Search

Eight kappa-opioid receptor agonists were examined for their effects in squirrel monkeys responding under a fixed interval 3-min schedule of stimulus termination. Six of these kappa-opioid receptor agonists decreased dose-dependently the total number of responses and with an order of potency consistent with kappa-opioid receptor interaction. Three of these kappa-opioid receptor agonists, bremazocine, U69,593 [[(5a,7a,8b)-(+)-N-[7-(1-pyrrolidinyl)-1-oxaspiro(4,5)dec-8-yl)] benzeneacetamide] and enadoline, were evaluated following pretreatment with 1.0 mg/kg of naltrexone or 3.0 mg/kg of norbinaltorphimine. The effects of the three agonists were antagonized significantly by naltrexone, but only those of bremazocine and U69,593 were antagonized significantly by norbinaltorphimine. Statistical analysis of the data averaged over six monkeys revealed that naltrexone was significantly more potent than norbinaltorphimine at antagonizing enadoline and U69,593, but naltrexone and norbinaltorphimine were equipotent at antagonizing bremazocine. Moreover, naltrexone was 8-fold more potent at antagonizing U69,593 and enadoline than at antagonizing bremazocine. These results suggest that under these conditions the effects of U69,593 and enadoline may be mediated, in part, by a different receptor population, perhaps a subtype of kappa-opioid receptors, from the one that mediates the effects of bremazocine.

Analgesic cross-tolerance between alpha 2-adrenoceptor and opioid receptor agonists was studied using the mouse tail-flick assay. Mice tolerant to clonidine (0.3 mg/kg s.c.) or xylazine (7 mg/kg s.c.) were cross-tolerant to morphine (5 mg/kg s.c.), nalorphine (70 mg/kg s.c.) and supraspinal [D-Ala2,MePhe4,Gly(ol)5]enkephalin (DAMGO; 4 ng i.c.v.), but not trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)- cyclohexyl] benzeneacetamide methanesulfonate (U50,488; 5 mg/kg s.c.), spinal DAMGO (10 ng i.t.), supraspinal [D-Pen2,D-Pen5]enkephalin (DPDPE; 9 micrograms i.c.v.) or spinal DPDPE (700 ng i.t.). In the complimentary studies, mice tolerant to morphine and nalorphine were cross-tolerant to both of the alpha 2-adrenoceptor agonists, but U50,488 tolerant mice were not. The results suggest differential interactions between alpha 2-adrenoceptor and mu 1-, mu 2-, delta-, kappa 1- and kappa 3-opioid analgesic circuitry.

On the basis of their in vivo activity and binding affinity, nalorphine and (-)SKF 10,047 were classified as mixed agonist/antagonist compounds. However, in isolated tissue preparations without a selective antagonist to block their agonist effect, the characterization of these compounds and the determination of their antagonist activity were very difficult. Nor-binaltorphimine, a selective kappa-opioid receptor antagonist, was used in the longitudinal muscle preparations of the guinea pig ileum to block the kappa-agonist activity of nalorphine and (-)SKF 10,047. In the absence of their kappa-agonist activity, we were able to determine the mu-antagonist activity using the mu-selective agonist DAMGO ([D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin). The pA2 values for nalorphine and (-)SKF 10,047 were 7.50 and 7.69, respectively.

Rats were given intracerebroventricular or intravenous injections of the quarternary opioid receptor antagonist N,N-diallyl-normorphinium (DANM. 100 microgram). Ten min later morphine (7.5 mg/kg) or loperamide (10 mg/kg) was injected subcutaneously. Intestinal propulsion was assessed by measuring the progress of radioactive chromium (Na51(2) CrO4, 0.5 microCi) along the small intestine. The radioactive chromium was instilled into the proximal duodenum 20 min after injection of morphine or loperamide, and the animals sacrificed 35 min after giving radioactive chromium. Morphine and loperamide both inhibit intestinal propulsion. DANM (100 microgram i.c.v.) reduces the effect of morphine but not loperamide. Intravenous injection of DANM does not reduce anti-propulsive action of morphine. By itself DANM neither increases nor decreases intestinal propulsion. These experiments indicate that morphine, when administered by a peripheral route, reduces small intestinal propulsion in the rat partly through an action on brain opioid receptors.

