Relevance of phosphorylation state to opioid responsiveness in opiate naive and tolerant/dependent tissue

Phosphorylation of unique C-terminal sites of the mu-opioid receptor variants 1B2 and 1C1 influences their Gs association following chronic morphine

We recently demonstrated in rat spinal cord that a regimen of escalating doses of systemic morphine, analogous to regimens used clinically for chronic pain management, selectively up-regulates the mu-opioid receptor (MOR) splice variants MOR-1B2 and MOR-1C1 mRNA and functional protein. This study investigated the potential relevance of up-regulating MOR-1B2 and MOR-1C1 to the ability of chronic morphine to shift MOR signaling from predominantly G_i /G_o inhibitory to G_s stimulatory. Specifically, we tested the hypotheses that chronic morphine induces phosphorylation of carboxyl terminal sites unique to MOR-1B2 and MOR-1C1, and that this phosphorylation is causally related to augmented association of these variants with G_s α. Hypotheses were validated by (i) abolition of the chronic morphine-induced increment in MOR-1C1 and MOR-1B2 association with G_s α by inhibitors of protein kinase A and Casein kinase 2, respectively; (ii) failure of chronic morphine to augment MOR variant G_s α interactions in Chinese hamster ovary cells transiently transfected with either rat MOR-1C1 or MOR-1B2 in which targeted protein kinase A and Casein kinase 2 serine phosphorylation sites, respectively, were mutated to alanine; (iii) abrogation of chronic morphine-induced augmented MOR G_s α association in spinal cord of male rats following intrathecal administration of dicer substrate small interfering RNAs targeting MOR-1B2/MOR-1C1 mRNA. The ability of chronic morphine to not only up-regulate-specific MOR variants but also their carboxyl terminal phosphorylation and consequent augmented association with G_s α may represent a novel component of opioid tolerance mechanisms, suggesting novel potential targets for tolerance abatement.

Protein kinase C-mediated phosphorylation of the μ-opioid receptor and its effects on receptor signaling

Phosphorylation of the μ opioid receptor (MOPr), mediated by several protein kinases, is a critical process in the regulation of MOPr signaling. Although G protein-coupled receptor kinases are known to play an essential role in the agonist-induced phosphorylation and desensitization of MOPr, evidence suggests that other protein kinases, especially protein kinase C (PKC), also participate in the regulation of MOPr signaling. In this study, we investigated the biochemical nature and downstream effects of PKC-mediated MOPr phosphorylation. We observed in vitro phosphorylation of the MOPr C terminus by purified PKC. Protein mass spectrometry and site-directed mutagenesis implicated Ser363 of MOPr as the primary substrate for PKC, and this was confirmed in Chinese hamster ovary cells stably expressing full-length MOPr using an antibody that specifically recognizes phosphorylated Ser363. Alanine mutation of Ser363 did not affect the affinity of MOPr-ligand binding and the efficiency of receptor G-protein coupling. However, the S363A mutation attenuated the desensitization of receptor G-protein coupling induced by phorbol 12-myristate. Our research thus has identified a specific PKC phosphorylation site in MOPr and demonstrated that PKC-mediated phosphorylation of MOPr induces receptor desensitization at the G protein coupling level.

Therapeutic targeting of "DARPP-32": a key signaling molecule in the dopiminergic pathway for the treatment of opiate addiction

The 32-kDa dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein (DARPP-32) is recognized to be critical to the pathogenesis of drug addiction. Opiates via the mu-receptor act on the dopaminergic system in the brain and modulates the expression of DARPP-32 phosphoprotein which is an important mediator of the activity of the extracellular signal-regulated kinase (ERK) signaling cascades, the activation of which represents an exciting nexus for drug-induced changes in neural long-term synaptic plasticity. Silencing of DARPP-32 using an siRNA against DARPP-32 may provide a novel gene therapy strategy to overcome drug addiction. In this study, we investigated the effect of the opiate (heroin) on D1 receptor (D1R) and DARPP-32 expression and additionally, evaluated the effects of DARPP-32-siRNA gene silencing on protein phosphatase-1 (PP-1), ERK, and cAMP response element-binding (CREB) gene expression in primary normal human astrocytes (NHA) cells in vitro. Our results indicate that heroin significantly upregulated both D1R and DARPP-32 gene expression, and that DARPP-32 silencing in the NHA cells resulted in the significant modulation of the activity of downstream effector molecules such as PP-1, ERK, and CREB which are known to play an important role in opiate abuse-induced changes in long-term neural plasticity. These findings have the potential to facilitate the development of DARPP32 siRNA-based therapeutics against drug addiction.

