Differential effects of self-administered cocaine in adolescent and adult rats on stimulus-reward learning

Unlocking the age-old secrets of reward and substance use

Although substance use is widespread across the lifespan from early adolescence to older adulthood, the prevalence of substance use disorder (SUD) differs between age groups. These age differences in SUD rates necessitate an investigation into how age moderates reward sensitivity, and consequently influences the risks and consequences related to substance use. This theoretical review integrates evidence from the literature to address the dynamic interplay between age and reward in the context of substance use. Overall, increasing evidence demonstrates that age moderates reward sensitivity and underlying reward system neurobiology. Reward sensitivity undergoes a non-linear trajectory across the lifespan. Low levels of reward sensitivity are associated with childhood and late adulthood. In contrast, high levels are associated with early to late adolescence, followed by a decline in the twenties. These fluctuations in reward sensitivity across the lifespan contribute to complex associations with substance use. This lends support to adolescence and young adulthood as vulnerable periods for the risk of subsequent SUD. More empirical research is needed to investigate reward sensitivity during SUD maintenance and recovery. Future research should also involve larger sample sizes and encompass a broader range of age groups, including older adults.

Cocaine reward and reinstatement in adolescent versus adult rodents

Adolescence is a critical juncture when initiation of drug use intersects with profound developmental changes in the brain. Adolescent drug use increases the risk to develop substance use disorders (SUDs) later in life, but the mechanisms that confer this vulnerability are not understood. SUDs are defined by cycles of use, abstinence, and relapse. Intense craving during drug-free periods is often triggered by cues and environmental contexts associated with previous use. In contrast to our understanding of stimuli that elicit craving and relapse in adults, the behavioral processes that occur during periods of abstinence and relapse in adolescents are poorly understood. The current mini-review will summarize findings from preclinical rodent studies that used cocaine conditioned place preference and operant cocaine self-administration to examine subsequent effects on reward, relapse and incubation of craving.

Role of adolescent-formed, context-drug-associations on reinstatement of drug-seeking behavior in rats

RATIONALE

Drug use during adolescence results in a lifelong risk to develop substance-use disorders. Adolescent rats are less reactive to cocaine-associated cues compared with adults; however, the contribution of adolescent-formed, context-drug-associations to elicit relapse-like behavior is underexplored. Although it is known that social isolation can impact drug-seeking behavior, the effects of housing conditions on context-induced, cocaine-seeking during adolescence vs adulthood are unknown.

OBJECTIVES

The present study compared the effect of adolescent vs adult-formed context-drug associations under different housing conditions (pair vs single) on cocaine-seeking behavior during adolescence or adulthood. This objective was accomplished using operant cocaine self-administration (Coc-SA) under a standard, non-abbreviated (Non-ABRV) or modified abbreviated (ABRV) paradigm.

METHODS

In experiment 1, adolescent and adult rats received Non-ABRV Coc-SA in a distinct context (2 h, 1×/day, 10 days), and extinction training (EXT) in a second context (1 h, 1×/day, 8 days) with reinstatement test (TEST) during adulthood in the cocaine-paired context. In experiments 2 and 3, rats received all behavioral phases during adolescence or adulthood: ABRV Coc-SA (2 h, 2×/day, 5 days), EXT (1 h, 4×/day, 2 days) with TEST in a cocaine-paired or novel, unpaired context. All experiments included pair and single-housing conditions.

RESULTS AND CONCLUSIONS

Age at cocaine exposure did not influence behavior in Non-ABRV or ABRV paradigms. Under Non-ABRV conditions, adolescent and adult single-housed rats had higher seeking behavior than pair housed. These data suggest that social isolation influences context-induced, cocaine-seeking regardless of age at drug exposure and provides a condensed, ABRV paradigm to investigate context-induced, cocaine-seeking behavior during adolescence.

Extinction and drug-induced reinstatement of cocaine seeking following self-administration or conditioned place preference in adolescent and adult rats

Adolescence marks a particularly vulnerable period to developing substance use disorders, and people who start using drugs in adolescence are more likely to relapse. A limited number of studies have investigated age difference in relapse following re-exposure to the drug after a period of abstinence. Using a cocaine self-administration paradigm, we showed no age difference in acquisition or extinction of self-administration. Interestingly, adolescent rats displayed impaired cocaine-primed reinstatement of cocaine seeking. Using the same dose as that self-administered in the first experiment, we then investigated age differences in acquisition and extinction of conditioned place preference, as well as locomotor sensitization. While there were no differences in locomotor activity or acquisition of preference, adolescents failed to extinguish their preference, even when the number of extinction sessions was doubled from what adults received. Taken together, these results suggest that while cocaine has similar rewarding and reinforcing effects regardless of age, adolescents may attribute stronger salience to the drug-associated context. In addition, re-exposure to cocaine itself may not be a strong relapse trigger in adolescence. Overall, these findings suggest that we should focus more on alleviating drug-context salience compared to re-exposure to substance in order to reduce relapse of drug seeking in adolescents.

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.

Methylphenidate has nonlinear dose effects on cued response inhibition in adults but not adolescents

Ongoing development of the dopamine system during adolescence may provide a partial mechanism for behavioral and psychiatric vulnerabilities. Despite early evidence for a hyperactive adolescent dopaminergic system, recent data suggest that adolescent dopamine may be functionally hypoactive compared to in adults. While this distinction has been established in response to dopaminergic drugs and natural rewards, little is known about age-related differences in cognitive efficacy of dopaminergic drugs. Using a recently established Cued Response Inhibition Task, we tested the effects of acute systemic methylphenidate, commonly known as Ritalin, on response inhibition and response initiation in adolescent and adults rats. First, we replicated previous data that adolescents are able to inhibit a response to a cue on par with adults, but are slower to produce a rewarded response after a stop cue. Next, we observed that methylphenidate modulated response inhibition in adult rats, with low dose (0.3mg/kg) improving inhibition, and high dose (3mg/kg) impairing performance. This dose-response pattern is commonly observed with psychostimulant cognitive modulation. In adolescents, however, methylphenidate had no effect on response inhibition at any dose. Latency of response initiation after the stop cue was not affected by methylphenidate in either adult or adolescent rats. These data establish that dose-response of a commonly prescribed psychostimulant medication is different in adolescents and adults. They further demonstrate that healthy adolescent response inhibition is not as sensitive to psychostimulants as in adults, supporting the idea that the dopamine system is hypoactive in adolescence. This article is part of a Special Issue entitled SI: Adolescent plasticity.

