An opioid agonist that does not induce mu-opioid receptor--arrestin interactions or receptor internalization.

G protein-coupled receptor desensitization and trafficking are important regulators of opioid receptor signaling that can dictate overall drug responsiveness in vivo. Furthermore, different mu-opioid receptor (muOR) ligands can lead to varying degrees of receptor regulation, presumably because of distinct structural conformations conferred by agonist binding. For example, morphine binding produces a muOR with low affinity for beta-arrestin proteins and limited receptor internalization, whereas enkephalin analogs promote robust trafficking of both beta-arrestins and the receptors. Here, we evaluate muOR trafficking in response to activation by a novel mu-selective agonist derived from the naturally occurring plant product, salvinorin A. It is interesting that this compound, termed herkinorin, does not promote the recruitment of beta-arrestin-2 to the muOR and does not lead to receptor internalization. Moreover, whereas G protein-coupled receptor kinase overexpression can promote morphine-induced beta-arrestin interactions and muOR internalization, such manipulations do not promote herkinorin-induced trafficking. Studies in mice have shown that beta-arrestin-2 plays an important role in the development of morphine-induced tolerance, constipation, and respiratory depression. Therefore, drugs that can activate the receptor without recruiting the arrestins may be a promising step in the development of opiate analgesics that distinguish between agonist activity and receptor regulation and may ultimately lead to therapeutics designed to provide pain relief without the adverse side effects normally associated with the opiate narcotics.

Relative opioid efficacy is determined by the complements of the G protein-coupled receptor desensitization machinery.

G protein-coupled receptor regulation by G protein-coupled receptor kinases and beta-arrestins can lead to desensitization and subsequent internalization of the receptor. In in vitro and cellular systems, beta-arrestins do not seem to play a major role in regulating micro opioid receptor (microOR) responsiveness. Removal of the betaarrestin2 (betaarr2) gene in mice leads paradoxically to enhanced and prolonged microOR-mediated antinociception. The betaarr2 knockout (betaarr2-KO) mice also fail to develop morphine antinociceptive tolerance in the hot-plate test, further indicating that the betaarr2 protein plays an essential role in microOR regulation in vivo. In this study, the contribution of betaarr2 to the regulation of the microOR was examined in both human embryonic kidney 293 cells and in betaarr2-KO mice after treatment with several opiate agonists. A green fluorescent protein tagged betaarr2 was used to assess receptor-betaarr2 interactions in living cells. Opiate agonists that induced robust betaarr2-green fluorescent protein translocation produced similar analgesia profiles in wild-type and betaarr2-KO mice, whereas those that do not promote robust betaarr2 recruitment, such as morphine and heroin, produce enhanced analgesia in vivo. In this report, we present a rationale to explain the seemingly paradoxical relationship between beta-arrestins and microOR regulation wherein morphine-like agonists fail to promote efficient internalization and resensitization of the receptor.

Glutamatergic modulation of hyperactivity in mice lacking the dopamine transporter.

In the brain, dopamine exerts an important modulatory influence over behaviors such as emotion, cognition, and affect as well as mechanisms of reward and the control of locomotion. The dopamine transporter (DAT), which reuptakes the released neurotransmitter into presynaptic terminals, is a major determinant of the intensity and duration of the dopaminergic signal. Knockout mice lacking the dopamine transporter (DAT-KO mice) display marked changes in dopamine homeostasis that result in elevated dopaminergic tone and pronounced locomotor hyperactivity. A feature of DAT-KO mice is that their hyperactivity can be inhibited by psychostimulants and serotonergic drugs. The pharmacological effect of these drugs occurs without any observable changes in dopaminergic parameters, suggesting that other neurotransmitter systems in addition to dopamine might contribute to the control of locomotion in these mice. We report here that the hyperactivity of DAT-KO mice can be markedly further enhanced when N-methyl-d-aspartate receptor-mediated glutamatergic transmission is blocked. Conversely, drugs that enhance glutamatergic transmission, such as positive modulators of l-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate glutamate receptors, suppress the hyperactivity of DAT-KO mice. Interestingly, blockade of N- methyl-d-aspartate receptors prevented the inhibitory effects of both psychostimulant and serotonergic drugs on hyperactivity. These findings support the concept of a reciprocal functional interaction between dopamine and glutamate in the basal ganglia and suggest that agents modulating glutamatergic transmission may represent an approach to manage conditions associated with dopaminergic dysfunction.

Mu-opioid receptor desensitization by beta-arrestin-2 determines morphine tolerance but not dependence.

