mTORC2/rictor signaling disrupts dopamine-dependent behaviors via defects in striatal dopamine neurotransmission

Organic cation transporter 2 contributes to SSRI antidepressant efficacy by controlling tryptophan availability in the brain

Selective serotonin reuptake inhibitors (SSRI) are common first-line treatments for major depression. However, a significant number of depressed patients do not respond adequately to these pharmacological treatments. In the present preclinical study, we demonstrate that organic cation transporter 2 (OCT2), an atypical monoamine transporter, contributes to the effects of SSRI by regulating the routing of the essential amino acid tryptophan to the brain. Contrarily to wild-type mice, OCT2-invalidated mice failed to respond to prolonged fluoxetine treatment in a chronic depression model induced by corticosterone exposure recapitulating core symptoms of depression, i.e., anhedonia, social withdrawal, anxiety, and memory impairment. After corticosterone and fluoxetine treatment, the levels of tryptophan and its metabolites serotonin and kynurenine were decreased in the brain of OCT2 mutant mice compared to wild-type mice and reciprocally tryptophan and kynurenine levels were increased in mutants' plasma. OCT2 was detected by immunofluorescence in several structures at the blood-cerebrospinal fluid (CSF) or brain-CSF interface. Tryptophan supplementation during fluoxetine treatment increased brain concentrations of tryptophan and, more discreetly, of 5-HT in wild-type and OCT2 mutant mice. Importantly, tryptophan supplementation improved the sensitivity to fluoxetine treatment of OCT2 mutant mice, impacting chiefly anhedonia and short-term memory. Western blot analysis showed that glycogen synthase kinase-3β (GSK3β) and mammalian/mechanistic target of rapamycin (mTOR) intracellular signaling was impaired in OCT2 mutant mice brain after corticosterone and fluoxetine treatment and, conversely, tryptophan supplementation recruited selectively the mTOR protein complex 2. This study provides the first evidence of the physiological relevance of OCT2-mediated tryptophan transport, and its biological consequences on serotonin homeostasis in the brain and SSRI efficacy.

Developmental Manganese Exposure Causes Lasting Attention Deficits Accompanied by Dysregulation of mTOR Signaling and Catecholaminergic Gene Expression in Brain Prefrontal Cortex

Elevated manganese (Mn) exposure is associated with attentional deficits in children, and is an environmental risk factor for attention deficit hyperactivity disorder (ADHD). We have shown that developmental Mn exposure causes lasting attention and sensorimotor deficits in a rat model of early childhood Mn exposure, and that these deficits are associated with a hypofunctioning catecholaminergic system in the prefrontal cortex (PFC), though the mechanistic basis for these deficits is not well understood. To address this, male Long-Evans rats were exposed orally to Mn (50 mg/kg/d) over PND 1-21 and attentional function was assessed in adulthood using the 5-Choice Serial Reaction Time Task. Targeted catecholaminergic system and epigenetic gene expression, followed by unbiased differential DNA methylation and gene regulation expression transcriptomics in the PFC, were performed in young adult littermates. Results show that developmental Mn exposure causes lasting focused attention deficits that are associated with reduced gene expression of tyrosine hydroxylase, dopamine transporter, and DNA methyltransferase 3a. Further, developmental Mn exposure causes broader lasting methylation and gene expression dysregulation associated with epigenetic regulation, inflammation, cell development, and hypofunctioning catecholaminergic neuronal systems. Pathway enrichment analyses uncovered mTOR and Wnt signaling pathway genes as significant transcriptomic regulators of the Mn altered transcriptome, and Western blot of total, C1 and C2 phospho-mTOR confirmed mTOR pathway dysregulation. Our findings deepen our understanding of the mechanistic basis of how developmental Mn exposure leads to lasting catecholaminergic dysfunction and attention deficits, which may aid future therapeutic interventions of environmental exposure associated disorders.

Significance Statement

Attention deficit hyperactivity disorder (ADHD) is associated with environmental risk factors, including exposure to neurotoxic agents. Here we used a rodent model of developmental manganese (Mn) exposure producing lasting attention deficits to show broad epigenetic and gene expression changes in the prefrontal cortex, and to identify disrupted mTOR and Wnt signaling pathways as a novel mechanism for how developmental Mn exposure may induce lasting attention and catecholaminergic system impairments. Importantly, our findings establish early development as a critical period of susceptibility to lasting deficits in attentional function caused by elevated environmental toxicant exposure. Given that environmental health threats disproportionately impact communities of color and low socioeconomic status, our findings can aid future studies to assess therapeutic interventions for vulnerable populations.

Canonical and Non-Canonical Antipsychotics' Dopamine-Related Mechanisms of Present and Next Generation Molecules: A Systematic Review on Translational Highlights for Treatment Response and Treatment-Resistant Schizophrenia

Schizophrenia is a severe psychiatric illness affecting almost 25 million people worldwide and is conceptualized as a disorder of synaptic plasticity and brain connectivity. Antipsychotics are the primary pharmacological treatment after more than sixty years after their introduction in therapy. Two findings hold true for all presently available antipsychotics. First, all antipsychotics occupy the dopamine D2 receptor (D2R) as an antagonist or partial agonist, even if with different affinity; second, D2R occupancy is the necessary and probably the sufficient mechanism for antipsychotic effect despite the complexity of antipsychotics' receptor profile. D2R occupancy is followed by coincident or divergent intracellular mechanisms, implying the contribution of cAMP regulation, β-arrestin recruitment, and phospholipase A activation, to quote some of the mechanisms considered canonical. However, in recent years, novel mechanisms related to dopamine function beyond or together with D2R occupancy have emerged. Among these potentially non-canonical mechanisms, the role of Na²⁺ channels at the dopamine at the presynaptic site, dopamine transporter (DAT) involvement as the main regulator of dopamine concentration at synaptic clefts, and the putative role of antipsychotics as chaperones for intracellular D2R sequestration, should be included. These mechanisms expand the fundamental role of dopamine in schizophrenia therapy and may have relevance to considering putatively new strategies for treatment-resistant schizophrenia (TRS), an extremely severe condition epidemiologically relevant and affecting almost 30% of schizophrenia patients. Here, we performed a critical evaluation of the role of antipsychotics in synaptic plasticity, focusing on their canonical and non-canonical mechanisms of action relevant to the treatment of schizophrenia and their subsequent implication for the pathophysiology and potential therapy of TRS.