Adolescent-onset of cocaine use is associated with heightened stress-induced reinstatement of cocaine seeking

Adolescent rats take cocaine more readily than adults, are more sensitive to lower doses of the drug and work harder for it. It remains unknown if adolescent-onset of cocaine use has long-term consequences on adult relapse liability. Therefore, we tested if self-administering cocaine during adolescence impacts subsequent stress-induced reinstatement to cocaine seeking and taking, after a prolonged drug-free period. Adolescent (~P42) or adult (P88) rats self-administered cocaine (0.6 or 1.2 mg/kg/infusion) for 7 or 10 days. Then, they underwent a prolonged drug-free period (21-40 days), after which they were tested for reinstatement of cocaine-seeking (i.e. responding in the absence of cocaine) induced by the stress hormone corticosterone, the pharmacological stressor yohimbine or electric footshock. Studies employed either single extinction session (within-session extinction/reinstatement) or repeated extinction prior to reinstatement (between-session extinction/reinstatement). Finally, in a separate set of experiments, rats underwent a prolonged drug-free period (~40 days) and were then allowed to self-administer cocaine again, using progressive-ratio procedures that appraise the reinforcing efficacy of cocaine. Rats with adolescent-onset of cocaine use showed greater stress-induced reinstatement of cocaine seeking than rats with adult-onset of cocaine use. This was observed across conditions, providing external validity to these results. Groups did not differ on drug taking in progressive-ratio tests. Our studies indicate that experiencing cocaine during adolescence renders subjects particularly responsive to the subsequent effects of stress on drug seeking. This heightened propensity for reinstatement puts adolescent-onset drug users at heightened risk for relapse.

Sex Differences in Behavioral Dyscontrol: Role in Drug Addiction and Novel Treatments

The purpose of this review is to discuss recent findings related to sex differences in behavioral dyscontrol that lead to drug addiction, and clinical implications for humans are discussed. This review includes research conducted in animals and humans that reveals fundamental aspects of behavioral dyscontrol. The importance of sex differences in aspects of behavioral dyscontrol, such as impulsivity and compulsivity, is discussed as major determinants of drug addiction. Behavioral dyscontrol during adolescence is also an important consideration, as this is the time of onset for drug addiction. These vulnerability factors additively increase drug-abuse vulnerability, and they are integral aspects of addiction that covary and interact with sex differences. Sex differences in treatments for drug addiction are also reviewed in terms of their ability to modify the behavioral dyscontrol that underlies addictive behavior. Customized treatments to reduce behavioral dyscontrol are discussed, such as (1) using natural consequences such as non-drug rewards (e.g., exercise) to maintain abstinence, or using punishment as a consequence for drug use, (2) targeting factors that underlie behavioral dyscontrol, such as impulsivity or anxiety, by repurposing medications to relieve these underlying conditions, and (3) combining two or more novel behavioral or pharmacological treatments to produce additive reductions in drug seeking. Recent published work has indicated that factors contributing to behavioral dyscontrol are an important target for advancing our knowledge on the etiology of drug abuse, intervening with the drug addiction process and developing novel treatments.

Cocaine self-administration enhances excitatory responses of pyramidal neurons in the rat medial prefrontal cortex to human immunodeficiency virus-1 Tat

The medial prefrontal cortex (mPFC) plays a critical role in reward-motivated behaviors. Repeated cocaine exposure dysregulates the dorsal mPFC, and this is thought to contribute to cocaine-seeking and relapse of abstinent abusers. Neuropathology of the mPFC also occurs in human immunodeficiency virus (HIV)-positive individuals, and this is exaggerated in those who also abuse cocaine. The impact of the comorbid condition on mPFC neuronal function is unknown. To fill this knowledge gap, we performed a behavioral and electrophysiological study utilising adult male rats that self-administered cocaine by pressing a lever for 14 once-daily operant sessions. Saline-yoked (SAL-yoked) rats served as controls. Cue reactivity (CR) was used to indicate drug-seeking, assessed by re-exposing the rats to cocaine-paired cues wherein non-reinforced lever pressing was quantified 1 day (CR1) and 18-21 days (CR2) after the 14th operant session. Only cocaine self-administration (COC-SA) rats showed CR. One day after CR2, brain slices were prepared for electrophysiological assessment. Whole-cell patch-clamp recordings of dorsal (prelimbic) mPFC pyramidal neurons from COC-SA rats showed a significant increase in firing evoked by depolarizing currents as compared with those from SAL-yoked control rats. Bath application of the toxic HIV-1 protein transactivator of transcription (Tat) also depolarized neuronal membranes and increased evoked firing. The Tat-induced excitation was greater in the neurons from withdrawn COC-SA rats than in controls. Tat also reduced spike amplitude, and this co-varied with cocaine-seeking during CR2. Taken together, these novel findings provide support at the neuronal level for the concept that the increased excitability of mPFC pyramidal neurons following cocaine self-administration drives drug-seeking and augments the neuropathophysiology caused by HIV-1 Tat.

Performance on a strategy set shifting task in rats following adult or adolescent cocaine exposure

RATIONALE

Neuropsychological testing is widespread in adult cocaine abusers, but lacking in teens. Animal models may provide insight into age-related neuropsychological consequences of cocaine exposure.

OBJECTIVES

The objective of the present study is to determine whether developmental plasticity protects or hinders behavioral flexibility after cocaine exposure in adolescent vs. adult rats.

METHODS

Using a yoked-triad design, one rat controlled cocaine delivery and the other two passively received cocaine or saline. Rats controlling cocaine delivery (1.0 mg/kg) self-administered for 18 sessions (starting P37 or P77), followed by 18 drug-free days. Rats next were tested in a strategy set shifting task, lasting 11-13 sessions.

RESULTS

Cocaine self-administration did not differ between age groups. During initial set formation, adolescent-onset groups required more trials to reach criterion and made more errors than adult-onset groups. During the set shift phase, rats with adult-onset cocaine self-administration experience had higher proportions of correct trials and fewer perseverative + regressive errors than age-matched yoked-controls or rats with adolescent-onset cocaine self-administration experience. During reversal learning, rats with adult-onset cocaine experience (self-administered or passive) required fewer trials to reach criterion, and the self-administering rats made fewer perseverative + regressive errors than yoked-saline rats. Rats receiving adolescent-onset yoked-cocaine had more trial omissions and longer lever press reaction times than age-matched rats self-administering cocaine or receiving yoked-saline.

