Comparison of the onset of hypoactivity and anxiety-like behavior during alcohol withdrawal in adolescent and adult rats

BACKGROUND: Early life alcohol use is associated with increased alcoholism risk. It has been suggested that alterations in the sensitivity of adolescents to the acute effects of ethanol may contribute to this risk by promoting excessive intake. However, an enhanced propensity for developing ethanol dependence or withdrawal-related behavior could also contribute to increased risk. The objective of these studies was to compare the appearance of ethanol withdrawal-related behaviors in adolescent and adult rats. METHODS: Male Sprague-Dawley rats were exposed to ethanol vapor (12 hr/day) for 12 or 14 days during adolescence or adulthood. In the first study, locomotor activity was assessed after 2, 4, 7, 10, and 14 days of ethanol exposure. In the second study, open field behavior was assessed after 5 or 12 days of ethanol exposure. In follow-up studies, changes in sucrose preference during ethanol withdrawal and motor activity during food restriction were assessed in adolescent rats. Withdrawal assessments were made 7 to 9 hr after daily exposure ended. RESULTS: Hypoactivity emerged rapidly in adolescent rats during ethanol withdrawal in activity tests, but comparable reductions were not found in adult rats. However, hypoactivity developed in both adolescents and adults in the novel open field. Enhanced anxiety-like behavior in the open field was not observed in either age group during withdrawal. Finally, sucrose preference was unchanged during ethanol withdrawal, and food restriction increased motor activity in adolescent rats. CONCLUSIONS: These data confirm that symptoms of withdrawal may be differentially expressed in adolescent and adult rats. However, discrepancies in hypoactivity between studies suggest that assessment in a novel versus familiar environment may influence the expression of withdrawal-related behaviors.     

The effects of continuous and intermittent ethanol exposure in adolesence on the aversive properties of ethanol during adulthood

Background:  Alcohol abuse among adolescents is prevalent. Epidemiological studies suggest that alcohol abuse during the adolescent developmental period may result in long-term changes such as an increased susceptibility to alcohol-related problems in adulthood. Laboratory findings suggest that alcohol exposure during the adolescent developmental period, as compared with adulthood, may differentially impact subsequent neurobehavioral responses to alcohol. The present study was designed to examine whether ethanol exposure, continuous versus intermittent, during the adolescent developmental period would alter the aversive properties of ethanol in adult C3H mice.
Methods:  Periadolescent (PD28) male C3H mice were exposed to 64 hours of continuous or intermittent ethanol vapor. As a comparison, adult (PD70) C3H mice were also exposed to 64 hours of continuous or intermittent ethanol vapor. Six weeks after ethanol exposure, taste aversion conditioning was carried out on both ethanol pre-exposed and ethanol-naive animals using a 1-trial, 1-flavor taste-conditioning procedure.
Results:  Ethanol exposure during the periadolescent period significantly attenuated a subsequent ethanol-induced conditioned taste aversion, as compared with control animals. Adult animals exposed to chronic ethanol vapor during adolescence showed less of an aversion to an ethanol-paired flavor than ethanol-naive adults. Intermittent exposure to ethanol vapor during periadolescence produced a greater attenuation.
Conclusion:  It is suggested that ethanol exposure during the periadolescent period results in long-term neurobehavioral changes, which lessen a conditioned aversion to ethanol in adulthood. It is suggested that this age-related effect may underlie the increased susceptibility to alcohol-related problems which is negatively correlated with the age of onset for alcohol abuse.     

Sensitivity and tolerance to autonomic effects of ethanol in adolescent and adult rats during repeated vapor inhalation sessions

