Place Conditioning: Age-Related Changes in the Rewarding and Aversive Effects of Alcohol

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.

Sensitivity to the initial rewarding effects of alcohol: Influence of age, sex, and β-endorphin

BACKGROUND

Alcohol use disorders (AUDs) are widespread, devastating and complex. About 20% of people who consume alcohol develop problem use, accounting for over 5% of worldwide deaths. While numerous animal models have facilitated understanding of the consequences of excessive drinking, translational models allow for experimental manipulation of factors thought to contribute to AUD liability.

METHODS

We employ a single-exposure conditioned place preference assay (SE-CPP) to investigate the influence of age, sex and the opioid peptide β-endorphin (bE) on the initial rewarding effects of ethanol, a strong predictor of AUDs. Adolescent (PND28-35) and adult (PND70-90) male and female, control C57BL/6J and bE-deficient mice were tested following a single injection of 1.5 g/kg of ethanol. Following the SE-CPP test, animals were deeply anesthetized, sacrificed, and perfused, and the brains were subsequently sectioned at 40 microns and processed for immunohistochemical localization of c-fos. One-sample, two-tailed t-tests were used to assess drug preference or aversion and the locomotor effects of alcohol.

RESULTS

In general, adults were more sensitive to the effects of alcohol than adolescents, and outcomes depended on sex and bE. For example, among females, adolescents were stimulated by the drug, but insensitive to locomotor effects as adults, while among males, adolescents were insensitive and adults sedated. Wild-type adolescents of both sexes failed to evince initial subjective reward from the drug, but bE-deficient adolescents, and all adult subjects, preferred a context once associated with ethanol over one that had been paired with saline. c-fos immunoreactivity in multiple brain regions was attenuated in bE-deficient animals, though influences of both sex and bE grew stronger with age.

CONCLUSIONS

This study demonstrates the utility of the SE-CPP paradigm for elucidating factors that contribute to the liability for AUDs, and supports the growing body of research that shows that sensitivity to the rewarding effects of alcohol changes during the course of development. Our results also suggest that developmental contributions are sex-dependent, and may also depend on the influence of endogenous opioid signaling.

Vagus nerve damage increases alcohol intake and preference in a nonpreferring rat line: Relationship to vagal regulation of the hypothalamic-pituitary-adrenal axis

BACKGROUND

Clinical and preclinical research indicates that gastric weight loss surgeries, such as Roux-en-Y gastric bypass surgery, can induce alcohol use disorder (AUD). While numerous mechanisms have been proposed for these effects, one relatively unexplored potential mechanism is physical damage to the gastric branch of the vagus nerve, which can occur during bypass surgery. Therefore, we hypothesized that direct damage to the gastric branch of the vagus nerve, without altering other aspects of gastric anatomy, could result in increased alcohol intake.

METHODS

To test this hypothesis, we compared alcohol intake and preference in multiple models in male Sprague-Dawley rats that received selective gastric branch vagotomy (VX) with rats who underwent sham surgery. Because the vagus nerve regulates hypothalamic-pituitary-adrenal (HPA) axis function, and alterations to HPA function are critical to the escalation of non-dependent alcohol intake, we also tested the hypothesis that gastric VX increases HPA function.

RESULTS

We found that VX increases alcohol intake and preference in the every-other-day, two-bottle choice test and increases preference for 1 g/kg alcohol in the conditioned place preference test. The effects were selective for alcohol, as sucrose intake and preference were not altered by VX. We also found that VX increases corticotropin releasing factor (CRF) mRNA in the paraventricular nucleus of the hypothalamus (PVN), increases putative PVN CRF neuronal action potential firing, and increases corticosterone levels.

CONCLUSIONS

Overall, these findings suggest that the vagus nerve may play a critical role in regulating HPA axis function via modulation of PVN CRF mRNA expression and putative PVN CRF neuronal activity. Furthermore, disruptions to vagal regulation of HPA axis function may increase alcohol intake and preference.

Methamphetamine and social rewards interact to produce enhanced conditioned place preference in male adolescent rats

Elucidating the influence of social context on drug reward is critical for understanding substance use disorders. Adolescents demonstrate enhanced sensitivity to drug and social rewards. However, the extent to which methamphetamine interacts with social reward in adolescents has not been thoroughly examined. Therefore, the present study used the conditioned place preference (CPP) model to examine the relationship between methamphetamine and social rewards in adolescent male rats. Sprague-Dawley rats (PND 30) were randomly assigned to one of the following four conditioning groups: saline alone (SA), methamphetamine alone (MA), saline with a social partner (SS) or methamphetamine with a social partner (MS). Testing occurred in a two-chamber biased apparatus across seven consecutive days using parameters presumed to be sub-threshold for establishing social- and methamphetamine-induced CPP. Similar to previous reports for nicotine and cocaine, the present results indicate that rats receiving methamphetamine with a social partner (i.e., MS) during conditioning demonstrated a significantly greater preference shift compared to all other groups. These findings further highlight the importance of social context in influencing the magnitude of drug reward during adolescence.

Chronic Ethanol Exposure during Adolescence Increases Voluntary Ethanol Consumption in Adulthood in Female Sprague Dawley Rats

Early alcohol use is a major concern due to the dramatic rise in alcohol use during adolescence. In humans, adolescent males and females consume alcohol at equivalent rates; however, in adulthood males are more likely to consume harmful levels of alcohol. In animal models, the long-term dose-dependent and sex-dependent effects of alcohol exposure during adolescence have not been readily assessed relative to exposure that is initiated in adulthood. The purpose of the present set of experiments was to determine if adolescent exposure to chronic ethanol would predispose male and female rats to greater ethanol intake in adulthood when compared to animals that were not exposed to chronic ethanol exposure until early adulthood. Male and female rats were chronically administered 0.75 g/kg or 1.5 g/kg ethanol or saline for 21 days during adolescence (postnatal day (PND) 30–50) or adulthood (PND 60–80). All rats subsequently underwent 14-days of abstinence (PND 51–64 or PND 81–94, respectively). Finally, all rats were given 30-min daily access to saccharin-sweetened ethanol or saccharin alone from PND 65–80 for adolescent-exposed rats and PND 95–110 for adult-exposed rats. Exposure to 0.75 g/kg ethanol did not alter ethanol or saccharin intake in adolescent-exposed or adult-exposed rats, regardless of sex. In contrast, chronic exposure to the higher 1.5 g/kg dose during adolescence increased ethanol intake in adulthood in female rats. However, there was no change in saccharin intake in animals exposed to 1.5 g/kg ethanol during adolescence or adulthood, regardless of sex. Additionally, there were no clear age- and ethanol-dependent changes in duration of loss of righting reflex and blood ethanol concentrations to a challenge administration of a higher dose of ethanol. The results of the present set of experiments indicate chronic exposure to a high dose of ethanol during adolescence in female rats did indeed predispose rats to consume more ethanol in adulthood. Given that these effects were only observed in adolescent-exposed female rats, these results support a unique vulnerability to the long-term consequences of adolescent ethanol exposure in female rats, an effect that is not merely mediated by the sweetener used in the ethanol solution.

Early-life ketamine exposure attenuates the preference for ethanol in adolescent Sprague-Dawley rats

Ketamine, a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, produces quick and effective antidepressant results in depressed juvenile and adult individuals. The long-term consequences of using ketamine in juvenile populations are not well known, particularly as it affects vulnerability to drugs of abuse later in life, given that ketamine is also a drug of abuse. Thus, the current study examined whether early-life ketamine administration produces long-term changes in the sensitivity to the rewarding effects of ethanol, as measured using the conditioned place preference (CPP) paradigm. On postnatal day (PD) 21, juvenile male and female rats were pretreated with ketamine (0.0 or 20 mg/kg) for 10 consecutive days (i.e., PD 21-30) and then evaluated for ethanol-induced CPP (0.0, 0.125, 0.5, or 2.0 g/kg) from PD 32-39. Results revealed that early-life ketamine administration attenuated the rewarding properties of ethanol in male rats, as ketamine pretreated rats failed to exhibit ethanol-induced CPP at any dose compared to saline pretreated rats, which showed an increased preference towards the ethanol-paired compartment in a dose-dependent manner. In females, ethanol-induced CPP was generally less robust compared to males, but ketamine pretreatment resulted in a rightward shift in the dose-response curve, given that ketamine pretreated rats needed a higher dose of ethanol compared to saline pretreated rats to exhibit ethanol-induced CPP. When considered together, the findings suggest that early use of ketamine does not appear to enhance the vulnerability to ethanol later in life, but in contrast, it may attenuate the rewarding effects of ethanol.

