Peripheral Analgesic Effect Research Articles

Sinomenine produces peripheral analgesic effects via inhibition of voltage-gated sodium currents

Sinomenium acutum has been used in traditional medicine to treat a painful disease such as rheumatic arthritis and neuralgia. Sinomenine, which is a main bioactive ingredient in Sinomenium acutum, has been reported to have an analgesic effect in diverse pain animal models. However little is known about the detailed mechanisms underlying peripheral analgesic effect of sinomenine. In the present study, we aimed to elucidate its cellular mechanism by using formalin-induced acute inflammatory pain model in mice. We found that intraperitoneal (i.p.) administration of sinomenine (50mg/kg) suppressed formalin-induced paw licking behavior in both the first and the second phase. Formalin-induced c-Fos protein expression was also suppressed by sinomenine (50mg/kg i.p.) in the superficial dorsal horn of spinal cord. Whole-cell patch-clamp recordings from small-sized dorsal root ganglion (DRG) neurons revealed that sinomenine reversibly increased the spike threshold and the threshold current intensity for evoking a single spike and decreased firing frequency of action potentials evoked in response to a long current pulse. Voltage-gated sodium currents (INa) were also significantly reduced by sinomenine in a dose-dependent manner (IC50=2.3±0.2mM). Finally, we confirmed that intraplantar application of sinomenine suppressed formalin-induced pain behavior only in the first phase, but not the second phase. Taken together, our results suggest that sinomenine has a peripheral analgesic effect by inhibiting INa.

Fatty-acid-binding protein inhibition produces analgesic effects through peripheral and central mechanisms.

BackgroundFatty-acid-binding proteins (FABPs) are intracellular carriers for endocannabinoids, N-acylethanolamines, and related lipids. Previous work indicates that systemically administered FABP5 inhibitors produce analgesia in models of inflammatory pain. It is currently not known whether FABP inhibitors exert their effects through peripheral or central mechanisms. Here, we examined FABP5 distribution in dorsal root ganglia and spinal cord and examined the analgesic effects of peripherally and centrally administered FABP5 inhibitors.ResultsImmunofluorescence revealed robust expression of FABP5 in lumbar dorsal root ganglia. FABP5 was distributed in peptidergic calcitonin gene-related peptide-expressing dorsal root ganglia and non-peptidergic isolectin B4-expressing dorsal root ganglia. In addition, the majority of dorsal root ganglia expressing FABP5 also expressed transient receptor potential vanilloid 1 (TRPV1) and peripherin, a marker of nociceptive fibers. Intraplantar administration of FABP5 inhibitors reduced thermal and mechanical hyperalgesia in the complete Freund’s adjuvant model of chronic inflammatory pain. In contrast to its robust expression in dorsal root ganglia, FABP5 was sparsely distributed in the lumbar spinal cord and intrathecal administration of FABP inhibitor did not confer analgesic effects. Administration of FABP inhibitor via the intracerebroventricular (i.c.v.) route reduced thermal hyperalgesia. Antagonists of peroxisome proliferator-activated receptor alpha blocked the analgesic effects of peripherally and i.c.v. administered FABP inhibitor while antagonism of cannabinoid receptor 1 blocked the effects of peripheral FABP inhibition and a TRPV1 antagonist blocked the effects of i.c.v. administered inhibitor. Although FABP5 and TRPV1 were co-expressed in the periaqueductal gray region of the brain, which is known to modulate pain, knockdown of FABP5 in the periaqueductal gray using adeno-associated viruses and pharmacological FABP5 inhibition did not produce analgesic effects.ConclusionsThis study demonstrates that FABP5 is highly expressed in nociceptive dorsal root ganglia neurons and FABP inhibitors exert peripheral and supraspinal analgesic effects. This indicates that peripherally restricted FABP inhibitors may serve as a new class of analgesic and anti-inflammatory agents.

