Antinociceptive effects of Nigella sativa oil and its major component, thymoquinone, in mice

The antinociceptive effects of Nigella sativa oil and its major component, thymoquinone, were examined in mice. The p.o. administration of N. sativa oil (50-400 mg/kg) dose-dependently suppressed the nociceptive response in the hot-plate test, tail-pinch test, acetic acid-induced writhing test and in the early phase of the formalin test. The systemic administration (2.5-10 mg/kg, p.o. and 1-6 mg/kg, i.p.) and the i.c.v. injection (1-4 microgram/mouse) of thymoquinone attenuated the nociceptive response in not only the early phase but also the late phase of the formalin test. Naloxone injected s.c. (1 mg/kg) significantly blocked N. sativa oil- and thymoquinone-induced antinociception in the early phase of the formalin test. Moreover, the i.c.v. injection of naloxone (10 microgram/mouse), the mu(1)-opioid receptor antagonist, naloxonazine (1-5 microgram/mouse), or the kappa-opioid receptor antagonist, nor-binaltorphimine (1-5 microgram/mouse), significantly reversed thymoquinone-induced antinociception in the early phase but not the late phase of the formalin test, whereas the delta-opioid receptor antagonist, naltrindole (1-5 ng/mouse, i.c.v.), had no effect on either phase. The antinociceptive effect of morphine was significantly reduced in thymoquinone- and N. sativa oil-tolerant mice, but not vice versa. These results suggest that N. sativa oil and thymoquinone produce antinociceptive effects through indirect activation of the supraspinal mu(1)- and kappa-opioid receptor subtypes.     

Analgesic effects of morphine and loperamide in the rat formalin test: interactions with NMDA receptor antagonists

To reveal peripheral components of opiate analgesia, effects of loperamide, opioid agonist which does not penetrate the blood-brain barrier, were examined in formalin and acute thermal pain tests in comparison with morphine. Formalin administration induces pain behaviour such licking/biting of injected paw expressed as two phases. The first phase is caused by C-fibre activation due to peripheral stimulation, the second phase attributed to ongoing input from peripheral site, leading to spinal hyperexcitability, which is dependent on N-methyl-D-aspartate (NMDA) receptor activation. Loperamide (3-10 mg/kg) and morphine (6 mg/kg) reduced formalin-induced nociceptive behaviours and these effects were reversed by naloxone methiodide (0.03-10 mg/kg), opioid receptor antagonist which poorly penetrates the blood-brain barrier. Loperamide action was enhanced only by centrally active NMDA receptor antagonists memantine (3 mg/kg) and CGP 37849 (3 mg/kg), but not by NMDA/glycineB receptor antagonists showing weak or no central nervous system (CNS) activity. Present results suggest that central NMDA receptor blockade may be necessary to enhance analgesia induced through peripheral opioid mechanisms in formalin-evoked nociception.     

Effects of morphine on the electrochemical signal modified by noxious stimuli in the nucleus raphe magnus of anesthetized rats.

The effects of noxious stimuli and morphine on the serotonergic system in the nucleus raphe magnus were examined by in vivo voltammetry studies, using anesthetized rats. The normal electrochemical signal of 280-300 mV was essentially due to the presence of 5-hydroxyindoles in the nucleus raphe magnus. Heating or pinching produced mean decreases of 21.9 +/- 5.2% and 18.0 +/- 6.1% of control, respectively in the 5-hydroxyindole signal. Non-noxious (brushing or warm water) stimulation did not affect the 5-hydroxyindole signal. A small dose of morphine (0.5 mg/kg, i.p.) enhanced the inhibition of the signal by noxious stimuli but large doses (2.0 or 5.0 mg/kg, i.p.) resulted in lesser reductions of the signal. The value of the 5-hydroxyindole signal was unaffected by morphine alone (0.5, 2.0 and 5.0 mg/kg). Effects of both small- and large-doses of morphine were antagonized by naloxone (1 mg/kg, i.v.). Allopurinol (20 mg/kg, i.p.), a xanthine oxidase inhibitor, decreased the steady signal (40.7 +/- 16.2%). After pretreatment with allopurinol, noxious stimuli-induced decreases, both with and without administration of morphine, were similar to those in nontreated rats. In brief, noxious stimulation was found to decrease 5-hydroxyindole signal in the nucleus raphe magnus; morphine enhanced or attenuated this decrease in the anesthesized rat.     