Dopamine Dynamics and Neurobiology of Non-Response to Antipsychotics, Relevance for Treatment Resistant Schizophrenia: A Systematic Review and Critical Appraisal

Treatment resistant schizophrenia (TRS) is characterized by a lack of, or suboptimal response to, antipsychotic agents. The biological underpinnings of this clinical condition are still scarcely understood. Since all antipsychotics block dopamine D2 receptors (D2R), dopamine-related mechanisms should be considered the main candidates in the neurobiology of antipsychotic non-response, although other neurotransmitter systems play a role. The aims of this review are: (i) to recapitulate and critically appraise the relevant literature on dopamine-related mechanisms of TRS; (ii) to discuss the methodological limitations of the studies so far conducted and delineate a theoretical framework on dopamine mechanisms of TRS; and (iii) to highlight future perspectives of research and unmet needs. Dopamine-related neurobiological mechanisms of TRS may be multiple and putatively subdivided into three biological points: (1) D2R-related, including increased D2R levels; increased density of D2Rs in the high-affinity state; aberrant D2R dimer or heteromer formation; imbalance between D2R short and long variants; extrastriatal D2Rs; (2) presynaptic dopamine, including low or normal dopamine synthesis and/or release compared to responder patients; and (3) exaggerated postsynaptic D2R-mediated neurotransmission. Future points to be addressed are: (i) a more neurobiologically-oriented phenotypic categorization of TRS; (ii) implementation of neurobiological studies by directly comparing treatment resistant vs. treatment responder patients; (iii) development of a reliable animal model of non-response to antipsychotics.

Psilocin suppresses methamphetamine-induced hyperlocomotion and acquisition of conditioned place preference via D2R-mediated ERK signaling

AIM

Psilocin is an active metabolite form of psilocybin and exerts psychoactive effects. Recent studies suggest that psilocin may have regulatory effects on abuse drugs, but the mechanisms remain unclear. In this study, we want to explore the effects of psilocin on methamphetamine (METH)-induced alterations of behavior in mice and its molecular mechanisms.

METHODS

Acute METH administration model and conditioned place preference (CPP) model were used to investigate the effects of psilocin on METH-induced alterations of behavior. Western blot was used to detect the expression of proteins.

RESULTS

In the acute 2 mg/kg METH administration model, 1 mg/kg psilocin counteracted METH-induced elevation of activity. In the 1 mg/kg METH-induced CPP model, 1 mg/kg psilocin inhibited CPP formation during the acquisition phase. However, psilocin did not impact METH extinction and relapse. Molecular results showed that the regulatory effect of psilocin on METH was underscored by altered expression of dopamine 2 receptor (D2R) and phosphorylated extra-cellular signal-regulated kinase (p-ERK) in the prefrontal cortex (PFC), nucleus accumbens (NAc), and ventral tegmental area (VTA). Trifluoperazine (TFP)-2HCl is a D2R inhibitor, and SCH772984 is a selective extra-cellular signal-regulated kinase (ERK) inhibitor that effectively inhibits ERK1/2 phosphorylation. The results indicated that 2 mg/kg TFP-2HCl and 10 mg/kg SCH772984 blocked METH-induced hyperactivity and acquisition of METH-induced CPP.

CONCLUSION

Psilocin has regulatory effects on METH-induced alterations of behavior in mice via D2R-mediated signal regulation of ERK phosphorylation.

Antidepressant-like effects of cinnamamide derivative M2 via D2 receptors in the mouse medial prefrontal cortex

Major depressive disorder is a global mental illness associated with severe mortality and disability. The dopaminergic system is involved in both the etiology and therapeutics of depression. Distinct functions of dopamine D1 and D2 receptor subtypes have attracted considerable research interest, and their roles in the pathogenesis of depression and interaction with antidepressants need to be comprehensively elucidated. Herein, we investigated the antidepressant effects of a candidate antidepressant from a cinnamamide derivative, M2, and examined underlying neural mechanisms. We observed that a single dose of M2 (30 mg/kg, ip) produced rapid antidepressant-like effects in mice subjected to the forced swim and tail suspension tests. Using whole-cell recordings in mouse coronal brain slices, we found that application of M2 (10-150 μM) concentration-dependently increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) of the pyramidal neurons in the medial prefrontal cortex (mPFC). Furthermore, M2-induced enhancement of sEPSC frequency was abolished by sulpiride (10 µM), a dopamine D2 receptor antagonist, but not by the dopamine receptor D1 antagonist, SCH23390 (10 μM). In addition, M2 administration significantly increased expression levels of synaptogenesis-related proteins, including p-mTOR and p-TrkB, in the mPFC at 30 min, and increased postsynaptic protein PSD-95 at 24 h. Our results demonstrated that M2 produces rapid antidepressant actions through a novel mechanism via dopamine D2 receptor-mediated enhancement of mPFC neurotransmission.