ANALGESIC ACTIVITY OF HYDROALCOHOLIC EXTRACT OF AERIAL PARTS OF MALVASTRUM COROMANDELIANUM

ABSTRACTObjective: The aim of this study is to explore the antinociceptive property of hydroalcoholic extract of Malvastrum coromandelianum. Pain is anunpleasant sensory and emotional experience associated with actual and potential tissue damage. Various types of pain are seen in humans, somatic,visceral, and neuropathic pain. M. coromandelianum belongs to Malvaceae family. Traditionally, it is used as an emollient, anti-inflammatory, analgesic,and antidysenteric.Methods: The antinociceptive effects of hydroalcoholic extract of M. coromandelianum (HEMC) were evaluated by tail-flick, hot-plate, and acetic acidinduced writhing method. Rats were divided into four groups. The first group which served as control was administered with aqueous 1% tragacanthsuspension. The second group received standard drug, morphine (5 mg/kg) orally as a suspension. Third and fourth group received HEMC (200 and400 mg/kg) and served as test drug treatment group, 30 minutes after treatment the reaction time and number of writhes was noted.Results: HEMC produced significant antinociceptive effects against thermally induced pain. In tail-flick method, the peak effect of HEMC 400 mg/kgwas shown at 60 minutes which is nearly equal to the peak effect of morphine 5 mg/kg. In hot-plate method, the HEMC showed significant analgesiceffect up to 3 hrs after the treatment, whereas morphine showed significant effect up to 6 hrs.Conclusion: On the basis of finding, it may conclude that the HEMC shows its central analgesic action probably through inhibition of central painreceptors, whereas peripheral analgesic effect may be mediated by prostaglandin inhibition.Keywords: Pain, Analgesia, Hydroalcoholic extract of Malvastrum coromandelianum, Morphine.

Efficacy of buprenorphine added to 2% lignocaine plus adrenaline 1:80,000 in providing postoperative analgesia after lower third molar surgery

A number of trials have examined the peripheral analgesic effect of opioids, known to have an anti-nociceptive effect at the central and/or spinal cord level. This study aimed to evaluate the efficacy of buprenorphine added to 2% lignocaine with adrenaline 1:80,000 in providing postoperative analgesia after lower third molar surgery. Sixty patients were randomized to three groups: group A received lignocaine 2% with adrenaline 1:80,000 for inferior alveolar nerve block (IANB), along with intramuscular (IM) injection of 1ml saline; group B received buprenorphine mixed with lignocaine 2% with adrenaline 1:80,000 for IANB (0.01mg buprenorphine/ml lignocaine with adrenaline), along with 1ml saline IM; group C received lignocaine 2% with adrenaline 1:80,000 for IANB, along with 0.03mg buprenorphine IM. Mean postoperative pain scores (visual analogue scale; when the patient first felt pain) were 6.0 for group A, 1.0 for group B, and 4.4 for group C. The mean duration of postoperative analgesia was 3.5h in groups A and C and 12h in group B. The mean number of postoperative analgesics consumed was 5.8 in groups A and C and 3.9 in group B. The addition of buprenorphine (0.03mg) to 2% lignocaine with adrenaline 1:80,000 significantly reduced the severity of postoperative pain and prolonged the duration of analgesia, thereby decreasing the need for postoperative analgesics.

Study The Analgesic And Anti-Inflammatory Activity Of Zingiber Officinale Rhizome Extract Comparison With Ibuprofen In Male Mice

The aim of the present study was to explore the probable analgesic and anti-inflammatory activity of ethanolic extract of Zingiber officinale by using Hot plate test , acetic acid – induced writhing and Carrageenan induced inflammation in male mice. Thirty Albino male mice (average weight 20g) were used and randomly divided in this study (six animals were used in each groups test for Zingiber officinale in doses of 100, 200 and 300 mg/kg B.W, Ibuprofen 20mg/kg B.W and control ( gave distilled water).The results were expressed as the Mean ±SEM and the statistical significance of differences between groups was analyzed by two Way Analysis of Variance (ANOVA) . The extract showed a significant inhibition of writhing response and increase in hot plate reaction time and also caused significant decrease P<0.05 in paw oedema that induced by Carrageenan comparable with Ibuprofen that standard drugs used. We can conclude from the outcome of the present work that Zingiber officinale extract exert an excellent analgesic and anti-inflammatory effect in the mice and exhibit peripheral and central analgesic effect and anti-inflammatory activity, which may be attributed to the various phytochemicals present in rhizomes of Zingiber officinale .

