NG-nitro-L-arginine prevents morphine tolerance.

NMDA receptor antagonists, such as MK-801, prevent the development of tolerance to morphine. Since many NMDA actions involve the production of nitric oxide, we examined the effects of a nitric oxide synthase inhibitor on morphine tolerance. The analgesic response to morphine (5 mg/kg s.c.) given daily diminishes from 60% in naive animals to 0% within 5 days. Coadministration of NG-nitro-L-arginine (8 mg/kg per day) along with morphine prevents the demonstration of appreciable tolerance for at least 11 days. These results suggest that morphine tolerance involves the activation of NMDA receptors followed by the subsequent release of nitric oxide.     

Acute exposure to saccharin reduces morphine analgesia in the rat: evidence for involvement of N-methyl-d-aspartate and peripheral opioid receptors

Rationale: Pairings of a sweet taste and injection of morphine result in a learned avoidance of that taste and learned analgesic tolerance. This avoidance is mediated by the drug’s peripheral effect, while learned tolerance involves activation of N-methyl-d-aspartate (NMDA) receptors. Exposure to a sweet taste also reduces morphine analgesia. We studied whether this taste-mediated reduction was reversed by an NMDA or peripheral opioid receptor antagonist. Objectives: To determine whether an intraoral infusion of saccharin would modulate morphine analgesia in rats, and to study the contribution of NMDA as well as peripheral opioid receptors to this modulation. Methods: Six experiments used the rat’s tail-flick response to study the effect of an intraoral infusion of a sodium saccharin solution on morphine analgesia, and the effects of the quaternary opioid receptor antagonist methylnaltrexone as well as the non-competitive NMDA receptor antagonist MK-801 on this modulation of analgesia. Results: An intraoral infusion of saccharin reduced the analgesic effects of an intraperitoneal (i.p.) injection of morphine across a range of doses (experiment 1a), which was not attributable to an influence on tail-skin temperature (experiment 1b). This reduction was mediated by opioid receptors in the periphery and activation of NMDA receptors because morphine analgesia was reinstated by an i.p. injection of either methylnaltrexone (experiment 2a) or MK-801 (experiment 3a), which was not due to the effect of methylnaltrexone (experiment 2b) or MK-801 (experiment 3b) on morphine analgesia in the absence of saccharin. Conclusions: These results document evidence for an antagonism of morphine analgesia by actions of the drug at peripheral opioid receptors and excitatory amino-acid activity at NMDA receptors. They are discussed with reference to the aversive motivational effects of peripheral opioid receptors and pain facilitatory circuits. Received: 12 June 1999 / Final version: 1 November 1999     

Low doses of alpha 2-adrenoceptor antagonists augment spinal morphine analgesia and inhibit development of acute and chronic tolerance

Background and purpose:

Ultra-low doses of opioid receptor antagonists augment spinal morphine antinociception and block the induction of tolerance. Considering the evidence demonstrating functional and physical interactions between the opioid and α₂-adrenoceptors, this study investigated whether ultra-low doses of α₂-adrenoceptor antagonists also influence spinal morphine analgesia and tolerance.

Experimental approach:

Effects of low doses of the competitive α₂-adrenoceptor antagonists—atipamezole (0.08, 0.8 ng), yohimbine (0.02, 2 ng), mirtazapine (0.02 ng) and idazoxan (0.08 ng) were investigated on intrathecal morphine analgesia, as well as acute and chronic morphine antinociceptive tolerance using the rat tail flick and paw pressure tests.

Key results:

At doses markedly lower than those producing α₂-adrenoceptor blockade, atipamezole, yohimbine, mirtazapine and idazoxan, prolonged the antinociceptive effects of morphine. When co-administered with repeated acute spinal injections of morphine, all four agents blocked the induction of acute tolerance. Co-injection of atipamezole with morphine for 5 days inhibited the development of tolerance in a chronic treatment paradigm. Spinal administration of atipamezole also reversed established antinociceptive tolerance to morphine as indicated by the restoration of morphine antinociceptive potency. The effects of atipamezole on spinal morphine tolerance were not influenced by treatment with 6-hydroxydopamine.

