Sensitivity to Ethanol Across Development in Rats: Comparison to [3H]Zolpidem Binding

Alcohol and stress exposure across the lifespan are key risk factors for Alzheimer's Disease and cognitive decline

Alzheimer's Disease and related dementias (ADRD) are an increasing threat to global health initiatives. Efforts to prevent the development of ADRD require understanding behaviors that increase and decrease risk of neurodegeneration and cognitive decline, in addition to uncovering the underlying biological mechanisms behind these effects. Stress exposure and alcohol consumption have both been associated with increased risk for ADRD in human populations. However, our ability to understand causal mechanisms of ADRD requires substantial preclinical research. In this review, we summarize existing human and animal research investigating the connections between lifetime stress and alcohol exposures and ADRD.

Influence of Age and Genetic Background on Ethanol Intake and Behavioral Response Following Ethanol Consumption and During Abstinence in a Model of Alcohol Abuse

Genetic background and age at first exposure have been identified as critical variables that contribute to individual vulnerability to drug addiction. Evidence shows that genetic factors may account for 40–70% of the variance in liability to addiction. Alcohol consumption by young people, especially in the form of binge-drinking, is becoming an alarming phenomenon predictive of future problems with drinking. Thus, the literature indicates the need to better understand the influence of age and genetic background on the development of alcohol dependence. To this aim, the inbred rat strains Lewis (LEW, addiction prone) and Fischer 344 (F344, addiction resistant) were used as a model of genetic vulnerability to addiction and compared with the outbred strain Sprague-Dawley (SD) in a two-bottle choice paradigm as a model of alcohol abuse. During a 9-week period, adolescent and adult male rats of the three strains were intermittently exposed to ethanol (20%) and water during three 24-h sessions/week. Adult and adolescent SD and LEW rats escalated their alcohol intake over time reaching at stable levels, while F344 rats did not escalate their intake, regardless of age at drinking onset. Among adolescents, only F344 rats consumed a higher total amount of ethanol than adults, although only SD and LEW rats escalated their intake. Adult LEW rats, albeit having a lower ethanol consumption as compared to SD rats but greater than F344, showed a more compulsive intake, consuming higher amounts of ethanol during the first hour of exposure, reaching a higher degree of ethanol preference when start drinking as adolescents. Behavioral analysis during the first hour of ethanol consumption revealed significant strain differences, among which noticeable the lack of sedative effect in the LEW strain, at variance with F344 and SD strains, and highest indices of withdrawal (most notable jumping) in LEW rats during the first hour of abstinence days. The present results underscore the importance of individual genetic background and early onset of alcohol use in the progression toward abuse and development of alcohol addiction.

Adolescent Alcohol Exposure Persistently Impacts Adult Neurobiology and Behavior

Adolescence is a developmental period when physical and cognitive abilities are optimized, when social skills are consolidated, and when sexuality, adolescent behaviors, and frontal cortical functions mature to adult levels. Adolescents also have unique responses to alcohol compared with adults, being less sensitive to ethanol sedative-motor responses that most likely contribute to binge drinking and blackouts. Population studies find that an early age of drinking onset correlates with increased lifetime risks for the development of alcohol dependence, violence, and injuries. Brain synapses, myelination, and neural circuits mature in adolescence to adult levels in parallel with increased reflection on the consequence of actions and reduced impulsivity and thrill seeking. Alcohol binge drinking could alter human development, but variations in genetics, peer groups, family structure, early life experiences, and the emergence of psychopathology in humans confound studies. As adolescence is common to mammalian species, preclinical models of binge drinking provide insight into the direct impact of alcohol on adolescent development. This review relates human findings to basic science studies, particularly the preclinical studies of the Neurobiology of Adolescent Drinking in Adulthood (NADIA) Consortium. These studies focus on persistent adult changes in neurobiology and behavior following adolescent intermittent ethanol (AIE), a model of underage drinking. NADIA studies and others find that AIE results in the following: increases in adult alcohol drinking, disinhibition, and social anxiety; altered adult synapses, cognition, and sleep; reduced adult neurogenesis, cholinergic, and serotonergic neurons; and increased neuroimmune gene expression and epigenetic modifiers of gene expression. Many of these effects are specific to adolescents and not found in parallel adult studies. AIE can cause a persistence of adolescent-like synaptic physiology, behavior, and sensitivity to alcohol into adulthood. Together, these findings support the hypothesis that adolescent binge drinking leads to long-lasting changes in the adult brain that increase risks of adult psychopathology, particularly for alcohol dependence.

