Dopamine D(1) and D(2) Receptors Differentially Regulate Rac1 and Cdc42 Signaling in the Nucleus Accumbens to Modulate Behavioral and Structural Plasticity After Repeated Methamphetamine Treatment

The Small G-Protein Rac1 in the Dorsomedial Striatum Promotes Alcohol-Dependent Structural Plasticity and Goal-Directed Learning in Mice

The small G-protein Ras-related C3 botulinum toxin substrate 1 (Rac1) promotes the formation of filamentous actin (F-actin). Actin is a major component of dendritic spines, and we previously found that alcohol alters actin composition and dendritic spine structure in the nucleus accumbens (NAc) and the dorsomedial striatum (DMS). To examine if Rac1 contributes to these alcohol-mediated adaptations, we measured the level of GTP-bound active Rac1 in the striatum of mice following 7 weeks of intermittent access to 20% alcohol. We found that chronic alcohol intake activates Rac1 in the DMS of male mice. In contrast, Rac1 is not activated by alcohol in the NAc and DLS of male mice or in the DMS of female mice. Similarly, closely related small G-proteins are not activated by alcohol in the DMS, and Rac1 activity is not increased in the DMS by moderate alcohol or natural reward. To determine the consequences of alcohol-dependent Rac1 activation in the DMS of male mice, we inhibited endogenous Rac1 by infecting the DMS of mice with an adeno-associated virus (AAV) expressing a dominant negative form of the small G-protein (Rac1-DN). We found that overexpression of AAV-Rac1-DN in the DMS inhibits alcohol-mediated Rac1 signaling and attenuates alcohol-mediated F-actin polymerization, which corresponded with a decrease in dendritic arborization and spine maturation. Finally, we provide evidence to suggest that Rac1 in the DMS plays a role in alcohol-associated goal-directed learning. Together, our data suggest that Rac1 in the DMS plays an important role in alcohol-dependent structural plasticity and aberrant learning.

Inhibition of SIRT1 in the nucleus accumbens attenuates heroin addiction-related behavior by decreasing D1 neuronal autophagy

This study aimed to investigate the effects of SIRT1 modulation on heroin addiction-like behavior and its possible biological mechanisms. Wild-type C57BL/6J and Sirt1loxp/loxp D1-Cre mice were used in this experiment, and Sirt1 loxp/loxp D1-Cre(-) mice were used as a control for conditional knockout mice. Mice were divided into saline control and heroin-dependent groups. Behavioral methods were used to record the withdrawal response, conditioned place preference (CPP) changes, and open field test results. Transmission electron microscopy (TEM) was used to observe the structure of autophagosomes in nucleus accumbens (NAc) neurons. The expression of SIRT1 and autophagy-related proteins and genes, such as LC3Ⅱ, ATG5 , and ATG7 , was detected in the NAc of each mouse group via western blot, real-time quantitative PCR (qPCR) analyzes, and immunofluorescence. The results of this experiment showed that compared with the saline group, mice in the wild-type heroin-dependent group showed marked withdrawal symptoms, with more autophagosomes observed in NAc via TEM. Compared with wild-type and Sirt1loxp/loxp D1-Cre(-) heroin-dependent groups, CPP formation was found to be reduced in the conditional knockout mouse group, with a significant decrease in spontaneous activity. Western blot, qPCR, and immunofluorescence results indicated that the expression of LC3Ⅱ, ATG-5, and ATG-7 was significantly reduced in the NAc of the Sirt1loxp/loxp D1-Cre(+) group. It was still, however, higher than that in the saline control group. These results suggest that inhibition of Sirt1 expression may prevent heroin-induced addiction-related behaviors via reducing D1 neuronal autophagy.

Lycium barbarum polysaccharide mitigated methamphetamine addiction and altered methamphetamine-induced gut microbiota dysbiosis

Methamphetamine (MA) is a highly addictive mental stimulant, and MA abuse remains a significant public health problem worldwide, while effective treatment options are limited. Lycium barbarum polysaccharide (LBP), a major effective component extracted from Lycium barbarum, has potential health-promoting effects on the nervous system; however, its role in MA dependence remains unclear. In this study, the conditioned place preference (CPP) of MA addiction in adult male mice was established to detect changes in gut microbiota profiles after LBP treatment through 16S rRNA gene sequencing. Our results found that LBP administration could alleviate MA-induced CPP and hyperactivity. Interestingly, LBP improved MA-induced gut microbiota dysbiosis by increasing some beneficial autochthonous genus abundances, such as Allobaculum, Gordonibacter, and Ileibacterium. MA exposure induced the co-occurrence network of intestinal microbiota to become weaker and more unstable when compared with the control group, while LBP changed the above effects when compared with the MA group. Bacterial gene function prediction showed that amphetamine addiction, cocaine addiction, and short-chain fatty acid metabolism were enriched. These findings reveal that LBP might regulate MA-induced gut microbiota and behavior changes, which showed potential therapeutic applicability in treating MA addiction by regulating the gut microbiota.

The small G-protein Rac1 in the dorsomedial striatum promotes alcohol-dependent structural plasticity and goal-directed learning in mice

The small G-protein Rac1 promotes the formation of filamentous actin (F-Actin). Actin is a major component of dendritic spines, and we previously found that alcohol alters actin composition and dendritic spine structure in the nucleus accumbens (NAc) and the dorsomedial striatum (DMS). To examine if Rac1 contributes to these alcohol-mediated adaptations, we measured the level of GTP-bound active Rac1 in the striatum of mice following 7 weeks of intermittent access to 20% alcohol. We found that chronic alcohol intake activates Rac1 in the DMS of male mice. In contrast, Rac1 is not activated by alcohol in the NAc and DLS of male mice, or in the DMS of female mice. Similarly, closely related small G-proteins are not activated by alcohol in the DMS, and Rac1 activity is not increased in the DMS by moderate alcohol or natural reward. To determine the consequences of alcohol-dependent Rac1 activation in the DMS of male mice, we inhibited endogenous Rac1 by infecting the DMS of mice with an AAV expressing a dominant negative form of the small G-protein (Rac1-DN). We found that overexpression of AAV-Rac1-DN in the DMS inhibits alcohol-mediated Rac1 signaling and attenuates alcohol-mediated F-actin polymerization, which corresponded with a decrease in dendritic arborization and spine maturation. Finally, we provide evidence to suggest that Rac1 in the DMS plays a role in alcohol-associated goal-directed learning. Together, our data suggest that Rac1 in the DMS plays an important role in alcohol-dependent structural plasticity and aberrant learning.

