Activation Of Mu-opioid Receptors Research Articles

Changes in extrasynaptic GABAA receptors in the periaqueductal gray with morphine tolerance (659.7)

Mu‐opioid receptors (MOPrs) reduce GABA‐mediated inhibition of periaqueductal gray (PAG) output neurons to activate the descending pain modulatory system and induce antinociception. The role of extrasynaptic GABAA receptors and tonic inhibition of PAG neurons in antinociception has not been explored. Rats were treated twice a day with saline or morphine (5 mg/kg, sc.) for 3 days. Superfusion of the GABAA antagonist, bicuculline (10 µM), inhibited miniature inhibitory postsynaptic currents and blocked a resting current, indicating the presence of both synaptic and extrasynaptic GABAA‐mediated currents in the PAG. Bicuculline reduced the holding current in saline rats 18 ± 4 pA (n = 10), with less effect in morphine rats (2 ± 1 pA, n = 6, p < 0.05). The bicuculline currents reversed at the expected chloride potential in chloride (‐24 ± 3 mV) and gluconate (‐84 ± 2 mV) intracellular solutions, suggesting the current is mediated by GABAA receptors. Tonic currents are often mediated by extrasynaptic GABAA receptors containing the delta‐subunit. THIP (10 µM), a GABAA delta subunit receptor agonist, induced currents in naïve and morphine treated slices (84 ± 22 pA, n= 8; 77 ± 24 pA, n = 6). The THIP induced currents were reduced by prior administration of met‐enkephalin (10 µM) (30 ± 7 pA, n = 5) compared to control (74 ± 16 pA, n = 7 , p < 0.05). These results suggest GABAA delta receptors may be modulated by activation of MOPrs in the PAG.Grant Funding Source: DA027625

Stress-induced activation of ventral tegmental mu-opioid receptors reduces accumbens dopamine tone by enhancing dopamine transmission in the medial pre-frontal cortex.

Endogenous opioids could play a major role in the mesocorticolimbic dopamine (DA) responses to stress challenge. However, there is still no direct evidence of an influence of endogenous opioids on any of these responses. We assessed whether and how endogenous opioids modulate fluctuations of mesocortical and mesoaccumbens DA tone in rats during a first experience with restraint stress. We first evaluated the effects of systemic naltrexone (NTRX) on DA outflow in the medial prefrontal cortex (mpFC) and in the nucleus accumbens (NAc) through dual-probe microdialysis. Second, we assessed the effect of perfusion, through reverse microdialysis, of direct DA receptor agonists in mpFC on NAc DA outflow in NTRX-pretreated stressed rats. Finally, we tested the effects of ventral tegmental area (VTA) perfusion of NTRX, the selective mu1 antagonist naloxonazine and the selective delta antagonist naltrindole on mpFC and NAc DA outflow in stressed rats, with multiple probe experiments. Systemic NTRX, at behaviorally effective doses, selectively prevented the increase of mpFC DA levels and the reduction of NAc DA levels observable during prolonged restraint. Local co-perfusion of D1 and D2 agonists in mpFC recovered inhibition of NAc DA in NTRX-pretreated restrained rats. Finally, intra-VTA perfusion of either NTRX or the mu1 antagonist, but not the delta antagonist, mimicked the effects of systemic NTRX. During prolonged experience with a novel unavoidable/uncontrollable stressor, endogenous opioids, through stimulation of mu1 receptors in the VTA, elevate mesocortical DA tone thus reducing DA tone in the NAc DA.

