Evidence for the implication of phosphoinositol signal transduction in mu-opioid inhibition of DNA synthesis

Preclinical and clinical studies into the bioactivity of low-dose naltrexone (LDN) for oncotherapy

Naltrexone (NTX) is a nonspecific opioid antagonist that exerts pharmacological effects on the opioid axis by blocking opioid receptors distributed in cytoplastic and nuclear regions. NTX has been used in opioid use disorder (OUD), immune-associated diseases, alcoholism, obesity, and chronic pain for decades. However, low-dose naltrexone (LDN) also exhibits remarkable inhibition of DNA synthesis, viability, and other functions in numerous cancers and is involved in immune remodeling against tumor invasion and chemical toxicity. The potential anticancer activity of LDN is a focus of basic research. Herein, we summarize the associated studies on LDN oncotherapy to highlight the potential mechanisms and prospective clinical applications.

Morphine and gp120 toxic interactions in striatal neurons are dependent on HIV-1 strain

A rigorously controlled, cell culture paradigm was used to assess the role of HIV-1 gp120 ± morphine in mediating opioid-HIV interactive toxicity in striatal neurons. Computerized time-lapse microscopy tracked the fate of individual neurons co-cultured with mixed-glia from mouse striata during opioid and gp120 exposure. Subpopulations of neurons and astroglia displayed μ-opioid receptor, CXCR4, and CCR5 immunoreactivity. While gp120 alone was or tended to be neurotoxic irrespective of whether X4-tropic gp120(IIIB), R5-tropic gp120(ADA), or dual-tropic gp120(MN) was administered, interactive toxicity with morphine differed depending on HIV-1 strain. For example, morphine only transiently exacerbated gp120(IIIB)-induced neuronal death; however, in combination with gp120(MN), morphine caused sustained increases in the rate of neuronal death compared to gp120(MN) alone that were prevented by naloxone. Alternatively, gp120(ADA) significantly increased the rate of neuron death, but gp120(ADA) toxicity was unaffected by morphine. The transient neurotoxic interactions between morphine and gp120(IIIB) were abrogated in the absence of glia suggesting that glia contribute significantly to the interactive pathology with chronic opiate abuse and neuroAIDS. To assess how mixed-glia might contribute to the neurotoxicity, the effects of morphine and/or gp120 on the production of reactive oxygen species (ROS) and on glutamate buffering were examined. All gp120 variants, and to a lesser extent morphine, increased ROS and/or decreased glutamate buffering, but together failed to show any interaction with morphine. Our findings indicate that HIV-1 strain-specific differences in gp120 are critical determinants in shaping both the timing and pattern of neurotoxic interactions with opioid drugs.

Global kinase screening. Applications of frontal affinity chromatography coupled to mass spectrometry in drug discovery

Utilizing frontal affinity chromatography with mass spectrometry detection (FAC-MS), we have identified novel applications in the discovery of small-molecule hits to protein targets that are difficult if not impossible to accomplish using traditional assays. We demonstrate for the first time an ability to distinguish between competitive ligands for the ATP and substrate sites of protein kinase C independently in the same experiment and show that ATP competitive ligands using a functionally inactive receptor tyrosine kinase can be identified. This ability of FAC-MS to simultaneously monitor binding at the ATP and substrate binding sites, as well as measure ligand binding to both active and inactive kinases, suggests that FAC-MS can be used as a "global kinase binding assay".

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.

Selective vulnerability of cerebellar granule neuroblasts and their progeny to drugs with abuse liability

Cerebellar development is shaped by the interplay of genetic and numerous environmental factors. Recent evidence suggests that cerebellar maturation is acutely sensitive to substances with abuse liability including alcohol, opioids, and nicotine. Assuming substance abuse disrupts cerebellar maturation, a central question is: what are the basic mechanisms underlying potential drug-induced developmental defects? Evidence reviewed herein suggests that the maturation of granule neurons and their progeny are intrinsically affected by several classes of substances with abuse liability. Although drug abuse is also likely to target directly other cerebellar neuron and glial types, such as Purkinje cells and Bergmann glia, findings in isolated granule neurons suggest that they are often the principle target for drug actions. Developmental events that are selectively disrupted by drug abuse in granule neurons and/or their neuroblast precursors include proliferation, migration, differentiation (including neurite elaboration and synapse formation), and programmed cell death. Moreover, different classes of drugs act through distinct molecular mechanisms thereby disrupting unique aspects of development. For example, drug-induced perturbations in: (i) neurotransmitter biogenesis; (ii) ligand and ion-gated receptor function and their coupling to intracellular effectors; (iii) neurotrophic factor biogenesis and signaling; and (iv) intercellular adhesion are all likely to have significant effects in shaping developmental outcome. In addition to identifying therapeutic strategies for drug abuse intervention, understanding the mechanisms by which drugs affect cellular maturation is likely to provide a better understanding of the neurochemical events that normally shape central nervous system development.

Effects of ginsenosides on carbachol-stimulated formation of inositol phosphates in rat cortical cell cultures

We examined the effect of ginseng total saponins (GTS) on phosphoinositide metabolism stimulated by activation of muscarinic receptor using rat cortical cultures. Carbachol stimulated formation of [3H]inositol phosphates ([3H]InsPs) by 3.3-fold over basal level in [3H]inositol-prelabeled cells. Pretreatment of GTS inhibited formation of [3H]InsPs evoked by carbachol by 70%-90%. Addition of GTS alone had no effect on the basal formation of [3H]InsPs. The inhibitory effect of the GTS on carbachol-stimulated formation of [3H]InsPs was dose- and time-dependent. IC50 was 6.0 +/- 2.8 microg/ml. We also examined the effect of GTS on [3H]InsP1, [3H]InsP2, or [3H]InsP3 formation evoked by carbachol. Although GTS had no effect on the basal [3H]InsP1, [3H]InsP2, or [3H]InsP3 formation, pretreatment of GTS inhibited [3H]InsP1, [3H]InsP2, or [3H]InsP3 formation evoked by carbachol, respectively. Addition of individual ginsenosides such as ginsenoside Rb1, Rc, Rd, Re, or Rg2 had no effect on the basal formation of [3H]InsPs, whereas pretreatment of ginsenoside Rb2, Rc, Rd, Re, Rf, Rg1 or Rg2 inhibited formation of [3H]InsPs evoked by carbachol by 79%-89%. The results suggest that the inhibitory effect of GTS and its individual ginsenosides on carbachol-stimulated formation of [3H]InsPs in cortical neurons could be one pharmacological action of Panax ginseng.

1alpha,25(OH)2D3 causes a rapid increase in phosphatidylinositol-specific PLC-beta activity via phospholipase A2-dependent production of lysophospholipid

1alpha,25(OH)(2)D(3) activates protein kinase C (PKC) in rat growth plate chondrocytes via mechanisms involving phosphatidylinositol-specific phospholipase C (PI-PLC) and phospholipase A(2) (PLA(2)). The purpose of this study was to determine if 1alpha,25(OH)(2)D(3) activates PI-PLC directly or through a PLA(2)-dependent mechanism. We determined which PLC isoforms are present in the growth plate chondrocytes, and determined which isoform(s) of PLC is(are) regulated by 1alpha,25(OH)(2)D(3). Inhibitors and activators of PLA(2) were used to assess the inter-relationship between these two phospholipid-signaling pathways. PI-PLC activity in lysates of prehypertrophic and upper hypertrophic zone (growth zone) cells that were incubated with 1alpha,25(OH)(2)D(3), was increased within 30s with peak activity at 1-3 min. PI-PLC activity in resting zone cells was unaffected by 1alpha,25(OH)(2)D(3). 1beta,25(OH)(2)D(3), 24R,25(OH)(2)D(3), actinomycin D and cycloheximide had no effect on PLC in lysates of growth zone cells. Thus, 1alpha,25(OH)(2)D(3) regulation of PI-PLC enzyme activity is stereospecific, cell maturation-dependent, and nongenomic. PLA(2)-activation (mastoparan or melittin) increased PI-PLC activity to the same extent as 1alpha,25(OH)(2)D(3); PLA(2)-inhibition (quinacrine, oleyloxyethylphosphorylcholine (OEPC), or AACOCF(3)) reduced the effect of 1alpha,25(OH)(2)D(3). Neither arachidonic acid (AA) nor its metabolites affected PI-PLC. In contrast, lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE) activated PI-PLC (LPE>LPC). 1alpha,25(OH)(2)D(3) stimulated PI-PLC and PKC activities via Gq; GDPbetaS inhibited activity, but pertussis toxin did not. RT-PCR showed that the cells express PLC-beta1a, PLC-beta1b, PLC-beta3 and PLC-gamma1 mRNA. Antibodies to PLC-beta1 and PLC-beta3 blocked the 1alpha,25(OH)(2)D(3) effect; antibodies to PLC-delta and PLC-gamma did not. Thus, 1alpha,25(OH)(2)D(3) regulates PLC-beta through PLA(2)-dependent production of lysophospholipid.

