Antinociception In Rats Research Articles

The Effects of Eating a Traditional High Fat/High Carbohydrate or a Ketogenic Diet on Sensitivity of Female Rats to Morphine.

Patients diagnosed with obesity are prescribed opioid medications at a higher rate than the general population; however, it is not known if eating a high fat diet might impact individual sensitivity to these medications. To explore the hypothesis that eating a high fat diet increases sensitivity of rats to the effects of morphine, 24 female Sprague-Dawley rats (n = 8/diet) ate either a standard (low fat) laboratory chow (17% kcal from fat), a high fat/low carbohydrate (ketogenic) chow (90.5% kcal from fat), or a traditional high fat/high carbohydrate chow (60% kcal from fat). Morphine-induced antinociception was assessed using a warm water tail withdrawal procedure, during which latency (in seconds) for rats to remove their tail from warm water baths was recorded following saline or morphine (0.32-56 mg/kg, i.p.) injections. Morphine was administered acutely and chronically (involving 18 days of twice-daily injections, increasing in 1/4 log dose increments every 3 days: 3.2-56 mg/kg, i.p., to induce dependence and assess tolerance). The adverse effects of morphine (i.e., tolerance, withdrawal, and changes in body temperature) were assessed throughout the study. Acute morphine induced comparable antinociception in rats eating different diets, and all rats developed tolerance following chronic morphine exposure. Observable withdrawal signs and body temperature were also comparable among rats eating different diets; however, withdrawal-induced weight loss was less severe for rats eating ketogenic chow. These results suggest that dietary manipulation might modulate the severity of withdrawal-related weight loss in ways that could be relevant for patients.

Activity of Mitragyna speciosa ("Kratom") Alkaloids at Serotonin Receptors.

Kratom alkaloids have mostly been evaluated for their opioid activity but less at other targets that could contribute to their physiological effects. Here, we investigated the in vitro and in vivo activity of kratom alkaloids at serotonin receptors (5-HTRs). Paynantheine and speciogynine exhibited high affinity for 5-HT1ARs and 5-HT2BRs, unlike the principal kratom alkaloid mitragynine. Both alkaloids produced antinociceptive properties in rats via an opioid receptor-independent mechanism, and neither activated 5-HT2BRs in vitro. Paynantheine, speciogynine, and mitragynine induced lower lip retraction and antinociception in rats, effects blocked by a selective 5-HT1AR antagonist. In vitro functional assays revealed that the in vivo 5-HT1AR agonistic effects may be due to the metabolites 9-O-desmethylspeciogynine and 9-O-desmethylpaynantheine and not the parent compounds. Both metabolites did not activate 5-HT2BR, suggesting low inherent risk of causing valvulopathy. The 5-HT1AR agonism by kratom alkaloids may contribute to the mood-enhancing effects associated with kratom use.

Blockade of α1 subtype GABAA receptors attenuates the development of tolerance to the antinociceptive effects of midazolam in rats.

Benzodiazepines bind to and act on α1-3 and α5-containing GABAA receptors. Previous studies suggest that different GABAA receptor α-subtypes mediate the various behavioral effects of benzodiazepines, which raises the possibility of combining benzodiazepines with subtype-selective GABAA receptor antagonists to improve the therapeutic profiles of benzodiazepines. This study examined the GABAA receptor subtype mediation of the tolerance to midazolam-induced antinociception in rats. Midazolam (3.2 mg/kg) significantly reduced the locomotion in rats which was prevented by the selective α1-preferring GABAA receptor antagonist β-carboline-3-carboxylate-t-butyl ester (βCCt) (3.2 mg/kg). Midazolam increased the paw withdrawal threshold as tested by the von Frey filament assay in the complete Freund's adjuvant-induced inflammatory pain model in rats, and this effect was not altered by βCCt or another α1-preferring GABAA receptor antagonist 3-propoxy-β-carboline hydrochloride (3PBC). Repeated treatment with midazolam in combination with vehicle, βCCt or 3PBC (twice daily) for 7 days led to a progressive increase of the ED50 values in the midazolam- and vehicle-treated rats, but not in other rats, suggesting the development of tolerance to midazolam but not to the combination of midazolam with α1-preferring GABAA receptor antagonists. These results suggest the essential role of the α1-subtype of GABAA receptors in mediating the development of tolerance to midazolam-induced antinociceptive effects and raise the possibility of increasing therapeutic profiles of benzodiazepines by selectively blocking specific α-subtypes of GABAA receptors.

