Chronic social defeat stress in prairie voles (Microtus ochrogaster): A preclinical model for the study of depression-related phenotypes

BACKGROUND

Stress-induced illnesses, like major depression, are among the leading causes of disability across the world. Consequently, there is a dire need for the validation of translationally-suited animal models incorporating social stress to uncover the etiology of depression. Prairie voles (Microtus ochrogaster) are more translationally relevant than many other rodent models as they display monogamous social and bi-parental behaviors. Therefore, we evaluated whether a novel social defeat stress (SDS) model in male prairie voles induces depression-relevant behavioral outcomes.

METHODS

Adult sexually-naïve male prairie voles experienced SDS bouts from a conspecific pair-bonded male aggressor, 10 min per day for 10 consecutive days. Non-stressed controls (same-sex siblings) were housed in similar conditions but never experienced physical stress. Twenty-four h later, voles were evaluated in social interaction, sucrose preference, and Morris water maze tests - behavioral endpoints validated to assess social withdrawal, anhedonia-related behavior, and spatial memory performance, respectively.

RESULTS

SDS-exposed voles displayed lower sociability and body weight, decreased preference for a sucrose solution, and impairment of spatial memory retrieval. Importantly, no differences in general locomotor activity were observed as a function of SDS exposure.

LIMITATIONS

This study does not include female voles in the experimental design.

CONCLUSIONS

We found that repeated SDS exposure, in male prairie voles, results in a depression-relevant phenotype resembling an anhedonia-like outcome (per reductions in sucrose preference) along with social withdrawal and spatial memory impairment - highlighting that the prairie vole is a valuable model with potential to study the neurobiology of social stress-induced depression-related outcomes.

Sex-Specific Alterations in Spatial Memory and Hippocampal AKT-mTOR Signaling in Adult Mice Pre-exposed to Ketamine and/or Psychological Stress During Adolescence

Background

Ketamine (KET) is administered to manage major depression in adolescent patients. However, the long-term effects of juvenile KET exposure on memory-related tasks have not been thoroughly assessed. We examined whether exposure to KET, psychological stress, or both results in long-lasting alterations in spatial memory in C57BL/6 mice. Furthermore, we evaluated how KET and/or psychological stress history influenced hippocampal protein kinase B-mechanistic target of rapamycin (AKT-mTOR)-related signaling.

Methods

On postnatal day 35, male and female mice underwent vicarious defeat stress (VDS), a form of psychological stress that reduces sociability in both sexes, with or without KET exposure (20 mg/kg/day, postnatal days 35-44). In adulthood (postnatal day 70), mice were assessed for spatial memory performance on a water maze task or euthanized for hippocampal tissue collection.

Results

Juvenile pre-exposure to KET or VDS individually increased the latency (seconds) to locate the escape platform in adult male, but not female, mice. However, juvenile history of concomitant KET and VDS prevented memory impairment. Furthermore, individual KET or VDS pre-exposure, unlike their combined history, decreased hippocampal AKT-mTOR signaling in adult male mice. Conversely, KET pre-exposure alone increased AKT-mTOR in the hippocampus of adult female mice. Lastly, rapamycin-induced decreases of mTOR in naïve adult female mice induced spatial memory retrieval deficits, mimicking adult male mice with a history of exposure to VDS or KET.

Conclusions

Our preclinical model shows how KET treatment for the management of adolescent psychological stress-induced sequelae does not impair spatial memory later in life. However, juvenile recreational KET misuse, like psychological stress history, results in long-term spatial memory deficits and hippocampal AKT-mTOR signaling changes in a sex-specific manner.

Viral-mediated expression of Erk2 in the nucleus accumbens regulates responses to rewarding and aversive stimuli

Second-messenger signaling within the mesolimbic reward circuit is involved in both the long-lived effects of stress and in the underlying mechanisms that promote drug abuse liability. To determine the direct role of kinase signaling within the nucleus accumbens, specifically mitogen-activated protein kinase 1 (ERK2), in mood- and drug-related behavior, we used a herpes-simplex virus to up- or down-regulate ERK2 in adult male rats. We then exposed rats to a battery of behavioral tasks including the elevated plus-maze, open field test, forced-swim test, conditioned place preference, and finally cocaine self-administration. Herein, we show that viral overexpression or knockdown of ERK2 in the nucleus accumbens induces distinct behavioral phenotypes. Specifically, over expression of ERK2 facilitated depression- and anxiety-like behavior while also increasing sensitivity to cocaine. Conversely, down-regulation of ERK2 attenuated behavioral deficits, while blunting sensitivity to cocaine. Taken together, these data implicate ERK2 signaling, within the nucleus accumbens, in the regulation of affective behaviors and modulating sensitivity to the rewarding properties of cocaine.

Sex and diet-dependent gene alterations in human and rat brains with a history of nicotine exposure

Introduction

Chronic nicotine exposure induces changes in the expression of key regulatory genes associated with metabolic function and neuronal alterations in the brain. Many bioregulatory genes have been associated with exposure to nicotine, but the modulating effects of sex and diet on gene expression in nicotine-exposed brains have been largely unexplored. Both humans and rodents display motivation for nicotine use and the emergence of withdrawal symptoms during abstinence. Research comparing pre-clinical models with human subjects provides an important opportunity to understand common biomarkers of the harmful effects of nicotine as well as information that may help guide the development of more effective interventions for nicotine cessation.

Methods

Human postmortem dorsolateral prefrontal cortex (dLPFC) tissue BA9 was collected from female and male subjects, smokers and non-smokers (N = 12 per group). Rat frontal lobes were collected from female and male rats that received a regular diet (RD) or a high-fat diet (HFD) (N = 12 per group) for 14 days following implantation of a osmotic mini-pump (Alzet) that delivered nicotine continuously. Controls (control-s) received a sham surgical procedure. RNA was extracted from tissue from human and rat samples and reversed-transcribed to cDNA. Gene expression of CHRNA10 (Cholinergic receptor nicotinic alpha 10), CERKL (Ceramide Kinase-Like), SMYD1 (SET and MYD Domin Containing 1), and FA2H (Fatty Acid 2-Hydrolase) in humans was compared to rats in each subset of groups and quantified by qPCR methods. Additionally, protein expression of FA2H was analyzed by immunohistochemistry (IHC) in human dLPFC.

