NLX-112, a highly selective 5-HT1A receptor biased agonist, does not exhibit misuse potential in male rats or macaques.

NLX-112 (a.k.a. F13640 or befiradol) exhibits nanomolar affinity, exceptional selectivity and biased agonism at serotonin 5-HT1A receptors. NLX-112 displays robust analgesic activity in a number of rodent models of pain, and is currently developed as a treatment for l-DOPA-induced dyskinesia (LID) in Parkinson's disease (PD) patients. Noteworthy, PD patients can suffer from comorbid chronic pain, thus necessitating the use of analgesic drugs, such as opioids, which have potential for misuse. Additionally, dopamine agonists used to treat PD can produce cocaine-like effects in preclinical assays of misuse potential. The present study investigated whether NLX-112 possesses misuse potential of its own using two behavioural assays routinely used for this purpose: intracranial self-stimulation (ICSS) in rats, and cocaine discrimination in macaque monkeys. In rats, low doses of NLX-112 (0.03 and 0.1 mg/kg p.o.) did not alter ICSS frequency-rate curves, while higher doses (0.3 and 1.0 mg/kg) shifted the curve to the right and flattened it, i.e., reduced ICSS. As expected, cocaine (10 mg/kg i.p.) shifted the curve to the left, i.e., facilitated ICSS, but NLX-112 (0.03 and 0.1 mg/kg p.o.) did not further enhance cocaine-induced facilitation of ICSS. In monkeys trained to discriminate cocaine (0.4 mg/kg i.m.) from saline, NLX-112 (0.01-0.1 mg/kg p.o.) did not substitute for cocaine. Taken together, these results suggest that NLX-112, at doses displaying anti-dyskinetic activity in rat, marmoset and macaque models of LID, is free from misuse potential. From a translational perspective, this is a desirable property for a compound destined to be used in PD patients, who can suffer from comorbid chronic pain necessitating the use of potentially misused analgesic drugs.

Effects of the δ opioid agonist AZD2327 upon operant behaviors and assessment of its potential for abuse.

AZD2327 is a brain-penetrant agonist at δ opioid receptors which has antidepressant and anxiolytic properties in a wide array of animal models. As part of the preclinical safety pharmacology assessment, a number of studies were conducted in order to characterize its behavioral effects and its potential for abuse, in order to enable testing in humans. AZD2327 produced only modest effects when tested in a multiple fixed-ratio differential reinforcement of low rate schedule in rats, and did not enhance the rate-suppressing effects of ethanol in the procedure. In a suppressed responding test, AZD2327 only reduced rates of unpunished responding. In drug discrimination studies, AZD2327 produced partial or no generalization from known drugs of abuse. In primates trained to self-administer cocaine, substitution with AZD2327 did not result in appreciable self-administration of AZD2327, indicating that it does not behave as a positive reinforcer under the present conditions. Following termination of repeated administration of AZD2327, no signs of physical dependence (withdrawal) were noted. Overall, the data suggest that AZD2327 does not possess a high potential for abuse, and appears to have only subtle behavioral effects as measured by operant behaviors.

Different types of GABA(A) receptors may mediate the anticonflict and response rate-decreasing effects of zaleplon, zolpidem, and midazolam in squirrel monkeys.

