Unidirectional cross-activation of GRPR by MOR1D uncouples itch and analgesia induced by opioids.

Spinal opioid-induced itch, a prevalent side effect of pain management, has been proposed to result from pain inhibition. We now report that the µ-opioid receptor (MOR) isoform MOR1D is essential for morphine-induced scratching (MIS), whereas the isoform MOR1 is required only for morphine-induced analgesia (MIA). MOR1D heterodimerizes with gastrin-releasing peptide receptor (GRPR) in the spinal cord, relaying itch information. We show that morphine triggers internalization of both GRPR and MOR1D, whereas GRP specifically triggers GRPR internalization and morphine-independent scratching. Providing potential insight into opioid-induced itch prevention, we demonstrate that molecular and pharmacologic inhibition of PLCß3 and IP3R3, downstream effectors of GRPR, specifically block MIS but not MIA. In addition, blocking MOR1D-GRPR association attenuates MIS but not MIA. Together, these data suggest that opioid-induced itch is an active process concomitant with but independent of opioid analgesia, occurring via the unidirectional cross-activation of GRPR signaling by MOR1D heterodimerization.

Mechanistic Analysis of Micro-Neurocircuits Underlying the Epileptogenic Zone in Focal Cortical Dysplasia Patients.

We aim to explore the microscopic neurophysiology of focal cortical dysplasia (FCD) induced epileptogenesis in specific macroscopic brain regions, therefore mapping a micro-macro neuronal network that potentially indicates the epileptogenic mechanism. Epileptic and relatively non-epileptic temporal neocortex specimens were resected from FCD and mesial temporal lobe epilepsy (mTLE) patients, respectively. Whole-cell patch-clamping was performed on cells from the seizure onset zone (SOZ) and non-SOZ inside the epileptogenic zone (EZ) of FCD patients, as well as the non-epileptic neocortex of mTLE patients. Microscopic data were recorded, including membrane characteristics, spontaneous synaptic activities, and evoked action potentials. Immunohistochemistry was also performed on parvalbumin-positive (PV+) interneurons. We found that SOZ interneurons exhibited abnormal neuronal expression and distribution as well as reduced overall function compared with non-SOZ and mTLE interneurons. The SOZ pyramidal cells experienced higher excitation but lower inhibition than the mTLE controls, whereas the non-SOZ pyramidal cells exhibited intermediate excitability. Action potential properties of both types of neurons also suggested more synchronized neuronal activity inside the EZ, particularly inside the SOZ. Together, our research provides evidence for a potential neurocircuit underlying SOZ epileptogenesis and non-SOZ seizure susceptibility. Further investigation of this microscopic network may promote understanding of the mechanism of FCD-induced epileptogenesis.

Kappa opioid receptor controls neural stem cell differentiation via a miR-7a/Pax6 dependent pathway.

Although the roles of opioid receptors in neurogenesis have been implicated in previous studies, the mechanism by which κ-opioid receptor (OPRK1) regulates adult neurogenesis remains elusive. We now demonstrate that two agonists of OPRK1, U50,488H and dynorphin A, inhibit adult neurogenesis by hindering neuronal differentiation of mouse hippocampal neural stem cells (NSCs), both in vitro and in vivo. This effect was blocked by nor-binaltorphimine (nor-BNI), a specific antagonist of OPRK1. By examining neurogenesis-related genes, we found that OPRK1 agonists were able to downregulate the expression of Pax6, Neurog2, and NeuroD1 in mouse hippocampal NSCs, in a way that Pax6 regulates the transcription of Neurog2 and Neurod1 by directly interacting with their promoters. Moreover, this effect of OPRK1 was accomplished by inducing expression of miR-7a, a miRNA that specifically targeted Pax6 by direct interaction with its 3'-UTR sequence, and thereby decreased the levels of Pax6, Neurog2, and NeuroD1, thus resulted in hindrance of neuronal differentiation of NSCs. Thus, by modulating Pax6/Neurog2/NeuroD1 activities via upregulation of miR-7a expression, OPRK1 agonists hinder the neuronal differentiation of NSCs and hence inhibit adult neurogenesis in mouse hippocampus.

