provide effective means of preventing central neuronal activation.

Oestrogen receptor-immunoreactive neurons in the trigeminal sensory system of male and cycling female rats

Many common craniofacial pain conditions are more prevalent in women than men and may be related to the phase of the menstrual cycle. Long-term effects of oestrogen in the nervous system are produced by receptor-mediated [oestrogen receptor alpha (ERalpha) and beta (ERbeta) isoforms] mechanisms; however, it is not known if the distribution of ER-positive neurons in the trigeminal system is similar in males and females. Quantitative immunocytochemistry was used to compare the distribution of ERalpha-labelled neurons in the trigeminal brainstem complex (TBC) and ganglion of male and female rats at different stages of the oestrous cycle. A high density of ERalpha-labelled neurons was seen in the superficial laminae (I-III) throughout the trigeminal subnucleus caudalis (Vc) and the upper cervical dorsal horn. Counts of ERalpha-positive neurons in laminae I-III were similar for prooestrous and dioestrous females, while males had fewer cells. The deeper laminae (IV-V) of the Vc and the cervical dorsal horn had few ERalpha-positive neurons in all groups. At the region surrounding the central canal at caudal levels of the Vc, prooestrous females had more ERalpha-positive neurons than dioestrous females or males. Few labelled cells were seen rostral to the trigeminal subnucleus interpolaris/caudalis transition region (Vi/Vc) in any group. In the trigeminal ganglion, prooestrous and dioestrous females had a moderate (8-10%) number of nuclear-labelled small or medium-sized neurons, while males had fewer labelled cells (4.5%). Qualitatively, the pattern of staining for ERbeta was similar, although weaker, than for ERalpha in the trigeminal dorsal horn or ganglion. These results were consistent with the hypothesis that oestrogen acts through trigeminal ganglion cells and caudal portions of the Vc to modulate sensory and autonomic aspects of craniofacial pain in a sex-related manner.

Central serotonin 3 receptors play an important role in the modulation of nociceptive neural activity of trigeminal subnucleus caudalis and nocifensive orofacial behavior in rats with persistent temporomandibular joint inflammation

The role of central serotonin 3 receptors on neural activities recorded from superficial laminae of trigeminal subnucleus caudalis/upper cervical spinal cord junction region was investigated using rats with (Complete Freund's Adjuvant day 7 group) or without (non-Complete Freund's Adjuvant group) persistent temporomandibular joint inflammation evoked by Complete Freund's Adjuvant for 7 days. We identified two types of units, Deep-wide dynamic range units and Skin-wide dynamic range units from extracellular recordings. Deep-wide dynamic range units have mechanoreceptive fields in the deep craniofacial tissues including masseter muscle but do not have cutaneous mechanoreceptive fields. Deep-wide dynamic range unit discharges evoked by the formalin injection into masseter muscle were significantly enhanced in the late phase in Complete Freund's Adjuvant day 7 group. Discharges of Skin-wide dynamic range units evoked by the noxious pinch stimulation to facial skin in Complete Freund's Adjuvant day 7 group were significantly enhanced compared with those in non-Complete Freund's Adjuvant group. Topical administration of central serotonin 3 receptor antagonist, tropisetron, onto trigeminal subnucleus caudalis/upper cervical spinal cord junction region significantly reduced both formalin-evoked Deep-wide dynamic range unit and pinch-evoked Skin-wide dynamic range unit discharges in non-Complete Freund's Adjuvant and Complete Freund's Adjuvant day 7 groups significantly. The inhibitory effects of tropisetron on pinch-evoked Skin-wide dynamic range unit discharges were prolonged in Complete Freund's Adjuvant day 7 group compared with those in non-Complete Freund's Adjuvant group. The role of central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region was also tested by orofacial formalin test in Complete Freund's Adjuvant day 7 group. Intracisternal administration of tropisetron decreased the orofacial nocifensive behavior in the late phase evoked by the injection of formalin into the masseter muscle. These results suggest that central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region are involved in mediating pronociceptive effects in both superficial and deep craniofacial tissues nociception during persistent temporomandibular joint inflammation.

