An emerging role for the Mammalian target of rapamycin in "pathological" protein translation: relevance to cocaine addiction

Dopamine D1 and NMDA receptor co-regulation of protein translation in cultured nucleus accumbens neurons

Protein translation is essential for some forms of synaptic plasticity. We used nucleus accumbens (NAc) medium spiny neurons (MSN), co-cultured with cortical neurons to restore excitatory synapses, to examine whether dopamine modulates protein translation in NAc MSN. FUNCAT was used to measure translation in MSNs under basal conditions and after disinhibiting excitatory transmission using the GABAA receptor antagonist bicuculline (2 hr). Under basal conditions, translation was not altered by the D1-class receptor (D1R) agonist SKF81297 or the D2-class receptor (D2R) agonist quinpirole. Bicuculline alone robustly increased translation. This was reversed by quinpirole but not SKF81297. It was also reversed by co-incubation with the D1R antagonist SCH23390, but not the D2R antagonist eticlopride, suggesting dopaminergic tone at D1Rs. This was surprising because no dopamine neurons are present. An alternative explanation is that bicuculline activates translation by increasing glutamate tone at NMDA receptors (NMDAR) within D1R/NMDAR heteromers, which have been described in other cell types. Supporting this, immunocytochemistry and proximity ligation assays revealed D1/NMDAR heteromers on NAc cells both in vitro and in vivo. Further, bicuculline's effect was reversed to the same extent by SCH23390 alone, the NMDAR antagonist APV alone, or SCH23390+APV. These results suggest that: 1) excitatory synaptic transmission stimulates translation in NAc MSNs, 2) this is opposed when glutamate activates D1R/NMDAR heteromers, even in the absence of dopamine, and 3) antagonist occupation of D1Rs within the heteromers prevents their activation. Our study is the first to suggest a role for D2 receptors and D1R/NMDAR heteromers in regulating protein translation.

Dopamine neuron morphology and output are differentially controlled by mTORC1 and mTORC2

The mTOR pathway is an essential regulator of cell growth and metabolism. Midbrain dopamine neurons are particularly sensitive to mTOR signaling status as activation or inhibition of mTOR alters their morphology and physiology. mTOR exists in two distinct multiprotein complexes termed mTORC1 and mTORC2. How each of these complexes affect dopamine neuron properties, and whether they have similar or distinct functions is unknown. Here, we investigated this in mice with dopamine neuron-specific deletion of Rptor or Rictor, which encode obligatory components of mTORC1 or mTORC2, respectively. We find that inhibition of mTORC1 strongly and broadly impacts dopamine neuron structure and function causing somatodendritic and axonal hypotrophy, increased intrinsic excitability, decreased dopamine production, and impaired dopamine release. In contrast, inhibition of mTORC2 has more subtle effects, with selective alterations to the output of ventral tegmental area dopamine neurons. Disruption of both mTOR complexes leads to pronounced deficits in dopamine release demonstrating the importance of balanced mTORC1 and mTORC2 signaling for dopaminergic function.

Ceftriaxone as a Novel Therapeutic Agent for Hyperglutamatergic States: Bridging the Gap Between Preclinical Results and Clinical Translation

Dysregulation of glutamate homeostasis is a well-established core feature of neuropsychiatric disorders. Extracellular glutamate concentration is regulated by glutamate transporter 1 (GLT-1). The discovery of a beta-lactam antibiotic, ceftriaxone (CEF), as a safe compound with unique ability to upregulate GLT-1 sparked the interest in testing its efficacy as a novel therapeutic agent in animal models of neuropsychiatric disorders with hyperglutamatergic states. Indeed, more than 100 preclinical studies have shown the efficacy of CEF in attenuating the behavioral manifestations of various hyperglutamatergic brain disorders such as ischemic stroke, amyotrophic lateral sclerosis (ALS), seizure, Huntington’s disease, and various aspects of drug use disorders. However, despite rich and promising preclinical data, only one large-scale clinical trial testing the efficacy of CEF in patients with ALS is reported. Unfortunately, in that study, there was no significant difference in survival between placebo- and CEF-treated patients. In this review, we discussed the translational potential of preclinical efficacy of CEF based on four different parameters: (1) initiation of CEF treatment in relation to induction of the hyperglutamatergic state, (2) onset of response in preclinical models in relation to onset of GLT-1 upregulation, (3) mechanisms of action of CEF on GLT-1 expression and function, and (4) non-GLT-1-mediated mechanisms for CEF. Our detailed review of the literature brings new insights into underlying molecular mechanisms correlating the preclinical efficacy of CEF. We concluded here that CEF may be clinically effective in selected cases in acute and transient hyperglutamatergic states such as early drug withdrawal conditions.

