Anxiogenic effects during withdrawal from acute ethanol in adolescent and adult rats

Ethanol withdrawal increases oxidative stress and reduces nitric oxide bioavailability in the vasculature of rats

We analyzed the effects of ethanol withdrawal on the vascular and systemic renin-angiotensin system (RAS) and vascular oxidative stress. Male Wistar rats were treated with ethanol 3-9% (v/v) for a period of 21 days. Ethanol withdrawal was induced by abrupt discontinuation of the treatment. Experiments were performed 48 h after ethanol discontinuation. Rats from the ethanol withdrawal group showed decreased exploration of the open arms of the elevated-plus maze (EPM) and increased plasma corticosterone levels. Ethanol withdrawal significantly increased systolic blood pressure and plasma angiotensin II (ANG II) levels without an effect on plasma renin activity (PRA), angiotensin converting enzyme (ACE) activity, or plasma angiotensin I (ANG I) levels. No differences in vascular ANG I, ANG II levels, and ACE activity/expression and AT1 and AT2 receptor expression were detected among the experimental groups. Plasma osmolality, as well as plasma sodium, potassium, and glucose levels were not affected by ethanol withdrawal. Ethanol withdrawal induced systemic and vascular oxidative stress, as evidenced by increased plasma thiobarbituric acid-reacting substances (TBARS) levels and the vascular generation of superoxide anion. Ethanol withdrawal significantly decreased plasma and vascular nitrate/nitrite levels. Major new findings of the present study are that ethanol withdrawal induces vascular oxidative stress and reduces nitric oxide (NO) levels in the vasculature. Additionally, our study provides novel evidence that ethanol withdrawal does not affect the vascular ANG II generating system while stimulating systemic RAS. These responses could predispose individuals to the development of cardiovascular diseases.

Intracranial self-stimulation reward thresholds during morphine withdrawal in rats bred for high (HiS) and low (LoS) saccharin intake

RATIONALE

Sweet preference is a marker of vulnerability to substance use disorders, and rats selectively bred for high (HiS) vs. low saccharin (LoS) intake display potentiated drug-seeking behaviors. Recent work indicated that LoS rats were more responsive to the negative effects of drugs in several assays.

OBJECTIVE

The current study used the intracranial self-stimulation (ICSS) procedure to investigate the anhedonic component of morphine withdrawal in male HiS and LoS rats.

METHODS

Rats were administered morphine (10mg/kg) or saline for 8 days. To evaluate withdrawal effects, reward thresholds were measured 24 and 28h following the 8th morphine injection (spontaneous withdrawal) and again for 4 days following daily acute morphine and naloxone (1mg/kg) administration (precipitated withdrawal).

RESULTS

24h following the final morphine injection, reward thresholds in LoS rats were significantly elevated compared to reward thresholds in LoS controls, indicating spontaneous withdrawal. This effect was not observed in HiS rats. LoS rats also showed greater elevations of reward thresholds on several days during naloxone-precipitated withdrawal compared to their HiS counterparts.

CONCLUSIONS

LoS rats were more sensitive to morphine withdrawal-mediated elevations in ICSS thresholds than HiS rats. While these differences were generally modest, our data suggest that severity of the negative affective component of opiate withdrawal may be influenced by genotypes related to addiction vulnerability.

Developmental consequences of fetal exposure to drugs: what we know and what we still must learn

Most drugs of abuse easily cross the placenta and can affect fetal brain development. In utero exposures to drugs thus can have long-lasting implications for brain structure and function. These effects on the developing nervous system, before homeostatic regulatory mechanisms are properly calibrated, often differ from their effects on mature systems. In this review, we describe current knowledge on how alcohol, nicotine, cocaine, amphetamine, Ecstasy, and opiates (among other drugs) produce alterations in neurodevelopmental trajectory. We focus both on animal models and available clinical and imaging data from cross-sectional and longitudinal human studies. Early studies of fetal exposures focused on classic teratological methods that are insufficient for revealing more subtle effects that are nevertheless very behaviorally relevant. Modern mechanistic approaches have informed us greatly as to how to potentially ameliorate the induced deficits in brain formation and function, but conclude that better delineation of sensitive periods, dose-response relationships, and long-term longitudinal studies assessing future risk of offspring to exhibit learning disabilities, mental health disorders, and limited neural adaptations are crucial to limit the societal impact of these exposures.

Dependence potential of tramadol: behavioral pharmacology in rodents

Tramadol is an opioid analgesic agent that has been the subject of a series of case reports suggesting potential for misuse or abuse. However, it is not a controlled substance and is not generally considered addictive in Korea. In this study, we examined the dependence potential and abuse liability of tramadol as well as its effect on the dopaminergic and serotonergic systems in rodents. In animal behavioral tests, tramadol did not show any positive effects on the experimental animals in climbing, jumping, and head twitch tests. However, in the conditioned place preference and self-administration tests, the experimental animals showed significant positive responses. Taken together, tramadol affected the neurological systems related to abuse liability and has the potential to lead psychological dependence.

Distinct effects of repeated restraint stress on basolateral amygdala neuronal membrane properties in resilient adolescent and adult rats

Severe and repeated stress has damaging effects on health, including initiation of depression and anxiety. Stress that occurs during development has long-lasting and particularly damaging effects on emotion. The basolateral amygdala (BLA) plays a key role in many affective behaviors, and repeated stress causes different forms of BLA hyperactivity in adolescent and adult rats. However, the mechanism is not known. Furthermore, not every individual is susceptible to the negative consequences of stress. Differences in the effects of stress on the BLA might contribute to determine whether an individual will be vulnerable or resilient to the effects of stress on emotion. The purpose of this study is to test the cellular underpinnings for age dependency of BLA hyperactivity after stress, and whether protective changes occur in resilient individuals. To test this, the effects of repeated stress on membrane excitability and other membrane properties of BLA principal neurons were compared between adult and adolescent rats, and between vulnerable and resilient rats, using in vitro whole-cell recordings. Vulnerability was defined by adrenal gland weight, and verified by body weight gain after repeated restraint stress, and fecal pellet production during repeated restraint sessions. We found that repeated stress increased the excitability of BLA neurons, but in a manner that depended on age and BLA subnucleus. Furthermore, stress resilience was associated with an opposite pattern of change, with increased slow afterhyperpolarization (AHP) potential, whereas vulnerability was associated with decreased medium AHP. The opposite outcomes in these two populations were further distinguished by differences of anxiety-like behavior in the elevated plus maze that were correlated with BLA neuronal excitability and AHP. These results demonstrate a substrate for BLA hyperactivity after repeated stress, with distinct membrane properties to target, as well as age-dependent factors that contribute to resilience to the effects of stress.

