Quantitative studies on the antagonism by naloxone of some narcotic and narcotic-antagonist analgesics

Sex differences in (-)-pentazocine antinociception: comparison to morphine and spiradoline in four rat strains using a thermal nociceptive assay

The present study examined the influence of sex on the antinociceptive effects of (-)-pentazocine, morphine and spiradoline in four rat strains, using a warm-water (50, 52 and 55 degrees C) tail-withdrawal procedure. In F344, Lewis, Sprague-Dawley (SD) and Wistar rats, baseline latencies decreased with increases in water temperature, and at each water temperature latencies were longer in males than in their female counterparts. Morphine and spiradoline produced maximal or near maximal antinociceptive effects in males and females of each strain. Whereas morphine was generally more potent in males, sex differences were not consistently observed with spiradoline. In contrast, there were marked sex differences with (-)-pentazocine, and in each strain (-)-pentazocine was more potent and produced a greater maximal effect in males. The magnitude of the sex differences varied markedly across strains, with (-)-pentazocine being 2.5-fold more potent in males of the F344 strain, but 11-fold more potent in males of the Wistar strain. When collapsed across nociceptive stimulus intensities, sex differences were largest in the Wistar and Lewis strains and smallest in the SD and F344 strains. The present findings indicate that there are marked sex differences in (-)-pentazocine antinociception, and that the magnitude of this effect is genotype dependent.

On the antinociceptive effect of fluoxetine, a selective serotonin reuptake inhibitor

Antidepressant drugs are reported to be used as co-analgesics in clinical management of migraine and neuropathic pain. The mechanism through which they alleviate pain remains unknown. The present study explores the possible mechanism of a selective serotonin reuptake inhibitor (SSRI) fluoxetine-induced antinociception in animals. Acetic acid-induced writhing, hot plate and tail-flick test were used to assess fluoxetine-induced antinociception. Fluoxetine (5-20 mg kg(-1), i.p.) produced a significant and dose-dependent antinociceptive effect against acetic acid-induced writhing in mice. Fluoxetine (20 mg kg(-1)) also exhibited antinociceptive effect in tail flick as well as hot plate assays. Further, i.c.v. administration of fluoxetine showed significant antinociception against writhing test in rats. However, fluoxetine (1 microg/10 microl/rat, i.c.v.) did not exhibit any antinociceptive effect in serotonin-depleted animals. Further, pindolol (10 mg kg(-1), i.p.) enhanced fluoxetine-induced antinociceptive effect. The antinociceptive effect of fluoxetine was sensitive to blockade by naloxone (5 mg kg(-1), i.p.) and naltrexone (5 mg kg(-1), i.p.). These data suggest that fluoxetine-induced antinociception involves both central opioid and the serotoninergic pathways.

Antinociceptive activity of the novel fentanyl analogue iso-carfentanil in rats

A large number of fentanyl analogues have been synthesized so far, both to establish the structure-activity-relationship (SAR) and to find novel, clinically useful antinociceptive drugs. In this study, the newly synthesized fentanyl analogue 3-carbomethoxy fentanyl (iso-carfentanil) was compared to fentanyl for its antinociceptive activity (tail-immersion test) in rats. It was revealed that the introduction of a 3-carbomethoxy group in the piperidine ring of fentanyl skeleton reduced the potency and shortened the duration of action of the parent compound, i.e., fentanyl. The antinociceptive potency of 3-carbomethoxy fentanyl is influenced mainly by the steric factor (voluminosity of the carbomethoxy group and the cis/trans isomerism), while the chemical nature of the group is probably irrelevant. This is in agreement with SAR studies of other 3-substituted fentanyl analogues. In contrast to potency, the duration of action is not affected by cis/trans isomerism. It is assumed that the time course of action of 3-carbomethoxy fentanyl is influenced by the nature of the carbomethoxy group. Since the potency and the duration of action of this novel antinociceptive compound are interesting from the aspect of SAR studies and have potential promise for clinical application, 3-carbomethoxy fentanyl deserves to be extensively evaluated.

Strong antinociceptive effect of incarvillateine, a novel monoterpene alkaloid from Incarvillea sinensis

Incarvillea sinensis is a wild plant distributed in northern China. The dried whole plant has been traditionally used to treat rheumatism and relieve pain as an ancient Chinese crude drug. To investigate its antinociceptive activity, we evaluated several fractions derived from the methanolic extract of Incarvillea sinensis in the formalin-induced pain model in mice. Incarvillateine, a novel monoterpene alkaloid, has been found to show significant antinociceptive activity. Here we report the antinociceptive activity of incarvillateine and compare its activity with that of morphine. Additionally, we suggest that its action may be related to influence on the central opioid pathways.

Interaction of opioids with antidepressant-induced antinociception

The antinociceptive activity of antidepressant drugs is poorly understood. In this study, using the acetic acid writhing test in mice, the antinociception produced by clomipramine (CLO), maprotiline (MAP), imipramine (IMI), and zimelidine (ZIM) was tested and correlated with opioid drugs. All the compounds displayed a significant dose-dependent antinociception, which was not antagonized by naloxone (NX) or naltrexone (NTX). The administration of morphine (M) plus CLO, MAP, IMI or ZIM resulted in a significant additive effect that was antagonized by 1 or 10 mg/kg NX or NTX, except in the case of IMI. This finding suggests that the additive effect seems to be partially due to activation of opioid receptors, except for the case of imipramine. However, aminophylline, a non-selective blocker of A1/A2 adenosine receptors, significantly antagonized the antinociceptive activity of CLO, IMI, MAP and ZIM, demonstrating an interaction at the level of adenosine receptors. This work suggests that the antinociceptive activity of antidepressants could be dependent on critical levels of free 5-HT and NE at receptor(s) site(s) in CNS and on their interaction with opioid and adenosine receptors.

