Mechanistic Understanding of Peptide Analogues, DALDA, [Dmt1]DALDA, and KGOP01, Binding to the Mu Opioid Receptor

Solving the Opioid Crisis: Respiratory Depression by Opioids as Critical End Point

In this study we investigated the mechanisms responsible for MAP kinase ERK1/2 activation following agonist activation of endogenous mu opioid receptors (MOR) normally expressed in cultured striatal neurons. Treatment with the MOR agonist fentanyl caused significant activation of ERK1/2 in neurons derived from wild type mice. Fentanyl effects were blocked by the opioid antagonist naloxone and were not evident in neurons derived from MOR knock-out (-/-) mice. In contrast, ERK1/2 activation by fentanyl was not evident in neurons from GRK3-/- mice or neurons pretreated with small inhibitory RNA for arrestin3. Consistent with this observation, treatment with the opiate morphine (which is less able to activate arrestin) did not elicit ERK1/2 activation in wild type neurons; however, transfection of arrestin3-(R170E) (a dominant positive form of arrestin that does not require receptor phosphorylation for activation) enabled morphine activation of ERK1/2. In addition, activation of ERK1/2 by fentanyl and morphine was rescued in GRK3-/- neurons following transfection with dominant positive arrestin3-(R170E). The activation of ERK1/2 appeared to be selective as p38 MAP kinase activation was not increased by either fentanyl or morphine treatment in neurons from wild type, MOR-/-, or GRK3-/- mice. In addition, U0126 (a selective inhibitor of MEK kinase responsible for ERK phosphorylation) blocked ERK1/2 activation by fentanyl. These results support the hypothesis that MOR activation of ERK1/2 requires opioid receptor phosphorylation by GRK3 and association of arrestin3 to initiate the cascade resulting in ERK1/2 phosphorylation in striatal neurons.

The role of opioids in cancer progression.

KOR agonists for the treatment and/or prevention of opioid use disorder and cocaine use disorder.

Amygdala opioids such as enkephalin appear to play some role in the control of anxiety and the anxiolytic effects of benzodiazepines, although the opioid receptor subtypes mediating such effects are unclear. This study compared the influences of mu-opioid receptor (MOR) activation in the central nucleus of the amygdala (CEA) on unconditioned fear or anxiety-like responses in two models, the elevated plus maze, and the defensive burying test. The role of MORs in the anxiolytic actions of the benzodiazepine agonist diazepam was also examined using both models. Either the MOR agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO), or the MOR antagonists Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP) or beta-funaltrexamine (FNA) were bilaterally infused into the CEA of rats before testing. The results show that microinjection of DAMGO in the CEA decreased open-arm time in the plus maze, whereas CTAP increased open-arm behaviors. In contrast, DAMGO injections in the CEA reduced burying behaviors and increased rearing following exposure to a predator odor, suggesting a shift in the behavioral response in this context. Amygdala injections of the MOR agonist DAMGO or the MOR antagonist CTAP failed to change the anxiolytic effects of diazepam in either test. Our results demonstrate that MOR activation in the central amygdala exerts distinctive effects in two different models of unconditioned fear or anxiety-like responses, and suggest that opioids may exert context-specific regulation of amygdalar output circuits and behavioral responses during exposure to potential threats (open arms of the maze) vs discrete threats (predator odor).

Physical exercise stimulates the release of endogenous opioid peptides supposed to be responsible for changes in mood, anxiety, and performance. Exercise alters sensitivity to these effects that modify the efficacy at the opioid receptor. Although there is evidence that relates exercise to neuropeptide expression in the brain, the effects of exercise on opioid receptor binding and signal transduction mechanisms downstream of these receptors have not been explored. Here, we characterized the binding and G protein activation of mu opioid receptor, kappa opioid receptor or delta opioid receptor in several brain regions following acute (7 days) and chronic (30 days) exercise. As regards short- (acute) or long-term effects (chronic) of exercise, overall, higher opioid receptor binding was observed in acute-exercise animals and the opposite was found in the chronic-exercise animals. The binding of [(35) S]GTPγS under basal conditions (absence of agonists) was elevated in sensorimotor cortex and hippocampus, an effect more evident after chronic exercise. Divergence of findings was observed for mu opioid receptor, kappa opioid receptor, and delta opioid receptor receptor activation in our study. Our results support existing evidence of opioid receptor binding and G protein activation occurring differentially in brain regions in response to diverse exercise stimuli. We characterized the binding and G protein activation of mu, kappa, and delta opioid receptors in several brain regions following acute (7 days) and chronic (30 days) exercise. Higher opioid receptor binding was observed in the acute exercise animal group and opposite findings in the chronic exercise group. Higher G protein activation under basal conditions was noted in rats submitted to chronic exercise, as visible in the depicted pseudo-color autoradiograms.

