Prolonged in vivo antagonism of central mu- and delta-opioid receptor activity by beta-funal trexamine

beta-Funaltrexamine (beta-FNA) was tested in the spinal cord and supraspinally against inhibition of reflex bladder contractions produced in the anesthetized rat by the opioid-receptor selective agonists [D-Ala2, MePhe4, Gly (ol)5]enkephalin (DAGO, mu-agonist) and [D-Pen2, D-Pen5] enkephalin (DPDPE, delta-agonist). All agents were microinjected either intracerebroventricularly (i.c.v.) or intrathecally (i.t.). beta-FNA (1-8 micrograms) produced long-lasting antagonism of both DAGO and DPDPE. Complete recovery from its effects was only observed some 24-32 h later. Higher doses of beta-FNA (4 and 8 micrograms i.t.) produced short-lived agonistic activity though the selectivity of this was not determined. It was concluded that beta-FNA was a potent, long-lasting antagonist at central opioid receptors in vivo but was unselective for the mu and delta opioid receptor.     

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.     

Effect of opioid agonist-antagonist interaction on morphine dependence in rats

Morphine dependence was induced by treatment with morphine-admixed food (0.25mg/g of food) for 7 days. Withdrawal was precipitated by injecting naloxone (0.5mg/kg, s.c.). Rats treated with morphine exhibited body weight loss upon the naloxone injection. When morphine-dependent rats were injected subcutaneously with morphine, codeine, meperidine and pentazocine 30 min before the naloxone injection, these drugs significantly suppressed the naloxone-precipitated loss of body weight in a dose-dependent manner. However, body weight loss induced through coadministration of naloxone and Mr-2266 BS were not suppressed by morphine pretreatment. These results suggest that opioids protect against naloxone-precipitated loss of body weight, and that mu and kappa opiate receptors play an important role in the protection against naloxone-precipitated withdrawal.     

Comparative effects of opioid peptides on respiration and analgesia in rats

Receptor mechanisms for opiate induced respiratory depression and analgesia (tail-flick) were studied by the ED50 ratios and the apparent pA2 values of the interactions of naloxone with the mu-agonists morphine and D-ala2-me-phe4-met (O)ol5-enkephalin (FK-33824), and the delta-agonists D-ala2-D-leu5-enkephalin (DADL) and tyr-D-ser-gly-phe-leu-thr. The apparent pA2 values of morphine, FK-33824 and DADL for analgesia were similar, whereas the apparent pA2 values of the mu-agonists for respiratory depression were significantly lower than those of the delta-agonists. The ratio between the ED50 of FK-33824 in analgesia and respiratory depression was much lower than that of DADL. It is concluded that different receptors mediate the opiate-induced respiratory depression. One difficulty with the delta-receptors being maximally involved in this action is the high degree of antagonism shown by naloxone on the respiratory effects of the delta-agonists.     

Phorbol ester suppression of opioid analgesia in rats

Protein kinase C (PKC) has been shown to be an important substrate in intracellular signal transduction. Very little is known concerning its possible role in mediating opiate-induced analgesia. In the present study, 12-O-tetradecanoylphorbol 13-acetate (TPA), a selective activator of PKC, was injected intrathecally (ith) to assess its influence on the analgesia induced by intrathecal injection of the mu opioid agonist PL017, the delta agonist DPDPE and the kappa agonist 66A-078. Radiant heat-induced tail flick latency (TFL) was taken as an index of nociception. TPA in the dose of 25-50 ng, which did not affect the baseline TFL, produced a marked suppression of opioid antinociception, with a higher potency in blocking mu and delta than the kappa effect. In addition, mu and delta agonists induced remarkable decreases in spinal cyclic AMP (cAMP) content whereas the kappa effect was weak. The results suggest a cross-talk between the PKC system and the signal transduction pathway subserving opioid analgesia.     

Significance of the hypothalamus opioid system in the mechanism of morphine analgesia

It has been demonstrated in experiments on conscious rabbits that microinjections of nalorphine to the paraventricular area of the hypothalamus blocked morphine analgesia assessed from the tail-flick test and evoked potential variation in the sensorimotor area of the brain cortex in response to nociceptive electrocutaneous stimulation. An analogous but less powerful effect was produced by nalorphine injected into the large raphe nucleus. It is assumed that morphine analgesia is primarily mediated by opioid structures of the hypothalamus.     

