Response of the Ubiquitin-Proteasome System to Memory Retrieval After Extended-Access Cocaine or Saline Self-Administration

The ubiquitin-proteasome system and learning-dependent synaptic plasticity - A 10 year update

Over 25 years ago, a seminal paper demonstrated that the ubiquitin-proteasome system (UPS) was involved in activity-dependent synaptic plasticity. Interest in this topic began to expand around 2008 following another seminal paper showing that UPS-mediated protein degradation controlled the "destabilization" of memories following retrieval, though we remained with only a basic understanding of how the UPS regulated activity- and learning-dependent synaptic plasticity. However, over the last 10 years there has been an explosion of papers on this topic that has significantly changed our understanding of how ubiquitin-proteasome signaling regulates synaptic plasticity and memory formation. Importantly, we now know that the UPS controls much more than protein degradation, is involved in plasticity underlying drugs of abuse and that there are significant sex differences in how ubiquitin-proteasome signaling is used for memory storage processes. Here, we aim to provide a critical 10-year update on the role of ubiquitin-proteasome signaling in synaptic plasticity and memory formation, including updated cellular models of how ubiquitin-proteasome activity could be regulating learning-dependent synaptic plasticity in the brain.

The roles of ubiquitin-proteasome system and regulator of G protein signaling 4 in behavioral sensitization induced by a single morphine exposure

AIMS

Opioid addiction is a major public health issue, yet its underlying mechanism is still unknown. The aim of this study was to explore the roles of ubiquitin-proteasome system (UPS) and regulator of G protein signaling 4 (RGS4) in morphine-induced behavioral sensitization, a well-recognized animal model of opioid addiction.

METHODS

We explored the characteristics of RGS4 protein expression and polyubiquitination in the development of behavioral sensitization induced by a single morphine exposure in rats, and the effect of a selective proteasome inhibitor, lactacystin (LAC), on behavioral sensitization.

RESULTS

Polyubiquitination expression was increased in time-dependent and dose-related fashions during the development of behavioral sensitization, while RGS4 protein expression was not significantly changed during this phase. Stereotaxic administration of LAC into nucleus accumbens (NAc) core inhibited the establishment of behavioral sensitization.

CONCLUSION

UPS in NAc core is positively involved in behavioral sensitization induced by a single morphine exposure in rats. Polyubiquitination was observed during the development phase of behavioral sensitization, while RGS4 protein expression was not significantly changed, indicating that other members of RGS family might be substrate proteins in UPS-mediated behavioral sensitization.

Persistent Neuroadaptations in the Nucleus Accumbens Core Accompany Incubation of Methamphetamine Craving in Male and Female Rats

Relapse is a major problem in treating methamphetamine use disorder. "Incubation of craving" during abstinence is a rat model for persistence of vulnerability to craving and relapse. While methamphetamine incubation has previously been demonstrated in male and female rats, it has not been demonstrated after withdrawal periods greater than 51 d and most mechanistic work used males. Here, we address both gaps. First, although methamphetamine intake was higher in males during self-administration training (6 h/d × 10 d), incubation was similar in males and females, with "incubated" craving persisting through withdrawal day (WD)100. Second, using whole-cell patch-clamp recordings in medium spiny neurons (MSNs) of the nucleus accumbens (NAc) core, we assessed synaptic levels of calcium-permeable AMPA receptors (CP-AMPARs), as their elevation is required for expression of incubation in males. In both sexes, compared with saline-self-administering controls, CP-AMPAR levels were significantly higher in methamphetamine rats across withdrawal, although this was less pronounced in WD100-135 rats than WD15-35 or WD40-75 methamphetamine rats. We also examined membrane properties and NMDA receptor (NMDAR) transmission. In saline controls, MSNs from males exhibited lower excitability than females. This difference was eliminated after incubation because of increased excitability of MSNs from males. NMDAR transmission did not differ between sexes and was not altered after incubation. In conclusion, incubation persists for longer than previously described and equally persistent CP-AMPAR plasticity in NAc core occurs in both sexes. Thus, abstinence-related synaptic plasticity in NAc is similar in males and females although other methamphetamine-related behaviors and neuroadaptations show differences.

