Autoshaping in adolescence enhances sign-tracking behavior in adulthood: impact on ethanol consumption

Propensity for risky choices despite lower cue reactivity in adolescent rats

Adolescence is a time of heightened risk-taking across species. Salient audiovisual cues associated with rewards are a common feature of gambling environments and have been connected to increased risky decision-making. We have previously shown that, in adult male rats, sign tracking - a behavioral measure of cue reactivity - predicts an individual's propensity for suboptimal risky choices in a rodent gambling task (rGT) with win-paired cues. However, adolescents perform less sign tracking than adult animals, suggesting that they are less cue-reactive than adults in some circumstances. Therefore, we investigated the performance of adolescent male rats on the rGT with win cues and examined its relationship with their sign-tracking behavior. We found that adolescents make more risky choices and fewer optimal choices on the rGT compared with adults, evidence of the validity of the rGT as a model of adolescent gambling behavior. We also confirmed that adolescents perform less sign tracking than adults, and we found that, unlike in adults, adolescents' sign tracking was unrelated to their risk-taking in the rGT. This implies that adolescent risk-taking is less likely than that of adults to be driven by reward-related cues. Finally, we found that adults trained on the rGT as adolescents retained an adolescent-like propensity toward risky choices, suggesting that early exposure to a gambling environment may have a long-lasting impact on risk-taking behavior.

Estradiol and progesterone in female reward-learning, addiction, and therapeutic interventions

Sex steroid hormones like estradiol (E2) and progesterone (P4) guide the sexual organization and activation of the developing brain and control female reproductive behavior throughout the lifecycle; importantly, these hormones modulate functional activity of not just the endocrine system, but most of the nervous system including the brain reward system. The effects of E2 and P4 can be seen in the processing of and memory for rewarding stimuli and in the development of compulsive reward-seeking behaviors like those seen in substance use disorders. Women are at increased risk of developing substance use disorders; however, the origins of this sex difference are not well understood and therapeutic interventions targeting ovarian hormones have produced conflicting results. This article reviews the contribution of the E2 and P4 in females to functional modulation of the brain reward system, their possible roles in origins of addiction vulnerability, and the development and treatment of compulsive reward-seeking behaviors.

Incubation of Oxycodone Craving Following Adult-Onset and Adolescent-Onset Oxycodone Self-Administration in Male Rats

Relapse is a major obstacle to curb the ongoing epidemic of prescription opioid abuse. We and others previously demonstrated that oxycodone seeking in adult rats progressively increases after abstinence from oxycodone self-administration (incubation of oxycodone craving). In humans, the onset of oxycodone use in adolescents may increase individuals' vulnerability to later opioid addiction. However, little is known about incubation of oxycodone craving after adolescent-onset oxycodone self-administration in rats. In the first study, we trained single-housed adolescent (postnatal day 35 at start) and adult (postnatal day 77 at start) male Sprague-Dawley rats to self-administer oxycodone (0.1 mg/kg/infusion, 6 h/day for 10 days) and then tested oxycodone relapse on both abstinence day 1 and day 15. Given that social experience is critical for neurobehavioral development in adolescents, we performed the second study using group-housed adolescent and adult rats. In both studies, we observed no age differences in oxycodone self-administration and incubated oxycodone seeking on abstinence day 15. However, on abstinence day 1, we observed decreased oxycodone seeking in adolescents compared with adults. This pattern of data led to elevated incubation slopes in adolescent rats compared with adult rats. Finally, group-housed rats exhibited attenuated oxycodone seeking compared with single-housed rats on abstinence day 15, but not on day 1. Taken together, these data suggest that adolescents may be resistant to oxycodone relapse during early abstinence, but this resistance dissipates quickly during the transition between adolescent and young adulthood. In addition, group-housing plays a protective role against incubated oxycodone craving.