The antinociceptive effect of morphine is reversed by okadaic acid in morphine-naive but not in morphine-tolerant mice

The activation of specific subtypes of serine/threonine protein phosphatases (PPs) plays a role in the antinociceptive effect of acute morphine, but it is not known whether these enzymes are involved in morphine-induced antinociception in morphine-tolerant animals. We evaluated the effects of both okadaic acid (a selective inhibitor of some serine/threonine PPs) and its inactive analogue L-norokadaone on the antinociception induced by morphine in morphine-naive and -tolerant female mice in the tail-flick test. Okadaic acid (0.01 and 1 pg/mouse, i.c.v.), but not L-norokadaone (1 pg/mouse, i.c.v.), antagonized in a dose-dependent way the antinociception induced by morphine (1-16 mg/kg, s.c.) in morphine-naive animals. However, both okadaic acid (0.01 and 1 pg/mouse, i.c.v.) and L-norokadaone (1 pg/mouse, i.c.v.) were unable to modify the antinociceptive effect of morphine in morphine-tolerant mice. These results suggest that in morphine-induced thermal analgesia, the role of serine/threonine PPs highly sensitive to okadaic acid is different in morphine-tolerant and morphine-naive female mice.

Phosphorylation: A molecular switch in opioid tolerance

Protein phosphorylation is a key posttranslational modification mechanism controlling the conformation and activity of many proteins. Increasing evidence has implicated an essential role of phosphorylation by several major protein kinases in promoting and maintaining opioid tolerance. We review some of the most recent studies on protein kinase C (PKC), cyclic AMP dependent protein kinase A (PKA), calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase G (PKG), and G protein receptor kinase (GRK). These kinases act as the molecular switches to modulate opioid tolerance. Pharmacological interventions at one or more of the protein kinases and phosphatases may provide valuable strategies to improve opioid analgesia by attenuating tolerance to these drugs.

Inhibitors of serine/threonine protein phosphatases antagonize the antinociception induced by agonists of alpha 2 adrenoceptors and GABAB but not kappa-opioid receptors in the tail flick test in mice

We previously reported that serine/threonine protein phosphatases (PPs) play a role in the antinociception induced by the mu-opioid receptor agonist morphine. In this study we evaluated the possible involvement of PPs on the antinociception induced by agonists of others G protein-coupled receptors in the tail flick test in mice. The subcutaneous administration of clonidine (0.25-4 mg/kg), baclofen (2-32 mg/kg) or U50,488H (2-16 mg/kg) (agonists of alpha(2) adrenoceptors, GABA(B) and kappa-opioid receptors, respectively) produced dose-dependent antinociception. The antinociceptive effects of clonidine and baclofen were antagonized in a dose-dependent way by the protein phosphatase inhibitors okadaic acid (0.001-10 pg/mouse, i.c.v.) and cantharidin (0.001-10 ng/mouse, i.c.v.), and okadaic acid was 1000 times more potent than cantharidin in producing this effect. The effects of these drugs appear to be specifically due to the blockade of PPs, since L-norokadaone (an analogue of okadaic acid that has no effect on PPs) did not modify clonidine- or baclofen-induced antinociception over the wide range of doses used (0.001-1000 pg/mouse, i.c.v.). On the other hand, the antinociception induced by activation of kappa-opioid receptors with U50,488H was not modified by okadaic acid or cantharidin. In conclusion, our data support the idea that serine/threonine PPs are differentially involved in the antinociceptive effects of several agonists of G protein-coupled receptors in mice.

Chronic morphine-induced loss of the facilitative interaction between vasoactive intestinal polypeptide and delta-opioid: involvement of protein kinase C and phospholipase Cbetas

This laboratory recently demonstrated a multiplicative interaction between the pelvic visceral afferent transmitter vasoactive intestinal polypeptide (VIP) and the delta-opioid receptor (DOR)-selective agonist [D-Pen2,5] enkephalin (DPDPE) to regulate cAMP levels in spinal cord [Brain Res. 959 (2003) 103]. Although DOR activation is required for the manifestation of the VIP-DPDPE facilitative interaction, its relevance to opioid antinociception remains unclear. The current study investigates whether or not the VIP-DPDPE facilitation of cAMP formation is subject to tolerance formation, a hallmark characteristic of opioid antinociception. Chronic morphine exposure abolishes the VIP-DPDPE facilitative interaction, consistent with its relevance to DOR antinociception. However, acute in vitro inhibition of protein kinase C (PKC) reinstates the VIP-DPDPE multiplicative interaction characteristic of opioid naïve spinal tissue. This suggests that its chronic morphine-induced loss requires a PKC phosphorylation. PKC phosphorylation negatively modulates phospholipase C (PLC)beta, enzymes intimately associated with phosphoinositide turnover and calcium trafficking. These are essential determinants of acute and chronic opioid effects. Accordingly, the effect of chronic morphine on their state of phosphorylation was also investigated. Central nervous system opioid tolerance is associated with the reciprocal phosphorylation (regulation) of two PLCbeta isoforms, PLCbeta1 and PLCbeta3. However, although chelerythrine reinstates the chronic morphine-induced loss of the multiplicative VIP-DPDPE interaction, it does not alter the associated changes in PLCbeta phosphorylation, possibly indicating different time courses of restitution of function and/or involvement of different kinases for different components of tolerance. These results could provide a mechanistic rubric for understanding positive modulation of opioid antinociception by afferent transmission.