The Analgesic Effect of Echinophora Platyloba Hydroalcoholic Extract in Male Rats

BACKGROUND AND OBJECTIVES: In traditional Iranian medicine, use of medicinal herbs is prevalent for the treatment and alleviation of pain and inflammation. Echinophora Platyloba is one of the major medicinal herbs with confirmed anti-cancer, anti-microbial, and anti-fungal effects. Therefore, in this study, we aimed to evaluate the analgesic effects of E. platyloba hydroalcoholic extracts in male rats. METHODS: In this experimental study, 42 rats were divided into six groups, i.e., control group, extract groups (80, 100, and 300 mg/kg via intraperitoneal injection), morphine group (1 mg/kg via intraperitoneal injection), and naloxone group (1 mg/kg of naloxone along with 300 mg/kg of the extract). To evaluate the analgesic effects of the extracts, rating, tail flick, and formalin tests were performed. FINDINGS: In the writhing test, doses of 100 and 300 mg/kg of the extracts could decrease the rating score from 43.3±22.61 to 27.31±2.34 (p<0.05) and 29.0±67.91 (p<0.01) in the control group, respectively. In addition, administration of 300 mg/kg of the extract in tail flick test decreased the latency time from 2.84±0.76 to 5.52±1.88 sec in the control group (p<0.05). Moreover, in formalin test, 300 mg/kg of the extract reduced the pain score in both acute and chronic phases from 2.18±0.22 to 0.74±0.13 in the control group (p<0.01). CONCLUSION: According to the present results, E. platyloba hydroalcoholic extracts have central and peripheral analgesic effects.

Piroxicam sulfonates biology-oriented drug synthesis (BIODS), characterization and anti-nociceptive screening

The present study was designed to synthesize aryl/alkyl sulfonyl substituted derivatives of piroxicam. The structures were elucidated through various spectroscopic procedures like 1H-NMR, 13C-NMR, MS, IR, UV and elemental analysis. The synthesized derivatives (I–VI) were screened for anti-nociceptive activity in BALB-c mice by dividing them in five groups (n = 6) for each compound. Test compounds significantly reduced the number of writhes in acetic acid test for peripheral analgesia, but no effect was observed in hot plate test for central analgesia. The compounds possessed good peripheral analgesic effect but no central effect. These compounds were also assessed for acute toxicity in different doses at 50, 100 and 150 mg/kg i.p., and no mortality was observed in animals. In conclusion, the piroxicam derivatives exhibited marked peripheral anti-nociceptive effect with considerable safety.

Biphalin preferentially recruits peripheral opioid receptors to facilitate analgesia in a mouse model of cancer pain - A comparison with morphine

The search for new drugs for cancer pain management has been a long-standing goal in basic and clinical research. Classical opioid drugs exert their primary antinociceptive effect upon activating opioid receptors located in the central nervous system. A substantial body of evidence points to the relevance of peripheral opioid receptors as potential targets for cancer pain treatment. Peptides showing limited blood-brain-barrier permeability promote peripheral analgesia in many pain models. In the present study we examined the peripheral and central analgesic effect of intravenously administered biphalin - a dimeric opioid peptide in a mouse skin cancer pain model, developed by an intraplantar inoculation of B16F0 melanoma cells. The effect of biphalin was compared with morphine - a golden standard in cancer pain management. Biphalin produced profound, dose-dependent and naloxone sensitive spinal analgesia. Additionally, the effect in the tumor-bearing paw was largely mediated by peripheral opioid receptors, as it was readily attenuated by the blood-brain-barrier-restricted opioid receptor antagonist - naloxone methiodide. On the contrary, morphine facilitated its analgesic effect primarily by activating spinal opioid receptors. Both drugs induced tolerance in B16F0 - implanted paws after chronic treatment, however biphalin as opposed to morphine, showed little decrease in its activity at the spinal level. Our results indicate that biphalin may be considered a future alternative drug in cancer pain treatment due to an enhanced local analgesic activity as well as lower tolerance liability compared with morphine.

Comparative analysis of analgesic and anti-inflammatory activity of bark and leaves of Acacia ferruginea DC.