Morphine is a powerful pain reliever, but also a potent inducer of tolerance and dependence. The development of opiate tolerance occurs on continued use of the drug such that the amount of drug required to elicit pain relief must be increased to compensate for diminished responsiveness. In many systems, decreased responsiveness to agonists has been correlated with the desensitization of G-protein-coupled receptors. In vitro evidence indicates that this process involves phosphorylation of G-protein-coupled receptors and subsequent binding of regulatory proteins called beta-arrestins. Using a knockout mouse lacking beta-arrestin-2 (beta arr2-/-), we have assessed the contribution of desensitization of the mu-opioid receptor to the development of morphine antinociceptive tolerance and the subsequent onset of physical dependence. Here we show that in mice lacking beta-arrestin-2, desensitization of the mu-opioid receptor does not occur after chronic morphine treatment, and that these animals fail to develop antinociceptive tolerance. However, the deletion of beta-arrestin-2 does not prevent the chronic morphine-induced up-regulation of adenylyl cyclase activity, a cellular marker of dependence, and the mutant mice still become physically dependent on the drug.

Mice lacking the norepinephrine transporter are supersensitive to psychostimulants.

The action of norepinephrine (NE) is terminated, in part, by its uptake into presynaptic noradrenergic neurons by the plasma-membrane NE transporter (NET), which is a target for antidepressants and psychostimulants. Disruption of the NET gene in mice prolonged the clearance of NE and elevated extracellular levels of this catecholamine. In a classical test for antidepressant drugs, the NET-deficient (NET-/-) animals behaved like antidepressant-treated wild-type mice. Mutants were hyper-responsive to locomotor stimulation by cocaine or amphetamine. These responses were accompanied by dopamine D2/D3 receptor supersensitivity. Thus altering NET expression significantly modulates midbrain dopaminergic function, an effect that may be an important component of the actions of antidepressants and psychostimulants.

Potentiated opioid analgesia in norepinephrine transporter knock-out mice.

Several studies have shown that activation of alpha(2)-adrenergic receptors (alpha(2)ARs) leads to mild analgesic effects. Tricyclic antidepressants (TCAs), such as desipramine (DMI), which block norepinephrine transporters (NETs), also produce mild antinociception. The coadministration of either alpha(2)AR agonists or TCAs with opiates produces synergistically potentiated antinociception. It has been postulated that the analgesic effects of TCAs are determined by their ability to inhibit norepinephrine reuptake via interactions with the NET. To test this idea, we studied mice lacking a functional NET in spontaneous and morphine-induced antinociceptive paradigms. Morphine (10 mg/kg, s.c. ) treatment produced greater analgesia, as assayed in the warm water tail-flick assay, in NET-knock-out (-KO) mice than in wild-type (WT) mice. As anticipated, yohimbine, an inhibitor of alpha(2)ARs, blocked this potentiation. Moreover, a warm water swim-stress paradigm, which is known to induce the release of endogenous opioids, produced greater antinociception in NET-KO than in the WT mice. Naloxone, an inhibitor of opioid receptors, blocked the development of the swim-evoked analgesia in both WT and NET-KO mice, confirming the involvement of the endogenous opioid system. In the NET-KO mice, DMI did not further enhance analgesia but was still able to produce inhibitory effects on the locomotor activity of these mutants, suggesting that the effects of this TCA are not exclusively via interactions with the NET. In summary, these results demonstrate in a genetic model that both endogenous and exogenous opiate-mediated analgesia can be enhanced by elimination of the NET, indicating that the interaction of TCAs with NET mediates these effects.

Mitogenic signaling via endogenous kappa-opioid receptors in C6 glioma cells: evidence for the involvement of protein kinase C and the mitogen-activated protein kinase signaling cascade.

As reports on G protein-coupled receptor signal transduction mechanisms continue to emphasize potential differences in signaling due to relative receptor levels and cell type specificities, the need to study endogenously expressed receptors in appropriate model systems becomes increasingly important. Here we examine signal transduction mechanisms mediated by endogenous kappa-opioid receptors in C6 glioma cells, an astrocytic model system. We find that the kappa-opioid receptor-selective agonist U69,593 stimulates phospholipase C activity, extracellular signal-regulated kinase 1/2 phosphorylation, PYK2 phosphorylation, and DNA synthesis. U69,593-stimulated extracellular signal-regulated kinase 1/2 phosphorylation is shown to be upstream of DNA synthesis as inhibition of signaling components such as pertussis toxin-sensitive G proteins, L-type Ca2+ channels, phospholipase C, intracellular Ca2+ release, protein kinase C, and mitogen-activated protein or extracellular signal-regulated kinase kinase blocks both of these downstream events. In addition, by overexpressing dominant-negative or sequestering mutants, we provide evidence that extracellular signal-regulated kinase 1/2 phosphorylation is Ras-dependent and transduced by Gbetagamma subunits. In summary, we have delineated major features of the mechanism of the mitogenic action of an agonist of the endogenous kappa-opioid receptor in C6 glioma cells.

