Identification of candidate genes and gene networks specifically associated with analgesic tolerance to morphine

Single nucleus transcriptomic analysis of rat nucleus accumbens reveals cell type-specific patterns of gene expression associated with volitional morphine intake

Opioid exposure is known to cause transcriptomic changes in the nucleus accumbens (NAc). However, no studies to date have investigated cell type-specific transcriptomic changes associated with volitional opioid taking. Here, we use single nucleus RNA sequencing (snRNAseq) to comprehensively characterize cell type-specific alterations of the NAc transcriptome in rats self-administering morphine. One cohort of male Brown Norway rats was injected with acute morphine (10 mg/kg, i.p.) or saline. A second cohort of rats was allowed to self-administer intravenous morphine (1.0 mg/kg/infusion) for 10 consecutive days. Each morphine-experienced rat was paired with a yoked saline control rat. snRNAseq libraries were generated from NAc punches and used to identify cell type-specific gene expression changes associated with volitional morphine taking. We identified 1106 differentially expressed genes (DEGs) in the acute morphine group, compared to 2453 DEGs in the morphine self-administration group, across 27 distinct cell clusters. Importantly, we identified 1329 DEGs that were specific to morphine self-administration. DEGs were identified in novel clusters of astrocytes, oligodendrocytes, and D1R- and D2R-expressing medium spiny neurons in the NAc. Cell type-specific DEGs included Rgs9, Celf5, Oprm1, and Pde10a. Upregulation of Rgs9 and Celf5 in D2R-expressing neurons was validated by RNAscope. Approximately 85% of all oligodendrocyte DEGs, nearly all of which were associated with morphine taking, were identified in two subtypes. Bioinformatic analyses identified cell type-specific upstream regulatory mechanisms of the observed transcriptome alterations and downstream signaling pathways, including both novel and previously identified molecular pathways. These findings show that volitional morphine taking is associated with distinct cell type-specific transcriptomic changes in the rat NAc and highlight specific striatal cell populations and novel molecular substrates that could be targeted to reduce compulsive opioid taking.

Morphine analgesia in male inbred genetic diversity mice recapitulates the among-individual variance in response to morphine in humans

Morphine is a widely used analgesic, but its use in clinical precision medicine is limited by the variance in response among individuals. Although previous studies have shown that individual differences in morphine can be explained in terms of pharmacodynamics and pharmacokinetics, genetic polymorphisms also play an important role. However, the genetic basis of different sensitivity and tolerance susceptibility to morphine remains ambiguous. Using 15 strains of inbred Genetic Diversity (GD) mice, a new resource with wide genetic and phenotypic variation, we demonstrated great variance in sensitivity to morphine analgesia and susceptibility to morphine tolerance between different GD strains. Among-individual variance in response to morphine analgesia in the population can be modeled in GD mice. Two loci respectively may be associated with the among-individual variance in morphine sensitivity and tolerance, confirming the role of genetic factors in among-individual different responses to morphine. These results indicate that GD mice may be a potential tool for the identification of new biomarkers to improve the clinical administration of morphine.

Epigenetic Alterations in Prescription Opioid Misuse: New Strategies for Precision Pain Management

Prescription opioids are used for some chronic pain conditions. However, generally, long-term therapy has unwanted side effects which may trigger addiction, overdose, and eventually cause deaths. Opioid addiction and chronic pain conditions have both been associated with evidence of genetic and epigenetic alterations. Despite intense research interest, many questions about the contribution of epigenetic changes to this typology of addiction vulnerability and development remain unanswered. The aim of this review was to summarize the epigenetic modifications detected in specific tissues or brain areas and associated with opioid prescription and misuse in patients who have initiated prescribed opioid management for chronic non-cancer pain. The review considers the effects of opioid exposure on the epigenome in central and peripheral tissues in animal models and human subjects and highlights the mechanisms in which opioid epigenetics may be involved. This will improve our current understanding, provide the basis for targeted, personalized pain management, and thus balance opioid risks and benefits in managing chronic pain.

Modeling SARS-CoV-2 infection in individuals with opioid use disorder with brain organoids

The COVID-19 pandemic has aggravated a preexisting epidemic: the opioid crisis. Much literature has shown that the circumstances imposed by COVID-19, such as social distancing regulations, medical and financial instability, and increased mental health issues, have been detrimental to those with opioid use disorder (OUD). In addition, unexpected neurological sequelae in COVID-19 patients suggest that COVID-19 compromises neuroimmunity, induces hypoxia, and causes respiratory depression, provoking similar effects as those caused by opioid exposure. Combined conditions of COVID-19 and OUD could lead to exacerbated complications. With limited human in vivo options to study these complications, we suggest that iPSC-derived brain organoid models may serve as a useful platform to investigate the physiological connection between COVID-19 and OUD. This mini-review highlights the advances of brain organoids in other neuropsychiatric and infectious diseases and suggests their potential utility for investigating OUD and COVID-19, respectively.

