Spinal Glucocorticoid Receptors Contribute to the Development of Morphine Tolerance in Rats

Background: Opioid analgesic tolerance is a pharmacologic phenomenon involving the mechanisms of cellular adaptation. Central glucocorticoid receptors (GRs) have been implicated in the cellular mechanism of neuronal plasticity that has many cellular steps in common with the mechanism of opioid tolerance. In a rat model of morphine tolerance, the authors examined the hypothesis that spinal GRs would play a significant role in the development of tolerance to the antinociceptive effect of morphine.

Methods: In experiment 1, each group of rats received the GR antagonist RU38486 (0.5 or 1 [mu]g), the mineralocorticoid receptor antagonist spironolactone (3 [mu]g), or a vehicle, given intrathecally with morphine (10 [mu]g) twice daily for 6 days. In experiment 2, four groups of rats were used, and each group received intrathecally 10 [mu]g morphine plus 5 [mu]mol GR antisense oligodeoxynucleotide, sense oligodeoxynucleotide, mixed-base oligodeoxynucleotide, or vehicle. Western blotting was used to examine the expression of GRs within the spinal cord dorsal horn. In experiment 3, the GR agonist dexamethasone (4 [mu]g) was given intrathecally twice daily in combination with 10 [mu]g morphine. For all experiments, the development of morphine antinociceptive tolerance was assessed using the tail-flick test.

Results: The development of tolerance to the antinociceptive effect of morphine was substantially attenuated when the GR antagonist RU38486 (1 > 0.5 [mu]g > vehicle) but not spironolactone was coadministered with morphine for 6 days. A single treatment with RU38486 did not affect morphine antinociception, nor did it reverse morphine tolerance on day 7. A similar reduction of morphine tolerance was observed in those rats treated with a GR antisense oligodeoxynucleotide but not a sense or mixed-base oligodeoxynucleotide. The administration of the GR antisense oligodeoxynucleotide also prevented GR up-regulation within the spinal cord dorsal horn. Moreover, the GR agonist dexamethasone facilitated the development of morphine tolerance.     

糖皮质激素受体在慢性吗啡耐受大鼠脊髓背角神经元凋亡中的作用

目的 评价糖皮质激素受体在慢性吗啡耐受大鼠脊髓背角神经元凋亡中的作用.方法 鞘内置管成功的健康雄性SD大鼠20只,体重300～350 g,随机分为4组(n=5):对照组(C组)、慢性吗啡耐受组(M组)、吗啡+糖皮质激素受体拮抗剂组(MR组)和吗啡+糖皮质激素受体激动剂组(MD组)分别于8:00和20:00鞘内注射生理盐水10μl、吗啡10μg、吗啡10μg+RU38486 2μg、吗啡10μg+地塞米松4μg,连续6 d.于每天8:00给药后30 min行甩尾实验,给药第7天处死大鼠,取L3～L5脊髓行TUNEL染色,光镜下观察脊髓背角神经元的凋亡情况,计算凋亡率.结果 地塞米松、RU38486分别对慢性吗啡耐受的形成起促进、抑制作用.与C组比较,M组和MD组脊髓背角神经元凋亡率升高(P＜0.05);与M组比较,MR组脊髓背角神经元凋亡率降低,MD组脊髓背角神经元凋亡率升高(P＜0.05).结论 糖皮质激素受体参与了慢性吗啡耐受形成中大鼠脊髓背角神经元凋亡的过程.     

