Peer-Reviewed Journal Publications (Reverse Chronological Order)

Prelimbic cortex is a common brain area activated during cue-induced reinstatement of cocaine and heroin seeking in a polydrug self-administration rat model

Eur J Neurosci. 2019;49:165–178.

Rubio FJ, Quintana‐Feliciano R, Warren BL, Li X, Witonsky KFR, Soto del Valle F, Selvam PV, Caprioli D, Venniro M, Bossert JM, Shaham Y, Hope BT

Many preclinical studies examined cue‐induced relapse to heroin and cocaine seeking in animal models, but most of these studies examined only one drug at a time. In human addicts, however, polydrug use of cocaine and heroin is common. We used a polydrug self‐administration relapse model in rats to determine similarities and differences in brain areas activated during cue‐induced reinstatement of heroin and cocaine seeking. We trained rats to lever press for cocaine (1.0 mg/kg/infusion, 3‐h/d, 18 d) or heroin (0.03 mg/kg/infusion) on alternating days (9 d for each drug); drug infusions were paired with either intermittent or continuous light cue. Next, the rats underwent extinction training followed by tests for cue‐induced reinstatement where they were exposed to either heroin‐ or cocaine‐associated cues. We observed cue‐selective reinstatement of drug seeking: the heroin cue selectively reinstated heroin seeking and the cocaine cue selectively reinstated cocaine seeking. We used Fos immunohistochemistry to assess cue‐induced neuronal activation in different subregions of the medial prefrontal cortex (mPFC), dorsal striatum (DS), nucleus accumbens (NAc), and amygdala. Fos expression results indicated that only the prelimbic cortex (PL) was activated by both heroin and cocaine cues; in contrast, no significant cue‐induced neuronal activation was observed in other brain areas. RNA in situ hybridization indicated that the proportion of glutamatergic and GABAergic markers in PL Fos‐expressing cells were similar for the heroin and cocaine cue‐activated neurons. Overall the results indicate that PL may be a common brain area involved in both heroin and cocaine seeking during polydrug use.

Parvalbumin-expressing neurons in the nucleus accumbens: a new player in amphetamine sensitization and reward

Neuropsychopharmacology, 2018, 43(5), 929-930. PMID: 29162905

Warren BL, Whitaker LR

Commentary on the publication: Wang X, Gallegos DA, Pogorelov VM, O'Hare JK, Calakos N, Wetsel WC et al (2017). Parvalbumin interneurons of the mouse nucleus accumbens are required for amphetamine-induced locomotor sensitization and conditioned place preference. Neuropsychopharmacology, 43, 953–963.

The Anterior Insular Cortex→Central Amygdala Glutamatergic Pathway Is Critical to Relapse after Contingency Management

Neuron, 2017, 96(2), 414-427. PMID: 29024664

Venniro M, Caprioli D, Zhang M, Whitaker LR, Zhang S, Warren BL, Cifani C, Marchant NJ, Yizhar O, Bossert JM, Chiamulera C, Morales M, Shaham Y

Despite decades of research on neurobiological mechanisms of psychostimulant addiction, the only effective treatment for many addicts is contingency management, a behavioral treatment that uses alternative non-drug reward to maintain abstinence. However, when contingency management is discontinued, most addicts relapse to drug use. The brain mechanisms underlying relapse after cessation of contingency management are largely unknown, and, until recently, an animal model of this human condition did not exist. Here we used a novel rat model, in which the availability of a mutually exclusive palatable food maintains prolonged voluntary abstinence from intravenous methamphetamine self-administration, to demonstrate that the activation of monosynaptic glutamatergic projections from anterior insular cortex to central amygdala is critical to relapse after the cessation of contingency management. We identified the anterior insular cortex-to-central amygdala projection as a new addiction- and motivation-related projection and a potential target for relapse prevention.

