Majonoside-R2, a major constituent of Vietnamese ginseng, attenuates opioid-induced antinociception

The effects of majonoside-R2 on antinociceptive responses caused by the mu-opioid receptor agonist morphine and the selective kappa-opioid receptor agonist U-50, 488H were examined by the tail-pinch test in mice. Intraperitoneal (IP) or intracerebroventricular (ICV) injection of majonoside-R2 (3.1-6.2 mg/kg, IP or 5-10 micrograms/mouse, ICV) and diazepam (0.1-0.5 mg/kg, IP or 0.5-1.0 microgram/mouse, ICV), as well as an opioid receptor antagonist naloxone (2 mg/kg, IP or 5 micrograms/mouse, ICV), dose-dependently attenuated the antinociception caused by subcutaneously administered morphine and U-50,488H. Moreover, when co-administered ICV or intrathecally (IT) with morphine (4 micrograms/mouse) or U-50,488H (60 micrograms/mouse), majonoside-R2 (5-20 micrograms/mouse) also exhibited antagonism against the antinociceptive action of these opioid receptor agonists in the tail-pinch test. The inhibitory effects of majonoside-R2 (10 micrograms/mouse, ICV) and diazepam (1 microgram/mouse, ICV) were reversed by flumazenil (2.5 micrograms/mouse, ICV), a selective benzodiazepine receptor antagonist, and picrotoxin (0.25 microgram/mouse, ICV), a GABA-gated chloride channel blocker. These results suggest that majonoside-R2 attenuates the opioid-induced antinociception by acting at the spinal and supraspinal levels, and that the GABAA receptor complex at the supraspinal level is involved in the effect of ICV administered majonoside-R2.     

Peripheral effects of opioids in a model of chronic intestinal inflammation in mice

The study describes a model of chronic intestinal inflammation in mice. Inflammation was induced by the administration of one dose of croton oil (CO) (acute CO) or two doses (chronic CO) of intragastric CO, whereas controls received saline (SS); GI transit was measured with charcoal. Chronic CO induced intestinal inflammation substantiated by optical microscopy, weight loss (20%) and a 25% increase in GI transit. The ED50 values in SS animals were 1.67 +/- 0.13 mg/kg for morphine and 0.038 +/- 0.006 mg/kg for fentanyl; chronic CO significantly decreased the ED50 values to 0.16 +/- 0.03 mg/kg (morphine) and 0.006 +/- 0.0005 mg/kg (fentanyl). Thus the potency of morphine increased 10.4 times and that of fentanyl 6.3 times. The effects of enkephalin, but not those of U-50488H, were also significantly enhanced during chronic CO. The antitransit effects of p.o. loperamide increased 11.7 times during chronic CO. All effects were reversed by specific antagonists. The fraction of the active opioid receptor that mediates the antitransit effects of morphine was evaluated using beta-funaltrexamine. In chronic CO, the doses of beta-funaltrexamine needed to antagonize 1 mg/kg of morphine were significantly higher than in SS and acute CO, and the ED50/KA ratio was 20 times lower. These results suggest an increase in the active concentration of mu-opioid receptors during chronic inflammation.     

Possible mechanisms of salt-induced hypertension in Dahl salt-sensitive rats

The effects of acute and chronic administration of cocaine on the antinociception and tolerance to the antinociceptive actions of mu-(morphine), kappa-(U-50,488H), and delta-([D-Pen2,D-Pen5]enkephalin; DPDPE), opioid receptor agonists were determined in male Swiss-Webster mice. Intraperitoneal injection of 40 mg/kg of cocaine by itself produced weak antinociceptive response as measured by the tail-fick test but the lower doses were ineffective. Administration of morphine (10 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (10 microg/mouse, ICV) produced antinociception in mice. Cocaine (20 mg/kg) potentiated the antinociceptive action of morphine and DPDPE but had no effect on U-50,488H-induced antinociception. Administration of morphine (20 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (20 microg/mouse, ICV) twice a day for 4 days resulted in the development of tolerance to their antinociceptive actions. Tolerance to the antinociceptive actions of morphine and U-50,488H was inhibited by concurrent treatment with 20 or 40 mg/kg doses of cocaine; however, tolerance to the antinociceptive action of DPDPE was not modified by cocaine. It is concluded that cocaine selectively potentiates the antinociceptive action of mu- and delta- but not of the kappa-opioid receptor agonist. On the other hand, cocaine inhibits the development of tolerance to the antinociceptive actions of mu- and kappa- but not of delta-opioid receptor agonists in mice.     