Increasing the antinociceptive effect of ingested glycinamide in female rats by increasing its palatability

BackgroundThese studies were undertaken to investigate whether the ingestion of glycinamide, a precursor of glycine, made more palatable by mixing with a chocolate suspension, improves antinociception in rats. MethodsTwo nociception threshold models were employed: the tail-flick latency and vocalization to tail shock, in restricted and freely-moving rats. Glycinamide in a highly palatable commercial chocolate aqueous suspension was provided for ad-lib ingestion after 24 hours of water deprivation. Antinociception threshold testing was performed before and 10, 30, 60, 90, 120, 180, and 240 minutes after the ingestion of the chocolate-glycinamide mixture. ResultsIngestion of the glycinamide-in-chocolate suspension induced antinociception based on the tail shock vocalization and tail-flick latency tests. Ingestion of the glycinamide-in-chocolate suspension induced an 80% elevation in the antinociceptive threshold that persisted for 4 hours. ConclusionsRats readily ingest the glycine precursor, glycinamide, in an aqueous chocolate mixture, which induces potent and prolonged antinociception.

The Lectin Isolated from the Alga Hypnea cervicornis Promotes Antinociception in Rats Subjected to Zymosan-Induced Arthritis: Involvement of cGMP Signalization and Cytokine Expression.

This study investigated the effects of the alga lectin Hypnea cervicornis agglutinin (HCA) on rat zymosan-induced arthritis (ZyA). Zymosan (50-500μg/25μL) or sterile saline (Sham) was injected into the tibio-tarsal joint of female Wistar rats (180-200g). Arthritic animals received morphine (4mg/kg, intraperitoneal), indomethacin (5mg/kg, intraperitoneal), or 2% lidocaine (100μL, subcutaneous). HCA (0.3-3mg/kg) was administered by intravenous route 30min before or 2h after zymosan. 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one (ODQ, 4μg, intra-articular) was given 30min prior HCA. Hypernociception was measured every hour until 6h, time in which animals were sacrificed for evaluation of leukocytes of the intra articular fluid and gene expression of TNF-α, IL-1, IL-10, and iNOS in the joint tissues using PCR techniques. Hypernociception was responsive to morphine and indomethacin, and its threshold was not altered by lidocaine. The post-treatment of HCA reduced both hypernociception and leukocyte influx. This antinociceptive effect was abolished either by ODQ and glibenclamide. HCA also reduced gene expression of iNOS and TNF-α. In conclusion, the antinociceptive effect of HCA in ZyA involves cyclic GMP signalization and selective modulation of cytokine expression.

Effectiveness and selectivity of a heroin conjugate vaccine to attenuate heroin, 6-acetylmorphine, and morphine antinociception in rats: Comparison with naltrexone

BackgroundOne emerging strategy to address the opioid crisis includes opioid-targeted immunopharmacotherapies. This study compared effectiveness of a heroin-tetanus toxoid (TT) conjugate vaccine to antagonize heroin, 6-acetylmorphine (6-AM), morphine, and fentanyl antinociception in rats. MethodsAdult male and female Sprague Dawley rats received three doses of active or control vaccine at weeks 0, 2, and 4. Vaccine pharmacological selectivity was assessed by comparing opioid dose-effect curves in 50 °C warm-water tail-withdrawal procedure before and after active or control heroin-TT vaccine. Route of heroin administration [subcutaneous (SC) vs. intravenous [IV)] was also examined as a determinant of vaccine effectiveness. Continuous naltrexone treatment (0.0032-0.032 mg/kg/h) effects on heroin, 6-AM, and morphine antinociceptive potency were also determined as a benchmark for minimal vaccine effectiveness. ResultsThe heroin-TT vaccine decreased potency of SC heroin (5-fold), IV heroin (3-fold), and IV 6-AM (3-fold) for several weeks without affecting IV morphine or SC and IV fentanyl potency. The control vaccine did not alter potency of any opioid. Naltrexone dose-dependently decreased antinociceptive potency of SC heroin, and treatment with 0.01 mg/kg/h naltrexone produced similar, approximate 8-fold decreases in potencies of SC and IV heroin, IV 6-AM, and IV morphine. The combination of naltrexone and active vaccine was more effective than naltrexone alone to antagonize SC heroin but not IV heroin. ConclusionsThe heroin-TT vaccine formulation examined is less effective, but more selective, than chronic naltrexone to attenuate heroin antinociception in rats. Furthermore, these results provide an empirical framework for future preclinical opioid vaccine research to benchmark effectiveness against naltrexone.