Estrogen Modulation of G-Protein-Coupled Receptor Activation of Potassium Channels in the Central Nervous System

Estrogen rapidly alters the excitability of hypothalamic neurons that are involved in regulating numerous homeostatic functions including reproduction, stress responses, feeding, and motivated behaviors. Neurosecretory neurons, such as gonadotropin-releasing hormone (GnRH) and dopamine neurons, and local circuitry neurons, such as pro-opiomelanocortin (POMC) and gamma-aminobutyric acid (GABA) neurons, are among those involved. We have identified membrane-initiated, rapid-signaling pathways through which 17beta-estradiol (E(2)) alters synaptic responses in these neurons using whole-cell patch recording in hypothalamic slices from ovariectomized female guinea pigs. E(2) rapidly uncouples micro -opioid and GABA(B) receptors from G-protein-gated inwardly rectifying K(+) (GIRK) channels in POMC and dopamine neurons as manifested by a reduction in the potency of micro -opioid and GABA(B) receptor agonists to activate these channels. These effects are mimicked by the selective E(2) receptor modulators raloxifene and 4OH-tamoxifen, the membrane impermeable E(2)-bovine serum albumin (BSA), but not by 17alpha-estradiol. Furthermore, the anti-estrogen ICI 182,780 antagonizes these rapid effects of E(2). Inhibitors of phospholipase C, protein kinase C, and protein kinase A block the actions of E(2), indicating that the E(2) receptor is G-protein-coupled to activation of this cascade. Conversely, estrogen enhances the efficacy of alpha1-adrenergic receptor agonists to inhibit apamin-sensitive small-conductance, Ca(2+)-activated K(+) (SK) currents in preoptic GABAergic neurons; it does so in both a rapid and sustained fashion. Finally, we observed a direct, steroid-induced hyperpolarization of GnRH neurons. These findings indicate that E(2) can modulate K(+) channels in hypothalamic (POMC, dopamine, GABA, GnRH) neurons that are involved in regulating numerous homeostatic functions through multiple intracellular signaling pathways.

Reciprocal modulation of phospholipase Cbeta isoforms: adaptation to chronic morphine

Phosphoinositide turnover and calcium mobilization are fundamental determinants of acute and chronic opioid effects. Phosphoinositide-specific phospholipase C (PLC) are key signaling enzymes that play a pivotal role in mediating opioid modulation of inositol trisphosphate production and cytosolic calcium distribution, substrates for many acute and chronic opioid effects. Notably, phosphorylation of the beta isoforms of PLC, by kinases that are up-regulated after chronic morphine, is a potent modality for their regulation. Direct assessment of PLCbeta1 and PLCbeta3 phosphorylation in the guinea pig longitudinal muscle myenteric plexus tissue revealed substantial alterations after the induction of opioid tolerance. Notably, the direction of this modulation is isoform-specific. Phosphorylation of PLCbeta1 is significantly reduced, whereas that of PLCbeta3 is substantially augmented, changes not accompanied by altered content of PLCbeta1 or PLCbeta3 protein. In contrast to chronic morphine, acute morphine treatment of opioid naïve longitudinal muscle myenteric plexus tissue attenuates PLCbeta3 phosphorylation, an effect also manifested by endogenous opioids that is reflected by the ability of acute naloxone to substantially augment PLCbeta3 phosphorylation. This indicates that PLCbeta phosphorylation is dynamically regulated. PLCbeta1 and PLCbeta3 activities are negatively modulated by phosphorylation. Thus, their concomitant reciprocal phosphorylation would alter the relative contribution of these isoforms to PLC/Ca2+ signaling, a significant shift in light of their differential regulatory characteristics. Reciprocal modulation of the phosphorylation (activity) of two isoforms within the same subclass of signaling enzyme, proteins that have a high degree of structural similarity and subserve the same biological function, represents an adaptation modality to chronic morphine that has heretofore not been recognized.

