Effects of fluoxetine and LY 367265 on tolerance to the analgesic effect of morphine in rats

Morphine is widely used to treat chronic pain, however its utility is hindered by the development of tolerance to its analgesic effects. The aim of this study was to investigate effects of fluoxetine, a specific serotonin (5-HT) reuptake inhibitor, and LY 367265, an inhibitor of the 5-HT transporter and 5-HT2A receptor antagonist, on tolerance induced to the analgesic effect of morphine in rats. The study was carried out on male Wistar Albino rats (weighing 170-190 g). To constitute morphine tolerance, animals received morphine (50 mg/kg; s.c.) once daily for 3 days. After last dose of morphine, injected on day 4, morphine tolerance was evaluated. The analgesic effects of fluoxetine (10 mg/ kg; i.p.), LY 367265 (3 mg/kg; i.p.) and morphine were considered at 30-min intervals by tail-flick and hot-plate tests. The results showed that fluoxetine and LY 367265 significantly attenuated the development and expression of morphine tolerance. The maximal antinociceptive effects were obtained 30 min after administration of fluoxetine and 60 min after administration of LY 367265. In conclusion, we observed that co-injection of morphine with fluoxetine and LY 367265 increased the analgesic effects of morphine and delayed development of tolerance to morphine analgesia.     

Intrathecal pertussis toxin attenuates the morphine withdrawal syndrome in normal but not in arthritic rats

The effect of intrathecal pertussis toxin on morphine dependence was studied in rats suffering from chronic pain (Freund's adjuvant-induced arthritis). Animals were rendered tolerant-dependent by subcutaneous implantation of 3 pellets of 75 mg morphine base each. In both, normal and arthritic animals, 1 microgram pertussis toxin reduced the analgesia induced by morphine in the tail-flick test. Naloxone (1 mg/kg, s.c.) precipitated a withdrawal syndrome in arthritic animals that was milder in respect to the one produced in normal rats. Pretreatment with pertussis toxin significantly diminished the incidence of withdrawal signs such as jumps, squeak on touch, chattering, ptosis, body shakes and diarrhoea in tolerant-dependent normal rats, while this effect could not be observed in animals suffering from chronic pain. This differential activity of the toxin could be due to the altered tonus of certain neurotransmitter systems that accompanies the chronic situation of pain.     

The effects of pituitary adenylate cyclase-activating polypeptide on acute and chronic morphine actions in mice

The effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on pain sensitivity, on morphine analgesia, on morphine tolerance and withdrawal were investigated in mice. The heat-radiant tail-flick test was used to assess antinociceptive threshold. Intracerebroventricular (i.c.v.) administration of PACAP alone had no effect on pain sensitivity but in a dose of 500 ng, it significantly diminished the analgesic effect of a single dose of morphine (2.25 mg/kg, s.c.). PACAP (500 ng, i.c.v.) significantly increased the chronic tolerance to morphine and enhanced the naloxone (1 mg/kg, s.c.)-precipitated withdrawal jumping. Theophylline (1 mg/kg, i.p.) pretreatment significantly enhanced the effect of PACAP on morphine analgesia but the effects of PACAP on tolerance and withdrawal were unaffected upon theophylline administration. On the grounds of our previous studies with vasoactive intestinal polypeptide (VIP), it appears that different receptors are involved in the effects of PACAP in acute and chronic morphine actions. Our results indicate that PACAP-induced actions likely participate in acute and chronic effects of morphine and suggest a potential role of PACAP in opioid analgesia, tolerance and withdrawal.     

Differential changes of laser evoked potentials,late auditory evoked potentials and P300 under morphine in chronic pain patients

The present study investigates the differential behavior of laser evoked brain potentials (LEPs), late auditory evoked potentials (AEP) and the endogenous P300 in response to morphine treatment, examined in 6 chronic pain patients. The main result was that in parallel with marked clinical pain relief, amplitudes of the long latency LEP positivity (P400) were significantly reduced under morphine. One patient suffering from extremely painful osteoporosis for 20 years exhibited a large middle latency component (N170) which was prominently attenuated by morphine. In contrast to LEP amplitude reductions, auditory N1 and P2 potentials appeared either unchanged or even enlarged during morphine treatment. Also P300 amplitude was slightly increased under morphine. Reaction time and mood scales also failed to indicate any sedation. Obviously, LEPs reflected specifically the analgesic morphine effect in this study, while stability or enhancement of AEPs and P300 during morphine treatment indicated lack of sedation or even improved perception and concentration due to the removal of persistent pain as a disruptive perceptual-cognitive stressor.     