Mu-opioid agonist inhibition of kappa-opioid receptor-stimulated extracellular signal-regulated kinase phosphorylation is dynamin-dependent in C6 glioma cells.

In previous studies we found that mu-opioids, acting via mu-opioid receptors, inhibit endothelin-stimulated C6 glioma cell growth. In the preceding article we show that the kappa-selective opioid agonist U69,593 acts as a mitogen with a potency similar to that of endothelin in the same astrocytic model system. Here we report that C6 cell treatment with mu-opioid agonists for 1 h results in the inhibition of kappa-opioid mitogenic signaling. The mu-selective agonist endomorphin-1 attenuates kappa-opioid-stimulated DNA synthesis, phosphoinositide turnover, and extracellular signal-regulated kinase phosphorylation. To investigate the role of receptor endocytosis in signaling, we have examined the effects of dynamin-1 and its GTPase-defective, dominant suppressor mutant (K44A) on opioid modulation of extracellular signal-regulated kinase phosphorylation in C6 cells. Overexpression of dynamin K44A in C6 cells does not affect kappa-opioid phosphorylation of extracellular signal-regulated kinase. However, it does block the inhibitory action on kappa-opioid signaling mediated by the kappa-opioid receptor. Our results are consistent with a growing body of evidence of the opposing actions of mu- and kappa-opioids and provide new insight into the role of opioid receptor trafficking in signaling.

Muscarinic supersensitivity and impaired receptor desensitization in G protein-coupled receptor kinase 5-deficient mice.

G protein-coupled receptor kinase 5 (GRK5) is a member of a family of enzymes that phosphorylate activated G protein-coupled receptors (GPCR). To address the physiological importance of GRK5-mediated regulation of GPCRs, mice bearing targeted deletion of the GRK5 gene (GRK5-KO) were generated. GRK5-KO mice exhibited mild spontaneous hypothermia as well as pronounced behavioral supersensitivity upon challenge with the nonselective muscarinic agonist oxotremorine. Classical cholinergic responses such as hypothermia, hypoactivity, tremor, and salivation were enhanced in GRK5-KO animals. The antinociceptive effect of oxotremorine was also potentiated and prolonged. Muscarinic receptors in brains from GRK5-KO mice resisted oxotremorine-induced desensitization, as assessed by oxotremorine-stimulated [5S]GTPgammaS binding. These data demonstrate that elimination of GRK5 results in cholinergic supersensitivity and impaired muscarinic receptor desensitization and suggest that a deficit of GPCR desensitization may be an underlying cause of behavioral supersensitivity.

Enhanced morphine analgesia in mice lacking beta-arrestin 2.

The ability of morphine to alleviate pain is mediated through a heterotrimeric guanine nucleotide binding protein (G protein)-coupled heptahelical receptor (GPCR), the mu opioid receptor (muOR). The efficiency of GPCR signaling is tightly regulated and ultimately limited by the coordinated phosphorylation of the receptors by specific GPCR kinases and the subsequent interaction of the phosphorylated receptors with beta-arrestin 1 and beta-arrestin 2. Functional deletion of the beta-arrestin 2 gene in mice resulted in remarkable potentiation and prolongation of the analgesic effect of morphine, suggesting that muOR desensitization was impaired. These results provide evidence in vivo for the physiological importance of beta-arrestin 2 in regulating the function of a specific GPCR, the muOR. Moreover, they suggest that inhibition of beta-arrestin 2 function might lead to enhanced analgesic effectiveness of morphine and provide potential new avenues for the study and treatment of pain, narcotic tolerance, and dependence.

Requirement of receptor internalization for opioid stimulation of mitogen-activated protein kinase: biochemical and immunofluorescence confocal microscopic evidence.