Significance Statement

Addiction, including alcohol use disorder, is characterized by molecular and cellular adaptations that promote maladaptive behaviors. We found that Rac1 was activated by alcohol in the dorsomedial striatum (DMS) of male mice. We show that alcohol-mediated Rac1 signaling is responsible for alterations in actin dynamics and neuronal morphology. We also present data to suggest that Rac1 is important for alcohol-associated learning processes. These results suggest that Rac1 in the DMS is an important contributor to adaptations that promote alcohol use disorder.

Synaptic remodeling of GluA1 and GluA2 expression in the nucleus accumbens promotes susceptibility to cognitive deficits concomitant with downstream GSK3β mediated neurotoxicity in female mice during abstinence from voluntary oral methamphetamine

Stimulant-use disorders can present with long-term cognitive and mental health deficits. Little is known about the underlying molecular mechanisms perpetuating sex differences in cognitive and behavioral deficits in preclinical models of addiction to stimulants such as methamphetamine (MA). The current study investigated the neurochemical shifts underlying sex disparities in MA-induced working memory deficits and an addictive phenotype following abstinence from chronic MA abuse. We used our previously reported mouse model of voluntary oral methamphetamine administration (VOMA) consisting of an acquisition phase (days 1-14) characterized by escalating doses of MA and a binge phase (days 14-28) characterized by static doses. Female VOMA mice exhibited sustained MA consumption during the binge phase, demonstrating sex-specific vulnerabilities to the maintenance of MA addiction. The 8-arm radial maze was used to test spatial working memory performance following abstinence from VOMA. Results indicate working memory deficits correlated to higher MA consumption in females only. Hippocampal and accumbal tissue were collected and analyzed by immunoblotting. Female VOMA mice had decreased GluA1, but not GluA2, in the hippocampus, which may perpetuate synaptic destabilization and working memory deficits. Female-specific increases in GluA1 and p-GSK3β expression in accumbal tissue suggest vulnerability toward abstinence-induced drug craving and heightened downstream neurotoxicity. Our study reveals female-specific neurochemical shifts in hippocampal and accumbal AMPA receptor signaling following abstinence from chronic MA consumption that may perpetuate female susceptibility to MA-induced cognitive deficits. These data demonstrate a novel molecular pathway that would exacerbate memory deficits and perpetuate an addictive phenotype in female populations following MA abuse.

Si-Ni-San alleviates early life stress-induced depression-like behaviors in adolescence via modulating Rac1 activity and associated spine plasticity in the nucleus accumbens

Background: Early life stress (ELS) is a major risk factor for depression in adolescents. The nucleus accumbens (NAc) is a key center of the reward system, and spine remodeling in the NAc contributes to the development of depression. The Si-Ni-San formula (SNS) is a fundamental prescription for treating depression in traditional Chinese medicine. However, little is known about the effects of SNS on behavioral abnormalities and spine plasticity in the NAc induced by ELS. Purpose: This study aimed to investigate the therapeutic effect and the modulatory mechanism of SNS on abnormal behaviors and spine plasticity in the NAc caused by ELS. Methods: We utilized a model of ELS that involved maternal separation with early weaning to explore the protective effects of SNS on adolescent depression. Depressive-like behaviors were evaluated by the sucrose preference test, the tail suspension test, and the forced swimming test; anxiety-like behaviors were monitored by the open field test and the elevated plus maze. A laser scanning confocal microscope was used to analyze dendritic spine remodeling in the NAc. The activity of Rac1 was detected by pull-down and Western blot tests. Viral-mediated gene transfer of Rac1 was used to investigate its role in ELS-induced depression-like behaviors in adolescence. Results: ELS induced depression-like behaviors but not anxiety-like behaviors in adolescent mice, accompanied by an increase in stubby spine density, a decrease in mushroom spine density, and decreased Rac1 activity in the NAc. Overexpression of constitutively active Rac1 in the NAc reversed depression-related behaviors, leading to a decrease in stubby spine density and an increase in mushroom spine density. Moreover, SNS attenuated depression-like behavior in adolescent mice and counteracted the spine abnormalities in the NAc induced by ELS. Additionally, SNS increased NAc Rac1 activity, and the inhibition of Rac1 activity weakened the antidepressant effect of SNS. Conclusion: These results suggest that SNS may exert its antidepressant effects by modulating Rac1 activity and associated spine plasticity in the NAc.