Functional Genomic Analysis of Amphetamine Sensitivity in Drosophila

Abuse of psychostimulants, including amphetamines (AMPHs), is a major public health problem with profound psychiatric, medical, and psychosocial complications. The actions of these drugs at the dopamine transporter (DAT) play a critical role in their therapeutic efficacy as well as their liability for abuse and dependence. To date, however, the mechanisms that mediate these actions are not well-understood, and therapeutic interventions for AMPH abuse have been limited. Drug exposure can induce broad changes in gene expression that can contribute to neuroplasticity and effect long-lasting changes in neuronal function. Identifying genes and gene pathways perturbed by drug exposure is essential to our understanding of the molecular basis of drug addiction. In this study, we used Drosophila as a model to examine AMPH-induced transcriptional changes that are DAT-dependent, as those would be the most relevant to the stimulatory effects of the drug. Using this approach, we found genes involved in the control of mRNA translation to be significantly upregulated in response to AMPH in a DAT-dependent manner. To further prioritize genes for validation, we explored functional convergence between these genes and genes we identified in a genome-wide association study of AMPH sensitivity using the Drosophila Genetic Reference Panel. We validated a number of these genes by showing that they act specifically in dopamine neurons to mediate the behavioral effects of AMPH. Taken together, our data establish Drosophila as a powerful model that enables the integration of behavioral, genomic and transcriptomic data, followed by rapid gene validation, to investigate the molecular underpinnings of psychostimulant action.

Effects of heroin self-administration and forced withdrawal on the expression of genes related to the mTOR network in the basolateral complex of the amygdala of male Lewis rats

RATIONALE

The development of substance use disorders involves long-lasting adaptations in specific brain areas that result in an elevated risk of relapse. Some of these adaptations are regulated by the mTOR network, a signalling system that integrates extracellular and intracellular stimuli and modulates several processes related to plasticity. While the role of the mTOR network in cocaine- and alcohol-related disorders is well established, little is known about its participation in opiate use disorders.

OBJECTIVES

To use a heroin self-administration and a withdrawal protocol that induce incubation of heroin-seeking in male rats and study the associated effects on the expression of several genes related to the mTOR system and, in the specific case of Rictor, its respective translated protein and phosphorylation.

RESULTS

We found that heroin self-administration elicited an increase in the expression of the genes Igf1r, Igf2r, Akt2 and Gsk3a in the basolateral complex of the amygdala, which was not as evident at 30 days of withdrawal. We also found an increase in the expression of Rictor (a protein of the mTOR complex 2) after heroin self-administration compared to the saline group, which was occluded at the 30-day withdrawal period. The activation levels of Rictor, measured by the phosphorylation rate, were also reduced after heroin self-administration, an effect that seemed more apparent in the protracted withdrawal group.

CONCLUSIONS

These results suggest that heroin self-administration under extended access conditions modifies the expression profile of activators and components of the mTOR complexes and show a putative irresponsive mTOR complex 2 after withdrawal from heroin use.

Autophagy as a gateway for the effects of methamphetamine: From neurotransmitter release and synaptic plasticity to psychiatric and neurodegenerative disorders

As a major eukaryotic cell clearing machinery, autophagy grants cell proteostasis, which is key for neurotransmitter release, synaptic plasticity, and neuronal survival. In line with this, besides neuropathological events, autophagy dysfunctions are bound to synaptic alterations that occur in mental disorders, and early on, in neurodegenerative diseases. This is also the case of methamphetamine (METH) abuse, which leads to psychiatric disturbances and neurotoxicity. While consistently altering the autophagy machinery, METH produces behavioral and neurotoxic effects through molecular and biochemical events that can be recapitulated by autophagy blockade. These consist of altered physiological dopamine (DA) release, abnormal stimulation of DA and glutamate receptors, as well as oxidative, excitotoxic, and neuroinflammatory events. Recent molecular insights suggest that METH early impairs the autophagy machinery, though its functional significance remains to be investigated. Here we discuss evidence suggesting that alterations of DA transmission and autophagy are intermingled within a chain of events underlying behavioral alterations and neurodegenerative phenomena produced by METH. Understanding how METH alters the autophagy machinery is expected to provide novel insights into the neurobiology of METH addiction sharing some features with psychiatric disorders and parkinsonism.

Distinct Regulation of Dopamine D3 Receptor in the Basolateral Amygdala and Dentate Gyrus during the Reinstatement of Cocaine CPP Induced by Drug Priming and Social Stress

Relapse in the seeking and intake of cocaine is one of the main challenges when treating its addiction. Among the triggering factors for the recurrence of cocaine use are the re-exposure to the drug and stressful events. Cocaine relapse engages the activity of memory-related nuclei, such as the basolateral amygdala (BLA) and the hippocampal dentate gyrus (DG), which are responsible for emotional and episodic memories. Moreover, D3 receptor (D3R) antagonists have recently arisen as a potential treatment for preventing drug relapse. Thus, we have assessed the impact of D3R blockade in the expression of some dopaminergic markers and the activity of the mTOR pathway, which is modulated by D3R, in the BLA and DG during the reinstatement of cocaine-induced conditioned place preference (CPP) evoked by drug priming and social stress. Reinstatement of cocaine CPP paralleled an increasing trend in D3R and dopamine transporter (DAT) levels in the BLA. Social stress, but not drug-induced reactivation of cocaine memories, was prevented by systemic administration of SB-277011-A (a selective D3R antagonist), which was able, however, to impede D3R and DAT up-regulation in the BLA during CPP reinstatement evoked by both stress and cocaine. Concomitant with cocaine CPP reactivation, a diminution in mTOR phosphorylation (activation) in the BLA and DG occurred, which was inhibited by D3R blockade in both nuclei before the social stress episode and only in the BLA when CPP reinstatement was provoked by a cocaine prime. Our data, while supporting a main role for D3R signalling in the BLA in the reactivation of cocaine memories evoked by social stress, indicate that different neural circuits and signalling mechanisms might mediate in the reinstatement of cocaine-seeking behaviours depending upon the triggering stimuli.

