Use of irreversible antagonists to determine the relative efficacy of mu-opioids in a pigeon drug discrimination procedure: comparison of beta-funaltrexamine and clocinnamox

The use of irreversible antagonists to assess opioid efficacy has proven fruitful for classifying opioids on the basis of high or low efficacy, but few studies have provided quantitative estimates of efficacy. The purpose of this study was to use beta-funaltrexamine (beta-FNA) and clocinnamox (C-CAM) in a drug discrimination procedure to examine the efficacy of fentanyl, morphine, l-methadone, sufentanil, and etorphine. In pigeons trained to discriminate 0.12 mg/kg fentanyl from water, dose-effect curves were determined for each opioid alone and after pretreatment with beta-FNA and C-CAM. Using quantitative analyses according to an extended model of Black and Leff (1983), apparent efficacy (tau) and affinity (KA) of each opioid was determined, as well as the degree of receptor inactivation (q) produced by each dose of each antagonist. beta-FNA and C-CAM produced dose- and time-dependent, rightward shifts in the dose-effect curves of each opioid, and analyses based on dose-ratios and tau values suggest a rank order of efficacy of etorphine > sufentanil = l-methadone > fentanyl = morphine. Marked differences in the profiles of antagonism produced by beta-FNA and C-CAM were also apparent, as C-CAM, but not beta-FNA, produced insurmountable antagonism. The q values for each antagonist were consistent with these data in indicating that C-CAM and beta-FNA can inactivate nearly 100 and 75% of the receptor population, respectively. In tests conducted in pigeons chronically treated with morphine, doses of beta-FNA that produced parallel, rightward shifts in untreated pigeons flattened the morphine dose-effect curve in morphine-treated pigeons. These results indicate that beta-FNA and C-CAM can differentiate opioids with high relative efficacy and yield comparable estimates of efficacy for various opioids. There are, however, limitations in the proportion of the receptor population that can by eliminated by beta-FNA.     

Relative efficacy of buprenorphine,nalbuphine and morphine in opioid-treated rhesus monkeys discriminating naltrexone

Efficacy is one determinant of whether a drug is an agonist or an antagonist under a particular set of conditions. Relative efficacy among the micro opioid receptor (MOR) ligands buprenorphine, nalbuphine, and morphine was examined in monkeys dependent on morphine (3.2 mg/kg/day) or l-alpha-acetylmethadol (LAAM) (1.0 mg/kg twice daily) and that discriminated naltrexone (0.0178 mg/kg) from saline. In morphine-treated monkeys, buprenorphine and not nalbuphine substituted for naltrexone. When administered before naltrexone in morphine-treated monkeys, morphine and nalbuphine shifted the naltrexone dose-effect curve to the right, while buprenorphine shifted the naltrexone dose-effect curve to the left. Under conditions of acute morphine deprivation, naltrexone-lever responding was slightly attenuated by buprenorphine and markedly attenuated by nalbuphine and morphine. In LAAM-treated monkeys, buprenorphine substituted completely for naltrexone in only one monkey, while nalbuphine and morphine failed to substitute in any monkey. When administered before naltrexone in LAAM-treated monkeys, buprenorphine, nalbuphine, and morphine dose dependently shifted the naltrexone dose-effect curve to the right, with the exception of one monkey in which buprenorphine shifted the naltrexone dose-effect curve to the left. These results demonstrate that a low efficacy MOR ligand can exert agonist or antagonist actions in the same animal depending on immediate pharmacologic history. The qualitatively different effects of buprenorphine in morphine- and LAAM-treated monkeys might be related to magnitude of dependence insofar as dependence can determine the efficacy required for agonist activity. Thus, buprenorphine has markedly different effects across different levels of opioid dependence.     

