Nucleus accumbens neuronal ensembles vary with cocaine reinforcement in male and female rats.

Neuronal ensembles in the medial prefrontal cortex mediate cocaine self-administration via projections to the nucleus accumbens. We have recently shown that neuronal ensembles in the prelimbic cortex form rapidly to mediate cocaine self-administration. However, the role of neuronal ensembles within the nucleus accumbens in initial cocaine-seeking behaviour remains unknown. Here, we sought to expand the current literature by testing the necessity of the cocaine self-administration ensemble in the nucleus accumbens core (NAcCore) 1 day after male and female rats acquire cocaine self-administration by using the Daun02 inactivation procedure. We found that disrupting the NAcCore ensembles after a no-cocaine reward-seeking test increased subsequent cocaine seeking, while disrupting NAcCore ensembles following a cocaine self-administration session decreased subsequent cocaine seeking. We then characterized neuronal cell type in the NAcCore using RNAscope in situ hybridization. In the no-cocaine session, we saw reduced dopamine D1 type neuronal activation, while in the cocaine self-administration session, we found preferential dopamine D1 type neuronal activity in the NAcCore.

Fos-expressing neuronal ensembles in rat infralimbic cortex encode initial and maintained oxycodone seeking in rats.

Neuronal ensembles within the infralimbic cortex (IL) and their projections to the nucleus accumbens (NAc) mediate opiate seeking in well-trained rats. However, it is unclear how early this circuitry is recruited during oxycodone self-administration. Here, we used retrograde labelling (CTb) and immunohistochemistry to identify NAc-projecting neurons in the IL that were activated during initial oxycodone seeking. Next, we sought to determine the role of IL neuronal ensembles in initial oxycodone self-administration. We used the Daun02 procedure in male and female Fos-LacZ rats to chemogenetically inactivate IL Fos-expressing neurons at different time points in oxycodone self-administration training: immediately after meeting criteria for acquisition of behaviour and following nine daily sessions with increasing schedules of reinforcement (FR1, FR2 and FR3) in which rats demonstrated stable oxycodone intake under increasing effort to self-administer. We found that Daun02 infusions attenuated oxycodone seeking at both the initial learning and well-trained time points. These results suggest that IL neuronal ensembles are formed during initial learning of oxycodone self-administration and required for the maintenance and expression of oxycodone seeking.

Fos-expressing neuronal ensemble in rat ventromedial prefrontal cortex encodes cocaine seeking but not food seeking in rats.

Neuronal ensembles in ventromedial prefrontal cortex (vmPFC) play a role in both cocaine and palatable food seeking. However, it is unknown whether similar or different vmPFC neuronal ensembles mediate food and cocaine seeking. Here, we used the Daun02 inactivation procedure to assess whether the neuronal ensembles mediating food and cocaine seeking can be functionally distinguished. We trained male and female Fos-LacZ rats to self-administer palatable food pellets and cocaine on alternating days for 18 days. We then exposed the rats to a brief nonreinforced food- or cocaine-seeking test to induce Fos and ß-gal in neuronal ensembles associated with food or cocaine seeking, respectively and infused Daun02 into vmPFC to ablate the ß-gal-expressing ensembles. Two days later, we tested the rats for food or cocaine seeking under extinction conditions. Although inactivation of the food-seeking ensemble did not influence food or cocaine seeking, inactivation of the cocaine-seeking ensemble reduced cocaine seeking but not food seeking. Results indicate that the neuronal ensemble activated by cocaine seeking in vmPFC is functionally separate from the ensemble activated by food seeking.

Separate vmPFC Ensembles Control Cocaine Self-Administration Versus Extinction in Rats.

Recent studies suggest that the ventral medial prefrontal cortex (vmPFC) encodes both operant drug self-administration and extinction memories. Here, we examined whether these opposing memories are encoded by distinct neuronal ensembles within the vmPFC with different outputs to the nucleus accumbens (NAc) in male and female rats. Using cocaine self-administration (3 h/d for 14 d) and extinction procedures, we demonstrated that vmPFC was similarly activated (indexed by Fos) during cocaine-seeking tests after 0 (no-extinction) or 7 extinction sessions. Selective Daun02 lesioning of the self-administration ensemble (no-extinction) decreased cocaine seeking, whereas Daun02 lesioning of the extinction ensemble increased cocaine seeking. Retrograde tracing with fluorescent cholera toxin subunit B injected into NAc combined with Fos colabeling in vmPFC indicated that vmPFC self-administration ensembles project to NAc core while extinction ensembles project to NAc shell. Functional disconnection experiments (Daun02 lesioning of vmPFC and acute dopamine D1-receptor blockade with SCH39166 in NAc core or shell) confirm that vmPFC ensembles interact with NAc core versus shell to play dissociable roles in cocaine self-administration versus extinction, respectively. Our results demonstrate that neuronal ensembles mediating cocaine self-administration and extinction comingle in vmPFC but have distinct outputs to the NAc core and shell that promote or inhibit cocaine seeking.SIGNIFICANCE STATEMENT Neuronal ensembles within the vmPFC have recently been shown to play a role in self-administration and extinction of food seeking. Here, we used the Daun02 chemogenetic inactivation procedure, which allows selective inhibition of neuronal ensembles identified by the activity marker Fos, to demonstrate that different ensembles for cocaine self-administration and extinction memories coexist in the ventral mPFC and interact with distinct subregions of the nucleus accumbens.