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

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

Saikosaponin D exerts antidepressant effect by regulating Homer1-mGluR5 and mTOR signaling in a rat model of chronic unpredictable mild stress

BACKGROUND

Many studies about depression have focused on the dysfunctional synaptic signaling in the hippocampus that drives the pathophysiology of depression. Radix Bupleuri has been used in China for over 2000 years to regulate liver-qi. Extracted from Radix Bupleuri, Saikosaponin D (SSD) is a pharmacologically active substance that has antidepressant effects. However, its underlying mechanism remains unknown.

MATERIALS AND METHODS

A chronic unpredictable mild stress (CUMS) paradigm was used as a rat model of depression. SD rats were randomly assigned to a normal control (NC) group or one exposed to a CUMS paradigm. Of the latter group, rats were assigned to four subgroups: no treatment (CUMS), fluoxetine-treated (FLU), high-dose and low-dose SSD-treated (SSDH and SSDL). SSD was orally administrated of 1.50 mg/kg and 0.75 mg/kg/days for three weeks in the SSDH and SSDL groups, respectively. Fluoxetine was administrated at a dose of 2.0 mg/kg/days. SSD's antidepressant effects were assessed using the open field test, forced swim test, and sucrose preference test. Glutamate levels were quantified by ELISA. Western blot and immunochemical analyses were conducted to quantify proteins in the Homer protein homolog 1 (Homer1)-metabotropic glutamate receptor 5 (mGluR5) and mammalian target of rapamycin (mTOR) pathways in the hippocampal CA1 region. To measure related gene expression, RT-qPCR was employed.

RESULTS

CUMS-exposed rats treated with SSD exhibited increases in food intake, body weight, and improvements in the time spent in the central are and total distance traveled in the OFT, and less pronounced pleasure-deprivation behaviors. SSD also decreased glutamate levels in CA1. In CA1 region of CUMS-exposed rats, SSD treatment increased mGluR5 expression while decreasing Homer1 expression. SSD also increased expressions of postsynaptic density protein 95 (PSD95) and synapsin I (SYP), and the ratios of p-mTOR/mTOR, p-p70S6k/p70S6k, and p-4E-BP1/4E-BP1 in the CA1 region in CUMS-exposed rats.

CONCLUSIONS

SSD treatment reduces glutamate levels in the CA1 region and promotes the expression of the synaptic proteins PSD-95 and SYP via the regulation of the Homer1-mGluR5 and downstream mTOR signaling pathways. These findings suggest that SSD could act as a natural neuroprotective agent in the prevention of depression.

Abnormal RasGRP1 Expression in the Post-Mortem Brain and Blood Serum of Schizophrenia Patients

Schizophrenia (SCZ) is a polygenic severe mental illness. Genome-wide association studies (GWAS) have detected genomic variants associated with this psychiatric disorder and pathway analyses have indicated immune system and dopamine signaling as core components of risk in dorsolateral-prefrontal cortex (DLPFC) and hippocampus, but the mechanistic links remain unknown. The RasGRP1 gene, encoding for a guanine nucleotide exchange factor, is implicated in dopamine signaling and immune response. RasGRP1 has been identified as a candidate risk gene for SCZ and autoimmune disease, therefore representing a possible point of convergence between mechanisms involving the nervous and the immune system. Here, we investigated RasGRP1 mRNA and protein expression in post-mortem DLPFC and hippocampus of SCZ patients and healthy controls, along with RasGRP1 protein content in the serum of an independent cohort of SCZ patients and control subjects. Differences in RasGRP1 expression between SCZ patients and controls were detected both in DLPFC and peripheral blood of samples analyzed. Our results indicate RasGRP1 may mediate risk for SCZ by involving DLPFC and peripheral blood, thus encouraging further studies to explore its possible role as a biomarker of the disease and/or a target for new medication.

α-Synuclein-induced dysregulation of neuronal activity contributes to murine dopamine neuron vulnerability

Pathophysiological damages and loss of function of dopamine neurons precede their demise and contribute to the early phases of Parkinson's disease. The presence of aberrant intracellular pathological inclusions of the protein α-synuclein within ventral midbrain dopaminergic neurons is one of the cardinal features of Parkinson's disease. We employed molecular biology, electrophysiology, and live-cell imaging to investigate how excessive α-synuclein expression alters multiple characteristics of dopaminergic neuronal dynamics and dopamine transmission in cultured dopamine neurons conditionally expressing GCaMP6f. We found that overexpression of α-synuclein in mouse (male and female) dopaminergic neurons altered neuronal firing properties, calcium dynamics, dopamine release, protein expression, and morphology. Moreover, prolonged exposure to the D2 receptor agonist, quinpirole, rescues many of the alterations induced by α-synuclein overexpression. These studies demonstrate that α-synuclein dysregulation of neuronal activity contributes to the vulnerability of dopaminergic neurons and that modulation of D2 receptor activity can ameliorate the pathophysiology. These findings provide mechanistic insights into the insidious changes in dopaminergic neuronal activity and neuronal loss that characterize Parkinson's disease progression with significant therapeutic implications.

Alteration in the mRNA expression profile of the autophagy-related mTOR pathway in schizophrenia patients treated with olanzapine

BACKGROUND

The mammalian target of rapamycin protein (mTOR) signaling pathway is involved in the pathogenesis of schizophrenia and the mechanism of extrapyramidal adverse reactions to antipsychotic drugs, which might be mediated by an mTOR-dependent autophagy impairment. This study aimed to examine the expression of mTOR pathway genes in patients with schizophrenia treated with olanzapine, which is considered an mTOR inhibitor and autophagy inducer.