Pharmacologic characterization of the sensitization to the rate-decreasing effects of naltrexone induced by acute opioid pretreatment in rats

The pharmacologic specificity of the sensitization to naltrexone induced by acute opioid pretreatment was studied in rats trained to lever-press on a multiple-trial, fixed-interval 3-min schedule of food reinforcement. Cumulative doses of naltrexone were given until responding was suppressed; control naltrexone ED50 values for decreasing response rates ranged from 5.0 to 22 mg/kg. Agonists were administered 4 hr before naltrexone challenge. Pretreatment with morphine (10 mg/kg) initially produced a 4-fold shift to the left of the naltrexone dose-effect curve, but after repeated weekly testing with various agonists, produced a 1700-fold shift. Pretreatment with other millimicrons agonists (i.e., 0.3 mg/kg of levorphanol, 0.06 mg/kg of fentanyl and 3.0 mg/kg of methadone) produced similarly large (100- to 250-fold) increases in sensitivity to the rate-decreasing effects of naltrexone. On the other hand, pretreatment with agonists selective for kappa (1.0 mg/kg of ethylketocyclazocine and 3.0 mg/kg of U-50,488) or sigma (10 mg/kg of [+]-N-allylnormetazocine) receptors, produced smaller (10-fold) changes in sensitization to naltrexone. Neither dextrorphan (3.0 mg/kg) nor pentobarbital (18 mg/kg) pretreatment altered sensitivity to naltrexone. Thus, the sensitization to naltrexone induced by acute opioid pretreatment was a stereoselective, opioid-specific effect, and appeared to be mediated primarily by a mu-opioid mechanism. With repeated testing, the effect of acute morphine pretreatment was comparable to that reported after chronic administration, thus supporting the hypothesis that this phenomenon reflects receptor-mediated changes that underlie the state of opioid physical dependence.     

Methocinnamox is a potent, long-lasting, and selective antagonist of morphine-mediated antinociception in the mouse: comparison with clocinnamox, beta-funaltrexamine, and beta-chlornaltrexamine

The irreversible mu-opioid antagonists beta-funaltrexamine (beta-FNA) and beta-chlornaltrexamine (beta-CNA) are important pharmacological tools but have a kappa-agonist activity and, in the latter case, low selectivity. This work examines whether clocinnamox (C-CAM) and the newer analog, methocinnamox (M-CAM), represent improved long-lasting antagonists for examining mu-opioid-mediated effects in vivo. beta-FNA, beta-CNA, C-CAM, and M-CAM were compared after systemic administration in mice and in vitro. beta-FNA and beta-CNA were effective agonists in the writhing assay, reversible by the kappa-antagonist norbinaltorphimine. Neither C-CAM nor M-CAM had agonist activity in vivo. M-CAM was devoid of agonist action at cloned opioid receptors. All four compounds depressed the dose-effect curve for the mu-agonist morphine in the warm-water tail-withdrawal test 1 h after administration; at 48 h, recovery was evident. In the writhing assay, the dose-effect curve for morphine was shifted in a parallel fashion in the order M-CAM > C-CAM > beta-CNA > or = beta-FNA. In comparison with their ability to shift the dose-effect curve for bremazocine (kappa) and BW373U86 (delta), beta-CNA was the least mu-selective, followed by C-CAM < beta-FNA < M-CAM. M-CAM (1.8 mg/kg) produced a 74-fold increase in the ED(50) of morphine while showing no effect on bremazocine or BW373U86 dose-response curves. In binding assays, C-CAM and M-CAM were 8-fold selective for mu- over kappa-receptors, whereas beta-FNA and beta-CNA were mu/delta-, but not mu/kappa, selective. However, ex vivo binding assays confirmed the mu-receptor selectivity of M-CAM. M-CAM is thus a potent, long-lasting, and specific antagonist at mu-receptors in vivo that lacks confounding agonist actions.     