1alpha,25-dihydroxyvitamin D(3) and 24R,25-dihydroxyvitamin D(3) modulate growth plate chondrocyte physiology via protein kinase C-dependent phosphorylation of extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase

Membrane-mediated increases in protein kinase C (PKC) activity and PKC-dependent physiological responses of growth plate chondrocytes to vitamin D metabolites depend on the state of endochondral maturation; 1alpha,25-dihydroxyvitamin D(3) [1alpha,25-(OH)(2)D(3)] regulates growth zone (GC) cells, whereas 24R,25-(OH)(2)D(3) regulates resting zone (RC) cells. Different mechanisms, including protein kinase A signaling, mediate the effects of 1alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3) on PKC, suggesting that different mechanisms may also regulate any MAPK involvement in the physiological responses. This study used confluent cultures of rat costochondral chondrocytes as a model. 1alpha,25-(OH)(2)D(3) stimulated MAPK specific activity in GC in a time- and dose-dependent manner, evident within 9 min. 24R,25-(OH)(2)D(3) stimulated MAPK in RC; increases were dose dependent, occurred after 9 min, and were greatest at 90 min. In both cells the effect was due to ERK1/2 activation (p42 > p44 in GC; p42 = p44 in RC). MAPK activation was dependent on PKC, but not protein kinase A. The effect of 1alpha,25-(OH)(2)D(3) required phospholipase C, and the effect of 24R,25-(OH)(2)D(3) required phospholipase D. Inhibition of cyclooxygenase activity reduced the effect of 1alpha,25-(OH)(2)D(3) on MAPK in GC and enhanced the effect of 24R,25-(OH)(2)D(3) in RC. Based on MAPK inhibition with PD98059, ERK1/2 MAPK mediated the effect of 24R,25-(OH)(2)D(3) on [(3)H]thymidine incorporation and [(35)S]sulfate incorporation by RC, but only partially mediated the effect of 1alpha,25-(OH)(2)D(3) on GC. ERK1/2 was not involved in the regulation of alkaline phosphatase specific activity by either metabolite. This paper supports the hypothesis that 1alpha,25-(OH)(2)D(3) regulates the physiology of GC via rapid membrane-mediated signaling pathways, and some, but not all, of the response to 1alpha,25-(OH)(2)D(3) is via the ERK family of MAPKs. In contrast, 24R,25-(OH)(2)D(3) exerts its effects on RC via PKC-dependent MAPK. Whereas 1alpha,25-(OH)(2)D(3) increases MAPK activity via phospholipase C and increased prostaglandin production, 24R,25-(OH)(2)D(3) increases MAPK via phospholipase D and decreased prostaglandin production. The cell specificity, metabolite stereospecificity, and the dependence on PKC argue for the participation of membrane receptors for 1alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3) in the regulation of ERK1/2 in the growth plate.

Therapeutic potential of PKC inhibitors in painful diabetic neuropathy

Diabetic neuropathy accompanied by anomalies in pain perception is one of the most frequent complications in insulin-dependent diabetes in humans. Many clinical and experimental studies have suggested that diabetes or hyperglycaemia alters pain sensitivity. In humans, diabetic neuropathy can be associated with burning, tactile hypersensitivity. Behavioural reactions of hyperalgesia in animal models of diabetes have been described. However, the aetiology of these disturbances is still unknown, although metabolic factors such as hyperglycaemia or neurotransmitter alteration may be involved. Activation of protein kinase C (PKC) has been implicated in changes in pain perception. Phorbol esters, which activate PKC, enhance the thermal hyperalgesia in diabetic mice and enhance nociceptive responses after tissue injury induced by formalin. Electrophysiological experiments have shown that activation of PKC leads to long-lasting enhancement of excitatory amino acid-mediated currents in dorsal horn neurones and trigeminal neurones. Thus, activation of PKC may underlie the neuronal sensitisation that produces hyperalgesia in diabetic neuropathy.

Inhibition of carbachol stimulated acid secretion by interleukin 1beta in rabbit parietal cells requires protein kinase C

BACKGROUND

Interleukin 1beta (IL-1beta) is a potent inhibitor of gastric acid secretion. Regulatory actions at several levels have previously been demonstrated, including direct inhibition of parietal cell acid secretion. Although IL-1beta may activate several intracellular signalling pathways, the mechanisms responsible for inhibition of carbachol stimulated acid secretion have not been determined.

AIMS

To investigate the roles of protein kinase C (PKC) and the sphingomyelinase signalling pathways in the regulation of acid secretion by IL-1beta.

METHODS

Rabbit parietal cells were obtained by collagenase-EDTA digestion and centrifugal elutriation. Acid secretion stimulated by carbachol and A23187 (to mimic elevations in intracellular calcium) was assessed by 14C aminopyrine uptake in response to IL-1beta, PKC, and sphingomyelinase manipulation.

RESULTS

IL-1beta inhibited carbachol and A23187 stimulated acid secretion in a dose dependent manner. The inhibitory actions were completely reversed by each of three different PKC inhibitors, staurosporine, H-7, and chelerythrine, as well as by PKC depletion with high dose phorbol ester pretreatment. IL-1beta did not downregulate parietal cell muscarinic receptor. IL-1beta significantly increased membrane PKC activity. Activation of the sphingomyelinase/ceramide pathway had no effect on basal or stimulated acid secretion. The inhibitory action of IL-1beta was independent of protein kinase A and protein kinase G activity.

CONCLUSIONS

IL-1beta directly inhibits parietal cell carbachol stimulated acid secretion. This action occurs distal to muscarinic receptor activation and elevations in intracellular calcium and requires PKC.

Adenosine receptor subtypes modulate two major functional pathways for hippocampal serotonin release

To clarify the mechanisms of interaction between adenosine A(1) receptor (A1-R) and adenosine A(2) receptor (A2-R) on neurotransmitter release, this study determined the functional interactions among adenosine receptors (AD-Rs), voltage-sensitive Ca(2+) channels (VSCCs), protein kinases (PKs), and synaptic proteins [N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptors] on hippocampal serotonin release using in vivo microdialysis in freely moving rat. Basal serotonin release was regulated by two functional complexes: N-type VSCC (N-VSCC)/calcium-phospholipid-dependent protein kinase (PKC)/syntaxin (major pathway) and P-type VSCC (P-VSCC)/cyclic AMP-dependent protein kinase (PKA)/synaptobrevin (minor pathway). However, K(+)-evoked serotonin release was regulated by N-VSCC/PKC/syntaxin (minor pathway) and P-VSCC/PKA/synaptobrevin (major pathway). A1-R antagonists increased basal serotonin release, which was reduced by inhibitors of N-VSCC, PKC, and syntaxin predominantly and by inhibitors of PKA and synaptobrevin weakly, but was not affected by P-VSCC inhibitor. In the presence of A1-R antagonist, A2-R agonists increased basal serotonin release, which was inhibited by inhibitors of P-VSCC, PKA, and synaptobrevin predominantly and reduced by inhibitors of N-VSCC, PKC, and syntaxin weakly. Under the condition of activation of adenylate cyclase in the absence of A1-R antagonists, A2-R agonists increased basal serotonin release. A1-R antagonist and A2-R agonist enhanced K(+)-evoked serotonin release, which was inhibited by inhibitors of P-VSCC, PKA, and synaptobrevin predominantly. These results suggest that an activation of A1-R suppresses serotonin release via inhibition of both N-VSCC/PKC/syntaxin and P-VSCC/PKA/synaptobrevin pathways, and an activation of A2-R stimulates serotonin release via enhancement of the P-VSCC/PKA/synaptobrevin pathway. Therefore, PKA activity plays an important role in the interaction between A1-R and A2-R on hippocampal serotonin release.