Cocaine and Its Abstinence Condition Modulate Striatal and Hippocampal Wnt Signaling in a Male Rat Model of Drug Self-Administration

Recent years have provided more and more evidence confirming the important role of Wnt/β-catenin signaling in the pathophysiology of mental illnesses, including cocaine use disorder. High relapse rates, which is a hallmark of drug addiction, prompt the study of changes in Wnt signaling elements (Wnt5a, Wnt7b, and Ctnnb1) in the motivational aspects of cocaine use and early drug-free period (3 days after the last exposure to cocaine). For this purpose, an animal model of intravenous cocaine self-administration and two types of drug-free period (extinction training and abstinence in the home cage) were used. The studies showed that chronic cocaine self-administration mainly disturbs the expression of Wnt5a and Ctnnb1 (the gene encoding β-catenin) in the examined brain structures (striatum and hippocampus), and the examined types of early abstinence are characterized by a different pattern of changes in the expression of these genes. At the same time, in cocaine self-administrated animals, there were no changes in the level of Wnt5a and β-catenin proteins at the tested time points. Moreover, exposure to cocaine induces a significant reduction in the striatal and hippocampal expression of miR-374 and miR-544, which can regulate Wnt5a levels post-transcriptionally. In summary, previous observations from experimenter-administered cocaine have not been fully validated in the cocaine self-administration model. Yoked cocaine administration appears to disrupt Wnt signaling more than cocaine self-administration. The condition of the cocaine-free period, the routes of drug administration, and the motivational aspect of drug administration play an important role in the type of drug-induced molecular changes observed. Furthermore, in-depth research involving additional brain regions is needed to determine the exact role of Wnt signaling in short-term and long-lasting plasticity as well as in the motivational aspects of cocaine use, and thus to assess its potential as a target for new drug therapy for cocaine use disorder.

Neuroadaptations and TGF-β signaling: emerging role in models of neuropsychiatric disorders

Neuropsychiatric diseases are manifested by maladaptive behavioral plasticity. Despite the greater understanding of the neuroplasticity underlying behavioral adaptations, pinpointing precise cellular mediators has remained elusive. This has stymied the development of pharmacological interventions to combat these disorders both at the level of progression and relapse. With increased knowledge on the putative role of the transforming growth factor (TGF- β) family of proteins in mediating diverse neuroadaptations, the influence of TGF-β signaling in regulating maladaptive cellular and behavioral plasticity underlying neuropsychiatric disorders is being increasingly elucidated. The current review is focused on what is currently known about the TGF-β signaling in the central nervous system in mediating cellular and behavioral plasticity related to neuropsychiatric manifestations.

Profiling prefrontal cortex protein expression in rats exhibiting an incubation of cocaine craving following short-access self-administration procedures

INTRODUCTION

Incubation of drug-craving refers to a time-dependent increase in drug cue-elicited craving that occurs during protracted withdrawal. Historically, rat models of incubated cocaine craving employed extended-access (typically 6 h/day) intravenous drug self-administration (IV-SA) procedures, although incubated cocaine craving is reported to occur following shorter-access IV-SA paradigms. The notoriously low-throughput of extended-access IV-SA prompted us to determine whether two different short-access IV-SA procedures akin to those in the literature result in qualitatively similar changes in glutamate receptor expression and the activation of downstream signaling molecules within prefrontal cortex (PFC) subregions as those reported previously by our group under 6h-access conditions.

METHODS

For this, adult, male Sprague-Dawley rats were trained to intravenously self-administer cocaine for 2 h/day for 10 consecutive days (2-h model) or for 6 h on day 1 and 2 h/day for the remaining 9 days of training (Mixed model). A sham control group was also included that did not self-administer cocaine.

RESULTS

On withdrawal day 3 or 30, rats were subjected to a 2-h test of cue-reinforced responding in the absence of cocaine and a time-dependent increase in drug-seeking was observed under both IV-SA procedures. Immunoblotting of brain tissue collected immediately following the cue test session indicated elevated phospho-Akt1, phospho-CaMKII and Homer2a/b expression within the prelimbic subregion of the PFC of cocaine-incubated rats. However, we failed to detect incubation-related changes in Group 1 metabotropic glutamate receptor or ionotropic glutamate receptor subunit expression in either subregion.