Conclusions and implications:

Low doses of competitive α₂-adrenoceptor antagonists can augment acute morphine analgesia and block or reverse tolerance to spinal administration of morphine. These actions are interpreted in terms of their interaction with an opioid-α₂-adrenoceptor complex, whose activity may have a function in the genesis of analgesic tolerance.     

大鼠蛛网膜下腔联合注射kappa受体激动剂和NMDA受体拮抗剂协同镇痛

以大鼠热辐射甩尾潜伏期为测痛指标,蛛网膜下腔(it)联合注射非镇痛剂量的kappa阿片受体激动剂强啡肽(dynorphin,Dyn)A-(1-13)5nmol或U50488H(trans-(±)-3,4-dichloro-N-methyl-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide)100nmol和N-methyl-D-aspartate(NMDA)受体拮抗剂DL-2-amino-5-phosphonovalericacid(APv)10 nmol 或 kynurenicacid (KYN) 50 nmol 有显著的协同镇痛效应,其效应与NMDA受体拮抗剂呈一定量效关系。Kappa阿片受体特异性拮抗剂nor-binaltorphi-mine(nor-BNI)15nmolit可完全翻转Dyn A-(1-13)5nmol和APv10nmol及U50488H100nmol和KYN50nmol的协同镇痛。说明协同作用是通过kappa受体和谷氨酸能神经元之间的相互作用实现的。     

Drugs used in the treatment of opioid tolerance and physical dependence: a review

Opioid drugs used in the treatment of severe pain are known to produce tolerance that requires a dose increase to maintain a sufficient analgesic effect. As this is connected with side effects such as respiratory depression, it is highly desirable to avoid or at least attenuate the development of tolerance. Closely related, but in some respect dissociable, is the phenomenon of physical dependence, which becomes apparent particularly in heroin withdrawal. Our knowledge about the mechanisms underlying tolerance has increased dramatically in recent years, but a final picture of the importance of each particular mechanism under in vivo conditions has not yet emerged. Recent studies suggest that the so-called receptor down-regulation is not the main mechanism in vivo. A desensitization on the basis of receptor decoupling, receptor internalization and increased alternative coupling to stimulatory G-proteins have been demonstrated. However, a functional antagonism of the opioid effects seems to be clinically most important, mediated by the activation of NMDA receptors, up-regulation of adenylyl cyclase and nitric oxide synthase. Drugs blocking these mechanisms are the most promising option in the treatment of tolerance. Namely, alpha2-adrenoreceptor agonists such as clonidine and NMDA antagonists such as ketamine or dextromethorphan have been used to minimize tolerance development during opioid treatment. Moreover, clinical strategies such as opioid rotation and multimodal analgesia, i.e. the simultaneous application of several analgetics of different type, have proven to be successful approaches.     

Effects of NMDA receptor channel blockers, dizocilpine and memantine, on the development of opiate analgesic tolerance induced by repeated morphine exposures or social defeats in mice

Development of tolerance to opiates involves various neurochemically and pharmacologically distinct processes. For instance, the diversity of opiate tolerance has been suggested by experiments comparing the establishment of diminished response to different effects of opiate agonists. Antagonists acting at N-methyl-d-aspartate (NMDA) receptors has become a very useful tool for studying opiate tolerance mechanisms since these drugs have been shown to retard the development of tolerance to analgesic properties of opiates. The present study compared the ability of two NMDA receptor channel blockers, dizocilpine and memantine, to affect the development of tolerance to morphine analgesia induced by repeated social defeat or by repeated morphine administrations. Male BALB/c mice were assessed for the tail-flick response before and after the defeat in five social confrontations, or before and after repeated morphine injections (20 mg/kg, s.c., once daily for 8 days). Repeated morphine injections were explicitly paired with environmental cues. Socially-defeated as well as morphine-treated mice developed significant tolerance to morphine analgesia. Separate groups of mice were exposed to repeated social confrontations or injections of morphine with each defeat or morphine injection followed by administration of either dizocilpine (0.03–0.3 mg/kg, i.p.) or low-affinity channel blocker memantine (3–30 mg/kg, i.p.). Both dizocilpine and memantine were effective in preventing the development of repeated morphine-induced tolerance to acute morphine analgesia. Treatments with NMDA receptor antagonists that retarded development of non-associative tolerance also suppressed the establishment of associative tolerance significantly. Social defeat-induced tolerance was prevented by dizocilpine but not by memantine. Our results suggest some degree of similarity in the mechanisms of morphine analgesic tolerance induced by pharmacological, contextual and social stimuli. Received: 13 December 1997 / Accepted: 15 April 1998     

