Distinct patterns of outcome valuation and amygdala-prefrontal cortex synaptic remodeling in adolescence and adulthood

Reward-predictive cues elicit excessive reward seeking in adolescent rats

Impulsive behavior during adolescence may stem from developmental imbalances between motivational and cognitive-control systems, producing greater urges to pursue reward and weakened capacities to inhibit such actions. Here, we developed a Pavlovian-instrumental transfer (PIT) protocol to assay rats' ability to suppress cue-motivated reward seeking based on changes in reward expectancy. Traditionally, PIT studies focus on how reward-predictive cues motivate instrumental reward-seeking behavior (lever pressing). However, cues signaling imminent reward delivery also elicit countervailing focal-search responses (food-port entry). We first examined how reward expectancy (cue-reward probability) influences expression of these competing behaviors. Adult male rats increased rates of lever pressing when presented with cues signaling lower probabilities of reward but focused their activity at the food cup on trials with cues that signaled higher probabilities of reward. We then compared adolescent and adult male rats in their responsivity to cues signaling different reward probabilities. In contrast to adults, adolescent rats did not flexibly adjust patterns of responding based on the expected likelihood of reward delivery but increased their rate of lever pressing for both weak and strong cues. These findings indicate that control over cue-motivated behavior is fundamentally dysregulated during adolescence, providing a model for studying neurobiological mechanisms of adolescent impulsivity.

Adolescent-onset vs. adult-onset cocaine use: Impact on cognitive functioning in animal models and opportunities for translation

Animal models are poised to make key contributions to the study of cognitive deficits associated with chronic cocaine use in people. Advantages of animal models include use of a longitudinal experimental design that can control for drug use history and onset-age, sex, drug consumption, and abstinence duration. Twenty-two studies were reviewed (13 in adult male rats, 5 in adolescent vs. adult male rats, 3 in adult male monkeys, and 1 in adult female monkeys), and it was demonstrated repeatedly that male animals with adult-onset cocaine self-administration exposure had impairments in sustained attention, decision making, stimulus-reward learning, working memory, and cognitive flexibility, but not habit learning and spatial learning and memory. These findings have translational relevance because adult cocaine users exhibit a similar range of cognitive deficits. In the limited number of studies available, male rats self-administering cocaine during adolescence were less susceptible than adults to impairment in cognitive flexibility, stimulus-reward learning, and decision making, but were more susceptible than adults to impairment in working memory, a finding also reported in the few studies performed in early-onset cocaine users. These findings suggest that animal models can help fill an unmet need for investigating important but yet-to-be-fully-addressed research questions in people. Research priorities include further investigation of differences between adolescents and adults as well as between males and females following chronic cocaine self-administration. A comprehensive understanding of the broad range of cognitive consequences of chronic cocaine use and the role of developmental plasticity can be of value for improving neuropsychological recovery efforts.

Sex-dependent effects of chronic intermittent voluntary alcohol consumption on attentional, not motivational, measures during probabilistic learning and reversal

Background

Forced alcohol (ethanol, EtOH) exposure has been shown to cause significant impairments on reversal learning, a widely-used assay of cognitive flexibility, specifically on fully-predictive, deterministic versions of this task. However, previous studies have not adequately considered voluntary EtOH consumption and sex effects on probabilistic reversal learning. The present study aimed to fill this gap in the literature.

Methods

Male and female Long-Evans rats underwent either 10 weeks of voluntary intermittent 20% EtOH access or water only (H2O) access. Rats were then pretrained to initiate trials and learn stimulus-reward associations via touchscreen response, and subsequently required to select between two visual stimuli, rewarded with probability 0.70 or 0.30. In the final phase, reinforcement contingencies were reversed.

Results

We found significant sex differences on several EtOH-drinking variables, with females reaching a higher maximum EtOH consumption, exhibiting more high-drinking days, and escalating their EtOH at a quicker rate compared to males. During early abstinence, EtOH drinkers (and particularly EtOH-drinking females) made more initiation omissions and were slower to initiate trials than H2O drinking controls, especially during pretraining. A similar pattern in trial initiations was also observed in discrimination, but not in reversal learning. EtOH drinking rats were unaffected in their reward collection and stimulus response times, indicating intact motivation and motor responding. Although there were sex differences in discrimination and reversal phases, performance improved over time. We also observed sex-independent drinking group differences in win-stay and lose-shift strategies specific to the reversal phase.

Conclusions

Females exhibit increased vulnerability to EtOH effects in early learning: there were sex-dependent EtOH effects on attentional measures during pretraining and discrimination phases. We also found sex-independent EtOH effects on exploration strategies during reversal. Future studies should aim to uncover the neural mechanisms for changes in attention and exploration in both acute and prolonged EtOH withdrawal.

