Biochemical demonstration of mu-opioid receptor association with Gsalpha: enhancement following morphine exposure

Plasticity of adenylyl cyclase-related signaling sequelae after long-term morphine treatment

Adaptations to long-term morphine treatment resulting in tolerance are protective by counteracting the consequences of sustained opioid receptor activation. Consequently, the manifestation of specific adenylyl cyclase (AC)-related neurochemical sequelae of long-term morphine treatment should depend on the consequences of short-term mu-opioid receptor (MOR) activation. We tested this by comparing complementary chemical sequelae of long-term morphine treatment among cells in which short-term MOR activation inhibited instead of stimulated AC activity. Short-term activation of MOR in Chinese hamster ovary (CHO) cells stably transfected with MOR (MOR-CHO) inhibits AC activity. Long-term morphine treatment of these cells increased AC and Gbeta phosphorylation, membrane protein kinase Cgamma (PKCgamma) translocation, and MOR G(s) association. All converge, shifting the consequences of short-term MOR activation from Galpha(i)/Galpha(o) inhibitory to AC stimulatory signaling. In contrast, overexpression of the Gbetagamma-stimulated AC isoform AC2 (which converted MOR-coupled inhibition to stimulation of AC) eliminated or reversed these adaptations to long-term morphine treatment; it negated the increase in Gbeta phosphorylation and PKCgamma translocation while reversing the increase in AC phosphorylation and MOR G(s) association. These adaptations greatly attenuated MOR-coupled stimulation of AC activity. Altered overexpression of AC protein per se was not a confounding factor because MOR-CHO overexpressing AC1, which is inhibited by short-term MOR activation, manifested adaptations to long-term morphine treatment qualitatively identical with those of MOR-CHO. These results reveal that adaptations elicited by long-term morphine treatment depend on the effects of short-term MOR activation. This dynamic and pliable nature of tolerance mechanisms could represent a new paradigm for pharmacotherapeutics.

High-affinity naloxone binding to filamin a prevents mu opioid receptor-Gs coupling underlying opioid tolerance and dependence

Ultra-low-dose opioid antagonists enhance opioid analgesia and reduce analgesic tolerance and dependence by preventing a G protein coupling switch (Gi/o to Gs) by the mu opioid receptor (MOR), although the binding site of such ultra-low-dose opioid antagonists was previously unknown. Here we show that with approximately 200-fold higher affinity than for the mu opioid receptor, naloxone binds a pentapeptide segment of the scaffolding protein filamin A, known to interact with the mu opioid receptor, to disrupt its chronic opioid-induced Gs coupling. Naloxone binding to filamin A is demonstrated by the absence of [³H]-and FITC-naloxone binding in the melanoma M2 cell line that does not contain filamin or MOR, contrasting with strong [³H]naloxone binding to its filamin A-transfected subclone A7 or to immunopurified filamin A. Naloxone binding to A7 cells was displaced by naltrexone but not by morphine, indicating a target distinct from opioid receptors and perhaps unique to naloxone and its analogs. The intracellular location of this binding site was confirmed by FITC-NLX binding in intact A7 cells. Overlapping peptide fragments from c-terminal filamin A revealed filamin A_2561-2565 as the binding site, and an alanine scan of this pentapeptide revealed an essential mid-point lysine. Finally, in organotypic striatal slice cultures, peptide fragments containing filamin A_2561-2565 abolished the prevention by 10 pM naloxone of both the chronic morphine-induced mu opioid receptor–Gs coupling and the downstream cAMP excitatory signal. These results establish filamin A as the target for ultra-low-dose opioid antagonists previously shown to enhance opioid analgesia and to prevent opioid tolerance and dependence.

Chronic morphine treatment up-regulates mu opioid receptor binding in cells lacking filamin A

We investigated the effects of morphine and other agonists on the human mu opioid receptor (MOP) expressed in M2 melanoma cells, lacking the actin cytoskeleton protein filamin A and in A7, a subclone of the M2 melanoma cells, stably transfected with filamin A cDNA. The results of binding experiments showed that after chronic morphine treatment (24 h) of A7 cells, MOP-binding sites were down-regulated to 63% of control, whereas, unexpectedly, in M2 cells, MOP binding was up-regulated to 188% of control naive cells. Similar up-regulation was observed with the agonists methadone and levorphanol. The presence of antagonists (naloxone or CTAP) during chronic morphine treatment inhibited MOP down-regulation in A7 cells. In contrast, morphine-induced up-regulation of MOP in M2 cells was further increased by these antagonists. Chronic morphine desensitized MOP in A7 cells, i.e., it decreased DAMGO-induced stimulation of GTPgammaS binding. In M2 cells DAMGO stimulation of GTPgammaS binding was significantly greater than in A7 cells and was not desensitized by chronic morphine. Pertussis toxin treatment abolished morphine-induced receptor up-regulation in M2 cells, whereas it had no effect on morphine-induced down-regulation in A7 cells. These results indicate that, in the absence of filamin A, chronic treatment with morphine, methadone or levorphanol leads to up-regulation of MOP, to our knowledge, the first instance of opioid receptor up-regulation by agonists in cell culture.