Selective and antagonist-dependent µ-opioid receptor activation by the combination of 2-{[2-(6-chloro-3,4-dihydro-1(2H)-quinolinyl)-2-oxoethyl]sulfanyl}-5-phenyl-4,6-(1H,5H)-pyrimidinedione and naloxone/naltrexone.

We identified, via high-throughput screening using a FLIPR® calcium assay, compound 1, which incorporated a dihydroquinolinyl-2-oxoethylsulfanyl-(1H,5H)-pyrimidinedione core and activated the µ-opioid receptor (MOR) in the presence of naloxone or naltrexone. A structure-activity relationship study of the analogs of 1 led to the design of compound 21, which activated MOR in the presence of naloxone with an EC50 of 3.3 ± 0.2 µM. MOR activation by the compound 21-antagonist pair was antagonist-dependent. Compound 21 did not affect the potency of the orthosteric agonist, morphine, toward MOR, indicating that it affected the function of MOR antagonists rather than that of the agonists. Computer modeling of the compound 21-MOR-naloxone complex revealed major interactions between compound 21 and MOR, including hydrogen bonding with Ser196, π-π stacking with Tyr149, and sulfur-aromatic interaction with Trp192. This study may pave the way for developing agents capable of safe and effective MOR modulation.

Characterizing the seizure onset zone and epileptic network using EEG-fMRI in a rat seizure model.

Accurate epileptogenic zone (EZ) or seizure onset zone (SOZ) localization is crucial for epilepsy surgery optimization. Previous animal and human studies on epilepsy have reported that changes in blood oxygen level-dependent (BOLD) signals induced by epileptic events could be used as diagnostic markers for EZ or SOZ localization. Simultaneous electroencephalography and functional magnetic resonance imaging (EEG-fMRI) recording is gaining interest as a non-invasive tool for preoperative epilepsy evaluation. However, EEG-fMRI studies have reported inconsistent and ambiguous findings. Therefore, it remains unclear whether BOLD responses can be used for accurate EZ or SOZ localization. In this study, we used simultaneous EEG-fMRI recording in a rat model of 4-aminopyridine-induced acute focal seizures to assess the spatial concordance between individual BOLD responses and the SOZ. This was to determine the optimal use of simultaneous EEG-fMRI recording in the SOZ localization. We observed a high spatial consistency between BOLD responses and the SOZ. Further, dynamic BOLD responses were consistent with the regions where the seizures were propagated. These results suggested that simultaneous EEG-fMRI recording could be used as a noninvasive clinical diagnostic technique for localizing the EZ or SOZ and could be an effective tool for mapping epileptic networks.

Naloxone Facilitates Contextual Learning and Memory in a Receptor-Independent and Tet1-Dependent Manner.

Opioids, like morphine and naloxone, regulate the proliferation and neuronal differentiation of neural stem cells (NSCs) in a receptor-independent and ten-eleven translocation methylcytosine dioxygenase (TET1)-dependent manner in vitro. Whether naloxone regulates hippocampal NSCs and contextual learning in vivo in a similar manner was determined. Naloxone infusion increased the Ki67 and Doublecortin positive cells in subgranular zone of wild type mice, which suggested the increased proliferation and differentiation of hippocampal NSCs in vivo and was consistent with the in vitro functions of naloxone. In addition, naloxone infusion also facilitated the contextual learning and memory of wild type mice. To determine the contribution of µ-opioid receptor (OPRM1) and TET1 to these functions of naloxone, several types of knockout mice were used. Since Tet1-/- mice have high deficiency in contextual learning and memory, Tet1+/- mice were used instead. The abilities of naloxone to regulate NSCs and to facilitate contextual learning were significantly impaired in Tet1+/- mice. In addition, these abilities of naloxone were not affected in Oprm1-/- mice. Therefore, naloxone facilitates contextual learning and memory in a receptor-independent and Tet1-dependent manner, which provides new understanding on the receptor-independent functions of opioids.

