Adolescent and adult heart rate responses to self-administered ethanol

Ovariectomy Exacerbates Acute Ethanol-Induced Tachycardia: Role of Nitric Oxide and NMDA Receptors in the Rostral Ventrolateral Medulla

Ethanol consumption influences cardiovascular functions. In humans, acute consumption of ethanol causes dose-dependent tachycardia. Our previous study showed that ethanol-induced tachycardia might involve decreased nitric oxide (NO) signaling in the brain's medulla. NMDA receptors, another important target of ethanol, are one of the upstream signals of nitric oxide. Reports showed the modulation of NMDA receptor function by estrogen or estrogen receptors. The present study aims to examine the hypothesis that depletion of estrogen by ovariectomy (OVX) might modulate ethanol-induced tachycardia by regulating NMDA receptor function and NO signaling in the cardiovascular regulatory nucleus of the brain. Ethanol (3.2 g/kg, 40% v/v, 10 mL/kg) or saline (10 mL/kg) was administered by oral gavage in sham or OVX female Sprague-Dawley (SD) rats. The blood pressure (BP) and heart rate (HR) were measured using the tail-cuff method. The levels of phosphoserine 896 of the GluN1 subunit (pGluN1-serine 896) and NMDA GluN1 subunits (GluN1) were determined by immunohistochemistry. The expressions of nitric oxide synthase (NOS) and estrogen receptors in the tissue were measured by Western blotting. Nitric oxide contents were measured as total nitrate-nitrite by colorimetric assay kit. In a 2-h observation, there was no significant change in BP between the saline and ethanol groups. However, compared with saline, ethanol caused an increase in HR (tachycardia) in sham control or OVX rats. Interestingly, ethanol produced more significant tachycardia in the OVX group than in the sham control group. Nitric oxide levels were lower in the area of the rostral ventrolateral medulla (RVLM) 60 min following ethanol administration in OVX compared with sham control, without significant changes in the expression of NOS and estrogen receptors (ERα and ERβ). In addition, a decrease in the immunoreactivity of pGluN1-serine 896, without significant changes in GluN1, was found in neurons of RVLM 40 min following ethanol administration in OVX compared with sham control. Our results suggest that depletion of estradiol (E2) by OVX might exacerbate the tachycardia following ethanol administration, the underlying mechanism of which might be associated with decreased NMDA receptor function and NO level in the RVLM.

Effects of voluntary adolescent intermittent alcohol exposure and social isolation on adult alcohol intake in male rats

Initiating alcohol use in adolescence significantly increases the likelihood of developing adult alcohol use disorder (AUD). However, it has been difficult to replicate adolescent alcohol exposure leading to increased adult alcohol intake across differing preclinical models. In the present study, differentially housed male rats (group vs. single cages) were used to determine the effects of voluntary intermittent exposure of saccharin-sweetened ethanol during adolescence on adult intake of unsweetened 20% ethanol. Adolescent male rats were assigned to group- or isolated-housing conditions and underwent an intermittent 2-bottle choice in adolescence (water only or water vs. 0.2% saccharin/20% ethanol), and again in adulthood (water vs. 20% ethanol). Intermittent 2-bottle choice sessions lasted for 24 h, and occurred three days per week, for five weeks. Rats were moved from group or isolated housing to single-housing cages for 2-bottle choice tests and returned to their original housing condition on off days. During adolescence, rats raised in isolated-housing conditions consumed significantly more sweetened ethanol than rats raised in group-housing conditions, an effect that was enhanced across repeated exposures. In adulthood, rats raised in isolated-housing conditions and exposed to sweetened ethanol during adolescence also consumed significantly higher levels of unsweetened 20% ethanol compared to group-housed rats. The effect was most pronounced over the first five re-exposure sessions. Housing conditions alone had little effect on adult ethanol intake. These preclinical results suggest that social isolation stress, combined with adolescent ethanol exposure, may play a key role in adult AUD risk.

