Chronic stress may facilitate the recruitment of habit- and addiction-related neurocircuitries through neuronal restructuring of the striatum

Lasting effects of adolescent social instability stress on dendritic morphology in the nucleus accumbens in female and male Long Evans rats

Social instability stress (SS) in adolescence in rats leads to long-lasting changes in social behaviour and reward-related behaviour relative to control rats. Given the role of the nucleus accumbens (NAc) in such behaviours, we investigated the morphology of medium spiny neurons (MSNs), which are most neurons in the NAc, in adult female and male rats exposed to SS in adolescence. Irrespective of sex, SS rats had increased number of dendritic spines in both the core and shell regions of the NAc (2.3 % and 18.1 % increase, respectively). In the core, SS rats had a 16 % reduction in the total dendritic lengths of MSNs, whereas in the shell, SS rats had a greater dendritic length closer to the soma, and particularly in SS female rats, whereas the opposite was found farther from the soma (SS 10.6 % > CTL overall). Although the extent to which such structural changes may underlie the enduring effects of SS in adolescence requires investigation, the results add to evidence that changes to the social environment in adolescence can determine adult neuronal structural.

Investigating Substance Use as a Coping Strategy Among Adolescent Psychiatric Inpatients: A Comparative Analysis Before and During the COVID-19 Pandemic

The COVID-19 pandemic resulted in significant changes in daily life, potentially impacting mental health and substance use behavior. Research on COVID-related changes in adolescent substance use have yielded mixed findings. The current cross-sectional chart review study compared rates of past-year substance use before and during COVID-19 among adolescent psychiatric inpatients, and investigated how motives for coping with COVID-19 changes were related to psychiatric acuity, and past-year substance use. Count models assessed if the number of past-year days of alcohol and cannabis use was higher among adolescents (n = 491, 11-18 years, 61% female) hospitalized during COVID-19 (3/14/20 to 4/5/21) versus adolescents hospitalized before COVID-19 (8/30/2019 to 3/13/20). For a subsample of COVID-19 inpatients (n = 124; 75% female), we evaluated psychiatric correlates of endorsing substances to cope with COVID-19 changes/rules. Results indicated adolescents admitted during COVID-19 reported significantly more past-year alcohol and cannabis use days than adolescents admitted before COVID-19. Adolescents endorsed using alcohol (19%), cannabis (33%), and e-cigarettes/vaping (25%) to cope with COVID-19. E-cigarette/vaping to cope with COVID-19 was significantly related to lifetime suicide attempt. Endorsing alcohol or cannabis to cope with COVID-19 was associated with a significantly greater number of past-year use days for each respective substance. Adolescent psychiatric inpatients admitted during COVID-19 reported more substance use days than adolescents admitted before COVID-19. Using substances to cope was linked to psychiatric correlates (e.g., suicidality). Assessing the presence and function of substance use in this population may be important to identify, treat, and prevent compounding negative outcomes during times of community stress.

Cumulative lifetime stressor exposure impairs stimulus-response but not contextual learning

Greater exposure to stressors over the life course is believed to promote striatum-dependent over hippocampus-dependent learning and memory processes under stressful conditions. However, little research in this context has actually assessed lifetime stressor exposure and, moreover, it remains unknown whether greater cumulative lifetime stressor exposure exerts comparable effects on striatum-dependent learning and hippocampus-dependent learning in non-stressful contexts. To investigate this issue, we used the Stress and Adversity Inventory for Adults (Adult STRAIN) and Multicued Search Task to investigate the relation between cumulative lifetime stressor exposure and striatum-dependent stimulus-response learning and hippocampus-dependent contextual learning under non-stressful conditions among healthcare professionals (N = 205; 157 females, 48 males; Age: M = 34.23, SD 9.3, range 20-59 years). Individuals with moderate, but not low, cumulative lifetime stressor exposure exhibited impaired learning for stimulus-response associations. In contrast, learning for context associations was unrelated to participants' lifetime stressor exposure profiles. These results thus provide first evidence that cumulative lifetime stressor exposure may have negative consequences on human striatum-dependent stimulus-response learning under non-stressful environmental conditions.

Acute stress facilitates habitual behavior in female rats

Instrumental behavior can reflect the influence of goal-directed and habitual systems. Contemporary research suggests that stress may facilitate control by the habitual system under conditions where the behavior would otherwise reflect control by the goal-directed system. However, it is unclear how stress modulates the influence of these systems on instrumental responding to achieve this effect, particularly in females. Here, we examine whether a mild psychogenic stressor experienced before acquisition training (Experiment 1), or prior to the test of expression (Experiment 2) would influence goal-directed and habitual control of instrumental responding in female rats. In both experiments, rats acquired an instrumental nose-poke response for a sucrose reward. This was followed by a reinforcer devaluation phase in which half the rats in Stressed and Non-Stressed conditions received pairings of the sucrose pellet with illness induced by lithium chloride until they rejected the pellet when offered. The remaining rats received a control treatment consisting of pellets and illness on separate days (Unpaired). Control by goal-directed and habitual systems was evaluated in a subsequent nonreinforced test of nose poking. The results of Experiment 1 indicated that the Non-Stressed Paired group reduced nose-poking compared to the Unpaired controls, identifying the response as goal directed, whereas the Stressed Paired and Unpaired groups made a similar number of nose pokes identifying the response as habitual despite a similar amount of training. Results from Experiment 2 indicated habitual control of nose-poke responding was present when stress was experienced just prior to the test. Collectively, these data suggest that stress may facilitate habitual control by altering the relative influence of goal-directed and habitual processes underpinning instrumental behavior. These results may be clinically relevant for understanding the contributions of stress to dysregulated instrumental behavior in compulsive pathologies.

Stress, associative learning, and decision-making

Exposure to acute and chronic stress has significant effects on the basic mechanisms of associative learning and memory. Stress can both impair and enhance associative learning depending on type, intensity, and persistence of the stressor, the subject's sex, the context that the stress and behavior is experienced in, and the type of associative learning taking place. In some cases, stress can cause or exacerbate the maladaptive behavior that underlies numerous psychiatric conditions including anxiety disorders, obsessive-compulsive disorder, post-traumatic stress disorder, substance use disorder, and others. Therefore, it is critical to understand how the varied effects of stress, which may normally facilitate adaptive behavior, can also become maladaptive and even harmful. In this review, we highlight several findings of associative learning and decision-making processes that are affected by stress in both human and non-human subjects and how they are related to one another. An emerging theme from this work is that stress biases behavior towards less flexible strategies that may reflect a cautious insensitivity to changing contingencies. We consider how this inflexibility has been observed in different associative learning procedures and suggest that a goal for the field should be to clarify how factors such as sex and previous experience influence this inflexibility.