Autophagy status as a gateway for stress-induced catecholamine interplay in neurodegeneration

The catecholamine-containing brainstem nuclei locus coeruleus (LC) and ventral tegmental area (VTA) are critically involved in stress responses. Alterations of catecholamine systems during chronic stress may contribute to neurodegeneration, including cognitive decline. Stress-related catecholamine alterations, while contributing to anxiety and depression, might accelerate neuronal degeneration by increasing the formation of toxic dopamine and norepinephrine by-products. These, in turn, may impair proteostasis within a variety of cortical and subcortical areas. In particular, the molecular events governing neurotransmission, neuroplasticity, and proteostasis within LC and VTA affect a variety of brain areas. Therefore, we focus on alterations of autophagy machinery in these nuclei as a relevant trigger in this chain of events. In fact, these catecholamine-containing areas are mostly prone to autophagy-dependent neurodegeneration. Thus, we propose a dynamic hypothesis according to which stress-induced autophagy alterations within the LC-VTA network foster a cascade towards early neurodegeneration within these nuclei.

The role of the mTOR pathway in models of drug-induced reward and the behavioural constituents of addiction

BACKGROUND

Exposure to drugs of abuse induces neuroadaptations in critical nodes of the so-called reward systems that are thought to mediate the transition from controlled drug use to the compulsive drug-seeking that characterizes addictive disorders. These neural adaptations are likely to require protein synthesis, which is regulated, among others, by the mechanistic target of the rapamycin kinase (mTOR) signalling cascade.

METHODS

We have performed a narrative review of the literature available in PubMed about the involvement of the mTOR pathway in drug-reward and addiction-related phenomena.

AIMS

The aim of this study was to review the underlying architecture of this complex intracellular network and to discuss the alterations of its components that are evident after exposure to drugs of abuse. The aim was also to delineate the effects that manipulations of the mTOR network have on models of drug reward and on paradigms that recapitulate some of the psychological components of addiction.

RESULTS

There is evidence for the involvement of the mTOR pathway in the acute and rewarding effects of drugs of abuse, especially psychostimulants. However, the data regarding opiates are scarce. There is a need to use sophisticated animal models of addiction to ascertain the real role of the mTOR pathway in this pathology and not just in drug-mediated reward. The involvement of this pathway in behavioural addictions and impulsivity should also be studied in detail in the future.

CONCLUSIONS

Although there is a plethora of data about the modulation of mTOR by drugs of abuse, the involvement of this signalling pathway in addictive disorders requires further research.

Rapid anti-depressant-like effects of ketamine and other candidates: Molecular and cellular mechanisms

Major depressive disorder takes at least 3 weeks for clinical anti‐depressants, such as serotonin selective reuptake inhibitors, to take effect, and only one‐third of patients remit. Ketamine, a kind of anaesthetic, can alleviate symptoms of major depressive disorder patients in a short time and is reported to be effective to treatment‐resistant depression patients. The rapid and strong anti‐depressant‐like effects of ketamine cause wide concern. In addition to ketamine, caloric restriction and sleep deprivation also elicit similar rapid anti‐depressant‐like effects. However, mechanisms about the rapid anti‐depressant‐like effects remain unclear. Elucidating the mechanisms of rapid anti‐depressant effects is the key to finding new therapeutic targets and developing therapeutic patterns. Therefore, in this review we summarize potential molecular and cellular mechanisms of rapid anti‐depressant‐like effects based on the pre‐clinical and clinical evidence, trying to provide new insight into future therapy.

Autophagy-Based Hypothesis on the Role of Brain Catecholamine Response During Stress

Stressful events, similar to abused drugs, significantly affect the homeostatic balance of the catecholamine brain systems while activating compensation mechanisms to restore balance. In detail, norepinephrine (NE)- and dopamine (DA)-containing neurons within the locus coeruleus (LC) and ventral tegmental area (VTA), are readily and similarly activated by psychostimulants and stressful events involving neural processes related to perception, reward, cognitive evaluation, appraisal, and stress-dependent hormonal factors. Brain catecholamine response to stress results in time-dependent regulatory processes involving mesocorticolimbic circuits and networks, where LC-NE neurons respond more readily than VTA-DA neurons. LC-NE projections are dominant in controlling the forebrain DA-targeted areas, such as the nucleus accumbens (NAc) and medial pre-frontal cortex (mPFC). Heavy and persistent coping demand could lead to sustained LC-NE and VTA-DA neuronal activity, that, when persisting chronically, is supposed to alter LC-VTA synaptic connections. Increasing evidence has been provided indicating a role of autophagy in modulating DA neurotransmission and synaptic plasticity. This alters behavior, and emotional/cognitive experience in response to drug abuse and occasionally, to psychological stress. Thus, relevant information to address the role of stress and autophagy can be drawn from psychostimulants research. In the present mini-review we discuss the role of autophagy in brain catecholamine response to stress and its dysregulation. The findings here discussed suggest a crucial role of regulated autophagy in the response and adaptation of LC-NE and VTA-DA systems to stress.

