Involvement of NMDA receptors in Zif/268 expression in the trigeminal nucleus caudalis following formalin injection into the rat whisker pad

Peripheral Purinergic Modulation in Pediatric Orofacial Inflammatory Pain Affects Brainstem Nitroxidergic System: A Translational Research

Physiology of orofacial pain pathways embraces primary afferent neurons, pathologic changes in the trigeminal ganglion, brainstem nociceptive neurons, and higher brain function regulating orofacial nociception. The goal of this study was to investigate the nitroxidergic system alteration at brainstem level (spinal trigeminal nucleus), and the role of peripheral P2 purinergic receptors in an experimental mouse model of pediatric inflammatory orofacial pain, to increase knowledge and supply information concerning orofacial pain in children and adolescents, like pediatric dentists and pathologists, as well as oro-maxillo-facial surgeons, may be asked to participate in the treatment of these patients. The experimental animals were treated subcutaneously in the perioral region with pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS), a P2 receptor antagonist, 30 minutes before formalin injection. The pain-related behavior and the nitroxidergic system alterations in the spinal trigeminal nucleus using immunohistochemistry and western blotting analysis have been evaluated. The local administration of PPADS decreased the face-rubbing activity and the expression of both neuronal and inducible nitric oxide (NO) synthase isoforms in the spinal trigeminal nucleus. These results underline a relationship between orofacial inflammatory pain and nitroxidergic system in the spinal trigeminal nucleus and suggest a role of peripheral P2 receptors in trigeminal pain transmission influencing NO production at central level. In this way, orofacial pain physiology should be elucidated and applied to clinical practice in the future.

N-Methyl-d-aspartate receptor open-channel blockers memantine and magnesium modulate nociceptive trigeminovascular neurotransmission in rats

Experimental and clinical studies suggest that the low-affinity N-methyl-d-aspartate (NMDA) receptor open-channel blockers Mg²⁺ and memantine are effective in reducing trigeminal nociceptive activation. The aim of this study was to investigate the apparent effectiveness of these channel blockers using a model of trigeminal activation in vivo. Rats were anaesthetized before electrically stimulating the dura mater adjacent the middle meningeal artery. Neurons responding to stimulation were recorded extracellularly using electrophysiological methods. l-Glutamate or NMDA, and Mg²⁺ , memantine, or sodium controls were applied locally using microiontophoresis. Microiontophoretic application of Mg²⁺ or memantine into the trigeminocervical complex inhibited mechanically and electrically stimulated craniovascular afferents,  and l-glutamate or NMDA-evoked neuronal activity at the second-order trigeminal synapse of craniovascular afferents. By contrast, intravenous administration of MgSO₄ (100 mg/kg) or memantine (10 mg/kg) did not significantly affect electrically stimulated afferent-evoked activity within the trigeminocervical complex. The Mg²⁺ and memantine concentrations achieved after systemic administration may not effectively inhibit activation of the trigeminocervical complex, perhaps providing an explanation for the relatively poor efficacy of these NMDA receptor open-channel blockers for headache treatment in clinical studies. Nevertheless, the present results suggest blocking of NMDA-receptor open channels inhibits nociceptive activation of the trigeminocervical complex. Further exploration of such channel blockers as a therapeutic strategy for primary head pain is warranted.

PTEN inhibition preserves trigeminal nucleus caudalis neuron activation through tyrosine phosphorylation of the NR2B subunit at Tyr1472 of the NMDA receptor in a rat model of recurrent migraine

OBJECTIVE

Activation of the trigeminal nucleus caudalis is believed to be involved in the pathomechanism of migraine. Evidence suggests that N-methyl-d-aspartate receptor subtype 2B tyrosine phosphorylation, originating from the trigeminal nucleus caudalis neuron dysfunction, might be a triggering mechanism for recurrent migraine. Phosphatase and tensin homolog is thought to have a neuroprotective effect in various neurologic diseases by regulating N-methyl-d-aspartate receptor subtype 2B or tyrosine phosphorylation. Thus, the aim of this study was to explore whether the recombinant adenovirus AdR-siPTEN attenuates neuron activation in the trigeminal nucleus caudalis in a rat model of recurrent migraine.

METHODS

Adenovirus-expressing siPTEN or RFP was independently injected into the spinal trigeminal nucleus of the rat model suffering from recurrent migraine by inflammatory soup stimulation the superior sagittal sinus of rats. Seven days later, tactile sensory testing was performed to detect the tactile threshold. Immunofluorescence, Immunohistochemistry, and western blot assay were done to measure PTEN, NR2B, NR2B-pTyr1472, and c-Fos levels in the trigeminal nucleus caudalis of recurrent migraine rats.

RESULTS

A significant increase (p < 0.05) in neuron c-Fos content, an indicator of neuron activation, was detected in the trigeminal nucleus caudalis in a rat model of recurrent migraine. However, neuron activation in the trigeminal nucleus caudalis was attenuated by pretreatment with AdR-siPTEN. Moreover, the attenuated effect was potentially mediated by tyrosine phosphorylation of the NR2B-p1472 tyrosine site in the trigeminal nucleus caudalis, as seen in rat brain slices.