Primed for addiction: A critical review of the role of microglia in the neurodevelopmental consequences of adolescent alcohol drinking

Alcohol is one of the most widely used recreational substances worldwide, with drinking frequently initiated during adolescence. The developmental state of the adolescent brain makes it vulnerable to initiating alcohol use, often in high doses, and particularly susceptible to alcohol-induced brain changes. Microglia, the brain parenchymal macrophages, have been implicated in mediating some of these effects, though the role that these cells play in the progression from alcohol drinking to dependence remains unclear. Microglia are uniquely positioned to sense and respond to central nervous system insult, and are now understood to exhibit innate immune memory, or "priming," altering their future functional responses based on prior exposures. In alcohol use disorders (AUDs), the role of microglia is debated. Whereas microglial activation can be pathogenic, contributing to neuroinflammation, tissue damage, and behavioral changes, or protective, it can also engage protective functions, providing support and mediating the resolution of damage. Understanding the role of microglia in adolescent AUDs is complicated by the fact that microglia are thought to be involved in developmental processes such as synaptic refinement and myelination, which underlie the functional maturation of multiple brain systems in adolescence. Thus, the role microglia play in the impact of alcohol use in adolescence is likely multifaceted. Long-term sequelae may be due to a failure to recover from EtOH-induced tissue damage, altered neurodevelopmental trajectories, and/or persistent changes to microglial responsivity and function. Here, we review critically the literature surrounding the effects of alcohol on microglia in models of adolescent alcohol misuse. We attempt to disentangle what is known about microglia from other neuroimmune effectors, to which we apply recent discoveries on the role of microglia in development and plasticity. Considered altogether, these studies challenge assumptions that proinflammatory microglia drive addiction. Alcohol priming microglia and thereby perturbing their homeostatic roles in neurodevelopment, especially during critical periods of plasticity such as adolescence, may have more serious implications for the neuropathogenesis of AUDs in adolescents.

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.

Impact of Sleep and Circadian Rhythms on Addiction Vulnerability in Adolescents

Sleep homeostasis and circadian function are important maintaining factors for optimal health and well-being. Conversely, sleep and circadian disruptions are implicated in a variety of adverse health outcomes, including substance use disorders. These risks are particularly salient during adolescence. Adolescents require 8 to 10 hours of sleep per night, although few consistently achieve these durations. A mismatch between developmental changes and social/environmental demands contributes to inadequate sleep. Homeostatic sleep drive takes longer to build, circadian rhythms naturally become delayed, and sensitivity to the phase-shifting effects of light increases, all of which lead to an evening preference (i.e., chronotype) during adolescence. In addition, school start times are often earlier in adolescence and the use of electronic devices at night increases, leading to disrupted sleep and circadian misalignment (i.e., social jet lag). Social factors (e.g., peer influence) and school demands further impact sleep and circadian rhythms. To cope with sleepiness, many teens regularly consume highly caffeinated energy drinks and other stimulants, creating further disruptions in sleep. Chronic sleep loss and circadian misalignment enhance developmental tendencies toward increased reward sensitivity and impulsivity, increasing the likelihood of engaging in risky behaviors and exacerbating the vulnerability to substance use and substance use disorders. We review the neurobiology of brain reward systems and the impact of sleep and circadian rhythms changes on addiction vulnerability in adolescence and suggest areas that warrant additional research.

Involvement of NMDA Receptors, Nitric Oxide, and GABA in Rostral Ventrolateral Medulla in Acute Ethanol-Induced Cardiovascular Responses in Rats

BACKGROUND

Consumption of ethanol (EtOH) (alcohol) has many effects on physiological functions, particularly those in the central nervous system (CNS) and cardiovascular system. Acute excessive intake of EtOH (alcohol intoxication) may cause hypotension and tachycardia. In this study, we examined the mechanistic involvement of glutamatergic N-methyl-d-aspartate (NMDA) receptors, nitric oxide (NO), and γ-aminobutyric acid (GABA) pathways in the CNS in acute EtOH-induced cardiovascular effects.

METHODS

EtOH was administered by intraperitoneal (IP) injection in Sprague-Dawley rats. The blood pressure (BP) and heart rate (HR) were measured in conscious and in urethane-anesthetized rats. Inhibitors were applied by intracerebroventricular (ICV) injection or by microinjection into rostral ventrolateral medulla (RVLM). Microdialysis was used to determine the level of glutamate, NO, and GABA in the RVLM.

RESULTS

IP injection of EtOH (3.2 g/kg) caused a significant decrease in BP in conscious and anesthetized rats and a late increase in HR in conscious rats. The cardiovascular effects of EtOH were significantly attenuated by ICV or by RVLM post treatment with ketamine (an NMDA receptor antagonist), N5-(nitroamidino)-L-2,5-diaminopentanoic acid (L-NNA; a NO synthase inhibitor), or bicuculline (a GABA receptor antagonist). EtOH caused an increase in the level of glutamate, NO, and GABA in the RVLM during the hypotensive responses. RVLM posttreatment with ketamine blocked the increase in NO and GABA levels; post treatment with L-NNA blocked the increase in GABA level.

CONCLUSIONS

Our results indicate that EtOH augmentation of glutamatergic NMDA receptors/NO/GABA pathways in the RVLM may participate in the hypotensive effects induced by acute administration of EtOH.