脊髓糖皮质激素受体在吗啡耐受大鼠PI3K/Akt信号通路中的作用

目的 探讨脊髓糖皮质激素受体(GR)在吗啡耐受大鼠磷脂酰肌醇-3激酶/蛋白激酶B(PI3K/Akt)信号通路中的作用.方法 健康雄性SD大鼠,8～ 10周龄,体重300 ～ 350 g,取鞘内置管成功的大鼠40只,采用随机数字表法,将大鼠随机分为4组(n=10):对照组(C组)鞘内注射生理盐水10μl;吗啡耐受组(M组)鞘内注射吗啡10 μg;地塞米松组(GR激动剂,DEX组)鞘内注射地塞米松4μg,30 min后注射吗啡10 μg; RU38486组(GR阻断剂,R组)鞘内注射RU38486 2 μg,30 min后注射吗啡10 μg.注射药物容量均为10μl,2次/d,连续7d.于每天第1次鞘内给药前和鞘内给药结束后1d测定甩尾潜伏期,计算最大抗伤害效应百分比( MPAE),最后一次甩尾潜伏期测定结束后,取脊髓背角组织,测定PI3K、Caspase-3的表达水平和Akt活性.结果 M组、DEX组和R组发生了吗啡耐受,C组未发生吗啡耐受.与C组比较,M组脊髓背角Akt活性降低,PI3K表达下调,Caspase-3表达上调(P＜0.01);与M组比较,DEX组MPAE和Akt活性降低,PI3K表达下调,Caspase-3表达上调,R组MPAE和Akt活性升高,PI3K表达上调,Caspase-3表达下调(P＜0.05).结论 脊髓GR可通过抑制PI3K/Akt信号通路参与吗啡耐受的形成.     

ERK-CREB信号通路在糖皮质激素受体介导大鼠慢性吗啡耐受中的作用

目的 评价细胞外信号调节激酶-cAMP反应元件结合蛋白(ERK- CREB)信号通路在糖皮质激素受体介导大鼠慢性吗啡耐受中的作用.方法 健康雄性SD大鼠,体重280～320 g,月龄2月,经枕骨大孔行鞘内置管.取36只鞘内置管成功的大鼠,采用随机数字表法,将大鼠随机分为6组(n=6):对照组(C组)、慢性吗啡耐受组(M组)、吗啡+地塞米松组(MD组)、吗啡+RU38486组(MR组)、地塞米松组(D组)和RU38486组(R组).分别鞘内注射生理盐水10 μl、吗啡10腭、吗啡10 μg+地塞米松4 μg、吗啡10 μg+ RU38486 2 μg、地塞米松4.μg、RU38486 2 μg,2次、d,连续6d.于给药前1d测定热甩尾潜伏期(TFL)基础值,随后于给药1、3、5d首次给药后30 min及最后1次给药后1 d(T1～4)测定TFL,计算最大镇痛效应百分比(MPE).最后1次测定TEL后取脊髓,采用免疫荧光染色法测定脊髓背角磷酸化ERK(pERK)和磷酸化CREB(pCREB)的表达.结果 与T1时比较,M组和MD组T3.4时MPE降低(P＜0.05).与C组比较,M组MPE升高,脊髓背角pERK和pCREB的表达上调(P＜0.05或0.01),R组和D组上述指标差异无统计学意义(P＞0.05).与M组比较,MR组MPE升高,脊髓背角pERK和pCREB表达上调,MD组脊髓背角pERK和pCREB表达下调,MPE降低(P＜0.05或0.01).结论 糖皮质激素受体介导大鼠慢性吗啡耐受的机制可能与抑制ERK-CREB信号通路有关.     

Repeated intrathecal injections of dezocine produce antinociception without evidence for neurotoxicity in the rat: a study of morphometric evaluation of spinal cord histology.

This study was designed to determine the antinociceptive and spinal cord histologic effects of a new agonist/antagonist opioid drug dezocine. This drug was injected intrathecally in rats at a dose of 50 or 125 micrograms twice daily for 14 days. The tail-flick test showed that the antinociceptive effect declined gradually, with no detectable effects by day 14. Quantitative histologic techniques and light and electron microscopy showed that neither dose, compared with vehicle, created any morphologic changes in the spinal cord that could be attributed to a neurotoxic or otherwise degenerative effect of the drug. In conclusion, dezocine is a drug that gives rise to sustained antinociceptive effects when administered intrathecally and causes no morphologic changes in the rat spinal cord that could be indicative of neurotoxic potential.     