Effects of NG-nitro-L-arginine on isolated rabbit afferent arterioles

We examined the effects of NG-nitro-L-arginine (L-NNA) on isolated rabbit afferent arterioles to confirm that nitric oxide is released at the resistance vessel level in the kidney. We microdissected the superficial afferent arterioles from the kidneys of New Zealand White rabbits. Each afferent arteriole was cannulated with a micropipette system, and the intraluminal pressure was set at 80 mmHg. By our methods, we found that norepinephrine (NE) decreased the lumen diameter of the afferent arterioles in a dose-dependent manner, and acetylcholine increased the lumen diameter of NE-constricted afferent arterioles. L-NNA (10(-4) M) gradually decreased the lumen diameter of afferent arterioles from 21.5 +/- 0.9 to 18.6 +/- 0.9 microns in 20 min, but NG-nitro-D-arginine (10(-4) M) did not affect them (from 21.8 +/- 1.3 to 21.8 +/- 1.5 microns). L-Arginine (10(-2) M) restored the lumen diameter of L-NNA-contracted afferent arterioles to the control levels. These findings indicate that the isolated afferent arteriole has the ability to release or to synthesize and release nitric oxide under basal conditions and that this basal release of nitric oxide plays an important role in the basal tone of the afferent arteriole.

[Synthesis of enantiomerically pure 6,10-epoxybenzocycloocten-7-amines with CNS activity]

In an oxa-Pictet-Spengler reaction the methyl (S)-phenyllactate 6 and methyl levulinate (7a) are condensed to the 2-benzopyrans cis-8a and trans-8a, which react with CH3I to yield the dimethyl ethers cis-9a and trans-9a. Cis-9a and trans-9a can be separated by medium pressure liquid chromatography. In the subsequent Dieckmann-Cyclisation cis-9a is transformed to the laevorotatory beta-ketoester (-)-10a, while the dextrorotatory enantiomer (+)-10a is obtained from trans-9a after C-3-epimerisation. With Eu(hfc)3 the ketone (-)-11, prepared by saponification and decarboxylation of (-)-10a, proves to be enantiomerically pure. By reductive amination, ketone (-)-11 is transformed to the amines (-)-12a and (-)-12b. Symptoms typical for central damping are caused after i.p. application of (-)-12a and (-)-12b to mice. In the mouse writhing-test (-)-12a HCl affords an ED50-value of 7.0 mg/kg, comparable with the ED50-value of tramadol.

Maintenance of acute morphine tolerance in mice by selective blockage of kappa opioid receptors with norbinaltorphimine

In this study we investigated the effect of the highly selective kappa opioid antagonist, norbinaltorphimine (norBNI) on the development of tolerance to a single dose of morphine. Mice were pretreated with 100 mg/kg of morphine sulfate (morphine), s.c. and 2 h later, norBNI (20 mg/kg s.c.) was administered and various times after this pretreatment, antinociceptive ED50 value of morphine was determined in the tail-flick assay. Twenty-four and 72 h after morphine injection, ED50 values of morphine were significantly increased by about 2.5-fold from those of their control mice that received saline instead of the tolerance-inducing dose of morphine. In a second set of experiments, animals were pretreated similarly with morphine and norBNI and 72 h after morphine injection, various opioid agonists were applied by the i.c.v. or i.t. route to see whether or not any cross-tolerance had developed to these agonists. The ED50 of i.c.v.-administered morphine was significantly greater than that of the non-pretreated controls. A small degree of cross-tolerance was observed with U-50,488H but not with DPDPE [D-Pen2,D-Pen5]enkephalin (DPDPE) at the supraspinal site. At the spinal site, tolerance to morphine was not observed. These results suggest that antagonism at kappa opioid sites after morphine administration, modulates positively the development of opioid tolerance.

Beta-funaltrexamine (beta-FNA) decreases deprivation and opioid-induced feeding

We studied the effect of the mu antagonist, beta-funaltrexamine (beta-FNA) on deprivation and opioid-induced feeding. Intracerebroventricular pre-treatment of 20 h deprived rats with 0.1, 1, 10 and 20 nmol of beta-FNA decreased feeding by 24%, 50%, 50% and 38% during the first hour. Central administration of beta-FNA (0.1, 1 and 10 nmol) also decreased feeding induced by the mu opioid agonist, DAMGO by 57%, 60% and 71%. Feeding induced by the delta agonist, DSLET, was decreased by pre-treatment with beta-FNA; but only during the 1-2 h time points, a time when relatively little food was ingested. Intraventricular injection of beta-FNA failed to alter feeding stimulated by the kappa opioid agonist, U-50,488H. These data further substantiate a role for the opioid receptor in deprivation and opioid-induced feeding.

Pharmacological interaction of opiates with various classes of centrally acting dopaminergic drugs

The comparative analgesic and sedative (narcosis potentiating) efficacy of mu and kappa opioids was studied as a function of time in rats and mice. The mu agonists, morphine and fentanyl, produced antinociceptive actions against both heat and chemical noxious agents, but the half-lives of their ED50s were longer in the writhing than in the hot plate test. The kappa agonist drugs, bremazocine, ethylketocyclazocine and pentazocine, proved to be inactive against heat nociception, and produced a potent, long-lasting analgesia in the acetic acid writhing test, similar to mu agonists. The combination of two mu agonists resulted in a synergistic interaction and a remarkable prolongation of antinociceptive action. When the kappa-drug bremazocine was coadministered with morphine, there was a significant prolongation of the duration of analgesic action, without any influence on the potency. The interactions of mu and kappa opioids with agonists and antagonists at dopamine receptors were also studied in narcosis. The time course of the naloxone-morphine antagonism in analgesiometric assays revealed similarities, when apparent pA2 values were estimated at the peak of agonist and antagonist activity, but it was different in the writhing test when the pA2 was determined 84 minutes after morphine administration (EDt1/2, half-life belonging to the ED50) while naloxone was given at its peak effect.