Conditioning the neuroimmune response to ethanol using taste and environmental cues in adolescent and adult rats

IMPACT STATEMENT

A combined odor and taste cue was paired with a binge-like ethanol exposure (4 g/kg intraperitoneal) using a single-trial learning paradigm. Re-exposure to the CS alone was sufficient to evoke a conditioned Interleukin (IL)-6 elevation in the amygdala in adolescents, an effect that was not observed in young adults. This demonstrates a particular sensitivity of adolescents to alcohol-associated cues and neuroimmune learning, whereas prior work indicated that adults require multiple pairings of ethanol to the CS in order to achieve a conditioned amygdala IL-6 response. While the role of immune conditioning has been studied in other drugs of abuse, these findings highlight a previously unknown aspect of alcohol-related learning. Given the emergent importance of the neuroimmune system in alcohol abuse, these findings may be important for understanding cue-induced reinstatement of alcohol intake among problem drinkers.

Oral Binge-Like Ethanol Pre-Exposure During Juvenile/Adolescent Period Attenuates Ethanol-Induced Conditioned Place Aversion in Rats

Aims

To determine if oral ethanol self-administration produces a conditioned place preference (CPP) and to determine if ethanol pre-exposure conditions during the juvenile/adolescent period alter the conditioned effects of ethanol and subsequent ethanol self-administration.

Short summary

Modified conditioned place preference paradigm allowed rats to orally self-administer ethanol followed by short duration exposure to conditioning chambers. Ethanol produced a conditioned place aversion even though rats self-administered ethanol following the final conditioning test. Juvenile/adolescent pre-exposure to ethanol decreased the place aversion but did not produce place preference.

Methods

Juvenile/adolescent rats consumed sweetened 5% ethanol in the home-cage either during continuous access or intermittent access with water restriction that promoted binge-like consumption. A control group had water access during the 4-week period. Adult rats were conditioned using a modified CPP paradigm wherein rats were water-restricted overnight before being placed in operant chambers to respond for 5% ethanol for 7 min. Following the operant session, rats were placed in the conditioning chamber for 8 min. After the conditioning post-test, rats self-administered ethanol during daily operant sessions.

Results

Ethanol produced a conditioned place aversion in water access rats and the continuous access rats. Binge-like ethanol consumption induced by intermittent access with water restriction abolished the place aversion, but did not allow place preference to develop. After conditioning, continuous access rats self-administered ethanol above ~0.6 g/kg which was similar to rats with binge-like experience via intermittent access.

Conclusions

Results suggest that oral ethanol self-administration elicits aversive properties in this model even though ethanol continues to maintain self-administration. Pre-exposure to ethanol during the juvenile/adolescent period may produce tolerance to ethanol's aversive properties only when consumed in a binge-like manner with water restriction. More exploration is needed to understand how behavioral history can influence sensitivity to ethanol's rewarding and aversive properties and subsequent ethanol consumption or self-administration.

Effects of environmental enrichment upon ethanol-induced conditioned place preference and pre-frontal BDNF levels in adolescent and adult mice

Environmental enrichment (EE) provides a non-pharmacological tool to alter drug-induced reward, yet its effects on ethanol-induced reward remain controversial. We analyzed adolescent vs. adult (mice) differences in the influence of EE on ethanol-induced conditioned place preference (CPP). The effects of these treatments on brain-derived neurotrophic factor (BDNF) levels in the prefrontal cortex were examined in a separate group of animals. Ethanol-induced CPP was found in adults, and it was similar in EE and in animals reared under standard housing conditions (SC). Adolescents kept under EE, but not those in SC, exhibited CPP. Among SC, but not among EE, adolescents, BDNF levels were significantly lower in those treated with ethanol than in those given vehicle. These results indicate that, compared to adults, adolescent exhibited reduced sensitivity to ethanol’s rewarding effects, yet the youth but not the adults exhibited sensitivity to the promoting effect of EE upon CPP by ethanol. Ethanol significantly reduced BDNF levels in adolescents reared under standard housing conditions, but not in adult mice nor in adolescents given EE housing conditions. The present results add to the plethora of adolescent-specific responses to ethanol or to environmental stimuli that may put the youth at risk for escalation of ethanol intake.

Prenatal ethanol exposure modifies locomotor activity and induces selective changes in Met-enk expression in adolescent rats

Several studies suggest that prenatal ethanol exposure (PEE) facilitates ethanol intake. Opioid peptides play a main role in ethanol reinforcement during infancy and adulthood. However, PEE effects upon motor responsiveness elicited by an ethanol challenge and the participation of opioids in these actions remain to be understood. This work assessed the susceptibility of adolescent rats to prenatal and/or postnatal ethanol exposure in terms of behavioral responses, as well as alcohol effects on Met-enk expression in brain areas related to drug reinforcement. Motor parameters (horizontal locomotion, rearings and stereotyped behaviors) in pre- and postnatally ethanol-challenged adolescents were evaluated. Pregnant rats received ethanol (2g/kg) or water during gestational days 17-20. Adolescents at postnatal day 30 (PD30) were tested in a three-trial activity paradigm (habituation, vehicle and drug sessions). Met-enk content was quantitated by radioimmunoassay in several regions: ventral tegmental area [VTA], nucleus accumbens [NAcc], prefrontal cortex [PFC], substantia nigra [SN], caudate-putamen [CP], amygdala, hypothalamus and hippocampus. PEE significantly reduced rearing responses. Ethanol challenge at PD30 decreased horizontal locomotion and showed a tendency to reduce rearings and stereotyped behaviors. PEE increased Met-enk content in the PFC, CP, hypothalamus and hippocampus, but did not alter peptide levels in the amygdala, VTA and NAcc. These findings suggest that PEE selectively modifies behavioral parameters at PD30 and induces specific changes in Met-enk content in regions of the mesocortical and nigrostriatal pathways, the hypothalamus and hippocampus. Prenatal and postnatal ethanol actions on motor activity in adolescents could involve activation of specific neural enkephalinergic pathways.

Prior Exposure to Alcohol Has No Effect on Cocaine Self-Administration and Relapse in Rats: Evidence from a Rat Model that Does Not Support the Gateway Hypothesis

The gateway hypothesis posits that initial exposure to legal drugs promotes subsequent addiction to illicit drugs. However, epidemiological studies are correlational and cannot rule out the alternative hypothesis of shared addiction vulnerability to legal and illegal drugs. We tested the gateway hypothesis using established rat alcohol exposure procedures and cocaine self-administration and reinstatement (relapse) procedures. We gave Wistar or alcohol-preferring (P) rats intermittent access to water or 20% alcohol in their homecage for 7 weeks (three 24-h sessions/week). We also exposed Wistar rats to air or intoxicating alcohol levels in vapor chambers for 14-h/day for 7 weeks. We then tested the groups of rats for acquisition of cocaine self-administration using ascending cocaine doses (0.125, 0.25, 0.5, 1.0 mg/kg/infusion) followed by a dose-response curve after acquisition of cocaine self-administration. We then extinguished lever pressing and tested the rats for reinstatement of drug seeking induced by cocaine-paired cues and cocaine priming (0, 2.5, 5, 10 mg/kg, i.p.). Wistar rats consumed moderate amounts of alcohol (4.6 g/kg/24 h), P rats consumed higher amounts of alcohol (7.6 g/kg/24 h), and Wistar rats exposed to alcohol vapor had a mean blood alcohol concentration of 176.2 mg/dl during the last week of alcohol exposure. Alcohol pre-exposure had no effect on cocaine self-administration, extinction responding, and reinstatement of drug seeking. Pre-exposure to moderate, high, or intoxicating levels of alcohol had no effect on cocaine self-administration and relapse to cocaine seeking. Our data do not support the notion that alcohol is a gateway drug to cocaine.