Pharmacological modulation of the delta opioid receptor (DOR) in conjunction with a mu‐opioid receptor (MOR) agonist produces antinociceptive effects with potentially a better side effect profile. MMP2200 (a.k.a.Lactomorphin) is a CNS permeable, glycosylated peptide and an agonist at both MORs and DORs. Previous reports characterized the MOR agonist effects of MMP2200; however, the DOR agonist effects of MMP2200in vivo have not been explored. DOR agonists, such as SNC80, have been shown to produce convulsions and discriminative stimulus effects in rodents. Therefore, based on the DOR agonist activity of MMP2200, we hypothesized that MMP2200 would produce DOR‐mediated convulsions in mice and SNC80‐like discriminative stimulus effects in rats. Convulsions were evaluated by continual observation for up to 60 min in an observation cage. For SNC80 discrimination experiments, rats were trained to discriminate an injection of 3.2 mg/kg SNC80 from saline under a fixed ratio 10 schedule of food reinforcement, and different doses of SNC80 or MMP2200 were substituted to evaluate percent generalization and response rates. MMP2200 produced brief, non‐lethal convulsions in C57BL/6N, but not ICR, mice that were blocked by the DOR antagonist naltrindole and by genetic deletion of DOR, indicating that MMP2200‐induced convulsions are DOR mediated. Following acute drug administration, mice were then given a second dose of the DOR agonist 24 h later. These mice were tolerant to convulsions induced by SNC80, but not to MMP2200‐induced convulsions. Tolerance to the convulsive effects of MMP2200 was apparent 8 h following the initial injection. SNC80 induced convulsions were enhanced in mice lacking arrestin 2 but were unaltered in mice lacking arrestin 3. However, MMP2200‐induced convulsions were attenuated in mice lacking either arrestin 2 or 3. In rats trained to discriminate SNC80, MMP2200 fully generalized to the discriminative stimulus effects of SNC80 and only slightly decreased rates of responding. Overall, these results demonstrate that the dual MOR/DOR agonist MMP2200 produces many behavioral effects that are similar to the prototypical DOR agonist SNC80. Interestingly, there are some distinct differences in these DOR agonist effects, which may be mediated by concurrent MOR agonist activity with MMP2200 or by activating differential intracellular signaling molecules downstream of DOR activation. Future studies will evaluate the effects of DOR agonists and signaling pathways mediating these effects.

Structure-activity relationship (SAR) studies of numerous opioid ligands have shown that introduction of a methyl or ethyl group on the tertiary amino group at position 17 of the epoxymorphinan skeleton generally results in a mu opioid receptor (MOR) agonist while introduction of a cyclopropylmethyl group typically leads to an antagonist. Furthermore, it has been shown that introduction of heterocyclic ring systems at position 6 can favor antagonism. However, it was reported that 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(2'-indolyl)acetamido]morphinan (INTA), which bears a cyclopropylmethyl group at position 17 and an indole ring at position 6, acted as a MOR agonist. We herein report a SAR study on INTA with a series of its complementary derivatives to understand how introduction of an indole moiety with α or β linkage at position 6 of the epoxymorphinan skeleton may influence ligand function. Interestingly, one of INTA derivatives, compound 15 (NAN) was identified as a MOR antagonist both in vitro and in vivo. Molecular modeling studies revealed that INTA and NAN may interact with different domains of the MOR allosteric binding site. In addition, INTA may interact with W293 and N150 residues found in the orthosteric site to stabilize MOR activation conformation while NAN does not. These results suggest that INTA and NAN may be bitopic ligands and the type of allosteric interactions with the MOR influence their functional activity. These insights along with our enriched comprehension of the "message-address" concept will to benefit future ligand design.

Activation of MORs in the VTA induces changes on cFos expression in different projecting regions: Effect of inflammatory pain

The development of safer mu opioid receptor (MOR) agonists with reduced side effects is a key focus of pain research. Some studies have suggested that MOR agonists with reduced β-arrestin 2 (βArr2) signaling (i.e. G-protein biased agonists) may have greater therapeutic windows. However, there have been a several conflicting reports, and it is not clear what role, if any, βArr2 signaling plays in MOR-mediated analgesia, tolerance, or side effects. Therefore, we used βArr2 knockout mice to systematically investigate the causal role of βArr2 signaling in antinociception, antinociceptive tolerance, respiratory depression, constipation, and reward induced by morphine and the two novel MOR agonists, kurkinorin and kurkinol. Kurkinorin and kurkinol exhibited potent antinociceptive effects that were reversed by MOR knockout. Unlike morphine or kurkinorin, our most G-protein biased agonist, kurkinol, showed no significant tolerance after seven days of ~2×ED50 dosing. However, in a chemotherapy-induced neuropathic pain model, all three compounds were ineffective after 20 days of ~ED50 dosing, indicative of tolerance. All compounds exhibited significant MOR-dependent side effects, though kurkinorin had reduced gastrointestinal and respiratory depressive effects compared to morphine despite exhibiting less G-protein bias. Knockout of βArr2 significantly increased antinociceptive potency for morphine and kurkinorin but not kurkinol, and otherwise had no significant impact on tolerance or any side effect tested. These results largely suggest that βArr2 signaling does not drive MOR-mediated antinociceptive tolerance, respiratory depression, constipation, or reward and do not support the development of G-protein biased compounds as a broadly effective strategy to reduce side effects.