METHODS

Thirty-two patients with acute schizophrenia who had been treated with olanzapine for four weeks (average dose 14.24 ± 4.35 mg/d) and 32 healthy volunteers were recruited. Before and after olanzapine treatment, the Positive and Negative Syndrome Scale (PANSS) was used to evaluate the symptoms of patients with schizophrenia, and the mRNA expression levels of mTOR pathway-related genes, including MTOR, RICTOR, RAPTOR, and DEPTOR, were detected in fasting venous blood samples from all subjects using real-time quantitative PCR.

RESULTS

The MTOR and RICTOR mRNA expression levels in patients with acute schizophrenia were significantly decreased compared with those of healthy controls and further significantly decreased after four weeks of olanzapine treatment. The DEPTOR mRNA expression levels in patients with acute schizophrenia were not significantly different from those of healthy controls but were significantly increased after treatment. The expression levels of the RAPTOR mRNA were not significantly different among the three groups. The pairwise correlations of MTOR, DEPTOR, RAPTOR, and RICTOR mRNA expression levels in patients with acute schizophrenia and healthy controls were significant. After olanzapine treatment, the correlations between the expression levels of the DEPTOR and MTOR mRNAs and between the DEPTOR and RICTOR mRNAs disappeared.

CONCLUSIONS

Abnormalities in the mTOR pathway, especially DEPTOR and mTORC2, might play important roles in the autophagy mechanism underlying the pathophysiology of schizophrenia and effects of olanzapine treatment.

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

BACKGROUND

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

METHODS

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

AIMS

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

RESULTS

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

CONCLUSIONS

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

Systems Biology Analysis of the Antagonizing Effects of HIV-1 Tat Expression in the Brain over Transcriptional Changes Caused by Methamphetamine Sensitization

Methamphetamine (Meth) abuse is common among humans with immunodeficiency virus (HIV). The HIV-1 regulatory protein, trans-activator of transcription (Tat), has been described to induce changes in brain gene transcription that can result in impaired reward circuitry, as well as in inflammatory processes. In transgenic mice with doxycycline-induced Tat protein expression in the brain, i.e., a mouse model of neuroHIV, we tested global gene expression patterns induced by Meth sensitization. Meth-induced locomotor sensitization included repeated daily Meth or saline injections for seven days and Meth challenge after a seven-day abstinence period. Brain samples were collected 30 min after the Meth challenge. We investigated global gene expression changes in the caudate putamen, an area with relevance in behavior and HIV pathogenesis, and performed pathway and transcriptional factor usage predictions using systems biology strategies. We found that Tat expression alone had a very limited impact in gene transcription after the Meth challenge. In contrast, Meth-induced sensitization in the absence of Tat induced a global suppression of gene transcription. Interestingly, the interaction between Tat and Meth broadly prevented the Meth-induced global transcriptional suppression, by maintaining regulation pathways, and resulting in gene expression profiles that were more similar to the controls. Pathways associated with mitochondrial health, initiation of transcription and translation, as well as with epigenetic control, were heavily affected by Meth, and by its interaction with Tat in anti-directional ways. A series of systems strategies have predicted several components impacted by these interactions, including mitochondrial pathways, mTOR/RICTOR, AP-1 transcription factor, and eukaryotic initiation factors involved in transcription and translation. In spite of the antagonizing effects of Tat, a few genes identified in relevant gene networks remained downregulated, such as sirtuin 1, and the amyloid precursor protein (APP). In conclusion, Tat expression in the brain had a low acute transcriptional impact but strongly interacted with Meth sensitization, to modify effects in the global transcriptome.

Genetic inactivation of mTORC1 or mTORC2 in neurons reveals distinct functions in glutamatergic synaptic transmission

Although mTOR signaling is known as a broad regulator of cell growth and proliferation, in neurons it regulates synaptic transmission, which is thought to be a major mechanism through which altered mTOR signaling leads to neurological disease. Although previous studies have delineated postsynaptic roles for mTOR, whether it regulates presynaptic function is largely unknown. Moreover, the mTOR kinase operates in two complexes, mTORC1 and mTORC2, suggesting that mTOR's role in synaptic transmission may be complex-specific. To better understand their roles in synaptic transmission, we genetically inactivated mTORC1 or mTORC2 in cultured mouse glutamatergic hippocampal neurons. Inactivation of either complex reduced neuron growth and evoked EPSCs (eEPSCs), however, the effects of mTORC1 on eEPSCs were postsynaptic and the effects of mTORC2 were presynaptic. Despite postsynaptic inhibition of evoked release, mTORC1 inactivation enhanced spontaneous vesicle fusion and replenishment, suggesting that mTORC1 and mTORC2 differentially modulate postsynaptic responsiveness and presynaptic release to optimize glutamatergic synaptic transmission.

Neurodevelopmental concepts of schizophrenia in the genome-wide association era: AKT/mTOR signaling as a pathological mediator of genetic and environmental programming during development

Normative brain development is contingent on the complex interplay between genes and environment. Schizophrenia (SCZ) is considered a highly polygenic, neurodevelopmental disorder associated with impaired neural circuit development, neurocognitive function and variations in neurotransmitter signaling systems, including dopamine. Significant evidence, accumulated over the last 30 years indicates a role for the in utero environment in SCZ pathophysiology. Emerging data suggests that changes in placental programming and function may mediate the link between genetic risk, early life complications (ELC) and adverse neurodevelopmental outcomes, with risk highlighted in key developmental drivers that converge on AKT/mTOR signaling. In this article we overview select risk genes identified through recent genome-wide association studies of SCZ including AKT3, miR-137, DRD2, and AKT1 itself. We propose that through convergence on AKT/mTOR signaling, these genes are critical factors directing both placentation and neurodevelopment, influencing risk for SCZ through dysregulation of placental function, metabolism and early brain development. We discuss association of risk genes in the context of their known roles in neurodevelopment, placental expression and their possible mechanistic links to SCZ in the broad context of the 'developmental origins of adult disease' construct. Understanding how common genetic variation impacts early fetal programming may advance our knowledge of disease etiology and identify early critical developmental windows for prevention and intervention.

