Possible involvement of the µ opioid receptor in the antinociception induced by sinomenine on formalin-induced nociceptive behavior in mice.

Sinomenine, an alkaloid originally isolated from the roots and the rhizome of Sinomenium acutum is used as a traditional Chinese herbal medicines for rheumatoid arthritis and neuralgia. The aims of this study were to investigate the effects of oral administration of shinomenine on formalin-induced nociceptive behavior in mice and the opioid receptor subtypes involved in the antinociceptive effects of sinomenine. Our findings showed that a single dose of oral-administrated sinomenine inhibited the formalin induced licking and biting responses in a dose-dependent manner. Intraperitoneal pretreatment with naloxone hydrochloride, an opioid receptor antagonist, and ß-funaltrexamine hydrochloride (ß-FNA), a selective µ-opioid receptor antagonist, significantly attenuated sinomenine induced antinociception, but not by naltrindole, a nonselective δ-opioid receptor antagonist and nor-binaltorphimine, a selective κ-opioid receptor antagonist. Furthermore, in western blot analysis, oral administration of sinomenine resulted in a significant blockage of spinal extracellular signal-regulated protein kinase (ERK1/2) activation induced by formalin. Naloxone hydrochloride and ß-FNA significantly reversed the blockage of spinal ERK1/2 activation induced by sinomenine. These results suggest that sinomenine-induced anti nociceptive effect and blockage of spinal ERK1/2 activation may be triggered by activation of µ-opioid receptors.

A delta opioid receptor agonist, KNT-127, in the prelimbic medial prefrontal cortex attenuates glial glutamate transporter blocker-induced anxiety-like behavior in mice.

We previously reported that systemic administration of a delta opioid receptor (DOP) agonist, KNT-127, produced a potent anxiolytic-like effect in rats. Interestingly, DOPs are highly distributed in the prelimbic medial prefrontal cortex (PL-PFC). In the present study, we investigated the effect of KNT-127 co-perfusion in the PL-PFC on anxiety-like behavior in mice, induced by a glial glutamate transporter inhibitor, (3S)-3-[[3-[[4-(Trifluoromethyl)benzoyl]amino]phenyl]methoxy]-l-aspartic acid (TFB-TBOA). Extracellular glutamate levels were measured in male C57BL/6N mice by in vivo microdialysis high-performance liquid chromatography/electrochemical detection, with behavior simultaneously assessed in the open field test. As expected, extracellular glutamate levels were significantly increased, and anxiety-like behavior was induced after local perfusion of TFB-TBOA in the PL-PFC. Uniquely, co-perfusion of KNT-127 in the PL-PFC diminished anxiety-like behavior induced by TFB-TBOA without affecting extracellular glutamate levels. Further, the effect of KNT-127 on anxiety-like behavior was antagonized by a selective DOP antagonist, naltrindole, suggesting that KNT-127 acts via DOPs. These findings do not support our preconceived hypothesis that KNT-127 in PL-PFC produces an anxiolytic-like effect via suppression of glutamatergic transmission. Hence, further studies are necessary to understand the mechanisms of DOP agonist-induced anxiolytic-like effects in the PL-PFC.

Comparison of the opioid receptor antagonist properties of naltrexone and 6 beta-naltrexol in morphine-naïve and morphine-dependent mice.