Cocaine and amphetamine attenuate the discriminative stimulus effects of naltrexone in opioid-dependent rhesus monkeys

This study tested the hypothesis that stimulants (indirect dopamine agonists) attenuate the discriminative stimulus of naltrexone in monkeys chronically treated with L-alpha-acetylmethadol (LAAM). Four rhesus monkeys (Macaca mulatta) received LAAM (1.0 mg/kg s.c.) twice daily and discriminated a withdrawal-precipitating dose of naltrexone (0.0178 mg/kg s.c.) from saline. Cocaine (0.1-1.78 mg/kg), amphetamine (0.32-1.78 mg/kg), haloperidol (0.01-0.1 mg/kg), sulpiride (1.0-10.0 mg/kg), propranolol (0.32-3.2 mg/kg), clonidine (0.001-0.1 mg/kg), desipramine (0.32-3.2 mg/kg), and imipramine (1.0-10.0 mg/kg) were given s.c. before cumulative doses of naltrexone. Cocaine and amphetamine antagonized the discriminative stimulus effects of naltrexone, each shifting the naltrexone dose-effect curve significantly (e.g., 100-fold) rightward or downward. In contrast, the dopamine antagonist haloperidol shifted the naltrexone dose-effect curve 5-fold leftward. Sulpiride, desipramine, clonidine, and propranolol had comparatively less effect on the naltrexone discriminative stimulus, whereas some doses of imipramine attenuated the naltrexone stimulus in a manner similar to that of cocaine and amphetamine. These results support the notion that multiple neurotransmitter systems are involved in the discriminative stimulus effects of opioid withdrawal. Furthermore, these data are consistent with reports that dopamine levels decrease during opioid withdrawal and provide evidence that enhancing dopamine or other monoamine levels may attenuate subjective effects of opioid withdrawal.     

Clocinnamox distinguishes opioid agonists according to relative efficacy in normal and morphine-treated rats trained to discriminate morphine

High doses of insurmountable antagonists or frequent administration of high doses of agonists are required to alter the potency of opioid agonists to produce discriminative stimuli. In the present study, insurmountable antagonism and repeated agonist treatment were combined to remove or disable a large enough proportion of mu-opioid receptors to alter the potency or maximal effect for four agonists in male Sprague-Dawley rats trained to discriminate 3.2 mg/kg morphine from saline under a fixed-ratio 15 schedule of food reinforcement. All agonists produced 88 to 100% morphine responding and were differentially sensitive to clocinnamox antagonism (fentanyl < morphine < or = buprenorphine = nalbuphine). Repeated treatment with 20 mg/kg per day morphine for 6 days decreased by 2- to 3-fold the potency of fentanyl, morphine, and buprenorphine to produce morphine responding. After morphine treatment, 3.2 mg/kg clocinnamox produced a 7-fold further decrease in morphine potency. Clocinnamox (10 mg/kg) produced a 7- and 12-fold further decrease in morphine and fentanyl potency, respectively, a reduction in the slope of the morphine dose-response curve, and a suppression of the maximal morphine responding for buprenorphine. Repeated treatment with 10 mg/kg per day morphine for 6 days failed to alter the potency of nalbuphine to produce morphine responding. In these morphine-treated rats, doses of 3.2 or 10 mg/kg clocinnamox suppressed the maximal morphine responding. Taken together, these data indicate that combined insurmountable antagonist and repeated agonist treatment produce additive effects at mu-opioid receptors to diminish discriminative stimulus effects in a manner predicted by the relative efficacy of opioid agonists.     

Buprenorphine and opioid antagonism, tolerance, and naltrexone-precipitated withdrawal

The dual antagonist effects of the mixed-action μ-opioid partial agonist/κ-opioid antagonist buprenorphine have not been previously compared in behavioral studies, and it is unknown whether they are comparably modified by chronic exposure. To address this question, the dose-related effects of levorphanol, trans-(-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide (U50,488), heroin, and naltrexone on food-maintained behavior in rhesus monkeys were studied after acute and chronic treatment with buprenorphine (0.3 mg/kg/day). In acute studies, the effects of levorphanol and U50,488 were determined at differing times after buprenorphine (0.003-10.0 mg/kg i.m.). Results show that buprenorphine produced similar, dose-dependent rightward shifts of the levorphanol and U50,488 dose-response curves that persisted for ≥ 24 h after doses larger than 0.1 mg/kg buprenorphine. During chronic treatment with buprenorphine, the effects of levorphanol, U50,488, heroin, and naltrexone were similarly determined at differing times (10 min to 48 h) after intramuscular injection. Overall, results show that buprenorphine produced comparable 3- to 10-fold rightward shifts in the U50,488 dose-response curve under both acute and chronic conditions, but that chronic buprenorphine produced larger (10- to ≥ 30-fold) rightward shifts in the heroin dose-effect function than observed acutely. Naltrexone decreased operant responding in buprenorphine-treated monkeys, and the position of the naltrexone dose-effect curve shifted increasingly to the left as the time after daily buprenorphine treatment increased from 10 min to 48 h. These results suggest that the μ-antagonist, but not the κ-antagonist, effects of buprenorphine are augmented during chronic treatment. In addition, the leftward shift of the naltrexone dose-effect function suggests that daily administration of 0.3 mg/kg buprenorphine is adequate to produce opioid dependence.     