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

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 per infusion, 3-hr/day, 18 day) or heroin (0.03 mg/kg per infusion) on alternating days (9 day 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, dorsal striatum, nucleus accumbens, 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 was 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.

Role of Dorsomedial Striatum Neuronal Ensembles in Incubation of Methamphetamine Craving after Voluntary Abstinence.

We recently developed a rat model of incubation of methamphetamine craving after choice-based voluntary abstinence. Here, we studied the role of dorsolateral striatum (DLS) and dorsomedial striatum (DMS) in this incubation. We trained rats to self-administer palatable food pellets (6 d, 6 h/d) and methamphetamine (12 d, 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/d) for 19 d. We used in situ hybridization to measure the colabeling of the activity marker Fos with Drd1 and Drd2 in DMS and DLS after the tests. Based on the in situ hybridization colabeling 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 d of voluntary abstinence than after 1 d (incubation of methamphetamine craving). The incubated response was associated with increased Fos expression in DMS but not in DLS; Fos was colabeled 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 the incubation of methamphetamine craving after voluntary abstinence and that DMS neuronal ensembles mediate this incubation. SIGNIFICANCE STATEMENT: In human addicts, abstinence is often self-imposed and relapse can be triggered by exposure to drug-associated cues that induce drug craving. We recently developed a rat model of incubation of methamphetamine craving after choice-based voluntary abstinence. Here, we used classical pharmacology, in situ hybridization, immunohistochemistry, and the Daun02 inactivation procedure to demonstrate a critical role of dorsomedial striatum neuronal ensembles in this new form of incubation of drug craving.

Bidirectional Modulation of Intrinsic Excitability in Rat Prelimbic Cortex Neuronal Ensembles and Non-Ensembles after Operant Learning.

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 activated selectively 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 after 10 d of operant food self-administration training (1 h/d). First, we used the Daun02 inactivation procedure in male FosLacZ-transgenic rats to ablate selectively 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 with 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- non-ensembles.SIGNIFICANCE STATEMENT Prefrontal cortex activity plays a critical role in operant learning, but the underlying cellular mechanisms are unknown. Using the chemogenetic Daun02 inactivation procedure, we found that a small number of strongly activated Fos-expressing neuronal ensembles in rat PLC play an important role in learned operant food seeking. Using GFP expression to identify Fos-expressing layer V pyramidal neurons in prelimbic cortex (PLC) of FosGFP-transgenic rats, we found that operant food self-administration led to increased intrinsic excitability in the behaviorally relevant Fos-expressing neuronal ensembles, but decreased intrinsic excitability in Fos- neurons using distinct cellular mechanisms.

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

UNLABELLED: 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. SIGNIFICANCE STATEMENT: A current popular hypothesis is that neuronal ensembles in different prefrontal cortex areas control reward-associated versus extinction-associated memories: the dorsal medial prefrontal cortex (mPFC) promotes reward seeking, whereas the ventral mPFC inhibits reward seeking. In this paper, we use the Daun02 chemogenetic inactivation procedure to demonstrate that Fos-expressing neuronal ensembles mediating both food reward and extinction memories intermingle within the same ventral mPFC area.

Neurons Internalize Functionalized Micron-Sized Silicon Dioxide Microspheres.

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.

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

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.

Context-induced reinstatement of methamphetamine seeking is associated with unique molecular alterations in Fos-expressing dorsolateral striatum neurons.

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.

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.

Altered gene expression and spine density in nucleus accumbens of adolescent and adult male mice exposed to emotional and physical stress.

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.

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.

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.

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.

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.

Prelimbic Ensembles Mediate Cocaine Seeking After Behavioral Acquisition and Once Rats Are Well-Trained.