(35)S]GTPγS binding and opioid tolerance and efficacy in mouse spinal cord

The present study examined efficacy of a series of opioid agonists and then using chronic in vivo treatment protocols, determined tolerance to opioid agonist stimulated [³⁵S]GTPγS (guanosine 5′-O-(3-[³⁵S] thio)triphosphate) binding in mouse spinal cord membranes and compared it directly to spinal analgesic tolerance. The [³⁵S]GTPγS binding assay was used to estimate efficacy (E_max and τ; Operational Model of Agonism) of a series of opioid agonists for G-protein activation in mouse spinal cord. The rank order of opioid agonist efficacy determined in the [³⁵S]GTPγS assay using the Operational Model and E_max was similar. These efficacy estimates correlated with historical analgesic efficacy estimates. For tolerance studies, mice were continuously treated s.c. for 7 days with morphine, oxycodone, hydromorphone, etorphine or fentanyl and [³⁵S]GTPγS studies were conducted in spinal cord membranes. Other mice were tested in i.t. analgesia dose response studies (tailflick). Tolerance to DAMGO ([D-Ala²,N-MePhe⁴,Gly-ol⁵]enkephalin) or morphine stimulated [³⁵S]GTPγS binding (decrease in E_max) was observed following etorphine and fentanyl treatment only. These treatment protocols downregulate μ-opioid receptor density whereas morphine, oxycodone and hydromorphone do not. Spinal analgesic tolerance was observed following all treatment protocols examined (morphine, oxycodone and etorphine). Opioid antagonist treatment that specifically upregulates (chronic naltrexone) or downregulates (clocinnamox) μ-opioid receptor density produced a corresponding change in opioid agonist stimulated [³⁵S]GTPγS binding. Although receptor downregulation and G-protein uncoupling are among potential mechanisms of opioid tolerance, the present results suggest that uncoupling in mouse spinal cord plays a minor role and that the [³⁵S]GTPγS assay is particularly responsive to changes in μ-opioid receptor density.     

NO/NOS系统与阿片系统相互作用的研究进展

一氧化氮 (NO)是与N 甲基 D 天 (门 )冬氨酸 (NM DA)受体活性相关的一种细胞内信使分子 ,在吗啡耐受和依赖中起作用。一氧化氮合酶 (NOS)抑制剂可以减弱或阻断吗啡的作用。NO/NOS系统也参与吗啡诱导的位置偏爱效应。NO/NOS系统作用与阿片受体的激活相关 ;NO/NOS系统也可能通过影响去甲肾上腺素神经元系统参与吗啡戒断症状表达。深入阐明NO/NOS系统和阿片系统之间的关系对于阿片药物镇痛治疗及阿片成瘾的戒毒治疗均具有积极的意义     

Pharmacological characterization of dihydromorphine, 6-acetyldihydromorphine and dihydroheroin analgesia and their differentiation from morphine