Endothelin receptor antagonists restore morphine analgesia in morphine tolerant rats

Several neurotransmitter mechanisms have been proposed to play a role in the development of morphine tolerance. The present study provides evidence for the first time that endothelin (ET) antagonists can restore morphine analgesia in morphine tolerant rats. Tolerance to morphine was induced by subcutaneous implantation of six morphine pellets during a 7-day period. The degree of tolerance to morphine was measured by determining analgesic response (tail-flick latency) and hyperthermic response to morphine sulfate (8 mg/kg, subcutaneously (s.c.)) in placebo and morphine pellet implanted rats. The maximal tail-flick latency in morphine pellet-vehicle treated rats (7.54 s) was significantly lower (P<0.05) when compared to placebo pellet-vehicle treated rats (10s), indicating that tolerance developed to the analgesic effect of morphine. In separate sets of experiments, ET antagonists, BQ123 (10 microg, intracerebroventricularly (i.c.v.)) and BMS182874 (50 microg, i.c.v.) were administered in placebo and morphine tolerant rats. BQ123 was injected twice daily for 7 days and once on day 8. BMS182874 was administered only on day 8. Morphine (8 mg/kg, s.c.) was administered 30min after BQ123 or BMS182874 administration. It was found that both BQ123 and BMS182874 potentiated morphine analgesia in placebo and morphine tolerant rats. BQ123 potentiated tail-flick latency by 30.0% in placebo tolerant rats and 94.5% in morphine tolerant rats compared to respective controls. BMS182874 potentiated tail-flick latency by 30.2% in placebo tolerant rats and 66.7% in morphine tolerant rats. Morphine-induced hyperthermic effect was also potentiated by BQ123 and BMS182874. The duration of analgesic action was also prolonged by BQ123 and BMS182874. The effect of BMS182874 was less as compared to BQ123. BQ123 and BMS182874 are selective ET(A) receptor antagonists. Therefore, it is concluded that ET(A) receptor antagonists restore morphine analgesia in morphine tolerant rats.     

Comparative effect of intraoperative propacetamol versus placebo on morphine consumption after elective reduction mammoplasty under remifentanil-based anesthesia: a randomized control trial [ISRCTN71723173]

BACKGROUND: Postoperative administration of paracetamol or its prodrug propacetamol has been shown to decrease pain with a morphine sparing effect. However, the effect of propacetamol administered intra-operatively on post-operative pain and early postoperative morphine consumption has not been clearly evaluated. In order to evaluate the effectiveness of analgesic protocols in the management of post-operative pain, a standardized anesthesia protocol without long-acting opioids is crucial. Thus, for ethical reasons, the surgical procedure under general anesthesia with remifentanil as the only intraoperative analgesic must be associated with a moderate predictable postoperative pain. METHODS: We were interested in determining the postoperative effect of propacetamol administered intraoperatively after intraoperative remifentanil. Thirty-six adult women undergoing mammoplasty with remifentanil-based anesthesia were randomly assigned to receive propacetamol 2 g or placebo one hour before the end of surgery. After remifentanil interruption and tracheal extubation in recovery room, pain was assessed and intravenous titrated morphine was given. The primary end-point was the cumulative dose of morphine administered in the recovery room. The secondary end-points were the pain score after tracheal extubation and one hour after, the delay for obtaining a Simplified Numerical Pain Scale (SNPS) less than 4, and the incidence of morphine side effects in the recovery room.For intergroup comparisons, categorical variables were compared using the chi-squared test and continuous variables were compared using the Student t test or Mann-Whitney U test, as appropriate. A p value less than 0.05 was considered as significant. RESULTS: In recovery room, morphine consumption was lower in the propacetamol group than in the placebo group (p = 0.01). Pain scores were similar in both groups after tracheal extubation and lower in the propacetamol group (p = 0.003) one hour after tracheal extubation. The time to reach a SNPS < 4 was significantly shorter in the propacetamol group (p = 0.02). The incidence of morphine related side effects did not differ between the two groups. CONCLUSIONS: Intraoperative propacetamol administration with remifentanil based-anesthesia improved significantly early postoperative pain by sparing morphine and shortening the delay to achieve pain relief.     