The Effects of Morphine, Baclofen, and Buspirone Alone and in Combination on Schedule-Controlled Responding and Hot Plate Antinociception in Rats.

Better therapeutic options are needed for pain. Baclofen, buspirone, and morphine are characterized as having analgesic properties. However, little is known about potential interactions between analgesic effects of these drugs when combined. Furthermore, it is not known if the magnitude of these potential interactions will be similar for all drug effects. Thus, we tested the effects of these drugs alone and in combination for their capacity to produce thermal antinociception and to decrease food-maintained responding. Four male and four female Sprague-Dawley rats responded for food under a fixed-ratio 10 schedule; afterward they were immediately placed on a 52°C hot plate. Morphine, baclofen, and buspirone were examined alone and in 1:1 combinations, based upon ED50 values. Morphine and baclofen effects were evaluated with the opioid antagonist naltrexone and the GABAB antagonist (3-Aminopropyl)(diethoxymethyl)phosphinic acid (CGP35348), respectively. Morphine, baclofen, and buspirone dose dependently decreased operant responding, with the calculated ED50 values being 7.09, 3.42, and 0.57 mg/kg, respectively. The respective antinociception ED50 values were 16.15, 8.75, and 2.20 mg/kg. Analysis of 1:1 combinations showed the effects of morphine plus baclofen to decrease schedule-controlled responding and to produce thermal antinociception were synergistic. Effects of morphine plus buspirone and baclofen plus buspirone to decrease schedule-controlled responding were additive. Effects of the two combinations to produce thermal antinociception were synergistic. Naltrexone and CGP35348 antagonized the effects of morphine and baclofen, respectively. Synergistic antinociceptive effects, in conjunction with additive effects on food-maintained responding, highlight the therapeutic utility of opioid and non-opioid drug combinations.

The effects of mitragynine and morphine on schedule-controlled responding and antinociception in rats.

Mitragyna speciosa (kratom) may hold promise as both an analgesic and treatment for opioid use disorder. Mitragynine, its primary alkaloid constituent, is an opioid receptor ligand. However, the extent to which the in vivo effects of mitragynine are mediated by opioid receptors, or whether mitragynine interacts with other opioid agonists, is not fully established. The effects of mitragynine and the prototypical opioid agonist morphine were compared for their capacity to decrease operant responding for food delivery, and to increase response latency to a thermal stimulus. Male and female Sprague-Dawley rats responded under a multiple cycle fixed ratio 10 schedule of food delivery and were tested on a hot plate (52°C) immediately after each cycle. Morphine and mitragynine were administered alone, in combination with each other, and in combination with the opioid antagonist naltrexone. Morphine and mitragynine dose-dependently decreased schedule-controlled responding; the ED50 values were 7.3 and 31.5mg/kg, respectively. Both drugs increased thermal antinociception; the ED50 value for morphine was 18.3. Further, doses of naltrexone that antagonized morphine did not antagonize mitragynine. Mitragynine (17.8mg/kg) did not alter the rate-decreasing or antinociceptive effects of morphine. The antinociceptive effects of mitragynine and morphine occur at doses larger than those that disrupt learned behavior. Opioid receptors do not appear to mediate the disruptive effects of mitragynine on learned behavior. Mitragynine had lesser antinociceptive effects than morphine, and these did not appear to be mediated by opioid receptors. The pharmacology of mitragynine includes a substantial non-opioid mechanism.