Pentazocine-induced antinociception is mediated mainly by μ-opioid receptors and compromised by κ-opioid receptors in mice

Pentazocine is a widely used mixed agonist-antagonist opioid. Previous animal studies have demonstrated that pentazocine-induced antinociception displayed a ceiling effect characterized by biphasic dose response with a increasing and then descending analgesia like a bell-shaped curve. This study attempted to clarify the mechanisms underlying such dose-response relationships. ddY and C57BL/6J mice received subcutaneous injection of saline or pentazocine (3, 10, 30, 56, or 100 mg · kg(-1)), at 120 min after subcutaneous injection of saline, a μ-opioid receptor antagonist clocinnamox mesylate (C-CAM) (5 mg · kg(-1)), a κ-opioid receptor antagonist nor-binaltorphimine (nor-BNI) (10 mg · kg(-1)), or the combination of C-CAM and nor-BNI. The antinociceptive effects of pentazocine were evaluated using tail pressure, hot plate, tail flick, and acetic acid writhing tests. Without pretreatment with an opioid receptor antagonist, the antinociceptive effects of pentazocine exhibited biphasic bell-shaped dose-response curves peaking at 30 mg · kg(-1). C-CAM completely and partly antagonized the antinociception induced by pentazocine at low (3-30 mg · kg(-1)) and high (56-100 mg · kg(-1)) doses, respectively. nor-BNI enhanced the antinociception by pentazocine at high doses and turned the later descending portion of the biphasic dose-response curves into a sigmoid curve. The combination of C-CAM and nor-BNI completely abolished the antinociception by pentazocine at all doses. Our results suggest pentazocine produces antinociception primarily via activation of μ-opioid receptors, but at high doses, this μ-opioid receptor-mediated antinociception is antagonized by concomitant activation of κ-opioid receptors. This provides the first reasonable hypothesis to explain the ceiling effects of pentazocine analgesia characterized by a biphasic dose response.     

Mirfentanil: pharmacological profile of a novel fentanyl derivative with opioid and nonopioid effects.

Mirfentanil [N-(2-pyrazinyl)-N-(1-phenethyl-4-piperidinyl)-2-furamide] was studied for its binding affinity in isolated neuronal membranes, and for its effects in vivo. In binding to opioid receptors in monkey brain membranes, mirfentanil was much more selective for mu sites (7.99 nM) than for either kappa (1428 nM) or delta (480 nM) sites as measured by displacement of [3H]DAMGO. [3H]U-69.593 or [3H]DPDPE, respectively. In morphine-treated pigeons discriminating among naltrexone, saline and morphine, mirfentanil failed to substitute for either training drug; in morphine-abstinent pigeons, mirfentanil reversed responding on the naltrexone key (i.e., reversed withdrawal). In morphine-treated monkeys discriminating between saline and naltrexone, mirfentanil substituted completely for naltrexone, and this effect was attenuated by an acute injection of morphine; mirfentanil also attenuated the withdrawal-reversing effects of alfentanil in morphine-abstinent monkeys. Administered i.v., mirfentanil maintained rates of self-administration responding only slightly below rates maintained by alfentanil, and this effect of mirfentanil was antagonized by quadazocine. Small doses of mirfentanil (0.032-0.32 mg/kg) antagonized the analgesic effects of alfentanil; larger doses of mirfentanil both antagonized the analgesic effects of alfentanil and produced analgesic effects when administered alone. The analgesic effects of mirfentanil were not attenuated by large doses of opioid antagonists. Mirfentanil had modest respiratory depressant effects that were not altered by quadazocine; however, mirfentanil antagonized the respiratory depressant effects of large doses of alfentanil. Both in vivo and in vitro, mirfentanil appears to have selectivity for opioid mu receptors. Moreover, at doses larger than those which exert opioid effects, mirfentanil has nonopioid analgesic effects.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparing the effects of anileridine, alphaprodine and fentanyl on schedule-controlled responding by pigeons