Bidirectional modulation of intrinsic excitability in rat prelimbic cortex neuronal ensembles and non-ensembles following operant learning

The Journal of Neuroscience, 2017, 37(36), 8845-8856. PMID: 28779019

Whitaker LR, Warren BL, Venniro M, Harte TC, McPherson KB, Beidel JM, Bossert JM, Shaham Y, Bonci A, Hope BT

Learned associations between environmental stimuli and rewards drive goal-directed learning and motivated behavior. These memories are thought to be encoded by alterations within specific patterns of sparsely distributed neurons called neuronal ensembles that are selectively activated by reward-predictive stimuli. Here we use the Fos promoter to identify strongly activated neuronal ensembles in rat prelimbic cortex (PLC) and assess altered intrinsic excitability following 10 days of operant food self-administration training (1-h/day). First, we used the Daun02 inactivation procedure in male FosLacZ transgenic rats to selectively ablate Fos-expressing PLC neurons that were active during operant food self-administration. Selective ablation of these neurons decreased food seeking. We then used male FosGFP transgenic rats to assess selective alterations of intrinsic excitability in Fos-expressing neuronal ensembles (FosGFP+) that were activated during food self-administration and compared these to alterations in less activated non-ensemble neurons (FosGFP-). Using whole cell recordings of layer V pyramidal neurons in an ex vivo brain slice preparation, we found that operant self-administration increased excitability of FosGFP+ neurons and decreased excitability of FosGFP- neurons. Increased excitability of FosGFP+ neurons was driven by increased steady-state input resistance. Decreased excitability of FosGFP- neurons was driven by increased contribution of small conductance calcium-activated potassium (SK) channels. Injections of the specific SK channel antagonist apamin into PLC increased Fos expression but had no effect on food seeking. Overall, operant learning increased intrinsic excitability of PLC Fos-expressing neuronal ensembles that play a role in food seeking but decreased intrinsic excitability of Fos-negative non-ensembles.

Mechanistic Resolution Required to Mediate Operant Learned Behaviors: Insights from Neuronal Ensemble-Specific Inactivation

Frontiers in Neuroscience, 2017, 11, 28. PMID: 28484375

Warren BL, Suto N, Hope BT

Many learned behaviors are directed by complex sets of highly specific stimuli or cues. The neural mechanisms mediating learned associations in these behaviors must be capable of storing complex cue information and distinguishing among different learned associations—we call this general concept “mechanistic resolution”. For many years, our understanding of the circuitry of these learned behaviors has been based primarily on inactivation of specific cell types or whole brain areas regardless of which neurons were activated during the cue-specific behaviors. However, activation of all cells or specific cell types in a brain area do not have enough mechanistic resolution to encode or distinguish high-resolution learned associations in these behaviors. Instead, these learned associations are likely encoded within specific patterns of sparsely distributed neurons called neuronal ensembles that are selectively activated by the cues. This review article focuses on studies of neuronal ensembles in operant learned responding to obtain food or drug rewards. These studies suggest that the circuitry of operant learned behaviors may need to be re-examined using ensemble-specific manipulations that have the requisite level of mechanistic resolution.

Neurons Internalize Functionalized Micron-Sized Silicon Dioxide Microspheres

Cellular and Molecular Neurobiology, 2017, PMID: 28260198

Wallace VJ, Cimbro R, Rubio FJ, Fortuno LV, Necarsulmer JC, Koivula PP, Henderson MJ, DeBiase LM,
Warren BL, Harvey BK, Hope BT

