Coupling specificity of NOP opioid receptors to pertussis-toxin-sensitive Galpha proteins in adult rat stellate ganglion neurons using small interference RNA

Nociceptin Receptor-Related Agonists as Safe and Non-addictive Analgesics

As clinical use of currently available opioid analgesics is often impeded by dose-limiting adverse effects, such as abuse liability and respiratory depression, new approaches have been pursued to develop safe, effective, and non-addictive pain medications. After the identification of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor more than 25 years ago, NOP receptor-related agonists have emerged as a promising target for developing novel and effective opioids that modulate the analgesic and addictive properties of mu-opioid peptide (MOP) receptor agonists. In this review, we highlight the effects of the NOP receptor-related agonists compared with those of MOP receptor agonists in experimental rodent and more translational non-human primate (NHP) models and the development status of key NOP receptor-related agonists as potential safe and non-addictive analgesics. Several lines of evidence demonstrated that peptidic and non-peptidic NOP receptor agonists produce potent analgesic effects by intrathecal delivery in NHPs. Moreover, mixed NOP/MOP receptor partial agonists (e.g., BU08028, BU10038, and AT-121) display potent analgesic effects when administered intrathecally or systemically, without eliciting adverse effects, such as respiratory depression, itch behavior, and signs of abuse liability. More importantly, cebranopadol, a mixed NOP/opioid receptor agonist with full efficacy at NOP and MOP receptors, produces robust analgesic efficacy with reduced adverse effects, conferring promising outcomes in clinical studies. A balanced coactivation of NOP and MOP receptors is a strategy that warrants further exploration and refinement for the development of novel analgesics with a safer and effective profile.

Nociceptin/Orphanin FQ Peptide Receptor-Related Ligands as Novel Analgesics

Despite similar distribution patterns and intracellular events observed in the nociceptin/ orphanin FQ peptide (NOP) receptor and other opioid receptors, NOP receptor activation displays unique pharmacological profiles. Several researchers have identified a variety of peptide and nonpeptide ligands to determine the functional roles of NOP receptor activation and observed that NOP receptor- related ligands exhibit pain modality-dependent pain processing. Importantly, NOP receptor activation results in anti-nociception and anti-hypersensitivity at the spinal and supraspinal levels regardless of the experimental settings in non-human primates (NHPs). Given that the NOP receptor agonists synergistically enhance mu-opioid peptide (MOP) receptor agonist-induced anti-nociception, it has been hypothesized that dual NOP and MOP receptor agonists may display promising functional properties as analgesics. Accumulating evidence indicates that the mixed NOP/opioid receptor agonists demonstrate favorable functional profiles. In NHP studies, bifunctional NOP/MOP partial agonists (e.g., AT-121, BU08028, and BU10038) exerted potent anti-nociception via NOP and MOP receptor activation; however, dose-limiting adverse effects associated with the MOP receptor activation, including respiratory depression, itch sensation, physical dependence, and abuse liability, were not observed. Moreover, a mixed NOP/opioid receptor agonist, cebranopadol, presented promising outcomes in clinical trials as a novel analgesic. Collectively, the dual agonistic actions on NOP and MOP receptors, with appropriate binding affinities and efficacies, may be a viable strategy to develop innovative and safe analgesics.

Lipid-Encapsulated Silica Nanobowls as an Efficient and Versatile DNA Delivery System