GABAergic neurons in the ventral tegmental area receive dual GABA/enkephalin‐mediated inhibitory inputs from the bed nucleus of the stria terminalis

Activation of mu-opioid receptor (MOR) disinhibits dopaminergic neurons in the ventral tegmental area (VTA) through inhibition of γ-aminobutyric acid (GABA)ergic neurons. This mechanism is thought to play a pivotal role in mediating reward behaviors. Here, we characterised VTA-projecting enkephalinergic neurons in the anterior division of the bed nucleus of the stria terminalis (BST) and investigated their targets by examining MOR expression in the VTA. In the BST, neurons expressing preproenkephalin mRNA were exclusively GABAergic, and constituted 37.2% of the total GABAergic neurons. Using retrograde tracer injected into the VTA, 21.6% of VTA-projecting BST neurons were shown to express preproenkephalin mRNA. Enkephalinergic projections from the BST exclusively formed symmetrical synapses onto the dendrites of VTA neurons. In the VTA, 74.1% of MOR mRNA-expressing neurons were GABAergic, with the rest being glutamatergic neurons expressing type-2 vesicular glutamate transporter mRNA. However, MOR mRNA was below the detection threshold in dopaminergic neurons. By immunohistochemistry, MOR was highly expressed on the extrasynaptic membranes of dendrites in GABAergic VTA neurons, including dendrites innervated by BST-VTA projection terminals. MOR was also expressed weakly on GABAergic and glutamatergic terminals in the VTA. Given that GABAA α1 is expressed at GABAergic BST-VTA synapses on dendrites of GABAergic neurons [T. Kudo etal. (2012) J. Neurosci., 32, 18035-18046], our results collectively indicate that the BST sends dual inhibitory outputs targeting GABAergic VTA neurons; GABAergic inhibition via 'wired' transmission, and enkephalinergic inhibition via 'volume' transmission. This dual inhibitory system provides the neural substrate underlying the potent disinhibitory control over dopaminergic VTA neurons exerted by the BST.

Opioid Receptor Gene Expression in Human Neuroblastoma SH-SY5Y Cells Following Tapentadol Exposure

Recent studies showed that combination of mu opioid receptor (MOP) agonism and monoamine reuptake inhibition may improve the therapeutic effect of opioids by reducing requirement for MOP activation. Tapentadol, showing such a combined mechanism of action, exhibits delayed analgesic tolerance development compared to pure MOP agonists. Here we investigated how opioid receptors are regulated following different schedules (two ranges of concentrations for 24 and 48 h) of tapentadol exposure in vitro in SH-SY5Y cells. MOP and nociceptin/orphaninFQ (NOP) receptor gene expressions were quantified using qReal-Time PCR. Moreover, studies were performed in U2 cells to assess tapentadol effect on MOP internalization compared with morphine and DAMGO. Ten and 100 nM tapentadol for 48 h induced a significant increase of MOP gene expression; cells exposed to 100 μM tapentadol for 24 and 48 h showed a significant increase of MOP mRNA levels. NOP gene expression showed a significant decrease following tapentadol at all low concentrations used after 24 h and at high concentrations (45 and 60 μM) after 24 h and (60 μM) after 48 h. Differently from DAMGO, tapentadol or morphine showed no effects on MOP internalization. This study suggests that tapentadol affects MOP and NOP gene expression and MOP internalization showing a pattern distinct from classical MOP agonists. Whether these differences can explain the improved therapeutic profile of tapentadol remains to be investigated.

Losing touch with opioids: New insights into a chemokine signaling cascade controlling morphine analgesia

The nigrostriatal dopamine system is implicated in the regulation of reward and motor activity. Dopamine (DA) release in dorsal striatum (DS) is controlled by the firing rate of DA neurons in substantia nigra pars compacta. However, influences at terminal level, such as those involving activation of mu opioid receptors (MORs), can play a key role in determining DA levels in striatum. Nonetheless, published data also suggest that the effect of opioid drugs on DA levels may differ depending on the DS subregion analyzed. In this study, in vivo microdialysis in rats was used to explore this regional dependence. Changes in basal DA levels induced by local retrodialysis application of DAMGO (selective MORs agonist) in three different subregions of DS along the rostro-caudal axis were studied. Our results indicate that whereas administration of 10 μM DAMGO into the rostral and caudal DS significantly reduced DA levels, in medial DS an increase in DA levels was observed. These data reveal a regional-dependent MOR modulation of DA release in DS, similar to that described in the ventral striatum. Our findings may lead to a better understanding of the nigrostriatal DA system regulation.