DISCUSSION

These results highlight further a role for Akt1-related signaling within the prelimbic cortex in driving incubated cocaine craving, and provide novel evidence supporting a potential role also for CaMKII-dependent signaling through glutamate receptors in this behavioral phenomenon.

Sex-Specific Linear Polyubiquitination Is a Critical Regulator of Contextual Fear Memory Formation

Strong evidence supports that protein ubiquitination is a critical regulator of fear memory formation. However, as this work has focused on protein degradation, it is currently unknown whether polyubiquitin modifications that are independent of the proteasome are involved in learning-dependent synaptic plasticity. Here, we present the first evidence that atypical linear (M1) polyubiquitination, the only ubiquitin chain that does not occur at a lysine site and is largely independent of the proteasome, is critically involved in contextual fear memory formation in the amygdala in a sex-specific manner. Using immunoblot and unbiased proteomic analyses, we found that male (49) and female (14) rats both had increased levels of linear polyubiquitinated substrates following fear conditioning, though none of these protein targets overlapped between sexes. In males, target protein functions involved cell junction and axonal guidance signaling, while in females the primary target was Adiponectin A, a critical regulator of neuroinflammation, synaptic plasticity, and memory, suggesting sex-dependent functional roles for linear polyubiquitination during fear memory formation. Consistent with these increases, in vivo siRNA-mediated knockdown of Rnf31, an essential component of the linear polyubiquitin E3 complex LUBAC, in the amygdala impaired contextual fear memory in both sexes without affecting memory retrieval. Collectively, these results provide the first evidence that proteasome-independent linear polyubiquitination is a critical regulator of fear memory formation, expanding the potential roles of ubiquitin-signaling in learning-dependent synaptic plasticity. Importantly, our data identify a novel sex difference in the functional role of, but not a requirement for, linear polyubiquitination in fear memory formation.

Ube2b-dependent degradation of DNMT3a relieves a transcriptional brake on opiate-induced synaptic and behavioral plasticity

Compelling evidence suggests that synaptic structural plasticity, driven by remodeling of the actin cytoskeleton, underlies addictive drugs-induced long-lasting behavioral plasticity. However, the signaling mechanisms leading to actin cytoskeleton remodeling remain poorly defined. DNA methylation is a critical mechanism used to control activity-dependent gene expression essential for long-lasting synaptic plasticity. Here, we provide evidence that DNA methyltransferase DNMT3a is degraded by the E2 ubiquitin-conjugating enzyme Ube2b-mediated ubiquitination in dorsal hippocampus (DH) of rats that repeatedly self-administrated heroin. DNMT3a degradation leads to demethylation in CaMKK1 gene promotor, thereby facilitating CaMKK1 expression and consequent activation of its downstream target CaMKIα, an essential regulator of spinogenesis. CaMKK1/CaMKIα signaling regulates actin cytoskeleton remodeling in the DH and behavioral plasticity by activation of Rac1 via acting Rac guanine-nucleotide-exchange factor βPIX. These data suggest that Ube2b-dependent degradation of DNMT3a relieves a transcriptional brake on CaMKK1 gene and thus activates CaMKK1/CaMKIα/βPIX/Rac1 cascade, leading to drug use-induced actin polymerization and behavior plasticity.

The diversity of linkage-specific polyubiquitin chains and their role in synaptic plasticity and memory formation

Over the last 20 years, a number of studies have provided strong support for protein degradation mediated by the ubiquitin-proteasome system in synaptic plasticity and memory formation. In this system, target substrates become covalently modified by the small protein ubiquitin through a series of enzymatic reactions involving hundreds of different ligases. While some substrates will acquire only a single ubiquitin, most will be marked by multiple ubiquitin modifications, which link together at specific lysine sites or the N-terminal methionine on the previous ubiquitin to form a polyubiquitin chain. There are at least eight known linkage-specific polyubiquitin chains a target protein can acquire, many of which are independent of the proteasome, and these chains can be homogenous, mixed, or branched in nature, all of which result in different functional outcomes and fates for the target substrate. However, as the focus has remained on protein degradation, much remains unknown about the role of these diverse ubiquitin chains in the brain, particularly during activity- and learning-dependent synaptic plasticity. Here, we review the different types and functions of ubiquitin chains and summarize evidence suggesting a role for these diverse ubiquitin modifications in synaptic plasticity and memory formation. We conclude by discussing how technological limitations have limited our ability to identify and elucidate the role of different ubiquitin chains in the brain and speculate on the future directions and implications of understanding linkage-specific ubiquitin modifications in activity- and learning-dependent synaptic plasticity.