Adolescent neurobehavioral characteristics, alcohol sensitivities, and intake: Setting the stage for alcohol use disorders?

The transition to adolescence is characterized by rapid biological transformations that include not only the hormonal and physiological changes of puberty but also dramatic changes in the brain as well. Similar neural and physiological changes are associated with the transition from immaturity to maturity across a variety of mammalian species, along with a variety of common adolescent-typical behavioral characteristics. Among the neural systems undergoing alterations during adolescence are those that modulate sensitivity to a variety of alcohol effects, potentially increasing the propensity for relatively high levels of adolescent alcohol use, which in turn may set the stage for later alcohol use disorders. This article reviews research on adolescent alcohol sensitivities and suggests possible implications of these findings for the frequent initiation and relatively high levels of alcohol intake seen at this age.

Periadolescent ethanol exposure reduces adult forebrain ChAT+IR neurons: correlation with behavioral pathology

Substance abuse typically begins in adolescence; therefore, the impact of alcohol during this critical time in brain development is of particular importance. Epidemiological data indicate that excessive alcohol consumption is prevalent among adolescents and may have lasting neurobehavioral consequences. Loss of cholinergic input to the forebrain has been demonstrated following fetal alcohol exposure and in adults with Wernicke-Korsakoff syndrome. In the present study, immunohistochemistry for choline acetyltransferase (ChAT) was determined to assess forebrain cholinergic neurons (Ch1-4), and behavioral changes following periadolescent alcohol exposure. Wistar rats were exposed to intermittent ethanol vapor (14 h on/10 h off/day) for 35 days from postnatal day (PD) 22 to PD 57 (average blood alcohol concentration (BAC): 163 mg%). Rats were withdrawn from vapor and assessed for locomotor activity, startle response, conflict behavior in the open field, and immobility in the forced swim test, as adults. Rats were then sacrificed at day 71/72 and perfused for histochemical analyses. Ethanol vapor-exposed rats displayed: increased locomotor activity 8 h after the termination of vapor delivery for that 24 h period at day 10 and day 20 of alcohol vapor exposure, significant reductions in the amplitude of their responses to prepulse stimuli during the startle paradigm at 24 h withdrawal, and at 2 weeks following withdrawal, less anxiety-like and/or more "disinhibitory" behavior in the open field conflict, and more immobility in the forced swim test. Quantitative analyses of ChAT immunoreactivity revealed a significant reduction in cell counts in the Ch1-2 and Ch3-4 regions of the basal forebrain in ethanol vapor-exposed rats. This reduction in cell counts was significantly correlated with less anxiety-like and/or more "disinhibitory" behavior in the open field conflict test. These studies demonstrate that behavioral measures of arousal, affective state, disinhibitory behavior, and ChAT+IR, are all significantly impacted by periadolescent ethanol exposure and withdrawal in Wistar rats.

Effect of the selective NMDA NR2B antagonist, ifenprodil, on acute tolerance to ethanol-induced motor impairment in adolescent and adult rats

BACKGROUND

Adolescent rats have been observed to be less sensitive than adults to a number of acute ethanol effects, including ethanol-induced motor impairment. These adolescent insensitivities may be related in part to the more rapid emergence of within session (acute) tolerance in adolescents than adults. Adolescent-related alterations in neural systems that serve as ethanol target sites, including changes in NMDA receptor subunit expression, may influence the responsiveness of adolescents to acute ethanol effects. This study explored the role of NMDA NR2B receptors in the development of acute tolerance to ethanol-induced motor impairment in male adolescent [postnatal day (P)28-30] and adult (P68-70) Sprague-Dawley rats.