The effects of anileridine, alphaprodine and fentanyl were studied on responding by pigeons under a multiple fixed-ratio, fixed-interval schedule of food presentation. Generally, all three drugs produced dose-related decreases in responding under both components of the multiple schedule, but rate increases were observed after low doses of anileridine and alphaprodine in some birds. Naloxone (1 mg/kg) antagonized the rate-increasing and rate-decreasing effects of doses of anileridine and alphaprodine of 10 mg/kg or less, whereas the effects of higher doses were not antagonized by naloxone. Likewise, chronic methadone or morphine (120 mg/kg/day p.o.) dosing produced only a slight cross-tolerance to the rate-decreasing effects of anileridine and alphaprodine. In contrast, naloxone (0.01, 0.1 and 1 mg/kg) and chronic methadone or morphine administration shifted the dose-effect curve for fentanyl to the right, indicating narcotic antagonism and methadone and morphine-induced cross-tolerance. These data indicate that the rate-decreasing effects of anileridine and alphaprodine are related only slightly to narcotic effects, whereas the rate-decreasing effects of fentanyl are primarily narcotic effects.     

Paradoxical effect of chronic fentanyl treatment on naltrexone-induced supersensitivity and upregulation.

The present study sought to evaluate the influence of chronic opioid antagonist treatment upon the discriminative stimulus and analgesic effects of the opioid receptor agonist fentanyl. Male Wistar rats were trained to discriminate fentanyl (0.04 mg/kg) from saline in a two-lever food reinforced paradigm. After acquisition of the discrimination, they were implanted with osmotic minipumps which delivered either naltrexone (0.07 mg/h) or distilled water, and the sensitivity of discrimination was assessed at various times after pump removal. The influence of chronic naltrexone treatment upon the antinociceptive effects of fentanyl was assessed in drug-naive (control) rats and in rats which had received fentanyl in the same dosage schedule as those in drug discrimination experiments. Chronic infusion of naltrexone for 7 days did not modify the dose-response curve for the fentanyl vs. saline discrimination. Algesiometric tests revealed a significant increase in the antinociceptive effect of fentanyl in control rats after naltrexone treatment. In contrast, such supersensitivity was not observed in rats which had previously received fentanyl injections. Autoradiographic data revealed a naltrexone-induced upregulation of mu opioid receptors in control animals. Paradoxically, this effect was significantly increased in fentanyl-pretreated rats. These data suggest that prior drug experience can affect the development of antagonist-induced supersensitivity to the behavioral actions of opioid agonists. Furthermore, it would appear that after chronic agonist treatment the phenomena of opioid receptor upregulation and functional supersensitivity are dissociated.     

Effects of quaternary naltrexone and chlordiazepoxide in squirrel monkeys with enhanced sensitivity to the behavioral effects of naltrexone

Dose-effect curves were determined for the effects of the opioid antagonist, naltrexone, on lever-pressing responses of squirrel monkeys maintained under a 30-response fixed-ratio schedule of food presentation. Cumulative doses of naltrexone up to 3 mg/kg i.m. had little or no effect on fixed-ratio responding, whereas higher doses reduced responding in all monkeys. After initial determinations of the dose-effect curve for naltrexone, repeated daily injections of high doses of naltrexone resulted in enhanced sensitivity to its behavioral effects. The cumulative dose-effect curve for naltrexone determined after termination of daily injections was shifted more than 3-fold to the left. In these same monkeys, the dose-effect curve for naltrexone methobromide, a quaternary derivative of naltrexone with limited access to the central nervous system, was similar to the initial dose-effect curve for naltrexone. Chlordiazepoxide given 1 hr before the experimental session shifted the naltrexone dose-effect curve back toward its initial position before the regimen of daily injections, but had no systematic effect on the dose-effect curve for quaternary naltrexone. Enhanced sensitivity to the behavioral effects of naltrexone in these experiments appears to depend on a central component of action and is attenuated by chlordiazepoxide.     

