Extracellular signal-regulated protein kinases 1 and 2 activation by addictive drugs: a signal toward pathological adaptation

Nuclear Calcium Signaling in D1 Receptor-Expressing Neurons of the Nucleus Accumbens Regulates Molecular, Cellular, and Behavioral Adaptations to Cocaine

BACKGROUND

The persistence of cocaine-evoked adaptations relies on gene regulations within the reward circuit, especially in the ventral striatum (i.e., nucleus accumbens [NAc]). Notably, activation of the ERK (extracellular signal-regulated kinase) pathway in the striatum is known to trigger a transcriptional program shaping long-term responses to cocaine. Nuclear calcium signaling has also been shown to control multiple forms of transcription-dependent neuroadaptations, but the dynamics and roles of striatal nuclear calcium signaling in preclinical models of addiction remain unknown.

METHODS

A genetically encoded cell type-specific nuclear calcium probe has been developed to monitor calcium dynamics in the nuclei of striatal neurons, including in freely moving mice. A cell type-specific inhibitor of nuclear calcium signaling combined with 3-dimensional imaging of neuronal morphology, immunostaining, and behavior was used to disentangle the roles of nuclear calcium in NAc medium spiny neurons (MSNs) expressing the dopamine D1 receptor (D1R) or D2 receptor (D2R) on cocaine-evoked responses.

RESULTS

The D1R-mediated potentiation of calcium influx through glutamate NMDA receptors, which shapes cocaine effects, also drives nuclear calcium transients. Fiber photometry revealed that cocaine-treated mice showed a sustained nuclear calcium increase in NAc D1R-MSNs. Disrupting nuclear calcium in D1R-MSNs, but not D2R-MSNs, blocked cocaine-evoked morphological changes of MSNs and gene expression and blunted cocaine's rewarding effects.

CONCLUSIONS

Our study unravels the dynamics and roles of cocaine-induced nuclear calcium signaling increases in D1R-MSNs on molecular, cellular, and behavioral adaptations to cocaine and represents a significant breakthrough because it could contribute to the development of innovative strategies with therapeutic potential to alleviate addiction symptoms.

How life events may confer vulnerability to addiction: the role of epigenetics

Substance use disorder (SUD) represents a large and growing global health problem. Despite the strong addictive potency of drugs of abuse, only a minority of those exposed develop SUDs. While certain life experiences (e.g., childhood trauma) may increase subsequent vulnerability to SUDs, mechanisms underlying these effects are not yet well understood. Given the chronic and relapsing nature of SUDs, and the length of time that can elapse between prior life events and subsequent drug exposure, changes in SUD vulnerability almost certainly involve long-term epigenetic dysregulation. To validate this idea, functional effects of specific epigenetic modifications in brain regions mediating reinforcement learning (e.g., nucleus accumbens, prefrontal cortex) have been investigated in a variety of animal models of SUDs. In addition, the effects of epigenetic modifications produced by prior life experiences on subsequent SUD vulnerability have been studied, but mostly in a correlational manner. Here, we review how epigenetic mechanisms impact SUD-related behavior in animal models and summarize our understanding of the relationships among life experiences, epigenetic regulation, and future vulnerability to SUDs. Despite variations in study design, epigenetic modifications that most consistently affect SUD-related behavior are those that produce predominantly unidirectional effects on gene regulation, such as DNA methylation and histone phosphorylation. Evidence explicitly linking environmentally induced epigenetic modifications to subsequent SUD-related behavior is surprisingly sparse. We conclude by offering several directions for future research to begin to address this critical research gap.

Endogenous Regulator of G protein Signaling 14 (RGS14) suppresses cocaine-induced emotionally motivated behaviors in female mice

Addictive drugs hijack the neuronal mechanisms of learning and memory in motivation and emotion processing circuits to reinforce their own use. Regulator of G-protein Signaling 14 (RGS14) is a natural suppressor of post-synaptic plasticity underlying learning and memory in the hippocampus. The present study used immunofluorescence and RGS14 knockout mice to assess the role of RGS14 in behavioral plasticity and reward learning induced by chronic cocaine in emotional-motivational circuits. We report that RGS14 is strongly expressed in discrete regions of the ventral striatum and extended amygdala in wild-type mice, and is co-expressed with D1 and D2 dopamine receptors in neurons of the nucleus accumbens (NAc). Of note, we found that RGS14 is upregulated in the NAc in mice with chronic cocaine history following acute cocaine treatment. We found significantly increased cocaine-induced locomotor sensitization, as well as enhanced conditioned place preference and conditioned locomotor activity in RGS14-deficient mice compared to wild-type littermates. Together, these findings suggest that endogenous RGS14 suppresses cocaine-induced plasticity in emotional-motivational circuits, implicating RGS14 as a protective agent against the maladaptive neuroplastic changes that occur during addiction.

Cell-type specific synaptic plasticity in dorsal striatum is associated with punishment-resistance compulsive-like cocaine self-administration in mice

Addiction-related compulsion-like behavior can be modeled in rodents with drug self-administration (SA) despite harmful consequences. Recent studies suggest that the potentiation of glutamatergic transmission at the orbitofrontal cortex (OFC) to dorsal striatum (DS) synapses drives the transition from controlled to compulsion-like SA. However, the timing of the induction of this synaptic plasticity remains elusive. Here, mice were first allowed to intravenously self-administer cocaine. When mice had to endure a risk of electrical foot shock, only a fraction persevered in cocaine SA. In these persevering mice, we recorded high A/N ratios (AMPA-R/NMDA-R: α-amino-3hydroxy-5-methyl-4-isoxazolepropionic acid receptor/N-methyl-D-aspartate receptor) in both types of spiny projection neurons (i.e., D1 and D2 dopamine receptor-expressing SPNs). By contrast, when we prepared slices at the end of the acquisition period, in all mice, the A/N was high in D1R- but not D2R-SPNs. These results indicate that the transition to compulsion-like cocaine SA emerges during the punishment sessions, where synapses onto D2R-SPNs are strengthened. In renouncing individuals, the cocaine-evoked strengthening in D1R-SPNs is lost. Our study thus reveals the cell-type specific sequence of the induction of plasticity that eventually may cause compulsion-like SA.

Segregation of D1 and D2 dopamine receptors in the striatal direct and indirect pathways: An historical perspective

The direct and indirect striatal pathways form a cornerstone of the circuits of the basal ganglia. Dopamine has opponent affects on the function of these pathways due to the segregation of the D1- and D2-dopamine receptors in the spiny projection neurons giving rise to the direct and indirect pathways. An historical perspective is provided on the discovery of dopamine receptor segregation leading to models of how the direct and indirect affect motor behavior.

Glycogen Synthase Kinase 3β Is a Key Regulator in the Inhibitory Effects of Accumbal Cocaine- and Amphetamine-Regulated Transcript Peptide 55-102 on Amphetamine-Induced Locomotor Activity

Microinjection of cocaine- and amphetamine-regulated transcript (CART) peptide 55-102 into the nucleus accumbens (NAcc) core significantly attenuates psychostimulant-induced locomotor activity. However, the molecular mechanism remains poorly understood. We examined the phosphorylation levels of Akt, glycogen synthase kinase 3β (GSK3β), and glutamate receptor 1 (GluA1) in NAcc core tissues obtained 60 min after microinjection of CART peptide 55-102 into this site, followed by systemic injection of amphetamine (AMPH). Phosphorylation levels of Akt at Thr308 and GSK3β at Ser9 were decreased, while those of GluA1 at Ser845 were increased, by AMPH treatment. These effects returned to basal levels following treatment with CART peptide 55-102. Furthermore, the negative regulatory effects of the CART peptide on AMPH-induced changes in phosphorylation levels and locomotor activity were all abolished by pretreatment with the S9 peptide, an artificially synthesized indirect GSK3β activator. These results suggest that the CART peptide 55-102 in the NAcc core plays a negative regulatory role in AMPH-induced locomotor activity by normalizing the changes in phosphorylation levels of Akt-GSK3β, and subsequently GluA1 modified by AMPH at this site. The present findings are the first to reveal GSK3β as a key regulator of the inhibitory role of the CART peptide in psychomotor stimulant-induced locomotor activity.