Microparticles have potential as neuron-specific delivery platforms and devices with many applications in neuroscience, pharmacology, and biomedicine. To date, most literature suggests that neurons are not phagocytic cells capable of internalizing microparticles larger than 0.5 μm. We report that neurons transport fluorescently labeled silica microspheres with diameters of 1–2 μm into neurons in vitro and in rat brain without having overt effects on cell viability. Using flow cytometry, fluorescence-activated cell sorting, and confocal and electron microscopy, we first found that SH-SY5Y human neuroblastoma cells internalized 1-μm silicon microspheres with surface charges of −70 mV (hydroxyl and carboxyl), −30 mV (amino), and +40 mV (ammonio). Uptake was rapid, within 2–4 h, and did not affect cell viability 48 h later. Flow cytometry assays indicate that SH-SY5Y cells internalize 1- and 1.5-μm microspheres at the same rate over a 24-h incubation period. Electron microscopy confirms that SH-SY5Y cells internalize 1-, 1.5-, and 2-μm microspheres. Confocal microscopy demonstrated that primary cortical neurons also internalized 1-, 1.5-, and 2-μm amino microspheres within 4 h. Finally, we injected 1-μm amino microspheres into rat striatum and found microspheres inside neurons. Overall, neurons can internalize microspheres up to 2 μm in diameter with a range of surface chemical groups and charges. These findings allow a host of neuroscience and neuroengineering applications including intracellular microdevices within neurons.

Role of dorsomedial striatum neuronal ensembles in incubation of methamphetamine craving after voluntary abstinence

The Journal of Neuroscience, 2016, 37(4), 1014-1027. PMID: 28123032

Caprioli D, Venniro M, Zhang M, Bossert JM, Warren BL, Hope BT, Shaham Y

We recently developed a rat model of incubation of methamphetamine craving after choice-based voluntary abstinence. Here, we studied the role of dorsolateral and dorsomedial striatum (DLS, DMS) in this incubation. We trained rats to self-administer palatable food pellets (6 days, 6-h/d) and methamphetamine (12 days, 6-h/d). We then assessed relapse to methamphetamine seeking under extinction conditions after 1 and 21 abstinence days. Between tests, the rats underwent voluntary abstinence (using a discrete choice procedure between methamphetamine and food; 20 trials/day) for 19 days. We used in situ hybridization to measure co-labeling of the activity marker Fos with Drd1 and Drd2 in DMS and DLS after the tests. Based on the in situ hybridization co-labeling results, we tested the causal role of DMS D1- and D2-family receptors, and DMS neuronal ensembles in ‘incubated' methamphetamine seeking, using selective dopamine receptor antagonists (SCH39166 or raclopride) and the Daun02 chemogenetic inactivation procedure, respectively. Methamphetamine seeking was higher after 21 days of voluntary abstinence than after 1 day (incubation of methamphetamine craving). The ‘incubated' response was associated with increased Fos expression in DMS but not DLS; Fos was co-labeled with both Drd1 and Drd2. DMS injections of SCH39166 or raclopride selectively decreased methamphetamine seeking after 21 abstinence days. In Fos-lacZ transgenic rats, selective inactivation of relapse test-activated Fos neurons in DMS on abstinence day 18 decreased incubated methamphetamine seeking on day 21. Results demonstrate a role of DMS dopamine D1 and D2-receptors in incubation of methamphetamine craving after voluntary abstinence and that DMS neuronal ensembles mediate this incubation.

Distinct Fos-Expressing Neuronal Ensembles in the Ventromedial Prefrontal Cortex Mediate Food Reward and Extinction Memories

The Journal of Neuroscience, 2016, 36(25), 6691-703. PMID: 27335401

Warren BL, Mendoza MP, Cruz FC, Leao RM, Caprioli D, Rubio FJ, Whitaker LR, McPherson KB,
Bossert JM, Shaham Y, Hope BT