Results

Humans with a history of smoking displayed decreased CHRNA10 (p = 0.0005), CERKL (p ≤ 0.0001), and SMYD1 (p = 0.0005) expression and increased FA2H (p = 0.0097) expression compared to non-smokers (p < 0.05). Similar patterns of results were observed in nicotine exposed vs. control rats. Interestingly, sex-related differences in gene expression for CERKL and FA2H were observed. In addition, ANCOVA analysis showed a significant effect of nicotine in a sex-different manner, including an increase in CERKL in male and female rats with RD or HFD. In rats exposed to an HFD, FA2H gene expression was lower in nicotine-treated rats compared to RD rats treated with nicotine. Protein expression of FA2H (p = 0.001) by IHC was significantly higher in smokers compared to non-smokers.

Conclusion

These results suggest that a history of long-term nicotine exposure in humans alters the expression of sphingolipid metabolism-related (CERKL, SMYD1, and FA2H) and neuronal (CHRNA10) marker genes similarly as compared to rats. Sex- and diet-dependent differences appear in nicotine-exposed rats, critical in regulating sphingolipid metabolism and nicotinic acetylcholine receptors. This research enhances the construct validity of rat models of nicotine usage by showing a similar pattern of changes in gene expression in human subjects with a smoking history.

Molecular, Circuit, and Stress Response Characterization of Ventral Pallidum Npas1-Neurons

Altered activity of the ventral pallidum (VP) underlies disrupted motivation in stress and drug exposure. The VP is a very heterogeneous structure composed of many neuron types with distinct physiological properties and projections. Neuronal PAS 1-positive (Npas1+) VP neurons are thought to send projections to brain regions critical for motivational behavior. While Npas1+ neurons have been characterized in the globus pallidus external, there is limited information on these neurons in the VP. To address this limitation, we evaluated the projection targets of the VP Npas1+ neurons and performed RNA-sequencing on ribosome-associated mRNA from VP Npas1+ neurons to determine their molecular identity. Finally, we used a chemogenetic approach to manipulate VP Npas1+ neurons during social defeat stress (SDS) and behavioral tasks related to anxiety and motivation in Npas1-Cre mice. We used a similar approach in females using the chronic witness defeat stress (CWDS). We identified VP Npas1+ projections to the nucleus accumbens, ventral tegmental area, medial and lateral habenula, lateral hypothalamus, thalamus, medial and lateral septum, and periaqueductal gray area. VP Npas1+ neurons displayed distinct translatome representing distinct biological processes. Chemogenetic activation of hM3D(Gq) receptors in VP Npas1+ neurons increased susceptibility to a subthreshold SDS and anxiety-like behavior in the elevated plus maze and open field while the activation of hM4D(Gi) receptors in VP Npas1+ neurons enhanced resilience to chronic SDS and CWDS. Thus, the activity of VP Npas1+ neurons modulates susceptibility to social stressors and anxiety-like behavior. Our studies provide new information on VP Npas1+ neuron circuitry, molecular identity, and their role in stress response.SIGNIFICANCE STATEMENT The ventral pallidum (VP) is a structure connected to both reward-related and aversive brain centers. It is a key brain area that signals the hedonic value of natural rewards. Disruption in the VP underlies altered motivation in stress and substance use disorder. However, VP is a very heterogeneous area with multiple neuron subtypes. This study characterized the projection pattern and molecular signatures of VP Neuronal PAS 1-positive (Npas1+) neurons. We further used tools to alter receptor signaling in VP Npas1+ neurons in stress to demonstrate a role for these neurons in stress behavioral outcomes. Our studies have implications for understanding brain cell type identities and their role in brain disorders, such as depression, a serious disorder that is precipitated by stressful events.

Acute and long-lasting effects of adolescent fluoxetine exposure on feeding behavior in Sprague-Dawley rats

The antidepressant medication fluoxetine (FLX) is frequently prescribed for the management of mood-related illnesses in the adolescent population-yet its long-term neurobehavioral consequences are not understood. To investigate how juvenile FLX exposure influences feeding behavior in adulthood, we conducted two experiments. In Experiment 1, adolescent male and female Sprague-Dawley rats were administered with 20 mg/kg/day FLX (postnatal day [PND] 35-49) and exposed to a binge access paradigm in adulthood (PND72+) to evaluate potential alterations for sweetened-fat preference. No long-term FLX-induced differences in preference for sweetened fat versus chow, nor total caloric intake, were noted; however, females displayed higher preference for sweetened fat compared to males. In Experiment 2, PND35 male rats received FLX (PND35-49) and were exposed to chronic variable stress (CVS) in adulthood (PND74-88). During treatment, FLX decreased body weight and intake (meal size), but not total meal number. Also, no differences in meal pattern parameters were observed after FLX completion. Likewise, no differences in meal pattern parameters to a palatable diet (45% fat, 17% sucrose) presented from PND74 to PND88, even after CVS, were observed. Our findings indicate that juvenile FLX reduces body weight gain acutely via reduced meal size intake; however, no long-term changes in ad libitum feeding behavior or binge access to a palatable stimulus are evident.

The BDNF-TrkB Pathway Acts Through Nucleus Accumbens D2 Expressing Neurons to Mediate Stress Susceptible Outcomes

Brain-derived neurotrophic factor (BDNF) has a critical role in stress response including neuropsychiatric disorders that are precipitated by stress, such as major depressive disorder (MDD). BDNF acts through its full-length BDNF receptor tyrosine kinase B (TrkB) to trigger a pro-plasticity effect. In contrast, the truncated isoform of the BDNF receptor (TrkB.t1) triggers an anti-plasticity effect. In stress outcomes, BDNF acting in the hippocampus has a stress resilience effect, and, inversely, in the nucleus accumbens (NAc), BDNF acts as a stress susceptible molecule. It is unknown if BDNF-TrkB acts on a specific NAc projection neuron, i.e., medium spiny neuron (MSN or spiny projection neuron), a subtype in stress outcomes. To determine this, we performed chronic social or vicarious witness defeat stress (CSDS or CWDS) in mice expressing TrkB.t1 in dopamine receptor 1 or 2 containing MSNs (D1- or D2-MSNs). Our results showed that TrkB.t1 overexpression in NAc D2-MSNs prevented the CSDS-induced social avoidance or other stress susceptible behaviors in male and female mice. We further showed that this overexpression in D2-MSNs blocked stress susceptible behavior induced by intra-NAc BDNF infusion. In contrast, our results demonstrate that overexpression of TrkB.t1 on NAc D1-MSNs facilitates the SDS susceptible behaviors. Our study provides enhanced details into the NAc cell subtype role of BDNF-TrkB signaling in stress outcomes.