CONCLUSIONS

Prior cocaine self-administration may impair memory to reduce proactive interference during set shifting and reversal learning in adult-onset but not adolescent-onset rats (developmental plasticity protective). Passive cocaine may disrupt aspects of executive function in adolescent-onset but not adult-onset rats (developmental plasticity hinders).

The effects of abused drugs on adolescent development of corticolimbic circuitry and behavior

Adolescence is a period of significant neurobiological change that occurs as individuals transition from childhood to adulthood. Because the nervous system is in a relatively labile state during this stage of development, it may be especially sensitive to experience-induced plasticity. One such experience that is relatively common to adolescents is the exposure to drugs of abuse, particularly alcohol and psychostimulants. In this review, we highlight recent findings on the long-lasting effects of exposure to these drugs during adolescence in humans as well as in animal models. Whenever possible, our focus is on studies that use comparison groups of adolescent- and adult-exposed subjects as this is a more direct test of the hypothesis that adolescence represents a period of enhanced vulnerability to the effects of drug-induced plasticity. Lastly, we suggest areas of future investigation that are needed and methodological concerns that should be addressed.

Adolescents are more vulnerable to cocaine addiction: behavioral and electrophysiological evidence

In humans, adolescence is a period of heightened propensity to develop cocaine addiction. It is unknown whether this is attributable to greater access and exposure to cocaine at this age, or whether the adolescent brain is particularly vulnerable to the addictive properties of cocaine. Here, we subjected male adolescent (P42) and adult (∼P88) rats to a wide range of cocaine self-administration procedures. In addition, to determine whether behavioral differences are associated with developmental differences in dopaminergic activity, we examined and manipulated the activity of dopamine neurons. Relative to adults, adolescent rats took cocaine more readily, were more sensitive to lower doses, showed greater escalation of cocaine intake, and were less susceptible to increases in price (i.e., were more "inelastic"). In parallel, adolescents also showed elevated activity of ventral tegmental area dopamine neurons, a feature known to be associated with increased self-administration behavior. Pharmacological manipulation of dopamine D2 receptor function with quinpirole (agonist) or eticlopride (antagonist), to alter dopamine neuron activity, eliminated age differences in cocaine self-administration. These data suggest a causal relationship between behavioral and electrophysiological determinants of cocaine addiction liability. In conclusion, adolescents show behavioral and electrophysiological traits of heightened addiction liability.

New perspectives on adolescent motivated behavior: attention and conditioning

Adolescence is a critical transition period, during which fundamental changes prepare the adolescent for becoming an adult. Heuristic models of the neurobiology of adolescent behavior have emerged, promoting the central role of reward and motivation, coupled with cognitive immaturities. Here, we bring focus to two basic sets of processes, attention and conditioning, which are essential for adaptive behavior. Using the dual-attention model developed by Corbetta and Shulman (2002), which identifies a stimulus-driven and a goal-driven attention network, we propose a balance that favors stimulus-driven attention over goal-driven attention in youth. Regarding conditioning, we hypothesize that stronger associations tend to be made between environmental cues and appetitive stimuli, and weaker associations with aversive stimuli, in youth relative to adults. An attention system geared to prioritize stimulus-driven attention, together with more powerful associative learning with appetitive incentives, contribute to shape patterns of adolescent motivated behavior. This proposed bias in attention and conditioning function could facilitate the impulsive, novelty-seeking and risk-taking behavior that is typical of many adolescents.

A role for the prefrontal cortex in heroin-seeking after forced abstinence by adult male rats but not adolescents

Adolescent drug abuse is hypothesized to increase the risk of drug addiction. Yet male rats that self-administer heroin as adolescents show attenuated drug-seeking after abstinence, compared with adults. Here we explore a role for neural activity in the medial prefrontal cortex (mPFC) in age-dependent heroin-seeking. Adolescent (35-day-old at start; adolescent-onset) and adult (86-day-old at start) male rats acquired lever-pressing maintained by heroin using a fixed ratio one reinforcement schedule (0.05 and 0.025 mg/kg per infusion). Following 12 days of forced abstinence, rats were tested for heroin-seeking over 1 h by measuring the number of lever presses on the active lever. Unbiased stereology was then used to estimate the number of Fos-ir(+) and Fos-ir(-) neurons in prelimbic and infralimbic mPFC. As before, adolescents and adults self-administered similar amounts of heroin, but subsequent heroin-seeking was attenuated in the younger rats. Similarly, the adolescent-onset group failed to show significant neural activation in the prelimbic or infralimbic mPFC during the heroin-seeking test, whereas the adult-onset heroin self-administration group showed two to six times more Fos-ir(+) neurons than their saline counterparts in both mPFC subregions. Finally, the overall number of neurons in the infralimbic cortex was greater in rats from the adolescent-onset groups than adults. The mPFC may thus have a key role in some age-dependent effects of heroin self-administration.

Exercise to reduce the escalation of cocaine self-administration in adolescent and adult rats

RATIONALE

Concurrent access to an exercise wheel decreases cocaine self-administration under short access (5 h/day for 5 days) conditions and suppresses cocaine-primed reinstatement in adult rats.

OBJECTIVE

The effect of exercise (wheel running) on the escalation of cocaine intake during long access (LgA, 6 h/day for 26 days) conditions was evaluated.

METHODS

Adolescent and adult female rats acquired wheel running, and behavior was allowed to stabilize for 3 days. They were then implanted with an iv catheter and allowed to self-administer cocaine (0.4 mg/kg, iv) during 6-h daily sessions for 16 days with concurrent access to either an unlocked or a locked running wheel. Subsequently, for ten additional sessions, wheel access conditions during cocaine self-administration sessions were reversed (i.e., locked wheels became unlocked and vice versa).

RESULTS

In the adolescents, concurrent access to the unlocked exercise wheel decreased responding for cocaine and attenuated escalation of cocaine intake irrespective of whether the locked or unlocked condition came first. However, cocaine intake increased when the wheel was subsequently locked for the adolescents that had initial access to an unlocked wheel. Concurrent wheel access either before or after the locked wheel access did not reduce cocaine intake in adults.

