Binding of BW 373U86, a non-peptidic delta opioid receptor agonist, is not regulated by guanine nucleotides and sodium

Delta Opioid Pharmacology in Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder that compromises multiple neurochemical substrates including dopamine, norepinephrine, serotonin, acetylcholine, and glutamate systems. Loss of these transmitter systems initiates a cascade of neurological deficits beginning with motor function and ending with dementia. Current therapies primarily address the motor symptoms of the disease via dopamine replacement therapy. Exogenous dopamine replacement brings about additional challenges since after years of treatment it almost invariably gives rise to dyskinesia as a side effect. Therefore there is a clear unmet clinical need for improved PD therapeutics. Opioid receptors and their respective peptides are expressed throughout the basal ganglia and cortex where monoaminergic denervation strongly contributes to PD pathology. Delta opioid receptors are of particular interest because of their dense localization in basal ganglia and because activating this system is known to enhance locomotor activity under a variety of conditions. This chapter will outline much of the work that has demonstrated the effectiveness of delta opioid receptor activation in models of PD and its neuroprotective properties. It also discusses some of the challenges that must be addressed before moving delta opioid receptor agonists into a clinical setting.

Coupling mode of receptors and G proteins

Signaling via G-protein-coupled receptors (GPCRs) is crucial to many physiological and pathophysiological processes in multicellular organisms, and GPCRs themselves are targets for important drugs. Classical cell supplementation experiments suggest a collision coupling model, in which receptors and G proteins diffuse randomly within the cell membrane and interact only if receptors are activated. This model is also backed by kinetic and live cell imaging data. According to the challenging theory, receptors and G proteins are precoupled--meaning they are forming stable complexes in the absence of agonist, which prevail during signaling. This model has been favored on the basis of copurification and coimmunoprecipitation of inactive receptors with G proteins and more recently by some approaches measuring energy transfer between labeled receptors and G proteins. This article reviews key findings regarding the receptor/G protein coupling mode, including most recent findings obtained by optical techniques.

Relationship between rate and extent of G protein activation: comparison between full and partial opioid agonists

Opioid agonists acting at their receptors alter intracellular events by initiating activation of various types of Gi/Go proteins. This can be measured by the binding of the stable GTP analog [(35)S]guanosine-5'-O-(3-thio)triphosphate ([(35)S]GTPgammaS). In this study agonist efficacy is defined by the degree to which an opioid stimulates the binding of [(35)S]GTPgammaS. This allows for a definition of full and partial agonists; a full agonist causing a greater stimulation of [(35)S]GTPgammaS binding than a partial agonist. The hypothesis that the rate of agonist-stimulated [(35)S]GTPgammaS binding is dependent upon agonist efficacy was tested using membranes from C6 glioma cells expressing mu- or delta-opioid receptors. At maximal concentrations the rate of agonist-stimulated [(35)S]GTPgammaS binding followed the efficacy of mu-agonists in stimulating [(35)S]GTPgammaS binding, i.e., [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin > morphine > meperidine > butorphanol > nalbuphine. At submaximal concentrations of mu- or delta-full agonists the [(35)S]GTPgammaS association rate was also reduced, such that the rate of [(35)S]GTPgammaS binding correlated with the extent of [(35)S]GTPgammaS bound, whether this binding was stimulated by a full agonist or a partial agonist. Agonists also stimulated [(35)S]GTPgammaS dissociation, showing that binding of this stable nucleotide was reversible. Comparison of the delta-agonists [D-Ser(2),Leu(5)]-enkephalin-Thr and (+/-)-4-((alpha-R*)-alpha-((2S*,5R*)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxylbenzyl)-N,N-diethylbenzamide, a compound with slow dissociation kinetics, showed the measured rate of G protein activation was not influenced by the agonist switching between receptors. The results are consistent with the idea that the active state(s) of the receptor induced by full or partial agonists is the same, but the number of activated receptors determines the rate of G protein activation.

Effects of guanine, inosine, and xanthine nucleotides on beta(2)-adrenergic receptor/G(s) interactions: evidence for multiple receptor conformations

The aim of our study was to examine the effects of different purine nucleotides [GTP, ITP, and xanthosine 5'-triphosphate (XTP)] on receptor/G protein coupling. As a model system, we used a fusion protein of the beta(2)-adrenergic receptor and the alpha subunit of the G protein G(s). GTP was more potent and efficient than ITP and XTP at inhibiting ternary complex formation and supporting adenylyl cyclase (AC) activation. We also studied the effects of several beta(2)-adrenergic receptor ligands on nucleotide hydrolysis and on AC activity in the presence of GTP, ITP, and XTP. The efficacy of agonists at promoting GTP hydrolysis correlated well with the efficacy of agonists for stimulating AC in the presence of GTP. This was, however, not the case for ITP hydrolysis and AC activity in the presence of ITP. The efficacy of ligands at stimulating AC in the presence of XTP differed considerably from the efficacies of ligands in the presence of GTP and ITP, and there was no evidence for receptor-regulated XTP hydrolysis. Our findings support the concept of multiple ligand-specific receptor conformations and demonstrate the usefulness of purine nucleotides as tools to study conformational states of receptors.