The present study examined the pharmacology of dihydromorphine, 6-acetyldihydromorphine and dihydroheroin (3,6-diacetyldihydromorphine). Like morphine, dihydromorphine and its acetylated derivatives all were highly selective mu-opioids in receptor binding assays. All the compounds were potent mu-selective analgesics, as shown by their sensitivity towards the mu-selective opioid receptor antagonists naloxonazine and beta-funaltrexamine. However, the actions of dihydromorphine and its analogs were readily distinguished from those of morphine, differences that were surprising in view of the very limited structural differences among them that consisted of only the reduction of the 7,8-double bond. Like heroin and morphine-6beta-glucuronide, the analgesic actions of dihydromorphine and its two acetylated derivatives were antagonized by 3-O-methylnaltrexone at a dose that was inactive against morphine analgesia. Antisense mapping also distinguished between morphine and the dihydromorphine compounds. Antisense oligodeoxynucleotides targeting exon 2 of the cloned MOR-1 gene decreased dihydromorphine analgesia and that of its acetylated derivatives, but not morphine analgesia. Conversely, the exon 1 antisense that effectively lowered morphine analgesia was inactive against dihydromorphine and its analogs. Finally, dihydromorphine and its analogs retained their analgesic activity in a mouse model of morphine tolerance, consistent with incomplete cross-tolerance. Together, these findings imply that the mu-opioid receptor mechanisms mediating the analgesic actions of dihydromorphine and its acetylated analogs are distinct from morphine and more similar to those of heroin and morphine-6beta-glucuronide.     

Augmentation of spinal morphine analgesia and inhibition of tolerance by low doses of mu- and delta-opioid receptor antagonists

BACKGROUND AND PURPOSE: Ultralow doses of naltrexone, a non-selective opioid antagonist, have previously been found to augment acute morphine analgesia and block the development of tolerance to this effect. Since morphine tolerance is dependent on the activity of micro and delta receptors, the present study investigated the effects of ultralow doses of antagonists selective for these receptor types on morphine analgesia and tolerance in tests of thermal and mechanical nociception. EXPERIMENTAL APPROACH: Effects of intrathecal administration of mu-receptor antagonists, CTOP (0.01 ng) or CTAP (0.001 ng), or a delta-receptor antagonist, naltrindole (0.01 ng), on spinal morphine analgesia and tolerance were evaluated using the tail-flick and paw-pressure tests in rats. KEY RESULTS: Both micro and delta antagonists augmented analgesia produced by a sub-maximal (5 microg) or maximal (15 microg) dose of morphine. Administration of the antagonists with morphine (15 microg) for 5 days inhibited the progressive decline of analgesia and prevented the loss of morphine potency. In animals exhibiting tolerance to morphine, administration of the antagonists with morphine produced a recovery of the analgesic response and restored morphine potency. CONCLUSIONS AND IMPLICATIONS: Combining ultralow doses of micro- or delta-receptor antagonists with spinal morphine augmented the acute analgesic effects, inhibited the induction of chronic tolerance and reversed established tolerance. The remarkably similar effects of micro- and delta-opioid receptor antagonists on morphine analgesia and tolerance are interpreted in terms of blockade of the latent excitatory effects of the agonist that limit expression of its full activity.     

Celecoxib induces tolerance in a model of peripheral inflammatory pain in rats

Celecoxib is a non-steroidal anti-inflammatory drug (NSAID) that selectively inhibits cyclooxygenase-2 (COX-2). Like most NSAIDs, celecoxib exhibits analgesic effects in models of inflammatory pain but these appear to be dependent on endogenous opioid release. Therefore, this study has assessed the ability of celecoxib to induce tolerance in rats, comparable to that induced by morphine.Rats were injected subcutaneously (s.c.) twice daily with divided doses of celecoxib, morphine or indomethacin. Inflammation was induced in one hind paw of rats by injecting prostaglandin E₂ (PGE₂; 200 ng) 30 min after drug administration, on days 1, 3, 5 and 6 or 7. Nociceptive thresholds to mechanical stimulation were measured 3 h after PGE₂ injection, on the same days. On days 6 or 7, analgesic effects of the full doses of test drugs were assessed.Celecoxib-induced tolerance, as did morphine, an effect not shown by another NSAID, indomethacin. Cross-tolerance between celecoxib and morphine was observed as they did not induce analgesia when animals were chronically treated with morphine or celecoxib, respectively. In addition, tolerance to celecoxib’s analgesic effects persisted for at least two days after the end of the chronic treatment with celecoxib. Naltrexone prevented induction of tolerance to morphine or celecoxib.The present results strengthen the possibility that celecoxib has also mechanisms of analgesia unrelated to COX inhibition but dependent on endogenous opioid release. Our results also imply the existence of a new class of analgesics without the deleterious effects of COX inhibitors.     

