Desensitization of the mu-opioid activation of phospholipase C in SH-SY5Y cells: the role of protein kinases C and A and Ca(2+)-activated K+ currents

Erythroxylum cuneatum prevented cellular adaptation in morphine-induced neuroblastoma cells

BACKGROUND

Chronic morphine stimulates prolonged stimulation of opioid receptors, especially µ-opioid subtype (MOR), which in turn signals cellular adaptation. However, the sudden termination of morphine after chronic intake causes withdrawal syndrome.

OBJECTIVES

Hence, this study was designed to find an alternative treatment for the morphine withdrawal using the alkaloid leaf extract of Erythroxylum cuneatum (E. cuneatum), done on morphine-exposed neuroblastoma cell lines.

METHODS

SK-N-SH, a commercialised neuroblastoma cell line, was used in two separate study designs; the antagonistic and pre-treatment of morphine. The antagonistic treatment was conducted through concurrent exposure of the cells to morphine and E. cuneatum or morphine and methadone for 24 h. The pre-treatment design was carried out by exposing the cells to morphine for 24 h, followed by 24 h exposures to E. cuneatum or methadone. The cytosolic fraction was collected and run for protein expression involved in cellular adaptation; mitogen-activated protein (MAP)/extracellular signal-regulated (ERK) kinase 1/2 (MEK 1/2), extracellular signal-regulated kinase 2 (ERK 2), cAMP-dependent protein kinase (PKA) and protein kinases C (PKC).

RESULTS

The antagonistic treatment showed the normal level of MEK 1/2, ERK 2, PKA and PKC by the combination treatment of morphine and E. cuneatum, comparable to the combination of morphine and methadone. Neuroblastoma cells exposed to morphine pre-treatment expressed a high level of MEK 1/2, ERK 2, PKA and PKC, while the treatments with E. cuneatum and methadone normalised the expression of the cellular adaptation proteins.

CONCLUSION

E. cuneatum exerted anti-addiction properties by lowering the levels of cellular adaptation proteins, and its effects are comparable to that of methadone (an established anti-addiction drug).

Opioid receptor trafficking and signaling: what happens after opioid receptor activation?

Prolonged opioid treatment leads to a comprehensive cellular adaptation mediated by opioid receptors, a basis to understand the development of opioid tolerance and dependence. However, the molecular mechanisms underlying opioid-induced cellular adaptation remain obscure. Recent advances in opioid receptor trafficking and signaling in cells have extensively increased our insight into the network of intracellular signal integration. This review focuses on those important intracellular biochemical processes that play critical roles in the development of opioid tolerance and dependence after opioid receptor activation, and tries to explain what happens after opioid receptor activation, and how the cellular adaptation develops from cell membrane to nucleus. Decades of research have delineated a network on opioid receptor trafficking and signaling, but the challenge remains to explain opioid tolerance and dependence from a single cellular signal network.

Tracking the opioid receptors on the way of desensitization

Opioid receptors belong to the super family of G-protein coupled receptors (GPCRs) and are the targets of numerous opioid analgesic drugs. Prolonged use of these drugs results in a reduction of their effectiveness in pain relief also called tolerance, a phenomenon well known by physicians. Opioid receptor desensitization is thought to play a major role in tolerance and a lot of work has been dedicated to elucidate the molecular basis of desensitization. As described for most of GPCRs, opioid receptor desensitization involves their phosphorylation by kinases and their uncoupling from G-proteins realized by arrestins. More recently, opioid receptor trafficking was shown to contribute to desensitization. In this review, our knowledge on the molecular mechanisms of desensitization and recent progress on the role of opioid receptor internalization, recycling or degradation in desensitization will be reported. A better understanding of these regulatory mechanisms would be helpful to develop new analgesic drugs or new strategies for pain treatment by limiting opioid receptor desensitization and tolerance.

