Yong C. Bae

Intratrigeminal ganglionic injection of LPA causes neuropathic pain-like behavior and demyelination in rats

ABSTRACT We have previously reported a novel method for producing chronic nociceptive behavior in rats following compression of the trigeminal ganglion. In the present study, we have further studied the role of demyelination in the development of prolonged nociceptive behavior in the trigeminal territory. For this purpose, lysophosphatidic acid (LPA) was injected into the trigeminal ganglia of male Sprague–Dawley rats weighing between 250 and 260 g. Under pentobarbital sodium anesthesia, the rats were mounted onto a stereotaxic frame and 3 μL of LPA (1 nmol) solution was injected into the trigeminal ganglion to produce demyelination. This treatment decreased the air‐puff thresholds both ipsilateral and contralateral to the injection site, which persisted until postoperative day 100 and returned to the preoperative levels 130 days after the LPA injection. The LPA injection also produced a significant ipsilateral hyper‐responsiveness to pin‐prick stimulation. The effects of DGPP, an LPA1/3 receptor antagonist, and Y‐27632, a Rho kinase inhibitor, upon LPA‐induced mechanical allodynia and hyperalgesia were also investigated. Pretreatment with DGPP blocked both mechanical allodynia and ipsilateral hyperalgesia. However, pretreatment with Y‐27632 blocked only ipsilateral and contralateral mechanical allodynia. These results thus indicate that a targeted blockade of LPA receptor and Rho kinase pathways are potentially important new treatments for demyelination‐induced trigeminal neuralgia‐like nociception.

Role of peripheral group I and II metabotropic glutamate receptors in IL-1β-induced mechanical allodynia in the orofacial area of conscious rats

&NA; The present study investigated the role of peripheral group I and II metabotropic glutamate receptors (mGluRs) in interleukin‐1β (IL‐1β)‐induced mechanical allodynia in the orofacial area. Experiments were carried out on Sprague–Dawley rats weighing between 230 and 280 g. After subcutaneous administration of 0.01, 0.1, 1, or 10 pg of IL‐1β, we examined withdrawal behavioral responses produced by 10 successive trials of a ramp of air‐puffs pressure applied ipsilaterally or contralaterally to the IL‐1β injection site. The thresholds of air puffs were measured 10, 30, 60, 120, or 180 min after 25 μl of IL‐1β was administered through an implanted tube. Subcutaneous injection of IL‐1β produced bilateral mechanical allodynia. While the IL‐1β‐induced mechanical allodynia was blocked by pretreatment with an IL‐1 receptor antagonist, the IL‐1β‐induced mirror‐image mechanical allodynia was not blocked by an IL‐1 receptor antagonist injected into the contralateral side. Subcutaneous administration of CPCCOEt or LY367385, an mGluR1 antagonist, or MPEP or SIB1893, an mGluR5 antagonist, 10 min prior to injection of IL‐1β abolished IL‐1β‐induced mechanical allodynia. Pretreatment with APDC or DCG4, a group II mGluR agonist, blocked the IL‐1β‐induced mechanical allodynia. The anti‐allodynic effect induced by APDC was inhibited by pretreatment with LY341495, a group II mGluR antagonist. These results suggest that peripheral group I and II mGluRs participate in IL‐1β‐induced mechanical allodynia in the orofacial area. Peripheral group I mGluR antagonists blocked the IL‐1β‐induced mechanical allodynia, while peripheral group II mGluR agonists produced anti‐allodynic effects on IL‐1β‐induced mechanical allodynia in the orofacial area of rats.

Behavioral evidence for the differential regulation of p-p38 MAPK and p-NF-κB in rats with trigeminal neuropathic pain

BackgroundWe investigated the differential regulation of p-p38 MAPK or p-NF-κB in male Sprague-Dawley rats with inferior alveolar nerve injury resulting from mal-positioned dental implants. For this purpose, we characterized the temporal expression of p-p38 MAPK or p-NF-κB in the medullary dorsal horn and examined changes in nociceptive behavior after a blockade of p-p38 MAPK or p-NF-κB pathways in rats with trigeminal neuropathic pain.ResultsUnder anesthesia, the left lower second molar was extracted and replaced with a mini dental implant to intentionally injure the inferior alveolar nerve. Western and immunofluorescence analysis revealed that p-p38 MAPK is upregulated in microglia following nerve injury and that this expression peaked on postoperative day (POD) 3 through 7. However, the activation of p-NF-κB in astrocyte peaked on POD 7 through 21. The intracisternal administration of SB203580 (1 or 10 μg), a p38 MAPK inhibitor, on POD 3 but not on POD 21 markedly inhibits mechanical allodynia and the p-p38 MAPK expression. However, the intracisternal administration of SN50 (0.2 or 2 ng), an NF-κB inhibitor, on POD 21 but not on POD 3 attenuates mechanical allodynia and p-NF-κB expression. Dexamethasone (25 mg/kg) decreases not only the activation of p38 MAPK but also that of NF-κB on POD 7.ConclusionsThese results suggest that early expression of p-p38 MAPK in the microglia and late induction of p-NF-κB in astrocyte play an important role in trigeminal neuropathic pain and that a blockade of p-p38 MAPK at an early stage and p-NF-κB at a late stage might be a potential therapeutic strategy for treatment of trigeminal neuropathic pain.