β-arrestin-2-biased agonism of delta opioid receptors sensitizes transient receptor potential vanilloid type 1 (TRPV1) in primary sensory neurons.

Despite advances in understanding the signaling mechanisms involved in the development and maintenance of chronic pain, the pharmacologic treatment of chronic pain has seen little advancement. Agonists at the mu opioid receptor (MOPr) continue to be vital in the treatment of many forms of chronic pain, but side-effects limit their clinical utility and range from relatively mild, such as constipation, to major, such as addiction and dependence. Additionally, chronic activation of MOPr results in pain hypersensitivity known as opioid-induced hyperalgesia (OIH), and we have shown recently that recruitment of β-arrestin2 to MOPr, away from transient potential vanilloid eceptor type 1 (TRPV1) in primary sensory neurons contributes to this phenomenon. The delta opioid receptor (DOPr) has become a promising target for the treatment of chronic pain, but little is known about the effects of chronic activation of DOPr on nociceptor sensitivity and OIH. Here we report that chronic activation of DOPr by the DOPr-selective agonist, SNC80, results in the sensitization of TRPV1 and behavioral signs of OIH via β-arrestin2 recruitment to DOPr and away from TRPV1. Conversely, chronic treatment with ARM390, a DOPr-selective agonist that does not recruit β-arrestin2, neither sensitized TRPV1 nor produced OIH. Interestingly, the effect of SNC80 to sensitize TRPV1 is species-dependent, as rats developed OIH but mice did not. Taken together, the reported data identify a novel side-effect of chronic administration of β-arrestin2-biased DOPr agonists and highlight the importance of potential species-specific effects of DOPr agonists.

Dopamine D<sub>1</sub>- and D<sub>2</sub>-Receptors in Immunostimulation under Activation of Mu-Opioid Receptors in Mice with Different Psychoemotional States

The purpose of the present study was to analyze the effect of activation of mu-opioid receptors (mu-OR) on the immune response under blockade of postsynaptic D1-and D2-receptors in mice of the C57BL/6J strain displaying either aggressive or depressive-like behaviors in the social conflict model. It is shown that activation of activation of mu-OR with a highly selective agonist DAGO (100 μg/kg) increased significantly IgM-immune response not only in C57BL/6J mice with an unchanged psychoemotional state but also in mice displaying aggressive or depressive-like behaviors in the social stress model (10 days of agonistic confrontations). Selective blockade of DA receptors of the D1-type with SCH-23390 (1.0 mg/kg with DAGO administration) caused a more pronounced elevation of IgM-immune response than DAGO alone while DAGO effect was completely blocked by prior administration of D2-receptor antagonist haloperidol (1.0 mg/kg). At the same time, both SCH-23390 and haloperidol prevented the immune response increase induced by DAGO injection in mice engaged in aggressive or depressive-like behaviors. Thus, in animals not subjected to social stress DAGO-induced immunostimulation is provided only by D2-receptors, whereas in animals with altered psychoemotional state mu-opioid immunostimulation is mediated by both types of DA receptors—D1 and D2. These data provide evidence for different impacts of the main subtypes of DA receptors in the mediation of immunomodulating effects of mu-opioid system under normal and stressful conditions.