Gabapentin and postoperative pain: a qualitative and quantitative systematic review, with focus on procedure

Background

Gabapentin is an antiepileptic drug used in a variety of chronic pain conditions. Increasing numbers of randomized trials indicate that gabapentin is effective as a postoperative analgesic. This procedure-specific systematic review aims to analyse the 24-hour postoperative effect of gabapentin on acute pain in adults.

Methods

Medline, The Cochrane Library and Google Scholar were searched for double-blind randomized placebo controlled trials of gabapentin for postoperative pain relief compared with placebo, in adults undergoing a surgical procedure. Qualitative analysis of postoperative effectiveness was evaluated by assessment of significant difference (P < 0.05) in pain relief using consumption of supplemental analgesic and pain scores between study groups. Quantitative analyses of combined data from similar procedures, were performed by calculating the weighted mean difference (WMD) of 24-hour cumulated opioid requirements, and the WMD for visual analogue scale (VAS) pain, (early (6 h) and late (24 h) postoperatively), between study groups. Side-effects (nausea, vomiting, dizziness and sedation) were extracted for calculation of their relative risk (RR).

Results

Twenty-three trials with 1529 patients were included. In 12 of 16 studies with data on postoperative opioid requirement, the reported 24-hour opioid consumption was significantly reduced with gabapentin. Quantitative analysis of five trials in abdominal hysterectomy showed a significant reduction in morphine consumption (WMD – 13 mg, 95% confidence interval (CI) -19 to -8 mg), and in early pain scores at rest (WMD – 11 mm on the VAS, 95% CI -12 to -2 mm) and during activity (WMD -8 mm on the VAS; 95% CI -13 to -3 mm), favouring gabapentin. In spinal surgery, (4 trials), analyses demonstrated a significant reduction in morphine consumption (WMD of – 31 mg (95%CI – 53 to -10 mg) and pain scores, early (WMD – 17 mm on the VAS; 95 % CI -31 to -3 mm) and late (WMD -12 mm on the VAS; 95% CI -23 to -1 mm) also favouring gabapentin treatment. Nausea was improved with gabapentin in abdominal hysterectomy (RR 0.7; 95 % CI 0.5 to 0.9). Other side-effects were unaffected.

Conclusion

Perioperative use of gabapentin has a significant 24-hour opioid sparing effect and improves pain score for both abdominal hysterectomy and spinal surgery. Nausea may be reduced in abdominal hysterectomy.     

Chronic morphine exposure increases the proportion of on-cells in the rostral ventromedial medulla in rats

Chronic opiate exposure produces tolerance and hypersensitivity to mechanical and thermal stimulation that involves increased pain facilitation from the rostral ventromedial medulla (RVM). The aim of the present study was to determine the effect of sustained systemic morphine exposure on RVM neurons. Three cell types in the RVM have been described: on-cells, off-cells and neutral cells. The activity of on-cells increases in response to noxious stimulation, whereas the activity of off-cells decreases following noxious stimulation. Neutral cells remain relatively unaffected. In lightly anesthetized rats, systematic exploration throughout the RVM using single-unit extracellular recordings was used to examine both the relative proportion and the neuronal properties of the different cell classes in chronic morphine and placebo treated animals. Seven days after implanting either morphine (150 mg, s.c.) or placebo pellets a total of four electrode penetrations through the RVM were made in each animal at identical coordinates along midline. Neuronal responses related to radiant heat-evoked paw withdrawals were recorded. When compared to placebo treated rats, chronic morphine increased the number of on-cells and decreased the number of neutral cells, while the number of off-cells remained unchanged. Chronic morphine exposure had no effect on the spontaneous or heat-evoked discharges in on-, off-, or neutral cells. These results indicate that chronic morphine may sensitize a subpopulation of RVM neurons to noxious stimulation, which would be expected to increase descending facilitation and promote tolerance and chronic morphine-induced paradoxical pain.     