Neurocognitive effects of stress: a metaparadigm perspective

Stressful experiences, both physical and psychological, that are overwhelming (i.e., inescapable and unpredictable), can measurably affect subsequent neuronal properties and cognitive functioning of the hippocampus. At the cellular level, stress has been shown to alter hippocampal synaptic plasticity, spike and local field potential activity, dendritic morphology, neurogenesis, and neurodegeneration. At the behavioral level, stress has been found to impair learning and memory for declarative (or explicit) tasks that are based on cognition, such as verbal recall memory in humans and spatial memory in rodents, while facilitating those that are based on emotion, such as differential fear conditioning in humans and contextual fear conditioning in rodents. These vertically related alterations in the hippocampus, procedurally observed after subjects have undergone stress, are generally believed to be mediated by recurrently elevated circulating hypothalamic-pituitary-adrenal (HPA) axis effector hormones, glucocorticoids, directly acting on hippocampal neurons densely populated with corticosteroid receptors. The main purposes of this review are to (i) provide a synopsis of the neurocognitive effects of stress in a historical context that led to the contemporary HPA axis dogma of basic and translational stress research, (ii) critically reappraise the necessity and sufficiency of the glucocorticoid hypothesis of stress, and (iii) suggest an alternative metaparadigm approach to monitor and manipulate the progression of stress effects at the neural coding level. Real-time analyses can reveal neural activity markers of stress in the hippocampus that can be used to extrapolate neurocognitive effects across a range of stress paradigms (i.e., resolve scaling and dichotomous memory effects issues) and understand individual differences, thereby providing a novel neurophysiological scaffold for advancing future stress research.

Chronic alcohol consumption shifts learning strategies and synaptic plasticity from hippocampus to striatum-dependent pathways

Introduction

The hippocampus and striatum have dissociable roles in memory and are necessary for spatial and procedural/cued learning, respectively. Emotionally charged, stressful events promote the use of striatal- over hippocampus-dependent learning through the activation of the amygdala. An emerging hypothesis suggests that chronic consumption of addictive drugs similarly disrupt spatial/declarative memory while facilitating striatum-dependent associative learning. This cognitive imbalance could contribute to maintain addictive behaviors and increase the risk of relapse.

Methods

We first examined, in C57BL/6 J male mice, whether chronic alcohol consumption (CAC) and alcohol withdrawal (AW) might modulate the respective use of spatial vs. single cue-based learning strategies, using a competition protocol in the Barnes maze task. We then performed in vivo electrophysiological studies in freely moving mice to assess learning-induced synaptic plasticity in both the basolateral amygdala (BLA) to dorsal hippocampus (dCA1) and BLA to dorsolateral striatum (DLS) pathways.

Results

We found that both CAC and early AW promote the use of cue-dependent learning strategies, and potentiate plasticity in the BLA → DLS pathway while reducing the use of spatial memory and depressing BLA → dCA1 neurotransmission.

Discussion

These results support the view that CAC disrupt normal hippocampo-striatal interactions, and suggest that targeting this cognitive imbalance through spatial/declarative task training could be of great help to maintain protracted abstinence in alcoholic patients.

Striatopallidal adenosine A2A receptor modulation of goal-directed behavior: Homeostatic control with cognitive flexibility

Dysfunction of goal-directed behaviors under stressful or pathological conditions results in impaired decision-making and loss of flexibility of thoughts and behaviors, which underlie behavioral deficits ranging from depression, obsessive-compulsive disorders and drug addiction. Tackling the neuromodulators fine-tuning this core behavioral element may facilitate the development of effective strategies to control these deficits present in multiple psychiatric disorders. The current investigation of goal-directed behaviors has concentrated on dopamine and glutamate signaling in the corticostriatal pathway. In accordance with the beneficial effects of caffeine intake on mood and cognitive dysfunction, we now propose that caffeine's main site of action - adenosine A2A receptors (A2AR) - represent a novel target to homeostatically control goal-directed behavior and cognitive flexibility. A2AR are abundantly expressed in striatopallidal neurons and colocalize and interact with dopamine D2, NMDA and metabotropic glutamate 5 receptors to integrate dopamine and glutamate signaling. Specifically, striatopallidal A2AR (i) exert an overall "break" control of a variety of cognitive processes, making A2AR antagonists a novel strategy for improving goal-directed behavior; (ii) confer homeostatic control of goal-directed behavior by acting at multiple sites with often opposite effects, to enhance cognitive flexibility; (iii) integrate dopamine and adenosine signaling through multimeric A2AR-D2R heterocomplexes allowing a temporally precise fine-tuning in response to local signaling changes. As the U.S. Food and Drug Administration recently approved the A2AR antagonist Nourianz® (istradefylline) to treat Parkinson's disease, striatal A2AR-mediated control of goal-directed behavior may offer a new and real opportunity for improving deficits of goal-directed behavior and enhance cognitive flexibility under various neuropsychiatric conditions. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Adolescent social isolation shifts the balance of decision-making strategy from goal-directed action to habitual response in adulthood via suppressing the excitatory neurotransmission onto the direct pathway of the dorsomedial striatum

Adverse experience, such as social isolation, during adolescence is one of the major causes of neuropsychiatric disorders that extend from adolescence into adulthood, such as substance addiction, obsessive-compulsive disorder, and eating disorders leading to obesity. A common behavioral feature of these neuropsychiatric disorders is a shift in the balance of decision-making strategy from goal-directed action to habitual response. This study has verified that adolescent social isolation directly shifts the balance of decision-making strategy from goal-directed action to habitual response, and that it cannot be reversed by simple regrouping. This study has further revealed that adolescent social isolation induces a suppression in the excitatory neurotransmission onto the direct-pathway medium spiny neurons of the dorsomedial striatum (DMS), and that chemogenetically compensating this suppression effect shifts the balance of decision-making strategy from habitual response back to goal-directed action. These findings suggest that the plasticity in the DMS causes the shift in the balance of decision-making strategy, which would potentially help to develop a general therapy to treat the various neuropsychiatric disorders caused by adolescent social isolation. Such a study is especially necessary under the circumstances that social distancing and lockdown have caused during times of world-wide, society-wide pandemic.

Elevated prefrontal dopamine interferes with the stress-buffering properties of behavioral control in female rats

Stress-linked disorders are more prevalent in women than in men and differ in their clinical presentation. Thus, investigating sex differences in factors that promote susceptibility or resilience to stress outcomes, and the circuit elements that mediate their effects, is important. In male rats, instrumental control over stressors engages a corticostriatal system involving the prelimbic cortex (PL) and dorsomedial striatum (DMS) that prevent many of the sequelae of stress exposure. Interestingly, control does not buffer against stress outcomes in females, and here, we provide evidence that the instrumental controlling response in females is supported instead by the dorsolateral striatum (DLS). Additionally, we used in vivo microdialysis, fluorescent in situ hybridization, and receptor subtype pharmacology to examine the contribution of prefrontal dopamine (DA) to the differential impact of behavioral control. Although both sexes preferentially expressed D1 receptor mRNA in PL GABAergic neurons, there were robust sex differences in the dynamic properties of prefrontal DA during controllable stress. Behavioral control potently attenuated stress-induced DA efflux in males, but not females, who showed a sustained DA increase throughout the entire stress session. Importantly, PL D1 receptor blockade (SCH 23390) shifted the proportion of striatal activity from the DLS to the DMS in females and produced the protective effects of behavioral control. These findings suggest a sex-selective mechanism in which elevated DA in the PL biases instrumental responding towards prefrontal-independent striatal circuitry, thereby eliminating the protective impact of coping with stress.