RATIONALE: The role of different types of GABA(A) receptors in mediating anticonflict and response rate-decreasing effects of benzodiazepines in primate species is not known. OBJECTIVE: To examine the behavioral effects of the benzodiazepine-site, GABA(A) agonists zolpidem, zaleplon, and midazolam in the presence of two antagonists, flumazenil and beta-carboline-3-carboxylate-t-butyl ester (beta-CCt) in squirrel monkeys. METHODS: Two schedules of operant responding were used: (1) a multiple fixed-ratio (FR) schedule of food presentation involving punished and nonpunished behavior, and (2) an FR schedule of stimulus shock-termination. RESULTS: Midazolam (0.03-1.0 mg/kg), zolpidem (0.1-3.0 mg/kg), and zaleplon (0.1-3.0 mg/kg) increased rates of punished responding and decreased rates of nonpunished responding under the multiple schedule. Pretreatment with flumazenil (0.3-1.0 mg/kg) antagonized the anticonflict and response rate-decreasing effects of all three agonists. Pretreatment with beta-CCt (3-10 mg/kg) antagonized the anticonflict and rate-decreasing effects of midazolam, as well as the rate-decreasing effects of zolpidem and zaleplon. However, beta-CCt did not antagonize the anticonflict effects of zolpidem and zaleplon; instead, these effects of zolpidem and zaleplon were apparently enhanced in the presence of beta-CCt. Under the schedule of stimulus shock-termination, both flumazenil and beta-CCt antagonized zolpidem and zaleplon; however, the effects of beta-CCt were less consistent than the effects of flumazenil. CONCLUSION: In nonhuman primates, different types of GABAA receptors may mediate the anticonflict and the response rate-decreasing effects of the nonselective GABAA agonist midazolam and the selective GABAA1 agonists zolpidem and zaleplon.

Acute dependence on, but not tolerance to, heroin and morphine as measured by respiratory effects in rhesus monkeys.

Acute dependence on and tolerance to heroin and morphine were assessed in rhesus monkeys using measures of respiration. Respiratory frequency (f) and minute volume (V(e)) were measured in monkeys breathing air or 5% CO(2) in air using a pressure-displacement plethysmograph. Cumulative doses of naltrexone (0.0001-1.0 mg/kg, i.m) did not alter these parameters in untreated monkeys. Twenty-four hours after a cumulative dose of heroin (1 mg/kg, i.m.), naltrexone produced an increase in both f and V(e) when monkeys were breathing air or 5% CO(2). Following 1 to 3 days of treatment with heroin (0.5 mg/kg/day, i.m.) or morphine (16 mg/kg/day, i.m.), naltrexone produced an increase in f and V(e) that was related to the dose of naltrexone and the number of days of agonist administration. Two days following termination of heroin administration, naltrexone-induced respiratory stimulation declined and had disappeared completely by the fifth day. In tolerance studies, heroin (0.032-0.5 mg/kg, i.m.) and morphine (1-16 mg/kg, i. m.) were injected cumulatively each day for three consecutive days. These drugs suppressed f and V(e) to nearly the same extent on day 3 as they had on day 1 of administration. These results suggest that dependence to morphine and heroin can be measured under conditions of acute 1 to 3 day administration conditions in primates using f and V(e) as reliable and quantitative indicators of opioid withdrawal. Under these conditions, tolerance does not occur.

Buprenorphine and methoclocinnamox: agonist and antagonist effects on respiratory function in rhesus monkeys.

Buprenorphine and methoclocinnamox are partial micro-opioid receptor agonists with potential use in the treatment of opioid abuse. The ability of these drugs to suppress respiration as well as their ability to antagonize the respiratory suppressant effects of morphine and heroin were tested in rhesus monkeys. Frequency (f), minute volume (V(e)) tidal volume (V(t)) in monkeys breathing air or 5% CO(2) in air were recorded using a pressure-displacement plethysmograph. Buprenorphine (0.001-10 mg/kg) produced a dose-dependent decrease in respiratory parameters that plateaued at a dose of 1 mg/kg in both air and 5% CO(2). Methoclocinnamox (0. 032-1 mg/kg) also produced dose-dependent respiratory depression that plateaued at a dose of 0.3 mg/kg in air, and was directly related to dose in 5% CO(2). Respiratory suppression produced by buprenorphine 1 and 10 mg/kg lasted for 3 and 7 days, respectively, whereas the suppression produced by the largest dose of methoclocinnamox (1 mg/kg, the solubility limit) lasted less than 24 h. Buprenorphine and methoclocinnamox antagonized morphine- and heroin-induced respiratory depression, and this antagonist effect was observed concomitantly with, as well as following, the mu-opioid receptor agonist effects of buprenorphine and methoclocinnamox. The mu-opioid receptor antagonist effects of buprenorphine (10 mg/kg) and methoclocinnamox (1 mg/kg) lasted for 2 weeks. These results suggest that buprenorphine and methoclocinnamox have a wide margin of safety in clinical use and that these two compounds have a prolonged, insurmountable, mu-opioid receptor antagonist effect after the disappearance of their agonist effects.