The aim of the present study was to investigate and compare the analgesic and anti-inflammatory activities of hydroalcoholic extracts of bark and leaves of Acacia ferruginea DC. Hydroalcoholic extracts of bark and leaves were evaluated for analgesic activity using hot plate method and acetic acid-induced writhing test, whereas the anti-inflammatory activity was evaluated by carrageenan-induced paw oedema method. Hydroalcoholic extract of the bark at the dose of 50 mg/kg (6.10 ± 0.30) and leaves at a dose of 100 mg/kg (5.72 ± 0.39) after 45 min exhibited significant (P < 0.001) analgesic activity in hot plate test, which was comparable to Tramadol (6.11 ± 0.31) at a dose of 10 mg/kg. However, in acetic acid-induced writhing test, hydroalcoholic extract of both bark (90%) and leaves (90.91%) showed maximum protection from acetic acid at the dose of 100 mg/kg as compared to standard drug (50.91%) at a dose of 5 mg/kg. In the evaluation of anti-inflammatory activity, hydroalcoholic extract of leaves at a dose of 400 mg/kg had significantly (74.68%) inhibited the inflammation as comparable to indomethacin (82.8%) after 3 h of induction of carrageenan. It is concluded that hydroalcoholic extracts of bark and leaves have central analgesic and peripheral analgesic effects, respectively. Both hydroalcoholic extracts of the bark and leaves significantly reduced the paw oedema at a dose of 400 mg/kg and exhibited anti-inflammatory activity.

The effect of dexmedetomidine added to preemptive (2% lignocaine with adrenaline) infiltration on intraoperative hemodynamics and postoperative pain after ambulatory maxillofacial surgeries under general anesthesia.

Background:Lignocaine + adrenaline; a local anesthetic agent; frequently used for perilesional infiltration, maintains the stable hemodynamics and decreases the postoperative pain after maxillofacial surgery. α2 agonists have peripheral analgesic effects. This prospective study was to evaluate the effectiveness of perilesional dexmedetomidine administered preincisionally in addition to conventional lignocaine adrenaline combinations for reconstructive maxillofacial surgery in an ambulatory care setting.Materials and Methods:76, American Society of Anesthesiologists I-II patients scheduled for unilateral traumatic maxillofacial surgeries were randomly allocated into group DL (n = 38) receiving 15 cc of 2% lignocaine + adrenaline (1:200,000) mixed with 1 μg/kg dexmedetomidine and group PL receiving 15 cc of 2% lignocaine + adrenaline with normal saline (placebo) via local wound infiltration 5 min prior to skin incision. Perioperative hemodynamics, time to first analgesic use, total analgesic need, bleeding, and side effects were recorded for each patient.Results:Dosage of supplemental propofol; total perioperative, postoperative, and postanesthesia care unit (PACU) fentanyl consumption was significantly lower (P = 0.0001, P= 0.0001, P= 0.0001, P= 0.004, respectively) in dexmedetomine treated group than placebo. Rescue analgesic requirement was significantly earlier in group PL than group DL. Group DL patients suffered from significantly less (P = 0.02) bleeding and surgeon's satisfaction score was also high in this group. Discharge from PACU was significantly earlier in group DL. Intraoperative hemodynamic parameters were significantly lower in group DL (P < 0.05) without any appreciable side effects.Conclusion:Thus, prior dexmedetomidine local infiltration at the site of maxillofacial trauma has significantly reduced bleeding from wound site; perioperative fentanyl, propofol consumption, and subsequently ensured earlier discharge from PACU, better surgeon's satisfaction score with better hemodynamic control and lesser side effects.

Evaluation of analgesic and antipyretic activities of Mahanimba (Melia azedarach Linn.) leaf and root powder.

Introduction:Mahanimba (Melia azedarach Linn.) is a deciduous tree of family Meliaceae and its root is mainly used in painful condition such as Gridhrasi (sciatica) in Ayurveda. Ethnomedicinal claims indicate that its leaves are used to treat fever, but its medicinal activities have not been proven by research.Aim:This study was aimed to evaluate the potential analgesic and antipyretic activities of M. azedarach L. leaf powder (MLP) and M. azedarach L. root powder (MRP).Materials and Methods:The root and leaves of M. azedarach were made into powder using guidelines mentioned in the Ayurvedic Pharmacopoeia of India. The analgesic activity of the test drugs was evaluated against acetic acid-induced writhing test in mice and radiant heat-induced pain in albino rats, and antipyretic activity was evaluated against Brewer’s yeast-induced pyrexia using Charles Foster rats.Results:In acetic acid induced writhing in mice, the test drugs MRP (1.3mg/kg) exhibit insignificant reduction in writhing reflex while MLP (1.3mg/kg) did not show any significant effect in comparison to the control group. MRP showed mild-to-moderate insignificant increase in latency of withdrawal response at 60 (26.74%) and 120 min (27.25%), while MLP did not show central analgesic effect in radiant heat model in rats. MRP showed a significant reduction in rectal temperature after 3 and 6 h, while MLP-treated group showed significant reduction after 6 h.Conclusion:MRP has mild-to-moderate peripheral and central analgesic effects, while MLP has not shown significant analgesic effects in both the experimental models. MRP has more pronounced antipyretic effect compared to MLP.