CONCLUSIONS

Wheel running reduced cocaine intake during LgA conditions in adolescent but not adult rats, and concurrent access to the running wheel was necessary. These results suggest that exercise prevents cocaine seeking and that this effect is more pronounced in adolescents than adults.

The effects of lobeline and naltrexone on methamphetamine-induced place preference and striatal dopamine and serotonin levels in adolescent rats with a history of maternal separation

Exposure to early life stress has been suggested to increase an individual's vulnerability to methamphetamine (MA) dependence. Although there is no cure for drug dependence, the opioid and vesicular monoamine transporter 2 (VMAT2) systems may be useful targets for treatment insofar as they play pivotal roles in the neurochemistry of addiction. Here we investigated the effects of naltrexone (opioid antagonist) and lobeline (VMAT2 inhibitor) on MA-induced place preference in adolescent rodents subjected to early life trauma (maternal separation, MS) and controls, as well as the effects on dopamine and serotonin levels in the striatum. We found: (1) maternal separation attenuated methamphetamine-induced place preference; (2) lobeline and naltrexone treatment had differential effects on serotonin and dopamine concentrations in the striatum, naltrexone increased serotonin levels in the maternally separated animals. The hypothesized effect of early adversity increasing MA-induced place preference may not be apparent in adolescence. However the data are consistent with the hypothesis that early life stress influences neurochemical pathways that predispose an individual to drug dependence.

Escalation of methamphetamine self-administration in adolescent and adult rats

BACKGROUND

Methamphetamine (METH) use has increased substantially in the last 10 years and poses a serious health concern, especially for young populations. Drug abuse primarily begins during adolescence, when uninhibited and excessive and drug intake is a common occurrence; thus, understanding the developmental patterns of addiction during this critical period is an essential step in its prevention. In the present study, the effect of age on the vulnerability to METH abuse was examined using a rat model of bingeing (i.e., escalation).

METHODS

Adolescent and adult rats were compared during short (ShA, 2-h) and long-access (LgA, 6-h) to METH self-administration. On postnatal (PN) days 23 (adolescents) and 90 (adults), rats were implanted with i.v. catheters and trained to lever press for infusions of METH (0.05mg/kg) during 2-h sessions. Once the rats reached a steady rate of METH self-administration, they were divided into ShA or LgA groups and allowed to self-administer METH for 15 additional days.

RESULTS

Results indicated that adolescent rats earned significantly more infusions than adults under the LgA condition, but the age groups did not differ during ShA. Adolescents, but not adults, also significantly increased (i.e., escalated) METH self-administration across the 15 days of testing under the LgA condition. Further analysis indicated excessive responding during infusions in the LgA METH-exposed adolescents compared to the other groups, suggesting elevated impulsivity or motivation for drug.

CONCLUSION

These results demonstrate that adolescents are more vulnerable to the escalation of METH than adults during LgA.

Dopamine neurons in the ventral tegmental area fire faster in adolescent rats than in adults

Adolescence may be a period of vulnerability to drug addiction. In rats, elevated firing activity of ventral tegmental area (VTA) dopamine neurons predicts enhanced addiction liability. Our aim was to determine if dopamine neurons are more active in adolescents than in adults and to examine mechanisms underlying any age-related difference. VTA dopamine neurons fired faster in adolescents than in adults as measured with in vivo extracellular recordings. Dopamine neuron firing can be divided into nonbursting (single spikes) and bursting activity (clusters of high-frequency spikes). Nonbursting activity was higher in adolescents compared with adults. Frequency of burst events did not differ between ages, but bursts were longer in adolescents than in adults. Elevated dopamine neuron firing in adolescent rats was also observed in cell-attached recordings in ex vivo brain slices. Using whole cell recordings, we found that passive and active membrane properties were similar across ages. Hyperpolarization-activated cation currents and small-conductance calcium-activated potassium channel currents were also comparable across ages. We found no difference in dopamine D2-class autoreceptor function across ages, although the high baseline firing in adolescents resulted in autoreceptor activation being less effective at silencing neurons. Finally, AMPA receptor-mediated spontaneous excitatory postsynaptic currents occurred at lower frequency in adolescents; GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents occurred at both lower frequency and smaller amplitude in adolescents. In conclusion, VTA dopamine neurons fire faster in adolescence, potentially because GABA tone increases as rats reach adulthood. This elevation of firing rate during adolescence is consistent with it representing a vulnerable period for developing drug addiction.

Heroin self-administration and reinstatement of heroin-seeking in adolescent vs. adult male rats

RATIONALE

Heroin abuse is prevalent among teenagers, and early onset drug use might predict long-term drug dependence. However, adolescent sensitivity to drug reinforcement has not been explored thoroughly in animal models.

OBJECTIVES

This study aimed to compare intravenous (i.v.) self-administration of heroin, as well as extinction and reinstatement of heroin-seeking, in adolescent vs. adult male rats.

METHODS

Adolescent (35 days old at start) and adult (86 days old at start) male Sprague-Dawley rats spontaneously acquired lever pressing maintained by i.v. heroin infusions. In experiment 1, self-administration was tested on a fixed ratio 1 schedule of reinforcement (0.05 and 0.025 mg/kg per infusion), followed by within-session extinction and reinstatement tests after 1 or 12 days of abstinence. In experiment 2, self-administration was tested on a progressive ratio schedule (0.0125-0.1 mg/kg per infusion), followed 12 days later by a single test of extinction responding in the presence of cues.

RESULTS

In experiment 1, adolescent rats self-administered more heroin than adults. After 1 or 12 days of abstinence, adolescents exhibited less heroin-seeking than adults, although levels of heroin-seeking increased over abstinence period for both age groups. In experiment 2, adolescents and adults reached the same maximal response ratio (breakpoint), although adolescents earned more infusions when response requirements were low. For extinction responding in the presence of cues, heroin-seeking was similar across ages.

CONCLUSIONS

Lower levels of heroin-seeking suggest that younger rats are less sensitive than adults to some residual effects of heroin intake.

Responding during signaled availability and nonavailability of iv cocaine and food in rats: age and sex differences

RATIONALE

Research suggests that age and sex are vulnerability factors for drug abuse. However, few studies have systematically examined their interaction.

