Neurotrophin-4 is required for tolerance to morphine in the mouse

Microglia P2X4R-BDNF signalling contributes to central sensitization in a recurrent nitroglycerin-induced chronic migraine model

Background

According to our previous study, microglia P2X4 receptors (P2X4Rs) play a pivotal role in the central sensitization of chronic migraine (CM). However, the molecular mechanism that underlies the crosstalk between microglia P2X4Rs and neurons of the trigeminal nucleus caudalis (TNC) is not fully understood. Therefore, the aim of this study is to examine the exact P2X4Rs signalling pathway in the development of central sensitization in a CM animal model.

Methods

We used an animal model with recurrent intermittent administration of nitroglycerin (NTG), which closely mimics CM. NTG-induced basal mechanical and thermal hypersensitivity were evaluated using a von Frey filament test and an increasing-temperature hot plate apparatus (IITC). We detected P2X4Rs, brain-derived neurotrophic factor (BDNF) and phosphorylated p38 mitogen-activated protein kinase (p-p38-MAPK) expression profiles in the TNC. We investigated the effects of a P2X4R inhibitor (5-BDBD) and an agonist (IVM) on NTG-induced hyperalgesia and neurochemical changes as well as on the expression of p-p38-MAPK and BDNF. We also detected the effects of a tropomyosin-related kinase B (TrkB) inhibitor (ANA-12) on the CM animal model in vivo. Then, we evaluated the effect of 5-BDBD and SB203580 (a p38-MAPK inhibitors) on the release and synthesis of BDNF in BV2 microglia cells treated with 50 μM adenosine triphosphate (ATP).

Results

Chronic intermittent administration of NTG resulted in chronic mechanical and thermal hyperalgesia, accompanied by the upregulation of P2X4Rs and BDNF expression. 5-BDBD or ANA-12 prevented hyperalgesia induced by NTG, which was associated with a significant inhibition of the NTG-induced increase in phosphorylated extracellular regulated protein kinases (p-ERK) and calcitonin gene related peptide (CGRP) release in the TNC. Repeated administration of IVM produced sustained hyperalgesia and significantly increased the levels of p-ERK and CGRP release in the TNC. Activating P2X4Rs with ATP triggered BDNF release and increased BDNF synthesis in BV2 microglia, and these results were then reduced by 5-BDBD or SB203580.

Conclusions

Our results indicated that the P2X4R contributes to the central sensitization of CM by releasing BDNF and promoting TNC neuronal hyper-excitability. Blocking microglia P2X4R-BDNF signalling may have an effect on the prevention of migraine chronification.

Expression of brain-derived neurotrophic factors, neurotrophin-3, and neurotrophin-4 in the nucleus accumbens during heroin dependency and withdrawal

Neurotrophins, brain-derived neurotrophic factors (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4), have been implicated in the modulation of heroin dependency. This study was designed to explore the expression alterations of BDNF, NT-3, and NT-4 in the context of heroin dependence and withdrawal in the rat nucleus accumbens (NAc). Heroin dependence was induced by a progressive intraperitoneal treatment of heroin. The results showed that the expression levels of BDNF and NT-4 were significantly decreased in the NAc of rats with heroin addiction in comparison with the control group, whereas there was a significant increase in BDNF and NT-4 expressions in the groups of rats with both naloxone-induced and spontaneous withdrawal. Moreover, NT-3 expression was markedly increased in the NAc of rats with heroin addiction and spontaneous withdrawal in comparison with the control group, but decreased in the NAc of rats with naloxone-induced withdrawal. These results indicated that chronic administration of heroin results in the alterations of BDNF, NT-3, and NT-4 expressions in the rat NAc. BDNF, NT-3, and NT-4 may play a critical role in the development of heroin dependency and withdrawal.

Neurotrophic Factors and Their Potential Applications in Tissue Regeneration

Neurotrophic factors are growth factors that can nourish neurons and promote neuron survival and regeneration. They have been studied as potential drug candidates for treating neurodegenerative diseases. Since their identification, there are more and more evidences to indicate that neurotrophic factors are also expressed in non-neuronal tissues and regulate the survival, anti-inflammation, proliferation and differentiation in these tissues. This mini review summarizes the characteristics of the neurotrophic factors and their potential clinical applications in the regeneration of neuronal and non-neuronal tissues.