Compression of the trigeminal ganglion produces prolonged nociceptive behavior in rats

The present study is the first demonstration of prolonged nociceptive behavior in the trigeminal region following compression of the trigeminal ganglion in rats. Experiments were carried out on male Sprague–Dawley rats mounted onto a stereotaxic frame under pentobarbital sodium anesthesia. For compression of the trigeminal ganglion, a 4% agar solution (8μl) was injected into the trigeminal ganglion through a stainless steel injector (24 gauge), which extended 2mm beyond the end of a guide cannula (21 gauge). Following agar injection, the injector and guide cannula were removed. In the control group, rats were sham operated without agar injection. Air‐puff thresholds (mechanical allodynia), pin prick responses (mechanical hyperalgesia), and spontaneous scratching behavior were examined 3 days before surgery and at 3, 7, 10, 14, 17, 21, 24, 30, and 40 days after surgery. Data were analyzed using a repeated measures ANOVA followed by multiple group comparisons using the LSD post‐hoc test. Air‐puff thresholds significantly decreased after compression of the trigeminal ganglion. Mechanical allodynia was established within 3 days and lasted beyond postoperative day 24. Mechanical hyperalgesia was also evident 3 days after compression and persisted until the 40th postoperative day. Although mechanical allodynia and hyperalgesia appeared bilaterally, the ipsilateral side was significantly more sensitive. Intraperitoneal treatment with carbamazepine significantly blocked mechanical allodynia produced by compression of the trigeminal ganglion. These findings suggest that prolonged nociceptive behavior following compression of the trigeminal ganglion may mimic trigeminal neuralgia in this animal model.

Central cyclooxygenase inhibitors reduced IL-1β-induced hyperalgesia in temporomandibular joint of freely moving rats

&NA; Microinjection of formalin (5%, 50 μl) into a temporomandibular joint (TMJ) causes noxious behavioral responses in freely moving rats. In the present study, we investigated the role of central cyclooxygenase (COX) pathways in IL‐1β‐induced hyperalgesia with formalin‐induced TMJ pain model. Intra‐articular injection of 100 pg or 1 ng of IL‐1β significantly facilitated formalin‐induced behavior by 130 or 174% in the number of scratches. Intracisternal administration of 100 pg or 1 ng of IL‐1β also significantly increased formalin‐induced behavior by 166 or 82% in the number of scratches. IL‐1β‐induced hyperalgesia was blocked by pretreatment with IL‐1 receptor antagonist. Intracisternal pretreatment with SC‐560, a selective COX‐1 inhibitor, or NS‐398, a selective COX‐2 inhibitor, abolished intra‐articular administration of IL‐1β‐induced hyperalgesic response. Intracisternal pretreatment with NS‐398, a selective COX‐2 inhibitor, abolished the intracisternal administration of IL‐1β‐induced hyperalgesic response, while pretreatment with SC‐560, a selective COX‐1 inhibitor, did not change IL‐1β‐induced hyperalgesic responses. On the other hand, pretreatment with acetaminophen, a tentative COX‐3 inhibitor, also abolished both intra‐articular and intracisternal administration of IL‐1β‐induced hyperalgesic responses. These results indicate that central COX‐2 plays important role in the central administration of IL‐1β‐induced hyperalgesia and that central COX‐1/2 pathways mediate peripheral administration of IL‐1β‐induced hyperalgesia in the TMJ. Central COX‐3 inhibitor seems to play an important role in the nociceptive process associated with both peripheral and central administration of IL‐1β‐induced hyperalgesia in TMJ. It is concluded that central acting of COX‐3 inhibitors may be of therapeutic value in the treatment of inflammatory pain in TMJ.