The escalation in ethanol consumption following chronic intermittent ethanol exposure is blunted in mice expressing ethanol-resistant GluN1 or GluN2A NMDA receptor subunits

N-Methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels essential for glutamatergic transmission and plasticity. NMDARs are inhibited by acute ethanol and undergo brain region-specific adaptations after chronic alcohol exposure. In previous studies, we reported that knock-in mice expressing ethanol-insensitive GluN1 or GluN2A NMDAR subunits display altered behavioral responses to acute ethanol and genotype-dependent changes in drinking using protocols that do not produce dependence. A key unanswered question is whether the intrinsic ethanol sensitivity of NMDARs also plays a role in determining behavioral adaptations that accompany the development of dependence. To test this, we exposed mice to repeated cycles of chronic intermittent ethanol (CIE) vapor known to produce a robust escalation in ethanol consumption and preference. As expected, wild-type mice showed a significant increase from baseline in ethanol consumption and preference after each of the four weekly CIE cycles. In contrast, ethanol consumption in male GluN2A(A825W) mice was unchanged following cycles 1, 2, and 4 of CIE with a modest increase appearing after cycle 3. Wild-type and GluN2A(A825W) female mice did not show a clear or consistent escalation in ethanol consumption or preference following CIE treatment. In male GluN1(F639A) mice, the increase in ethanol consumption observed with their wild-type littermates was delayed until later cycles of exposure. These results suggest that the acute ethanol sensitivity of NMDARs especially those containing the GluN2A subunit may be a critical factor in the escalation of ethanol intake in alcohol dependence.

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.

Adolescents exhibit reduced Pavlovian biases on instrumental learning

Multiple learning systems allow individuals to flexibly respond to opportunities and challenges present in the environment. An evolutionarily conserved "Pavlovian" learning mechanism couples valence and action, promoting a tendency to approach cues associated with reward and to inhibit action in the face of anticipated punishment. Although this default response system may be adaptive, these hard-wired reactions can hinder the ability to learn flexible "instrumental" actions in pursuit of a goal. Such constraints on behavioral flexibility have been studied extensively in adults. However, the extent to which these valence-specific response tendencies bias instrumental learning across development remains poorly characterized. Here, we show that while Pavlovian response biases constrain flexible action learning in children and adults, these biases are attenuated in adolescents. This adolescent-specific reduction in Pavlovian bias may promote unbiased exploration of approach and avoidance responses, facilitating the discovery of rewarding behavior in the many novel contexts that adolescents encounter.

Mapping sign-tracking and goal-tracking onto human behaviors

As evidenced through classic Pavlovian learning mechanisms, environmental cues can become incentivized and influence behavior. These stimulus-outcome associations are relevant in everyday life but may be particularly important for the development of impulse control disorders including addiction. Rodent studies have elucidated specific learning profiles termed 'sign-tracking' and 'goal-tracking' which map onto individual differences in impulsivity and other behaviors associated with impulse control disorders' etiology, course, and relapse. Whereas goal-trackers are biased toward the outcome, sign-trackers fixate on features that are associated with but not necessary for achieving an outcome; a pattern of behavior that often leads to escalation of reward-seeking that can be maladaptive. The vast majority of the sign- and goal-tracking research has been conducted using rodent models and very few have bridged this concept into the domain of human behavior. In this review, we discuss the attributes of sign- and goal-tracking profiles, how these are manifested neurobiologically, and how these distinct learning styles could be an important tool for clinical interventions in human addiction.

Translations in Stimulus-Stimulus Pairing: Autoshaping of Learner Vocalizations

Stimulus-stimulus pairing (SSP) is a procedure used by behavior analysis practitioners that capitalizes on respondent conditioning processes to elicit vocalizations. These procedures usually are implemented only after other, more customary methods (e.g., standard echoic training via modeling) have been exhausted. Unfortunately, SSP itself has mixed research support, probably because certain as-yet-unidentified procedural variations are more effective than others. Even when SSP produces (or increases) vocalizations, its effects can be short-lived. Although specific features of SSP differ across published accounts, fundamental characteristics include presentation of a vocal stimulus proximal with presentation of a preferred item. In the present article, we draw parallels between SSP procedures and autoshaping, review factors shown to affect autoshaping, and interpret autoshaping research for suggested SSP tests and applications. We then call for extended use and reporting of SSP in behavior-analytic treatments. Finally, three bridges created by this article are identified: basic-applied, respondent-operant, and behavior analysis with other sciences.