Alteration of calcitonin gene related peptide and its receptor binding sites during the development of tolerance to mu and delta opioids

Calcitonin gene related peptide (CGRP), one of the most abundant peptides in the spinal cord, is localized in primary afferents and released following nociceptive stimuli. Its colocalization and corelease with substance P, a well-known nociceptive neuropeptide, support the importance of CGRP in pain mechanisms. However, its distinctive function in that regard remains to be fully established. Recently, we reported that increases in CGRP-like immunostaining and decrements in specific 125I-labelled human CGRP alpha ([125I]hCGRP alpha) binding sites in the spinal cord were correlated with the development of tolerance to the spinal antinociceptive action of a mu opioid agonist, morphine. The goal of the present study was to investigate whether the development of tolerance to other classes of opioids, namely, delta and kappa agonists, can also alter CGRP-like immunostaining and receptors in the rat spinal cord. The antinociceptive effects of all opioids were monitored by the tail-immersion test. Tolerance to their antinociceptive properties was induced by the infusion for 7 days of mu (morphine sulfate, 7.5 micrograms/h), delta D([D-Pen2,D-Pen5]enkephalin (DPDPE), 2.0 micrograms/h), and kappa (U-50488H, 10.0 micrograms/h) related agonists at the spinal level (L4), using osmotic minipumps. We confirmed that rats chronically treated with morphine showed significant decreases in [125I]CGRP alpha binding in laminae I, II, and III of the L4 spinal cord, while CGRP-like immunostaining was increased in these same laminae. Similar effects were observed following a treatment with the delta agonist, DPDPE, while the kappa agonist, U-50488H, apparently only slightly decreased [125I]CGRP alpha] binding in lamina II. Binding in other laminae and CGRP-like immunostaining were not affected. These results suggest a specific interaction between spinal CGRP systems and the development of tolerance to the spinal antinociceptive effects of mu- and delta-related agonists.     

Interaction between sufentanil and U-50488H with respect to antinociception and respiratory depression in rats

BACKGROUND: Opiate receptors have been argued to differentially regulate analgesia and respiratory depression. In order to validate possible interactions between the opiate mu- and kappa-receptors, interactions between sufentanil and U-50488H were studied in rats. METHODS: Rats equipped with an arterial catheter were tested in the tail flick latency (TFL) test after intravenous treatment with sulentanil (a mu-agonist), U-50488H (a kappa-agonist) or fixed ratio combinations of both drugs. Simultaneously, respiratory changes were monitored by blood gas analysis. RESULTS: The ED50s of sufentanil for a TFL > 6.0 and > or = 10.0 s were 0.0002 and 0.00059 mg/kg. For U-50488H the corresponding values were 1.53 and 8.11 mg/kg. Using a fixed dose ratio of 1/10,000, an additivity was demonstrated between sufentanil and U-50488H in terms of antinociception. With regard to respiratory parameters, PaCO2 significantly increased after all doses of sufentanil early after treatment. At the higher doses tested, there was also a decrease in PaO2 and O2 saturation. For U-50488H only the highest doses resulted in an early and small shift in PaCO2. The combination of sufentanil/U-50488H resulted in only a small increase in PaCO2 at the highest dose regimen tested. CONCLUSION: The results presented here demonstrate that drug mixtures of sufentanil and U-50488H can be additive with respect to antinociception with additionally less risk for respiratory side-effects, as compared with sufentanil alone. Therefore, a combination of mu- and kappa-opiate-receptor agonists might be more beneficial than each agent alone.     