Nonmesoporous Janus silica nanobowls (NBs) are unique in that they possess two different nonporous surfaces per particle for loading biological molecules and can thus be designed with multifunctional properties. Although silica NBs have been successfully employed for both targeted therapeutic and diagnostic applications, their ability to deliver DNA has not yet been fully explored. The purpose of this study was to design and develop an in vitro transfection agent that would exploit the distinct characteristics of the silica NB. First, we determined that the NB surface can be linked to either supercoiled cDNA plasmids or vectorless, linear cDNA constructs. Additionally, the linearized cDNA can be functionalized and chemisorbed on NBs to obtain a controlled release. Second, the successful transfection of cells studied was dependent on lipid coating of the NB (LNBs). Although both NBs and LNBs were capable of undergoing endocytosis, NBs appeared to remain within vesicles as shown by transmission electron microscopy (TEM). Third, fluorescence microscopy and Western blotting assays revealed that transfection of four different cell lines and acutely isolated rat sensory neurons with LNBs loaded with either linear or supercoiled cDNA constructs coding for the fluorescent protein, clover and tdTomato, resulted in protein expression. Fourth, two separate opioid receptor-ion channel signaling pathways were functionally reconstituted in HEK cells transfected with LNBs loaded with three separate cDNA constructs. Overall, these results lay the foundation for the use and further development of LNBs as in vitro transfection agents.

Therapeutic potentials of NOP and MOP receptor coactivation for the treatment of pain and opioid abuse

Following the identification of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) as an endogenous ligand for the NOP receptor, ample evidence has revealed unique functional profiles of the N/OFQ-NOP receptor system. NOP receptors are expressed in key neural substrates involved in pain and reward modulation. In nonhuman primates (NHPs), NOP receptor activation effectively exerts antinociception and anti-hypersensitivity at the spinal and supraspinal levels. Moreover, NOP receptor activation inhibits dopaminergic transmission and synergistically enhances mu-opioid peptide (MOP) receptor-mediated analgesia. In this article, we have discussed the functional profiles of ligands with dual NOP and MOP receptor agonist activities and highlight their optimal functional efficacy for pain relief and drug abuse treatment. Through coactivation of NOP and MOP receptors, bifunctional NOP/MOP receptor "partial" agonists (e.g., AT-121, BU08028, and BU10038) reveal a wider therapeutic window with fewer side effects. These newly developed ligands potently induce antinociception without MOP receptor agonist-associated side effects such as abuse potential, respiratory depression, itching sensation, and physical dependence. In addition, in both rodent and NHP models, bifunctional NOP/MOP receptor agonists can attenuate reward processing and/or the reinforcing effects of opioids and other abused drugs. While a mixed NOP/opioid receptor "full" agonist cebranopadol is undergoing clinical trials, bifunctional NOP/MOP "partial" agonists exhibit promising therapeutic profiles in translational NHP models for the treatment of pain and opioid abuse. This class of drugs demonstrates the therapeutic advantage of NOP and MOP receptor coactivation, indicating a greater potential for future development.

Electrophysiological Actions of N/OFQ

Whilst the nociceptin/orphanin FQ (N/OFQ) receptor (NOP) has similar intracellular coupling mechanisms to opioid receptors, it has distinct modulatory effects on physiological functions such as pain. These actions range from agonistic to antagonistic interactions with classical opioids within the spinal cord and brain, respectively. Understanding the electrophysiological actions of N/OFQ has been crucial in ascertaining the mechanisms by which these agonistic and antagonistic interactions occur. These similarities and differences between N/OFQ and opioids are due to the relative location of NOP versus opioid receptors on specific neuronal elements within these CNS regions. These mechanisms result in varied cellular actions including postsynaptic modulation of ion channels and presynaptic regulation of neurotransmitter release.