METHODS

Motor-impairing effects of ethanol on the stationary inclined plane and blood ethanol concentrations (BECs) were examined following challenge at each age with a functionally equivalent ethanol dose (adolescents: 2.25 g/kg; adults: 1.5 g/kg). Data were collected at two postinjection intervals (10 or 60 minutes) to compare rate of recovery from ethanol intoxication with BEC declines using the Radlow approach (Radlow, 1994) and changes in motor impairment/BEC ratios over time for assessing acute tolerance.

RESULTS

Both vehicle-treated adolescent and adult animals showed similar acute tolerance development to the motor-impairing effects of ethanol at these functionally equivalent doses on the stationary inclined plane, as indexed by an increasing time-dependent dissociation between BECs and ethanol-induced motor impairment, with motor impairment declining faster than BECs, as well as by significant declines in motor impairment/BEC ratios over time. Acute tolerance development was reliably blocked by administration of the NR2B antagonist, ifenprodil, (5.0 mg/kg), in adult rats, whereas adolescents were affected by a higher dose (10.0 mg/kg).

CONCLUSIONS

These data support the suggestion that alterations in NMDA receptor systems occurring during adolescence may contribute to reduced sensitivity to ethanol by enhancing the expression of acute tolerance development in adolescents relative to 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.

Ontogeny of ethanol-induced motor impairment following acute ethanol: assessment via the negative geotaxis reflex in adolescent and adult rats

Adolescent rats have been observed to be less sensitive than adults to a number of ethanol effects that may serve as feedback cues to reduce further ethanol intake. Among these findings are a few reports of attenuated sensitivities of adolescents to ethanol-induced motor impairment. The purpose of the present study was to further explore potential age-related differences in ethanol-induced motor impairment in both male and female adolescent (postnatal day [P]28-32), and adult (P68-72) Sprague-Dawley rats using an inclined plane assessment of the negative geotaxis reflex. Adult males displayed significant motor impairment at 1.5 g/kg, whereas adolescent males required higher doses, showing significant motor impairment only at doses of 2.25 g/kg ethanol or greater. Intoxicated practice did not significantly influence level of motor impairment at either age. When female rats of both ages were separately analyzed in terms of their response to ethanol, a dose of 1.5 g/kg ethanol was found to significantly impair adults, whereas adolescent females showed significant motor impairment when challenged with 2.25 g/kg but not 1.5 g/kg ethanol. Yet when the 1.5 g/kg data of females at the two ages were directly compared, no significant age difference was seen at this dose. These data document an attenuated sensitivity of adolescent relative to adult rats to the motor impairing effects of ethanol using a stationary inclined plane test, an effect particularly robust in male animals, and demonstrates the utility of this test for assessment of motor coordination in adolescent and adult rats.

Sensitization to social anxiolytic effects of ethanol in adolescent and adult Sprague-Dawley rats after repeated ethanol exposure

Ontogenetic studies using a social interaction paradigm have shown that adolescent rats are less sensitive to anxiolytic properties of acute ethanol than their adult counterparts. It is not known, however, whether adaptations to these anxiolytic effects on repeated experiences with ethanol would be similar in adolescents and adults. The present study investigated sensitivity to the anxiolytic effects of ethanol in adolescent and adult male and female Sprague-Dawley rats after 7 days of exposure (postnatal day [P] 27-33 for adolescents and P62-68 for adults) to 1g/kg ethanol or saline (intraperitoneally]) and in animals left nonmanipulated during this time. Anxiolytic effects of ethanol (0, 0.75, 1.0, 1.25, and 1.5g/kg for adolescents and 0, 0.25, 0.5, 0.75, 1.0, and 1.25g/kg for adults in experiments 1 and 2, respectively) were examined 48h after the last exposure using a modified social interaction test under unfamiliar test circumstances. At both ages, repeated ethanol exposure resulted in the development of apparent sensitization to anxiolytic effects of ethanol, indexed by enhancement of social investigation and transformation of social avoidance into social indifference or preference and expression of tolerance to the socially inhibiting effects induced by higher ethanol doses. Evidence for the emergence of sensitization in adults and tolerance at both ages was seen not only after chronic ethanol but also after chronic saline exposure, suggesting that chronic manipulation per se may be sufficient to alter the sensitivity of both adolescents and adults to socially relevant effects of ethanol.