Epigenetic Mechanisms of Opioid Addiction

Opioid use kills tens of thousands of Americans each year, devastates families and entire communities, and cripples the health care system. Exposure to opioids causes long-term changes to brain regions involved in reward processing and motivation, leading vulnerable individuals to engage in pathological drug seeking and drug taking that can remain a lifelong struggle. The persistence of these neuroadaptations is mediated in part by epigenetic remodeling of gene expression programs in discrete brain regions. Although the majority of work examining how epigenetic modifications contribute to addiction has focused on psychostimulants such as cocaine, research into opioid-induced changes to the epigenetic landscape is emerging. This review summarizes our knowledge of opioid-induced epigenetic modifications and their consequential changes to gene expression. Current evidence points toward opioids promoting higher levels of permissive histone acetylation and lower levels of repressive histone methylation as well as alterations to DNA methylation patterns and noncoding RNA expression throughout the brain's reward circuitry. Additionally, studies manipulating epigenetic enzymes in specific brain regions are beginning to build causal links between these epigenetic modifications and changes in addiction-related behavior. Moving forward, studies must leverage advanced chromatin analysis and next-generation sequencing approaches combined with bioinformatics pipelines to identify novel gene networks regulated by particular epigenetic modifications. Improved translational relevance also requires increased focus on volitional drug-intake models and standardization of opioid exposure paradigms. Such work will significantly advance our understanding of how opioids cause persistent changes to brain function and will provide a platform on which to develop interventions for treating opioid addiction.

Novel biomarkers to assess in utero effects of maternal opioid use: First steps toward understanding short- and long-term neurodevelopmental sequelae

Maternal opioid use disorder is common, resulting in significant neonatal morbidity and cost. Currently, it is not possible to predict which opioid-exposed newborns will require pharmacotherapy for neonatal abstinence syndrome. Further, little is known regarding the effects of maternal opioid use disorder on the developing human brain. We hypothesized that novel methodologies utilizing fetal central nervous system-derived extracellular vesicles isolated from maternal blood can address these gaps in knowledge. Plasma from opioid users and controls between 9 and 21 weeks was precipitated and extracellular vesicles were isolated. Mu opioid and cannabinoid receptor levels were quantified. Label-free proteomics studies and unbiased small RNA next generation sequencing was performed in paired fetal brain tissue. Maternal opioid use disorder increased mu opioid receptor protein levels in extracellular vesicles independent of opioid equivalent dose. Moreover, cannabinoid receptor levels in extracellular vesicles were upregulated with opioid exposure indicating cross talk with endocannabinoids. Maternal opioid use disorder was associated with significant changes in extracellular vesicle protein cargo and fetal brain micro RNA expression, especially in male fetuses. Many of the altered cargo molecules and micro RNAs identified are associated with adverse clinical neurodevelopmental outcomes. Our data suggest that assays relying on extracellular vesicles isolated from maternal blood extracellular vesicles may provide information regarding fetal response to opioids in the setting of maternal opioid use disorder. Prospective clinical studies are needed to evaluate the association between extracellular vesicle biomarkers, risk of neonatal abstinence syndrome and neurodevelopmental outcomes.

Comparison and Functional Genetic Analysis of Striatal Protein Expression Among Diverse Inbred Mouse Strains

C57BL/6J (B6) and DBA/2J (D2) inbred mouse strains are highly variable genetically and differ in a large number of behavioral traits related to striatal function, including depression, anxiety, stress response, and response to drugs of abuse. The genetic basis of these phenotypic differences are, however, unknown. Here, we present a comparison of the striatal proteome between B6 and D2 and relate differences at the protein level to strain differences at the mRNA level. We also leverage a recombinant inbred BXD population derived from B6 and D2 strains to investigate the role of genetic variation on the regulation of mRNA and protein levels. Finally, we test the hypothesis that differential protein expression contributes to differential behavioral responses between the B6 and D2 strain. We detected the expression of over 2,500 proteins in membrane-enriched protein fractions from B6 and D2 striatum. Of these, 160 proteins demonstrated significant differential expression between B6 and D2 strains at a 10% false discovery level, including COMT, GABRA2, and cannabinoid receptor 1 (CNR1)—key proteins involved in synaptic transmission and behavioral response. Similar to previous reports, there was little overlap between protein and transcript levels (25%). However, the overlap was greater (51%) for proteins demonstrating genetic regulation of cognate gene expression. We also found that striatal proteins with significantly higher or lower relative expression in B6 and D2 were enriched for dopaminergic and glutamatergic synapses and processes involved in synaptic plasticity [e.g., long-term potentiation (LTP) and long-term depression (LTD)]. Finally, we validated higher expression of CNR1 in B6 striatum and demonstrated greater sensitivity of this strain to the locomotor inhibiting effects of the CNR1 agonist, Δ9-tetrahydrocannabinol (THC). Our study is the first comparison of differences in striatal proteins between the B6 and D2 strains and suggests that alterations in the striatal proteome may underlie strain differences in related behaviors, such as drug response. Furthermore, we propose that genetic variants that impact transcript levels are more likely to also exhibit differential expression at the protein level.