家兔隔区和伏核内钙、镁离子对抗电针镇痛与吗啡镇痛

本文通过脑内慢性埋植套管向家兔一侧隔区或伏核内注射微量(10 nmol)CaCl₂或MgCl₂,可显著对抗吗啡镇痛和电针镇痛。注入核外则无效。家兔一侧隔区或伏核内注入阳离子螯合剂CDTA(20 nmol)加强吗啡镇痛和电针镇痛。文中就Ca²⁺,Mg²⁺作用的相似性,电针镇痛与吗啡镇痛机理的相似性,以及伏核和隔区在上述镇痛中的重要性进行了讨论。     

Associative and non-associative mechanisms of morphine analgesic tolerance are neurochemically distinct in the rat spinal cord

 Opiate tolerance involves both associative and non-associative changes. However, procedures designed to distinguish between these two processes have rarely been employed when investigating the physiological basis of such plasticity. The present experiments assessed some of the mechanisms contributing to both associative and non-associative decreases in morphine analgesic potency following repeated IV morphine administration (4 days, 5 mg/kg per day). For one group of rats, testing for morphine analgesia (tailflick) occurred in a context that had been repeatedly paired with morphine administration. Another group of rats, exposed equally to the testing context, handling procedures and morphine, was tested for morphine analgesia in a context that was specifically unpaired with prior drug. Although both of these groups showed a decrease in the drug effect following repeated administrations, those rats tested in the morphine-associated context were significantly more tolerant than the unpaired group. We evaluated the spinal cord involvement of NMDA receptors, as well as the peptide neurotensin in these two types of tolerance. NMDA receptors appeared to mediate non-associative changes in drug potency, as rats tested in either context were less tolerant when morphine administration was preceded with the non-competitive NMDA antagonist, MK-801 (2.5 and 5 nmol). Spinal neurotensin antagonism with [d-Trp¹¹]neurotensin (3 pmol) selectively abolished associative tolerance. These findings provide information about the mechanisms of opiate tolerance and support the distinction between associative and non-associative processes underlying these changes. Received: 20 December 1995 / Final version: 11 June 1996     

Analgesic/μ- & δ-opioids: Co-Administration of μ- and δ-Opioid Agonists Reduces the Side-Effects of the μ-opioid Agonist

Summary

Novelty: A combination of a μ-opioid agonist (opiate) and a δ-opioid agonist (enkephalin derivative) is disclosed, which shows potentiation of the analgesic, sedative, antitussive, and gastrointestinal motility modalities of opiate action, with a claimed reduction of tolerance and dependence.

Biology: Dose-response data for antinociceptive activity (hot plate trial) for the opiate/enkephalin combination are presented showing a significant and useful leftward shift in the presence of the enkephalin. For example, the ED₅₀ for morphine alone was 1.0 nmol upon intracerebroventricular (KK) administration and was 0.2 nmol in combination with 1.5 nmol DPDPE. Naloxone-precipitated dependence symptoms are not potentiated, though still present.

Chemistry: Preferred opiates are morphine, oxymorphine, codeine, methadone etc. Preferred enkephalins are [D-Pen², D-Pen⁵] enkephalin (DPDPE) and [D-Pen², L-Pen⁵] enkephalin (DPLPE).     