Involvement of spinal lipoxygenase metabolites in hyperalgesia and opioid tolerance

This study investigated role of spinal lipoxygenase metabolites in induction of hyperalgesia and development of opioid analgesic tolerance. In the rat, nociception was measured using formalin and tail-flick tests. Intrathecal administration of leukotriene receptor agonist (LTB4) augmented the second phase of the formalin response and marginally increased sensitivity to acute thermal stimulation in the tail-flick test, responses suppressed by 6-(6-(3R-hydroxy-1E,5Z-undecadien-1-yl)-2-pyridinyl)-1,5S-hexanediol (U75302), a leukotriene BLT receptor antagonist. Treatment with 15-hydroxyperoxyeicosatetranoic acid (HPETE) increased phase II formalin activity, but had no effect on tail-flick responses. 12-HPETE failed to produce an effect in either nociceptive test. In the second part of this study, chronic spinal morphine for 5 days produced progressive decline in morphine antinociception and loss in analgesic potency. These effects were attenuated by co-administration of morphine with selective and nonselective lipoxygenase inhibitors. These results suggest involvement of lipoxygenase metabolites in both pain modulation and induction of opioid tolerance at the spinal level.     

芋螺毒素ω-SO3单次及连续给药对福尔马林致大鼠炎性疼痛的镇痛作用

目的评价单次及连续鞘内给药ω-SO3对福尔马林致大鼠炎性疼痛的镇痛作用。方法采用福尔马林致大鼠炎性疼痛模型,通过大鼠鞘内置管术分别单次及多次鞘内给药,观察ω-SO3对炎性疼痛急性期和持续期的镇痛强度和有效时间以及连续给药对其镇痛作用可能的影响;通过大鼠自发活动实验评价ω-SO3单次鞘内给药可能引起的中枢副反应。结果在大鼠福尔马林炎性疼痛模型上,单次鞘内给药ω-SO3产生剂量及时间依赖性的镇痛作用,其抑制炎性疼痛急性期和持续期的ED50值分别为1.79和0.41ng.g-1,比吗啡的镇痛作用强并且有效时间长;以ED80剂量每日鞘内给药2次连续5d后,ω-SO3仍然产生与单次给药类似的镇痛强度,而吗啡的镇痛作用降低,说明产生镇痛耐受,更有意义的是ω-SO3对吗啡镇痛耐受大鼠仍具有镇痛作用。在上述镇痛剂量范围内,ω-SO3单次鞘内给药对大鼠自发活动没有明显的影响。结论ω-SO3对福尔马林致炎性疼痛的急性期和持续期均具良好的镇痛作用,其镇痛作用强,有效时间长,连续给药不产生自身镇痛耐受,并且对吗啡不产生交叉耐受。     

黄芪总苷的镇痛作用及其作用机制

目的：研究黄芪总甙（astragalosides,AST)的镇痛作用及其作用机制。方法：采用小鼠福尔马林致痛模型,对痛反应进行评分。通过福尔马林试验前30min皮下注射吗啡和纳络酮或福尔马林试验前20min腹腔内注射L－精氨酸和L－NAME以研究阿片肽和一氧化氮在此疼痛模型中的作用并对AST的作用与吗啡和L－NAME进行比较。结果：AST 20,40和80mg/kg可显著降低小鼠福尔马林致痛后第二时相的疼痛反应（P＜0．01）。AST 40mg/kg最大镇痛作用见于给药后4h(第二时相疼痛反应的抑制率为34．4％）,吗啡5mg/kg对两个时相的疼痛反应均有明显抑制作用（P＜0．01）,此抑制作用能被纳络酮2mg/kg拮抗（P＜0．01）,而AST的镇痛作用不受纳络酮影响。L－精氨酸（400或800mg/kg)可部分抑制AST的作用（P＜0．01）。结论：AST所具有的镇痛作用不是通过内源性阿片肽系统介导,而可能与抑制NO等参与疼痛反应的炎症介质的生成有关。     

黄芪总甙的镇痛作用及其作用机制(英文)