Association of ADHD and Obesity in Hispanic Children on the US-Mexico Border: A Retrospective Analysis

Pediatric obesity and Attention Deficit Hyperactivity Disorder (ADHD) are rising health concerns in the United States, especially among Hispanic children and adolescents. Research on Hispanic children and adolescents indicates disproportionately higher prevalence rates of obesity in this community but scant data on ADHD prevalence rates. In contrast, a plethora of research studies across the general population examines the relationship between childhood obesity and ADHD. In addition, there is a lack of research that examines the role of ethnicity and sub-ethnic group correlations in ADHD, particularly in the Hispanic population. Existing studies in the general population indicate ADHD may be a risk factor for being overweight compared to normal controls. The objective of the present study is to examine the prevalence of obesity in children with ADHD compared to children in the general population in a predominately Hispanic sample on the US-Mexico border. A total of 7,270 pediatric medical records were evaluated. The retrospective analysis included Body Mass Index (BMI) and related health variables, and ethnicity and showed that children with ADHD are more likely to be underweight. In conclusion, no significant relationship existed between obesity and ADHD among Hispanic children on the US-Mexico Border, and instead we found the opposite correlation.

Adolescent fluoxetine exposure increases ERK-related signaling within the prefrontal cortex of adult male Sprague-Dawley rats

There has been a disproportionate increase in fluoxetine (FLX) prescription rates within the juvenile population. Thus, we evaluated how adolescent FLX exposure alters expression/phosphorylation of proteins from the extracellular signal regulated kinase (ERK)-1/2 cascade within the adult prefrontal cortex (PFC). Male Sprague-Dawley rats were exposed to FLX (20 mg/kg) for 15 consecutive days (postnatal-day [PD] 35-49). At PD70 (adulthood), we examined protein markers for ERK1/2, ribosomal S6 kinase (RSK), and mammalian target of rapamycin (mTOR). FLX-pretreatment decreased body weight, while increasing PFC phosphorylation of ERK1/2 and RSK, as well as total mTOR protein expression in adulthood. We provide first-line evidence that juvenile FLX-pretreatment induces long-term decreases in body weight-gain, along with neurobiological changes in the adult PFC - highlighting that early-life antidepressant exposure increases ERK-related signaling markers in later life.

Autophagy Induction and Accumulation of Phosphorylated Tau in the Hippocampus and Prefrontal Cortex of Adult C57BL/6 Mice Subjected to Adolescent Fluoxetine Treatment

BACKGROUND

Fluoxetine (FLX) represents the antidepressant of choice for the management of pediatric mood-related illnesses. Accumulating preclinical evidence suggests that ontogenic FLX exposure leads to deregulated affect-related phenotypes in adulthood. Mood-related symptomatology constitutes a risk-factor for various neurological disorders, including Alzheimer's disease (AD), making it possible for juvenile FLX history to exacerbate the development of neurodegenerative diseases.

OBJECTIVE

Because AD is characterized by the pathological accumulation of hyperphosphorylated tau, which can result from impaired function of protein degradation pathways, such as autophagy and the ubiquitin-proteasome system (UPS), we evaluated the long-term effects of adolescent FLX exposure on these pathways, using mice as a model system.

METHODS

We subjected C57BL/6 adolescent male mice to FLX (20 mg/kg/day) from postnatal day (PD) 35 to PD49. Twenty-one days after the last FLX injection (i.e., adulthood; PD70), mice were euthanized and, using immunoblotting analysis, we evaluated protein markers of autophagy (Beclin-1, LC3-II, p62) and the UPS (K48-pUb), as well as AD-associated forms of phosphorylated tau, within the hippocampus and prefrontal cortex.

RESULTS

Juvenile FLX pre-exposure mediated long-term changes in the expression of protein markers (increased LC3-II and decreased p62) that is consistent with autophagy activation, particularly in the prefrontal cortex. Furthermore, FLX history induced persistent accumulation of AD-associated variants of tau in both the hippocampus and prefrontal cortexConclusion: Adolescent FLX treatment may have enduring effects in the neuronal protein degradation machinery, which could adversely influence clearance of abnormal proteins, potentially predisposing individuals to developing AD in later life.

Adolescent Fluoxetine Exposure Induces Persistent Gene Expression Changes in the Hippocampus of Adult Male C57BL/6 Mice

Mood-related disorders have a high prevalence among children and adolescents, posing a public health challenge, given their adverse impact on these young populations. Treatment with the selective serotonin reuptake inhibitor fluoxetine (FLX) is the first line of pharmacological intervention in pediatric patients suffering from affect-related illnesses. Although the use of this antidepressant has been deemed efficacious in the juvenile population, the enduring neurobiological consequences of adolescent FLX exposure are not well understood. Therefore, we explored for persistent molecular adaptations, in the adult hippocampus, as a function of adolescent FLX pretreatment. To do this, we administered FLX (20 mg/kg/day) to male C57BL/6 mice during adolescence (postnatal day [PD] 35-49). After a 21-day washout period (PD70), whole hippocampal tissue was dissected. We then used qPCR analysis to assess changes in the expression of genes associated with major intracellular signal transduction pathways, including the extracellular signal-regulated kinase (ERK), the phosphatidylinositide-3-kinase (PI3K)/AKT pathway, and the wingless (Wnt)-dishevelled-GSK3β signaling cascade. Our results show that FLX treatment results in long-term dysregulation of mRNA levels across numerous genes from the ERK, PI3K/AKT, and Wnt intracellular signaling pathways, along with increases of the transcription factors CREB, ΔFosB, and Zif268. Lastly, FLX treatment resulted in persistent increases of transcripts associated with cytoskeletal integrity (β-actin) and caspase activation (DIABLO), while decreasing genes associated with metabolism (fucose kinase) and overall neuronal activation (c-Fos). Collectively, these data indicate that adolescent FLX exposure mediates persistent alterations in hippocampal gene expression in adulthood, thus questioning the safety of early-life exposure to this antidepressant medication.

Extrahypothalamic oxytocin neurons drive stress-induced social vigilance and avoidance

Oxytocin increases the salience of both positive and negative social contexts and it is thought that these diverse actions on behavior are mediated in part through circuit-specific action. This hypothesis is based primarily on manipulations of oxytocin receptor function, leaving open the question of whether different populations of oxytocin neurons mediate different effects on behavior. Here we inhibited oxytocin synthesis in a stress-sensitive population of oxytocin neurons specifically within the medioventral bed nucleus of the stria terminalis (BNSTmv). Oxytocin knockdown prevented social stress-induced increases in social vigilance and decreases in social approach. Viral tracing of BNSTmv oxytocin neurons revealed fibers in regions controlling defensive behaviors, including lateral hypothalamus, anterior hypothalamus, and anteromedial BNST (BNSTam). Oxytocin infusion into BNSTam in stress naïve mice increased social vigilance and reduced social approach. These results show that a population of extrahypothalamic oxytocin neurons plays a key role in controlling stress-induced social anxiety behaviors.