The impact of mouse strain-specific spatial and temporal immune responses on the progression of neuropathic pain

The present study was designed to investigate the correlation between the spatial and temporal aspects of immune responses and genetic heterogeneity in the progression of peripheral neuropathic pain. To address this issue, we first screened four inbred mouse strains (C57BL/6J, C3H/He, DBA/2, and A/J mice) to identify high- and low-responder strains to mechanical hypersensitivity induced by partial sciatic nerve ligation (pSNL). Among these strains, the C57BL/6J strain showed the highest vulnerability to pSNL-induced mechanical hypersensitivity, whereas the C3H/HeSlc strain was most resistant. C3H/HeSlc mice exhibited a significant increase in CD206-immunoreactivity (anti-inflammatory macrophages) in the dorsal root ganglia (DRG) at 3 and 7 days, and lower Iba1-immunoreactivity (microglia) in the spinal cord from 3 to 14 days after pSNL than C57BL/6J mice. These phenomena might be associated with a decrease in the production of inflammatory factors (interleukin-1β, interleukin-6, and CX3CL1) in the DRG and the poor responsiveness of spinal microglia (i.e. microglial production of IL1β, CCL2, and TNFα) against CX3CL1 in C3H/HeSlc mice. Behavioral experiments using bone marrow (BM) chimeric mice derived by crossing C3H/HeSlc and C57BL/6J strains showed that the strength of mechanical hypersensitivity 3 days following pSNL was inversely correlated with the increase in the ratio of anti-inflammatory/pro-inflammatory DRG macrophages, which was based on the BM-derived hematopoietic cells from donor mice. By contrast, the intensity of Iba1-immunoreactivity (microglia) in the spinal cord was dependent on the phenotypes of recipient mice, but not affected by the phenotypes of BM-derived donor hematopoietic cells. These findings suggest that the strain-specific aspects of DRG macrophages and spinal microglia might be related to the early and late phases of pSNL-induced mechanical hypersensitivity, respectively. This study presents a greater understanding of the differences in neuropathic pain among genetically heterogeneous inbred mouse strains, and provides further insights into the spatial and temporal roles of the immune system in the pathogenesis of neuropathic pain.

The multiple PDZ domain protein Mpdz/MUPP1 regulates opioid tolerance and opioid-induced hyperalgesia

BACKGROUND

Opioids are a mainstay for the treatment of chronic pain. Unfortunately, therapy-limiting maladaptations such as loss of treatment effect (tolerance), and paradoxical opioid-induced hyperalgesia (OIH) can occur. The objective of this study was to identify genes responsible for opioid tolerance and OIH.

RESULTS

These studies used a well-established model of ascending morphine administration to induce tolerance, OIH and other opioid maladaptations in 23 strains of inbred mice. Genome-wide computational genetic mapping was then applied to the data in combination with a false discovery rate filter. Transgenic mice, gene expression experiments and immunoprecipitation assays were used to confirm the functional roles of the most strongly linked gene. The behavioral data processed using computational genetic mapping and false discovery rate filtering provided several strongly linked biologically plausible gene associations. The strongest of these was the highly polymorphic Mpdz gene coding for the post-synaptic scaffolding protein Mpdz/MUPP1. Heterozygous Mpdz +/- mice displayed reduced opioid tolerance and OIH. Mpdz gene expression and Mpdz/MUPP1 protein levels were lower in the spinal cords of low-adapting 129S1/Svlm mice than in high-adapting C57BL/6 mice. Morphine did not alter Mpdz expression levels. In addition, association of Mpdz/MUPP1 with its known binding partner CaMKII did not differ between these high- and low-adapting strains.

CONCLUSIONS

The degrees of maladaptive changes in response to repeated administration of morphine vary greatly across inbred strains of mice. Variants of the multiple PDZ domain gene Mpdz may contribute to the observed inter-strain variability in tolerance and OIH by virtue of changes in the level of their expression.