Effects of NOP-Related Ligands in Nonhuman Primates

The nociceptin/orphanin FQ peptide (NOP) receptor-related ligands have been demonstrated in preclinical studies for several therapeutic applications. This article highlights (1) how nonhuman primates (NHP) were used to facilitate the development and application of positron emission tomography tracers in humans; (2) effects of an endogenous NOP ligand, nociceptin/orphanin FQ, and its interaction with mu opioid peptide (MOP) receptor agonists; and (3) promising functional profiles of NOP-related agonists in NHP as analgesics and treatment for substance use disorders. NHP models offer the most phylogenetically appropriate evaluation of opioid and non-opioid receptor functions and drug effects. Based on preclinical and clinical data of ligands with mixed NOP/MOP receptor agonist activity, several factors including their intrinsic efficacies for activating NOP versus MOP receptors and different study endpoints in NHP could contribute to different pharmacological profiles. Ample evidence from NHP studies indicates that bifunctional NOP/MOP receptor agonists have opened an exciting avenue for developing safe, effective medications with fewer side effects for treating pain and drug addiction. In particular, bifunctional NOP/MOP partial agonists hold a great potential as (1) effective spinal analgesics without itch side effects; (2) safe, nonaddictive analgesics without opioid side effects such as respiratory depression; and (3) effective medications for substance use disorders.

Gγ7 proteins contribute to coupling of nociceptin/orphanin FQ peptide (NOP) opioid receptors and voltage-gated Ca(2+) channels in rat stellate ganglion neurons

The nociceptin/orphanin FQ peptide (NOP) opioid receptors regulate neurotransmitter release via inhibition of voltage-gated Ca(2+) channels (CaV2.2) in sympathetic and sensory neurons. Stimulation of NOP receptors by its endogenous agonist, nociception (Noc), leads to membrane-delimited, voltage-dependent (VD) block of CaV2.2 channel currents mediated by Gβγ protein subunits. Previously we reported that the pertussis toxin-sensitive Gαi1 and Gβ2/β4 isoforms mediate the functional coupling of NOP opioid receptors with CaV channels in rat stellate ganglion (SG) sympathetic neurons. In the present report we extended our studies by identifying the Gγ subunit that forms the heterotrimer within this signaling pathway. Small interference RNA (or siRNA) was employed to silence the expression of the natively expressed Gγ subunits. Initial PCR assays indicated that SG neurons expressed seven Gγ subunits. Silencing Gγ3 subunits did not alter signaling between NOP receptors and Ca(2+) channels. However, after Gγ7 isoforms were silenced, the Noc-mediated inhibition of CaV channels was significantly decreased when compared to SG neurons transfected with scrambled siRNA. We observed that Gγ10 and Gγ11 mRNA levels increased 2.5- and 2.7-fold, respectively, after Gγ7 subunits were silenced. However, this compensatory increase in mRNA expression did not appear to fully rescue the NOP receptor coupling efficiency. Additionally, both Gγ2 and Gγ5 levels increased 50 and 75%, respectively, while Gγ3 and Gγ4 expression levels remained relatively unchanged. Taken together, our findings suggest that the Gαi1/Gβ2(β4)/Gγ7 heterotrimeric G protein complex determines the NOP receptor-mediated modulation of CaV channels in SG neurons.

Central N/OFQ-NOP Receptor System in Pain Modulation

Two decades have passed since the peptide, nociceptin/orphanin FQ (N/OFQ), and its cognate (NOP) receptor were discovered. Although NOP receptor activation causes a similar pattern of intracellular actions as mu-opioid (MOP) receptors, NOP receptor-mediated pain modulation in rodents are more complicated than MOP receptor activation. This review highlights the functional evidence of spinal, supraspinal, and systemic actions of NOP receptor agonists for regulating pain. In rodents, effects of the N/OFQ-NOP receptor system in spinal and supraspinal sites for modulating pain are bidirectional depending on the doses, assays, and pain modalities. The net effect of systemically administered NOP receptor agonists may depend on relative contribution of spinal and supraspinal actions of the N/OFQ-NOP receptor signaling in rodents under different pain states. In stark contrast, NOP receptor agonists produce only antinociception and antihypersensitivity in spinal and supraspinal regions of nonhuman primates regardless of doses and assays. More importantly, NOP receptor agonists and a few bifunctional NOP/MOP receptor agonists do not exhibit reinforcing effects (abuse liability), respiratory depression, itch pruritus, nor do they delay the gastrointestinal transit function (constipation) in nonhuman primates. Depending upon their intrinsic efficacies for activating NOP and MOP receptors, bifunctional NOP/MOP receptor agonists warrant additional investigation in primates regarding their side effect profiles. Nevertheless, NOP receptor-related agonists display a much wider therapeutic window as compared to that of MOP receptor agonists in primates. Both selective NOP receptor agonists and bifunctional NOP/MOP receptor agonists hold great potential as effective and safe analgesics without typical opioid-associated side effects in humans.