Long-term ethanol self-administration induces ΔFosB in male and female adolescent, but not in adult, Wistar rats

Early-onset ethanol consumption predicts later development of alcohol use disorders. Age-related differences in reactivity to ethanol's effects may underlie this effect. Adolescent rats are more sensitive and less sensitive than adults to the appetitive and aversive behavioral effects of ethanol, respectively, and more sensitive to the neurotoxic effects of experimenter-administered binge doses of ethanol. However, less is known about age-related differences in the neural consequences of self-administered ethanol. ΔFosB is a transcription factor that accumulates after chronic drug exposure and serves as a molecular marker of neural plasticity associated with the transition to addiction. We analyzed the impact of chronic (18 two-bottle choice intake sessions spread across 42days, session length: 18h) ethanol [or only vehicle (control group)] self-administration during adolescence or adulthood on the induction of ΔFosB in several brain areas, anxiety-like behavior, and ethanol-induced locomotor activity and conditioned place preference (CPP) in Wistar rats. Adolescent rats exhibited a progressive escalation of ethanol intake and preference, whereas adult rats exhibited a stable pattern of ingestion. Few behavioral differences in the open field or light-dark test were observed after the intake test. Furthermore, ethanol self-administration did not promote the expression of ethanol-induced CPP. There were, however, large age-related differences in the neural consequences of ethanol drinking: a significantly greater number of ethanol-induced ΔFosB-positive cells was found in adolescents vs. adults in the prelimbic cortex, dorsolateral striatum, nucleus accumbens core and shell, and central amygdala nucleus capsular and basolateral amygdala, with sex-related differences found at central amygdala. This greater ethanol-induced ΔFosB induction may represent yet another age-related difference in the sensitivity to ethanol that may put adolescents at higher risk for problematic ethanol use.

Post-weaning Environmental Enrichment, But Not Chronic Maternal Isolation, Enhanced Ethanol Intake during Periadolescence and Early Adulthood

This study analyzed ethanol intake in male and female Wistar rats exposed to maternal separation (MS) during infancy (postnatal days 1-21, PD1-21) and environmental enrichment (EE) during adolescence (PD 21-42). Previous work revealed that MS enhances ethanol consumption during adulthood. It is still unknown if a similar effect is found during adolescence. Several studies, in turn, have revealed that EE reverses stress experiences, and reduces ethanol consumption and reinforcement; although others reported greater ethanol intake after EE. The interactive effects between these treatments upon ethanol's effects and intake have yet to be explored. We assessed chronic ethanol intake and preference (12 two-bottle daily sessions, spread across 30 days, 1st session on PD46) in rats exposed to MS and EE. The main finding was that male - but not female - rats that had been exposed to EE consumed more ethanol than controls given standard housing, an effect that was not affected by MS. Subsequent experiments assessed several factors associated with heightened ethanol consumption in males exposed to MS and EE; namely taste aversive conditioning and hypnotic-sedative consequences of ethanol. We also measured anxiety response in the light-dark box and in the elevated plus maze tests; and exploratory patterns of novel stimuli and behaviors indicative of risk assessment and risk-taking, via a modified version of the concentric square field (CSF) test. Aversive conditioning, hypnosis and sleep time were similar in males exposed or not to EE. EE males, however, exhibited heightened exploration of novel stimuli and greater risk taking behaviors in the CSF test. It is likely that the promoting effect of EE upon ethanol intake was due to these effects upon exploratory and risk-taking behaviors.

Behavioral sensitization to ethanol: Neural basis and factors that influence its acquisition and expression

Ethanol-induced behavioral sensitization (EBS) was first described in 1980, approximately 10 years after the phenomenon was described for psychostimulants. Ethanol acts on γ-aminobutyric acid (GABA) and glutamate receptors as an allosteric agonist and antagonist, respectively, but it also affects many other molecular targets. The multiplicity of factors involved in the behavioral and neurochemical effects of ethanol and the ensuing complexity may explain much of the apparent disparate results, found across different labs, regarding ethanol-induced behavioral sensitization. Although the mesocorticolimbic dopamine system plays an important role in EBS, we provide evidence of the involvement of other neurotransmitter systems, mainly the glutamatergic, GABAergic, and opioidergic systems. This review also analyses the neural underpinnings (e.g., induction of cellular transcription factors such as cyclic adenosine monophosphate response element binding protein and growth factors, such as the brain-derived neurotrophic factor) and other factors that influence the phenomenon, including age, sex, dose, and protocols of drug administration. One of the reasons that make EBS an attractive phenomenon is the assumption, firmly based on empirical evidence, that EBS and addiction-related processes have common molecular and neural basis. Therefore, EBS has been used as a model of addiction processes. We discuss the association between different measures of ethanol-induced reward and EBS. Parallels between the pharmacological basis of EBS and acute motor effects of ethanol are also discussed.

Cocaine self-administration punished by intravenous histamine in adolescent and adult rats

Adolescence is a transitional phase marked by a heightened vulnerability to substances of abuse. It has been hypothesized that both increased sensitivity to reward and decreased sensitivity to aversive events may drive drug-use liability during this phase. To investigate possible age-related differences in sensitivity to the aversive consequences of drug use, adolescent and adult rats were compared on self-administration of cocaine before, during, and after a 10-day period in which an aversive agent, histamine, was added to the cocaine solution. Adult and adolescent female rats were trained to self-administer intravenous cocaine (0.4 mg/kg/infusion) over 10 sessions (2 h/session; 2 sessions/day). Histamine (4 mg/kg/infusion) was then added directly into the cocaine solution for the next 10 sessions. Finally, the cocaine/histamine solution was replaced with a cocaine-only solution, and rats continued to self-administer cocaine (0.4 mg/kg) for 20 sessions. Compared with adolescent rats, adult rats showed a greater decrease in cocaine self-administration when it was punished with intravenous histamine compared with their baseline cocaine self-administration rates. These results suggest that differences in the sensitivity to negative consequences of drug use may partially explain developmental differences in drug use vulnerability.

The ontogeny of ethanol aversion

Recent work has suggested separate developmental periods within the broader framework of adolescence, with data suggesting distinct alterations and vulnerabilities within these intervals. While previous research has suggested reduced sensitivity to the aversive effects of alcohol in adolescence relative to adults, a more detailed ontogeny of this effect has yet to be conducted. The adolescent brain undergoes significant transitions throughout adolescence, including in regions linked with drug reward and aversion. The current study aimed to determine the ontogeny of ethanol aversion by utilizing a conditioned taste aversion procedure at six different ages to test the hypothesis that the transitions into, through, and out of adolescence are associated with ontogenetic alterations in sensitivity to the aversive properties of ethanol. Non-deprived animals given Boost® as the conditioned stimulus (CS) were used in Experiment 1, whereas Experiment 2 used water-restricted animals provided with a saccharin/sucrose solution as the CS. In both experiments, an attenuated sensitivity to the aversive properties of ethanol was evident in adolescents compared to adults, although more age differences were apparent in water deprived animals than when a highly palatable CS was given to ad libitum animals. Overall, the data suggest an attenuated sensitivity to the aversive properties of ethanol that is most pronounced during pre- and early adolescence, declining thereafter to reach the enhanced aversive sensitivity of adults.

Glutamatergic transmission in drug reward: implications for drug addiction

Individuals addicted to drugs of abuse such as alcohol, nicotine, cocaine, and heroin are a significant burden on healthcare systems all over the world. The positive reinforcing (rewarding) effects of the above mentioned drugs play a major role in the initiation and maintenance of the drug-taking habit. Thus, understanding the neurochemical mechanisms underlying the reinforcing effects of drugs of abuse is critical to reducing the burden of drug addiction in society. Over the last two decades, there has been an increasing focus on the role of the excitatory neurotransmitter glutamate in drug addiction. In this review, pharmacological and genetic evidence supporting the role of glutamate in mediating the rewarding effects of the above described drugs of abuse will be discussed. Further, the review will discuss the role of glutamate transmission in two complex heterogeneous brain regions, namely the nucleus accumbens (NAcc) and the ventral tegmental area (VTA), which mediate the rewarding effects of drugs of abuse. In addition, several medications approved by the Food and Drug Administration that act by blocking glutamate transmission will be discussed in the context of drug reward. Finally, this review will discuss future studies needed to address currently unanswered gaps in knowledge, which will further elucidate the role of glutamate in the rewarding effects of drugs of abuse.