Oral methylnalorphine ( methylnalorphinium ) caused a dose-dependent selective inhibition of inflammatory hyperalgesia (measured in the rat by a modified version of the Randall- Selitto test) without affecting the oedema. When subcutaneously injected, repeated doses of morphine for 5 days caused progressive analgesic tolerance. Tolerance was not observed after similar treatment with methylnalorphinium or methylmorphinium . Animals displaying analgesic tolerance to systemic morphine did not exhibit tolerance to the local ( intraplantar ) injection of morphine, methylnalorphinium or methylmorphinium . In contrast with nalorphine and other opiates, methylnalorphinium did not reduce intestinal transit in mice. Methylnalorphinium , a mixed opiate agonist-antagonist devoid of central effects, might be considered the prototype of an ideal peripheral analgesic since it was orally active, did not affect intestinal transit and did not cause analgesic tolerance.

Fentanyl (10 and 30 mug/kg), a narcotic analgesic, induced in cats a decrease in blood pressure and heart rate and reduced spontaneous splanchnic nerve activity. Fentanyl reduced the pressor response to medullary stimulation, but did not change the pressor response to hypothalamic or cervical spinal cord stimulation. Fentanyl reduced the potential evoked in the splanchnic nerve by stimulation at low frequency of a pressor area of the medulla oblongata. The potentials evoked in the splanchnic nerve by hypothalamic or cervical spinal cord stimulation were only slightly changed. Nalorphine (0.5 mg/kg) or naloxone (30 mug/kg) induced a recovery in blood pressure, heart rate and spontaneous splanchnic discharges which had been reduced by fentanyl, but nalorphine or naloxone did not restore pressor response to medullary stimulation or potentials evoked in the splanchnic nerve by medullary stimulation, which had been decreased by fentanyl.

The sedative effect of xylazine and its interaction with antagonists of alpha-adrenoceptors or opiate receptors was examined in chicks. The duration of the sleep-like behavioral state induced by xylazine was determined by measuring the time interval during which the chicks failed to exhibit the righting reflex. In these chicks, intramuscular administration of xylazine (0.3-4.8 mg/kg) induced a loss of the righting reflex, the duration of which was dose-dependent. Both alpha-adrenoceptor antagonists of the alpha2 type, i.e. yohimbine (0.1-1.0 mg/kg) and compound 170150 (0.1-1.0 mg/kg) (a benzodioxane derivative), and opiate receptor antagonists, i.e. naloxone (0.4-1.6 mg/kg) and nalorphine (3.0-30.0 mg/kg), effectively inhibited the sedative effect of xylazine while prazosin (0.1-10.0 mg/kg), an alpha 1-adrenoceptor antagonist, failed to antagonize xylazine-induced sedation. These findings suggest that in addition to the stimulation of central alpha 2-adrenoceptors, activation of an endogenous opiate mechanism may be involved in the sedative effect of xylazine.

Acute treatment of rats with morphine (10 mg/kg) resulted in a marked reduction of motor response to inescapable electric footshock (EFS). Nalorphine (2mg/kg) antagonized this action of morphine. Pretreatment with synthetic ACTH 1-24 (10 IU) 60 min prior to testing also inhibited this morphine-induced reduction, whereas other ACTH-like peptides, lacking corticotrophic activity, were ineffective. ACTH 1-24 had no effect on the response of adrenalectomized rats to EFS after morphine. In intact rats dexamethasone pretreatment 4 hr prior to testing also antagonized the action of morphine on EFS. Taken together these findings suggest that ACTH 1-24 interferes with the antinociceptive action of morphine and that the integrity of the adrenal is essential for demonstration of this antagonism.