Increased Goal Tracking in Adolescent Rats Is Goal-Directed and Not Habit-Like

When a cue is paired with reward in a different location, some animals will approach the site of reward during the cue, a behavior called goal tracking, while other animals will approach and interact with the cue itself: a behavior called sign tracking. Sign tracking is thought to reflect a tendency to transfer incentive salience from the reward to the cue. Adolescence is a time of heightened sensitivity to rewards, including environmental cues that have been associated with rewards, which may account for increased impulsivity and vulnerability to drug abuse. Surprisingly, however, studies have shown that adolescents are actually less likely to interact with the cue (i.e., sign track) than adult animals. We reasoned that adolescents might show decreased sign tracking, accompanied by increased apparent goal tracking, because they tend to attribute incentive salience to a more reward-proximal "cue": the food magazine. On the other hand, adolescence is also a time of enhanced exploratory behavior, novelty-seeking, and behavioral flexibility. Therefore, adolescents might truly express more goal-directed reward-seeking and less inflexible habit-like approach to a reward-associated cue. Using a reward devaluation procedure to distinguish between these two hypotheses, we found that adolescents indeed exhibit more goal tracking, and less sign tracking, than a comparable group of adults. Moreover, adolescents' goal tracking behavior is highly sensitive to reward devaluation and therefore goal-directed and not habit-like.

Adolescent but not adult Sprague-Dawley rats display goal-directed responding after reward devaluation

Alcohol drinking is typically initiated in adolescence, with use sometimes escalating to problematic levels. Escalation of drinking is often associated with a shift in drinking motives, with goal-directed initial use later transitioning to more habitual behavior. This study assessed whether adolescents are more sensitive than adults to habit formation when indexed via insensitivity to reward devaluation in an operant task for food reward. Adolescent and adult Sprague-Dawley rats were trained on either a random ratio (RR) or random interval (RI) schedule before undergoing devaluation. Adolescent animals on both schedules increased the number of lever presses across all training days. In contrast, adults in the RR group increased the number of lever presses across days whereas RI adults remained relatively stable. In response to pellet devaluation, only adolescents exhibited reduced responding, suggestive of goal-directed behavior, whereas no age differences were evident following control (home cage chow) devaluation. Contrary to our hypothesis, adolescents (but not adults) displayed goal-directed responding indexed via sensitivity to reward devaluation. These findings suggest that adolescents are not necessarily more likely to develop habits than adults, and hence other factors may contribute to the greater propensity of adolescents to engage in and escalate alcohol use.

Sex Differences in Ethanol Reward Seeking Under Conflict in Mice

BACKGROUND

Alcohol use disorders are characterized by inflexible alcohol seeking that occurs despite adverse consequences. Males and females are differentially sensitive to ethanol (EtOH) reward, but it is unclear whether sex differences in EtOH seeking under reward-aversion conflict are present.

METHODS

To investigate sex differences in EtOH seeking under conflict, adult male and female C57BL/6J mice underwent chronic intermittent EtOH (CIE) exposure by vapor inhalation or served as air-exposed controls. After CIE, mice were trained in a modified EtOH conditioned place preference paradigm. During 3 conditioning sessions, 2 g/kg EtOH was administered prior to confinement in the "EtOH-paired" chamber. On alternating days, saline was injected prior to confinement in the "saline-paired" chamber. After conditioning, mice experienced a footshock in the EtOH-paired chamber. EtOH-seeking behavior was assessed before and after footshock.

RESULTS

Control and CIE-exposed males reduced the time spent in and increased latency to enter the reward-paired chamber following footshock. Control females did not alter EtOH-seeking behavior following footshock. CIE-exposed females spent more time in the EtOH-paired chamber at baseline. However, following a footshock, CIE-exposed females significantly reduced the time spent in and increased latency to enter the EtOH-paired chamber.