Emergence of sex differences in the development of substance use and abuse during adolescence

Substance use and abuse begin during adolescence. Male and female adolescent humans initiate use at comparable rates, but males increase use faster. In adulthood, more men than women use and abuse addictive drugs. However, some women progress more rapidly from initiation of use to entry into treatment. In animal models, adolescent males and females consume addictive drugs similarly. However, reproductively mature females acquire self-administration faster, and in some models, escalate use more. Sex/gender differences exist in neurobiologic factors mediating both reinforcement (dopamine, opioids) and aversiveness (CRF, dynorphin), as well as intrinsic factors (personality, psychiatric co-morbidities) and extrinsic factors (history of abuse, environment especially peers and family) which influence the progression from initial use to abuse. Many of these important differences emerge during adolescence, and are moderated by sexual differentiation of the brain. Estradiol effects which enhance both dopaminergic and CRF-mediated processes contribute to the female vulnerability to substance use and abuse. Testosterone enhances impulsivity and sensation seeking in both males and females. Several protective factors in females also influence initiation and progression of substance use including hormonal changes of pregnancy as well as greater capacity for self-regulation and lower peak levels of impulsivity/sensation seeking. Same sex peers represent a risk factor more for males than females during adolescence, while romantic partners increase risk for women during this developmental epoch. In summary, biologic factors, psychiatric co-morbidities as well as personality and environment present sex/gender-specific risks as adolescents begin to initiate substance use.

Consequences of repeated ethanol exposure during early or late adolescence on conditioned taste aversions in rats

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Adolescent Intermittent Ethanol (AIE) resulted in attenuated CTA in adulthood.

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AIE in late adolescence resulted in greater CTA attenuation 2 days post-exposure.

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All AIE animals were equally insensitive when tested 3 weeks post-exposure.

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No metabolic tolerance was noted following AIE.

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AIE results in a decreased sensitivity to aversive effects of ethanol in adulthood.

Alcohol use is prevalent during adolescence, yet little is known about possible long-lasting consequences. Recent evidence suggests that adolescents are less sensitive than adults to ethanol's aversive effects, an insensitivity that may be retained into adulthood after repeated adolescent ethanol exposure. This study assessed whether intermittent ethanol exposure during early or late adolescence (early-AIE or late-AIE, respectively) would affect ethanol conditioned taste aversions 2 days (CTA1) and >3 weeks (CTA2) post-exposure using supersaccharin and saline as conditioning stimuli (CS), respectively. Pair-housed male Sprague-Dawley rats received 4 g/kg i.g. ethanol (25%) or water every 48 h from postnatal day (P) 25–45 (early AIE) or P45-65 (late AIE), or were left non-manipulated (NM). During conditioning, 30 min home cage access to the CS was followed by 0, 1, 1.5, 2 or 2.5 g/kg ethanol i.p., with testing 2 days later. Attenuated CTA relative to controls was seen among early and late AIE animals at both CTA1 and CTA2, an effect particularly pronounced at CTA1 after late AIE. Thus, adolescent exposure to ethanol was found to induce an insensitivity to ethanol CTA seen soon after exposure and lasting into adulthood, and evident with ethanol exposures not only early but also later in adolescence.

Repeated administration of a synthetic cannabinoid receptor agonist differentially affects cortical and accumbal neuronal morphology in adolescent and adult rats

Recent studies demonstrate a differential trajectory for cannabinoid receptor expression in cortical and sub-cortical brain areas across postnatal development. In the present study, we sought to investigate whether chronic systemic exposure to a synthetic cannabinoid receptor agonist causes morphological changes in the structure of dendrites and dendritic spines in adolescent and adult pyramidal neurons in the medial prefrontal cortex (mPFC) and medium spiny neurons (MSN) in the nucleus accumbens (Acb). Following systemic administration of WIN 55,212-2 in adolescent (PN 37-40) and adult (P55-60) male rats, the neuronal architecture of pyramidal neurons and MSN was assessed using Golgi-Cox staining. While no structural changes were observed in WIN 55,212-2-treated adolescent subjects compared to control, exposure to WIN 55,212-2 significantly increased dendritic length, spine density and the number of dendritic branches in pyramidal neurons in the mPFC of adult subjects when compared to control and adolescent subjects. In the Acb, WIN 55,212-2 exposure significantly decreased dendritic length and number of branches in adult rat subjects while no changes were observed in the adolescent groups. In contrast, spine density was significantly decreased in both the adult and adolescent groups in the Acb. To determine whether regional developmental morphological changes translated into behavioral differences, WIN 55,212-2-induced aversion was evaluated in both groups using a conditioned place preference paradigm. In adult rats, WIN 55,212-2 administration readily induced conditioned place aversion as previously described. In contrast, adolescent rats did not exhibit aversion following WIN 55,212-2 exposure in the behavioral paradigm. The present results show that synthetic cannabinoid administration differentially impacts cortical and sub-cortical neuronal morphology in adult compared to adolescent subjects. Such differences may underlie the disparate development effects of cannabinoids on behavior.

Viral over-expression of D1 dopamine receptors in the prefrontal cortex increase high-risk behaviors in adults: comparison with adolescents

RATIONALE

Adolescents are often described as "lacking brakes" resulting in an increase in several behaviors associated with risk for addiction. Prefrontal cortex dopamine and cortico-limbic interaction play an important role in addiction, and we have previously shown that the dopamine D1 receptor is elevated on prelimbic prefrontal output neurons in adolescent rats. We hypothesized that a constellation of risk-related behaviors is mediated by prefrontal output neuron expression of D1.

OBJECTIVES

We aimed to determine the role of the dopamine D1 receptor in behavioral and neural correlates of risk for addiction that are often observed in adolescents. Therefore, high-risk behaviors as well as subcortical D2 receptor expression were investigated in adult animals with experimentally elevated D1 on prefrontal glutamatergic neurons.

METHODS

A lentiviral vector that selectively expressed the D1 receptor within glutamate neurons was injected in the prelimbic prefrontal cortex of adult male rats. Place conditioning to cocaine, alcohol, and nicotine, as well as delay discounting, novelty preferences, anxiety, cocaine self-administration, and sucrose preferences were assessed.

RESULTS

Virally mediated D1 over-expression in adults leads to stronger drug-cue associations and greater consumption of sweet solutions, elevates bias towards immediate satisfaction rather than delaying gratification, decreases anxiety, and causes rats to work harder for and take more cocaine. Furthermore, elevated cortical D1 reduces D2 receptors in the accumbens (a putative risk marker).

CONCLUSIONS

Together, these data suggest a common mechanism for increased motivational drive to seek and consume substances with hedonic value, consistent with adolescent addictive processes.

Effect of age on methylphenidate-induced conditioned taste avoidance and related BDNF/TrkB signaling in the insular cortex of the rat

RATIONALE

Drug use and abuse is thought to be a function of the balance between its rewarding and aversive effects, such that the rewarding effects increase the likelihood of use while the drug's dissociable aversive effects limit it. Adolescents exhibit a shift in this balance toward reward, which may ultimately lead to increased use. Importantly, recent work shows that adolescents are also protected from the aversive effects of many abusable drugs as measured by conditioned taste avoidance (CTA). However, such effects of methylphenidate (MPH, widely prescribed to adolescents with ADHD) have not been characterized.

OBJECTIVES

The effect of age on MPH-induced CTA was assessed. In addition, MPH-induced changes in brain-derived neurotrophic factor (BDNF) activity in the insular cortex (IC) and central nucleus of the amygdala (CeA), known to be important to CTA, were examined and related to CTAs in adolescents and adults.

METHODS

CTAs induced by MPH (0, 10, 18, and 32 mg/kg) were assessed in adolescent (n = 34) and adult (n = 33) male Sprague Dawley rats. Following MPH CTA, IC and CeA tissue was probed for differences in BDNF and tropomyosin-related kinase receptor-B (TrkB) using Western blots.

RESULTS

Blunted expression of MPH CTA was observed in the adolescents versus adults, which correlated with generally attenuated adolescent BDNF/TrkB activity in the IC, but the drug effects ran contrary to the expression of CTA.