In operant learning, initial reward-associated memories are thought to be distinct from subsequent extinction-associated memories. Memories formed during operant learning are thought to be stored in “neuronal ensembles.” Thus, we hypothesize that different neuronal ensembles encode reward- and extinction-associated memories. Here, we examined prefrontal cortex neuronal ensembles involved in the recall of reward and extinction memories of food self-administration. We first trained rats to lever press for palatable food pellets for 7 d (1 h/d) and then exposed them to 0, 2, or 7 daily extinction sessions in which lever presses were not reinforced. Twenty-four hours after the last training or extinction session, we exposed the rats to either a short 15 min extinction test session or left them in their homecage (a control condition). We found maximal Fos (a neuronal activity marker) immunoreactivity in the ventral medial prefrontal cortex of rats that previously received 2 extinction sessions, suggesting that neuronal ensembles in this area encode extinction memories. We then used the Daun02 inactivation procedure to selectively disrupt ventral medial prefrontal cortex neuronal ensembles that were activated during the 15 min extinction session following 0 (no extinction) or 2 prior extinction sessions to determine the effects of inactivating the putative food reward and extinction ensembles, respectively, on subsequent nonreinforced food seeking 2 d later. Inactivation of the food reward ensembles decreased food seeking, whereas inactivation of the extinction ensembles increased food seeking. Our results indicate that distinct neuronal ensembles encoding operant reward and extinction memories intermingle within the same cortical area.

Vicarious social defeat stress: Bridging the gap between physical and emotional stress

J Neuro Methods, 2016, 258, 94-103. PMID: 26545443

Sial OK*, Warren BL*, Alcantara LF, Parise EM, Bolaños-Guzmán CA

Background: Animal models capable of differentiating the neurobiological intricacies between physical and emotional stress are scarce. Current models rely primarily on physical stressors (e.g., chronic unpredictable or mild stress, social defeat, learned helplessness), and neglect the impact of psychological stress alone. This is surprising given extensive evidence that a traumatic event needs not be directly experienced to produce enduring perturbations on an individual's health and psychological well-being. Post-traumatic stress disorder (PTSD), a highly debilitating neuropsychiatric disorder characterized by intense fear of trauma-related stimuli, often occurs in individuals that have only witnessed a traumatic event.
New method: By modifying the chronic social defeat stress (CSDS) paradigm to include a witness component (witnessing the social defeat of another mouse), we demonstrate a novel behavioral paradigm capable of inducing a robust behavioral syndrome reminiscent of PTSD in emotionally stressed adult mice.
Results: We describe the vicarious social defeat stress (VSDS) model that is capable of inducing a host of behavioral deficits that include social avoidance and other depressive- and anxiety-like phenotypes in adult male mice. VSDS exposure induces weight loss and spike in serum corticosterone (CORT) levels. A month after stress, these mice retain the social avoidant phenotype and have an increased CORT response when exposed to subsequent stress.
Comparison with existing method(s): The VSDS is a novel paradigm capable of inducing emotional stress by isolating physical stress/confrontation in mice.
Conclusions: The VSDS model can be used to study the short- and long-term neurobiological consequences of exposure to emotional stress in mice.

Context-Induced Reinstatement of Methamphetamine Seeking Is Associated with Unique Molecular Alterations in Fos-Expressing Dorsolateral Striatum Neurons

The Journal of Neuroscience, 2015, 35(14), 5625-5639. PMID: 25855177

Rubio FJ, Liu QR, Li X, Cruz F, Leao RM, Warren BL, Kambhampati S, Babin K, McPherson K, Cimbro R,
Bossert JM, Shaham Y, Hope BT