Apparent pA2 values of benzodiazepine antagonists and partial agonists in monkeys.

Drugs that bind to benzodiazepine recognition sites of gamma-aminobutyric acid type A receptor complexes may function as agonists in some behavioral assays and as antagonists in other behavioral assays. The present studies compared the effects of the benzodiazepines midazolam, flumazenil, bretazenil, Ro 41-7812, and Ro 42-8773 and the beta-carboline, beta-carboline-3-carboxylate-t-butyl ester (beta-CCt) under two different types of schedule-controlled responding in squirrel monkeys. One group of monkeys responded under a fixed-ratio schedule of stimulus-shock termination, and a second group of monkeys responded under a multiple fixed-ratio schedule of food presentation involving suppressed and nonsuppressed behavior. Under the schedule of stimulus-shock termination, midazolam produced dose-related decreases in response rate, and these effects were surmountably antagonized by flumazenil, bretazenil, Ro 41-7812, Ro 42-8773, and beta-CCt. Schild plot analysis of these data revealed the following mean pA(2) values: flumazenil, 7.18; bretazenil, 7.62; Ro 41-7812, 7. 06; Ro 42-8773, 6.95. Apparent pA(2) values were not calculated for beta-CCt because the CL of the slope of the Schild plot included positive values. Under the multiple schedule, midazolam, bretazenil, and Ro 42-8773 dose-dependently increased rates of suppressed responding, whereas flumazenil, Ro 41-7812, and beta-CCt had no significant rate-altering effects. Flumazenil antagonized the antisuppressant effects of midazolam and bretazenil; however, individual variability in these effects prohibited the determination of apparent pA(2) values. These results indicate that in vivo pA(2) values may be determined for benzodiazepine-site ligands. These results further demonstrate that some benzodiazepine-site ligands, e. g., bretazenil and Ro 42-8773, may function as both agonists and as competitive antagonists in vivo.

Clocinnamox dose-dependently antagonizes morphine-analgesia and [3H]DAMGO binding in rats.

Clocinnamox is a long-lasting, nonequilibrium, mu-opioid receptor antagonist in mice and monkeys. The present studies examined the in vivo and ex vivo effects of clocinnamox in rats. Under control conditions, morphine dose-dependently increased tail-withdrawal latencies from 50 degrees C water, with a mean ED50 of 7.3 +/- 1.1 mg/kg. Clocinnamox antagonized the antinociceptive effects of morphine. 1.0 mg/kg clocinnamox displaced the morphine dose-response curve 4-fold to the right of the control curve and 10 mg/kg clocinnamox eliminated morphine's antinociceptive effects at doses up to 1000 mg/kg for at least seven days. There was a gradual recovery of the antinociceptive response to morphine; however, the morphine dose-response curve did not return to its original position by five weeks after 10 mg/kg clocinnamox. Whole brain membranes were prepared from separate groups of rats for determination of binding parameters of [3H][D-Ala2, N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO). Clocinnamox dose-dependently decreased [3H]DAMGO binding ex vivo and the decreased binding was a result of changes in Bmax. The control Bmax for [3H]DAMGO was 234 +/- 8 fmol/mg protein; in membranes prepared from rats pretreated with 10 mg/kg clocinnamox, the Bmax value for [3H]DAMGO was 54 +/- 2 fmol/mg protein. The Bmax values for [3H]DAMGO binding after an injection of 10 mg/kg clocinnamox returned towards control values gradually, four weeks after clocinnamox the Bmax was 178 +/- 10 fmol/mg protein. These results suggest that clocinnamox is a long-lasting, nonequilibrium mu-opioid receptor antagonist in rats.

Ventilation in morphine-maintained rhesus monkeys. I: Effects of naltrexone and abstinence-associated withdrawal.