Long-lasting neurobehavioral alterations in burn-injured mice resembling post-traumatic stress disorder in humans

OBJECTIVE

To establish an animal model for posttraumatic stress disorder in burn-injured patients.

METHODS

Thermal-injured mice with 15% total body surface area were subjected to a series of neurobehavioral tests at 1 and 3 months postburn. Brains were collected for analysis of key molecules expression, spleens for T cell function analysis, and blood for biochemistry and hormones detection.

RESULTS

Comparison with sham mice, burn mice showed extremely high locomotion in homecage, open field, and forced swimming tests, indicating a hyper-arousal state. Burn mice exhibited improved spatial memory in Morris Water Maze test and heightened context fear memory in context fear conditioning, suggesting re-experiencing behavior. Although burn mice showed pronounced passive avoidance in the step-through test, their active avoidance capability in response to the conditional stimulus in the shuttle box test was relatively deteriorated. Likewise, the retention of cue-feared memory was impaired in fear conditioning test. The above negative alterations in mood were recapitulated in open-field test, in which the burn mice displayed an anxiety-like behavior with less time spent in the center. However, no sign of depression was found in the forced swimming and sucrose preference tests. The negative mood of burn mice was reinforced by a deficit in sociality and preference for social novelty in social interaction test. These neurobehavioral alterations were associated with an increased expression of brain-derived neurotrophic factor along with a remarkable microgliosis and a moderate astrocytosis in the brain of burn vs. sham mice. Moreover, a prominent Th2 switch and consequent increased nuclear NF-κB translocation were seen in the splenic T cells from burn relative to sham mice.

CONCLUSIONS

We conclude that even mild burn injury could lead to long-lasting cognitive and effective alterations in mice. These findings shed light on the interactions among neuropsychology, neurobiology, and immunology throughout the recovery period of burn injury.

Autism-linked dopamine transporter mutation alters striatal dopamine neurotransmission and dopamine-dependent behaviors

The precise regulation of synaptic dopamine (DA) content by the dopamine transporter (DAT) ensures the phasic nature of the DA signal, which underlies the ability of DA to encode reward prediction error, thereby driving motivation, attention, and behavioral learning. Disruptions to the DA system are implicated in a number of neuropsychiatric disorders, including attention deficit hyperactivity disorder (ADHD) and, more recently, Autism Spectrum Disorder (ASD). An ASD-associated de novo mutation in the SLC6A3 gene resulting in a threonine to methionine substitution at site 356 (DAT T356M) was recently identified and has been shown to drive persistent reverse transport of DA (i.e. anomalous DA efflux) in transfected cells and to drive hyperlocomotion in Drosophila melanogaster. A corresponding mutation in the leucine transporter, a DAT-homologous transporter, promotes an outward-facing transporter conformation upon substrate binding, a conformation possibly underlying anomalous dopamine efflux. Here we investigated in vivo the impact of this ASD-associated mutation on DA signaling and ASD-associated behaviors. We found that mice homozygous for this mutation display impaired striatal DA neurotransmission and altered DA-dependent behaviors that correspond with some of the behavioral phenotypes observed in ASD.

Prolonged increase in ser31 tyrosine hydroxylase phosphorylation in substantia nigra following cessation of chronic methamphetamine

Methamphetamine (MA) exposure may increase the risk of motor or cognitive impairments similar to Parkinson's disease (PD) by middle age. Although damage to nigrostriatal or mesoaccumbens dopamine (DA) neurons may occur during or early after MA exposure, overt PD-like symptoms at a younger age may not manifest due to compensatory mechanisms to maintain DA neurotransmission. One possible compensatory mechanism is increased tyrosine hydroxylase (TH) phosphorylation. In the rodent PD 6-OHDA model, nigrostriatal lesion decreases TH protein in both striatum and substantia nigra (SN). However, DA loss in the SN is significantly less than that in the striatum. An increase in ser31 TH phosphorylation in the SN may increase TH activity in response to TH loss. To determine if similar compensatory mechanisms may be engaged in young mice after MA exposure, TH expression, phosphorylation, and DA tissue content were evaluated, along with dopamine transporter expression, 21 days after cessation of MA (24 mg/kg, daily, 14 days). DA tissue content was unaffected by the MA regimen in striatum, nucleus accumbens, SN, or ventral tegmental area (VTA), despite decreased TH protein in SN and VTA. In the SN, but not striatum, ser31 phosphorylation increased over 2-fold. This suggests that increased ser31 TH phosphorylation may be an inherent compensatory mechanism to attenuate DA loss against TH loss, similar to that in an established PD model. These results also indicate the somatodendritic compartments of DA neurons are more vulnerable to TH protein loss than terminal fields following MA exposure.