The Primary Motor Cortex Stimulation Attenuates Cold Allodynia in a Chronic Peripheral Neuropathic Pain Condition in <i>Rattus norvegicus</i>

Background: The primary motor cortex (M1) stimulation (MCS) is a useful tool for attenuation of the peripheral neuropathic pain in patients with pharmacologically refractory pain. Furthermore, that neurological procedure may also cause antinociception in rodents with neuropathic pain. Cold allodynia is a frequent clinical finding in patients with neuropathic pain, then, we evaluated if an adapted model of neuropathy induced by chronic constriction injury (CCI) of the ischiadicus nervus (sciatic nerve) produces cold allodynia in an animal model of chronic pain. In addition, we also investigated the effect of the electrical stimulation of the M1 on chronic neuropathic pain condition in laboratory animals. Methods: Male Wistar rats were used. An adapted model of peripheral mononeuropathy induced by CCI was carried out by placing a single loose ligature around the right sciatic nerve. The acetone test was used to evaluate the cold allodynia in CCI or Sham (without ligature) rats. The MCS (M1) was performed at low-frequency (20 μA, 100 Hz) during 15 s by deep brain stimulation (DBS-Thomas Recording device) 21 days after CCI or Sham procedures. The cold allodynia was measured before and immediately after the neurostimulation of M1 in the following time-window: 0, 15 and 30 min after MCS. Results: Cold allodynia threshold increased in animals with chronic neuropathic pain submitted to the acetone test 21 days after the CCI surgery. The M1-stimulation by DBS procedure decreased the cold allodynia immediately and until 30 min after M1-stimulation in rats with chronic neuropathic pain. Conclusion: The current proposal for a CCI model by a single loose ligature of the sciatic nerve can be employed as an experimental model of chronic neuropathic pain in rats submitted to peripheral nervous system injury. The M1-stimulation produced antinociception in rats with chronic neuropathic pain. Thus, we reinforced that the MCS decreases cold allodynia in laboratory animals submitted to persistent sciatic nerve constriction and can be a more reasonable procedure for the treatment of chronic intractable neuropathic pain.

Evaluation of Swimming Exercise in Combination with non‐Steroidal Anti‐Inflammatory Drugs on Inflammatory Nociception in Rats

Pain is a growing health problem with increasing prevalence and burden worldwide. Major approaches for pain control include pharmacological and non‐pharmacological interventions. Pharmacological treatment is achieved by the use of analgesics, such as non‐steroidal anti‐inflammatory drugs (NSAIDs) and opioids. However, these medications are usually accompanied with many side effects upon prolonged use. Physical exercise is an example of non‐pharmacological interventions for pain control. In this study, we examined the effect of one of the most commonly prescribed NSAIDs, ibuprofen, on nociception threshold, with or without swimming exercise in rats. Two swimming protocols were used; Prophylactic group and treatment group. Nociception was induced by the intraplantar injection of Complete Freund's Adjuvant (CFA). Nociception threshold was measured using von Frey filaments in an up‐down protocol. The goal of this study was to determine the effect of swimming exercise on nociception threshold and ibuprofen‐induced antinociception in inflammatory pain rat model. We hypothesized that swimming exercise will enhance the ibuprofen‐induced antinociception in rats. Ibuprofen produced an antinociception effect only at the highest dose (32 mg/kg), with a duration that lasted for 17 days. A dose of 32 mg/kg ibuprofen produced significant increase in pain threshold in swimming treatment and prophylactic swimming groups when compared to ibuprofen alone group. Moreover, Ibuprofen produced significant antinociception after 10 mg/kg drug dose compared to vehicle in the prophylactic swimming group only. The duration of mechanical allodynia after CA injection was decreased in both swimming groups to 7 days only. In conclusion, the combination of ibuprofen and swimming exercise was shown to be effective in controlling nociception in a rat inflammatory pain model. The combination of exercise and ibuprofen could be suggested as an effective intervention to help control chronic pain. In addition, prophylactic swimming can increase the potency of analgesic medications, by lowering the administered drug dose, and thus, lowering the risk for ibuprofen‐induced side effects.Support or Funding InformationJUST 130/2016This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Mechanisms of imidazoline I2 receptor agonist-induced antinociception in rats: involvement of monoaminergic neurotransmission.