The experiments concerned the effects of glucuronate or sulfate conjugation at the 6-position of nalorphine on the analgesic and antagonistic activities and also on the development of tolerance and physical dependence. Nalorphine-3-and 6-sulfate ester were synthesized for the first time. The analgesic effect of nalorphine-6-sulfate and -glucuronide was higher than that of nalorphine when assessed in the acetic acid writhing test. However, these 6-conjugates exhibited less potent agonistic activity in the test with guinea-pig ileum muscle strip and revealed no analgesic effect in the tail pinch test. The antagonistic activity of these 6-conjugates to morphine analgesia was lower on their s.c. injection, but higher on i.c.v. injection than that of nalorphine. The development of tolerance to the analgesia caused by nalorphine was not affected by the 6-modifications. Frequent withdrawal signs were seen in mice treated chronically with anlorphine-6-conjugates by challenging with naloxone while mice treated with nalorphine showed no such signs. This potent enhancing effect of 6-conjugation on the development of physical dependence was suggested to be also the case with morphine. These changes of potency due to conjugation were interpreted as due to the altered interaction with multiple opioid receptors.

Methylxanthines increase central noradrenergic turnover by an unknown mechanism. Isobutyl methylxanthine (IBMX, 0.5-16 mg/kg i.v.) was found in this study to increase firing rats of noradrenergic neurons in the locus coeruleus, supporting increased impulse flow as one possible mechanism for increased noradrenergic turnover. Naloxone (1 mg/kg i.p.) pretreatment potentiated the effects of IBMX, while clonidine (0.01 mg/kg i.v.) reversed increases in activity. Some of the behavioral effects of methylxanthines are consistent with increased noradrenergic activity.

The proposed kappa opioid receptor agonists ethylketocyclazocine (EK), nalorphine, bremazocine and U-50,488H were evaluated for their ability to produce contractions of isolated, vascularly perfused canine small intestinal segments. Responses to these agonists were compared to those of morphine and phenazocine, a mu benzomorphan. Morphine (0.04-25 micrograms) and phenazocine (0.01-3.0 micrograms) both produced naloxone-reversible contractions, suggesting that the responses were mediated largely by mu opioid receptors. In contrast, the proposed kappa agonists were ineffective in producing intestinal stimulation, with only EK (1-100 micrograms) showing minimal but significant activity at very high doses. We suggest that the effects of EK may be mediated through mu opioid receptors and that kappa receptors appear not to be involved in the contractile response of the dog small intestine to opioids.

C-fiber reflexes were recorded from an S1 ventral root in the acute decerebrate low spinal cat following stimulation of the ipsilateral superficial peroneal nerve or application of radiant heat to the metacarpel footpad. Naloxone when administered i.v. increased the electrically evoked C-fiber reflex to 158% (+/- 23.8% S.E.M.) of control 10 min after administration; whereas, naltrexone, 0.0025 mg/kg, increased the C-fiber reflex to 206 +/- 26.1% (S.E.M.) of control. Naloxone in a dose of 0.050 mg/kg increased the radiant heat evoked ventral root reflext to 161 +/- 19.5% of control. Nalorphine, 1 mg/kg, facilitated the electrically evoked C-fiber reflex to 282 +/- 75% of control. These findings that naloxone and naltrexone facilitated these reflexes in doses too small to have non-specific excitatory effects and that nalorphine facilitated the C-fiber reflex at a dose level that is depressant to the flexor reflex in the chronic spinal dog are consistent with a hypothesis that these effects are due to antagonism of a naturally occurring opiate-like inhibitory substance.

