GPR55 and GPR119 Receptors Contribute to the Processing of Neuropathic Pain in Rats

Orphan G-protein-coupled receptors (GPCR) comprise a large number of receptors which are widely distributed in the nervous system and represent an opportunity to identify new molecular targets in pain medicine. GPR55 and GPR119 are two orphan GPCR receptors whose physiological function is unclear. The aim was to explore the participation of spinal GPR55 and GPR119 in the processing of neuropathic pain in rats. Mechanical allodynia was evaluated using von Frey filaments. Protein localization and modulation were measured by immunohistochemistry and western blotting, respectively. Intrathecal administration of CID16020046 (selective GPR55 antagonist) or AS1269574 (selective GPR119 agonist) produced a dose-dependent antiallodynic effect, whereas O1062 (GPR55 agonist) and G-protein antagonist peptide dose-dependently prevented the antiallodynic effect of CID16020046 and AS1269574, respectively. Both GPR55 and GPR119 receptors were expressed in spinal cord, dorsal root ganglia and sciatic nerve, but only GPR119 was downregulated after 14 days of spinal nerve ligation. Data suggest that GPR55 and GPR119 participate in the processing of neuropathic pain and could be useful targets to manage neuropathic pain disorders.

Effect of Pentoxifylline on Diabetic Neuropathic Pain in Rats

Background and Aim Diabetic neuropathy (DN) is the most common diabetic complication, which has a lifetime prevalence of about 50%. About 20 to 30% of patients with DN suffer from neuropathic pain. This study was designed to investigate the role of early and late treatment of pentoxifylline (PTX) in management of diabetic neuropathic pain (DNP). Method Diabetic rats were administered PTX (50 and 200 mg/kg/day, in drinking water) either one week after STZ injection and for 7 weeks or 6 weeks after STZ injection and for 2 weeks. Mechanical allodynia were assessed weekly, Iba-1 and GFAP immunorectivity, spinal NFκB & spinal sciatic TNF-α were estimated at the end of the study. Epidermal thickness of the food pad and Na+/K+-ATPase activity in sciatic nerve were measured as an outcome measure for predegenerative markers of DN. Results The results of the present study suggest that activated microglia and not astrocytes are involved in the development of experimental diabetic neuropathy. PTX inhibit neuroimmune activation of microglia, reduce the levels of proinflammatory cytokines and improve epidermal thickness of the food pad and Na+/K+-ATPase activity in sciatic nerve. Only PTX 200 mg/kg/day when administered early and late treatment in diabetic rats produced antiallodynic effect. Conclusion PTX is a fundamental drug whose antiallodynic effect depends on the pathogenetic mechanisms and has the ability to halt or inhibit the progression of the disease. The earlier the use of PTX in the developmental phase of mechanical hyperalgesia the more the antiallodynic effect of the drug.

Antinociceptive effect of melatonin in the animal model of Parkinson’s Disease

Several animal experimental and clinical studies have shown the effectiveness of melatonin in the treatment of some symptoms of Parkinson's disease (PD). However, the antinociceptive effect of melatonin against pain associated to PD has not been fully investigated. Thus, the present study investigated the possible antiallodynic and antinociceptive effects of acute and chronic melatonin treatments in Parkinsonian model of rats. This model was created by unilateral injection of 6-hydroxydopamine (6-OHDA) into the left medial forebrain bundle (MFB). The electronic von Frey test was used to analyze the antiallodynic effect of melatonin on this PD animal model. In addition, c-Fos immunostaining was also used as a marker of nociception to evaluate the neuronal activity related to the nociception processing. The results showed that unilateral injection of 6-OHDA induced a significant decrease in paw withdrawal threshold in both ipsilateral and contralateral paws, which indicate mechanical allodynia induction. This allodynia was transitorily reversed by apomorphine as a dopamine agonist. Melatonin treatment significantly increased threshold of allodynia. Melatonin administration of both acutely or chronically significantly downregulated the c-Fos expression of neurons in 6-OHDA treated animals. In conclusion, 6-OHDA treatment can induces a bilateral mechanical hypernociception in rats while melatonin treatment produces profound antinociceptive effect. This finding paves the way to use melatonin as an antinociceptive agent for PD clinically.

Investigation of the Combination of Pregabalin with Duloxetine or Amitriptyline on the Pharmacokinetics and Antiallodynic Effect During Neuropathic Pain in Rats