Development of tolerance to the analgesic activity of mu agonists after continuous infusion of morphine, meperidine or fentanyl in rats.

We investigated the role of intrinsic activity in the ability of mu-opioid agonists to produce tolerance to the analgesic effects of other mu agonists. Dose-response curves were generated for each test drug before and 24 hr after 1 week s.c. agonist infusions, using a tail-flick procedure in male rats. Morphine tolerance was dose-dependent over a range of doses, 0.3 to 1.4 mg/kg/hr, infused for 7 days by osmotic pump; 0.8 mg/kg/hr (1/4 acute ED50/hr) shifted the morphine dose-response curve roughly 2-fold to the right. Morphine, fentanyl or meperidine were then infused for 1 week to induce tolerance; doses were based on equipotent acute s.c. doses (1/4 ED50/hr). Drugs tested for analgesic activity, using a cumulative dosing procedure, included morphine, fentanyl, meperidine, methadone, buprenorphine, etorphine and levorphanol. The relative potency (RP) was calculated for each animal by dividing the preinfusion analgesic ED50 by the postinfusion ED50. The potency of morphine, fentanyl, meperidine and levorphanol decreased after a 7-day infusion of 0.8 mg/kg/hr morphine (RP = 0.44-0.70), but the potency of etorphine (RP = 0.85) and methadone (RP = 0.78) were not significantly changed. Chronic infusions of 6.25 mg/kg/hr meperidine produced more tolerance than did morphine (RP = 0.11-0.51). No significant change in analgesic potency was seen in six of the seven test drugs after infusions of 0.01 mg/kg/hr fentanyl (RP = 0.81-1.27). Buprenorphine did not produce an analgesic response in rats that received infusions of any of the three mu agonists. The RP of drugs with low intrinsic activity was lower than the RP of high efficacy drugs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacological mechanisms of action of flupirtine: a novel, centrally acting, nonopioid analgesic evaluated by its discriminative effects in the rat

Rats were trained to discriminate the novel analgesic flupirtine (10.0 mg/kg i.p., 10 min) from no drug under a two-choice fixed-ratio 5 shock-termination schedule. Flupirtine yielded a dose-response curve with an ED50 of 3.87 mg/kg. The opioid analgesics pentazocine, codeine and tramadol failed to produce flupirtine appropriate responding. The opioid antagonist naltrexone did not antagonize the discriminative effects of flupirtine. The mixed alpha-1/alpha-2 adrenergic agonist clonidine and the highly specific alpha-2 adrenergic agonist UK-14304, both partially and dose-dependently produced flupirtine appropriate responding. The mixed alpha-1/alpha-2 antagonist yohimbine and the highly specific alpha-2 antagonists idazoxan and L-654,284 all partially and dose-dependently antagonized flupirtine appropriate responding. Neither of the alpha-1 agonists phenylephrine or ST 587 produced flupirtine appropriate responding, nor did the alpha-1 antagonist prazosin antagonize flupirtine responding. It is concluded that the discriminative effects of flupirtine are neither of opioid nor of alpha-1 adrenergic type, but are primarily mediated through alpha-2 adrenergic mechanisms.     