Role of opioid receptors in cardioprotection of cold-restraint stress and morphine

Since cold exposure confers cardioprotection, the present study attempted to determine the role of opioid receptors (OR). Stress with cold exposure and restraint for 3 h, shown previously to induce peptic ulcer in a synergistic manner, attenuated infarct size induced by myocardial ischemia and reperfusion in the isolated perfused rat heart from 36.64 ± 1.8 to 22.85 ± 2.6%. This is similar to protecting the rat with morphine at 8 mg/kg, which also attenuated the infarct size from 36.26 ± 1.6 to 20.30 ± 2.1%. The effects of cold-restraint or morphine were abolished by naloxone, a non-selective OR antagonist; nor-binaltorphimine, a selective -OR antagonist; naltrindole, a selective -OR antagonist, or CTOP, a selective -OR antagonist. The effects were also attenuated by blockade of protein kinase C or the mitochondrial K_ATP channel. The finding is first evidence that all three OR subtypes mediate cardioprotection of cold-restraint stress in the rat.     

"Paradoxical" analgesia and aggravated morphine dependence induced by opioid antagonists

Chronic treatment with naloxone (Nx) or naltrexone (Ntx) induces paradoxical analgesia. In the present study, the effects of chronic treatment with opioid receptor antagonists, such as nor-binaltorphimine (nor-BNI) for kappa and naltrindole (NTI) for delta receptors, on analgesic response using the hot plate test and on morphine physical dependence in rats were examined. The hot plate latency was significantly increased by pretreatment with Nx (5 mg/kg, s.c.), nor-BNI (20 mg/kg, i.p.) or NTI (20 mg/kg, i.p.) for 5 days. After chronic pretreatment with these antagonists, the rats were treated with morphine-admixed food (0.5 mg/g of food) for 3 days. Chronic pretreatment with Nx and NTI significantly increased Nx precipitated body weight loss in morphine dependent rats, while chronic pretreatment with nor-BNI produced small increase. These results indicate that chronic treatment with nor-BNI or NTI as well as with Nx induces obviously paradoxical analgesia, and that chronic blockade of mu or delta may enhance the development of physical dependence on morphine.     

Modulation of delta opioid agonist-induced antinociception by repeated morphine pretreatment in rhesus monkeys

AimsRepeated treatment with morphine increases antinociceptive effects of delta opioid agonists in rodents by a mechanism that may involve increased cell-surface expression of delta receptors. The present study evaluated effects of repeated morphine treatment on behavioral effects of the delta agonist SNC80 and the mu agonist fentanyl in rhesus monkeys.Main methodsIn an assay of schedule-controlled responding, three monkeys responded for food reinforcement under a fixed-ratio 30 schedule. In an assay of thermal nociception, tail-withdrawal latencies were evaluated in three monkeys using thermal stimulus intensities of 48 and 54 °C. In both assays, the effects of SNC80 (0.032–3.2 mg/kg) and fentanyl (0.001–0.056 mg/kg) were evaluated after repeated treatment with saline or a regimen of morphine doses modeled on the regimen that enhanced delta agonist antinociception and apparent delta receptor availability in previous rodent studies.Key findingsBoth SNC80 and fentanyl dose-dependently decreased rates of schedule-controlled responding, and repeated morphine treatment did not significantly alter these effects. In the assay of thermal nociception, SNC80 had little effect on tail-withdrawal latencies from water heated to 48 or 54 °C, whereas fentanyl increased tail-withdrawal latencies at both temperatures. Repeated morphine tended to increase the antinociceptive effects of SNC80 and to decrease the antinociceptive effects of fentanyl, but these effects of repeated morphine were small and were significant only at the higher stimulus intensity (54 °C).SignificanceThese results provide limited support for the proposition that prior stimulation of mu receptors selectively increases the antinociceptive effects of delta agonists in rhesus monkeys.     

Retention of supraspinal delta-like analgesia and loss of morphine tolerance in delta opioid receptor knockout mice

Gene targeting was used to delete exon 2 of mouse DOR-1, which encodes the delta opioid receptor. Essentially all 3H-[D-Pen2,D-Pen5]enkephalin (3H-DPDPE) and 3H-[D-Ala2,D-Glu4]deltorphin (3H-deltorphin-2) binding is absent from mutant mice, demonstrating that DOR-1 encodes both delta1 and delta2 receptor subtypes. Homozygous mutant mice display markedly reduced spinal delta analgesia, but peptide delta agonists retain supraspinal analgesic potency that is only partially antagonized by naltrindole. Retained DPDPE analgesia is also demonstrated upon formalin testing, while the nonpeptide delta agonist BW373U69 exhibits enhanced activity in DOR-1 mutant mice. Together, these findings suggest the existence of a second delta-like analgesic system. Finally, DOR-1 mutant mice do not develop analgesic tolerance to morphine, genetically demonstrating a central role for DOR-1 in this process.     

