Chronic methadone treatment shows a better cost/benefit ratio than chronic morphine in mice.

Chronic treatment of pain with opiate drugs can lead to analgesic tolerance and drug dependence. Although all opiate drugs can promote tolerance and dependence in practice, the severity of those unwanted side effects differs depending on the drug used. Although each opiate drug has its own unique set of pharmacological profiles, methadone is the only clinically used opioid drug that produces substantial receptor endocytosis at analgesic doses. Here, we examined whether moderate doses of methadone carry any benefits over chronic use of equianalgesic morphine, the prototypical opioid. Our data show that chronic administration of methadone produces significantly less analgesic tolerance than morphine. Furthermore, we found significantly reduced precipitated withdrawal symptoms after chronic methadone treatment than after chronic morphine treatment. Finally, using a novel animal model with a degrading µ-opioid receptor we showed that, although endocytosis seems to protect against tolerance development, endocytosis followed by receptor degradation produces a rapid onset of analgesic tolerance to methadone. Together, these data indicated that opioid drugs that promote receptor endocytosis and recycling, such as methadone, may be a better choice for chronic pain treatment than morphine and its derivatives that do not.

Morphine-induced receptor endocytosis in a novel knockin mouse reduces tolerance and dependence.

Opioid drugs, such as morphine, are among the most effective analgesics available. However, their utility for the treatment of chronic pain is limited by side effects including tolerance and dependence. Morphine acts primarily through the mu-opioid receptor (MOP-R) , which is also a target of endogenous opioids. However, unlike endogenous ligands, morphine fails to promote substantial receptor endocytosis both in vitro, and in vivo. Receptor endocytosis serves at least two important functions in signal transduction. First, desensitization and endocytosis act as an "off" switch by uncoupling receptors from G protein. Second, endocytosis functions as an "on" switch, resensitizing receptors by recycling them to the plasma membrane. Thus, both the off and on function of the MOP-R are altered in response to morphine compared to endogenous ligands. To examine whether the low degree of endocytosis induced by morphine contributes to tolerance and dependence, we generated a knockin mouse that expresses a mutant MOP-R that undergoes morphine-induced endocytosis. Morphine remains an excellent antinociceptive agent in these mice. Importantly, these mice display substantially reduced antinociceptive tolerance and physical dependence. These data suggest that opioid drugs with a pharmacological profile similar to morphine but the ability to promote endocytosis could provide analgesia while having a reduced liability for promoting tolerance and dependence.

A novel knock-in mouse reveals mechanistically distinct forms of morphine tolerance.

The role of µ-opioid receptor (MOR) down-regulation in opioid tolerance remains controversial. In this study, we used a novel knock-in mouse to examine how changing the extent of MOR down-regulation alters the development of morphine tolerance. These mice express a mutant MOR, degrading MOR (DMOR), that differs from the wild-type (WT) MOR in two ways: 1) unlike the recycling WT MOR, the mutant DMOR is targeted for degradation after its internalization, thus facilitating down-regulation; and 2) unlike the WT MOR, DMOR is efficiently internalized in response to morphine activation. We found that both WT MOR and DMOR mice develop tolerance to morphine, but DMOR mice exhibit a more rapid onset of tolerance and show receptor down-regulation. WT MOR mice develop morphine tolerance more slowly but even once profoundly tolerant show no receptor down-regulation. Furthermore, WT mice show significantly more morphine dependence than DMOR mice after long-term treatment as indicated by withdrawal. Taken together these data indicate that tolerance mediated by receptor down-regulation manifests differently both at the behavioral and biochemical level than does the actual morphine tolerance that occurs in WT mice and that loss of receptor function is not a major contributor to morphine tolerance in WT MOR mice.

Identification of the First Marine-Derived Opioid Receptor "Balanced" Agonist with a Signaling Profile That Resembles the Endorphins.

Opioid therapeutics are excellent analgesics, whose utility is compromised by dependence. Morphine (1) and its clinically relevant derivatives such as OxyContin (2), Vicodin (3), and Dilaudid (4) are "biased" agonists at the µ opioid receptor (OR), wherein they engage G protein signaling but poorly engage ß-arrestin and the endocytic machinery. In contrast, endorphins, the endogenous peptide agonists for ORs, are potent analgesics, show reduced liability for tolerance and dependence, and engage both G protein and ß-arrestin pathways as "balanced" agonists. We set out to determine if marine-derived alkaloids could serve as novel OR agonist chemotypes with a signaling profile distinct from morphine and more similar to the endorphins. Screening of 96 sponge-derived extracts followed by LC-MS-based purification to pinpoint the active compounds and subsequent evaluation of a mini library of related alkaloids identified two structural classes that modulate the ORs. These included the following: aaptamine (10), 9-demethyl aaptamine (11), demethyl (oxy)-aaptamine (12) with activity at the δ-OR (EC50: 5.1, 4.1, 2.3 µM, respectively) and fascaplysin (17), and 10-bromo fascaplysin (18) with activity at the µ-OR (EC50: 6.3, 4.2 µM respectively). An in vivo evaluation of 10 using δ-KO mice indicated its previously reported antidepressant-like effects are dependent on the δ-OR. Importantly, 17 functioned as a balanced agonist promoting both G protein signaling and ß-arrestin recruitment along with receptor endocytosis similar to the endorphins. Collectively these results demonstrate the burgeoning potential for marine natural products to serve as novel lead compounds for therapeutic targets in neuroscience research.

Chronic ethanol potentiates the effect of neuropeptide s in the basolateral amygdala and shows increased anxiolytic and anti-depressive effects.

Alleviating anxiety and depression is pivotal for reducing the risk of relapse in alcoholics. Currently available anxiolytic treatments are limited by side effects, including reduced efficacy in alcoholics, addiction, and sedation. We examined whether the neuropeptide S receptor (NPSR) was effective at controlling ethanol consumption and the anxiety and depression produced by forced abstinence from ethanol. We found that the anxiolytic and anti-depressant effects of NPS are enhanced in acute ethanol abstinent mice. In addition, we found that NPS reduced ethanol consumption and is not in and of itself rewarding. We also provide evidence that ethanol consumption increases the ability of NPS to modulate neuronal activity in the basolateral amygdala. Finally, we found that local injection of NPS in the basolateral amygdala promotes anxiolysis after chronic ethanol consumption, thereby providing insight into the molecular mechanism underlying the changes in behavioral response to NPS. In light of the improved anxiolytic efficacy and benign side effects of NPS in ethanol-withdrawn animals, the NPSR may prove a suitable target for reducing relapse in alcoholism.