It has been proposed that on chronic morphine treatment the micro-opioid receptor becomes constitutively active, and as a consequence, the opioid withdrawal response arises from a reduction in the level of this constitutively active receptor. In support of this, the putative micro-opioid receptor inverse agonist naltrexone has been shown to precipitate more severe withdrawal behavior in mice than the putative neutral receptor antagonist 6 beta-naltrexol. In the present study naltrexone and 6 beta-naltrexol were compared in NIH Swiss mice to test the hypothesis that their differential ability to precipitate withdrawal is due to differences in their in vivo opioid receptor antagonist potencies caused by differential access to micro-opioid receptors in the central nervous system and not necessarily by intrinsic differences in their opioid receptor activity. In naïve mice both compounds had similar potencies to antagonize morphine-induced antinociception in the hot plate and warm-water tail-withdrawal assays when measured under equilibrium conditions and afforded similar calculated apparent in vivo micro-opioid receptor affinities. In morphine-dependent mice both compounds precipitated withdrawal jumping but naltrexone was between 10- and 100-fold more potent than 6 beta-naltrexol. A similar potency difference was seen for other withdrawal behaviors. Both naltrexone and 6 beta-naltrexol at 1 mg/kg reversed antinociception induced by the long-lasting micro-opioid receptor agonist BU72 in the warm-water tail-withdrawal assay, but antagonism by naltrexone was 6-fold more rapid in onset at equal doses. Since the compounds have similar affinity for the micro-opioid receptor in vivo, the results suggest that the differences observed between the ability of naltrexone and 6 beta-naltrexol to precipitate withdrawal in the mouse may be explained by differential onset of receptor antagonist action.

Quantitative contribution of CYP2D6 and CYP3A to oxycodone metabolism in human liver and intestinal microsomes.

Oxycodone undergoes N-demethylation to noroxycodone and O-demethylation to oxymorphone. The cytochrome P450 (P450) isoforms capable of mediating the oxidation of oxycodone to oxymorphone and noroxycodone were identified using a panel of recombinant human P450s. CYP3A4 and CYP3A5 displayed the highest activity for oxycodone N-demethylation; intrinsic clearance for CYP3A5 was slightly higher than that for CYP3A4. CYP2D6 had the highest activity for O-demethylation. Multienzyme, Michaelis-Menten kinetics were observed for both oxidative reactions in microsomes prepared from five human livers. Inhibition with ketoconazole showed that CYP3A is the high affinity enzyme for oxycodone N-demethylation; ketoconazole inhibited >90% of noroxycodone formation at low substrate concentrations. CYP3A-mediated noroxycodone formation exhibited a mean K(m) of 600 +/- 119 microM and a V(max) that ranged from 716 to 14523 pmol/mg/min. Contribution from the low affinity enzyme(s) did not exceed 8% of total intrinsic clearance for N-demethylation. Quinidine inhibition showed that CYP2D6 is the high affinity enzyme for O-demethylation with a mean K(m) of 130 +/- 33 microM and a V(max) that ranged from 89 to 356 pmol/mg/min. Activity of the low affinity enzyme(s) accounted for 10 to 26% of total intrinsic clearance for O-demethylation. On average, the total intrinsic clearance for noroxycodone formation was 8 times greater than that for oxymorphone formation across the five liver microsomal preparations (10.5 microl/min/mg versus 1.5 microl/min/mg). Experiments with human intestinal mucosal microsomes indicated lower N-demethylation activity (20-50%) compared with liver microsomes and negligible O-demethylation activity, which predict a minimal contribution of intestinal mucosa in the first-pass oxidative metabolism of oxycodone.

The antinociceptive potencies of oxycodone, noroxycodone and morphine after intracerebroventricular administration to rats.

The antinociceptive activity of noroxycodone (NOR) was determined and its potency compared with that of oxycodone (OXY) and morphine (MOR) after intracerebroventricular (icv) administration to Sprague-Dawley rats. The antinociceptive potencies of OXY and NOR relative to MOR were 0.44 and 0.17, respectively. Administration of naloxone (55 nmol icv) abolished the antinociceptive response produced by the subsequent administration of OXY (227 nmol icv), indicating that the antinociceptive effects of OXY are mediated by opioid receptors. In addition, the same dose of naloxone (55 nmol icv) markedly reduced the antinociceptive effects of NOR (332 nmol icv) and MOR (93 nmol icv) indicating the involvement of opioid receptors in the antinociceptive activities of NOR and MOR. NOR, administered by the icv route, also produced excitatory effects throughout the antinociceptive dose range, the severity of which was reduced but not abolished by prior administration of naloxone (55 nmol icv). As excitatory effects have not been observed in patients receiving OXY, it is unlikely that NOR contributes to the analgesic activity of OXY administered systemically to humans.