Sex differences in ethanol intake and sensitivity to aversive effects during adolescence and adulthood

Aims: The present experiments examined sex differences in ethanol intake and in the influence of a social context on aversive properties of ethanol in adolescent and adult Sprague-Dawley rats. Methods: Experiment 1 examined ethanol intake, with animals receiving daily 2-h access to ethanol and water for 8 days. Experiment 2 assessed the aversive effects of ethanol using a conditioned taste aversion (CTA) paradigm, with animals placed either alone or with a same-sex, same-age peer during the ethanol intoxication phase of conditioning. Results: Ethanol intake varied with both age and sex, although the sex differences emerging at each age were opposite in nature. Adolescent males consumed more ethanol relative to their body weights than adolescent females and adults of both sexes, whereas adult females generally consumed more than adult males. The CTA test revealed no sex differences in aversive effects of ethanol in adults, whereas adolescent males were less sensitive to the aversive properties of ethanol than adolescent females when intoxication occurred in the presence of a peer. Ethanol-induced CTA was evident in adults at lower doses than in adolescents. Conclusions: These results suggest that age differences in ethanol intake in males and sex differences in intake during adolescence may be associated in part with the relative insensitivity of the male adolescents to ethanol's aversive properties, especially when intoxication occurred in a social context. However, the elevated ethanol intake observed in adult females relative to their male counterparts appears to be unrelated to the aversive properties of ethanol.

Gender differences in factors influencing alcohol use and drinking progression among adolescents

While prevalence rates for alcohol use and related disorders differ widely between adult men and women, male and female adolescents do not exhibit the same disparity in alcohol consumption. Previous research and reviews do not address the emergence of differences in drinking patterns that occur during late adolescence. Therefore, a developmental perspective is presented for understanding how various risk and protective factors associated with problematic drinking affect diverging alcohol trajectories as youth move into young adulthood. This review examines factors associated with risk for developing an alcohol use disorder in adolescent girls and boys separately. Findings indicate that certain biological (i.e., genetic risk, neurological abnormalities associated with P300 amplitudes) and psychosocial (i.e., impact of positive drinking expectancies, personality characteristics, and deviance proneness) factors appear to impact boys and girls similarly. In contrast, physiological and social changes particular to adolescence appear to differentially affect boys and girls as they transition into adulthood. Specifically, boys begin to manifest a constellation of factors that place them at greater risk for disruptive drinking: low response to alcohol, later maturation in brain structures and executive function, greater estimates of perceived peer alcohol use, and socialization into traditional gender roles. On an individual level, interventions which challenge media-driven stereotypes of gender roles while simultaneously reinforcing personal values are suggested as a way to strengthen adolescent autonomy in terms of healthy drinking decisions. Moreover, parents and schools must improve consistency in rules and consequences regarding teen drinking across gender to avoid mixed messages about acceptable alcohol use for boys and girls.

Early ethanol consumption predicts relapse-like behavior in adolescent male rats

BACKGROUND

Alcohol abuse disorders emerge over time with repeated consumption of ethanol, but not all ethanol drinkers develop these disorders. There are pre-existing characteristics that indicate which drinkers are most likely to abuse alcohol. Adolescence, novelty seeking, and high stress reactivity are among the characteristics of the most vulnerable individuals. In addition, an individual's response to his or her first exposure to the drug influences future consumption. We assessed an array of behavioral and hormonal characteristics in adolescent (28-day-old) male rats before exposure to ethanol, and then determined which rats were most prone to high levels of alcohol drinking.