The effects of naltrexone on ventilation were examined in three rhesus monkeys maintained on 3.2 mg/kg/day morphine. Before the onset of the daily morphine-dosing regimen, naltrexone had only modest effects on ventilation; a dose of 32 mg/kg increased ventilatory rate in the presence of normal air to 36 +/- 1 breaths/min, from a baseline rate of 25 +/- 1 breaths/min. Naltrexone did not affect other measures of ventilation in the presence of normal air or 5% CO2. Subsequent to the onset of the daily morphine injection regimen, naltrexone dose-dependently increased ventilatory rate at doses 4 orders of magnitude lower (0.001-0.01 mg/kg) than those effective in nondependent monkeys. A dose of 0.01 mg/kg naltrexone in morphine-maintained monkeys increased ventilatory rate in the presence of normal air to 52 +/- 4 breaths/min. Naltrexone also dose-dependently increased ventilatory rate in the presence of 3% and 5% CO2; tidal volume was not affected by naltrexone administration. Doubling the maintenance dose of morphine to 6.4 mg/ kg/day further increased the ventilatory effects of naltrexone. Withholding the maintenance dose of morphine also increased ventilatory rate without affecting tidal volumes, in a manner similar to that seen after naltrexone administration. These results are consistent with the view that changes in ventilation can be used to measure precipitated and abstinence-associated opioid withdrawal in monkeys.

Ventilation in morphine-maintained rhesus monkeys. II: Tolerance to the antinociceptive but not the ventilatory effects of morphine.

The antinociceptive and ventilatory effects of morphine and other opioid agonists were determined in three rhesus monkeys during a period of morphine maintenance, as well as before and after the chronic exposure to morphine. Before the onset of the daily dosing regimen, morphine increased tail-withdrawal latencies from 50 degrees C water, with an ED50 of 6.4 +/- 2.1 mg/kg. Daily injection of 3.2 mg/kg morphine produced a rightward displacement of the morphine dose-response curve, increasing the ED50 of morphine to 28.4 +/- 12.3 mg/kg. Doubling the daily morphine dose to 6.4 mg/kg resulted in a further shift to the right of the dose-response curve of morphine. After cessation of the daily dosing regimen, the morphine dose-response curve for producing antinociceptive effects returned toward baseline. The antinociceptive effects of the kappa opioid agonist, ethylketazocine, were similar during the period of daily exposure to morphine, and after cessation of the daily dosing regimen. Before the onset of the daily dosing regimen, morphine, ethylketazocine, fentanyl, butorphanol and nalbuphine decreased ventilation in the presence of air or air mixed with CO2. The baseline ED50 value of morphine for decreasing minute volume in the presence of 5% CO2 was 2.9 +/- 0.8 mg/kg. The ventilatory effects of morphine and other mu opioid agonists tested were not attenuated during the daily morphine-dosing regimen. After 40 weeks of daily injections of 3.2 mg/kg morphine, the ED50 of morphine for decreasing minute volume in 5% CO2 was 2.3 +/- 1.0 mg/kg, and when the daily dose was doubled to 6.4 mg/kg morphine, the ED50 of morphine was 1.5 +/- 0.5 mg/kg. The ventilatory depressant effects of the daily injection 3.2 mg/kg morphine were also unchanged during morphine maintenance. The differential development of tolerance to the antinociceptive and ventilatory effects of morphine demonstrates a separation of these two mu opioid agonist effects in rhesus monkeys.

Sensitization and tolerance to the discriminative stimulus effects of mu-opioid agonists.