Health Care Costs Must Come Down

The endogenous opioid system plays a crucial role in stress-induced analgesia. Mu-opioid receptors (MORs), one of the major opioid receptors, are expressed widely in subpopulations of cells throughout the CNS. However, the potential roles of MORs expressed in glutamatergic (MOR~Glut~) and γ-aminobutyric acidergic (MOR~GABA~) neurons in stress-induced analgesia remain unclear. By examining tail-flick latencies to noxious radiant heat of male mice, here we investigated the contributions of MOR~GABA~ and MOR~Glut~ to behavioral analgesia and activities of neurons projecting from periaqueductal gray (PAG) to rostral ventromedial medulla (RVM) induced by a range of time courses of forced swim exposure. The moderate but not transitory or prolonged swim exposure induced a MOR-dependent analgesia, although all of these three stresses enhanced β-endorphin release. Selective deletion of MOR~GABA~ but not MOR~Glut~ clearly attenuated analgesia and blocked the enhancement of activities of PAG-RVM neurons induced by moderate swim exposure. Under transitory swim exposure, in contrast, selective deletion of MOR~Glut~ elicited an analgesia behavior via strengthening the activities of PAG-RVM neurons. These results indicate that MOR-dependent endogenous opioid signaling participates in nociceptive modulation in a wide range, not limited to moderate, of stress intensities. Endogenous activation of MOR~GABA~ exerts analgesia, whereas MOR~Glut~ produces antianalgesia. More importantly, with an increase of stress intensities, the efficiencies of MORs on nociception shifts from balance between MOR~Glut~ and MOR~GABA~ to biasing toward MOR~GABA~-mediated processes. Our results point to the cellular dynamic characteristics of MORs expressed in excitatory and inhibitory neurons in pain modulation under various stress intensities.

HINT1 protein cooperates with cannabinoid 1 receptor to negatively regulate glutamate NMDA receptor activity

BackgroundG protein-coupled receptors (GPCRs) are the targets of a large number of drugs currently in therapeutic use. Likewise, the glutamate ionotropic N-methyl-D-aspartate receptor (NMDAR) has been implicated in certain neurological disorders, such as neurodegeration, neuropathic pain and mood disorders, as well as psychosis and schizophrenia. Thus, there is now an important need to characterize the interactions between GPCRs and NMDARs. Indeed, these interactions can produce distinct effects, and whereas the activation of Mu-opioid receptor (MOR) increases the calcium fluxes associated to NMDARs, that of type 1 cannabinoid receptor (CNR1) antagonizes their permeation. Notably, a series of proteins interact with these receptors affecting their responses and interactions, and then emerge as novel therapeutic targets for the aforementioned pathologies.ResultsWe found that in the presence of GPCRs, the HINT1 protein influences the activity of NMDARs, whereby NMDAR activation was enhanced in CNR1+/+/HINT1-/- cortical neurons and the cannabinoid agonist WIN55,212-2 provided these cells with no protection against a NMDA insult. NMDAR activity was normalized in these cells by the lentiviral expression of HINT1, which also restored the neuroprotection mediated by cannabinoids. NMDAR activity was also enhanced in CNR1-/-/HINT1+/+ neurons, although this activity was dampened by the expression of GPCRs like the MOR, CNR1 or serotonin 1A (5HT1AR).ConclusionsThe HINT1 protein plays an essential role in the GPCR-NMDAR connection. In the absence of receptor activation, GPCRs collaborate with HINT1 proteins to negatively control NMDAR activity. When activated, most GPCRs release the control of HINT1 and NMDAR responsiveness is enhanced. However, cannabinoids that act through CNR1 maintain the negative control of HINT1 on NMDAR function and their protection against glutamate excitotoxic insult persists.

Novel fentanyl-based dual μ/δ-opioid agonists for the treatment of acute and chronic pain