Role of neuronal nitric oxide synthase (nNOS) at medulla in tachycardia induced by repeated administration of ethanol in conscious rats

Background

Intake of ethanol (alcohol) has been shown to influence cardiovascular function; the underlying brain mechanism remains unclear. Noting that nitric oxide (NO) system in the CNS is involved in the regulation of cardiovascular function, the present study examined the role of NO in medulla in ethanol-induced cardiovascular changes.

Methods

Ethanol was administered by oral gavage at dose of 3.2 g/kg once every day for 8 consecutive days. Changes in blood pressure (BP) and heart rate (HR) in response to ethanol were measured by radiotelemetry method in freely moving female Sprague-Dawley rats. NO modulators were applied by intracerebroventricular (ICV) injection. The protein levels of nitric oxide synthase (NOS) and NO content in rostroventral medulla were measured by Western blot and nitrate/nitrite colorimetric assay kit, respectively.

Results

Ethanol intake had little effects on basal BP and HR following 8 consecutive day treatments. A significant increase in HR but not BP following ethanol intake was observed at 6th and 8th, but not at 1st and 4th day treatments as compared with saline group. A decrease in the protein expression of neuronal NOS (nNOS) but not inducible NOS or endothelial NOS and a decline in the level of NO in the medulla 30 min after ethanol administration was observed at 8th day treatment. ICV treatment with NO donors attenuated ethanol-induced tachycardia effects at 8th day treatment. Ethanol produced significantly tachycardia responses when ICV nNOS inhibitors were given at 1st day treatment.

Conclusion

Our results suggest that medulla nNOS/NO pathways play an important role in ethanol regulation of HR.

Ethanol-induced autonomic responses and risk taking increase in young adults with a positive family history of alcohol problems

The mechanisms that underlie the greater prevalence of alcohol use disorders in individuals with a positive family history (FH+) of alcohol abuse are still under investigation. These subjects may exhibit differential sensitivity to alcohol's effects on psychomotor stimulation and impulsivity. Alcohol-induced psychomotor stimulation, measured as the heart rate (HR) response, is a proxy for the positive rewarding effects of the drug. We analyzed alcohol-induced effects on time perception (Time Production Task), risk taking (Balloon Analogue Risk Task [BART]), and HR in FH+ and FH- participants. In the FH+ and FH- groups, women and men received 0.6 and 0.7g/kg alcohol, respectively. The alcohol dose yielded a breath alcohol concentration of 0.08% throughout the experiment. The control groups received placebo, and the subjective perception of alcohol intoxication was assessed. Alcohol intoxication significantly increased HR and the adjusted average number of pumps on the BART (a measure of risk taking) in FH+ men and women but not in FH- participants. Behavioral impulsivity was unaffected by alcohol or a FH of alcohol abuse. FH- but not FH+ participants who received alcohol reported significantly greater subjective perception of alcohol's effects than their placebo counterparts. These results indicate that FH+ individuals presented heightened sensitivity to alcohol-induced HR stimulation and alcohol-induced risk taking compared with their FH- counterparts. FH+ subjects, however, were insensitive to the subjective effects of alcohol. This idiosyncratic response pattern may be a likely pathway by which a FH of alcohol problems promotes alcohol drinking.

Negative Affect and Excessive Alcohol Intake Incubate during Protracted Withdrawal from Binge-Drinking in Adolescent, But Not Adult, Mice

Binge-drinking is common in underage alcohol users, yet we know little regarding the biopsychological impact of binge-drinking during early periods of development. Prior work indicated that adolescent male C57BL6/J mice with a 2-week history of binge-drinking (PND28-41) are resilient to the anxiogenic effects of early alcohol withdrawal. Herein, we employed a comparable Drinking-in-the-Dark model to determine how a prior history of binge-drinking during adolescence (EtOH^adolescents) influences emotionality (assayed with the light-dark box, marble burying test, and the forced swim test) and the propensity to consume alcohol in later life, compared to animals without prior drinking experience. For additional comparison, adult mice (EtOH^adults) with comparable drinking history (PND56-69) were subdivided into groups tested for anxiety/drinking either on PND70 (24 h withdrawal) or PND98 (28 days withdrawal). Tissue from the nucleus accumbens shell (AcbSh) and central nucleus of the amygdala (CeA) was examined by immunoblotting for changes in the expression of glutamate-related proteins. EtOH^adults exhibited some signs of hyperanxiety during early withdrawal (PND70), but not during protracted withdrawal (PND98). In contrast, EtOH^adolescents exhibited robust signs of anxiety-l and depressive-like behaviors when tested as adults on PND70. While all alcohol-experienced animals subsequently consumed more alcohol than mice drinking for the first time, alcohol intake was greatest in EtOH^adolescents. Independent of drinking age, the manifestation of withdrawal-induced hyperanxiety was accompanied by reduced Homer2b expression within the CeA and increased Group1 mGlu receptor expression within the AcbSh. The present data provide novel evidence that binge-drinking during adolescence produces a state characterized by profound negative affect and excessive alcohol consumption that incubates with the passage of time in withdrawal. These data extend our prior studies on the effects of subchronic binge-drinking during adulthood by demonstrating that the increase in alcoholism-related behaviors and glutamate-related proteins observed in early withdrawal dissipate with the passage of time. Our results to date highlight a critical interaction between the age of binge-drinking onset and the duration of alcohol withdrawal in glutamate-related neuroplasticity within the extended amygdala of relevance to the etiology of psychopathology, including pathological drinking, in later life.