Discriminative stimulus effects of naltrexone in morphine-treated rhesus monkeys

Three adult, female rhesus monkeys responded under a fixed-ratio 5 schedule of stimulus-shock termination while discriminating between s.c. injections of saline and naltrexone (0.01 mg/kg). In addition, monkeys received daily injections of morphine (1.78 or 3.2 mg/kg) 3 hr before experimental sessions. With increasing doses of naltrexone monkeys switched in a dose-related manner from the saline to the naltrexone lever; complete generalization (greater than 80% responding on the naltrexone lever) occurred in all monkeys at doses of naltrexone larger than 0.0032 mg/kg. Doses of naltrexone that produced responding on the drug lever also produced effects typically observed during opioid withdrawal (e.g., miosis and salivation). Compounds with opioid mu antagonist effects under other conditions (e.g., nalorphine) substituted for naltrexone whereas a wide variety of opioid agonists (e.g., morphine, U-50,488 and butorphanol) as well as nonopioids (e.g., pentobarbital and ketamine) produced responding predominantly on the saline lever. Monkeys also switched to the naltrexone lever in a time-related manner after the daily injection of morphine with complete generalization occurring between 8 and 27 hr after morphine. Among a variety of opioid and nonopioid compounds, including drugs used in the treatment of opioid abuse (e.g., clonidine), only compounds with mu agonist actions under other conditions produced a switch in responding from the naltrexone to the saline lever in 27-hr morphine-abstinent monkeys. These results suggest single, daily injections of morphine are sufficient to produce dependence in rhesus monkeys and indicate further drug discrimination studies in morphine-treated rhesus monkeys might be pharmacologically more specific than observational procedures.     

Analgesic effect of the direct D2 dopamine receptor agonist RU 24926 and cross tolerance with morphine

Summary— The direct D₂ dopamine receptor agonist RU 24926, administered subcutaneously to mice, elicited, starting at the dose of 0.125 mg/kg, a dose dependent analgesic effect, assessed as the jump latency from a hot plate (55°C). The analgesic effect induced by 0.25 mg/kg RU 24926 was dose dependently antagonized by the preferential D₂ dopamine receptor antagonist haloperidol (ID₅₀ = 15.1 ± 3.3 μg/kg sc) as well as by the opioid receptor antagonist naloxone (ID₅₀ = 0.59 ± 0.17 mg/kg sc). The reversion of RU 24926-induced analgesia by naloxone was not accompanied by a reversion of hypothermia. Semi-chronic administration of RU 24926 (2.5 mg/kg, sc, 3 times a day for 3 days) completely desensitized to the analgesic effect induced by a 0.25 mg/kg test dose of RU 24926 and partially reduced the analgesic effect of low doses of morphine (0.5, 1, 1.5 mg/kg). Conversely, semi-chronic administration of morphine (32 mg/kg sc, twice daily for 4 days) completely desensitized the analgesic effect induced by a 2 mg/kg test dose of morphine and partially reduced the analgesic effect of RU 24926 (0.25, 0.5 and 1 mg/kg). The RU 24926-induced analgesia did not seem to be the nonspecific consequence of its hypothermic effect since: i) the time course of hypothermia was shorter than that of analgesia, ii) the analgesic effect was observed in inbred C3H mice which have a low sensitivity to the hypothermic effect of direct agonists of D₂ dopamine receptors and iii) in mice semi-chronically treated with morphine and thereby made tolerant to the RU 24926-induced analgesia, the hypothermic effect of RU 24926 was found unchanged. It is suggested that the stimulation of D₂ dopamine receptors, without obvious link with the resultant hypothermia, releases an opioid material which induces analgesia.     

Analgesia produced by normal doses of opioid antagonists alone and in combination with morphine

In a recent study [30] it was reported that naloxone, at doses normally employed for opioid antagonism, produced a dose-dependent analgesia in BALB/c mice in the formalin test. We report here that another opioid antagonists, naltrexone, also produces analgesia under these conditions. Female BALB/c mice were injected subcutaneously with naltrexone (0.01-1.0 mg/kg) or saline alone and tested for analgesia using the formalin test. Naltrexone produced a statistically significant dose-dependent analgesia, with an ED50 of 0.05 mg/kg and almost total analgesia at doses of 0.1 mg/kg or greater. To determine the relationship between naloxone analgesia and better documented forms of opioid analgesia, BALB/c mice were injected with naloxone or saline following the administration of a pre-determined ED50 for morphine and tested for analgesia using the tail-flick and formalin tests. Naloxone antagonized morphine analgesia in the tail-flick test at both doses used (0.3 and 10 mg/kg). In the formalin test, however, naloxone attenuated morphine analgesia at the lower doses (0.1 and 0.3 mg/kg) and potentiated morphine analgesia at the highest dose (10 mg/kg). The implications of this finding are discussed.     