BACKGROUND: It is during adolescence that most drinkers initiate ethanol intake, with some of this use being excessive. One possible contributor to the increased ethanol consumption often seen during adolescence in humans and in various animal models is age differences in ethanol sensitivity and tolerance. The present study examined the impact of age on ethanol-related alterations in the autonomic nervous system. METHODS: Sensitivity to the initial ethanol challenge and chronic tolerance as well as acute and protracted withdrawal-like phenomena were assessed in male adolescent and adult Sprague-Dawley rats, using implanted telemetry probes with ethanol delivered via vapor inhalation. RESULTS: Both ages showed similar ethanol-induced tachycardia and activity suppression; however, adolescents were found to be more sensitive than adults to the hypothermic effect of ethanol, data opposite other results from our laboratory and elsewhere using intragastric intubations or intraperitoneal administrations of ethanol. Although little tolerance to ethanol's tachycardic or activity suppressant effects was seen after repeated ethanol inhalation sessions, chronic tolerance to ethanol's hypothermic effect developed faster in adults than in adolescents. A withdrawal-like syndrome, characterized by bradycardia and hypoactivity, typically emerged during the dark phase of the diurnal cycle after ethanol vapor exposure sessions. These effects were observed in animals of both ages, with the bradycardic effect more pronounced in adolescents. CONCLUSIONS: In contrast to results indicating that adolescents may be less sensitive than adults to ethanol's hypothermic effect when ethanol is administered via bolus injection/intubation, adolescents appear more sensitive and develop tolerance to ethanol's hypothermic effects more slowly than adults when ethanol is administered at a more moderate rate via vapor inhalation.     

Age- and sex-dependent effects of footshock stress on subsequent alcohol drinking and acoustic startle behavior in mice selectively bred for high-alcohol preference

Background: Exposure to stress during adolescence is known to be a risk factor for alcohol‐use and anxiety disorders. This study examined the effects of footshock stress during adolescence on subsequent alcohol drinking in male and female mice selectively bred for high‐alcohol preference (HAP1 lines). Acoustic startle responses and prepulse inhibition (PPI) were also assessed in the absence of, and immediately following, subsequent footshock stress exposures to determine whether a prior history of footshock stress during adolescence would produce enduring effects on anxiety‐related behavior and sensorimotor gating. Methods: Alcohol‐naïve, adolescent (male, n = 27; female, n = 23) and adult (male, n = 30; female, n = 30) HAP1 mice were randomly assigned to a stress or no stress group. The study consisted of 5 phases: (1) 10 consecutive days of exposure to a 30‐minute footshock session, (2) 1 startle test, (3) one 30‐minute footshock session immediately followed by 1 startle test, (4) 30 days of free‐choice alcohol consumption, and (5) one 30‐minute footshock session immediately followed by 1 startle test. Results: Footshock stress exposure during adolescence, but not adulthood, robustly increased alcohol drinking behavior in both male and female HAP1 mice. Before alcohol drinking, females in both the adolescent and adult stress groups showed greater startle in phases 2 and 3; whereas males in the adolescent stress group showed greater startle only in phase 3. After alcohol drinking, in phase 5, enhanced startle was no longer apparent in any stress group. Males in the adult stress group showed reduced startle in phases 2 and 5. PPI was generally unchanged, except that males in the adolescent stress group showed increased PPI in phase 3 and females in the adolescent stress group showed decreased PPI in phase 5. Conclusions: Adolescent HAP1 mice appear to be more vulnerable to the effects of footshock stress than adult mice, as manifested by increased alcohol drinking and anxiety‐related behavior in adulthood. These results in mice suggest that stress exposure during adolescence may increase the risk for developing an alcohol‐use and/or anxiety disorder in individuals with a genetic predisposition toward high alcohol consumption.     

Altered EEG responses to ethanol in adult rats exposed to ethanol during adolescence

BACKGROUND: Adolescent ethanol (EtOH) exposure is a significant health concern due to the potential long-term effects of EtOH on the developing brain. However, few studies have assessed how exposure to EtOH during adolescence influences the response of adults to EtOH after a long period of withdrawal. This study was designed to assess long-term changes in EEG activity after EtOH challenge in adult rats exposed to EtOH during adolescence. METHODS: Male Sprague Dawley rats (n = 24) were exposed to EtOH vapor for 5 days (12 hr/day) between postnatal days 35 and 40. After maturing to adulthood, rats were implanted with cortical, amygdalar, and hippocampal electrodes. Then EEG activity after EtOH challenge (0.0-1.5 g/kg) was assessed. RESULTS: There were no EEG differences between groups under baseline or vehicle conditions, but EtOH did have differential behavioral and electrophysiological effects when adolescent ethanol-exposed rats were compared with controls. After 1.5 g/kg EtOH, ethanol-exposed rats displayed decreased behavioral indexes of intoxication. In addition, EtOH significantly increased 4 to 6 Hz power in the hippocampus and parietal cortex of the control group but had no effect on 4 to 6 Hz power in the ethanol-exposed group in either of these brain regions. EtOH produced maximal increases in cortical EEG variability in control rats after 1.5 g/kg EtOH but produced maximal increases in cortical EEG variability in ethanol-exposed rats after 1.0 g/kg EtOH. As a result, ethanol-induced increases in EEG variability after 1.5 g/kg ethanol were blunted in the ethanol-exposed group compared with the control group. CONCLUSIONS: These data demonstrate persistent and brain-region-specific changes in the neurobehavioral effects of acute EtOH challenge in adult rats exposed to EtOH during adolescence in the absence of baseline neurophysiological differences. Decreased EEG responses to high doses of EtOH combined with decreased behavioral measures of intoxication suggest that adolescent ethanol exposure produces long-lasting tolerance to the sedative effects of ethanol.     