Supraspinal and spinal cord opioid receptors are responsible for antinociception following intrathecal morphine injections

BACKGROUND AND OBJECTIVE: The clinical practice of spinal morphine administration for pain relief is based on observations in animals that opioid receptors exist in the spinal cord and intrathecal injections of opioids in those species (mostly rats) lead to antinociceptive effects. Clinicians are well aware that administration of spinal opioids is associated with side-effects, such as nausea and respiratory depression, that indicate supraspinal spread of the drug administered. Those observations call into question how much of the observed pain relief is due to action of the drug in the brain. This study investigated the spinal cord actions of morphine given intrathecally to rats in a model that allows investigation of drug-receptor interaction at the spinal cord level. Experiments were performed on male Wistar rats with chronically implanted lumbar subarachnoid catheters. METHODS: Nociceptive thresholds were measured in rats given morphine intrathecally alone and in combination with intrathecal injections of selective opioid receptor antagonists: beta-funaltrexamine (mu), naltrindole (delta) and nor-binaltorphimine (kappa). RESULTS: Intrathecal morphine caused dose-related antinociceptive effects that were reversed totally by naloxone. Intrathecal beta-funaltrexamine and naltrindole did not reverse the effects of intrathecal morphine. However, intrathecal nor-binaltorphimine did reverse the electrical current threshold effects of morphine but not tail flick latency. CONCLUSIONS: Antinociception following intrathecal morphine involves spinal and supraspinal opioid receptors. The tail flick effect described in rat experiments involves actions at opioid receptors in the brain that override any action that may be caused by combination of morphine with mu-opioid receptors in the spinal cord.     

Intrathecal catheterization influences tolerance to chronic morphine in rats

We evaluated the antinociceptive effects of acute and chronic morphine administered spinally via lumbar puncture in intrathecally catheterized and sham-surgery rats. The effects of acute morphine did not differ between groups. Catheterized rats developed tolerance to chronic morphine more rapidly, compared with sham and naive rats. Therefore, catheter presence facilitated development of opioid antinociceptive tolerance. Spinal astrogliosis, determined by measurement of 3-dimensional cell volumes, was observed in catheterized rats as indicated by significantly larger cell volumes compared with surgery-naive controls. Gliosis induced by chronic intrathecal morphine administered to surgery-naive animals was comparable to that observed in saline-treated catheterized rats.     

关节炎慢性吗啡耐受大鼠脊髓背角谷氨酸转运体3型的表达

目的观察谷氨酸转运体3型(EAAT3)在关节炎慢性吗啡耐受大鼠脊髓背角表达的变化。方法36只健康雄性SD大鼠随机分为六组,每组6只,行鞘内置管。其中的四组制成佐剂性关节炎模型,分别经鞘内给予生理盐水(A组)、吗啡10μg/kg(B组)、吗啡20μg/kg(C组)、吗啡10μg/kg加纳洛酮10μg/kg(D组);另外两组非致炎大鼠分别经鞘内给予生理盐水(E组)、吗啡20μg/kg(F组)。各组给药均为每日2次,连续7d。动态检测大鼠50%缩爪阈值,以免疫组化方法检测脊髓背角EAAT3表达。结果B、C组关节炎大鼠在鞘内给予吗啡第7天形成较稳定的吗啡耐受,其脊髓背角EAAT3表达下降。结论脊髓EAAT3可能参与炎性痛大鼠慢性吗啡耐受的形成机制。     