OBJECTIVE

The purpose of the present study was to examine male and female, adult and adolescent rats under a procedure that measures responding during periods of signaled availability and nonavailability of iv cocaine and food reinforcers.

METHODS

Adolescent and adult rats lever pressed for iv infusions of cocaine or food pellets under a procedure with three components of signaled availability of the reinforcer alternating with two components of signaled nonavailability. Adolescent rats were removed and then later retested under the same conditions as adults, and a group of adult rats was also removed and retested after a similar number of days. A subset of rats earning cocaine infusions under the initial test was later retested with food pellets under the same behavioral task to assess the influence of prior cocaine exposure on subsequent responding for a nondrug reinforcer.

RESULTS

Adolescents (vs. adults) made more responses during periods of signaled iv cocaine availability and nonavailabiltiy, and adult females responded more than adult males during these periods. Responding during periods of signaled nonavailability of iv cocaine and food did not differ between the initial and subsequent retest conditions in adult rats. Further, adult males and females exposed to cocaine during adolescence responded more during periods of food availability compared to cocaine-naïve adults.

CONCLUSION

These results indicate that sex and age are vulnerability factors in cocaine abuse, and cocaine exposure during critical developmental stages can have long-lasting effects.

Role of individual and developmental differences in voluntary cocaine intake in rats

RATIONALE

Early-onset drug taking is associated with increased likelihood of addiction, but it is unclear whether early onset is causal in development of addiction. Many other factors are associated with increased risk of addiction and also promote early intake. Here, a rodent model is used to explore the causality of early onset in development of self-administration and addiction-like behavior and to examine factors that promote self-administration.

METHODS

We used cocaine self-administration to examine drug taking and addiction-like behavior in adolescent and adult rats a priori characterized for their locomotor responses to novelty and cocaine and behavior in the light-dark task.

RESULTS

Adolescent animals initially sought more cocaine than adults. However, as the adolescents matured, their intake fell and they did not differ from adults in terms of unreinforced lever-pressing, extinction or reinstatement behavior. For both age groups, self-administration was positively correlated with the locomotor response to novelty, the locomotor response to cocaine, and with time in light in the light-dark task. The rats that were insensitive to cocaine's locomotor effects and that spent the least time in light in the light-dark task sought the least cocaine, appearing to be "protected" from the reinforcing effects of cocaine. There was no difference between the two age groups in appearance of this "protected" phenotype.

CONCLUSIONS

These results suggest that early onset of drug taking may promote increased use, but does not promote progression to addiction-like behavior. Furthermore, protective factors, such as innate anxiety and insensitivity to cocaine's pharmacological effects, function across developmental stages.

Cognitive enhancers for facilitating drug cue extinction: insights from animal models

Given the success of cue exposure (extinction) therapy combined with a cognitive enhancer for reducing anxiety, it is anticipated that this approach will prove more efficacious than exposure therapy alone in preventing relapse in individuals with substance use disorders. Several factors may undermine the efficacy of exposure therapy for substance use disorders, but we suspect that neurocognitive impairments associated with chronic drug use are an important contributing factor. Numerous insights on these issues are gained from research using animal models of addiction. In this review, the relationship between brain sites whose learning, memory and executive functions are impaired by chronic drug use and brain sites that are important for effective drug cue extinction learning is explored first. This is followed by an overview of animal research showing improved treatment outcome for drug addiction (e.g. alcohol, amphetamine, cocaine, heroin) when explicit extinction training is conducted in combination with acute dosing of a cognitive-enhancing drug. The mechanism by which cognitive enhancers are thought to exert their benefits is by facilitating consolidation of drug cue extinction memory after activation of glutamatergic receptors. Based on the encouraging work in animals, factors that may be important for the treatment of drug addiction are considered.

Residual performance impairments in adult rats trained on an object discrimination task subsequent to cocaine administration during adolescence

The present study was conducted to determine whether cognitive impairments in adult rats treated with cocaine during adolescence demonstrated in previous investigations extend to tests of object discrimination learning. Accordingly, 30-day-old male Long-Evans rats were injected subcutaneously with either 10 or 20 mg/kg cocaine or received control injections of saline for 7-8 consecutive days. An extended abstinence period was then introduced (mean = 70.7 ± 9.8 days) before subjects, who were now young adults (mean = 106.3 ± 10.2 days old), were assessed for acquisition of a two-choice object discrimination task. Using a correctional learning procedure conducted in a water maze, subjects were trained (eight trials per day for 10 days) to approach one of two multi-dimentional 'junk' objects. Although all animals acquired the discrimination to a reasonable extent, cocaine-treated subjects exhibited lower percentages of correct choices over the course of training (10 mg/kg = 59.6 ± 7.2% and 20 mg/kg = 59.4 ± 4.9%) relative to the saline control group (67.5 ± 4.9%). Further analyses revealed that saline-treated subjects acquired proficient discrimination performance earlier during the course of training, achieving an approximate 72% performance rate after only 3 days of training. This was in contrast to the two cocaine-treated groups needing 7 days of training to achieve comparable levels of performance. In addition, saline-treated subjects required significantly fewer trials (20.8 ± 8.9) than either cocaine-treated group (10 mg/kg = 52.2 ± 11.9 and 20 mg/kg = 63.3 ± 8.7) to reach an 87.5% correct response criterion (i.e. 7-correct-out-of-8-consecutive-trials) and performed at a higher above-chance level (13.5%) than either cocaine-treated group (3.6% and 5.3% for the 10 and 20 mg/kg cocaine groups, respectively). These findings demonstrate the existence of cognitive impairments in adulthood subsequent to cocaine exposure during adolescence despite a prolonged drug-free interval. Speculation regarding the neurobiological basis for this effect, especially with regard to alterations to prefrontal circuitry, is provided.