Modulation of Na, K-ATPase activity by prostaglandin E1 and [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin

Adenylyl cyclase is activated by prostaglandin E and inhibited by mu-opioids. Since cAMP-related events influence the activity of the Na Pump and its biochemical correlate Na,K-ATPase in many systems, we tested the hypothesis that prostaglandin E1 and [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO), a mu-opioid agonist, have opposing actions on Na,K-ATPase activity. Studies were conducted with alamethicin-permeabilized SH-SY5Y human neuroblastoma cells. Prostaglandin E1 (1 microM) transiently inhibited Na,K-ATPase activity for 10-15 min. A direct activator of protein kinase A, 8-Br-cAMP (150 and 500 microM), also inhibited, but more rapidly and for a shorter duration. Both DAMGO (1 microM) and Rp-adenosine 3',5'-cyclic monophosphorothioate (500 microM), a protein kinase A-inhibitor, reversed the inhibitory effect of prostaglandin E1. DAMGO alone (1 microM) stimulated Na,K-ATPase activity up to nearly three-fold control activity. The stimulatory action of DAMGO was blocked by cyclosporine A (2 microM), an inhibitor of calcineurin, and was dependent on Ca2+ entry through nifedipine-sensitive Ca2+ channels. In the presence of 1 mM EGTA, DAMGO inhibited Na,K-ATPase activity. DAMGO-induced inhibition was blocked by the inositol 1,4,5-trisphosphate receptor antagonist xestospongin C (1 microM). Na,K-ATPase is poised to modulate neuronal excitability through its roles in maintaining the membrane potential and transmembrane ion gradients. The differential effects of prostaglandin E1 and opioids on Na,K-ATPase activity may be related to their actions in hyperalgesia.

SSeCKS immunolabeling in rat primary sensory neurons

SSeCKS (src suppressed C kinase substrate) is a protein kinase C substrate that may play a role in tumor suppression. Recently described in fibroblasts, testes and mesangial cells, SSeCKS may have a function in the control of cell signaling and cytoskeletal arrangement. To investigate the distribution of SSeCKS throughout the nervous system, representative sections of brain, spinal cord and dorsal root ganglia were processed using immunofluorescence. Labeling of central axonal collaterals of primary sensory neurons was observed in the dorsal horn at all spinal levels. SSeCKS-immunoreactivity was also observed in the cerebellum, medulla and sensory ganglia (including trigeminal ganglia). The pattern and distribution of anti-SSeCKS labeling in dorsal root ganglia and the dorsal horn of the spinal cord was similar to that observed for other markers of small primary sensory neurons. Therefore, the coexistence of SSeCKS with substance P, CGRP and acid phosphatase was examined in sections of sensory ganglia, spinal cord and medulla using double immunofluorescent labeling for SSeCKS and substance P/CGRP or sequential SSeCKS immunofluorescence and acid phosphatase/fluoride-resistant acid phosphatase enzyme histochemistry. A small portion of the SSeCKS-labeled cell bodies appeared to represent a subpopulation of substance P (4.8%) and CGRP (4.7%) containing neurons, while 45.0% contained fluoride-resistant acid phosphatase reactivity. These results indicate that SSeCKS has a restricted distribution within the nervous system and that expression of this protein may reflect the specific signaling requirements of a distinct population of nociceptive sensory neurons.

Angiotensin II stimulates PGF(2 alpha)release in cultured neonatal rat ventricular myocytes via L-type calcium channels

Angiotensin II (Ang II) has been shown to cause Prostaglandin F(2 alpha)(PGF(2 alpha)) release in neonatal rat ventricular myocytes and smooth muscle cells. In these cells, Ang II has also been shown to regulate growth. We used neonatal rat ventricular myocytes to investigate the role of calcium in maintenance of Ang II-induced PGF(2 alpha)release. The amount of PGF(2 alpha)produced was determined by radioimmunoassay. Ang II-induced PGF(2 alpha)release. Pretreatment of neonatal rat ventricular myocytes with different doses (10(-8)M, 10(-7)M, 10(-6)M and 10(-5)M) of diltiazm (voltage-sensitive L-type calcium channel blocker) produced significant inhibition in Ang II-induced PGF(2 alpha)release. Inhibition was first noted at 10(-8)M and was complete at 10(-6)M. Conversely, pretreatment of neonatal rat ventricular myocytes with different doses (10(-8)M, 10(-7)M, 10(-6)M and 10(-5)M) of calcium channel blockers (conotoxin; voltage-sensitive N-type calcium channel blocker or thapsigargin; intracellular calcium channel blocker) produced no changes in Ang II-induced PGF(2 alpha)release. These results strongly suggest that Ang II-induced PGF(2 alpha)release in neonatal rat ventricular myocytes is maintained, at least in part, via increase in extracellular calcium influx.