Magnesium sulfate potentiates morphine antinociception at the spinal level

Intrathecal magnesium sulfate coinfusion with morphine increases antinociception in normal rats; however, because magnesium also delays the onset of tolerance, it is not clear whether this additional antinociception is a result of potentiated analgesia or tolerance abatement. We examined the antinociceptive interaction of intrathecal (IT) bolus magnesium sulfate and morphine in morphine naive rats and those with mechanical allodynia after a surgical incision. After intrathecal catheter implantation, rats were given preinjections of magnesium or saline, followed by injections of morphine or saline. In morphine na?ve rats, IT bolus magnesium sulfate 281 and 375 microg followed by IT morphine 0.25 or 0.5 nmol enhanced peak antinociception and area under the response versus time curve two-to-three-fold in the tail-flick test as compared with morphine alone. Likewise, in rats with incisional pain, IT bolus magnesium sulfate 188 and 375 microg followed by morphine 0.5 nmol reduced mechanical allodynia, whereas morphine 0.5 nmol alone did not. This study suggests that IT magnesium sulfate potentiates morphine at a spinal site of action. Implications: Magnesium sulfate potentiates morphine analgesia when coadministered intrathecally in normal rats, and in an animal model of mechanical allodynia after a surgical incision. These results suggest that intrathecal administration of magnesium sulfate may be a useful adjunct to spinal morphine analgesia.     

The effects of intrathecal morphine encapsulated in L- and D-dipalmitoylphosphatidyl choline liposomes on acute nociception in rats

Liposomes can serve as a sustained-release carrier system, permitting the spinal delivery of large opioid doses restricting the dose for acute systemic uptake. We evaluated the antinociceptive effects of morphine encapsulated in liposomes of two isomeric phospholipids, L-dipalmitoylphosphatidyl choline (L-DPPC) and D-dipalmitoylphosphatidyl choline (D-DPPC), in comparison with morphine in saline. Sprague-Dawley rats with chronic lumbar intrathecal catheters were tested for their acute nociceptive response using a hindpaw thermal escape test. Their general behavior, motor function, pinna reflex, and corneal reflex were also examined. The duration of antinociception was longer in both liposomal morphine groups than in the free morphine group. The peak antinociceptive effects were observed within 30 min after intrathecal morphine, L-DPPC or D-DPPC morphine injection. The rank order of the area under the effect-time curve for antinociception was L-DPPC morphine > D-DPPC morphine > morphine. The 50% effective dose was: 2.7 microg (morphine), 4.6 microg (L-DPPC morphine), and 6.4 microg (D-DPPC morphine). D-DPPC morphine had less side effects for a given antinociceptive AUC than morphine. In conclusion, L-DPPC and D-DPPC liposome encapsulation of morphine prolonged the antinociceptive effect on acute thermal stimulation and could decrease side effects, compared with morphine alone. Implications: Two isomers of liposome (L-dipalmitoylphosphatidyl choline and D-dipalmitoylphosphatidyl choline) encapsulation of morphine prolonged the analgesic effect on acute thermal-induced pain when administered intrathecally and could decrease side effects, compared with morphine alone.     

Antinociceptive interactions between alpha 2-adrenergic and opiate agonists at the spinal level in rodents

This study was undertaken to evaluate the antinociceptive interactions of alpha 2 adrenergic and opiate receptors at the spinal level. Morphine and clonidine were administered intrathecally (i.t.) by lumbar puncture to rats either alone or in the presence of either i.t. yohimbine, an alpha 2 antagonist, or systemic naloxone, an opioid antagonist. The effect of tolerance to systematically administered morphine on responses to i.t. morphine and clonidine was examined in mice. Antinociception was determined by observing the response to a clamp applied to the tail (Haffner test) in mice and by the tail-flick test in rats; log dose-response curves for antinociception were generated for morphine, clonidine, and each drug combination. Morphine and clonidine both produced dose-dependent antinociception when given i.t. in both species. The i.t. administration of yohimbine attenuated the antinociceptive effect of both clonidine and morphine, but naloxone attenuated only the response to morphine. Further, a sub-analgetic dose of i.t. clonidine potentiated the effect of i.t. morphine. In morphine-tolerant mice, i.t. morphine was not efficacious whereas clonidine retained full efficacy, although potency was slightly diminished. Thus, it appears that alpha 2 adrenoceptor-mediated antinociception is independent of opiate receptor mechanisms. Clinical use of intrathecal combinations of alpha 2 adrenergic and opiate receptor agonists to increase analgesia and use of intrathecal alpha 2 agonists for pain relief in patients tolerant to opiates might deserve evaluation.     