CONCLUSIONS

Nondependent female mice exhibited aversion-resistant alcohol seeking to a greater degree than males. Chronic EtOH exposure did not impact EtOH seeking in males. In females, CIE enhanced EtOH seeking in the absence of conflict, but reduced EtOH seeking after an aversive experience. While these sex-specific effects of CIE are not present when reward seeking is assessed in the absence of an aversive experience, multiple factors may underlie the differences in reward seeking despite adverse consequences, including reward- and aversion-related learning and decision making under conflict. These data highlight the importance of considering sex as a variable influencing EtOH seeking and provide a greater understanding of how sex interacts with EtOH exposure to alter behavior.

Modelling Differential Vulnerability to Substance Use Disorder in Rodents: Neurobiological Mechanisms

Despite the prevalence of drug use within society, only a subset of individuals actively taking addictive drugs lose control over their intake and develop compulsive drug-seeking and intake that typifies substance use disorder (SUD). Although research in this field continues to be an important and dynamic discipline, the specific neuroadaptations that drive compulsive behaviour in humans addicted to drugs and the neurobiological mechanisms that underlie an individual's innate susceptibility to SUD remain surprisingly poorly understood. Nonetheless, it is clear from research within the clinical domain that some behavioural traits are recurrently co-expressed in individuals with SUD, thereby inviting the hypothesis that certain behavioural endophenotypes may be predictive, or at least act in some way, to modify an individual's probability for developing this disorder. The analysis of such endophenotypes and their catalytic relationship to the expression of addiction-related behaviours has been greatly augmented by experimental approaches in rodents that attempt to capture diagnostically relevant aspects of this progressive brain disorder. This work has evolved from an early focus on aberrant drug reinforcement mechanisms to a now much richer account of the putatively impaired cognitive control processes that ultimately determine individual trajectories to compulsive drug-related behaviours. In this chapter we discuss the utility of experimental approaches in rodents designed to elucidate the neurobiological and genetic underpinnings of so-called risk traits and how these innate vulnerabilities collectively contribute to the pathogenesis of SUD.

Sex differences in incentive motivation and the relationship to the development and maintenance of alcohol use disorders

Despite considerable evidence of higher rates of alcohol use disorders (AUDs) in men than in women, there is a dearth of research into the underlying causes of this disparity. As the gap in high risk drinking between men and women closes, it is critical to disentangle the biological factors that may place men and women at different risk for the development of AUDs as well as AUD-associated health problems. While sex differences in alcohol drinking have been reported in animal models and in human alcoholics, it increasingly seems that consummatory behavior may be dissociated from propensity toward inflexible and cue-elicited drug seeking and taking that characterize alcohol use disorders. While much of this work was initially performed in males a growing, yet limited, body of literature suggests that there are sex differences in both cue reactivity, and further, the relationship between cue reactivity and the maintenance of addictive behavior, indicating that males may be at greater risk for the development of a subset of addiction-related behaviors independent of alcohol consumption. Here, we will review the current literature on sex effects on the relationship between incentive motivation and addictive behavior and discuss unanswered questions that we expect will inform the development of individualized and sex-specific treatment and prevention strategies for AUDs. We believe that a greater understanding of how sex interacts with in cue reactivity to independently mediate the drug taking and risk for the development of uncontrolled drug or alcohol-seeking and -taking will inform the development of individualized treatment and prevention strategies for addiction.

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 and Adolescent Ethanol Exposure Influence Pavlovian Conditioned Approach

BACKGROUND

Alcohol use among adolescents is widespread and a growing concern due to long-term behavioral deficits, including altered Pavlovian behavior, that potentially contribute to addiction vulnerability. We tested the hypothesis that adolescent intermittent ethanol (AIE) exposure alters Pavlovian behavior in males and females as measured by a shift from goal-tracking to sign-tracking. Additionally, we investigated GLT-1, an astrocytic glutamate transporter, as a potential contributor to a sign-tracking phenotype.