Mu-opioid signaling modulates biphasic expression of TrkB and IκBα genes and neurite outgrowth in differentiating and differentiated human neuroblastoma cells

Chronic opioid exposure leads to changes in gene expression (functional changes), resulting in structural changes in neural circuits that are linked to eventually behavioral changes. Little is known about the cellular and molecular mechanisms of how such changes occur. In this study, we found that mu-opioid [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) and morphine exposure led to dynamic changes in neural differentiation- and growth-associated genes, IκBα and NTRK2 (TrkB), in differentiating and differentiated human neuroblastoma SH-SY5Y cells. Chromatin immunoprecipitation-polymerase chain reaction (ChIP-PCR) analysis revealed that binding of NF-κB/p65 to the IκBα promoter in living cells was temporally altered when the cells were exposed to morphine. The changes in gene expression correlated with the changes in neurite length of the RA-differentiating and RA-differentiated neuron-like cells. Our findings for the first time showed that TrkB signaling and NF-κB/IκBα signaling temporally correlated with each other in response to single-dose and repeated mu-opioid treatment in differentiating and differentiated human neuron-like cells. The findings from this human cell study in vitro indicate that both relatively high single-dose and chronic opioid exposure may induce the structural changes in the developing human brain and the adult brain by altering the expression of neuronal differentiation- and neurite outgrowth-related genes IκBa and TrkB in vivo.

Cerebrolysin Attenuates Heat Shock Protein (HSP 72 KD) Expression in the Rat Spinal Cord Following Morphine Dependence and Withdrawal: Possible New Therapy for Pain Management

The possibility that pain perception and processing in the CNS results in cellular stress and may influence heat shock protein (HSP) expression was examined in a rat model of morphine dependence and withdrawal. Since activation of pain pathways result in exhaustion of growth factors, we examined the influence of cerebrolysin, a mixture of potent growth factors (BDNF, GDNF, NGF, CNTF etc,) on morphine induced HSP expression. Rats were administered morphine (10 mg/kg, s.c. /day) for 12 days and the spontaneous withdrawal symptoms were developed by cessation of the drug administration on day 13^th that were prominent on day 14^th and continued up to day 15^th (24 to 72 h periods). In a separate group of rats, cerebrolysin was infused intravenously (5 ml/kg) once daily from day one until day 15^th. In these animals, morphine dependence and withdrawal along with HSP immunoreactivity was examined using standard protocol. In untreated group mild HSP immunoreaction was observed during morphine tolerance, whereas massive upregulation of HSP was seen in CNS during withdrawal phase that correlated well with the withdrawal symptoms and neuronal damage. Pretreatment with cerebrolysin did not affect morphine tolerance but reduced the HSP expression during this phase. Furthermore, cerebrolysin reduced the withdrawal symptoms on day 14^th to 15^th. Taken together these observations suggest that cellular stress plays an important role in morphine induced pain pathology and exogenous supplement of growth factors, i.e. cerebrolysin attenuates HSP expression in the CNS and induce neuroprotection. This indicates a new therapeutic role of cerebrolysin in the pathophysiology of drugs of abuse, not reported earlier.

Adeno-associated virus harboring fusion gene NT4-ant-shepherdin induce cell death in human lung cancer cells

To further enhance anticancer effect of Shepherdin and overcome limitation of peptide therapy, recombinant adeno-associated virus (rAAV) was constructed with following strategies: therapeutic peptide secretory expression and adeno-associated virus gene transfer system. MTT assay and flow cytometric analysis revealed that rAAV harboring fusion gene NT4-Ant-Shepherdin significantly suppressed A549 cell growth in a time-dependent manner and induced apoptosis. In the infected A549 cells, survivin expression level decreased strongly while Caspase-3/7 activities increased significantly. These results indicated that rAAV harboring fusion gene NT4-Ant-Shepherdin may be a novel strategy in lung cancer peptide therapy.

NT4(Si)-p53(N15)-antennapedia induces cell death in a human hepatocellular carcinoma cell line

AIM: To construct the recombinant lentivirus expression plasmid, pLenti6/V5-NT4 p53(N15)-antennapedia (Ant), and study its effect on HepG2 cells.