The role of spinal opioid receptors in antinociceptive effects produced by intrathecal administration of hydromorphone and buprenorphine in the rat

The intrathecal administration of morphine has been the standard therapy to control long-term intractable pain. Recently, a panel of pain therapy experts suggested that because of the lack of efficacy or because of the side effects produced by morphine in some patients, other drugs, such as hydromorphone and buprenorphine, should be investigated for their analgesic properties. We designed this study to compare the efficacy of intrathecal hydromorphone and buprenorphine to suppress thermal nociception in male Sprague-Dawley rats. An additional objective was to understand whether hydromorphone and buprenorphine bind and act as agonists to mu-, delta-, and kappa-spinal opioid receptors. Intrathecally-administered hydromorphone and buprenorphine produced a dose- and time-dependent increase in the tail-flick response latency in rats. The 50% effective dose value for the antinociceptive effect of buprenorphine and hydromorphone were 4 and 69.5 nmol/L, respectively. Both drugs act as agonists to mu-opioid receptors, as determined by their ability to displace [(3)H]-DAMGO from the spinal opioid receptors and by the ability of an opioid receptor antagonist, naloxone, to reverse their antinociceptive effects. Buprenorphine also has an agonistic effect on the kappa-opioid receptors. For the first time, we report that intrathecal buprenorphine is approximately 17 times more effective than hydromorphone in inhibiting thermal pain, and buprenorphine produces its antinociceptive effect by acting as an agonist at both mu- and kappa-spinal opioid receptors. Naloxone administered intrathecally was effective in preventing the antinociceptive effects of subsequent intrathecal injections of buprenorphine. IMPLICATIONS: Hydromorphone and buprenorphine are two important drugs used for pain relief. We observed that intrathecal buprenorphine is 17 times more potent than hydromorphone to inhibit pain in rats. Both drugs exert their effects through specific spinal opioid receptors.     

Antinociceptive effect of morphine, but not mu opioid receptor number, is attenuated in the spinal cord of diabetic rats

BACKGROUND: The mechanisms of decreased analgesic potency of mu opioids in diabetic neuropathic pain are not fully known. The authors recently found that G protein activation stimulated by the mu opioid agonist is significantly reduced in the spinal cord dorsal horn in diabetes. In the current study, they determined potential changes in the number and binding affinity of mu opioid receptors in the spinal cord in diabetic rats. METHODS: Rats were rendered diabetic with an intraperitoneal injection of streptozotocin. The nociceptive withdrawal threshold was measured before and after intrathecal injection of morphine by applying a noxious pressure stimulus to the hind paw. The mu opioid receptor was determined with immunocytochemistry labeling and a specific mu opioid receptor radioligand, [3H]-(D-Ala2,N-Me-Phe4,Gly-ol5)-enkephalin ([3H]-DAMGO), in the dorsal spinal cord obtained from age-matched normal and diabetic rats 4 weeks after streptozotocin treatment. RESULTS: The antinociceptive effect of intrathecal morphine (2-10 microg) was significantly reduced in diabetic rats, with an ED50 about twofold higher than that in normal rats. However, both the dissociation constant (3.99 +/- 0.22 vs. 4.01 +/- 0.23 nm) and the maximal specific binding (352.78 +/- 37.26 vs. 346.88 +/- 35.23 fmol/mg protein) of [3H]-DAMGO spinal membrane bindings were not significantly different between normal and diabetic rats. The mu opioid receptor immunoreactivity in the spinal cord dorsal horn also was similar in normal and diabetic rats. CONCLUSIONS: The reduced analgesic effect of intrathecal morphine in diabetes is probably due to impairment of mu opioid receptor-G protein coupling rather than reduction in mu opioid receptor number in the spinal cord dorsal horn.     