Low doses of cannabinoids enhance the antinociceptive effects of intracisternally administered mGluRs groups II and III agonists in formalin-induced TMJ nociception in rats

Abstract This study provides the first demonstration that central cannabinoids modulate the antinociceptive actions of metabotropic glutamate receptors (mGluRs) on formalin‐induced temporomandibular joint (TMJ) nociception. Noxious scratching behavior induced by formalin injection in the TMJ was used as a model of pain. Intracisternal injection of 30 μg of WIN 55,212‐2, a non‐subtype selective cannabinoid receptor agonist, attenuated the number of scratches by 75% as compared with the vehicle‐treated group, whereas vehicle alone or 3 or 10 μg of WIN 55,212‐2 had no effect. To explore the postulated interaction between central cannabinoid receptors and mGluRs, effects of combined administration of sub‐analgesic doses of WIN 55,212‐2 and group II or III mGluR agonists were tested. Group II or III mGluRs agonists were administered intracisternally 10 min after intracisternal administration of WIN 55,212‐2. Neither 100 nmol APDC, a group II mGluRs agonist, nor L‐AP4, a group III mGluR agonist, altered nociceptive behavior when given alone but significantly inhibited the formalin‐induced nociceptive behavior in the presence of a sub‐threshold dose (3 μg) of WIN 55,212‐2. The ED50 value of APDC or L‐AP4 was significantly reduced upon co‐treatment with WIN 55,212‐2 than in the vehicle‐treated group, highlighting the important therapeutic potential of the combined administration of group II or III mGluR agonists with cannabinoids to effectively treat inflammatory pain associated with the TMJ. Potentiating effects of group II or III mGluRs agonists will likely permit the administration of cannabinoids at doses that do not achieve significant accumulation to produce undesirable motor dysfunction.

Differential regulation of peripheral IL-1β-induced mechanical allodynia and thermal hyperalgesia in rats

Summary Mechanical allodynia is mediated by sensitized N‐methyl‐d‐aspartate/&agr;‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid through protein kinase A in large fibers, whereas thermal hyperalgesia is mediated by sensitized transient receptor potential vanilloid 1 through protein kinase C in small fibers. ABSTRACT This study examined the differential mechanisms of mechanical allodynia and thermal hyperalgesia after injection of interleukin (IL) 1&bgr; into the orofacial area of male Sprague‐Dawley rats. The subcutaneous administration of IL‐1&bgr; produced both mechanical allodynia and thermal hyperalgesia. Although a pretreatment with iodoresiniferatoxin (IRTX), a transient receptor potential vanilloid 1 (TRPV1) antagonist, did not affect IL‐1&bgr;‐induced mechanical allodynia, it significantly abolished IL‐1&bgr;‐induced thermal hyperalgesia. On the other hand, a pretreatment with D‐AP5, an N‐methyl‐d‐aspartate (NMDA) receptor antagonist, and NBQX, an &agr;‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) receptor antagonist, blocked IL‐1&bgr;‐induced mechanical allodynia. Pretreatment with H89, a protein kinase A (PKA) inhibitor, blocked IL‐1&bgr;‐induced mechanical allodynia but not thermal hyperalgesia. In contrast, pretreatment with chelerythrine, a protein kinase C (PKC) inhibitor, inhibited IL‐1&bgr;‐induced thermal hyperalgesia. Subcutaneous injections of 2% lidocaine, a local anesthetic agent, blocked IL‐1&bgr;‐induced thermal hyperalgesia but not IL‐1&bgr;‐induced mechanical allodynia. In the resiniferatoxin (RTX)‐pretreated rats, a subcutaneous injection of IL‐1&bgr; did not produce thermal hyperalgesia due to the depletion of TRPV1 in the primary afferent fibers. Double immunofluorescence revealed the colocalization of PKA with neurofilament 200 (NF200) and of PKC with the calcitonin gene‐related peptide (CGRP) in the trigeminal ganglion. Furthermore, NMDA receptor 1 (NR1) and TRPV1 predominantly colocalize with PKA and PKC, respectively, in the trigeminal ganglion. These results suggest that IL‐1&bgr;‐induced mechanical allodynia is mediated by sensitized peripheral NMDA/AMPA receptors through PKA‐mediated signaling in the large‐diameter primary afferent nerve fibers, whereas IL‐1&bgr;‐induced thermal hyperalgesia is mediated by sensitized peripheral TRPV1 receptors through PKC‐mediated signaling in the small‐diameter primary afferent nerve fibers.