Developmental rodent models of fear and anxiety: from neurobiology to pharmacology

Anxiety disorders pose one of the biggest threats to mental health in the world, and they predominantly emerge early in life. However, research of anxiety disorders and fear-related memories during development has been largely neglected, and existing treatments have been developed based on adult models of anxiety. The present review describes animal models of anxiety disorders across development and what is currently known of their pharmacology. To summarize, the underlying mechanisms of intrinsic 'unlearned' fear are poorly understood, especially beyond the period of infancy. Models using 'learned' fear reveal that through development, rats exhibit a stress hyporesponsive period before postnatal day 10, where they paradoxically form odour-shock preferences, and then switch to more adult-like conditioned fear responses. Juvenile rats appear to forget these aversive associations more easily, as is observed with the phenomenon of infantile amnesia. Juvenile rats also undergo more robust extinction, until adolescence where they display increased resistance to extinction. Maturation of brain structures, such as the amygdala, prefrontal cortex and hippocampus, along with the different temporal recruitment and involvement of various neurotransmitter systems (including NMDA, GABA, corticosterone and opioids) are responsible for these developmental changes. Taken together, the studies described in this review highlight that there is a period early in development where rats appear to be more robust in overcoming adverse early life experience. We need to understand the fundamental pharmacological processes underlying anxiety early in life in order to take advantage of this period for the treatment of anxiety disorders.

Associations between a history of binge drinking during adolescence and self-reported responses to alcohol in young adult Native and Mexican Americans

BACKGROUND

Binge drinking during adolescence is common and may predict increased drinking in young adulthood and enhanced risk of alcohol dependence. Variation in level of response to the hedonic and adverse effects of alcohol is in part an inherited factor that may also influence its use, abuse, and dependence. This study investigated, in young adults, whether an association could be demonstrated between variation in self-reported responses to alcohol and a history of binge drinking during adolescence.

METHODS

Young adult (18 to 30 years, n = 790) Native Americans and Mexican Americans were recruited from the community and completed a structured diagnostic interview. Response to alcohol was indexed using the expectation version of the Subjective High Assessment Scale (SHAS-E). An adolescent history of regular binge drinking was defined as drinking 5 or more drinks for boys and 4 or more drinks for girls per drinking occasion at least once a month during their highest drinking period prior to the age of 18.

RESULTS

An adolescent history of regular binge drinking was found to be associated with a lower level of self-reported responses to the negative aspects of alcohol intoxication (feeling terrible) as well as to the overall level of intoxication, but not to the positive impressions of intoxication (feeling great) on the SHAS-E. A history of regular adolescent binge drinking was also correlated with less feelings of the "terrible" and "total" effects of alcohol, as indexed by the SHAS-E, in a linear regression model that included several diagnostic and demographic variables such as a history of conduct disorder and current levels of drinking.

CONCLUSIONS

These data suggest that a history of adolescent binge drinking is associated with a reduction in the self-reported level of intoxication in young adulthood, a factor that could theoretically lead to increased risk of alcohol dependence.

Developmental differences in the effects of alcohol and stress on heart rate variability

Adolescent rats differ in their responses to stress and ethanol from their adult counterparts, although not much is known about the contribution of the autonomic nervous system (ANS) to these differences. This study assessed the impact of stress, ethanol, and their combination on parameters of heart rate variability (HRV) in adolescent and adult male Sprague-Dawley rats. Animals were habituated to the testing box and neck sensors (MouseOX, STARR Life Sciences Corp.) used for recording heart rate (HR). After 8-10min of baseline recording, animals were restrained for 90min or returned home, followed by intraperitoneal injection of 0, 0.5, 1.0, or 1.5g/kg ethanol. The 8-10min test recording occurred 30min post-injection. Ethanol-related decreases in LF (an index of sympathetic activity) were evident under non-stressed conditions in adolescents but only after stress in adults, perhaps in part due to apparent ethanol-induced sympathetic deactivation in adolescents. Parasympathetic tone, indexed by HF, was unaffected by both ethanol and stress in adolescents, while again both the 1.0 and 1.5g/kg ethanol doses decreased HF in adults following stress. Ethanol also decreased low frequency/high frequency tone (LF/HF), an index of sympathovagal balance, only in adolescents, with no decrease evident in adults. Further, stressed adults, and not adolescents, had significantly lower CORT and PROG values than their non-stressed counterparts. Taken together, these results demonstrate notable age differences in the ANS response to ethanol under stressful vs. non-stressful circumstances, reflected by ethanol-mediated autonomic effects that were more pronounced following stressor exposure in adults but under non-stressed conditions in adolescents.

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.

Long-term effects of exposure to methamphetamine in adolescent rats

BACKGROUND

Flexible cognition is a set of processes mediated by the prefrontal cortex (PFC), an area of the brain that continues to develop during adolescence and into adulthood. Adult rodents exhibit impairments specific to reversal learning across various dosing regimens of methamphetamine (mAMPH). For adolescent rodents, ongoing PFC development can be assessed by discrimination reversal learning, a task dependent on frontostriatal integrity. The task may also index an increased vulnerability for mAMPH sampling in adulthood.

METHODS

The purpose of the present study was to investigate the long-term effects of escalating, adolescent mAMPH exposure on reversal learning, a PFC-dependent task (Experiment 1) and the likelihood of later sampling of mAMPH in adulthood (Experiment 2).

RESULTS

Unlike previous research in adult-treated rats, our results show more generalized learning impairments after adolescent mAMPH exposure to include both attenuated visual discrimination as well as reversal learning. Additionally, we found that rats pre-exposed to mAMPH during adolescence consumed significantly more drug in adulthood. Intake of mAMPH was positively correlated with this learning. Taken together, these findings show that even modest exposure to mAMPH during adolescence may induce general learning impairments in adulthood, and an enduring sensitivity to the effects of mAMPH.

Effect of sex on ethanol consumption and conditioned taste aversion in adolescent and adult rats

RATIONALE

Vulnerability to alcoholism is determined by many factors, including the balance of pleasurable vs. aversive alcohol-induced sensations: pleasurable sensations increase intake, while aversive sensations decrease it. Female sex and adolescent age are associated with lower sensitivity to intake-reducing effects and more rapid development of alcohol abuse.

OBJECTIVES

This study assessed voluntary drinking and the aversive effects of alcohol to determine whether these measures are inversely related across the sexes and development.

METHODS

Voluntary drinking of 20 % ethanol in an every-other-day (EOD) availability pattern and the dose-response relationship of ethanol conditioned taste aversion (CTA) were assessed in male and female adolescent and adult rats.

RESULTS

CTA was sex specific in adult but not adolescent rats, with adult females exhibiting less aversion. Voluntary ethanol consumption varied according to age and individual differences but was not sex specific. Adolescents initially drank more than adults, exhibited greater day-to-day variation in consumption, were more susceptible to the alcohol deprivation effect, and took longer to establish individual differences in consumption patterns.

CONCLUSIONS

These results show that the emergence of intake patterns differs between adolescents and adults. Adolescents as a group initiate drinking at high levels but decrease intake as they mature. A subset of adolescents maintained high drinking levels into adulthood. In contrast, most adults consumed at steady, low levels, but a small subset quickly established and maintained high-consumption patterns. Adolescents also showed marked deprivation-induced increases. Sex differences were not observed in EOD drinking during either adolescence or adulthood.