目的:研究黄芪总甙(astragalosides,AST)的镇痛作用及其作用机制。方法:采用小鼠福尔马林致痛模型,对痛反应进行评分。通过福尔马林试验前30min皮下注射吗啡和纳络酮或福尔马林试验前20min腹腔内注射L-精氨酸和L-NAME以研究阿片肽和一氧化氮在此疼痛模型中的作用并对AST的作用与吗啡和L-NAME进行比较。结果:AST20,40和80mg/kg可显著降低小鼠福尔马林致痛后第二时相的疼痛反应(P<0.01)。AST 40mg/kg最大镇痛作用见于给药后4h(第二时相疼痛反应的抑制率为34.4％)。吗啡5mg/kg对两个时相的疼痛反应均有明显抑制作用(P<0.01),此抑制作用能被纳络酮2mg/kg拮抗(P<0.01),而AST的镇痛作用不受纳络酮影响。L-精氨酸(400或800mg/kg)可部分抑制AST的作用(P<0.01)。结论:AST所具有的镇痛作用不是通过内源性阿片肽系统介导,而可能与抑制NO等参与疼痛反应的炎症介质的生成有关。     

Behavioural effects of the novel AMPA/GluR5 selective receptor antagonist NS1209 after systemic administration in animal models of experimental pain

The effects of systemic administration of the novel AMPA/GluR5 selective receptor antagonist NS1209 in animal models of experimental pain have been tested and compared with the AMPA receptor antagonist NBQX and the opiate morphine. In the mouse hot plate test, NS1209 (3-30 mg/kg, s.c. and i.p.) and morphine (3-30 mg/kg, s.c.) significantly increased the nociceptive response latency, whereas NBQX (3-30 mg/kg, i.p.) was ineffective. In the rat formalin test, a model of persistent pain, NS1209 (3 and 6 mg/kg, i.p.) and morphine (0.5 and 3 mg/kg, s.c.) produced dose-dependent reductions in second phase nociceptive behaviours, although NBQX (10 and 20 mg/kg, i.p.) was without effect. In the chronic constriction injury model of neuropathic pain, NS1209 (3 and 6 mg/kg, i.p.), NBQX (10 and 20 mg/kg, i.p.) and morphine (3 and 6 mg/kg, s.c.) all reduced mechanical allodynia and hyperalgesia responses to von Frey hair and pin prick stimulation of the injured hindpaw. NS1209 and morphine also reduced cold hypersensitivity in response to ethyl chloride stimulation of the injured hindpaw. At the doses associated with anti-nociceptive actions, no effects on motor performance as determined by the rotarod test were observed for any of the drugs tested. Thus, systemic administration of NS1209 at non-ataxic doses has marked analgesic actions comparable to those of morphine in a range of animal models of experimental pain.     

Metamizol potentiates morphine antinociception but not constipation after chronic treatment

This work evaluates the antinociceptive and constipating effects of the combination of 3.2 mg/kg s.c. morphine with 177.8 mg/kg s.c. metamizol in acutely and chronically treated (once a day for 12 days) rats. On the 13th day, antinociceptive effects were assessed using a model of inflammatory nociception, pain-induced functional impairment model, and the charcoal meal test was used to evaluate the intestinal transit. Simultaneous administration of morphine with metamizol resulted in a markedly antinociceptive potentiation and an increasing of the duration of action after a single (298+/-7 vs. 139+/-36 units area (ua); P<0.001) and repeated administration (280+/-17 vs. 131+/-22 ua; P<0.001). Antinociceptive effect of morphine was reduced in chronically treated rats (39+/-10 vs. 18+/-5 au) while the combination-induced antinociception was remained similar as an acute treatment (298+/-7 vs. 280+/-17 au). Acute antinociceptive effects of the combination were partially prevented by 3.2 mg/kg naloxone s.c. (P<0.05), suggesting the partial involvement of the opioidergic system in the synergism observed. In independent groups, morphine inhibited the intestinal transit in 48+/-4% and 38+/-4% after acute and chronic treatment, respectively, suggesting that tolerance did not develop to the constipating effects. The combination inhibited intestinal transit similar to that produced by morphine regardless of the time of treatment, suggesting that metamizol did not potentiate morphine-induced constipation. These findings show a significant interaction between morphine and metamizol in chronically treated rats, suggesting that this combination could be useful for the treatment of chronic pain.     