A matter of habit? Stressful life events and cognitive flexibility in 15-month-olds

Exposure to chronic stress is associated with habitual learning in adults. We studied the origins of this association by examining the link between stressful life events and infant cognitive flexibility. The final sample consisted of N = 72 fifteen-month-old infants and their mothers. Mothers completed a survey on pre- and postnatal negative life events. To assess chronic stress physiologically, infant and maternal hair cortisol concentrations were determined for cortisol accumulation during the past 3 months. Each infant participated in two cognitive tasks in the laboratory. An instrumental learning task tested infants' ability to disengage from a habituated action when this action became ineffective (Seehagen et al., 2015). An age-adequate version of the A-not-B task tested infants' ability to find a toy at location B after repeatedly finding it at location A. Correlations between cortisol concentrations and postnatal negative life events (number, perceived impact) did not yield significance. Infant and maternal hair cortisol concentrations were not correlated. Infants' ability to shift to a new action in either task, controlled for acute stress, correlated neither with pre- and postnatal negative life events nor with cortisol concentrations. Taken together, these results indicate that the potential link between long-term stress exposure and cognitive flexibility might not be present in samples with low levels of psychosocial stress.

Convergent actions of stress and stimulants via epigenetic regulation of neural circuitry

The dorsal striatum integrates prior and current information to guide appropriate decision-making. Chronic stress and stimulant exposure interferes with decision-making, and can confer similar cognitive and behavioral inflexibilities. This review examines the literature on acute and chronic regulation of the epigenome by stress and stimulants. Recent evidence suggests that exposures to stress and stimulants share similarities in the manners in which they regulate the dorsal striatum epigenome through DNA methylation, transposable element activity, and histone post-translational modifications. These findings suggest that chronic stress and stimulant exposure leads to the accumulation of epigenetic modifications that impair immediate and future neuron function and activity. Such epigenetic mechanisms represent potential therapeutic targets for ameliorating convergent symptoms of stress and addiction.

Habit Formation and the Effect of Repeated Stress Exposures on Cognitive Flexibility Learning in Horses

Horse training exposes horses to an array of cognitive and ethological challenges. Horses are routinely required to perform behaviours that are not aligned to aspects of their ethology, which may delay learning. While horses readily form habits during training, not all of these responses are considered desirable, resulting in the horse being subject to retraining. This is a form of cognitive flexibility and is critical to the extinction of habits and the learning of new responses. It is underpinned by complex neural processes which can be impaired by chronic or repeated stress. Domestic horses may be repeatedly exposed to multiples stressors. The potential contribution of stress impairments of cognitive flexibility to apparent training failures is not well understood, however research from neuroscience can be used to understand horses’ responses to training. We trained horses to acquire habit-like responses in one of two industry-style aversive instrumental learning scenarios (moving away from the stimulus-instinctual or moving towards the stimulus-non-instinctual) and evaluated the effect of repeated stress exposures on their cognitive flexibility in a reversal task. We measured heart rate as a proxy for noradrenaline release, salivary cortisol and serum Brain Derived Neurotrophic Factor (BDNF) to infer possible neural correlates of the learning outcomes. The instinctual task which aligned with innate equine escape responses to aversive stimuli was acquired significantly faster than the non-instinctual task during both learning phases, however contrary to expectations, the repeated stress exposure did not impair the reversal learning. We report a preliminary finding that serum BDNF and salivary cortisol concentrations in horses are positively correlated. The ethological salience of training tasks and cognitive flexibility learning can significantly affect learning in horses and trainers should adapt their practices where such tasks challenge innate equine behaviour.

Principles for humanistic responsiveness to children and adolescents coping with the pandemic

Pandemic stress can be a dehumanizing experience, especially for children and adolescents, and can be described in terms of the research paradigm of chronic variable stress (CVS). Humanistic counseling speaks to this particular kind of stress. This article presents eight principles for humanistic responsiveness for working with children and adolescents.

Impact of putamen inhibition by DREADDs on schedule-induced drinking in rhesus monkeys

The putamen is a nucleus within the sensory-motor striatal network that is involved in automatic, habitual actions. Schedule-induced polydipsia (SIP) is highly automated behavior, reliably occurring under intermediate interval schedules of reinforcement. The effect of putamen inhibition in mediating SIP of water and ethanol (4% w/v) under a Fixed Time 5-min (FT-5 min) schedule for food delivery was tested in 12 rhesus monkeys (6 male, 6 female). Water and ethanol SIP sessions ended after set volumes were consumed. Baseline patterns of SIP intake differed between water and ethanol SIP in volume but not in pattern of drinking. Activation of the designer receptor exclusively activated by designer drug (DREADD: hM4Di) with deschloroclozapine (DCZ; 300 μg/kg, i.m.) administered 30 min prior to the onset of the SIP session, for four consecutive sessions. DCZ administration increased the postpellet drink volume and reduced the time to drink both water and ethanol. Although the effect of DCZ treatment was similar for increasing SIP with either water or ethanol, post-DCZ return to baseline SIP rates of differed, perhaps highlighting the effect of a state dependency with ethanol SIP. Overall, the study shows that targeting the putamen with the inhibitory DREADD produces a reversible, reproducible and reliable increase in adjunctive drinking.

Adolescent social isolation induced alterations in nucleus accumbens glutamate signalling

Exposure to adversity during early childhood and adolescence increases an individual's vulnerability to developing substance use disorder. Despite the knowledge of this vulnerability, the mechanisms underlying it are still poorly understood. Excitatory afferents to the nucleus accumbens (NAc) mediate responses to both stressful and rewarding stimuli. Understanding how adolescent social isolation alters these afferents could inform the development of targeted interventions both before and after drug use. Here, we used social isolation rearing as a model of early life adversity which we have previously demonstrated increases vulnerability to cocaine addiction-like behaviour. The current study examined the effect of social isolation rearing on presynaptic glutamatergic transmission in NAc medium spiny neurons in both male and female mice. We show that social isolation rearing alters presynaptic plasticity in the NAc by decreasing the paired-pulse ratio and the size of the readily releasable pool of glutamate. Optogenetically activating the glutamatergic input from the ventral hippocampus to the NAc is sufficient to recapitulate the decreases in paired-pulse ratio and readily releasable pool size seen following electrical stimulation of all NAc afferents. Further, optogenetically inhibiting the ventral hippocampal afferent during electrical stimulation eliminates the effect of early life adversity on the paired-pulse ratio or readily releasable pool size. In summary, we demonstrate that social isolation rearing leads to alterations in glutamate transmission driven by projections from the ventral hippocampus. These data suggest that targeting the circuit from the ventral hippocampus to the nucleus accumbens could provide a means to reverse stress-induced plasticity.