Substance use disorder (SUD) is a chronic relapsing condition characterized by continued use of drugs despite negative consequences. SUD is thought to involve disordered learning and memory wherein drug-paired cues gain increased salience, and ultimately drive craving and relapse. These types of associations are thought to be encoded within sparsely distributed sets of neurons, called neuronal ensembles, that drive encoded behaviors through synchronous activity of the participant neurons. We have previously found that Fos-expressing neuronal ensembles within the prefrontal cortex are required for well-trained cocaine seeking. However, less is known about how quickly cortical neuronal ensembles form during the initiation of cocaine seeking behavior. Here, we seek to further elucidate the role of Fos-expressing neuronal ensembles within the prelimbic cortex (PL) after the initial acquisition of cocaine self-administration (SA), or, after 10 days of additional SA training (well-trained). We trained Fos-LacZ transgenic rats to lever press for cocaine under an FR1 schedule of reinforcement. Once rats met acquisition criteria for cocaine self-administration, we ablated Fos-expressing neuronal ensembles in the PL using the Daun02 inactivation method, either 1 or 10 days after the rats met the acquisition criteria. Targeted ablation of Fos-expressing neuronal ensembles in the PL attenuated active lever pressing both 1 day and 10 days after rats acquired cocaine self-administration. Together, this suggests that Fos-expressing neuronal ensembles rapidly form in the PL and continue to mediate maintained cocaine seeking behavior.

Extracellular signal-regulated kinase-2 within the ventral tegmental area regulates responses to stress.

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.

Food-Seeking Behavior Is Mediated by Fos-Expressing Neuronal Ensembles Formed at First Learning in Rats.

Neuronal ensembles in the infralimbic cortex (IL) develop after prolonged food self-administration training. However, rats demonstrate evidence of learning the food self-administration response as early as day 1, with responding quickly increasing to asymptotic levels. Since the contribution of individual brain regions to task performance shifts over the course of training, it remains unclear whether IL ensembles are gradually formed and refined over the course of extensive operant training, or whether functionally-relevant ensembles might be recruited and formed as early as the initial acquisition of food self-administration behavior. Here, we aimed to determine the role of IL ensembles at the earliest possible point after demonstrable learning of a response-outcome association. We first allowed rats to lever press for palatable food pellets and stopped training rats once their behavior evidenced the response-outcome association (learners). We compared their food-seeking behavior and neuronal activation (Fos protein expression) to similarly trained rats that did not form this association (non-learners). Learners had greater food-seeking behavior and neuronal activation within the medial prefrontal cortex (mPFC), suggesting that mPFC subregions might encode initial food self-administration memories. To test the functional relevance of mPFC Fos-expressing ensembles to subsequent food seeking, we tested region-wide inactivation of the IL using muscimol+baclofen and neuronal ensemble-specific ablation using the Daun02 inactivation procedure. Both region-wide inactivation and ensemble-specific inactivation of the IL significantly decreased food seeking. These data suggest that IL neuronal ensembles form during initial learning of food self-administration behavior, and furthermore, that these ensembles play a functional role in food seeking.

Adolescent fluoxetine treatment mediates a persistent anxiety-like outcome in female C57BL/6 mice that is ameliorated by fluoxetine re-exposure in adulthood.

The objective of this study was to evaluate whether juvenile fluoxetine (FLX) exposure induces long-term changes in baseline responses to anxiety-inducing environments, and if so, whether its re-exposure in adulthood would ameliorate this anxiety-like phenotype. An additional goal was to assess the impact of adolescent FLX pretreatment, and its re-exposure in adulthood, on serotonin transporters (5-HTT) and brain-derived-neurotrophic-factor (BDNF)-related signaling markers (TrkB-ERK1/2-CREB-proBDNF-mBDNF) within the hippocampus and prefrontal cortex. To do this, female C57BL/6 mice were exposed to FLX in drinking water during postnatal-days (PD) 35-49. After a 21-day washout-period (PD70), mice were either euthanized (tissue collection) or evaluated on anxiety-related tests (open field, light/dark box, elevated plus-maze). Juvenile FLX history resulted in a persistent avoidance-like profile, along with decreases in BDNF-signaling markers, but not 5-HTTs or TrkB receptors, within both brain regions. Interestingly, FLX re-exposure in adulthood reversed the enduring FLX-induced anxiety-related responses across all behavioral tasks, while restoring ERK2-CREB-proBDNF markers to control levels and increasing mBDNF within the prefrontal cortex, but not the hippocampus. Collectively, these results indicate that adolescent FLX history mediates neurobehavioral adaptations that endure into adulthood, which are indicative of a generalized anxiety-like phenotype, and that this persistent effect is ameliorated by later-life FLX re-exposure, in a prefrontal cortex-specific manner.