Can I Get a Witness? Using Vicarious Defeat Stress to Study Mood-Related Illnesses in Traditionally Understudied Populations

The chronic social defeat stress model has been instrumental in shaping our understanding of neurobiology relevant to affect-related illnesses, including major depressive disorder. However, the classic chronic social defeat stress procedure is limited by its exclusive application to adult male rodents. We have recently developed a novel vicarious social defeat stress procedure wherein one mouse witnesses the physical defeat bout of a conspecific from the safety of an adjacent compartment. This witness mouse develops a similar behavioral phenotype to that of the mouse that physically experiences social defeat stress, modeling multiple aspects of major depressive disorder. Importantly, this new procedure allows researchers to perform vicarious social defeat stress in males or females and in juvenile mice, which typically are excluded from classic social defeat experiments. Here we discuss several recent advances made using this procedure and how its application provides a new preclinical approach to study the neurobiology of psychological stress-induced phenotypes.

Enduring effects of adolescent ketamine exposure on cocaine- and sucrose-induced reward in male and female C57BL/6 mice

Ketamine has shown promising antidepressant efficacy for adolescent treatment-resistant depression. However, the potential enduring consequences of ketamine exposure have not been thoroughly evaluated. Thus, we examined if juvenile ketamine treatment results in long-lasting changes for the rewarding properties of sucrose and cocaine in adulthood, across three separate experiments. In Experiment 1, adolescent male and female C57BL/6 mice received ketamine (20 mg/kg) for 15 consecutive days (Postnatal Day [PD] 35-49). Twenty-one days later (PD70; adulthood) we examined their behavioral responsivity to sucrose (1%) on a two-bottle choice design, or cocaine (0, 5, 10 mg/kg) using the conditioned place preference (CPP) test. We found that juvenile ketamine-pretreatment increased preference for sucrose and environments paired with cocaine in male, but not female, adult mice. This long-term outcome was not observed when male and female mice received ketamine as adults (PD70-84) and tested for sucrose and cocaine preference 21-days later (Experiment 2). Similarly, in Experiment 3, no long-lasting differences in these measures were observed when adolescent male mice were exposed to concomitant ketamine and social stressors (PD35-44), namely the social defeat or vicarious defeat stress paradigms-procedures that mediated a depression-related phenotype (along with a ketamine antidepressant-like response). Collectively, we demonstrate that in the absence of physical or psychological stress, adolescent ketamine exposure increases later life preference for the rewarding properties of sucrose and cocaine in a sex- and age-specific manner. As such, this preclinical work provides awareness for the potential long-term behavioral consequences associated with juvenile ketamine exposure.

Early-life ketamine exposure attenuates the preference for ethanol in adolescent Sprague-Dawley rats

Ketamine, a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, produces quick and effective antidepressant results in depressed juvenile and adult individuals. The long-term consequences of using ketamine in juvenile populations are not well known, particularly as it affects vulnerability to drugs of abuse later in life, given that ketamine is also a drug of abuse. Thus, the current study examined whether early-life ketamine administration produces long-term changes in the sensitivity to the rewarding effects of ethanol, as measured using the conditioned place preference (CPP) paradigm. On postnatal day (PD) 21, juvenile male and female rats were pretreated with ketamine (0.0 or 20 mg/kg) for 10 consecutive days (i.e., PD 21-30) and then evaluated for ethanol-induced CPP (0.0, 0.125, 0.5, or 2.0 g/kg) from PD 32-39. Results revealed that early-life ketamine administration attenuated the rewarding properties of ethanol in male rats, as ketamine pretreated rats failed to exhibit ethanol-induced CPP at any dose compared to saline pretreated rats, which showed an increased preference towards the ethanol-paired compartment in a dose-dependent manner. In females, ethanol-induced CPP was generally less robust compared to males, but ketamine pretreatment resulted in a rightward shift in the dose-response curve, given that ketamine pretreated rats needed a higher dose of ethanol compared to saline pretreated rats to exhibit ethanol-induced CPP. When considered together, the findings suggest that early use of ketamine does not appear to enhance the vulnerability to ethanol later in life, but in contrast, it may attenuate the rewarding effects of ethanol.

Dorsal Hippocampus ERK2 Signaling Mediates Anxiolytic-Related Behavior in Male Rats

Background

Anxiety disorders are the most common neuropathologies worldwide, but the precise neuronal mechanisms that underlie these disorders remain unknown. The hippocampus plays a role in mediating anxiety-related responses, which can be modeled in rodents using behavioral assays, such as the elevated plus maze. Yet, the molecular markers that underlie affect-related behavior on the elevated plus maze are not well understood.

Methods

We used herpes simplex virus vector delivery to overexpress extracellular signal-regulated kinase-2, a signaling molecule known to be involved in depression and anxiety, within the dorsal hippocampus of adult Sprague-Dawley male rats. Three days post virus delivery, we assessed anxiety-like responses on the elevated plus maze or general locomotor activity on the open field test.

Results

When compared to controls, rats overexpressing extracellular signal-regulated kinase-2 in the dorsal hippocampus displayed an anxiolytic-like phenotype, per increases in time spent in the open arms, and less time in the closed arms, of the elevated plus maze. Furthermore, no changes in locomotor activity as a function of virus infusion were observed on the open field test between the experimental groups.

Conclusion

This investigation demonstrates that virus-mediated increases of extracellular signal-regulated kinase-2 signaling, within the hippocampus, plays a critical role in decreasing anxiogenic responses on the rat elevated plus maze. As such, our data provide construct validity, at least in part, to the molecular mechanisms that mediate anxiolytic-like behavior in rodent models for the study of anxiety.

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

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

Upregulation of hippocampal extracellular signal-regulated kinase (ERK)-2 induces antidepressant-like behavior in the rat forced swim test

The hippocampus mediates responses to affect-related behavior in preclinical models of pharmacological antidepressant efficacy, such as the forced swim test. However, the molecular mechanisms that regulate escape-directed behavior in this preclinical model of despair are not well understood. Here, using viral-mediated gene transfer, we assessed how overexpression of extracellular signal-regulated protein kinase (ERK)-2 within the dorsal hippocampus influenced behavioral reactivity to inescapable swimming stress in adult male Sprague-Dawley rats. When compared to controls, rats overexpressing hippocampal ERK-2 displayed increases in the time to initially adopt a posture of immobility, along with decreases in total time spent immobile, without influencing general locomotor activity. Collectively, the results indicate that hippocampal upregulation of ERK-2 increases escape-directed behavior in the rat forced swim test, thus providing insight into the neurobiological mechanisms that mediate antidepressant efficacy. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Fluoxetine exposure in adolescent and adult female mice decreases cocaine and sucrose preference later in life

BACKGROUND

Preclinical evidence from male subjects indicates that exposure to psychotropic medications, during early development, results in long-lasting altered responses to reward-related stimuli. However, it is not known if exposure to the antidepressant fluoxetine, in female subjects specifically, changes sensitivity to natural and drug rewards, later in life.