Effects of beta-funaltrexamine in normal and morphine-dependent rhesus monkeys: observational studies

The behavioral effects of the opioid receptor alkylating agent beta-funaltrexamine (beta-FNA) were assessed in normal (drug-naive) and morphine-dependent rhesus monkeys. In normal monkeys, beta-FNA (10 mg/kg s.c.) produced muscle relaxation and stupor, which could be reversed by the opioid antagonist Win 44,441. Given as a 48-hr pretreatment, beta-FNA antagonized the behavioral effects of acute morphine, but not those of two kappa agonists, ethylketazocine and Mr 2033 (UM 1072). In morphine-dependent monkeys, beta-FNA (10 mg/kg, s.c. and 0.003 mg i.c.v.) precipitated severe abstinence which lasted for 3 days. beta-FNA was more than 13,000 times more potent in precipitating withdrawal after i.c.v. than s.c. administration, whereas naltrexone and Win 44,441 were equipotent by these routes. Deprivation-induced abstinence (14 hr) and withdrawal of similar severity precipitated by naltrexone, Win 44,441 or naloxonazine were suppressed completely by 17.5 mg/kg of morphine. In contrast, 320 mg/kg of morphine failed to suppress completely a withdrawal syndrome of the same severity elicited by s.c. or i.c.v. beta-FNA. These data are consistent with the view that beta-FNA has reversible opioid agonist and insurmountable mu selective antagonist activity in the rhesus monkey.     

Behavioral pharmacology of the mu/delta opioid glycopeptide MMP2200 in rhesus monkeys

H(2)N-Tyr-D-Thr-Gly-Phe-Leu-Ser-(O-beta-D-lactose)-CONH(2) (MMP2200) is a novel glycopeptide opioid agonist with similar affinities for mu and delta receptors. Glycosylation promoted brain penetration and production of centrally mediated behavioral effects in mice; however, it is unknown whether the magnitude of enhanced brain penetration is sufficient to permit central mediation of drug effects and production of synergistic mu/delta antinociceptive interactions after systemic administration in primates. To address this issue, the present study compared the effects of MMP2200 and the mu-agonist morphine in four behavioral procedures in rhesus monkeys. In an assay of thermal nociception, morphine (1.0-5.6 mg/kg) produced dose-dependent antinociception, whereas MMP2200 (10-56 mg/kg) was ineffective. In an assay of capsaicin-induced thermal allodynia, both morphine (0.01-1.0 mg/kg) and MMP2200 (0.032-3.2 mg/kg) produced dose-dependent antiallodynic effects. MMP2200-induced antiallodynia was blocked by the moderately mu-selective antagonist naltrexone (0.01 mg/kg), the delta-selective antagonist naltrindole (1.0 mg/kg), and the peripherally selective opioid antagonist quaternary naltrexone (0.32 mg/kg). In an assay of schedule-controlled behavior, both morphine (0.01-1.0 mg/kg) and MMP2200 (10-56 mg/kg) decreased response rates. Morphine effects were antagonized by naltrexone (0.001-0.01 mg/kg); however, the effects of MMP2200 were not antagonized by either naltrexone (0.01 mg/kg) or naltrindole (1.0 mg/kg). In an assay of drug self-administration, morphine (0.0032-0.32 mg/kg/injection) produced reinforcing effects, whereas MMP2200 (0.032-0.32 mg/kg/injection) did not. These results suggest that systemically administered MMP2200 acted as a peripheral, mu/delta-opioid agonist with limited distribution to the central nervous system in rhesus monkeys. These results also suggest the existence of species differences in the pharmacokinetics and brain penetration of glycopeptides.