SWI/SNF chromatin remodeler complex within the reward pathway is required for behavioral adaptations to stress

Enduring behavioral changes upon stress exposure involve changes in gene expression sustained by epigenetic modifications in brain circuits, including the mesocorticolimbic pathway. Brahma (BRM) and Brahma Related Gene 1 (BRG1) are ATPase subunits of the SWI/SNF complexes involved in chromatin remodeling, a process essential to enduring plastic changes in gene expression. Here, we show that in mice, social defeat induces changes in BRG1 nuclear distribution. The inactivation of the Brg1/Smarca4 gene within dopamine-innervated regions or the constitutive inactivation of the Brm/Smarca2 gene leads to resilience to repeated social defeat and decreases the behavioral responses to cocaine without impacting midbrain dopamine neurons activity. Within striatal medium spiny neurons, Brg1 gene inactivation reduces the expression of stress- and cocaine-induced immediate early genes, increases levels of heterochromatin and at a global scale decreases chromatin accessibility. Altogether these data demonstrate the pivotal function of SWI/SNF complexes in behavioral and transcriptional adaptations to salient environmental challenges.

Repeated exposure to social stressors in rodents results in behavioural changes. Here the authors show that behavioural adaptations to stress are associated with nuclear organization changes through SWI/SNF chromatin remodeler in specific neuronal populations of the mesolimbic system.

Cell-Type-Specific Adaptions in Striatal Medium-Sized Spiny Neurons and Their Roles in Behavioral Responses to Drugs of Abuse

Drug addiction is defined as a compulsive pattern of drug-seeking- and taking- behavior, with recurrent episodes of abstinence and relapse, and a loss of control despite negative consequences. Addictive drugs promote reinforcement by increasing dopamine in the mesocorticolimbic system, which alters excitatory glutamate transmission within the reward circuitry, thereby hijacking reward processing. Within the reward circuitry, the striatum is a key target structure of drugs of abuse since it is at the crossroad of converging glutamate inputs from limbic, thalamic and cortical regions, encoding components of drug-associated stimuli and environment, and dopamine that mediates reward prediction error and incentive values. These signals are integrated by medium-sized spiny neurons (MSN), which receive glutamate and dopamine axons converging onto their dendritic spines. MSN primarily form two mostly distinct populations based on the expression of either DA-D1 (D1R) or DA-D2 (D2R) receptors. While a classical view is that the two MSN populations act in parallel, playing antagonistic functional roles, the picture seems much more complex. Herein, we review recent studies, based on the use of cell-type-specific manipulations, demonstrating that dopamine differentially modulates dendritic spine density and synapse formation, as well as glutamate transmission, at specific inputs projecting onto D1R-MSN and D2R-MSN to shape persistent pathological behavioral in response to drugs of abuse. We also discuss the identification of distinct molecular events underlying the detrimental interplay between dopamine and glutamate signaling in D1R-MSN and D2R-MSN and highlight the relevance of such cell-type-specific molecular studies for the development of innovative strategies with potential therapeutic value for addiction. Because drug addiction is highly prevalent in patients with other psychiatric disorders when compared to the general population, we last discuss the hypothesis that shared cellular and molecular adaptations within common circuits could explain the co-occurrence of addiction and depression. We will therefore conclude this review by examining how the nucleus accumbens (NAc) could constitute a key interface between addiction and depression.

A Rational Design of α-Helix-Shaped Peptides Employing the Hydrogen-Bond Surrogate Approach: A Modulation Strategy for Ras-RasGRF1 Interaction in Neuropsychiatric Disorders

In the last two decades, abnormal Ras (rat sarcoma protein)-ERK (extracellular signal-regulated kinase) signalling in the brain has been involved in a variety of neuropsychiatric disorders, including drug addiction, certain forms of intellectual disability, and autism spectrum disorder. Modulation of membrane-receptor-mediated Ras activation has been proposed as a potential target mechanism to attenuate ERK signalling in the brain. Previously, we showed that a cell penetrating peptide, RB3, was able to inhibit downstream signalling by preventing RasGRF1 (Ras guanine nucleotide-releasing factor 1), a neuronal specific GDP/GTP exchange factor, to bind Ras proteins, both in brain slices and in vivo, with an IC₅₀ value in the micromolar range. The aim of this work was to mutate and improve this peptide through computer-aided techniques to increase its inhibitory activity against RasGRF1. The designed peptides were built based on the RB3 peptide structure corresponding to the α-helix of RasGRF1 responsible for Ras binding. For this purpose, the hydrogen-bond surrogate (HBS) approach was exploited to maintain the helical conformation of the designed peptides. Finally, residue scanning, MD simulations, and MM-GBSA calculations were used to identify 18 most promising α-helix-shaped peptides that will be assayed to check their potential activity against Ras-RasGRF1 and prevent downstream molecular events implicated in brain disorders.

Disrupting D1-NMDA or D2-NMDA receptor heteromerization prevents cocaine's rewarding effects but preserves natural reward processing

Addictive drugs increase dopamine in the nucleus accumbens (NAc), where it persistently shapes excitatory glutamate transmission and hijacks natural reward processing. Here, we provide evidence, from mice to humans, that an underlying mechanism relies on drug-evoked heteromerization of glutamate N-methyl-d-aspartate receptors (NMDAR) with dopamine receptor 1 (D1R) or 2 (D2R). Using temporally controlled inhibition of D1R-NMDAR heteromerization, we unraveled their selective implication in early phases of cocaine-mediated synaptic, morphological, and behavioral responses. In contrast, preventing D2R-NMDAR heteromerization blocked the persistence of these adaptations. Interfering with these heteromers spared natural reward processing. Notably, we established that D2R-NMDAR complexes exist in human samples and showed that, despite a decreased D2R protein expression in the NAc, individuals with psychostimulant use disorder display a higher proportion of D2R forming heteromers with NMDAR. These findings contribute to a better understanding of molecular mechanisms underlying addiction and uncover D2R-NMDAR heteromers as targets with potential therapeutic value.

Heteromerization of dopaminergic receptors in the brain: Pharmacological implications

Dopamine exerts its physiological effects through two subtypes of receptors, i.e. the receptors of the D₁ family (D_1R and D_5R) and the D₂ family (D_2R, D_3R, and D_4R), which differ in their pattern of distribution, affinity, and signaling. The D₁-like subfamily (D_1R and D_5R) are coupled to Gα_s/olf proteins to activate adenylyl cyclase whereas the D₂-like receptors are coupled to Gα_i/o subunits and suppress the activity of adenylyl cyclase. Dopamine receptors are capable of forming homodimers, heterodimers, and higher-order oligomeric complexes, resulting in a change in the individual protomers' recognition, signaling, and pharmacology. Heteromerization has the potential to modify the canonical pharmacological features of individual monomeric units such as ligand affinity, activation, signaling, and cellular trafficking through allosteric interactions, reviving the field and introducing a new pharmacological target. Since heteromers are expressed and formed in a tissue-specific manner, they could provide the framework to design selective and effective drug candidates, such as brain-penetrant heterobivalent drugs and interfering peptides, with limited side effects. Therefore, heteromerization could be a promising area of pharmacology research, as it could contribute to the development of novel pharmacological interventions for dopamine dysregulated brain disorders such as addiction, schizophrenia, cognition, Parkinson's disease, and other motor-related disorders. This review is articulated based on the three criteria established by the International Union of Basic and Clinical Pharmacology for GPCR heterodimers (IUPHAR): evidence of co-localization and physical interactions in native or primary tissue, presence of a new physiological and functional property than the individual protomers, and loss of interaction and functional fingerprints upon heterodimer disruption.

Dopamine and glutamate receptors control social stress-induced striatal ERK1/2 activation

Stress has been acknowledged as one of the main risk factors for the onset of psychiatric disorders. Social stress is the most common type of stressor encountered in our daily lives. Uncovering the molecular determinants of the effect of stress on the brain would help understanding the complex maladaptations that contribute to pathological stress-related mental states. We examined molecular changes in the reward system following social defeat stress in mice, as increasing evidence implicates this system in sensing stressful stimuli. Following acute or chronic social defeat stress, the activation (i.e. phosphorylation) of extracellular signal-regulated kinases ERK1 and ERK2 (pERK1/2), markers of synaptic plasticity, was monitored in sub-regions of the reward system. We employed pharmacological antagonists and inhibitory DREADD to dissect the sequence of events controlling pERK1/2 dynamics. The nucleus accumbens (NAc) showed marked increases in pERK1/2 following both acute and chronic social stress compared to the dorsal striatum. Increases in pERK1/2 required dopamine D1 receptors and GluN2B-containing NMDA receptors. Paraventricular thalamic glutamatergic inputs to the NAc are required for social stress-induced pERK1/2. The molecular adaptations identified here could contribute to the long-lasting impact of stress on the brain and may be targeted to counteract stress-related psychopathologies.