Effect of 7-nitroindazole on tolerance to morphine, U-50,488H and [D-Pen2, D-Pen5] enkephalin in mice

The effects of 7-nitroindazole (7-NI), an inhibitor of the neuronal nitric oxide synthase (nNOS) which does not increase blood pressure, on tolerance to the antinociceptive activity of mu-(morphine), kappa-(U-50,488H) and delta-([D-Pen2, D-Pen5]enkephalin, DPDPE) opioid receptor agonists were determined in mice. Male Swiss-Webster mice were made tolerant by twice daily injections of morphine (20 mg/kg, s.c.), U-50,488H (25 mg/kg, i.p.) or DPDPE (20 micrograms/mouse, i.c.v.) for 4 days. When tested on day 5, tolerance to their antinociceptive activity was evidenced by decreased response in chronic drug treated mice in comparison to vehicle-injected mice. Concurrent administration of 7-NI (20, 40 or 80 mg/kg, i.p.) with DPDPE did not modify the development of tolerance to the antinociceptive action of DPDPE. However, 7-NI (40 or 80 mg/kg, i.p.) inhibited the development of tolerance to the antinociceptive activity of morphine and U-50,488H but the lower dose of 7-NI (20 mg/kg, i.p.) was not effective. Chronic administration of 7-NI by itself did not modify the acute response to morphine, U-50,488H or DPDPE. It is concluded that a specific inhibitor of nNOS can inhibit tolerance to the antinociceptive activity of mu- and kappa- but not of delta-opioid receptor agonists in mice.     

ACEA-1328, an NMDA receptor antagonist, increases the potency of morphine and U50,488H in the tail flick test in mice

The effect of 5-nitro-6,7-dimethyl-1,4-dihydro-2,3-quinoxalinedione (ACEA-1328), a competitive and systemically bioavailable NMDA receptor/glycine site antagonist, was examined on opioid-induced antinociception in the tail flick test. Swiss Webster mice were injected with ACEA-1328 either alone or in combination with morphine or (+/-)-trans-U-50488 methanesulfonate (U50,488H), a mu- and a kappa-opioid receptor agonist, respectively, and tested for antinociception. Systemic administration of ACEA-1328 alone increased the tail flick latencies with an ED50 of approximately 45 mg kg-1. Concurrent administration of ACEA-1328 with morphine, or U50,488H, at doses that did not affect tail flick latencies, potentiated the antinociceptive effect of the opioid analgesics and vice versa. Naloxone, an opioid receptor antagonist, while not modifying the effect of ACEA-1328, did block the augmentation, suggesting that opioid receptors might be involved in the latter effect. 5-Aza-7-chloro-4-hydroxy-3-(m-phenoxyphenyl)quinoline-2(1H)-one (ACEA-0762), a selective NMDA receptor/glycine site antagonist, also showed enhancement of the antinociceptive effect of morphine and U50,488H. However, concurrent administration of 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzol[f]quinoxaline (NBQX), a selective non-NMDA receptor antagonist, with morphine did not alter the antinociceptive potency of the opioid analgesic. Overall, the data suggest that ACEA-1328 may increase the potency of the opioid analgesics by antagonising the glycine site associated with the NMDA receptor.     

Effects of N-methyl-D-aspartate receptor antagonists on discriminative stimulus effects of naloxone in morphine-dependent rats using the Y-maze drug discrimination paradigm