BACKGROUND: Amitriptyline, duloxetine, and pregabalin are among the most pharmacotherapeutic, effective treatments for neuropathic pain control. However, the evaluation of synergism by combining these treatments is still poorly investigated. OBJECTIVES: To evaluate the pharmacokinetics of the combination of pregabalin plus duloxetine and pregabalin plus amitriptyline, as well as the effect of these on neuropathic pain on rodent model. STUDY DESIGN: The experimental study. SETTING: The research took place in the research laboratories at the Federal University of Alfenas after ethics committee approval. METHODS: This study used male Wistar rats weighing between 220 and 250 g. The animals were randomly divided into the following groups: monotherapy (pregabalin, amitriptyline, duloxetine), combined therapy (pregabalin + amitriptyline, pregabalin + duloxetine), and vehicle (ultrapure water). Pharmacokinetic analysis of pregabalin or combination (pregabalin + amitriptyline or pregabalin + duloxetine) in the plasma were performed by ultraperformance liquid chromatography tandem mass spectrometry. Neuropathic pain was induced by sciatic nerve constriction (chronic constriction injury [CCI]) model, and nociceptive threshold was measured by von Frey filaments test. In addition, to evaluate the influence of the treatments on the motor coordination, the rotarod test was used. RESULTS: The pharmacokinetic disposition of pregabalin was changed in the association with amitriptyline, presenting a clearance reduction and consequently an increase in bioavailability. Furthermore, after the 14th day of CCI, pregabalin was administered orally and induced antiallodynic effect after 1, 2:15, 4, and 8 hours of its administration and showed the greatest antiallodynic effect after 4 hours of its administration. Moreover, this effect was prolonged (up to 8 hours) by combination with amitriptyline. Additionally, pregabalin and pregabalin + duloxetine showed a hypoalgesic effect in sham rats. In addition, the rotarod test results showed that drugs did not influence the motor coordination of the rats. LIMITATIONS: Potential competition mechanisms during the excretion of pregabalin, when pregabalin was combined with amitriptyline, were not investigated in this study. CONCLUSIONS: The data demonstrated that combined therapy of pregabalin plus amitriptyline improved the bioavailability of pregabalin and potentiated the efficacy of the antiallodynic effect of pregabalin alone, proving to be advantageous for the treatment of sciatic neuropathic pain. KEY WORDS: Neuropathic pain, pregabalin, duloxetine, amitriptyline, pharmacokinetic, antiallodynic effect, combined therapy, rats

Antiallodynic and anti-inflammatory effects of intrathecal R-PIA in a rat model of vincristine-induced peripheral neuropathy

Background: Studies investigating the correlation between spinal adenosine A1 receptors and vincristine-induced peripheral neuropathy (VIPN) are limited. This study explored the role of intrathecal N6-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA) in the rat model of VIPN. Methods: Vincristine (100 μg/kg) was intraperitoneally administered for 10 days (two 5-day cycles with a 2-day pause) and VIPN was induced in rats. Pain was assessed by evaluating mechanical hyperalgesia, mechanical dynamic allodynia, thermal hyperalgesia, cold allodynia, and mechanical static allodynia. Biochemically, tumor necrosis factor-alpha (TNF-α) level and myeloperoxidase (MPO) activity were measured in the tissue from beneath the sciatic nerve.Results: Vincristine administration resulted in the development of cold allodynia, mechanical hyperalgesia, thermal hyperalgesia, mechanical dynamic allodynia, and mechanical static allodynia. Intrathecally administered R-PIA (1.0 and 3.0 μg/10 μl) reversed vincristine-induced neuropathic pain (cold and mechanical static allodynia). The attenuating effect peaked 15 min after intrathecal administration of R-PIA after which it decreased until 180 min. However, pretreatment with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 μg/10 μl) 15 min before intrathecal R-PIA administration significantly attenuated the antiallodynic effect of R-PIA. This antiallodynic effect of intrathecal R-PIA may be mediated through adenosine A1 receptors in the spinal cord. Intrathecally administered R-PIA also attenuated vincristine-induced increases in TNF-α level and MPO activity. However, pretreatment with intrathecal DPCPX significantly reversed this attenuation.Conclusions: These results suggest that intrathecally administered R-PIA attenuates cold and mechanical static allodynia in a rat model of VIPN, partially due to its anti-inflammatory actions.

Antiallodynic Effect of Intrathecal Korean Red Ginseng in Cisplatin-Induced Neuropathic Pain Rats

Background: Chemotherapy-induced neuropathic pain (CINP) is a serious side effect of chemotherapy. Korean Red Ginseng (KRG) is a popular herbal medicine in Asian countries. We examined the therapeutic potential of intrathecally administered KRG for CINP and clarified the mechanisms of action with regard to 5-hydroxytryptamine (5-HT)7 receptor at the spinal level. Methods: CINP was evoked by intraperitoneal injection of cisplatin in male Sprague-Dawley rats. After examining the effects of intrathecally administered KRG on CINP, 5-HT receptor antagonist (dihydroergocristine [DHE]) was pretreated to determine the involvement of 5-HT receptor. In addition, intrathecal 5-HT7 receptor antagonist (SB269970) was administered to define the role of 5-HT7 receptor on the effect of KRG. 5-HT7 receptor mRNA expression levels and 5-HT concentrations were examined in the spinal cord. Results: Intrathecally administered KRG produced a limited, but a dose-dependent, antiallodynic effect. Intrathecally administered DHE antagonized the antiallodynia caused by KRG. Furthermore, intrathecal SB269970 also reversed the effect of KRG. No changes in 5-HT7 receptor mRNA expression were seen in the dorsal horn of the spinal cord after cisplatin injection. After injecting cisplatin, 5-HT levels were decreased in the spinal cord, whereas those of 5-HT were increased by intrathecal KRG. Conclusions: Intrathecally administered KRG decreased CINP. In addition, spinal 5-HT7 receptors contributed to the antiallodynic effect of KRG.