Analgesic effects of 3-methoxybenzamide in rats

The i.p. injection of 3-methoxybenzamide (3-MBA) in rats produces a dose-related elevation of the threshold for response to a painful stimulus. Metoclopramide, also a substituted benzamide, has analgesic activity that is attenuated by bromocriptine, a dopamine receptor agonist, and by the narcotic antagonist, naloxone, suggesting involvement of dopamine and opiate receptors in the action of this drug. The involvement of these receptors in the analgesic action of 3-MBA has been examined using L-dopa and naloxone. Neither significantly altered the analgesic action. Although the results are preliminary, the analgesic action of 3-MBA would not seem to occur via opiate or dopamine receptors.

Differential sensitivity of antinociceptive tests to opioid agonists and partial agonists

1. The antinociceptive activity of a range of opioid agonists and agonist-antagonist analgesics was determined in mice by use of the 55 degrees C hot plate and abdominal constriction assays. 2. Opioid agonists were approximately 10 times more effective in the abdominal constriction assay. 3. The agonist-antagonists produced analgesia only in the abdominal constriction assay, and antagonized the antinociceptive action of opioid agonists in the 55 degrees C hot plate test. 4. These differences were shown to be attributable to the different levels of stimulus employed in the two tests. 5. By comparing the antagonist potencies of the agonist-antagonists in the 55 degrees C hot plate test with their antinociceptive ED50 values in the abdominal constriction assay, an index of intrinsic activity was calculated.

Effect of morphine and naloxone on a defensive response of the crab Chasmagnathus granulatus

Male crabs (Chasmagnathus granulatus) exhibited a defensive response (DR) to an electric shock (8 V, 50 Hz, 1 sec). The DR so elicited was used as a model for studying the antinociceptive effect of morphine. Injections of morphine-HCl (MP) (25, 50, 100 and 150 micrograms/g) were administered and the DR was examined at 2, 7.5, 15, 30, 45 and 75 min post-injection. (a) MP produced a dose-dependent reduction of the crab's sensitivity to the nociceptive stimulus. (b) A 100 micrograms/g dose of MP caused a 50% response inhibition with an injection-shock interval of 30 minutes, but no inhibition occurred when the same dose was administered with 1.6 micrograms/g of naloxone-HCl, suggesting that MP acts through an opiate receptor. (c) The ED50 at 2 min post-injection was roughly 33 micrograms/g and the threshold dose was estimated to be 6.8 micrograms/g. These doses are lower than ED50 values reported for other arthropods (90 to 930 micrograms/g) and approach those of vertebrates. (d) The peak MP effect was reached quickly, within 2 min post-injection. The duration of the MP effect was calculated to be 45.0-75.0 min depending on the dose, and an indirect estimate of half-life elimination was 15.7 min. These values are remarkably lower than those reported for vertebrates. The shorter duration of the MP peak effect is attributable to a greater permeability of the arthropod blood-brain barrier as compared to that of vertebrates.

The significance of multiple CNS opioid receptor types: a review of critical considerations relating to technical details and anatomy in the study of central opioid actions

The study of the CNS actions of opioids is complicated by the presence of both multiple opioid receptors and endogenous ligands in the brain. The recent descriptions of opioid isoreceptors, of tonic opioid systems, and of multiple opioid receptors on a single neuron are further technical details which must be considered. In the use of various opiates and opioid peptides to study physiological systems, the multiple opioid affinities of these compounds, as well as potential non-opioid actions, must be controlled for in the experimental design. In conjunction with the multiple receptor affinities of various opiates is the problem of receptor dualism with some drugs; particularly with the agonist/antagonist analgesics. Species differences in the relative proportions of different opioid receptor populations also limit any generalizations of a finding in one species. These limitations in the study of opioid receptors will be discussed with reference to previous neurochemical, neuroendocrine, electrophysiological and behavioral reports of multiple opioid receptors.

Heterogeneous distribution and upregulation of mu, delta and kappa opioid receptors in the amygdala

Levels of mu, delta and kappa opioid receptors in 4 subnuclei of the rat amygdala were determined by quantitative autoradiography following chronic treatment with naloxone or saline. A different distribution of each receptor subtype was observed, with mu binding greatest in the lateral nucleus (La), delta greatest in the basolateral (Bl), and kappa greatest in the medial (Me). Levels of all 3 receptors were very low in the central nucleus. Receptor upregulation following chronic naloxone treatment was also anatomically heterogeneous. Increases in mu receptors were statistically significant in the Me, Bl and La, while increases in delta and kappa receptors were significant only in the Bl.