Sex-Specific Role for Egr3 in Nucleus Accumbens D2-Medium Spiny Neurons Following Long-Term Abstinence From Cocaine Self-administration

BACKGROUND

We previously showed that the transcription factor Egr3 (early growth response 3) is oppositely regulated in nucleus accumbens (NAc) cell subtypes 24 hours following cocaine exposure and bidirectionally mediates cocaine-related behaviors in male rodents. Overexpressing Egr3 in D₂ receptor-containing medium spiny neurons (D2-MSNs) before drug exposure reduces the rewarding and psychomotor sensitization effects of cocaine. However, it is unknown if Egr3 plays a role in long-term neuroadaptations in the NAc and relapse to cocaine seeking.

METHODS

We measured EGR3 protein levels in the NAc following 20 days of forced abstinence from intravenous cocaine self-administration in 10-week-old Sprague Dawley rats and C57BL/6 mice. In 8- to 10-week-old A2A-Cre mice, we used virally mediated Egr3 overexpression in NAc D2-MSNs to test the role of Egr3 on operant responding during seeking, extinction, and drug-induced reinstatement of cocaine self-administration. To evaluate if Egr3 contributed to sex differences to cocaine relapse, we conducted these procedures in both male and female rodents.

RESULTS

We found that EGR3 expression was reduced only in female rodents after 20 days of forced abstinence. Additionally, we showed that our self-administration paradigm in mice recapitulated the sex differences in cocaine intake and relapse demonstrated in humans and rats. Finally, whereas Egr3 overexpression in D2-MSNs during forced abstinence facilitated extinction and blunted drug-induced reinstatement in female mice, it had the opposite effect in male mice.

CONCLUSIONS

We showed that the immediate early gene Egr3 has long-term effects on drug-related behaviors. Our work suggests that changes in Egr3 expression in D2-MSNs contributes to sex differences in cocaine relapse.

Ubiquitin-proteasomal regulation of chromatin remodeler INO80 in the nucleus accumbens mediates persistent cocaine craving

Neuroadaptations in the nucleus accumbens (NAc) underlie cue-induced cocaine craving that intensifies ("incubates") during abstinence and is believed to contribute to persistent relapse vulnerability. Changes in gene expression often govern perpetual behavioral abnormalities, but epigenetic plasticity during prolonged abstinence from drug exposure is poorly understood. We examined how E3 ubiquitin ligase TRIM3 dysregulates chromatin remodeler INO80 to mediate cocaine craving during prolonged abstinence. We found that INO80 expression increased in the NAc on abstinence day 30 (AD30) but not on AD1 following cocaine self-administration. Furthermore, TRIM3, which mediates degradation of INO80, was reduced on AD30, along with TRIM3-INO80 interaction. Viral-mediated gene transfer of INO80 or TRIM3 governed cocaine craving during prolonged abstinence. Lastly, chromatin immunoprecipitation followed by massively parallel DNA sequencing identified INO80-mediated transcriptional regulation of predicted pathways associated with cocaine plasticity. Together, these results demonstrate a novel ubiquitin-proteasomal-epigenetic mechanism by which TRIM3-INO80 mediates cocaine craving during prolonged abstinence.