The biochemical analysis of methadone modulation on morphine-induced tolerance and dependence in the rat brain

We have recently demonstrated that the combination of methadone and morphine enhances the ability of morphine to induce mu-opioid peptide (MOP) receptor endocytosis. As a result, rats receiving both drugs show reduced morphine tolerance and dependence. In the present study, we identify the biochemical basis for the protective effect of the drug combination. In rats treated with morphine alone, the inhibitory effect of DAMGO on forskolin-stimulated adenylyl cyclase activity was significantly reduced in a brain-region-selective manner. Importantly, these reductions were prevented in animals receiving the drug combination. We found that these changes were not due to alterations in MOP receptor density, or MOP receptor-G protein coupling, as no significant change in these parameters was observed. Together these data demonstrate that neither changes in receptor number nor function are required for morphine tolerance and dependence. Rather, brain-region-selective changes in adenylyl cyclase signal transduction are critical, and both these biochemical changes and the behavioral effects are prevented by facilitating endocytosis of the MOP receptor.

Integrins regulate opioid receptor signaling in trigeminal ganglion neurons

The binding of integrins to the extracellular matrix results in focal organization of the cytoskeleton and the genesis of intracellular signals that regulate vital neuronal functions. Recent evidence suggests that integrins modulate G-protein-coupled receptor (GPCR) signaling in hippocampal neurons. In this study we evaluated the hypothesis that integrins regulate the mu opioid receptor in rat trigeminal ganglion neurons. For these studies, primary cultures of adult rat trigeminal ganglion neurons were used to demonstrate the colocalization of beta1 and beta3 integrins with mu opioid receptor in caveolin-1-rich membrane fractions, and at focal adhesions sites generated by integrin ligand binding. Furthermore, we show that the mu opioid receptor agonist, DAMGO ([D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin), inhibits cyclic AMP (cAMP) accumulation in response to prostaglandin E2 (PGE(2)) stimulation in bradykinin-primed, but not unprimed, cultured trigeminal ganglia neurons. Application of soluble GRGDS (Gly-Arg-Gly-Asp-Ser) peptides that bind specific integrins (i.e. RGD-binding integrins) completely abolished the DAMGO effect in bradykinin-primed trigeminal ganglia neurons, but did not alter bradykinin-mediated hydrolysis of phosphatidylinositol. Likewise, monospecific anti-beta1 and anti-beta3 integrin subunit antibodies blocked this DAMGO effect in bradykinin-primed trigeminal ganglia neurons. Indeed, application of anti-beta1 integrin subunit actually reversed DAMGO signaling, resulting in increased cAMP accumulation in these cells. This suggests that the relative amounts of specific activated integrins at focal adhesions may govern signaling by the mu opioid receptor, perhaps by altering interactions with G proteins (e.g. Galphai vs. Galphas). Collectively, these data provide the first evidence that specific integrins regulate opioid receptor signaling in sensory neurons.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Mu- and kappa-opioids induce the differentiation of embryonic stem cells to neural progenitors

Growth factors, hormones, and neurotransmitters have been implicated in the regulation of stem cell fate. Since various neural precursors express functional neurotransmitter receptors, which include G protein-coupled receptors, it is anticipated that they are involved in cell fate decisions. We detected mu-opioid receptor (MOR-1) and kappa-opioid receptor (KOR-1) expression and immunoreactivity in embryonic stem (ES) cells and in retinoic acid-induced ES cell-derived, nestin-positive, neural progenitors. Moreover, these G protein-coupled receptors are functional, since [D-Ala(2),MePhe(4),Gly-ol(5)]enkephalin, a MOR-selective agonist, and U69,593, a KOR-selective agonist, induce a sustained activation of extracellular signal-regulated kinase (ERK) signaling throughout a 24-h treatment period in undifferentiated, self-renewing ES cells. Both opioids promote limited proliferation of undifferentiated ES cells via the ERK/MAP kinase signaling pathway. Importantly, biochemical and immunofluorescence data suggest that [D-Ala(2),MePhe(4),Gly-ol(5)]enkephalin and U69,593 divert ES cells from self-renewal and coax the cells to differentiate. In retinoic acid-differentiated ES cells, opioid-induced signaling features a biphasic ERK activation profile and an opioid-induced, ERK-independent inhibition of proliferation in these neural progenitors. Collectively, the data suggest that opioids may have opposite effects on ES cell self-renewal and ES cell differentiation and that ERK activation is only required by the latter. Finally, opioid modulation of ERK activity may play an important role in ES cell fate decisions by directing the cells to specific lineages.

Post-opioid receptor adaptations to chronic morphine; Altered functionality and associations of signaling molecules

Opioid desensitization/tolerance mechanisms have largely focused on adaptations that occur on the level of the mu-opioid receptor (MOR) itself. These include opioid receptor phosphorylation and ensuing trafficking events. Recent research, however, has revealed additional adaptations that occur downstream from the opioid receptor, which involve covalent modification of signaling molecules and altered associations among them. These include augmented isoform-specific synthesis of adenylyl cyclase (AC) and their phosphorylation as well as augmented phosphorylation of the G(beta) subunit of G(beta gamma). The aggregate effect of these changes is to shift mu-opioid receptor-coupled signaling from predominantly G(i alpha) inhibitory to (G(i)-derived) G(beta gamma) stimulatory AC signaling. Most recently, chronic morphine has been shown to enhance the association (interaction) between MOR and G(s), which should provide an additional avenue for offsetting inhibitory MOR signaling sequelae. The unfolding complexity of chronic morphine-induced sequelae demands an evolving broader and more encompassing perspective on opioid tolerance-producing mechanisms. This should facilitate understanding tolerance within the context of physiological plasticity that is activated by chronic exposure to drugs of abuse. Additional research is required to integrate the various tolerance-producing adaptations that have been elucidated to date. Specifically, the relative contribution to opioid tolerance of identified adaptations is still unknown as is the extent to which they vary among different regions of the central nervous system.