METHODS

Male and female Sprague-Dawley rats were exposed to AIE (5 g/kg, intragastric) or water intermittently 2 days on and 2 days off from postnatal day (P) 25 to 54. Around P70, animals began 20 daily sessions of Pavlovian conditioned approach (PCA), where they learned that a cue predicted noncontingent reward delivery. Lever pressing indicated interaction with the cue, or sign-tracking, and receptacle entries indicated approach to the reward delivery location, or goal-tracking. To test for effects of AIE on nucleus accumbens (NAcc) excitatory signaling, we isolated membrane subfractions and measured protein levels of the glutamate transporter GLT-1 after animals completed behavior as a measure of glutamate homeostasis.

RESULTS

Females exhibited elevated sign-tracking compared to males with significantly more lever presses, faster latency to first lever press, and greater probability to lever press in a trial. AIE significantly increased lever pressing while blunting goal-tracking, as indicated by fewer cue-evoked receptacle entries, slower latency to receptacle entry, and lower probability to enter the receptacle in a trial. No significant sex-by-exposure interactions were observed in sign- or goal-tracking metrics. Moreover, we found no significant effects of sex or exposure on membrane GLT-1 expression in the NAcc.

CONCLUSIONS

Females exhibited enhanced sign-tracking compared to males, while AIE decreased goal-tracking compared to control exposure. Our findings support the hypothesis that adolescent binge ethanol can shift conditioned behavior from goal- to cue-directed in PCA, especially in females.

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.

Elucidating the role of D4 receptors in mediating attributions of salience to incentive stimuli on Pavlovian conditioned approach and conditioned reinforcement paradigms

The power of drug-associated cues to instigate drug 'wanting' and consequently promote drug seeking is a corner stone of contemporary theories of addiction. Gambling disorder has recently been added to the pantheon of addictive disorders due to the phenomenological similarities between the diseases. However, the neurobiological mechanism that may mediate increased sensitivity towards conditioned stimuli in addictive disorders is unclear. We have previously demonstrated using a rodent analogue of a simple slot machine that the dopamine D4 receptor is critically engaged in controlling animals' attribution of salience to stimuli associated with reward in this paradigm, and consequently may represent a target for the treatment of gambling disorder. Here, we investigated the role of acute administration of a D4 receptor agonist on animals' responsivity to conditioned stimuli on both a Pavlovian conditioned approach (autoshaping) and a conditioned reinforcement paradigm. Following training on one of the two tasks, separate cohorts of rats (male and female) were administered a dose of PD168077 shown to be maximally effective at precipitating errors in reward expectancy on the rat slot machine task (10mg/kg). However, augmenting the activity of the D4 receptors in this manner did not alter behaviour on either task. These data therefore provide novel evidence that the D4 receptor does not alter incentive motivation in response to cues on simple behavioural tasks.

Pavlovian Conditioned Approach Training in Rats

Cues that are contingently paired with unconditioned, rewarding stimuli can acquire rewarding properties themselves through a process known as the attribution of incentive salience, or the transformation of neutral stimuli into attractive, "wanted' stimuli capable of motivating behavior. Pavlovian conditioned approach (PCA) develops after the response-independent presentation of a conditioned stimulus (CS; e.g., a lever) that predicts the delivery of an unconditioned stimulus (US; e.g., a food pellet) and can be used to measure incentive salience. During training, three patterns of conditioned responses (CRs) can develop: sign-tracking behavior (CS-directed CR), goal-tracking behavior (US-directed CR), and an intermediate response (both CRs). Sign-trackers attribute incentive salience to reward-related cues and are more vulnerable to cue-induced reinstatement of drug-seeking as well as other addiction-related behaviors, making PCA a potentially valuable procedure for studying addiction vulnerability. Here, we describe materials and methods used to elicit PCA behavior from rats as well as analyze and interpret PCA behavior in individual experiments.