Striatal Dopamine Induced ERK Phosphorylation Is Altered in Mouse Models of Monogenic Dystonia

BACKGROUND

Similar to some monogenic forms of dystonia, levodopa-induced dyskinesia is a hyperkinetic movement disorder with abnormal nigrostriatal dopaminergic neurotransmission. Molecularly, it is characterized by hyper-induction of phosphorylation of extracellular signal-related kinase in response to dopamine in medium spiny neurons of the direct pathway.

OBJECTIVES

The objective of this study was to determine if mouse models of monogenic dystonia exhibit molecular features of levodopa-induced dyskinesia.

METHODS

Western blotting and quantitative immunofluorescence was used to assay baseline and/or dopamine-induced levels of the phosphorylated kinase in the striatum in mouse models of DYT1, DYT6, and DYT25 expressing a reporter in dopamine D1 receptor-expressing projection neurons. Cyclic adenosine monophosphate (cAMP) immunoassay and adenylyl cyclase activity assays were also performed.

RESULTS

In DYT1 and DYT6 models, blocking dopamine reuptake with cocaine leads to enhanced extracellular signal-related kinase phosphorylation in dorsomedial striatal medium spiny neurons in the direct pathway, which is abolished by pretreatment with the N-methyl-d-aspartate antagonist MK-801. Phosphorylation is decreased in a model of DYT25. Levels of basal and stimulated cAMP and adenylyl cyclase activity were normal in the DYT1 and DYT6 mice and decreased in the DYT25 mice. Oxotremorine induced increased abnormal movements in the DYT1 knock-in mice.

CONCLUSIONS

The increased dopamine induction of extracellular signal-related kinase phosphorylation in 2 genetic types of dystonia, similar to what occurs in levodopa-induced dyskinesia, and its decrease in a third, suggests that abnormal signal transduction in response to dopamine in the postsynaptic nigrostriatal pathway might be a point of convergence for dystonia and other hyperkinetic movement disorders, potentially offering common therapeutic targets. © 2021 International Parkinson and Movement Disorder Society.

Cocaine-Dependent Acquisition of Locomotor Sensitization and Conditioned Place Preference Requires D1 Dopaminergic Signaling through a Cyclic AMP, NCS-Rapgef2, ERK, and Egr-1/Zif268 Pathway

Elucidation of the mechanism of dopamine signaling to ERK that underlies plasticity in dopamine D1 receptor-expressing neurons leading to acquired cocaine preference is incomplete. NCS-Rapgef2 is a novel cAMP effector, expressed in neuronal and endocrine cells in adult mammals, that is required for D1 dopamine receptor-dependent ERK phosphorylation in mouse brain. In this report, we studied the effects of abrogating NCS-Rapgef2 expression on cAMP-dependent ERK→Egr-1/Zif268 signaling in cultured neuroendocrine cells; in D1 medium spiny neurons of NAc slices; and in either male or female mouse brain in a region-specific manner. NCS-Rapgef2 gene deletion in the NAc in adult mice, using adeno-associated virus-mediated expression of cre recombinase, eliminated cocaine-induced ERK phosphorylation and Egr-1/Zif268 upregulation in D1-medium spiny neurons and cocaine-induced behaviors, including locomotor sensitization and conditioned place preference. Abrogation of NCS-Rapgef2 gene expression in mPFC and BLA, by crossing mice bearing a floxed Rapgef2 allele with a cre mouse line driven by calcium/calmodulin-dependent kinase IIα promoter also eliminated cocaine-induced phospho-ERK activation and Egr-1/Zif268 induction, but without effect on the cocaine-induced behaviors. Our results indicate that NCS-Rapgef2 signaling to ERK in dopamine D1 receptor-expressing neurons in the NAc, but not in corticolimbic areas, contributes to cocaine-induced locomotor sensitization and conditioned place preference. Ablation of cocaine-dependent ERK activation by elimination of NCS-Rapgef2 occurred with no effect on phosphorylation of CREB in D1 dopaminoceptive neurons of NAc. This study reveals a new cAMP-dependent signaling pathway for cocaine-induced behavioral adaptations, mediated through NCS-Rapgef2/phospho-ERK activation, independently of PKA/CREB signaling.SIGNIFICANCE STATEMENT ERK phosphorylation in dopamine D1 receptor-expressing neurons exerts a pivotal role in psychostimulant-induced neuronal gene regulation and behavioral adaptation, including locomotor sensitization and drug preference in rodents. In this study, we examined the role of dopamine signaling through the D1 receptor via a novel pathway initiated through the cAMP-activated guanine nucleotide exchange factor NCS-Rapgef2 in mice. NCS-Rapgef2 in the NAc is required for activation of ERK and Egr-1/Zif268 in D1 dopaminoceptive neurons after acute cocaine administration, and subsequent enhanced locomotor response and drug seeking behavior after repeated cocaine administration. This novel component in dopamine signaling provides a potential new target for intervention in psychostimulant-shaped behaviors, and new understanding of how D1-medium spiny neurons encode the experience of psychomotor stimulant exposure.

Established and In-trial GPCR Families in Clinical Trials: A Review for Target Selection

The largest family of drug targets in clinical trials constitute of GPCRs (G-protein coupled receptors) which accounts for about 34% of FDA (Food and Drug Administration) approved drugs acting on 108 unique GPCRs. Factors such as readily identifiable conserved motif in structures, 127 orphan GPCRs despite various de-orphaning techniques, directed functional antibodies for validation as drug targets, etc. has widened their therapeutic windows. The availability of 44 crystal structures of unique receptors, unexplored non-olfactory GPCRs (encoded by 50% of the human genome) and 205 ligand receptor complexes now present a strong foundation for structure-based drug discovery and design. The growing impact of polypharmacology for complex diseases like schizophrenia, cancer etc. warrants the need for novel targets and considering the undiscriminating and selectivity of GPCRs, they can fulfill this purpose. Again, natural genetic variations within the human genome sometimes delude the therapeutic expectations of some drugs, resulting in medication response differences and ADRs (adverse drug reactions). Around ~30 billion US dollars are dumped annually for poor accounting of ADRs in the US alone. To curb such undesirable reactions, the knowledge of established and currently in clinical trials GPCRs families can offer huge understanding towards the drug designing prospects including "off-target" effects reducing economical resource and time. The druggability of GPCR protein families and critical roles played by them in complex diseases are explained. Class A, class B1, class C and class F are generally established family and GPCRs in phase I (19%), phase II(29%), phase III(52%) studies are also reviewed. From the phase I studies, frizzled receptors accounted for the highest in trial targets, neuropeptides in phase II and melanocortin in phase III studies. Also, the bioapplications for nanoparticles along with future prospects for both nanomedicine and GPCR drug industry are discussed. Further, the use of computational techniques and methods employed for different target validations are also reviewed along with their future potential for the GPCR based drug discovery.

From Signaling Molecules to Circuits and Behaviors: Cell-Type-Specific Adaptations to Psychostimulant Exposure in the Striatum

Addiction is characterized by a compulsive pattern of drug seeking and consumption and a high risk of relapse after withdrawal that are thought to result from persistent adaptations within brain reward circuits. Drugs of abuse increase dopamine (DA) concentration in these brain areas, including the striatum, which shapes an abnormal memory trace of drug consumption that virtually highjacks reward processing. Long-term neuronal adaptations of gamma-aminobutyric acidergic striatal projection neurons (SPNs) evoked by drugs of abuse are critical for the development of addiction. These neurons form two mostly segregated populations, depending on the DA receptor they express and their output projections, constituting the so-called direct (D₁ receptor) and indirect (D₂ receptor) SPN pathways. Both SPN subtypes receive converging glutamate inputs from limbic and cortical regions, encoding contextual and emotional information, together with DA, which mediates reward prediction and incentive values. DA differentially modulates the efficacy of glutamate synapses onto direct and indirect SPN pathways by recruiting distinct striatal signaling pathways, epigenetic and genetic responses likely involved in the transition from casual drug use to addiction. Herein we focus on recent studies that have assessed psychostimulant-induced alterations in a cell-type-specific manner, from remodeling of input projections to the characterization of specific molecular events in each SPN subtype and their impact on long-lasting behavioral adaptations. We discuss recent evidence revealing the complex and concerted action of both SPN populations on drug-induced behavioral responses, as these studies can contribute to the design of future strategies to alleviate specific behavioral components of addiction.