Nociceptin/orphanin FQ receptor and pain: Feasibility of the fourth opioid family member

The pharmacological management of chronic pain is a major therapeutic problem. The need of repeated treatments reduces the usefulness of classical analgesic drugs, like μ opioid receptor (MOP) agonists, characterized by tolerance development, side effects and abuse. Moreover, the pathological persistence of pain modifies nociceptive signals and pain-devoted structure activity weakening MOP agonists and making difficult the research of new active molecules. Nociceptine/orphanin FQ (N/OFQ) and its receptor (NOP) offers a peculiar opioid system able to interact with MOP receptors and made more sensitive by chronic pain conditions. The pain reliever efficacy of NOP agonists against persistent pain, mainly neuropathic pain, has been highlighted after intrathecal infusions in rats and non human primates (NHPs). The differences emerged between the effects of NOP stimulation in rodents and NHPs allow to hypothesize the relevance of NOP modulators in higher organisms strongly encouraging the development of compounds active by a systemic route. Possible applicative perspectives are (i) selective NOP agonists as such, (ii) NOP modulation as adjuvant of MOP-based treatments, or (iii) mixed non-selective agonists vs NOP and classical opioid receptors.

Pharmacological Profile of Nociceptin/Orphanin FQ Receptors Interacting with G-Proteins and β-Arrestins 2

Nociceptin/orphanin FQ (N/OFQ) controls several biological functions by selectively activating an opioid like receptor named N/OFQ peptide receptor (NOP). Biased agonism is emerging as an important and therapeutically relevant pharmacological concept in the field of G protein coupled receptors including opioids. To evaluate the relevance of this phenomenon in the NOP receptor, we used a bioluminescence resonance energy transfer technology to measure the interactions of the NOP receptor with either G proteins or β-arrestin 2 in the absence and in presence of increasing concentration of ligands. A large panel of receptor ligands was investigated by comparing their ability to promote or block NOP/G protein and NOP/arrestin interactions. In this study we report a systematic analysis of the functional selectivity of NOP receptor ligands. NOP/G protein interactions (investigated in cell membranes) allowed a precise estimation of both ligand potency and efficacy yielding data highly consistent with the known pharmacological profile of this receptor. The same panel of ligands displayed marked differences in the ability to promote NOP/β-arrestin 2 interactions (evaluated in whole cells). In particular, full agonists displayed a general lower potency and for some ligands an inverted rank order of potency was noted. Most partial agonists behaved as pure competitive antagonists of receptor/arrestin interaction. Antagonists displayed similar values of potency for NOP/Gβ₁ or NOP/β-arrestin 2 interaction. Using N/OFQ as reference ligand we computed the bias factors of NOP ligands and a number of agonists with greater efficacy at G protein coupling were identified.

Functional plasticity of the N/OFQ-NOP receptor system determines analgesic properties of NOP receptor agonists