Isolating the incentive salience of reward-associated stimuli: value, choice, and persistence

Sign- and goal-tracking are differentially associated with drug abuse-related behavior. Recently, it has been hypothesized that sign- and goal-tracking behavior are mediated by different neurobehavioral valuation systems, including differential incentive salience attribution. Herein, we used different conditioned stimuli to preferentially elicit different response types to study the different incentive valuation characteristics of stimuli associated with sign- and goal-tracking within individuals. The results demonstrate that all stimuli used were equally effective conditioned stimuli; however, only a lever stimulus associated with sign-tracking behavior served as a robust conditioned reinforcer and was preferred over a tone associated with goal-tracking. Moreover, the incentive value attributed to the lever stimulus was capable of promoting suboptimal choice, leading to a significant reduction in reinforcers (food) earned. Furthermore, sign-tracking to a lever was more persistent than goal-tracking to a tone under omission and extinction contingencies. Finally, a conditional discrimination procedure demonstrated that sign-tracking to a lever and goal-tracking to a tone were dependent on learned stimulus–reinforcer relations. Collectively, these results suggest that the different neurobehavioral valuation processes proposed to govern sign- and goal-tracking behavior are independent but parallel processes within individuals. Examining these systems within individuals will provide a better understanding of how one system comes to dominate stimulus–reward learning, thus leading to the differential role these systems play in abuse-related behavior.

Effects of ethanol exposure during adolescence or in adulthood on Pavlovian conditioned approach in Sprague-Dawley rats

Human studies have shown that adolescents who repeatedly use alcohol are more likely to be dependent on alcohol and are more likely to suffer from psychological problems later in life. There has been limited research examining how ethanol exposure in adolescence might contribute to later abuse or addiction in adulthood. The present experiment examined effects of intermittent ethanol exposure during adolescence on sign-tracking behavior in adulthood, indexed by a Pavlovian conditioned approach (PCA) task wherein an 8s lever presentation served as a cue predicting subsequent delivery of a flavored food pellet. Although no response was required for food delivery, after multiple pairings, 1 of 2 different responses often emerged during the lever presentation: goal tracking (head entries into the food trough) or sign tracking (engagement with the lever when presented). Sign tracking is thought to reflect the attribution of incentive salience to reward-paired cues and has been previously correlated with addiction-like behaviors. Following the last PCA session, blood samples were collected for analysis of post-session corticosterone levels. Sixty-two rats (n = 10-12/group) were pseudo-randomly assigned to 1 of 2 intragastric (i.g.) exposure groups (water or 4 g/kg ethanol) or a non-manipulated (NM) control group. Animals were intubated with ethanol or water every other session from postnatal session (PND) 28-48 or PND 70-90. Rats were then tested in adulthood (PND 71-79 or PND 113-122) on the PCA task. Animals exposed chronically to ethanol during adolescence exhibited significantly higher levels of sign-tracking behavior in adulthood than NM and water-treated animals, and showed higher corticosterone than NM control animals. These effects were not seen after comparable ethanol exposure in adulthood. These results suggest that adolescent alcohol exposure has long-term consequences on the expression of potential addiction-relevant behaviors in adulthood.

Environmental manipulations alter age differences in attribution of incentive salience to reward-paired cues

Cues repeatedly paired with rewards often themselves become imbued with enhanced motivational value, or incentive salience. During Pavlovian conditioned approach procedures, a cue repeatedly preceding reward delivery often elicits conditioned responses at either the reward delivery location ("goal-tracking") or the cue itself ("sign-tracking"). Sign-tracking behavior is thought to reflect the individual differences in attribution of incentive salience to reward-paired cues that may contribute to addiction vulnerability. Adolescent rats typically demonstrate less sign-tracking behavior than adult rats, a surprising finding given that adolescence is hypothesized to be a time of heightened addiction vulnerability. Given evidence that adult sign-tracking behavior can be influenced by environmental conditions, the present study compared the effects of isolate housing and food deprivation on expression of sign-tacking and goal-tracking behavior in adolescent and adult male rats across eight days of a Pavlovian conditioned approach procedure. Pair-housed adults exhibited more sign-tracking behavior than pair-housed adolescents; however, this age difference was not apparent in isolate-housed subjects. Adolescents often appeared more sensitive than adults to both food restriction- and isolate housing-induced changes in behavior, with food restriction promoting an increase in sign-tracking among isolate-housed adolescents and an increase in goal-tracking among pair-housed adolescents. For adults, food restriction resulted in a modest increase in overall expression of both sign- and goal-tracking behavior. To the extent that sign-tracking behavior reflects attribution of incentive salience to reward-paired cues, results from the present study provide evidence that reactivity to rewards during adolescence is strongly related to the nature of the surrounding environment.