Involvement of protein kinase C in the adaptive changes of cholinergic neurons to sympathetic denervation in the guinea pig myenteric plexus

Supersensitivity to muscarinic, kappa- and mu-opioid agents modulating cholinergic neurons in the guinea pig colon develops after chronic sympathetic denervation. A possible role for protein kinase C (PKC) in contributing to development of these sensitivity changes was investigated. The PKC activator, phorbol-12-myristate-13-acetate (PMA), enhanced acetylcholine (ACh) overflow in preparations obtained from normal animals. The facilitatory effect of PMA was significantly reduced after prolonged exposure to the phorbol ester and by the PKC inhibitors, chelerythrine and calphostin C. Subsensitivity to the facilitatory effect of PMA developed after chronic sympathetic denervation. In this experimental condition, immunoblot analysis revealed reduced levels of PKC in myenteric plexus synaptosomes. The facilitatory effect of the muscarininc antagonist, scopolamine, on ACh overflow was significantly reduced by the phospolipase C (PLC) inhibitor, U73122, chelerythrine and calphostin C, both in normal and denervated animals. However, in both experimental groups, PLC antagonists and PKC antagonists did not affect the inhibitory effect of the muscarinic agonist, oxotremorine-M on ACh overflow. The inhibitory effects of U69593 (kappa-opioid receptor agonist) and DAMGO (mu-opioid receptor agonist) on ACh overflow significantly increased in the presence of U73122, chelerythrine and calphostin C in preparations obtained from normal animals, but not in those obtained from sympathetically denervated animals. These results indicate that activation of PKC enhances ACh release in the myenteric plexus of the guinea pig colon. At this level, chronic sympathetic denervation entails a reduced efficiency of the enzyme. In addition, PKC is involved in the inhibitory modulation of ACh release mediated by muscarinic-, kappa- and mu-opioid receptors, although with different modalities. Muscarinic receptors inhibit PKC activity, whereas kappa- and mu-opioid receptors increase PKC activity. Both the inhibitory and the facilitatory effect on PKC involve modulation of PLC activity. The possibility that the change in PKC activity represents one of the biochemical mechanisms at the basis of development of sensitivity changes to opioid and muscarinic agents after chronic sympathetic denervation is discussed.

Effects of serine/threonine protein phosphatase inhibitors on morphine-induced antinociception in the tail flick test in mice

The aim of this study was to evaluate the effects of serine/threonine protein phosphatase (PP) inhibitors on morphine-induced antinociception in the tail flick test in mice, and on [3H]naloxone binding to the forebrain crude synaptosome fraction. Neither okadaic acid nor cantharidin (1-10000 nM) displaced [3H]naloxone from its specific binding sites, which indicates that they do not interact at the opioid receptor level. The i.c.v. administration of very low doses of okadaic acid (0.001-1 pg/mouse) and cantharidin (0.001-1 ng/mouse), which inhibit PP2A, produced a dose-dependent antagonism of the antinociception induced by morphine (s.c.). However, L-nor-okadaone (0.001 pg/mouse-1 ng/mouse, i.c.v.), an analogue of okadaic acid lacking activity against protein phosphatases, did not affect the antinociceptive effect of morphine. On the other hand, high doses of okadaic acid (10 ng/mouse, i.c.v.) and cantharidin (1 microg/mouse, i.c.v.), which also block PP1, and calyculin-A (0.1 fg/mouse-1 ng/mouse, i.c.v.), which inhibits equally both PP1 and PP2A, did not modify the morphine-induced antinociception. These results suggest that the activation of type 2A serine/threonine protein phosphatases may play a role in the antinociceptive effect of morphine, and that PP1 might counterbalace this activity.

Rapid effects of estrogen on G protein-coupled receptor activation of potassium channels in the central nervous system (CNS)

Estrogen rapidly alters the excitability of hypothalamic neurons that are involved in regulating numerous homeostatic functions including reproduction, stress responses, feeding and motivated behaviors. Some of the neurons include neurosecretory neurons such as gonadotropin-releasing hormone (GnRH) and dopamine neurons, and local circuitry neurons such as proopiomelanocortin (POMC) and gamma-aminobutyric acid (GABA) neurons. We have elucidated several non-genomic pathways through which the steroid alters synaptic responses in these hypothalamic neurons. We have examined the modulation by estrogen of the coupling of various receptor systems to inwardly-rectifying and small-conductance, Ca(2+)-activated K(+) (SK) channels using intracellular sharp-electrode and whole-cell recording techniques in hypothalamic slices from ovariectomized female guinea pigs. Estrogen rapidly uncouples mu-opioid receptors from G protein-gated inwardly-rectifying K(+) (GIRK) channels in POMC neurons and GABA(B) receptors from GIRK channels in dopamine neurons as manifested by a reduction in the potency of mu-opioid and GABA(B) receptor agonists to hyperpolarize their respective cells. This effect is blocked by inhibitors of protein kinase A (PKA) and protein kinase C (PKC). In addition, after 24h following steroid administration in vivo, the GABA(B)/GIRK channel uncoupling observed in GABAergic neurons of the preoptic area is associated with reduced agonist efficacy. Conversely, estrogen enhances the efficacy of alpha(1)-adrenergic receptor agonists to inhibit apamin-sensitive SK currents in these preoptic GABAergic neurons, and does so in both a rapid and sustained fashion. Finally, we observed a direct, steroid-induced hyperpolarization of GnRH neurons. These findings indicate a richly complex yet coordinated steroid modulation of K(+) channel activity in hypothalamic (POMC, dopamine, GABA, GnRH) neurons that are involved in regulating numerous homeostatic functions.