The involvement of midbrain astrocyte in the development of morphine tolerance

Aims

Systemic administration of opiate analgesics such as morphine remains the most effective treatment for alleviating severe pain across a range of conditions including acute pain. However, chronic or repeated administration of opiate analgesics results in the development of analgesic tolerance. Glial cells such as microglia and astrocytes are known to release various inflammatory cytokines and neurotrophic factors leading to regulation of neuronal function. Recently, glial cells were reported to play important roles in the development of analgesic tolerance to morphine. Here, we focused on the involvement of midbrain glial cells, particularly astrocytes, in the development of analgesic tolerance to morphine.

Main methods

Mice were treated with morphine (10 mg/kg, s.c.) or vehicle once a day for 5 days. Pentoxifylline (an inhibitor of glial activation; 20 mg/kg, i.p. or 50 and 100 μg/mouse, i.c.v.) was administered 30 min before morphine treatment. Flavopiridol (a cyclin-dependent kinase inhibitor; 5 nmol/mouse, i.c.v.) was administered 10 min before and 10 h after morphine treatment. The analgesic effect of morphine was measured using the tail flick method.

Key findings

The development of analgesic tolerance to morphine was gradually observed during daily treatment of morphine for 5 days in mice. On days 1 and 3 after repeated morphine treatment, astrocyte marker glial fibrillary acidic protein expression levels were significantly increased, as determined by western blot analyses. These phenomena were significantly inhibited following pre-treatment with pentoxifylline or flavopiridol.

Significance

We demonstrated that midbrain astrocytes play an important role in the development of analgesic tolerance to morphine.     

Quantitative analysis of the ultrasonic vocalization responses elicited in adjuvant-induced arthritic rats for screening analgesic drugs

Adjuvant-induced arthritic (AIA) rats have been developed as a chronic pain model to evaluate the effects of analgesic drugs. The purpose of the present study was to examine whether there is dose-dependent inhibition of the emission of ultrasonic vocalization (USV) responses by analgesic drugs in AIA rats. It was demonstrated that morphine (1.25-5.0 mg/kg, s.c.) and ketoprofen (2.5-10.0 mg/kg, s.c.) dose-dependently inhibit USV responses. These results suggest that the USV responses elicited in AIA rats are useful for the quantitative evaluation of analgesic drugs.     

Selective REM Sleep Deprivation Improves Expectation-Related Placebo Analgesia

The placebo effect is a neurobiological and psychophysiological process known to influence perceived pain relief. Optimization of placebo analgesia may contribute to the clinical efficacy and effectiveness of medication for acute and chronic pain management. We know that the placebo effect operates through two main mechanisms, expectations and learning, which is also influenced by sleep. Moreover, a recent study suggested that rapid eye movement (REM) sleep is associated with modulation of expectation-mediated placebo analgesia. We examined placebo analgesia following pharmacological REM sleep deprivation and we tested the hypothesis that relief expectations and placebo analgesia would be improved by experimental REM sleep deprivation in healthy volunteers. Following an adaptive night in a sleep laboratory, 26 healthy volunteers underwent classical experimental placebo analgesic conditioning in the evening combined with pharmacological REM sleep deprivation (clonidine: 13 volunteers or inert control pill: 13 volunteers). Medication was administered in a double-blind manner at bedtime, and placebo analgesia was tested in the morning. Results revealed that 1) placebo analgesia improved with REM sleep deprivation; 2) pain relief expectations did not differ between REM sleep deprivation and control groups; and 3) REM sleep moderated the relationship between pain relief expectations and placebo analgesia. These results support the putative role of REM sleep in modulating placebo analgesia. The mechanisms involved in these improvements in placebo analgesia and pain relief following selective REM sleep deprivation should be further investigated.     

多瑞吉在带状疱疹疼痛治疗阿片类药物转换中的应用

目的:探讨多瑞吉在带状疱疹疼痛治疗阿片类药物转换中的应用。方法:选择37例住院治疗的带状疱疹疼痛患者,年龄>45岁、VAS评分≥4分、所有病人常规抗病毒治疗、增加免疫力等常规治疗,予硬膜外腔置管间断注入消炎镇痛药物并持续泵吗啡,根据疼痛调整至止痛剂量,转换为多瑞吉贴剂后出院。疼痛控制后逐渐减药,每半个月减量半贴多瑞吉,对病人的疼痛评分、生活质量及并发症进行评估。结果:有1例病人应药物副反应出组,其余病人硬膜外泵吗啡后均在一周左右控制疼痛,等效转换为多瑞吉,定时定量减药,无疼痛反复,成瘾戒断等情况。结论:带状疱疹疼痛采用硬膜外间断注药持续泵吗啡迅速达到无痛后,转换为等效剂量的多瑞吉,定时定量减药,安全有效。     