METHODS

The assessments consisted of measures of anxiety (elevated plus maze), response to novelty (open field locomotion, novel object exploration), and circulating corticosterone levels after mild restraint and after the elevated plus maze task. After this test battery, the rats were placed in lickometer cages nightly (5 pm to 9 am) for evaluation of fluid consumption. Rats were first habituated to the cages with water in the lickometer bottles, and then given 10% (v/v) ethanol for 3 nights as the only available fluid. After this forced ethanol exposure, the rats were allowed to choose between 8% ethanol and water for 10 consecutive nights. After 2 nights of abstinence, the rats were again placed in the lickometer cages and given a choice between 8% ethanol and water to assess ethanol consumption in response to alcohol deprivation, a measure of relapse-like behavior.

RESULTS

Ethanol consumption on the third day of forced consumption was significantly correlated with ethanol consumption on days 8 to 10 of the choice phase, which in turn was significantly correlated to relapse-like consumption. Preference for ethanol was also significantly correlated with early consumption. Novel object exploration, open field activity, open arm time in the elevated plus maze, initial water consumption, and circulating corticosterone levels did not significantly predict deprivation-stimulated consumption.

CONCLUSIONS

These results suggest that consumption during early exposure to ethanol establishes a pattern leading to development of increased alcohol consumption and preference in adolescent male rats. In addition, they represent an animal model of the well-described observation that humans who consume large quantities of ethanol during early exposure are the most likely to repeat heave drinking behavior. Furthermore, early consumption is distinct from novelty seeking, anxiety, and stress hormone levels which are also thought to contribute to vulnerability to alcoholism.

Decreased sensitivity in adolescent vs. adult rats to the locomotor activating effects of toluene

Volatile organic solvent (inhalant) abuse continues to be a major health concern throughout the world. Of particular concern is the abuse of inhalants by adolescents because of its toxicity and link to illicit drug use. Toluene, which is found in many products such as glues and household cleaners, is among the most commonly abused organic solvents. While studies have assessed outcomes of exposure to inhalants in adult male animals, there is little research on the neurobehavioral effects of inhalants in female or younger animals. In attempt to address these shortcomings, we exposed male and female Long-Evans rats to 20 min of 0, 2000, 4000, or 8000 parts per million (ppm) inhaled toluene for 10 days in rats aged postnatal (PN) day 28-39 (adolescent), PN44-PN55, or >PN70 (adult). Animals were observed individually in 29-l transparent glass cylindrical jars equipped with standard photocells that were used to measure locomotor activity. Toluene significantly increased activity as compared to air exposure in all groups of male and female rats with the magnitude of locomotor stimulation produced by 4000 ppm toluene being significantly greater for female adults than during any age of adolescence. The results demonstrate that exposure to abuse patterns of high concentrations of toluene through inhalation can alter spontaneous locomotor behavior in rats and that the expression of these effects appears to depend upon the postnatal age of testing and sex of the animal.

Age-dependent variation in behavior following acute ethanol administration in male and female adolescent rhesus macaques (Macaca mulatta)

BACKGROUND

There has been considerable focus on the adolescent stage of development in the study of alcohol use and the etiology of alcohol-related problems. Because adolescence is a process of dynamic change rather than a discrete or static stage of development, it is important to consider ontogenetic changes in the response to ethanol within the adolescent time period. In rodents, levels of ethanol-induced motor impairment have been shown to increase from early to late adolescence. This study investigated associations between behavior following acute ethanol administration and age, rearing condition (mother-reared vs nursery-reared), and serotonin transporter (rh5-HTTLPR) genotype in a sample of alcohol-naïve adolescent rhesus macaques.

METHODS

Rhesus macaques (n=97; 41 males, 56 females), ranging in age from 28 to 48 months, were administered intravenous (IV) doses of ethanol (2.2 g/kg for males, 2.0 g/kg for females) twice in 2 separate testing sessions. A saline/ethanol group (n=16; 8 males, 6 females) was administered saline in 1 testing session and ethanol in the second session. Following each IV injection, subjects underwent a 30-minute general motor behavioral assessment. Behavior in the saline/ethanol group was compared between the saline and ethanol-testing sessions using analysis of variance. Behavioral data for the larger study sample were averaged between the 2 testing sessions and summarized using factor analysis. Rotated factor scores were used as dependent variables in multiple regression analyses to test for relationships between behavior and age, rearing condition, and rh5-HTTLPR genotype.