Weaning-induced development of delta-opioid receptors in rat brain: differential effects of guanine nucleotides and sodium upon ligand-receptor recognition

1. We have previously shown that weaning at day 21 increases delta-opioid receptor binding in the brain at day 25, which might be due to stimulation of the development of a delta-opioid receptor subtype or activation of G-protein coupling processes. 2. We have addressed the possibility that weaning stimulates coupling of the delta-receptor by homogenate binding studies with four agonist and one antagonist radioligand in the presence of a GTP analogue and Na+ in brain tissue from weaned and non-weaned animals. 3. Saturation studies with three agonist ligands ([3H]-deltorphin I, [3H]-S-Atc-Ile(5,6)deltorphin I and [3H]-R-Atc-Ile(5,6)deltorphin II) showed higher levels of maximal binding in brains from 25-day weaned than in brains from non-weaned rats. The magnitude of the effects of GMPPNP and Na+ in decreasing this binding was ligand dependent and in each case was significantly more marked in brains from weaned animals. GMPPNP and Na+ were completely without effect on Bmax for, [3H]-S-Atc-Ile(5,6)deltorphin I and [3H]-R-Atc-Ile(5,6)deltorphin II in brains from non-weaned rats. 4. [3H]-Ile(5,6)deltorphin II and [3H]-naltrindole showed no differences in labelling between weaned and non-weaned groups and both groups responded similarly to the effects of GMPPNP and Na+ treatment. 5. GMPPNP and Na+ had small effects on binding affinity (K(D)) for some of the agonist radioligands which were similar in both weaned and non-weaned groups. 6. Weaning induced increases in binding of delta-receptors in 25-day rats can be explained in terms of the way delta-agonist radioligands recognize the receptor environment.

Characterization of the delta-opioid receptor found in SH-SY5Y neuroblastoma cells

The delta-opioid receptor found in SH-SY5Y cells was characterized in terms of binding profile and ability to mediate the inhibition of forskolin-stimulated cAMP accumulation. Both DPDPE ([D-Pen2,D-Pen5]enkephalin) and deltorphin II, compounds reported to be selective for the delta 1- and delta 2-opioid receptor respectively, were potent agonists in these cells. Binding studies indicated that naltrindole benzofuran (NTB) had significantly higher affinity than 7-benzylidenenaltrexone (BNTX); however, both compounds have high affinity for the delta-opioid receptor found in SH-SY5Y cells. Naltrindole benzofuran was found to be a potent antagonist, with an IC50 of less than 1 nM, while 7-benzylidene naltrexone was found to be a relatively weak antagonist, requiring greater than 100 nM to inhibit 50% of agonist activity. Binding to intact SH-SY5Y cells was compared to binding to cell membranes and guinea-pig brain membranes. In each case, binding affinities were very similar. These studies suggest that the receptor found in SH-SY5Y cells could probably be classified as a delta 2-opioid receptor. However, the very similar binding characteristics of SH-SY5Y cells and guinea-pig brain membranes call into question the ability to label delta 1-opioid receptors.

Development of a common 3D pharmacophore for delta-opioid recognition from peptides and non-peptides using a novel computer program

A unified three-dimensional (3D) pharmacophore for recognition of the delta-opioid receptor by families of structurally diverse delta-opioid ligands, including peptides and non-peptides, has been determined. An additional structural feature required for delta-selectivity was also characterized using a subset of these ligands that are highly selective for the delta-opioid receptor. To obtain these pharmacophores, we have used a recently developed computer program that performs systematic and automated comparisons of molecules to determine whether any common 3D relationships exist among candidate recognition moieties in high-affinity analogs. All the low-energy conformations of each ligand are included in these comparisons. The program developed should be applicable in general to molecular superimposition problems in rational drug design and to develop both 3D recognition and activation pharmacophores for any receptor for which high- and low-affinity analogs and agonists and antagonists have been identified.

Binding of [3H](+)-BW373U86 to delta-opioid receptors in rat brain membranes

A tritiated form of the non-peptide delta-opioid receptor agonist (+)-BW373U86 ((+)-4-((alpha-R)-alpha-((2S,5R)-4-allyl-2, 5-dimethyl-l-piperazinyl)-3-hydroxybenzyl)-N,N-diethylbenzamide) was synthesized and its binding characteristics studied. [3H](+)-BW373U86 bound with subnanomolar affinity to rat brain membranes and was displaced most effectively by ligands selective for delta-opioid receptors. Naltrindole, naltriben, and 7-benzylidenenaltrexone exhibited apparent inhibition constants of 0.06, 1.54, and 4.49 nM, respectively, while mu- or kappa-selective ligands showed little affinity for this site. [3H](+)-BW373U86 binding was sensitive to the presence of guanine nucleotides; GDP caused a 3-fold decrease and 5'-guanylyl-imidodiphosphate (Gpp[NH]p) caused a 25% increase in binding affinity.

Increased expression of opioid delta receptors by deoxy conformation heme proteins in NG108-15 cells

Adaptations to prolonged hypoxia include an increase in the expression of proteins that may facilitate survival. One mechanism by which hypoxia increases protein expression involves a change of heme proteins from oxygenated to deoxygenated conformations. In the present study, we tested the hypothesis that treatment of NG108-15 cells with metallic cations, which are known to induce a deoxygenated conformation of heme proteins, would increase delta opioid receptor (DOR) expression. Cells were treated with cobalt and nickel, which induce deoxygenated heme protein conformation, or zinc as a control for 48 h prior to quantifying DOR expression. Cobalt and nickel, but not zinc, significantly increased DOR expression. Heme synthesis inhibitors would block the synthesis of cobalt-substituted heme proteins which are locked in a deoxygenated conformation. The cobalt-induced increase in DOR expression was blocked by the heme synthesis inhibitor, 4,6-dioxoheptanoic acid. These experiments indicate that deoxygenated conformation heme proteins, which are thought to partially mimic hypoxia, increase DOR expression. The increase in DOR expression suggests that the DOR gene may be hypoxia-sensitive. Further, the increase in DOR expression suggests a potential adaptation strategy to hypoxia and may represent one of the first findings of physiological regulation of DOR expression.