Approximately one third of the adult U.S. population suffers from some type of on-going, chronic pain annually, and many more will have some type of acute pain associated with trauma or surgery. First-line therapies for moderate to severe pain include prescriptions for common mu opioid receptor agonists such as morphine and its various derivatives. The epidemic use, misuse and diversion of prescription opioids have highlighted just one of the adverse effects of mu opioid analgesics. Alternative approaches include novel opioids that target delta or kappa opioid receptors, or compounds that interact with two or more of the opioid receptors. AimsHere we report the pharmacology of a newly synthesized bifunctional opioid agonist (RV-Jim-C3) derived from combined structures of fentanyl and enkephalin in rodents. RV-Jim-C3 has high affinity binding to both mu and delta opioid receptors. Main methodsMice and rats were used to test RV-Jim-C3 in a tailflick test with and without opioid selective antagonist for antinociception. RV-Jim-C3 was tested for anti-inflammatory and antihypersensitivity effects in a model of formalin-induced flinching and spinal nerve ligation. To rule out motor impairment, rotarod was tested in rats. Key findingsRV-Jim-C3 demonstrates potent-efficacious activity in several in vivo pain models including inflammatory pain, antihyperalgesia and antiallodynic with no significant motor impairment. SignificanceThis is the first report of a fentanyl-based structure with delta and mu opioid receptor activity that exhibits outstanding antinociceptive efficacy in neuropathic pain, reducing the propensity of unwanted side effects driven by current therapies that are unifunctional mu opioid agonists.

Spinal Endomorphin 2 Antinociception and the Mechanisms That Produce It Are Both Sex- and Stage of Estrus Cycle–Dependent in Rats

Endomorphin 2 (EM2) is the predominant endogenous mu-opioid receptor (MOR) ligand in the spinal cord. Given its endogenous presence, antinociceptive responsiveness to the intrathecal application of EM2 most likely reflects its ability to modulate nociception when released in situ. In order to explore the physiological pliability of sex-dependent differences in spinal MOR-mediated antinociception, we investigated the antinociception produced by intrathecal EM2 in male, proestrus female, and diestrus female rats. Antinociception was reflected by changes in tail flick latency to radiant heat. In females, the spinal EM2 antinociceptive system oscillated between analgesically active and inactive states. During diestrus, when circulating estrogens are low, spinal EM2 antinociceptive responsiveness was minimal. In contrast, during proestrus, when circulating estrogens are high, spinal EM2 antinociception was robust and comparable in magnitude to that manifest by males. Furthermore, in proestrus females, spinal EM2 antinociception required spinal dynorphin and kappa-opioid receptor activation, concomitant with MOR activation. This is required for neither spinal EM2 antinociception in males nor the antinociception elicited in proestrus females by spinal sufentanil or [d-Ala2,N-methyl-Phe4,Gly-ol5]-enkephalin, which are prototypic MOR-selective nonpeptide and peptide agonists, respectively. These results reveal that spinal EM2 antinociception and the signaling mechanisms used to produce it fundamentally differ in males and females. PerspectiveThe inability to mount spinal EM2 antinociception during defined stages of the estrus (and presumably menstrual) cycle and impaired transition from spinal EM2 analgesically nonresponsive to responsive physiological states could be causally associated with the well-documented greater severity and frequency of chronic intractable pain syndromes in women vs men.

AGH is a new hemoglobin alpha-chain fragment with antinociceptive activity

Limited proteolysis of certain proteins leads to the release of endogenous bioactive peptides. Hemoglobin-derived peptides such as hemorphins and hemopressins are examples of intracellular protein-derived peptides that have antinociceptive effects by modulating G-protein coupled receptors activities. In the present study, a previously characterized substrate capture assay that uses a catalytically inactive form of the thimet oligopeptidase was combined with isotopic labeling and mass spectrometry in order to identify new bioactive peptides. Indeed, we have identified the peptide AGHLDDLPGALSAL (AGH), a fragment of the hemoglobin alpha-chain, which specifically bind to the inactive thimet oligopeptidase in the substrate capture assay. Previous peptidomics studies have identified the AGH as well as many other natural peptides derived from hemoglobin alpha-chain containing this sequence, further suggesting that AGH is a natural endogenous peptide. Pharmacological assays suggest that AGH inhibits peripheral inflammatory hyperalgesic responses through indirect activation of mu opioid receptors, without having a central nervous system activity. Therefore, we have successfully used the substrate capture assay to identify a new endogenous bioactive peptide derived from hemoglobin alpha-chain.