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

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

Adolescent Alcohol Exposure: Burden of Epigenetic Reprogramming, Synaptic Remodeling, and Adult Psychopathology

Adolescence represents a crucial phase of synaptic maturation characterized by molecular changes in the developing brain that shape normal behavioral patterns. Epigenetic mechanisms play an important role in these neuromaturation processes. Perturbations of normal epigenetic programming during adolescence by ethanol can disrupt these molecular events, leading to synaptic remodeling and abnormal adult behaviors. Repeated exposure to binge levels of alcohol increases the risk for alcohol use disorder (AUD) and comorbid psychopathology including anxiety in adulthood. Recent studies in the field clearly suggest that adolescent alcohol exposure causes widespread and persistent changes in epigenetic, neurotrophic, and neuroimmune pathways in the brain. These changes are manifested by altered synaptic remodeling and neurogenesis in key brain regions leading to adult psychopathology such as anxiety and alcoholism. This review details the molecular mechanisms underlying adolescent alcohol exposure-induced changes in synaptic plasticity and the development of alcohol addiction-related phenotypes in adulthood.

Binge drinking impacts dorsal striatal response during decision making in adolescents

Adolescence is a time of both increased risk taking and increased vulnerability to the neurotoxic effects of alcohol. However, it is unclear whether brain functioning abnormalities in adolescent binge drinkers are a result of alcohol use itself or whether they represent premorbid risk characteristics. The current study addresses this question by using a modified version of the Wheel of Fortune (WOF) task, during functional magnetic resonance imaging (fMRI), at both baseline, while all subjects were alcohol-naïve, and revisit, when half of the subjects had emerged into regular binge drinking (n=13) and half remained alcohol and substance-naïve (n=13). Region of interest (ROI) analysis revealed that during decision making, there was a significant binge-drinking related reduction in brain activation in the dorsal striatum, an effect associated with degree of recent use. Furthermore, whole-brain analysis revealed a decrease in fronto-parietal brain activation prior to initiation of alcohol use, in adolescents who went on to binge drink. Additionally, there were numerous regions, both cortical and subcortical, in which there was a significant time-related developmental change, across groups. These results demonstrate how abnormalities in decision-making related circuitry might both lead to and perpetuate alcohol drinking behavior. These findings help aid in our ability to disentangle consequences of binge drinking from potential risk markers for future binge drinking, and may help guide future prevention and intervention strategies.

Adolescent rats are resistant to the development of ethanol-induced chronic tolerance and ethanol-induced conditioned aversion

The analysis of chronic tolerance to ethanol in adult and adolescent rats has yielded mixed results. Tolerance to some effects of ethanol has been reported in adolescents, yet other studies found adults to exhibit greater tolerance than adolescents or comparable expression of the phenomena at both ages. Another unanswered question is how chronic ethanol exposure affects subsequent ethanol-mediated motivational learning at these ages. The present study examined the development of chronic tolerance to ethanol's hypothermic and motor stimulating effects, and subsequent acquisition of ethanol-mediated odor conditioning, in adolescent and adult male Wistar rats given every-other-day intragastric administrations of ethanol. Adolescent and adult rats exhibited lack of tolerance to the hypothermic effects of ethanol during an induction phase; whereas adults, but not adolescents, exhibited a trend towards a reduction in hypothermia at a challenge phase (Experiment 1). Adolescents, unlike adults, exhibited ethanol-induced motor activation after the first ethanol administration. Adults, but not adolescents, exhibited conditioned odor aversion by ethanol. Subsequent experiments conducted only in adolescents (Experiment 2, Experiment 3 and Experiment 4) manipulated the context, length and predictability of ethanol administration. These manipulations did not promote the expression of ethanol-induced tolerance. This study indicated that, when moderate ethanol doses are given every-other day for a relatively short period, adolescents are less likely than adults to develop chronic tolerance to ethanol-induced hypothermia. This resistance to tolerance development could limit long-term maintenance of ethanol intake. Adolescents, however, exhibited greater sensitivity than adults to the acute motor stimulating effects of ethanol and a blunted response to the aversive effects of ethanol. This pattern of response may put adolescents at risk for early initiation of ethanol intake.