新斯的明经穴位注射对大鼠关节疼痛的镇痛作用及机制研究

目的：新斯的明可经关节腔注射发挥外周镇痛作用，但关节注射存在技术难度和并发症风险。本研究目的为制备大鼠关节疼痛动物模型，经穴位注射新斯的明，分析镇痛效果及作用机制，为新斯的明外周注射治疗关节疼痛的临床应用提供临床前研究依据。方法：采用雄性Wistar大鼠经踝关节注射单碘醋酸钠生理盐水溶液制备关节疼痛模型，经行为学观察及痛阈测量确认模型制备成功。实验组经后三里穴位注射新斯的明，对照组注射等体积生理盐水。注射后测量两组实验动物舔足次数和热辐射痛阈，取血清样本测量前炎症因子白介素1-1β (IL-1β)和肿瘤坏死因子-α (TNF-α)水平。结果：与对照组相比，实验组动物经穴位新斯的明注射后的舔足次数减少，热辐射痛阈提高，血清IL-1β和TNF-α水平降低，两组间上述指标结果差异具有统计学意义(p < 0.01)。结论：研究表明经穴位于关节周围注射新斯的明，可提高关节疼痛实验动物热辐射痛阈，降低体内前炎症因子表达水平，产生镇痛作用。 Objective: Neostigmine could provide peripheral analgesic effects via intra-articular injection, but with obvious technical difficulty and potential complications. The purpose of this study is to inves-tigate the analgesic effect and mechanism of acupoint injection of neostigmine in a rat model of joint pain. Methods: Animal models of ankle joint pain were prepared via intra-articular injection of monosodium iodoacetate in male Wistar rats. The experimental group received acupoint injection of neostigmine while the control group received acupoint injection of normal saline. The numbers of hind paw licking and radiation pain threshold were recorded, and serum IL-1β and TNF-α were measured using ELISA. Results: Compared with the control group, the animals in the experimental group presented less hind paw licking, higher radiation threshold and lower levels of serum IL-1β and TNF-α (p < 0.01). Conclusion: Acupoint injection of neostigmine elevated the radiation pain threshold and lowered the level of proinflammatory pain mediators, providing effective analgesia in the rat model of joint pain.

Peripheral morphine reduces acute pain in inflamed tissue after third molar extraction: A double-blind, randomized, active-controlled clinical trial

BackgroundThe clinical use of peripheral analgesic effects of opioids has been investigated in numerous controlled clinical trials. The majorities of these have tested the local, intra-articular administration of morphine in knee surgery and have demonstrated marginal postoperative analgesia. ObjectiveWe examined direct morphine infiltration of the surgical site in a clinical model of tooth pain under two different conditions. Eighty-eight patients undergoing surgical tooth removal entered into the two prospective, parallel, randomized, double-blind studies. MethodsPatients undergoing surgical tooth removal received a standard local anaesthetic solution (articaine plus epinephrine) before surgery.Patients were assigned to an injection of peripheral 2mg morphine either into non-inflamed (Trial I) or inflamed (Trial II) submucous tissue before the surgery. Patients who received an intramuscular morphine in the upper arm were concomitantly given 1ml isotonic saline (NaCl) as a submucous injection. Patients who received a submucous injection of morphine peripherally were concomitantly given an intramuscular injection (IM) of 1ml of NaCl in the upper arm. Postoperative pain intensity was assessed by the numeric rating scale every 30min for the first 2h and then every hour for the next 8h after surgery. In addition, patients recorded the occurrence of side effects and the supplemental consumption of ibuprophen and codeine+paracetamol combination tablets. ResultsOf the eighty-eight original participants, nine patients (4 patients in Trial I and 5 patients in Trial II) were withdrawn for protocol noncompliance and loss at follow-up. Thirty-one patients in trial I and forty-eight patients in trial II were analyzed. Patients receiving 2mg morphine into non-inflamed tissue did not show any further reduction in pain scores and pain medication consumption compared to IM morphine group (Trial I).In patients receiving 2mg morphine into inflamed tissue, pain scores at rest were reduced to a similar extent in both groups at all measurement times up to 10h in the follow-up (Trial II). At the same time, in the area under the curves pain scores on swallowing between 2 and 6h in the peripheral morphine group (5.2±5.6) were significantly lower than in the IM morphine group (9.3±7.3, p=0.03), demonstrating the marginal analgesic efficacy of additional morphine. Sedation scores were significantly higher in the peripheral morphine group only 1h after surgery in Trial I (p=0.008). The time to first analgesic intake was similar between groups. No serious side effects were reported. ConclusionsOur results showed in patients undergoing surgical tooth removal that injection of 2mg of morphine into inflamed tissue results in significantly lower pain scores on swallowing in the early postoperative state while administration into non-inflamed tissue is not effective. ImplicationsOur studies indicate that the peripheral administration of opioids, at the doses and conditions set out for these two studies, produces significant analgesia by a pharmacologically specific mechanism that is active in chronically, but not acutely, inflamed tissue. Thus, consistent with preclinical experimental studies, the requirement of an inflammatory process for the occurrence of the peripheral opioid effects is also found in the clinical setting.