The Role of Stress in Obsessive-Compulsive Disorder: A Narrative Review

Obsessive-compulsive disorder (OCD) has a worldwide prevalence of 2%-3%. Characterized by the presence of either one or two core symptoms-obsessions and compulsions-it generally runs a chronic course and may cause serious functional impairment. Though previously thought to be of psychogenic origin, the pathophysiology of OCD is now understood to be more complex. A multitude of environmental factors have been shown to contribute to the development of OCD, including infection, neonatal complications, childhood trauma, occurrence of stressful events, and brain injury. It has also been proposed that genetic vulnerability may play a role in OCD pathology, although candidate genes have yet to be identified. Likewise, although it is widely accepted that stress plays a role in OCD pathophysiology, the mechanisms remain unclear. Observations from the clinics indicate that stress may serve as both a triggering and aggravating factor, meaning it can prompt symptoms to appear while also contributing to their exacerbation. Additionally, dysfunction of the hypothalamic-pituitary-adrenal axis and impaired stress response have been identified in OCD patients. In this review, we analyze the role of stress in the pathophysiology of OCD, complemented by relevant findings from recent animal studies.

Interactive relations between maternal prenatal stress, fetal brain connectivity, and gestational age at delivery

Studies reporting significant associations between maternal prenatal stress and child outcomes are frequently confounded by correlates of prenatal stress that influence the postnatal rearing environment. The major objective of this study is to identify whether maternal prenatal stress is associated with variation in human brain functional connectivity prior to birth. We utilized fetal fMRI in 118 fetuses [48 female; mean age 32.9 weeks (SD = 3.87)] to evaluate this association and further addressed whether fetal neural differences were related to maternal health behaviors, social support, or birth outcomes. Community detection was used to empirically define networks and enrichment was used to isolate differential within- or between-network connectivity effects. Significance for χ2 enrichment was determined by randomly permuting the subject pairing of fetal brain connectivity and maternal stress values 10,000 times. Mixtures modelling was used to test whether fetal neural differences were related to maternal health behaviors, social support, or birth outcomes. Increased maternal prenatal negative affect/stress was associated with alterations in fetal frontoparietal, striatal, and temporoparietal connectivity (β = 0.82, p < 0.001). Follow-up analysis demonstrated that these associations were stronger in women with better health behaviors, more positive interpersonal support, and lower overall stress (β = 0.16, p = 0.02). Additionally, magnitude of stress-related differences in neural connectivity was marginally correlated with younger gestational age at delivery (β = -0.18, p = 0.05). This is the first evidence that negative affect/stress during pregnancy is reflected in functional network differences in the human brain in utero, and also provides information about how positive interpersonal and health behaviors could mitigate prenatal brain programming.

Corticosterone in the dorsolateral striatum facilitates the extinction of stimulus-response memory

Glucocorticoid hormones are crucially involved in modulating mnemonic processing of stressful or emotionally arousing experiences. They are known to enhance the consolidation of new memories, including those that extinguish older memories. In this study, we investigated whether glucocorticoids facilitate the extinction of a striatum-dependent, and behaviorally more rigid, stimulus-response memory. For this, male rats were initially trained for six days on a stimulus-response task in a T-maze to obtain a reward after making an egocentric right-turn body response, regardless of the starting position in this maze. This training phase was followed by three extinction sessions in which right-turn body responses were not reinforced. Corticosterone administration into the dorsolateral region of the striatum after the first extinction session dose-dependently enhanced the consolidation of extinction memory: Rats administered the higher dose of corticosterone (30 ng), but not lower doses (5 or 10 ng), exhibited significantly fewer right-turn body responses and had longer latencies compared to vehicle-treated animals on the second and third extinction sessions. Co-administration of the glucocorticoid receptor antagonist RU 486 (10 ng) prevented the corticosterone effect, indicating that glucocorticoids enhance the extinction of stimulus-response memory via activation of the glucocorticoid receptor. Corticosterone administration into the dorsomedial striatum did not affect extinction memory. These findings indicate that stress-response mechanisms involving corticosterone actions in the dorsolateral striatum facilitate the extinction of stimulus-response memory that might allow for the development of an opportune behavioral strategy.

Place vs. Response Learning: History, Controversy, and Neurobiology

The present article provides a historical review of the place and response learning plus-maze tasks with a focus on the behavioral and neurobiological findings. The article begins by reviewing the conflict between Edward C. Tolman's cognitive view and Clark L. Hull's stimulus-response (S-R) view of learning and how the place and response learning plus-maze tasks were designed to resolve this debate. Cognitive learning theorists predicted that place learning would be acquired faster than response learning, indicating the dominance of cognitive learning, whereas S-R learning theorists predicted that response learning would be acquired faster, indicating the dominance of S-R learning. Here, the evidence is reviewed demonstrating that either place or response learning may be dominant in a given learning situation and that the relative dominance of place and response learning depends on various parametric factors (i.e., amount of training, visual aspects of the learning environment, emotional arousal, et cetera). Next, the neurobiology underlying place and response learning is reviewed, providing strong evidence for the existence of multiple memory systems in the mammalian brain. Research has indicated that place learning is principally mediated by the hippocampus, whereas response learning is mediated by the dorsolateral striatum. Other brain regions implicated in place and response learning are also discussed in this section, including the dorsomedial striatum, amygdala, and medial prefrontal cortex. An exhaustive review of the neurotransmitter systems underlying place and response learning is subsequently provided, indicating important roles for glutamate, dopamine, acetylcholine, cannabinoids, and estrogen. Closing remarks are made emphasizing the historical importance of the place and response learning tasks in resolving problems in learning theory, as well as for examining the behavioral and neurobiological mechanisms of multiple memory systems. How the place and response learning tasks may be employed in the future for examining extinction, neural circuits of memory, and human psychopathology is also briefly considered.

Long-Term Impacts of Post-weaning Social Isolation on Nucleus Accumbens Function

Adolescence is a period of incredible change, especially within the brain's reward circuitry. Stress, including social isolation, during this time has profound effects on behaviors associated with reward and other neuropsychiatric disorders. Because the Nucleus Accumbens (NAc), is crucial to the integration of rewarding stimuli, the NAc is especially sensitive to disruptions by adolescent social isolation stress. This review highlights the long-term behavioral consequences of adolescent social isolation rearing on the NAc. It will discuss the cellular and molecular changes within the NAc that might underlie the long-term effects on behavior. When available sex-specific effects are discussed. Finally by mining publicly available data we identify, for the first time, key transcriptional profiles induced by adolescence social isolation in genes associated with dopamine receptor 1 and 2 medium spiny neurons and genes associated with cocaine self-administration. Together, this review provides a comprehensive discussion of the wide-ranging long-term impacts of adolescent social isolation on the dopaminergic system from molecules through behavior.

[Neurobiological effects of childhood abuse and neglect as a predisposing factor to substance use disorder]

The article presents data from research literature on the neurobiological consequences of abuse and neglect in childhood. It is shown that early life stress affects the trajectory of brain development, changes the patterns of cognitive behavioral control and reception of rewards, potentially increasing the risk of substance misuse. The combination of this factor with an immature prefrontal cortex can dramatically increase the liability to the development of substance dependence.