CONCLUSION

These results suggest that, phosphatase and tensin homolog might be a novel and promising candidate for future treatment or prophylaxis of recurrent migraine by attenuating neuron activation in the trigeminal nucleus caudalis.

Stimulation-induced expression of immediate early gene proteins in the dorsal horn is increased in neuropathy

BACKGROUND AND AIMS

Peripheral neuropathic pain is described as a pain state caused by an injury or dysfunction of the nervous system, and could have clinical manifestations such as hyperalgesia, allodynia and spontaneous pain. The development of neuropathic pain may depend on long-term forms of neuronal plasticity in the spinal cord (SC). Expression of the immediate early gene proteins (IEGPs) Arc, Zif268, and c-Fos are implicated in establishment of long-term potentiation (LTP) induced by conditioning stimulation (CS) of primary afferent fibres. However, the impact of the neuropathic state (Bennett's model) on CS-induced expression of IEGPs has not been studied. The aim of this study was to compare the levels of Arc, c-Fos and Zif268 immunoreactivity prior to and after conditioning stimulation in animals with developed neuropathic pain, with sham operated, non-ligated controls.

METHODS

Twenty-four animals were divided equally into the neuropathic and non-neuropathic groups. Neuropathic pain was induced in all animals by conducting a loose ligation of the sciatic nerve with Chromic Catgut 4.0 sutures 7 days prior to conditioning stimulation or sham operation. The loose ligation was performed by placing sutures around the sciatic nerve compressing the nerve slightly just enough to reduce but not completely diminish the perineural circulation. A state of neuropathy was confirmed by a significant decrease in mechanical withdrawal threshold measured by von Frey's fibres. Immunohistochemical analysis was performed on transverse sections obtained from the L3-L5 segments of the SC at 2 and 6h post-CS and IEGP positive cells were counted in lamina I and II of the dorsal horn. During statistical analyses, the groups were compared by means of analysis of variance (univariate general linear model). If significant differences were found, each set of animals was compared with the sham group with post hoc Tukey's multiple comparison test.

RESULTS

Strikingly, all IEGPs exhibited a significant increase in immunoreactivity at both time points compared to time-matched, sham operated controls. Maximal IEGP expression was found 2h after CS in neuropathic rats, and there was a smaller but still significant increase 6h after CS. The unstimulated side of the dorsal horn in stimulated animals did not show any significant change of the number of IEGP positive cells and was approximately at the same level as sham operated animals. The number of IEGP positive cells in sham operated controls (non-neuropathic and non-stimulated animals) showed same immunoreactivity in 2 and 6h post sham operation.

CONCLUSIONS AND IMPLICATIONS

The neurophysiological process of neuropathic pain development is complex and needs to be studied further in order to clarify its nature and components. This present study is meant to reveal a step towards further understanding the role of Arc, c-Fos and Zif268 in neuropathic pain. Moreover, this study might contribute to the knowledge base for further research on better therapeutic possibilities for neuropathic pain.

CGRP infusion in unanesthetized rats increases expression of c-Fos in the nucleus tractus solitarius and caudal ventrolateral medulla, but not in the trigeminal nucleus caudalis

BACKGROUND AND AIMS

Calcitonin gene-related peptide (CGRP) and glyceryl trinitrate (GTN) infusion in migraineurs provokes headache resembling spontaneous migraine, and CGRP receptor antagonists are effective in the treatment of acute migraine. We hypothesized that CGRP infusion would increase molecular markers of neuronal activation in migraine-relevant tissues of the rat.

METHODS

CGRP was infused intravenously (i.v.) in freely moving rats to circumvent factors like anesthesia, acute surgery and severe hypotension, the three confounding factors for c-Fos expression. The trigeminal nucleus caudalis (TNC) was isolated at different time points after CGRP infusion. The level of c-Fos mRNA and protein expression in TNC were analyzed by qPCR and immunohistochemistry. c-Fos-stained nuclei were also counted in the nucleus tractus solitarius (NTS) and caudal ventrolateral medulla (CVLM), integrative sites in the brain stem for processing cardiovascular signals. We also investigated Zif268 protein expression (another immediate early gene) in TNC. The protein expression of p-ERK, p-CREB and c-Fos was analyzed in dura mater, trigeminal ganglion (TG) and TNC samples using Western blot.

RESULTS

CGRP infusion caused a significant dose-dependent fall in mean arterial blood pressure. No significant activation of c-Fos in the TNC at mRNA and protein levels was observed after CGRP infusion. A significant increase in c-Fos protein was observed in the NTS and CVLM in the brain stem. Zif268 expression in the TNC was also not changed after CGRP infusion. p-ERK was increased in the dura mater 30 minutes after CGRP infusion.

CONCLUSION

CGRP infusion increased the early expression of p-ERK in the dura mater but did not increase c-Fos and Zif268 expression in the TNC. The rats may, thus, differ from migraine patients, in whom infusion of CGRP caused headache and a delayed migraine attack. The rat CGRP infusion model with c-Fos or Zif268 as neuronal pain markers in TNC is unsuitable for antimigraine drug testing.