Immunohistochemical study of opioid receptors after chronic morphine treatment in rats

Previously, we have used the biochemical receptor binding method to investigate whether down-regulation of the opioid receptor is a mechanism for morphine tolerance, and we were led to a negative conclusion. In the current study, we further used immunohistochemistry to reinvestigate this issue. Male Sprague-Dawley rats (250-300 g) were chronically treated with morphine s.c. for 2, 4 or 6 days, using an escalating dosage paradigm (5-45 mg), which resulted in a 1.8 to 4.0-fold increase in AD50. Rat brains were removed, frozen, coronally sectioned (14 microm) and processed for mu- or delta-opioid receptor immunohistochemistry using the Avidin-Biotin Complex (ABC) method. No significant decrease in mu-opioid receptor (MOR) immunodensity was found in most of the brain regions, which were enriched with MOR after chronic treatment with morphine except for the anteroventral thalamic nucleus in the ventrolateral part (AVVL). No significant change in delta-opioid receptor (DOR) immunodensity after chronic treatment with morphine was found either. Therefore, our conclusion is that down regulation of opioid receptors may not be an important mechanism for morphine tolerance.     

Role of peripheral and central opioid activity in analgesia induced by restraint stress

Rats subjected to prolonged restraint showed an increase in tail flick latency which outlasted the period of restraint by 15 min. This restraint could be blocked but not reversed by 1 mg/kg of naltrexone hydrochloride given subcutaneously. Naltrexone methobromide, administered subcutaneously in doses of 10 or 25 mg/kg, did not block the analgesia indicating that peripheral opioid receptors were probably not involved. Naltrexone hydrochloride was shown to have no effect on brain tryptophan uptake in restrained rats, a neurochemical event which had previously been shown to be critical to restraint-induced analgesia.     

芬太尼和吗啡在全麻开胸术后病人静脉自控镇痛作用效果的对比研究

目的:探讨芬太尼和吗啡用于开胸术后镇痛效果及不良反应的观察.方法:356例全麻开胸手术病人,随机分成吗啡治疗组(M组)和芬太尼治疗组(F组),每组178例.F组采用芬太尼100μg+利多卡因200mg+异丙嗪25mg,M组采用吗啡20rag+利多卡因200mg+异丙嗪25mg,分别用生理盐水稀释至100ml,术毕接电子泵行静脉自控镇痛(PCA).分别记录手术后6、12、24、48h四个时相点两组病人的视觉模拟疼痛评分(VAS评分)、镇静评分(Ramsay评分)以及恶心、呕吐、瘙痒、呼吸抑制等不良反应的发生情况.结果:各时相点F组VAS评分均低于M组,差异显著(p<0.05),而两组间血压、心率、心率、呼吸频率及SpO2无明显差异.各时相点恶心及呕吐评分F组优于M组,两组间差别显著(p<0.05),镇静效果两组相当.两组均未发生严重不良反应.结论:芬太尼用于全麻开胸术后病人的镇痛效果显著强于吗啡,镇静效果与吗啡相当,但不良反应明显小于吗啡,疗效确切,安全性高,值得在推广使用.     

Effects of morphine and opioid peptides on plasma levels of atrial natriuretic peptide

The effect of opiate ligand administration on plasma levels of atrial natriuretic peptide (ANP) was studied in awake, freely moving Sprague-Dawley rats. Prior to and following the intracerebroventricular (icv) or central venous (iv) injection of morphine (MS), leu-enkephalin (Leu-enk), dynorphin (Dyn) or beta-endorphin (B-endor), plasma samples were obtained for measurement of ANP concentrations by radioimmunoassay. MS was 10 times more potent when given icv than when given iv to increase plasma ANP levels. Icv injection of Leu-enk decreased plasma ANP concentrations. Dyn and B-endor administration (iv or icv) did not alter the plasma concentration of ANP. These effects of MS and Leu-enk on plasma concentrations of ANP appear to be mediated through actions on the central nervous system. MS, Leu-enk, B-endor, and Dyn given icv, produced elevations of plasma norepinephrine (NE) and epinephrine (Epi) concentrations. When MS was given icv, mean Epi and NE plasma levels increased 10-50 times the increases noted with B-endor, Leu-enk and Dyn. A role of catecholamines in mediating MS-stimulated ANP release is supported by the observation that ganglionic blockade with chlorisondamine significantly attenuated the increase of plasma ANP levels. MS, but not B-endor, Leu-enk and Dyn, acts within the brain to increase plasma levels of ANP. MS-induced elevations of plasma ANP levels may be dependent on an intact autonomic nervous system.     