The analgesic activity of some opioid peptides which display a relative selectivity for either the mu-receptor subtype, [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAGO) or the delta-receptor subtype. [D-Ala2, D-Leu5]enkephalin (DADLE), [D-Ser2, Leu5]enkephalyl-Thr (DSLET) and [D-Thr2, Leu5]enkephalyl-Thr (DTLET) is highly correlated with their affinity for central or peripheral mu- but not delta-receptors. Moreover their analgesic effects as well as those elicited by degrading enzyme inhibitors (bestatin + thiorphan) of endogenous enkephalins were easily antagonized by naloxone with similar pA2 values but not by the delta-antagonist ICI 154,129. Therefore the analgesia produced by opioid peptides including endogenous enkephalins is likely connected to mu-receptor stimulation. Finally, there was no obvious potentiation by delta-agonists of the analgesia resulting from either administration of the mu-agonist morphine or endogenous enkephalins. This suggested that in the hot plate test, there is no modulation of the effect resulting from mu-receptor stimulation by a delta-receptor interaction. Likewise, enkephalinergic activity such as that due to thiorphan + bestatin does not appear to be regulated through mu- or delta-receptor stimulation.

The effects of increased doses (0.05-02 mg/kg) of morphine and nalorphine on colonic motility were investigated in 15-22 h fasted dogs fitted with two strain gauge transducers on the transverse colon at 8 and 15 cm from the ileo-colonic junction. These effects were compared to those obtained after previous administration of naloxone (0.3 mg/kg), atropine (0.1 mg/kg) and methysergide (0.1 mg/kg). Both morphine and nalorphine, at a dose of 0.1 mg/kg increased the colonic motility index by 285 and 248% from 0 to 60 min respectively after their administration, these effects lasting 3.2 and 2.6 h. These stimulatory effects were abolished by previous administration of naloxone (0.3 mg/kg) or atropine (0.1 mg/kg) and limited to 60 min after methysergide (0.1 mg/kg). It was concluded that nalorphine may be considered as a potent morphinomimetic substance upon colonic motility and that cholinergic and serotonergic receptors were involved in the colonic response to morphine.

Nalorphine, butorphanol and oxilorphan were compared in their ability to increase urinary output in the normally hydrated rat and to antagonize the increased urinary output produced by the full kappa-agonist, bremazocine. Nalorphine, butorphanol and oxilorphan increased 5-h cumulative urine output compared to controls, but the maximal effect was less than half the amount excreted after injection with 0.08 mg/kg of bremazocine. The effects of bremazocine were antagonized by nalorphine, butorphanol and oxilorphan. These effects of nalorphine, butorphanol and oxilorphan satisfy the criteria for classifying these agents as partial agonists at the kappa-opioid receptor responsible for mediating the effect of increased urinary output.

The purpose of this study was to investigate further the kappa opioid receptor selectivity of the field-stimulated isolated rabbit vas deferens preparation and to study the profile of a series of kappa agonists in this tissue. Agonists acting at mu, delta and sigma receptors were without detectable effect in the rabbit vas deferens. But a number of kappa agonists, including bremazocine, tifluadom, ethylketocyclazocine, ketocyclazocine, U-50,488 and Win 42,610 all depressed contractions, producing parallel dose-response curves. Mr 2034 generally produced a shallower dose-response curve and achieved a lower maximum effect, thus acting like a partial agonist. The effect of ethylketocyclazocine was not reduced by the irreversible mu antagonist, beta-funaltrexamine, confirming that it is not acting via mu receptors. Another group of drugs, including nalorphine, butorphanol and proxorphan, which produce an agonist action via kappa receptors in the guinea-pig ileum and mouse vas deferens, were antagonists in the rabbit vas deferens, suggesting that this tissue will only respond to high efficacy kappa agonists.

Pronounced resting tremor was produced in unanesthetized cats by injecting morphine (25-110 mug) into the caudate nucleus. The effects of morphine were antagonized by intracaudate (i.c.) injections of nalorphine (81-263 mug). Tremor activity was also inhibited by i.c. injections of dopamine (61-145 mug), Ca2+ (24-40 mug), scopolamine (88-121 mug) and hemicholinium-3 (HC-3; 73-129 mug) while serotonin (125 mug) was ineffective. Tremor inhibition by HC-3 was reversed by i.c. doses of acetylcholine (15-30 mug) which were subthreshold for tremor production in the absence of morphine. Morphine (55-110 mug) further increased the intensity of ongoing tremor activity induced by physostigmine (111 mug i.c.) I.c. injections of nalorphine antagonized the motor effects of morphine without affecting physostigmime tremor. Tremor production by morphine is attributed to a reduction in dopamine function which allows cholinergic activity in the caudate nucleus to predominate.