Reduced cerebellar brain activity during reward processing in adolescent binge drinkers

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Adolescent binge drinkers have reduced cerebellar activity during reward outcome.

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Average drinks consumed/drinking day was negatively related to brain activity.

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Salience of rewards may be blunted because of alcohol-induced neurotoxicity.

Due to ongoing development, adolescence may be a period of heightened vulnerability to the neurotoxic effects of alcohol. Binge drinking may alter reward-driven behavior and neurocircuitry, thereby increasing risk for escalating alcohol use. Therefore, we compared reward processing in adolescents with and without a history of recent binge drinking. At their baseline study visit, all participants (age = 14.86 ± 0.88) were free of heavy alcohol use and completed a modified version of the Wheel of Fortune (WOF) functional magnetic resonance imaging task. Following this visit, 17 youth reported binge drinking on ≥3 occasions within a 90 day period and were matched to 17 youth who remained alcohol and substance-naïve. All participants repeated the WOF task during a second visit (age = 16.83 ± 1.22). No significant effects were found in a region of interest analysis of the ventral striatum, but whole-brain analyses showed significant group differences in reward response at the second study visit in the left cerebellum, controlling for baseline visit brain activity (p/α < 0.05), which was negatively correlated with mean number of drinks consumed/drinking day in the last 90 days. These findings suggest that binge drinking during adolescence may alter brain activity during reward processing in a dose-dependent manner.

Operant self-administration of sweetened ethanol and time course of blood ethanol levels in adolescent and adult male Long-Evans rats

BACKGROUND

Little is known regarding mechanisms regulating ethanol (EtOH) self-administration during adolescence or if the mechanisms differ from adults. One of the best models of abuse liability is operant self-administration. Therefore, we characterized operant sweetened EtOH self-administration behavior in adolescent and adult rats.

METHODS

Adolescent (36 days old at first EtOH exposure) and adult male Long-Evans rats were first trained to self-administer 10% sucrose (10S) in an appetitive/consummatory operant model for 1 week, and then the drinking solution was switched to 10% sucrose plus 10% EtOH (10S10E) for 2 weeks. Next, rats were switched to a fixed ratio 2 schedule, and this was followed by 1 session using a progressive ratio schedule of reinforcement. Last, rats were tested for cue-induced reinstatement of lever-pressing behavior under extinction conditions after 13 days of abstinence. Blood EtOH concentration (BEC) of sweetened EtOH (self-administered or intragastric [IG] administration of 1 g/kg) was determined via gas chromatography. Control rats drank only 10S.

RESULTS

Consumption of sweetened EtOH was not different between adolescents and adults under any schedule tested, reaching 1 g/kg in 20 minutes in the appetitive/consummatory model. Appetitive behavior directed at sweetened EtOH was less focused in adolescents versus adults. No age differences were found in motivation for sweetened EtOH. Cue-induced reinstatement of EtOH-seeking behavior after abstinence also did not differ by age. In control groups, no age difference was found in appetitive behavior or the amount of sucrose consumed, although adults exhibited greater cue-induced reinstatement. BEC after self-administration or IG administration of sweetened EtOH was higher in adults than adolescents.

CONCLUSIONS

Consumption and motivation for sweetened EtOH are similar in adolescents and adults, although adolescents are more vulnerable to the effects of EtOH consumption on appetitive behavior. The IG results suggest larger volume of distribution and higher first-pass metabolism of sweetened EtOH in adolescents versus adults, which may limit the reinforcing effects of EtOH in some adolescents. Overall, we have begun to establish an operant sweetened EtOH self-administration model in adolescent rats.