The present study assessed the ability of various site-selective N-methyl-D-aspartate (NMDA) receptor antagonists to affect the discriminative stimulus properties of naloxone in morphine-dependent rats. Adult male Wistar rats were trained to discriminate 0.1 mg/kg of s.c. naloxone from saline using a Y-maze shock-avoidance procedure. Naloxone-appropriate responding was exhibited as a function of naloxone dose (0.01-1.0 mg/kg, ED50 = 0.03 mg/kg) and was also observed when morphine treatment temporarily was discontinued (8-96 hr, peak at 24 hr). Discriminative stimulus effects of naloxone (0.1-3.0 mg/kg) were antagonized by morphine (10-100 mg/kg). Ligands of peripheral opioid receptors failed to either substitute for naloxone (methylnaloxone, 0.1-3.0 mg/kg) or attenuate naloxone's stimulus effects (loperamide, 1-30 mg/kg). In rats treated with the training dose of naloxone, administration of dizocilpine (0.03-0.3 mg/kg) and D-CPPene (1-10 mg/kg) decreased levels of naloxone-appropriate responding, whereas memantine (1-30 mg/kg), ACEA-1021 (10 and 50 mg/kg) and eliprodil (3-30 mg/kg) seemed to have little or no effects. Meanwhile, all NMDA receptor antagonists produced a decrease in the occurrence of two or more of the following opioid withdrawal signs: weight loss, forelimb tremor, ptosis, diarrhea and "wet-dog"-like shaking. Additionally, dizocilpine (0.1 mg/kg), D-CPPene (5.6 mg/kg) and ACEA-1021 (50 mg/kg) but not memantine (10 mg/kg) or eliprodil (30 mg/kg) significantly reduced the naloxone-appropriate escape area selection when administered during the period of suspended morphine treatment 24 hr after the last morphine injection. Thus, NMDA receptor antagonists appear to inhibit the discriminative stimulus effects of both naloxone-precipitated and spontaneous morphine withdrawal, and this ability depends on the type of antagonist applied.     

Synthesis and trazodone-like analgesic activity of 4-phenyl-6-aryl-2-[3-(4-arylpiperazin-1-yl)propyl]pyridazin -3-ones

A series of 4,6-diaryl pyridazinones, chemically related to trazodone, ws synthesized and evaluated for analgesic activity. With ED50 values ranging from 8.4 to 46.7 mg kg(-1) i.p. in the phenylbenzoquinone-induced writhing test (PBQ test), most compounds were several times more potent than acetaminophen (ED50 = 231.3 mg kg(-1) i.p.) and noramidopyrine (ED50 = 68.5 mg kg(-1) i.p.). A multiple linear regression analysis demonstrated a correlation between antinociceptive activity and lipophilicity, as well as electronic and steric factors. The most active pyridazinones 2c and 2j exhibited minimal sedative and neurotoxic effects at the dose of 25 mg kg(-1) i.p. They were devoid of activity in the hot plate test and their analgesic activity was not significantly reversed by naloxone in the PBQ test. The antinociceptive response induced by morphine (0.15 mg kg(-1) s.c.) in the PBQ test was greatly potentiated by 2c and 2j administered at the low doses of 1 and 2.5 mg kg(-1) i.p., respectively. On the other hand, their analgesic effects were enhanced synergistically by 5-hydroxytryptophan combined with carbidopa. All these data imply that a significant part of the antinociceptive effect induced by 2c and 2j may involve both opioid and serotonergic pathways. In addition, these two pyridazinones did not exhibit any antidepressant properties in the forced swimming test, nor did they potentiate yohimbine-induced toxicity.     

Study of the antinociceptive effect of tetrahydro-papaveroline derivatives. Interaction with opioids

The antinociceptive effect of racemic tetrahydropapaveroline (THP), of its two R(+)- and S(-) enantiomers, of 1-2-dehydro-THP and of 1-carboxy-THP was assessed using different pain tests in mice. None of these drugs possessed a significant activity in the hot-plate and tail-flick tests. However, after i.p. injection, they reduced the number of abdominal writhes induced by phenylbenzoquinone, with ED50 values of 51 +/- 7, 73 +/- 9 and 79 +/- 7 mg/kg for the most potent compounds: 1,2-dehydro-THP, +/- THP and -THP, respectively. This activity was not antagonized by naloxone (1 mg/kg, s.c.). However combination of inactive doses of these three compounds (32 mg/kg, i.p.) and of morphine (0.5 mg/kg, s.c.) led to a significant antinociceptive effect (83 to 85% of reduction of the number of writhes). This synergistic potentiation confirmed with the combination of +/- THP (16 mg/kg, i.p.) and morphine (0.5 mg/kg, s.c.) was totally inhibited by naloxone (1 mg/kg, s.c.). These results, although excluding a direct agonistic effect of THP derivatives on opiate receptors, suggest an indirect interaction of these drugs with the endogenous opioid system.     