Despite high sequence similarity between NOP (nociceptin/orphanin FQ opioid peptide) and opioid receptors, marked differences in endogenous ligand selectivity, signal transduction, phosphorylation, desensitization, internalization and trafficking have been identified; underscoring the evolutionary difference between NOP and opioid receptors. Activation of NOP receptors affects nociceptive transmission in a site-specific manner, with antinociceptive effects prevailing after peripheral and spinal activation, and pronociceptive effects after supraspinal activation in rodents. The net effect of systemically administered NOP receptor agonists on nociception is proposed to depend on the relative contribution of peripheral, spinal and supraspinal activation, and this may depend on experimental conditions. Functional expression and regulation of NOP receptors at peripheral and central sites of the nociceptive pathway exhibits a high degree of plasticity under conditions of neuropathic and inflammatory pain. In rodents, systemically administered NOP receptor agonists exerted antihypersensitive effects in models of neuropathic and inflammatory pain. However, they were largely ineffective in acute pain while concomitantly evoking severe motor side effects. In contrast, systemic administration of NOP receptor agonists to non-human primates (NHPs) exerted potent and efficacious antinociception in the absence of motor and sedative side effects. The reason for this species difference with respect to antinociceptive efficacy and tolerability is not clear. Moreover, co-activation of NOP and μ-opioid peptide (MOP) receptors synergistically produced antinociception in NHPs. Hence, both selective NOP receptor as well as NOP/MOP receptor agonists may hold potential for clinical use as analgesics effective in conditions of acute and chronic pain.

Alteration of the mu opioid receptor: Ca2+ channel signaling pathway in a subset of rat sensory neurons following chronic femoral artery occlusion

The exercise pressor reflex, a crucial component of the cardiovascular response under physiological and pathophysiological states, is activated via metabolic and mechanical mediators that originate from contracting muscles and stimulate group III and IV afferents. We reported previously that stimulation of mu opioid receptors (MOR), expressed in both afferents, led to a significant attenuation of the reflex in rats whose femoral arteries had been occluded for 72 h. The present study examined the effect of arterial occlusion on the signaling components involved in the opioid-mediated modulation of Ca(2+) channels in rat dorsal root ganglion neurons innervating the triceps surae muscles. We focused on neurons that were transfected with cDNA coding for enhanced green fluorescent protein whose expression is driven by the voltage-gated Na(+) channel 1.8 (Na(V)1.8) promoter region, a channel expressed primarily in nociceptive neurons. With the use of a small interference RNA approach, our results show that the pertussis toxin-sensitive Gα(i3) subunit couples MOR with Ca(2+) channels. We observed a significant leftward shift of the MOR agonist [D-Ala2-N-Me-Phe4-Glycol5]-enkephalin concentration-response relationship in neurons isolated from rats with occluded arteries compared with those that were perfused freely. Femoral occlusion did not affect Ca(2+) channel density or the fraction of the main Ca(2+) channel subtype. Furthermore, Western blotting analysis indicated that the leftward shift did not result from either increased Gα(i3) or MOR expression. Finally, all neurons from both groups exhibited an inward current following exposure of the transient potential receptor vanilloid 1 (TRPV1) agonist, 8-methyl-N-vanillyl-6-nonenamide. These findings suggest that sensory neurons mediating the exercise pressor reflex express Na(V)1.8 and TRPV1 channels, and femoral occlusion alters the MOR pharmacological profile.

Pharmacogenomic study of the role of the nociceptin/orphanin FQ receptor and opioid receptors in diabetic hyperalgesia

Targeting functionally independent receptors may provide synergistic analgesic effects in neuropathic pain. To examine the interdependency between different opioid receptors (µ-opioid peptide [MOP], δ-opioid peptide [DOP] and κ-opioid peptide [KOP]) and the nociceptin/orphanin FQ peptide (NOP) receptor in streptozotocin (STZ)-induced diabetic polyneuropathy, nocifensive activity was measured using a hot plate test in wild-type and NOP, MOP, DOP and KOP receptor knockout mice in response to the selective receptor agonists Ro65-6570, morphine, SNC-80 and U50488H, or vehicle. Nocifensive activity was similar in non-diabetic wild-type and knockout mice at baseline, before agonist or vehicle administration. STZ-induced diabetes significantly increased heat sensitivity in all mouse strains, but MOP, DOP and KOP receptor knockouts showed a smaller degree of hyperalgesia than wild-type mice and NOP receptor knockouts. For each agonist, a significant antihyperalgesic effect was observed in wild-type diabetic mice (all P<0.05 versus vehicle); the effect was markedly attenuated in diabetic mice lacking the cognate receptor compared with wild-type diabetic mice. Morphine was the only agonist that demonstrated near-full antihyperalgesic efficacy across all non-cognate receptor knockouts. Partial or near-complete reductions in efficacy were observed with Ro65-6570 in DOP and KOP receptor knockouts, with SNC-80 in NOP, MOP and KOP receptor knockouts, and with U50488H in NOP and DOP receptor knockouts. There was no evidence of NOP and MOP receptor interdependency in response to selective agonists for these receptors. These findings suggest that concurrent activation of NOP and MOP receptors, which showed functional independence, may yield an effective and favorable therapeutic analgesic profile.