Dopamine D1 and Glutamate Receptors Co-operate With Brain-Derived Neurotrophic Factor (BDNF) and TrkB to Modulate ERK Signaling in Adult Striatal Slices

In the striatum, the input nucleus of the basal ganglia, the extracellular-signal-regulated kinase (ERK) pathway, necessary for various forms of behavioral plasticity, is triggered by the combined engagement of dopamine D1 and ionotropic glutamate receptors. In this study, we investigated the potential crosstalk between glutamatergic, dopaminergic, and brain-derived neurotrophic factor (BDNF)-TrkB inputs to ERK cascade by using an ex vivo model of mouse striatal slices. Our results confirmed that the concomitant stimulation of D1 and glutamate receptors is necessary to activate ERK in striatal medium spiny neurons (MSNs). Moreover, we found that ERK activation is significantly enhanced when BDNF is co-applied either with glutamate or the D1 agonist SKF38393, supporting the idea of possible integration between BDNF, glutamate, and D1R-mediated signaling. Interestingly, ERK activation via BDNF-TrkB is upregulated upon blockade of either AMPAR/NMDAR or D1 receptors, suggesting a negative regulatory action of these two neurotransmitter systems on BDNF-mediated signaling. However, the observed enhancement of ERK1/2 phosphorylation does not result in corresponding downstream signaling changes at the nuclear level. Conversely, the TrkB antagonist cyclotraxin B partially prevents glutamate- and D1-mediated ERK activation. Altogether, these results suggest a complex and unexpected interaction among dopaminergic, glutamatergic, and BDNF receptor systems to modulate the ERK pathway in striatal neurons.

Glycogen Synthase Kinase 3β in the Ventral Hippocampus is Important for Cocaine Reward and Object Location Memory

The ventral hippocampus is a component of the neural circuitry involved with context-associated memory for reward and generation of appropriate behavioral responses to context. Glycogen synthase kinase 3 beta (GSK3β) has been linked to the maintenance of synaptic plasticity, contextual memory retrieval, and is involved in the reconsolidation of cocaine-associated contextual memory. In this study, the effects of targeted downregulation of GSK3β in the ventral hippocampus were examined on a series of behavioral tests for assessing drug reward-context association and non-reward related memory. The Cre/loxP site-specific recombination system was used to knockdown GSK3β through bilateral stereotaxic delivery of an adeno-associated virus expressing Cre-recombinase (AAV-Cre) into the ventral hippocampus of adult mice homozygous for a floxed GSK3β allele. GSK3β floxed mice injected with AAV-Cre had a loss of 56-75% of GSK3β in the ventral hippocampus and displayed diminished development of cocaine conditioned place preference, but not morphine place preference as compared with wild-type mice injected with AAV-Cre or GSK3β floxed mice injected with a control virus, AAV-GFP. Impaired object location memory was observed in mice with GSK3β downregulation in the ventral hippocampus, but novel object recognition remained intact. These results indicate that GSK3β signaling in the ventral hippocampus is differentially involved in the formation of place-drug reward association dependent upon drug class. Additionally, ventral hippocampal GSK3β signaling is important in detection of discrete spatial cues, but not recognition memory for objects.

Early epigenomic and transcriptional changes reveal Elk-1 transcription factor as a therapeutic target in Huntington's disease

Huntington's disease (HD) is a chronic neurodegenerative disorder characterized by a late clinical onset despite ubiquitous expression of the mutant Huntingtin gene (HTT) from birth. Transcriptional dysregulation is a pivotal feature of HD. Yet, the genes that are altered in the prodromal period and their regulators, which present opportunities for therapeutic intervention, remain to be elucidated. Using transcriptional and chromatin profiling, we found aberrant transcription and changes in histone H3K27acetylation in the striatum of R6/1 mice during the presymptomatic disease stages. Integrating these data, we identified the Elk-1 transcription factor as a candidate regulator of prodromal changes in HD. Exogenous expression of Elk-1 exerted beneficial effects in a primary striatal cell culture model of HD, and adeno-associated virus-mediated Elk-1 overexpression alleviated transcriptional dysregulation in R6/1 mice. Collectively, our work demonstrates that aberrant gene expression precedes overt disease onset in HD, identifies the Elk-1 transcription factor as a key regulator linked to early epigenetic and transcriptional changes in HD, and presents evidence for Elk-1 as a target for alleviating molecular pathology in HD.

Nuclear Receptor Nr4a1 Regulates Striatal Striosome Development and Dopamine D1 Receptor Signaling

The GABAergic medium-size spiny neuron (MSN), the striatal output neuron, may be classified into striosome, also known as patch, and matrix, based on neurochemical differences between the two compartments. At this time, little is known regarding the regulation of the development of the two compartments. Nr4a1, primarily described as a nuclear receptor/immediate early gene involved in the homeostasis of the dopaminergic system, is a striosomal marker. Using Nr4a1-overexpressing and Nr4a1-null mice, we sought to determine whether Nr4a1 is necessary and/or sufficient for striosome development. We report that in vivo and in vitro, Nr4a1 and Oprm1 mRNA levels are correlated. In the absence of Nr4a, there is a decrease in the percentage of striatal surface area occupied by striosomes. Alterations in Nr4a1 expression leads to dysregulation of multiple mRNAs of members of the dopamine receptor D₁ signal transduction system. Constitutive overexpression of Nr4a1 decreases both the induction of phosphorylation of ERK after a single cocaine exposure and locomotor sensitization following chronic cocaine exposure. Nr4a1 overexpression increases MSN excitability but reduces MSN long-term potentiation. In the resting state, type 5 adenylyl cyclase (AC5) activity is normal, but the ability of AC5 to be activated by Drd1 G-protein-coupled receptor inputs is decreased. Our results support a role for Nr4a1 in determination of striatal patch/matrix structure and in regulation of dopaminoceptive neuronal function.

Dopamine signaling in the striatum

The striatum integrates dopamine-mediated reward signals to generate appropriate behavior in response to glutamate-mediated sensory cues. Such associative learning relies on enduring neural plasticity in striatal GABAergic spiny projection neurons which, when altered, can lead to the development of a wide variety of pathological states. Considerable progress has been made in our understanding of the intracellular signaling mechanisms in dopamine-related behaviors and pathologies. Through the prism of the regulation of histone H3 and ribosomal protein S6 phosphorylation, we review how dopamine-mediated signaling events regulate gene transcription and mRNA translation. Particularly, we focus on the intracellular cascades controlling these phosphorylations downstream of the modulation of dopamine receptors by psychostimulants, antipsychotics and l-DOPA. Finally, we highlight the importance to precisely determine in which neuronal populations these signaling events occur in order to understand how they participate in remodeling neural circuits and altering dopamine-related behaviors.

LSD Administered as a Single Dose Reduces Alcohol Consumption in C57BL/6J Mice

There is a substantive clinical literature on classical hallucinogens, most commonly lysergic acid diethylamide (LSD) for the treatment of alcohol use disorder. However, there has been no published research on the effect of LSD on alcohol consumption in animals. This study evaluated the effect of LSD in mice using a two-bottle choice alcohol drinking paradigm. Adult male C57BL/6J mice were exposed to ethanol to develop preference and divided into three groups of equal ethanol consumption, and then treated with single intraperitoneal injection of saline or 25 or 50 μg/kg LSD and offered water and 20% ethanol. The respective LSD-treated groups were compared to the control group utilizing a multilevel model for repeated measures. In mice treated with 50 μg/kg LSD ethanol consumption was reduced relative to controls (p = 0.0035), as was ethanol preference (p = 0.0024), with a group mean reduction of ethanol consumption of 17.9% sustained over an interval of 46 days following LSD administration. No significant effects on ethanol consumption or preference were observed in mice treated with 25 μg/kg LSD. Neither total fluid intake nor locomotor activity in the LSD-treated groups differed significantly from controls. These results suggest that classical hallucinogens in the animal model merit further study as a potential approach to the identification of targets for drug discovery and investigation of the neurobiology of addiction.