Comparison of the fatty acid compositions in intraepithelial and infiltrating lesions of the cervix: part II, free fatty acid profiles

Clinical studies report a low incidence of intestinal side effects with transdermally administered fentanyl (TTS-fentanyl) in comparison with oral morphine. To support these clinical data, analgesic and intestinal effects of both opioids were compared in rats. After subcutaneous injection, analgesia in the tail withdrawal reaction test was obtained at a peak effect dose of 0.032 mg/kg with fentanyl and 8.0 mg/kg with morphine. This analgesic dose exceeded the ED50 for inhibition of castor oil-induced diarrhea only slightly (1.1 x) in the case of fentanyl (0.028 mg/kg) but markedly (36 x) in the case of morphine (0.22 mg/kg). To reverse completely the antidiarrheal effect of equivalent analgesic doses of the opioids (their ED50S for analgesia lasting 2 hours), much more naloxone was required in the case of morphine (5.4 mg/kg) than in the case of fentanyl (0.19 mg/kg). After oral administration, the difference between both opioids was less pronounced. Analgesia was obtained at 0.85 mg/kg with fentanyl and 32 mg/kg with morphine. This analgesic dose only slightly (1.7 x) exceeded the antidiarrheal dose in the case of fentanyl (0.49 mg/kg) but significantly (6.2 x) in the case of morphine (5.2 mg/ kg). To reverse completely the antidiarrheal effect of equivalent analgesic oral doses of the opioids (their ED50S for analgesia lasting 2 hours), more naloxone was required in the case of morphine (11 mg/kg) than in the case of fentanyl (2.0 mg/kg). Rapid penetration of fentanyl into the brain is thought to be responsible for small dissociation between the analgesic and intestinal effect of this lipophilic opioid. The present data provide preclinical evidence to support the relatively low incidence of intestinal side effects observed clinically with the use of TTS-fentanyl in comparison with orally administered morphine.     

Dysfunctional voiding in women: which muscles are responsible?

We studied the effect of mixed agonist-antagonist opioids (nalbuphine and pentazocine) and a kappa opioid agonist (U50488H) on gastric emptying and gastrointestinal transit, and their interactions with morphine in rats. In each group, nalbuphine (0.01-30 mg kg-1), pentazocine (1-30 mg kg-1), U50488H (1-100 mg kg(-1)1) or saline was injected i.p. at 0 min. Another four groups of rats received morphine 13.4 mg kg-1 (ED75) and one of the following substances: saline, nalbuphine, pentazocine or U50488H. In both groups, at 30 min, radiolabelled saline 1 ml was infused into the stomach; at 1 h, gastric emptying and gastrointestinal transit were calculated by measuring the radioactivity in the gastrointestinal tract. Slopes for dose-response curves were determined. Nalbuphine significantly, but only weakly, delayed gastric emptying (P < 0.0005) and gastrointestinal transit (P < 0.01). Pentazocine markedly inhibited both, whereas U50488H did not significantly inhibit either. The slopes of the dose-response curves for nalbuphine, but not for pentazocine, on both gastric emptying and gastrointestinal transit were significantly less steep than those for morphine. Nalbuphine significantly antagonized the inhibitory effect of morphine on gastric emptying (P = 0.005) and gastrointestinal transit (P = 0.02), whereas pentazocine and U50488H did not. Nalbuphine and pentazocine delay gastric emptying and gastrointestinal transit, possibly by different mechanisms.     