The neurobiology of opiate tolerance, dependence and sensitization: mechanisms of NMDA receptor-dependent synaptic plasticity

Long-term administration of opiates leads to changes in the effects of these drugs, including tolerance, sensitization and physical dependence. There is, as yet, incomplete understanding of the neural mechanisms that underlie these phenomena. Tolerance, sensitization and physical dependence can be considered adaptive processes similar to other experience-dependent changes in the brain, such as learning and neural development. There is considerable evidence demonstrating that N-methyl-D-aspartate (NMDA) receptors and downstream signaling cascades may have an important role in different forms of experience-dependent changes in the brain and behavior. This review will explore evidence indicating that NMDA receptors and downstream messengers may be involved in opiate tolerance, sensitization and physical dependence. This evidence has been used to develop a cellular model of NMDA receptor/opiate interactions. According to this model, mu opioid receptor stimulation leads to a protein kinase C-mediated activation of NMDA receptors. Activation of NMDA receptors leads to influx of calcium and activation of calcium-dependent processes. These calcium-dependent processes have the ability to produce critical changes in opioid-responsive neurons, including inhibition of opioid receptor/second messenger coupling. This model is similar to cellular models of learning and neural development in which NMDA receptors have a central role. Together, the evidence suggests that the mechanisms that underlie changes in the brain and behavior produced by long-term opiate use may be similar to other central nervous system adaptations. The experimental findings and the resulting model may have implications for the treatment of pain and addiction.

Estrogen modulation of K(+) channel activity in hypothalamic neurons involved in the control of the reproductive axis

Here we report on the progress we have made in elucidating the mechanisms through which estrogen alters synaptic responses in hypothalamic neurons. We examined the modulation by estrogen of the coupling of various receptor systems to inwardly rectifying and small conductance, Ca(2+)-activated K(+) (SK) channels. We used intracellular sharp-electrode and whole-cell recordings in hypothalamic slices from ovariectomized female guinea pigs. Estrogen rapidly uncouples mu-opioid receptors from G protein-gated inwardly rectifying K(+) (GIRK) channels in beta-endorphin neurons, manifest by a reduction in the potency of mu-opioid receptor agonists to hyperpolarize these cells. This effect is blocked by inhibitors of protein kinase A and protein kinase C. Estrogen also uncouples gamma-aminobutyric acid (GABA)(B) receptors from the same population of GIRK channels coupled to mu-opioid receptors. At 24 h after steroid administration, the GABA(B)/GIRK channel uncoupling observed in GABAergic neurons of the preoptic area (POA) is associated with reduced agonist efficacy. Conversely, estrogen enhances the efficacy of alpha(1)-adrenergic receptor agonists to inhibit apamin-sensitive SK currents in these POA GABAergic neurons, and does so in both a rapid and sustained fashion. Finally, we observed a direct, steroid-induced hyperpolarization of both arcuate and POA neurons, among which gonadotropin-releasing hormone (GnRH) neurons are particularly sensitive. These findings indicate a richly complex yet coordinated steroid modulation of K(+) channel activity that serves to control the excitability of hypothalamic neurons involved in regulating the reproductive axis.

Different requirements for cAMP response element binding protein in positive and negative reinforcing properties of drugs of abuse

Addiction is a complex process that relies on the ability of an organism to integrate positive and negative properties of drugs of abuse. Therefore, studying the reinforcing as well as aversive components of drugs of abuse in a single model system will enable us to understand the role of final common mediators, such as cAMP response element-binding protein (CREB), in the addiction process. To this end, we analyzed mice with a mutation in the alpha and Delta isoforms of the CREB gene. Previously we have shown that CREB(alphaDelta) mutant mice in a mixed genetic background show attenuated signs of physical dependence, as measured by the classic signs of withdrawal. We have generated a uniform genetically stable F1 hybrid (129SvEv/C57BL/6) mouse line harboring the CREB mutation. We have found the functional activity of CREB in these F1 hybrid mice to be dramatically reduced compared with their wild-type littermates. These mice maintain a reduced withdrawal phenotype after chronic morphine. We are now poised to examine a number of complex behavioral phenotypes related to addiction in a well defined CREB-deficient mouse model. We demonstrate that the aversive properties of morphine are still present in CREB mutant mice despite a reduction of physical withdrawal. On the other hand, these mice do not respond to the reinforcing properties of morphine in a conditioned place preference paradigm. In contrast, CREB mutant mice demonstrate an enhanced response to the reinforcing properties of cocaine compared with their wild-type controls in both conditioned place preference and sensitization behaviors. These data may provide the first paradigm for differential vulnerability to various drugs of abuse.