Role of progesterone in nicotine addiction: evidence from initiation to relapse

Nicotine addiction continues to be the main cause of preventable death in developed countries. Women and teen girls appear to be more vulnerable on certain aspects of nicotine addiction compared with men and boys. While the mechanism of gender differences in nicotine addiction is not yet clear, evidence suggests that while estrogen may underlie enhanced vulnerability in females, progesterone may protect females. Thus, progesterone may have therapeutic use for tobacco addiction, especially in female smokers. A greater understanding of the role of progesterone in nicotine addiction is important not only from a treatment standpoint, but also from a prevention standpoint: hormone transition phases, such as those that occur at adolescence, and during pregnancy and following birth, as well as following hormonal manipulation (e.g., using methods of hormonal birth control), may all contribute to changes in vulnerability to nicotine addiction. In this review, we summarize recent evidence from clinical and preclinical studies examining the role of progesterone in nicotine addiction focusing on its role during initiation of use and during later phases of the addiction process as a potential relapse prevention treatment. We conclude with future directions including further examination of progesterone as a potential intervention and treatment of nicotine addiction.

The emergence of gonadal hormone influences on dopaminergic function during puberty

Adolescence is the developmental epoch during which children become adults-intellectually, physically, hormonally and socially. Brain development in critical areas is ongoing. Adolescents are risk-taking and novelty-seeking and they weigh positive experiences more heavily and negative experiences less than adults. This inherent behavioral bias can lead to risky behaviors like drug taking. Most drug addictions start during adolescence and early drug-taking is associated with an increased rate of drug abuse and dependence. The hormonal changes of puberty contribute to physical, emotional, intellectual and social changes during adolescence. These hormonal events do not just cause maturation of reproductive function and the emergence of secondary sex characteristics. They contribute to the appearance of sex differences in non-reproductive behaviors as well. Sex differences in drug use behaviors are among the latter. The male predominance in overall drug use appears by the end of adolescence, while girls develop the rapid progression from first use to dependence (telescoping) that represent a female-biased vulnerability. Sex differences in many behaviors including drug use have been attributed to social and cultural factors. A narrowing gap in drug use between adolescent boys and girls supports this thesis. However, some sex differences in addiction vulnerability reflect biologic differences in brain circuits involved in addiction. The purpose of this review is to summarize the contribution of sex differences in the function of ascending dopamine systems that are critical to reinforcement, to briefly summarize the behavioral, neurochemical and anatomical changes in brain dopaminergic functions related to addiction that occur during adolescence and to present new findings about the emergence of sex differences in dopaminergic function during adolescence.

Time-dependent increases in cue-induced reinstatement of sucrose seeking after sucrose self-administration in adolescence

Previously we reported that time-dependent increases in cue-induced reinstatement of cocaine seeking were attenuated in rats self-administering cocaine as adolescents, compared with adults (Li and Frantz (2009)). Now using sucrose self-administration, we report time-dependent increases in cue-induced reinstatement of sucrose seeking are similar across age groups, suggesting that age differences in reinstatement of cocaine seeking depend on specific effects of cocaine, not a compromised ability among younger rats to associate cues with rewards.

The effects of adolescent methylphenidate self-administration on responding for a conditioned reward, amphetamine-induced locomotor activity, and neuronal activation

BACKGROUND

Abuse of methylphenidate (Ritalin) is rising, particularly during adolescence and early adulthood, but the long-term effects of its abuse during adolescence are unclear.

METHODS

In experiment 1, we examined the effect of adolescent methylphenidate self-administration (0.0625 mg/infusion), as compared with cocaine self-administration (0.125 mg/infusion), under a fixed ratio 1 schedule of reinforcement in male Sprague-Dawley rats during adolescence (postnatal day (PND) 32-47) on adult dopamine-mediated behaviors (PND >70). These included responding for a conditioned reward (CR), a measure of incentive motivation, and amphetamine-induced locomotor activity. In experiment 2, we aimed to replicate and enhance the effects observed in experiment 1, and we also examined the effects of methylphenidate self-administration during adolescence on adult amphetamine-induced zif268 messenger ribonucleic acid (mRNA) expression.

RESULTS

Adolescent rats self-administered both cocaine and methylphenidate. There was no effect of adolescent drug self-administration on adult baseline or amphetamine-induced responding for a CR. However, both adolescent methylphenidate and cocaine self-administration increased amphetamine-induced locomotion. Adolescent methylphenidate self-administration also enhanced amphetamine-induced zif268 mRNA expression in the nucleus accumbens.

CONCLUSIONS

Our findings suggest that repeated, behaviorally contingent exposure to methylphenidate during adolescence enhances responsivity to the locomotor-stimulating and neuronal activating effects of amphetamine but not incentive motivation.

Reinstatement of cocaine seeking induced by drugs, cues, and stress in adolescent and adult rats

RATIONALE

In human and animal studies, adolescence marks a period of increased vulnerability to the initiation and subsequent abuse of drugs. Adolescents may be especially vulnerable to relapse, and a critical aspect of drug abuse is that it is a chronically relapsing disorder. However, little is known of how vulnerability factors such as adolescence are related to conditions that induce relapse, triggered by the drug itself, drug-associated cues, or stress.

OBJECTIVE

The purpose of this study was to compare adolescent and adult rats on drug-, cue-, and stress-induced reinstatement of cocaine-seeking behavior.

METHODS

On postnatal days 23 (adolescents) and 90 (adults), rats were implanted with intravenous catheters and trained to lever press for i.v. infusions of cocaine (0.4 mg/kg) during two daily 2-h sessions. The rats then self-administered i.v. cocaine for ten additional sessions. Subsequently, visual and auditory stimuli that signaled drug delivery were unplugged, and rats were allowed to extinguish lever pressing for 20 sessions. Rats were then tested on cocaine-, cue-, and yohimbine (stress)-induced cocaine seeking using a within-subject multicomponent reinstatement procedure.

RESULTS

Results indicated that adolescents had heightened cocaine seeking during maintenance and extinction compared to adults. During reinstatement, adolescents (vs adults) responded more following cocaine- and yohimbine injections, while adults (vs adolescents) showed greater responding following presentations of drug-associated cues.

CONCLUSION

These results demonstrated that adolescents and adults differed across several measures of drug-seeking behavior, and adolescents may be especially vulnerable to relapse precipitated by drugs and stress.

Effects of self-administered cocaine in adolescent and adult male rats on orbitofrontal cortex-related neurocognitive functioning

RATIONALE

Deficits in amygdala-related stimulus-reward learning are produced following 18 drug-free days of cocaine self-administration or its passive delivery in rats exposed during adulthood. No deficits in stimulus-reward learning are produced by cocaine exposure initiated during adolescence.