Cdc42GAP deficiency contributes to the Alzheimer's disease phenotype

Alzheimer's disease, the most common cause of dementia, is a chronic degenerative disease with typical pathological features of extracellular senile plaques and intracellular neurofibrillary tangles and a significant decrease in the density of neuronal dendritic spines. Cdc42 is a member of the small G protein family that plays an important role in regulating synaptic plasticity and is regulated by Cdc42GAP, which switches Cdc42 from active GTP-bound to inactive GDP-bound states regulating downstream pathways via effector proteins. However, few studies have focused on Cdc42 in the progression of Alzheimer's disease. In a heterozygous Cdc42GAP mouse model that exhibited elevated Cdc42-GTPase activity accompanied by increased Cdc42-PAK1-cofilin signalling, we found impairments in cognitive behaviours, neuron senescence, synaptic loss with depolymerization of F-actin and the pathological phenotypes of Alzheimer's disease, including phosphorylated tau (p-T231, AT8), along with increased soluble and insoluble Aβ1-42 and Aβ1-40, which are consistent with typical Alzheimer's disease mice. Interestingly, these impairments increased significantly with age. Furthermore, the results of quantitative phosphoproteomic analysis of the hippocampus of 11-month-old GAP mice suggested that Cdc42GAP deficiency induces and accelerates Alzheimer's disease-like phenotypes through activation of GSK-3β by dephosphorylation at Ser9, Ser389 and/or phosphorylation at Tyr216. In addition, overexpression of dominant-negative Cdc42 in the primary hippocampal and cortical neurons of heterozygous Cdc42GAP mice reversed synaptic loss and tau hyperphosphorylation. Importantly, the Cdc42 signalling pathway, Aβ1-42, Aβ1-40 and GSK-3β activity were increased in the cortical sections of Alzheimer's disease patients compared with those in healthy controls. Together, these data indicated that Cdc42GAP is involved in regulating Alzheimer's disease-like phenotypes such as cognitive deficits, dendritic spine loss, phosphorylated tau (p-T231, AT8) and increased soluble and insoluble Aβ1-42 and Aβ1-40, possibly through the activation of GSK-3β, and these impairments increased significantly with age. Thus, we provide the first evidence that Cdc42 is involved in the progression of Alzheimer's disease-like phenotypes, which may provide new targets for Alzheimer's disease treatment.

CNTN1 in the Nucleus Accumbens is Involved in Methamphetamine-Induced Conditioned Place Preference in Mice

Methamphetamine (Meth), a commonly used central nervous system stimulant, is highly addictive. Currently, there is no effective treatment for Meth dependence and abuse, although cell adhesion molecules (CAMs) have been shown to play an important role in the formation and remodeling of synapses in the nervous system while also being involved in addictive behavior. Contactin 1 (CNTN1) is a CAM that is widely expressed in the brain; nevertheless, its role in Meth addiction remains unclear. Therefore, in the present study, we established mouse models of single and repeated Meth exposure and subsequently determined that CNTN1 expression in the nucleus accumbens (NAc) was upregulated in mice following single or repeated Meth exposure, whereas CNTN1 expression in the hippocampus was not significantly altered. Intraperitoneal injection of the dopamine receptor 2 antagonist haloperidol reversed Meth-induced hyperlocomotion and upregulation of CNTN1 expression in the NAc. Additionally, repeated Meth exposure also induced conditioned place preference (CPP) in mice and upregulated the expression levels of CNTN1, NR2A, NR2B, and PSD95 in the NAc. Using an AAV-shRNA-based approach to specifically silence CNTN1 expression in the NAc via brain stereotaxis reversed Meth-induced CPP and decreased the expression levels of NR2A, NR2B, and PSD95 in the NAc. These findings suggest that CNTN1 expression in the NAc plays an important role in Meth-induced addiction, and the underlying mechanism may be related to the expression of synapse-associated proteins in the NAc. The results of this study improved our understanding of the role of cell adhesion molecules in Meth addiction.

Dopamine D1 receptor in orbitofrontal cortex to dorsal striatum pathway modulates methamphetamine addiction

The orbitofrontal cortex (OFC)-dorsal striatum (DS) is an important neural circuit that contributes to addictive behavior, including compulsive reinforcement, yet the specific types of neurons that play a major role still need to be further elucidated. Here, we used a place conditioning paradigm to measure the conditioned responses to methamphetamine (MA). The results demonstrated that MA increases the expression of c-Fos, synaptic plasticity in OFC and DS. Patch-clamp recording showed that MA activated projection neurons from the OFC to the DS, and chemogenetic manipulation of neuronal activity in OFC-DS projection neurons affects conditioned place preference (CPP) scores. And the combined patch-electrochemical technique was used to detect the DA release in OFC, the data indicated that the DA release was increased in MA group. Additionally, SCH23390, a D1R antagonist, was used to verify the function of D1R projection neurons, showing that SCH23390 reversed MA addiction-like behavior. Collectively, these findings provide evidence for the D1R neuron is sufficient to regulate MA addiction in the OFC-DS pathway, and the study provides new insight into the underlying mechanism of pathological changes in MA addiction.

The potential therapeutic roles of Rho GTPases in substance dependence

Rho GTPases family are considered to be molecular switches that regulate various cellular processes, including cytoskeleton remodeling, cell polarity, synaptic development and maintenance. Accumulating evidence shows that Rho GTPases are involved in neuronal development and brain diseases, including substance dependence. However, the functions of Rho GTPases in substance dependence are divergent and cerebral nuclei-dependent. Thereby, comprehensive integration of their roles and correlated mechanisms are urgently needed. In this review, the molecular functions and regulatory mechanisms of Rho GTPases and their regulators such as GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs) in substance dependence have been reviewed, and this is of great significance for understanding their spatiotemporal roles in addictions induced by different addictive substances and in different stages of substance dependence.

Cdc42 signaling regulated by dopamine D2 receptor correlatively links specific brain regions of hippocampus to cocaine addiction

BACKGROUND

Hippocampus plays critical roles in drug addiction. Cocaine-induced modifications in dopamine receptor function and the downstream signaling are important regulation mechanisms in cocaine addiction. Rac regulates actin filament accumulation while Cdc42 stimulates the formation of filopodia and neurite outgrowth. Based on the region specific roles of small GTPases in brain, we focused on the hippocampal subregions to detect the regulation of Cdc42 signaling in long-term morphological and behavioral adaptations to cocaine.