Adolescent alcohol exposure and persistence of adolescent-typical phenotypes into adulthood: a mini-review

Alcohol use is typically initiated during adolescence, which, along with young adulthood, is a vulnerable period for the onset of high-risk drinking and alcohol abuse. Given across-species commonalities in certain fundamental neurobehavioral characteristics of adolescence, studies in laboratory animals such as the rat have proved useful to assess persisting consequences of repeated alcohol exposure. Despite limited research to date, reports of long-lasting effects of adolescent ethanol exposure are emerging, along with certain common themes. One repeated finding is that adolescent exposure to ethanol sometimes results in the persistence of adolescent-typical phenotypes into adulthood. Instances of adolescent-like persistence have been seen in terms of baseline behavioral, cognitive, electrophysiological and neuroanatomical characteristics, along with the retention of adolescent-typical sensitivities to acute ethanol challenge. These effects are generally not observed after comparable ethanol exposure in adulthood. Persistence of adolescent-typical phenotypes is not always evident, and may be related to regionally specific ethanol influences on the interplay between CNS excitation and inhibition critical for the timing of neuroplasticity.

Adolescents and alcohol: acute sensitivities, enhanced intake, and later consequences

Adolescence is an evolutionarily conserved developmental period characterized by notable maturational changes in the brain along with various age-related behavioral characteristics, including the propensity to initiate alcohol and other drug use and consume more alcohol per occasion than adults. After a brief review of adolescent neurobehavioral function from an evolutionary perspective, the paper will turn to assessment of adolescent alcohol sensitivity and consequences, with a focus on work from our laboratory. After summarizing evidence showing that adolescents differ considerably from adults in their sensitivity to various effects of alcohol, potential contributors to these age-typical sensitivities will be discussed, and the degree to which these findings are generalizable to other drugs and to human adolescents will be considered. Recent studies are then reviewed to illustrate that repeated alcohol exposure during adolescence induces behavioral, cognitive, and neural alterations that are highly specific, replicable, persistent and dependent on the timing of the exposure. Research in this area is in its early stages, however, and more work will be necessary to characterize the extent of these neurobehavioral alterations and further determine the degree to which observed effects are specific to alcohol exposure during adolescence.

Effects of ethanol exposure on nervous system development in zebrafish

Alcohol (ethanol) is a teratogen that adversely affects nervous system development in a wide range of animal species. In humans numerous congenital abnormalities arise as a result of fetal alcohol exposure, leading to a spectrum of disorders referred to as fetal alcohol spectrum disorder (FASD). These abnormalities include craniofacial defects as well as neurological defects that affect a variety of behaviors. These human FASD phenotypes are reproduced in the rodent central nervous system (CNS) following prenatal ethanol exposure. While the study of ethanol effects on zebrafish development has been more limited, several studies have shown that different strains of zebrafish exhibit differential susceptibility to ethanol-induced cyclopia, as well as behavioral deficits. Molecular mechanisms underlying the effects of ethanol on CNS development also appear to be shared between rodent and zebrafish. Thus, zebrafish appear to recapitulate the observed effects of ethanol on human and mouse CNS development, indicating that zebrafish can serve as a complimentary developmental model system to study the molecular basis of FASD. Recent studies examining the effect of ethanol exposure on zebrafish nervous system development are reviewed, with an emphasis on attempts to elucidate possible molecular pathways that may be impacted by developmental ethanol exposure. Recent work from our laboratories supports a role for perturbed extracellular matrix function in the pathology of ethanol exposure during zebrafish CNS development. The use of the zebrafish model to assess the effects of ethanol exposure on adult nervous system function as manifested by changes in zebrafish behavior is also discussed.

Age-related differences in the disposition of nicotine and metabolites in rat brain and plasma

INTRODUCTION

Studies have evaluated the behavioral and neurochemical impact of nicotine administration in rodents. However, the distribution of nicotine and metabolites in rat brain and plasma as a function of age has not been investigated. This is a significant issue because human adolescents have a greater risk for developing nicotine addiction than adults, and reasons underlying this observation have not been fully determined. Thus, in this present study, we evaluated the impact of the transition from adolescence (postnatal day [PND 40]) to adulthood (PND 90) on nicotine distribution in rats.

METHODS

PND 40, 60, and 90 rats received a single injection of (-) nicotine (0.8 mg/kg, subcutaneously). Liquid chromatography tandem-mass spectrometry was used to measure concentration of nicotine and metabolites in selected biological matrices.

RESULTS

Nicotine, cotinine, and nornicotine were detected in rat striata and frontal cortex 30 min, 1 hr, 2 hr, and 4 hr after a single administration. These and several additional metabolites (nicotine-1'-oxide, cotinine-N-oxide, norcotinine, and trans-3'-hydroxycotinine) were also detected in plasma at these same timepoints. The mean concentration of nicotine in brain and plasma was lower in PND 40 versus PND 90 rats. In contrast, the mean concentration of nornicotine was higher in the plasma and brain of PND 40 versus PND 90 rats.