Morphine tolerance in arthritic rats and serotonergic system

To understand whether chronic inflammation alters the development of morphine tolerance, the tail-flick test was used to evaluate the analgesic effect of morphine (75 mg tablet, s.c.) in the arthritic rats at the day 9-12 after the inoculation with Freund's adjuvant. Spinal cord monoamines and amino acid neurotransmitters were concomitantly measured. Chronic inflammation attenuated the antinociceptive effect of morphine as tolerance developed faster in the arthritic rats compared to the vehicle-treated controls. In addition, ratio of 5-hydroxyindole-3-acetic acid/5-hydroxytryptamine (5-HIAA/5-HT) increased in the lumbar spinal cord of arthritic rats without any change in the concentrations of norepinephrine, glutamate, aspartate or GABA. Interestingly, increased serotonin turnover in the spinal cord was observed in both control and arthritic rats 24 hours after morphine treatment. Overall, the results suggest a significant role of serotonin up-regulation in the spinal cord during chronic pain and the development of morphine tolerance.     

A Randomized Controlled Trial on the Effect of Tapentadol and Morphine on Conditioned Pain Modulation in Healthy Volunteers

Background

Modulatory descending pathways, originating at supraspinal sites that converge at dorsal horn neurons, influence pain perception in humans. Defects in descending pain control are linked to chronic pain states and its restoration may be a valuable analgesic tool. Conditioned pain modulation (CPM) is a surrogate marker of descending inhibition that reduces the perception of pain from a primary test stimulus during application of a conditioning stimulus. Here the effects of the analgesics tapentadol, a combined mu-opioid receptor agonist and noradrenaline reuptake inhibitor, and morphine, a strong mu-opioid receptor agonist, were tested on CPM in a randomized, double-blind, placebo-controlled crossover trial in 12 healthy pain-free volunteers, to understand possible differences in mechanism of action between these opioids.

Methods and Results

On three occasions CPM responses were obtained 60-90 and 120-150 min following intake of tapentadol (100 mg immediate release tablet), morphine (40 mg immediate release tablet) or placebo. At both time points, CPM was detectable after treatment with placebo and tapentadol (peak pain ratings reduced by 20-30% after application of the conditioning stimulus) but not after morphine. Compared to placebo morphine displayed significantly less CPM: mean treatment difference 18.2% (95% CI 3.4 to 32.9%) at 60-90 min after drug intake and 19.5% (95% CI 5.7 to 33.2%) at 120-150 min after drug intake (p = 0.001). No difference in CPM between placebo and tapentadol was detected: mean treatment difference 1.5% (95% CI -11.6 to 14.6%) at 60-90 min after drug intake and 1.5% (95% CI -16.0 to 18.9%) at 120-150 min after drug intake (p = 0.60).

Conclusions

Our data show that in volunteers morphine affects CPM, while tapentadol was without effect despite identical experimental conditions. These data confirm that tapentadol’s main mechanism of action is distinct from that of morphine and likely related to the effect of adrenergic stimulation on descending controls.

Trial Registration

Netherlands Trial Register NTR2716     

Comparative effects of Pro-Leu-Gly-NH2 and cyclo(Leu-Gly) administered orally on the development of tolerance to the analgesic effect of morphine in the rat

Comparative effects of Pro-Leu-Gly-NH2 (MIF) and cyclo(Leu-Gly) (CLG) administered orally at different stages of chronic morphine treatment on the development of tolerance to the analgesic effect of morphine in the rat were determined. Male Sprague-Dawley rats were implanted with either 6 placebo or morphine pellets during a 7-day period. Implantation of morphine pellets resulted in the development of a high degree of tolerance as evidenced by a decrease in the analgesic response to morphine. Administration of CLG (8 and 16 mg/kg/day) on day 5, 6 and 7 of implantation inhibited the development of tolerance to morphine but 4 and 32 mg/kg doses had no effect. Further, CLG (2 mg/kg/day for 7 days) inhibited the development of tolerance but higher doses (4 and 8 mg/kg) had no effect. MIF (26 and 52 mg/kg) administered orally on the last three days of the implantation schedule inhibited the development of tolerance to morphine. MIF (6.5 mg/kg/day for 7 days) inhibited the development of tolerance but the higher doses had no effect. Concurrent administration of MIF (6.5 mg/kg) and CLG (2 mg/kg) for seven days failed to inhibit the development of tolerance. A single dose of MIF or CLG administered a day before the assessment of tolerance did not affect the morphine tolerance. Thus, even after a significant degree of tolerance to morphine had developed, neuropeptides like MIF and CLG given orally, in appropriate doses, can inhibit development of tolerance to morphine and restore the analgesic effect of morphine.     