Adenosine receptor subtypes modulate two major functional pathways for hippocampal serotonin release

To clarify the mechanisms of interaction between adenosine A(1) receptor (A1-R) and adenosine A(2) receptor (A2-R) on neurotransmitter release, this study determined the functional interactions among adenosine receptors (AD-Rs), voltage-sensitive Ca(2+) channels (VSCCs), protein kinases (PKs), and synaptic proteins [N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptors] on hippocampal serotonin release using in vivo microdialysis in freely moving rat. Basal serotonin release was regulated by two functional complexes: N-type VSCC (N-VSCC)/calcium-phospholipid-dependent protein kinase (PKC)/syntaxin (major pathway) and P-type VSCC (P-VSCC)/cyclic AMP-dependent protein kinase (PKA)/synaptobrevin (minor pathway). However, K(+)-evoked serotonin release was regulated by N-VSCC/PKC/syntaxin (minor pathway) and P-VSCC/PKA/synaptobrevin (major pathway). A1-R antagonists increased basal serotonin release, which was reduced by inhibitors of N-VSCC, PKC, and syntaxin predominantly and by inhibitors of PKA and synaptobrevin weakly, but was not affected by P-VSCC inhibitor. In the presence of A1-R antagonist, A2-R agonists increased basal serotonin release, which was inhibited by inhibitors of P-VSCC, PKA, and synaptobrevin predominantly and reduced by inhibitors of N-VSCC, PKC, and syntaxin weakly. Under the condition of activation of adenylate cyclase in the absence of A1-R antagonists, A2-R agonists increased basal serotonin release. A1-R antagonist and A2-R agonist enhanced K(+)-evoked serotonin release, which was inhibited by inhibitors of P-VSCC, PKA, and synaptobrevin predominantly. These results suggest that an activation of A1-R suppresses serotonin release via inhibition of both N-VSCC/PKC/syntaxin and P-VSCC/PKA/synaptobrevin pathways, and an activation of A2-R stimulates serotonin release via enhancement of the P-VSCC/PKA/synaptobrevin pathway. Therefore, PKA activity plays an important role in the interaction between A1-R and A2-R on hippocampal serotonin release.

Angiotensin II increases vasopressin-stimulated facilitated urea permeability in rat terminal IMCDs

Angiotensin II receptors are present along the rat inner medullary collecting duct (IMCD), although their physiological role is unknown. Because urea is one of the major solutes transported across the terminal IMCD, we measured angiotensin II's effect on urea permeability. In the perfused rat terminal IMCD, angiotensin II had no effect on basal urea permeability but significantly increased vasopressin-stimulated urea permeability by 55%. Angiotensin II, both without and with vasopressin, also increased the amount of (32)P incorporated into urea transporter (UT)-A1 in inner medullary tissue exposed to these hormones ex vivo. Because angiotensin II activates protein kinase C, we tested the effect of staurosporine (SSP). In the absence of angiotensin II, SSP had no effect on vasopressin-stimulated urea permeability in the perfused terminal IMCD. However, SSP completely and reversibly blocked the angiotensin II-mediated increase in vasopressin-stimulated urea permeability. SSP and chelerythrine reduced the angiotensin II-stimulated (32)P incorporation into UT-A1 in inner medullary tissue exposed ex vivo. We conclude that angiotensin II increases vasopressin-stimulated facilitated urea permeability and (32)P incorporation into the 97- and 117-kDa UT-A1 proteins via a protein kinase C-mediated signaling pathway. These data suggest that angiotensin II augments vasopressin-stimulated facilitated urea transport in the rat terminal IMCD and may play a physiological role in the urinary concentrating mechanism by augmenting the maximal response to vasopressin.

Evidence for transduction of mu but not kappa opioid modulation of extracellular signal-regulated kinase activity by G(z) and G(12) proteins

Chronic treatment with micro or kappa opioid agonists (>/=2 h) inhibits EGF-induced ERK activation in opioid receptor overexpressing COS-7 cells. Although acute mu and kappa opioids activate ERK via a pertussis toxin-sensitive G protein, pertussis toxin insensitivity of the chronic mu (but not kappa) action was observed. Here, we tested several pertussis toxin-insensitive G proteins as candidates to transduce acute and/or chronic opioid modulation of ERK. Overexpressed Galpha(z) (but not Galpha(12)) transduced acute mu (but not kappa) ERK activation in pertussis toxin-treated COS-7 cells. Chronic mu (but not kappa) inhibited EGF stimulation of ERK in pertussis toxin-treated cells overexpressing Galpha(z) or Galpha(12). Transfection of Galpha(13) or Galpha(q) blocked inhibition under the same conditions. Overexpressed interfering and non-interfering Galpha(z) mutants differentially affected mu inhibition of ERK consistent with G(z) transduction. In this and prior studies, Galpha(z) and Galpha(12) immunoreactivity were detected in untransfected COS-7 cells, suggesting that these G proteins may be endogenous mediators of chronic mu inhibitory actions on ERK.

Opioid regulation of gonadotropin release: role of signal transduction cascade

The present investigation elucidates the opioidergic modulation of gonadotropin releasing hormone release mechanism by signal transduction cascade in discrete brain regions from estrogen-progesterone primed ovariectomized rats. The effects of mu-opioid agonist morphine and its antagonist naloxone followed by morphine were studied (in two different groups of rats) on protein kinase A, adenosine 3',5' cyclic monophosphate, protein kinase C and calcium/calmodulin protein kinase-II as well as phospholipase C, phospholipase A(2), diacylglycerol and inositol 1,4, 5-triphosphate. Significant decline in phosphoinositide metabolism was observed after morphine treatment as depicted by decrease in phospholipase C and phospholipase A2 activities as well as inositol 1,4,5-triphosphate and diacylglycerol contents from discrete brain regions. Protein kinase A activity showed translocation from membrane bound to cytosolic form along with a decrease in its activator adenosine 3',5'-cyclic monophosphate levels in morphine-treated group. Calcium/calmodulin dependent protein kinase II activity also declined, whereas, protein kinase C activity increased in the cytosolic fraction after 45 min of morphine administration. Naloxone was seen to counteract the changes induced by morphine in most of the brain regions studied. Morphine also suppressed luteinizing hormone levels, whereas, follicle stimulating hormone level did not change. The present investigation provides evidence for opioidergic mediated suppression of gonadotropin release through the downregulation of signal transduction cascade.

Opioids intrinsically inhibit the genesis of mouse cerebellar granule neuron precursors in vitro: differential impact of mu and delta receptor activation on proliferation and neurite elongation

Although opioids are known to affect neurogenesis in vivo, it is uncertain the extent to which opioids directly or indirectly affect the proliferation, differentiation or death of neuronal precursors. To address these questions, the intrinsic role of the opioid system in neurogenesis was systematically explored in cerebellar external granular layer (EGL) neuronal precursors isolated from postnatal mice and maintained in vitro. Isolated neuronal precursors expressed proenkephalin-derived peptides, as well as specific mu and delta, but negligible kappa, opioid receptors. The developmental effects of opioids were highly selective. Morphine-induced mu receptor activation inhibited DNA synthesis, while a preferential delta2-receptor agonist ([D-Ala2]-deltorphin II) or Met-enkephalin, but not the delta1 agonist [D-Pen2, D-Pen5]-enkephalin, inhibited differentiation within the same neuronal population. If similar patterns occur in the developing cerebellum, spatiotemporal differences in endogenous mu and delta opioid ligand-receptor interactions may coordinate distinct aspects of granule neuron maturation. The data additionally suggest that perinatal exposure to opiate drugs of abuse directly interfere with cerebellar maturation by disrupting normal opioid signalling and inhibiting the proliferation of granule neuron precursors.

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.

Inhibitory effect of the kinase inhibitor chelerythrine on acetylcholine-induced current in PC12 cells

In order to analyze the effect of protein kinase C(PKC) on nicotinic acetylcholine receptor in pheochromocytoma (PC12) cells by the whole-cell clamp technique, chelerythrine, a well-known inhibitor of PKC, was used to investigate the influence of PKC on acetylcholine (ACh)-induced current. When cells were preincubated with chelerythrine (0.1-10 microM) for 5 min, an inhibitory effect of chelerythrine on the peak of ACh-induced current was found. This effect was concentration-dependent, voltage-independent, and time-dependent within 1-6 min and reversible. However, intracellular dialysis with 0.1-5 microM PKCI 19-31, a specific pseudosubstrate PKC inhibitor, did not affect the inhibitory effect of chelerythrine. These results suggest that chelerythrine has an inhibitory effect on ACh-induced current in PC12 cells and that this effect is independent of its inhibition on PKC, may represent a new pharmacological effect of chelerythrine, and is mediated by an alternative mechanism.

Response of Djun and Dfos mRNA abundance to signal transduction pathways in cultured cells of Drosophila melanogaster

The mammalian proto-oncogenes c-jun and c-fos are situated at the end of multiple signal transduction pathways and activation of their products Jun and Fos, components of the transcription factor AP-1, are able to regulate gene transcription in response to extracellular stimuli. Djun and Dfos, the products of the Drosophila proto-oncongenes Djun and Dfos, are similar in size and sequence to their mammalian counterparts c-Jun and c-Fos and are related to their mammalian counterparts by their antigenic properties. However, very little is known about how they are regulated through signal transduction pathways. This paper has investigated the response of their mRNA abundance levels to three signal transduction pathways in Drosophila cultured cells. Various agonists and antagonists that stimulate and inhibit specific enzymes in the pathways have been tested. The results suggest that Djun and Dfos mRNA are continuously expressed and their abundance levels are transiently regulated by multiple signaling pathways, the peak response coming at 1-2 hours after perturbation. Dfos is more highly regulated than Djun which is only modulated. The receptor tyrosine kinase pathways positively regulate Dfos and Djun. The cAMP-mediated pathway positively regulates Dfos but negatively regulates Djun. The protein kinase C-activated pathway does not affect Djun whereas it negatively regulates Dfos.