Administration of myr(+)-G(i2)alpha subunits prevents acute tolerance (tachyphylaxis) to mu-opioid effects in mice

The administration of efficacious doses of morphine or beta-endorphin causes acute tolerance (tachyphylaxis) to the effects of additional administrations of these opioids. Mice intracerebroventricularly (icv)-injected with biologically active myristoylated (myr(+))-G(i2)alpha subunits developed no tachyphylaxis to morphine antinociception in the tail-flick test. This treatment increased the potency of opioid-induced analgesia during the declining phase. Moreover, animals showing tachyphylaxis to opioid effects exhibited normal responses to the agonists after icv-administration of myr(+)-G(i2)alpha subunits. In morphine tolerant/dependent mice, an icv dose of 12 pmol/mouse myr(+)-G(i2)alpha subunits facilitated complete restoration of morphine antinociception in only 4 or 5 days instead of the 10 to 11 days required for post-dependent mice. This was observed when myr(+)-G alpha subunits were injected within the first 24 h of chronic morphine administration -- but not later when long-term tolerance takes place. These results suggest that during the course of an opioid effect a progressive reduction of receptor-regulated G-proteins occurs, and hence tachyphylaxis develops. Exogenous administration of myr(+)-G alpha subunits may be of therapeutic potential in improving agonist activity and accelerating the recovery of post-dependent receptors.

Calcium and the anaesthetist

Calcium plays a central role in a large number of physiological actions that are essential for life. It is important therefore that the anaesthetist understands calcium pathophysiology. In this review, the physiology, regulation, clinical features, causes and treatment of alterations in circulating calcium will be discussed. In addition, the effects that acid-base status, massive blood transfusion and cardiopulmonary bypass may have on circulating calcium will be highlighted. Finally, the role that calcium plays in ischaemic/reperfusion injury and myocardial stunning will be summarised.

[Mechanisms of opioid tolerance and opioid dependence]

OBJECTIVE

Prescription of opiates to non cancer chronic pain patients is controversial, partly because of the risk of tolerance and dependence development. The two objectives of that review were: a) to identify the factors which may explain the variability of tolerance and dependence in clinical practice; b) to analyse the cellular mechanisms of occurrence of those phenomenons.

DATA SOURCES AND EXTRACTION

To our own file, we added articles retrieved in the Medline database, using, alone or in combination, following key-words (opiate, tolerance, dependence, opiate receptor, pain treatment, cAMP, cGMP, NO, NMDA, protein kinase, gene). Out of nearly 450 articles, we selected less than 200.

DATA SYNTHESIS

Tolerance, defined as loss of opioid efficacy with time, is extremely variable and depends on pain mechanisms, intrinsic efficacy and administration modality of the opioid, as well as co-administration of other agents. Physical dependence is a consequence of the intrinsic and extrinsic adaptations concerning structures as locus coeruleus, paragigantocellular nucleus, spinal cord. Acute and chronic application of opiates and withdrawal give rise to cellular adaptations which depend on the nature and efficacy of the opiate, the type of receptor and second messengers, as well as the type of cell line under study. These cellular mechanisms have consequences on neuronal excitability and gene expression. They constitute a model of cellular tolerance and dependence, but cannot explain the subtelties encountered in clinical practice.

Met-enkephalin-induced mobilization of intracellular Ca2+ in rat intracardiac ganglion neurones

The effects of Met-enkephalin on Ca2+-dependent K+ channel activity were investigated using the cell-attached patch recording technique on isolated parasympathetic neurones of rat intracardiac ganglia. Large-conductance, Ca2+-dependent K+ channels (BK(Ca)) were examined as an assay of agonist-induced changes in the intracellular free calcium ion concentration ([Ca2+]i). These BK(Ca) channels had a conductance of approximately 200 pS and were charybdotoxin- and voltage-sensitive. Caffeine (5 mM), used as a control, evoked a large increase in BK(Ca) channel activity, which was inhibited by 10 microM ryanodine. Met-enkephalin (10 microM) evoked a similar increase in BK(Ca) channel activity, which was dependent on the presence of extracellular Ca2+ and inhibited by either ryanodine (10 microM) or naloxone (1 microM). In Fura-2-loaded intracardiac neurones, Met-enkephalin evoked a transient increase in [Ca2+]i. Met-enkephalin-induced mobilization of intracellular Ca+ may play a role in neuronal excitability and firing behaviour in mammalian intracardiac ganglia.

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.