Discriminative stimulus effects of cyclorphan: selective antagonism with naltrexone.

The opioid antagonist, naltrexone, was used to identify some of the receptor mechanisms responsible for the discriminative stimulus effects of cyclorphan in the pigeon. Subjects were trained to discriminate 10 mg/kg IM injections of either morphine or dextrorphan from saline injections in a two key drug discrimination procedure in which responding was maintained by food presentation. The dextrorphan-trained birds generalized to l-cyclorphan at 10 mg/kg; naltrexone did not alter the l-cyclorphan dose-response curve for this effect. In the morphine-trained group, l-cyclorphan produced only partial generalization, and naltrexone greatly increased the dose of l-cyclorphan necessary to produce this effect. These results are consistent with the conclusion that in morphine-trained pigeons the partial generalization to l-cyclorphan is mediated by opioid receptors. Moreover, limited intrinsic efficacy at mu opioid receptors may be the characteristic of l-cyclorphan that prevents full generalization in morphine-trained pigeons. d-Cyclorphan produced partial generalization in both groups, but the involvement of opioid receptor mechanisms could not be confirmed, as 1 mg/kg naltrexone did not antagonize d-cyclorphan in either group.

Immobilization of white-tailed deer by etorphine and xylazine and its antagonism by nalmefene and yohimbine.

White-tailed deer (Odocoileus virginianus) were immobilized with either 4.0 mg etorphine hydrochloride (ETOR) or 3.5 mg ETOR and 50.0 mg xylazine (XYL). Deer immobilized with ETOR only were given 4.0 mg nalmefene hydrochloride (NAL), a new opioid antagonist, 20 min after induction. Deer immobilized with ETOR and XYL received 3.5 mg NAL and 0.125 mg/kg yohimbine hydrochloride (YOH). The dose of 4.0 mg ETOR did not provide acceptable immobilization and was discontinued. A NAL:ETOR ratio of 1:1 was insufficient for complete and sustained antagonism of ETOR. Subsequently, deer were immobilized with ETOR and XYL as before which was then antagonized with 35.0 mg NAL and 0.125 mg/kg YOH. The 10:1 ratio of NAL:ETOR appeared to provide complete antagonism with no evidence of renarcotization. Although more study is required, NAL could become a useful antagonist for opioid-induced immobilizations.

Beta-endorphin-(1-27) is a naturally occurring antagonist to etorphine-induced analgesia.

The potent opioid peptide beta-endorphin is found in the brain and pituitary with two related fragments, beta-endorphin-(1-27) and beta-endorphin-(1-26). The fragments retain substantial opioid-receptor binding activity but are virtually inactive analgesically. beta-Endorphin-(1-27) inhibits beta-endorphin-induced and etorphine-induced analgesia when coinjected intracerebroventricularly into mice. Antagonism by competition at the same site(s) is suggested from parallel shifts of the dose-response curves of etorphine or beta-endorphin in the presence of beta-endorphin-(1-27). Its potency is 4-5 times greater than that of the opiate antagonist naloxone. beta-Endorphin-(1-26) does not antagonize the antinociceptive action of etorphine or beta-endorphin in doses up to 500 pmol per animal.

An etorphine-evoked vagal reflex in rats is inhibited by naloxone, N-methylnaloxone, and SMS 201-995.