Deciphering intracellular localization and physiological role of nociceptin and nocistatin

Nociceptin and nocistatin are endogenous ligands of G protein coupled receptor family. Numerous techniques have been used to study the diverse parameters including, localization, distribution and ultrastructure of these peptides. The majority of the study parameters are based on their physiological roles in different organ systems. The present study presents an overview of the different methods used for the study of nociceptin, nocistatin and their receptors. Nociceptin has been implicated in many physiological functions including, nociception, locomotion, stressed-induced analgesia, learning and memory, neurotransmitter and hormone release, renal function, neuronal differentiation, sexual and reproductive behavior, uterine contraction, feeding, anxiety, gastrointestinal motility, cardiovascular function, micturition, cough, hypoxic-ischemic brain injury, diuresis and sodium balance, temperature regulation, vestibular function, and mucosal transport. It has been noted that the use of light and electron microscopy was less frequent, though it may be one of the most promising tools to study the intracellular localization of these neuropeptides. In addition, more studies on the level of circulating nociceptin and nocistatin are also necessary for investigating their clinical roles in health and disease. A variety of modern tools including physiological, light and electron microscopy (EM) are needed to decipher the extent of intracellular localization, tissue distribution and function of these peptides. The intracellular localization of nociceptin and nocistatin will require a high resolution transmission EM capable of identifying these peptides and other supporting molecules that co-localize with them. A tracing technique could also elucidate a possible migratory ability of nociceptin and nocistatin from one cellular compartment to the other.

Gβ2 and Gβ4 participate in the opioid and adrenergic receptor-mediated Ca2+ channel modulation in rat sympathetic neurons

Cardiac function is regulated in part by the sympathetic branch of the autonomic nervous system via the stellate ganglion (SG) neurons. Neurotransmitters, such as noradrenaline (NA), and neuropeptides, including nociceptin (Noc), influence the excit ability of SG neurons by modulating Ca(2+) channel function following activation of the adrenergic and nociceptin/orphanin FQ peptide (NOP) opioid receptors, respectively. The regulation of Ca(2+) channels is mediated by Gβγ, but the specific Gβ subunit that modulates the channels is not known. In the present study, small interference RNA (siRNA) was employed to silence the natively expressed Gβ proteins in rat SG tissue and to examine the coupling specificity of adrenergic and NOP opioid receptors to Ca(2+) channels employing the whole-cell variant of the patch-clamp technique.Western blotting analysis showed that Gβ1, Gβ2 and Gβ4 are natively expressed. The knockdown of Gβ2 or Gβ4 led to a significant decrease of the NA- and Noc-mediated Ca(2+)current inhibition, while Gβ1 silencing was without effect. However, sustaining low levels of Gβ2 resulted in an increased expression of Gβ4 and a concomitant compensation of both adrenergic and opioid signalling pathways modulating Ca(2+) channels. Conversely, Gβ4-directed siRNA was not accompanied with a compensation of the signalling pathway. Finally, the combined silencing of Gβ2 and Gβ4 prevented any additional compensatory mechanisms.Overall, our studies suggest that in SG neurons, Gβ2 and Gβ4 normally maintain the coupling of Ca(2+) channels with the receptors, with the latter subtype responsible for maintaining the integrity of both pathways.

