G protein activation and cyclic AMP modulation by naloxone benzoylhydrazone in distinct layers of rat olfactory bulb

Ligand based conformational space studies of the μ-opioid receptor

BACKGROUND

G protein-coupled receptors (GPCRs) comprise a family of membrane proteins that can be activated by a variety of external factors. The μ-opioid receptor (MOR), a class A GPCR, is the main target of morphine. Recently, enhanced sampling molecular dynamics simulations of a constitutively active mutant of MOR in its apo form allowed us to capture the novel intermediate states of activation, as well as the active state. This prompted us to apply the same techniques to wild type MOR in complex with ligands, in order to explore their contributions to the receptor conformational changes in the activation process.

METHODS

MOR was modeled in complex with agonists (morphine, BU72), a partial agonist (naloxone benzoylhydrazone) and an antagonist (naloxone). Replica exchange with solute tempering (REST2) molecular dynamics simulations were carried out for all systems. Trajectory frames were clustered, and the activation state of each cluster was assessed by two different methods.

RESULTS

Cluster sizes and activation indices show that while agonists stabilized structures in a higher activation state, the antagonist behaved oppositely. Morphine tends to drive the receptor towards increasing R165-T279 distances, while naloxone tends to increase the NPxxYA motif conformational change.

CONCLUSIONS

Despite not observing a full transition between inactive and active states, an important conformational change of transmembrane helix 5 was observed and associated with a ligand-driven step of the process.

GENERAL SIGNIFICANCE

The activation process of GPCRs is widely studied but still not fully understood. Here we carried out a step forward in the direction of gaining more details of this process.

Molecular Imaging of Opioid and Dopamine Systems: Insights Into the Pharmacogenetics of Opioid Use Disorders

Opioid use in the United States has steadily risen since the 1990s, along with staggering increases in addiction and overdose fatalities. With this surge in prescription and illicit opioid abuse, it is paramount to understand the genetic risk factors and neuropsychological effects of opioid use disorder (OUD). Polymorphisms disrupting the opioid and dopamine systems have been associated with increased risk for developing substance use disorders. Molecular imaging studies have revealed how these polymorphisms impact the brain and contribute to cognitive and behavioral differences across individuals. Here, we review the current molecular imaging literature to assess how genetic variations in the opioid and dopamine systems affect function in the brain's reward, cognition, and stress pathways, potentially resulting in vulnerabilities to OUD. Continued research of the functional consequences of genetic variants and corresponding alterations in neural mechanisms will inform prevention and treatment of OUD.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

G protein-coupled receptor heteromerization: a role in allosteric modulation of ligand binding

It is becoming increasingly recognized that G protein-coupled receptors physically interact. These interactions may provide a mechanism for allosteric modulation of receptor function. In this study, we examined this possibility by using an established model system of a receptor heteromer consisting of μ and δ opioid receptors. We examined the effect of a number of μ receptor ligands on the binding equilibrium and association and dissociation kinetics of a radiolabeled δ receptor agonist, [(3)H]deltorphin II. We also examined the effect of δ receptor ligands on the binding equilibrium and association and dissociation kinetics of a radiolabeled μ receptor agonist, [(3)H][d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin ([(3)H]DAMGO). We show that μ receptor ligands are capable of allosterically enhancing δ receptor radioligand binding and vice versa. Thus, there is strong positive cooperativity between the two receptor units with remarkable consequences for ligand pharmacology. We find that the data can be simulated by adapting an allosteric receptor model previously developed for small molecules, suggesting that the ligand-occupied protomers function as allosteric modulators of the partner receptor's activity.

Proteinase-activated receptors 1 and 2 in rat olfactory system: layer-specific regulation of multiple signaling pathways in the main olfactory bulb and induction of neurite retraction in olfactory sensory neurons

Proteinase-activated receptors (PARs) are a family of four G protein-coupled receptors that are widely distributed in the CNS and involved in neural cell proliferation, differentiation and survival. The olfactory system undergoes continuous neurogenesis throughout life and may represent a critical target of PAR cellular actions. In the present study we investigated the functional activity of PAR1 and PAR2 in microdissected tissue preparations of olfactory nerve-glomerular layer (ON-GL), external plexiform layer (EPL) and granule cell layer (GRL) of the rat main olfactory bulb and in primary cultures of olfactory neuroepithelial cells. Activation of either PAR1 or PAR2 regulated multiple signaling pathways, including activation of pertussis-toxin sensitive Gi/o proteins, inhibition of cyclic AMP formation, stimulation of Gq/11-mediated phosphoinositide (PI) hydrolysis, phosphorylation of Ca2+/calmodulin-dependent protein kinase II and activation of the monomeric G protein Rho, predominantly in ON-GL, whereas only activation of Rho was detected in the deeper layers. Olfactory nerve lesion by nasal irrigation with ZnSO4 induced a marked decrease of PAR signaling in ON-GL. In primary cultures of olfactory neurons, double immunofluorescence analysis showed the localization of PAR1 and PAR2 in cells positive for olfactory-marker protein and neuron-specific enolase. Cell exposure to either nanomolar concentrations of thrombin and trypsin or PAR-activating peptides caused rapid neurite retraction. This study provides the first characterization of the laminar distribution of PAR1 and PAR2 signaling in rat olfactory bulb, demonstrates the presence of the receptors in olfactory sensory neurons and suggests a role of PARs in olfactory sensory neuron neuritogenesis.