Alteration of spinal protein kinase C expression and kinetics in morphine, but not clonidine, tolerance

Antinociceptive synergism between spinally administered morphine and clonidine decreases to an additive interaction in morphine- and clonidine-tolerant mice. Spinally administered protein kinase C (PKC) inhibitors also decrease the synergism to addition. To determine whether chronic morphine or clonidine treatment alters spinal PKC activity, the present studies measured PKC activity and expression of PKC isoform proteins in spinal cord cytosol and membrane fractions. Mice were treated for 4 days with either placebo pellets, morphine pellets, s.c. saline, or s.c. clonidine. Morphine pellet-implanted mice were tolerant to morphine-induced tail flick antinociception, but not cross-tolerant to clonidine. Clonidine-pretreated mice were tolerant to clonidine, but not cross-tolerant to morphine. Induction of morphine tolerance produced a 2-fold lower Km value for PKC (8.24 +/- 1.67 microM in placebo pellet vs 4.43 +/- 1.24 microM in morphine pellet) in cytosol, but not membrane fractions from spinal cord. Vmax values were not different. No difference in Km or Vmax values was found between proteins from saline- and clonidine-pretreated animals. Immunoreactive cPKCalpha, betaI, and gamma isoforms decreased 14, 26, and 17%, respectively, in cytosol from morphine-tolerant animals. No difference in PKC isoforms was found in the membranes or in fractions from clonidine-tolerant mice. Morphine tolerance, but not clonidine tolerance, enhanced PKC activity while decreasing protein expression.

Modulation of the induction of ornithine decarboxylase by some opioid receptor agonists in immune cells and cardiomyocytes

The ability of natural and synthetic opioids to modulate the induction of ornithine decarboxylase (ODC) was investigated in immune cells and cardiomyocytes in culture. In particular, Leu-enkephalin, which shows preference for delta-receptors, enhanced ODC activity in both thymocytes and cardiomyocytes, whereas the effect of U-50488H, a synthetic kappa-selective agonist, was cell-specific. In thymocytes, U-50488H markedly inhibited the induction of the enzyme elicited by the mitogen concanavalin A (Con A) or by a combined treatment with PMA and A23187, and also reduced basal ODC activity. However the drug did not affect ODC induced by other stimuli. The inhibition of the induction of ODC activity was accompanied by a reduction of ODC mRNA level and an acceleration of ODC turnover. The action of U-50488H in thymocytes does not appear to be mediated by kappa or other classical opioid receptors lacking both stereospecificity and antagonist sensitivity, but may involve a pertussis toxin-sensitive G protein. Splenocytes also showed the ODC inhibiting effect of U-50488H, although they were less sensitive compared to thymocytes. In contrast, U-50488H enhanced ODC activity in cardiomyocytes and this effect was blocked by a specific kappa-antagonist. In conclusion, these results indicate that some opioid agonists can modulate ODC expression in non neural cells. In particular, kappa-opioid receptors may be involved in the U-50488H action in cardiomyocytes, and a distinct site, linked to inhibition of cell proliferation, may operate in immune cells.

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.

1,25(OH)2D3 regulates protein kinase C activity through two phospholipid-dependent pathways involving phospholipase A2 and phospholipase C in growth zone chondrocytes

We have previously shown that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a major role in growth zone chondrocyte (GC) differentiation and that this effect is mediated by protein kinase C (PKC). The aim of the present study was to identify the signal transduction pathway used by 1,25(OH)2D3 to stimulate PKC activation. Confluent, fourth passage GC cells from costochondral cartilage were used to evaluate the mechanism of PKC activation. Treatment of GC cultures with 1,25(OH)2D3 elicited a dose-dependent increase in both inositol-1,4,5-trisphosphate and diacylglycerol (DAG) production, suggesting a role for phospholipase C and potentially for phospholipase D. Addition of dioctanoylglycerol to plasma membranes isolated from GCs increased PKC activity. Neither pertussis toxin nor choleratoxin had an inhibitory effect on PKC activity in control or 1,25(OH)2D3-treated GCs, indicating that neither Gi nor Gs proteins were involved. Phospholipase A2 inhibitors, quinacrine, OEPC (selective for secretory phospholipase A2), and AACOCF3 (selective for cytosolic phospholipase A2), and the cyclooxygenase inhibitor indomethacin decreased PKC activity, while the phospholipase A2 activators melittin and mastoparan increased PKC activity in GC cultures. Arachidonic acid and prostaglandin E2, two downstream products of phospholipase A2 action, also increased PKC activity. These results indicate that 1,25(OH)2D3-dependent stimulation of PKC activity is regulated by two distinct phospholipase-dependent mechanisms: production of DAG, primarily via phospholipase C and production of arachidonic acid via phospholipase A2.

Opioid receptor and peptide mRNA expression in proliferative zones of fetal rat central nervous system

There is increasing evidence to suggest that opioid peptides may have widespread effects as regulators of growth. To evaluate the hypothesis that endogenous opioids control cellular proliferation during neural development, we have used in situ hybridization to examine opioid peptide and receptor mRNA expression in neuroepithelial zones of fetal rat brain and spinal cord. Our data show that proenkephalin mRNA is widely expressed in forebrain germinal zones and choroid plexus during the second half of gestation. In contrast, prodynorphin mRNA expression is restricted to the periventricular region of the ventral spinal cord. Little µ or delta receptor mRNA expression was detected in any regions of neuronal proliferation prior to birth. However, kappa receptor mRNA is widely expressed in hindbrain germinal zones during the 3rd week of gestation. Our present findings support the hypothesis that endogenous opioids may regulate proliferation of both neuronal and non-neuronal cells during central nervous system development. Given the segregated expression of proenkephalin mRNA in forebrain neuroepithelium and kappa receptor mRNA within hindbrain, different opioid mechanisms may regulate cell division in rostral and caudal brain regions.Key words: enkephalin, dynorphin, ontogeny, neurogenesis.

Opioid modulation of extracellular signal-regulated protein kinase activity is ras-dependent and involves Gbetagamma subunits

Although it is well-established that G protein-coupled receptor signaling systems can network with those of tyrosine kinase receptors by several mechanisms, the point(s) of convergence of the two pathways remains largely undelineated, particularly for opioids. Here we demonstrate that opioid agonists modulate the activity of the extracellular signal-regulated protein kinase (ERK) in African green monkey kidney COS-7 cells transiently cotransfected with mu-, delta-, or kappa-opioid receptors and ERK1- or ERK2-containing plasmids. Recombinant proteins in transfected cells were characterized by binding assay or immunoblotting. On treatment with corresponding mu- ([D-Ala2,Me-Phe4,Gly-ol5]enkephalin)-, delta- ([D-Pen2,D-Pen5]enkephalin)-, or kappa- (U69593)-selective opioid agonists, a dose-dependent, rapid stimulation of ERK1 and ERK2 activity was observed. This activation was inhibited by specific antagonists, suggesting the involvement of opioid receptors. Pretreatment of cells with pertussis toxin abolished ERK1 and ERK2 activation by agonists. Cotransfection of cells with dominant negative mutant N17-Ras or with a betagamma scavenger, CD8- beta-adrenergic receptor kinase-C, suppressed opioid stimulation of ERK1 and ERK2. When epidermal growth factor was used to activate ERK1, chronic (>2-h) opioid agonist treatment resulted in attenuation of the stimulation by the growth factor. This inhibition was blocked by the corresponding antagonists and CD8- beta-adrenergic receptor kinase-C cotransfection. These results suggest a mechanism involving Ras and betagamma subunits of Gi/o proteins in opioid agonist activation of ERK1 and ERK2, as well as opioid modulation of epidermal growth factor-induced ERK activity.