INVOLVEMENT OF HYPOGASTRIC AND PELVIC NERVES FOR CONVEYING CYSTITIS INDUCED NOCICEPTION IN CONSCIOUS RATS

PURPOSE: We determined the sites of the antinociceptive action of morphine in the experimental model of cyclophosphamide induced cystitis and investigated the afferent nerve fibers involved in nociception transmission originating from the bladder. MATERIALS AND METHODS: Cyclophosphamide (200 mg./kg.) given intraperitoneally was used to induce cystitis in male rats and their behavior was observed and scored. The effect of 2 mg./kg. systemic morphine given intravenously on cyclophosphamide induced behavioral modifications was tested when administered alone and after 100 microg. naloxone per rat given intrathecally at the L1 to L2 or L6 to S1 level. The spinal antinociceptive effect of morphine was also tested when administered intrathecally alone at 10, 100 and 200 microg. per rat at L1 to L2, alone at 100 microg. per rat at L1 to L2 or L6 to S1, alone at 100 microg. per rat at L1 to L2 and L6 to S1 simultaneously, alone at 200 microg. per rat at L1 to L2 and after 100 microg. naloxone per rat given intrathecally at L6 to S1 at 100 microg. per rat at L1 to L2. RESULTS: Cyclophosphamide induced marked modifications in the behavior of the rats, including a decreased breathing rate, eye closing and specific postures. Morphine given intravenously reversed these behavioral disorders and this reversal was completely prevented by pretreatment with intrathecal naloxone. A dose of 100 microg. per rat given intrathecally also reversed these behavioral disorders by about 25% at the L1 to L2 and L6 to S1 levels. In addition, a dose of 100 microg. morphine per rat administered intrathecally and simultaneously at L1 to L2 and L6 to S1 produced an effect equal to the sum of those observed when administered separately, that is about 50%, whereas morphine at an intrathecal dose of 200 microg. at L1 to L2 produced the same effect as 100 microg. given intrathecally at the same level or at L6 to S1 (25%). Also, 100 microg. naloxone per rat administered intrathecally at L6 to S1 prevented the effect of 100 microg. morphine at L1 to L2. CONCLUSIONS: These results confirm the previously reported antinociceptive effect of systemic morphine in this model of cyclophosphamide cystitis, suggest that this antinociceptive action is completely located at the spinal site and most importantly demonstrate by the pharmacological approach and behavioral analysis that nociceptive sensations originating from the bladder are conveyed by hypogastric and pelvic nerves in this cyclophosphamide cystitis model in the conscious rat.     

Peripheral nerve injury alters the alpha2 adrenoceptor subtype activated by clonidine for analgesia

BACKGROUND: Previous studies suggest that the alpha adrenoceptor subtype is the target for spinally administered alpha -adrenergic agonists, clonidine, for pain relief. However, ST 91, a preferential alpha adrenoceptor subtype agonist, induces antinociception, and intrathecally administered alpha antisense oligodeoxynucleotide decreases antinociception induced by clonidine in the rat, suggesting non-A sites may be important as well. Therefore, the authors examined the subtype of alpha adrenoceptor activated by clonidine and ST 91 in normal rats and those with nerve injury-induced hypersensitivity. METHODS: The same mechanical stimulus was applied to normal rats and those following spinal nerve ligation, and the effect of intrathecal clonidine and ST 91 on withdrawal threshold to the stimulus was determined. To further examine subtypes, animals were spinally pretreated with vehicle, BRL 44408 (an alpha subtype-preferring antagonist), and ARC 239 (an alpha subtype-preferring antagonist). RESULTS: In normal animals, clonidine's effect was diminished by pretreatment with either antagonist, whereas ST 91's antinociceptive effect was solely blocked by pretreatment with ARC 239. In nerve-injured animals, the antihypersensitivity action of both clonidine and ST 91 was blocked by administration of ARC 239, whereas BRL 44408 was ineffective. CONCLUSIONS: These data agree with previous studies supporting that the alpha adrenoceptor is important to the antinociceptive effect of clonidine in normal animals. Nerve injury alters this and results in a total reliance on alpha adrenoceptors.     