The discriminative stimulus effects of several mu-opioid agonists were examined under conditions of opioid sensitization or tolerance, i.e., before and after 1-week SC infusions of naloxone or mu-opioid agonists. Rats were trained to discriminate 3.0 mg/kg morphine from saline using a two-lever, discrete trial, shock-avoidance/escape procedure. The rats generalized completely to morphine, fentanyl, meperidine, buprenorphine, and etorphine, and partially to pentazocine. A 7-day infusion of naloxone (0.3 mg/kg per h) potentiated the discriminative stimulus effects of all of these drugs. The magnitude of the increased potency varied indirectly with the efficacy of the mu-opioid agonists; potency ratios (pre-infusion ED50/post-infusion ED50) ranged from 1.58 (etorphine) to 3.58 (pentazocine). Stimulus generalization to morphine, fentanyl, and meperidine also was examined following infusions of equieffective doses of each of these three drugs. Differences among drugs were generally small, and failed to reach statistical significance. Nonetheless, the induction of mu-opioid tolerance did seem to vary with the efficacy of the three mu-opioid agonists. Thus, meperidine (6.25 mg/kg per h), which has the lowest efficacy of the drugs infused, produced the greatest shift to the right of the stimulus-generalization curves of these three drugs; the post-meperidine PR ranged between 0.40 and 0.61. Fentanyl (0.1 mg/kg per h), a drug with a higher efficacy at mu-opioid receptors, did not produce tolerance to the discriminative stimulus effects of morphine, fentanyl, or meperidine; potency ratios ranged from 0.50 to 0.75. Potency ratios for buprenorphine, etorphine, fentanyl, meperidine, and morphine after 7-day morphine infusions (0.75 mg/kg per h) ranged from 0.38 (buprenorphine) to 0.80 (etorphine). Morphine induced significant tolerance only to the discriminative stimulus effects of fentanyl. Our results suggest that different cellular mechanisms underlie the development of tolerance and sensitization to the discriminative stimulus effects of mu-opioid agonists.

Naltrexone in vivo protects mu receptors from inactivation by beta-funaltrexamine, but not kappa receptors from inactivation by nor-binaltorphimine.

The ability of the competitive opioid antagonist, naltrexone, to protect opioid receptors from inactivation by the nonequilibrium antagonists, beta-funaltrexamine (beta-FNA) and nor-binaltorphimine (nor-BNI), was examined in vivo. Male rats were injected SC with 10 mg/kg naltrexone or saline, 30 min before being injected intracisternally (IC) with water, 10 micrograms beta-FNA, or 1.0 or 10 micrograms nor-BNI. The rats were tested for analgesic responses to either U69,593 (nor-BNI groups) or morphine (beta-FNA groups), on a 50 degrees C hot plate, 24 h later. Morphine produced dose-related increases in the latency to paw lick in rats that received water (IC) (mean ED50 = 3.2 mg/kg). Little or no analgesia occurred after 1.0-30 mg/kg of morphine in animals that had received saline (SC) and 10 micrograms beta-FNA (IC) 24 h earlier. Pretreatment with 10 mg/kg naltrexone attenuated the antagonist effects of beta-FNA (morphine ED50 = 10.8 mg/kg). U69,593 also produced analgesia in animals that received water (IC) (ED50 = 0.97 mg/kg). This analgesia was dose-dependently blocked by nor-BNI for up to 7 days. Naltrexone did not inhibit the actions of nor-BNI. Thus, naltrexone prevented inactivation of mu receptors by beta-FNA but not inactivation of kappa receptors by nor-BNI, suggesting that antagonist interactions with mu receptors are different from those with kappa receptors.

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)

Apparent pA2 value of naltrexone is not changed in rats following continuous exposure to morphine or naloxone.

Chronic opioid antagonist administration increases opioid binding sites and potentiates behavioral responses to morphine. Conversely, chronic opioid agonist administration attenuates behavioral responses to morphine, though this is not necessarily accompanied by a parallel loss of binding sites. We examined the possibility that the in vivo affinity of the mu receptors might be altered as a consequence of the continuous administration of either naloxone or morphine. Rats were implanted sc with naloxone- or morphine-filled osmotic pumps; control animals were implanted with sham pumps. One week later, 24 hr after removing the osmotic pumps, cumulative dose-response curves for fentanyl analgesia were generated in the presence of 0.0, 0.03, 0.1, or 0.3 mg/kg naltrexone, using a tail-flick procedure. The analgesic ED50 (with 95% C. L.) of fentanyl in sham implanted animals, following saline pretreatment was 0.027 mg/kg (0.019, 0.039). The potency of fentanyl was decreased in rats infused with morphine, ED50 = 0.051 mg/kg (0.028, 0.093), and increased in rats that received naloxone, ED50 = 0.018 mg/kg (0.015, 0.022). The mean apparent pA2 value for naltrexone (with 95% C.L.) in the control group was 7.7 (7.5, 7.9). No differences were detected in animals that had received either naloxone or morphine for 7 days, pA2 = 7.8 (7.5, 8.1) and 7.4 (7.3, 7.6), respectively. Our results indicate that there is no change in the apparent affinity of the mu-receptor following continuous exposure to either an opioid agonist or antagonist, at a time when the analgesic potency of the agonist is decreased or increased, respectively.