A radiolabeled-ligand-binding technique for the characterization of opioid receptors in the intact mouse vas deferens

The mouse vas deferens has served as a useful bioassay for examining the properties of opiate receptor subtypes. However, recent data indicate that the response of the vas deferens to opiates may be mediated by one or more of the several opiate receptors found in this preparation. Although a number of techniques can be utilized to assess the relative contribution of these receptors to the response of the mouse vas deferens to opiates (e.g., selective tolerance and naloxone antagonism studies), a radiolabeled-binding technique would provide an independent means of more completely characterizing the opiate receptor profiles in this preparation. Up to the present, however, there has been only limited success in developing a binding assay utilizing crude membrane fractions of the mouse vas deferens. To circumvent these problems, we have developed a binding technique utilizing the intact vas deferens. In contrast to results obtained with membrane fractions, we found highly specific (90-95%) and saturable binding of D-[2-3H]alanine, 5-D-leucine enkephalin, a ligand selective for delta opiate receptors, to the intact vas. Scatchard analyses indicated a single class of binding sites with an apparent Kd of 1.5 nM and a Bmax of approximately 12 pmol/2 vas. The selectivity of binding was also examined. Naltrexone was 40 times less potent than unlabeled 2-D-alanine, 5-D-leucine enkephalin in displacing binding, whereas morphine and ethylketocyclazocine were 300 and 500 times less effective, respectively. This technique, coupled with the mouse vas deferens bioassay, should provide a more complete characterization of opioid receptor populations than has heretofore been possible.

Restraint stress enhances morphine-induced analgesia in the rat without changing apparent affinity of receptor

Antagonism of morphine analgesia (tail-flick assay) by naloxone was assessed quantitatively by in vivo "apparent" pA2 determination in unstressed rats and in rats subjected to restraint stress. Restrained rats had a higher baseline tail-flick latency than did unstressed (unrestrained) animals, and were more sensitive to the analgesic effect of morphine, as reflected in lower morphine ED50s. There was no significant difference between apparent pA2 values of unstressed and restrained rats using pA2 regression line analysis. This suggests that while stress enhances the analgesic effect of morphine, it does not appreciably alter opiate receptor affinity for naloxone under the conditions of this study.

One- and two-receptor models of opioid drug action

The history of models for opioid drug receptors is traced and two currently proposed models are compared, the first envisaging two rigid receptors, one for agonists and one for antagonists, while the second model postulates one single flexible receptor which in its native conformation fits opioid antagonists but is distorted when it binds to agonists. Important recent experiments are satisfactorily interpreted by both models but the flexible one-receptor model requires fewer assumptions and explains more facts.

Mu and delta, but not kappa, opioid agonists induce contractions of the canine small intestine in vivo

Extraluminal strain gage transducers were sutured along the transverse axis of the duodenum in order to monitor circular muscle contractile activity in the pentobarbital anesthetized dog. Administration by intravenous bolus of a variety of mu- and delta-directed opioid ligands resulted in a dose-dependent increase in duodenal contractile activity. In contrast, all kappa-directed ligands were devoid of stimulatory activity. Naloxone reversed the effects of normorphine and [Met5]enkephalin but was 20 times more effective against normorphine than [Met5]enkephalin. Based on the inactivity of all kappa ligands examined and the differential potency of naloxone against [Met5]enkephalin and normorphine, we suggest that this model may be useful in the classification of opioid ligands as to their receptor selectivity in vivo. Further, these data indicate that the stimulation of duodenal contractile activity is not mediated by enteric kappa receptors.

Picenadol

Picenadol is a unique opioid mixed agonist-antagonist analgesic currently under clinical evaluation. Structurally, picenadol is a 4-phenylpiperidine derivative and a racemic mixture whose mixed agonist-antagonist properties are a consequence of the d-isomer being a potent opiate agonist, whereas the l-isomer is an opioid antagonist. In the mouse writhing and rat tail heat tests, the analgesic potency of picenadol is estimated to be 1/3 that of morphine. Picenadol itself has weak antagonist activity, whereas the antagonist potency of the l-isomer is approx. 1/10 that of nalorphine. Evaluation of picenadol's affinity for opioid receptors reveals that picenadol, unlike other mixed agonist-antagonists has high affinity for both the mu and delta receptors but a markedly lower affinity for the kappa receptor. Extensive pharmacological investigations show picenadol to have a low potential to produce opiate-like side effects, including a low liability for abuse and physical dependence.

Quantitative analysis of the interaction between the agonist and antagonist isomers of picenadol (LY150720) on electric shock titration in the squirrel monkey

The opioid mixed agonist-antagonist picenadol (LY150720) is a racemate whose resolution results in a highly stereospecific separation of opioid agonist and antagonist activity. Attenuation of the antinociceptive effects of the agonist (dextro) isomer LY136596 by the antagonist (levo) isomer LY136595 was evaluated quantitatively in squirrel monkeys responding under a schedule of electric shock titration through the use of a dose-ratio analysis. LY136596 (0.3-3.0 mg/kg) produced a dose-related increase in the intensity at which monkeys maintained the shock. Increases in shock intensity produced by LY136596 were antagonized by LY136595 (0.1-10.0 mg/kg); dose-response curves for LY136596 were shifted to the right in a parallel manner by increasing doses of LY136595. An apparent pA2 (Schild) plot obtained from these data yielded a line with a slope of -0.60 +/- 0.05 and an apparent pA2 value of 5.67 +/- 0.07. These data support previous suggestions that the antinociceptive activity of picenadol (LY150720) resides in the d-isomer (LY136596) and that the l-isomer (LY136595) acts to limit the analgesic efficacy of the racemate.