Morphine self-administration alters the expression of translational machinery genes in the amygdala of male Lewis rats

BACKGROUND

Addiction is a chronic disorder with a high risk of relapse. The neural mechanisms mediating addictions require protein synthesis, which could be relevant for the development of more effective treatments. The mTOR signaling pathway regulates protein synthesis processes that have recently been linked to the development of drug addiction.

AIMS

To assess the effects of morphine self-administration and its subsequent extinction on the expression of several genes that act in this pathway, and on the levels of specific phosphoproteins (Akt, Gsk3α/β, mTOR, PDK1 and p70 S6 kinase) in the amygdala, nucleus accumbens, and the prefrontal cortex.

METHODS

Male Lewis rats underwent morphine self-administration (1 mg/kg) for 19 days. They subsequently were submitted to extinction training for 15 days. Rats were killed either after self-administration or extinction, their brains extracted, and gene expression or phosphoprotein levels were assessed.

RESULTS

We found an increase in Raptor and Eif4ebp2 expression in the amygdala of rats that self-administered morphine, even after extinction. The expression of Insr in the amygdala of control animals decreased over time while the opposite effect was seen in the rats that self-administered morphine.

CONCLUSIONS

Our results suggest that morphine self-administration affects the gene expression of some elements of the translational machinery in the amygdala.

Quantitative Systems Pharmacological Analysis of Drugs of Abuse Reveals the Pleiotropy of Their Targets and the Effector Role of mTORC1

Existing treatments against drug addiction are often ineffective due to the complexity of the networks of protein-drug and protein-protein interactions (PPIs) that mediate the development of drug addiction and related neurobiological disorders. There is an urgent need for understanding the molecular mechanisms that underlie drug addiction toward designing novel preventive or therapeutic strategies. The rapidly accumulating data on addictive drugs and their targets as well as advances in machine learning methods and computing technology now present an opportunity to systematically mine existing data and draw inferences on potential new strategies. To this aim, we carried out a comprehensive analysis of cellular pathways implicated in a diverse set of 50 drugs of abuse using quantitative systems pharmacology methods. The analysis of the drug/ligand-target interactions compiled in DrugBank and STITCH databases revealed 142 known and 48 newly predicted targets, which have been further analyzed to identify the KEGG pathways enriched at different stages of drug addiction cycle, as well as those implicated in cell signaling and regulation events associated with drug abuse. Apart from synaptic neurotransmission pathways detected as upstream signaling modules that “sense” the early effects of drugs of abuse, pathways involved in neuroplasticity are distinguished as determinants of neuronal morphological changes. Notably, many signaling pathways converge on important targets such as mTORC1. The latter emerges as a universal effector of the persistent restructuring of neurons in response to continued use of drugs of abuse.

Autophagy pathways in drug abusers after forensic autopsy: LC3B, ph-mTOR and p70S6K analysis

INTRODUCTION

Autophagy plays a role in various central nervous system diseases. Little is known about its molecular activation in drug addiction. Our aim was to investigate the signalling pathways of autophagy in brain tissues from drug abusers.

METHODS

Twenty-five drug abusers with acute lethal intoxication and 10 controls were medico-legally autopsied. Brain-tissue samples from the parietal cortex and cerebellum were obtained. Expression of LC3B, phospho-mTOR (ph-mTOR) and phospho70S6 Kinase (p70S6K) was identified in tissue microarrays, with three tissue spots per case. Blood, urine or vitreous humour were tested in all cases to identify the acute intoxication. Hair analysis was performed in 14 cases to confirm chronic intoxication; the remaining cases had a documented medical history of chronic abuse.

RESULTS

The autophagy marker LC3B was always positive on both the cortex and the cerebellum, stratified as strongly in 18 (72%) cases and weakly positive in seven (28%) cases. ph-mTOR was negative in all cases. The p70S6K molecule showed positivity in 14 (56%) cases on cortex tissue. The cerebellum was always negative, except for Purkinje cells. Drug abusers had statistically more double positive cases (LC3B-p70S6K) than controls ( p=0.0094).

CONCLUSION

Autophagy pathways were activated in our series, and 56% of drug abusers showed simultaneous LC3B-p70S6K immunoexpression on tissue from the parietal cortex and cerebellum. This may be of value in autopsy practice as an indicator of brain damage due to drug abuse and could serve as alternative or additional double sensitive diagnostic method to detect drug-related deaths using a tissue-based rationale.