RESULTS

During the ethanol-testing session, behaviors indicative of motor impairment (stumbles, falls, sways, bumping the wall, and unsuccessful jumps) were frequently observed in the saline/ethanol group, while they did not occur under the saline-testing session. Factor analysis of behavior following ethanol administration in the larger study sample yielded 3 factors: Ataxia, Impaired Jumping Ability, and Stimulation. Significant negative correlations between age and Ataxia were found for both males and females. Females also exhibited positive correlations between age and Impaired Jumping Ability and age and Stimulation. No significant correlations were found with either rearing condition or rh5-HTTLPR genotype.

CONCLUSIONS

These findings suggest that ontogenetic changes during adolescence in the behavioral response to ethanol differ between rodents and primates. Furthermore, sex differences in the behavioral response to ethanol appear to develop during adolescence.

Ontogeny of acute tolerance to ethanol-induced social inhibition in Sprague-Dawley rats

BACKGROUND

Adolescent rats are less sensitive than adults to a number of acute effects of ethanol, including ethanol-induced social inhibition. Adolescent insensitivity to the suppressing effects of ethanol on social interactions could be related in part to age differences in compensatory responses, including acute tolerance, that serve to counteract these inhibitory effects of ethanol. The present study explored ontogenetic development of acute tolerance within 30 minutes after administration of a relatively low ethanol dose, using ethanol-induced social impairment as the target response measure.

METHODS

Overall social activity was examined following challenge with 1 g/kg ethanol (intraperitoneally) at 2 postinjection intervals (5 or 30 minutes) in early [postnatal day (P) 28], mid (P35), or late (P42) adolescent or adult (P70) group-housed male and female Sprague-Dawley rats (Experiment 1). Blood and brain ethanol concentrations (BECs and BrECs) were assessed in separate groups of animals 5 or 30 minutes after ethanol administration (Experiment 2). Expression of acute tolerance was examined by assessing the relationship between BrECs and the degree of social impairment in individual animals at P28, P35, P42, and P70 during early recovery period (up to 30 minutes) following acute ethanol challenge (Experiment 3).

RESULTS

Effects of ethanol on overall social activity were age-dependent and time-dependent. Whereas all age groups showed equivalent ethanol-induced social inhibition 5 minutes after injection, testing at 30 minutes revealed marked age differences. Social inhibition was still pronounced at this time in adults, but was diminished in an age-related manner at younger ages (Experiment 1). In contrast to the ontogenetic differences in rates of decline in social impairment across time, decreases in brain and blood ethanol levels over time were similar across age (Experiment 2). Only P28 and P35 adolescents showed acute tolerance to ethanol-induced social inhibition, as indexed by an increasing time-dependent dissociation between BrECs and ethanol-induced social impairment, with social impairment declining faster than BrECs (Experiment 3).

CONCLUSIONS

This is the first study to document enhanced acute tolerance in adolescent rats relative to adult animals at nonhypnotic doses of ethanol. The greater expression of acute tolerance in young animals may reflect an enhanced predisposition of their nervous systems to respond rapidly to even modest doses of ethanol with compensatory adaptations. A greater propensity of early adolescents to develop acute tolerance may contribute to their resistance to adverse effects of ethanol, thereby permitting heavy drinking at this age and placing early adolescents at higher risk for extensive alcohol use.