METHODS

Genetically modified mouse models of Cdc42, dopamine receptor D1 (D1R) and D2 (D2R) and expressed Cdc42 point mutants that are defective in binding to and activation of its downstream effector molecules PAK and N-WASP were generated, respectively, in CA1 or dentate gyrus (DG) subregion.

RESULTS

Cocaine induced upregulation of Cdc42 signaling activity. Cdc42 knockout or mutants blocked cocaine-induced increase in spine plasticity in hippocampal CA1 pyramidal neurons, leading to a decreased conditional place preference (CPP)-associated memories and spatial learning and memory in water maze. Cdc42 knockout or mutants promoted cocaine-induced loss of neurogenesis in DG, leading to a decreased CPP-associated memories and spatial learning and memory in water maze. Furthermore, by using D1R knockout, D2R knockout, and D2R/Cdc42 double knockout mice, we found that D2R, but not D1R, regulated Cdc42 signaling in cocaine-induced neural plasticity and behavioral changes.

CONCLUSIONS

Cdc42 acts downstream of D2R in the hippocampus and plays an important role in cocaine-induced neural plasticity through N-WASP and PAK-LIMK-Cofilin, and Cdc42 signaling pathway correlatively links specific brain regions (CA1, dentate gyrus) to cocaine-induced CPP behavior.

D1 receptor-expressing neurons in ventral tegmental area alleviate mouse anxiety-like behaviors via glutamatergic projection to lateral septum

Dopamine (DA) acts as a key regulator in controlling emotion, and dysfunction of DA signal has been implicated in the pathophysiology of some psychiatric disorders, including anxiety. Ventral tegmental area (VTA) is one of main regions with DA-producing neurons. VTA DAergic projections in mesolimbic brain regions play a crucial role in regulating anxiety-like behaviors, however, the function of DA signal within VTA in regulating emotion remains unclear. Here, we observe that pharmacological activation/inhibition of VTA D1 receptors will alleviate/aggravate mouse anxiety-like behaviors, and knockdown of VTA D1 receptor expression also exerts anxiogenic effect. With fluorescence in situ hybridization and electrophysiological recording, we find that D1 receptors are functionally expressed in VTA neurons. Silencing/activating VTA D1 neurons bidirectionally modulate mouse anxiety-like behaviors. Furthermore, knocking down D1 receptors in VTA DA and glutamate neurons elevates anxiety-like state, but in GABA neurons has the opposite effect. In addition, we identify the glutamatergic projection from VTA D1 neurons to lateral septum is mainly responsible for the anxiolytic effect induced by activating VTA D1 neurons. Thus, our study not only characterizes the functional expression of D1 receptors in VTA neurons, but also uncovers the pivotal role of DA signal within VTA in mediating anxiety-like behaviors.

Low and high dose methamphetamine differentially regulate synaptic structural plasticity in cortex and hippocampus

Psychostimulants, such as methamphetamine (METH) can induce structural remodeling of synapses by remodeling presynaptic and postsynaptic morphology. Escalating or long-lasting high dose METH accounts for neurodegeneration by targeting multiple neurotransmitters. However, the effects of low dose METH on synaptic structure and the modulation mechanism remain elusive. This study aims to assess the effects of low dose (2 mg/kg) and high dose (10 mg/kg) of METH on synaptic structure alternation in hippocampus and prefrontal cortex (PFC) and to reveal the underlying mechanism involved in the process. Low dose METH promoted spine formation, synaptic number increase, post-synaptic density length elongation, and memory function. High dose of METH induced synaptic degeneration, neuronal number loss and memory impairment. Moreover, high dose, but not low dose, of METH caused gliosis in PFC and hippocampus. Mechanism-wise, low dose METH inactivated ras-related C3 botulinum toxin substrate 1 (Rac1) and activated cell division control protein 42 homolog (Cdc42); whereas high dose METH inactivated Cdc42 and activated Rac1. We provided evidence that low and high doses of METH differentially regulate synaptic plasticity in cortex and hippocampus.

Protective Effects of Resveratrol on Adolescent Social Isolation-Induced Anxiety-Like Behaviors via Modulating Nucleus Accumbens Spine Plasticity and Mitochondrial Function in Female Rats

Social isolation (SI) is a major risk factor for mood disorders in adolescents. The nucleus accumbens (NAc) is an important reward center implicated in psychiatric disorders. Resveratrol (RSV) is one of the most effective natural polyphenols with anti-anxiety and depression effects. However, little is known about the therapeutic effects and mechanisms of RSV on behavioral abnormality of adolescent social stress. Therefore, this study aimed to investigate the underlying mechanism of RSV on the amelioration of SI-induced behavioral abnormality. We found that SI induced anxiety-like behavior and social dysfunction in isolated female rats. Moreover, SI reduced mitochondrial number and ATP levels and increased thin spine density in the NAc. RNA sequencing results showed that SI changed the transcription pattern in the NAc, including 519 upregulated genes and 610 downregulated genes, especially those related to mitochondrial function. Importantly, RSV ameliorated behavioral and spine abnormalities induced by SI and increased NAc ATP levels and mitochondria number. Furthermore, RSV increased the activity of cytochrome C oxidase (COX) and upregulated mRNA levels of Cox5a, Cox6a1 and Cox7c. These results demonstrate that the modulation of spine plasticity and mitochondrial function in the NAc by RSV has a therapeutic effect on mood disorders induced by social isolation.