Bile diversion, a bariatric surgery, and bile acid signaling reduce central cocaine reward

The gut-to-brain axis exhibits significant control over motivated behavior. However, mechanisms supporting this communication are poorly understood. We reveal that a gut-based bariatric surgery chronically elevates systemic bile acids and attenuates cocaine-induced elevations in accumbal dopamine. Notably, this surgery reduces reward-related behavior and psychomotor sensitization to cocaine. Utilizing a knockout mouse model, we have determined that a main mediator of these post-operative effects is the Takeda G protein-coupled bile acid receptor (TGR5). Viral restoration of TGR5 in the nucleus accumbens of TGR5 knockout animals is sufficient to restore cocaine reward, centrally localizing this TGR5-mediated modulation. These findings define TGR5 and bile acid signaling as pharmacological targets for the treatment of cocaine abuse and reveal a novel mechanism of gut-to-brain communication.

mTORC2 in the dorsomedial striatum of mice contributes to alcohol-dependent F-Actin polymerization, structural modifications, and consumption

Actin is highly enriched at dendritic spines, and actin remodeling plays an essential role in structural plasticity. The mammalian target of rapamycin complex 2 (mTORC2) is a regulator of actin polymerization. Here, we report that alcohol consumption increases F-actin content in the dorsomedial striatum (DMS) of mice, thereby altering dendritic spine morphology in a mechanism that requires mTORC2. Specifically, we found that excessive alcohol consumption increases mTORC2 activity in the DMS, and that knockdown of Rictor, an essential component of mTORC2 signaling, reduces actin polymerization, and attenuates the alcohol-dependent alterations in spine head size and the number of mushroom spines. Finally, we show that knockdown of Rictor in the DMS reduces alcohol consumption, whereas intra-DMS infusion of the mTORC2 activator, A-443654, increases alcohol intake. Together, these results suggest that mTORC2 in the DMS facilitates the formation of F-actin, which in turn induces changes in spine structure to promote and/or maintain excessive alcohol intake.

Roles for neuronal and glial autophagy in synaptic pruning during development

The dendritic protrusions known as spines represent the primary postsynaptic location for excitatory synapses. Dendritic spines are critical for many synaptic functions, and their formation, modification, and turnover are thought to be important for mechanisms of learning and memory. At many excitatory synapses, dendritic spines form during the early postnatal period, and while many spines are likely being formed and removed throughout life, the net number are often gradually "pruned" during adolescence to reach a stable level in the adult. In neurodevelopmental disorders, spine pruning is disrupted, emphasizing the importance of understanding its governing processes. Autophagy, a process through which cytosolic components and organelles are degraded, has recently been shown to control spine pruning in the mouse cortex, but the mechanisms through which autophagy acts remain obscure. Here, we draw on three widely studied prototypical synaptic pruning events to focus on two governing principles of spine pruning: 1) activity-dependent synaptic competition and 2) non-neuronal contributions. We briefly review what is known about autophagy in the central nervous system and its regulation by metabolic kinases. We propose a model in which autophagy in both neurons and non-neuronal cells contributes to spine pruning, and how other processes that regulate spine pruning could intersect with autophagy. We further outline future research directions to address outstanding questions on the role of autophagy in synaptic pruning.

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

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

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

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

Identification of candidate genes involved in the etiology of sporadic Tourette syndrome by exome sequencing

Tourette Syndrome (TS) is a neurodevelopmental disorder characterized by chronic motor and vocal tics. Although there is a large genetic contribution, the genetic architecture of TS remains unclear. Exome sequencing has successfully revealed the contribution of de novo mutations in sporadic cases with neuropsychiatric disorders such as autism and schizophrenia. Here, using exome sequencing, we investigated de novo mutations in individuals with sporadic TS to identify novel risk loci and elucidate the genetic background of TS. Exome analysis was conducted for sporadic TS cases: nine trio families and one quartet family with concordant twins were investigated. Missense mutations were evaluated using functional prediction algorithms, and their population frequencies were calculated based on three public databases. Gene expression patterns in the brain were analyzed using the BrainSpan Developmental Transcriptome. Thirty de novo mutations, including four synonymous and four missense mutations, were identified. Among the missense mutations, one in the rapamycin-insensitive companion of mammalian target of rapamycin (RICTOR)-coding gene (rs140964083: G > A, found in one proband) was predicted to be hazardous. In the three public databases analyzed, variants in the same SNP locus were absent, and variants in the same gene were either absent or present at an extremely low frequency (3/5,008), indicating the rarity of hazardous RICTOR mutations in the general population. The de novo variant of RICTOR may be implicated in the development of sporadic TS, and RICTOR is a novel candidate factor for TS etiology.

Deletion of Rictor in catecholaminergic neurons alters locomotor activity and ingestive behavior

While the etiology of depression is not fully understood, increasing evidence from animal models suggests a role for the ventral tegmental area (VTA) in pathogenesis. In this paper, we investigate the potential role of VTA mechanistic target of rapamycin 2 (TORC2) signaling in mediating susceptibility to chronic social defeat stress (CSDS), a well-established mouse model of depression. Utilizing genetic and viral knockout of Rictor (rapamycin-insensitive companion of target of rapamycin), a requisite component of TORC2, we demonstrate that decreasing Rictor-dependent TORC2 signaling in catecholaminergic neurons, or within the VTA specifically, does not alter susceptibility to CSDS. Opiate abuse and mood disorders are often comorbid, and previous data demonstrate a role for VTA TORC2 in mediating opiate reward. Thus, we also investigated its potential role in mediating changes in opiate reward following CSDS. Catecholaminergic deletion of Rictor increases water, sucrose, and morphine intake but not preference in a two-bottle choice assay in stress-naïve mice, and these effects are maintained after stress. VTA-specific knockout of Rictor increases water and sucrose intake after physical CSDS, but does not alter consummatory behavior in the absence of stress. These findings suggest a novel role for TORC2 in mediating stress-induced changes in consummatory behaviors that may contribute to some aspects of mood disorders.