Powerful inhibitory action of mu opioid receptors (MOR) on cholinergic interneuron excitability in the dorsal striatum

Cholinergic interneurons (ChIs) of dorsal striatum play a key role in motor control and in behavioural learning. Neuropeptides regulate cholinergic transmission and mu opioid receptor (MOR) activation modulates striatal acetylcholine release. However, the mechanisms underlying this effect are yet uncharacterized. Here, we examined the electrophysiological responses of ChIs to the selective MOR agonist, DAMGO {[D-Ala2-MePhe4-Gly(ol)5] enkephalin}. We observed a robust, dose-dependent inhibition of spontaneous firing activity (0.06–3 μM) which was reversible upon drug washout and blocked by the selective antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) (1 μM). Voltage-clamp analysis of the reversal potential of the DAMGO effect did not provide univocal results, indicating the involvement of multiple membrane conductances. The MOR-dependent effect persisted in the presence of GABAA and ionotropic glutamate receptor antagonists, ruling out an indirect effect. Additionally, it depended upon G-protein activation, as it was prevented by intrapipette GDP-β-S. Because D2 dopamine receptors (D2R) and MOR share a common post-receptor signalling pathway, occlusion experiments were performed with maximal doses of both D2R and MOR agonists. The D2R agonist quinpirole decreased spike discharge, which was further reduced by adding DAMGO. Then, D2R or MOR antagonists were used to challenge the response to the respective agonists, DAMGO or quinpirole. No cross-effect was observed, suggesting that the two receptors act independently.Our findings demonstrate a postsynaptic inhibitory modulation by MOR on ChIs excitability. Such opioidergic regulation of cholinergic transmission might contribute to shape information processing in basal ganglia circuits, and represent a potential target for pharmacological intervention.

Curvilinear relationships between mu-opioid receptor labeling and undirected song in male European starlings (Sturnus vulgaris)

Female-directed communication in male songbirds has been reasonably well studied; yet, relatively little is known about communication in other social contexts. Songbirds also produce song that is not clearly directed towards another individual (undirected song) when alone or in flocks. Although the precise functions of undirected song may differ across species, this type of song is considered important for flock maintenance, song learning or practice. Past studies show that undirected song is tightly coupled to analgesia and positive affective state, which are both mediated by opioid activity. Furthermore, labeling for the opioid met-enkephalin in the medial preoptic nucleus (POM) correlates positively with undirected song production. We propose that undirected song is facilitated and maintained by opioid receptor activity in the POM and other brain regions involved in affective state, analgesia, and social behavior. To provide insight into this hypothesis, we used immunohistochemistry to examine relationships between undirected song and mu-opioid receptors in male starlings. Polynomial regression analyses revealed significant inverted-U shaped relationships between measures of undirected song and mu-opioid receptor labeling in the POM, medial bed nucleus of the stria terminalis (BSTm), and periaqueductal gray (PAG). These results suggest that low rates of undirected song may stimulate and/or be maintained by mu-opioid receptor activity; however, it may be that sustained levels of mu-opioid receptor activity associated with high rates of undirected song cause mu-opioid receptor down-regulation. The results indicate that mu-opioid receptor activity in POM, BSTm, and PAG may underlie previous links identified between undirected song, analgesia, and affective state.

Nitric Oxide and Zinc-Mediated Protein Assemblies Involved in Mu Opioid Receptor Signaling