Age differences to methylphenidate-NAc neuronal and behavioral recordings from freely behaving animals

Methylphenidate (MPD) is a psychostimulant that is widely prescribed to treat attention deficit-hyperactivity disorder, but it is abused recreationally as well. The nucleus accumbens (NAc) is part of the motivation circuit implicated in drug-seeking behaviors. The NAc neuronal activity was recorded alongside the behavioral activity from young and adult rats to determine if there are significant differences in the response to MPD. The same dose of MPD elicits behavioral sensitization in some animals and behavioral tolerance in others. In adult animals, higher doses of MPD resulted in a greater ratio of tolerance/sensitization. Animals who responded to chronic MPD with behavioral sensitization usually exhibited further increases in their NAc neuronal firing rates as well. Different upregulations of transcription factors (ΔFOSB/CREB), variable proportions of D1/D2 dopamine receptors, and modulation from other brain areas may predispose certain animals to express behavioral and neuronal sensitization versus tolerance to MPD.

Silencing the Tlr4 Gene Alleviates Methamphetamine-Induced Hepatotoxicity by Inhibiting Lipopolysaccharide-Mediated Inflammation in Mice

Methamphetamine (METH) is a stimulant drug. METH abuse induces hepatotoxicity, although the mechanisms are not well understood. METH-induced hepatotoxicity was regulated by TLR4-mediated inflammation in BALB/c mice in our previous study. To further investigate the underlying mechanisms, the wild-type (C57BL/6) and Tlr4^−/− mice were treated with METH. Transcriptomics of the mouse liver was performed via RNA-sequencing. Histopathological changes, serum levels of metabolic enzymes and lipopolysaccharide (LPS), and expression of TLR4-mediated proinflammatory cytokines were assessed. Compared to the control, METH treatment induced obvious histopathological changes and significantly increased the levels of metabolic enzymes in wild-type mice. Furthermore, inflammatory pathways were enriched in the liver of METH-treated mice, as demonstrated by expression analysis of RNA-sequencing data. Consistently, the expression of TLR4 pathway members was significantly increased by METH treatment. In addition, increased serum LPS levels in METH-treated mice indicated overproduction of LPS and gut microbiota dysbiosis. However, antibiotic pretreatment or silencing Tlr4 significantly decreased METH-induced hepatic injury, serum LPS levels, and inflammation. In addition, the dampening effects of silencing Tlr4 on inflammatory pathways were verified by the enrichment analysis of RNA-sequencing data in METH-treated Tlr4^−/− mice compared to METH-treated wild-type mice. Taken together, these findings implied that Tlr4 silencing, comparable to antibiotic pretreatment, effectively alleviated METH-induced hepatotoxicity by inhibiting LPS-TLR4-mediated inflammation in the liver.

Knockout of Dopamine D3 Receptor Gene Blocked Methamphetamine-Induced Distinct Changes of Dopaminergic and Glutamatergic Synapse in the Nucleus Accumbens Shell of Mice

Structural plasticity changes in the brain are thought to underlie, at least partially, drug-induced persistent changes in behavior. Our previous study reported that increased synaptic density in the nucleus accumbens shell (NAcsh) correlates with and may contribute to behavioral sensitization induced by methamphetamine (METH). However, the distinct changes of dopaminergic and glutamatergic synapses and the modulating effects of dopamine D3 receptor remain unclear. In the current study, we used immunohistochemistry electron-microscopy and immunofluorescence to detect the changes of dopamine D1, D2, and glutamate NR2B-positive synapses and cells in the NAcsh of METH-sensitized wild type (WT) and knockout of dopamine D3 receptor gene (D3^–/–) mice. We found that METH induced long-term behavioral sensitization in WT mice, which was accompanied by an increased number and rate of dopamine D1 receptor-positive synapses and cells, as well as glutamate NR2B-positive synapses and cells. In contrast, the number and rate of dopamine D2 receptor-positive synapses and cells were significantly decreased in the NAcsh of METH-sensitized WT mice. D3^–/– mice exhibited attenuated acute locomotor responses and behavioral sensitization to METH compared with WT mice. Moreover, the knockout of dopamine D3 receptor gene inhibited METH-induced changes of dopaminergic and glutamatergic synapses in the NAcsh of METH-sensitized mice. Taken together, our results suggest that METH induced distinct changes of dopaminergic and glutamatergic synapses and cells in the NAcsh of mice, which was blocked by the knockout of dopamine D3 receptor gene, and may contribute to, at least partially, METH-induced behavior sensitization as well as the modulating effect of the dopamine D3 receptor.

Dopamine modulates visual threat processing in the superior colliculus via D2 receptors

Innate defensive responses, unlearned behaviors improving individuals’ chances of survival, have been found to involve the dopamine (DA) system. In the superior colliculus (SC), known for its role in defensive behaviors to visual threats, neurons expressing dopaminergic receptors of type 1 (Drd1+) and of type 2 (Drd2+) have been identified. We hypothesized that SC neurons expressing dopaminergic receptors may play a role in promoting innate defensive responses. Optogenetic activation of SC Drd2+ neurons, but not Drd1+ neurons, triggered defensive behaviors. Chemogenetic inhibition of SC Drd2+ neurons decreased looming-induced defensive behaviors, as well as pretreatment with the pharmacological Drd2+ agonist quinpirole, suggesting an essential role of Drd2 receptors in the regulation of innate defensive behavior. Input and output viral tracing revealed SC Drd2+ neurons mainly receive moderate inputs from the locus coeruleus (LC). Our results suggest a sophisticated regulatory role of DA and its receptor system in innate defensive behavior.