The competitive alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor antagonist LY293558 attenuates and reverses analgesic tolerance to morphine but not to delta or kappa opioids

Antagonists of the NMDA type of excitatory amino acid (EAA) receptor attenuate or reverse the development of tolerance to the analgesic effects of the mu opioid agonist morphine, the delta-1 opioid agonist DPDPE but not the kappa-1 agonist U50,488H or the kappa-3 agonist naloxone benzoylhydrazone. The role of the AMPA subtype of EAA receptor in analgesic tolerance was examined using LY293558, a selective competitive antagonist that is active after systemic administration. Administration of morphine, DPDPE, or U50,488H three times daily for 3 days according to an escalating dosing schedule resulted in analgesic tolerance as indicated by an increase in analgesic ED50 values using the tail-flick test in mice. Analgesic tolerance was attenuated when mice received a continuous subcutaneous infusion of LY293558 at doses of 30, 45 or 60 mg/kg/24 hr via an osmotic pump concurrent with the morphine treatment. Continuous subcutaneous infusion of LY293558 (45 mg/kg/24 hr) also reversed established morphine tolerance. In contrast, continuous subcutaneous infusion of the highest dose of LY293558 (60 mg/kg/24 hr) was ineffective in preventing the development of analgesic tolerance to DPDPE or U50,488H. Continuous subcutaneous infusion of LY293558 (60 mg/kg/24 hr) for 3 days protected mice from generalized convulsions produced by the selective AMPA agonist ATPA, indicating that the dosage of LY293558 that attenuated morphine tolerance was effective as an antagonist at AMPA receptors. These results demonstrate that AMPA receptors may play a role in the development and maintenance of morphine, but not DPDPE or U50,488H, analgesic tolerance.     

Naloxone-induced supersensitivity of oxytocin neurones to opioid antagonists

Here we report that a single administration of naloxone to conscious rats produces no significant increase in oxytocin release, but when repeated 3-4 days later results in a large release of oxytocin. Plasma oxytocin concentrations were measured in conscious and urethane-anaesthetized rats pretreated with naloxone or isotonic saline on Day 1. On Days 2, 3 or 4, a second dose of naloxone was given, producing an increase in oxytocin secretion in naloxone-pretreated groups (P < 0.05 vs. controls) on Day 3 and 4, but not on Day 2. The specificity of the opioid antagonist supersensitivity was determined by injection of the kappa-antagonist nor-binaltorphimine (nor-BNI). Pretreated rats (naloxone, saline or nor-BNI, Day 1) received an additional acute nor-BNI injection (Day 4) which increased plasma oxytocin concentration in the three groups. However, this increase was higher in naloxone-pretreated rats with no differences between the nor-BNI- and saline-pretreated animals. Measurements of electrical activity of single supraoptic nucleus oxytocin neurons and of plasma oxytocin concentration (Day 4) showed that naloxone modestly enhanced the responsiveness of oxytocin neurons to cholecystokinin (CCK) in naloxone-pretreated rats (by comparison with saline-pretreated rats), but had only a small effect on basal firing rate that did not differ between naloxone-pretreated rats and saline-pretreated rats. To investigate whether naloxone-pretreatment modified the effect of morphine on CCK-induced oxytocin release, on Day 4 CCK was injected i.v. with or without morphine. Morphine at a dose of 0.1 mg/kg did not affect CCK-induced oxytocin release, whereas 1 mg/kg of morphine blocked this release in both saline- and naloxone-pretreated rats. The results suggest that naloxone induces opioid antagonist supersensitivity on oxytocin secretion, mainly by up-regulating kappa-opioid mechanisms on oxytocin nerve terminals in the posterior pituitary.     

Differential effects of specific delta and kappa opioid receptor antagonists on the bidirectional dose-dependent effect of systemic naloxone in arthritic rats, an experimental model of persistent pain