Tyrosine Phosphorylation of the μ-Opioid Receptor Regulates Agonist Intrinsic Efficacy

The mu-opioid receptor (MOR) contains four highly conserved cytoplasmic tyrosine residues that may serve to regulate receptor activity. For Xenopus laevis oocytes coexpressing the rat MOR and the heteromultimeric potassium channel, K(IR)3.1/3.2, pretreatment with insulin produced both a 40% suppression in the basal channel conductance and potentiation of response to the mu-opioid agonist [D-Ala(2),methyl-Phe(4),Gly(5)-ol]enkephalin (DAMGO) to 155% of matched, untreated control cells. Insulin-induced potentiation of the DAMGO response was concentration-dependent and reversed after 1 h. Insulin pretreatment increased the maximal effect of DAMGO, but did not change its EC(50) value. Potentiation of the DAMGO response did not result from a recruitment of MOR to the cell surface, as measured by specific binding of the opioid peptide antagonist [(3)H]d-Phe((3)H)-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH(2) (cyclic) to whole-oocytes, but instead the potentiation was probably caused by an increase in intrinsic efficacy of G protein coupling. The involvement of tyrosine residues on the putative intracellular loops of the MOR was demonstrated with four point-mutated receptors, replacing tyrosine with phenylalanine to create MOR(Y96F), MOR(Y106F), MOR(Y166F), and MOR(Y336F). None of these mutations significantly altered the EC(50) value for DAMGO compared with wild-type MOR, and insulin pretreatment still potentiated the effect of 1 microM DAMGO in oocytes containing either MOR(Y96F) or MOR(Y336F) to 137 +/- 10 and 124 +/- 8%, respectively. However, insulin did not significantly potentiate the DAMGO response with oocytes containing either MOR(Y106F) or MOR(Y166F), suggesting that these two sites were responsible for the insulin-induced opioid potentiation. The tyrosine-kinase inhibitors genistein (100 microM) or K-252a (20 microM) did not block the insulin-induced potentiation of the DAMGO response, but coincubation of insulin with either the MAP kinase inhibitor PD98,059 (20 microM) or phosphatase inhibitor orthovanadate (30 microM) completely blocked the potentiation. The results suggest the hypothesis that the potentiation was caused by dephosphorylation of the two tyrosines in MOR. To test this hypothesis, we measured the recovery rates after insulin treatment. As predicted, tyrosine kinase inhibition by K-252a significantly slowed the reversal and phosphatase inhibition by orthovanadate significantly accelerated the recovery. These findings support a rapid modulatory role for insulin on opioid signal transduction, possibly through the dephosphorylation of the MOR at tyrosines 106 and 166 by an insulin-activated MAP kinase/protein tyrosine phosphatase cascade. We conclude that tyrosine phosphorylation of the mu-opioid receptor regulates receptor-G protein coupling efficacy.

Chronic morphine induces the concomitant phosphorylation and altered association of multiple signaling proteins: a novel mechanism for modulating cell signaling

Traditional mechanisms thought to underlie opioid tolerance include receptor phosphorylation/down-regulation, G-protein uncoupling, and adenylyl cyclase superactivation. A parallel line of investigation also indicates that opioid tolerance development results from a switch from predominantly opioid receptor G(i alpha) inhibitory to G(beta gamma) stimulatory signaling. As described previously, this results, in part, from the increased relative abundance of G(beta gamma)-stimulated adenylyl cyclase isoforms as well as from a profound increase in their phosphorylation [Chakrabarti, S., Rivera, M., Yan, S.-Z., Tang, W.-J. & Gintzler, A. R. (1998) Mol. Pharmacol. 54, 655-662; Chakrabarti, S., Wang, L., Tang, W.-J. & Gintzler, A. R. (1998) Mol. Pharmacol. 54, 949--953]. The present study demonstrates that chronic morphine administration results in the concomitant phosphorylation of three key signaling proteins, G protein receptor kinase (GRK) 2/3, beta-arrestin, and G(beta), in the guinea pig longitudinal muscle myenteric plexus tissue. Augmented phosphorylation of all three proteins is evident in immunoprecipitate obtained by using either anti-GRK2/3 or G(beta) antibodies, but the phosphorylation increment is greater in immunoprecipitate obtained with G(beta) antibodies. Analyses of coimmunoprecipitated proteins indicate that phosphorylation of GRK2/3, beta-arrestin, and G(beta) has varying consequences on their ability to associate. As a result, increased availability of and signaling via G(beta gamma) could occur without compromising the membrane content (and presumably activity) of GRK2/3. Induction of the concomitant phosphorylation of multiple proteins in a multimolecular complex with attendant modulation of their association represents a novel mechanism for increasing G(beta gamma) signaling and opioid tolerance formation.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) alleviates pain and restores opioid efficacy after nerve injury in rats