The capacity of the narcotic antagonists naloxone and nalorphine and the benzodiazepine derivatives diazepam and oxazepam to increase the LD50s of the narcotic analgesics morphine and methadone administered at convulsant doses was eveluated in the mouse. Naloxone produced a dose-related increase in the LD50s of both morphine and methadone. Iiazepam and oxazepam were also effective in increasing the LD50s of the narcotics; this effect was additive with that of naloxone. However, the maximal increase in the LD50s of the narcotics produced by pretreatment with naloxone alone was not increased further by the combined pretreatment of naloxone and a benzodiazepine. The anticonvulsant trimethadione did not elevate the LD50s of methadone, nor did it potentiate the effects of naloxone. These results suggest that the benzodiazepines may reduce the lethality of narcotic analgesics administered at high doses by a mechanism other than by an anticonvulsant effect alone. Therefore, the present results support the conclusion that the capacity to increase the convulsant LD50 of the narcotic analgesics is a general property of the narcotic antagonists.

Diallylnormorphine (DANM) the quaternary N-allyl derivative of nalorphine, administered to rats, 8 mg/kg i.p., 20 min before i.v. injection of the potent opiate buprenorphine (1 microgram/kg, tritrium-labeled), reduced in vivo binding of the latter (63% of controls at 30 min) in small intestine longitudinal muscle including myenteric plexus, but not in cerebrum. Naloxone, 1 mg/kg s.c., had the same effect of buprenorphine binding in the two sites (52 and 54% of controls respectively). The marked slowing in the transit of a forced charcoal meal through the small intestine of rats given 10 mg/kg morphine, i.v. was prevented to comparable extents by DANM and naloxone; the latter also abolished the delay in hot plate reaction induced in these animals by morphine which, however, retained considerable antinociceptive effect in DANM-pretreated rats.

Corticosteroids and ascorbic acid in the adrenal glands of adult cats have been investigated after single or repeated administration of morphine. Also plasma levels and urinary excretion of corticosteroids were determined. A significant increase in the content of corticosteroids in the glands and plasma was found after initial injection of morphine. After 7 days of consecutive morphine treatment a fall of corticosteroids in the glands was observed; after 2 weeks of daily injections the content of adrenal corticosteroids was significantly lower than in the control animals but no change was found in the plasma. Administration of the drug during 1 month led to a highly significant decrease of corticosteroids in the glands as well as in plasma. No significant change in adrenal ascorbic acids was found whether the adrenal corticosteroids were higher or lower than in the control cats. Urinary corticosteroids were high during the first week of morphine treatment but thereafter the excretion declined progressively and was lower than the control level after 13 days of treatment. The significant decrease of corticosteroids observed after repeated administration of morphine and the rise in adrenal corticosteroids found after the injection of nalorphine to the morphinized animals suggest that some kind of morphine dependence had been developed in the cats after repeated administration of the drug.

Neonatal mice were treated with d,1-methadone, 1-alpha-acetylmethadol (LAAM) or the narcotic antagonists, naloxone, nalorphine or levallorphan. Litter mates were injected with normal saline solution and handled in the same way. Treatment began on the second postpartum day and continued daily or on alternate days for up to 6 weeks. Injection of methadone in dosages of 2 mg/kg to mg/kg inhibited weight gain in a log dose-related fashion. LAAM, 1 mg/kg or 2 mg/kg also retarded weight gain. Mice gained weight normally when naloxone, 10 mg/kg was injected with methadone, 2 mg/kg. Furthermore the daily injection of d-methadone, 4 mg/kg, did not inhibit weight gain nor did any of the narcotic antagonists. There findings indicate that growth inhibition induced by methadone is a stereospecific, opioid effect.