Anti-inflammatory and analgesic properties in a rodent model of a (1-->3),(1-->6)-linked beta-glucan isolated from Pleurotus pulmonarius

A glucan was extracted with hot water from the basidiomycete Pleurotus pulmonarius and shown to have a (1-->3)-linked beta-D-glucopyranosyl main-chain substituted at O-6 of every third unit by single beta-D-glucopyranosyl non-reducing end units. This was shown by mono- and bidimensional nuclear magnetic resonance (NMR) spectroscopy, methylation analysis, and a controlled Smith degradation. The glucan was tested for its effects on the acetic acid-induced writhing reaction in mice, a typical model for quantifying inflammatory pain. It caused a marked and dose-dependent anti-inflammatory response, demonstrated by the inhibition of leukocyte migration to injured tissues (82 +/- 6%) with an ID50 of 1.19 (0.74-1.92) mg/kg. Furthermore, animals previously treated with the glucan (3 mg/kg i.p.), showed a reduction of 85 +/- 5% of writhes, after receiving the acetic acid injection. Furthermore, in the formalin test, the glucan (3-30 mg/kg, i.p.) also caused significant inhibition of both the early (neurogenic pain) and the late phases (inflammatory pain) of formalin-induced licking. However, it was more potent and effective in relation to the late phase of the formalin test, with mean ID(50) values for the neurogenic and the inflammatory phases of > 30 and 12.9 (6.7-24.6) mg/kg and the inhibitions observed were 43 +/- 5% and 96 +/- 4%, respectively. These data showed that the glucan had potent anti-inflammatory and analgesic (antinociceptive) activities, possibly by the inhibition of pro-inflammatory cytokines.     

Effect of chronic and acute administration of fluoxetine and its additive effect with morphine on the behavioural response in the formalin test in rats

Serotonergic systems are involved in the central regulation of nociceptive sensitivity. Fluoxetine, a selective inhibitor of the reuptake of serotonin (5-hydroxytryptamine, 5-HT), was administered orally (0.16, 0.32, 0.8 mg kg(-1) daily for 7 days), intraperitoneally (0.04, 0.08, 0.16 mg kg(-1) day(-1) for 7 days and a single dose of 0.32 mg kg(-1)) and intracerebroventricularly (10 microg/rat) to rats and nociceptive sensitivity was evaluated using the formalin test (50 microL of 2.5% formalin injected subcutaneously). The effect of fluoxetine was also studied in the presence of 5,7-dihydroxytryptamine creatinine sulfate (5,7-DHT) and after co-administration with morphine. Oral (0.8 mg kg(-1)), intraperitoneal (0.16 and 0.32 mg kg(-1)) and intracerebroventricular (10 microg/rat) fluoxetine induced antinociception in the late phase of the formalin test. Furthermore, intrathecal administration of 5-HT (100 microg/rat) induced an analgesic effect. The analgesic effect of fluoxetine (0.16 and 0.32 mg kg(-1), i.p.) and 5-HT (100 microg/rat, i.t.) was abolished by pre-treatment with 5,7-DHT (100 microg/rat, i.t.). In addition, the analgesic effect of 5-HT (100 microg/rat, i.t.) was decreased by pre-treatment with naloxone (2 mg kg(-1), i.p.). Morphine (5 mg kg(-1), i.p.) induced analgesia that was increased by fluoxetine (0.32 mg kg(-1), i.p.). These results suggest that fluoxetine has an antinociceptive effect in tonic inflammatory pain through functional alteration of the serotonergic system and also potentiates the analgesic effect of morphine.     

Standardization of the rat paw formalin test for the evaluation of analgesics

Administration of 5% formalin into the rat or guinea pig hind paw evokes two spontaneous responses: flinching/shaking and licking/biting of the injected paw. The temporal and behavioral characteristics of these objective endpoints are described. Additionally, several practical suggestions aimed at standardizing this test for the evaluation of analgesics are presented. The early/acute and late/tonic (0–10 and 20–35 min post-formalin, respectively) phases of flinching were used to quantitate antinociception in the rat. PD 117302, the kappa selective agonist, was three times more potent than morphine against tonic flinching after SC administration. Formalin may therefore be a noxious stimulus of choice in the evaluation of kappa agonists. Morphine was only twice as potent against tonic flinching as against acute flinching or the tail-dip reflex to water (50° C). In contrast, PD 117302 was 27 times less potent on early phase and was inactive in the tail-dip test. Thus, while morphine is essentially equipotent across tests, PD 117302 shows a spectrum of activity with impressive potency and efficacy being obtained against tonic pain. Kappa receptors may therefore be prominently involved in tonic pain states. Aspirin given orally was not consistently antinociceptive in either phase of the formalin test. Spinal transection completely abolished late phase responding but only partly attenuated flinching in the early phase. This suggests that the relative involvement of spinal (as opposed to supraspinal) processing of noxious inputs may, at least in part, be a function of stimulus intensity and underlie the differences in antinociceptive potency observed in this work.     