Peripheral purinergic receptor modulation influences the trigeminal ganglia nitroxidergic system in an experimental murine model of inflammatory orofacial pain

ATP plays an important role as an endogenous pain mediator generating and/or modulating pain signaling from the periphery to the central nervous system. The aim of this study was to analyze the role of peripheral purinergic receptors in modulation of the nitroxidergic system at a trigeminal ganglia level by monitoring changes in nitric oxide synthase isoforms. We also evaluated Fos-positive neurons in brainstem (spinal trigeminal nucleus) and pain-related behavior. We found that local administration of the P2 purinergic receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) decreased face-rubbing activity, nitric oxide synthase isoform expression in trigeminal ganglia, and Fos expression in spinal trigeminal nucleus after subcutaneous injection of formalin. These results suggest a role for peripheral P2 purinergic receptors in orofacial pain transmission through modulation of the nitroxidergic system. .

Glutamate pharmacology and metabolism in peripheral primary afferents: physiological and pathophysiological mechanisms

In addition to using glutamate as a neurotransmitter at central synapses, many primary sensory neurons release glutamate from peripheral terminals. Primary sensory neurons with cell bodies in dorsal root or trigeminal ganglia produce glutaminase, the synthetic enzyme for glutamate, and transport the enzyme in mitochondria to peripheral terminals. Vesicular glutamate transporters fill neurotransmitter vesicles with glutamate and they are shipped to peripheral terminals. Intense noxious stimuli or tissue damage causes glutamate to be released from peripheral afferent nerve terminals and augmented release occurs during acute and chronic inflammation. The site of action for glutamate can be at the autologous or nearby nerve terminals. Peripheral nerve terminals contain both ionotropic and metabotropic excitatory amino acid receptors (EAARs) and activation of these receptors can lower the activation threshold and increase the excitability of primary afferents. Antagonism of EAARs can reduce excitability of activated afferents and produce antinociception in many animal models of acute and chronic pain. Glutamate injected into human skin and muscle causes acute pain. Trauma in humans, such as arthritis, myalgia, and tendonitis, elevates glutamate levels in affected tissues. There is evidence that EAAR antagonism at peripheral sites can provide relief in some chronic pain sufferers.

The Antinociceptive Effect of Sigma-1 Receptor Antagonist, BD1047, in a Capsaicin Induced Headache Model in Rats

Intracranial headaches, including migraines, are mediated by nociceptive activation of the trigeminal nucleus caudalis (TNC), but the precise mechanisms are poorly understood. We previously demonstrated that selective blockage of spinal sigma-1 receptors (Sig-1R) produces a prominent antinociceptive effect in several types of pain models. This study evaluates whether the Sig-1R antagonist (BD1047) has an antinociceptive effect on capsaicin (a potent C-fiber activator) induced headache models in rats. Intracisternal infusion of capsaicin evoked pain behavior (face grooming), which was significantly attenuated by BD1047 pretreatment. BD1047 consistently reduced capsaicin-induced Fos-like immunoreactivity (Fos-LI), a neuronal activator, in the TNC in a dose-dependent manner. Moreover, capsaicin-induced phosphorylation of N-methyl-D-aspartate receptor subunit 1 was reversed by BD1047 pretreatment in the TNC. These results indicate that the Sig-1R antagonist has an inhibitory effect on nociceptive activation of the TNC in the capsaicin-induced headache animal model.

Selective contribution of Egr1 (zif/268) to persistent inflammatory pain

The zinc finger transcription factor Egr1 is critical for coupling extracellular signals to changes in cellular gene expression. Expression of Egr1, as well as other immediate early genes, is up-regulated in response to a number of noxious stimuli. Activity-dependent activation of Egr1 has been reported in forebrain regions, including the anterior cingulate cortex (ACC), after peripheral injury. However, no study has reported a direct contribution of Egr1 to behavioral nociceptive responses. Here, we use Egr1 knockout mice to show that Egr1 is selectively required for behavioral responses to persistent inflammatory pain. Behavioral responses to peripheral inflammation were significantly reduced in Egr1 knockout mice, whereas responses to acute noxious stimuli were normal. In addition, inflammation triggered an up-regulation of Egr1 expression in the ACC of wild-type mice. Last, synaptic potentiation induced by theta (theta) burst stimulation in the ACC was significantly reduced or blocked in Egr1 knockout mice. Our study suggests that the transcription factor Egr1 plays a selective role in nociceptive behavioral responses to persistent inflammatory pain but not to acute noxious stimuli.

PERSPECTIVE

Chronic pain diminishes the quality of life. Here, we show that the immediate early gene Egr1 plays a role in chronic inflammatory, but not acute, pain. Egr1 knockout mice showed reduced nociceptive behaviors to persistent inflammatory pain and inflammation increased Egr1 expression in the anterior cingulate cortex of wild-type mice.