Previously, we implicated the opioid receptor (OR), Gbetagamma subunits, and Ras in the opioid activation of extracellular signal-regulated protein kinase (ERK), a member of the mitogen-activated protein (MAP) kinase family involved in mitogenic signaling. We now report that OR endocytosis also plays a role in the opioid stimulation of ERK activity. COS-7 and HEK-293 cells were cotransfected with the cDNA of delta-, mu;-, or kappa-OR, dynamin wild-type (DWT), or the dominant suppressor mutant dynamin K44A, which blocks receptor endocytosis. The activation of ERK by opioid agonists in the presence of DWT was detected. In contrast, parallel ectopic coexpression of the K44A mutant with OR, followed by agonist treatment, resulted in a time-dependent attenuation of ERK activation. Immunofluorescence confocal microscopy of delta-OR and DWT-cotransfected COS-7 cells revealed that agonist exposure for 10 min resulted in an ablation of cell surface delta-OR immunoreactivity (IR) and an intensification of cytoplasmic (presumably endosomal) staining as seen in the absence of overexpressed DWT. After 1 hr of delta-agonist exposure the cells displayed substantial internalization of delta-OR IR. If the cells were cotransfected with delta-OR and dynamin mutant K44A, OR IR was retained on the cell surface even after 1 hr of delta-agonist treatment. Parallel immunofluorescence confocal microscopy, using an anti-ERK antibody, showed that agonist-induced time-dependent ERK IR trafficking into perinuclear and nuclear loci was impaired in the internalization-defective cells. Thus, both biochemical and immunofluorescence confocal microscopic evidence supports the hypothesis that the opioid activation of ERK requires receptor internalization in transfected mammalian cells.

Brain opioid receptor adaptation and expression after prenatal exposure to buprenorphine.

Previous in vivo studies revealed that buprenorphine can down-regulate mu and up-regulate delta2 and kappa1 opioid receptors in adult and neonatal rat brain. To assess gestational effects of buprenorphine on offspring, pregnant rats were also administered this drug and opioid receptor binding parameters (Kd and Bmax values) were measured by homologous binding assays of postnatal day 1 (P1) brain membranes. Buprenorphine concentrations of 2.5 mg/kg injected into dams elicited an up-regulation of kappa1 opioid receptors as detected with the kappa1-selective agonist 3H-U69593. Parallel studies with the mu-selective agonist [D-ala2, mephe4,gly-ol5] enkephalin revealed a buprenorphine-induced down-regulation in receptor density at 0.3, 0.6 or 2.5 mg/kg drug treatment. A greater down-regulation of mu receptors for P1 males than for their female counterparts was observed. Buprenorphine did not cause a reduction in binding affinity in these experiments. Changes in opioid receptor adaptation induced by buprenorphine were further supported by data from cross-linking of 125I-beta-endorphin to brain membrane preparations. RT-PCR analysis of opioid receptor expression was also estimated in P1 brains. However, significant changes in neither mu nor kappa receptor message were detected in P1 brains as a result of prenatal buprenorphine treatment under the conditions of these experiments. Since buprenorphine is being evaluated in clinical trials for the treatment of heroin abuse, the in utero actions of the drug have ramifications for its use in the treatment of maternal drug abuse.

Evidence for kappa- and mu-opioid receptor expression in C6 glioma cells.

The astrocytoma cell line rat C6 glioma has been used as a model system to study the mechanism of various opioid actions. Nevertheless, the type of opioid receptor(s) involved has not been established. Here we demonstrate the presence of high-affinity U69,593, endomorphin-1, morphine, and beta-endorphin binding in desipramine (DMI)-treated C6 cell membranes by performing homologous and heterologous binding assays with [3H]U69,593, [3H]morphine, or 125I-beta-endorphin. Naive C6 cell membranes displayed U69,593 but neither endomorphin-1, morphine, nor beta-endorphin binding. Cross-linking of 125I-beta-endorphin to C6 membranes gave labeled bands characteristic of opioid receptors. Moreover, RT-PCR analysis of opioid receptor expression in control and DMI-treated C6 cells indicate that both kappa- and mu-opioid receptors are expressed. There does not appear to be a significant difference in the level of mu nor kappa receptor expression in naive versus C6 cells treated with DMI over a 20-h period. Collectively, the data indicate that kappa- and mu-opioid receptors are present in C6 glioma cells.

Kappa-opioid receptor binding varies inversely with tumor grade in human gliomas.

BACKGROUND: Opioid agonists can inhibit cell proliferation in various neural tumor cell lines, including rat gliomas. Because opioid antimitogenic effects are mediated by opioid receptors, it was of interest to the authors to determine opioid receptor levels in human brain tumors. METHODS: Specimens obtained at craniotomy from 30 patients with glioma and nonneoplastic brain disorders were evaluated for their kappa-opioid receptor binding. Kd and Bmax values were estimated from homologous competition binding curves with the kappa1-selective radioligand [3H]U69,593. RESULTS: Receptor binding density was greatest in nonneoplastic brain tissue, less in Grade 2 and 3 astrocytoma, and least in glioblastoma multiforme. CONCLUSIONS: These results suggest that opioid receptor-based stratification of grade may have clinical utility in distinguishing glioblastoma multiforme from lower grade astrocytomas, and thereby may facilitate diagnosis and treatment.