OBJECTIVES

To determine if age of initiating cocaine exposure differentially affects behavioral functioning of an additional memory system linked to cocaine addiction, the orbitofrontal cortex.

MATERIALS AND METHODS

A yoked-triad design (n = 8) was used. One rat controlled cocaine delivery and the other two passively received cocaine or saline. Rats controlling drug delivery (1.0 mg/kg) self-administered cocaine from either P37-P59 or P77-P99, and then underwent 18 drug-free days (P60-P77 vs. P100-P117). Rats next were tested for acquisition of odor-delayed win-shift behavior conducted over 15 sessions (P78-P96 vs. P118-P136).

RESULTS

Cocaine self-administration did not differ between adults and adolescents. During the test phase of the odor-delayed win-shift task (relatively difficult task demands), rats from both drug-onset ages showed learning deficits. Rats with cocaine self-administration experience committed more errors and had longer session latencies compared to rats passively receiving saline or cocaine. Rats with adolescent-onset cocaine self-administration experience showed an additional learning deficit by requiring more sessions to reach criterion levels for task acquisition compared to same-aged passive saline controls or rats with adult-onset cocaine self-administration experience. Rats passively receiving cocaine did not differ from the passive saline control from either age group.

CONCLUSIONS

Rats with adolescent-onset cocaine self-administration experience were more impaired in an orbitofrontal cortex-related learning task than rats with adult-onset cocaine self-administration experience.

Are adolescents more vulnerable to drug addiction than adults? Evidence from animal models

BACKGROUND AND RATIONALE

Epidemiological evidence suggests that people who begin experimenting with drugs of abuse during early adolescence are more likely to develop substance use disorders (SUDs), but this correlation does not guarantee causation. Animal models, in which age of onset can be tightly controlled, offer a platform for testing causality. Many animal models address drug effects that might promote or discourage drug intake and drug-induced neuroplasticity.

METHODS

We have reviewed the preclinical literature to investigate whether adolescent rodents are differentially sensitive to rewarding, reinforcing, aversive, locomotor, and withdrawal-induced effects of drugs of abuse.

RESULTS AND CONCLUSIONS

The rodent model literature consistently suggests that the balance of rewarding and aversive effects of drugs of abuse is tipped toward reward in adolescence. However, increased reward does not consistently lead to increased voluntary intake: age effects on voluntary intake are drug and method specific. On the other hand, adolescents are consistently less sensitive to withdrawal effects, which could protect against compulsive drug seeking. Studies examining neuronal function have revealed several age-related effects but have yet to link these effects to vulnerability to SUDs. Taken together, the findings suggest factors which may promote recreational drug use in adolescents, but evidence relating to pathological drug-seeking behavior is lacking. A call is made for future studies to address this gap using behavioral models of pathological drug seeking and for neurobiologic studies to more directly link age effects to SUD vulnerability.

Attenuated incubation of cocaine seeking in male rats trained to self-administer cocaine during periadolescence

RATIONALE AND OBJECTIVES

Although onset of drug use during adolescence appears to increase long-term vulnerability to drug dependence in humans, relatively little is known about extinction and reinstatement of drug seeking after periadolescent onset of drug self-administration in laboratory animals. Furthermore, although cue-induced reinstatement of cocaine seeking increases progressively during abstinence from cocaine self-administration in adult subjects, this "incubation of cocaine craving" remains unexplored after adolescent drug intake in animal models.

MATERIALS AND METHODS

We allowed periadolescent (postnatal day (PND) 35 at start) and adult (PND 83-95 at start) male Wistar rats to self-administer cocaine (0.36 mg/kg/infusion) in 2-h daily sessions on a fixed ratio 1 schedule of reinforcement over 14 days. Then, we compared extinction and cue-induced or cocaine priming-induced reinstatement (10 mg/kg cocaine, intraperitoneal) of cocaine seeking in both age groups after 30 days of abstinence in home cages. In separate cohorts, we tested for time-dependent increases in cue-induced reinstatement over approximately 1, 14, 30, or 60 days of abstinence in both age groups.

RESULTS

Adolescent and adult rats self-administered similar amounts of cocaine. Subsequent cue-induced reinstatement was lower in the adolescent-onset group after a 30-day abstinence period, but cocaine priming-induced reinstatement did not differ across ages. Also, extinction responding and time-dependent increases in cue-induced reinstatement (incubation) were less pronounced in rats that took cocaine as adolescents compared with adults.

CONCLUSIONS

Surprisingly, these results may reflect resistance among adolescent subjects to some enduring effects of drug self-administration, such as reward learning.

Age-dependent morphine intake and cue-induced reinstatement, but not escalation in intake, by adolescent and adult male rats

Despite increasing rates of opioid abuse by human adolescents, few laboratory experiments address adolescent vulnerability to opiates. We examined intravenous morphine self-administration after adolescent- vs. adult-onset, followed by extinction and cue-induced reinstatement. Adolescent male Sprague-Dawley rats [postnatal day (P) 35 at start] and adults (P91) acquired lever pressing maintained by 0.375 mg/kg/infusion morphine on a fixed ratio one schedule of reinforcement. Subjects were subsequently divided into short or long daily access conditions (ShAcc, 1-h vs. LgAcc, 8-h; 18 sessions). After extinction, cue-induced reinstatement was recorded over 1 h. During the first six 1-h acquisition sessions and continuing throughout ShAcc conditions, adolescent-onset rats self-administered less morphine than adults, an effect commonly interpreted as higher drug sensitivity. In contrast under LgAcc conditions, escalation of morphine intake was similar across ages. Extinction of drug-seeking was similar across ages, although rats from LgAcc conditions pressed more than ShAcc conditions. Notably, cue-induced reinstatement was less robust in rats that began morphine self-administration during adolescence vs. adulthood. Although increased sensitivity of younger rats to morphine reinforcement under ShAcc conditions might help explain opioid abuse by human adolescents, lower rates of reinstatement in younger rats might suggest that adolescent development includes some protective factors that dampen the long-term impact of early drug intake.