Context-induced reinstatement of drug seeking is a well established animal model for assessing the neural mechanisms underlying context-induced drug relapse, a major factor in human drug addiction. Neural activity in striatum has previously been shown to contribute to context-induced reinstatement of heroin, cocaine, and alcohol seeking, but not yet for methamphetamine seeking. In this study, we found that context-induced reinstatement of methamphetamine seeking increased expression of the neural activity marker Fos in dorsal but not ventral striatum. Reversible inactivation of neural activity in dorsolateral but not dorsomedial striatum using the GABA agonists muscimol and baclofen decreased context-induced reinstatement. Based on our previous findings that Fos-expressing neurons play a critical role in conditioned drug effects, we assessed whether context-induced reinstatement was associated with molecular alterations selectively induced within context-activated Fos-expressing neurons. We used fluorescence-activated cell sorting to isolate reinstatement-activated Fos-positive neurons from Fos-negative neurons in dorsal striatum and used quantitative PCR to assess gene expression within these two populations of neurons. Context-induced reinstatement was associated with increased expression of the immediate early genes Fos and FosB and the NMDA receptor subunit gene Grin2a in only Fos-positive neurons. RNAscope in situ hybridization confirmed that Grin2a, as well as Grin2b, expression were increased in only Fos-positive neurons from dorsolateral, but not dorsomedial, striatum. Our results demonstrate an important role of dorsolateral striatum in context-induced reinstatement of methamphetamine seeking and that this reinstatement is associated with unique gene alterations in Fos-expressing neurons.

Individual differences in the peripheral immune system promote resilience versus susceptibility to social stress

PNAS, 2014, 111(45), 16136-41. PMID: 25331895

Hodes G, Pfau ML, Leboeuf M, Golden S, Christoffel D, Bregman Dana, Rebusi N, Heshmati M,
Aleyasin H, Warren BL, Sarah H, Lapidus K, Stelzhammer V, Wong E, Bahn S, Bolanos-Guzman CA,
Murrough J, Merad Miriam, Russo SJ

Depression and anxiety disorders are associated with increased release of peripheral cytokines; however, their functional relevance remains unknown. Using a social stress model in mice, we find preexisting individual differences in the sensitivity of the peripheral immune system that predict and promote vulnerability to social stress. Cytokine profiles were obtained 20 min after the first social stress exposure. Of the cytokines regulated by stress, IL-6 was most highly up-regulated only in mice that ultimately developed a susceptible behavioral phenotype following a subsequent chronic stress, and levels remained elevated for at least 1 mo. We confirmed a similar elevation of serum IL-6 in two separate cohorts of patients with treatment-resistant major depressive disorder. Before any physical contact in mice, we observed individual differences in IL-6 levels from ex vivo stimulated leukocytes that predict susceptibility versus resilience to a subsequent stressor. To shift the sensitivity of the peripheral immune system to a pro- or antidepressant state, bone marrow (BM) chimeras were generated by transplanting hematopoietic progenitor cells from stress-susceptible mice releasing high IL-6 or from IL-6 knockout (IL-6−/−) mice. Stress-susceptible BM chimeras exhibited increased social avoidance behavior after exposure to either subthreshold repeated social defeat stress (RSDS) or a purely emotional stressor termed witness defeat. IL-6−/− BM chimeric and IL-6−/− mice, as well as those treated with a systemic IL-6 monoclonal antibody, were resilient to social stress. These data establish that preexisting differences in stress-responsive IL-6 release from BM-derived leukocytes functionally contribute to social stress-induced behavioral abnormalities.

Altered Gene Expression and Spine Density in Nucleus Accumbens of Adolescent and Adult Male Mice Exposed to Emotional and Physical Stress

Developmental Neuroscience, 2014, 36(3-4), 250-60. PMID: 24943326

Warren BL, Sial OK, Alcantara LF, Greenwood MA, Brewer JS, Rozofsky JP, Parise EM, Bolaños-Guzmán CA

