TMJ-responsive Neurons Research Articles

Estrogen Status Gates Effects of Kappa-Opioid Receptor on Temporomandibular Joint-Responsive Neurons at the Spinomedullary Junction in Female Rats.

To determine whether estrogen status alters κ-opioid inhibition of nociceptive processing by affecting temporomandibular joint (TMJ) input to neurons in the trigeminal subnucleus caudalis [Vc]/C1-2 region at the spinomedullary junction in female rats. TMJ-responsive neurons were recorded in laminae I-II of the Vc/C1-2 region at the spinomedullary junction of ovariectomized female rats treated for 2 days with low-dose estradiol (LE group; 2 mg/day) or high-dose estradiol (HE group; 20 mg/day). Under isoflurane anesthesia, TMJ neurons were activated by adenosine triphosphate (ATP; 1 mM, 20 μl), which was injected into the joint space before and after cumulative doses of a κ-opioid receptor (KOR) agonist (U50488) given systemically (0.03, 0.3, and 3 mg/kg, intravenously) or by local application to the dorsal surface of the Vc/C1-2 region (1 and 10 nmol/30 μl). Analysis of variance and Newman-Keuls test were performed to compare the data. Systemic U50488 caused a dose-related inhibition of ATP-evoked neuronal activity in HE rats and reduced the size of the neuronal cutaneous receptive field (RF), while effects in LE rats were not significant. Systemic U50488 reduced the spontaneous activity of TMJ-responsive neurons to similar levels in LE and HE groups. Locally applied U50488 inhibited ATP-evoked neuronal activity in HE rats, but not in LE rats. Systemic and local administration of the KOR antagonist nor-binaltorphinine (nor-BNI) partially reversed the decrease in Rmag induced by U50488, but had no effect on neurons from LE rats. These results indicate that KOR-dependent effects on TMJ-responsive neurons in the superficial laminae of the Vc/C1-2 region in female rats are differentially modified by high and low estrogen status. The site of action for estrogen-induced modulation of TMJ neuronal activity by KOR likely includes second-order neurons in the Vc/C1-2 region.

Local group I mGluR antagonists reduce TMJ-evoked activity of trigeminal subnucleus caudalis neurons in female rats

Group I metabotropic glutamate receptors (mGluR1 and mGluR5) are functionally linked to estrogen receptors and play a key role in the plasticity of central neurons. Estrogen status strongly influences sensory input from the temporomandibular joint (TMJ) to neurons at the spinomedullary (Vc/C1–2) region. This study tested the hypothesis that TMJ input to trigeminal subnucleus caudalis/upper cervical cord (Vc/C1–2) neurons involved group I mGluR activation and depended on estrogen status. TMJ-responsive neurons were recorded in superficial laminae at the Vc/C1–2 region in ovariectomized (OvX) female rats treated with low-dose estradiol (2μg/day, LE) or high-dose estradiol (20μg/day, HE) for 2days. TMJ-responsive units were activated by adenosine triphosphate (ATP, 1mM) injected into the joint space. Receptor antagonists selective for mGluR1 (CPCCOEt) or mGluR5 (MPEP) were applied topically to the Vc/C1–2 surface at the site of recording 10min prior to the intra-TMJ ATP stimulus. In HE rats, CPCCOEt (50 and 500μM) markedly reduced ATP-evoked unit activity. By contrast, in LE rats, a small but significant increase in neural activity was seen after 50μM CPCCOEt, while 500μM caused a large reduction in activity that was similar in magnitude as that seen in HE rats. Local application of MPEP produced a significant inhibition of TMJ-evoked unit activity independent of estrogen status. Neither mGluR1 nor mGluR5 antagonism altered the spontaneous activity of TMJ units in HE or LE rats. High-dose MPEP caused a small reduction in the size of the convergent cutaneous receptive field in HE rats, while CPCCOEt had no effect. These data suggest that group I mGluRs play a key role in sensory integration of TMJ nociceptive input to the Vc/C1–2 region and are largely independent of estrogen status.