CGS8216 noncompetitively antagonizes the discriminative effects of diazepam in rats

The benzodiazepine antagonist properties of CGS8216 were evaluated in rats trained to discriminate between saline and 1.0 mg/kg of diazepam in a two-choice, stimulus-shock termination procedure. CGS8216 (0.3 to 100 mg/kg) administered alone, either s.c., p.o. or i.p., occasioned only saline-appropriate responding. When administered concomitantly with a constant 1.0 mg/kg dose of diazepam, CGS8216 produced dose-related decreases in drug-appropriate responding. CGS8216 was most potent by the i.p. route, and approximately tenfold less potent by the oral route. CGS8216 was dermatotoxic after s.c. administration. CGS8216 i.p. had a long duration of action. A dose of 30 mg/kg completely antagonized the discriminative effects of the 1.0 mg/kg training dose of diazepam when the antagonist was administered 8 hr before the start of the test session. In order to determine the type of antagonism by CGS8216, the dose-effect curve for diazepam was redetermined in the presence of varying doses of CGS8216 (0.3 to 3.0 mg/kg, i.p.). CGS8216 produced a dose-related rightward shift in the diazepam dose-effect curve, but also decreased the slope and appeared to decrease the maximal effect. These results are consistent with the interpretation that CGS8216 antagonizes diazepam in a noncompetitive manner. It may do so because either it interacts with a subpopulation of benzodiazepine receptors, it functions as a pseudo-irreversible antagonist due to its high affinity, or because it is an antagonist with agonist properties.

Opiate receptor in praying mantis: effect of morphine and naloxone

A praying mantis displays a "frightening reaction" called deimatic reaction (DR), any time that it is faced with a patterned visual stimulus that represents a potential damage for the insect. Results of the present paper show that the DR could be also elicited by an actual noxious (an electrical shock) and that this response is similar to that elicited by a potential nociceptive stimulus (a patterned visual stimulus). The DR elicited by the electric shock was used as a model for studying the analgesic effect of opiates. The mantis was placed in an apparatus that allowed us to give the insect an electrical shock and to measure the strength of its DR. During a first session the voltage threshold necessary to induce a full DR was determined, and then, the insect was injected with a certain solution. The voltage threshold was tested one, two and four hours after injection. Mantises that were injected with only distilled water showed no changes in their voltage threshold during the three tests. Injections of 300, 350 and 400 micrograms/g of morphine-HCl increased the voltage threshold in both a time-dependent and a dose related manner. A dose of 350 micrograms/g of morphine-HCl produced 50% of response inhibition after two hours of injections and is referred to as the median antinoxious dose ( AD50 ). Sixteen micrograms/g of naloxone given in conjunction with an AD50 of morphine, partially blocked the effect of morphine during the first hour and fully blocked it during the second hour. Thirty-two micrograms/g of naloxone fully blocked the morphine effect during the first and the second hour. However, more than 48 micrograms/g of naloxone alone also increased the voltage threshold in insects, similar to those described for vertebrates.

Agonist-antagonistic interactions of pentazocine with morphine studied in mice

Interactions between the antinociceptive effects of pentazocine and morphine were studied in mice. In the tail-pressure test, the antinociceptive effect of pentazocine, 4.75 to 9.5 mg/kg, SC, was synergistic to that of morphine, 0.69 to 1.38 mg/kg, SC. In the acetic acid writhing test, the effect was also synergistic with pentazocine, 7.13 to 9.5 mg/kg, SC, and morphine, 1.03 to 1.38 mg/kg, SC. In the tail-pinch test, larger doses of morphine than those above were required to suppress the nociceptive response, and simultaneous administration of pentazocine, 2.38 to 19.0 mg/kg, SC, and morphine, 2.75 mg/kg, SC, produced antagonistic effects. Pentazocine, 19.0 mg/kg, completely antagonized the effect of morphine, 2.75 mg/kg, with simultaneous administration at these doses always nearly equipotent to administration of pentazocine alone. These results suggested that when pentazocine and morphine are simultaneously administered, pentazocine synergizes or antagonizes to antinociceptive effects of morphine depending on the dose sizes of morphine and pentazocine, and that the relative saturation levels of morphine and pentazocine at the receptor may be important factors in determining whether the interaction of pentazocine with morphine is antagonistic or synergistic.

Gossypin-induced analgesia in mice

A flavonoid, gossypin, was evaluated for its analgesic action by using acetic acid-induced writhing in mice and was compared with morphine. Gossypin inhibited writhing in a dose-dependent manner. This action was antagonised by naloxone. The pA2 values for morphine-naloxone and gossypin-naloxone were almost identical suggesting a definite involvement of opiate receptors in the analgesic action of gossypin.

The stinging response of the honeybee: effects of morphine, naloxone and some opioid peptides

Changes in responsiveness for the stinging reaction of honeybees fixed in a holder after receiving 3 electrical shocks delivered with 1 min interval, was registered and used as measurement for the effect of 2 microliter of different solutions injected. Every shock consisted of a train of pulses of 1 msec each, delivered for 2 sec at a frequency of 100 Hz. Injection of morphine-HCl (50 to 200 n-moles/bee) produced a dose dependent reduction of the honeybee stinging response to the electrical shocks. The morphine dose that produced a 50% inhibition of the response (D50) was 148 n-moles/bee (927 micrograms/g), i.e., a value far greater than that reported for vertebrates in behavioral test of analgesia. Naloxone 1.1 micrograms/g produces a significant reduction of morphine D50 effect and at 4-5 micrograms/g, a full disinhibition. Thus, whereas the D50 of morphine for honeybees is far greater than that for vertebrates, the doses of naloxone that antagonize morphine are similar for bees and vertebrates. Possible explanations of this difference are mentioned. Injections of met-enkephalin, leu-enkephalin, kyotorphin and (D-Ala2) methionine-enkephalinamide, given in doses of 200 n-moles/bee, an amount greater than that of the morphine D50, exhibited no effect on the stinging response.