双向阿片功能调节剂：胍丁胺

近年研究表明某些药物尽管不能与阿片受体发生相互作用，但能对阿片药理作用产生重要的调节．特别是有些药物能对阿片功能产生双向调节作用，即增强阿片镇痛，对抗阿片耐受和躯体依赖．我们将这些不与阿片受体发生作用，但具有双向调节阿片功能的药物称之为双向阿片功能调节剂(biphasic opioid function modulator，BOFM)．基于我们的研究工作，可以认定胍丁胺就是一个典型的双向阿片功能调节剂．胍丁胺本身有弱的镇痛作用，它能增强吗啡镇痛，对抗吗啡耐受和依赖：胍丁胺产生上述作用的主要机制与抑制阿片长期作用下在阿片受体信号转导系统产生的代偿性适应过程相关．     

A biphasic opioid function modulator: agmatine

Recently it has been revealed that some agents that are not able to interact with opioid receptors play an important role in regulating the pharmacological actions of opioids. Especially, some of them show biphasic modulation on opioid functions, which enhance opioid analgesia, but inhibit tolerance to and substance dependence on opioids. We would like to call these agents which do not interact with opioid receptors, but do have biphasic modulation on opioid functions as biphasic opioid function modulator (BOFM). Mainly based on our results, agmatine is a typical BOFM. Agmatine itself was a weak analgesic which enhanced analgesic action of morphine and inhibited tolerance to and dependence on opioid. The main mechanisms of agmatine were related to inhibition of the adaptation of opioid receptor signal transduction induced by chronic treatment of opioid.     

Chronic opioid antagonist treatment facilitates nonopioid, stress-induced analgesia

Chronic exposure to opioid antagonists produces increases brain opioid receptors and enhances morphine analgesia. Since opioid antagonists could affect both opioid and nonopioid analgesic systems, the present study evaluated whether chronic opioid antagonist treatment with naltrexone alters the nonopioid analgesia produced by cold-water swims (CWS). Rats were implanted (SC) with two, 30 mg naltrexone pellets. The pellets were removed 8 days later or left in place and rats tested 24 hr later for analgesia (tail-flick) following a 3.5 min CWS or morphine (3 mg/kg, SC). As expected, morphine analgesia was potentiated in rats with naltrexone pellets removed, but was blocked in rats tested with the naltrexone still implanted. In contrast, naltrexone pretreatment potentiated CWS analgesia, irrespective of whether the pellets were removed or left in place. These findings confirm the nonopioid nature of CWS analgesia and indicate that chronic treatment with an opioid antagonist can affect both opioid and nonopioid analgesic mechanisms.     

Behavioral interactions caused by combined administration of morphine and MK-801 in rats

Rationale: The NMDA antagonist MK-801 reportedly blocks experience-dependent changes in sensitivity to morphine, including tolerance to its analgesic actions and sensitization to its locomotor-stimulating effects. However, evidence in the existing literature suggests that some of MK-801’s effects are additive (or synergistic) with those of morphine. Objectives: Experiments were conducted to characterize the effects of acute and repeated administration of the combination of MK-801 and morphine on analgesia, locomotor activity, and drug discrimination in rats. Methods: In each experiment, rats were first tested repeatedly after treatment with the combination of MK-801 and morphine, and then after treatment with either drug alone. Results: The analgesic effects of MK-801 combined with morphine were greater than those of morphine alone, but tolerance to the combination of drugs developed at a similar rate as to morphine alone. The locomotor-stimulating effects of MK-801 combined with morphine were also greater than those of either drug alone, and locomotor sensitization developed to the combination of drugs but not to either drug alone at the low doses used. Rats learned to discriminate a combination of MK-801 and morphine from vehicle as quickly as they learned to discriminate morphine alone from vehicle, but those trained with the combination of MK-801 and morphine responded primarily at the vehicle-appropriate lever when given either drug alone. Conclusions: Since behavioral adaptations readily occur in the presence of MK-801, it appears that NMDA antagonists fail to invariably block the cellular plasticity that underlies such adaptations. Rather, the expression of adaptations in drug sensitivity appears related, at least in part, to the continued presence of the discriminative stimulus cues that are present during conditioning. Although NMDA receptors are important for some forms of cellular plasticity, the present studies illustrate the difficulty in interpreting behavioral studies in which MK-801 is given with morphine. Received: 24 November 1999 / Accepted: 25 March 2000     