Striatopallidal adenosine A2A receptors in the nucleus accumbens confer motivational control of goal-directed behavior

The ability to learn the reward-value and action-outcome contingencies in dynamic environment is critical for flexible adaptive behavior and development of effective pharmacological control of goal-directed behaviors represents an important challenge for improving the deficits in goal-directed behavior which may underlie seemingly disparate symptoms across psychiatric disorders. Adenosine A_2A receptor (A_2AR) is emerging as a novel neuromodulatory target for controlling goal-directed behavior for its unique neuromodulatory features: the ability to integrate dopamine and glutamate signaling, the "brake" constraint of various cognitive processes and the balanced control of goal-directed and habit actions. However, the contribution and circuit mechanisms of the striatopallidal A_2ARs in nucleus accumbens (NAc) to control of goal-directed behavior remain to be determined. Here, we employed newly developed opto-A_2AR and the focal A_2AR knockdown strategies to demonstrate the causal role of NAc A_2AR in control of goal-directed behavior. Furthermore, we dissected out multiple distinct behavioral mechanisms underlying which NAc A_2ARs control goal-directed behavior: (i) NAc A_2ARs preferentially control goal-directed behavior at the expense of habit formation. (ii) NAc A_2ARs modify the animals' sensitivity to the value of the reward without affecting the action-outcome contingency. (iii) A_2AR antagonist KW6002 promotes instrumental actions by invigorating motivation. (iv) NAc A_2ARs facilitate Pavlovian incentive value transferring to instrumental action. (v) NAc A_2ARs control goal-directed behavior probably not through NAc-VP pathway. These insights into the behavioral and circuit mechanisms for NAc A_2AR control of goal-directed behavior facilitate translational potential for A_2AR antagonists in reversal of deficits in goal-directed decision-making associated with multiple neuropsychiatric disorders.

Threat-induced modulation of hippocampal and striatal memory systems during navigation of a virtual environment

The brain is composed of multiple memory systems that mediate distinct types of navigation. The hippocampus is important for encoding and retrieving allocentric spatial cognitive maps, while the dorsal striatum mediates procedural memories based on stimulus-response (S-R) associations. These memory systems are differentially affected by emotional arousal. In particular, rodent studies show that stress typically impairs hippocampal spatial memory while it spares or sometimes enhances striatal S-R memory. The influence of emotional arousal on these separate navigational memory systems has received less attention in human subjects. We investigated the effect of dynamic changes in anticipatory anxiety on hippocampal spatial and dorsal striatal S-R memory systems while participants attempted to solve a virtual eight-arm radial maze. In Experiment 1, participants completed a hippocampus-dependent spatial version of the eight-arm radial maze that required allocentric spatial memory to successfully navigate the environment. In Experiment 2, participants completed a dorsal striatal S-R version of the maze where no allocentric spatial cues were present, requiring the use of S-R navigation. Anticipatory anxiety was modulated via threat of receiving an unpleasant electrical shock to the wrist during memory retrieval. Results showed that threat of shock was associated with more errors and increased use of non-spatial navigational strategies in the hippocampal spatial task, but did not influence memory performance in the striatal S-R task. Findings indicate a dissociation regarding the influence of anticipatory anxiety on memory systems that has implications for understanding how fear and anxiety contribute to memory-related symptoms in human psychopathologies.

A potential role for microglia in stress- and drug-induced plasticity in the nucleus accumbens: A mechanism for stress-induced vulnerability to substance use disorder

Stress is an important risk factor for the development of substance use disorder (SUD). Exposure to both stress and drugs abuse lead to changes in synaptic plasticity and stress-induced alterations in synaptic plasticity may contribute to later vulnerability to SUD. Recent developmental neuroscience studies have identified microglia as regulators of synaptic plasticity. As both stress and drugs of abuse lead to microglial activation, we propose this as a potential mechanism underlying their ability to change synaptic plasticity. This review focuses on three components of synaptic plasticity: spine density, brain-derived neurotrophic factor (BDNF) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor expression. Their roles in addiction, stress, and development will be reviewed, as well as possible mechanisms by which microglia could regulate their function. Potential links between stress, vulnerability to addiction, and microglial activity will be explored.

Dopamine signals related to appetitive and aversive events in paradigms that manipulate reward and avoidability

Using environmental cues to acquire good and avoid harmful things is critical for survival. Rewarding and aversive outcomes both drive behavior through reinforcement learning and sometimes occur together in the environment, but it remains unclear how these signals are encoded within the brain and if signals for positive and negative reinforcement are encoded similarly. Recent studies demonstrate that the dopaminergic system and interconnected brain regions process both positive and negative reinforcement necessary for approach and avoidance behaviors, respectively. Here, we review these data with a special focus on behavioral paradigms that manipulate both expected reward and the avoidability of aversive events to reveal neural correlates related to value, prediction error encoding, motivation, and salience.

Voluntary Wheel Running: A Useful Rodent Model for Investigating the Mechanisms of Stress Robustness and Neural Circuits of Exercise Motivation

Despite evidence that exercise reduces the negative impacts of stressor exposure and promotes stress robustness, health and well-being, most people fail to achieve recommended levels of physical activity. One reason for this failure could be our fundamental lack of understanding the brain motivational and motor circuits underlying voluntary exercise behavior. Wheel running is an animal model used to reveal mechanisms of exercise-induced stress robustness. Here we detail the strengths and weakness of wheel running as a model; and propose that running begins as a purposeful, goal-directed behavior that becomes habitual with continued access. This fresh perspective could aid in the development of novel strategies to motivate and sustain exercise behavior and maximize the stress-robust phenotype.

CRMP2 mediates GSK3β actions in the striatum on regulating neuronal structure and mania-like behavior

BACKGROUND

Genetic and physiological studies have implicated the striatum in bipolar disorder (BD). Although Glycogen synthase kinase 3 beta (GSK3β) has been suggested to play a role in the pathophysiology of BD since it is inhibited by lithium, it remains unknown how GSK3β activity might be involved. Therefore we examined the functional roles of GSK3β and one of its substrates, CRMP2, within the striatum.

METHODS

Using CRISPR-Cas9 system, we specifically ablated GSK3β in the striatal neurons in vivo and in vitro. Sholl analysis was performed for the structural studies of medium spiny neurons (MSNs) and amphetamine-induced hyperlocomotion was measured to investigate the effects of gene ablations on the mania-like symptom of BD.

RESULTS

GSK3β deficiency in cultured neurons and in neurons of adult mouse brain caused opposite patterns of neurite changes. Furthermore, specific knockout of GSK3β in the MSNs of the indirect pathway significantly suppressed amphetamine-induced hyperlocomotion. We demonstrated that these phenotypes of GSK3β ablation were mediated by CRMP2, a major substrate of GSK3β.

LIMITATIONS

Amphetamine-induced hyperlocomotion only partially recapitulate the symptoms of BD. It requires further study to examine whether abnormality in GSK3β or CRMP2 is also involved in depression phase of BD. Additionally, we could not confirm whether the behavioral changes observed in GSK3β-ablated mice were indeed caused by the cellular structural changes observed in the striatal neurons.

CONCLUSION

Our results demonstrate that GSK3β and its substrate CRMP2 critically regulate the neurite structure of MSNs and their functions specifically within the indirect pathway of the basal ganglia network play a critical role in manifesting mania-like behavior of BD. Moreover, our data also suggest lithium may exert its effect on BD through a GSK3β-independent mechanism, in addition to the GSK3β inhibition-mediated mechanism.