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Optogenetic activation of SC Drd2 + neurons, but not Drd1 + , induces defensive behaviors

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Repeated activation of SC Drd2 + provokes aversive memory and depression-like behavior

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Chemogenetic and pharmacological inhibition of SC Drd2 + impaired defensive behaviors

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Monosynaptic tracing revealed SC Drd2 + neurons mainly receive TH + projections from LC

Regulation of Cdc42 signaling by the dopamine D2 receptor in a mouse model of Parkinson's disease

Substantial spine loss in striatal medium spiny neurons (MSNs) and abnormal behaviors are common features of Parkinson's disease (PD). The caudate putamen (CPu) mainly contains MSNs expressing dopamine D1 receptor (dMSNs) and dopamine D2 receptor (iMSNs) exerting critical effects on motor and cognition behavior. However, the molecular mechanisms contributing to spine loss and abnormal behaviors in dMSNs and iMSNs under parkinsonian state remain unknown. In the present study, we revealed that Cell division control protein 42 (Cdc42) signaling was significantly decreased in the caudate putamen (CPu) in parkinsonian mice. In addition, overexpression of constitutively active Cdc42 in the CPu reversed spine abnormalities and improved the behavior deficits in parkinsonian mice. Utilizing conditional dopamine D1 receptor (D1R) or D2 receptor (D2R) knockout mice, we found that such a decrease under parkinsonian state was further reduced by conditional knockout of the D2R but not D1R. Moreover, the thin spine loss in iMSNs and deficits in motor coordination and cognition induced by conditional knockout of D2R were reversed by overexpression of constitutively active Cdc42 in the CPu. Additionally, conditional knockout of Cdc42 from D2R‐positive neurons in the CPu was sufficient to induce spine and behavior deficits similar to those observed in parkinsonian mice. Overall, our results indicate that impaired Cdc42 signaling regulated by D2R plays an important role in spine loss and behavioral deficits in PD.

Methamphetamine induces intestinal injury by altering gut microbiota and promoting inflammation in mice

Methamphetamine (METH) is a psychostimulant abused worldwide. Its abuse induces intestinal toxicity. Moreover, the gut microbiota is altered by drugs, which induces intestinal injury. Whether gut microbiota mediates METH-induced intestinal toxicity remains to be validated. In the present study, wild-type and TLR4^-/- mice were treated with METH. Gut microbiota was determined using 16S rRNA gene sequencing. Transcriptomics of the intestinal mucosa was performed by RNA-Sequencing. Blood levels of pro-inflammatory cytokines and lipopolysaccharide (LPS), the intestinal barrier, and inflammation were also assessed. METH treatment weakened the intestinal barrier and increased pro-inflammatory cytokines and LPS levels in the blood. Moreover, METH treatment significantly decreased the diversity of probiotics but increased the abundance of pathogenic gut microbiota, contributing to the over-production of LPS and disruption of intestinal barrier. Inflammatory pathways were enriched in the intestinal mucosa of METH-treated mice by KEGG analysis. Consistently, activation of the TLR4 pathway was determined in METH-treated mice, which confirmed intestinal inflammation. However, pretreatment with antibiotics or Tlr4 silencing significantly alleviated METH-induced gut microbiota dysbiosis, LPS over-production, intestinal inflammation, and disruption of the intestinal barrier. These findings suggested that the gut microbiota and LPS-mediated inflammation took an important role in METH-induced intestinal injury. Taken together, these findings suggest that METH-induced intestinal injury is mediated by gut microbiota dysbiosis and LPS-associated inflammation.

A novel microRNA, novel-m009C, regulates methamphetamine rewarding effects

Methamphetamine (METH) is a widely abused psychostimulant, whose hyper-rewarding property is believed to underlie its addictive effect, but the molecular mechanism regulating this effect remains unclear. We previously reported that decreased expression of a novel microRNA (miRNA), novel-m009C, is implicated in the regulation of METH hyperlocomotion. Here, we found that novel-m009C may be homologous to hsa-miR-604. Its expression is consistently downregulated in the nucleus accumbens (NAc) of mice when exposed to METH and cocaine, whereas significant alterations in novel-m009C expression were not observed in the NAc of mice subjected to other rewarding and psychiatric stimuli, such as sucrose, morphine and MK-801. We further found the substantial reduction in novel-m009C expression may be regulated by both dopamine receptor D1 (D1R) and D2 (D2R). Increasing novel-m009C levels in the NAc attenuated METH-induced conditioned place preference (CPP) and hyperlocomotion, whereas inhibiting novel-m009C expression in the NAc enhanced these effects but did not change the preference of mice for a natural reward, i.e., sucrose. These effects may involve targeting of genes important for the synaptic transmission, such as Grin1 (NMDAR subunit 1). Our findings demonstrate an important role for NAc novel-m009C in regulating METH reward, reveal a novel molecular regulator of the actions of METH on brain reward circuitries and provide a new strategy for treating METH addiction based on the modulation of small non-coding RNAs.

Methamphetamine abuse disturbs the dopaminergic system to impair hippocampal-based learning and memory: An overview of animal and human investigations

Diverse intellectual functions including memory are some important aspects of cognition. Dopamine is a neurotransmitter of the catecholamine family, which contributes to the experience of pleasure and/or emotional states but also plays crucial roles in learning and memory. Methamphetamine is an illegal drug, the abuse of which leads to long lasting pathological manifestations in the brain. Chronic methamphetamine-induced neurotoxicity results in an alteration of various parts of the memory systems by affecting learning processes, an effect attributed to the structural similarities of this drug with dopamine. An evolving field of research established how cognitive deficits in abusers arise and how they could possibly trigger neurodegenerative disorders. Thus, the drugs-induced tenacious neurophysiological changes of the dopamine system trigger cognitive deficits, thereby affirming the influence of this addictive drug on learning, memory and executive function in human abusers. Here we present an overview of the effects of methamphetamine abuse on cognitive functions, dopaminergic transmission and hippocampal integrity as they have been validated in animals and in humans during the past 20 years.