Involvement of Cholinergic and Opioid Receptor Mechanisms in Nicotine-Induced Antinociception

Abstract: In this work we have studied the influences of nicotinic agents on the antinociception of morphine in formalin test. Nicotine (0.001-0.1 mg/kg) induced antinociception in mice in a dose-dependent manner in the early phase of formalin test, and also potentiated the morphine effect. The nicotinic receptor antagonist, mecamylamine (0.5 mg/kg), but not hexamethonium decreased the antinociception induced by nicotine (0.1 mg/kg) in both phases. The muscarinic receptor antagonist atropine (5 and 10 mg/kg) also decreased the response of nicotine. Mecamylamine, hexamethonium or atropine did not alter morphine antinociceptive response, while naloxone decreased responses induced by nicotine or morphine. The antagonists by themselves did not elicit any response in formalin test, however, high doses of mecamylamine tend to increase pain response. It is concluded that central cholinergic and opioid receptor mechanisms may be involved in nicotine-induced antinociception.     

Morphine-induced antinociception in the formalin test: sensitization and interactions with D1 and D2 dopamine receptors and nitric oxide agents

In this study, the effects of dopamine receptor antagonists and nitric oxide agents on morphine-induced sensitization in the formalin test in mice have been investigated. Repeated daily intraperitoneal administration of morphine (30 mg/kg for 3 days) followed by a 11-day wash out period increased morphine-induced antinociception in the formalin test, which may be due to sensitization. The antinociceptive response to higher doses of morphine (6 and 9 mg/kg) but not 3 mg/kg was significantly increased in sensitized animals compared with control groups. Pretreatment of animals with an opioid receptor antagonist, naloxone (4 mg/kg), during repeated administration of morphine, attenuated the morphine-induced sensitization. In the second part of the study, the animals received SCH23390 (D1 receptor antagonist), sulpiride (D2 receptor antagonist), L-Arg (nitric oxide precursor) and NG-nitro-L-Arg methylester (nitric oxide synthase inhibitor) during repeated morphine administration, to evaluate the role of dopamine receptor antagonists and nitric oxide agents in this phenomenon. Pretreatment of animals with NG-nitro-L-Arg methylester (20 mg/kg) and sulpiride (100 mg/kg) during morphine sensitization decreased the antinociceptive response to higher doses of morphine in the formalin test. It is concluded that D2 dopamine receptor and nitric oxide mechanisms may be involved at least partly in morphine-induced sensitization in the formalin test.     

Analgesic effects of mGlu1 and mGlu5 receptor antagonists in the rat formalin test

mGlu1 and mGlu5 receptors have been implicated in pain associated with inflammation. In the present study, the formalin test was used to measure sustained pain with components of tissue injury. The aims of the present study were to assess: (i) the role of mGlu1 and mGlu5 receptors in inflammatory pain using selective antagonist EMQMCM, 1.25-5 mg/kg, as the mGlu1 receptor antagonist, and MPEP or MTEP, 2.5-10 mg/kg, as mGlu5 receptor antagonist; (ii) the possible interaction between mGlu1 and mGlu5 receptor antagonists and morphine; and (iii) whether tolerance develops to the analgesic effects of these antagonists after prolonged treatment. EMQMCM, MTEP and MPEP significantly reduced the manifestation of both phases of formalin response. However, all these mGlu receptor antagonists did not affect the withdrawal latencies in a model of acute pain (Hargreaves test), which has a different underlying mechanism. In the present study, the suppressive effect on formalin-induced pain behaviour was much stronger when mGlu1 and mGlu5 receptor antagonists were co-injected compared to administration of a single antagonist, but this effect was not seen when mGlu receptor antagonist was co-administered with morphine. This is in contrast to the pronounced inhibitory effects after co-treatment with morphine and the uncompetitive NMDA receptor antagonist memantine. The present study also provides the first direct in vivo evidence that prolonged administration of MTEP (5 mg/kg) over 7 days leads to the development of tolerance to its antinociceptive effects. Such tolerance was not observed when EMQMCM (5 mg/kg) was administered in the same manner. In conclusion, these results provide additional arguments for the role of group I mGlu receptors in pain with inflammatory conditions.     