AIMS

The aim of this work was to investigate if exposure to fluoxetine mediates enduring changes in sensitivity to the rewarding properties of cocaine and sucrose, using female mice as a model system.

METHODS

We exposed C57BL/6 female mice to fluoxetine (250 mg/L in their drinking water) for 15 consecutive days, either during adolescence (postnatal day 35-49) or adulthood (postnatal day 70-84). Twenty-one days later, mice were examined on their behavioral reactivity to cocaine (0, 2.5, 5, 7.5 mg/kg) using the conditioned place preference paradigm, or assessed on the two-bottle sucrose (1%) test.

RESULTS

We found that regardless of age of antidepressant exposure, female mice pre-exposed to fluoxetine displayed reliable conditioning to the cocaine-paired compartment. However, when compared to respective age-matched controls, antidepressant pre-exposure decreased the magnitude of conditioning at the 5 and 7.5 mg/kg cocaine doses. Furthermore, fluoxetine pre-exposure reduced sucrose preference without altering total liquid intake.

CONCLUSIONS

The data suggest that pre-exposure to fluoxetine, during adolescence or adulthood, results in a prolonged decrease in sensitivity to the rewarding properties of both natural and drug rewards in female C57BL/6 mice.

Vicarious Social Defeat Stress Induces Depression-Related Outcomes in Female Mice

BACKGROUND

Stress is a prevailing risk factor for mood-related illnesses, wherein women represent the majority of those affected by major depression. Despite the growing literature suggesting that affective disorders can arise after a traumatic event is vicariously experienced, this relationship remains understudied in female subjects at the preclinical level. Thus, the objective of the current investigation was to examine whether exposure to emotional and/or psychological stress (ES) mediates depression-related outcomes in female mice.

METHODS

Female C57BL/6 mice (8 weeks old, null parity) vicariously experienced the defeat bout of a male conspecific, by a male CD1 aggressor, for 10 consecutive days. Twenty-four hours after the last stress exposure, female mice were tested in the social interaction, sucrose preference, tail suspension, or elevated plus maze tests. Furthermore, we examined whether ketamine and chlordiazepoxide, pharmacological agents used to treat mood-related disorders in the clinical population, would reverse the ES-induced social dysfunction.

RESULTS

When compared with control mice, female mice exposed to ES displayed decreased social behavior and preference for sucrose, along with increased immobility in the tail suspension test. Also, they displayed higher levels of blood serum corticosterone, as well as decreased body weight. Lastly, the ES-induced avoidance-like phenotype was ameliorated by both ketamine and chlordiazepoxide.

CONCLUSIONS

Our data indicate that female mice exposed to ES display a behavioral and physiologic profile that mimics symptoms of depression in the clinical population. As such, this experimental model may be adopted to examine vicarious stress-induced mood-related disorders, as well as pharmacological antidepressant response, in a sex-specific manner.

Social defeat stress induces depression-like behavior and alters spine morphology in the hippocampus of adolescent male C57BL/6 mice

Social stress, including bullying during adolescence, is a risk factor for common psychopathologies such as depression. To investigate the neural mechanisms associated with juvenile social stress-induced mood-related endophenotypes, we examined the behavioral, morphological, and biochemical effects of the social defeat stress model of depression on hippocampal dendritic spines within the CA1 stratum radiatum. Adolescent (postnatal day 35) male C57BL/6 mice were subjected to defeat episodes for 10 consecutive days. Twenty-four h later, separate groups of mice were tested on the social interaction and tail suspension tests.

Hippocampi were then dissected and Western blots were conducted to quantify protein levels for various markers important for synaptic plasticity including protein kinase M zeta (PKMζ), protein kinase C zeta (PKCζ), the dopamine-1 (D1) receptor, tyrosine hydroxylase (TH), and the dopamine transporter (DAT). Furthermore, we examined the presence of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptor subunit GluA2 as well as colocalization with the post-synaptic density 95 (PSD95) protein, within different spine subtypes (filopodia, stubby, long-thin, mushroom) using an immunohistochemistry and Golgi-Cox staining technique. The results revealed that social defeat induced a depression-like behavioral profile, as inferred from decreased social interaction levels, increased immobility on the tail suspension test, and decreases in body weight. Whole hippocampal immunoblots revealed decreases in GluA2, with a concomitant increase in DAT and TH levels in the stressed group. Spine morphology analyses further showed that defeated mice displayed a significant decrease in stubby spines, and an increase in long-thin spines within the CA1 stratum radiatum. Further evaluation of GluA2/PSD95 containing-spines demonstrated a decrease of these markers within long-thin and mushroom spine types. Together, these results indicate that juvenile social stress induces GluA2- and dopamine-associated dysregulation in the hippocampus – a neurobiological mechanism potentially underlying the development of mood-related syndromes as a consequence of adolescent bullying.

Fluoxetine exposure during adolescence increases preference for cocaine in adulthood

Currently, there is a high prevalence of antidepressant prescription rates within juvenile populations, yet little is known about the potential long-lasting consequences of such treatments, particularly on subsequent responses to drugs of abuse. To address this issue at the preclinical level, we examined whether adolescent exposure to fluoxetine (FLX), a selective serotonin reuptake inhibitor, results in changes to the sensitivity of the rewarding properties of cocaine in adulthood. Separate groups of male c57bl/6 mice were exposed to FLX (0 or 20 mg/kg) for 15 consecutive days either during adolescence (postnatal days [PD] 35–49) or adulthood (PD 65–79). Twenty-one days after FLX treatment, behavioral responsivity to cocaine (0, 2.5, 5, 10, or 20 mg/kg) conditioned place preference was assessed. Our data shows that mice pretreated with FLX during adolescence, but not during adulthood, display an enhanced dose-dependent preference to the environment paired with cocaine (5 or 10 mg/kg) when compared to age-matched saline pretreated controls. Taken together, our findings suggest that adolescent exposure to FLX increases sensitivity to the rewarding properties of cocaine, later in life.