Naloxone regulates the differentiation of neural stem cells via a receptor-independent pathway.

The abilities of opioids to activate downstream signaling pathways normally depend on the binding between opioids and their receptors. However, opioids may also function in a receptor-independent manner, especially in neural stem cells (NSCs) in which the expression of opioid receptors and endogenous opioid agonists is low. When two opioids, morphine and naloxone, were used during the early stage of NSC differentiation, increased neurogenesis was observed. However, naloxone methiodide, a membrane impenetrable analog of naloxone, did not affect the NSC differentiation. The abilities of morphine and naloxone to facilitate neurogenesis were also observed in opioid receptor-knockout NSCs. Therefore, morphine and naloxone promote neurogenesis in a receptor-independent manner at least during the early stage. In addition, the receptor-independent functions of opioids were not observed in methylcytosine dioxygenase ten-eleven translocation 1 (Tet1) knockout NSCs. When the expression of opioid receptors increased and the expression of Tet1 decreased during the late stage of NSC differentiation, morphine, but not naloxone, inhibited neurogenesis via traditional receptor-dependent and miR181a-Prox1-Notch-related pathway. In summary, the current results demonstrated the time-dependent effects of opioids during the differentiation of NSCs and provided additional insight on the complex functions of opioids.

Automatic detection of interictal ripples on scalp EEG to evaluate the effect and prognosis of ACTH therapy in patients with infantile spasms.

OBJECTIVE: We aimed to explore the feasibility of using scalp-recorded high-frequency oscillations (HFOs) to evaluate the efficacy and prognosis of adrenocorticotropic hormone (ACTH) treatment in patients with infantile spasms. METHODS: Thirty-nine children with infantile spasms were enrolled and divided into seizure-free and non-seizure-free groups after ACTH treatment. Patients who were seizure-free were further divided into relapse and non-relapse subgroups based on the observations made during a 6-month follow-up period. Scalp ripples were detected and compared during the interictal periods before and after 2 weeks of treatment. RESULTS: After ACTH treatment, the number and channels of ripples were significantly lower, whereas the percentage decrease in the number, spectral power, and channels of ripples was significantly higher in the seizure-free group than in the non-seizure-free group. In addition, the relapse subgroup showed higher number and spectral power and wider distribution of ripples than did the non-relapse subgroup. Changes in HFOs in terms of number, spectral power, and channel of ripples were closely related to the severity of epilepsy and can indicate disease susceptibility. SIGNIFICANCE: Scalp HFOs can be used as an effective biomarker to monitor the effect and evaluate the prognosis of ACTH therapy in patients with infantile spasms.

Opioid receptors: toward separation of analgesic from undesirable effects.

The use of opioid analgesics for pain has always been hampered by their many side effects; in particular, the addictive liability associated with chronic use. Recently, attempts to develop analgesic agents with reduced side effects have targeted either the putative opioid receptor splice variants or the receptor hetero-oligomers. This review discusses the potential for receptor splice variant- and the hetero-oligomer-based discovery of new opioid analgesics. We also examine an alternative approach of using receptor mutants for pain management. Finally, we discuss the role of the biased agonism observed and the recently reported opioid receptor crystal structures in guiding the future development of opioid analgesics.

Epigenetic Activation of µ-Opioid Receptor Gene via Increased Expression and Function of Mitogen- and Stress-Activated Protein Kinase 1.