METHODS: Plasmid pLenti6/V5-NT4 p53(N15)-Ant was constructed incorporating the following functional regions, including signal peptide sequence and pro-region of neurotrophin 4, N-terminal residues 12-26 of p53 and 17 amino acid drosophila carrier protein, Ant. Hepatocellular carcinoma (HepG2) cells were used for transfection. 3-[4,5-dimethyl-thiazol-2yl]-2,5 diphenyl tetrazolium bromide (MTT) assay, lactate dehydrogenase (LDH) release assay, transmission electron microscopy (TEM) and flow cytometric analysis (FCM) were employed to investigate the effects of LV-NT4(Si)-p53(N15)-Ant in vitro on HepG2 cells. In vivo experiment was also performed to investigate the inhibitory effect of LV-NT4(Si)-p53(N15)-Ant on tumor growth in nude mice.

RESULTS: LV-NT4(Si)-p53(N15)-Ant significantly suppressed the growth of HepG2 cells. MTT assay showed that the growth of HepG2 cells was mucj more significantly inhibited by LV-NT4(Si)-p53(N15)-Ant than by LV-EGFP. The inhibition rate for HepG2 cell growth in the two groups was 46.9% and 94.5%, respectively, 48 h after infection with LV-NT4(Si)-p53(N15)-Ant, and was 33.9% and 95.8%, respectively, 72 h after infection with LV-NT4(Si)-p53(N15)-Ant (P < 0.01). Light microscopy and TEM showed morphological changes in HepG2 cells infected with LV-NT4(Si)-p53(N15)-Ant, but no significant changes in HepG2 cells infected with LV-EGFP. Changes were observed in ultra-structure of HepG2 cells infected with LV-NT4(Si)-p53(N15)-Ant, with degraded membranes, resulting in necrosis. LDH release from HepG2 cells was analyzed at 24, 48, 72 and 96 h after infection with LV-NT4(Si)-p53(N15)-Ant and LV-EGFP, which showed that LDH release was significantly higher in LV-NT4(Si)-p53(N15)-Ant treatment group (682 IU/L) than in control group (45 IU/L, P < 0.01). The longer the time was after infection, the bigger the difference was in LDH release. FCM analysis showed that LV-NT4(Si)-p53(N15)-Ant could induce two different kinds of cell death: necrosis and apoptosis, with apoptosis being the minor type and necrosis being the main type, suggesting that LV-NT4(Si)-p53(N15)-Ant exerts its anticancer effect on HepG2 cells by inducing necrosis. The in vivo study showed that LV-NT4(Si)-p53(N15)-Ant significantly inhibited tumor growth with an inhibition rate of 66.14% in terms of tumor size and weight.

CONCLUSION: LV-NT4(Si)-p53(N15)-Ant is a novel recombinant lentivirus expression plasmid and can be used in gene therapy for cancer.

Effect of KEPI (Ppp1r14c) deletion on morphine analgesia and tolerance in mice of different genetic backgrounds: when a knockout is near a relevant quantitative trait locus

We previously identified KEPI as a morphine-regulated gene using subtractive hybridization and differential display PCR. Upon phosphorylation by protein kinase C, KEPI becomes a powerful inhibitor of protein phosphatase 1. To gain insights into KEPI functions, we created KEPI knockout (KO) mice on mixed 129S6xC57BL/6 genetic backgrounds. KEPI maps onto mouse chromosome 10 close to the locus that contains the mu-opioid receptor (Oprm1) and provides a major quantitative trait locus for morphine effects. Analysis of single nucleotide polymorphisms in and near the Oprm1 locus identified a doubly-recombinant mouse with C57BL/6 markers within 1 Mb on either side of the KEPI deletion. This strategy minimized the amount of 129S6 DNA surrounding the transgene and documented the C57BL/6 origin of the Oprm1 gene in this founder and its offspring. Recombinant KEPIKO mice displayed (a) normal analgesic responses and normal locomotion after initial morphine treatments, (b) accelerated development of tolerance to analgesic effects of morphine, (c) elevated activity of protein phosphatase 1 in thalamus, (d) attenuated morphine reward as assessed by conditioned place preference. These data support roles for KEPI action in adaptive responses to repeated administration of morphine that include analgesic tolerance and drug reward.