Nociceptin/orphanin FQ activates protein kinase C, and this effect is mediated through phospholipase C/Ca2+ pathway

The effect of nociceptin/orphanin FQ (N/OFQ) on protein kinase C (PKC) was investigated in Chinese hamster ovary cells stably expressing opioid receptor-like (ORL1) receptor (CHO-ORL1 cells). N/OFQ significantly activated PKC in CHO-ORL1 cells with EC50 of 0.2 nM. This response was blocked by PKC inhibitors chelerythrine and Gö 6976, and by pretreatment of cells with pertussis toxin (PTX). The inhibition of PKC activation by N/OFQ was also achieved by use of Ca(2+)-chelators and phospholipase C (PLC) inhibitor U-73122. These results indicate that N/OFQ can effectively activate PKC via ORL1 receptor, and suggest the activation involve the PLC/Ca2+ system.

Opioids potentiate transmitter release from SK-N-SH human neuroblastoma cells by modulating N-type calcium channels

Opioids induce dual (inhibitory and excitatory) regulation of depolarization-evoked [3H]dopamine release in SK-N-SH cells through either mu or delta receptors. The potentiation of dopamine release by opioid agonists is mediated by N-type voltage-dependent calcium channels and does not involve Gi/Go proteins. Removal of the excitatory opioid effect by blockade with omega-conotoxin, an N-channel antagonist, reveals the inhibitory effect of opioids on release, thus suggesting that both modulatory effects of opioids are exerted in parallel.

Modulation of protein kinase C and cAMP-dependent protein kinase by delta-opioid

Modulation of protein kinase C (PKC) and cAMP-dependent protein kinase (PKA) activities by delta-opioid receptor specific agonist [D-Pen2, D-Pen5]-enkephalin (DPDPE) was investigated in neuroblastoma x glioma hybrid NG 108-15 cells. DPDPE activated PKC in a dose-dependent manner, with the maximal response at 5 min. The DPDPE-stimulated PKC activation could be blocked by naltrindole. The activation of PKC by DPDPE was dependent on Ca2+ and was inhibited by chelerythrine chloride (10 microM), but not by H89 (1 microM). Pretreatment of NG 108-15 cells with pertussis toxin (100 ng/ml for 24 h) completely abolished DPDPE-stimulated PKC activation. In contrast to the result from the acute treatment with DPDPE, which had no significant effect on PKA activity, chronic treatment of DPDPE (1 microM for 24 h) increased PKA activity, but reduced the basal activity of PKC. These results demonstrated that DPDPE differentially modulated PKC and PKA activities via a receptor-mediated, PTX sensitive pathway.

The effects of recombinant rat mu-opioid receptor activation in CHO cells on phospholipase C, [Ca2+]i and adenylyl cyclase

1. The rat mu-opioid receptor has recently been cloned yet its second messenger coupling remains unclear. The endogenous mu-opioid receptor in SH-SY5Y cells couples to phospholipase C (PLC), increases [Ca2+]i and inhibits adenylyl cyclase (AC). We have examined the effects of mu-opioid agonists on inositol(1,4,5)trisphosphate (Ins(1,4,5)P3), [Ca2+]i and adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation in Chinese hamster ovarian (CHO) cells transfected with the cloned mu-opioid receptor. 2. Opioid receptor binding was assessed with [3H]-diprenorphine ([3H]-DPN) as a radiolabel. Ins(1,4,5)P3 and cyclic AMP were measured by specific radioreceptor assays. [Ca2+]i was measured fluorimetrically with Fura-2. 3. Scatchard analysis of [3H]-DPN binding revealed that the Bmax varied between passages. Fentanyl (10 pM 1 microM) dose-dependently displaced [3H]-DPN, yielding a curve which had a Hill slope of less than unity (0.6 +/- 0.1), and was best fit to a two site model, with pK1 values (% of sites) of 9.97 +/- 0.4 (27 +/- 4.8%) and 7.68 +/- 0.07 (73 +/- 4.8%). In the presence of GppNHp (100 microM) and Na+ (100 mM), the curve was shifted to the right and became steeper (Hill slope = 0.9 +/- 0.1) with a pK1 value of 6.76 +/- 0.04. 4. Fentanyl (0.1 nM-1 microM) had no effect on basal, but dose-dependently inhibited forskolin (1 microM)-stimulated, cyclic AMP formation (pIC50 -7.42 +/- 0.23), in a pertussis toxin (PTX; 100 ng ml-1 for 24 h)-sensitive and naloxone-reversible manner (K1 = 1.7 nM). Morphine (1 microM) and [D-Ala2, MePhe4, gly(ol)5]-enkephalin (DAMGO, 1 microM) also inhibited forskolin (1 microM)-stimulated cyclic AMP formation, whilst [D-Pen2, D-Pen5], enkephalin (DPDPE, 1 microM) did not. 5. Fentanyl (0.1 nM-10 microM) caused a naloxone (1 microM)-reversible, dose-dependent stimulation of Ins(1,4,5)P3 formation, with a pEC50 of 7.95 +/- 0.15 (n-5), PTX (100 ng ml-1 for 24 h) abolished, whilst Ni2 (2.5 mM) inhibited (by 52%), the fentanyl-induced Ins(1,4,5)P3 response. Morphine (1 microM) and DAMGO (1 microM), but not DPDPE (1 microM), also stimulated Ins(1,4,5)P3 formation. Fentanyl (1 microM) also caused an increase in [Ca2+]i (80 +/- 16.4 nM, n-6), reaching a maximum at 26.8 +/- 2.5 s. The increase in [Ca2+]i remained elevated until sampling ended (200 s) and was essentially abolished by the addition of naloxone (1 microM). Pre-incubation with naloxone (1 microM, 3 min) completely abolished fentanyl-induced increases in [Ca2+]i. 6. In conclusion, the cloned mu-opioid receptor when expressed in CHO cells stimulates PLC and inhibits AC, both effects being mediated by a PTX-sensitive G-protein. In addition, the receptor couples to an increase in [Ca2+]i. These findings are consistent with the previously described effector-second messenger coupling of the endogenous mu-opioid receptor.

