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 microL 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.
The present study is the first to investigate the participation of central cyclooxygenase (COX) pathways in modulating the antinociceptive effects of intracisternally administered cannabinoid on nociception induced by inflammation of the temporomandibular joint (TMJ) in freely moving rats. Following intra-articular injection of 5% formalin in the TMJ, nociceptive scratching behavior was recorded for nine successive 5-min intervals in Sprague-Dawley rats. Intracisternal injection of 30 microg of WIN 55,212-2, a synthetic non-subtype-selective CB1/2 agonist, administered 20 min prior to formalin injection significantly reduced the number of scratches and duration of scratching induced by formalin compared with the vehicle-treated group. Antinociceptive effect of WIN 55,212-2 was blocked by intracisternal injection of 10 microg of AM251, a CB1 receptor-selective antagonist, but not by AM630, a CB2 receptor-selective antagonist. A 10 microg dose of WIN 55,212-2 that was ineffective in producing antinociception became effective following intracisternal administration of NS-398, a selective COX-2 inhibitor; indomethacin, a non-selective COX 1/2 inhibitor; acetaminophen, a putative COX-3 inhibitor, but not following pretreatment with the selective COX-1 inhibitor, SC-560. The ED(50) value of WIN 55,212-2 in the NS-398-treated group was significantly lower than that in the vehicle-treated group. Importantly, administration of low doses of COX inhibitors alone did not attenuate nociception. These results indicate that inhibition of central COX pathways, presumably via COX-2 inhibition, reduces inflammatory pain by enhancing the cannabinoid-induced antinociceptive effect. Based on our observations, combined administration of cannabinoids with COX inhibitors may hold a therapeutic promise in the treatment of inflammatory TMJ pain.
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 (8microl) 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.
The present study investigated the role of peripheral opioid receptors in mustard oil-induced nociceptive behavior and inflammation in the masseter muscles of lightly anesthetized rats. Experiments were carried out on male Sprague-Dawley rats weighing between 300 and 400 g. After initial anesthesia with sodium pentobarbital (40 mg/kg, i.p.), one femoral vein was cannulated and connected to an infusion pump for the intravenous infusion of sodium pentobarbital. The rate of infusion was adjusted to provide a constant level of anesthesia. Mustard oil (MO, 30 microl) was injected into the mid-region of the left masseter muscle via a 30-gauge needle. Intramuscularly-administered morphine significantly reduced shaking behavior but not MO-induced inflammation. Intramuscular pretreatment with naloxone, an opioid receptor antagonist, reversed antinociception produced by intramuscularly-administered morphine, while intracisternal administration of naloxone did not affect the antinociception of peripheral morphine. Pretreatment with d-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a mu opioid receptor antagonist, but not naltrindole, a delta opioid receptor antagonist, nor norbinaltorphimine (nor-BNI), a kappa opioid receptor antagonist, reversed intramuscularly-administered morphine-induced antinociception. These results indicate that intramuscularly-administered morphine produces antinociception in craniofacial muscle nociception and that this intramuscularly-administered morphine-induced antinociception is mediated by a peripheral mu opioid receptor. Our observations further support the clinical approach of administering opioids in the periphery for the treatment of craniofacial muscle nociception.
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