1. Nerve injury often produces long-lasting spontaneous pain, hyperalgesia and allodynia that are refractory to treatment, being only partially relieved by clinical analgesics, and often insensitive to morphine. With the aim of assessing its therapeutic potential, we examined the effect of antisense oligonucleotide knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) in neuropathic rats. 2. We chronically infused rats intrathecally with either vehicle, or 50 microg day(-1) antisense or missense oligonucleotides beginning either 3 days prior to or 5 days after nerve injury. Cold, heat and mechanical sensitivity was assessed prior to any treatment and again every few days after nerve injury. 3. Here we show that knockdown of mGluR(1) significantly reduces cold hyperalgesia, heat hyperalgesia and mechanical allodynia in the ipsilateral (injured) hindpaw of neuropathic rats. 4. Moreover, we show that morphine analgesia is reduced in neuropathic rats, but not in sham-operated rats, and that knockdown of mGluR(1) restores the analgesic efficacy of morphine. 5. We also show that neuropathic rats are more sensitive to the excitatory effects of intrathecally injected N-methyl-D-aspartate (NMDA), and have elevated protein kinase C (PKC) activity in the spinal cord dorsal horn, two effects that are reversed by knockdown of mGluR(1). 6. These results suggest that activity at mGluR(1) contributes to neuropathic pain through interactions with spinal NMDA receptors and PKC, and that knockdown of mGluR(1) may be a useful therapy for neuropathic pain in humans, both to alleviate pain directly, and as an adjunct to opioid analgesic treatment.

Opioid tolerance and the emergence of new opioid receptor-coupled signaling

Multiple cellular adaptations are elicited by chronic exposure to opioids. These include diminution of spare opioid receptors, decreased opioid receptor density, and G-protein content and coupling thereof. All imply that opioid tolefance is a manifestation of a loss of opioid function, i.e., desensitization. Recent observations challenge the exclusiveness of this formulation and indicate that opioid tolerance also results from qualitative changes in opioid signaling. In this article, Gintzler and Chakrabarti discuss the evidence that suggests that opioid tolerance results not only from impaired opioid receptor functionality, but also from altered consequences of coupling. Underlying the latter are fundamental changes in the nature of effectors that are coupled to the opioid receptor/G-protein signaling pathway. These molecular changes include the upregulation of adenylyl cyclase isoforms of the type II family as well as a substantial increase in their phosphorylation state. As a result, there is a shift in opioid receptor/G-protein signaling from predominantly Gialpha inhibitory to Gbetagamma stimulatory following chronic in vivo morphine exposure. These adaptations to chronic morphine indicate the plasticity of opioid-signal transduction mechanisms and the ability of chronic morphine to augment new signaling strategies.

Role of protein kinase C (PKC) in agonist-induced mu-opioid receptor down-regulation: I. PKC translocation to the membrane of SH-SY5Y neuroblastoma cells is induced by mu-opioid agonists

Agonist-induced down-regulation of opioid receptors appears to require the phosphorylation of the receptor protein. However, the identities of the specific protein kinases that perform this task remain uncertain. Protein kinase C (PKC) has been shown to catalyze the phosphorylation of several G protein-coupled receptors and potentiate their desensitization toward agonists. However, it is unknown whether opioid receptor agonists induce PKC activation under physiological conditions. Using cultured SH-SY5Y neuroblastoma cells, which naturally express mu- and delta-opioid receptors, we investigated whether mu-opioid receptor agonists can activate PKC by measuring enzyme translocation to the membrane fraction. PKC translocation and opioid receptor densities were simultaneously measured by 3H-phorbol ester and [3H]diprenorphine binding, respectively, to correlate alterations in PKC localization with changes in receptor binding sites. We observed that mu-opioid agonists have a dual effect on membrane PKC density depending on the period of drug exposure. Exposure for 2-6 h to [D-Ala2,N-Me-Phe4,Gly-ol]enkephalin or morphine promotes the translocation of PKC from the cytosol to the plasma membrane. Longer periods of opioid exposure (>12 h) produce a decrease in membrane-bound PKC density to a level well below basal. A significant decrease in [3H]diprenorphine binding sites is first observed at 2 h and continues to decline through the last time point measured (48 h). The opioid receptor antagonist naloxone attenuated both opioid-mediated PKC translocation and receptor down-regulation. These results demonstrate that opioids are capable of activating PKC, as evidenced by enhanced translocation of the enzyme to the cell membrane, and this finding suggests that PKC may have a physiological role in opioid receptor plasticity.