Stimulatory effects of opioids on transmitter release and possible cellular mechanisms: overview and original results

Opiates and opioid peptides carry out their regulatory effects mainly by inhibiting neuronal activity. At the cellular level, opioids block voltage-dependent calcium channels, activate potassium channels and inhibit adenylate cyclase, thus reducing neurotransmitter release. An increasing body of evidence indicates an additional opposite, stimulatory activity of opioids. The present review summarizes the potentiating effects of opioids on transmitter release and the possible cellular events underlying this potentiation: elevation of cytosolic calcium level (by either activating Ca2+ influx or mobilizing intracellular stores), blockage of K+ channels and stimulation of adenylate cyclase. Biochemical, pharmacological and molecular biology studies suggest several molecular mechanisms of the bimodal activity of opioids, including the coupling of opioid receptors to various GTP-binding proteins, the involvement of different subunits of these proteins, and the activation of several intracellular signal transduction pathways. Among the many experimental preparations used to study the bimodal opioid activity, the SK-N-SH neuroblastoma cell line is presented here as a suitable model for studying the complete chain of events leading from binding to receptors down to regulation of transmitter release, and for elucidating the molecular mechanism involved in the stimulatory effects of opioid agonists.

The stimulatory effects of opioids and their possible role in the development of tolerance

Opioids have stimulatory as well as the traditional inhibitory effects on neurotransmission, but the underlying mechanisms are poorly understood. Here, Darren Smart and David Lambert review the stimulatory effects of opioids on second messengers, including inositol (1,4,5)-trisphosphate (IP3), protein kinase C (PKC), Ca2+, and cAMP, and propose that these coordinated changes at the cellular level underlie the facilitatory effects of opioids on neurotransmission. The evidence for a possible role for these stimulatory effects, particularly the activation of PKC by opioids, in the development of tolerance is also discussed.

Tyr-D-Arg2-Phe-sarcosine4 activates phospholipase C-coupled mu2-opioid receptors in SH-SY5Y cells

The dermorphin analogue Tyr-D-Arg2-Phe-sarcosine4 acts as a mu1-opioid receptor agonist, but as a mu2-opioid receptor antagonist, in vivo, yet the biochemical effects of Tyr-D-Arg2-Phe-sarcosine4 are unknown. Therefore, we characterized the effects of Tyr-D-Arg2-Phe-sarcosine4 on the mu-opioid receptor-mediated stimulation of inositol(1,4,5)trisphosphate, and inhibition of cAMP, in SH-SY5Y cells. We report here for the first time that Tyr-D-Arg2-Phe-sarcosine4 has no effect on basal cAMP or inositol(1,4,5)trisphosphate formation, but reversed the effects of fentanyl on these second messengers, consistent with Tyr-D-Arg2-Phe-sarcosine4 acting as a mu2-opioid receptor antagonist, and confirming that the mu-opioid receptors in SH-SY5Y cells are of the mu2 subtype.