The novelty-seeking phenotype modulates the long-lasting effects of intermittent ethanol administration during adolescence

The aim of the present study was to investigate if a novelty-seeking phenotype mediates the long-lasting consequences of intermittent EtOH intoxication during adolescence. The hole board test was employed to classify adolescent mice as High- or Low-Novelty Seekers. Subsequently, animals were administered ethanol (1.25 or 2.5 g/kg) on two consecutive days at 48-h intervals over a 14-day period. Anxiety levels--measured using the elevated plus maze- spontaneous motor activity and social interaction test were studied 3 weeks later. A different set of mice underwent the same procedure, but received only the 2.5 g/kg dose of ethanol. Three weeks later, in order to induce CPP, the same animals were administered 1 or 6 mg/kg of cocaine or 1 or 2.5 mg/kg MDMA. The results revealed a decrease in aggressive behaviors and an anxiolytic profile in HNS mice and longer latency to explore the novel object by LNS mice. Ethanol exposure enhanced the reinforcing effects of cocaine and MDMA in both groups when CPP was induced with a sub-threshold dose of the drugs. The extinguished cocaine-induced CPP (1 and 6 mg/kg) was reinstated after a priming dose in HNS animals only. Our results confirm that intermittent EtOH administration during adolescence induces long-lasting effects that are manifested in adult life, and that there is an association between these effects and the novelty-seeking phenotype.

GABAergic contributions to alcohol responsivity during adolescence: insights from preclinical and clinical studies

There is a considerable body of literature demonstrating that adolescence is a unique age period, which includes rapid and dramatic maturation of behavioral, cognitive, hormonal and neurobiological systems. Most notably, adolescence is also a period of unique responsiveness to alcohol effects, with both hyposensitivity and hypersensitivity observed to the various effects of alcohol. Multiple neurotransmitter systems are undergoing fine-tuning during this critical period of brain development, including those that contribute to the rewarding effects of drugs of abuse. The role of developmental maturation of the γ-amino-butyric acid (GABA) system, however, has received less attention in contributing to age-specific alcohol sensitivities. This review integrates GABA findings from human magnetic resonance spectroscopy studies as they may translate to understanding adolescent-specific responsiveness to alcohol effects. Better understanding of the vulnerability of the GABA system both during adolescent development, and in psychiatric conditions that include alcohol dependence, could point to a putative mechanism, boosting brain GABA, that may have increased effectiveness for treating alcohol use disorders.

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.

Developmental differences in EEG and sleep responses to acute ethanol administration and its withdrawal (hangover) in adolescent and adult Wistar rats

Age-related differences in sensitivity to the acute effects of alcohol may play an important role in the increased risk for the development of alcoholism seen in teens that begin drinking at an early age. The present study evaluated the acute and protracted (hangover) effects of ethanol in adolescent (P33-P40) and adult (P100-P107) Wistar rats, using the cortical electroencephalogram (EEG). Six minutes of EEG was recorded during waking, 15 min after administration of 0, 1.5, or 3.0 g/kg ethanol, and for 3 h at 20 h post ethanol, during the rats' next sleep cycle. Significantly higher overall frontal and parietal cortical power was seen in a wide range of EEG frequencies in adolescent rats as compared to adult rats in their waking EEG. Acute administration of ethanol did not produce differences between adolescents and adults on behavioral measures of acute intoxication. However, it did produce a significantly less intense acute EEG response to ethanol in the theta frequencies in parietal cortex in the adolescents as compared to the adults. At 20 h following acute ethanol administration, during the rats' next sleep cycle, a decrease in slow-wave frequencies (1-4 Hz) was seen and the adolescent rats were found to display more reduction in the slow-wave frequencies than the adults did. The present study found that adolescent rats, as compared to adults, demonstrate low sensitivity to acute ethanol administration in the theta frequencies and more susceptibility to disruption of slow-wave sleep during hangover. These studies may lend support to the idea that these traits may contribute to increased risk for alcohol use disorders seen in adults who begin drinking in their early teenage years.

Controversies about the enhanced vulnerability of the adolescent brain to develop addiction

Adolescence, defined as a transition phase toward autonomy and independence, is a natural time of learning and adjustment, particularly in the setting of long-term goals and personal aspirations. It also is a period of heightened sensation seeking, including risk taking and reckless behaviors, which is a major cause of morbidity and mortality among teenagers. Recent observations suggest that a relative immaturity in frontal cortical neural systems may underlie the adolescent propensity for uninhibited risk taking and hazardous behaviors. However, converging preclinical and clinical studies do not support a simple model of frontal cortical immaturity, and there is substantial evidence that adolescents engage in dangerous activities, including drug abuse, despite knowing and understanding the risks involved. Therefore, a current consensus considers that much brain development during adolescence occurs in brain regions and systems that are critically involved in the perception and evaluation of risk and reward, leading to important changes in social and affective processing. Hence, rather than naive, immature and vulnerable, the adolescent brain, particularly the prefrontal cortex, should be considered as prewired for expecting novel experiences. In this perspective, thrill seeking may not represent a danger but rather a window of opportunities permitting the development of cognitive control through multiple experiences. However, if the maturation of brain systems implicated in self-regulation is contextually dependent, it is important to understand which experiences matter most. In particular, it is essential to unveil the underpinning mechanisms by which recurrent adverse episodes of stress or unrestricted access to drugs can shape the adolescent brain and potentially trigger life-long maladaptive responses.

Dependence potential of quetiapine: behavioral pharmacology in rodents

Quetiapine is an atypical or second-generation antipsychotic agent and has been a subject of a series of case report and suggested to have the potential for misuse or abuse. However, it is not a controlled substance and is not generally considered addictive. In this study, we examined quetiapine’s dependence potential and abuse liability through animal behavioral tests using rodents to study the mechanism of quetiapine. Molecular biology techniques were also used to find out the action mechanisms of the drug. In the animal behavioral tests, quetiapine did not show any positive effect on the experimental animals in the climbing, jumping, and conditioned place preference tests. However, in the head twitch and self-administration tests, the experimental animals showed significant positive responses. In addition, the action mechanism of quetiapine was found being related to dopamine and serotonin release. These results demonstrate that quetiapine affects the neurological systems related to abuse liability and has the potential to lead psychological dependence, as well.

Combined exposure to tobacco smoke and ethanol during adolescence leads to short- and long-term modulation of anxiety-like behavior

BACKGROUND

Tobacco smoking is associated with alcohol drinking and consumption of both drugs typically begins during adolescence. Since anxiety is considered a relevant factor for both smoking and drinking due to its motivating force for a continued consumption, anxiety alterations shared by these two drugs could explain their co-use and co-abuse.