Rho Signaling in Synaptic Plasticity, Memory, and Brain Disorders

Memory impairments are associated with many brain disorders such as autism, Alzheimer's disease, and depression. Forming memories involves modifications of synaptic transmission and spine morphology. The Rho family small GTPases are key regulators of synaptic plasticity by affecting various downstream molecules to remodel the actin cytoskeleton. In this paper, we will review recent studies on the roles of Rho proteins in the regulation of hippocampal long-term potentiation (LTP) and long-term depression (LTD), the most extensively studied forms of synaptic plasticity widely regarded as cellular mechanisms for learning and memory. We will also discuss the involvement of Rho signaling in spine morphology, the structural basis of synaptic plasticity and memory formation. Finally, we will review the association between brain disorders and abnormalities of Rho function. It is expected that studying Rho signaling at the synapse will contribute to the understanding of how memory is formed and disrupted in diseases.

Synergistic Impairment of the Neurovascular Unit by HIV-1 Infection and Methamphetamine Use: Implications for HIV-1-Associated Neurocognitive Disorders

The neurovascular units (NVU) are the minimal functional units of the blood-brain barrier (BBB), composed of endothelial cells, pericytes, astrocytes, microglia, neurons, and the basement membrane. The BBB serves as an important interface for immune communication between the brain and peripheral circulation. Disruption of the NVU by the human immunodeficiency virus-1 (HIV-1) induces dysfunction of the BBB and triggers inflammatory responses, which can lead to the development of neurocognitive impairments collectively known as HIV-1-associated neurocognitive disorders (HAND). Methamphetamine (METH) use disorder is a frequent comorbidity among individuals infected with HIV-1. METH use may be associated not only with rapid HIV-1 disease progression but also with accelerated onset and increased severity of HAND. However, the molecular mechanisms of METH-induced neuronal injury and cognitive impairment in the context of HIV-1 infection are poorly understood. In this review, we summarize recent progress in the signaling pathways mediating synergistic impairment of the BBB and neuronal injury induced by METH and HIV-1, potentially accelerating the onset or severity of HAND in HIV-1-positive METH abusers. We also discuss potential therapies to limit neuroinflammation and NVU damage in HIV-1-infected METH abusers.

Astrocyte-selective STAT3 knockdown rescues methamphetamine withdrawal-disrupted spatial memory in mice via restoring the astrocytic capacity of glutamate clearance in dCA1

Methamphetamine (METH) is a common abused drug. METH-triggered glutamate (Glu) levels in dorsal CA1 (dCA1) could partially explain the etiology of METH-caused abnormal memory, but the synaptic mechanism remains unclear. Here, we found that METH withdrawal disrupted spatial memory in mice, accompanied by the increases in Glu levels and postsynaptic neuronal activities at dCA1 synapses. METH withdrawal weakened the capacity of Glu clearance in astrocytes, as indicated by increasing the A1-like astrocytes and phosphorylated signal transducer and activator of transcription 3 (p-STAT3), decreasing the Glu transporter 1(GLT-1, also known as EAAT2 or SLC1A2), Glu-aspartate-transporter (GLAST also known as EAAT1 or SLC1A3) and astrocytic glutamine synthase (GS), but failed to affect the presynaptic Glu release from dCA3 within dCA1. Moreover, we identified that in vitro A1-like astrocytes exhibited an increased STAT3 activation and the impaired capacity of Glu clearance. Most importantly, selective knockdown of astrocytic STAT3 in vivo in dCA1 restored the astrocytic capacity of Glu clearance, normalized Glu levels at dCA1 synapses, and finally rescued METH withdrawal-disrupted spatial memory in mice. Thus, astrocytic Glu clearance system, especially STAT3, serves as a novel target for future therapies against METH neurotoxicity.

Pharmacological and Genetic Evidence of Dopamine Receptor 3-Mediated Vasoconstriction in Isolated Mouse Aorta

Dopamine receptors (DRs) are generally considered as mediators of vasomotor functions. However, when used in pharmacological studies, dopamine and/or DR agonists may not discriminate among different DR subtypes and may even stimulate alpha1 and beta-adrenoceptors. Here, we tested the hypothesis that D2R and/or D3R may specifically induce vasoconstriction in isolated mouse aorta. Aorta, isolated from wild-type (WT) and D3R-/- mice, was mounted in a wire myograph and challenged with cumulative concentrations of phenylephrine (PE), acetylcholine (ACh), and the D3R agonist 7-hydrxy-N,N-dipropyl-2-aminotetralin (7-OH-DPAT), with or without the D2R antagonist L741,626 and the D3R antagonist SB-277011-A. The vasoconstriction to PE and the vasodilatation to ACh were not different in WT and D3R-/-; in contrast, the contractile responses to 7-OH-DPAT were significantly weaker in D3R-/-, though not abolished. L741,626 did not change the contractile response induced by 7-OH-DPAT in WT or in D3R-/-, whereas SB-277011-A significantly reduced it in WT but did not in D3R-/-. D3R mRNA (assessed by qPCR) was about 5-fold more abundant than D2R mRNA in aorta from WT and undetectable in aorta from D3R-/-. Following transduction with lentivirus (72-h incubation) delivering synthetic microRNAs to specifically inactivate D2R (LV-miR-D2) or D3R (LV-miR-D3), the contractile response to 7-OH-DPAT was unaffected by LV-miR-D2, while it was significantly reduced by LV-miR-D3. These data indicate that, at least in mouse aorta, D3R stimulation induces vasoconstriction, while D2R stimulation does not. This is consistent with the higher expression level of D3R. The residual vasoconstriction elicited by high concentration D3R agonist in D3R-/- and/or in the presence of D3R antagonist is likely to be unrelated to DRs.