In the rat, chronic intermittent ethanol exposure during adolescence alters the ethanol sensitivity of tonic inhibition in adulthood

Background: Alcohol drinking by adolescents is a major public health concern. Adolescents tend to drink in a chronic, intermittent, that is, “binge,” pattern, and such patterns of ethanol exposure are associated with increased risk of neurotoxicity and the development of alcohol use disorders ( Crews et al., 2000 ; Hunt, 1993 ). Both adolescent humans and rats are more sensitive to acute ethanol‐induced memory impairment than adults ( Acheson et al., 1998 ; Markwiese et al., 1998 ). Furthermore, in rats, chronic intermittent ethanol (CIE) exposure during adolescence produces a long‐lasting, perhaps permanent, maintenance of the adolescent high sensitivity to ethanol’s amnestic effects ( White et al., 2000a ). We have previously shown that acute ethanol increases tonic inhibitory current mediated by extrasynaptic GABA_A receptors more efficaciously in dentate granule cells (DGCs) from adolescent than adult rats ( Fleming et al., 2007 ). In this study, we determined if CIE during adolescence produced long‐lasting changes in this tonic current. Methods: Adolescent rats were subjected to a CIE exposure regimen and allowed to mature to full adulthood. Whole‐cell voltage‐clamp measurements of tonic inhibitory current and mean phasic current were made in vitro in hippocampal brain slices. Results:  CIE exposure during adolescence increased the ethanol sensitivity of tonic inhibitory current mediated by extrasynaptic GABA_A receptors and decreased the ethanol sensitivity of phasic, synaptic GABA_A receptor‐mediated current in adult DGCs. Conclusions: CIE exposure during adolescence produces long‐lasting changes in the function and ethanol sensitivity of extrasynaptic GABA_A receptors in DGCs. These changes appear to “lock‐in” and maintain the high adolescent sensitivity to ethanol in these cells. Furthermore, greater ethanol enhancement of tonic inhibition in the hippocampal formation after CIE is consistent with the greater sensitivity to ethanol‐induced memory impairment after adolescent CIE. This finding represents the first demonstration of a long‐term, memory‐related cellular effect of CIE during adolescence, and the “lock‐in” of adolescent ethanol sensitivity that these results suggest could represent a conceptual step forward in understanding the vulnerability of the adolescent brain to alcohol.     

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.     

Differential effects of acute alcohol on prepulse inhibition and event-related potentials in adolescent and adult Wistar rats

Background: Previous studies have demonstrated that adolescent and adult rats show differential sensitivity to many of the acute effects of alcohol. We recently reported evidence of developmental differences in the effects of acute alcohol on the cortical electroencephalogram. However, it is unclear whether developmental differences are also observed in other neurophysiological and neurobehavioral measurements known to be sensitive to alcohol exposure. The present study determined the age‐related effects of acute alcohol on behavioral and event‐related potential (ERP) responses to acoustic startle (AS) and prepulse inhibition (PPI). Methods: Male adolescent and adult Wistar rats were implanted with cortical recording electrodes. The effects of acute alcohol (0.0, 0.75, and 1.5 g/kg) on behavioral and ERP responses to AS and PPI were assessed. Results: Acute alcohol (0.75 and 1.5 g/kg) significantly reduced the behavioral and electrophysiological response to AS in adolescent and adult rats. Both 0.75 and 1.5 g/kg alcohol significantly enhanced the behavioral response to PPI in adolescent, but not in adult rats. During prepulse + pulse trials, 1.5 g/kg alcohol significantly increased the N10 pulse response in the adolescent frontal cortex. Acute alcohol (0.75 and 1.5 g/kg) also increased the N1 ERP pulse response to prepulse stimuli in frontal and parietal cortices in adult rats, but not in adolescent rats. Conclusions: These data suggest that alcohol’s effect on behavioral and electrophysiological indices of AS do not differ between adults and adolescents whereas developmental stage does appear to significantly modify alcohol‐influenced response to PPI.     