Epigenetic Effects Induced by Methamphetamine and Methamphetamine-Dependent Oxidative Stress

Methamphetamine is a widely abused drug, which possesses neurotoxic activity and powerful addictive effects. Understanding methamphetamine toxicity is key beyond the field of drug abuse since it allows getting an insight into the molecular mechanisms which operate in a variety of neuropsychiatric disorders. In fact, key alterations produced by methamphetamine involve dopamine neurotransmission in a way, which is reminiscent of spontaneous neurodegeneration and psychiatric schizophrenia. Thus, understanding the molecular mechanisms operated by methamphetamine represents a wide window to understand both the addicted brain and a variety of neuropsychiatric disorders. This overlapping, which is already present when looking at the molecular and cellular events promoted immediately after methamphetamine intake, becomes impressive when plastic changes induced in the brain of methamphetamine-addicted patients are considered. Thus, the present manuscript is an attempt to encompass all the molecular events starting at the presynaptic dopamine terminals to reach the nucleus of postsynaptic neurons to explain how specific neurotransmitters and signaling cascades produce persistent genetic modifications, which shift neuronal phenotype and induce behavioral alterations. A special emphasis is posed on disclosing those early and delayed molecular events, which translate an altered neurotransmitter function into epigenetic events, which are derived from the translation of postsynaptic noncanonical signaling into altered gene regulation. All epigenetic effects are considered in light of their persistent changes induced in the postsynaptic neurons including sensitization and desensitization, priming, and shift of neuronal phenotype.

Long-term ethanol exposure: Temporal pattern of microRNA expression and associated mRNA gene networks in mouse brain

Long-term alcohol use can result in lasting changes in brain function, ultimately leading to alcohol dependence. These functional alterations arise from dysregulation of complex gene networks, and growing evidence implicates microRNAs as key regulators of these networks. We examined time- and brain region-dependent changes in microRNA expression after chronic intermittent ethanol (CIE) exposure in C57BL/6J mice. Animals were sacrificed at 0, 8, and 120h following the last exposure to four weekly cycles of CIE vapor and we measured microRNA expression in prefrontal cortex (PFC), nucleus accumbens (NAC), and amygdala (AMY). The number of detected (395–419) and differentially expressed (DE, 42–47) microRNAs was similar within each brain region. However, the DE microRNAs were distinct among brain regions and across time within each brain region. DE microRNAs were linked with their DE mRNA targets across each brain region. In all brain regions, the greatest number of DE mRNA targets occurred at the 0 or 8h time points and these changes were associated with microRNAs DE at 0 or 8h. Two separate approaches (discrete temporal association and hierarchical clustering) were combined with pathway analysis to further characterize the temporal relationships between DE microRNAs and their 120h DE targets. We focused on targets dysregulated at 120h as this time point represents a state of protracted withdrawal known to promote an increase in subsequent ethanol consumption. Discrete temporal association analysis identified networks with highly connected genes including ERK1/2 (mouse equivalent Mapk3, Mapk1), Bcl2 (in AMY networks) and Srf (in PFC networks). Similarly, the cluster-based analysis identified hub genes that include Bcl2 (in AMY networks) and Srf in PFC networks, demonstrating robust microRNA-mRNA network alterations in response to CIE exposure. In contrast, datasets utilizing targets from 0 and 8h microRNAs identified NF-kB-centered networks (in NAC and PFC), and Smad3-centered networks (in AMY). These results demonstrate that CIE exposure results in dynamic and complex temporal changes in microRNA-mRNA gene network structure.

Detection of phasic dopamine by D1 and D2 striatal medium spiny neurons

KEY POINTS

Brief dopamine events are critical actors of reward-mediated learning in the striatum; the intracellular cAMP-protein kinase A (PKA) response of striatal medium spiny neurons to such events was studied dynamically using a combination of biosensor imaging in mouse brain slices and in silico simulations. Both D1 and D2 medium spiny neurons can sense brief dopamine transients in the sub-micromolar range. While dopamine transients profoundly change cAMP levels in both types of medium spiny neurons, the PKA-dependent phosphorylation level remains unaffected in D2 neurons. At the level of PKA-dependent phosphorylation, D2 unresponsiveness depends on protein phosphatase-1 (PP1) inhibition by DARPP-32. Simulations suggest that D2 medium spiny neurons could detect transient dips in dopamine level.

ABSTRACT

The phasic release of dopamine in the striatum determines various aspects of reward and action selection, but the dynamics of the dopamine effect on intracellular signalling remains poorly understood. We used genetically encoded FRET biosensors in striatal brain slices to quantify the effect of transient dopamine on cAMP or PKA-dependent phosphorylation levels, and computational modelling to further explore the dynamics of this signalling pathway. Medium-sized spiny neurons (MSNs), which express either D1 or D2 dopamine receptors, responded to dopamine by an increase or a decrease in cAMP, respectively. Transient dopamine showed similar sub-micromolar efficacies on cAMP in both D1 and D2 MSNs, thus challenging the commonly accepted notion that dopamine efficacy is much higher on D2 than on D1 receptors. However, in D2 MSNs, the large decrease in cAMP level triggered by transient dopamine did not translate to a decrease in PKA-dependent phosphorylation level, owing to the efficient inhibition of protein phosphatase 1 by DARPP-32. Simulations further suggested that D2 MSNs can also operate in a 'tone-sensing' mode, allowing them to detect transient dips in basal dopamine. Overall, our results show that D2 MSNs may sense much more complex patterns of dopamine than previously thought.

Rapid Synaptogenesis in the Nucleus Accumbens Is Induced by a Single Cocaine Administration and Stabilized by Mitogen-Activated Protein Kinase Interacting Kinase-1 Activity

BACKGROUND

Repeated cocaine exposure produces new spine formation in striatal projection neurons (SPNs) of the nucleus accumbens. However, an acute exposure to cocaine can trigger long-lasting synaptic plasticity in SPNs leading to behavioral alterations. This raises the intriguing question as to whether a single administration of cocaine could enduringly modify striatal connectivity.

METHODS

A three-dimensional morphometric analysis of presynaptic glutamatergic boutons and dendritic spines was performed on SPNs 1 hour and 1 week after a single cocaine administration. Time-lapse two-photon microscopy in adult slices was used to determine the precise molecular-events sequence responsible for the rapid spine formation.

RESULTS

A single injection triggered a rapid synaptogenesis and persistent increase in glutamatergic connectivity in SPNs from the shell part of the nucleus accumbens, specifically. Synapse formation occurred through clustered growth of active spines contacting pre-existing axonal boutons. Spine growth required extracellular signal-regulated kinase activation, while spine stabilization involved transcription-independent protein synthesis driven by mitogen-activated protein kinase interacting kinase-1, downstream from extracellular signal-regulated kinase. The maintenance of new spines driven by mitogen-activated protein kinase interacting kinase-1 was essential for long-term connectivity changes induced by cocaine in vivo.

CONCLUSIONS

Our study originally demonstrates that a single administration of cocaine is able to induce stable synaptic rewiring in the nucleus accumbens, which will likely influence responses to subsequent drug exposure. It also unravels a new functional role for cocaine-induced extracellular signal-regulated kinase pathway independently of nuclear targets. Finally, it reveals that mitogen-activated protein kinase interacting kinase-1 has a pivotal role in cocaine-induced connectivity.