Phytochemicals, antioxidant, antinociceptive and anti-inflammatory potential of the aqueous extract of Teucrium stocksianum bioss

BackgroundDespite availability of a substantial number of potent synthetic drugs, medicinal plants are still playing a key role in the discovery of novel and effective drug molecules. Numerous researchers are focusing on the plant based medicines due to its strong safety profiles. Teucrium species exhibit profound antidiabetic, analgesic and spasmolytic activities. The methanolic extract and essential oil of Teucrium stocksianum possess strong antinociceptive activity. The aim of the current research study was to determine the phytochemicals, antioxidant, analgesic and anti-inflammatory potential of the aqueous extract of Teucrium stocksianum Bioss (AETS).MethodPhytochemical screening was carried out according to standard procedures. The antioxidant potential of the extract was ascertained with the stable organic free radical (2, 2-diphenyl-1-picryl-hydrazyl). Three different pain models, including acetic acid induced writhing, formalin induced paw licking and tail immersion tests were carried out for the determination of antinociceptive potential, while the anti-inflammatory activity was evaluated through carrageenan induced paw edema test in mice. The antinociceptive and anti-inflammatory potentials of AETS were assessed at 100, 200 and 300 mg/kg body weight, while acute toxicity were observed at 1500 mg/kg body weight in various groups of mice.ResultsPhytochemical screening has shown the occurrence of flavonoids saponins, reducing sugars, terpenoids and tannins. AETS exhibited profound antioxidant activity and has shown maximum activity (60.06 ± 0.846) at 250 μg/ml. In the three pain models AETS displayed marked dose dependent antinociceptive potential. AETS exhibited 63.5, 67.61 and 64 % activity in acetic acid induced, formalin induced paw licking and tail immersion tests respectively. The antinociceptive effect of AETS and reference standard drug TramadolR was significantly reversed by Naloxone, endorsed the central analgesic potential of AETS. Similarly the extract also reversed the paw edema in dose dependent manner. AETS displayed significant (53.81 %) anti-inflammatory effects at a dose of 300 mg/kg that persisted till 5th h. In acute toxicity test AETS was found safe at 1500 mg/kg body weight.ConclusionsAETS exhibited profound antioxidant activity. The test sample displayed marked antinociceptive potential in all the test procedures, indicating the peripheral and central analgesic effects of AETS. The plant extract also displayed marked anti-inflammatory activity at all test doses.

Activation of κ Opioid Receptors in Cutaneous Nerve Endings by Conorphin-1, a Novel Subtype-Selective Conopeptide, Does Not Mediate Peripheral Analgesia.

Selective activation of peripheral κ opioid receptors (KORs) may overcome the dose-limiting adverse effects of conventional opioid analgesics. We recently developed a vicinal disulfide-stabilized class of peptides with subnanomolar potency at the KOR. The aim of this study was to assess the analgesic effects of one of these peptides, named conorphin-1, in comparison with the prototypical KOR-selective small molecule agonist U-50488, in several rodent pain models. Surprisingly, neither conorphin-1 nor U-50488 were analgesic when delivered peripherally by intraplantar injection at local concentrations expected to fully activate the KOR at cutaneous nerve endings. While U-50488 was analgesic when delivered at high local concentrations, this effect could not be reversed by coadministration with the selective KOR antagonist ML190 or the nonselective opioid antagonist naloxone. Instead, U-50488 likely mediated its peripheral analgesic effect through nonselective inhibition of voltage-gated sodium channels, including peripheral sensory neuron isoforms NaV1.8 and NaV1.7. Our study suggests that targeting the KOR in peripheral sensory nerve endings innervating the skin is not an alternative analgesic approach.

