Function of delta opioid receptors in cultured cells

Opioid-Induced Reductions in Amygdala Lateral Paracapsular GABA Neuron Circuit Activity

Opioid use and withdrawal evokes behavioral adaptations such as drug seeking and anxiety, though the underlying neurocircuitry changes are unknown. The basolateral amygdala (BLA) regulates these behaviors through principal neuron activation. Excitatory BLA pyramidal neuron activity is controlled by feedforward inhibition provided, in part, by lateral paracapsular (LPC) GABAergic inhibitory neurons, residing along the BLA/external capsule border. LPC neurons express µ-opioid receptors (MORs) and are potential targets of opioids in the etiology of opioid-use disorders and anxiety-like behaviors. Here, we investigated the effects of opioid exposure on LPC neuron activity using immunohistochemical and electrophysiological approaches. We show that LPC neurons, and other nearby BLA GABA and non-GABA neurons, express MORs and δ-opioid receptors. Additionally, DAMGO, a selective MOR agonist, reduced GABA but not glutamate-mediated spontaneous postsynaptic currents in LPC neurons. Furthermore, in LPC neurons, abstinence from repeated morphine-exposure in vivo (10 mg/kg/day, 5 days, 2 days off) decrease the intrinsic membrane excitability, with a ~75% increase in afterhyperpolarization and ~40-50% enhanced adenylyl cyclase-dependent activity in LPC neurons. These data show that MORs in the BLA are a highly sensitive targets for opioid-induced inhibition and that repeated opioid exposure results in impaired LPC neuron excitability.

Pain Perception and Management: Where do We Stand?

BACKGROUND

Pain is often flammable, sharp and sometimes described as an electrical shock. It can be categorized in three different ways as nociceptive, neuropathic and inflammatory. Nociceptive pain always originates in specific situations such as in trauma. Neuropathic pain results in nerve damage. In inflammatory pain, inflammatory mediators are involved in the sensitization of nociceptors. It is important to control the pain as it affects the individual physically, mentally, and socially.

OBJECTIVE

Recognizing pain physiopathology and pain pathways, defining the relationship between receptor and transmitter is critical in developing new treatment strategies. In this review, current information on the definitions, classifications, and physiological and chemical mechanisms involved in pain are reviewed.

METHODS

Various search engines were used to gather related articles/information. Only peer-reviewed journals were considered. Additional, books/chapters of standard publishers were also included in the article.

RESULTS

With a better understanding of the physiological and chemical mechanisms that play a role in pain, significant improvements have been made in pain treatment. Various oral or intravenous drugs, local injection treatments, physical and occupational therapy, electrical stimulation, alternative medicine applications, psychological support, and surgical applications are routinely performed in the treatment, dependent upon the type, severity and cause of the pain.

CONCLUSION

Improved understanding of pain physiopathology will serve as the basis for future improvements in the delivery of efficacious and reliable treatments, and is likely to rely on novel technological innovations.

Dietary fatty acids and membrane protein function

In recent years, there has been growing public awareness of the potential health benefits of dietary fatty acids, and of the distinction between the effects of the omega6 and omega3 polyunsaturated fatty acids that are concentrated in vegetable and fish oils, respectively. A part of the biologic effectiveness of the two families of polyunsaturated fatty acids resides in their relative roles as precursors of the eicosanoids. However, we are also beginning to appreciate that as the major components of the hydrophobic core of the membrane bilayer, they can interact with and directly influence the functioning of select integral membrane proteins. Among the most important of these are the enzymes, receptors, and ion channels that are situated in the plasma membrane of the cell, since they carry out the communication and homeostatic processes that are necessary for normal cell function. This review examines current information regarding the effects of diet-induced changes in plasma membrane fatty acid composition on several specific enzymes (adenylate cyclase, 5'-nucleotidase, Na(+)/K(+)-ATPase) and cell-surface receptors (opiate, adrenergic, insulin). Dietary manipulation studies have demonstrated a sensitivity of each to a fatty acid environment that is variably dependent on the nature of the fatty acid(s) and/or source of the membrane. The molecular mechanisms appear to involve fatty acid-dependent effects on protein conformation, on the "fluidity" and/or thickness of the membrane, or on protein synthesis. Together, the results of these studies reinforce the concept that dietary fats have the potential to regulate physiologic function and to further our understanding of how this occurs at a membrane level.