Chronic stress increases susceptibility to food addiction by increasing the levels of DR2 and MOR in the nucleus accumbens

Background: Stress-related obesity might be related to the suppression of the hypothalamic-pituitary- adrenocortical axis and dysregulation of the metabolic system. Chronic stress also induces the dysregulation of the reward system and increases the risk of food addiction, according to recent clinical findings. However, few studies have tested the effect of chronic stress on food addiction in animal models. Purpose: The objective of this study was to identify whether chronic stress promotes food addiction or not and explore the possible mechanisms. Method: We applied adaily 2 hrsflashing LED irradiation stress to mice fed chow or palatable food to mimic the effect of chronic stress on feeding. After 1 month of chronic stress exposure, we tested their binge eating behaviors, cravings for palatable food, responses for palatable food, and compulsive eating behaviors to evaluate the effect of chronic stress on food addiction-like behaviors. We detected changes in the levels of various genes and proteins in the nucleus accumbens (NAc), ventral tegmental area (VTA) and lateral hypothalamus using qPCR and immunofluorescence staining, respectively. Results: Behaviors results indicated chronic stress obviously increased food addiction score (FAS) in the palatable food feeding mice. Moreover, the FAS had astrong relationship with the extent of the increase in body weight. Chronic stress increased the expression of corticotropin-releasing factor receptor 1(CRFR1) was increased in the NAc shell and core but decreased in the VTA of the mice fed with palatable food. Chronic stress also increased expression of both dopamine receptor 2 (DR2) and mu-opioid receptor (MOR) in the NAc. Conclusion: Chronic stress aggravates the FAS and contributed to the development of stress-related obesity. Chronic stress drives the dysregulation of the CRF signaling pathway in the reward system and increases the expression of DR2 and MOR in the nucleus accumbens.

Behavioral and neural mechanisms underlying habitual and compulsive drug seeking

Addiction is characterized by compulsive drug use despite negative consequences. Here we review studies that indicate that compulsive drug use, and in particular punishment resistance in animal models of addiction, is related to impaired cortical control over habitual behavior. In humans and animals, instrumental behavior is supported by goal-directed and habitual systems that rely on distinct corticostriatal networks. Chronic exposure to addictive drugs or stress has been shown to bias instrumental response strategies toward habit learning, and impair prefrontal cortical (PFC) control over responding. Moreover, recent work has implicated prelimbic PFC hypofunction in the punishment resistance that has been observed in a subset of animals with an extended history of cocaine self-administration. This may be related to a broader role for prelimbic PFC in mediating adaptive responding and behavioral flexibility, including exerting goal-directed control over behavior. We hypothesize that impaired cortical control and reduced flexibility between habitual and goal-directed systems may be critically involved in the development of maladaptive, compulsive drug use.

Transient Prepubertal Mifepristone Treatment Normalizes Deficits in Contextual Memory and Neuronal Activity of Adult Male Rats Exposed to Maternal Deprivation

Early life adversity is a well-known risk factor for behavioral dysfunction later in life, including the formation of contextual memory; it is also (transiently) accompanied by hyperactivity of the stress system. We tested whether mifepristone (MIF) treatment, which among other things blocks glucocorticoid receptors (GRs), during the prepubertal period [postnatal days (PND)26-PND28] normalizes memory deficits in adult male rats exposed to 24-h maternal deprivation (MD) at PND3. MD reduced body weight gain and increased basal corticosterone (CORT) levels during the PND26, but not in adulthood. In adulthood, contextual memory formation of MD compared to noMD (i.e., control) male rats was significantly impaired. This impairment was fully prevented by MIF treatment at PND26-PND28, whereas MIF by itself did not affect behavior. A second behavioral test, a rodent version of the Iowa Gambling Task (rIGT), revealed that flexible spatial learning rather than reward-based aspects of performance was impaired by MD; the deficit was prevented by MIF. Neuronal activity as tested by c-Fos staining in the latter task revealed changes in the right hippocampal-dorsomedial striatal pathway, but not in prefrontal areas involved in reward learning. Follow-up electrophysiological recordings measuring spontaneous glutamate transmission showed reduced frequency of miniature postsynaptic excitatory currents in adult CA1 dorsal hippocampal and enhanced frequency in dorsomedial striatal neurons from MD versus noMD rats, which was not seen in MIF-treated rats. We conclude that transient prepubertal MIF treatment normalizes hippocampus-striatal-dependent contextual memory/spatial learning deficits in male rats exposed to early life adversity, possibly by normalizing glutamatergic transmission.

Impaired Latent Inhibition in GDNF-Deficient Mice Exposed to Chronic Stress

Increased reactivity to stress is maladaptive and linked to abnormal behaviors and psychopathology. Chronic unpredictable stress (CUS) alters catecholaminergic neurotransmission and remodels neuronal circuits involved in learning, attention and decision making. Glial-derived neurotrophic factor (GDNF) is essential for the physiology and survival of dopaminergic neurons in substantia nigra and of noradrenergic neurons in the locus coeruleus. Up-regulation of GDNF expression during stress is linked to resilience; on the other hand, the inability to up-regulate GDNF in response to stress, as a result of either genetic or epigenetic modifications, induces behavioral alterations. For example, GDNF-deficient mice exposed to chronic stress exhibit alterations of executive function, such as increased temporal discounting. Here we investigated the effects of CUS on latent inhibition (LI), a measure of selective attention and learning, in GDNF-heterozygous (HET) mice and their wild-type (WT) littermate controls. No differences in LI were found between GDNF HET and WT mice under baseline experimental conditions. However, following CUS, GDNF-deficient mice failed to express LI. Moreover, stressed GDNF-HET mice, but not their WT controls, showed decreased neuronal activation (number of c-Fos positive neurons) in the nucleus accumbens shell and increased activation in the nucleus accumbens core, both key regions in the expression of LI. Our results add LI to the list of behaviors affected by chronic stress and support a role for GDNF deficits in stress-induced pathological behaviors relevant to schizophrenia and other psychiatric disorders.

Impaired Spatial Memory and Enhanced Habit Memory in a Rat Model of Post-traumatic Stress Disorder

High levels of emotional arousal can impair spatial memory mediated by the hippocampus, and enhance stimulus-response (S-R) habit memory mediated by the dorsolateral striatum (DLS). The present study was conducted to determine whether these memory systems may be similarly affected in an animal model of post-traumatic stress disorder (PTSD). Sprague-Dawley rats were subjected to a “single-prolonged stress” (SPS) procedure and 1 week later received training in one of two distinct versions of the plus-maze: a hippocampus-dependent place learning task or a DLS-dependent response learning task. Results indicated that, relative to non-stressed control rats, SPS rats displayed slower acquisition in the place learning task and faster acquisition in the response learning task. In addition, extinction of place learning and response learning was impaired in rats exposed to SPS, relative to non-stressed controls. The influence of SPS on hippocampal spatial memory and DLS habit memory observed in the present study may be relevant to understanding some common features of PTSD, including hippocampal memory deficits, habit-like avoidance responses to trauma-related stimuli, and greater likelihood of developing drug addiction and alcoholism.