Opioids are among the most effective analgesics in controlling the perception of intense pain, although their continuous use decreases their potency due to the development of tolerance. The glutamate N-methyl-D-aspartate (NMDA) receptor system is currently considered to be the most relevant functional antagonist of morphine analgesia. In the postsynapse of different brain regions the C terminus of the mu-opioid receptor (MOR) associates with NR1 subunits of NMDARs, as well as with a series of signaling proteins, such as neural nitric oxide synthase (nNOS)/nitric oxide (NO), protein kinase C (PKC), calcium and calmodulin-dependent kinase II (CaMKII) and the mitogen-activated protein kinases (MAPKs). NO is implicated in redox signaling and PKC falls under the regulation of zinc metabolism, suggesting that these signaling elements might participate in the regulation of MOR activity by the NMDAR. In this review, we discuss the influence of redox signaling in the mechanisms whose plasticity triggers opioid tolerance. Thus, the MOR C terminus assembles a series of signaling proteins around the homodimeric histidine triad nucleotide-binding protein 1 (HINT1). The NMDAR NR1 subunit and the regulator of G protein signaling RGSZ2 bind HINT1 in a zinc-independent manner, with RGSZ2 associating with nNOS and regulating MOR-induced production of NO. This NO acts on the RGSZ2 zinc finger, providing the zinc ions that are required for PKC/Raf-1 cysteine-rich domains to simultaneously bind to the histidines present in the HINT1 homodimer. The MOR-induced activation of phospholipase β (PLCβ) regulates PKC, which increases the reactive oxygen species (ROS) by acting on NOX/NADPH, consolidating the long-term PKC activation required to regulate the Raf-1/MAPK cascade and enhancing NMDAR function. Thus, RGSZ2 serves as a Redox Zinc Switch that converts NO signals into Zinc signals, thereby modulating Redox Sensor Proteins like PKCγ and Raf-1. Accordingly, redox-dependent and independent processes weave together to situate the MOR under the negative control of the NMDAR.

Opioid Modulation of Ventral Pallidal Afferents to Ventral Tegmental Area Neurons

Activation of mu opioid receptors within the ventral tegmental area (VTA) can produce reward through the inhibition of GABAergic inputs. GABAergic neurons in the ventral pallidum (VP) provide a major input to VTA neurons. To determine the specific VTA neuronal targets of VP afferents and their sensitivity to mu opioid receptor agonists, we virally expressed channel rhodopsin (ChR2) in rat VP neurons and optogenetically activated their terminals in the VTA. Light activation of VP neuron terminals elicited GABAergic IPSCs in both dopamine (DA) and non-DA VTA neurons, and these IPSCs were inhibited by the mu opioid receptor agonist DAMGO. In addition, using a fluorescent retrograde marker to identify VTA-projecting VP neurons, we found them to be hyperpolarized by DAMGO. Both of these actions decrease GABAergic input onto VTA neurons, revealing two mechanisms by which endogenous or exogenous opioids can activate VTA neurons, including DA neurons.

Repeated cannabinoid administration induces ΔFosB and sensitizes mu opioid receptor activity in the nucleus accumbens

CB1 cannabinoid receptors (CB1Rs) mediate the central effects of Δ9‐tetrahydrocannabinol (THC), the main psychoactive constituent in marijuana. CB1Rs and endocannabinoids are expressed in striatum and regulate reward, motivation and motor activity. We reported that inducible transgenic expression of ΔFosB in D1/dynorphin medium spiny striatal neurons enhanced mu opioid receptor (MOR) signaling in the nucleus accumbens (NAc) and enhanced rewarding effects of morphine, without affecting MOR expression. Moreover, repeated THC administration induces the transcription factor ΔFosB in the NAc, but inducible transgenic expression of ΔFosB did not enhance CB1R signaling in this region. Here we show that repeated THC administration induced ΔFosB in both NAc and dorsal striatum. Interestingly, repeated THC administration also enhanced MOR‐mediated G‐protein activation in the NAc, whereas CB1R‐mediated G‐protein activity was not increased. Neither THC nor inducible transgenic expression of ΔFosB altered MOR‐mediated G‐protein activity in dorsal striatum. Finally, inducible transgenic expression of ΔcJun, a dominant negative inhibitor of ΔFosB‐mediated transcription, blocked THC‐induced enhancement of MOR‐mediated G‐protein activity in NAc. These results suggest that sensitization of MOR activity by repeated THC could be mediated by ΔFosB. Support: R01‐ DA014277 and R01‐DA030404 from NIDA