The synergistic effects of vitamin D metabolites and transforming growth factor-beta on costochondral chondrocytes are mediated by increases in protein kinase C activity involving two separate pathways

Transforming growth factor-beta (TGFbeta), as well as the vitamin D3 metabolites 1,25-dihydroxyvitamin D3 (1,25) and 24,25-dihydroxyvitamin D3 (24,25), regulate chondrocyte differentiation and maturation during endochondral bone formation. Both the growth factor and secosteroids also affect protein kinase C (PKC) activity, although each has its own unique time course of enzyme activation. Vitamin D3 metabolite effects are detected soon after addition to the media, whereas TGFbeta effects occur over a longer term. The present study examines the interrelation between the effects of 1,25, 24,25, and TGFbeta on chondrocyte differentiation, matrix production, and proliferation. We also examined whether the effect is hormone-specific and maturation-dependent and whether the effect of combining hormone and growth factor is mediated by PKC. This study used a chondrocyte culture model developed in our laboratory that allows comparison of chondrocytes at two stages of differentiation: the more mature growth zone (GC) cells and the less mature resting zone chondrocyte (RC) cells. Only the addition of 24,25 with TGFbeta showed synergistic effects on RC alkaline phosphatase-specific activity (ALPase). No similar effect was found when 24,25 plus TGFbeta was added to GC cells or when 1,25 plus TGFbeta were added to GC or RC cells. The addition of 1,25 plus TGFbeta and 24,25 plus TGFbeta to GC and RC cells, respectively, produced a synergistic increase in [35S]sulfate incorporation and had an additive effect on [3H]thymidine incorporation. To examine the signal transduction pathway involved in producing the synergistic effect of 24,25 and TGFbeta on RC cells, the level of PKC activity was examined. Addition of 24,25 and TGFbeta for 12 h produced a synergistic increase in PKC activity. Moreover, a similar effect was found when 24,25 was added for only the last 90 min of a 12-h incubation. However, a synergistic effect could not be found when 24,25 was added for the last 9 min or the first 90 min of incubation. To further understand how 24,25 and TGFbeta may mediate the observed synergistic increase in PKC activity, the pathways potentially leading to activation of PKC were examined. It was found that 24,25 affects PKC activity through production of diacylglycerol, not through activation of G protein, whereas TGFbeta only affected PKC activity through G protein. The results of the present study indicate that vitamin D metabolites and TGFbeta produced a synergistic effect that is maturation-dependent and hormone-specific. Moreover, the synergistic effect between 24,25 and TGFbeta was mediated by activation of PKC through two parallel pathways: 24,25 through diacylglycerol production and TGFbeta through G protein activation.

Activation of the human histamine H2 receptor is linked to cell proliferation and c-fos gene transcription

We examined whether histamine could regulate cell proliferation and expression of the early response gene c-fos in HEK-293 cells stably transfected with the human H2 receptor (HEK-H2). Histamine stimulated [3H]thymidine incorporation [50% effective concentration (EC50) = 3.6 x 10(-6) M] in HEK-H2 cells in a cimetidine-sensitive manner and increased c-fos mRNA in a time-dependent fashion, reaching maximal induction after 30 min. Histamine induced luciferase activity in HEK-H2 cells transiently transfected with a construct containing the luciferase reporter gene (Luc) coupled to the serum response element (SRE) of the c-fos gene promoter (EC50 = 1.5 x 10(-6) M) or a plasmid containing the SRE core fragment (bases -320 to -298). The protein kinase C (PKC) inhibitor staurosporine and long-term pretreatment of HEK cells with phorbol ester inhibited the effect of histamine on PKC activation, SRE-Luc activity, and [3H]thymidine incorporation. We have demonstrated that activation of the human H2 receptor can lead to induction of c-fos gene transcription and cell proliferation through a PKC-dependent mechanism.

Substance P modulates sensory action potentials in the lamprey via a protein kinase C-mediated reduction of a 4-aminopyridine-sensitive potassium conductance

We have examined the effects of the tachykinin substance P on the action potential of lamprey mechanosensory dorsal cells. Substance P increased the spike duration and reduced the afterhyperpolarization. These effects were mimicked by stimulation of the dorsal root, which contains tachykinin-like immunoreactive fibres. The tachykinin antagonist spantide II blocked the effects of both substance P and dorsal root stimulation. The spike broadening was voltage-dependent, and was due to the reduction of a 4-aminopyridine-sensitive potassium conductance. The spike broadening was mimicked by G-protein activators and blocked by the G-protein inhibitor GDPbetaS. Pertussis toxin did not block the effects of substance P. The spike broadening was blocked by the protein kinase C and cAMP-dependent protein kinase inhibitor H7, and by the specific protein kinase C antagonist chelerythrine, but not by the cAMP and cGMP-dependent protein kinase inhibitor H8. The phorbol ester phorbol 12,13-dibutyrate mimicked and blocked the effects of substance P, supporting the role of protein kinase C in the spike modulation. The adenylate cyclase activator forskolin and the cAMP agonist SpcAMPs mimicked but did not block the effects of substance P on the spike duration, suggesting that protein kinase A also modulates the dorsal cell action potential, but that substance P acts independently of this pathway. Substance P also increased the excitability of the dorsal cells. This effect was blocked by 4-AP, PDBu and chelerythrine, but not by H8, suggesting that the increase in excitability shares the same intracellular and effector pathways as the spike broadening.

A factor with a zinc- and phorbol ester-binding domain is necessary for endosome fusion

An inhibitory effect of several zinc chelators on endosome fusion reconstituted in an in vitro system has been recently reported (A. Aballay et al., 1995, Biochem. J. 312, 919-923). The factor that requires zinc for its activity is still unknown. Since the regulatory domain of protein kinase C (PKC) contains cysteine-rich motifs which coordinate zinc, we suspected that PKC or a PKC-like protein might be that factor. To test this hypothesis, we studied the effect of calphostin C, a specific inhibitor of PKC that interacts with the cysteine-rich motif, and PMA (phorbol 12-myristate 13-acetate), an activator of several PKC isoforms that bind to the same region, on endosome fusion. Calphostin C inhibited endosome fusion in a zinc-regulated manner, whereas PMA enhanced endosome fusion. Moreover, fusion was strongly stimulated when both PMA and zinc were added together to zinc-depleted fusion reactions. Inhibitors of the catalytic domain of PKC had no effect on the assay suggesting that the kinase activity is not required. In contrast, a glutathione S-transferase fusion protein containing a cysteine-rich region of the regulatory domain of PKCgamma inhibited endosome fusion in a PMA-dependent manner. Western blot analysis demonstrated the presence of proteins containing PKC-like cysteine-rich regions that are released from endosomal fractions by zinc chelators. These results indicate that the previously proposed zinc-dependent factor required for endosome fusion could be either a PKC isoform or a protein containing the phorbol ester-binding domain of PKC.

Attenuation of precipitated morphine withdrawal symptoms by acute i.c.v. administration of a group II mGluR agonist

1. We previously showed that chronic i.c.v. antagonism of metabotropic glutamate receptors (mGluRs) concurrently with s.c. morphine significantly attenuated precipitated withdrawal symptoms. Conversely, acute i.c.v. injection of a selective group II mGluR antagonist just before the precipitation of withdrawal exacerbated abstinence symptoms. 2. In the present study, we showed that acute i.c.v. administration of the non-selective mGluR agonist 1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD), as well as the group II selective agonist (2S,1'R,2'R,3'R)-2-(2'.3'-dicarboxycyclopropyl)glycine (DCG-IV), significantly attenuated the severity of precipitated withdrawal symptoms. 3. From these results we hypothesize that chronic opioid treatment may indirectly induce a desensitization of group II mGluRs, which contributes to the development of dependence.

G proteins and opioid receptor-mediated signalling

Most opioid receptor-mediated functions appear to be mediated through G protein interactions, therefore an understanding of opioid signalling requires knowledge of those interactions. This review chronicles the studies examining these interactions for all the opioid receptor subtypes, both in vivo and in vitro.

Study of protein kinase C antagonists on cortical granule exocytosis and cell-cycle resumption in fertilized mouse eggs

Although pharmacological agonists of protein kinase C (PKC) stimulate some events of mammalian egg activation, including cortical granule (CG) exocytosis, it is not known if these events are dependent on PKC activation during the normal process of fertilization. In order to examine the potential role of PKC in CG exocytosis, this study investigated whether PKC agonists faithfully mimic CG release and whether PKC antagonists block fertilization-induced CG release in mature mouse eggs. Phorbol ester (TPA, 2.5 ng/ml) treatment resulted in an atypical pattern of CG release in which there was a greater net loss of CGs in the equatorial region of the egg than in the region opposite the spindle. This pattern also was in contrast to that during fertilization, in which CG release occurred randomly throughout the cortex. Fertilization experiments utilized two different PKC inhibitors, bisindolylmaleimide (5 microM) and chelerytherine (0.8 microM), targeted to both the "conserved" substrate and ATP binding domains of PKC. Simultaneous use of both inhibitors at maximal concentrations (compatible with fertilization and above their IC50S) resulted in no detectable inhibition of CG release in treated fertilized eggs compared to controls. In addition no inhibition of anaphase onset was observed in treated fertilized eggs. Activity of the inhibitors was verified by demonstrating that they blocked the induction of CG loss by TPA. Moreover, 1 microM staurosporine, a potent but less specific antagonist of PKC, also did not block CG loss whereas the metaphase-anaphase transition was temporarily inhibited. The results indicate that TPA does not faithfully mimic CG release in fertilized eggs, that a role for PKC in CG release at fertilization remains to be established, and that other calcium-dependent effectors may be involved in CG exocytosis.