Intracisternal administration of COX inhibitors attenuates mechanical allodynia following compression of the trigeminal ganglion in rats

The purpose of the present study was to investigate the role of central cyclooxygenase (COX) pathways in the modulation of mechanical allodynia following compression of the left trigeminal ganglion. Experiments were carried out on male Sprague-Dawley rats mounted onto a stereotaxic frame under anesthesia. For compression, a 4% agar solution (10 microl) was injected into the trigeminal ganglion. In the control group, rats were sham operated without agar injections. Ipsilateral and contralateral air-puff thresholds significantly decreased following trigeminal ganglion compression. Mechanical allodynia was established within 3 days and lasted beyond postoperative day 30, returning to preoperative levels at approximately 55 days following compression. Intracisternal administration of indomethacin, a non-selective COX inhibitor, SC-560, a selective COX-1 inhibitor, or NS-398, a selective COX-2 inhibitor, significantly inhibited mechanical allodynia. The individual anti-allodynic effects of the three COX inhibitors persisted for 6 h and returned to pretreatment values within 24 h. Based on these results, the blockade of central COX pathways may comprise a potential new therapeutic tool for the treatment of trigeminal ganglion compression-induced nociception.

Blockade of spinal glutamate recycling produces paradoxical antinociception in rats with orofacial inflammatory pain

In our current study, we investigated the role of spinal glutamate recycling in the development of orofacial inflammatory pain. DL-threo-β-benzyloxyaspartate (TBOA) or methionine sulfoximine (MSO) was administered intracisternally to block spinal glutamate transporter and glutamine synthetase activity in astroglia. Intracisternal administration of high dose TBOA (10 μg) produced thermal hyperalgesia in naïve rats but significantly attenuated the thermal hyperalgesia in rats that had been pretreated with interleukin (IL)-1β or Complete Freunds Adjuvant (CFA). In contrast, intracisternal injection of MSO produced anti-hyperalgesic effects against thermal stimuli in CFA-treated rats only. To confirm the paradoxical antinociceptive effects of TBOA and MSO, we examined changes in c-Fos expression in the medullary dorsal horn produced by thermal stimulation in naïve, IL-1β-, or CFA-treated rats, after intracisternal injections of TBOA and MSO. Intracisternal administration of TBOA significantly increased c-Fos immunoreactivity in naïve rats. In contrast, intracisternal administration of TBOA significantly decreased the up-regulation of c-Fos immunoreactivity in the medullary dorsal horn of IL-1β- and CFA-treated rats. However, intracisternal injection of MSO blocked the up-regulation of c-Fos immunoreactivity in CFA-treated rats only. We also investigated the effects of botulinum toxin type A (BoNT-A) on TBOA-induced paradoxical antinociception in CFA-treated rats, as BoNT-A inhibits the release of neurotransmitters, including glutamate. BoNT-A treatment reversed behavioral responses produced by intracisternal administration of TBOA in CFA-treated rats. These results suggest that the paradoxical responses produced by blocking glutamate transporters under inflammatory pain conditions are mediated by the modulation of glutamate release from presynaptic terminals. Moreover, blockade of glutamate reuptake could represent a new therapeutic target for the treatment of chronic inflammatory pain conditions.

A novel trigeminal neuropathic pain model: Compression of the trigeminal nerve root produces prolonged nociception in rats

We demonstrate the establishment of a novel animal model for trigeminal neuropathic pain following compression of the trigeminal nerve root, which produces prolonged nociceptive behavior and demyelination of the trigeminal nerve root. Under anesthesia, male Sprague-Dawley rats (200-230 g) were mounted onto a stereotaxic frame and injections of a 4% agar solution (10 μl) were given to achieve compression of the trigeminal nerve root. A sham operation was performed using identical procedures but without agar injections. Nociceptive behavior was examined 3 days before and then at 3, 7, 10, 14, 17, 21, 24, 30, 40, 55, and 70 days after the surgery. Compression of the trigeminal nerve root caused mechanical allodynia, hyperalgesia, and cold hypersensitivity. Mechanical allodynia was established within 3 days and recovered to preoperative levels on postoperative day (POD) 40. Mechanical hyperalgesia and cold hypersensitivity persisted until 55 days following compression. The compression produced focal demyelination in the trigeminal nerve root. In the medullary dorsal horn, phospho-p38 (p-p38) mitogen-activated protein kinase (MAPK) was found to be exclusively expressed in the microglia on POD 14. Furthermore, intraperitoneal administration of carbamazepine (50mg/kg) significantly blocked mechanical allodynia and reduced p38 MAPK activation induced by the compression of the trigeminal nerve root. Our findings suggest that prolonged nociceptive behavior following compression of the trigeminal nerve root may mimic trigeminal neuralgia in this animal model and that the activation of p38 MAPK in the microglia contributes to pain hypersensitivity in rats that have undergone compression of the trigeminal nerve root.