Novelty-induced locomotion is positively associated with cocaine ingestion in adolescent rats; anxiety is correlated in adults

The present studies assessed the roles of sex, age, novelty-seeking and plus-maze behavior on cocaine drinking in rats. Cocaine/saccharin solution was available in three daily, 5-hour sessions then a saccharin-only solution was also available in following sessions. In the one-bottle drinking phase, early and late adolescent males, post-natal day 28 (PN28) and PN42, consumed more cocaine/saccharin solution than young adults (PN65), but females did not exhibit significant age differences. Adolescents of both sexes consumed more cocaine/saccharin than adults during choice drinking. Saccharin availability in the two-bottle trials decreased cocaine/saccharin consumption in PN28 and PN65 rats. After a drug-free period, cocaine-stimulated locomotion was lower in cocaine/saccharin drinking than saccharin-only males, indicating tolerance. We tested the hypothesis that individual differences in pre-screened behavioral traits would correlate with cocaine/saccharin consumption in PN28 and PN65 male rats. High locomotor responses to novelty were associated with greater cocaine/saccharin drinking in adults in one-bottle sessions. In the subsequent choice drinking phase, correlations were age-specific. Adolescents with high novelty-induced locomotion and adults that spent less time on open arms of the elevated plus-maze drank more cocaine/saccharin. Thus, behavioral phenotypes correlated with individual differences in cocaine/saccharin consumption in an age-related manner.

Impaired object recognition following prolonged withdrawal from extended-access cocaine self-administration

Cocaine addicts have a number of cognitive deficits that persist following prolonged abstinence. These include impairments in executive functions dependent on the prefrontal cortex, as well as deficits on learning and memory tasks sensitive to hippocampal function. Recent preclinical studies using non-human animals have demonstrated that cocaine treatment can produce persistent deficits in executive functions, but there is relatively little evidence that treatment with cocaine produces persistent deficits in performance on hippocampal-dependent tasks. We recently demonstrated that extended (but not limited) access to self-administered cocaine is especially effective in producing persistent deficits on a test of cognitive vigilance, and therefore, we used this procedure to examine the effects of limited or extended access to cocaine self-administration on recognition memory performance, which is sensitive to hippocampal function. We found that extended access to cocaine produced deficits in recognition memory in rats that persisted for at least 2 weeks after the cessation of drug use. We conclude that the deficits in learning and memory observed in cocaine addicts may be at least in part due to repeated drug use, rather than just due to a pre-existing condition, and that in studying the neural basis of such deficits procedures involving extended access to self-administered cocaine may be especially useful.

Delayed extinction and stronger reinstatement of cocaine conditioned place preference in adolescent rats, compared to adults

Adolescence is a transitional period during development that is associated with a greater likelihood of addiction to drugs than any other age. One possibility for this observation is that learned associations between the rewarding experience of drugs and drug-related cues may produce greater motivational salience, and thus are more difficult to extinguish. Using an unbiased place-conditioning paradigm with two doses of cocaine (10 mg/kg or 20 mg/kg), the authors show here that adolescents require 75 +/- 17% more extinction trials than adults to extinguish cocaine place-preferences. Furthermore, once extinguished, adolescents display a greater preference for a previously cocaine-paired environment upon drug-primed reinstatement compared with adults. These results suggest that adolescent vulnerability to addiction involves robust memories for drug-associated cues that are difficult to extinguish. Therefore, drug-addicted adolescents may have a higher risk of relapse than adults, leading to greater prevalence of addiction in this population.

Cocaine increases stimulated dopamine release more in periadolescent than adult rats

The neural mechanisms responsible for the enhanced adolescent vulnerability for initiating drug abuse are unclear. We investigated whether age differences in dopamine neurotransmission could explain cocaine's enhanced psychomotor effects in the periadolescent rat. Electrical stimulation of the medial forebrain bundle of anesthetized post-natal age 28 days (PN28) and PN65 rats elicited dopamine release in caudate nucleus and nucleus accumbens core before and after 15 mg/kg cocaine i.p. Extracellular dopamine concentrations were greater in PN65 than PN28 caudate following 20 and 60 Hz stimulations and in the PN65 nucleus accumbens following 60 Hz stimulations. Cocaine increased dopamine concentrations elicited by 20 Hz stimulations 3-fold in the adult, but almost 9-fold in periadolescent caudate. Dopamine release rate was lower in the periadolescent caudate although total dopamine clearance was similar to that of adults. The periadolescent caudate achieved adult levels of clearance by compensating for a lower V(max) with higher uptake affinity. Tighter regulation of extracellular dopamine by the higher uptake/release ratio in periadolescents led to greater increases after cocaine. In nucleus accumbens, dopamine release and V(max) were lower in periadolescents than adults, but uptake affinity and cocaine effects were similar. Immaturity of dopamine neurotransmission in dorsal striatum may underlie enhanced acute responses to psychostimulants in adolescent rats and suggests a mechanism for the greater vulnerability of adolescent humans to drug addiction.

Acquisition and maintenance of cocaine self-administration in adolescent rats: effects of sex and gonadal hormones

RATIONALE

Previous work has shown that adult female rats are more sensitive than adult male rats to the reinforcing effects of cocaine, an effect that appears to be due, at least in part, to ovarian hormones.

OBJECTIVE

In this study, we examine sex differences in cocaine self-administration during adolescence, a period of marked hormonal change.

MATERIALS AND METHODS

Adolescent male and female Sprague-Dawley rats were trained to self-administer cocaine (0.75 mg/kg per infusion) under a fixed ratio 1 schedule (i.e., each response was reinforced by an infusion of cocaine) beginning on postnatal day 30. After acquisition, responding was assessed under a progressive-ratio schedule until postnatal day 50 with blood sampling occurring before the first five sessions to determine the relationship between gonadal hormones (i.e., estradiol, progesterone, and testosterone) and motivation for cocaine. Estrous cycle phase was monitored throughout the study. Separate groups of adolescent male and female rats were compared on the acquisition of and progressive-ratio responding for sucrose reinforcement.

RESULTS

Females acquired cocaine self-administration more readily than did males, and a greater percentage of females acquired self-administration. Under progressive-ratio testing conditions, adolescent females responded at higher levels than adolescent males to obtain cocaine infusions, and in females, responding was positively associated with levels of estradiol and greatest during estrus. No sex differences were observed for sucrose reinforcement.

CONCLUSION

These findings suggest that sex differences are relevant during adolescence with evidence implicating circulating estradiol level as a factor that contributes to the enhanced sensitivity in females to the reinforcing effects of cocaine.