METHODS

Here, we investigated the short- and long-term effects of adolescent tobacco smoke and/or ethanol exposure on anxiety levels. From postnatal day 30-45, Swiss mice were exposed to tobacco smoke (SMK--whole body exposure, 8 h/day) and/or ethanol (ETOH--25% solution, 2g/kg i.p. injected every other day) as follows: (1) SMK+ETOH exposure; (2) SMK exposure; (3) ETOH exposure; (4) Control. Anxiety levels were assessed with the elevated plus maze and open field tests.

RESULTS

By the end of exposure, SMK female mice presented an anxiolytic response in the elevated plus maze and this response was intensified by co-exposure to ethanol. A short-term deprivation from SMK elicited an anxiogenic state in females in this maze. Although neither smoke nor ethanol effects persisted one month post-exposure, SMK+ETOH male and female mice exhibited an anxiogenic response in the open field.

CONCLUSION

Adolescent female mice are more susceptible to the anxiolytic effects of SMK. The stronger effect in SMK+ETOH group suggests that, in females, the combined exposure leads to lower anxiety levels. Anxiety levels do not seem to be relevant during a short-term SMK+ETOH deprivation, however, increased anxiety during long-term smoking and drinking deprivation demonstrate late-emergent effects both in males and females.

Developmental imaging genetics: linking dopamine function to adolescent behavior

Adolescence is a period of development characterized by numerous neurobiological changes that significantly influence behavior and brain function. Adolescence is of particular interest due to the alarming statistics indicating that mortality rates increase two to three-fold during this time compared to childhood, due largely to a peak in risk-taking behaviors resulting from increased impulsivity and sensation seeking. Furthermore, there exists large unexplained variability in these behaviors that are in part mediated by biological factors. Recent advances in molecular genetics and functional neuroimaging have provided a unique and exciting opportunity to non-invasively study the influence of genetic factors on brain function in humans. While genes do not code for specific behaviors, they do determine the structure and function of proteins that are essential to the neuronal processes that underlie behavior. Therefore, studying the interaction of genotype with measures of brain function over development could shed light on critical time points when biologically mediated individual differences in complex behaviors emerge. Here we review animal and human literature examining the neurobiological basis of adolescent development related to dopamine neurotransmission. Dopamine is of critical importance because of (1) its role in cognitive and affective behaviors, (2) its role in the pathogenesis of major psychopathology, and (3) the protracted development of dopamine signaling pathways over adolescence. We will then focus on current research examining the role of dopamine-related genes on brain function. We propose the use of imaging genetics to examine the influence of genetically mediated dopamine variability on brain function during adolescence, keeping in mind the limitations of this approach.

Effects of acute or chronic ethanol exposure during adolescence on behavioral inhibition and efficiency in a modified water maze task

Ethanol is well known to adversely affect frontal executive functioning, which continues to develop throughout adolescence and into young adulthood. This is also a developmental window in which ethanol is misused by a significant number of adolescents. We examined the effects of acute and chronic ethanol exposure during adolescence on behavioral inhibition and efficiency using a modified water maze task. During acquisition, rats were trained to find a stable visible platform onto which they could escape. During the test phase, the stable platform was converted to a visible floating platform (providing no escape) and a new hidden platform was added in the opposite quadrant. The hidden platform was the only means of escape during the test phase. In experiment 1, adolescent animals received ethanol (1.0g/kg) 30min before each session during the test phase. In experiment 2, adolescent animals received chronic intermittent ethanol (5.0g/kg) for 16 days (PND30 To PND46) prior to any training in the maze. At PND72, training was initiated in the same modified water maze task. Results from experiment 1 indicated that acute ethanol promoted behavioral disinhibition and inefficiency. Experiment 2 showed that chronic intermittent ethanol during adolescence appeared to have no lasting effect on behavioral disinhibition or new spatial learning during adulthood. However, chronic ethanol did promote behavioral inefficiency. In summary, results indicate that ethanol-induced promotion of perseverative behavior may contribute to the many adverse behavioral sequelae of alcohol intoxication in adolescents and young adults. Moreover, the long-term effect of adolescent chronic ethanol exposure on behavioral efficiency is similar to that observed after chronic exposure in humans.

Puberty and gonadal hormones: role in adolescent-typical behavioral alterations

This article is part of a Special Issue "Puberty and Adolescence". Adolescence is characterized by a variety of behavioral alterations, including elevations in novelty-seeking and experimentation with alcohol and other drugs of abuse. Some adolescent-typical neurobehavioral alterations may depend upon pubertal rises in gonadal hormones, whereas others may be unrelated to puberty. Using a variety of approaches, studies in laboratory animals have not revealed clear relationships between pubertal-related changes and adolescent- or adult-typical behaviors that are not strongly sexually dimorphic. Data reviewed suggest surprisingly modest influences of gonadal hormones on alcohol intake, alcohol preference and novelty-directed behaviors. Gonadectomy in males (but not females) increased ethanol intake in adulthood following surgery either pre-pubertally or in adulthood, with these increases in intake largely reversed by testosterone replacement in adulthood, supporting an activational role of androgens in moderating ethanol intake in males. In contrast, neither pre-pubertal nor adult gonadectomy influenced sensitivity to the social inhibitory or aversive effects of ethanol when indexed via conditioned taste aversions, although gonadectomy at either age altered the microstructure of social behavior of both males and females. Unexpectedly, the pre-pubertal surgical manipulation process itself was found to increase later ethanol intake, decrease sensitivity to ethanol's social inhibitory effects, attenuate novelty-directed behavior and lower social motivation, with gonadal hormones being necessary for these long-lasting effects of early surgical perturbations.

Lower anxiogenic effects of serotonin agonists are associated with lower activation of amygdala and lateral orbital cortex in adolescent male rats

There has been controversy over use of selective serotonin reuptake inhibitors (SSRIs) to treat affective disorders in children and adolescents due to clinical reports of increased risk for suicidal ideation and behavior during treatment, and animal studies showing changes in adult anxiety- and depressive-like behaviors after repeated treatment during adolescence. However, the acute effect of serotonergic drugs on affective behavior during adolescence is poorly understood. We investigated serotonergic modulation of anxiety-like behavior in adolescent (PN28-32) and adult (PN67-73) male rats using the SSRI fluoxetine, the 5-HT(1A) agonist 8-OH DPAT, and the 5-HT₂ agonist mCPP. Acute treatment with fluoxetine (10 mg/kg, i.p.) produced greater anxiogenic effects in adults than adolescents in the light/dark (LD) test for anxiety-like behavior, but fluoxetine (2.5, 5, and 10 mg/kg, i.p.) increased extracellular serotonin in the medial prefrontal cortex similarly in both ages. Adults were also more sensitive to the anxiogenic effects of 8-OH DPAT (0.25 and 0.5 mg/kg, i.p.), but not mCPP (0.5 and 1 mg/kg, i.p.), in the LD test. Fluoxetine (10 mg/kg) stimulated greater increases in c-Fos expression across the extended amygdala in adults than in adolescents, and 8-OH DPAT (0.5 mg/kg) produced greater increases in c-Fos in the lateral orbital cortex and central nucleus of the amygdala in adults. These data show that lower anxiogenic effects of acute SSRIs in adolescents are associated with lesser activation of cortical and amygdala brain regions. This immaturity could contribute to the different profile of behavioral effects observed in adolescents and adults treated with SSRIs.