Although the antinociceptive efficacies of imidazoline I2 receptor agonists have been established, the exact post-receptor mechanisms remain unknown. This study tested the hypothesis that monoaminergic transmission is critical for I2 receptor agonist-induced antinociception. von Frey filaments were used to assess antinociceptive effects of two I2 receptor agonists, 2-BFI and CR4056 on chronic constriction injury (CCI)-induced neuropathic pain or complete Freund's adjuvant (CFA)-induced inflammatory pain in rats. Rectal temperature was measured to assess hypothermic effects of 2-BFI. A two-lever drug discrimination paradigm in which rats were trained to discriminate 5.6mg·kg-1 2-BFI (i.p.) from its vehicle was used to examine the discriminative stimulus effects of 2-BFI. In each experiment, pharmacological mechanisms were investigated by combining 2-BFI or CR4056 with various pharmacological manipulations of the monoaminergic system including selective reuptake inhibition, monoamine depletion and monoamine receptor antagonism. In the CCI model, selective reuptake inhibitors of 5-HT (fluoxetine) or noradrenaline (desipramine), but not dopamine (GBR12909), enhanced 2-BFI-induced antinociception. Selective depletion of 5-HT or noradrenaline almost abolished 2-BFI-induced antinociception. 5-HT1A , 5-HT2A and α1 -adrenoceptor antagonists, but not other monoaminergic antagonists, attenuated 2-BFI and CR4056-induced antinociception in CCI and/or CFA models. However, none of these monoamine receptor antagonists significantly altered 2-BFI-induced hypothermia or discriminative stimulus effects. Antinociception induced by I2 receptor agonists was mediated by serotonergic and noradrenergic mechanisms with 5-HT1A , 5-HT2A and α1 -adrenoceptor being particularly important. In contrast, the hypothermic and discriminative stimulus effects of I2 receptor agonists were mediated by distinct, independent mechanisms.

Role of orexin-2 and CB1 receptors within the periaqueductal gray matter in lateral hypothalamic-induced antinociception in rats.

Orexin plays an important role in pain modulation. Orexin-1 and orexin-2 receptors (Ox1r and Ox2r) are found at high density in the ventrolateral periaqueductal gray matter (vlPAG). Our previous study showed that chemical stimulation of the lateral hypothalamus with carbachol induces antinociception in the tail-flick test, a model of acute pain, and Ox1r-mediated antinociception in the vlPAG is modulated by the activity of vlPAG CB1 receptors. In the current study, TCS OX2 29, an Ox2r antagonist (5, 15, 50, 150, and 500 nmol/l), was microinjected into the vlPAG 5 min before the administration of carbachol (125 nmol/l). TCS OX2 29 dose dependently reduced carbachol-induced antinociception. In a second set of experiments, animals were treated with carbachol 5 min after intra-vlPAG administration of 15 nmol/l TCS OX2 29 and 1 nmol/l AM251 (a selective CB1 receptor antagonist), or 150 nmol/l TCS OX2 29 and 10 nmol/l AM251. The findings showed that the antinociceptive effect of orexin is partially mediated by activation of vlPAG Ox2 receptors. Furthermore, the administration of ineffective doses of Ox2 and CB1 receptor antagonists reduced the lateral hypothalamus-induced antinociception. It seems that Ox2 and CB1 receptors act through different pathways and Ox2r-mediated antinociception is not dependent on CB1 receptor activity.

The synthetic peptide PnPP-19 induces peripheral antinociception via activation of NO/cGMP/KATP pathway: Role of eNOS and nNOS