Nicotine-induced and D1-receptor-dependent dendritic remodeling in a subset of dorsolateral striatum medium spiny neurons

Nicotine is one of the most addictive substances known, targeting multiple memory systems, including the ventral and dorsal striatum. One form of neuroplasticity commonly associated with nicotine is dendrite remodeling. Nicotine-induced dendritic remodeling of ventral striatal medium spiny neurons (MSNs) is well-documented. Whether MSN dendrites in the dorsal striatum undergo a similar pattern of nicotine-induced structural remodeling is unknown. A morphometric analysis of Golgi-stained MSNs in rat revealed a natural asymmetry in dendritic morphology across the mediolateral axis, with larger, more complex MSNs found in the dorsolateral striatum (DLS). Chronic nicotine produced a lasting (at least 21day) expansion in the dendritic complexity of MSNs in the DLS, but not dorsomedial striatum (DMS). Given prior evidence that MSN subtypes can be distinguished based on dendritic morphology, MSNs were segregated into morphological subpopulations based on the number of primary dendrites. Analysis of these subpopulations revealed that DLS MSNs with more primary dendrites were selectively remodeled by chronic nicotine exposure and remodeling was specific to the distal-most portions of the dendritic arbor. Co-administration of the dopamine D1 receptor (D1R) antagonist SCH23390 completely reversed the selective effects of nicotine on DLS MSN dendrite morphology, supporting a causal role for dopamine signaling at D1 receptors in nicotine-induced dendrite restructuring. Considering the functional importance of the DLS in shaping and expressing habitual behavior, these data support a model in which nicotine induces persistent and selective changes in the circuit connectivity of the DLS that may promote and sustain addiction-related behavior.

Glucocorticoid administration into the dorsolateral but not dorsomedial striatum accelerates the shift from a spatial toward procedural memory

Glucocorticoid stress hormones are known to enhance the consolidation of hippocampus-dependent spatial and contextual memory. Recent findings indicate that glucocorticoids also enhance the consolidation of procedural memory that relies on the dorsal striatum. The dorsal striatum can be functionally subdivided into the dorsolateral striatum (DLS), which is primarily implicated in shaping procedural memories, and the dorsomedial striatum (DMS), which is engaged in spatial memory. Here, we investigated the hypothesis that posttraining glucocorticoid administration into the DLS promotes the formation of a procedural memory that will normally take place only with extensive training. Male Wistar rats were trained to find a reward in a cross maze that can be solved through either place or response learning. Rats received four trials per day for 5days, a probe trial on Day 6, further training on Days 7-13, and an additional probe trial on Day 14. On Days 2-4 of training, they received posttraining infusions of corticosterone (10 or 30ng) or vehicle into either the DLS or DMS. Rats treated with vehicle into either the DLS or DMS displayed place learning on Day 6 and response learning on Day 14, indicating a shift in control of learned behavior toward a habit-like procedural strategy with extended training. Rats administered corticosterone (10ng) into the DLS displayed response learning on both Days 6 and 14, indicating an accelerated shift to response learning. In contrast, corticosterone administered posttraining into the DMS did not significantly alter the shift from place to response learning. These findings indicate that glucocorticoid administration into the DLS enhances memory consolidation of procedural learning and thereby influences the timing of the switch from the use of spatial/contextual memory to habit-like procedural memory to guide behavior.

Low lifetime stress exposure is associated with reduced stimulus-response memory

Exposure to stress throughout life can cumulatively influence later health, even among young adults. The negative effects of high cumulative stress exposure are well-known, and a shift from episodic to stimulus–response memory has been proposed to underlie forms of psychopathology that are related to high lifetime stress. At the other extreme, effects of very low stress exposure are mixed, with some studies reporting that low stress leads to better outcomes, while others demonstrate that low stress is associated with diminished resilience and negative outcomes. However, the influence of very low lifetime stress exposure on episodic and stimulus–response memory is unknown. Here we use a lifetime stress assessment system (STRAIN) to assess cumulative lifetime stress exposure and measure memory performance in young adults reporting very low and moderate levels of lifetime stress exposure. Relative to moderate levels of stress, very low levels of lifetime stress were associated with reduced use and retention (24 h later) of stimulus–response (SR) associations, and a higher likelihood of using context memory. Further, computational modeling revealed that participants with low levels of stress exhibited worse expression of memory for SR associations than those with moderate stress. These results demonstrate that very low levels of stress exposure can have negative effects on cognition.

Histone Lysine Demethylases of JMJD2 or KDM4 Family are Important Epigenetic Regulators in Reward Circuitry in the Etiopathology of Depression

Major depressive disorder (MDD) is debilitating mental illness and is one of the leading contributors to global burden of disease, but unfortunately newer and better drugs are not forthcoming. The reason is lack of complete understanding of molecular mechanisms underlying the development of this disorder. Recent research shows dysregulation in epigenetic regulatory mechanisms, particularly the transcriptionally repressive di- and tri-methylation of histone 3 lysine 9 (H3K9me2/me3) in nucleus accumbens (NAc), a critical region of the reward pathway involved in the development of anhedonia, the hallmark of depression. However, the role of histone lysine demethylases, which can remove methylation from H3K9, in particular Jumonji domain containing demethylases 2 or Jmjd2 family, has not been studied. Using social defeat stress-induced mouse model of depression, this study uncovered that transcripts of most of the Jmjd2 members were unchanged after 5 days of defeat during the onset of depression, but were downregulated after 10 days of defeat in full-blown depression. Blocking the Jumonji domain containing demethylases by chronic administration of inhibitors dimethyloxalylglycine (DMOG) and ML324 resulted in depression-like phenotype even in absence of stress exposure, which was associated with an increase in transcriptionally repressive epigenetic marks H3K9me2/me3 in NAc, causing altered neuroplastic changes as reported in NAc in depression models. Thus, we report for the first time that Jmjd2 class demethylases are critical epigenetic regulators involved in etiopathology of depression and related disorders and activation of these demethylases can be a good strategy in the treatment of MDD and related psychiatric disorders.

Sensitive periods of substance abuse: Early risk for the transition to dependence

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Early substance use dramatically increases the risk of substance use disorder (SUD).

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Although many try drugs, only a small percentage transition to SUD.

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High reactivity of reward, habit, and stress systems increase risk.

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Identification of early risk enables targeted, preventative interventions for SUD.

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Prevention must start before the sensitive adolescent period to maximize resilience.

Early adolescent substance use dramatically increases the risk of lifelong substance use disorder (SUD). An adolescent sensitive period evolved to allow the development of risk-taking traits that aid in survival; today these may manifest as a vulnerability to drugs of abuse. Early substance use interferes with ongoing neurodevelopment to induce neurobiological changes that further augment SUD risk. Although many individuals use drugs recreationally, only a small percentage transition to SUD. Current theories on the etiology of addiction can lend insights into the risk factors that increase vulnerability from early recreational use to addiction. Building on the work of others, we suggest individual risk for SUD emerges from an immature PFC combined with hyper-reactivity of reward salience, habit, and stress systems. Early identification of risk factors is critical to reducing the occurrence of SUD. We suggest preventative interventions for SUD that can be either tailored to individual risk profiles and/or implemented broadly, prior to the sensitive adolescent period, to maximize resilience to developing substance dependence. Recommendations for future research include a focus on the juvenile and adolescent periods as well as on sex differences to better understand early risk and identify the most efficacious preventions for SUD.