Limbic activation to novel versus familiar food cues predicts food preference and alcohol intake

Expectation of salient rewards and novelty seeking are processes implicated in substance use disorders but the neurobiological substrates underlying these associations are not well understood. To better understand the regional circuitry of novelty and reward preference, rats were conditioned to pair unique cues with bacon, an initially novel food, or chow, a familiar food. In the same animals, after training, cue-induced brain activity was measured, and the relationships between activity and preference for three rewards, the conditioned foods and ethanol (EtOH), were separately determined. Activity in response to the food paired cues was measured using brain glucose metabolism (BGluM). Rats favoring bacon-paired (BAP) cues had increased BGluM in mesocorticolimbic brain regions after exposure to these cues, while rats favoring chow-paired (CHP) cues showed relative deactivation in these regions. Rats exhibiting BAP cue-induced activation in prefrontal cortex (PFC) also consumed more EtOH while rats with cortical activation in response to CHP cues showed lower EtOH consumption. Additionally, long-term stable expression levels of PFC Grin2a, a subunit of the NMDA receptor, correlated with individual differences in EtOH preference insomuch that rats with high EtOH preference had enduringly low PFC Grin2a mRNA expression. No other glutamatergic, dopaminergic or endocannabinoid genes studied showed this relationship. Overall, these results suggest that natural variation in mesocorticolimbic sensitivity to reward-paired cues underlies behavioral preferences for and vulnerability to alcohol abuse, and support the notion of common neuronal circuits involved in food- and drug-seeking behavior. The findings also provide evidence that PFC NMDA-mediated glutamate signaling may modulate these associations.

The effect of nicotine on sign-tracking and goal-tracking in a Pavlovian conditioned approach paradigm in rats

RATIONALE

Nicotine (NIC) potently increases operant responding for non-NIC reinforcers, and this effect may depend on drug-mediated increases in incentive motivation. According to this hypothesis, NIC should also potently increase approach to Pavlovian-conditioned stimuli associated with rewards.

OBJECTIVE

The present studies explored the effects of NIC on Pavlovian-conditioned approach responses.

METHOD

To do so, liquid dippers were used to deliver an unconditioned stimulus (US; 0.1 ml sucrose) after presentation of a conditioned stimulus (CS; 30 s illumination of a stimulus light)-both the CS and US were presented in receptacles equipped to monitor head entries.

RESULTS

In experiment 1, the CS and US were presented in the same receptacle, but NIC pretreatment (0.4 mg/kg base) did not increase conditioned approach responses. Delivery of the sucrose US was then shifted to receptacle in a different location. All rats learned to approach the new US location (goal-tracking) at similar rates. Approach to the CS receptacle (sign-tracking) declined for saline-pretreated rats, but NIC pretreatment increased sign-tracking. In experiment 2, NIC pretreatment increased sign-tracking when the CS and US were spatially separated during acquisition. In experiment 3, NIC pretreatments were replaced with saline, but the effect of NIC persisted for an additional 24 test sessions.

CONCLUSION

The findings suggest that NIC increases incentive motivation and that this effect is long-lasting, persisting beyond the pharmacological effects of NIC.

Quantifying individual variation in the propensity to attribute incentive salience to reward cues