Inhibition of temporomandibular joint input to medullary dorsal horn neurons by 5HT3 receptor antagonist in female rats

Repeated forced swim (FS) conditioning enhances nociceptive responses to temporomandibular joint (TMJ) stimulation in female rats. The basis for FS-induced TMJ hyperalgesia remains unclear. To test the hypothesis that serotonin 3 receptor (5HT3R) mechanisms contribute to enhanced TMJ nociception after FS, ovariectomized female rats were treated with estradiol and subjected to FS for three days. On day 4, rats were anesthetized with isoflurane and TMJ-responsive neurons were recorded from superficial and deep laminae at the trigeminal subnucleus caudalis/upper cervical (Vc/C1–2) region and electromyographic (EMG) activity was recorded from the masseter muscle. Only Vc/C1–2 neurons activated by intra-TMJ injections of ATP were included for further analysis. Although neurons in both superficial and deep laminae were activated by ATP, only neurons in deep laminae displayed enhanced responses after FS. Local application of the 5HT3R antagonist, ondansetron (OND), at the Vc/C1–2 region reduced the ATP-evoked responses of neurons in superficial and deep laminae and reduced the EMG response in both sham and FS rats. OND also decreased the spontaneous firing rate of neurons in deep laminae and reduced the high-threshold convergent cutaneous receptive field area of neurons in superficial and deep laminae in both sham and FS rats. These results revealed that central application of a 5HT3R antagonist, had widespread effects on the properties of TMJ-responsive neurons at the Vc/C1–2 region and on jaw muscle reflexes under sham and FS conditions. It is concluded that 5HT3R does not play a unique role in mediating stress-induced hyperalgesia related to TMJ nociception.

Menstrual cycle change in thalamic glutamate decarboxylase and GABA in response to TMJ pain in rats (726.4)

Women report TMJ pain more often than men and epidemiological evidence suggests the pain response varies between men and women and over the menstrual cycle. Recent studies show TMJ responsive neurons projecting to the thalamus have different activities in male and females. Using a rat model our lab performed whole genome array screens on thalamic tissue of cycling rats. The results indicate that the transcript of both glutamate decarboxylase 1 and 2 (GAD67 and GAD65, respectively) are significantly up‐regulated in the thalamus of proestrus (high estrogen, progesterone and luteinizing hormone concentration) versus diestrus/metestrus (low estrogen, progesterone and luteinizing hormone concentration) rats. On the other hand, GABA expression (by ELISA assay) did not significantly change in the thalamic tissue during the proestrus phase. The differences in response of the enzyme and its product could be explained by the distribution of each enzyme within individual neurons. In conclusion, sex steroids could alter thalamic GABAergic signaling to effect TMJ hyperalgesia through regulation of GAD.Grant Funding Source: Supported by NIH R01 NIDCR grant: DE022129 Estrogen and TMJ Pain

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 (C1–2) and deeper laminae (Lam III/V) at C1–2 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.

Rapid Estrogenic Effects on TMJ-responsive Brainstem Neurons

Estrogen status is a risk factor for temporomandibular muscle and joint disorders (TMJD) and other craniofacial pain conditions. The basis for estrogen modulation of pain is poorly understood and has often been attributed to long-term genomic effects. However, estrogens also act rapidly through membrane-initiated mechanisms to alter neural activity. To assess if estrogens act rapidly to affect TMJ-responsive neurons, we applied 17β-estradiol (E2) directly at the spinomedullary (Vc/C1-2) region, the initial brainstem site for synaptic integration of TMJ sensory signals, while recording single neuron activity. In ovariectomized female rats, E2 rapidly (within 10 minutes) and reversibly reduced TMJ-evoked neural activity at the Vc/C1-2 region. The effect was estrogen receptor (ER) subtype-specific, since ERβ agonists inhibited, while an ERβ agonist enhanced, evoked activity. A membrane-mediated mechanism was indicated, since the membrane-impermeable analogue, E2-BSA, mimicked the inhibitory effect of E2 and was prevented by an ER antagonist. This study demonstrated that E2 acted rapidly, through membrane-mediated pathways, and locally at the Vc/C1-2 region, to modulate sensory signals from the TMJ region. These results were consistent with the hypothesis that estrogens can act rapidly at the level of the trigeminal brainstem complex to influence sensory integration of TMJ-related information.

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/C1-2) junction in naïve ovariectomized (OvX) female rats treated for 2 days with high-dose (20 μg/day; HE2) or low-dose estradiol (2 μg/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/C1-2 neurons. The MAPK/ERK inhibitor (PD98059, 0.01–1 mM) was applied topically to the dorsal Vc/C1-2 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 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.

H 2O 2 production in lung mitochondria of broilers with pulmonary hypertension syndrome (PHS) : [formula omitted], D. Cawthon, R.F. Wideman, Jr. and W. Bottje, Dept. of Poultry Science, Uni. of Arkansas, Fayetteville, AR 72701

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 (C1–2) and deeper laminae (Lam III/V) at C1–2 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.