Backgroundand purpose: The peptide PnPP-19, derived from the spider toxin PnTx2-6 (renamed as δ-CNTX-Pn1c), potentiates erectile function by activating the nitrergic system. Since NO has been studied as an antinociceptive molecule and PnPP-19 is known to induce peripheral antinociception, we intended to evaluate whether PnPP-19 could induce peripheral antinociception through activation of this pathway. Experimental approachNociceptive thresholds were measured by paw pressure test. PGE2 (2 μg/paw) was administered intraplantarly together with PnPP-19 and inhibitors/blockers of NOS, guanylyl cyclase and KATP channels. The nitrite concentration was accessed by Griess test. The expression and phosphorylation of eNOS and nNOS were determined by western blot. Key resultsPnPP-19 (5, 10 and 20 μg/paw) induced peripheral antinociception in rats. Administration of NOS inhibitor (L-NOarg), selective nNOS inhibitor (L-NPA), guanylyl cyclase inhibitor (ODQ) and the blocker of KATP (glibenclamide) partially inhibited the antinociceptive effect of PnPP-19 (10 μg/paw). Tissue nitrite concentration increased after PnPP-19 (10 μg/paw) administration. Expression of eNOS and nNOS remained the same in all tested groups, however the phosphorylation of nNOS Ser852 (inactivation site) increased and phosphorylation of eNOS Ser1177 (activation site) decreased after PGE2 injection. Administration of PnPP-19 reverted this PGE2-induced effect. Conclusions and implicationsThe peripheral antinociceptive effect induced by PnPP-19 is resulting from activation of NO-cGMP-KATP pathway. Activation of eNOS and nNOS might be required for such effect. Our results suggest PnPP-19 as a new drug candidate to treat pain and reinforce the importance of nNOS and eNOS activation, as well as endogenous NO release, for induction of peripheral antinociception.

Descending antinociception induced by secondary somatosensory cortex stimulation in experimental neuropathy: role of the medullospinal serotonergic pathway.

Stimulation of the secondary somatosensory cortex (S2) has attenuated pain in humans and inflammatory nociception in animals. Here we studied S2 stimulation-induced antinociception and its underlying mechanisms in an experimental animal model of neuropathy induced by spinal nerve ligation (SNL). Effect of S2 stimulation on heat-evoked limb withdrawal latency was assessed in lightly anesthetized rats that were divided into three groups based on prior surgery and monofilament testing before induction of anesthesia: 1) sham-operated group and 2) hypersensitive and 3) nonhypersensitive (mechanically) SNL groups. In a group of hypersensitive SNL animals, a 5-HT1A receptor agonist was microinjected into the rostroventromedial medulla (RVM) to assess whether autoinhibition of serotonergic cell bodies blocks antinociception. Additionally, effect of S2 stimulation on pronociceptive ON-cells and antinociceptive OFF-cells in the RVM or nociceptive spinal wide dynamic range (WDR) neurons were assessed in anesthetized hypersensitive SNL animals. S2 stimulation induced antinociception in hypersensitive but not in nonhypersensitive SNL or sham-operated animals. Antinociception was prevented by a 5-HT1A receptor agonist in the RVM. Antinociception was associated with decreased duration of heat-evoked response in RVM ON-cells. In spinal WDR neurons, heat-evoked discharge was delayed by S2 stimulation, and this antinociceptive effect was prevented by blocking spinal 5-HT1A receptors. The results indicate that S2 stimulation suppresses nociception in SNL animals if SNL is associated with tactile allodynia-like hypersensitivity. In hypersensitive SNL animals, S2 stimulation induces antinociception mediated by medullospinal serotonergic pathways acting on the spinal 5-HT1A receptor, and partly through reduction of the RVM ON-cell discharge.NEW & NOTEWORTHY Stimulation of S2 cortex, but not that of an adjacent cortical area, induced descending heat antinociception in rats with the spinal nerve ligation-induced model of neuropathy. Antinociception was bilateral, and it involved suppression of pronociceptive medullary cells and activation of serotonergic pathways that act on the spinal 5-HT1A receptor. S2 stimulation failed to induce descending antinociceptive effect in sham-operated controls or in nerve-ligated animals that had not developed mechanical hypersensitivity.

Antinociception induced by galanin in anterior cingulate cortex in rats with acute inflammation

The present study was performed to explore the role of galanin in nociceptive modulation in anterior cingulate cortex (ACC) of rats with acute inflammation, and the changes in galanin and galanin receptor 2 (Gal R2) expressions in rats with acute inflammation. Intra-ACC injection of galanin induced antinociception in rats with acute inflammation, the antinociceptive effects induced by galanin were attenuated significantly by intra-ACC injection of the Gal R2 antagonist M871, indicating an involvement of Gal R2 in nociceptive modulation in ACC in rats with acute inflammation. Furthermore, we found that both the galanin mRNA expression and galanin content increased significantly in ACC in rats with acute inflammation than that in normal rats. Moreover, both the mRNA levels of Gal R2 and the content of Gal R2 in ACC increased significantly in rats with acute inflammation than that in normal rats. These results demonstrated that galanin induced antinociception in ACC in rats with acute inflammation. And there were changes in the expression of galanin and Gal R2 in rats with acute inflammation.