N-Desmethylclozapine, a major clozapine metabolite, acts as a selective and efficacious delta-opioid agonist at recombinant and native receptors

The present study examined the effects of N-desmethylclozapine (NDMC), a biologically active metabolite of the atypical antipsychotic clozapine, at cloned human opioid receptors stably expressed in Chinese hamster ovary (CHO) cells and at native opioid receptors present in NG108-15 cells and rat brain. In CHO cells expressing the delta-opioid receptor (CHO/DOR), NDMC behaved as a full agonist both in stimulating [(35)S]GTPgammaS binding (pEC(50)=7.24) and in inhibiting cyclic AMP formation (pEC(50)=6.40). NDMC inhibited [(3)H]naltrindole binding to CHO/DOR membranes with competition curves that were modulated by guanine nucleotides in an agonist-like manner. Determination of intrinsic efficacies by taking into consideration both the maximal [(35)S]GTPgammaS binding stimulation and the extent of receptor occupancy at which half-maximal effect occurred indicated that NDMC had an efficacy value equal to 82% of that of the full delta-opioid receptor agonist DPDPE, whereas clozapine and the other clozapine metabolite clozapine N-oxide displayed much lower levels of agonist efficacy. NDMC exhibited poor agonist activity and lower affinity at the kappa-opioid receptor and was inactive at mu-opioid and NOP receptors. In NG108-15 cells, NDMC inhibited cyclic AMP formation and stimulated the phosphorylation of extracellular signal-regulated kinase 1/2 by activating the endogenously expressed delta-opioid receptor. Moreover, in membranes of different brain regions, NDMC stimulated [(35)S]GTPgammaS binding and regulated adenylyl cyclase activity and the effects were potently antagonized by naltrindole. These data demonstrate for the first time that NDMC acts as a selective and efficacious delta-opioid receptor agonist and suggest that this unique property may contribute, at least in part, to the clinical actions of the atypical antipsychotic clozapine.

Agonist activity of naloxone benzoylhydrazone at recombinant and native opioid receptors

1. In the present study, we examined the pharmacological activity of the putative kappa3-opioid receptor agonist naloxone benzoylhydrazone (NalBzoH) at recombinant human opioid receptors individually expressed in Chinese hamster ovary (CHO) cells and native opioid receptors present in rat striatum. 2. At the mu-opioid receptor (MOR), NalBzoH stimulated guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding (pEC50=8.59) and inhibited cyclic AMP accumulation (pEC50=8.74) with maximal effects (Emax) corresponding to 55 and 65% of those obtained with the MOR agonist DAMGO, respectively. The MOR antagonist CTAP blocked the stimulatory effects of NalBzoH and DAMGO with similar potencies. 3. At the kappa-opioid receptor (KOR), NalBzoH stimulated [35S]GTPgammaS binding (pEC50=9.70) and inhibited cyclic AMP formation (pEC50=9.45) as effectively as the selective KOR agonist (-)-U-50,488. The NalBzoH effect was blocked by the KOR antagonist nor-binaltorphimine (nor-BNI) (pKi=10.30). 4. In CHO cells expressing the delta-opioid receptor (DOR), NalBzoH increased [35S]GTPgammaS binding (pEC50=8.49) and inhibited cyclic AMP formation (pEC50=8.61) almost as effectively as the DOR agonist DPDPE. Naltrindole (NTI), a selective DOR antagonist, completely blocked the response to NalBzoH (pKi of 10.40). 5. In CHO cells expressing the nociceptin/orphanin FQ (N/OFQ) receptor (NOP), NalBzoH failed to exert agonist effects and antagonized the agonist-induced receptor activation. 6. When compared to other opioid receptor ligands, NalBzoH showed an efficacy that was lower than that of morphine at MOR, but higher at KOR and DOR. 7. In rat striatum, NalBzoH enhanced [35S]GTPgammaS binding and inhibited adenylyl cyclase activity. These effects were antagonized by either CTAP, nor-BNI or NTI, each antagonist blocking a fraction of the NalBzoH response. 8. These data demonstrate that NalBzoH displays agonist activity at MOR, DOR and KOR expressed either in a heterologous cell system or in a native environment.

Has the sun set on kappa3-opioid receptors?

Mu-opioid receptor agonists are a mainstay of clinical analgesia, despite the significant unwanted effects and dependence liability associated with drugs like morphine. The quest for opioids that produce analgesia with fewer undesirable effects has lead to the putative identification of multiple opioid receptor subtypes, despite the identification of only four opioid-related receptor genes. One such putative receptor subtype is the kappa3 receptor, activation of which supposedly produces analgesia in animals. In the present issue of this Journal, Olianas and co-workers have demonstrated that the prototypic kappa3 agonist naloxone benzoylhydrazone is actually a partial agonist at the cloned mu, delta, and kappa opioid receptors and an antagonist at opioid-like NOP receptors. Together with a recent study that showed that high-affinity naloxone benzoylhydrazone binding is abolished in triple mu/delta/kappa receptor knockout mice, the present study provides strong evidence that in vivo effects attributed to kappa3 receptor activation probably just reflect the combined actions of a particularly nonselective opioid drug. Indeed, molecular identification of any of the proposed subtypes of mu, delta, and kappa opioid receptors has proven elusive, suggesting that it is perhaps time to retire the notion of opioid receptor subtypes until definitive evidence for their existence is provided.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.