Dopamine synthesis in alcohol drinking-prone and -resistant mouse strains

Alcoholism is a prevalent and debilitating neuropsychiatric disease, and much effort has been aimed at elucidating the neurobiological mechanisms underlying maladaptive alcohol drinking in an effort to design rational treatment strategies. In preclinical literature, the use of inbred mouse lines has allowed for the examination of ethanol effects across vulnerable and resistant phenotypes. C57BL/6J mice consistently show higher rates of ethanol drinking compared to most mouse strains. Conversely, DBA/2J mice display low rates of ethanol consumption. Given that the reinforcing and rewarding effects of ethanol are thought to be in part mediated by its actions on dopamine neurotransmission, we hypothesized that alcohol-preferring C57BL/6J and alcohol-avoiding DBA/2J mice would display basal differences in dopamine system function. By administering an L-aromatic acid decarboxylase inhibitor and measuring L-Dopa accumulation via high-performance liquid chromatography as a measure of tyrosine hydroxylase activity, we found no difference in dopamine synthesis between mouse strains in the midbrain, dorsal striatum, or ventral striatum. However, we did find that quinpirole-induced inhibition of dopamine synthesis was greater in the ventral striatum of C57BL/6J mice, suggesting increased presynaptic D2-type dopamine autoreceptor sensitivity. To determine whether dopamine synthesis or autoreceptor sensitivity was altered by a history of ethanol, we exposed C57BL/6J mice to one or two weekly cycles of chronic intermittent ethanol (CIE) exposure and withdrawal. We found that there was an attenuation of baseline dopamine synthesis in the ventral striatum after two cycles of CIE. Finally, we examined tissue content of dopamine and dopamine metabolites across recombinant inbred mice bred from a C57BL/6J × DBA/2J cross (BXD). We found that low dopaminergic activity, as indicated by high dopamine/metabolite ratios, was positively correlated with drinking. Together, these findings show differential autoreceptor effects on dopamine synthesis between C57BL/6J and DBA/2J mice, and suggest that decreased dopaminergic activity is associated with excessive drinking.

Exercise increases mTOR signaling in brain regions involved in cognition and emotional behavior

Exercise can enhance learning and memory and produce resistance against stress-related psychiatric disorders such as depression and anxiety. In rats, these beneficial effects of exercise occur regardless of exercise controllability: both voluntary and forced wheel running produce stress-protective effects. The mechanisms underlying these beneficial effects of exercise remain unknown. The mammalian target of rapamycin (mTOR) is a translation regulator important for cell growth, proliferation, and survival. mTOR has been implicated in enhancing learning and memory as well as antidepressant effects. Moreover, mTOR is sensitive to exercise signals such as metabolic factors. The effects of exercise on mTOR signaling, however, remain unknown. The goal of the present study was to test the hypothesis that exercise, regardless of controllability, increases levels of phosphorylated mTOR (p-mTOR) in brain regions important for learning and emotional behavior. Rats were exposed to 6 weeks of either sedentary (locked wheel), voluntary, or forced wheel running conditions. At 6 weeks, rats were sacrificed during peak running and levels of p-mTOR were measured using immunohistochemistry. Overall, both voluntary and forced exercise increased p-mTOR-positive neurons in the medial prefrontal cortex, striatum, hippocampus, hypothalamus, and amygdala compared to locked wheel controls. Exercise, regardless of controllability, also increased numbers of p-mTOR-positive glia in the striatum, hippocampus, and amygdala. For both neurons and glia, the largest increase in p-mTOR positive cells was observed after voluntary running, with forced exercise causing a more modest increase. Interestingly, voluntary exercise preferentially increased p-mTOR in astrocytes (GFAP+), while forced running increased p-mTOR in microglia (CD11+) in the inferior dentate gyrus. Results suggest that mTOR signaling is sensitive to exercise, but subtle differences exist depending on exercise controllability. Increases in mTOR signaling could contribute to the beneficial effects of exercise on cognitive function and mental health.

Rictor/mTORC2 loss in osteoblasts impairs bone mass and strength

Mammalian target of rapamycin (mTOR) is a Ser/Thr kinase conserved through evolution that coordinates extra cellular signals associated with cell growth. Main functions of mTOR present in the form of two complexes, namely mTORC1 and mTORC2, which are distinct in their unique components, raptor and rictor. In the current study, using a Cre/loxp system, we found an anabolic effect of mTORC2 signaling on skeleton. Osteoblast differentiation was reduced, with down-regulation of mTORC2 signaling activity in primary cultures of osteoblasts that did not contain rictor. Mice with a specific deletion of rictor in mature osteoblasts showed a significant reduction in lean mass and bone mineral density by dual energy x-ray absorptiometry analysis. Micro-computed tomography, histomorphometric, and molecular biological analyses revealed a marked impairment of the cortical bone mass and microarchitecture, as well as minor changes in trabecular bone, of the Rictorob(-/-) mice. Cortical bone mass and thickness of the femoral mid-shaft were dramatically reduced, with unusual increases in porosity and marrow area in Rictorob(-/-) mice. Thinner trabeculae were found in the L4 vertebrae with relatively normal structural indices of trabecular numbers and separation. A lower rate of bone turnover was observed, as the consequence of the decreased individual osteoblast activity and bone resorption. Furthermore, these changes were associated with significantly decreased bone biomechanical properties. In conclusion, expression of rictor in osteoblasts is essential for the maintenance of normal bone remodeling and microarchitecture, especially for the maintenance of the cortical bone.