Signaling Mechanism of Cannabinoid Receptor-2 Activation-Induced β-Endorphin Release.

Activation of cannabinoid receptor-2 (CB2) results in β-endorphin release from keratinocytes, which then acts on primary afferent neurons to inhibit nociception. However, the underlying mechanism is still unknown. The CB2 receptor is generally thought to couple to Gi/o to inhibit cAMP production, which cannot explain the peripheral stimulatory effects of CB2 receptor activation. In this study, we found that in a keratinocyte cell line, the Gβγ subunits from Gi/o, but not Gαs, were involved in CB2 receptor activation-induced β-endorphin release. Inhibition of MAPK kinase, but not PLC, abolished CB2 receptor activation-induced β-endorphin release. Also, CB2 receptor activation significantly increased intracellular Ca(2+). Treatment with BAPTA-AM or thapsigargin blocked CB2 receptor activation-induced β-endorphin release. Using a rat model of inflammatory pain, we showed that the MAPK kinase inhibitor PD98059 abolished the peripheral effect of the CB2 receptor agonist on nociception. We thus present a novel mechanism of CB2 receptor activation-induced β-endorphin release through Gi/o-Gβγ-MAPK-Ca(2+) signaling pathway. Our data also suggest that stimulation of MAPK contributes to the peripheral analgesic effect of CB2 receptor agonists.

The effects of morphine and methylnaltrexone on gastrointestinal pain in healthy male participants.

Opioid antagonists are increasingly used to abolish the gastrointestinal side effects of opioids. However, they can potentially interfere with local analgesia exerted via opioid receptors in the gut. Thus, in the current study we aimed to explore the effect of rectal morphine before and after blocking opioid receptors outside the central nervous system with methylnaltrexone (MNTX). In this randomized, placebo controlled, cross-over study 15 healthy male participants received the following drugs at three separate sessions: (i) placebo, (ii) 30 mg morphine administered per rectum, or (iii) 12 mg MNTX given subcutaneously before 30 mg rectal morphine. At baseline and after drug administration peripheral and central effects of the drugs were assessed by experimental pain to the skin, muscle, rectum and pupillometry. Compared to placebo there was no local effect of morphine on mechanical rectal distension. In contrast, an increase in tolerated volume was seen following MNTX/morphine administration (p < 0.001), starting 7 min after dosing. Both morphine and MNTX/morphine had a central effect manifested as an increase in mechanical muscle pressure thresholds (both p < 0.001) and a decrease in pupil diameter (both p < 0.001). These effects occurred 30 min after dosing. No peripheral analgesic effect of morphine was found. Methodological shortcomings may have contributed to the lack of peripheral analgesia and thus, a peripheral morphine effect on rectal pain cannot be excluded. On the other hand, the combination of MNTX and morphine exerted a local effect on rectal distensions and seems to improve analgesia.

Dexmedetomidine inhibits Tetrodotoxin-resistant Nav1.8 sodium channel activity through Gi/o-dependent pathway in rat dorsal root ganglion neurons.