Males and Females Differ in the Subcellular and Brain Region Dependent Regulation of Proteasome Activity by CaMKII and Protein Kinase A

The ubiquitin-proteasome system (UPS) controls the degradation of ~90% of short-lived proteins in cells and is involved in activity- and learning-dependent synaptic plasticity in the brain. Calcium/calmodulin dependent protein kinase II (CaMKII) and Protein Kinase A (PKA) can regulate activity of the proteasome. However, there have been a number of conflicting reports regarding under what conditions CaMKII and PKA regulate proteasome activity in the brain. Furthermore, this work has been done exclusively in males, leaving questions about whether these kinases also regulate the proteasome in females. Here, using subcellular fractionation protocols in combination with in vitro pharmacology and proteasome activity assays, we investigated the conditions under which CaMKII and PKA regulate proteasome activity in the brains of male and female rats. In males, nuclear proteasome chymotrypsin activity was regulated by PKA in the amygdala but CaMKII in the hippocampus. Conversely, in females CaMKII regulated nuclear chymotrypsin activity in the amygdala, but not hippocampus. Additionally, in males CaMKII and PKA regulated proteasome trypsin activity in the cytoplasm of hippocampal, but not amygdala cells, while in females both CaMKII and PKA could regulate this activity in the nucleus of cells in both regions. Proteasome peptidylglutamyl activity was regulated by CaMKII and PKA activity in the nuclei of amygdala and hippocampus cells in males. However, in females PKA regulated nuclear peptidylglutamyl activity in the amygdala, but not hippocampus. Collectively, these results suggest that CaMKII- and PKA-dependent regulation of proteasome activity in the brain varies significantly across subcellular compartments and between males and females.

Drp1 Mitochondrial Fission in D1 Neurons Mediates Behavioral and Cellular Plasticity during Early Cocaine Abstinence

Altered brain energy homeostasis is a key adaptation occurring in the cocaine-addicted brain, but the effect of cocaine on the fundamental source of energy, mitochondria, is unknown. We demonstrate an increase of dynamin-related protein-1 (Drp1), the mitochondrial fission mediator, in nucleus accumbens (NAc) after repeated cocaine exposure and in cocaine-dependent individuals. Mdivi-1, a demonstrated fission inhibitor, blunts cocaine seeking and locomotor sensitization, while blocking c-Fos induction and excitatory input onto dopamine receptor-1 (D1) containing NAc medium spiny neurons (MSNs). Drp1 and fission promoting Drp1 are increased in D1-MSNs, consistent with increased smaller mitochondria in D1-MSN dendrites after repeated cocaine. Knockdown of Drp1 in D1-MSNs blocks drug seeking after cocaine self-administration, while enhancing the fission promoting Drp1 enhances seeking after long-term abstinence from cocaine. We demonstrate a role for altered mitochondrial fission in the NAc, during early cocaine abstinence, suggesting potential therapeutic treatment of disrupting mitochondrial fission in cocaine addiction.

Ethanol withdrawal limits fear memory reactivation-induced molecular events associated with destabilization phase: Influence of d-cycloserine

A 1-day fear memory in ethanol withdrawn (ETOH) rats is resistant to destabilization-reconsolidation process. However, d-cycloserine (DCS) reverts this disturbance. Considering that the formation of pathological fear memories in humans often occurs long time before the requirement of an intervention, the study of older memories is relevant in ETOH rats. In addition, the resistance to destabilization and DCS effect on this memory phase at molecular level in ETOH rats have not been corroborated yet. Firstly, we examined the effect of a pharmacological intervention after reactivation on reconsolidation of a 7-day fear memory in ETOH rats. Then, and considering that enhanced GluN2B expression and ubiquitin-proteasome system (UPS) activity are involved in destabilization, we evaluated them following reactivation in ETOH rats. Furthermore, DCS effect on such destabilization markers was examined. It was found that the pharmacological intervention after reactivation did not affect the 7-day fear memory in ETOH rats with DCS reversing this resistance. Memory reactivation increased GluN2B expression, polyubiquitination levels and proteasome activity in the basolateral amygdala complex (BLA) of control (CON) rats only; without affecting these molecular events in ETOH rats. Finally, ETOH rats treated with DCS and CON animals displayed elevated and similar UPS activities in the BLA after reactivation. In conclusion, the reactivation of an older fear memory formed during ethanol withdrawal does not trigger the molecular events associated with destabilization, and DCS facilitates this memory phase by enhancing the UPS activity.