Chronic tolerance to the social consequences of ethanol in adolescent and adult Sprague-Dawley rats

Adolescence is a time when experimentation with ethanol becomes normative, with high levels of use becoming apparent in some adolescents. Little is known, however, as to whether ethanol adaptations emerging in adolescents with repeated ethanol use are similar to those emerging in adults. The presents study used a rodent model to investigate the development of chronic tolerance to ethanol-induced alterations in social behavior. The study focused both on ethanol-induced social facilitations, typically evident in adolescents but not adult animals at low doses of ethanol, as well as the inhibition of social behavior occurring at higher doses in both adolescent and adult rats. Adolescent and adult male and female Sprague-Dawley rats were injected intraperitoneally with either isotonic saline or 1 g/kg ethanol for 7 consecutive days: postnatal day (P) 27-33 for adolescents and P62-68 for adults. Acute effects of ethanol (0, 0.25, 0.5, 0.75, and 1 g/kg) on social behavior, social motivation (measured in terms of social preference), and locomotor activity were assessed 48 h after the last chronic exposure using a modified social interaction test in a familiar environment. Adolescents chronically exposed to ethanol developed tolerance to ethanol-induced social facilitation. Animals of both ages likewise developed chronic tolerance to ethanol-induced social inhibition. Metabolic tolerance emerged in adults, as indexed by a decrease in blood ethanol concentrations after chronic ethanol at this age, whereas only functional tolerance was evident in adolescents. Unexpectedly, chronic ethanol diminished baseline levels of social preference in adolescents, but made them more responsive to ethanol-induced enhancement of social preference. Chronic ethanol exposure in adulthood, however, only induced tolerance to the suppressing effects of higher ethanol doses on social preference. Thus, whereas adolescents and adult both develop adaptations following repeated exposure to ethanol, adolescents are more vulnerable to the disruptive effects of chronic ethanol exposure on social preference than their more mature counterparts.

Periadolescent exposure to ethanol and diazepam alters the aversive properties of ethanol in adult mice

Evidence suggests that the developing adolescent brain may be especially vulnerable to long-term neurobehavioral consequences following ethanol exposure and withdrawal. In the present study, we examined the long-term effect of adolescent ethanol withdrawal on a subsequent EtOH-induced conditioned taste aversion (CTA). Periadolescent and adult C3H mice were exposed to 64 h of continuous (single withdrawal) or intermittent (multiple withdrawal) ethanol vapor. Following each ethanol exposure, animals received either 0, 1, 2, or 3 mg/kg diazepam (DZP) in an attempt to counteract the possible effect of ethanol withdrawal. About 6 weeks following ethanol and DZP treatment, animals were tested for an EtOH-induced CTA. As expected, exposure to EtOH during adolescence attenuated the EtOH-induced CTA as compared to controls. Unexpectedly, administration of DZP during withdrawal did not spare but rather mimicked the attenuation of the EtOH-induced CTA seen in animals exposed to ethanol in adolescence. This attenuation was not evident when EtOH and/or DZP was administered in adulthood. Given the similar mode of action of EtOH and DZP on the GABA system, the principal implication of the present findings is that the intoxicating effect of ethanol on the developing brain can result in long-term changes in the aversive properties of EtOH.

SB242084, flumazenil, and CRA1000 block ethanol withdrawal-induced anxiety in rats

Anxiety-like behaviors are integral features of withdrawal from chronic ethanol exposure. In the experiments in the current study, we tested the hypothesis that anxiety can be regulated independently of other withdrawal signs and thus may be responsive to selective pharmacological agents. For 17 days, rats were fed ethanol (8-12 g/kg/day) in a liquid diet. Between 5 and 6 h after cessation of ethanol treatment, rats were tested in either the social interaction or plus-maze test of anxiety-like behavior after treatment with drugs hypothesized to have anxiolytic action. SB242084, flumazenil, and CRA1000-antagonists for 5-hydroxytryptamine (serotonin) (5-HT) 2C (5-HT(2C)), benzodiazepine, and corticotropin-releasing factor type 1 (CRF(1)) receptors, respectively-attenuated decreased social interaction without concomitant effects on activity measures. In contrast, ifenprodil, MDL 72222, and zolpidem-antagonists for N-methyl-d-aspartate (NMDA) and 5-HT(3) receptors, and agonist for benzodiazepine type 1 receptors, respectively-did not share this effect. Results for SB242084, flumazenil, and ifenprodil in the elevated plus-maze test were comparable to those in the social interaction test. These results support the suggestion that multiple neuronal systems (CRF(1), 5-HT(2C), and benzodiazepine receptors) contribute to the ethanol withdrawal sign of decreased social interaction. Furthermore, the selective effects of pharmacological agents on social interaction seem to indicate that this behavior can be dissociated from other signs. Because anxiety may be a complicating factor in alcohol withdrawal and relapse, future studies of this type are needed to provide focus for the effort to define selective and novel antianxiety agents for these disorders.