Cross-tolerance between antinociception induced by swim-stress and morphine in formalin test

The present study investigated cross-tolerance between antinociception induced by water swim-stress and morphine in the formalin test. Intraperitoneal administration of morphine (3, 6 and 9 mg/kg) induced dose-dependent antinociception in both phases of the formalin test. Mice treated with a lower dose of morphine (25 mg/kg), once daily for 3 days, showed tolerance to antinociception induced by a lower test dose of morphine (3 mg/kg). Similar repeated treatments with a higher dose of morphine (50 mg/kg) produced tolerance to antinociception induced by different test doses of morphine (3, 6 and 9 mg/kg). Exposure to water swim-stress, once daily for 2 or 3 days in order to induce tolerance, also decreased morphine-induced antinociception. Swim-stress exposure for 2 or 3 days also tends to potentiate tolerance induced by a lower dose of morphine. Acute swim-stress of different durations (0.5, 1 and 3 min) induced antinociception in both phases of the formalin test, which was not reduced by naloxone, but showed even more antinociception in the second phase. The response to swim stress was decreased in mice treated with higher doses of morphine, but not those animals that received swimming stress (3 min) once daily for 2-3 days, in order to induce habituation to swim-stress-induced antinociception. The results may indicate a possible cross-tolerance between antinociception induced by morphine and by swim stress.     

Profile of spinal and supra-spinal antinociception of (-)-linalool

We previously reported that administration of (-)-linalool, the naturally occurring enantiomer in essential oils, induced a significant reduction in carrageenin-induced oedema and in acetic acid-induced writhing. The latter effect was completely antagonised by the muscarinic receptor antagonist atropine and by the opioid receptor antagonist naloxone. To further characterise the antinociceptive profile of (-)-linalool, we studied its effect in the hot plate and the formalin in tests. In addition, to determine the possible involvement of the cholinergic, opioidergic and dopaminergic systems, we tested the effects of atropine, pirenzepine, a muscarinic M1 receptor antagonist, naloxone, sulpiride, a dopamine D2 receptor antagonist and (R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH-23390), a dopamine D1 receptor antagonist on (-)-linalool-induced antinociception. Moreover, since K(+) channels seem to play an important role in the mechanisms of pain modulation, we examined the effect of glibenclamide, an ATP-sensitive K(+) channel inhibitor on (-)-linalool-induced antinociception. The administration of (-)-linalool (100 and 150 mg/kg, s.c.) increased the reaction time in the hot-plate test. Moreover, (-)-linalool (50 and 100 mg/kg) produced a significant reduction in the early acute phase of the formalin model, but not in the late tonic phase. The highest dose (150 mg/kg) caused a significant antinociceptive effect on both phases. The antinociceptive effects of (-)-linalool were decreased by pre-treatment with atropine, naloxone, sulpiride and glibenclamide but not by pirenzepine and SCH-23390. These results are in agreement with the demonstrated pharmacological properties of linalool, mainly its cholinergic, local anaesthetic activity and its ability to block NMDA receptors. Furthermore, a key role seems to be played by K(+) channels, whose opening might be the consequence of a stimulation of muscarinic M2, opioid or dopamine D2 receptors.     