Patterns of Ethanol Intake in Preadolescent, Adolescent, and Adult Wistar Rats Under Acquisition, Maintenance, and Relapse-Like Conditions

Background: Animal behavioral models of voluntary ethanol consumption represent a valuable tool to investigate the relationship between age and propensity to consume alcohol using an experimental methodology. Although adolescence has been considered as a critical age, few are the studies that consider the preadolescence age. This study examines the ethanol consumption/preference and the propensity to show an alcohol deprivation effect (ADE) after a short voluntary ethanol exposure from a developmental perspective. Methods: Three groups of heterogeneous Wistar rats of both sexes with ad libitum food and water were exposed for 10 days to 3 ethanol solutions at 3 different ontogenetic periods: preadolescence (PN19), adolescence (PN28), and adulthood (PN90). Ethanol intake (including circadian rhythm), ethanol preference, water and food consumption, and ADE were measured. Results: During the exposure, the 3 groups differed in their ethanol intake; the greatest amount of alcohol (g/kg) was consumed by the preadolescent rats while the adolescents showed a progressive decrease in alcohol consumption as they approached the lowest adult levels by the end of the assessed period. The pattern of ethanol consumption was not fully explained in terms of hyperphagia and/or hyperdipsia at early ages, and showed a wholly circadian rhythm in adolescent rats. After an abstinence period of 7 days, adult rats showed an ADE measured both as an increment in ethanol consumption and preference, whereas adolescent rats only showed an increment in ethanol preference. Preadolescent rats decreased their consumption and their preference remained unchanged. Conclusions: In summary, using a short period of ethanol exposure and a brief deprivation period the results revealed a direct relationship between chronological age and propensity to consume alcohol, being the adolescence a transition period from the infant to the adult pattern of alcohol consumption. Preadolescent animals showed the highest ethanol consumption level. The ADE was only found in adult animals for both alcohol consumption and preference, whereas adolescents showed an ADE only for preference. No effect of sex was detected in any phase of the experiment.     

Adolescent C57BL/6J (but not DBA/2J) Mice Consume Greater Amounts of Limited‐Access Ethanol Compared to Adults and Display Continued Elevated Ethanol Intake into Adulthood

Background: Alcohol use is common during the adolescent period, a time at which a number of crucial neurobiological, hormonal, and behavioral changes occur ( Spear, 2000 ). In order to more fully understand the complex interaction between alcohol use and these age‐typical neurobiological changes, animal models must be utilized. Rodents experience a developmental period similar to that of adolescence. Although rat models have shown striking adolescent‐specific differences in sensitivity to ethanol, little work has been done in mice despite the fact that the C57BL/6J (B6) and DBA2/J (D2) mice have been shown to markedly differ in ethanol preference drinking and exhibit widely different sensitivities to ethanol. Methods: The current study examined ethanol intake in adolescent and adult B6 and D2 mice using a limited access alcohol exposure paradigm called Drinking in the Dark (DID). Additionally, the effect of adolescent (or adult) ethanol exposure on subsequent adult ethanol intake was examined by re‐exposing the mice to the same paradigm once the adolescents reached adulthood. We hypothesized that adolescent (P25–45) mice would exhibit greater binge‐like alcohol intake compared to adults (P60–80), and that B6 mice would exhibit greater binge‐like alcohol intake compared to D2 mice. Moreover, we predicted that relative difference in binge‐like alcohol intake between adolescents and adults would be greater in D2 mice. Results: Adolescent B6 mice consumed more ethanol than adults in the DID model. There was no difference between adolescent and adult D2 mice. Conclusions: This work adds to the literature suggesting that adolescents will consume more ethanol than adults and that this exposure can result in altered adult intake. However, this effect seems largely dependent upon genotype. Future work will continue to examine age‐related differences in ethanol intake, preference, and sensitivity in inbred mouse strains.     