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

Elevated signs of anxiety are observed in adult rodents during withdrawal from chronic as well as acute ethanol exposure. To determine whether adolescents, in addition to their insensitivity to a number of acute ethanol effects, might likewise be hyposensitive to these anxiogenic manifestations of withdrawal from an acute ethanol challenge, the behavior of adolescent and adult male Sprague-Dawley rats was assessed in an elevated plus maze (EPM) 18 h following intraperitoneal challenge with 4 g/kg ethanol. Adult but not adolescent animals demonstrated evidence of anxiety in the plus maze during acute ethanol withdrawal. To ensure that this finding did not merely reflect age differences in ethanol clearance, clearance times at each age were determined, with additional adolescents tested at the same time postclearance as the adults were previously. Adolescents still failed to demonstrate anxiogenic signs of withdrawal. Suppression of activity during the withdrawal test, however, was evident in animals of both ages. A relative resistance to the anxiogenic effects associated with acute ethanol withdrawal during adolescence could serve as a permissive factor for development of binge drinking patterns among human adolescents.

Physiological and behavioral effects of acute ethanol hangover in juvenile, adolescent, and adult rats

This study examined differential responding of juvenile, adolescent, and adult rats after intoxication from an acute alcohol challenge. Experiment I generated blood ethanol curves for subjects 25, 35, or 110 days postnatal, after doses of 2.0 or 4.0 g/kg, assessing elimination rates and time of drug clearance. Experiment 2 compared ethanol's initial hypothermic and delayed hyperthermic effect across age by 48-hr temperature measurement with telemetry. At clearance or 24 hr after alcohol exposure, Experiment 3 tested subjects for changes in acoustic startle reactivity and ultrasonic vocalization (USV). Younger rats showed an absent or reduced tendency for residual hyperthermia, and adults showed alterations in USV observed as aftereffects of intoxication, despite greater initial blood alcohol levels and ethanol hypothermia in the former. The lesser ethanol hangover effects in weanlings and adolescents may be due in part to faster ethanol elimination at these ages compared with adults.

Synchronous postnatal increase in alpha1 and gamma2L GABA(A) receptor mRNAs and high affinity zolpidem binding across three regions of rat brain

The objective of this study was to correlate postnatal changes in levels of mRNAs encoding predominant GABA(A) receptor subunits with a functional index of receptor development. This study is the first to quantify the temporal relationship between postnatal changes in predominant GABA(A) receptor mRNAs and zolpidem-sensitive GABA(A) receptor subtypes. In Experiment 1, we measured zolpidem displacement of 3H-flunitrazepam from rat cerebral cortex, hippocampus, and cerebellum at 0, 6, 14, 21, 29, and 90 postnatal days. Three independent 3H-flunitrazepam sites with high (K(i)=2. 7+/-0.6 nM), low (K(i)=67+/-4.8 nM), and very low (K(i)=4.1+/-0.9 mM) affinities for zolpidem varied in regional and developmental expression. In Experiment 2, we used RNAse protection assays to quantify levels of alpha1, alpha2, beta1, beta2, gamma2S and gamma2L mRNAs in the above regions at the same postnatal ages. Although there was a high degree of regional variation in the developmental expression of zolpidem-sensitive GABA(A) receptors and subunit mRNAs, a dramatic increase in high affinity zolpidem binding sites and alpha1 mRNA levels occurred within all three regions during the second postnatal week. Furthermore, a temporal overlap was observed between the rise in alpha1 mRNA and high affinity zolpidem binding and a more prolonged increase in gamma2L in each region. These results point to the inclusion of the alpha1 and gamma2L subunits in a GABA(A) receptor subtype with a high zolpidem affinity and suggest that a global signal may influence the emergence of this subtype in early postnatal life.