24,25-(OH)2D3 regulates protein kinase C through two distinct phospholipid-dependent mechanisms

We have previously shown that 24,25-(OH)2D3 plays a major role in resting zone (RC) chondrocyte differentiation and that this vitamin D metabolite regulates protein kinase C (PKC). The aim of the present study was to identify the signal transduction pathway used by 24,25-(OH)2D3 to stimulate PKC activation. Confluent, fourth passage RC cells from rat costochondral cartilage were used to evaluate the mechanism of PKC activation. Treatment of RC cultures with 24,25-(OH)2D3 for 90 min produced a dose-dependent increase in diacylglycerol (DAG). Addition of R59022, a diacylglycerol kinase inhibitor, significantly increased PKC activity in cultures treated with 24,25-(OH)2D3. Addition of dioctanoylglycerol (DOG) to plasma membranes isolated from RC increased PKC activity 447-fold. Addition of pertussis toxin or cholera toxin to control cultures elevated basal PKC activity. When added together with 10(-9) M 24,25-(OH)2D3, there was an additive effect on PKC activity but in cultures treated with 10(-8) M 24,25-(OH)2D3, only the hormone-dependent stimulation of PKC was observed. The phospholipase C inhibitor, U73-122, had no effect on PKC activity, indicating that the DAG produced in response to 24,25-(OH)2D3 is not derived from phosphatidylinositol. Addition of the tyrosine kinase inhibitor, genistein, also had no effect on 24,25-(OH)2D3-stimulated PKC, further supporting the hypothesis that phospholipase C is not involved in the mechanism and that phospholipase D is responsible for the increase in DAG production. Phospholipase A2 inhibitors, quinacrine and AACOCF3, and the cyclooxygenase inhibitor indomethacin increased PKC activity in the RC cultures. Exogenous PGE2, one of the downstream products of phospholipase A2 action, inhibited PKC activity. These results suggest that 24,25-(OH)2D3 regulates PKC activity by two distinct phospholipid-dependent mechanisms: production of DAG via phospholipase D and inhibition of the production of PGE2 via inhibition of phospholipase A2 and cyclooxygenase.

2,3-Butanedione monoxime modifies the glycine-gated chloride current of acutely isolated murine hypothalamic neurons

In this study, we explored the effect of the chemical phosphatase 2,3-butanedione monoxime (BDM) on glycine current (IGly) of murine ventromedial hypothalamic neurons. Co-application of 0.01 to 67 mM BDM increased IGly decay rate with little change of the peak amplitude. This effect was both rapid in onset and offset and required the presence of the agonist. Pretreatment with BDM alone did not alter-IGly decay. In addition, dialysis of neurons with 500 microM ATP-gamma-S did not alter the acute effect of BDM. Thus, this effect may result from open channel block rather than BDM-induced dephosphorylation of the receptor/channel protein. In contrast to the acute effect described above, relatively prolonged (i.e., greater than 80 s) pretreatment with BDM reduced peak IGly. The phorbol ester (PDBu), a protein kinase C (PKC) activator, mimicked this effect of BDM. Furthermore, chelerythrine, a specific PKC inhibitor, prevented this effect of BDM on peak IGly. Thus, activation of PKC may mediate this attenuating effect of BDM on IGly. For a sub-population of these pretreated neurons, there was a subsequent potentiation of IGly which followed the initial suppressant effect. This potentiation may be due to a phosphatase effect of BDM, since it was observed more frequently when neurons were also pretreated with the protein kinase inhibitors H7 or chelerythrine. These findings suggest that BDM alters protein kinase activity and acts as a phosphatase to regulate the activity of the glycine receptor/channel complex.

Inhibition of the expression of ornithine decarboxylase by some kappa-opioidergic receptor ligands in difluoromethylornithine-resistant L1210 cells

In difluoromethylornithine resistant L1210 cells stimulated to growth from quiescence, the selective kappa-opioidergic agonist trans-(+/-)-3,4-dichloro-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneaceta mid e (U-50488H) caused a dose dependent inhibition of the induction of ODC activity, with a half-maximal effect at about 1 microM. U-50488H also provoked reduction of ODC mRNA level and increase of ODC turnover, as well as inhibition of cell growth. U-69593, another kappa-selective agonist, was only slightly effective. The action of U-50488H on ODC induction was not blocked by naloxone, beta-chlornaltrexamine or by the kappa-selective opioid antagonists Mr1452 and nor-binaltorphimine (nBNI). Actually Mr1452 and nBNI exerted some inhibitory effect. Furthermore, the separated enantiomers (+) and (-) of U-50488H were similarly effective. The (-)cis-(1S,2R)-U50488 stereoisomer, exhibiting low affinity for kappa and high affinity for sigma receptors and carbetapentane, another sigma ligand, also inhibited ODC induction, although less effectively than U-50488H. None of several other opioid ligands tested had significant effects on ODC induction. In conclusion, the inhibition of ODC expression by U-50488H does not involve classical, enantiospecific opioid receptors; rather, these results suggest the involvement of a distinct site of action linked to inhibition of lymphoid cell proliferation.

mu-Opioid receptor-induced Ca2+ mobilization and astroglial development: morphine inhibits DNA synthesis and stimulates cellular hypertrophy through a Ca(2+)-dependent mechanism

Morphine, a preferential mu-opioid receptor agonist, alters astroglial development by inhibiting cell proliferation and by promoting cellular differentiation. Although morphine affects cellular differentiation through a Ca(2+)-dependent mechanism, few studies have examined whether Ca2+ mediates the effect of opioids on cell proliferation, or whether a particular Ca2+ signal transduction pathway mediates opioid actions. Moreover, it is uncertain whether one or more opioid receptor types mediates the developmental effects of opioids. To address these questions, the present study examined the role of mu-opioid receptors and Ca2+ mobilization in morphine-induced astrocyte development. Morphine (1 microM) and non-morphine exposed cultures enriched in murine astrocytes were incubated in Ca(2+)-free media supplemented with < 0.005, 0.3, 1.0, or 3.0 mM Ca2+ ([Ca2+]o), or in unmodified media containing Ca2+ ionophore (A23187), nifedipine (1 microM), dantrolene (10 microM), thapsigargin (100 nM), or L-glutamate (100 microM) for 0-72 h. mu-Opioid receptor expression was examined immunocytochemically using specific (MOR1) antibodies. Intracellular Ca2+ ([Ca2+]i) was measured by microfluorometric analysis using fura-2. Astrocyte morphology and bromodeoxyuridine (BrdU) incorporation (DNA synthesis) were assessed in glial fibrillary acidic protein (GFAP) immunoreactive astrocytes. The results showed that morphine inhibited astroglial growth by activating mu-opioid receptors. Astrocytes expressed MOR1 immunoreactivity and morphine's actions were mimicked by the selective mu agonist PL017. In addition, morphine inhibited DNA synthesis by mobilizing [Ca2+]i in developing astroglia. At normal [Ca2+]o, morphine attenuated DNA synthesis by increasing [Ca2+]i; low [Ca2+]o (0.3 mM) blocked this effect, while treatment with Ca2+ ionophore or glutamate mimicked morphine's actions. At extremely low [Ca2+]o (< 0.005 mM), morphine paradoxically increased BrdU incorporation. Although opioids can increase [Ca2+]i in astrocytes through several pathways, not all affect DNA synthesis or cellular morphology. Nifedipine (which blocks L-type Ca2+ channels) did not prevent morphine-induced reductions in BrdU incorporation or cellular differentiation, while thapsigargin (which depletes IP3-sensitive Ca2+ stores) severely affected inhibited DNA synthesis and cellular differentiation-irrespective of morphine treatment. However, dantrolene (an inhibitor of Ca(2+)-dependent Ca2+ release) selectively blocked the effects of morphine. Collectively, the findings suggest that opioids suppress astroglial DNA synthesis and promote cellular hypertrophy by inhibiting Ca(2+)-dependent Ca2+ release from dantrolene-sensitive intracellular stores. This implies a fundamental mechanism by which opioids affect central nervous system maturation.