Attenuation of opioid analgesic tolerance in p75 neurotrophin receptor null mutant mice

Repeated exposure to opioid drugs can lead to the development of tolerance, which manifests as a reduction in analgesic potency, and physical dependence, a response indicated by a withdrawal syndrome. Accumulating evidence suggests that the nerve growth factor (NGF) family of neurotrophins may have an important modulatory role in the induction of opioid analgesia and opioid addiction. Because neurotrophins universally bind the p75 neurotrophin receptor (p75NTR), we investigated whether the activity of this receptor is involved in the development of opioid analgesic tolerance and physical dependence. We found that in both the wild-type and p75NTR-/- mice an acute systemic (i.p.) injection of morphine produced a maximal analgesic response as measured by the thermal tail-immersion test. Repeated injection of morphine over 5 days in wild-type mice resulted in a progressive decline of the analgesic effect and a concomitant loss of the agonist potency, reflecting development of morphine tolerance. However, the loss of morphine analgesia was not observed in p75NTR-/- mice. In the second part of this study, mice were given escalating doses of systemic (i.p.) morphine over 5 days and subsequently challenged with the opioid receptor antagonist naloxone. This challenge precipitated a robust withdrawal syndrome that was comparable in wild-type mice and p75NTR-/- mice. The findings suggest that p75NTR activity plays a critical role in the development of opioid analgesic tolerance but not in the induction or the expression of opioid physical dependence.

Characterizing intercellular signaling peptides in drug addiction

Intercellular signaling peptides (SPs) coordinate the activity of cells and influence organism behavior. SPs, a chemically and structurally diverse group of compounds responsible for transferring information between neurons, are broadly involved in neural plasticity, learning and memory, as well as in drug addiction phenomena. Historically, SP discovery and characterization has tracked advances in measurement capabilities. Today, a suite of analytical technologies is available to investigate individual SPs, as well as entire intercellular signaling complements, in samples ranging from individual cells to entire organisms. Immunochemistry and in situ hybridization are commonly used for following preselected SPs. Discovery-type investigations targeting the transcriptome and proteome are accomplished using high-throughput characterization technologies such as microarrays and mass spectrometry. By integrating directed approaches with discovery approaches, multiplatform studies fill critical gaps in our knowledge of drug-induced alterations in intercellular signaling. Throughout the past 35 years, the National Institute on Drug Abuse has made significant resources available to scientists that study the mechanisms of drug addiction. The roles of SPs in the addiction process are highlighted, as are the analytical approaches used to detect and characterize them.

Counteracting effect of papaverine on morphine inhibition of gastrointestinal transit in mice

Oral papaverine has been shown to be capable of antagonizing the constipation induced by a single dose of oral morphine. The primary aim of the present study was to ascertain whether papaverine is also capable of counteracting morphine-induced decrease of upper gastrointestinal transit (UGT) after repeated parenteral administration of the opioid. We next investigated the mechanisms(s) responsible for the counteracting effect of papaverine, by analysing whether this effect was changed by pretreatment with N(G)-nitro-L-arginine methyl ester (L-NAME), dexamethasone, indomethacin or capsaicin. Papaverine, co-administered with morphine, counteracted the morphine-induced decrease in UGT in mice pretreated with morphine for 3 days but did not do so in naive animals. The counteracting effect of papaverine was antagonized by L-NAME, but not by indomethacin. In mice pretreated with both morphine and dexamethasone, papaverine failed to antagonize the effect of morphine. Capsaicin pretreatment completely abolished the effect of a single dose of morphine, the effect being partially restored by the 3 days pretreatment with morphine. In mice pretreated with both capsaicin and morphine, the UGT decrease elicited by morphine was lower than in the other experimental groups and was not modified by papaverine. Our results show that papaverine can counteract the morphine inhibition of UGT in mice repeatedly exposed to the opioid. Papaverine exerts its action through a nitric oxide synthase-mediated mechanism; this mechanism is only effective after repeated morphine administration and does not operate when capsaicin-sensitive afferent neurones are ablated.