Cocaine increases dopaminergic connectivity in the nucleus accumbens

The development of addictive behavior is associated with functional and structural plasticity in the mesocorticolimbic pathway. Increased connectivity upon cocaine administration has been inferred from increases in dendritic spine density, but without observations of presynaptic elements. Recently, we established a method that enables analyses of both dendritic spines and glutamatergic boutons and presented evidence that cocaine induces changes in striatal connectivity. As the pharmacological and behavioral effects of cocaine directly implicate dopaminergic neurons and their afferents, a remaining question is whether dopaminergic striatal innervations also undergo structural plasticity. To address this issue, we generated transgenic mice in which the fluorophore tdTomato is expressed under the promoter of the dopamine transporter gene. In these mice, specific labeling of dopaminergic boutons was observed in the striatum. Of note, the accordance of our results for control mice with previous electron microscopy studies confirms that our method can be used to decipher the spatial organization of boutons in relation to dendritic elements. Following repeated cocaine administration that led to behavioral locomotor sensitization, an increased density of dopaminergic boutons was observed 1 day later in the nucleus accumbens shell specifically, and not in other striatal regions. Combined labeling of dopaminergic boutons and striatal dendrites showed that cocaine significantly increased the percentage of dendritic spines associated with a dopaminergic bouton. Our results show that chronic cocaine administration induces structural plasticity of dopaminergic boutons that could participate in dopamine-dependent neuronal adaptations in the striatum.

NCS-Rapgef2, the Protein Product of the Neuronal Rapgef2 Gene, Is a Specific Activator of D1 Dopamine Receptor-Dependent ERK Phosphorylation in Mouse Brain

The neuritogenic cAMP sensor (NCS), encoded by the Rapgef2 gene, links cAMP elevation to activation of extracellular signal-regulated kinase (ERK) in neurons and neuroendocrine cells. Transducing human embryonic kidney (HEK)293 cells, which do not express Rapgef2 protein or respond to cAMP with ERK phosphorylation, with a vector encoding a Rapgef2 cDNA reconstituted cAMP-dependent ERK activation. Mutation of a single residue in the cyclic nucleotide-binding domain (CNBD) conserved across cAMP-binding proteins abrogated cAMP-ERK coupling, while deletion of the CNBD altogether resulted in constitutive ERK activation. Two types of mRNA are transcribed from Rapgef2 in vivo. Rapgef2 protein expression was limited to tissues, i.e., neuronal and endocrine, expressing the second type of mRNA, initiated exclusively from an alternative first exon called here exon 1’, and an alternative 5’ protein sequence leader fused to a common remaining open reading frame, which is termed here NCS-Rapgef2. In the male mouse brain, NCS-Rapgef2 is prominently expressed in corticolimbic excitatory neurons, and striatal medium spiny neurons (MSNs). Rapgef2-dependent ERK activation by the dopamine D1 agonist SKF81297 occurred in neuroendocrine neuroscreen-1 (NS-1) cells expressing the human D1 receptor and was abolished by deletion of Rapgef2. Corticolimbic [e.g., dentate gyrus (DG), basolateral amygdala (BLA)] ERK phosphorylation induced by SKF81297 was significantly attenuated in CamK2α-Cre^+/−; Rapgef2^cko/cko male mice. ERK phosphorylation in nucleus accumbens (NAc) MSNs induced by treatment with SKF81297, or the psychostimulants cocaine or amphetamine, was abolished in male Rapgef2^cko/cko mice with NAc NCS-Rapgef2-depleting AAV-Synapsin-Cre injections. We conclude that D1-dependent ERK phosphorylation in mouse brain requires NCS-Rapgef2 expression.

Blockade of the 5-HT transporter contributes to the behavioural, neuronal and molecular effects of cocaine

BACKGROUND AND PURPOSE

The psychostimulant cocaine induces complex molecular, cellular and behavioural responses as a consequence of inhibiting presynaptic dopamine, noradrenaline and 5-HT transporters. To elucidate 5-HT transporter (SERT)-specific contributions to cocaine action, we evaluated cocaine effects in the SERT Met172 knock-in mouse, which expresses a SERT coding substitution that eliminates high-affinity cocaine recognition.

EXPERIMENTAL APPROACH

We measured the effects of SERT Met172 on cocaine antagonism of 5-HT re-uptake using ex vivo synaptosome preparations and in vivo microdialysis. We assessed SERT dependence of cocaine actions behaviourally through acute and chronic locomotor activation, sensitization, conditioned place preference (CPP) and oral cocaine consumption. We used c-Fos, quantitative RT-PCR and RNA sequencing methods for insights into cellular and molecular networks supporting SERT-dependent cocaine actions.

KEY RESULTS

SERT Met172 mice demonstrated functional insensitivity for cocaine at SERT. Although they displayed wild-type levels of acute cocaine-induced hyperactivity or chronic sensitization, the pattern of acute motor activation was different, with a bias toward thigmotaxis. CPP was increased, and a time-dependent elevation in oral cocaine consumption was observed. SERT Met172 mice displayed relatively higher levels of neuronal activation in the hippocampus, piriform cortex and prelimbic cortex (PrL), accompanied by region-dependent changes in immediate early gene expression. Distinct SERT-dependent gene expression networks triggered by acute and chronic cocaine administration were identified, including PrL Akt and nucleus accumbens ERK1/2 signalling.

CONCLUSION AND IMPLICATIONS

Our studies reveal distinct SERT contributions to cocaine action, reinforcing the possibility of targeting specific aspects of cocaine addiction by modulation of 5-HT signalling.

ERK/MAPK Signaling Is Required for Pathway-Specific Striatal Motor Functions

The ERK/MAPK intracellular signaling pathway is hypothesized to be a key regulator of striatal activity via modulation of synaptic plasticity and gene transcription. However, prior investigations into striatal ERK/MAPK functions have yielded conflicting results. Further, these studies have not delineated the cell-type-specific roles of ERK/MAPK signaling due to the reliance on globally administered pharmacological ERK/MAPK inhibitors and the use of genetic models that only partially reduce total ERK/MAPK activity. Here, we generated mouse models in which ERK/MAPK signaling was completely abolished in each of the two distinct classes of medium spiny neurons (MSNs). ERK/MAPK deletion in D1R-MSNs (direct pathway) resulted in decreased locomotor behavior, reduced weight gain, and early postnatal lethality. In contrast, loss of ERK/MAPK signaling in D2R-MSNs (indirect pathway) resulted in a profound hyperlocomotor phenotype. ERK/MAPK-deficient D2R-MSNs exhibited a significant reduction in dendritic spine density, markedly suppressed electrical excitability, and suppression of activity-associated gene expression even after pharmacological stimulation. Our results demonstrate the importance of ERK/MAPK signaling in governing the motor functions of the striatal direct and indirect pathways. Our data further show a critical role for ERK in maintaining the excitability and plasticity of D2R-MSNs.SIGNIFICANCE STATEMENT Alterations in ERK/MAPK activity are associated with drug abuse, as well as neuropsychiatric and movement disorders. However, genetic evidence defining the functions of ERK/MAPK signaling in striatum-related neurophysiology and behavior is lacking. We show that loss of ERK/MAPK signaling leads to pathway-specific alterations in motor function, reduced neuronal excitability, and the inability of medium spiny neurons to regulate activity-induced gene expression. Our results underscore the potential importance of the ERK/MAPK pathway in human movement disorders.

Phospho-ERK and sex steroids in the mPOA: involvement in male mouse sexual behaviour

This paper aimed to investigate the mechanisms triggering ERK phosphorylation and its functional role in male sexual behaviour. ERK1/2-phosphorylated form was detected in the medial preoptic area of the hypothalamus (mPOA) during the sexual stimulation of naive and sexually experienced males who were killed 5 min after the first intromission. This mating-induced ERK phosphorylation was increased in sexually experienced males compared to that in naive mice. The functional role of the ERK1/2 pathway activation during sexual behaviour was explored with the administration of a MEK inhibitor, SL-327 (30 mg/kg, i.p.), 45 min before the contact with a receptive female. Inhibition of ERK phosphorylation was found to decrease sexual motivation in both naive and experienced males without altering their copulatory ability. The mechanisms potentially involved in this rapid ERK1/2 pathway activation were specified ex vivo on hypothalamic slices. A thirty-minute incubation with 100 nM of testosterone (T), dihydrotestosterone (DHT) or oestradiol (E2) led to ERK phosphorylation. No changes were observed after incubation with testosterone 3-(O-carboxymethyl)oxime-BSA (T-BSA), an impermeable to the plasma membrane form of testosterone. All these results indicate that ERK phosphorylation within the mPOA could be a key player in the motivational signalling pathway and considered as an index of sexual motivation. They also demonstrate the involvement of oestrogen receptor (ER) and androgen receptor (AR) transduction pathways in steroid-dependent ERK activation.