Nucleus accumbens medium spiny neuron subtypes mediate depression-related outcomes to social defeat stress

BACKGROUND

The nucleus accumbens is a critical mediator of depression-related outcomes to social defeat stress. Previous studies demonstrate distinct neuroplasticity adaptations in the two medium spiny neuron (MSN) subtypes, those enriched in dopamine receptor D1 versus dopamine receptor D2, in reward and reinforcement leading to opposing roles for these MSNs in these behaviors. However, the distinct roles of nucleus accumbens MSN subtypes, in depression, remain poorly understood.

METHODS

Using whole-cell patch clamp electrophysiology, we examined excitatory input to MSN subtypes and intrinsic excitability measures in D1-green fluorescent protein and D2-green fluorescent protein bacterial artificial chromosome transgenic mice that underwent chronic social defeat stress (CSDS). Optogenetic and pharmacogenetic approaches were used to bidirectionally alter firing of D1-MSNs or D2-MSNs after CSDS or before a subthreshold social defeat stress in D1-Cre or D2-Cre bacterial artificial chromosome transgenic mice.

RESULTS

We demonstrate that the frequency of excitatory synaptic input is decreased in D1-MSNs and increased in D2-MSNs in mice displaying depression-like behaviors after CSDS. Enhancing activity in D1-MSNs results in resilient behavioral outcomes, while inhibition of these MSNs induces depression-like outcomes after CSDS. Bidirectional modulation of D2-MSNs does not alter behavioral responses to CSDS; however, repeated activation of D2-MSNs in stress naïve mice induces social avoidance following subthreshold social defeat stress.

CONCLUSIONS

Our studies uncover novel functions of MSN subtypes in depression-like outcomes. Notably, bidirectional alteration of D1-MSN activity promotes opposite behavioral outcomes to chronic social stress. Therefore, targeting D1-MSN activity may provide novel treatment strategies for depression or other affective disorders.

Social defeat stress induces a depression-like phenotype in adolescent male c57BL/6 mice

Abstract Exposure to stress is highly correlated with the emergence of mood-related illnesses. Because major depressive disorder often emerges in adolescence, we assessed the effects of social defeat stress on responses to depressive-like behaviors in juvenile mice. To do this, postnatal day (PD) 35 male c57BL/6 mice were exposed to 10 days of social defeat stress (PD35-44), while control mice were handled daily. Twenty-four hours after the last episode of defeat (PD45), separate groups of mice were tested in the social interaction, forced swimming, sucrose preference, and elevated plus-maze behavioral assays (n = 7-12 per group). Also, we examined body weight gain across days of social defeat and levels of blood serum corticosterone 40 min after the last episode of defeat stress. Our data indicates that defeated mice exhibited a depressive-like phenotype as inferred from increased social avoidance, increased immobility in the forced swim test, and reduced sucrose preference (a measure of anhedonia), when compared to non-defeated controls. Defeated mice also displayed an anxiogenic-like phenotype when tested on the elevated plus-maze. Lastly, stressed mice displayed lower body weight gain, along with increased blood serum corticosterone levels, when compared to non-stressed controls. Overall, we show that in adolescent male c57BL/6 mice, social defeat stress induces a depression- and anxiety-like phenotype 24 h after the last episode of stress. These data suggest that the social defeat paradigm may be used to examine the etiology of stress-induced mood-related disorders during adolescence.

Effects of psychotropic drugs on second messenger signaling and preference for nicotine in juvenile male mice

RATIONALE

A common treatment strategy for pediatric attention deficit/hyperactivity disorder (ADHD) and major depressive disorder (MDD) is combined methylphenidate (MPH) and fluoxetine (FLX). This has raised concerns because MPH + FLX treatment may have pharmacodynamic properties similar to cocaine, potentially increasing drug abuse liability.

OBJECTIVES

To examine the short- and long-term consequences of repeated vehicle, MPH, FLX, MPH + FLX, and cocaine treatment on gene expression in juvenile (postnatal days [PD] 20-34) and adult (PD 70-84) male mice. We further assessed whether juvenile drug treatment influenced subsequent sensitivity for nicotine in adulthood.

METHODS

Juvenile and adult C57BL/6J mice received vehicle, MPH, FLX, MPH + FLX, or cocaine twice-daily for 15 consecutive days. Mice were sacrificed 24 h or 2 months after the last drug injection to assess drug-induced effects on the extracellular signal-regulated protein kinase-1/2 (ERK) pathway within the ventral tegmental area. Subsequent sensitivity for nicotine (0.05, 0.07, and 0.09 mg/kg) was measured using the place-conditioning paradigm (CPP) 24 h and 2 months after juvenile drug exposure.

RESULTS

MPH + FLX, or cocaine exposure in juvenile mice increased mRNA expression of ERK2 and its downstream targets (CREB, cFos, and Zif268), and increased protein phosphorylation of ERK2 and CREB 2 months after drug exposure. Similar mRNA findings were observed in the adult-treated mice. Findings on gene expression 24 h following drug treatment were variable. Juvenile drug exposure increased preference for nicotine when tested in adulthood.

CONCLUSIONS

Early-life MPH + FLX, or cocaine exposure similarly disrupts the ERK pathway, a signaling cascade implicated in motivation and mood regulation, and increases sensitivity for nicotine in adulthood.

Fluoxetine exposure during adolescence alters responses to aversive stimuli in adulthood

The mechanisms underlying the enduring neurobiological consequences of antidepressant exposure during adolescence are poorly understood. Here, we assessed the long-term effects of exposure to fluoxetine (FLX), a selective serotonin reuptake inhibitor, during adolescence on behavioral reactivity to emotion-eliciting stimuli. We administered FLX (10 mg/kg, bi-daily, for 15 d) to male adolescent [postnatal day 35 (P35) to P49] C57BL/6 mice. Three weeks after treatment (P70), reactivity to aversive stimuli (i.e., social defeat stress, forced swimming, and elevated plus maze) was assessed. We also examined the effects of FLX on the expression of extracellular signal-regulated kinase (ERK) 1/2-related signaling within the ventral tegmental area (VTA) of adolescent mice and Sprague Dawley rats. Adolescent FLX exposure suppressed depression-like behavior, as measured by the social interaction and forced swim tests, while enhancing anxiety-like responses in the elevated plus maze in adulthood. This complex behavioral profile was accompanied by decreases in ERK2 mRNA and protein phosphorylation within the VTA, while stress alone resulted in opposite neurobiological effects. Pharmacological (U0126) inhibition, as well as virus-mediated downregulation of ERK within the VTA mimicked the antidepressant-like profile observed after juvenile FLX treatment. Conversely, overexpression of ERK2 induced a depressive-like response, regardless of FLX pre-exposure. These findings demonstrate that exposure to FLX during adolescence modulates responsiveness to emotion-eliciting stimuli in adulthood, at least partially, via long-lasting adaptations in ERK-related signaling within the VTA. Our results further delineate the role ERK plays in regulating mood-related behaviors across the lifespan.