Chronic-intermittent ethanol exposure during adolescence prevents normal developmental changes in sensitivity to ethanol-induced motor impairments

BACKGROUND: Recent evidence indicates that adolescent and adult rats are differentially sensitive to many of the acute effects of ethanol. Little is known about the neurobehavioral consequences of repeated ethanol exposure during adolescence relative to adulthood. In the present study we examined the impact of repeated ethanol exposure during adolescence and adulthood on subsequent sensitivity to ethanol-induced motor impairments. METHODS: The tilting plane test was used to assess the impact of acute ethanol (3.0 g/kg) on motor coordination before (test 1), 2 days after (test 2), and 16 days after (test 3) a 20 day period of chronic-intermittent ethanol (CIE; 5.0 g/kg intraperitoneally every 48 hrs for 20 days) or isovolumetric chronic-intermittent saline (CIS) treatment in adolescent (postnatal (PD) 30) and adult (PD 70) rats. RESULTS: Adolescent subjects were less sensitive than adults to the effects of ethanol on motor coordination during test 1. Adolescent CIS-treated subjects exhibited an increase in sensitivity to ethanol-induced motor impairments that reached adult levels by test 2 (PD 51) and remained stable between test 2 and test 3 (PD 65). Adolescent CIE-treated subjects did not exhibit this age-related increase in sensitivity. In this group, the effects of acute ethanol remained unchanged across the three testing days. Adult CIE-treated subjects exhibited a small degree of tolerance between test 1 and test 2 (PD 91) that was no longer evident during test 3 (PD 105). In adult CIS-treated subjects, the effects of acute ethanol remained stable across the three testing days. Blood ethanol concentrations assessed during testing suggest that age-related changes in sensitivity to ethanol-induced motor impairments are not related to changes in ethanol metabolism. CONCLUSIONS: The data suggest that CIE exposure during adolescence has a lasting impact on sensitivity to ethanol-induced motor impairments. This effect might stem from a disruption of normal developmental processes.     

Adolescent and adult heart rate responses to self-administered ethanol

Background: Despite the fact that adolescent rats have repeatedly been found to consume more ethanol than adult rats in a variety of ethanol access paradigms, the exact cause of the increase in ethanol consumption during adolescence is not known. One possibility is that age differences in sensitivity to ethanol’s rewarding effects may contribute to the elevated intake seen among adolescents. Human studies have shown that autonomic effects of ethanol, particularly ethanol‐induced tachycardia, are correlated with the positive hedonic properties of the drug and, hence, may serve as a biomarker for reward. Methods: In this experiment, a limited‐access self‐administration paradigm was used to examine the autonomic effects of ethanol in outbred male adolescent and in adult Sprague‐Dawley rats under circumstances likely to reveal the rewarding value of ethanol. Results: The results indicated that voluntary ethanol consumption was greater in adolescent than adult rats and that only adolescents consumed enough of the saccharin‐sweetened ethanol solution to show a tachycardic effect greater than that seen in response to saccharin alone. Conclusions: To the extent that these tachycardic properties of ethanol are associated with the rewarding/hedonic properties of ethanol as previously reported in humans, these findings support the suggestion that adolescent animals may have found the ethanol‐containing solution to be more rewarding than the saccharin solution. A similar effect was not seen in adults, findings consistent with the notion that adult rats may not consume enough ethanol under these circumstances to experience its positive rewarding properties.     

Intermittent ethanol exposure in adolescent rats: dose-dependent impairments in trace conditioning