New Morphine Analogs Produce Peripheral Antinociception within a Certain Dose Range of Their Systemic Administration.

Growing data support peripheral opioid antinociceptive effects, particularly in inflammatory pain models. Here, we examined the antinociceptive effects of subcutaneously administered, recently synthesized 14-O-methylmorphine-6-O-sulfate (14-O-MeM6SU) compared with morphine-6-O-sulfate (M6SU) in a rat model of inflammatory pain induced by an injection of complete Freund's adjuvant and in a mouse model of visceral pain evoked by acetic acid. Subcutaneous doses of 14-O-MeM6SU and M6SU up to 126 and 547 nmol/kg, respectively, produced significant and subcutaneous or intraplantar naloxone methiodide (NAL-M)-reversible antinociception in inflamed paws compared with noninflamed paws. Neither of these doses significantly affected thiobutabarbital-induced sleeping time or rat pulmonary parameters. However, the antinociceptive effects of higher doses were only partially reversed by NAL-M, indicating contribution of the central nervous system. In the mouse writhing test, 14-O-MeM6SU was more potent than M6SU after subcutaneous or intracerebroventricular injections. Both displayed high subcutaneous/intracerebroventricular ED50 ratios. The antinociceptive effects of subcutaneous 14-O-MeM6SU and M6SU up to 136 and 3043 nmol/kg, respectively, were fully antagonized by subcutaneous NAL-M. In addition, the test compounds inhibited mouse gastrointestinal transit in antinociceptive doses. Taken together, these findings suggest that systemic administration of the novel compound 14-O-MeM6SU similar to M6SU in specific dose ranges shows peripheral antinociception in rat and mouse inflammatory pain models without central adverse effects. These findings apply to male animals and must be confirmed in female animals. Therefore, titration of systemic doses of opioid compounds with limited access to the brain might offer peripheral antinociception of clinical importance.

Functional interaction between orexin-1 and CB1 receptors in the periaqueductal gray matter during antinociception induced by chemical stimulation of the lateral hypothalamus in rats.

Chemical stimulation of the lateral hypothalamus (LH) with carbachol induces antinociception which is antagonized by blockade of orexin receptors in some pain modulatory sites in the tail-flick test. In this study, we evaluated the role of orexin-1 and CB1 receptors in the periaqueductal gray matter (PAG), a critical pain modulatory site, in mediation of antinociceptive responses induced by LH stimulation in rats. One hundred thirty-two adult male albino Wistar rats weighing 180-250g were unilaterally implanted with two separate cannulae into the LH and ventrolateral PAG (vlPAG). Intra-vlPAG administration of SB334867, as a selective orexin-1 receptor antagonist (0.5, 1.5, 5, 15 and 50nM), or AM251, as a selective CB1 receptor antagonist (1, 3, 10, 30 and 100nM), was performed just 5min before carbachol (125nM) microinjection into the LH. Our findings showed that SB334867 or AM251 administration dose dependently prevented the development of LH-induced antinociception in rats. Treatment with two antagonists at the same time could not intensify their effects in comparison with separate administration of antagonists. It seems that antinociceptive effect of intra-LH administration of carbachol is mediated, at least partially, through the activation of orexin-1 and CB1 receptors in the vlPAG. This work demonstrates a pain modulatory role of the orexinergic system via the PAG in hypothalamic-mediated analgesia suggesting that orexins can be advantageously targeted to achieve analgesia. WHAT DOES THIS STUDY ADD?: OX1 receptor antagonist (SB334867) administration into the ventrolateral periaqueductal gray matter (vlPAG) dose dependently blocked the carbachol-induced antinociception. CB1 receptor antagonist (AM251) microinjection in the vlPAG prevented carbachol-induced antinociception in a dose-dependent manner. Concurrent administration of SB334867 and AM251 into the vlPAG did not reinforce the antinociceptive responses.

Administration of orexin receptor 1 antagonist into the rostral ventromedial medulla increased swim stress-induced antinociception in rat.