Dissociation of cold-water swin and morphine analgesia in Brattleboro rats with diabetes insipidus

The present study examined whether vasopressin mediated the analgesic response to morphine and cold-water stress by comparing the analgesic responses of homozygous Brattleboro rats with diabetes insipidus with those of normal rats of the same strain of similar weight. Brattleboro rats exhibited hypersensitive responses to foot shock which were brought back to within normal limits by systemic administration of arginine vasopressin and desamino-D-arginine vasopressin. While Brattleboro rats failed to display an analgesic response to cold-water swim stress, they displayed a normal analgesic response to morphine. These results provide further evidence for dissociation of pain-inhibitory mechanisms into opioid and non-opioid components, and suggests that vasopressin might be involved in the elucidation of the latter component.     

Differential effects of exercise on brain opioid receptor binding and activation in rats

Physical exercise stimulates the release of endogenous opioid peptides supposed to be responsible for changes in mood, anxiety, and performance. Exercise alters sensitivity to these effects that modify the efficacy at the opioid receptor. Although there is evidence that relates exercise to neuropeptide expression in the brain, the effects of exercise on opioid receptor binding and signal transduction mechanisms downstream of these receptors have not been explored. Here, we characterized the binding and G protein activation of mu opioid receptor, kappa opioid receptor or delta opioid receptor in several brain regions following acute (7 days) and chronic (30 days) exercise. As regards short‐ (acute) or long‐term effects (chronic) of exercise, overall, higher opioid receptor binding was observed in acute‐exercise animals and the opposite was found in the chronic‐exercise animals. The binding of [³⁵S]GTPγS under basal conditions (absence of agonists) was elevated in sensorimotor cortex and hippocampus, an effect more evident after chronic exercise. Divergence of findings was observed for mu opioid receptor, kappa opioid receptor, and delta opioid receptor receptor activation in our study. Our results support existing evidence of opioid receptor binding and G protein activation occurring differentially in brain regions in response to diverse exercise stimuli.

     

GABA in morphine analgesia and tolerance

Drugs affecting various steps of GABA transmission exhibit analgesia in a variety of experimental models in animals; this analgesic response generally requires high doses of the drugs and does not appear to be opiate-like since the GABAergic analgesia is naloxone-insensitive and lacks dependence liability. The outcome of the analgesia response is variable when opiate and GABAergic drugs are administered together; however, directly acting GABA receptor stimulants and GABA-transaminase inhibitors generally enhance the analgesic effect of opiates. The development of newer GABAergic drugs with greater potency and specificity may offer an alternative to opiate analgesics. The results obtained over the years, on the possible involvement of the GABA system in morphine tolerance and dependence are equivocal. Studies on region-specific changes in opiate-GABA interaction as well as opiate-GABA-benzodiazepine interaction are needed to further elucidate the role of GABA on opiate system.     

Acupuncture analgesia in rats and its changes under the effect of morphine and naloxone

It was established in chronic experiments on rats that electric acupuncture of the acupuncture point noticeably decreases pain reaction to electric stimulation of the tail. Morphine given in a subanalgetic dose (5 mg/kg) potentiated acupuncture analgesia, while 5 mg/kg of naloxone completely abolished it. Potential mechanisms of analgesia realization during electric acupuncture are discussed.     

3-Carboxamido analogues of morphine and naltrexone. synthesis and opioid receptor binding properties.

In response to the unexpectedly high affinity for opioid receptors observed in a novel series of cyclazocine analogues where the prototypic 8-OH was replaced by a carboxamido group, we have prepared the corresponding 3-CONH(2) analogues of morphine and naltrexone. High affinity (K(i)=34 and 1.7nM) for mu opioid receptors was seen, however, the new targets were 39- and 11-fold less potent than morphine and naltrexone, respectively.