Increased analgesic potency of mu agonists after continuous naloxone infusion in rats.

Chronic opioid antagonist administration increases opioid binding sites and potentiates behavioral responses to morphine. The present study was designed to examine in rats the temporal and dosage parameters of naloxone-induced potentiation of morphine analgesia and the effect of continuous infusion of naloxone on the analgesic potency of other mu agonists. Cumulative dose-response curves were generated in a tail-flick procedure for each drug tested. Naloxone-filled osmotic pumps were then implanted s.c. for 1 week after which the rats were retested with the agonist. The potency ratio of morphine (ED50 before naloxone/ED50 after naloxone) showed orderly increases over a range of naloxone doses (0.03-1.0 mg/kg/hr) and increased, then decreased, over a range of time points (6-72 hr) after removal of the osmotic pumps. The relative potency of morphine was 2.1 at 24 hr after a 7-day infusion of 0.3 mg/kg/hr of naloxone. These parameters were then used in tests of other mu agonists. Five drugs produced maximum increases in tail-flick latencies before naloxone. Of these, naloxone increased the analgesic potency of fentanyl, methadone and levorphanol (relative potencies ranged from 1.7-2.1) but not of etorphine and propoxyphene. The naloxone infusion increased the maximum analgesic effect of meperidine, profadol and pentazocine, but had no effect on the analgesic activity of buprenorphine, butorphanol, ethylketocyclazocine and nalbuphine. Our results demonstrate that 7-day naloxone infusion increases the analgesic potency of some, but not all, opioids with mu agonist activity.

Assessment of relative intrinsic activity of mu-opioid analgesics in vivo by using beta-funaltrexamine.

Morphine is the prototypic mu-opioid analgesic; however, in certain situations in vitro, morphine behaves as a partial agonist. To assess the relative intrinsic activity of morphine and three other mu-opioid analgesics in vivo, beta-funaltrexamine (beta-FNA), an irreversible antagonist selective for the mu receptor, was used to reduce the effective receptor reserve. By using a stereotaxic device, 1.25 to 20 micrograms of beta-FNA was infused into the lateral ventricle of rats. Twenty-four hours later, animals were tested in the tail-flick assay with cumulative doses of morphine, levorphanol, methadone or fentanyl. Pretreatment with 2.5 micrograms of beta-FNA induced parallel rightward shifts of both the morphine and levorphanol dose-effect curves and 5.0 micrograms of beta-FNA reduced the maximum analgesic effect of these agonists. Methadone surmounted the antagonism of 5.0 micrograms of beta-FNA; 10 micrograms was required to reduce the maximum analgesic effect of methadone. Fentanyl overcame the blockade induced by both 5.0 and 10 micrograms of beta-FNA. Only with a pretreatment dose of 20 micrograms of beta-FNA was the maximum analgesic effect of fentanyl reduced. Thus, when a certain proportion of mu receptors is inactivated, i.e., with 5.0 micrograms of beta-FNA, fentanyl and methadone have the capacity to surmount the blockade, whereas morphine and levorphanol do not. This suggests that fentanyl and methadone have higher intrinsic efficacies than do morphine and levorphanol. Thus, a strategy used widely in vitro was applied successfully in vivo to assess relative intrinsic activities of a series of mu-opioid agonists.