Co-localization of mu and delta opioid receptors on SK-N-SH cells detected by fluorescence microscopy using labeled anti-idiotypic antibodies

Selective fluorescence labeling of opioid receptor subclasses on SK-N-SH cultured cells has been accomplished using labeled polyclonal anti-idiotypic antibodies along with subclass-selective opioid agonists (DPDPE, delta-selective; DAMGO, mu-selective) as blocking reagents. Labeling of the cells was examined using conventional fluorescence microscopy. Co-localization of mu- and delta-opioid receptors on SK-N-SH cells has been studied by double labeling fluorescence experiments. In agreement with our own, and other workers', previous observations on NG108-15 cells, a subpopulation of viable cells in asynchronous cultures are labeled. Among those SK-N-SH cells that are labeled, both subclasses of receptors are seen. On the basis of sequential blocking experiments we interpret our combined results to be consistent with a model where mu- and delta- binding sites reside on different subunits of a multimeric complex.

Opioid receptor-coupled second messenger systems

Although pharmacological data provide strong evidence for different types of opioid receptors (e.g., mu, delta, and kappa), they share many common properties in their ability to couple to second messenger systems. All opioid receptor types are coupled to G-proteins, since agonist binding is diminished by guanine nucleotides and agonist-stimulated GTPase activity has been identified in several preparations. Moreover, all three types inhibit adenylyl cyclase. This second messenger system has been identified for opioid receptors in both isolated brain membranes and in transformed cell culture. Studies with chronic treatment with opioid agonists suggest that the coupling of receptors with G-proteins and second messenger effectors may play important roles in development of opioid tolerance.

Demonstration and characterization of zeta (zeta), a growth-related opioid receptor, in a neuroblastoma cell line

Endogenous opioids and opioid receptors (i.e. endogenous opioid systems) are involved in carcinogenesis. Using homogenates of S20Y neuroblastoma (NB) cells grown in culture, the binding of a growth-selective ligand, [Met5]enkephalin, was examined to ascertain the zeta (zeta) opioid receptor. Specific and saturable binding of [3H]-[Met5]enkephalin was detected in NB cells; the data were consistent with a single binding site. Scatchard analysis yielded a Kd of 1.6 nM and a binding capacity (Bmax) of 48.1 fmol/mg protein; 14,000 receptors per cell were estimated. Binding was dependent on protein concentration, time, temperature, and pH, and was sensitive to 100 nM, but not 5 nM, Na+, Ca2+, and Mg2+; GppNHp at concentrations of 100-500 mM had little effect on binding. Optimal binding required protease inhibitors, and pretreatment of the tumor cell homogenates with trypsin markedly reduced [3H]-[Met5]enkephalin binding, suggesting that the binding site was proteinaceous in character. Displacement experiments indicated that [Met5]enkephalin was the most potent displacer of [3H]-[Met5]enkephalin. Cell density (log, confluence, postconfluence) did not alter the Kd or Bmax. This study serves as the first demonstration and characterization of the zeta (zeta) opioid receptor in tissue culture cells. The homogeneous nature of NB cell cultures, along with the enrichment in receptor number, provides an excellent model system to isolate and purify the zeta receptor.

Altered post-receptorial signal transduction mechanism under various stimulation in polymorphonuclear granulocytes of Alzheimer's disease

The cyclic nucleotide changes were studied under 10(-6) M isoproterenol (IP), 10(-6) M carbachol and 10(-8) M Met-enkephalin (Met-enk) stimulations in polymorphonuclear granulocytes (PMNLs) of middle-aged (aged 35-52 years) and elderly (aged 61-97 years) healthy subjects, as well as of patients with Alzheimer's disease (AD) (aged 58-65 years). From our results we can conclude that in the case of middle-aged healthy subjects only the IP caused a marked cAMP elevation while in elderly and AD all the applied substances stimulated the cAMP at different degrees. Concerning the cGMP levels in PMNLs, we observed a marked increase under carbachol and Met-enk stimulation, in middle-aged subjects, while in the elderly a weak change was obtained by carbachol. In AD practically no change of cGMP levels could be obtained. Thus, the main features of AD are a cAarP response to Met-enk and an abolition of a GarP response to carbachol. We can conclude that in PMNLs of elderly and patients with AD we assist to an altered post-receptorial signal transduction mechanism, which seems to be even more marked in the case of AD comparing to normal aging.