Chronic inhibition of intracellular Ca2+ release or protein kinase C activation significantly reduces the development of morphine dependence

We have previously shown that chronic antagonism of metabotropic glutamate receptors in the brain attenuates naloxone-precipitated withdrawal symptoms in rats treated chronically with subcutaneous (s.c.) morphine. Several subtypes of metabotropic glutamate receptors are directly linked, through a guanine nucleotide regulatory protein, to the phosphatidylinositol (p.i.) second messenger system. In the present investigation, we assessed the effect of inhibiting the products of p.i. hydrolysis on the development of opioid dependence. Thus, concurrently with subcutaneous morphine, we infused intracerebroventricularly (i.c.v.) in rats, various doses of chelerythrine, which selectively inhibits the activation of protein kinase C, and thapsigargin, which inhibits the release of intracellular Ca2+ when given chronically. Both chelerythrine and thapsigargin reduced the severity of naloxone-precipitated abstinence symptoms when infused i.c.v. at a dose of 10 nmol/day. A single injection of either chelerythrine or thapsigargin immediately prior to the precipitation of withdrawal failed to decrease the severity of abstinence symptoms. Our results suggest that by chronically inhibiting activity of the phosphatidylinositol system, the development of morphine dependence can be attenuated.

Regulation of Plant Defense Response to Fungal Pathogens: Two Types of Protein Kinases in the Reversible Phosphorylation of the Host Plasma Membrane H+-ATPase

The role of reversible phosphorylation of the host plasma membrane H+-ATPase in signal transduction during the incompatible interaction between tomato cells and the fungal pathogen Cladosporium fulvum was investigated. Tomato cells (with the Cf-5 resistance gene) or isolated plasma membranes from Cf-5 cells treated with elicitor preparations from race 2.3 or 4 of C. fulvum (containing the avr5 gene product) showed a marked dephosphorylation of plasma membrane H+-ATPase. Similar treatment with elicitor preparations from races 5 and 2.4.5.9.11 (lacking the avr5 gene product) showed no change in dephosphorylation. Elicitor (race 4) treatment of cells, but not of isolated plasma membranes, for 2 hr resulted in rephosphorylation of the ATPase via Ca2+-dependent protein kinases. The initial (first hour) rephosphorylation was enhanced by protein kinase C (PKC) activators and was prevented by PKC inhibitors. Activity of a second kinase appeared after 1 hr and was responsible for the continuing phosphorylation of the H+-ATPase. This latter Ca2+-dependent kinase was inhibited by a calmodulin (CaM) antagonist and by an inhibitor of Ca2+/CaM-dependent protein kinase II. The activation of the Ca2+/CaM-dependent protein kinase depended on the prior activation of the PKC-like kinase.

Effects of benzophenanthridine alkaloids on the phosphorylation of an approximately 44 kDa protein present in a mitochondrial fraction of the rat heart

Chelerythrine and sanguinarine, benzophenanthridine alkaloids that are known to have a wide variety of biologic actions including inhibitory activity against the phosphorylation of proteins, were tested for their effects on the phosphorylation of a specific approximately 44 kDa protein present in the mitochondrial fraction of the rat heart. The concentrations required for 50% inhibition were determined to be 90.3 and approximately 200 microM for chelerythrine and sanguinarine, respectively, while the median-effect concentrations were 71 and 98 microM for chelerythrine and 186 microM for sanguinarine. The combination index values, determined from median-effect plots, for the combination of chelerythrine and taurine in a ratio of 1:100 were greater than 1, which indicates that chelerythrine plus taurine is antagonistic. Both chelerythrine and sanguinarine had biphasic (i.e. stimulation and inhibition) effects on the phosphorylation of the approximately 44 kDa protein. It was determined that the biphasic effect for checlerythrine depended upon the time of preincubation at 37 degrees of chelerythrine with the mitochondrial preparation. Preincubation times of 0.5 and 1 min produced 70 and 82% stimulation, while longer preincubation times of 2-22 min resulted in inhibition of the phosphorylation reaction by 40-95%. Dithiothreitol (DTT), a reducing agent, prevented the inhibitory effect of chelerythrine. Glutathione was less effective in protecting the phosphorylation of the approximately 44 kDa protein. It is suggested that the minimum bond of chelerythrine reacts with the thiol group on DTT, thereby preventing chelerythrine from reacting with thiol groups on the kinase responsible for phosphorylating the approximately 44 kDa protein. The inhibitory effects of taurine were only partially eliminated by DTT.

Inositol phosphate formation in mice prenatally exposed to drugs: relation to muscarinic receptors and postreceptor effects

Mice were exposed to phenobarbital or heroin [diacetylmorphine (DAM)] prenatally by feeding the mother phenobarbital on gestation day 9-18; DAM was injected into the mother on gestation days 9-18. At the age of 50 days, mice exposed to phenobarbital or DAM prenatally were examined for long-term biochemical changes in the postsynaptic septohippocampal system as measured by alterations in formation of the second messenger inositol phosphate (i.p.). A significant increase in i.p. formation in response to carbachol was found after prenatal exposure to DAM. An increase in i.p. formation in response to 20 mM KCl alone or in the additional presence of 10 mM carbachol or 1mM physostigmine was found after prenatal exposure to phenobarbital or DAM. In addition, a significant increase in IP formation in response to sodium fluoride was found after prenatal exposure to phenobarbital or DAM. It is suggested that an increase in G-protein activation and in the second messenger formation accompanies the early drug-induced upregulation of the muscarinic receptors found in our previous studies.

Primary astroglial cultures derived from several rat brain regions differentially express mu, delta and kappa opioid receptor mRNA

The existence of opioid receptors within glial cell membranes has been proposed by several laboratories based on biochemical and radioligand binding data. The recent cloning of the mu, delta and kappa receptors has enabled us to directly examine the issue of opioid receptor expression in rat brain astroglia by using solution hybridization/ribonuclease protection assays to analyze the total RNA obtained from primary cultures of cortical, striatal, cerebellar, hippocampal and hypothalamic astrocytes. The results indicate that all five glial cultures expressed mu, delta and kappa receptor mRNA. The rank order of receptor mRNA abundance, expressed collectively across all five cultures, was determined to be delta > or = kappa >> mu. An analysis of the glial distribution profile for each receptor type revealed that mu receptor mRNA levels were the most abundantly expressed in cortical cultures, while the greatest levels of delta receptor mRNA were found in the cortical and hypothalamic cultures, and significant kappa receptor mRNA levels were produced by the cortical, hypothalamic and cerebellar cultures. Furthermore, the five glial cultures each expressed different levels of total opioid receptor (mu + delta + kappa) mRNA. The rank order of total opioid receptor mRNA expression across different astroglial cultures was found to be cortex > hypothalamus > cerebellum = hippocampus > striatum. An analysis of the relative expression profiles for mu, delta and kappa receptor mRNA within each culture revealed that all cultures manifested relatively high levels of delta and kappa receptor mRNA, but relatively low levels of mu receptor mRNA. Generally, cortical, hippocampal and hypothalamic cultures were characterized by comparable levels of delta and kappa receptor mRNA, and little, if any, mu receptor mRNA. However, striatal cultures were characterized by a high level of delta receptor mRNA which was approximately twice and four times that of the kappa and mu receptor mRNA, respectively. In contrast, cerebellar cultures expressed predominantly kappa receptor mRNA at a level which was almost twice that of the delta receptor mRNA, and expressed very little mu receptor mRNA. These data show that primary astroglial cultures not only express mu, delta and kappa receptor mRNAs, but they do so in a manner dependent upon receptor type and brain region. This suggests a regional heterogeneity of astrocytes with respect to opioid receptor expression, a characteristic previously described only for neurons. Furthermore, it suggests the existence of an additional anatomical component in CNS opioid systems.

Dynorphins modulate DNA synthesis in fetal brain cell aggregates

Previously, opioid peptide analogues, beta-endorphin, and synthetic opiates were found to inhibit DNA synthesis in 7-day fetal rat brain cell aggregates via kappa- and mu-opioid receptors. Here dynorphins and other endogenous opioid peptides were investigated for their effect on DNA synthesis in rat and guinea pig brain cell aggregates. At 1 microM, all dynorphins tested and beta-endorphin inhibited [3H]thymidine incorporation into DNA by 20-38% in 7-day rat brain cell aggregates. The putative epsilon-antagonist beta-endorphin (1-27) did not prevent the effect of beta-endorphin, suggesting that the epsilon-receptor is not involved in opioid inhibition of DNA synthesis. The kappa-selective antagonist norbinaltorphimine blocked dynorphin A or B inhibition of DNA synthesis, implicating a kappa-opioid receptor. In dose-dependency studies, dynorphin B was three orders of magnitude more potent than dynorphin A in the attenuation of thymidine incorporation, indicative of the mediation of its action by a discrete kappa-receptor subtype. The IC50 value of 0.1 nM estimated for dynorphin B is in the physiological range for dynorphins in developing brain. In guinea pig brain cell aggregates, the kappa-receptor agonists U50488, U69593, and dynorphin B reduced thymidine incorporation by 40%. When 21-day aggregates were treated with dynorphins, a 33-86% enhancement of thymidine incorporation was observed. Because both 7- and 21-day aggregates correspond to stages in development when glial cell proliferation is prevalent and glia preferentially express kappa-receptors in rat brain, these findings support the hypothesis that dynorphins modulate glial DNA synthesis during brain ontogeny.