Long-term plasticity of corticostriatal synapses is modulated by pathway-specific co-release of opioids through κ-opioid receptors

KEY POINTS

Both endogenous opioids and opiate drugs of abuse modulate learning of habitual and goal-directed actions, and can also modify long-term plasticity of corticostriatal synapses. Striatal projection neurons of the direct pathway co-release the opioid neuropeptide dynorphin which can inhibit dopamine release via κ-opioid receptors. Theta-burst stimulation of corticostriatal fibres produces long-term potentiation (LTP) in striatal projection neurons when measured using whole-cell patch recording. Optogenetic activation of direct pathway striatal projection neurons inhibits LTP while reducing dopamine release. Because the endogenous release of opioids is activity dependent, this modulation of synaptic plasticity represents a negative feedback mechanism that may limit runaway enhancement of striatal neuron activity in response to drugs of abuse.

ABSTRACT

Synaptic plasticity in the striatum adjusts behaviour adaptively during skill learning, or maladaptively in the case of addiction. Just as dopamine plays a critical role in synaptic plasticity underlying normal skill learning and addiction, endogenous and exogenous opiates also modulate learning and addiction-related striatal plasticity. Though the role of opioid receptors in long-term depression in striatum has been characterized, their effect on long-term potentiation (LTP) remains unknown. In particular, direct pathway (dopamine D1 receptor-containing; D1R-) spiny projection neurons (SPNs) co-release the opioid neuropeptide dynorphin, which acts at presynaptic κ-opioid receptors (KORs) on dopaminergic afferents and can negatively regulate dopamine release. Therefore, we evaluated the interaction of co-released dynorphin and KOR on striatal LTP. We optogenetically facilitate the release of endogenous dynorphin from D1R-SPNs in brain slice while using whole-cell patch recording to measure changes in the synaptic response of SPNs following theta-burst stimulation (TBS) of cortical afferents. Our results demonstrate that TBS evokes corticostriatal LTP, and that optogenetic activation of D1R-SPNs during induction impairs LTP. Additional experiments demonstrate that optogenetic activation of D1R-SPNs reduces stimulation-evoked dopamine release and that bath application of a KOR antagonist provides full rescue of both LTP induction and dopamine release during optogenetic activation of D1R-SPNs. These results suggest that an increase in the opioid neuropeptide dynorphin is responsible for reduced TBS LTP and illustrate a physiological phenomenon whereby heightened D1R-SPN activity can regulate corticostriatal plasticity. Our findings have important implications for learning in addictive states marked by elevated direct pathway activation.

The extracellular signal-regulated kinase 1/2 pathway in neurological diseases: A potential therapeutic target (Review)

Signaling pathways are critical modulators of a variety of physiological and pathological processes, and the abnormal activation of some signaling pathways can contribute to disease progression in various conditions. As a result, signaling pathways have emerged as an important tool through which the occurrence and development of diseases can be studied, which may then lead to the development of novel drugs. Accumulating evidence supports a key role for extracellular signal-regulated kinase 1/2 (ERK1/2) signaling in the embryonic development of the central nervous system (CNS) and in the regulation of adult brain function. ERK1/2, one of the most well characterized members of the mitogen-activated protein kinase family, regulates a range of processes, from metabolism, motility and inflammation, to cell death and survival. In the nervous system, ERK1/2 regulates synaptic plasticity, brain development and repair as well as memory formation. ERK1/2 is also a potent effector of neuronal death and neuroinflammation in many CNS diseases. This review summarizes recent findings in neurobiological ERK1/2 research, with a special emphasis on findings that clarify our understanding of the processes that regulate the plethora of isoform-specific ERK functions under physiological and pathological conditions. Finally, we suggest some potential therapeutic strategies associated with agents acting on the ERK1/2 signaling to prevent or treat neurological diseases.

Activity-Regulated Cytoskeleton-Associated Protein Accumulates in the Nucleus in Response to Cocaine and Acts as a Brake on Chromatin Remodeling and Long-Term Behavioral Alterations

BACKGROUND

Addiction relies on persistent alterations of neuronal properties, which depends on gene regulation. Activity-regulated cytoskeleton-associated protein (Arc) is an immediate early gene that modulates neuronal plasticity underlying learning and memory. Its role in cocaine-induced neuronal and behavioral adaptations remains elusive.

METHODS

Acute cocaine-treated mice were used for quantitative reverse-transcriptase polymerase chain reaction, immunocytochemistry, and confocal imaging from striatum. Live imaging and transfection assays for Arc overexpression were performed from primary cultures. Molecular and behavioral adaptations to cocaine were studied from Arc-deficient mice and their wild-type littermates.

RESULTS

Arc messenger RNA and proteins are rapidly induced in the striatum after acute cocaine administration, via an extracellular-signal regulated kinase-dependent de novo protein synthesis. Although detected in dendrites, Arc accumulates in the nucleus in active zones of transcription, where it colocalizes with phospho-Ser10-histone H3, an important component of nucleosomal response. In vitro, Arc overexpression downregulates phospho-Ser10-histone H3 without modifying extracellular-signal regulated kinase phosphorylation in the nucleus. In vivo, Arc-deficient mice display decreased heterochromatin domains, a high RNA-polymerase II activity and enhanced c-Fos expression. These mice presented an exacerbated psychomotor sensitization and conditioned place preference induced by low doses of cocaine.

CONCLUSIONS

Cocaine induces the rapid induction of Arc and its nuclear accumulation in striatal neurons. Locally, it alters the nucleosomal response, and acts as a brake on chromatin remodeling and gene regulation. These original observations posit Arc as a major homeostatic modulator of molecular and behavioral responses to cocaine. Thus, modulating Arc levels may provide promising therapeutic approaches in drug addiction.

Profiling the MAPK/ERK dependent and independent activity regulated transcriptional programs in the murine hippocampus in vivo

Activity-dependent alteration of the transcriptional program is central for shaping neuronal connectivity. Constitutively expressed transcription factors orchestrate the initial response to neuronal stimulation and serve as substrates for second messenger-regulated kinase signalling cascades. The mitogen-activated protein kinase ERK conveys signalling from the synapse to the nucleus but its genetic signature following neuronal activity has not been revealed. The goal of the present study was to identify ERK dependent and independent activity regulated transcriptional programs in the murine hippocampus. We used generalized seizures combined with the pharmacological intervention of MEK activation as an in vivo model to determine the complete transcriptional program initiated by ERK after neuronal activity. Our survey demonstrates that the induction of a large number of activity-regulated genes, including Arc/Arg3.1, Arl5b, Gadd45b, Homer1, Inhba and Zwint, is indeed dependent on ERK phosphorylation. In contrast, expression of a small group of genes, including Npas4, Arl4d, Errfi1, and Rgs2, is only partially dependent or completely independent (Ppp1r15a) of this signalling pathway. Among the identified transcripts are long non-coding (lnc) RNAs and induction of LincPint and splice variants of NEAT1 are ERK dependent. Our survey provides a comprehensive analysis of the transcriptomic response conveyed by ERK signalling in the hippocampus.

TGR5, Not Only a Metabolic Regulator

G-protein-coupled bile acid receptor, Gpbar1 (TGR5), is a member of G-protein-coupled receptor (GPCR) superfamily. High levels of TGR5 mRNA were detected in several tissues such as small intestine, stomach, liver, lung, especially in placenta and spleen. TGR5 is not only the receptor for bile acids, but also the receptor for multiple selective synthetic agonists such as 6α-ethyl-23(S)-methyl-cholic acid (6-EMCA, INT-777) and a series of 4-benzofuranyloxynicotinamde derivatives to regulate different signaling pathways such as nuclear factor κB (NF-κB), AKT, and extracellular signal-regulated kinases (ERK). TGR5, as a metabolic regulator, is involved in energy homeostasis, bile acid homeostasis, as well as glucose metabolism. More recently, our group and others have extended the functions of TGR5 to more than metabolic regulation, which include inflammatory response, cancer and liver regeneration. These findings highlight TGR5 as a potential drug target for different diseases. This review summarizes the basic information of TGR5 and its new functions.