Rodent Models of Adaptive Value Learning and Decision-Making

Real-world decisions are rarely as straightforward as choosing between clearly "good" vs. "bad" options. More often, options must be evaluated carefully because they differ in relative value. For example, we typically learn about (and make decisions between) options in comparison, where one outcome may be more costly or risky than the other. Several neuropsychiatric conditions are characterized by atypical evaluation of effort and risk costs, including major depression, schizophrenia, autism, obsessive-compulsive disorder, and substance use disorders. Aberrant value learning and decision-making have long been considered a cognitive-behavioral endophenotype of these disorders and can be modeled in rodents. This chapter presents two general methodological domains that the experimenter can manipulate in animal decision-making tasks: risk and effort. Here, we present detailed methods of rodent tasks frequently employed within these domains: probabilistic reversal learning (PRL) and effort choice. These tasks recruit regions within rodent frontal cortex, the amygdala, and the striatum, and performance is heavily modulated by dopamine, making these assays highly valid measures in the study of behavioral and substance addictions, in particular.

Increased sign-tracking behavior in adolescent rats

An autoshaping procedure was used to test the notion that conditioned stimuli (CSs) gain greater incentive salience during adolescence than young adulthood under conditions of social isolation rearing and food restriction. Rats were single-housed and placed on food restriction during 10 daily training sessions in which a lever (CS⁺ ) was presented then followed immediately by a food unconditioned stimulus (US). A second lever (CS^- ) was presented on intermixed trials and was not reinforced. Despite the fact that food delivery was not contingent on the rats' behavior, all rats exhibited behaviors directed towards the lever (i.e., sign-tracking). In the adolescent group, the rate of lever pressing and the percentage of trials with a lever press were higher than in young adults. Initially, group differences were observed when rats were retrained when the adolescents had reached young adulthood. These findings support the hypothesis that cues that come to predict reward become imbued with excessive motivational value in adolescents, perhaps contributing to the hyper-responsiveness to reward-related stimuli typically observed during this period of development.

Sex-specific enhancement of palatability-driven feeding in adolescent rats

It has been hypothesized that brain development during adolescence perturbs reward processing in a way that may ultimately contribute to the risky decision making associated with this stage of life, particularly in young males. To investigate potential reward dysfunction during adolescence, Experiment 1 examined palatable fluid intake in rats as a function of age and sex. During a series of twice-weekly test sessions, non-food-deprived rats were given the opportunity to voluntarily consume a highly palatable sweetened condensed milk (SCM) solution. We found that adolescent male, but not female, rats exhibited a pronounced, transient increase in SCM intake (normalized by body weight) that was centered around puberty. Additionally, adult females consumed more SCM than adult males and adolescent females. Using a well-established analytical framework to parse the influences of reward palatability and satiety on the temporal structure of feeding behavior, we found that palatability-driven intake at the outset of the meal was significantly elevated in adolescent males, relative to the other groups. Furthermore, although we found that there were some group differences in the onset of satiety, they were unlikely to contribute to differences in intake. Experiment 2 confirmed that adolescent male rats exhibit elevated palatable fluid consumption, relative to adult males, even when a non-caloric saccharin solution was used as the taste stimulus, demonstrating that these results were unlikely to be related to age-related differences in metabolic need. These findings suggest that elevated palatable food intake during adolescence is sex specific and driven by a fundamental change in reward processing. As adolescent risk taking has been hypothesized as a potential result of hypersensitivity to and overvaluation of appetitive stimuli, individual differences in reward palatability may factor into individual differences in adolescent risky decision making.

Rigid patterns of effortful choice behavior after acute stress in rats

Physical effort is a common cost of acquiring rewards, and decreased effort is a feature of many neuropsychiatric disorders. Stress affects performance on several tests of cognition and decision making in both humans and nonhumans. Only a few recent reports show impairing effects of stress in operant tasks involving effort and cognitive flexibility. Brain regions affected by stress, such as the medial prefrontal cortex and amygdala, are also implicated in mediating effortful choices. Here, we assessed effort-based decision making after an acute stress procedure known to induce persistent impairment in shuttle escape and elevated plasma corticosterone. In these animals, we also probed levels of polysialyted neural cell adhesion molecule (PSA-NCAM), a marker of structural plasticity, in medial frontal cortex and amygdala. We found that animals that consistently worked for high magnitude rewards continued to do so, even after acute shock stress. We also found that PSA-NCAM was increased in both regions after effortful choice experience but not after shock stress alone. These findings are discussed with reference to the existing broad literature on cognitive effects of stress and in the context of how acute stress may bias effortful decisions to a rigid pattern of responding.