CONCLUSIONS

Nicotine and metabolite distribution differs between adolescent and adult rats. These data suggest that adolescent rats metabolize nicotine to some metabolites faster than adult rats. Further studies are needed to investigate the potential correlation between age, drug distribution, and nicotine addiction.

ADOLESCENTS AND ALCOHOL

The high levels of alcohol consumption characteristic of adolescence may be in part biologically based, given that elevated consumption levels are also evident during this developmental transition in other mammalian species as well. Studies conducted using a simple animal model of adolescence in the rat has shown adolescents to be more sensitive than adults to social facilitatory and rewarding effects of alcohol, but less sensitive to numerous alcohol effects that may serve as cues to limit intake. These age-specific alcohol sensitivities appear related to differential rates of development of neural systems underlying different alcohol effects as well as to an ontogenetic decline in rapid brain compensations to alcohol, termed "acute tolerance". In contrast, these adolescent-typical sensitivities to alcohol do not appear to be notably influenced by pubertally-related increases in gonadal hormones. Although data are sparse, there are hints that similar alcohol sensitivities may also be seen in human adolescents, with this developmentally decreased sensitivity to alcohol's intoxicating effects possibly exacerbated by genetic vulnerabilities also characterized by an insensitivity to alcohol intoxication, thereby perhaps permitting especially high levels of alcohol consumption among vulnerable youth.

Effects of ethanol administration on corticosterone levels in adolescent and adult rats

Adolescent humans and rodents have been shown to consume more alcohol than their adult counterparts. Given that corticosterone (CORT) has been shown to be related to the intake of several drugs of abuse, this study assessed the ontogenetic effects of low-moderate doses of ethanol on CORT increases and recovery. Despite no significant differences in baseline (home cage) CORT levels, CORT responses to ethanol were greater in females than in males and in adult females than in adolescent females; males, however, showed less marked age differences in CORT levels after ethanol consumption. Adolescent blood ethanol concentrations (BECs) were lower than those of adults, although these BEC differences appear insufficient to account for the ontogenetic differences in CORT levels. Collectively, these findings suggest that it is unlikely that age differences in CORT elevations provide a major contribution to the ontogenetic differences in alcohol intake seen between adolescents and adults.

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.

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.

"Binge" drinking experience in adolescent mice shows sex differences and elevated ethanol intake in adulthood

Binge drinking, defined as achieving blood ethanol concentrations (BEC) of 80 mg%, has been increasing in adolescents and was reported to predispose later physical dependence. The present experiments utilized an animal model of binge drinking to compare the effect of ethanol "binge" experience during adolescence or adulthood on subsequent ethanol intake in male and female C57BL/6 mice. Adolescent and adult mice were initially exposed to the scheduled high alcohol consumption procedure, which produces BECs that exceed the levels for binge drinking following a 30-min ethanol session every third day. Ethanol intake and BECs were significantly higher in the adolescent ( approximately 3 g/kg, 199 mg%) versus adult ( approximately 2 g/kg, 135 mg%) mice during the first three ethanol sessions, but were more equivalent during the final two ethanol sessions (1.85-2.0 g/kg, 129-143 mg%). Then, separate groups of the ethanol-experienced mice were tested with ethanol naïve adolescent and adult mice for 2-h limited access (10% and 20% solutions) or 24-h (5%, 10% and 20% solutions) ethanol preference drinking. Limited access ethanol intake was significantly higher in female versus male mice, but was not altered by age or ethanol experience. In contrast, 24-h ethanol intake was significantly higher in the adolescent versus adult mice and in female versus male mice. Furthermore, binge drinking experience in the adolescent mice significantly increased subsequent ethanol intake, primarily due to intake in female mice. Thus, adolescent binge drinking significantly increased unlimited ethanol intake during adulthood, with female mice more susceptible to this effect.

Adolescence as a critical window for developing an alcohol use disorder: current findings in neuroscience

PURPOSE OF REVIEW

Alcohol consumption during adolescence greatly increases the likelihood that an alcohol use disorder will develop later in life. Elucidating how alcohol impacts the adolescent brain is paramount to understanding how alcohol use disorders arise. This review focuses on recent work addressing alcohol's unique effect on the adolescent brain.

RECENT FINDINGS

The unique and dynamic state of the developing adolescent brain is discussed with an emphasis on the developmentally distinct effect of alcohol on the dopaminergic reward system and corticolimbic structure and function. Reward neurocircuitry undergoes significant developmental shifts during adolescence, making it particularly sensitive to alcohol in ways that could promote excessive consumption. In addition, developing corticolimbic systems, including the prefrontal cortex and hippocampus, exhibit enhanced vulnerability to alcohol-induced damage. Disruption of white matter integrity, neurotoxicity and inhibition of adult neurogenesis may underlie alcohol-mediated cognitive dysfunction and lead to decreased behavioral control over consumption.