Since the discovery of µ-opioid receptor (MOR) gene two decades ago, various regulatory factors have been shown to interact with the MOR promoter and modulate transcript levels. However, the majority of early transcriptional studies on MOR gene have not addressed how intracellular signaling pathways mediate extracellular modulators. In this study, we demonstrate that MOR epigenetic regulation requires multiple coordinated signals converging at the MOR promoter, involving mitogen-activated protein kinase (MAPK) activation and mitogen- and stress-activated protein kinase 1 (MSK1)-ranges of intracellular signaling pathways similar to those activated by opioid agonists. Inhibiting p38 MAPK or extracellular signal-regulated kinase (ERK) 1/2 MAPK (upstream activators of MSK1) reduced MOR expression levels; accordingly, the functional role of MSK1, but not MSK2, was demonstrated using genetic approaches. However, for maximal MSK1 effect, an open chromatin configuration was required, because in vitro CpG methylation of the MOR promoter abolished MSK1 activity. Finally, endogenous MSK1 levels concomitantly increased to regulate MOR gene expression during neuronal differentiation of P19 cells, suggesting a conserved role of this kinase in the epigenic activation of MOR in neurons. Taken together, our findings indicate that the expression of MOR gene requires the activity of intracellular signaling pathways that have been implicated in the behavioral outcomes of opioid drugs, which suggests that an autoregulatory mechanism may function in opioid systems.

Post-Transcriptional Regulation of the Human Mu-Opioid Receptor (MOR) by Morphine-Induced RNA Binding Proteins hnRNP K and PCBP1.

Expression of the mu-opioid receptor (MOR) protein is controlled by extensive transcriptional and post-transcriptional processing. MOR gene expression has previously been shown to be altered by a post-transcriptional mechanism involving the MOR mRNA untranslated region (UTR). Here, we demonstrate for the first time the role of heterogeneous nuclear ribonucleic acids (hnRNA)-binding protein (hnRNP) K and poly(C)-binding protein 1 (PCBP1) as post-transcriptional inducers in MOR gene regulation. In the absence of morphine, a significant level of MOR mRNA is sustained in its resting state and partitions in the translationally inactive polysomal fraction. Morphine stimulation activates the downstream targets hnRNP K and PCPB1 and induces partitioning of the MOR mRNA to the translationally active fraction. Using reporter and ligand binding assays, as well as RNA EMSA, we reveal potential RNP binding sites located in the 5'-untranslated region of human MOR mRNA. In addition, we also found that morphine-induced RNPs could regulate MOR expression. Our results establish the role of hnRNP K and PCPB1 in the translational control of morphine-induced MOR expression in human neuroblastoma (NMB) cells as well as cells stably expressing MOR (NMB1). J. Cell. Physiol. 232: 576-584, 2017. © 2016 Wiley Periodicals, Inc.

Mapping the naloxone binding sites on the mu-opioid receptor using cell-based photocrosslinkers.

Naloxone is an alkaloid antagonist that acts as an antidote to opioids through the mu-opioid receptor (MOR), a G protein-coupled receptor. However, its binding site on the MOR remains unknown. To investigate the binding interfaces necessary for naloxone and MOR, available structural information was combined with a cell-based photocrosslinking approach. Computer prediction revealed that four binding sites on MOR were required for naloxone binding. In addition, in the photocrosslinking approach, an amber stop codon was used to replace the sense codon of the MOR at 266 selected individual positions, in order to introduce the photoreactive amino acid p-benzoyl-l-phenylalanine (BzF) into MOR to evaluate the results of the computer analysis. The BzF-incorporated MOR mutant genes were expressed in CHO cells, in which MOR retained the ability to interact with its ligands, such as morphine, and exhibited MOR-dependent activation of ERK signaling following morphine stimulation. Notably, after treatment with tritium-labeled naloxone and exposure to UV light, we observed naloxone crosslinking with BzF replacement at hydrophobic residues and some polar/uncharged residues in the computer-predicted sites 1 and 3, indicating that these two sites in the MOR interact with naloxone. In conclusion, these results indicate that MOR has two naloxone binding sites and that the hydrophobic and polar/uncharged residues within these sites are important for naloxone binding.

1-(2,4-Dibromophenyl)-3,6,6-trimethyl-1,5,6,7-tetrahydro-4H-indazol-4-one: A Novel Opioid Receptor Agonist with Less Accompanying Gastrointestinal Dysfunction than Morphine.