Effect of persistent monoarthritis of the temporomandibular joint region on acute mustard oil-induced excitation of trigeminal subnucleus caudalis neurons in male and female rats

The effect of persistent inflammation of the temporomandibular (TMJ) region on Fos-like immunoreactivity (Fos-LI) evoked by acute noxious stimulation of the same or opposite TMJ was assessed in male and cycling female rats. Two weeks after inflammation of the TMJ by complete Freund's adjuvant (CFA, 25 microg) the selective small fiber excitant, mustard oil (MO, 20%), was injected into the arthritic or opposite TMJ under barbiturate anesthesia. MO stimulation of the arthritic TMJ increased Fos-LI ipsilateral, but not contralateral, to MO compared to naïve subjects in superficial laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C2) junction independent of sex hormone status. Unexpectedly, MO stimulation of the opposite TMJ in arthritic rats also produced a greater Fos-LI response ipsilateral to MO than naïve animals. Fos-LI produced in the dorsal paratrigeminal region (dPa5) and Vc/C2 junction after MO stimulation of the normal TMJ was significantly greater in proestrous than diestrous females or male monoarthritic rats. In contrast to naïve animals, Fos-LI was produced in deep laminae at the Vc/C2 junction ipsilateral to MO in CFA-treated animals independent of the site of prior CFA inflammation or sex hormone status. These results indicated that persistent monoarthritis of the TMJ region enhanced the excitability of trigeminal brainstem neurons to subsequent TMJ injury that occurred bilaterally in multiple regions of the lower trigeminal brainstem complex and depended on sex hormone status.

Role of capsaicin-sensitive primary afferent inputs from the masseter muscle in the C1 spinal neurons responding to tooth-pulp stimulation in rats

The aim of the present study was to demonstrate the convergence of inputs from masseter muscle (MM) and tooth pulp (TP) onto C1 spinal neurons and to determine whether the afferent fibers express the functional vanilloid receptor (VR1). Extracellular single-unit recordings were made from 61 C1 units responding to TP electrical stimulation with a constant temporal relationship to a digastric electromyogram signal in pentobarbital anesthetized rats. Eighty-four percent of C1 neurons responding to TP stimulation also responded to the ipsilateral MM stimulation. Of these neurons, 61% were considered to be afferent inputs from Adelta-fibers and the remaining units (39%) were C-fibers, based on calculation of the nerve conduction velocity. Intramuscular injection of capsaicin (0.05 and 0.1%) produced a reduction in a MM-induced C1 neuronal activity in a dose-dependent manner and this effect was antagonized by pretreatment with an antagonist of VR1, capsazepine. Some of these units were also excited by noxious heat stimulation (> 43 degrees C). The trigeminal root ganglion (TRG) neurons that innervated the MM were retrogradely labeled with Fluorogold (FG) and the small-diameter FG-labeled TRG neurons expressed the immunoreactivity for VR1. After intramuscular mustard oil injection (noxious chemical stimulation), the C1 neuronal activity induced by both touch and pinch stimuli was enhanced and their receptive field sizes were significantly expanded. These changes were reversed within 15-20 min. These results suggest that there may be the convergence of noxious afferents inputs from the MM and TP afferents on the same C1 neurons in rats, and that the afferent fibers expressing the functional VR1 may contribute to the hyperalgesia and/or referred pain associated with temporomandibular joint disorder.

Craniofacial inputs to upper cervical dorsal horn: implications for somatosensory information processing

The aim of this study was to characterize the properties of somatosensory neurons in the first 2 cervical spinal dorsal horns (C1 and C2 DHs) and compare them with those previously described for the rostral subnucleus caudalis (rVc). A total of 74 nociceptive neurons classified as wide-dynamic-range (WDR) or nociceptive-specific (NS), as well as 72 low-threshold mechanoreceptive (LTM) neurons, was studied in urethane/chloralose-anesthetized rats. The majority of LTM neurons were located in laminae III/IV and had a small mechanoreceptive field (RF) that included the posterior face and cervical tissues. In contrast, the nociceptive neurons were located in laminae I/II or V/VI, and the RF of each C1 and C2 DH nociceptive neuron included a part of the face and in 47% of them the RF included a region supplied by upper cervical afferents. There was a gradual caudal shift in the neuronal RF from nasal/intraoral tissues towards the neck as recording sites progressed from rVc to C1 and C2 DHs. In contrast to LTM neurons, many C1 and C2 DH nociceptive neurons received mechanosensitive convergent afferent inputs from cervical and craniofacial deep tissues (e.g., tongue muscles or temporomandibular joint), and over 50% could be activated by hypoglossal (XII) nerve electrical stimulation. We propose that C1 and C2 DHs represent part of the caudal extension of the Vc, and that Vc and C1 and C2 DHs may act together as one functional unit to process nociceptive information from craniofacial and cervical tissues, including that from deep craniofacial tissues.