CONCLUSIONS

Adolescents are protected from the aversive effects of MPH versus adults, but further work is needed to characterize the possible involvement of BDNF/TrkB.

Effects of repeated exposure to morphine in adolescent and adult male C57BL/6J mice: age-dependent differences in locomotor stimulation, sensitization, and body weight loss

RATIONALE

Given evidence for age-related differences in the effects of drugs of abuse, surprisingly few preclinical studies have explored effects of opioids in adolescents (versus adults).

OBJECTIVES

This study compared the motor stimulating and ataxic effects of repeatedly administered morphine in adolescent, late adolescent, and adult mice.

METHODS

Mice were treated with saline or morphine (10-100 mg/kg, i.p.) once per day for 4 days, and morphine (3.2-56 mg/kg)-induced locomotion was assessed 3 days or 5 weeks later. Different mice were treated repeatedly with morphine and ataxia was measured.

RESULTS

Acute administration of morphine increased locomotion more in adolescents than in adults. Repeated morphine enhanced morphine-induced locomotion, assessed 3 days later, to a similar extent in each age group (minimum effective dose 17.8 mg/kg). This sensitization was still evident 5 weeks later when the adolescents had become adult, but was smaller and occurred at a higher dose (56 mg/kg). In animals treated repeatedly with morphine as adults, sensitization was no longer apparent 5 weeks later. Intermittent morphine was at least 10-fold less potent to produce body weight loss in adolescents than in adults. Repeated morphine did not alter morphine-induced ataxia at any age.

CONCLUSIONS

Compared with adults, adolescents were more sensitive to the acute locomotor stimulating effects of morphine and to its long-lasting locomotor sensitizing effects, consistent with overactivity of dopamine systems during adolescence. In contrast, adolescents were less sensitive than adults to body weight loss induced by intermittent morphine, an effect indicative of morphine withdrawal in adult rodents.

Ethanol conditioned place preference and alterations in ΔFosB following adolescent nicotine administration differ in rats exhibiting high or low behavioral reactivity to a novel environment

This study determined the effects of adolescent nicotine administration on adult alcohol preference in rats exhibiting high or low behavioral reactivity to a novel environment, and ascertained whether nicotine altered ΔFosB in the ventral striatum (vStr) and prefrontal cortex (PFC) immediately after drug administration or after rats matured to adulthood. Animals were characterized as exhibiting high (HLA) or low (LLA) locomotor activity in the novel open field on postnatal day (PND) 31 and received injections of saline (0.9%) or nicotine (0.56 mg free base/kg) from PND 35 to 42. Ethanol-induced conditioned place preference (CPP) was assessed on PND 68 following 8 days conditioning in a biased paradigm; ΔFosB was measured on PND 43 or PND 68. Following adolescent nicotine exposure, HLA animals demonstrated a CPP when conditioned with ethanol; LLA animals were unaffected. Further, adolescent nicotine exposure for 8 days increased levels of ΔFosB in limbic regions in both HLA and LLA rats, but this increase persisted into adulthood only in LLA animals. Results indicate that adolescent nicotine exposure facilitates the establishment of an ethanol CPP in HLA rats, and that sustained elevations in ΔFosB are not necessary or sufficient for the establishment of an ethanol CPP in adulthood. These studies underscore the importance of assessing behavioral phenotype when determining the behavioral and cellular effects of adolescent nicotine exposure.

Female rats display enhanced rewarding effects of ethanol that are hormone dependent

BACKGROUND

Ethanol (EtOH) abuse is a major health and economic concern, particularly for females who appear to be more sensitive to the rewarding effects of EtOH. This study compared sex differences to the rewarding and aversive effects of EtOH using place-conditioning procedures in rats.

METHODS

Separate groups of adult (male, female, ovariectomized [OVX] female) and adolescent (male and female) rats received EtOH (0, 0.5, 1.0, 2.0, or 2.5 g/kg, intraperitoneal) and were confined to their initially nonpreferred side of our conditioning apparatus for 30 minutes. On alternate days, they received saline and were confined to the other side. Following 5 drug pairings, the rats were retested for preference behavior. Separate cohorts of the same groups of rats were injected with a similar dose range of EtOH, and blood EtOH levels (BELs) were compared 30 minutes later.

RESULTS

EtOH produced rewarding or aversive effects in a dose-dependent manner. An intermediate dose of EtOH (1.0 g/kg) produced rewarding effects in adult female, but not in male or OVX female rats, suggesting that ovarian hormones facilitate the rewarding effects of EtOH. Similarly, this intermediate dose of EtOH produced rewarding effects in adolescent female, but not in male rats. The highest dose of EtOH (2.5 g/kg) produced aversive effects that were similar across all adult groups. However, the aversive effects of EtOH were lower in adolescents than adults, suggesting that adolescents are less sensitive to the aversive effects of EtOH. The aversive effects of EtOH did not vary across the estrous cycle in intact adult females. There were also no group differences in BELs, suggesting that our results are not related to EtOH metabolism.

CONCLUSION

Our results in rats suggest that human females may be more vulnerable to EtOH abuse due to enhanced rewarding effects of this drug that are mediated by the presence of ovarian hormones.

Ethanol inhibits excitatory neurotransmission in the nucleus accumbens of adolescent mice through GABAA and GABAB receptors

Age-related differences in various acute physiological and behavioral effects of alcohol have been demonstrated in humans and in other species. Adolescents are more sensitive to positive reinforcing properties of alcohol than adults, but the cellular mechanisms that underlie such a difference are not clearly established. We, therefore, assessed age differences in the ability of ethanol to modulate glutamatergic synaptic transmission in the mouse nucleus accumbens (NAc), a brain region importantly involved in reward mechanisms. We measured field excitatory postsynaptic potentials/population spikes (fEPSP/PS) in NAc slices from adolescent (22-30 days old) and adult (5-8 months old) male mice. We found that 50mM ethanol applied in the perfusion solution inhibits glutamatergic neurotransmission in the NAc of adolescent, but not adult, mice. This effect is blocked by the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline and by the GABAB receptor antagonist CGP 55845. Furthermore, bicuculline applied alone produces a stronger increase in the fEPSP/PS amplitude in adult mice than in adolescent mice. Activation of GABAA receptors with muscimol produces a stronger and longer lasting depression of neurotransmission in adolescent mice as compared with adult mice. Activation of GABAB receptors with SKF 97541 also depresses neurotransmission more strongly in adolescent than in adult mice. These results demonstrate that an increased GABA receptor function associated with a reduced inhibitory tone underlies the depressant action of ethanol on glutamatergic neurotransmission in the NAc of adolescent mice.

Use of the light/dark test for anxiety in adult and adolescent male rats

The light/dark (LD) test is a commonly used rodent test of unconditioned anxiety-like behavior that is based on an approach/avoidance conflict between the drive to explore novel areas and an aversion to brightly lit, open spaces. We used the LD test to investigate developmental differences in behavior between adolescent (postnatal day (PN) 28-34) and adult (PN67-74) male rats. We investigated whether LD behavioral measures reflect anxiety-like behavior similarly in each age group using factor analysis and multiple regression. These analyses showed that time in the light compartment, percent distance in the light, rearing, and latency to emerge into the light compartment were measures of anxiety-like behavior in each age group, while total distance traveled and distance in the dark compartment provided indices of locomotor activity. We then used these measures to assess developmental differences in baseline LD behavior and the response to anxiogenic drugs. Adolescent rats emerged into the light compartment more quickly than adults and made fewer pokes into the light compartment. These age differences could reflect greater risk taking and less risk assessment in adolescent rats than adults. Adolescent rats were less sensitive than adults to the anxiogenic effects of the benzodiazepine inverse agonist N-methyl-β-carboline-3-carboxamide (FG-7142) and the α₂ adrenergic antagonist yohimbine on anxiety-like behaviors validated by factor analysis, but locomotor variables were similarly affected. These data support the results of the factor analysis and indicate that GABAergic and noradrenergic modulation of LD anxiety-like behavior may be immature during adolescence.