Chronopharmacology of morphine in mice

Dosing-time-dependent changes in the effect and toxicity of morphine were examined in mice housed under alternating 12 h light (07:00 to 19:00 h) and dark (19:00 to 07:00 h) cycles. Morphine (0.5 mg/kg) was injected intraperitoneally (i.p.) in animals to assess its beneficial effect (i.e., protection against the kaolin-induced, bradykinin-mediated, writhing reaction) and its toxicity (i.e., alteration of the hepatic enzymes of aspartate aminotransferase [AST] alanine aminotransferase [ALT], and glutathione [GSH] in separate experiments). The magnitude of the analgesic effect of morphine depended on dosing time, with minimum effect at 02:00 h and maximum effect at 14:00 h. The serum hepatic enzyme levels of AST and ALT increased after dosing morphine (100 mg/kg) at 02:00 and 14:00 h. Time courses of these enzymes did not differ between the two trials. However, hepatic GSH, which is involved in the detoxification of chemical compounds, significantly decreased after i.p. morphine injection at 02:00 but not at 14:00 h. Overall, the results suggest that the analgesic effect of morphine is greater after dosing during the resting than during the activity phase of mice that have been induced with bradykinin-mediated pain. Drug-induced hepatic damage as inferred by GSH alteration, however, may be greater after dosing during the active phase.     

Biological time-dependent difference in effect of peroxynitrite demonstrated by the mouse hot plate pain model

We previously demonstrated the rhythmic pattern of L-arginine/nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) cascade in nociceptive processes. The coupled production of excess NO and superoxide leads to the formation of an unstable intermediate peroxynitrite, which is primarily responsible for NO-mediated toxicity. In the present study, we evaluated the biological time-dependent effects of exogenously administered peroxynitrite on nociceptive processes and peroxynitrite-induced changes in the analgesic effect of morphine using the mouse hot-plate pain model. Experiments were performed at four different times of day (1, 7, 13, and 19 hours after lights on, i.e., HALO) in mice of both sexes synchronized to a 12 h:12 h light-dark cycle. Animals were injected intraperitoneally (i.p.) with saline or 10 mg/kg morphine 30 min before and 0.001 mg/kg peroxynitrite 30 sec before hot-plate testing, respectively. The analgesic effect of morphine exhibited significant biological time-dependent differences in the thermally-induced algesia; whereas, administration of peroxynitrite alone exhibited either significant algesic or analgesic effect, depending on the circadian time of its injection. Concomitant administration of peroxynitrite and morphine reduced morphine-induced analgesia at three of the four different study time points. In conclusion, peroxynitrite displayed nociceptive and antinociceptive when administered alone according to the circadian time of treatment, while it diminished analgesic activity when administered in combination with morphine at certain biological times.     

Announcements

Dosing‐time–dependent changes in the effect and toxicity of morphine were examined in mice housed under alternating 12 h light (07:00 to 19:00 h) and dark (19:00 to 07:00 h) cycles. Morphine (0.5 mg/kg) was injected intraperitoneally (i.p.) in animals to assess its beneficial effect (i.e., protection against the kaolin‐induced, bradykinin‐mediated, writhing reaction) and its toxicity (i.e., alteration of the hepatic enzymes of aspartate aminotransferase [AST] alanine aminotransferase [ALT], and glutathione [GSH] in separate experiments). The magnitude of the analgesic effect of morphine depended on dosing time, with minimum effect at 02:00 h and maximum effect at 14:00 h. The serum hepatic enzyme levels of AST and ALT increased after dosing morphine (100 mg/kg) at 02:00 and 14:00 h. Time courses of these enzymes did not differ between the two trials. However, hepatic GSH, which is involved in the detoxification of chemical compounds, significantly decreased after i.p. morphine injection at 02:00 but not at 14:00 h. Overall, the results suggest that the analgesic effect of morphine is greater after dosing during the resting than during the activity phase of mice that have been induced with bradykinin‐mediated pain. Drug‐induced hepatic damage as inferred by GSH alteration, however, may be greater after dosing during the active phase.     