Occurrence of the opiate alkaloid-selective mu3 receptor in mammalian microglia, astrocytes and Kupffer cells

Evidence is presented for occurrence of opiate alkaloid-selective, opioid-peptide-insensitive receptor binding sites, labeled with [3H]morphine, in primary cultures of cat microglia and cat astrocytes, as well as on highly purified preparations of rat Kupffer cells. These receptors have been designated mu3 on the basis of their close similarity to receptors first found to be present on human peripheral blood monocytes. Exposure of the microglia to morphine and etorphine caused marked quantifiable changes in cellular morphology, including assumption of a more rounded shape and retraction of cytoplasmic processes; in contrast, several opioid peptides were without effect on morphology. The effects of morphine on microglial morphology were blocked by the opiate antagonist naloxone. These effects of drugs on morphology were as predicted for action via the mu3 receptor. Opiate alkaloid binding sites previously detected on the rat C6 glioma cell line were also characterized here as of the mu3 receptor subtype. It is proposed that mu3 receptors have broad distribution in different macrophage cell types of bone marrow lineage, including microglia and Kupffer cells. Furthermore, these receptors are not restricted to cells of bone marrow lineage, since they are also present on astrocytes.

Paradoxical stimulatory effect of the kinase inhibitor chelerythrine on the phosphorylation of a approximately 20 K M(r) protein present in the mitochondrial fraction of rat retina

In order to characterize the phosphorylation of a approximately 20k M(r) protein present in the mitochondrial fraction of the rat retina, chelerythrine chloride, a well-known protein kinase C inhibitor, was tested for activity. Instead of the expected inhibition of the kinase reaction by chelerythrine the phosphorylation of the approximately 20k M(r) protein was stimulated by a factor of 3 at 150 microM. This unique stimulatory action of chelerythrine could be eliminated by the addition of 10 mM dithiothreitol. A suggested mechanism of action for dithiothreitol in the elimination of the increased phosphorylation of the approximately 20k M(r) protein by chelerythrine is the addition of the thiol group of dithiothreitol to the iminium bond of chelerythrine. Taurine, a known inhibitor of the phosphorylation of retinal proteins was also tested in combination with chelerythrine for its effects on the phosphorylation of the approximately 20k M(r) protein. A non-competitive relationship was observed when chelerytrhine was used as the variable activator and taurine as the fixed inhibitor (30 mM). The stimulatory effect of chelerythrine on the phosphorylation of proteins was not limited to retinal tissue but was also observed in the P2 fraction of brain cortex. Chelerythrine demonstrated only inhibitory effects on the phosphorylation of proteins in a heart mitochondrial fraction.

LiCl uncouples signal transduction in morphine-induced supraspinal antinociception in mice

1. The present study examined whether LiCl antagonism of morphine-induced antinociception in mice occurs at mu-opioid receptors. 2. The antinociceptive ED50 value of intracerebroventricular morphine was maximally increased compared to controls 18 hr after LiCl (10 mmol/kg, s.c.) and remained significantly less (P < 0.05) 7 and 14 days after once-daily LiCl treatment. 3. There was no significant difference in [3H]-[D-Ala2,N-MePhe4,Gly- ol5]enkephalin affinity or receptor density compared to controls (KD = 0.43 nM, Bmax = 54.8 +/- 9.3 pM). 4. These results suggest that LiCl's effect is not on mu-opioid receptors, but rather on some distal site.

Opioids inhibit endothelin-mediated DNA synthesis, phosphoinositide turnover, and Ca2+ mobilization in rat C6 glioma cells

Opioid agonists inhibit DNA synthesis in C6 rat glioma cells that express opioid receptors, induced by desipramine (DMI). This inhibition was not observed in cells that were not treated with DMI, and thus did not express opioid-binding sites. Endothelin, a known mitogen, increased thymidine incorporation dose dependently (up to 1.7-fold) in DMI-treated C6 cells. This increase was reversed by an anti-idiotypic antibody to opioid receptors, Ab2AOR, which has opioid agonist properties. The opioid antagonist naltrexone blocked the inhibition caused by Ab2AOR. Endothelin also stimulated phosphoinositide (PI) turnover and this effect was inhibited by morphine (50%) or by Ab2AOR (72%) in DMI-treated but not in DMI-untreated C6 cells. These actions of morphine and Ab2AOR were reversed by naltrexone. The inhibition of PI turnover and of thymidine incorporation by Ab2AOR or morphine was insensitive to pertussis toxin (PTX). Since PI turnover is known to induce Ca2+ mobilization, it was of interest to examine the effects of the applied opioids on intracellular Ca2+ concentrations. Endothelin increased the concentration of cytosolic free Ca2+ in the cells while Ab2AOR, morphine, and beta-endorphin reversed the endothelin-induced Ca2+ mobilization in DMI-treated but not in DMI-untreated C6 cells. The effect of these agonists was also blocked by naltrexone. The results indicate that glial cells can be a target of an opioid receptor-mediated antimitogenic action and that an abatement in PI turnover and Ca2+ mobilization may be associated with this mechanism.

Opioids inhibit endothelin-mediated DNA synthesis, phosphoinositide turnover, and Ca2+ mobilization in rat C6 glioma cells

Opioid agonists inhibit DNA synthesis in C6 rat glioma cells that express opioid receptors, induced by desipramine (DMI). This inhibition was not observed in cells that were not treated with DMI, and thus did not express opioid-binding sites. Endothelin, a known mitogen, increased thymidine incorporation dose dependently (up to 1.7-fold) in DMI-treated C6 cells. This increase was reversed by an anti-idiotypic antibody to opioid receptors, Ab2AOR, which has opioid agonist properties. The opioid antagonist naltrexone blocked the inhibition caused by Ab2AOR. Endothelin also stimulated phosphoinositide (PI) turnover and this effect was inhibited by morphine (50%) or by Ab2AOR (72%) in DMI-treated but not in DMI-untreated C6 cells. These actions of morphine and Ab2AOR were reversed by naltrexone. The inhibition of PI turnover and of thymidine incorporation by Ab2AOR or morphine was insensitive to pertussis toxin (PTX). Since PI turnover is known to induce Ca2+ mobilization, it was of interest to examine the effects of the applied opioids on intracellular Ca2+ concentrations. Endothelin increased the concentration of cytosolic free Ca2+ in the cells while Ab2AOR, morphine, and beta-endorphin reversed the endothelin-induced Ca2+ mobilization in DMI-treated but not in DMI-untreated C6 cells. The effect of these agonists was also blocked by naltrexone. The results indicate that glial cells can be a target of an opioid receptor-mediated antimitogenic action and that an abatement in PI turnover and Ca2+ mobilization may be associated with this mechanism.

mu-Opioid receptor stimulation of inositol (1,4,5)trisphosphate formation via a pertussis toxin-sensitive G protein

The cellular mechanisms underlying opioid action remain to be fully determined, although there is now growing indirect evidence that some opioid receptors may be coupled to phospholipase C. Using SH-SY5Y human neuroblastoma cells (expressing both mu- and delta-opioid receptors), we demonstrated that fentanyl, a mu-preferring opioid, caused a dose-dependent (EC50 = 16 nM) monophasic increase in inositol (1,4,5)trisphosphate mass formation that peaked at 15 s and returned to basal within 1-2 min. This response was of similar magnitude (25.4 +/- 0.8 pmol/mg of protein for 0.1 microM fentanyl) to that found in the plateau phase (5 min) following stimulation with 1 mM carbachol (18.3 +/- 1.4 pmol/mg of protein), and was naloxone-, but not naltrindole- (a delta antagonist), reversible. Further studies using [D-Ala2, MePhe4, Gly(ol)5]enkephalin and [D-Pen2,5]enkephalin confirmed that the response was specific for the mu receptor. Incubation with Ni2+ (2.5 mM) or in Ca(2+)-free buffer abolished the response, as did pretreatment (100 ng/ml for 24 h) with pertussis toxin (control plus 0.1 microM fentanyl, 26.9 +/- 1.5 pmol/mg of protein; pertussis-treated plus 0.1 microM fentanyl, 5.1 +/- 1.3 pmol/mg of protein). In summary, we have demonstrated a mu-opioid receptor-mediated activation of phospholipase C, via a pertussis toxin-sensitive G protein, that is Ca(2+)-dependent. This stimulatory effect of opioids on phospholipase C, and the potential inositol (1,4,5)trisphosphate-mediated rises in intracellular Ca2+, could play a part in the cellular mechanisms of opioid action.