BACKGROUND: Recent studies have revealed that the adolescent brain may be especially vulnerable to ethanol-induced toxicity. Corticolimbic regions are more severely damaged following ethanol exposure during adolescence than during adulthood. The consequences of adolescent ethanol exposure on cognition however, have only recently begun to be explored. METHODS: Male and female rats were administered 0, 1.5, 2.5 or 4.5 g/kg ethanol (20% v/v) by acute intragastric gavage during adolescence (postnatal days [PD] 28, 30, 32 and 34). On PD 40, half of the subjects in each dose group were given 5 pairings of a 10-sec flashing light (CS; conditioned stimulus) immediately followed by mild footshock (US; unconditioned stimulus), a procedure known as delay conditioning. The other half were also given 5 CS-US pairings, but the US was presented 10 sec after CS offset, a procedure known as trace conditioning. All subjects were tested for CS-elicited freezing 24 h later. RESULTS: There was no effect of adolescent ethanol exposure on delay conditioned responding, with all subjects demonstrating comparable levels of CS-elicited freezing. In contrast, the amount of freezing in the trace conditioned subjects was negatively correlated with prior ethanol dose. Specifically, exposure to 2.5 or 4.5 g/kg during adolescence resulted in a deficit in trace conditioned responding. CONCLUSIONS: These data indicate that intermittent exposure to ethanol during adolescence results in impairment in hippocampal-dependent trace conditioning that persists beyond the period of ethanol exposure. Delay conditioning was unaffected by prior ethanol treatment, indicating that there was no difficulty in detecting the CS or US, or in the ability to engage in freezing behavior. These results suggest that the adolescent brain may be particularly vulnerable to the effects of repeated exposure to ethanol that can have consequences for nonspatial, hippocampal-dependent cognitive abilities.     

Binge pattern ethanol exposure in adolescent and adult rats: differential impact on subsequent responsiveness to ethanol

BACKGROUND: Recent evidence indicates that adolescent animals are more sensitive than adults to the disruptive effects of acute ethanol exposure on spatial learning. It is not yet known whether adolescent animals are also more sensitive than adults to the enduring neurobehavioral effects of repeated ethanol exposure. In this study, animals were exposed to ethanol in a binge-pattern during either adolescence or adulthood. At a time when all subjects were adults, spatial working memory was examined in the absence and presence of an acute ethanol challenge. METHODS: Rats were exposed to ethanol (5.0 g/kg intraperitoneally) or isovolumetric saline at 48 hr intervals over 20 days. Exposure began on either postnatal day 30 (adolescent group) or 70 (adult group). Twenty days after the final injection, a time at which all animals were adults, the subjects were tested on an elevated plus maze and then were trained to perform a spatial working memory task on an eight-arm radial maze. At the beginning of each session of training on the working memory task, subjects retrieved food rewards on four of the eight arms. After a delay, subjects were placed on the maze and allowed to retrieve food from the remaining four arms. RESULTS: Prior exposure to ethanol did not influence behavior on the plus maze. Performance of the groups did not differ during acquisition of the spatial working memory task with a 5 min delay or during subsequent testing with a 1 hr delay. However, animals treated with ethanol during adolescence exhibited larger working memory impairments during an ethanol challenge (1.5 g/kg intraperitoneally) than subjects in the other three groups. CONCLUSIONS: The findings indicate that binge pattern exposure to ethanol during adolescence enhances responsiveness to the memory-impairing effects of ethanol in adulthood.     

Behavioral Consequences of Repeated Nicotine During Adolescence in Alcohol-Preferring AA and Alcohol-Avoiding ANA Rats

Background: Epidemiological studies suggest that exposure to nicotine at adolescent age is associated with increased potential to use alcohol and that genetic predisposition may further increase the risk. The present study addressed adolescent vulnerability to repeated nicotine exposure and its influence on subsequent ethanol self‐administration by investigating interactions between nicotine‐induced behavioral sensitization and voluntary ethanol consumption in alcohol preferring AA (Alko Alcohol) and alcohol nonpreferring ANA (Alko Non‐Alcohol) rat lines selected for differential ethanol intake. Methods: Adolescent and adult rats received 10 injections of nicotine (0.5 mg/kg s.c.), given every second day from postnatal day (Pnd) 27 and 75, respectively. Nicotine‐induced (0.5 mg/kg) locomotor activity was measured acutely after the first injection, and after the repeated treatment with nicotine on Pnds 52 and 86 in the adolescent groups and on Pnd 99 in the adult groups. After this, acquisition of voluntary ethanol (10% v/v) consumption as well as nicotine‐induced (0.5 mg/kg) ethanol intake was measured in the AA rats. Results: Adolescent AA rats were more sensitive than adolescent ANA rats to the locomotor effects of nicotine. They were also stimulated more than adult AA rats, but such a difference was not found among ANA rats. Adolescent and adult rats did not differ in their susceptibility to nicotine‐induced behavioral sensitization. Genetic predisposition to ethanol self‐administration did not interact with development of behavioral sensitization in either adolescents or adults. Acquisition of ethanol intake was enhanced in the adolescent groups relative to the adult groups in a manner that was independent of the nicotine treatment. An increase in ethanol intake was found after challenging animals with nicotine, and this effect was enhanced in the nicotine‐treated adolescent group. Conclusions: These findings provide no or little support for the views that adolescent animals are more sensitive to the neurobehavioral effects of repeated exposure to nicotine and that exposure to nicotine in adolescence may contribute to enhanced vulnerability to ethanol abuse. Furthermore, genetic predisposition to high or low ethanol self‐administration does not seem to be a factor that influences individual vulnerability to the neurobehavioral effects of repeated administration of nicotine.     