Molecular mechanisms underlying alcohol-drinking behaviours

The main characteristic of alcohol use disorder is the consumption of large quantities of alcohol despite the negative consequences. The transition from the moderate use of alcohol to excessive, uncontrolled alcohol consumption results from neuroadaptations that cause aberrant motivational learning and memory processes. Here, we examine studies that have combined molecular and behavioural approaches in rodents to elucidate the molecular mechanisms that keep the social intake of alcohol in check, which we term 'stop pathways', and the neuroadaptations that underlie the transition from moderate to uncontrolled, excessive alcohol intake, which we term 'go pathways'. We also discuss post-transcriptional, genetic and epigenetic alterations that underlie both types of pathways.

G protein-coupled receptor kinases as regulators of dopamine receptor functions

Actions of the neurotransmitter dopamine in the brain are mediated by dopamine receptors that belong to the superfamily of G protein-coupled receptors (GPCRs). Mammals have five dopamine receptor subtypes, D1 through D5. D1 and D5 couple to Gs/olf and activate adenylyl cyclase, whereas D2, D3, and D4 couple to Gi/o and inhibit it. Most GPCRs upon activation by an agonist are phosphorylated by GPCR kinases (GRKs). The GRK phosphorylation makes receptors high-affinity binding partners for arrestin proteins. Arrestin binding to active phosphorylated receptors stops further G protein activation and promotes receptor internalization, recycling or degradation, thereby regulating their signaling and trafficking. Four non- visual GRKs are expressed in striatal neurons. Here we describe known effects of individual GRKs on dopamine receptors in cell culture and in the two in vivo models of dopamine-mediated signaling: behavioral response to psychostimulants and L-DOPA- induced dyskinesia. Dyskinesia, associated with dopamine super-sensitivity of striatal neurons, is a debilitating side effect of L-DOPA therapy in Parkinson's disease. In vivo, GRK subtypes show greater receptor specificity than in vitro or in cultured cells. Overexpression, knockdown, and knockout of individual GRKs, particularly GRK2 and GRK6, have differential effects on signaling of dopamine receptor subtypes in the brain. Furthermore, deletion of GRK isoforms in select striatal neuronal types differentially affects psychostimulant-induced behaviors. In addition, anti-dyskinetic effect of GRK3 does not require its kinase activity: it is mediated by the binding of its RGS-like domain to Gαq/11, which suppresses Gq/11 signaling. The data demonstrate that the dopamine signaling in defined neuronal types in vivo is regulated by specific and finely orchestrated actions of GRK isoforms.

Impairment of cocaine-mediated behaviours in mice by clinically relevant Ras-ERK inhibitors

Ras-ERK signalling in the brain plays a central role in drug addiction. However, to date, no clinically relevant inhibitor of this cascade has been tested in experimental models of addiction, a necessary step toward clinical trials. We designed two new cell-penetrating peptides - RB1 and RB3 - that penetrate the brain and, in the micromolar range, inhibit phosphorylation of ERK, histone H3 and S6 ribosomal protein in striatal slices. Furthermore, a screening of small therapeutics currently in clinical trials for cancer therapy revealed PD325901 as a brain-penetrating drug that blocks ERK signalling in the nanomolar range. All three compounds have an inhibitory effect on cocaine-induced ERK activation and reward in mice. In particular, PD325901 persistently blocks cocaine-induced place preference and accelerates extinction following cocaine self-administration. Thus, clinically relevant, systemically administered drugs that attenuate Ras-ERK signalling in the brain may be valuable tools for the treatment of cocaine addiction.

Repeated ventral midbrain neurotensin injections sensitize to amphetamine-induced locomotion and ERK activation: A role for NMDA receptors

Previous studies have shown that activation of ventral midbrain NMDA receptors is required to initiate sensitization by amphetamine. In view of the recent evidence that neurotensin modulates ventral midbrain glutamate neurotransmission, we tested the hypothesis that neurotensin is acting upstream to glutamate to initiate sensitization to the behavioral and neurochemical effects of amphetamine. During a first testing phase, adult male rats implanted with bilateral ventral midbrain cannulae were injected every second day for three days with D-[Tyr¹¹]neurotensin (1.5 nmol/side), the preferred NMDA GluN2A/B antagonist, CPP (40 or 120 pmol/side), the selective GluN2B antagonist, Ro04-5595 (200 or 1200 pmol/side), CPP (40 or 120 pmol/side) + D-[Tyr¹¹]neurotensin (1.5 nmol/side) or Ro04-5595 (200 or 1200 pmol/side) + D-[Tyr¹¹]neurotensin (1.5 nmol/side) and locomotor activity was measured immediately after the injection. Five days after the last central injection, the locomotor response or the expression of phosphorylated extracellular signal-regulated kinases 1/2 (pERK1/2) in neurons of different limbic nuclei was measured following a systemic injection of amphetamine sulfate (0.75 mg/kg, i.p.). Results show that amphetamine induced significantly stronger locomotor activity and pERK1/2 expression in the nucleus accumbens shell and infralimbic cortex in neurotensin pre-exposed animals than in controls (vehicle pre-exposed). These sensitization effects initiated by neurotensin were prevented by CPP, but not Ro04-5595. These results support the hypothesis that neurotensin is stimulating glutamate neurotransmission to initiate neural changes that sub-serve amphetamine sensitization and that glutamate is acting on NMDA receptors that are mostly likely composed of GluN2A, but not GluN2B, subunits. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

The Brain on Drugs: From Reward to Addiction

Advances in neuroscience identified addiction as a chronic brain disease with strong genetic, neurodevelopmental, and sociocultural components. We here discuss the circuit- and cell-level mechanisms of this condition and its co-option of pathways regulating reward, self-control, and affect. Drugs of abuse exert their initial reinforcing effects by triggering supraphysiologic surges of dopamine in the nucleus accumbens that activate the direct striatal pathway via D1 receptors and inhibit the indirect striato-cortical pathway via D2 receptors. Repeated drug administration triggers neuroplastic changes in glutamatergic inputs to the striatum and midbrain dopamine neurons, enhancing the brain's reactivity to drug cues, reducing the sensitivity to non-drug rewards, weakening self-regulation, and increasing the sensitivity to stressful stimuli and dysphoria. Drug-induced impairments are long lasting; thus, interventions designed to mitigate or even reverse them would be beneficial for the treatment of addiction.

Gene network analysis shows immune-signaling and ERK1/2 as novel genetic markers for multiple addiction phenotypes: alcohol, smoking and opioid addiction

Background

Addictions to alcohol and tobacco, known risk factors for cancer, are complex heritable disorders. Addictive behaviors have a bidirectional relationship with pain. We hypothesize that the associations between alcohol, smoking, and opioid addiction observed in cancer patients have a genetic basis. Therefore, using bioinformatics tools, we explored the underlying genetic basis and identified new candidate genes and common biological pathways for smoking, alcohol, and opioid addiction.

Results

Literature search showed 56 genes associated with alcohol, smoking and opioid addiction. Using Core Analysis function in Ingenuity Pathway Analysis software, we found that ERK1/2 was strongly interconnected across all three addiction networks. Genes involved in immune signaling pathways were shown across all three networks. Connect function from IPA My Pathway toolbox showed that DRD2 is the gene common to both the list of genetic variations associated with all three addiction phenotypes and the components of the brain neuronal signaling network involved in substance addiction. The top canonical pathways associated with the 56 genes were: 1) calcium signaling, 2) GPCR signaling, 3) cAMP-mediated signaling, 4) GABA receptor signaling, and 5) G-alpha i signaling.

Conlusions

Cancer patients are often prescribed opioids for cancer pain thus increasing their risk for opioid abuse and addiction. Our findings provide candidate genes and biological pathways underlying addiction phenotypes, which may be future targets for treatment of addiction. Further study of the variations of the candidate genes could allow physicians to make more informed decisions when treating cancer pain with opioid analgesics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0167-x) contains supplementary material, which is available to authorized users.

Modulation of the glutamatergic transmission by Dopamine: a focus on Parkinson, Huntington and Addiction diseases

Dopamine (DA) plays a major role in motor and cognitive functions as well as in reward processing by regulating glutamatergic inputs. In particular in the striatum the release of DA rapidly influences synaptic transmission modulating both AMPA and NMDA receptors. Several neurodegenerative and neuropsychiatric disorders, including Parkinson, Huntington and addiction-related diseases, manifest a dysregulation of glutamate and DA signaling. Here, we will focus our attention on the mechanisms underlying the modulation of the glutamatergic transmission by DA in striatal circuits.