In an attempt to determine the opioid receptor class(es) which underly the two opposing effects of naloxone in models of persistent pain, we tested the action of the selective delta antagonist naltrindole, and that of the kappa antagonist MR-2266 on the bidirectional effect of systemic naloxone in arthritic rats. As a nociceptive test, we used the measure of the vocalization thresholds to paw pressure. The antagonists were administered at a dose (1 mg/kg i.v. naltrindole, 0.2 mg/kg i.v. MR-2266), without action per se but which prevents the analgesic effect of the delta agonist DTLET (3 mg/kg, i.v.) or the kappa agonist U-69,593 (1.5 mg/kg, i.v.) respectively, and does not influence the effect of morphine (1 mg/kg i.v.) or the mu agonist DAMGO (2 mg/kg, i.v.) in these animals. In arthritic rats injected with the delta antagonist, the paradoxical antinociceptive effect produced by 3 micrograms/kg i.v. naloxone was not significantly modified (maximal vocalization thresholds (% of control) were 146 +/- 9% versus 161 +/- 7% in the control group). By contrast, the hyperalgesic effect produced by 1 mg/kg i.v. naloxone was significantly reduced (maximal vocalization thresholds were 87 +/- 4% versus 69 +/- 5% in the control group). In rats injected with the kappa antagonist, the antinociceptive effect of the low dose of naloxone was almost abolished (mean vocalization thresholds were 115 +/- 3% versus 169 +/- 7%) whereas the hyperalgesic effect of naloxone 1 mg/kg i.v. was not significantly modified (mean vocalization thresholds = 70 +/- 3% and 65 +/- 3%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential influence of D1 and D2 dopamine receptors on acute opiate withdrawal in guinea-pig isolated ileum

1. The effects exerted by D1 and D2 dopamine agonists and antagonists on the acute opiate withdrawal induced by mu- and kappa-receptor agonists were investigated in vitro. 2. Following a 4 min in vitro exposure to morphine (moderately selective mu-agonist), [D-Ala2, Me-Phe4, Gly-ol5]enkephalin (DAMGO, highly selective mu-agonist) or U-50488H (highly selective kappa-agonist) the guinea-pig isolated ileum exhibited a strong contracture after the addition of naloxone. 3. The non-selective dopamine receptor antagonist haloperidol when added before or after the opioid agonists, was able dose-dependently to prevent or to reverse the naloxone-induced contracture after exposure to mu- (morphine and DAMGO) and kappa- (U-50488H) opioid agonists. The non-selective dopamine receptor agonist, apomorphine, was able to exert the same effects only at the highest concentration used. 4. The selective D2 dopamine receptor antagonist, sulpiride, was also able to reduce dose-dependently both mu- and kappa-opioid withdrawal, whereas the D1-receptor selective antagonist SCH 23390 did not affect either mu- or kappa-opioid withdrawal. 5. Bromocriptine, a D2 selective dopamine receptor agonist was able to increase significantly, and in a concentration-dependent manner, the naloxone-induced contracture by mu- and kappa-opioid agonists, whereas SKF 38393, a D1 selective dopamine receptor agonist, increased only the withdrawal after morphine or U50-488H. 6. Our data indicate that both D1 and D2 dopamine agonists and antagonists are able to influence opiate withdrawal in vitro, suggesting an important functional interaction between the dopaminergic system and opioid withdrawal at both the mu- and kappa-receptor level. 7. Furthermore, the ability of sulpiride to block strongly opiate withdrawal when compared to SCH 23390, as well as the effect of bromocriptine to increase opiate withdrawal suggest that D2 dopamine receptors may be primarily involved in the control of opiate withdrawal.     

Ligand binding profiles of U-69, 593-sensitive and-insensitive sites in human cerebral cortex membranes: evidence of kappa opioid receptors heterogeneity

Receptor binding studies were performed to characterize the properties of subtypes of kappa opioid receptors in membrane preparations of human cerebral cortex. [3H]U69,593 ([3H]U69), a selective kappa 1-agonist, and [3H]diprenorphine ([3H]DIP), a non-selective opioid antagonist, in the presence of 1 microM each of DAMGO, DPDPE and U-69 to block mu-, delta-, and kappa 1-sites, labeled single population of binding sites, respectively. [3H]U-69 binding sites (KD = 3.8 +/- 0.2 nM, Bmax = 6.3 +/- 0.2 fmol/mg protein) had a binding profile that correspond to kappa 1-receptor. That is, dynorphin A (1-13) (Dyn A), bremazocine (BZC), U50,488H (U50), (-)ethylketocyclazocine (EKC) and nor-binaltorphimine (nor-BNI) bound to this site with high affinities. [3H]DIP labeled binding sites (Kd = 7.3 +/- 0.2 nM, Bmax = 102 +/- 9 fmol/mg protein) that were not sensitive to U-50, but to BZC, EKC and nor-BNI. These results indicate that kappa 1 and Kappa 2 opioid receptors exist in human cerebral cortex with different ligand binding profiles.     