Withdrawal From Cocaine Self-administration Alters the Regulation of Protein Translation in the Nucleus Accumbens

BACKGROUND

Cue-induced cocaine craving incubates during abstinence from cocaine self-administration. Expression of incubation ultimately depends on elevation of homomeric GluA1 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors in the nucleus accumbens (NAc). This adaptation requires ongoing protein translation for its maintenance. Aberrant translation is implicated in central nervous system diseases, but nothing is known about glutamatergic regulation of translation in the drug-naïve NAc or after incubation.

METHODS

NAc tissue was obtained from drug-naïve rats and from rats after 1 or >40 days of abstinence from extended-access cocaine or saline self-administration. Newly translated proteins were labeled using 35S-Met/Cys or puromycin. We compared basal overall translation and its regulation by metabotropic glutamate receptor 1 (mGlu1), mGlu5, and N-methyl-D-aspartate receptors (NMDARs) in drug-naïve, saline control, and cocaine rats, and we compared GluA1 and GluA2 translation by immunoprecipitating puromycin-labeled proteins.

RESULTS

In all groups, overall translation was unaltered by mGlu1 blockade (LY367385) but increased by mGlu5 blockade (MTEP). NMDAR blockade (AVP) increased overall translation in drug-naïve and saline control rats but not in cocaine/late withdrawal rats. Cocaine/late withdrawal rats exhibited greater translation of GluA1 (but not GluA2), which was not further affected by NMDAR blockade.

CONCLUSIONS

Our results suggest that increased GluA1 translation contributes to the elevated homomeric GluA1 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor levels in the NAc that mediate incubation. Additional contributions to incubation-related plasticity may result from loss of the braking influence on translation normally exerted by NMDARs. Apart from elucidating incubation-related adaptations, we found a suppressive effect of mGlu5 on NAc translation regardless of drug exposure, which is opposite to results obtained in the hippocampus and points to heterogeneity of translational regulation between brain regions.

Ionotropic and metabotropic glutamate receptors regulate protein translation in co-cultured nucleus accumbens and prefrontal cortex neurons

The regulation of protein translation by glutamate receptors and its role in plasticity have been extensively studied in the hippocampus. In contrast, very little is known about glutamatergic regulation of translation in nucleus accumbens (NAc) medium spiny neurons (MSN), despite their critical role in addiction-related plasticity and recent evidence that protein translation contributes to this plasticity. We used a co-culture system, containing NAc MSNs and prefrontal cortex (PFC) neurons, and fluorescent non-canonical amino acid tagging (FUNCAT) to visualize newly synthesized proteins in neuronal processes of NAc MSNs and PFC pyramidal neurons. First, we verified that the FUNCAT signal reflects new protein translation. Next, we examined the regulation of translation by group I metabotropic glutamate receptors (mGluRs) and ionotropic glutamate receptors by incubating co-cultures with agonists or antagonists during the 2-h period of non-canonical amino acid labeling. In NAc MSNs, basal translation was modestly reduced by blocking Ca²⁺-permeable AMPARs whereas blocking all AMPARs or suppressing constitutive mGluR5 signaling enhanced translation. Activating group I mGluRs with dihydroxyphenylglycine increased translation in an mGluR1-dependent manner in NAc MSNs and PFC pyramidal neurons. Disinhibiting excitatory transmission with bicuculline also increased translation. In MSNs, this was reversed by antagonists of mGluR1, mGluR5, AMPARs or NMDARs. In PFC neurons, AMPAR or NMDAR antagonists blocked bicuculline-stimulated translation. Our study, the first to examine glutamatergic regulation of translation in MSNs, demonstrates regulatory mechanisms specific to MSNs that depend on the level of neuronal activation. This sets the stage for understanding how translation may be altered in addiction.

VTA mTOR Signaling Regulates Dopamine Dynamics, Cocaine-Induced Synaptic Alterations, and Reward

Mechanistic target of rapamycin (mTOR) regulates long-term synaptic plasticity, learning, and memory by controlling dendritic protein synthesis. The mTOR inhibitor rapamycin has been shown to attenuate the behavioral effects of drugs of abuse, including cocaine. Using viral vectors to selectively delete mTOR in the ventral tegmental area (VTA) in adult male mTOR^loxP/loxP mice, we investigated the role of mTOR in regulating neuronal morphology, basal synaptic transmission, dopamine dynamics, and cocaine-induced synaptic plasticity and rewarding effects. We find that targeted deletion of mTOR in the VTA had no significant effects on soma size and dendritic morphology of VTA neurons but significantly decreased dopamine release and reuptake in the nucleus accumbens (NAc) shell, a major target region. Western blot analysis revealed that mTOR deletion led to decreases in phosphorylated tyrosine hydroxylase (pTH-Ser40) levels in the VTA and dopamine transporter expression in the NAc. mTOR deletion had no significant effects on basal excitatory transmission in VTA dopamine neurons but caused an increase in GABAergic inhibition because of an increase in VTA GABAergic neuron firing. Furthermore, mTOR deletion attenuated conditioned place preference to cocaine and cocaine-induced potentiation of excitation and reduction of GABAergic inhibition in VTA dopamine neurons. Taken together, these results suggest that loss of mTOR in the VTA shifts the balance of excitatory and inhibitory synaptic transmission and decreases dopamine release and reuptake in the NAc. In addition, VTA mTOR signaling regulates cocaine-cue associative learning and cocaine-induced synaptic plasticity in VTA dopamine neurons.