Effects of the kappa opioid receptor antagonist, nor-binaltorphimine, on ethanol intake: impact of age and sex

The kappa opioid receptor (KOR) antagonist, nor-binaltorphimine (nor-BNI), was used to investigate the role of the KOR system in mediating ethanol intake. On P25 (adolescent) or P67 (adult) male and female rats were individually housed and given ad libitum access to food and water. The experimental procedure was initiated on P28 or P70: animals were given 30 min/day access to a 10% ethanol/supersaccharin solution every other day (3 baseline exposures). On the day after the final baseline test, rats were injected with nor-BNI (0, 2.5, 5, 10 mg/kg), with testing initiated 24 hr later (30-min access every other day, 3 test exposures). Nor-BNI (10 mg/kg) increased ethanol intake in adult males, whereas the same dose decreased intake in adult females, suggesting pronounced sex differences in KOR-associated mediation of ethanol intake in adulthood. There was no impact of nor-BNI in adolescent animals of either sex, suggesting that the KOR may play less of a role in modulating ethanol intake during adolescence.

Bupropion induced changes in exploratory and anxiety-like behaviour in NMRI male mice depends on the age

The aim of this study was to assess the effects of the antidepressant bupropion on anxiety and novelty-seeking in adolescent mice of different ages and adults. Behavioural differences between early adolescent, late adolescent and adult NMRI mice were measured both in the elevated plus-maze and the hole-board tasks following acute administration of bupropion (5, 10, 15, 20mg/kg) or saline. In the plus maze test, early and late adolescent mice treated with bupropion (10, 15mg/kg, respectively) had lower percentages of entries in the open-arms compared to their vehicle controls. Adult mice treated with bupropion did not differ from their vehicle controls. These results suggest that the effect of this drug on anxiety-like behaviour in mice depends on the age, showing adolescents an anxiogenic-like profile. In the hole-board, adolescents showed more elevated levels of novelty-seeking than adults, exhibiting shorter latency to the first head-dip (HD) and a higher number of HD's. Bupropion increases the latency to the first HD and decreases the number of HD's in all age-groups, indicating a decline in exploratory tendency. Findings reveal that the age can modulate the behaviour displayed by mice in both animal models, and that adolescents are more sensitive to bupropion's anxiogenic effects.

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.

Long-term effects of chronic intermittent ethanol exposure in adolescent and adult rats: radial-arm maze performance and operant food reinforced responding

Background

Adolescence is not only a critical period of late-stage neurological development in humans, but is also a period in which ethanol consumption is often at its highest. Given the prevalence of ethanol use during this vulnerable developmental period we assessed the long-term effects of chronic intermittent ethanol (CIE) exposure during adolescence, compared to adulthood, on performance in the radial-arm maze (RAM) and operant food-reinforced responding in male rats.

Methodology/Principal Findings

Male Sprague Dawley rats were exposed to CIE (or saline) and then allowed to recover. Animals were then trained in either the RAM task or an operant task using fixed- and progressive- ratio schedules. After baseline testing was completed all animals received an acute ethanol challenge while blood ethanol levels (BECs) were monitored in a subset of animals. CIE exposure during adolescence, but not adulthood decreased the amount of time that animals spent in the open portions of the RAM arms (reminiscent of deficits in risk-reward integration) and rendered animals more susceptible to the acute effects of an ethanol challenge on working memory tasks. The operant food reinforced task showed that these effects were not due to altered food motivation or to differential sensitivity to the nonspecific performance-disrupting effects of ethanol. However, CIE pre-treated animals had lower BEC levels than controls during the acute ethanol challenges indicating persistent pharmacokinetic tolerance to ethanol after the CIE treatment. There was little evidence of enduring effects of CIE alone on traditional measures of spatial and working memory.

Conclusions/Significance

These effects indicate that adolescence is a time of selective vulnerability to the long-term effects of repeated ethanol exposure on neurobehavioral function and acute ethanol sensitivity. The positive and negative findings reported here help to further define the nature and extent of the impairments observed after adolescent CIE and provide direction for future research.

Differential effects of binge drinking on learning and memory in emerging adults

Alterations in memory function due to alcohol exposure have been observed in both animal models and human populations. The human literature on neurocognitive consequences of binge alcohol use in emerging adults has not systematically investigated its potential negative impacts on visuospatial memory. For instance, these impacts have not yet been assessed using a human analogue of the Morris Water Maze Task (WMT), a key memory measure in the animal literature. Accordingly, this study compared performance between emerging adult binge drinkers (BD, n=22) and age- and sex-matched light drinkers (LD, n=29) using the Morris WMT, as well as verbal memory using the California Verbal Learning Test (CVLT). Emerging adult BD demonstrated worse performance on verbal learning and memory relative to LD. However, no significant group differences were observed on spatial learning and memory. Furthermore, no sex differences or interactions with drinking status were observed on either memory domain. These data suggest that in emerging adults who are at a heightened risk for alcohol abuse disorders, but who do not yet meet diagnostic criteria, verbal learning is uniquely impacted by the neurotoxic effects of binge drinking, whereas spatial learning is relatively spared between bouts of intoxication.

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

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

Attenuated effects of experimenter-administered heroin in adolescent vs. adult male rats: physical withdrawal and locomotor sensitization

OBJECTIVES

Early onset of heroin use during adolescence might increase chances of later drug addiction. Prior work from our laboratory suggests, however, that adolescent male rats are actually less sensitive than adults to some enduring effects of heroin self-administration. In the present study, we tested two likely correlates of sensitivity to behavioral reinforcement in rats: physical withdrawal and locomotor sensitization.

METHODS

Adolescent (35 days old at start) and adult (79 days old) male Sprague-Dawley rats were administered escalating doses of heroin, increasing from 1.0 to 8.0 mg/kg (i.p.) every 12 h, across 13 days. Somatic signs of spontaneous withdrawal were scored 12 and 24 h after the last injection, and then every 24 h for 5 days; locomotion was recorded concurrently. Challenge injections of heroin (1 mg/kg i.p.) were given at four points: as the first of the escalating doses (day 1), at days 7 and 13 during the escalating regimen, and after 12 days of forced abstinence. Body mass and food intake were measured throughout experimentation.

RESULTS

A heroin withdrawal syndrome was not observed among adolescents as it was among adults, including somatic signs as well as reduced locomotion, body mass, and food intake. On the other hand, heroin-induced locomotor sensitization did not differ across ages.