Stressful early life experiences are implicated in lifelong health. However, little is known about the consequences of emotional stress (ES) or physical stress (PS) on neurobiology. Therefore, the following set of experiments was designed to assess changes in transcription and translation of key proteins within the nucleus accumbens (NAc). Male adolescent (postnatal day 35) or adult (8-week-old) mice were exposed to ES or PS using a witness social defeat paradigm. Then, 24 h after the last stress session, we measured levels of specific mRNAs and proteins within the NAc. Spine density was also assessed in separate groups of mice. Exposure to ES or PS disrupted extracellular signal-related kinase 2 (ERK2), reduced transcription of ΔFosB and had no effect on cAMP response element-binding protein (CREB) mRNA. Western blots revealed that exposure to ES or PS decreased ERK2 phosphorylation in adolescents, whereas the same stress regimen increased ERK2 phosphorylation in adults. Exposure to ES or PS had no effect on ΔFosB or CREB phosphorylation. ES and PS increased spine density in the NAc of adolescent exposed mice, but only exposure to PS increased spine density in adults. Together, these findings demonstrate that exposure to ES or PS is a potent stressor in adolescent and adult mice and can disturb the integrity of the NAc by altering transcription and translation of important signaling molecules in an age-dependent manner. Furthermore, exposure to ES and PS induces substantial synaptic plasticity of the NAc.

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

Stress, 2014, 17(3), 247-55. PMID: 24689732

Iñiguez SD, Riggs LM, Nieto SJ, Dayrit G, Zamora NN, Shawhan KL, Cruz B, Warren BL

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

Psychopharmacology, 2014 231(8): 1479-92. PMID: 24452697

Alcantara LF, Warren BL, Parise EM, Iñiguez SD, Bolaños-Guzmán CA

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 Journal of Neuroscience, 2014 34(3), 1007-21. PMID: 24431458

Iñiguez SD, Alcantara LF, Warren BL, Riggs LM, Parise EM, Vialou VF, Wright KN, Dayrit G, Nieto SJ,
Wilkinson MB, Lobo MK, Neve RL, Nestler EJ, Bolaños-Guzmán CA

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

Biological Psychiatry, 2013, 74(10): 750-9. PMID: 23790225

Parise EM, Alcantara LF, Warren BL, Wright KN, Hadad R, Sial OK, Kroeck KG, Iñiguez SD, Kroeck K,
Bolaños-Guzmán CA

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

Biological Psychiatry, 2013, 73(1), 7-14. PMID: 22795644

Warren BL, Vialou VF, Iñiguez SD, Alcantara LF, Wright KN, Feng J, Kennedy PJ, LaPlant Q, Shen, L,
Nestler EJ, Bolaños-Guzmán CA

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.

Juvenile Administration of Concomitant Methylphenidate and Fluoxetine Alters Behavioral Reactivity to Reward- and Mood-Related Stimuli and Disrupts Ventral Tegmental Area Gene Expression in Adulthood

The Journal of Neuroscience, 2011, 31(28), 10347-58. PMID: 21753012

Warren BL, Iñiguez SD, Alcantara LF, Wright KN, Weakley SK, Bolaños-Guzmán CA

There is a rise in the concurrent use of methylphenidate (MPH) and fluoxetine (FLX) in pediatric populations. However, the long-term neurobiological consequences of combined MPH and FLX treatment (MPH + FLX) during juvenile periods are unknown. We administered saline (VEH), MPH, FLX, or MPH + FLX to juvenile Sprague Dawley male rats from postnatal day 20 to 34, and assessed their reactivity to reward- and mood-related stimuli 24 h or 2 months after drug exposure. We also assessed mRNA and protein levels within the ventral tegmental area (VTA) to determine the effect of MPH, FLX, or MPH + FLX on the extracellular signal-regulated protein kinase-1/2 (ERK) pathway—a signaling cascade implicated in motivation and mood regulation. MPH + FLX enhanced sensitivity to drug (i.e., cocaine) and sucrose rewards, as well as anxiety (i.e., elevated plus maze)- and stress (i.e., forced swimming)-eliciting situations when compared with VEH-treated rats. MPH + FLX exposure also increased mRNA of ERK2 and its downstream targets cAMP response element-binding protein (CREB), BDNF, c-Fos, early growth response protein-1 (Zif268), and mammalian target of rapamycin (mTOR), and also increased protein phosphorylation of ERK2, CREB, and mTOR 2 months after drug exposure when compared with VEH-treated rats. Using herpes simplex virus-mediated gene transfer to block ERK2 activity within the VTA, we rescued the MPH and FLX-induced behavioral deficits seen in the forced-swimming task 2 months after drug treatment. These results indicate that concurrent MPH + FLX exposure during preadolescence increases sensitivity to reward-related stimuli while simultaneously enhancing susceptibility to stressful situations, at least in part, due to long-lasting disruptions in ERK signaling within the VTA.