Ethanol and opioid receptor signalling

Ethanol may modulate endogenous opioid systems by disrupting opioid receptor signalling. Low concentrations of ethanol slightly potentiate mu-opioid receptor binding by increasing receptor Bmax, and, in some cases, chronic ethanol exposure decreases the density or affinity of the mu-opioid receptors. By contrast, high concentrations of ethanol acutely decrease delta-opioid receptor binding by decreasing receptor affinity, whereas chronic exposure of animals and neuronal cell lines to lower concentrations of ethanol leads to possibly adaptive increases in the density or affinity of the delta-opioid receptors. In the neuronal cell line NG108-15, ethanol does not up-regulate the delta-opioid receptor by blocking receptor degradation or endocytosis, but protein synthesis is required for this response. Up-regulation of the delta-opioid receptor renders ethanol-treated NG108-15 cells 3.5-fold more sensitive to opioid inhibition of adenylyl cyclase. Long-term treatment with ethanol also increases maximal opioid inhibition in NG108-15 cells, possibly by decreasing levels of G alpha s and its mRNA. Ethanol differentially modulates signal transduction proteins in three additional neuronal cell lines, N18TG2, N4TG1, and N1E-115. Ethanol-treated N18TG2 cells show the least up-regulation of the delta-opioid receptor, little heterologous desensitization of adenylyl cyclase, and no changes in G alpha s or G alpha i. By contrast, ethanol-treated N1E-115 cells show the largest up-regulation of the delta-opioid receptor, the most heterologous desensitization of adenylyl cyclase, and concentration-dependent decreases in G alpha s and increases in G alpha i. Further analysis of these related neuronal cell lines may help to identify the molecular elements that endow some, but not all, neuronal cells with the capacity to adapt to ethanol.

Characterization of opioid binding sites in murine neuroblastoma

The binding of [3H] [D-Ala2, MePhe4, Gly-ol5]enkephalin ([3H]DAGO), [3H]D-Ala2,D-Leu5]enkephalin ([3H]DADLE) and (+/-)-[3H]ethylketocyclazocine ([3H]EKC) to neurotumor tissues derived from S20Y neuroblastoma cells transplanted into A/Jax mice was examined. Specific and saturable binding to [3H]DADLE and [3H]EKC was detected, and the data fit a single homogeneous binding site for each ligand. Scatchard analysis for [3H]DADLE and [3H]EKC yielded Kd values of 0.65 and 0.45 nM, respectively, and Bmax values of 9.2 and 116 fmol/mg protein. Binding was dependent on time, temperature, and pH, and was sensitive to Na+ and guanine nucleotides. Pretreatment of the tumor homogenates with trypsin markedly reduced binding to both ligands, suggesting that the binding sites were proteinaceous in character. Displacement experiments indicated that delta (delta) receptor related compounds (e.g. DPDPE, ICI 174,864) avidly displaced [3H]DADLE, whereas kappa (kappa) related compounds (e.g. U50,488, dynorphin) markedly competed with [3H]EKC. Mu (mu) receptor drugs (e.g. DAGO, beta-FNA, morphine) were not potent in displacing either [3H]DADLE or [3H]EKC. These results are the first to characterize opioid binding sites in tumor tissue. The function of these sites is unclear, but previous evidence as to the growth regulatory properties of endogenous opioid systems may suggest that either one, or both, binding sites may be involved in carcinogenic events.

Opiate-inhibited adenylate cyclase in rat brain membranes depleted of Gs-stimulated adenylate cyclase

Opiate agonists inhibit adenylate cyclase in brain membranes, but under normal conditions the maximal inhibition is small (10-15%). When rat brain membranes were preincubated at pH 4.5, washed, and then assayed for adenylate cyclase at pH 7.4, stimulation of activity by agents (fluoride, guanylyl-5'-imidodiphosphate, cholera toxin) that act through the stimulatory GTP-binding coupling protein (Gs) protein was lost. At the same time, inhibition of basal adenylate cyclase by opiate agonists was increased to a maximum of 30-40%. Opiate inhibition was maximal at low magnesium concentrations (less than 5 mM), required guanine nucleotides, and decreased the Vmax, not Km, of the enzyme. Incubation of membranes with pertussis toxin lowered the apparent affinity for agonists in inhibiting activity. The delta opioid agonists were more potent than mu agonists, and the Ke values for naloxone in blocking agonist inhibition were similar for both mu and delta agonists (50-90 nM). These results suggest that inhibition of adenylate cyclase in brain is not mediated by mu opiate receptors, but whether classic high-affinity delta and kappa receptors are involved with this enzyme cannot be confirmed by these experiments.