Modulation of silent and constitutively active nociceptin/orphanin FQ receptors by potent receptor antagonists and Na+ ions in rat sympathetic neurons

The pharmacology of G protein-coupled receptors can be influenced by factors such as constitutive receptor activation and Na(+) ions. In this study, we examined the coupling of natively and heterologously expressed nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptors with voltage-dependent Ca(2+) channels after exposure to four high-affinity NOP receptor blockers [[Nphe(1)Arg(14)Lys(15)]N/OFQ-NH(2) (UFP-101), 1-[1-(cyclooctylmethyl)-1,2,3,6-tetrahydro-5-(hydroxymethyl)-4-pyridinyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one (Trap-101), 1-benzyl-N-{3-[spiroisobenzofuran-1(3H),4'-piperidin-1-yl]propyl}pyrrolidine-2-carboxamide (compound 24), and N-(4-amino-2-methylquinolin-6-yl)-2-(4-ethylphenoxymethyl)benzamide hydrochloride (JTC-801)] in sympathetic neurons. The enhanced tonic inhibition of Ca(2+) currents in the absence of agonists, indicative of constitutively active NOP receptors in transfected neurons, was abolished after pretreatment with pertussis toxin. In control neurons, the four antagonists did not exert any effects when applied alone but significantly blocked the N/OFQ-mediated Ca(2+) current inhibition. Exposure of transfected neurons to UFP-101 resulted in partial agonist effects. In contrast, Trap-101, compound 24, and JTC-801 exerted inverse agonism, as measured by the loss of tonic Ca(2+) current inhibition. In experiments designed to measure the N/OFQ concentration-response relationship under varying Na(+) concentrations, a leftward shift of IC(50) values was observed after Na(+) exposure. Although similar N/OFQ efficacies were measured with all solutions, a significant decrease of Hill coefficient values was obtained with increasing Na(+) concentrations. Examination of the allosteric effects of Na(+) on heterologously overexpressed NOP receptors showed that the tonic Ca(2+) current inhibition was abolished in the presence of the monovalent cation. These results demonstrate that constitutively active NOP receptors exhibit differential blocker pharmacology and allosteric regulation by Na(+). Data are also presented demonstrating that heterologously expressed mu opioid receptors in sympathetic neurons are similarly modulated.

Molecular reconstruction of mGluR5a-mediated endocannabinoid signaling cascade in single rat sympathetic neurons

Endocannabinoids (eCB) such as 2-arachidonylglycerol (2-AG) are lipid metabolites that are synthesized in a postsynaptic neurons and act upon CB(1) cannabinoid receptors (CB(1)R) in presynaptic nerve terminals. This retrograde transmission underlies several forms of short and long term synaptic plasticity within the CNS. Here, we constructed a model system based on isolated rat sympathetic neurons, in which an eCB signaling cascade could be studied in a reduced, spatially compact, and genetically malleable system. We constructed a complete eCB production/mobilization pathway by sequential addition of molecular components. Heterologous expression of four components was required for eCB production and detection: metabotropic glutamate receptor 5a (mGluR5a), Homer 2b, diacylglycerol lipase alpha, and CB(1)R. In these neurons, application of l-glutamate produced voltage-dependent modulation of N-type Ca(2+) channels mediated by activation of CB(1)R. Using both molecular dissection and pharmacological agents, we provide evidence that activation of mGluR5a results in rapid enzymatic production of 2-AG followed by activation of CB(1)R. These experiments define the critical elements required to recapitulate retrograde eCB production and signaling in a single peripheral neuron. Moreover, production/mobilization of eCB can be detected on a physiologically relevant time scale using electrophysiological techniques. The system provides a platform for testing candidate molecules underlying facilitation of eCB transport across the plasma membrane.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).