Differential effects of intrathecal midazolam on morphine-induced antinociception in the rat: role of spinal opioid receptors.

The antinociceptive effects of an intrathecally administered benzodiazepine agonist midazolam, alone and in combination with morphine, were examined in the rat by using the tail-flick test. The duration of antinociceptive effect produced by midazolam was significantly less (P less than 0.05) than that produced by morphine. Low doses of midazolam (10 micrograms) and morphine (10 micrograms) produced a synergistic effect in prolonging antinociceptive effect. However, at higher doses (20 or 30 micrograms), these drugs reduced the extent of antinociception produced by each other. Naloxone administration prevented antinociception produced by these drugs, indicating interactions between midazolam and opioid receptors. Midazolam had dual effects on the binding of opioid ligands to the spinal opioid receptors. At low dose, it potentiated the displacement of [3H]naloxone by morphine. At higher doses, midazolam inhibited the binding of opioid ligands to their spinal receptors in the following order: kappa greater than delta greater than mu. These results indicate that differential antinociceptive effects of midazolam on morphine-induced antinociception involve interaction of this benzodiazepine with spinal opioid receptors.     

A single dose of intrathecal morphine in rats induces long-lasting hyperalgesia: the protective effect of prior administration of ketamine

An active pronociceptive process involving N-methyl-D-aspartate (NMDA) receptor activation is initiated by opioid administration, leading to opioid-induced pain sensitivity. Experimental observations in rats have reported reduction of baseline nociceptive threshold after prolonged spinal opioid administration. In this study we sought to determine whether a single dose of intrathecal morphine can induce hyperalgesia in uninjured rats and to assess the effects of pretreatment with the NMDA-antagonist ketamine on nociceptive thresholds. Sensitivity to nociceptive stimuli (paw pressure test) was assessed for several days after an acute intrathecal injection of morphine (5 microg and 10 microg) in male Sprague-Dawley rats. The effects of subcutaneously administered NMDA-receptor antagonist ketamine (10 mg/kg) before intrathecally administered morphine were also evaluated. A single intrathecal injection of morphine led to a biphasic effect on nociception; early analgesia associated with an increase in the nociceptive threshold lasting 3-5 h was followed by delayed hyperalgesia associated with a decrease in the nociceptive threshold lasting 1-2 days. Subcutaneous ketamine did not significantly modify the early analgesic component but almost completely prevented the delayed decrease in nociceptive threshold after intrathecal administration of morphine. A single intrathecal injection of morphine in rats produces a delayed and sustained hyperalgesia linked to the development of opioid-induced pain sensitivity.     

The effects of intrathecal gabapentin on spinal morphine tolerance in the rat tail-flick and paw pressure tests