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.

Targeting the intracellular signaling "STOP" and "GO" pathways for the treatment of alcohol use disorders

In recent years, research has identified the molecular and neural substrates underlying the transition of moderate "social" consumption of alcohol to the characteristic alcohol use disorder (AUD) phenotypes including excessive and compulsive alcohol use which we define in the review as the GO signaling pathways. In addition, growing evidence points to the existence of molecular mechanisms that keep alcohol consumption in check and that confer resilience for the development of AUD which we define herein as the STOP signaling pathways. In this review, we focus on examples of the GO and the STOP intracellular signaling pathways and discuss our current knowledge of how manipulations of these pathways may be used for the treatment of AUD.

Essential role of D1R in the regulation of mTOR complex1 signaling induced by cocaine

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that is involved in neuronal adaptions that underlie cocaine-induced sensitization and reward. mTOR exists in two functionally distinct multi-component complexes known as mTORC1 and mTORC2. In this study, we show that increased mTORC1 activity induced by cocaine is mediated by the dopamine D1 receptor (D1R). Specifically, cocaine treatment increased the phosphorylation on residues Thr2446 and Ser2481 but not on Ser2448 in the nucleus accumbens (NAc) and that this increase in phosphorylated mTOR levels was also apparent when complexed with its binding partner Raptor. Furthermore, the increase in phosphorylated mTOR levels, as well as phosphorylated 4E-BP1 and S6K, downstream targets of mTORC1 were blocked with SCH23390 treatment. Similar results were also observed in the dopamine-transporter knockout mice as the increase in phosphorylated mTOR Thr2446 and Ser2481 was blocked by SCH23390 but not with raclopride. To further validate D1R role in mTORC1 signaling, decrease in phosphorylated mTOR levels were observed in D1R knockout mice, whereas administration of SKF81297 elevated phosphorylated mTOR in the NAc. Lastly deletion of mTOR or Raptor in D1R expressing neurons reduced cocaine-induced locomotor activity. Together, our data supports a mechanism whereby mTORC1 signaling is activated by cocaine administration through the stimulation of D1R.

Distinct miRNA expression in dorsal striatal subregions is associated with risk for addiction in rats

Recently, we published data using an animal model that allowed us to characterize animals into two groups, addiction vulnerable and addiction resilient, where we identified that addiction/relapse vulnerability was associated with deficits in synaptic plasticity-associated gene expression in the dorsal striatum (DS). Notable was the strong reduction in expression for activity-regulated cytoskeleton-associated protein (Arc) considered a master regulator of synaptic plasticity. In the present study, we confirmed that Arc messenger RNA was significantly decreased in the DS, but importantly, we identified that this reduction was restricted to the dorsomedial (DMS) and not dorsolateral striatum (DLS). There is recent evidence of microRNA (miRNA)-associated posttranscriptional suppression of Arc and animal models of addiction have identified a key role for miRNA in the regulation of addiction-relevant genes. In further support of this link, we identified several differentially expressed miRNA with the potential to influence addiction-relevant plasticity genes, including Arc. A key study recently reported that miR-212 expression is protective against compulsive cocaine-seeking. Supporting this hypothesis, we found that miR-212 expression was significantly reduced in the DMS but not DLS of addiction-vulnerable animals. Together, our data provide strong evidence that miRNA promote ongoing plasticity deficits in the DS of addiction-vulnerable animals.

Selective suppression of excessive GluN2C expression rescues early epilepsy in a tuberous sclerosis murine model

Tuberous sclerosis complex (TSC), caused by dominant mutations in either TSC1 or TSC2 tumour suppressor genes is characterized by the presence of brain malformations, the cortical tubers that are thought to contribute to the generation of pharmacoresistant epilepsy. Here we report that tuberless heterozygote Tsc1^+/− mice show functional upregulation of cortical GluN2C-containing N-methyl-D-aspartate receptors (NMDARs) in an mTOR-dependent manner and exhibit recurrent, unprovoked seizures during early postnatal life (<P19). Seizures are generated intracortically in the granular layer of the neocortex. Slow kinetics of aberrant GluN2C-mediated currents in spiny stellate cells promotes excessive temporal integration of persistent NMDAR-mediated recurrent excitation and seizure generation. Accordingly, specific GluN2C/D antagonists block seizures in Tsc1^+/− mice in vivo and in vitro. Likewise, GluN2C expression is upregulated in TSC human surgical resections, and a GluN2C/D antagonist reduces paroxysmal hyperexcitability. Thus, GluN2C receptor constitutes a promising molecular target to treat epilepsy in TSC patients.