Dnmt3a regulates emotional behavior and spine plasticity in the nucleus accumbens

Nature Neuroscience, 2010, 13(9), 1137-43. PMID: 2072844

LaPlant Q, Vialou V, Covington HE 3rd, Dumitriu D, Feng J, Warren BL¸ Maze I, Dietz DM, Watts EL,
Iñiguez SD, Koo JW, Mouzon E, Renthal W, Hollis F, Wang H, Noonan MA, Ren Y, Eisch AJ, Bolaños
CA, Kabbaj M, Xiao G, Neve RL, Hurd YL, Oosting RS, Fan G, Morrison JH, Nestler EJ

Despite abundant expression of DNA methyltransferases (Dnmts) in brain, the regulation and behavioral role of DNA methylation remain poorly understood. We found that Dnmt3a expression was regulated in mouse nucleus accumbens (NAc) by chronic cocaine use and chronic social defeat stress. Moreover, NAc-specific manipulations that block DNA methylation potentiated cocaine reward and exerted antidepressant-like effects, whereas NAc-specific Dnmt3a overexpression attenuated cocaine reward and was pro-depressant. On a cellular level, we found that chronic cocaine use selectively increased thin dendritic spines on NAc neurons and that DNA methylation was 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.

Extracellular Signal-Regulated Kinase-2 within the Ventral Tegmental Area Regulates Responses to Stress

The Journal of Neuroscience, 2010, 30(22), 7652-63. PMID: 20519540

Iñiguez SD, Vialou V, Warren BL, Cao JL, Alcantara LF, Davis LC, Manojlovic Z, Neve RL, Russo SJ, Han
MH, Nestler EJ, Bolaños-Guzmán CA

Neurotrophic factors and their signaling pathways have been implicated in the neurobiological adaptations in response to stress and the regulation of mood-related behaviors. A candidate signaling molecule implicated in mediating these cellular responses is the extracellular signal-regulated kinase (ERK1/2), although its functional role in mood regulation remains to be fully elucidated. Here we show that acute (1 d) or chronic (4 weeks) exposure to unpredictable stress increases phosphorylation of ERK1/2 and of two downstream targets (ribosomal S6 kinase and mitogen- and stress-activated protein kinase 1) within the ventral tegmental area (VTA), an important substrate for motivated behavior and mood regulation. Using herpes simplex virus-mediated gene transfer to assess the functional significance of this ERK induction, we show that overexpressing ERK2 within the VTA increases susceptibility to stress as measured in the forced swim test, responses to unconditioned nociceptive stimuli, and elevated plus maze in Sprague Dawley male rats, and in the tail suspension test and chronic social defeat stress procedure in C57BL/6 male mice. In contrast, blocking ERK2 activity in the VTA produces stress-resistant behavioral responses in these same assays and also blocks a chronic stress-induced reduction in sucrose preference. The effects induced by ERK2 blockade were accompanied by decreases in the firing frequency of VTA dopamine neurons, an important electrophysiological hallmark of resilient-like behavior. Together, these results strongly implicate a role for ERK2 signaling in the VTA as a key modulator of responsiveness to stress and mood-related behaviors.

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

Behavioral Brain Research, 2010, 214(2), 460-4. PMID: 20561901

Iñiguez SD, Warren BL, Neve RL, Russo SJ, Nestler EJ, Bolaños-Guzmán CA

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.