CONCLUSION

Reduced withdrawal is consistent with the attenuated reinforcing effects of heroin among adolescent male rats that we reported previously. Thus, it is possible that adolescent rats could reveal important neuroprotective factors for use in treatment of heroin dependence.

Binge ethanol intoxication heightens subsequent ethanol intake in adolescent, but not adult, rats

A question still to be answered is whether ethanol initiation has a greater effect on ethanol consumption if it occurs during adolescence than in adulthood. This study assessed the effect of ethanol initiation during adolescence or adulthood on voluntary ethanol consumption when animals were still within the same age range. Adolescent or adult rats were given 5, 2, or 0 ethanol exposures. The animals were tested for ethanol consumption through two-bottle choice tests, before undergoing a 1-week deprivation. A two-bottle assessment was conducted after the deprivation. Adolescents, but not adults, given two ethanol administrations during initiation exhibited significantly higher ethanol intake during the pre-deprivation period. These adolescents also exhibited a threefold increase in ethanol intake after 7 days of drug withdrawal, when compared with controls. These findings suggest that very brief experience with binge ethanol intoxication in adolescence, but not in adulthood, impacts later predisposition to drink.

Adolescent alcohol exposure alters the rat adult hypothalamic-pituitary-adrenal axis responsiveness in a sex-specific manner

Exposure to alcohol during adolescence exerts long-term effects on the adult brain stress circuits, causing many changes that persist into adulthood. Here we examined the consequences of adolescent intermittent ethanol (AIE, administered from postnatal day (PND) 28-42) on the hypothalamic-pituitary-adrenal (HPA) axis-related brain circuitry of rats challenged with intragastric (ig) administration of alcohol in adulthood (PND 70-71). Both male and female adolescent rats were exposed to alcohol vapors, while controls did not receive the drug, to assess whether AIE alters adult alcohol response in a sex-specific manner. We demonstrated that AIE increased paraventricular nucleus (PVN) Avp mRNA levels during late (PND 42) but not middle (PND 36) adolescence in males. While an alcohol challenge administered to 70-71-day-old rats increased Crf mRNA levels in males and Avp mRNA levels in females, AIE blunted both effects. These results suggest that AIE produced long-lasting changes in the responsiveness of the HPA axis to a subsequent alcohol challenge in a sex-specific manner. Furthermore, AIE altered adrenergic brain stem nuclei involved in stress responses in adulthood, resulting in increased numbers of phenylethanolamine N-methyltransferase (PNMT) neurons in male C2 and female C1 regions. This tended to enhance activation of the male C2 nucleus upon alcohol challenge. Collectively, these results suggest that AIE exerts long-term effects on the ability of the PVN to respond to an alcohol challenge in adulthood, possibly mediated by catecholaminergic input from the brain stem to the PVN.

Alcohol intoxications during adolescence increase motivation for alcohol in adult rats and induce neuroadaptations in the nucleus accumbens

Adolescent alcohol binge drinking constitutes a major vulnerability factor to develop alcoholism. However, mechanisms underlying this susceptibility remain unknown. We evaluated the effect of adolescent binge-like ethanol intoxication on vulnerability to alcohol abuse in Sprague-Dawley rats. To model binge-like ethanol intoxication, every 2 days, rats received an ethanol injection (3.0 g/kg) for 2 consecutive days across 14 days either from postnatal day 30 (PND30) to 43 (early adolescence) or from PND 45 to PND 58 (late adolescence). In young adult animals, we measured free ethanol consumption in the two-bottle choice paradigm, motivation for ethanol in the operant self-administration task and both ethanol's rewarding and aversive properties in the conditioned place preference (CPP) and taste aversion (CTA) paradigms. While intermittent ethanol intoxications (IEI) during late adolescence had no effect on free-choice 10% ethanol consumption, we found that IEI during early adolescence promoted free-choice 10% ethanol consumption, enhanced motivation for ethanol in the self-administration paradigm and induced a loss of both ethanol-induced CPP and CTA in young adults. No modification in either sucrose self-administration or amphetamine-induced CPP was observed. As the nucleus accumbens (Nac) is particularly involved in addictive behavior, we analyzed IEI-induced long-term neuroadaptations in the Nac using c-Fos immunohistochemistry and an array of neurotransmission-related genes. This vulnerability to ethanol abuse was associated with a lower c-Fos immunoreactivity in the Nac and enduring alterations of the expression of Penk and Slc6a4, 2 neurotransmission-related genes that have been shown to play critical roles in the behavioral effects of ethanol and alcoholism.

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.

Acute illness-induced behavioral alterations are similar to those observed during withdrawal from acute alcohol exposure

Exposure to an immunogen results in a constellation of behavioral changes collectively referred to as "sickness behaviors," with alterations in cytokine expression previously shown to contribute to this sickness response. Since behaviors observed during ethanol withdrawal are strikingly similar to sickness behaviors, we hypothesized that behavioral manifestations of ethanol withdrawal might be an expression of sickness behaviors induced by ethanol-related changes in peripheral and/or central cytokine expression. Accordingly, behaviors exhibited during a modified social investigation test were first characterized in male rats following an acute injection of lipopolysaccharide (LPS; 100μg/kg). Subsequently, behavioral changes after either a high (4-g/kg; Experiment 2) or low dose (0.5g/kg; Experiment 3) of ethanol were also examined in the same social investigation test, as well as in the forced-swim test (FST; Experiment 4). Results from these experiments demonstrated similar reductions in both exploration and social investigatory behavior during acute illness and ethanol withdrawal, while a seemingly paradoxical decrease in immobility was observed in the FST during acute ethanol withdrawal. In follow-up studies, neither indomethacin (Experiment 5) nor interleukin-1 receptor antagonist (Experiment 6) pre-exposure reversed the ethanol withdrawal-induced behavioral changes observed in this social investigation test. Taken together, these studies demonstrate that the behavioral sequelae of acute illness and ethanol withdrawal are similar in nature, while antagonist studies suggest that these behavioral alterations are not reversed by blockade of IL-1 receptors or inhibition of prostaglandin synthesis. Though a direct mechanistic link between cytokines and the expression of acute ethanol withdrawal-related behaviors has yet to be found, future studies examining the involvement of brain cytokines as potential mediators of ethanol effects are greatly needed.