Mice carrying a schizophrenia-associated mutation of the Arhgap10 gene are vulnerable to the effects of methamphetamine treatment on cognitive function: association with morphological abnormalities in striatal neurons

We recently found a significant association between exonic copy-number variations in the Rho GTPase activating protein 10 (Arhgap10) gene and schizophrenia in Japanese patients. Special attention was paid to one patient carrying a missense variant (p.S490P) in exon 17, which overlapped with an exonic deletion in the other allele. Accordingly, we generated a mouse model (Arhgap10 S490P/NHEJ mice) carrying a missense variant and a coexisting frameshift mutation. We examined the spatiotemporal expression of Arhgap10 mRNA in the brain and found the highest expression levels in the cerebellum, striatum, and nucleus accumbens (NAc), followed by the frontal cortex in adolescent mice. The expression levels of phosphorylated myosin phosphatase-targeting subunit 1 and phosphorylated p21-activated kinases in the striatum and NAc were significantly increased in Arhgap10 S490P/NHEJ mice compared with wild-type littermates. Arhgap10 S490P/NHEJ mice exhibited a significant increase in neuronal complexity and spine density in the striatum and NAc. There was no difference in touchscreen-based visual discrimination learning between Arhgap10 S490P/NHEJ and wild-type mice, but a significant impairment of visual discrimination was evident in Arhgap10 S490P/NHEJ mice but not wild-type mice when they were treated with methamphetamine. The number of c-Fos-positive cells was significantly increased after methamphetamine treatment in the dorsomedial striatum and NAc core of Arhgap10 S490P/NHEJ mice. Taken together, these results suggested that schizophrenia-associated Arhgap10 gene mutations result in morphological abnormality of neurons in the striatum and NAc, which may be associated with vulnerability of cognition to methamphetamine treatment.

Decreased Neuronal Excitability in Medial Prefrontal Cortex during Morphine Withdrawal is associated with enhanced SK channel activity and upregulation of small GTPase Rac1

Rationale: Neuroadaptations in the medial prefrontal cortex (mPFC) and Nucleus Accumbens (NAc) play a role in the disruption of control-reward circuits in opioid addiction. Small Conductance Calcium-Activated Potassium (SK) channels in the mPFC have been implicated in neuronal excitability changes during morphine withdrawal. However, the mechanism that modulates SK channels during withdrawal is still unknown.

Methods: Rats were exposed for one week to daily morphine injections (10 mg·kg^-1 s.c.) followed by conditional place preference (CPP) assessment. One week after withdrawal, electrophysiological, morphological and molecular biological methods were applied to investigate the effects of morphine on SK channels in mPFC, including infralimbic (IL), prelimbic (PrL) cortices and NAc (core and shell). We verified the hypothesis that Rac1, a member of Rho family of small GTPases, implicated in SK channel regulation, modulate SK channel neuroadaptations during opiate withdrawal.

Results: One week after morphine withdrawal, the neuronal excitability of layer 5 pyramidal neurons in IL was decreased, but not in PrL. Whereas, the excitability was increased in NAc-shell, but not in NAc-core. In mPFC, the expression of the SK3 subunit was enhanced after one-week of withdrawal compared to controls. In the IL, Rac1 signaling was increased during withdrawal, and the Rac1 inhibitor NSC23766 disrupted SK current, which increased neuronal firing. Suppression of Rac1 inhibited morphine-induced CPP and expression of SK channels in IL.

Conclusions: These findings highlight the potential value of SK channels and the upstream molecule Rac1, which may throw light on the therapeutic mechanism of neuromodulation treatment for opioid dependence.

The Role of Rac GTPase in Dendritic Spine Morphogenesis and Memory

The ability to form memories in the brain is needed for daily functions, and its impairment is associated with human mental disorders. Evidence indicates that long-term memory (LTM)-related processes such as its consolidation, extinction and forgetting involve changes of synaptic efficacy produced by alterations in neural transmission and morphology. Modulation of the morphology and number of dendritic spines has been proposed to contribute to changes in neuronal transmission mediating such LTM-related processes. Rac GTPase activity is regulated by synaptic activation and it can affect spine morphology by controlling actin-regulatory proteins. Recent evidence shows that changes in Rac GTPase activity affect memory consolidation, extinction, erasure and forgetting and can affect spine morphology in brain areas that mediate these behaviors. Altered Rac GTPase activity is associated with abnormal spine morphology and brain disorders. By affecting Rac GTPase activity we can further understand the roles of spine morphogenesis in memory. Moreover, manipulation of Rac GTPase activity may serve as a therapeutic tool for the treatment of memory-related brain diseases.

Dopamine outside the brain: The eye, cardiovascular system and endocrine pancreas

Dopamine (DA) and DA receptors (DR) have been extensively studied in the central nervous system (CNS), but their role in the periphery is still poorly understood. Here we summarize data on DA and DRs in the eye, cardiovascular system and endocrine pancreas, three districts where DA and DA-related drugs have been studied and the expression of DR documented. In the eye, DA modulates ciliary blood flow and aqueous production, which impacts on intraocular pressure and glaucoma. In the cardiovascular system, DA increases blood pressure and heart activity, mostly through a stimulation of adrenoceptors, and induces vasodilatation in the renal circulation, possibly through D1R stimulation. In pancreatic islets, beta cells store DA and co-release it with insulin. D1R is mainly expressed in beta cells, where it stimulates insulin release, while D2R is expressed in both beta and delta cells (in the latter at higher level), where it inhibits, respectively, insulin and somatostatin release. The formation of D2R-somatostatin receptor 5 heteromers (documented in the CNS), might add complexity to the system. DA may exert both direct autocrine effects on beta cells, and indirect paracrine effects through delta cells and somatostatin. Bromocriptine, an FDA approved drug for diabetes, endowed with both D1R (antagonistic) and D2R (agonistic) actions, may exert complex effects, resulting from the integration of direct effects on beta cells and paracrine effects from delta cells. A full comprehension of peripheral DA signaling deserves further studies that may generate innovative therapeutic drugs to manage conditions such as glaucoma, cardiovascular diseases and diabetes.