MicroRNAs Are Involved in the Development of Morphine-Induced Analgesic Tolerance and Regulate Functionally Relevant Changes in Serpini1

Long-term opioid treatment results in reduced therapeutic efficacy and in turn leads to an increase in the dose required to produce equivalent pain relief and alleviate break-through or insurmountable pain. Altered gene expression is a likely means for inducing long-term neuroadaptations responsible for tolerance. Studies conducted by our laboratory (Tapocik et al., 2009) revealed a network of gene expression changes occurring in canonical pathways involved in neuroplasticity, and uncovered miRNA processing as a potential mechanism. In particular, the mRNA coding the protein responsible for processing miRNAs, Dicer1, was positively correlated with the development of analgesic tolerance. The purpose of the present study was to test the hypothesis that miRNAs play a significant role in the development of analgesic tolerance as measured by thermal nociception. Dicer1 knockdown, miRNA profiling, bioinformatics, and confirmation of high value targets were used to test the proposition. Regionally targeted Dicer1 knockdown (via shRNA) had the anticipated consequence of eliminating the development of tolerance in C57BL/6J (B6) mice, thus supporting the involvement of miRNAs in the development of tolerance. MiRNA expression profiling identified a core set of chronic morphine-regulated miRNAs (miR's 27a, 9, 483, 505, 146b, 202). Bioinformatics approaches were implemented to identify and prioritize their predicted target mRNAs. We focused our attention on miR27a and its predicted target serpin peptidase inhibitor clade I (Serpini1) mRNA, a transcript known to be intricately involved in dendritic spine density regulation in a manner consistent with chronic morphine's consequences and previously found to be correlated with the development of analgesic tolerance. In vitro reporter assay confirmed the targeting of the Serpini1 3'-untranslated region by miR27a. Interestingly miR27a was found to positively regulate Serpini1 mRNA and protein levels in multiple neuronal cell lines. Lastly, Serpini1 knockout mice developed analgesic tolerance at a slower rate than wild-type mice thus confirming a role for the protein in analgesic tolerance. Overall, these results provide evidence to support a specific role for miR27a and Serpini1 in the behavioral response to chronic opioid administration (COA) and suggest that miRNA expression and mRNA targeting may underlie the neuroadaptations that mediate tolerance to the analgesic effects of morphine.

Endogenous cholinergic neurotransmission contributes to behavioral sensitization to morphine

Neuroplasticity in the mesolimbic dopaminergic system is critical for behavioral adaptations associated with opioid reward and addiction. These processes may be influenced by cholinergic transmission arising from the laterodorsal tegmental nucleus (LDTg), a main source of acetylcholine to mesolimbic dopaminergic neurons. To examine this possibility we asked if chronic systemic morphine administration affects expression of genes in ventral and ventrolateral periaqueductal gray at the level of the LDTg using rtPCR. Specifically, we examined gene expression changes in the area of interest using Neurotransmitters and Receptors PCR array between chronic morphine and saline control groups. Analysis suggested that chronic morphine administration led to changes in expression of genes associated, in part, with cholinergic neurotransmission. Furthermore, using a quantitative immunofluorescent technique, we found that chronic morphine treatment produced a significant increase in immunolabeling of the cholinergic marker (vesicular acetylcholine transporter) in neurons of the LDTg. Finally, systemic administration of the nonselective and noncompetitive neuronal nicotinic antagonist mecamylamine (0.5 or 2 mg/kg) dose-dependently blocked the expression, and to a lesser extent the development, of locomotor sensitization. The same treatment had no effect on acute morphine antinociception, antinociceptive tolerance or dependence to chronic morphine. Taken together, the results suggest that endogenous nicotinic cholinergic neurotransmission selectively contributes to behavioral sensitization to morphine and this process may, in part, involve cholinergic neurons within the LDTg.

Whole-genome expression profile in zebrafish embryos after chronic exposure to morphine: identification of new genes associated with neuronal function and mu opioid receptor expression

BACKGROUND

A great number of studies have investigated changes induced by morphine exposure in gene expression using several experimental models. In this study, we examined gene expression changes during chronic exposure to morphine during maturation and differentiation of zebrafish CNS.

RESULTS

Microarray analysis showed 254 genes whose expression was identified as different by at least 1.3 fold change following chronic morphine exposure as compared to controls. Of these, several novel genes (grb2, copb2, otpb, magi1b, grik-l, bnip4 and sox19b) have been detected for the first time in an experimental animal model treated with morphine. We have also identified a subset of genes (dao.1, wls, bnip4 and camk1γb) differentially expressed by chronic morphine exposure whose expression is related to mu opioid receptor gene expression. Altered expression of copb2, bnip4, sox19b, otpb, dao.1, grik-l and wls is indicative of modified neuronal development, CNS patterning processes, differentiation and dopaminergic neurotransmission, serotonergic signaling pathway, and glutamatergic neurotransmission. The deregulation of camk1γb signaling genes suggests an activation of axonogenesis and dendritogenesis.

CONCLUSIONS

Our study identified different functional classes of genes and individual candidates involved in the mechanisms underlying susceptibility to morphine actions related to CNS development. These results open new lines to study the treatment of pain and the molecular mechanisms involved in addiction. We also found a set of zebrafish-specific morphine-induced genes, which may be putative targets in human models for addiction and pain processes.