BACKGROUND: The authors investigated the pharmacology and signaling pathways of the opioid receptors modulated by compound 1, 1-(2,4-dibromophenyl)-3,6,6-trimethyl-1,5,6,7-tetrahydro-4H-indazol-4-one. METHODS: In vitro studies of compound 1 were assessed by using a radioligand-binding assay (n = 3), a cyclic adenosine monophosphate assay (n = 3), a ß-arrestin assay (n = 3), an internalization assay (n = 3), and an immunohistochemistry (n = 8). In vivo studies of compound 1 were characterized using a tail-flick test (n = 5 to 6), tail-clip test (n = 7), von Frey hair test (n = 5), and charcoal meal test (n = 5). RESULTS: Compound 1 elicited robust effects in µ-opioid (mean ± SD; binding affinity: 15 ± 2 nM; cyclic adenosine monophosphate assay: 24 ± 6 nM), δ-opioid (82 ± 7 nM; 1.9 ± 0.1 µM), and κ-opioid (76 ± 9 nM; 1.4 ± 0.5 µM) receptor-expressing cells. Compound 1 acts as a full agonist of ß-arrestin-2 recruitment in µ-opioid (1.1 ± 0.3 µM) and δ-opioid (9.7 ± 1.9 µM) receptor-expressing cells. Compound 1 caused less gastrointestinal dysfunction (charcoal meal test: morphine: 82 ± 5%; compound 1: 42 ± 5%) as well as better antinociception in mechanical pain hypersensitivity (tail-clip test: morphine: 10 ± 3 s; compound 1: 19 ± 1 s) and in cancer-induced pain (von Frey hair test: morphine: 0.1 ± 0.1 g; compound 1: 0.3 ± 0.1 g) than morphine at equi-antinociceptive doses. CONCLUSIONS: Compound 1 produced antinociception with less gastrointestinal dysfunction than morphine.

Spinal or supraspinal phosphorylation deficiency at the MOR C-terminus does not affect morphine tolerance in vivo.

The development of tolerance to morphine, one of the most potent analgesics, in the management of chronic pain is a significant clinical problem and its mechanisms are poorly understood. Morphine exerts its pharmacological effects via the µ-opioid receptor (MOR). Tolerance is highly connected to G-protein-coupled receptors (GPCR) phosphorylation and desensitization increase. Because morphine desensitization previously has been shown to be MOR phosphorylation- and ß-arrestin2-independent (in contrast to agonists such as fentanyl), we examined the contribution of phosphorylation of the entire C-terminus to the development of antinociceptive tolerance to the partial (morphine) and full (fentanyl) MOR agonists in vivo. In MOR knockout (MORKO) mice, we delivered via lentivirus the genes encoding the wild-type MOR (WTMOR) or a phosphorylation-deficient MOR (Cterm(-S/T)MOR) in which all of the serine and threonine residues were mutated to alanine into the ventrolateral periaqueductal grey matter (vlPAG) or lumbar spinal cord (SC), structures that are involved in nociception. We compared the analgesic ED50 in WTMOR- and Cterm(-S/T)MOR-expressing MORKO mice before and after morphine or fentanyl tolerance was induced. Morphine acute antinociception was partially restored in WTMOR- or Cterm(-S/T)MOR-transferred MORKO mice. Fentanyl acute antinociception was observed only in MORKO mice with the transgenes expressed in the SC. Morphine antinociceptive tolerance was not affected by expressing Cterm(-S/T)MOR in the vlPAG or SC of MORKO mice. Fentanyl-induced tolerance in MORKO mice expressing WTMOR or Cterm(-S/T)MOR, is greater than morphine-induced tolerance. Thus, MOR C-terminus phosphorylation does not appear to be critical for morphine tolerance in vivo.

Effects of addictive drugs on adult neural stem/progenitor cells.