Intragastric administration of cyclo(Leu-Gly) inhibits the development of tolerance to the analgesic effect of morphine in the rat

The effect of intragastric administration of cyclo(Leu-Gly), a cyclic dipeptide derived from melanotropin release inhibiting factor (Pro-Leu-Gly-NH2), on the development of tolerance to the analgesic effect of morphine in the rat was determined. The tolerance to morphine in the rat was induced by subcutaneous implantation of four morphine pellets during a 3-day period. The rats which served as controls were implanted with placebo pellets. The analgesic response to a challenge dose of morphine was determined by the tail-flick test. The tail-flick latencies were determined before and then every 30 min for 180 min. The analgesic response was computed by determining the area under the time-response curve. Implantation of morphine pellets resulted in the development of tolerance as evidenced by decreased analgesic response to morphine in morphine pellet implanted rats as compared to placebo pellet implanted rats. Chronic intragastric administration of cyclo(Leu-Gly) (4 to 16 mg/kg) inhibited the development of tolerance to morphine. A dose of 8 mg/kg of cyclo(Leu-Gly) completely blocked the tolerance to morphine. The study provides for the first time evidence that intragastric administration of a cyclic peptide can inhibit the development of tolerance to morphine, and that effective neuropeptides and their analogs can be developed as potential drugs to inhibit opiate-induced tolerance.     

First-dose analgesic effect of the cyclo-oxygenase-2 selective inhibitor lumiracoxib in osteoarthritis of the knee: a randomized, double-blind, placebo-controlled comparison with celecoxib [NCT00267215]

Cyclo-oxygenase-2 selective inhibitors are frequently used to manage osteoarthritis. We compared the analgesic efficacy of the novel cyclo-oxygenase-2 selective inhibitor lumiracoxib (Prexige) versus placebo and celecoxib in patients with knee osteoarthritis. This seven day, double-blind, placebo and active comparator controlled, parallel group study included 364 patients aged > or = 50 years with moderate-to-severe symptomatic knee osteoarthritis. Patients received lumiracoxib 400 mg/day (four times the recommended chronic dose in osteoarthritis; n = 144), placebo (n = 75), or celecoxib 200 mg twice daily (n = 145). The primary variable was actual pain intensity difference (100 mm visual-analogue scale) between baseline and the mean of three hour and five hour assessments after the first dose. Actual pain intensity difference, average and worst pain, pain relief and functional status (Western Ontario and McMaster Universities Osteoarthritis Index [WOMAC]) were measured over seven days. Patients also completed a global evaluation of treatment effect at study end or premature discontinuation. For the primary variable, the superiority of lumiracoxib versus placebo, the noninferiority of lumiracoxib versus celecoxib, and the superiority of lumiracoxib versus celecoxib were assessed by closed test procedure adjusting for multiplicity, thereby maintaining the overall 5% significance level. In addition, celecoxib was assessed versus placebo in a predefined exploratory manner to assess trial sensitivity. Lumiracoxib provided better analgesia than placebo 3-5 hours after the first dose (P = 0.004) through to study end. The estimated difference between lumiracoxib and celecoxib 3-5 hours after the first dose was not significant (P = 0.185). Celecoxib was not significantly different from placebo in this analysis (P = 0.069). At study end 13.9% of lumiracoxib-treated patients reported complete pain relief versus 5.5% and 5.3% of celecoxib and placebo recipients, respectively. WOMAC total and subscales improved for both active treatments versus placebo except for difficulty in performing daily activities, for which celecoxib just failed to achieve significance (P = 0.056). In the patient's global evaluation of treatment effect, 58.1% of patients receiving lumiracoxib rated treatment as 'excellent' or 'good', versus 48.6% of celecoxib and 25.3% of placebo patients. Lumiracoxib was well tolerated. The overall incidence of adverse events was similar across treatment groups.