Analysis of candidate genes for morphine preference quantitative trait locus Mop2

Compared to DBA/2J (D2), C57BL/6J (B6) inbred mice exhibit strong morphine preference when tested using a two-bottle choice drinking paradigm. A morphine preference quantitative trait locus (QTL), Mop2, was originally mapped to proximal chromosome (Chr) 10 using a B6xD2 F2 intercross population, confirmed with reciprocal congenic strains and fine mapped with recombinant congenic strains. These efforts identified a ∼ 10-Million base pair (Mbp) interval, underlying Mop2, containing 35 genes. To further reduce the interval, mice from the D2.B6-Mop2-P1 congenic strain were backcrossed to parental D2 mice and two new recombinant strains of interest were generated: D2.B6-Mop2-P1.pD.dB and D2.B6-Mop2-P1.pD.dD. Results obtained from testing these strains in the two-bottle choice drinking paradigm suggest that the gene(s) responsible for the Mop2 QTL is one or more of 22 remaining within the newly defined interval (∼ 7.6 Mbp) which includes Oprm1 and several other genes related to opioid pharmacology. Real-time qRT-PCR analysis of Oprm1 and opioid-related genes Rgs17, Ppp1r14c, Vip, and Iyd revealed both between-strain and within-strain expression differences in comparisons of saline- and morphine-treated B6 and D2 mice. Analysis of Rgs17 protein levels also revealed both between-strain and within-strain differences in comparisons of saline- and morphine-treated B6 and D2 mice. Results suggest that the Mop2 QTL represents the combined influence of multiple genetic variants on morphine preference in these two strains. Relative contributions of each variant remain to be determined.

Molecular mechanisms underlying the enhanced analgesic effect of oxycodone compared to morphine in chemotherapy-induced neuropathic pain

Oxycodone is a μ-opioid receptor agonist, used for the treatment of a large variety of painful disorders. Several studies have reported that oxycodone is a more potent pain reliever than morphine, and that it improves the quality of life of patients. However, the neurobiological mechanisms underlying the therapeutic action of these two opioids are only partially understood. The aim of this study was to define the molecular changes underlying the long-lasting analgesic effects of oxycodone and morphine in an animal model of peripheral neuropathy induced by a chemotherapic agent, vincristine. Using a behavioural approach, we show that oxycodone maintains an optimal analgesic effect after chronic treatment, whereas the effect of morphine dies down. In addition, using DNA microarray technology on dorsal root ganglia, we provide evidence that the long-term analgesic effect of oxycodone is due to an up-regulation in GABA_B receptor expression in sensory neurons. These receptors are transported to their central terminals within the dorsal horn, and subsequently reinforce a presynaptic inhibition, since only the long-lasting (and not acute) anti-hyperalgesic effect of oxycodone was abolished by intrathecal administration of a GABA_B receptor antagonist; in contrast, the morphine effect was unaffected. Our study demonstrates that the GABA_B receptor is functionally required for the alleviating effect of oxycodone in neuropathic pain condition, thus providing new insight into the molecular mechanisms underlying the sustained analgesic action of oxycodone.

Expression, covariation, and genetic regulation of miRNA Biogenesis genes in brain supports their role in addiction, psychiatric disorders, and disease

The role of miRNA and miRNA biogenesis genes in the adult brain is just beginning to be explored. In this study we have performed a comprehensive analysis of the expression, genetic regulation, and co-expression of major components of the miRNA biogenesis pathway using human and mouse data sets and resources available on the GeneNetwork web site (genenetwork.org). We found a wide range of variation in expression in both species for key components of the pathway—Drosha, Pasha, and Dicer. Across species, tissues, and expression platforms all three genes are generally well-correlated. No single genetic locus exerts a strong and consistent influence on the expression of these key genes across murine brain regions. However, in mouse striatum, many members of the miRNA pathway are correlated—including Dicer, Drosha, Pasha, Ars2 (Srrt), Eif2c1 (Ago1), Eif2c2 (Ago2), Zcchc11, and Snip1. The expression of these genes may be partly influenced by a locus on Chromosome 9 (105.67–106.32 Mb). We explored ~1500 brain phenotypes available for the C57BL/6J × DBA/2J (BXD) genetic mouse population in order to identify miRNA biogenesis genes correlated with traits related to addiction and psychiatric disorders. We found a significant association between expression of Dicer and Drosha in several brain regions and the response to many drugs of abuse, including ethanol, cocaine, and methamphetamine. Expression of Dicer, Drosha, and Pasha in most of the brain regions explored is strongly correlated with the expression of key members of the dopamine system. Drosha, Pasha, and Dicer expression is also correlated with the expression of behavioral traits measuring depression and sensorimotor gating, impulsivity, and anxiety, respectively. Our study provides a global survey of the expression and regulation of key miRNA biogenesis genes in brain and provides preliminary support for the involvement of these genes and their product miRNAs in addiction and psychiatric disease processes.