Characterization of the opiate receptor population mediating inhibition of VIP-induced secretion from the small intestine of the rat

Net water transport was measured from the jejunum of anaesthetized rats by a re-circulation technique. Several narcotic analgesics were administered intravenously and assessed for activity in reversing net fluid secretion induced by intra-arterial infusion of vasoactive intestinal peptide (VIP). The mu-opiate agonists, morphine, RX 783006 and FK 33-824 produced full reversal of the secretory phase of the VIP response, but failed to restore totally fluid transport to the control level of net absorption. The kappa-agonist, ethylketocyclazocine, caused only partial reversal of VIP-induced secretion while the more selective kappa-agonist, MR 2034, produced a small though non-significant antisecretory effect at high doses. The delta-agonist, D-Ala2-D-Leu5-enkephalin also had negligible antisecretory activity. Naloxone caused a parallel displacement to the right of the antisecretory dose-response line to morphine. The 'in vivo pA2' value of naloxone was 7.14. The results are compared with previously published antinociceptive activities of the opiate agonists and in vivo pA2 values of naloxone. It is concluded that stimulation of mu-opiate receptors mediates inhibition of VIP-induced fluid secretion from the rat jejunal mucosa.

A comparison of the sites at which pentazocine and morphine act to produce analgesia

Sites in the brain stem at which microinjected morphine can produce analgesia have been investigated for sensitivity to microinjections of pentazocine, which has been proposed to act at receptors different to those mediating the effects of morphine. Microinjection of 10 micrograms of pentazocine into the periaqueductal grey matter (PAG), the nucleus reticularis gigantocellularis (NRGC) and nucleus reticularis paragigantocellularis (NRPG) produced analgesia as determined by the tail flick response to noxious heat. Microinjection of pentazocine into nucleus raphe magnus (NRM) did not produce any discernible change in the nociceptive threshold measured with the tail flick test. Using the pain pressure test, analgesia was observed following microinjections of pentazocine into NRGC and NRPG, but not following microinjections into PAG or NRM. Morphine (3 and 5 micrograms) microinjected into PAG, NRM, NRGC or NRPG produced analgesia as determined by both heat and pressure tests. The analgesia produced by injection of pentazocine into the NRGC or NRPG was comparable to the analgesia produced by microinjection of 3 micrograms of morphine into these areas. The analgesia produced by injection of pentazocine into PAG was significantly less than that produced by 3 micrograms of morphine injected into PAG. Pretreatment with naloxone did not affect the analgesia produced by microinjection of pentazocine into NRGC or NRPG, but did antagonize the analgesia produced by injection of pentazocine into PAG. Naloxone blocked the analgesic effects of microinjected morphine. Analgesia produced by systemically given pentazocine was significantly reduced following microinjection of naloxone into PAG or NRM but not into NRGC or NRPG. The present data provide further evidence that the effects of pentazocine, a kappa agonist drug may be mediated by mechanisms different to those mediating the action of morphine, a mu agonist.

Receptor binding, antagonist, and withdrawal precipitating properties of opiate antagonists

A number of opiate antagonists and the dextro isomers of some of these drugs were studied for antagonism of acute opiate effects on ilea isolated from opiate-naive guinea pigs, precipitation of a withdrawal contraction of ilea isolated from morphine-dependent guinea pigs, precipitation of withdrawal in morphine-dependent rhesus monkeys and stereospecific displacement of 3H-etorphine binding to rat-brain membranes. With the exception of d-naloxone, all of the compounds displaced 3H-etorphine. With the exception of d-naloxone, nalorphine, and quaternary nalorphine, all of the antagonists caused a contraction of ilea isolated from morphine-dependent guinea pigs. Moreover, the IC 50 values of the compounds for displacing 3H-etorphine binding were well correlated with both their Ke values for antagonism in the ileum (r = 0.95) and with their EC 50 values for precipitating a contraction in this preparation (r = 0.92). Generally, the concentration of antagonist necessary to precipitate half maximal contracture was 30-fold greater than the Ke value of the antagonist. Most of the opiate antagonists also precipitated withdrawal when administered to morphine-dependent rhesus monkeys and their in vivo potencies were well correlated with their in vitro potencies in ileum (with Ke: r = 0.95; with EC 50: r = 0.99) and in displacing 3H-etorphine (r = 0.95). The quaternary derivative of naltrexone, however, was an effective opiate antagonist only in vitro, and was ineffective in precipitating withdrawal in morphine-dependent rhesus monkeys. These results suggest that the receptor sites labeled by 3H-etorphine are the same as those involved in antagonism of acute opiate actions and in precipitation of withdrawal.

The antinociceptive activity of meptazinol depends on both opiate and cholinergic mechanisms

1 Antinociceptive responses to meptazinol, morphine and oxotremorine and the effects of pretreatment with naloxone or scopolamine on these responses in mice and rats, were examined. 2 Meptazinol evoked larger increases in nociceptive thresholds in the mouse than in the rat, whereas morphine induced large increases in both species. Oxotremorine was both a more potent and a more effective antinociceptive agent in the mouse than in the rat. 3 Antinociceptive responses to meptazinol were consistently inhibited in animals pretreated with naloxone, whereas scopolamine attenuated the effects of meptazinol in some, particularly the mouse tail immersion test, but not in all of the procedures used. 4 Naloxone inhibited all antinociceptive responses to morphine, and scopolamine inhibited all responses to oxotremorine. However, there was no significant interaction between naloxone and oxotremorine or between scopolamine and various opioid analgesic agents. 5 These results indicate that meptazinol, unlike established opioid drugs, may induce antinociception by a dual action on opiate and cholinergic mechanisms.