E3 Ubiquitin-Protein Ligase SMURF1 in the Nucleus Accumbens Mediates Cocaine Seeking

BACKGROUND

Substance use disorder is a neurobiological disease characterized by episodes of relapse despite periods of withdrawal. It is thought that neuroadaptations in discrete brain areas of the reward pathway, including the nucleus accumbens, underlie these aberrant behaviors. The ubiquitin-proteasome system degrades proteins and has been shown to be involved in cocaine-induced plasticity, but the role of E3 ubiquitin ligases, which conjugate ubiquitin to substrates, is unknown. Here, we examined E3 ubiquitin-protein ligase SMURF1 (SMURF1) in neuroadaptations and relapse behavior during withdrawal following cocaine self-administration.

METHODS

SMURF1 and downstream targets ras homolog gene family, member A (RhoA), SMAD1/5, and Runt-related transcript factor 2 were examined using Western blotting (n = 9-11/group), quantitative polymerase chain reaction (n = 6-9/group), co-immunoprecipitation (n = 9-11/group), tandem ubiquitin binding entities affinity purification (n = 5-6/group), and quantitative chromatin immunoprecipitation (n = 3-6/group) (2 rats/sample). Viral-mediated gene transfer (n = 7-12/group) and intra-accumbal microinjections (n = 9-10/group) were used to examine causal roles of SMURF1 and substrate RhoA, respectively, in cue-induced cocaine seeking.

RESULTS

SMURF1 protein expression was decreased, while SMURF1 substrates RhoA and SMAD1/5 were increased, in the nucleus accumbens on withdrawal day 7, but not on withdrawal day 1, following cocaine self-administration. Viral-mediated gene transfer of Smurf1 or constitutive activation of RhoA attenuated cue-induced cocaine seeking, while catalytically inactive Smurf1 enhanced cocaine seeking. Furthermore, SMURF1-regulated, SMAD1/5-associated transcription factor Runt-related transcript factor 2 displayed increased binding at promoter regions of genes previously associated with cocaine-induced plasticity.

CONCLUSIONS

SMURF1 is a key mediator of neuroadaptations in the nucleus accumbens following cocaine exposure and mediates cue-induced cocaine seeking during withdrawal.

Genome-wide transcriptional profiling of central amygdala and orbitofrontal cortex during incubation of methamphetamine craving

Methamphetamine (Meth) seeking progressively increases after withdrawal (incubation of Meth craving), but the transcriptional mechanisms that contribute to this incubation are unknown. Here we used RNA-sequencing to analyze transcriptional profiles associated with incubation of Meth craving in central amygdala (CeA) and orbitofrontal cortex (OFC), two brain areas previously implicated in relapse to drug seeking. We trained rats to self-administer either saline (control condition) or Meth (10 days; 9 h/day, 0.1 mg/kg/infusion). Next, we collected brain tissue from CeA and OFC on withdrawal day 2 (when Meth seeking is low and non-incubated) and on day 35 (when Meth seeking is high and incubated), for subsequent RNA-sequencing. In CeA, we identified 10-fold more differentially expressed genes (DEGs) on withdrawal day 35 than day 2. These genes were enriched for several biological processes, including protein ubiquitination and histone methylation. In OFC, we identified much fewer expression changes than in CeA, with more DEGs on withdrawal day 2 than on day 35. There was a significant overlap between upregulated genes on withdrawal day 2 and downregulated genes on withdrawal day 35 in OFC. Our analyses highlight the CeA as a key region of transcriptional regulation associated with incubation of Meth seeking. In contrast, transcriptional regulation in OFC may contribute to Meth seeking during early withdrawal. Overall, these findings provide a unique resource of gene expression data for future studies examining transcriptional mechanisms in CeA that mediate Meth seeking after prolonged withdrawal.

Protein Translation in the Nucleus Accumbens Is Dysregulated during Cocaine Withdrawal and Required for Expression of Incubation of Cocaine Craving