Intracerebroventricular injection of orexin-A (hypocretin-1) antagonist has been shown to inhibit stress-induced analgesia. However the locations of central sites that may mediate these effects have not been totally demonstrated. This study was performed to investigate the role of rostral ventromedial medulla (RVM) orexin receptor 1 in stress-induced analgesia (SIA). Forced swim stress in water was employed to adult male rats (200-250 g). Nociceptive responses were measured by formalin test (50 µl injection of formalin 2% subcutaneously into hind paw) and, pain related behaviors were monitored for 90 min following intra-microinjection of SB-334867 (orexin receptor 1 antagonist) into RVM. Exposure to swimming stress test after administration of SB-334867 into RVM significantly reduces the formalin-induced nociceptive behaviors in phase1, interphase, and phase 2 in rats. The result demonstrated the involvement of OXR1 in antinociceptive behaviors induced by swim stress in RVM.

PnPP-19, a spider toxin peptide, induces peripheral antinociception through opioid and cannabinoid receptors and inhibition of neutral endopeptidase.

The synthetic peptide PnPP-19 has been studied as a new drug candidate to treat erectile dysfunction. However, PnTx2-6, the spider toxin from which the peptide was designed, induces hyperalgesia. Therefore, we intended to investigate the role of PnPP-19 in the nociceptive pathway. Nociceptive thresholds were measured by paw pressure test. PnPP-19 was administered intraplantarly alone or with selective cannabinoid or opioid receptor antagonists. The hydrolysis of PnPP-19 by neutral endopeptidase (NEP) (EC 3.4.24.11), an enzyme that cleaves enkephalin, was monitored by HPLC and the cleavage sites were deduced by LC-MS. Inhibition by PnPP-19 and Leu-enkephalin of NEP enzyme activity was determined spectrofluorimetrically. PnPP-19 (5, 10 and 20 μg per paw) induced peripheral antinociception in rats. Specific antagonists of μ opioid receptors (clocinnamox), δ opioid receptors (naltrindole) and CB1 receptors (AM251) partly inhibited the antinociceptive effect of PnPP-19. Inhibition of fatty acid amide hydrolase by MAFP or of anandamide uptake by VDM11 enhanced PnPP-19-induced antinociception. NEP cleaved PnPP-19 only after a long incubation, and Ki values of 35.6 ± 1.4 and 14.6 ± 0.44 μmol·L(-1) were determined for PnPP-19 and Leu-enkephalin respectively as inhibitors of NEP activity. Antinociception induced by PnPP-19 appears to involve the inhibition of NEP and activation of CB1, μ and δ opioid receptors. Our data provide a greater understanding of the antinociceptive effects of PnPP-19. This peptide could be useful as a new antinociceptive drug candidate.

Role of fosaprepitant, a neurokinin Type 1 receptor antagonist, in morphine-induced antinociception in rats.

Objectives:Opioids such as morphine form the cornerstone in the treatment of moderate to severe pain. However, opioids also produce serious side effects such as tolerance. Fosaprepitant is a substance P (SP) receptor antagonist, which is used for treating chemotherapy-induced nausea and vomiting. SP is an important neuropeptide mediating transmission of pain at the spinal level. Thus, it was hypothesized that combining morphine with fosaprepitant would increase the antinociceptive effect of morphine. The objectives were to evaluate the effect of fosaprepitant on morphine-induced antinociception in rats and to investigate its mechanism of action.Methods:Sprague-Dawley rats were injected with morphine (10 mg/kg twice daily) and/or fosaprepitant (30 mg/kg once daily) for 7 days. Pain threshold was assessed by the hot plate test. Expression of SP and calcitonin gene-related peptide (CGRP) in the spinal cords of these rats was evaluated by immunohistochemistry.Results:Morphine administration resulted in an antinociceptive effect compared to the control group (day 1 and to a lesser extent on day 4). The decreased antinociception despite continued morphine treatment indicated development of tolerance. Co-administration of fosaprepitant attenuated tolerance to morphine (days 1 and 3) and increased the antinociceptive effect compared to control group (days 1–4). Expression of SP was increased in the morphine + fosaprepitant group.Conclusions:The results show that fosaprepitant attenuates the development of tolerance to morphine and thereby, increases the antinociceptive effect. This is likely linked to decreased release of SP from presynaptic terminals.