Repeated ketamine exposure induces an enduring resilient phenotype in adolescent and adult rats

BACKGROUND

Major depressive disorder afflicts up to 10% of adolescents. However, nearly 50% of those afflicted are considered nonresponsive to available treatments. Ketamine, a noncompetitive N-methyl-D-aspartate receptor antagonist has shown potential as a rapid-acting and long-lasting treatment for major depressive disorder in adults. Thus, the effectiveness and functional consequences of ketamine exposure during adolescence were explored.

METHODS

Adolescent male rats (postnatal day [PD] 35) received two ketamine (0, 5, 10, or 20 mg/kg) injections, 4 hours apart, after exposure to day 1 of the forced swim test (FST). The next day, rats were reexposed to the FST to assess ketamine-induced antidepressant-like responses. Separate groups were exposed to chronic unpredictable stress to confirm findings from the FST. After these initial experiments, adolescent naive rats were exposed to either 1 or 15 consecutive days (PD35-49) of ketamine (20 mg/kg) twice daily. Ketamine's influence on behavioral reactivity to rewarding (i.e., sucrose preference) and aversive (i.e., elevated plus-maze, FST) circumstances was then assessed 2 months after treatment. To control for age-dependent effects, adult rats (PD75-89) were exposed to identical experimental conditions.

RESULTS

Ketamine (20 mg/kg) reversed the chronic unpredictable stress-induced depression-like behaviors in the FST. Repeated ketamine exposure resulted in anxiolytic- and antidepressant-like responses 2 months after drug exposure. None of the ketamine doses used were capable of inducing drug-seeking behaviors as measured by place preference conditioning.

CONCLUSIONS

Repeated ketamine exposure induces enduring resilient-like responses regardless of age of exposure. These findings point to ketamine, and its repeated exposure, as a potentially useful antidepressant during adolescence.

Neurobiological sequelae of witnessing stressful events in adult mice

BACKGROUND

It is well known that exposure to severe stress increases the risk for developing mood disorders. However, most chronic stress models in rodents involve at least some form of physically experiencing traumatic events.

METHODS

This study assessed the effects of a novel social stress paradigm that is insulated from the effects of physical stress. Specifically, adult male C57BL/6J mice were exposed to either emotional (ES) or physical stress (PS) for 10 minutes per day for 10 days. The ES mice were exposed to the social defeat of a PS mouse by a larger, more aggressive CD-1 mouse from the safety of an adjacent compartment.

RESULTS

Like PS mice, ES mice exhibited a range of depression- and anxiety-like behaviors both 24 hours and 1 month after the stress. Increased levels of serum corticosterone, part of the stress response, accompanied these behavioral deficits. Based on previous work that implicated gene expression changes in the ventral tegmental area (a key brain reward region) in the PS phenotype, we compared genome-wide mRNA expression patterns in this brain region of ES and PS mice using RNA-seq. We found significant overlap between these conditions, which suggests several potential gene targets for mediating the behavioral abnormalities observed.

CONCLUSIONS

These findings demonstrate that witnessing traumatic events is a potent stress in adult male mice capable of inducing long-lasting neurobiological perturbations.

Post-training cocaine exposure facilitates spatial memory consolidation in C57BL/6 mice

In this study, we examined the ability of post-training injections of cocaine to facilitate spatial memory performance using the Morris water maze (MWM). We also investigated the role that hippocampal protein kinase A (PKA) and extracellular signal-regulated kinase 1/2 (ERK) signaling may play in cocaine-mediated spatial memory consolidation processes. Male and female C57BL/6 mice were first trained in a MWM task (eight consecutive trials) then injected with cocaine (0, 1.25, 2.5, 5, or 20 mg/kg), and memory for the platform location was retested after a 24 h delay. Cocaine had a dose-dependent effect on spatial memory performance because only the mice receiving 2.5 mg/kg cocaine displayed a significant reduction in latency to locate the platform. No sex differences in MWM performance were observed; however, females showed higher hippocampal levels of PKA when compared with males. A second experiment demonstrated that 2.5 mg/kg cocaine enhanced MWM performance only when administered within 2, but not 4 h after spatial training. We also found that cocaine (2.5 mg/kg) increased ERK2 phosphorylation within the hippocampus and one of its downstream targets (ribosomal S6 kinase), a mechanism that may be responsible, at least in part, for the enhanced cocaine-mediated spatial memory performance. Overall, these data demonstrate that a low dose of cocaine (2.5 mg/kg) administered within 2 h after training facilitates MWM spatial memory performance in C57BL/6 mice.

Dnmt3a regulates emotional behavior and spine plasticity in the nucleus accumbens

Despite abundant expression of DNA methyltransferases (Dnmt’s) in brain, the regulation and behavioral role of DNA methylation remain poorly understood. We find that Dnmt3a expression is regulated in mouse nucleus accumbens (NAc) by chronic cocaine and chronic social defeat stress. Moreover, NAc specific manipulations that block DNA methylation potentiate cocaine reward and exert antidepressant-like effects, whereas NAc specific Dnmt3a overexpression attenuates cocaine reward and is pro-depressant. On a cellular level, we show that chronic cocaine selectively increases thin dendritic spines on NAc neurons and that DNA methylation is both necessary and sufficient to mediate these effects. These data establish the importance of Dnmt3a in the NAc in regulating cellular and behavioral plasticity to emotional stimuli.

Short- and long-term functional consequences of fluoxetine exposure during adolescence in male rats

BACKGROUND

Fluoxetine (FLX), a selective serotonin reuptake inhibitor, is prescribed for the treatment of major depressive disorder in young populations. Here, we explore the short- and long-term consequences of adolescent exposure to FLX on behavioral reactivity to emotion-eliciting stimuli.

METHODS

Adolescent male rats received FLX (10 mg/kg) twice daily for 15 consecutive days (postnatal days 35-49). The influence of FLX on behavioral reactivity to rewarding and aversive stimuli was assessed 24 hours (short-term) or 3 weeks after FLX treatment (long-term). A separate group of adult rats was also treated with FLX (postnatal days 65-79) and responsiveness to forced swimming was assessed at identical time intervals as with the adolescents.