Tuberous sclerosis complex (TSC) is a rare genetic condition characterized by epileptic seizures that start in infancy. Here, the authors show that these seizures are modulated by GluN2C-containing NMDA receptors in the cortex of a mouse model of TSC, and that suppressing their activity attenuates seizures.

mTOR complex 1: a key player in neuroadaptations induced by drugs of abuse

The mammalian (or mechanistic) target of rapamycin (mTOR) complex 1 (mTORC1) is a serine and threonine kinase that regulates cell growth, survival, and proliferation. mTORC1 is a master controller of the translation of a subset of mRNAs. In the central nervous system mTORC1 plays a crucial role in mechanisms underlying learning and memory by controlling synaptic protein synthesis. Here, we review recent evidence suggesting that the mTORC1 signaling pathway promotes neuroadaptations following exposure to a diverse group of drugs of abuse including stimulants, cannabinoids, opiates, and alcohol. We further describe potential molecular mechanisms by which drug-induced mTORC1 activation may alter brain functions. Finally, we propose that mTORC1 is a focal point shared by drugs of abuse to mediate drug-related behaviors such as reward seeking and excessive drug intake, and offer future directions to decipher the contribution of the kinase to mechanisms underlying addiction. Recent studies suggesting that exposure to diverse classes of drugs of abuse as well as exposure to drug-associated memories lead to mTORC1 kinase activation in the limbic system. In turn, mTORC1 controls the onset and the maintenance of pathological neuroadaptions that underlie several features of drug addiction such as drug seeking and relapse. Therefore, we propose that targeting mTORC1 and its effectors is a promising strategy to treat drug disorders.

mTORC1 inhibition in the nucleus accumbens 'protects' against the expression of drug seeking and 'relapse' and is associated with reductions in GluA1 AMPAR and CAMKIIα levels

The mechanistic target of rapamycin complex 1 (mTORC1) is necessary for synaptic plasticity, as it is critically involved in the translation of synaptic transmission-related proteins, such as Ca(2+)/Calmodulin-dependent kinase II alpha (CAMKIIα) and AMPA receptor subunits (GluAs). Although recent studies have implicated mTORC1 signaling in drug-motivated behavior, the ineffectiveness of rapamycin, an mTORC1 inhibitor, in suppressing cocaine self-administration has raised questions regarding the specific role of mTORC1 in drug-related behaviors. Here, we examined mTORC1's role in three drug-related behaviors: cocaine taking, withdrawal, and reinstatement of cocaine seeking, by measuring indices of mTORC1 activity and assessing the effect of intra-cerebroventricular rapamycin on these behaviors in rats. We found that withdrawal from cocaine self-administration increased indices of mTORC1 activity in the nucleus accumbens (NAC). Intra-cerebroventricular rapamycin attenuated progressive ratio (PR) break points and reduced phospho-p70 ribosomal S6 kinase, GluA1 AMPAR, and CAMKIIα levels in the NAC shell (NACsh) and core (NACc). In a subsequent study, we treated rats with intra-NACsh infusions of rapamycin (2.5 μg/side/day for 5 days) during cocaine self-administration and then tracked the expression of addiction-relevant behaviors through to withdrawal and extinction. Rapamycin reduced drug seeking in signaled non-drug-available periods, PR responding, and cue-induced reinstatement, with these effects linked to reduced mTORC1 activity, total CAMKIIα, and GluA1 AMPAR levels in the NACsh. Together, these data highlight a role for mTORC1 in the neural processes that control the expression and maintenance of drug reward, including protracted relapse vulnerability. These effects appear to involve a role for mTORC1 in the regulation of GluA1 AMPARs and CAMKIIα in the NACsh.

A protein synthesis-dependent mechanism sustains calcium-permeable AMPA receptor transmission in nucleus accumbens synapses during withdrawal from cocaine self-administration

Extended-access cocaine self-administration results in withdrawal-dependent incubation of cocaine craving. Rats evaluated after ∼1 month of withdrawal from such regimens ("incubated rats") exhibit changes in medium spiny neurons (MSNs) of the nucleus accumbens (NAc) that include accumulation of Ca(2+)-permeable AMPA receptors (CP-AMPARs) and a switch in group I metabotropic glutamate receptor (mGluR)-mediated suppression of synaptic transmission from mGluR5-dependent to mGluR1-dependent. To determine the role of protein synthesis in mediating these adaptations, we conducted whole-cell patch-clamp recordings in NAc core MSNs of "incubated rats" in the presence of translational inhibitors (anisomycin, cycloheximide, rapamycin) or the transcriptional inhibitor actinomycin-D. The contribution of CP-AMPARs to synaptic transmission was determined by the rectification index and the sensitivity to the CP-AMPAR antagonist 1-naphthyl acetyl spermine. We found that CP-AMPAR-mediated transmission in the NAc of "incubated rats" was reduced to levels comparable to those found in saline control rats when brain slices were treated with translational inhibitors, whereas actinomycin-D had no effect. We also investigated the effect of protein translation inhibitors on the switch of mGluR function in MSNs of "incubated rats" using the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine in combination with either an mGluR1 (LY367385) or an mGluR5 (3-[(2-methyl-4-thiazolyl)ethynyl]pyridine) antagonist. Data revealed that inhibition of protein translation eliminated the mGluR1-mediated inhibition and restored the mGluR5 responsiveness to a state functionally similar to that of saline control rats. Together, these results suggest that aberrant regulation of local protein synthesis contributes to the maintenance of adaptations accrued at NAc MSN synapses during incubation of cocaine craving.