IntroductionTianeptine is approved in some countries to treat depression and anxiety. In addition to its activity on serotonin and glutamate neurotransmission, tianeptine has been proven to be a mu-opioid receptor (MOR) agonist, but only a few preclinical studies have characterized the opioid-like behavioral effects of tianeptine.MethodsIn this study, we tested tianeptine activity on G protein activation using the [S35] GTPγS binding assay in brain tissue from MOR+/+ and MOR−/− mice. Then, to determine whether tianeptine behavioral responses are MOR-dependent, we characterized the analgesic, locomotor, and rewarding responses of tianeptine in MOR+/+ and MOR−/− mice using tail immersion, hot plate, locomotor, and conditioned place preference tests.ResultsUsing the [S35] GTPγS binding assay, we found that tianeptine signaling is mediated by MOR in the brain with properties similar to those of DAMGO (a classic MOR agonist). Furthermore, we found that the MOR is necessary for tianeptine's analgesic (tail immersion and hot plate), locomotor, and rewarding (conditioned place preference) effects. Indeed, these behavioral effects could only be measured in MOR+/+ mice but not in MOR−/− mice. Additionally, chronic administration of tianeptine induced tolerance to its analgesic and hyperlocomotor effects.DiscussionThese findings suggest that tianeptine's opioid-like effects require MOR and that chronic use could lead to tolerance.

Rejection sensitivity and mu opioid receptor dynamics associated with mood alterations in response to social feedback

The canonical two neuron model of opioid reward posits that mu opioid receptor (MOR) activation produces reward by disinhibiting midbrain ventral tegmental area (VTA) dopamine neurons through inhibition of local GABAergic interneurons. Although indirect evidence supports the neural circuit postulated by this model, its validity has been called into question by growing evidence for VTA neuronal heterogeneity and the recent demonstration that MOR agonists inhibit GABAergic terminals in the VTA arising from extrinsic neurons. In addition, VTA MOR reward can be dopamine-independent. To directly test the assumption that MOR activation directly inhibits local GABAergic neurons, we investigated the properties of rat VTA GABA neurons directly identified with either immunocytochemistry for GABA or GAD65/67, or in situ hybridization for GAD65/67 mRNA. Utilizing co-labeling with an antibody for the neural marker NeuN and in situ hybridization against GAD65/67, we found that 23±3% of VTA neurons are GAD65/67(+). In contrast to the assumptions of the two neuron model, VTA GABAergic neurons are heterogeneous, both physiologically and pharmacologically. Importantly, only 7/13 confirmed VTA GABA neurons were inhibited by the MOR selective agonist DAMGO. Interestingly, all confirmed VTA GABA neurons were insensitive to the GABAB receptor agonist baclofen (0/6 inhibited), while all confirmed dopamine neurons were inhibited (19/19). The heterogeneity of opioid responses we found in VTA GABAergic neurons, and the fact that GABA terminals arising from neurons outside the VTA are inhibited by MOR agonists, make further studies essential to determine the local circuit mechanisms underlying VTA MOR reward.

Heroin is rapidly metabolized to morphine that in turn is transformed in morphine-3-glucuronide (M3G), an inactive metabolite, and morphine-6-glucuronide (M6G), a potent mu-opioid receptor (MOR) agonist. We have found that heroin addicts exhibit higher M6G/M3G ratios relative to morphine-treated control subjects. We have also shown that heroin-treated rats exhibit measurable levels of M6G (which is usually undetectable in this species) and reduced levels of M3G. We investigated the role of MOR in these effects of heroin, by examining the effects of methadone, a MOR agonist, and of naltrexone, a MOR antagonist, on morphine glucuronidation. We also investigated the effects of alcohol, which is known to alter drug metabolism and is frequently coabused by heroin addicts. Morphine glucuronidation was studied in liver microsomes obtained from rats exposed daily for 10 days to saline, heroin (10 mg/kg, i.p.), naltrexone (20-40 mg/kg, i.p.), heroin + naltrexone (10 mg/kg+20-40 mg/kg, i.p.), methadone (5-20 mg/kg, i.p.), or 10% ethanol. Heroin induced the synthesis of M6G and decreased the synthesis of M3G. Naltrexone exhibited intrinsic modulatory activity on morphine glucuronidation, increasing the synthesis of M3G via a low-affinity/high-capacity reaction characterized by positive cooperativity. The rate of M3G synthesis in the heroin + naltrexone groups was not different from that of the naltrexone groups. Methadone and ethanol induced a modest increase in M3G synthesis and had no effect on M6G synthesis. The effects of heroin on morphine glucuronidation are not shared by methadone or alcohol (two drugs that figure prominently in the natural history of heroin addiction) and do not appear to depend on the activation of MOR.

Herkinorin dilates cerebral vessels via kappa opioid receptor and cyclic adenosine monophosphate (cAMP) in a piglet model