microRNAs and the adolescent brain: Filling the knowledge gap

Over two decades ago the discovery of microRNAs (miRNA) broadened our understanding of the diverse molecular pathways mediating post-transcriptional control over gene expression. These small non-coding RNAs dynamically fluctuate, temporally and spatially, throughout the lifespan of all organisms. The fundamental role that miRNAs have in shaping embryonic neurodevelopment provides strong evidence that adolescent brain remodeling could be rooted in the changing miRNA landscape of the cell. Few studies have directly measured miRNA gene expression changes in the brain across pubertal development, and even less is known about the functional impact of those miRNAs on the maturational processes that occur in the developing adolescent brain. This review summarizes miRNA biogenesis and function in the brain in the context of normal (i.e. not diseased) physiology. These landmark studies can guide predictions about the role of miRNAs in facilitating maturation of the adolescent brain. However, there are clear indicators that adolescence/puberty is a unique life stage, suggesting miRNA function during adolescence is distinct from those in any other previously described system.

Reward-centricity and attenuated aversions: An adolescent phenotype emerging from studies in laboratory animals

Adolescence is an evolutionarily conserved developmental period, with neural circuits and behaviors contributing to the detection, procurement, and receipt of rewards bearing similarity across species. Studies with laboratory animals suggest that adolescence is typified by a "reward-centric" phenotype-an increased sensitivity to rewards relative to adults. In contrast, adolescent rodents are reportedly less sensitive to the aversive properties of many drugs and naturally aversive stimuli. Alterations within the mesocorticolimbic dopamine and endocannabinoid systems likely contribute to an adolescent reward-sensitive, yet aversion-resistant, phenotype. Although early hypotheses postulated that developmental changes in dopaminergic circuitry would result in a "reward deficiency" syndrome, evidence now suggests the opposite: that adolescents are uniquely poised to seek out hedonic stimuli, experience greater "pleasure" from rewards, and consume rewarding stimuli in excess. Future studies that more clearly define the role of specific brain regions and neurotransmitter systems in the expression of behaviors toward reward- and aversive-related cues and stimuli are necessary to more fully understand an adolescent-proclivity for and vulnerability to rewards and drugs of potential abuse.

Sex differences, learning flexibility, and striatal dopamine D1 and D2 following adolescent drug exposure in rats

Corticostriatal circuitry supports flexible reward learning and emotional behavior from the critical neurodevelopmental stage of adolescence through adulthood. It is still poorly understood how prescription drug exposure in adolescence may impact these outcomes in the long-term. We studied adolescent methylphenidate (MPH) and fluoxetine (FLX) exposure in rats and their impact on learning and emotion in adulthood. In Experiment 1, male and female rats were administered MPH, FLX, or saline (SAL), and compared with methamphetamine (mAMPH) treatment beginning in postnatal day (PND) 37. The rats were then tested on discrimination and reversal learning in adulthood. In Experiment 2, animals were administered MPH or SAL also beginning in PND 37 and later tested in adulthood for anxiety levels. In Experiment 3, we analyzed striatal dopamine D1 and D2 receptor expression in adulthood following either extensive learning (after Experiment 1) or more brief emotional measures (after Experiment 2). We found sex differences in discrimination learning and attenuated reversal learning after MPH and only sex differences in adulthood anxiety. In learners, there was enhanced striatal D1, but not D2, after either adolescent MPH or mAMPH. Lastly, also in learners, there was a sex x treatment group interaction for D2, but not D1, driven by the MPH-pretreated females, who expressed significantly higher D2 levels compared to SAL. These results show enduring effects of adolescent MPH on reversal learning in rats. Developmental psychostimulant exposure may interact with learning to enhance D1 expression in adulthood, and affect D2 expression in a sex-dependent manner.

Hijacking translation in addiction

Two studies suggest that the reduced activity of a translation initiation factor called eIF2α might be partly responsible for the increased risk of drug addiction seen in adolescents.

The basolateral amygdala in reward learning and addiction

Sophisticated behavioral paradigms partnered with the emergence of increasingly selective techniques to target the basolateral amygdala (BLA) have resulted in an enhanced understanding of the role of this nucleus in learning and using reward information. Due to the wide variety of behavioral approaches many questions remain on the circumscribed role of BLA in appetitive behavior. In this review, we integrate conclusions of BLA function in reward-related behavior using traditional interference techniques (lesion, pharmacological inactivation) with those using newer methodological approaches in experimental animals that allow in vivo manipulation of cell type-specific populations and neural recordings. Secondly, from a review of appetitive behavioral tasks in rodents and monkeys and recent computational models of reward procurement, we derive evidence for BLA as a neural integrator of reward value, history, and cost parameters. Taken together, BLA codes specific and temporally dynamic outcome representations in a distributed network to orchestrate adaptive responses. We provide evidence that experiences with opiates and psychostimulants alter these outcome representations in BLA, resulting in long-term modified action.