Exposure to drug-associated cues can induce drug craving and relapse in abstinent addicts. Cue-induced craving that progressively intensifies ("incubates") during withdrawal from cocaine has been observed in both rats and humans. Building on recent evidence that aberrant protein translation underlies incubation-related adaptations in the NAc, we used male rats to test the hypothesis that translation is dysregulated during cocaine withdrawal and/or when rats express incubated cocaine craving. We found that intra-NAc infusion of anisomycin, a general protein translation inhibitor, or rapamycin, an inhibitor of mammalian target of rapamycin, reduced the expression of incubated cocaine craving, consistent with previous results showing that inhibition of translation in slices normalized the adaptations that maintain incubation. We then examined signaling pathways involved in protein translation using NAc synaptoneurosomes prepared after >47 d of withdrawal from cocaine or saline self-administration, or after withdrawal plus a cue-induced seeking test. The most robust changes were observed following seeking tests. Most notably, we found that eukaryotic elongation factor 2 (eEF2) and eukaryotic initiation factor 2α (eIF2α) are dephosphorylated when cocaine rats undergo a cue-induced seeking test; both effects are consistent with increased translation during the test. Blocking eIF2α dephosphorylation and thereby restoring its inhibitory influence on translation, via intra-NAc injection of Sal003 just before the test, substantially reduced cocaine seeking. These results are consistent with dysregulation of protein translation in the NAc during cocaine withdrawal, enabling cocaine cues to elicit an aberrant increase in translation that is required for the expression of incubated cocaine craving.SIGNIFICANCE STATEMENT Cue-induced cocaine craving progressively intensifies (incubates) during withdrawal in both humans and rats. This may contribute to persistent vulnerability to relapse. We previously demonstrated a role for protein translation in synaptic adaptations in the NAc closely linked to incubation. Here, we tested the hypothesis that translation is dysregulated during cocaine withdrawal, and this contributes to incubated craving. Analysis of signaling pathways regulating translation suggested that translation is enhanced when "incubated" rats undergo a cue-induced seeking test. Furthermore, intra-NAc infusions of drugs that inhibit protein translation through different mechanisms reduced expression of incubated cue-induced cocaine seeking. These results demonstrate that the expression of incubation depends on an acute increase in translation that may result from dysregulation of several pathways.

Consequences of Adolescent Exposure to the Cannabinoid Receptor Agonist WIN55,212-2 on Working Memory in Female Rats

Marijuana is a prevalent illicit substance used by adolescents, and several studies have indicated that adolescent use can lead to long-term cognitive deficits including problems with attention and memory. However, preclinical animal studies that observe cognitive deficits after cannabinoid exposure during adolescence utilize experimenter administration of doses of cannabinoids that may exceed what an organism would choose to take, suggesting that contingency and dose are critical factors that need to be addressed in translational models of consequences of cannabinoid exposure. Indeed, we recently developed an adolescent cannabinoid self-administration paradigm in male rats, and found that prior adolescent self-administration of the cannabinoid receptor agonist WIN55,212-2 (WIN) resulted in improved working memory performance in adulthood. In addition, the doses self-administered were not as high as those that are found to produce memory deficits. However, given known sex differences in both drug self-administration and learning and memory processes, it is possible that cannabinoid self-administration could have different cognitive consequences in females. Therefore, we aimed to explore the effects of self-administered vs. experimenter-administered WIN in adolescent female rats on adult cognitive function. Female rats were trained to self-administer WIN daily throughout adolescence (postnatal day 34–59). A control group self-administered vehicle solution. The acute effects of adolescent WIN self-administration on memory were determined using a short-term spatial memory test 24 h after final SA session; and the long-term effects on cognitive performance were assessed during protracted abstinence in adulthood using a delayed-match-to-sample working memory task. In a separate experiment, females were given daily intraperitoneal (IP) injections of a low or high dose of WIN, corresponding to self-administered and typical experimenter-administered doses, respectively, or its vehicle during adolescence and working memory was assessed under drug-free conditions in adulthood. While self-administration of WIN in adolescence had no significant effects on short-term spatial memory or adult working memory, experimenter administration of WIN resulted in improved adult working memory performance that was more pronounced in the low dose group. Thus, low-dose adolescent WIN exposure, whether self-administered or experimenter-administered, results in either improvements or no change in adult working memory performance in female rats, similar to previous results found in males.