Neuroplasticity, axonal guidance and micro-RNA genes are associated with morphine self-administration behavior

Neuroadaptations in the ventral striatum (VS) and ventral midbrain (VMB) following chronic opioid administration are thought to contribute to the pathogenesis and persistence of opiate addiction. In order to identify candidate genes involved in these neuroadaptations, we utilized a behavior-genetics strategy designed to associate contingent intravenous drug self-administration with specific patterns of gene expression in inbred mice differentially predisposed to the rewarding effects of morphine. In a Yoked-control paradigm, C57BL/6J mice showed clear morphine-reinforced behavior, whereas DBA/2J mice did not. Moreover, the Yoked-control paradigm revealed the powerful consequences of self-administration versus passive administration at the level of gene expression. Morphine self-administration in the C57BL/6J mice uniquely up- or down-regulated 237 genes in the VS and 131 genes in the VMB. Interestingly, only a handful of the C57BL/6J self-administration genes (<3%) exhibited a similar expression pattern in the DBA/2J mice. Hence, specific sets of genes could be confidently assigned to regional effects of morphine in a contingent- and genotype-dependent manner. Bioinformatics analysis revealed that neuroplasticity, axonal guidance and micro-RNAs (miRNAs) were among the key themes associated with drug self-administration. Noteworthy were the primary miRNA genes H19 and micro-RNA containing gene (Mirg), processed, respectively, to mature miRNAs miR-675 and miR-154, because they are prime candidates to mediate network-like changes in responses to chronic drug administration. These miRNAs have postulated roles in dopaminergic neuron differentiation and mu-opioid receptor regulation. The strategic approach designed to focus on reinforcement-associated genes provides new insight into the role of neuroplasticity pathways and miRNAs in drug addiction.

Systems biology approaches to finding novel pain mediators

Chronic pain represents a major health burden; this maladaptive pain state occurs as a consequence of hypersensitivity within the peripheral and central components of the somatosensory system. High throughput technologies (genomics, transciptomics, lipidomics, and proteomics) are now being applied to tissue derived from pain patients as well as experimental pain models to discover novel pain mediators. The use of clustering, meta-analysis and other techniques can help refine potential candidates. Of particular importance are systems biology methods, such as co-expression network generating algorithms, which infer potential associations/interactions between molecules and build networks based on these interactions. Protein-protein interaction networks allow the lists of potential targets generated by these different platforms to be analyzed in their biological context. Outputs from these different methods must also be related to the clinical pain phenotype. The improved and standardized phenotyping of pain symptoms and sensory signs enables much better subject stratification. Our hope is that, in the future, the use of computational approaches to integrate datasets including sensory phenotype as well as the outputs of high throughput technologies will help define novel pain mediators and provide insights into the pathogenesis of chronic pain.

Natural and engineered coding variation in antidepressant-sensitive serotonin transporters

The presynaptic serotonin (5-HT) transporter (SERT) is a key regulator of 5-HT signaling and is a major target for antidepressant medications and psychostimulants. In recent years, studies of natural and engineered genetic variation in SERT have provided new opportunities to understand structural dimensions of drug interactions and regulation of the transporter, to explore 5-HT contributions to antidepressant action, and to assess the impact of SERT-mediated 5-HT contributions to neuropsychiatric disorders. Here we review three examples from our recent studies where genetic changes in SERT, identified or engineered, have led to new models, findings, and theories that cast light on new dimensions of 5-HT action in the CNS and periphery. First, we review our work to identify specific residues through which SERT recognizes antagonists, and the conversion of this knowledge to the creation of mice lacking high-affinity antidepressant and cocaine sensitivity. Second, we discuss our studies of functional coding variation in SERT that exists in commonly used strains of inbred mice, and how this variation is beginning to reveal novel 5-HT-associated phenotypes. Third, we review our identification and functional characterization of multiple, hyperactive SERT coding variants in subjects with autism. Each of these activities has driven the development of new model systems that can be further exploited to understand the contribution of 5-HT signaling to risk for neuropsychiatric disorders and their treatment.

5-hydroxytryptamine type 3 receptor modulates opioid-induced hyperalgesia and tolerance in mice

BACKGROUND

Opioid-induced hyperalgesia (OIH) and tolerance are challenging maladaptations associated with opioids in managing pain. Recent genetic studies and the existing literature suggest the 5-hydroxytryptamine type 3 (5-HT3) receptor participates in these phenomena. The location of the relevant receptor populations and the interactions between the 5-HT3 system and other systems controlling OIH and tolerance have not been explored, however. We hypothesized that 5-HT3 receptors modulate OIH and tolerance, and that this modulation involves the control of expression of multiple neurotransmitter and receptor systems.