CME--what works?

AIM: To compare the inhibitory effects of 3 opioid receptor agonists, (D-Ala2, NMe-Phe4, Gly-ol)-enkephalin (DAGO), (D-Pen2,5)-enkephalin (D-PEN), and trans-(+/-)-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzeneacetamide methanesulfonate (U-50488H) in different concentrations on synaptic transmission. METHODS: The excitatory postsynaptic potentials (EPSP) in slice preparation of nucleus accumbens of rats were recorded using electric stimulation of the olfactory tubercle area and intracellular micropipettes filled with potassium acetate (3 mol.L-1). RESULTS: Superfusion of DAGO, D-PEN, and U-50488H (1 mumol.L-1) reduced the amplitude of EPSP and the inhibitory effect on EPSP were reversed by superfusing naloxone (Nal, 1 mumol.L-1), in which the DAGO-induced reduction of synaptic transmission was the most effective. The depolarizing responses to microiontophoretic injection of glutamate were reduced by superfusing DAGO in 19 neurons of slice preparation of nucleus accumbens. CONCLUSION: The inhibitory effects of DAGO, D-PEN, and U-50488H on EPSP were in a concentration-dependent manner, and the mechanism of opioid agonists (at least DAGO) reducing EPSP was related to a decrease of postsynaptic transmission mediated by glutamate.     

Gabapentin enhances the antinociceptive effects of spinal morphine in the rat tail-flick test

The antinociceptive effects of the combination of spinal morphine and gabapentin were evaluated in the tail-flick test in rats. The intrathecal coadministration of a subantinociceptive dose of morphine at 0.2 microgram and gabapentin at 300 micrograms produced significant antinociception. Pretreatment with spinal gabapentin at 300 micrograms shifted the dose-response curve of spinal morphine to the left with a decrease in morphine ED50 value from 1.06 micrograms to 0.34 microgram. The antinociceptive effects produced by the combination of a subantinociceptive dose of morphine and gabapentin were reversed by spinal naloxone at 30 micrograms but were not reversed by spinal bicuculline at 0.3 microgram. Furthermore, the concurrent administration of spinal naloxone at 30 micrograms with the combination of morphine and gabapentin blocked antinociception, while the concurrent administration of spinal bicuculline at 0.3 microgram failed to prevent antinociception. These results indicate that the combination of spinal gabapentin and morphine produces an enhancement of antinociception that appears to involve the spinal mu opioid receptors. Furthermore, repeated administration of gabapentin for 3 days did not affect the enhancing effect of gabapentin on the antinociceptive effect of morphine, indicating that tolerance did not develop to gabapentin's ability to enhance morphine antinociception.     

Cross-tolerance to acute administration of mu and kappa opioid agonists at the spinal cord level in the rat

Development of tolerance and cross-tolerance after acute administration of the mu agonist morphine and the kappa agonist U-50,488H was assessed in rats, through recording of a C-fiber-evoked spinal nociceptive reflex. Rats rendered tolerant to morphine (a single dose of 1 mg/kg i.p.) showed, after a 5-hour period, tolerance to morphine and cross-tolerance to the kappa-opioid receptor agonist U-50,488H, as revealed by depressed C-reflex responsiveness. In contrast, pretreatment with U-50,488H (a single dose of 1 mg/kg i.p.) rendered tolerant the rats to U-50,488H, but the animals did not develop cross-tolerance to morphine. Results indicate that acute administration of mu and kappa ligands leads to development of unidirectional cross-tolerance in rat spinal cord. This points to limitations in using alternated mu and kappa opioid agonists to bypass the problem of development of opioid tolerance in chronic pain complaints.