Short- and long-term effects of juvenile stressor exposure on the expression of GABAA receptor subunits in rats

During the juvenile period rodents are particularly sensitive to stressors. Aversive events encountered during this period may have enduring effects that are not evident among animals initially stressed as adults. Interestingly, experiencing stressor during juvenile period was found to elicit a biphasic behavioral pattern over the course of development. During the juvenile period, the expression of several GABAA receptor subunits is subject to elevated plasticity, rendering the GABAergic system sensitive to stressors. In the present investigation, animals were exposed to a juvenile variable stressor regimen (JUV-S) at 27-29 postnatal days (PND): 27 PND-acute swim stress (10 min), 28 PND-elevated platform stress (3 sessions×30 min each), and 29 PND-restraint (2 h). One hour following the last exposure to stressor or in adulthood (60 PND), anxiety-related behaviors were assessed in a 5-min elevated plus maze test. The western blotting technique was used to evaluate whether the juvenile stress induced behavioral pattern will be accompanied by respective changes in GABAA α1, α2, and α3 protein expression in male rats. Our findings further established that juvenile stressor elicits hyper-reactivity when rats were tested as juveniles, whereas rats exhibited reduced activity and increased anxiety when tested as adults. Additionally, the effects of juvenile stressor on α1, α2, and α3 were more pronounced among juvenile stressed rats that were challenged as adults compared with rats that were only challenged as juveniles. Interestingly, the stress-induced modulation of the subunits was particularly evident in the amygdala, a brain region closely associated with anxiety. Thus, age- and region-specific alterations of the α subunits may contribute to the age-specific behavioral alterations observed following juvenile stress exposure.

Escalation of methamphetamine self-administration in adolescent and adult rats

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Effect of alcohol consumption in prenatal life, childhood, and adolescence on child development

The effects of alcohol consumption in adults are well described in the literature, while knowledge about the effects of alcohol consumption in children is more limited and less systematic. The present review shows how alcohol consumption may negatively influence the neurobiological and neurobehavioral development of humans. Three different periods of life have been considered: the prenatal term, childhood, and adolescence. For each period, evidence of the short-term and long-term effects of alcohol consumption, including neurodevelopmental effects and associations with subsequent alcohol abuse or dependence, is presented.

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

RATIONALE

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Enhancement of the hypothalamic-pituitary-adrenal axis but not cytokine responses to stress challenges imposed during withdrawal from acute alcohol exposure in Sprague-Dawley rats

RATIONALE

Alcohol withdrawal is associated with reduced activity, increased anxiety, and other signs of distress.

OBJECTIVE

The goal of the current studies was to determine whether acute ethanol exposure would alter hypothalamic-pituitary-adrenal (HPA) axis reactivity and cytokine responses to stress challenges imposed during the withdrawal period.

METHODS

Male Sprague-Dawley rats were intubated with 4 g/kg of ethanol to simulate acute binge-like ethanol intake. After characterizing the blood ethanol concentrations (BECs; Experiment 1), exploratory activity in a novel environment was explored at 10, 14 and 18 h after ethanol (Experiment 2) to characterize altered activity patterns indicative of withdrawal. In Experiment 3, rats were exposed to footshock during withdrawal to examine whether prior ethanol exposure would alter cytokine and HPA axis responses to stress. Experiments 4 and 5 investigated HPA axis sensitivity and gene expression changes during restraint imposed during withdrawal.

RESULTS

Prior ethanol exposure produced a period of stress hyper-reactivity evidenced by an enhanced HPA axis response (increased corticosterone and adrenocorticotropic hormone) observed during withdrawal. While this hyper-reactivity in response to two different stress challenges (novel environment and restraint) was accompanied by profound behavioral changes indicative of withdrawal, no alterations in cytokine changes evoked by stress were observed.

CONCLUSIONS

Taken together, these findings provide support for the hypothesis that alcohol withdrawal enhances HPA axis reactivity to stress challenges, though not likely as the result of heightened inflammatory signaling, and may have implications for understanding the mechanisms by which stress impacts relapse drinking in humans.

Altered anxiety-like behavior and long-term potentiation in the bed nucleus of the stria terminalis in adult mice exposed to chronic social isolation, unpredictable stress, and ethanol beginning in adolescence

Alcohol and chronic stress exposure, especially during adolescence, can lead to an increased risk in adulthood of developing alcohol use disorders. To date, however, no study has assessed the potential long-term effects of chronic intermittent and unpredictable ethanol (EtOH) exposure in mice chronically stressed beginning in adolescence on brain function and anxiety-like behaviors in adulthood. In particular, alterations in function of the bed nucleus of the stria terminalis (BNST), a brain region heavily implicated in anxiety-related behaviors and altered plasticity following EtOH exposure, may play a key role in the pathological responses to chronic stress and EtOH. In the present study, adolescent and adult C57Bl/6J mice were exposed to a regimen of chronic social isolation and unpredictable stressors and EtOH (or air [sham]; CSI-CUS-EtOH and CSI-CUS-Sham, respectively) for 8-10 weeks. In adulthood, mice were tested for altered anxiety-like behavior (elevated plus maze [EPM] and modified social interaction [SI] test). Following behavioral testing, mice were reexposed to CSI-CUS-EtOH (and CSI-CUS-Sham for controls) for an additional 3 days. Four to six hours following the final EtOH (or air) exposure, field potential recordings of the dorsal-lateral (dl)BNST were performed. Mice first exposed during adolescence to CSI-CUS-EtOH displayed lower levels of anxiety-like behavior on the EPM compared with mice first exposed to CSI-CUS-EtOH during adulthood and control mice only exposed to CSI-CUS-Sham, regardless of age of first exposure. However, mice first exposed to CSI-CUS-EtOH during adulthood displayed lower levels of anxiety-like behavior on the SI test compared with mice first exposed during adolescence and control CSI-CUS-Sham mice. CSI-CUS-EtOH exposure, regardless of age, produced blunted expression of long-term potentiation (LTP) in the dlBNST compared with CSI-CUS-Sham mice. This study demonstrates age-dependent effects of chronic unpredictable ethanol exposure in chronically stressed mice on anxiety-like behaviors during adulthood. Further, CSI-CUS-EtOH exposure results in blunted LTP expression in the adult dlBNST.

The ontogeny of anxiety-like behavior in rats from adolescence to adulthood

In human beings, susceptibility to anxiety disorders can be relatively high during adolescence. Understanding the ontogeny of anxiety-like behavior in laboratory rodents has implications for developing anxiolytic drugs that are suitable for this age group. Given the dearth of information about adolescent rodents, this study examined the response of both male and female adolescent, late adolescent, young adult, and older adult rats to three tests of anxiety-like behavior: the emergence test (ET), open field (OF), and elevated plus-maze (EPM). The results showed that adolescent rats exhibited a higher anxiety-like response than adults on each test; the amount of locomotion in the OF and percentage of time spent on the open arms of the EPM increased across the age groups, while older adult rats made the fewest start box re-entries in the ET. These results support the hypothesis that adolescent rats have a more pronounced response to stressors than do adults.

Chronic intermittent ethanol exposure in early adolescent and adult male rats: effects on tolerance, social behavior, and ethanol intake

BACKGROUND

Given the prevalence of alcohol use in adolescence, it is important to understand the consequences of chronic ethanol exposure during this critical period in development. The purpose of this study was to assess possible age-related differences in susceptibility to tolerance development to ethanol-induced sedation and withdrawal-related anxiety, as well as voluntary ethanol intake after chronic exposure to relatively high doses of ethanol during adolescence or adulthood.