Effects of NMDA receptor antagonists on acute mu-opioid analgesia in the rat

Mixed research findings have led to a debate regarding the effect of N-methyl-D-aspartate (NMDA) receptor antagonists on opiate analgesia. NMDA antagonists have been found in various studies to enhance, to inhibit, or to have no effect on opiate analgesia. The present research used a single protocol to explore the effects of six NMDA receptor antagonists on acute morphine (3.0 mg/kg s.c.) and fentanyl (0.05 mg/kg s.c.) analgesia in adult male Sprague-Dawley rats. NMDA receptor antagonists were selected based on their abilities to block various sites on the NMDA receptor complex, including the noncompetitive antagonists MK-801 (0.1 and 0.3 mg/kg i.p.), dextromethorphan (10.0 and 30.0 mg/kg i.p.), and memantine (3.0 and 10.0 mg/kg i.p.), a glycine site antagonist, (+)-HA-966 (10.0 and 30.0 mg/kg i.p.), a competitive antagonist, LY235959 (1.0 and 3.0 mg/kg i.p.), and a polyamine site antagonist, ifenprodil (1.0 and 3.0 mg/kg i.p.). Analgesia was assessed using the tail-flick test. A single dose of each opiate was used. The low doses of the antagonists, which are known to produce significant neural and behavioral actions at NMDA receptors, had no effect on morphine or fentanyl analgesia. At the higher doses, morphine analgesia was significantly enhanced by LY235959 (3.0 mg/kg), and fentanyl analgesia was significantly enhanced by LY235959 (3.0 mg/kg), dextromethorphan (30.0 mg/kg), and (+)-HA-966 (30.0 mg/kg). Enhancement of analgesia occurred without any apparent adverse side effects. None of the NMDA antagonists affected tail-flick responses on their own, except the higher dose of LY235959 (3.0 mg/kg), which produced a mild analgesic effect. Because no consistent effects were observed, the data suggest that NMDA receptors are not involved in acute mu-opioid analgesia. The mechanisms underlying the enhancement of opiate analgesia by selected NMDA antagonists, such as LY235959, dextromethorphan, and (+)-HA-966, remain to be determined.     

Involvement of Gi/o proteins and GIRK channels in the potentiation of morphine-induced spinal analgesia in acutely inflamed mice

The analgesic efficacy of opiates can be enhanced in inflammatory states due to peripheral and spinal alterations. We describe here that the analgesic effect induced by intrathecal (i.t.) morphine assessed by measuring thermal withdrawal latencies is enhanced in carrageenan-inflamed mice. The spinal μ-opioid receptor (MOR) population is not up-regulated as demonstrated by Western blot assays. In contrast, behavioural experiments show the involvement of changes in transduction mechanisms activated by spinal opioid receptors. The i.t. administration of the nitric oxide (NO) synthase inhibitor l-NMMA (3–30 μg) antagonised with a similar potency and efficacy morphine-induced analgesia in inflamed and non-inflamed mice, discarding that an increase in NO release could be responsible of the enhancement of morphine-induced analgesia. The analgesic effects evoked by the i.t. administration of the direct G_i/o protein activator mastoparan (0.03–10 μg), but not those induced by the N-type calcium channel blocker ω-conotoxin GVIA (3–30 ng), were potentiated in inflamed mice, suggesting that postsynaptic and not presynaptic mechanisms could be involved. Furthermore, the inhibitory effects on morphine-induced analgesia produced by the G_i/o protein inhibitor pertussis toxin (0.1–17 ng) or the G-coupled inwardly rectifying potassium (GIRK) channels inhibitor tertiapin-Q (0.75–750 ng) were greatly enhanced in inflamed mice. These results suggest that differences in the transduction mechanism activated by MOR at postsynaptic level, probably related with GIRK channels activity, could participate in the potentiation of morphine-induced spinal analgesia in acutely inflamed mice.