Individual differences in cocaine conditioned taste aversion are developmentally stable and independent of locomotor effects of cocaine

Drugs of abuse induce complex motivational states in their users which have been shown to vary developmentally. In addition to developmental variation, interindividual variation in the rewarding and aversive effects of drugs of abuse is an important consideration. A rat model was used to assess whether the conditioned rewarding/aversive effects of cocaine were maintained as individuals matured from adolescence into adulthood. We tested rats in the cocaine conditioned taste aversion task as adolescents and again in adulthood. We observed a wide range of approach/avoidance behaviors in this task, and also observed that the relative interindividual differences in approach/avoidance are remarkably stable across the two developmental stages. Furthermore, we observed that these interindividual differences are not attributable to individual differences in cocaine-induced locomotor effects or individual differences in blood or brain cocaine levels. Taken together, these findings indicate that sensitivity to cocaine’s motivational effects is stable across development and part of a unique neurological process.

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.

Stress, κ manipulations, and aversive effects of ethanol in adolescent and adult male rats

Elevated ethanol use during adolescence, a potentially stressful developmental period, is accompanied by insensitivity to many aversive effects of ethanol relative to adults. Given evidence that supports a role for stress and the kappa opioid receptor (KOR) system in mediating aversive properties of ethanol and other drugs, the present study assessed the role of KOR antagonism by nor-binaltorphimine (nor-BNI) on ethanol-induced conditioned taste aversion (CTA) in stressed (exposed to repeated restraint) and non-stressed male rats (Experiment 1), with half of the rats pretreated with nor-BNI before stressor exposure. In Experiment 2, CTA induced by the kappa agonist U62,066 was also compared in stressed and non-stressed adolescents and adults. A highly palatable solution (chocolate Boost) was used as the conditioned stimulus (CS), thereby avoiding the need for water deprivation to motivate consumption of the CS during conditioning. No effects of stress on ethanol-induced CTA were found, with all doses eliciting aversions in adolescents and adults in both stress conditions. However, among stressed subjects, adults given nor-BNI before the repeated stressor displayed blunted ethanol aversion relative to adults given saline at that time. This effect of nor-BNI was not seen in adolescents, findings that support a differential role for the KOR involvement in ethanol CTA in stressed adolescents and adults. Results from Experiment 2 revealed that all doses of U62,066 elicited aversions in non-stressed animals of both ages that were attenuated in stressed animals, findings that support a modulatory role for stress in aversive effects of KOR activation. Collectively, these results suggest that although KOR sensitivity appears to be reduced in stressed subjects, this receptor system does not appear to contribute to age differences in ethanol-induced CTA under the present test circumstances.

Ethanol-induced locomotor activity in adolescent rats and the relationship with ethanol-induced conditioned place preference and conditioned taste aversion

Adolescent rats exhibit ethanol-induced locomotor activity (LMA), which is considered an index of ethanol's motivational properties likely to predict ethanol self-administration, but few studies have reported or correlated ethanol-induced LMA with conditioned place preference (CPP) by ethanol at this age. The present study assessed age-related differences in ethanol's motor stimulating effects and analyzed the association between ethanol-induced LMA and conventional measures of ethanol-induced reinforcement. Experiment 1 compared ethanol-induced LMA in adolescent and adult rats. Subsequent experiments analyzed ethanol-induced CPP and conditioned taste aversion (CTA) in adolescent rats evaluated for ethanol-induced LMA. Adolescent rats exhibit a robust LMA after high-dose ethanol. Ethanol-induced LMA was fairly similar across adolescents and adults. As expected, adolescents were sensitive to ethanol's aversive reinforcement, but they also exhibited CPP. These measures of ethanol reinforcement, however, were not related to ethanol-induced LMA. Spontaneous LMA in an open field was, however, negatively associated with ethanol-induced CTA.

Increased sensitivity to alcohol induced changes in ERK Map kinase phosphorylation and memory disruption in adolescent as compared to adult C57BL/6J mice

Adolescence is a critical period of brain development that is accompanied by increased probability of risky behavior, such as alcohol use. Emerging research indicates that adolescents are differentially sensitive to the behavioral effects of acute ethanol as compared to adults but the neurobiological mechanisms of this effect remain to be fully elucidated. This study was designed to evaluate effects of acute ethanol on extracellular signal-regulated kinase phosphorylation (p-ERK1/2) in mesocorticolimbic brain regions. We also sought to determine if age-specific effects of ethanol on p-ERK1/2 are associated with ethanol-induced behavioral deficits on acquisition of the hippocampal-dependent novel object recognition (NOR) test. Adolescent and adult C57BL/6J mice were administered acute ethanol (0 0.5, 1, or 3g/kg, i.p.). Brains were removed 30-min post injection and processed for analysis of p-ERK1/2 immunoreactivity (IR). Additional groups of mice were administered ethanol (0 or 1g/kg) prior to the NOR test. Analysis of p-ERK1/2 IR showed that untreated adolescent mice had significantly higher levels of p-ERK1/2 IR in the nucleus accumbens shell, basolateral amygdala (BLA), central amygdala (CeA), and medial prefrontal cortex (mPFC) as compared to adults. Ethanol (1g/kg) selectively reduced p-ERK1/2 IR in the dentate gyrus and increased p-ERK1/2 IR in the BLA only in adolescent mice. Ethanol (3g/kg) produced the same effects on p-ERK1/2 IR in both age groups with increases in CeA and mPFC, but a decrease in the dentate gyrus, as compared to age-matched saline controls. Pretreatment with ethanol (1g/kg) disrupted performance on the NOR test specifically in adolescents, which corresponds with the ethanol-induced inhibition of p-ERK1/2 IR in the hippocampus. These data show that adolescent mice have differential expression of basal p-ERK1/2 IR in mesocorticolimbic brain regions. Acute ethanol produces a unique set of changes in ERK1/2 phosphorylation in the adolescent brain that are associated with disruption of hippocampal-dependent memory acquisition.

Changes in histone acetylation in the prefrontal cortex of ethanol-exposed adolescent rats are associated with ethanol-induced place conditioning

Alcohol drinking during adolescence can induce long-lasting effects on the motivation to consume alcohol. Abnormal plasticity in reward-related processes might contribute to the vulnerability of adolescents to drug addiction. We have shown that binge-like ethanol treatment in adolescent rats induces alterations in the dopaminergic system and causes histone modifications in brain reward regions. Considering that histone acetylation regulates transcriptional activity and contributes to drug-induced alterations in gene expression and behavior, we addressed the hypothesis that ethanol is capable of inducing transcriptional changes by histone modifications in specific gene promoters in adolescent brain reward regions, and whether these events are associated with acquisition of place conditioning. After treating juvenile and adult rats with intermittent ethanol administration, we found that ethanol treatment upregulates histone acetyl transferase (HAT) activity in adolescent prefrontal cortex and increases histone (H3 or H4) acetylation and H3(K4) dimethylation in the promoter region of cFos, Cdk5 and FosB. Inhibition of histone deacetylase by sodium butyrate before ethanol injection enhances both up-regulation of HAT activity and histone acetylation of cFos, Cdk5 and FosB. Furthermore, co-administration of sodium butyrate with ethanol prolongs the extinction of conditioned place aversion and increased the reinstatement effects of ethanol in ethanol-treated adolescents, but not in ethanol-treated adult rats. These results indicate that ethanol exposure during adolescence induces chromatin remodeling, changes histone acetylation and methylation, and modify the effects of ethanol on place conditioning. They also suggest that epigenetic mechanisms might open up avenues to new treatments for binge drinking-induced drug addiction during adolescence.

Investigation of age-specific behavioral and proteomic changes in an animal model of chronic ethanol exposure

Alcohol use during adolescence represents a major health concern given that this is a period in which the brain continues to undergo critical developmental changes. Much behavioral research has been conducted in animal models of alcohol exposure, and a vulnerable period in adolescence has been identified that suggests lasting effects of ethanol exposure during adolescence. However, identification of molecular changes underlying the behavioral outcomes observed as a result from exposure to ethanol during adolescence remains a major technical challenge. In this chapter, we describe a method that allows for assessment of the effects of chronic ethanol exposure during adolescence relative to adulthood through global-scale analysis of protein expression as well as evaluation of behavioral responsivity in adolescent and adult rats. Results from this type of analysis can facilitate identification of age-specific molecular markers associated with behavioral changes following treatment with ethanol or in other animal models of drug abuse.