Activation of the lipoxygenase pathway in the methionine enkephalin induced respiratory burst in human polymorphonuclear leukocytes

In comparative studies of f-met-Leu-Phe (FMLP) and methionine enkephalin (ME) induced polymorphonuclear leukocyte (PMNL) stimulation the following results were obtained: (i) both FMLP and ME increased the intracellular killing (IK) capability of human PMNLs probably through NADPH oxidase activation, (ii) the ME-induced respiratory burst (RB) differed from the chemotactic peptide FMLP-triggered superoxide generation because the former was not accompanied by the activation of the glutathione system and the duration of the superoxide production was prolonged. The reaction was dependent on lipoxygenation, was potentiated by indomethacin (IM) and was inhibited by nordihidro-guairetic acid (NDGA), (iii) both 14C-arachidonic acid (14C-AA) release and leukotriene B4 (LTB4) synthesis of ME-treated PMNLs were elevated as compared to those of FMLP triggered cells. Our results suggest that lipoxygenation and even an increased LTB4 synthesis are involved in the ME-induced RB of leukocytes.

Effects of membrane polyunsaturated fatty acids on opiate peptide inhibition of basal and prostaglandin E1-stimulated cyclic AMP formation in intact N1E-115 neuroblastoma cells

The effects of membrane polyunsaturated fatty acids (PUFA) on opiate peptide-mediated inhibition of basal and prostaglandin E1-stimulated cyclic AMP formation were examined in intact N1E-115 neuroblastoma cells. Addition of opiate peptides such as methionine 5-enkephalin (metEnk) to control cultures and to cultures that had been supplemented for 48 hr with 50 microM linoleic acid resulted in dose-dependent decreases in cAMP formation; these decreases were blocked by naloxone. Maximum inhibition of basal cyclase activity was 50-55% in both control and PUFA-enriched cells; however, half-maximal inhibition required ten times more metEnk in supplemented cultures than in controls. This is consistent with our observation that the affinity of binding of [tyrosyl-3',5'-3H(N)](2-D-alanine-5-D-leucine)enkephalin ([3H]DADLE) to intact PUFA-enriched cells was lower than that to control cells. Receptor density was not modified as a result of supplementation. Addition of prostaglandin E1 (PGE1) to the cells produced rapid dose-dependent increases in cAMP formation. Maximum responses were higher in PUFA-enriched than in control cells (1924 and 972 pmol cAMP formed/mg protein respectively). Also, the apparent value for EC50 for PGE1 was consistently lower in supplemented cultures. MetEnk reduced PGE1-stimulated cAMP formation by 45-55% in both control and supplemented cells, and values for IC50 were similar (approximately 30 nM) in both. In the presence of the opiate peptide, values for EC50 for PGE1 were similar in control and PUFA-enriched cultures (0.07 and 0.09 microM respectively). The data from these studies suggest that membrane PUFA increase the efficiency of coupling of receptors that stimulate cAMP formation and decrease the efficiency of those that mediate inhibition.

Concentration-dependent effect of met-enkephalin on human polymorphonuclear leukocytes

Met-enkephalin in 10(-9)-10(-7)M concentrations exerts an ADCC-stimulating effect on human PMNLs through naloxone-sensitive opiate receptors, elevating the cytoplasmic free Ca2+ and cGMP levels. In higher, 10(-6)-10(-5)M concentrations ME has a cAMP-elevating effect and a rapid 45Ca2+ influx was observed. This latter effect of ME, which is also associated with the suppression of ADCC activity, was abolished by the enkephalinase inhibitor, puromycin. A strong relationship is suggested between the suppressing effect of metenkephalin, enkaphalinase and the protein kinase C system.

Impaired coupling of naloxone sensitive opiate receptors to adenylate cyclase in PMNLs of aged male subjects

Met-enkephalin (Met-enk) in 10(-9)-10(-7) M concentrations enhanced the extracellular cytotoxicity of polymorphonuclear leukocytes (PMNL) of young adult humans partly by stimulation of the "respiratory burst" in these cells. Meanwhile adenylate cyclase was inhibited and guanylate cyclase was stimulated. All the observed effects were abolished by 10(-5) M naloxone. On the other hand, a positive receptor coupling to adenylate cyclase was found when Met-enk was added in higher (10(-6)-10(-5) M) concentrations to PMNLs. The elevated cAMP level resulted in decreased extracellular cytotoxicity of PMNLs by a naloxone insensitive way. In PMNLs obtained from healthy aged male subjects, Met-enk induced in all of the applied concentrations an increased cAMP level and no change in cGMP level, with subsequent decrease of cytotoxicity, i.e. an impaired negative coupling of naloxone sensitive opiate receptors was detected with aging.