METHODS

C57BL/6 mice were exposed to a standardized 4-day morphine administration protocol. The 5-HT3 antagonist ondansetron was administered either during or after the conclusion of morphine administration. Mechanical testing was used to quantify OIH, and thermal tail-flick responses were used to measure morphine tolerance. In other experiments spinal cord and dorsal root ganglion tissues were harvested for analysis of messenger RNA concentrations by real-time polymerase chain reaction or immunochemistry analysis.

RESULTS

The results showed that (1) systemic or intrathecal injection of ondansetron significantly prevented and reversed OIH, but not local intraplantar injection; (2) systemic or intrathecal injection of ondansetron prevented and reversed tolerance; and (3) ondansetron blocked morphine-induced increases of multiple genes relevant to OIH and tolerance in dorsal root ganglion and spinal cord.

CONCLUSIONS

Morphine acts via a 5-HT3-dependent mechanism to support multiple maladaptations to the chronic administration of morphine. Furthermore, the use of 5-HT3 receptor antagonists may provide a new avenue to prevent or reverse OIH and tolerance associated with chronic opioid use.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 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).

Inter-strain differences of serotonergic inhibitory pain control in inbred mice

BACKGROUND

Descending inhibitory pain control contributes to the endogenous defense against chronic pain and involves noradrenergic and serotonergic systems. The clinical efficacy of antidepressants suggests that serotonin may be particularly relevant for neuropathic pain conditions. Serotonergic signaling is regulated by synthesis, metabolisms, reuptake and receptors.

RESULTS

To address the complexity, we used inbred mouse strains, C57BL/6J, 129 Sv, DBA/2J and Balb/c, which differ in brain serotonin levels. Serotonin analysis after nerve injury revealed inter-strain differences in the adaptation of descending serotonergic fibers. Upregulation of spinal cord and midbrain serotonin was apparent only in 129 Sv mice and was associated with attenuated nerve injury evoked hyperalgesia and allodynia in this strain. The increase of dorsal horn serotonin was blocked by hemisectioning of descending fibers but not by rhizotomy of primary afferents indicating a midbrain source. Para-chlorophenylalanine-mediated serotonin depletion in spinal cord and midbrain intensified pain hypersensitivity in the nerve injury model. In contrast, chronic inflammation of the hindpaw did not evoke equivalent changes in serotonin levels in the spinal cord and midbrain and nociceptive thresholds dropped in a parallel manner in all strains.

CONCLUSION

The results suggest that chronic nerve injury evoked hypernociception may be contributed by genetic differences of descending serotonergic inhibitory control.

Genetics of pain, opioids, and opioid responsiveness

Pain is an integral part of the defense mechanisms required for survival. Several hereditary syndromes of complete or almost complete insensitivity to pain have been identified and include channelopathy-associated pain insensitivity, of which the most likely candidate gene is the α-subunit of the voltage-gated sodium channel known as Na(v)1.7. Five hereditary sensory and autonomic neuropathy syndromes have been described. Variable pain sensitivity in the general population has been linked to common variants of the μ-opioid receptor and of the catecholamine-O-methyltransferase genes potentially leading to increased opioid tonus. Variants of the guanosine triphosphate cyclohydrolase 1/dopa-responsive dystonia gene appear to regulate nociception. Other candidate genes are the transient receptor potential cation channel, subfamily 5 member 1, gene and the melanocortin-1 receptor gene. Candidate genes for predicting opioid efficacy are drug-metabolizing enzymes and transporters-including cytochrome P450, uridine 5'-diphosphate-glucuronosyltransferases, and adenosine triphosphate-binding cassette transporters-that are involved in opioid metabolism. Most current knowledge on the genetic regulation of pain has been derived from animal models developed mainly in mice. Genomics has the potential to contribute to therapeutic advances with the promising approach of using small interfering RNA in the control of neuropathic pain. Knowledge of the genetic factors that affect opioid efficacy, metabolism, and adverse effects has the potential for personalizing both acute and chronic pain management, and for designing more useful opiate pain medications with lower adverse event profiles.

TLX1 and NOTCH coregulate transcription in T cell acute lymphoblastic leukemia cells

BACKGROUND

The homeobox gene TLX1 (for T-cell leukemia homeobox 1, previously known as HOX11) is inappropriately expressed in a major subgroup of T cell acute lymphoblastic leukemia (T-ALL) where it is strongly associated with activating NOTCH1 mutations. Despite the recognition that these genetic lesions cooperate in leukemogenesis, there have been no mechanistic studies addressing how TLX1 and NOTCH1 functionally interact to promote the leukemic phenotype.