METHODS

Juvenile/adolescent and adult male Sprague-Dawley rats were assigned to one of five 10-day exposure conditions: chronic ethanol (4 g/kg every 48 hours), chronic saline (equivalent volume every 24 hours), chronic saline/acutely challenged with ethanol (4 g/kg on day 10), nonmanipulated/acutely challenged with ethanol (4 g/kg on day 10), or nonmanipulated. For assessment of tolerance development, duration of the loss of righting reflex (LORR) and blood ethanol concentrations (BECs) upon regaining of righting reflex (RORR) were tested on the first and last ethanol exposure days in the chronic ethanol group, with both saline and nonmanipulated animals likewise challenged on the last exposure day. Withdrawal-induced anxiety was indexed in a social interaction test 24 hours after the last ethanol exposure, with ethanol-naïve chronic saline and nonmanipulated animals serving as controls. Voluntary intake was assessed 48 hours after the chronic exposure period in chronic ethanol, chronic saline and nonmanipulated animals using an 8-day 2 bottle choice, limited-access ethanol intake procedure.

RESULTS

In general, adolescent animals showed shorter durations of LORR and higher BECs upon RORR than adults on the first and last ethanol exposure days, regardless of chronic exposure condition. Adults, but not adolescents, developed chronic tolerance to the sedative effects of ethanol, tolerance that appeared to be metabolic in nature. Social deficits were observed after chronic ethanol in both adolescents and adults. Adolescents drank significantly more ethanol than adults on a gram per kilogram basis, with intake uninfluenced by prior ethanol exposure at both ages.

CONCLUSIONS

Adolescents and adults may differ in their ability and/or propensity to adapt to chronic ethanol exposure, with adults, but not adolescents, developing chronic metabolic tolerance. However, this chronic exposure regimen was sufficient to disrupt baseline levels of social behavior at both ages. Taken together, these results suggest that, despite the age-related differences in tolerance development, adolescents are as susceptible as adults to consequences of chronic ethanol exposure, particularly in terms of disruptions in social behavior. Whether these effects would last into adulthood remains to be determined.

Neurobiology of the adolescent brain and behavior: implications for substance use disorders

OBJECTIVE

Adolescence is a developmental period that entails substantial changes in risk-taking behavior and experimentation with alcohol and drugs. Understanding how the brain is changing during this period relative to childhood and adulthood and how these changes vary across individuals are key in predicting risk for later substance abuse and dependence.

METHOD

This review discusses recent human imaging and animal work in the context of an emerging view of adolescence as characterized by a tension between early emerging "bottom-up" systems that express exaggerated reactivity to motivational stimuli and later maturing "top-down" cognitive control regions. Behavioral, clinical, and neurobiological evidences are reported for dissociating these two systems developmentally. The literature on the effects of alcohol and its rewarding properties in the brain is discussed in the context of these two systems.

RESULTS

Collectively, these studies show curvilinear development of motivational behavior and the underlying subcortical brain regions, with a peak inflection from 13 to 17 years. In contrast, prefrontal regions, important in top-down regulation of behavior, show a linear pattern of development well into young adulthood that parallels that seen in behavioral studies of impulsivity.

CONCLUSIONS

The tension or imbalance between these developing systems during adolescence may lead to cognitive control processes being more vulnerable to incentive-based modulation and increased susceptibility to the motivational properties of alcohol and drugs. As such, behavior challenges that require cognitive control in the face of appetitive cues may serve as useful biobehavioral markers for predicting which teens may be at greater risk for alcohol and substance dependence.

Ontogenetic differences in adolescent and adult C57BL/6J and DBA/2J mice: anxiety-like, locomotor, and consummatory behaviors

Adolescence is a highly conserved period during which mammals undergo a number of hormonal, biological, and behavioral changes [Spear [2000] Neurosci. Biobehav. Rev. 24: 417-463]. Ethical constraints limit the research that can be done in human adolescents. Rodents provide a useful model of at least some of the features of adolescence, including increases in body growth, differences in sleep/wake, and eating patterns, as well as differences in risk-taking, novelty seeking, and exploratory behaviors. Much of the available developmental research has utilized rats; however, the use of inbred mouse strains provides a unique means to assess the genetic factors involved in behavioral differences during adolescence. We assessed differences between adults and adolescents in anxiety-like, locomotor, and consummatory behaviors using two commonly used inbred strains of mice, the DBA/2J and C57BL/6J strains. Age and genotype-dependent differences were found in all three behaviors measured, suggesting both factors are important determinants of behavior in mice.

Ethanol-induced conditioned taste aversion in male sprague-dawley rats: impact of age and stress

BACKGROUND

Age-specific characteristics may contribute to the elevation in ethanol intake commonly reported among adolescents compared to adults. This study was designed to examine age-related differences in sensitivity to ethanol's aversive properties using a conditioned taste aversion (CTA) procedure with sucrose serving as the conditioned stimulus (CS). Given that ontogenetic differences in responsiveness to stressors have been previously reported, the role of stressor exposure on the development of CTA was also assessed.

METHODS

Experiment 1 examined the influence of 5 days of prior restraint stress exposure on the expression of CTA in a 2-bottle test following 1 pairing of a sucrose solution with ethanol. In Experiment 2, the effects of 7 days of social isolation on the development of CTA were observed using a 1-bottle test following multiple sucrose-ethanol pairings.

RESULTS

This study revealed age-related differences in the development of ethanol-induced CTA. In Experiment 1, adolescents required a higher dose of ethanol than adults to demonstrate an aversion. In Experiment 2, adolescents required not only a higher ethanol dose but also more pairings of ethanol with the sucrose CS. No effects of prior stressor exposure were observed in either experiment.

CONCLUSIONS

Together, these experiments demonstrate an adolescent-specific insensitivity to the aversive properties of ethanol that elicit CTA, a pattern not influenced by repeated restraint stress or housing in social isolation. This age-related insensitivity to the dysphoric effects of ethanol is consistent with other work from our laboratory, adding further to the evidence that adolescent rats are less susceptible to negative consequences of ethanol that may serve as cues to curb consumption.

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.

Sensitivity to ethanol and other hedonic stimuli in an animal model of adolescence: implications for prevention science?

Age-related patterns of sensitivity to appetitive and aversive stimuli seemingly have deep evolutionary roots, with marked developmental transformations seen during adolescence in a number of relatively ancient brain systems critical for motivating and directing reward-related behaviors. Using a simple animal model of adolescence in the rat, adolescents have been shown to be more sensitive than their adult counterparts to positive rewarding effects of alcohol, other drugs, and certain natural stimuli, while being less sensitive to the aversive properties of such stimuli. Adolescent-typical alcohol sensitivities may be exacerbated further by a history of prior stress or alcohol exposure as well as by genetic vulnerabilities, permitting relatively high levels of adolescent alcohol use and perhaps an increased probability for the emergence of abuse disorders. A number of potential (albeit tentative) implications of these basic research findings for prevention science are considered.

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.