Age- and sex-related differences in the acquisition and reinstatement of ethanol CPP in mice

Many people begin to experiment with alcohol during adolescence, an important developmental period during which sex differences in the effects of ethanol appear. In the present study we evaluated the effect of ethanol (0, 0.625, 1.25 or 2.5 g/kg) on the acquisition of a conditioned place preference (CPP) in early and late adolescent male and female mice. In addition, we assessed the capacity of ethanol to induce reinstatement of the CPP after its extinction. CPP was induced in early and late adolescent females with 2.5 g/kg, and in early adolescent males with 1.25 or 2.5 g/kg of ethanol. No CPP was observed in late adolescent males. Priming with ethanol reinstated the CPP induced by the highest dose in early adolescent male and early and late adolescent female mice. Our data suggest that early adolescents of both sex and late adolescent females are particularly vulnerable to the effects of ethanol.

The developmental psychopathology of motivation in adolescence

Adolescence is a key period for the emergence of psychopathology, with many psychiatric disorders having their modal age-of-onset during this period. Relative to other periods of the lifespan, susceptibility to a number of psychiatric disorders is greatest during adolescence, particularly in females. In addition, disorders which emerge during adolescence appear to be more enduring and serious than those with a later onset. Although these psychiatric conditions may appear different from each other in terms of their associated behavioral signs or symptoms, this review will argue that they involve common alterations in motivational processes or disturbances in reward processing, although the direction of such changes (hypersensitivity vs. hyposensitivity to reward) and the stage of processing affected (reward anticipation vs. receipt) may differ across broader groupings of disorder. Recent behavioral, neuropsychological and neuroimaging research on reward processing in children, adolescents, and adults with these conditions will be described and evaluated. In addition, this article will consider what these studies tell us about their etiology and highlight gaps in our knowledge base. The review will also attempt to explain why adolescence is a period of elevated risk for the development of psychopathology, by discussing normative changes in reward processing in humans and animals.

Cannabinoid exposure in pubertal rats increases spontaneous ethanol consumption and NMDA receptor associated protein levels

Recent evidence suggests an involvement of the endocannabinoid system in the regulation of emotional behaviour and ethanol intake. Here we investigated age-specific acute behavioural effects of the cannabinoid receptor agonist WIN 55,212-2 (WIN) on anxiety-related behaviour and voluntary ethanol consumption in rats. Animals were treated with WIN (1.2 mg/kg)/vehicle at puberty onset on postnatal day (PD) 40, or at adulthood (PD 100). Animals were tested in the elevated plus-maze (EPM) and the light/dark emergence test (EMT) and for the initial response to alcohol in a free-choice ethanol consumption paradigm. Acute WIN treatment increased anxiety-related behaviours, and this effect was found to be partially more pronounced in pubertal than adult rats. Additionally, increased intake of higher ethanol solutions after cannabinoid treatment was only observed in pubertal rats. These drug-induced behavioural changes during puberty are paralleled by induction of the NR1 subunit of the NMDA receptor in the medial prefrontal cortex and the striatum. Moreover, pubertal but not adult WIN administration increased the levels of the scaffold protein Homer in these brain regions. Enhanced CB₁ receptor levels in the reinforcement system were also observed in pubertal compared to adult rats. These data support the notion that puberty is a highly vulnerable period for the aversive effects of cannabinoid exposure. In particular, augmented ethanol intake in pubertal cannabinoid-exposed animals might be related to some extent to increased emotional behaviour and in particular to enhanced NMDA and CB₁ receptor signalling.

Ethanol-induced modulation of synaptic output from the dorsolateral striatum in rat is regulated by cholinergic interneurons

The striatum is the largest input nucleus to the basal ganglia and associated with reward-based behavior. We assessed whether acute ethanol (EtOH) exposure could modulate synaptic efficacy in the dorsolateral striatum of juvenile Wistar rats. Since acute EtOH administration can both increase and decrease the probability of release of different neurotransmitters from synaptic terminals, we used field potential recordings to evaluate the net effect of EtOH on striatal output. We showed that 50mM EtOH but not 20, 80 or 100mM, depresses population spike (PS) amplitude in the dorsolateral striatum. This depression of synaptic output is insensitive to the N-methyl-d-aspartic acid (NMDA) receptor inhibitor DL-2-amino-5-phosphonopentanoic acid (AP-5, 50μM), but is blocked in slices treated with glycine receptor antagonists (strychnine, 1μM; PMBA, 50μM), nicotinic acetylcholine receptor antagonists (mecamylamine, 10μM; methyllycaconitine citrate (MLA), 40nM), or GABA(A) receptor inhibitors (picrotoxin, 100μM; bicuculline, 2μM, 20μM). A long-term facilitation of synaptic output, which is more pronounced in slices from adult Wistar rats, is detected following EtOH washout (50, 80, 100mM). This long-term enhancement of PS amplitude is regulated by cholinergic interneurons and completely blocked by mecamylamine, MLA or the non-selective muscarinic antagonist scopolamine (10μM). Administration of 100mM EtOH significantly depresses PS amplitude in scopolamine-treated slices, suggesting that EtOH exerts dual actions on striatal output that are initiated instantly upon drug wash-on. In conclusion, EtOH modulates striatal microcircuitry and neurotransmission in a way that could be of importance for understanding the intoxicating properties as well as the acute reward sensation of EtOH.

Ethanol induces second-order aversive conditioning in adolescent and adult rats

Alcohol abuse and dependence are considered public health problems, with an etiological onset often occurring during late childhood and adolescence, and understanding age-related differences in ethanol sensitivity is important. Low to moderate ethanol doses (0.5 and 2.0 g/kg, intragastrically [i.g.]) induce single-trial, appetitive second-order place conditioning (SOC) in adolescent, but not adult, rats. Recent studies have demonstrated that adolescents may be less sensitive than adults to the aversive properties of ethanol, reflected by conditioned taste aversion. The present study assessed the aversive motivational effects of high-dose ethanol (3.0 and 3.25 g/kg, i.g., for adolescents and adults, respectively) using SOC. Experiment 1 revealed similar blood and brain ethanol levels in adolescent and adult rats given 3.0 and 3.25 g/kg ethanol, respectively. In Experiment 2, animals received ethanol or vehicle paired with intraoral pulses of sucrose (conditioned stimulus 1 [CS1]). After one, two, or three conditioning trials, the rats were presented with the CS1 while in a distinctive chamber (CS2). When tested for CS2 preference, ethanol-treated animals exhibited reduced preference for the CS2 compared with controls. This result, indicative of ethanol-mediated aversive place conditioning, was similar for adolescents and adults; for females and males; and after one, two, or three training trials. In conjunction with previous results, the present study showed that, in adolescent rats subjected to SOC, ethanol's hedonic effects vary from appetitive to aversive as the ethanol dose increases. Adolescent and adult animals appear to perceive the postingestive effects of high-dose ethanol as similarly aversive when assessed by SOC.

High ethanol dose during early adolescence induces locomotor activation and increases subsequent ethanol intake during late adolescence

Adolescent initiation of ethanol consumption is associated with subsequent heightened probability of ethanol use disorders. The present study examined the relationship between motivational sensitivity to ethanol initiation in adolescent rats and later ethanol intake. Experiment 1 determined that ethanol induces locomotor activation shortly after administration but not if tested at a later post-administration interval. In Experiment 2, adolescent rats were assessed for ethanol-induced locomotor activation on postnatal Day 28. These animals were then evaluated for ethanol-mediated conditioned taste aversion and underwent a 16-day-long ethanol intake protocol. Ethanol-mediated aversive effects were unrelated to ethanol locomotor stimulation or subsequent ethanol consumption patterns. Ethanol intake during late adolescence was greatest in animals initiated to ethanol earliest at postnatal Day 28. Females that were more sensitive to ethanol's locomotor-activating effects showed a transient increase in ethanol self-administration. Blood ethanol concentrations during initiation were not related to ethanol-induced locomotor activation. Adolescent rats appeared sensitive to the locomotor-stimulatory effects of ethanol. Even brief ethanol exposure during adolescence may promote later ethanol intake.