Differential modulation of TMJ neurons in superficial laminae of trigeminal subnucleus caudalis/upper cervical cord junction region of male and cycling female rats by morphine

Sex differences in the cellular responses to morphine were examined in an animal model of temporomandibular joint (TMJ) pain. TMJ-responsive neurons were recorded in the superficial laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C(2)) junction region, the initial site of synaptic integration for TMJ afferents, in male and cycling female rats under barbiturate anesthesia. Unit activity was evoked by local injection of bradykinin into the TMJ capsule at 30 min intervals and the effects of morphine sulfate (0.03-3 mg/kg, i.v.) were assessed by a cumulative dose regimen. Morphine caused a dose-related inhibition of bradykinin-evoked unit activity in males and diestrous females in a naloxone-reversible manner, while evoked unit activity in proestrous females was not reduced. The apparent sex hormone-related aspect of morphine analgesia was selective for evoked unit activity, since the spontaneous activity of TMJ units was reduced similarly in all groups, while the convergent cutaneous receptive field area of TMJ units did not change in any group. These results were consistent with the hypothesis that sex hormone status interacts with pain control systems to modify neural activity at the level of the Vc/C(2) junction region relevant for TMD pain.

Convergence of nociceptive information from temporomandibular joint and tooth pulp afferents on C1 spinal neurons in the rat

The aim of the present study was to test the hypothesis that there is a convergence of afferent inputs from the temporomandibular joint (TMJ) on C1 spinal neurons responding to electrical stimulation of the tooth pulp (TP). In 14 pentobarbital anesthetized rats, the extracellular single unit activity of 31 C1 spinal neurons and the amplitude in a digastric muscle electromyogram (n = 31) increased proportionally during 1.0-3.5 times the threshold for the jaw-opening reflex (JOR). Of 31 C1 spinal neurons responsive to TP afferents, 28 (approximately 90%) were also excited by electrical stimulation of the ipsilateral TMJ capsule. All neurons tested were divided into three categories of nociceptive specific, wide dynamic range and non-responsive as to their responsiveness to mechanical stimuli (pin prick and touch) of the somatic receptive field (skin of the face, neck, jaw and upper forearm) and TMJ capsule. Nineteen (68%) of 28 C1 spinal neurons received nociceptive information from C fibers of the TMJ capsule. These results suggest that there is a convergence of noxious information from the TMJ and TP afferents on the same C1 spinal neurons, which importantly contribute to pain perception from the TMJ region.

TNF alpha-p55 receptors: medullary brainstem immunocytochemical localization in normal and vagus nerve-transected rats

Tumor necrosis factor alpha (TNF(alpha)) is a potent modulator of autonomic reflex mechanisms that control the stomach. Evidence suggests that TNF(alpha) action directly on vago-vagal reflex control circuits causes the autonomic misregulation of digestion manifested as gastrointestinal stasis, nausea, and emesis associated with illness. Neurophysiological studies indicated that TNF(alpha) may have effects on vagal afferents in the solitary nucleus, as well as neurons of the solitary nucleus (NST) and dorsal motor nucleus (DMN) of the vagus. The aim of this study was to determine the location of the TNFR1 receptor (p55) in the medulla using immunocytochemical methods. We devised a technique for localizing the p55 receptor using heat-induced antigen recovery in fixed tissue sections. This protocol allowed us to demonstrate that dense p55-immunoreactivity (p55-ir) is constitutively present on central (but not peripheral) vagal afferents in the solitary tract (ST) and nucleus; p55-ir is also present on afferents entering the spinal trigeminal nucleus. Unilateral supra-nodose vagotomy eliminated p55-ir from ipsilateral central vagal afferents. Virtually all neurons in the brainstem appeared to express p55-ir at a low level, i.e., just above background. However, vagotomy caused a dramatic up-regulation of p55-ir in vagal motor neurons. This increase in p55-ir in axotomized neurons may play a pivotal role in the connection between the occurrence of the injury and the initiation of apoptotic processes resulting in elimination of damaged neurons.