Regulation of Tyrosine Hydroxylase Expression and Phosphorylation in Dopamine Transporter-Deficient Mice

Tyrosine hydroxylase (TH) and dopamine transporters (DATs) regulate dopamine (DA) neurotransmission at the biosynthesis and reuptake steps, respectively. Dysfunction or loss of these proteins occurs in impaired locomotor or addictive behavior, but little is known about the influence of DAT expression on TH function. Differences in TH phosphorylation, DA tissue content, l-DOPA biosynthesis, and DA turnover exist between the somatodendritic and terminal field compartments of nigrostriatal and mesoaccumbens pathways. We examined whether differential DAT expression affects these compartmental differences in DA regulation by comparing TH expression and phosphorylation at ser31 and ser40. In heterozygous DAT knockout (KO) (+/-) mice, DA tissue content and DA turnover were unchanged relative to wild-type mice, despite a 40% reduction in DAT protein expression. In DAT KO (-/-) mice, DA turnover increased in all DA compartments, but DA tissue content decreased (90-96%) only in terminal fields. TH protein expression and phosphorylation were differentially affected within DA pathway compartments by relative expression of DAT. TH protein decreased (∼74%), though to a significantly lesser extent than DA, in striatum and nucleus accumbens (NAc) in DAT -/- mice, with no decrease in substantia nigra or ventral tegmental area. Striatal ser31 TH phosphorylation and recovery of DA relative to TH protein expression in DAT +/- and DAT -/- mice decreased, whereas ser40 TH phosphorylation increased ∼2- to 3-fold in striatum and NAc of DAT -/- mice. These results suggest that DAT expression affects TH expression and phosphorylation largely in DA terminal field compartments, further corroborating evidence for dichotomous regulation of TH between somatodendritic and terminal field compartments of the nigrostriatal and mesoaccumbens pathways.

Mammalian Target of Rapamycin: Its Role in Early Neural Development and in Adult and Aged Brain Function

The kinase mammalian target of rapamycin (mTOR) integrates signals triggered by energy, stress, oxygen levels, and growth factors. It regulates ribosome biogenesis, mRNA translation, nutrient metabolism, and autophagy. mTOR participates in various functions of the brain, such as synaptic plasticity, adult neurogenesis, memory, and learning. mTOR is present during early neural development and participates in axon and dendrite development, neuron differentiation, and gliogenesis, among other processes. Furthermore, mTOR has been shown to modulate lifespan in multiple organisms. This protein is an important energy sensor that is present throughout our lifetime its role must be precisely described in order to develop therapeutic strategies and prevent diseases of the central nervous system. The aim of this review is to present our current understanding of the functions of mTOR in neural development, the adult brain and aging.

Glucagon-like peptide 1 receptor activation regulates cocaine actions and dopamine homeostasis in the lateral septum by decreasing arachidonic acid levels

Agonism of the glucagon-like peptide 1 (GLP-1) receptor (GLP-1R) has been effective at treating aspects of addictive behavior for a number of abused substances, including cocaine. However, the molecular mechanisms and brain circuits underlying the therapeutic effects of GLP-1R signaling on cocaine actions remain elusive. Recent evidence has revealed that endogenous signaling at the GLP-1R within the forebrain lateral septum (LS) acts to reduce cocaine-induced locomotion and cocaine conditioned place preference, both considered dopamine (DA)-associated behaviors. DA terminals project from the ventral tegmental area to the LS and express the DA transporter (DAT). Cocaine acts by altering DA bioavailability by targeting the DAT. Therefore, GLP-1R signaling might exert effects on DAT to account for its regulation of cocaine-induced behaviors. We show that the GLP-1R is highly expressed within the LS. GLP-1, in LS slices, significantly enhances DAT surface expression and DAT function. Exenatide (Ex-4), a long-lasting synthetic analog of GLP-1 abolished cocaine-induced elevation of DA. Interestingly, acute administration of Ex-4 reduces septal expression of the retrograde messenger 2-arachidonylglycerol (2-AG), as well as a product of its presynaptic degradation, arachidonic acid (AA). Notably, AA reduces septal DAT function pointing to AA as a novel regulator of central DA homeostasis. We further show that AA oxidation product γ-ketoaldehyde (γ-KA) forms adducts with the DAT and reduces DAT plasma membrane expression and function. These results support a mechanism in which postsynaptic septal GLP-1R activation regulates 2-AG levels to alter presynaptic DA homeostasis and cocaine actions through AA.

Impaired mTORC2 signaling in catecholaminergic neurons exaggerates high fat diet-induced hyperphagia

Objective

Food intake is highly regulated by central homeostatic and hedonic mechanisms in response to peripheral and environmental cues. Neutral energy balance stems from proper integration of homeostatic signals with those “sensing” the rewarding properties of food. Impairments in brain insulin signaling causes dysregulation of feeding behaviors and, as a consequence, hyperphagia. Here, we sought to determine how the mammalian target of rapamycin complex 2 (mTORC2), a complex involved in insulin signaling, influences high fat feeding.

Methods

Rictor is a subunit of mTORC2, and its genetic deletion impairs mTORC2 activity. We used Cre-LoxP technology to delete Rictorin tyrosine hydroxylase (TH) expressing neurons (TH Rictor KO). We assessed food intake, body weight, body composition and DA dependent behaviors.

Results

TH Rictor KO mice display a high-fat diet specific hyperphagia, yet, when on low-fat diet, their food intake is indistinguishable from controls. Consistently, TH Rictor KO become obese only while consuming high-fat diet. This is paralleled by reduced brain DA content, and disruption of DA dependent behaviors including increased novelty-induced hyperactivity and exaggerated response to the psycho stimulant amphetamine (AMPH).

Conclusions

Our data support a model in which mTORC2 signaling within catecholaminergic neurons constrains consumption of a high-fat diet, while disruption causes high-fat diet-specific exaggerated hyperphagia. In parallel, impaired mTORC2 signaling leads to aberrant striatal DA neurotransmission, which has been associated with obesity in human and animal models, as well as with escalating substance abuse. These data suggest that defects localized to the catecholaminergic pathways are capable of overriding homeostatic circuits, leading to obesity, metabolic impairment, and aberrant DA-dependent behaviors.