Methadone conformation and opioid activity

The inactive methadone analog threo-5-methylmethadone has a solid-state conformation in which the nitrogen is antiperiplanar to the tertiary carbon C(4). Since threo-5-methylmethadone exhibits no opioid agonism either in vivo or in vitro, methadone analogs probably do not have this conformation when bound to an opioid receptor. The potent agonist (-)-erythro-5-methylmethadone has a solid-state conformation in which the nitrogen atom is rotated back toward the phenyl rings on the quarternary carbon, suggesting that this unusual conformation is the active one.

Interaction of naloxone with mu- and delta-opioid agonists on the respiration of rats

The respiratory depression induced by two mu-opiate agonists morphine and Tyr-D-Ala-Gly-N-Me-Phe-Met(O)-ol (FK-33824), and two delta-agonists Tyr-D-Ala-Gly-Phe-D-Leu (DADLE) and Tyr-D-Ser-Gly-Phe-Leu-Thr (D-Ser2-Thr6) was studied in rats by using the intracerebroventricular route. The four opioids caused a dose-dependent depression of respiratory frequency down to apnea but high doses of morphine elicited motor activation and seizure activity. FK-33824 was the most potent, followed by DADLE, D-Ser2-Thr6 and morphine. The in vivo apparent pA2 values were determined for naloxone against FK-33824, DADLE and D-Ser2-Thr6. The pA2 value of naloxone interacting with the mu-agonist FK-33824 was significantly lower than those obtained against the two delta-agonists. It is proposed that different types of opiate receptors are involved in the opiate-induced respiratory depression.

Contractile effect of morphine and related opioid alkaloids, beta-endorphin and methionine enkephalin on the isolated colon from Long Evans rats

1 Morphine and related synthetic surrogates as well as beta-endorphin and methionine enkephalin caused a contractile response of the longitudinal musculature of the terminal colon of Long Evans rats.2 The muscular contraction caused by the narcotic analgesics exhibited stereospecificity, with levorphanol being about 50 times more potent than dextrorphan and (-)-methadone 4 times more potent than (+)-methadone. In addition, the rank order in potency of a homologous series of N-alkyl substituted norketobemidones demonstrated that the activity of these compounds in eliciting contractile responses corresponded to that for analgesic efficacy in the rat and also correlated to the ability of these derivatives to inhibit the muscular twitch evoked by electrical stimulation of the guinea-pig ileum.3 Naloxone blocked the contractile response of the opiates following competitive kinetics; the naloxone pA(2) values for morphine, etorphine, levorphanol and methadone were very close, in spite of the marked differences in potency of these agents.4 The contractile effect of morphine on the rat colon was abolished by incubation of the tissues with tetrodotoxin 2.0 x 10(-7) M or by decreasing the external Ca(2+) level 100 fold. Increasing the external Ca(2+) concentration caused an apparent non-competitive antagonism of the response to morphine.5 Pretreatment of the tissues with hexamethonium 8.3 x 10(-5) M caused a modest antagonism of the morphine effect while atropine 5.8 x 10(-7) M did not significantly modify the morphine contractile effect. In contrast, methysergide 10(-5) M caused a 10 fold increase in the morphine EC(50).6 Colons from rats rendered tolerant-dependent on morphine were markedly less sensitive to the contractile effects of morphine than those from placebo-treated controls. Tolerance to morphine was also accompanied by an increased sensitivity to the contractile effects of 5-hydroxytryptamine (5-HT).7 A marked increase in the spontaneous muscular activity of segments of the terminal colon of rats chronically treated with morphine was found to occur upon removal of the residual morphine in the tissues by repetitive washings. The spontaneous activity was arrested by applications of morphine, suggesting that physical dependence can be demonstrated in vitro in this particular preparation.8 It is concluded that the opiate-induced contractile response is mediated via stereospecific, naloxone-sensitive, opiate receptors and that the muscular response involves the activation of a 5-HT neurone in the nerve terminals of the colon.

Comparison of the antinociceptive activity of intraventricularly administered acetylcholine to narcotic antinociception

Intraventricularly administered acetylcholine inhibits mouse tail-flick latency in a dose-dependent manner with an ED50 of about 20 microgram. This antinociceptive activity is not mimicked by biogenic amine neurotransmitters (i.e. norepinephrine, epinephrine, dopamine, serotonin or histamine) and is not markedly affected by selective depletors of brain catecholamines or serotonin. However, pretreatment with reserpine or tetrabenazine dramatically reduces acetylcholine-induced antinociception. Tolerance develops rapidly to the antinociceptive effects of acetylcholine. Cross-tolerance to morphine in acetylcholine-tolerance mice is minimal, but the antinociceptive activity of acetylcholine is markedly reduced in mice chronically pretreated with morphine. Acetylcholine-induced antinociception differs from narcotic antinociception in the reversed stereoselectivity of several narcotic antagonists and in the in vivo pA2 values for inhibition by naloxone. Therefore, the antinociceptive activity of intraventricularly administered acetylcholine cannot be described as a specific narcotic action.

Receptor binding and antinociceptive properties of phencyclidine opiate-like derivatives

The relative potencies of a new series of phencyclidine (PCP) analogs for the displacement of [3H] morphine binding from rat brain homogenates are well correlated with the relative antinociceptive potencies in the test of writhing induced by acetic acid (0.6%). One group of compounds exerts a completely naloxone-reversible analgesic effect, while the effects of a second group are partially reversed by naxolone. These findings and the structural differences between the two groups suggest that their analgesic is mediated through different opiate receptors.