Analgesic tolerance to opioids has been described in both experimental and clinical conditions and may limit the clinical utility of these drugs. We have previously shown that systemic gabapentin (GBP), a non-opioid drug, prevents and reverses tolerance to systemic morphine in the rat. In this study, we investigated the effect of intrathecal GBP on spinal morphine tolerance. Studied rats were given 7 days of intrathecal injections with saline (10 microL), GBP (300 microg), morphine (15 microg), or a GBP-morphine combination, and analgesic testing using tail-flick and paw-pressure tests was conducted before and 30 min after the drug injection. On Day 8, an antinociceptive dose-response curve was constructed and the 50% effective dose (ED(50)) values for morphine (given alone) were calculated for each study group. Coinjection of GBP with morphine blocked the development of tolerance, as shown by the preservation of morphine analgesia over 7 days as well as by a concomitant decrease in ED(50) values on Day 8, as compared with the morphine-alone group. Although additive analgesia over Days 1-7 cannot be ruled out, ED(50) reductions in the GBP-morphine combination group indeed suggest some suppression of tolerance. These data support previous evidence that GBP prevents opioid tolerance and, more specifically, indicate that intrathecal GBP prevents the development of spinal opioid tolerance. Future studies are required to examine the respective roles of supraspinal and peripheral sites of GBP-morphine interaction and to investigate the mechanisms underlying the action of GBP on opioid tolerance.     

Pharmacology of methadone and its isomers

Methadone is a synthetic opioid analgesic that is used as an alternate to morphine and hydromorphone for patients with severe pain. It is increasingly being used in opioid rotation schedules. Methadone has an asymmetric carbon atom resulting in 2 enantiomeric forms, the d and l isomers. The racemic mixture (dl-methadone) is the form commonly used clinically. Recent studies have revealed the pharmacological activity of the d-methadone isomer. We found that the d isomer of methadone has N-methyl-D-aspartate (NMDA) receptor antagonist activity both in vitro and in vivo. Studies were designed to examine the ability of d-methadone to attenuate the development of morphine tolerance and to modify NMDA-induced hyperalgesia in rats. Repeated dosing with intrathecal morphine produced a 38-fold increase in the morphine ED50 value. This decrease in the potency of morphine was completely prevented by the coadministration of intrathecal d-methadone at 160 microg/rat. In addition, the decrease in thermal paw withdrawal latency induced by the intrathecal administration of 1.64 microg/rat NMDA was completely blocked by pretreatment with 160 microg/rat d-methadone. Thus, the same dose of intrathecal d-methadone that attenuates the development of spinal morphine tolerance blocks NMDA-induced hyperalgesia in rats. These results support the CONCLUSIONS: that d-metha-done affects the development of morphine tolerance and NMDA-induced hyperalgesia by virtue of its NMDA receptor antagonist activity.     

Interaction between the spinal melanocortin and opioid systems in a rat model of neuropathic pain

BACKGROUND: The authors recently demonstrated that administration of the melanocortin-4 receptor antagonist SHU9119 decreased neuropathic pain symptoms in rats with a sciatic chronic constriction injury. The authors hypothesised that there is a balance between tonic pronociceptive effects of the spinal melanocortin system and tonic antinociceptive effects of the spinal opioid system. Therefore, they investigated a possible interaction between these two systems and tested whether opioid effectiveness could be increased through modulation of the spinal melanocortin system activity. METHODS: In chronic constriction injury rats, melanocortin and opioid receptor ligands were administered through a lumbar spinal catheter, and their effects on mechanical allodynia were assessed by von Frey probing. RESULTS: Naloxone (10-100 microg) dose-dependently increased allodynia (percent of maximum possible effect of -67 +/- 9%), which is in agreement with a tonic antinociceptive effect of the opioid system. SHU9119 decreased allodynia (percent of maximum possible effect of 60 +/- 13%), and this effect could be blocked by a low dose of naloxone (0.1 microg), which by itself had no effect on withdrawal thresholds. Morphine (1-10 microg) dose-dependently decreased allodynia (percent of maximum possible effect of 73 +/- 14% with the highest dose tested). When 0.5 microg SHU9119 (percent of maximum possible effect of 47 +/- 14%) was given 15 min before morphine, there was an additive antiallodynic effect of both compounds. CONCLUSIONS: Together, these data confirm that there is an interaction between the spinal melanocortin and opioid systems and that combined treatment with melanocortin-4 receptor antagonists and opioids might possibly contribute to the treatment of neuropathic pain.