BackgroundSystemically administered dexmedetomidine (DEX), a selective α2 adrenergic receptor (α2-AR) agonists, produces analgesia and sedation. Peripherally restricted α2-AR antagonist could block the analgesic effect of systemic DEX on neuropathic pain, with no effect on sedation, indicating peripheral analgesic effect of DEX. Tetrodotoxin-resistant (TTX-R) sodium channel Nav1.8 play important roles in the conduction of nociceptive sensation. Both α2-AR and Nav1.8 are found in small nociceptive DRG neurons. We, therefore, investigated the effects of DEX on the Nav1.8 currents in acutely dissociated small-diameter DRG neurons.ResultsWhole-cell patch-clamp recordings demonstrated that DEX concentration-dependently suppressed TTX-R Nav1.8 currents in small-diameter lumbar DRG neurons. DEX also shifted the steady-state inactivation curves of Nav1.8 in a hyperpolarizing direction and increased the threshold of action potential and decrease electrical and chemical stimuli-evoked firings in small-diameter DRG neurons. The α2-AR antagonist yohimbine or α2A-AR antagonist BRL44408 but not α2B-AR antagonist imiloxan blocked the inhibition of Nav1.8 currents by DEX. Immunohistochemistry results showed that Nav1.8 was predominantly expressed in peripherin-positive small-diameter DRG neurons, and some of them were α2A-AR-positive ones. Our electrophysiological recordings also demonstrated that DEX-induced inhibition of Nav1.8 currents was prevented by intracellular application of G-protein inhibitor GDPβ-s or Gi/o proteins inhibitor pertussis toxin (PTX), and bath application of adenylate cyclase (AC) activator forskolin or membrane-permeable cAMP analogue 8-Bromo-cAMP (8-Br-cAMP). PKA inhibitor Rp-cAMP could mimic DEX-induced inhibition of Nav1.8 currents.ConclusionsWe established a functional link between α2-AR and Nav1.8 in primary sensory neurons utilizing the Gi/o/AC/cAMP/PKA pathway, which probably mediating peripheral analgesia of DEX.

Phytochemical Constituents and Analgesic Activity of Ethyl Acetate Fraction of Punicagranatum L (Punicaceae)

Purpose: To investigate the active fraction of pomegranate fruit extract and screen it for analgesic activity.Methods: The analgesic activity of pomegranate ethyl acetate fraction (EtOAc) was examined using three models of pain: writhing, hot tail flick and plantar tests. EtOAc was administered by oral gavage in doses of 100, 150 and 200 mg/kg, p.o., for all the tests and compared to aspirin (100 mg/kg, p.o.) which was used as standard drug. Phytochemical studies of EtOAc were carried out by high performance liquid chromatography (HPLC) with ultraviolet (UV) detection and mass spectrometry (MS).Results: In the writhing test, the index of pain inhibition (IPI) was 41 % for EtOAc (200 mg/kg, p.o.) and 56 % for aspirin. In the hot tail flick test, EtOAc (200 mg/kg, p.o.) showed analgesia reaching its peak at 60 min with maximum possible analgesia (MPA) of 30.5 %, compared with 43.8 % for aspirin. Plantar test showed that pain was reduced by EtOAc in a dose-dependent manner and compared well with aspirin at 100 mg/kg, p.o., dose. The 200 mg/kg dose showed the highest effect, prolonging withdrawal latency in the left hind paw to 11.9 ± 0.3 compared to aspirin with 13.4 ± 0.2 (p &lt; 0.001). HPLC analysis of EtOAc revealed the presence of gallic acid, ellagic acid and punicalagins A &amp; B. Confirmation of their structures was achieved by mass spectroscopy.Conclusion: EtOAc has a central and peripheral analgesic effect that is most likely due to the presence of gallic acid and ellagic acid.Keywords: Analgesia, Pomegranate, Gallic acid, Ellagic acid, Punicalagins, Phytochemical constituents

Peripheral Analgesic and Anti-Inflammatory Effects of Smilax canariensis in an Animal Model

Smilax canariensis Brouss. ex Willd. is an endemic plant of the Canary Islands. Its rhizomes, leaves and stems have been traditionally used in Canary folk medicine to treat a wide variety of conditions including pain. Our objective is to investigate the analgesic and anti-inflammatory activities of different extracts of S. canariensis in Swiss mice, using established biological models for pain and inflammation, such as phenylquinone writhing test, formalin test, tail-flick test and mouse paw edema induced by carrageenan. Oral administration of S. canariensis extracts significantly reduce writhing episodes evoked by phenylquinone injection in a dose-dependent manner; and higher doses result in a reduction of pain similar to or higher than that of the reference drug piroxicam (59.56%; p < 0.01). The extracts also cause a marked dose-dependent inhibition of for-malin-induced pain in the second phase but only minimal inhibition of tail-flick behavior, suggesting that S. canariensis is not a centrally acting analgesic. Finally, in the carrageenan-induced hind paw edema model, the extracts show a moderate anti-inflammatory effect, the most active being the ethyl acetate fraction at 200 mg/kg p.o. (33.33%; p < 0.05). Our results suggest that S. canariensis extracts have clear dose-dependent peripheral analgesic effects, which lends support to the traditional use of this medicinal plant to treat pain associated with inflammatory or other processes.