Delta opioid receptors in human neuroblastoma cell lines

Opioid receptor sites were detectable in 4 out of 9 human neuroblastoma cell lines tested, in the human retinoblastoma line Y79 NHT C10 and in the mouse neuroblastoma line Neuro 2A. All of these cell lines expressed delta sites, while only one coexpressed mu sites (SK-N-SH). Together with delta sites previously found in rodent neuroblastoma lines, these results suggest that the expression of delta sites is under less stringent control than that of mu and chi sites. A large number of delta sites (greater than 10,000 sites per cell) is expressed in IMR-32 and NMB neuroblastoma lines. Agonist binding was sensitive to Na+ and guanine nucleotides. The delta sites in IMR-32 and NMB cells were further characterized with delta selective ligands and [3H]DADL tracer. Their delta binding affinities were identical to those of the mu and delta cell line SK-N-SH; therefore the presence of mu sites does not appear to affect the binding behavior of the delta sites by any potential interaction among the binding proteins. Further, close correlations were found when comparing ligand binding in the human neuroblastoma cell lines with those of mouse neuroblastoma cells and rodent brain, an indication that the delta receptor is highly preserved among different species.

Intact brain cells: a novel model system for studying opioid receptor binding

This paper presents the use of a novel tissue preparation to study opioid receptor binding in viable intact cells derived from whole brains of adult rats. Mechanically dissociated and sieved cells, which were not homogenized at any stage of the experimental protocol, and iso-osmotic physiological buffer were used in these experiments. This system was adapted in order to avoid mechanical and chemical disruption of cell membranes, cytoskeletal ultrastructure or receptor topography by homogenization or by the use of non-physiological buffers, and to mimic in vivo binding conditions as much as possible. Using [3H]naloxone as the radioligand, our studies showed saturable and stereospecific high-affinity binding of this opioid antagonist in intact cells, which in turn showed consistently high viability. [3H]Naloxone binding was also linear over a wide range of tissue concentrations. This technique provides a very promising model for future studies of the binding of opioids and of many other classes of drugs to brain tissue receptors in a more physiologically relevant system than those commonly used to date.

Influence of delta-opioid receptors on production of labeled methionine5-enkephalin in murine neuroblastoma cells

Synthesis of methionine5-enkephalin by intact cells of murine neuroblastoma clone N1E-115 has been demonstrated both immunocytochemically and biochemically. In addition, N1E-115 cells possess homogeneous enkephalin (delta) receptors which inhibit prostaglandin E1-induced intracellular cyclic AMP formation. An assay was developed for measuring de novo synthesis of methionine5-enkephalin by pulsing cells in culture with radioactive methionine and isolating this pentapeptide to radiochemical purity by a procedure that included immunoaffinity chromatography specific for oxidized methionine5-enkephalin. This assay indicated that production of radiolabeled-methionine5-enkephalin was increased upon lengthy exposure of intact N1E-115 cells in the late logarithmic phase of growth to a nonproteolyzable analog of methionine5-enkephalin. This increase in synthesis of intracellular methionine5-enkephalin relative to control cells was prevented by prior incubation of the clone with naloxone, indicating that the response was mediated by the delta receptor.

Xorphanol

Xorphanol is a new mixed agonist-antagonist from the morphinan class of analgesics. On the basis of animal experiments, the physical dependence liability of xorphanol is predicted to be of a low order in man. Conceptually, xorphanol is of interest since in vitro experiments have revealed anti-naloxone properties and resistance to antagonism by opioid antagonists. At the practical level, xorphanol is a well tolerated, orally active analgesic that provides effective pain relief clinically.

Interaction of Leu-enkephalin with isolated enterocytes from guinea pig: binding to specific receptors and stimulation of cAMP accumulation

The specific binding of Leu-enkephalin and the stimulatory effect of the peptide on cAMP accumulation have been assessed in isolated enterocytes of guinea pig. The binding was reversible as well as time and temperature dependent. Two classes of binding sites could be defined: a class with a relatively high affinity (Kd = 0.7 microM) that represented 1% of total binding capacity, and another class with low affinity (Kd = 55.5 microM). The stimulation of cAMP accumulation was also shown to depend on time and temperature and was potentiated by a phosphodiesterase inhibitor. Half-maximal stimulation of cAMP accumulation was observed at 119 microM and maximal stimulation (27-fold basal level) at 300 microM Leu-enkephalin. Both steps of the interaction were not modified by Na+ but exhibited a high specificity since modification in the structure of Leu-enkephalin resulted in an important loss of binding affinity and stimulatory activity.