Intravenous injection of opiates in rats is known to produce a rapid fall in heart rate, blood pressure, and respiratory frequency. Etorphine, a potent opiate agonist which reaches central and peripheral receptors, administered at 1.1 nmol/kg i.v., evoked these characteristic effects in the urethane-anaesthetized rat. Three opiate antagonists, with somewhat different properties, have been used to assess the site of action of etorphine-induced bradycardia, hypotension, and inhibition of respiration. The antagonists used were naloxone hydrochloride, N-methylnaloxone bromide, and SMS 201-995. Low doses of naloxone blocked the cardiovascular and respiratory effects of etorphine. N-Methylnaloxone blocked the bradycardia, hypotension and the initial phase of apnea produced by etorphine but not the subsequent slowing of respiration. SMS 201-995 blocked the bradycardia and partially antagonized the hypotension and inhibition of respiratory rate produced by etorphine. These results indicate that N-methylnaloxone and SMS 201-995 can block the peripheral receptors which mediate opiate-induced bradycardia. Naloxone blocks both the central and peripheral actions of etorphine and so more completely antagonizes the cardiovascular and respiratory effects of etorphine.

Use of etorphine-acepromazine and diprenorphine in reversible neuroleptanalgesia of rats.

The death rate due to neuroleptanalgesia (0.35%) was significantly lower than for barbiturate anaesthesia (1.59%). Complications were few even when animals received multiple, repeated, anaesthesia, and depth of anaesthesia could be adjusted easily. Further, it could be reversed within minutes by the specific antagonist diprenorphine.

Studies on the mechanism of post-etorphine catalepsy. Modyfying effect of amphetamine, apomorphine and dihydroxy-phenylalanine (L-dopa) on etorphine-induced concentrations of dopamine and noradrenaline in the rat central nervous system.

Studies on the mechanism of post-etorphine catalepsy. Modyfying of amphetamine, apomorphine and dihydroxyphenyl-alanine (L-DOPA) on etorphine-induced concentrations of dopamine and noradrenaline in the rat central nervous system. Acta Physiol. Pol., 1979, 30 (2): 279--287. During stereotypy induced with amphetamine, apomorphine and 1-dihydroxyphenylalanine (L-DOPA) increased concentrations of dopamine (DA) and noradrenaline (NA) were found in the motor centres of the central nervous system (CNS). In post-etorphine catalepsy the concentrations of DA and NA were also increased in the frontal cortex, striopallidum, pons and cerebellum and in the lumbosacral spinal cord. However, these stereotypy-inducing agents used in premedication of post-etorphine catalepsy delayed significantly its onset and reduced its duration.

Clinical observations on the antagonism of immobilon by naloxone in dogs.

The effectiveness of naloxone hydrochloride in reversing Immobilon anaesthesia was evaluated in 14 dogs. Although a dose rate of 0.02 mg per kg body-weight briefly reversed the respiratory and cardiovascular depression, a dose of at least 0.6 mg per kg body-weight was required before full recovery of consciousness occurred. The action of naloxone was found to be relatively short lived and relapse tended to occur after 10 to 15 minutes. The implications of these findings are discussed in relation to the use of naloxone in the event of accidental self-administration of Immobilon in man.

On the sites of antitussive action of d-3-methyl-N-methylmorphinan (AT-17).

The sites of antitussive action of d-3-methyl-N-methylmorphinan (AT-17) were studied. It was assumed from the following results that AT-17 acts on the cough centre per se. a) When AT-17 was given by the routes leading to the brain stem such as the cerebello-medullar cistern, far smaller doses were sufficient to obtain the same effect as that by i.v. administration. b) It showed neither effect on the afferent pathway for cough reflex nor influence on pulmonary stretch receptors. c) Decerebration exerted no influence on the antitussive efficacy. d) It definitely depressed the potentials of both the recurrent and internal intercostal nerves evoked by the superior laryngeal nerve stimulation. e) Micro-injection of AT-17 into the bilateral solitary nuclei or their adjacent regions inhibited coughs induced by peripheral stimulation. f) In deafferentated and decerebrate cats, AT-17 increased the spontaneous discharges of the phrenic nerve, whereas codeine decreased them.