SUMMARY

In adolescents, alcohol interacts extensively with reward neurocircuitry and corticolimbic structure and function in ways that promote maladaptive behaviors that lead to addiction. Future work is needed to further understand the mechanisms involved in these interactions. Therapeutic strategies that restore proper reward neurochemistry or reverse alcohol-induced neurodegeneration could prove useful in preventing emergence of alcohol use disorders.

Conditioned taste aversion learning: implications for animal models of drug abuse

Drugs of abuse are typically discussed in terms of their rewarding effects and how these effects mediate drug taking. However, these drugs produce aversive effects that could have an important role in the overall acceptability of a drug and its likelihood of being self-administered. Rewarding and aversive effects, then, could be interpreted as separate behavioral effects, with the balance of the two determining overall drug acceptability. Interestingly, the role of aversive effects on drug acceptability in the self-administration preparation has received limited attention in this context. This chapter examines the aversive effects of drugs and discusses their role in drug taking. If these aversive effects serve a protective function, manipulations that alter or decrease these effects could have implications for drug taking. Several factors have been reported to alter conditioned taste aversion (CTA) learning, a preparation used in the assessment of the aversive effects of drugs in general. Two of these factors, drug history and strain, are reviewed here. By reviewing these, we intend to demonstrate the protective nature of aversive effects in the initiation and escalation of drug taking and to provide evidence that reductions in aversive effects could produce changes in patterns of drug self-administration that could lead to an increased vulnerability to abuse drugs by altering the reward-aversion balance. The aim of this chapter is not to question the importance of rewarding effects in self-administration but rather to provide evidence that aversive effects are an important factor that needs to be considered in discussions of drug-taking behavior.

Human and laboratory rodent low response to alcohol: is better consilience possible?

If people are brought into the laboratory and given alcohol, there are pronounced differences among individuals in many responses to the drug. Some participants in alcohol challenge protocols show a cluster of 'low level of responses to alcohol' determined by observing post-drinking-related changes in subjective, motor and physiological effects at a given dose level. Those individuals characterized as having low level of response (LR) to alcohol have been shown to be at increased risk for a lifetime diagnosis of alcohol dependence (AD), and this relationship between low LR and AD appears to be in part genetic. LR to alcohol is an area where achieving greater consilience between the human and the rodent phenotypes would seem to be highly likely. However, despite extensive data from both human and rodent studies, few attempts have been made to evaluate the human and animal data systematically in order to understand which aspects of LR appear to be most directly comparable across species and thus the most promising for further study. We review four general aspects of LR that could be compared between humans and laboratory animals: (1) behavioral measures of subjective intoxication; (2) body sway; (3) endocrine responses; and (4) stimulant, autonomic and electrophysiological responses. None of these aspects of LR provide completely face-valid direct comparisons across species. Nevertheless, one of the most replicated findings in humans is the low subjective response, but, as it may reflect either aversively valenced and/or positively valenced responses to alcohol as usually assessed, it is unclear which rodent responses are analogous. Stimulated heart rate appears to be consistent in animal and human studies, although at-risk subjects appear to be more rather than less sensitive to alcohol using this measure. The hormone and electrophysiological data offer strong possibilities of understanding the neurobiological mechanisms, but the rodent data in particular are rather sparse and unsystematic. Therefore, we suggest that more effort is still needed to collect data using refined measures designed to be more directly comparable in humans and animals. Additionally, the genetically mediated mechanisms underlying this endophenotype need to be characterized further across species.

Motivational systems in adolescence: possible implications for age differences in substance abuse and other risk-taking behaviors

Adolescence is an evolutionarily conserved developmental phase characterized by hormonal, physiological, neural and behavioral alterations evident widely across mammalian species. For instance, adolescent rats, like their human counterparts, exhibit elevations in peer-directed social interactions, risk-taking/novelty seeking and drug and alcohol use relative to adults, along with notable changes in motivational and reward-related brain regions. After reviewing these topics, the present paper discusses conditioned preference and aversion data showing adolescents to be more sensitive than adults to positive rewarding properties of various drugs and natural stimuli, while less sensitive to the aversive properties of these stimuli. Additional experiments designed to parse specific components of reward-related processing using natural rewards have yielded more mixed findings, with reports of accentuated positive hedonic sensitivity during adolescence contrasting with studies showing less positive hedonic affect and reduced incentive salience at this age. Implications of these findings for adolescent substance abuse will be discussed.