Neural stem/progenitor cells (NSPCs) undergo a series of developmental processes before giving rise to newborn neurons, astrocytes and oligodendrocytes in adult neurogenesis. During the past decade, the role of NSPCs has been highlighted by studies on adult neurogenesis modulated by addictive drugs. It has been proven that these drugs regulate the proliferation, differentiation and survival of adult NSPCs in different manners, which results in the varying consequences of adult neurogenesis. The effects of addictive drugs on NSPCs are exerted via a variety of different mechanisms and pathways, which interact with one another and contribute to the complexity of NSPC regulation. Here, we review the effects of different addictive drugs on NSPCs, and the related experimental methods and paradigms. We also discuss the current understanding of major signaling molecules, especially the putative common mechanisms, underlying such effects. Finally, we review the future directions of research in this area.

Morphine Promotes Astrocyte-Preferential Differentiation of Mouse Hippocampal Progenitor Cells via PKCε-Dependent ERK Activation and TRBP Phosphorylation.

Previously we have shown that morphine regulates adult neurogenesis by modulating miR-181a maturation and subsequent hippocampal neural progenitor cell (NPC) lineages. Using NPCs cultured from PKCε or ß-arrestin2 knockout mice and the MAPK/ERK kinase inhibitor U0126, we demonstrate that regulation of NPC differentiation via the miR-181a/Prox1/Notch1 pathway exhibits ligand-dependent selectivity. In NPCs, morphine and fentanyl activate ERK via the PKCε- and ß-arrestin-dependent pathways, respectively. After fentanyl exposure, the activated phospho-ERK translocates to the nucleus. Conversely, after morphine treatment, phospho-ERK remains in the cytosol and is capable of phosphorylating TAR RNA-binding protein (TRBP), a cofactor of Dicer. This augments Dicer activity and promotes the maturation of miR-181a. Furthermore, using NPCs transfected with wild-type TRBP, SΔA, and SΔD TRBP mutants, we confirmed the crucial role of TRBP phosphorylation in Dicer activity, miR-181a maturation, and finally the morphine-induced astrocyte-preferential differentiation of NPCs. Thus, morphine modulates the lineage-specific differentiation of NPCs by PKCε-dependent ERK activation with subsequent TRBP phosphorylation and miR-181a maturation.

Activation of delta-opioid receptor contributes to the antinociceptive effect of oxycodone in mice.

Oxycodone has been used clinically for over 90 years. While it is known that it exhibits low affinity for the multiple opioid receptors, whether its pharmacological activities are due to oxycodone activation of the opioid receptor type or due to its active metabolite (oxymorphone) that exhibits high affinity for the mu-opioid receptors remains unresolved. Ross and Smith (1997) reported the antinociceptive effects of oxycodone (171nmol, i.c.v.) are induced by putative kappa-opioid receptors in SD rat while others have reported oxycodone activities are due to activation of mu- and/or delta-opioid receptors. In this study, using male mu-opioid receptor knock-out (MOR-KO) mice, we examined whether delta-opioid receptor was involved in oxycodone antinociception. Systemic subcutaneous (s.c.) administration of oxycodone (above 40mg/kg) could induce a small but significant antinociceptive effect in MOR-KO mice by the tail flick test. Delta-opioid receptor antagonist (naltrindole, 10mg/kg or 20mg/kg, i.p.) could block this effect. When oxycodone was injected directly into the brain of MOR-KO mice by intracerebroventricular (i.c.v.) route, oxycodone at doses of 50nmol or higher could induce similar level of antinociceptive responses to those observed in wild type mice at the same doses by i.c.v. Delta-opioid receptor antagonists (naltrindole at 10nmol or ICI 154,129 at 20µg) completely blocked the supraspinal antinociceptive effect of oxycodone in MOR-KO mice. Such oxycodone antinociceptive responses were probably not due to its active metabolites oxymorphone because (a) the relative low level of oxymorphone was found in the brain after systemically or centrally oxycodone injection using LC/MS/MS analysis; (b) oxymorphone at a dose that mimics the level detected in the mice brain did not show any significant antinocieption effect; (c) oxycodone exhibits equal potency as oxymorphone albeit being a partial agonist in regulating [Ca(2+)]I transients in a clonal cell line expressing high level of mu-opioid receptor. These data suggest that oxycodone by itself can activate both the mu- and delta-opioid receptors and that delta-opioid receptors may contribute to the central antinociceptive effect of oxycodone in mice.