An acetaldehyde-sequestering agent inhibits appetitive reinforcement and behavioral stimulation induced by ethanol in preweanling rats

Ethanol's motivational consequences have been related to the actions of acetaldehyde, a metabolic product of ethanol oxidation. The present study assessed the role of acetaldehyde in the motivational effects of ethanol on preweanling rats. In Experiment 1 pups (postnatal days 13-14, PD 13-14) were given systemic administration of D-penicillamine (DP, a drug that sequesters acetaldehyde: 0, 25, 50 or 75 mg/kg) before pairings of 1.0 g/kg ethanol and a rough surface (sandpaper, conditioned stimulus, CS). At test, pups given sandpaper-ethanol pairings exhibited greater preference for the CS than unpaired controls, but this preference was not expressed by pups given DP. Pre-training administration of 25 or 50 mg/kg DP completely blocked the expression of ethanol-mediated appetitive conditioning. D-penicillamine did not alter blood ethanol levels. Subsequent experiments revealed that ethanol-induced activation was blocked by central (intra-cisterna magna injections, volume: 1 μl, dose: 0 or 75 μg) but not systemic treatment with DP (0, 25, 50 or 75 mg/kg; ip). These results indicate that: (a) preweanling rats are sensitive to the reinforcing effect of ethanol, and (b) that this effect is associated with the motor activating effect of the drug. These effects seem to be mediated by the first metabolite of ethanol, acetaldehyde.

Aversive effects of ethanol in adolescent versus adult rats: potential causes and implication for future drinking

BACKGROUND

Many people experiment with alcohol and other drugs of abuse during their teenage years. Epidemiological evidence suggests that younger initiates into drug taking are more likely to develop problematic drug seeking behavior, including binge and other high-intake behaviors. The level of drug intake for any individual depends on the balance of rewarding and aversive effects of the drug in that individual. Multiple rodent studies have demonstrated that aversive effects of drugs of abuse are reduced in adolescent compared to adult animals. In this study, we addressed 2 key questions: First, do reduced aversive effects of ethanol in younger rats correlate with increased ethanol consumption? Second, are the reduced aversive effects in adolescents attributable to reduced sensitivity to ethanol's physiologic effects?

METHODS

Adolescent and adult rats were tested for ethanol conditioned taste aversion (CTA) followed by a voluntary drinking period, including postdeprivation consumption. Multivariate regression was used to assess correlations. In separate experiments, adolescent and adult rats were tested for their sensitivity to the hypothermic and sedative effects of ethanol, and for blood ethanol concentrations (BECs).

RESULTS

We observed that in adolescent rats but not adults, taste aversion was inversely correlated with postdeprivation consumption. Adolescents also exhibited a greater increase in consumption after deprivation than adults. Furthermore, the age difference in ethanol CTA was not attributable to differences in hypothermia, sedation, or BECs.

CONCLUSIONS

These results suggest that during adolescence, individuals that are insensitive to aversive effects are most likely to develop problem drinking behaviors. These results underscore the importance of the interaction between developmental stage and individual variation in sensitivity to alcohol.

Naloxone blocks ethanol-mediated appetitive conditioning and locomotor activation in adolescent rats

Age-related differences in ethanol sensitivity could put adolescents at risk for developing alcohol-related problems. Little information exists, however, about adolescent sensitivity to ethanol's appetitive effects and the neurobiological mechanisms underlying ethanol reinforcement during this developmental stage. The present study assessed the role of the opioid system in adolescent rats in an appetitive second-order schedule of ethanol reinforcement and ethanol-induced locomotor stimulation. On postnatal day 32 (PD32), animals were pretreated with the general opioid antagonist naloxone (0.0, 0.75, 1.50, or 2.5 mg/kg) and then given pairings of ethanol (0.0 or 2.0 g/kg, intragastrically) with intraoral pulses of water (conditioned stimulus 1 [CS₁], first-order conditioning phase). CS₁ delivery occurred 30-45 min after ethanol administration when the effect of ethanol was assumed to be appetitive. On PD33, adolescents were exposed to CS₁ (second-order conditioning phase) while in a chamber featuring distinctive exteroceptive cues (CS₂). Preference for CS₂ was then tested. Adolescents given CS₁-ethanol pairings exhibited greater preference for CS₂ than controls, indicating ethanol-mediated reinforcement, but only when not pretreated with naloxone. Blood alcohol levels during conditioning were not altered by naloxone. Experiment 2 revealed that ethanol-induced locomotor activation soon after administration, and naloxone dose-dependently suppressed this stimulating effect. The present study indicates that adolescent rats are sensitive to ethanol's reinforcing and locomotor-stimulating effects. Both effects of ethanol appear to be mediated by endogenous opioid system activation.

The interaction of gestational and postnatal ethanol experience on the adolescent and adult odor-mediated responses to ethanol in observer and demonstrator rats

BACKGROUND

Gestational ethanol exposure enhances the adolescent reflexive sniffing response to ethanol odor. Postnatal exposures of naïve animals as either an observer (i.e., conspecific) or demonstrator (i.e., intoxicated peer) using a social transmission of food odor preference paradigm also yields enhanced odor-mediated responses. Studies on the interaction of fetal and postnatal exposures using the social transmission paradigm have been limited to the responses of observers. When combined, the enhanced response is greater than either form of exposure alone and, in observer females, yields adult persistence. The absence of a male effect is noteworthy, given that chemosensory mechanisms are suggested to be an important antecedent factor in the progression of ethanol preference. Observers gain odor information on the breath of the demonstrator through social interaction. Demonstrators experience the pharmacologic properties of ethanol along with retronasal and hematogenic olfaction. Thus, we tested whether augmentation of the fetal ethanol-induced behavioral response with postnatal exposure as a demonstrator differed from that as an observer. We also examined whether re-exposure as a demonstrator yields persistence in both sexes.

METHODS

Pregnant dams were fed an ethanol containing or control liquid diet throughout gestation. Progeny received four ethanol or water exposures: one every 48 hours through either intragastric infusion or social interaction with the infused peer beginning on P29. The reflexive behavioral sniffing response to ethanol odor was tested at postnatal (P) day 37 or P90, using whole-body plethysmography.

RESULTS

When tested in either adolescence or adulthood - fetal ethanol exposed adolescent ethanol observers and demonstrators significantly differed in their odor-mediated response to ethanol odor both between themselves and from their respective water controls. Nonetheless, adolescent ethanol re-exposure as a demonstrator, like an observer, enhanced the reflexive sniffing response to ethanol odor at both testing ages by augmenting the known effects of prior fetal ethanol experience. At each age, the magnitude of the enhanced odor response in demonstrators was similar to that of observers. Interestingly, only re-exposure as a demonstrator resulted in persistence of the behavioral response into adulthood in both sexes.

CONCLUSIONS

The method of ethanol re-exposure plays an important role in prolonging the odor-mediated effects of fetal exposure. While ethanol odor-specific exposure through social interaction is important, additional factors such as the pairing of retronasal and hematogenic olfaction with ethanol's intoxicating properties appear necessary to achieve persistence in both sexes.

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.

Genetic and environmental influences on ethanol consumption: perspectives from preclinical research

BACKGROUND

Alcohol use disorders (abuse and dependence, AUD) are multifactorial phenomena, depending on the interplay of environmental and genetic variables.

METHOD

This review describes current developments in animal research that may help (a) develop gene therapies for the treatment of alcoholism, (b) understand the permissive role of stress on ethanol intake, and (c) elucidate why exposure to ethanol early in life is associated with a greater risk of AUD.

RESULTS

The polymorphisms found in liver alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) affect the elimination of ethanol and the susceptibility to ethanol intake. A highly active ADH protects against alcoholism, an effect related to a presteady state burst in arterial acetaldehyde. Social stressors, such as repeated early maternal separation or social defeat, exert a permissive effect on ethanol intake, perhaps by altering the normal development of the hypothalamic-pituitary-adrenal axis. Ethanol exposure during gestation, infancy, or adolescence increases the likelihood of AUD later in life. Early perception of ethanol's positive and negative (anti-anxiety) reinforcing effects may play a role in this phenomenon.

CONCLUSIONS

The review underscores the advantages of using preclinical animal models of AUD and highlights points of intersection between the topics to help design a more integrated approach for the study of alcohol-related problems.