Sex differences in discriminative stimulus effects of morphine in the rat

Nine female and ten male rats were trained to discriminate 3.0mg/kg s.c. morphine from saline. The six female rats that acquired and maintained the morphine discrimination did so in significantly fewer sessions than the eight males did (28 +/- 5 vs 51 +/- 9 sessions, respectively), and the ED(50) for morphine substitution was significantly lower in females (0.69 +/- 0.15 vs 1.28 +/- 0.20mg/kg). The time course of morphine substitution was approximately equivalent in females and males. The μ agonist fentanyl completely substituted for morphine in both sexes, with no sex difference in potency to substitute for morphine. The μ agonist buprenorphine partially or completely substituted for morphine in all females and five of six males, but at a lower dose in females (ED(50) 0.009 +/- 0.002 vs 0.019 +/- 0.006mg/kg). The delta agonist BW373U86 partially substituted for morphine in both sexes, with no potency differences; the kappa agonist U69,593 and the non-opioid cocaine did not substitute for morphine in either sex. On a test of spontaneous locomotor activity, morphine increased locomotion to a slightly but not significantly greater extent in males than in females. Morphine also produced significantly greater hotplate antinociception in males than in females. Further drug discrimination training with a lower dose of morphine, 1.0mg/kg, decreased the ED(50) for morphine substitution in females and males to 0.26 +/- 0.06 vs 0.45 +/- 0.11mg/kg, respectively (not significant). In a separate group of age-matched rats, there was no sex difference in brain or plasma levels of morphine measured via HPLC 20min post-injection, the pretreatment time used to examine behavioral effects of morphine. The HPLC results, plus the fact that sex differences were not the same for all behavioral effects of morphine, suggest that sex differences in discriminative stimulus effects of morphine are not due to differential pharmacokinetics. The possibility that sex differences in morphine discrimination reflect sex differences in opioid receptor pharmacology, or differential reinforcement between morphine and saline levers for males but not females, is discussed.     

Dexketoprofen-induced antinociception in animal models of acute pain: synergy with morphine and paracetamol

The antinociceptive activity of dexketoprofen was studied in mice using the acetic acid writhing test (acute tonic pain), the tail flick test (acute phasic pain) and the formalin assay (inflammatory pain). Isobolographic analysis was used to study the antinociceptive interactions between morphine and paracetamol co-administered with dexketoprofen. In the writhing test, the intraperitoneal administration of dexketoprofen or ketoprofen resulted in parallel dose-response curves with equal efficacy, but higher relative potency for dexketoprofen. In the tail flick test, the curves were parallel with similar efficacy and potency. The administration of morphine or paracetamol in both tests resulted in dose-response curves not parallel with that of dexketoprofen, which showed a potency between morphine and paracetamol. In the formalin assay, the antinociceptive activity of morphine during phase I was 122, 295 and 1695 times higher than dexketoprofen, ketoprofen and paracetamol, respectively. Isobolographic analysis demonstrated that the combination of sub-analgesic doses of dexketoprofen with morphine or with paracetamol was strongly synergic in all three tests. Synergistic drug combinations should improve effective pharmacological treatment of pain, minimizing drug specific adverse effects. These findings are undoubtedly worthy of additional controlled clinical trials in severe pain syndromes.     

The effect of cyclosporin A on morphine tolerance and dependence: involvement of L-arginine/nitric oxide pathway

Cyclosporin A is known to decrease nitric oxide (NO) production in nervous tissues. The effects of systemic cyclosporine A on the induction and expression of morphine tolerance and dependence, acute morphine-induced antinociception, and the probable involvement of the L-arginine/nitric oxide pathway in these effects were assessed in mice. Cyclosporin A (20 mg/kg), N(G)-nitro-L-arginine methyl ester (L-NAME) (10 mg/kg) and a combination of the two at lower and per se non-effective doses (5 and 3 mg/kg, respectively) showed a similar pattern of action, inhibiting the induction of tolerance to morphine-induced antinociception and increasing the antinociception threshold in the expression phase of morphine tolerance. These agents also inhibited the expression of morphine dependence as assessed by naloxone-precipitated withdrawal signs, while having no effect on the induction of morphine dependence. L-Arginine, at a per se non-effective dose (60 mg/kg), inhibited the effects of Cyclosporin A. Moreover, acute administration of Cyclosporin A (20 mg/kg) or L-NAME (10 mg/kg) enhanced the antinociception induced by acute administration of morphine (5 mg/kg), while chronic pretreatment with Cyclosporin A (20 mg/kg) or L-NAME (10 mg/kg) for 2 days (twice daily) did not affect morphine-induced antinociception. The inducible nitric oxide synthase inhibitor, aminoguanidine (100 mg/kg), did not alter morphine antinociception, tolerance or dependence. In conclusion, decreasing NO production through constitutive nitric oxide synthase may be a mechanism through which cyclosporin A differentially modulates morphine tolerance, dependence and antinociception.