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

The discriminative stimulus effects of several μ-opioid agonists were examined under conditions of opioid sensitization or tolerance, i.e., before and after 1-week SC infusions of naloxone or μ-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 μ-opioid agonists; potency ratios (pre-infusion ED 50 /post-infusion ED 50 ) 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 μ-opioid tolerance did seem to vary with the efficacy of the three μ-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 μ-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 μ-opioid agonists.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46342/1/213_2005_Article_BF02244991.pd

The Intriguing Effects of Substituents in the N-Phenethyl Moiety of Norhydromorphone: A Bifunctional Opioid from a Set of “Tail Wags Dog” Experiments

This work is licensed under a Creative Commons Attribution 4.0 International License.(−)-N-Phenethyl analogs of optically pure N-norhydromorphone were synthesized and pharmacologically evaluated in several in vitro assays (opioid receptor binding, stimulation of [35S]GTPγS binding, forskolin-induced cAMP accumulation assay, and MOR-mediated β-arrestin recruitment assays). “Body” and “tail” interactions with opioid receptors (a subset of Portoghese’s message-address theory) were used for molecular modeling and simulations, where the “address” can be considered the “body” of the hydromorphone molecule and the “message” delivered by the substituent (tail) on the aromatic ring of the N-phenethyl moiety. One compound, N-p-chloro-phenethynorhydromorphone ((7aR,12bS)-3-(4-chlorophenethyl)-9-hydroxy-2,3,4,4a,5,6-hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinolin-7(7aH)-one, 2i), was found to have nanomolar binding affinity at MOR and DOR. It was a potent partial agonist at MOR and a full potent agonist at DOR with a δ/μ potency ratio of 1.2 in the ([35S]GTPγS) assay. Bifunctional opioids that interact with MOR and DOR, the latter as agonists or antagonists, have been reported to have fewer side-effects than MOR agonists. The p-chlorophenethyl compound 2i was evaluated for its effect on respiration in both mice and squirrel monkeys. Compound 2i did not depress respiration (using normal air) in mice or squirrel monkeys. However, under conditions of hypercapnia (using air mixed with 5% CO2), respiration was depressed in squirrel monkeys.NIDA grant P30 DA13429NIDA grant DA039997NIDA grant DA018151NIDA grant DA035857NIDA grant DA047574NIH Intramural Research Programs of the National Institute on Drug AbuseNational Institute of Alcohol Abuse and AlcoholismNIH Intramural Research Programs of the National Institute on Drug AbuseNIH Intramural Research Program through the Center for Information TechnologyNIH Intramural Research Programs of the National Institute on Drug Abus

Probing the Carboxyester Side Chain in Controlled Deactivation (−)-Δ8-Tetrahydrocannabinols

We recently reported on a controlled deactivation/detoxification approach for obtaining cannabinoids with improved druggability. Our design incorporates a metabolically labile ester group at strategic positions within the THC structure. We have now synthesized a series of (−)-Δ8-THC analogues encompassing a carboxyester group within the 3-alkyl chain in an effort to explore this novel cannabinergic chemotype for CB receptor binding affinity, in vitro and in vivo potency and efficacy, as well as controlled deactivation by plasma esterases. We have also probed the chain’s polar characteristics with regard to fast onset and short duration of action. Our lead molecule, namely 2-[(6aR,10aR)-6a,7,10,10a-tetrahydro-1-hydroxy-6,6,9-trimethyl-6H-dibenzo[b,d]pyran-3-yl]-2-methyl-propanoic acid 3-cyano-propyl ester (AM7438), showed picomolar affinity for CB receptors and is deactivated by plasma esterases while the respective acid metabolite is inactive. In further in vitro and in vivo experiments, the compound was found to be a remarkably potent and efficacious CB1 receptor agonist with relatively fast onset/offset of action

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

Modulating GABA modulators

Benzodiazepines produce a broad spectrum of behavioral effects, which may be clinically desirable or undesirable, by positively or negatively modulating the effects of GABA at GABA(A) receptors. Over the past 20 years, much effort has been devoted towards identifying new compounds with limited undesirable effects. Most of this work has focused on developing drugs either with lower intrinsic activity than drugs such as diazepam, or with different binding profiles at subtypes of GABA(A) receptors. However, the benzodiazepine binding site is only one of multiple binding sites contained within the GABA(A) receptor complex, and other endogenous or exogenous compounds also may positively or negatively modulate the effects of GABA. Despite the availability of ligands for each of these distinct binding sites, very little research has examined the effects of GABA modulators given in combination. This may be due, in part, to the noncompelling results of those few studies which, depending on the particular drugs, have demonstrated site-selective antagonism or only additive effects, suggesting that each site modulates the effects of GABA independently. In this issue, McMahon and France challenge this view by showing that low-efficacy benzodiazepine ligands will effectively antagonize midazolam and, at the same doses, will enhance the effects of a neuroactive steroid. These studies raise interesting questions regarding the nature of the interaction between the benzodiazepine and neurosteroid binding sites on GABA(A) receptors

Discovery of a Potent Highly Biased MOR Partial Agonist among Diastereomeric C9-Hydroxyalkyl-5-phenylmorphans

All possible diastereomeric C9-hydroxymethyl-, hydroxyethyl-, and hydroxypropyl-substituted 5-phenylmorphans were synthesized to explore the three-dimensional space around the C9 substituent in our search for potent MOR partial agonists. These compounds were designed to lessen the lipophilicity observed with their C9-alkenyl substituted relatives. Many of the 12 diastereomers that were obtained were found to have nanomolar or subnanomolar potency in the forskolin-induced cAMP accumulation assay. Almost all these potent compounds were fully efficacious, and three of those chosen for in vivo evaluation, 15, 21, and 36, were all extremely G-protein biased; none of the three compounds recruited beta-arrestin2. Only one of the 12 diastereomers, 21 (3-((1S,5R,9R)-9-(2-hydroxyethyl)-2-phenethyl-2-azabicyclo[3.3.1]nonan-5-yl)phenol), was a MOR partial agonist with good, but not full, efficacy (Emax = 85%) and subnanomolar potency (EC50 = 0.91 nM) in the cAMP assay. It did not have any KOR agonist activity. This compound was unlike morphine in that it had a limited ventilatory effect in vivo. The activity of 21 could be related to one or more of three well-known theories that attempt to predict a dissociation of the desired analgesia from the undesirable opioid-like side-effects associated with clinically used opioids. In accordance with the theories, 21 was a potent MOR partial agonist, it was highly G-protein biased and did not attract beta-arrestin2, and it was found to have both MOR and DOR agonist activity. All the other diastereomers that were synthesized were either much less potent than 21 or had either too little or too much efficacy for our purposes. It was also noted that a C9-methoxymethyl compound with 1R,5S,9R stereochemistry (41) was more potent than the comparable C9-hydroxymethyl compound 11 (EC50 = 0.65 nM for 41 vs. 2.05 nM for 11). Both 41 and 11 were fully efficacious