The role of the ubiquitin proteasome system in the memory process

Quite intuitive is the notion that memory formation and consolidation is orchestrated by protein synthesis because of the synaptic plasticity necessary for those processes. Nevertheless, recent advances have begun accumulating evidences of a high requirement for protein degradation on the molecular mechanisms of the memory process in the mammalian brain. Because degradation determines protein half-life, degradation has been increasingly recognized as an important intracellular regulatory mechanism. The proteasome is the main player in the degradation of intracellular proteins. Proteasomal substrates are mainly degraded after a post-translational modification by a poly-ubiquitin chain. Latter process, namely poly-ubiquitination, is highly regulated at the step of the ubiquitin molecule transferring to the protein substrate mediated by a set of proteins whose genes represent almost 2% of the human genome. Understanding the role of polyubiquitin-mediated protein degradation has challenging researchers in many fields of investigation as a new source of targets for therapeutic intervention, e.g. E3 ligases that transfer ubiquitin moieties to the substrate. The goal of present work was to uncover mechanisms underlying memory processes regarding the role of the ubiquitin-proteasome system (UPS). For that purpose, preceded of a short review on UPS and memory processes a top-down systems biology approach was applied to establish central proteins involved in memory formation and consolidation highlighting their cross-talking with the UPS. According to that approach, the pattern of expression of several elements of the UPS were found overexpressed in regions of the brain involved in processing cortical inputs.

BAZ1B in Nucleus Accumbens Regulates Reward-Related Behaviors in Response to Distinct Emotional Stimuli

ATP-dependent chromatin remodeling proteins are being implicated increasingly in the regulation of complex behaviors, including models of several psychiatric disorders. Here, we demonstrate that Baz1b, an accessory subunit of the ISWI family of chromatin remodeling complexes, is upregulated in the nucleus accumbens (NAc), a key brain reward region, in both chronic cocaine-treated mice and mice that are resilient to chronic social defeat stress. In contrast, no regulation is seen in mice that are susceptible to this chronic stress. Viral-mediated overexpression of Baz1b, along with its associated subunit Smarca5, in mouse NAc is sufficient to potentiate both rewarding responses to cocaine, including cocaine self-administration, and resilience to chronic social defeat stress. However, despite these similar, proreward behavioral effects, genome-wide mapping of BAZ1B in NAc revealed mostly distinct subsets of genes regulated by these chromatin remodeling proteins after chronic exposure to either cocaine or social stress. Together, these findings suggest important roles for BAZ1B and its associated chromatin remodeling complexes in NAc in the regulation of reward behaviors to distinct emotional stimuli and highlight the stimulus-specific nature of the actions of these regulatory proteins.

SIGNIFICANCE STATEMENT

We show that BAZ1B, a component of chromatin remodeling complexes, in the nucleus accumbens regulates reward-related behaviors in response to chronic exposure to both rewarding and aversive stimuli by regulating largely distinct subsets of genes.

Cocaine and ethanol target 26S proteasome activity and gene expression in neuroblastoma cells

BACKGROUND

Ethanol and cocaine are widely abused drugs triggering long-lasting changes in neuronal circuits and synaptic transmission through the regulation of enzyme activity and gene expression. Compelling evidence indicates that the ubiquitin-proteasome system plays a role in the molecular changes induced by addictive substances, impacting on several mechanisms implicated in abuse. The goal of these studies was to evaluate the effects of cocaine or ethanol on proteasome activity in neuroblastoma cells. Moreover, the gene expression of specific subunits was assessed.

METHODS

Chymotrypsin-like activity was measured after 2 h, 24 h, and 48 h exposure to 5 μM cocaine or 40 mM ethanol. Proteasome subunit transcripts were evaluated by qPCR at the same time-points.

RESULTS

Treatments modified proteasome function in opposite directions, since cocaine increased and ethanol reduced chymotrypsin-like activity. Interestingly, we observed gene expression alterations induced by these drugs. In the core particle, the β1 and α5 subunits were mainly up-regulated by cocaine, whereas α6 transcripts were mostly decreased. β2 and β5 did not change. Similarly, ethanol exposure generally increased β1 and α5 mRNAs. Moreover, the β2 subunit was significantly up-regulated by ethanol only. The β5 and α6 subunits were not altered. In the regulatory particle, Rpt3 was increased by cocaine exposure, whereas it was reduced by ethanol. No significant Rpn9 alterations were observed.

CONCLUSIONS

These findings support the notion that addictive substances regulate proteasome function, contributing to the dysregulations related to drug abuse since the availability of adequate subunit amounts is necessary for proper complex assembly and function.