Innocuous jaw movements increase c-fos expression in trigeminal sensory nuclei produced by masseter muscle inflammation

Muscle tenderness and pain during movements are prominent symptoms associated with persistent jaw muscle pain. However, there is virtually no information on how trigeminal neurons respond to jaw movements (JM) or muscle palpation in the presence of muscle tissue injury or myositis. In this study, we investigated the effects of innocuous JM in the presence of acute masseteric inflammation on postsynaptic responses in the trigeminal brainstem nuclei by examining the expression of c-fos. In one group of rats, unilateral injections of an inflammatory substance, mustard oil (MO: 20%, 25 microl) were made into a masseter muscle. In another group, controlled and systematic JM were provided following MO injection. Three additional groups of rats were used to control for anesthetic, JM, and injection procedure. MO injected in the masseter muscle induced a high level of Fos protein expression in four principal trigeminal regions: the subnucleus caudalis (Vc), the ventral and dorsal regions of the Vc/Vi (subnucleus interpolaris) transition zone, and the paratrigeminal nucleus (PTN). Movements following MO injection consistently produced a significantly greater level of Fos expression in all these areas, especially in the Vc/Vi transition region and caudal Vc on the ipsilateral side. Importantly, movements also induced a significantly greater level of Fos expression in the caudal Vc on the contralateral side. The present results provide the first documentation that innocuous JM in the presence of muscle inflammation significantly increase the MO-induced c-fos expression in the trigeminal brainstem nuclei, which may explain the greater pain experienced during movement of inflamed or injured muscles.

Response properties of TMJ units in superficial laminae at the spinomedullary junction of female rats vary over the estrous cycle

Neurons responsive to stimulation of the temporomandibular joint (TMJ) region were recorded from superficial laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C(2)) junction region of cycling female rats under barbiturate anesthesia. To determine if receptive field (RF) properties or sensitivity to algesic chemicals of TMJ units vary over the estrous cycle, animals were selected from proestrous (high estrogen) or early diestrous (low estrogen) stages. More than 90% of TMJ units from each group received convergent nociceptive input [wide dynamic range (WDR) or nociceptive specific (NS)-like] from facial skin. The cutaneous high-threshold RF areas of WDR units from proestrous rats were 30% larger than diestrous units, while RF areas of NS units were similar. Bradykinin (BK, 0.1-10 microM) injection into the TMJ region excited a high percentage of units (>80% of total) from both groups in a dose-related manner. However, BK-evoked response magnitude (R(mag), +140%) and duration (+64%) were greater for proestrous than diestrous units. Both WDR and NS-like TMJ units of proestrous females displayed enhanced BK-evoked R(mag) values and response duration. Glutamate or mustard oil excitation of TMJ units was not affected by stage of the estrous cycle. Several TMJ units from proestrous and diestrous females were activated antidromically from the contralateral posterior thalamus, indicating that projection and nonprojection units were included in the sample population. These results were consistent with the hypothesis that factors related to stage of the estrous cycle modify the processing of deep craniofacial inputs by superficial dorsal horn neurons at the spinomedullary junction, a key region for the initial integration of sensory signals from the TMJ.

Sex differences in amino acid release from rostral trigeminal subnucleus caudalis after acute injury to the TMJ region

The neurological basis for painful temporomandibular disorders (TMD) and the higher prevalence of TMD pain in women than men is not known. To better define the circuitry and neurochemical mechanisms in the lower brainstem associated with noxious sensory inputs from the temporomandibular joint (TMJ) region a microdialysis method was used to measure the release of amino acid transmitters from the ventral trigeminal subnucleus interpolaris/caudalis transition region (Vi/Vc-vl). The irritant chemical, mustard oil, was injected into the TMJ region (TMJ-MO) under barbiturate anesthesia in males and normal cycling female rats. Males displayed significant increases in glutamate, serine, and glycine within 15 min after TMJ-MO and increases in citrulline occurred after a delay of 15-30 min. TMJ-MO did not enhance amino acid release in diestrus or proestrus females. GABA release was not affected by TMJ-MO in males or females. Pretreatment with morphine (3 mg/kg, i.v.) prevented the increase in amino acid release seen after TMJ-MO in males. Amino acid release at the Vi/Vc-vl transition region evoked by TMJ-MO also was prevented by prior microinjection of the GABA(A) receptor agonist, muscimol, into the most caudal portion of Vc suggesting this region acted as a critical relay for nociceptive inputs from the TMJ region. These results suggest that glutamatergic mechanisms acting at the Vi/Vc-vl transition region contribute to processing of nociceptive signals that arise from the TMJ region. These results also are consistent with the hypothesis that central neural mechanisms that integrate nociceptive inputs from deep craniofacial tissues are different in males and females.