The irreversible narcotic antagonistic and reversible agonistic properties of the fumaramate methyl ester derivative of naltrexone

The fumaramate methyl ester derivatives of naltrexone (beta-FNA) and oxymorphone (beta-FOA) were both found to be reversible agonists on the guinea pig ileal longitudinal muscle preparation. In addition, beta-FNA possessed on irreversible antagonistic effect against morphine whereas beta-FOA had no such capacity. Analysis by pA2 values revealed that beta-FOA resembled pure agonists like morphine and enkephalin while beta-FNA resembled the mixed agonist-antagonists like nalorphine and pentazocine. The antagonism by beta-FNA was very selective in that it antagonized pure agonists but had little or no effect on the effects of either mixed agonist-antagonists, ethylketocyclazocine or other non-opiate-type agonists like norepinephrine.

Discriminative stimulus effects of prototype opiate receptor agonists in monkeys

This study was performed to clarify the apparent differences in the cellular substrates mediating the discriminative stimulus effects of morphine and cyclazocine. Squirrel monkeys trained to discriminate between i.m. injections of saline and either morphine (3.0 mg/kg) or cyclazocine (0.1 mg/kg) in a two-choice avoidance paradigm were tested for stimulus generalization to ethylketocyclazocine and SKF 10,047, proposed agonists of receptors mediating the effects of cyclazocine. Both drugs completely mimicked the stimulus effects of cyclazocine but not those of morphine. Naltrexone (0.3 mg/kg) produced a 3-fold shift to the right of the cyclazocine dose-response curve but did not completely block the cyclazocine-like stimulus effects of either SKF 10,047 or ethylketocyclazocine. SKF 10,047 was a competitive antagonist at the receptors mediating the stimulus effects of morphine. These data are consistent with a multiple opiate receptor model, and provide further evidence that the stimulus effects of morphine and cyclazocine are subserved by different cellular substrates.

Opiate antagonist receptor binding in vivo: evidence for a new receptor binding model

The in vivo accumulation and retention of the opiate antagonist tracers [3H]diprenorphine and [3H]naloxone at cerebral opiate receptor sites in rats exceed that expected from their known in vitro receptor affinities. The [3H]diprenorphine serum and brain levels can be stimulated with a pharmacokinetic model that contains the receptors in a micro-compartment. The receptor micro-compartment consists of a population of binding sites next to a diffusion boundary which restricts ligand diffusion away from the receptor. Such an arrangement introduces a delay in the binding equilibrium of potent antagonists with the receptor sites and an increase in the apparent in vivo receptor affinity at subsaturating doses of the ligand; at saturating ligand concentrations these functions of the receptor micro-compartment are abolished. A physiological interpretation of the receptor micro-compartment could be the location of clustered opiate receptor sites on the exterior cell surface next to the synaptic cleft as the diffusion boundary. This kinetic approach involving a combination of pharmacokinetics and drug-receptor interactions permits the quantitative analysis of receptor site availability in the intact animal. Our results support the hypothesis that only one receptor population affects the in vivo disposition of the antagonist tracers, while they do not exclude the presence of low affinity binding sites that have been observed with the use of [3H]naloxone in vitro. Moreover, the binding site population observed in vivo may be responsible for mediating opiate agonist analgesia.

Analgesic and other pharmacological activities of a new narcotic antagonist analgesic (-)-1-(3-methyl-2-butenyl)-4-[2-(3-hydroxyphenyl)-1-phenylethyl]-piperazine and its enantiomorph in experimental animals

Of 1-chclohexyl-4-[2-(3-hydroxyphenyl)-1-phenylethyl]piperazine (I) and its 1-(3-methyl-2-butenyl) derivative (II), the S(+)-isomers were analgesically more active than either their +(-)-isomers or their racemates, having 15 to 44 times the potency of morphine in mice and rats. R(-)-I had comparable analgesic activity to morphine R(-)-II to pentazocine in mice, rats and dogs and they were nearly equipotent with pentazocine in reversing some actions of morphine. The S(+)-isomers and racemates lacked this action. R(-)-II required about 10 times more naloxone to reverse its analgesic activity than was needed to antagonise the S(+)-isomers, morphine and pentazocine. The S(+)-isomers and racemates produce a typical Straub tail reaction and increased spontaneous locomotor activity in mice, but the R(-)-isomers did not. R(-)-II had no significant physical dependence liability in mice, rats and monkeys. From these results, it is suggested that the compounds show an uncommon steroselectivity in comparison with morphine and its surrogates, and that R(-)-II is worth investigating further as a narcotic antagonist analgesic.

Stimulus effects of morphine in the monkey: quantitative analysis of antagonism

The ability of narcotic antagonists to block the discriminative stimulus effects of 3.0 mg/kg (IM) of morphine was evaluated quantitatively in the squirrel monkey using a two-choice discrete-trial avoidance paradigm. The time-course and relative potency of naloxone and naltrexone for antagonizing morphine's stimulus effects in the squirrel monkey was similar to those determined in rats and pigeons. Complete blockade of morphine's effects was attained with 0.03 mg/kg of either antagonist when given simultaneously with morphine, but only naltrexone completely blocked the stimulus effects of morphine when the pretreatment interval was extended to 12 and 18 hr. A Schild plot derived from the degree of antagonism of graded doses of morphine by graded doses of naltrexone (0.003-0.1 mg/kg) yielded a line with a slope of -0.63 +/- 0.2 and an apparent pA2 value of 8.25 +/- 0.2. These results demonstrate the feasibility of quantitatively assessing the drug-receptor interactions that subserve the discriminative stimulus effects of morphine in the squirrel monkey.