If reward-associated cues acquire the properties of incentive stimuli they can come to powerfully control behavior, and potentially promote maladaptive behavior. Pavlovian incentive stimuli are defined as stimuli that have three fundamental properties: they are attractive, they are themselves desired, and they can spur instrumental actions. We have found, however, that there is considerable individual variation in the extent to which animals attribute Pavlovian incentive motivational properties ("incentive salience") to reward cues. The purpose of this paper was to develop criteria for identifying and classifying individuals based on their propensity to attribute incentive salience to reward cues. To do this, we conducted a meta-analysis of a large sample of rats (N = 1,878) subjected to a classic Pavlovian conditioning procedure. We then used the propensity of animals to approach a cue predictive of reward (one index of the extent to which the cue was attributed with incentive salience), to characterize two behavioral phenotypes in this population: animals that approached the cue ("sign-trackers") vs. others that approached the location of reward delivery ("goal-trackers"). This variation in Pavlovian approach behavior predicted other behavioral indices of the propensity to attribute incentive salience to reward cues. Thus, the procedures reported here should be useful for making comparisons across studies and for assessing individual variation in incentive salience attribution in small samples of the population, or even for classifying single animals.

A food predictive cue must be attributed with incentive salience for it to induce c-fos mRNA expression in cortico-striatal-thalamic brain regions

Cues associated with rewards acquire the ability to engage the same brain systems as rewards themselves. However, reward cues have multiple properties. For example, they not only act as predictors of reward capable of evoking conditional responses (CRs), but they may also acquire incentive motivational properties. As incentive stimuli they can evoke complex emotional and motivational states. Here we sought to determine whether the predictive value of a reward cue is sufficient to engage brain reward systems, or whether the cue must also be attributed with incentive salience. We took advantage of the fact that there are large individual differences in the extent to which reward cues are attributed with incentive salience. When a cue (conditional stimulus, CS) is paired with delivery of food (unconditional stimulus, US), the cue acquires the ability to evoke a CR in all rats; that is, it is equally predictive and supports learning the CS-US association in all. However, only in a subset of rats is the cue attributed with incentive salience, becoming an attractive and desirable incentive stimulus. We used in situ hybridization histochemistry to quantify the ability of a food cue to induce c-fos mRNA expression in rats that varied in the extent to which they attributed incentive salience to the cue. We found that a food cue induced c-fos mRNA in the orbitofrontal cortex, striatum (caudate and nucleus accumbens), thalamus (paraventricular, intermediodorsal and central medial nuclei), and lateral habenula, only in rats that attributed incentive salience to the cue. Furthermore, patterns of "connectivity" between these brain regions differed markedly between rats that did or did not attribute incentive salience to the food cue. These data suggest that the predictive value of a reward cue is not sufficient to engage brain reward systems-the cue must also be attributed with incentive salience.

Development of anticipatory 50 kHz USV production to a social stimuli in adolescent and adult male Sprague-Dawley rats

This study examined ontogenetic differences in anticipatory 50 kHz ultrasonic vocalization (USV) production to social interactions in male Sprague-Dawley rats. Adults increased USVs across days when tested socially but not when left alone (Experiment 1), and displayed anticipatory USVs to return to the cage-mate (Experiment 2). Adolescents did not display evidence of anticipatory USVs. To the extent that anticipatory USVs index incentive salience, this suggests an adolescent attenuation of incentive salience of social interactions.

Rewards, aversions and affect in adolescence: emerging convergences across laboratory animal and human data

The adolescent transition is associated with increases in reward- and sensation-seeking, peer-directed social interactions, and risk-taking, with exploratory use of alcohol and other drugs often beginning at this time. These age-related behaviors may have biological roots embedded in the evolutionary past, with similar adolescent-typical characteristics evident across a variety of mammalian species. Drawing across human behavioral and fMRI data and studies conducting in laboratory animals, this review examines processing of rewards, aversions, and affect in adolescence. Evidence for both hyper- and hypo-reactivity during adolescence in the processing of rewards is reviewed, along with possible contributors to these differences. Indications of sometimes heightened reward reactivity during adolescence are contrasted with frequent attenuations in adolescent sensitivity to aversive stimuli. At the same time, adolescents appear particularly prone to becoming emotionally aroused, especially in social contexts. Emerging evidence hints that exaggerated adolescent reactivity in reward and affective systems may be promoted in part by unusual strong cross-reactivity between these systems during adolescence. Such age-related propensities may promote adolescent risk taking, especially in social and exciting contexts, and contribute to adolescent-typical propensities to attach greater benefit and less cost to risky behaviors such as alcohol and drug use than individuals at other ages.