The Use of Bifunctional NOP/Mu and NOP Receptor Selective Compounds for the Treatment of Pain, Drug Abuse, and Psychiatric Disorders

The NOP receptor, the fourth receptor in the opioid receptor family, is found throughout the brain and is involved in a variety of CNS systems and pathways. The endogenous ligand for NOP receptors, nociceptin/orphanin FQ (now called N/OFQ), was originally thought to increase a painful stimulus since intracerebroventricular (i.c.v.) administration of this heptadecapeptide led to a decrease in tail-flick and hot-plate latency in mice. Further studies suggested that N/OFQ blocks opiate analgesia when administered i.c.v. but potentiates opiate analgesia and has antinociceptive activity when administered intrathecally. I.c.v. administration of N/OFQ has other beneficial actions including inhibition of reward induced by several different abused drugs, as well as anti-anxiety activity. Recent work has demonstrated that individual small molecules that activate both NOP and mu receptors possess mu-mediated antinociceptive activity with reduced reward, as determined by conditioned place preference tests. Furthermore, selective NOP receptor agonists appear to be active in certain chronic pain models and reduce both drug craving and anxiety. NOP receptor antagonists may also have therapeutic benefits since both peptide and small molecule antagonists have anti-depressant activity in two different animal models. Therefore, both selective NOP receptor compounds and non-selective compounds, with both NOP receptor and mu opioid receptor activity, appear to have potential for clinical use for several neurological and psychiatric disorders including acute and chronic pain, drug abuse, anxiety and depression.

Impact of A118G Polymorphism on the Mu Opioid Receptor Function in Pain

Mu Opioid Receptor (MOR) activation by exogenous or endogenous agonists causes reduction of pain threshold after a noxious stimulus, relieving pain sensation. MOR is encoded by OPRM1 gene and its messenger RNA suffers extensible modifications by alternative splicing and single nucleotide polymorphisms (SNPs). A118G (N40D) is the most frequent encoding MOR SNP in humans. In this review we discuss the impact of this polymorphism at molecular, cellular and clinical levels. Since some SNPs are unequally distributed among human populations, we also discuss the utility of A118G as an ethnicity marker among worldwide human populations. As an example, we evaluate A118G frequency in an Argentinean human population and compare it with worldwide frequencies extracted from HapMap database.

Food reward-sensitive interaction of ghrelin and opioid receptor pathways in mesolimbic dopamine system

Ghrelin is a stomach-derived orexigenic peptide. The goal of the study was to investigate the roles of mu and kappa opioid receptors in systemic ghrelin-mediated regulation of the mesolimbic dopamine system. To evaluate the interaction of systemic ghrelin with values of food reward, rats were exposed to food removal, regular food or palatable food after systemic ghrelin administration. Extracellular dopamine levels were quantified in the nucleus accumbens (NAc) and receptor-specific compounds were infused into the ventral tegmental area (VTA) using dual-probe microdialysis. Consumption of regular or palatable food without systemic ghrelin administration induced an increase in dopamine levels in the NAc via activation of mu opioid receptors in the VTA. Systemic ghrelin administration (3 nmol, i.v.) followed by no food induced a decrease in dopamine levels via activation of kappa opioid receptors in the VTA. Systemic ghrelin administration followed by consumption of regular food induced an increase in dopamine levels via preferential activation of mu opioid receptors, whereas systemic ghrelin administration followed by consumption of palatable food suppressed the increase in dopamine levels via preferential activation of kappa opioid receptors. Thus, natural food reward and systemic ghrelin activate mu and kappa opioid receptor pathways in the VTA, respectively, resulting in opposite influences on dopamine release in the NAc. Furthermore, systemic ghrelin induces switching of the dominant opioid receptor pathway for highly rewarding food from mu to kappa, resulting in suppression of the mesolimbic dopamine system. These novel findings might provide insights into the neural pathways involved in eating disorders.