Morphine-induced analgesic tolerance, locomotor sensitization and physical dependence do not require modification of mu opioid receptor, cdk5 and adenylate cyclase activity

Acute morphine administration produces analgesia and reward, but prolonged use may lead to analgesic tolerance in patients chronically treated for pain and to compulsive intake in opioid addicts. Moreover, long-term exposure may induce physical dependence, manifested as somatic withdrawal symptoms in the absence of the drug. We set up three behavioral paradigms to model these adaptations in mice, using distinct regimens of repeated morphine injections to induce either analgesic tolerance, locomotor sensitization or physical dependence. Interestingly, mice tolerant to analgesia were not sensitized to hyperlocomotion, whereas sensitized mice displayed some analgesic tolerance. We then examined candidate molecular modifications that could underlie the development of each behavioral adaptation. First, analgesic tolerance was not accompanied by mu opioid receptor desensitization in the periaqueductal gray. Second, cdk5 and p35 protein levels were unchanged in caudate-putamen, nucleus accumbens and prefrontal cortex of mice displaying locomotor sensitization. Finally, naloxone-precipitated morphine withdrawal did not enhance basal or forskolin-stimulated adenylate cyclase activity in nucleus accumbens, prefrontal cortex, amygdala, bed nucleus of stria terminalis or periaqueductal gray. Therefore, the expression of behavioral adaptations to chronic morphine treatment was not associated with the regulation of micro opioid receptor, cdk5 or adenylate cyclase activity in relevant brain areas. Although we cannot exclude that these modifications were not detected under our experimental conditions, another hypothesis is that alternative molecular mechanisms, yet to be discovered, underlie analgesic tolerance, locomotor sensitization and physical dependence induced by chronic morphine administration.

Specific alterations of tyrosine hydroxylase immunopositive cells in the retina of NT-4 knock out mice

To assess the effect of NT-4 deprivation on maturation of retinal circuitry, we investigated a mouse with targeted deletion of the gene encoding nt-4 (nt-4(-/-)). In particular, we studied neurons immunostained by an antibody recognizing tyrosine hydroxylase (TH), the rate limiting enzyme for dopamine (DA) synthesis. We found that TH immunopositive processes were altered in the retina of nt-4(-/-). Alteration of TH immunopositive processes in nt-4(-/-) mice resulted in changes of DA turnover, as assessed by high-pressure liquid chromatography measurements. These findings suggest that retinal NT-4 plays a role in the morphological maturation of dopaminergic retinal cells.

Pathophysiological mechanisms for actions of the neurotrophins

Neurotrophins provide trophic and tropic support for different neuronal subpopulations in the developing and adult nervous systems. Expression of the neurotrophins and their receptors can be altered in several different disease or injury states that impact upon the functions in the central and peripheral nervous systems. The intracellular signals used by the neurotrophins are triggered by ligand binding to the cell surface Trk and p75NTR receptors. In general, signals emanating from Trk receptors support survival, growth and synaptic strengthening, while those emanating from p75NTR induce apoptosis, attenuate growth and weaken synaptic signaling. Mature neurotrophins are the preferred ligand for Trk proteins while p75NTR binds preferentially to the proneurotrophins and serves as a signaling component of the receptor complex for growth inhibitory molecules of central nervous system myelin [ie, myelin-associated glycoprotein (MAG), oligodendrocyte-myelin glycoprotein (OMgP) and Nogo]. The functional antagonism between Trk and p75NTR signaling may significantly impact the pathogenesis of human neurodevelopmental and neurodegenerative diseases and further complicate therapeutic uses of exogenous neurotrophins. The potential for each is discussed in this review.

Additive Effect of Dextromethorphan on the Inhibitory Effect of Anti-NT4 on Morphine Tolerance