RESULTS

Fluoxetine exposure during adolescence resulted in long-lasting decreases in behavioral reactivity to forced swimming stress and enhanced sensitivity to sucrose and to anxiety-eliciting situations in adulthood. The FLX-induced anxiety-like behavior was alleviated by re-exposure to FLX in adulthood. Fluoxetine treatment during adolescence also impaired sexual copulatory behaviors in adulthood. Fluoxetine-treated adult rats did not show changes in behavioral reactivity to forced swim stress as observed in those treated during adolescence and tested in adulthood.

CONCLUSIONS

Treating adolescent rats with FLX results in long-lived complex outputs regulated by the emotional valence of the stimulus, the environment in which it is experienced, and the brain circuitry likely being engaged by it. Our findings highlight the need for further research to improve our understanding of the alterations that psychotropic exposure may induce on the developing nervous system and the potential enduring effects resulting from such treatments.

Viral-mediated expression of extracellular signal-regulated kinase-2 in the ventral tegmental area modulates behavioral responses to cocaine

Chronic exposure to cocaine increases the activity of extracellular signal-regulated kinase (ERK1/2) in the ventral tegmental area (VTA), a neural substrate for drugs of abuse. However, the functional significance of changes in ERK1/2 activity in this brain region is unknown. Using herpes simplex virus-mediated gene transfer to regulate ERK2 activity within the VTA in male rats, we show that overexpressing ERK2 increases preference for environments previously paired with low doses of cocaine and enhances cocaine-induced locomotion, whereas blocking ERK2 activity blocks cocaine-induced place conditioning and locomotor activity. These results demonstrate that ERK2-signaling within the VTA is a key modulator of functional responses to cocaine.

CREB regulation of nucleus accumbens excitability mediates social isolation-induced behavioral deficits

Here, we characterized behavioral abnormalities induced by prolonged social isolation in adult rodents. Social isolation induced both anxiety- and anhedonia-like symptoms and decreased cAMP response element-binding protein (CREB) activity in the nucleus accumbens shell (NAcSh). All of these abnormalities were reversed by chronic, but not acute, antidepressant treatment. However, although the anxiety phenotype and its reversal by antidepressant treatment were CREB-dependent, the anhedonia-like symptoms were not mediated by CREB in NAcSh. We found that decreased CREB activity in NAcSh correlated with increased expression of certain K(+) channels and reduced electrical excitability of NAcSh neurons, which was sufficient to induce anxiety-like behaviors and was reversed by chronic antidepressant treatment. Together, our results describe a model that distinguishes anxiety- and depression-like behavioral phenotypes, establish a selective role of decreased CREB activity in NAcSh in anxiety-like behavior, and provide a mechanism by which antidepressant treatment alleviates anxiety symptoms after social isolation.

Methylphenidate potentiates morphine-induced antinociception, hyperthermia, and locomotor activity in young adult rats

The goal of this study was to determine if the exaggerated morphine-induced conditioned place preference (CPP) response seen in adult rats after preweanling methylphenidate exposure is unique to reward-mediated behaviors or is indicative of generalized changes in opioid-mediated behaviors. Rats were exposed to saline or methylphenidate (2.0 or 5.0 mg/kg) for 10 consecutive days starting on postnatal (PD) 11 with testing beginning on PD 60. In Experiment 1, morphine-induced (0, 2.5, 5.0 or 10.0 mg/kg) antinociception was assessed using the tail immersion and hot plate tasks. In Experiment 2, morphine-induced (0, 2.5, 5.0, or 10.0 mg/kg) hyperthermia and locomotor activity were measured. Morphine caused an increase in antinociception, with early methylphenidate (5.0 mg/kg) exposure potentiating the effects of 5.0 mg/kg morphine. Rectal temperatures were elevated after morphine, with the greatest increase occurring in male rats. Methylphenidate potentiated the hyperthermic effects of morphine (10.0 mg/kg) but only in males. Moderate doses (2.5 and 5.0 mg/kg) of morphine increased the locomotor activity of adult rats, while a higher dose (10.0 mg/kg) decreased locomotion. Interestingly, methylphenidate-pretreated females showed increased locomotor activity relative to controls. These results suggest that early methylphenidate exposure induces general changes in opioid system functioning that are not specific to reward-mediated behaviors.

AKT signaling within the ventral tegmental area regulates cellular and behavioral responses to stressful stimuli

BACKGROUND

The neurobiological mechanisms by which only a minority of stress-exposed individuals develop psychiatric diseases remain largely unknown. Recent evidence suggests that dopaminergic neurons of the ventral tegmental area (VTA) play a key role in the manifestation of stress vulnerability.

METHODS

Using a social defeat paradigm, we segregated susceptible mice (socially avoidant) from unsusceptible mice (socially interactive) and examined VTA punches for changes in neurotrophic signaling. Employing a series of viral vectors, we sought to causally implicate these neurotrophic changes in the development of avoidance behavior.

RESULTS

Susceptibility to social defeat was associated with a significant reduction in levels of active/phosphorylated AKT (thymoma viral proto-oncogene) within the VTA, whereas chronic antidepressant treatment (in mice and humans) increased active AKT levels. This defeat-induced reduction in AKT activation in susceptible mice was both necessary and sufficient to recapitulate depressive behaviors associated with susceptibility. Pharmacologic reductions in AKT activity also significantly raised the firing frequency of VTA dopamine neurons, an important electrophysiologic hallmark of the susceptible phenotype.

CONCLUSIONS

These studies highlight a crucial role for decreases in VTA AKT signaling as a key mediator of the maladaptive cellular and behavioral response to chronic stress.

Insulin receptor substrate-2 in the ventral tegmental area regulates behavioral responses to cocaine

Neurotrophic factor signaling modulates cellular and behavioral responses to drugs of abuse. Among other biochemical adaptations, chronic exposure to abused drugs decreases the expression of insulin receptor substrate-2 (IRS-2; a protein involved in neurotrophic signaling) in the ventral tegmental area (VTA), a neural substrate for many drugs of abuse. Using viral-mediated gene transfer to locally alter the activity of IRS-2, the authors show that overexpression of IRS-2 in the VTA results in an enhanced preference for environments previously paired with cocaine, as measured by the place conditioning paradigm, whereas blockade of IRS-2 activity results in avoidance of cocaine-paired compartments. In addition, IRS-2 overexpression leads to enhanced cocaine-induced locomotor activity, and blockade of IRS-2 expression significantly blunts behavioral responses to cocaine. These results demonstrate that levels of IRS-2 in the VTA regulate responsiveness to the behavioral effects of cocaine.