Role of protein kinase C (PKC) in agonist-induced mu-opioid receptor down-regulation: II. Activation and involvement of the alpha, epsilon, and zeta isoforms of PKC

Phosphorylation of specific amino acid residues is believed to be crucial for the agonist-induced regulation of several G protein-coupled receptors. This is especially true for the three types of opioid receptors (mu, delta, and kappa), which contain consensus sites for phosphorylation by numerous protein kinases. Protein kinase C (PKC) has been shown to catalyze the in vitro phosphorylation of mu- and delta-opioid receptors and to potentiate agonist-induced receptor desensitization. In this series of experiments, we continue our investigation of how opioid-activated PKC contributes to homologous receptor down-regulation and then expand our focus to include the exploration of the mechanism(s) by which mu-opioids produce PKC translocation in SH-SY5Y neuroblastoma cells. [D-Ala2,N-Me-Phe4,Gly-ol]enkephalin (DAMGO)-induced PKC translocation follows a time-dependent and biphasic pattern beginning 2 h after opioid addition, when a pronounced translocation of PKC to the plasma membrane occurs. When opioid exposure is lengthened to >12 h, both cytosolic and particulate PKC levels drop significantly below those of control-treated cells in a process we termed "reverse translocation." The opioid receptor antagonist naloxone, the PKC inhibitor chelerythrine, and the L-type calcium channel antagonist nimodipine attenuated opioid-mediated effects on PKC and mu-receptor down-regulation, suggesting that this is a process partially regulated by Ca2+-dependent PKC isoforms. However, chronic exposure to phorbol ester, which depletes the cells of diacylglycerol (DAG) and Ca2+-sensitive PKC isoforms, before DAMGO exposure, had no effect on opioid receptor down-regulation. In addition to expressing conventional (PKC-alpha) and novel (PKC-epsilon) isoforms, SH-SY5Y cells also contain a DAG- and Ca2+-independent, atypical PKC isozyme (PKC-zeta), which does not decrease in expression after prolonged DAMGO or phorbol ester treatment. This led us to investigate whether PKC-zeta is similarly sensitive to activation by mu-opioids. PKC-zeta translocates from the cytosol to the membrane with kinetics similar to those of PKC-alpha and epsilon in response to DAMGO but does not undergo reverse translocation after longer exposure times. Our evidence suggests that direct PKC activation by mu-opioid agonists is involved in the processes that result in mu-receptor down-regulation in human neuroblastoma cells and that conventional, novel, and atypical PKC isozymes are involved.

Chronic morphine augments adenylyl cyclase phosphorylation: relevance to altered signaling during tolerance/dependence

Despite the demonstration that chronic morphine increases phosphorylation of multiple substrate proteins, their identity has, for the most part, remained elusive. Thus far, chronic morphine has not been shown to increase the phosphorylation of any identified effector protein. This is the first demonstration that persistent activation of opioid receptors has profound effects on phosphorylation of adenylyl cyclase (AC). A dramatic increase in phosphorylation of AC (type II family) was observed in ileum longitudinal muscle myenteric plexus preparations obtained from chronic morphine-treated guinea pigs. Analogous results were obtained when AC was immunoprecipitated using two differentially directed AC antibodies. The magnitude of the augmented AC phosphorylation was substantially attenuated by chelerythrine, a protein kinase C-selective inhibitor. These results suggest the potential relevance of increased phosphorylation (protein kinase C-mediated) of AC to opioid tolerant/dependent mechanisms. Because phosphorylation of AC isoforms (type II family) can significantly increase their stimulatory responsiveness to Gsalpha and Gbetagamma, this mechanism could underlie, in part, the predominance of opioid AC stimulatory signaling observed in opioid tolerant/dependent tissue. Moreover, in light of the fact that many G protein-coupled receptors signal through common effector proteins, this effect provides a mechanism for divergent consequences of chronic morphine treatment and could explain the well documented complexity of changes that accompany the opioid tolerant/dependent state.

Differential effect of chronic morphine on mRNA encoding adenylyl cyclase isoforms: relevance to physiological sequela of tolerance/dependence

In opiate naive longitudinal muscle myenteric plexus tissue, facilitation (GS-mediated) and inhibition (Gi-mediated) of adenylyl cyclase (AC) activity is observed in response to low (nM) and high (microM) concentrations of sufentanil, respectively. Following chronic in vivo exposure to morphine, previously inhibitory concentrations produce excitatory effects. The present study was undertaken to explore the potential relevance of AC isoform-specific regulation to the qualitative change in opioid responsiveness following chronic morphine. Following persistent activation of opiate receptors, levels of AC I mRNA remain unchanged but that of AC IV is significantly augmented (approximately 37%, P < 0.05). AC I and IV are differentially regulated by G alpha i and G beta gamma. The former is inhibited by G alpha i and G beta gamma whereas the latter is relatively insensitive to G alpha i and is stimulated by G beta gamma. Thus, an increase in AC IV mRNA could represent a shift from inhibitory to stimulatory opiate receptor-G protein signalling, as has been observed following chronic morphine. These results indicate that persistent activation of opiate receptors can induce selective changes in the abundance (activity) of AC isoforms. This could explain, in part, some of the adaptations that occur following chronic in vivo morphine exposure.

Endogenous opiates: 1996

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress, tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.