Phasic dopamine release in the rat nucleus accumbens predicts approach and avoidance performance

Dopamine (DA) is critical for reward processing, but significantly less is known about its role in punishment avoidance. Using a combined approach-avoidance task, we measured phasic DA release in the nucleus accumbens (NAc) of rats during presentation of cues that predicted reward, punishment or neutral outcomes and investigated individual differences based on avoidance performance. Here we show that DA release within a single microenvironment is higher for reward and avoidance cues compared with neutral cues and positively correlated with poor avoidance behaviour. We found that DA release delineates trial-type during sessions with good avoidance but is non-selective during poor avoidance, with high release correlating with poor performance. These data demonstrate that phasic DA is released during cued approach and avoidance within the same microenvironment and abnormal processing of value signals is correlated with poor performance.

Early and Persistent Dendritic Hypertrophy in the Basolateral Amygdala following Experimental Diffuse Traumatic Brain Injury

In the pathophysiology of traumatic brain injury (TBI), the amygdala remains understudied, despite involvement in processing emotional and stressful stimuli associated with anxiety disorders, such as post-traumatic stress disorder (PTSD). Because the basolateral amygdala (BLA) integrates inputs from sensory and other limbic structures coordinating emotional learning and memory, injury-induced changes in circuitry may contribute to psychiatric sequelae of TBI. This study quantified temporal changes in dendritic complexity of BLA neurons after experimental diffuse TBI, modeled by midline fluid percussion injury. At post-injury days (PIDs) 1, 7, and 28, brain tissue from sham and brain-injured adult, male rats was processed for Golgi, glial fibrillary acidic protein (GFAP), or silver stain and analyzed to quantify BLA dendritic branch intersections, activated astrocytes, and regional neuropathology, respectively. Compared to sham, brain-injured rats at all PIDs showed enhanced dendritic branch intersections in both pyramidal and stellate BLA neuronal types, as evidenced by Sholl analysis. GFAP staining in the BLA was significantly increased at PID1 and 7 in comparison to sham. However, the BLA was relatively spared from neuropathology, demonstrated by an absence of argyrophilic accumulation over time, in contrast to other brain regions. These data suggest an early and persistent enhancement of dendritic complexity within the BLA after a single diffuse TBI. Increased dendritic complexity would alter information processing into and through the amygdala, contributing to emotional symptoms post-TBI, including PTSD.

Psychological and Neurobiological Correlates of Food Addiction

Food addiction (FA) is loosely defined as hedonic eating behavior involving the consumption of highly palatable foods (ie, foods high in salt, fat, and sugar) in quantities beyond homeostatic energy requirements. FA shares some common symptomology with other pathological eating disorders, such as binge eating. Current theories suggest that FA shares both behavioral similarities and overlapping neural correlates to other substance addictions. Although preliminary, neuroimaging studies in response to food cues and the consumption of highly palatable food in individuals with FA compared to healthy controls have shown differing activation patterns and connectivity in brain reward circuits including regions such as the striatum, amygdala, orbitofrontal cortex, insula, and nucleus accumbens. Additional effects have been noted in the hypothalamus, a brain area responsible for regulating eating behaviors and peripheral satiety networks. FA is highly impacted by impulsivity and mood. Chronic stress can negatively affect hypothalamic-pituitary-adrenal axis functioning, thus influencing eating behavior and increasing desirability of highly palatable foods. Future work will require clearly defining FA as a distinct diagnosis from other eating disorders.

Differential Gambling Motivations and Recreational Activity Preferences Among Casino Gamblers

This study investigated three different types of gamblers (recreational, problem, and pathological gamblers) to determine differences in gambling motivations and recreational activity preferences among casino gamblers. We collected data from 600 gamblers recruited in an actual gambling environment inside a major casino in South Korea. Findings indicate that motivational factors of escape, sightseeing, and winning were significantly different among these three types of gamblers. When looking at motivations to visit the casino, pathological gamblers were more likely to be motivated by winning, whereas recreational gamblers were more likely to be motivated by scenery and culture in the surrounding casino area. Meanwhile, the problem gamblers fell between these two groups, indicating higher preferences for non-gambling activities than the pathological gamblers. As this study builds upon a foundational previous study by Lee et al. (Psychiatry Investig 6(3):141-149, 2009), the results of this new study were compared with those of the previous study to see if new developments within a resort-style casino contribute to changes in motivations and recreational activity preferences.

Depression and substance use comorbidity: What we have learned from animal studies

Depression and substance use disorders are often comorbid, but the reasons for this are unclear. In human studies, it is difficult to determine how one disorder may affect predisposition to the other and what the underlying mechanisms might be. Instead, animal studies allow experimental induction of behaviors relevant to depression and drug-taking, and permit direct interrogation of changes to neural circuits and molecular pathways. While this field is still new, here we review animal studies that investigate whether depression-like states increase vulnerability to drug-taking behaviors. Since chronic psychosocial stress can precipitate or predispose to depression in humans, we review studies that use psychosocial stressors to produce depression-like phenotypes in animals. Specifically, we describe how postweaning isolation stress, repeated social defeat stress, and chronic mild (or unpredictable) stress affect behaviors relevant to substance abuse, especially operant self-administration. Potential brain changes mediating these effects are also discussed where available, with an emphasis on mesocorticolimbic dopamine circuits. Postweaning isolation stress and repeated social defeat generally increase acquisition or maintenance of drug self-administration, and alter dopamine sensitivity in various brain regions. However, the effects of chronic mild stress on drug-taking have been much less studied. Future studies should consider standardizing stress-induction protocols, including female subjects, and using multi-hit models (e.g. genetic vulnerabilities and environmental stress).

Chronic variable stress and intravenous methamphetamine self-administration - Role of individual differences in behavioral and physiological reactivity to novelty

Stress is a contributing factor to the development and maintenance of addiction in humans. However, few studies have shown that stress potentiates the rewarding and/or reinforcing effects of methamphetamine in rodent models of addiction. The present study assessed the effects of exposure to 14 days of chronic variable stress (CVS), or no stress as a control (CON), on the rewarding and reinforcing effects of methamphetamine in adult rats using the conditioned place preference (Experiment 1) and intravenous self-administration (Experiment 2) paradigms. In Experiment 2, we also assessed individual differences in open field locomotor activity, anxiety-like behavior in the elevated plus maze (EPM), and physiological responses to a novel environment as possible predictors of methamphetamine intake patterns. Exposure to CVS for 14 days did not affect overall measures of methamphetamine conditioned reward or reinforcement. However, analyses of individual differences and direct vs. indirect effects revealed that rats exhibiting high physiological reactivity and locomotor activity in the EPM and open field tests self-administered more methamphetamine and reached higher breakpoints for drug reinforcement than rats exhibiting low reactivity. In addition, CVS exposure significantly increased the proportion of rats that exhibited high reactivity, and high reactivity was significantly correlated with increased levels of methamphetamine intake. These findings suggest that individual differences in physiological and locomotor reactivity to novel environments, as well as their interactions with stress history, predict patterns of drug intake in rodent models of methamphetamine addiction. Such predictors may eventually inform future strategies for implementing individualized treatment strategies for amphetamine use disorders.