RESULTS

Global gene expression profiling after downregulation of TLX1 and inhibition of the NOTCH pathway in ALL-SIL cells revealed that TLX1 synergistically regulated more than 60% of the NOTCH-responsive genes. Structure-function analysis demonstrated that TLX1 binding to Groucho-related TLE corepressors was necessary for maximal transcriptional regulation of the NOTCH-responsive genes tested, implicating TLX1 modulation of the NOTCH-TLE regulatory network. Comparison of the dataset to publicly available biological databases indicated that the TLX1/NOTCH-coregulated genes are frequently targeted by MYC. Gain- and loss-of-function experiments confirmed that MYC was an essential mediator of TLX1/NOTCH transcriptional output and growth promotion in ALL-SIL cells, with TLX1 contributing to the NOTCH-MYC regulatory axis by posttranscriptional enhancement of MYC protein levels. Functional classification of the TLX1/NOTCH-coregulated targets also showed enrichment for genes associated with other human cancers as well as those involved in developmental processes. In particular, we found that TLX1, NOTCH and MYC coregulate CD1B and RAG1, characteristic markers of early cortical thymocytes, and that concerted downregulation of the TLX1 and NOTCH pathways resulted in their irreversible repression.

CONCLUSIONS

We found that TLX1 and NOTCH synergistically regulate transcription in T-ALL, at least in part via the sharing of a TLE corepressor and by augmenting expression of MYC. We conclude that the TLX1/NOTCH/MYC network is a central determinant promoting the growth and survival of TLX1+ T-ALL cells. In addition, the TLX1/NOTCH/MYC transcriptional network coregulates genes involved in T cell development, such as CD1 and RAG family members, and therefore may prescribe the early cortical stage of differentiation arrest characteristic of the TLX1 subgroup of T-ALL.

Pain-related behaviors and neurochemical alterations in mice expressing sickle hemoglobin: modulation by cannabinoids

Sickle cell disease causes severe pain. We examined pain-related behaviors, correlative neurochemical changes, and analgesic effects of morphine and cannabinoids in transgenic mice expressing human sickle hemoglobin (HbS). Paw withdrawal threshold and withdrawal latency (to mechanical and thermal stimuli, respectively) and grip force were lower in homozygous and hemizygous Berkley mice (BERK and hBERK1, respectively) compared with control mice expressing human hemoglobin A (HbA-BERK), indicating deep/musculoskeletal and cutaneous hyperalgesia. Peripheral nerves and blood vessels were structurally altered in BERK and hBERK1 skin, with decreased expression of mu opioid receptor and increased calcitonin gene-related peptide and substance P immunoreactivity. Activators of neuropathic and inflammatory pain (p38 mitogen-activated protein kinase, STAT3, and mitogen-activated protein kinase/extracellular signal-regulated kinase) showed increased phosphorylation, with accompanying increase in COX-2, interleukin-6, and Toll-like receptor 4 in the spinal cord of hBERK1 compared with HbA-BERK. These neurochemical changes in the periphery and spinal cord may contribute to hyperalgesia in mice expressing HbS. In BERK and hBERK1, hyperalgesia was markedly attenuated by morphine and cannabinoid receptor agonist CP 55940. We show that mice expressing HbS exhibit characteristics of pain observed in sickle cell disease patients, and neurochemical changes suggestive of nociceptor and glial activation. Importantly, cannabinoids attenuate pain in mice expressing HbS.

Qualitative differences between C57BL/6J and DBA/2J mice in morphine potentiation of brain stimulation reward and intravenous self-administration

RATIONALE

The C57BL/6J (C57) and DBA/2J (DBA) mice are the most common genotypes used to identify chromosomal regions and neurochemical mechanisms of interest in opioid addiction. Unfortunately, outside of the oral two-bottle choice procedure, limited and sometimes controversial evidence is available for determining their relative sensitivity to the rewarding effects of morphine.

OBJECTIVES

The purpose of this study was to utilize classically accepted models of drug abuse liability to determine relative susceptibility to the rewarding effects of morphine.

METHODS

The ability of morphine or amphetamine to potentiate lateral hypothalamic brain stimulation and intravenous morphine self-administration (across three doses in a fixed ratio schedule and at the highest dose in progressive ratio schedules) was investigated in both genotypes.

RESULTS

In both measures, C57 and DBA mice differed dramatically in their response to morphine. Morphine potentiated rewarding stimulation in the C57 mice but antagonized it in the DBA mice. Consistent with these findings, intravenous morphine did not serve as a positive reinforcer in DBA mice under conditions that were effective in the C57 mice using a fixed ratio schedule and failed to sustain levels of responding sufficient to maintain a constant rate of drug intake under a progressive ratio schedule. In contrast, amphetamine potentiated the rewarding effects of brain stimulation similarly in the two genotypes.

CONCLUSIONS

These findings provide strong evidence that morphine is rewarding in the C57 genotype and not in the DBA genotype. Understanding their relative susceptibility is important given the prominence of these genotypes in candidate gene identification and gene mapping.