Short- and Long-Term Functional Consequences of Fluoxetine Exposure During Adolescence in Male Rats

Biological Psychiatry, 2010, 67(11), 1057-66. PMID: 20172503

Iñiguez SD, Warren BL, Bolaños-Guzmán CA

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.

ΔFosB in brain reward circuits mediates resilience to stress and antidepressant responses

Nature Neuroscience, 2010, 13(6), 743-752. PMID: 20473292

Vialou V, Robison AJ, LaPlant QC, Covington HE III, Dietz DM, Ohnishi YN, Mouzon E, Rush AJ III,
Watts EL, Wallace DL, Iñiguez SD, Ohnishi YH, Steiner MA, Warren BL, Krishnan V, Bolaños-Guzmán
CA, Neve RL, Ghose S, Berton O, Tamminga CA, Nestler EJ

In contrast with the many studies of stress effects on the brain, relatively little is known about the molecular mechanisms of resilience, the ability of some individuals to escape the deleterious effects of stress. We found that the transcription factor ΔFosB mediates an essential mechanism of resilience in mice. Induction of ΔFosB in the nucleus accumbens, an important brain reward-associated region, in response to chronic social defeat stress was both necessary and sufficient for resilience. ΔFosB induction was also required for the standard antidepressant fluoxetine to reverse behavioral pathology induced by social defeat. ΔFosB produced these effects through induction of the GluR2 AMPA glutamate receptor subunit, which decreased the responsiveness of nucleus accumbens neurons to glutamate, and through other synaptic proteins. Together, these findings establish a previously unknown molecular pathway underlying both resilience and antidepressant action.

Nicotine Exposure during Adolescence Induces a Depression-Like State in Adulthood

Neuropsychopharmacology, 2009, 34(6), 1609-1624. PMID: 19092782

Iñiguez SD*, Warren BL*, Parise E, Alcantara LF, Schuh B, Maffeo ML, Manojlovic Z, Bolaños-Guzmán CA

There is a strong link between tobacco consumption and mood disorders. It has been suggested that afflicted individuals smoke to manage mood, however, there is evidence indicating that tobacco consumption can induce negative mood. This study was designed to investigate whether nicotine exposure during adolescence influences emotionality/behavioral functioning later in life. Adolescent (postnatal days, PD 30–44) male rats were treated with twice-daily injections of nicotine (0, 0.16, 0.32, or 0.64 mg/kg) for 15 consecutive days, and their behavioral reactivity to various behavioral paradigms (the elevated plus maze (EPM), sucrose preference, locomotor activity in the open field, and forced swim test (FST) was assessed 24 h (short term) or 1-month (long term) after exposure. Separate groups of adult rats received nicotine (0.32 mg/kg) to control for age-dependent effects. We report that nicotine exposure during adolescence—but not adulthood—leads to a depression-like state manifested in decreased sensitivity to natural reward (sucrose), and enhanced sensitivity to stress- (FST) and anxiety-eliciting situations (EPM) later in life. Our data show that behavioral dysregulation can emerge 1 week after drug cessation, and that a single day of nicotine exposure during adolescence can be sufficient to precipitate a depression-like state in adulthood. We further demonstrate that these deficits can be normalized by subsequent nicotine (0.32 mg/kg) or antidepressant (ie fluoxetine or bupropion; 10 mg/kg) treatment in adulthood. These data suggest that adolescent exposure to nicotine results in a negative emotional state rendering the organism significantly more vulnerable to the adverse effects of stress. Within this context, our findings, together with others indicating that nicotine exposure during adolescence enhances risk for addiction later in life, could serve as a potential model of comorbidity.

Insulin Receptor Substrate-2 in the Ventral Tegmental Area Regulates Behavioral Responses to Cocaine

Behavioral Neuroscience, 2008, 122(5), 1172-1177. PMID: 18639865

Iñiguez SD, Warren BL, Neve RL, Nestler EJ, Russo SJ, Bolaños-Guzmán CA

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.