It has been proposed that opioid tolerance is a model of neuronal plasticity similar to learning and memory. Recent evidence suggests that neurotrophins may be involved in synaptic development and plasticity. Observations indicate that neurotrophin 4 (NT4) is required for the synaptic plasticity mediating both tolerance and memory. Also there are lines of evidence to indicate that NMDA receptors are involved in the neural plasticity underlying the development of opiate tolerance. Neurotrophins affect central transmission postsynaptically by enhancing NMDA receptor responsiveness. So we used the clinically available NMDA receptor antagonist, dextromethorphan, and the neurotrophin 4 antibody, anti-NT4, concomitantly and alone to investigate their effects on morphine tolerance. Tolerance was induced by injecting morphine (7 and 10 mg/kg i.p.) once per day for 4 days. Anti-NT4 (1 microg/rat i.c.v.) was administered 15 min before morphine. Results showed that chronic concomitant treatment of anti-NT4 with morphine in both doses inhibited the development of morphine tolerance. Also acute treatment of anti-NT4 significantly reversed the tolerance that was induced by morphine 7 mg/kg but failed to reverse the tolerance of morphine 10 mg/kg. Dextromethorphan in both doses (10 or 30 mg/kg) has an additive effect on the inhibitory effect of anti-NT4 on the reversal of morphine tolerance (7 mg/kg). These findings provide additional support for the hypothesis that NMDA receptor and NT4 may be involved in neural plasticity underlying opiate tolerance.

Neurotrophins: mediators and modulators of pain

The neurotrophin family of neurotrophic factors are well-known for their effects on neuronal survival and growth. Over the past decade, considerable evidence has accumulated from both humans and animals that one neurotrophin, nerve growth factor (NGF), is a peripheral pain mediator, particularly in inflammatory pain states. NGF is upregulated in a wide variety of inflammatory conditions, and NGF-neutralizing molecules are effective analgesic agents in many models of persistent pain. Such molecules are now being evaluated in clinical trials. NGF regulates the expression of a second neurotrophin, brain-derived neurotrophic factor (BDNF), in nociceptors. BDNF is released when nociceptors are activated, and it acts as a central modulator of pain. The chapter reviews the evidence for these roles (and briefly the effects of other neurotrophins), the range of conditions under which they act, and their mechanism of action.

Sustained activation of phosphatidylinositol 3-kinase/Akt/nuclear factor kappaB signaling mediates G protein-coupled delta-opioid receptor gene expression

Expression of the delta-opioid receptor gene (dor) is tightly controlled during neuronal differentiation and developmental stages. Such distinct temporal and spatial expression of dor during development suggests a role for the delta-opioid receptor in early developmental events. However, little is known about intracellular signaling pathways that control dor expression. A well established cell line model for the study of gene expression during neuronal differentiation is the rat adrenal pheochromocytoma PC12 cell line. Here we found that the constitutively activated TrkA/phosphatidylinositol 3-kinase/Akt (protein kinase B)/NF-kappaB survival cascade mediates dor expression during nerve growth factor (NGF)-induced differentiation of PC12h cells. Biochemical experiments showed that constitutive phosphorylation of Akt and IkappaBalpha correlates with NGF-induced dor expression. Overexpression of the transcriptional activator NF-kappaB/p65 increased dor promoter activity. Overexpression of the NF-kappaB signaling super inhibitor mutant IkappaBalpha (S32A/S36A) abolished the effect of p65 and blocked NGF-induced activation of NF-kappaB signaling, resulting in a significant reduction in dor promoter activity. Treatment with SN50, an NF-kappaB-specific nuclear translocation peptide inhibitor, inhibited the translocation of NF-kappaB, resulting in a reduction of dor mRNA. The gel shift assay supported the fact that there exists an NF-kappaB-binding site on the dor promoter. RNA interference experiments using NF-kappaB/p65 small interfering RNA confirmed that NF-kappaB signaling is required for dor expression. Our findings not only provide a new mechanistic explanation for NGF-induced dor expression but also shed some light on the molecular mechanism of the temporal and spatial expression of dor and the roles of the delta-opioid receptor during neuronal differentiation.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 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).

Mu opioid receptor: a gateway to drug addiction

Mu opioid receptors mediate positive reinforcement following direct (morphine) or indirect (alcohol, cannabinoids, nicotine) activation, and our understanding of mu receptor function is central to the development of addiction therapies. Recent data obtained in native neurons confirm that mu receptor signaling and regulation are strongly agonist-dependent. Current functional mapping reveals morphine-activated neurons in the extended amygdala and early genomic approaches have identified novel mu receptor-associated proteins. A classification of about 30 genes either promoting or counteracting the addictive properties of morphine is proposed from the analysis of knockout mice data. The targeting of effectors or regulatory proteins, beyond the mu receptor itself, might provide valuable strategies to treat addictive disorders.