Opioid doses required for pain management in lung cancer patients with different cholesterol levels: negative correlation between opioid doses and cholesterol levels.

BACKGROUND: Pain management has been considered as significant contributor to broad quality-of-life improvement for cancer patients. Modulating serum cholesterol levels affects analgesia abilities of opioids, important pain killer for cancer patients, in mice system. Thus the correlation between opioids usages and cholesterol levels were investigated in human patients with lung cancer. METHODS: Medical records of 282 patients were selected with following criteria, 1) signed inform consent, 2) full medical records on total serum cholesterol levels and opioid administration, 3) opioid-naïve, 4) not received/receiving cancer-related or cholesterol lowering treatment, 5) pain level at level 5-8. The patients were divided into different groups basing on their gender and cholesterol levels. Since different opioids, morphine, oxycodone, and fentanyl, were all administrated at fixed low dose initially and increased gradually only if pain was not controlled, the percentages of patients in each group who did not respond to the initial doses of opioids and required higher doses for pain management were determined and compared. RESULTS: Patients with relative low cholesterol levels have larger percentage (11 out of 28 in female and 31 out of 71 in male) to not respond to the initial dose of opioids than those with high cholesterol levels (0 out of 258 in female and 8 out of 74 in male). Similar differences were obtained when patients with different opioids were analyzed separately. After converting the doses of different opioids to equivalent doses of oxycodone, significant correlation between opioid usages and cholesterol levels was also observed. CONCLUSIONS: Therefore, more attention should be taken to those cancer patients with low cholesterol levels because they may require higher doses of opioids as pain killer.

Morphine drives internal ribosome entry site-mediated hnRNP K translation in neurons through opioid receptor-dependent signaling.

Heterogeneous nuclear ribonucleoprotein K (hnRNP K) binds to the promoter region of mu-opioid receptor (MOR) to regulate its transcriptional activity. How hnRNP K contributes to the analgesic effects of morphine, however, is largely unknown. We provide evidence that morphine increases hnRNP K protein expression via MOR activation in rat primary cortical neurons and HEK-293 cells expressing MORs, without increasing mRNA levels. Using the bicistronic reporter assay, we examined whether morphine-mediated accumulation of hnRNP K resulted from translational control. We identified potential internal ribosome entry site elements located in the 5' untranslated regions of hnRNP K transcripts that were regulated by morphine. This finding suggests that internal translation contributes to the morphine-induced accumulation of hnRNP K protein in regions of the central nervous system correlated with nociceptive and antinociceptive modulatory systems in mice. Finally, we found that down-regulation of hnRNP K mediated by siRNA attenuated morphine-induced hyperpolarization of membrane potential in AtT20 cells. Silencing hnRNP K expression in the spinal cord increased nociceptive sensitivity in wild-type mice, but not in MOR-knockout mice. Thus, our findings identify the role of translational control of hnRNP K in morphine-induced analgesia through activation of MOR.

Effect of Opioid on Adult Hippocampal Neurogenesis.

During the past decade, the study of the mechanisms and functional implications of adult neurogenesis has significantly progressed. Many studies focus on the factors that regulate proliferation and fate determination of adult neural stem/progenitor cells, including addictive drugs such as opioid. Here, we review the most recent works on opiate drugs' effect on different developmental stages of adult hippocampal neurogenesis, as well as the possible underlying mechanisms. We conclude that opiate drugs in general cause a loss of newly born neural progenitors in the subgranular zone of dentate gyrus, by either modulating proliferation or interfering with differentiation and maturation. We also discuss the consequent impact of regulation of adult neurogenesis in animal's opioid addiction behavior. We further look into the future directions in studying the convergence between the adult neurogenesis field and opioid addiction field, since the adult-born granular cells were shown to play a role in neuroplasticity and may help to reduce the vulnerability to drug craving and relapse.