Long-term effects of cocaine experience on neuroplasticity in the nucleus accumbens core of addiction-prone rats

Repeated exposure to drugs of abuse is associated with structural plasticity in brain reward pathways. Rats selectively bred for locomotor response to novelty differ on a number of neurobehavioral dimensions relevant to addiction. This unique genetic animal model was used here to examine both pre-existing differences and long-term consequences of repeated cocaine treatment on structural plasticity. Selectively bred high-responder (bHR) and low-responder (bLR) rats received repeated saline or cocaine injections for 9 consecutive days. Escalating doses of cocaine (7.5, 15 and 30 mg/kg) were administered on the first (day 1) and last (day 9) days of treatment and a single injection of the intermediate dose (15 mg/kg) was given on days 2-8. Motor activity in response to escalating doses of cocaine was compared on the first and last days of treatment to assess the acute and sensitized response to the drug. Following prolonged cocaine abstinence (28 days), spine density was examined on terminal dendrites of medium spiny neurons in the nucleus accumbens core. Relative to bLRs, bHRs exhibited increased psychomotor activation in response to both the acute and repeated effects of cocaine. There were no differences in spine density between bHR and bLR rats under basal conditions or following repeated saline treatment. However, spine density differed markedly between these two lines following prolonged cocaine abstinence. All spine types were decreased in cocaine-treated bHRs, while only mushroom spines were decreased in bLRs that received cocaine. Changes in spine density occurred specifically near the branch point of terminal dendrites. These findings indicate that structural plasticity associated with prolonged cocaine abstinence varies markedly in two selected strains of rats that vary on numerous traits relevant to addiction. Thus, genetic factors that contribute to individual variation in the behavioral response to cocaine also influence cocaine-induced structural plasticity.

Intracerebroventricular administration of ouabain, a Na/K-ATPase inhibitor, activates mTOR signal pathways and protein translation in the rat frontal cortex

Intracerebroventricular (ICV) injection of ouabain, a specific Na/K-ATPase inhibitor, induces behavioral changes in rats in a putative animal model of mania. The binding of ouabain to Na/K-ATPase affects signaling molecules in vitro, including ERK1/2 and Akt, which promote protein translation. We have also reported that ERK1/2 and Akt in the brain are involved in the ouabain-induced hyperactivity of rats. In this study, rats were given an ICV injection of ouabain, and then their frontal cortices were examined to determine the effects of ouabain on the mTOR/p70S6K/S6 signaling pathway and protein translation, which are important in modifications of neural circuits and behavior. Rats showed ouabain-induced hyperactivity up to 8h following injection, and increased phosphorylation levels of mTOR, p70S6K, S6, eIF4B, and 4E-BP at 1, 2, 4, and 8h following ouabain injection. Immunohistochemical analyses revealed that increased p-S6 immunoreactivity in the cytoplasm of neurons by ouabain was evident in the prefrontal, cingulate, and orbital cortex. These findings suggested increased translation initiation in response to ouabain. The rate of protein synthesis was measured as the amount of [(3)H]-leucine incorporation in the cell-free extracts of frontal cortical tissues, and showed a significant increase at 8h after ouabain injection. These results suggest that ICV injection of ouabain induced activation of the protein translation initiation pathway regulated by ERK1/2 and Akt, and prolonged hyperactivity in rats. In conclusion, protein translation pathway could play an important role in ouabain-induced hyperactivity in a rodent model of mania.

Consolidation and translation regulation

mRNA translation, or protein synthesis, is a major component of the transformation of the genetic code into any cellular activity. This complicated, multistep process is divided into three phases: initiation, elongation, and termination. Initiation is the step at which the ribosome is recruited to the mRNA, and is regarded as the major rate-limiting step in translation, while elongation consists of the elongation of the polypeptide chain; both steps are frequent targets for regulation, which is defined as a change in the rate of translation of an mRNA per unit time. In the normal brain, control of translation is a key mechanism for regulation of memory and synaptic plasticity consolidation, i.e., the off-line processing of acquired information. These regulation processes may differ between different brain structures or neuronal populations. Moreover, dysregulation of translation leads to pathological brain function such as memory impairment. Both normal and abnormal function of the translation machinery is believed to lead to translational up-regulation or down-regulation of a subset of mRNAs. However, the identification of these newly synthesized proteins and determination of the rates of protein synthesis or degradation taking place in different neuronal types and compartments at different time points in the brain demand new proteomic methods and system biology approaches. Here, we discuss in detail the relationship between translation regulation and memory or synaptic plasticity consolidation while focusing on a model of cortical-dependent taste learning task and hippocampal-dependent plasticity. In addition, we describe a novel systems biology perspective to better describe consolidation.