Age-related changes in the acoustic startle reflex in Fischer 344 and Long Evans rats

The behavioral consequences of age-related changes in the auditory system were studied in Fischer 344 (F344) rats as a model of fast aging and in Long Evans (LE) rats as a model of normal aging. Hearing thresholds, the strength of the acoustic startle responses (ASRs) to noise and tonal stimuli, and the efficiency of the prepulse inhibition (PPI) of ASR were assessed in young-adult, middle-aged, and aged rats of both strains. Compared with LE rats, F344 rats showed larger age-related hearing threshold shifts, and the amplitudes of their startle responses were mostly lower. Both rat strains demonstrated a significant decrease of startle reactivity during aging. For tonal stimuli, this decrease occurred at an earlier age in the F344 rats: middle-aged F344 animals expressed similar startle reactivity as aged F344 animals, whereas middle-aged LE animals had similar startle reactivity as young-adult LE animals. For noise stimuli, on the other hand, a similar progression of age-related ASR changes was found in both strains. No significant relationship between the hearing thresholds and the ASR amplitudes was found within any age group. Auditory PPI was less efficient in F344 rats than in LE rats. An age-related reduction of the PPI of ASR was observed in rats of both strains; however, a significant reduction of PPI occurred only in aged rats. The results indicate that the ASR may serve as an indicator of central presbycusis.     

Estradiol treatment and hormonal fluctuations during the estrous cycle modulate the expression of estrogen receptors in the auditory system and the prepulse inhibition of acoustic startle response

Estrogens' effects on hearing are documented across species, but the responsible molecular mechanisms remain unknown. The presence of estrogen receptors (ER) throughout the auditory system offers a potential pathway of direct estrogenic effects on auditory function, but little is known about how each ER's expression is regulated by the overall hormonal status of the body. In the present study, we determined the effects of ovariectomy and chronic 17β-estradiol treatment on mRNA and protein expression of ERα and ERβ in peripheral (cochlea) and central (inferior colliculus) auditory structures of mice, as well as on auditory-related behavior using the acoustic startle response (ASR), prepulse inhibition (PPI), and habituation of the startle response. 17β-Estradiol treatment down-regulated ERα but not ERβ and increased PPI and latency of the ASR. Neither the magnitude nor the habituation of ASR was affected. Furthermore, ER's mRNA and protein expression in the inner ear were analyzed throughout the estrous cycle (proestrus, estrus, metestrus, and diestrus), revealing a negative correlation of circulating estrogens with ERα expression, whereas ERβ was stable. Our findings show that ER not only are present in both the peripheral and central auditory system but also that circulating estrogen levels down-regulate ERα expression in the auditory system and affect PPI and the latency of ASR, suggesting a key role of ERα as a hormone-induced modulator of the auditory system and behavior.     

Chronic intermittent injections of high-dose ethanol during adolescence produce metabolic, hypnotic, and cognitive tolerance in rats

BACKGROUND: Many humans are first exposed to ethanol during adolescence, the time at which they are most likely to binge drink ethanol. Chronic intermittent ethanol (CIE) exposure produces ethanol tolerance in adolescent rodents. Recent studies suggested that adolescent animals administered CIE experienced increased cognitive impairment following an ethanol challenge. These studies further explore development of ethanol tolerance caused by CIE in adolescence, and whether CIE during adolescence leads to altered ethanol response in adulthood. METHODS: Beginning postnatal day (P) 30, adolescent rats were administered 5.0 g/kg ethanol or saline every 48 hours for 20 days. In experiment I, animals were tested for differential weight gain. In experiment II, loss of righting reflex (LORR) was observed after each injection, then at completion of pretreatment all animals were tested with 5.0 g/kg ethanol and LORR was observed. In experiment III, blood ethanol levels were observed and elimination rates calculated after the first and fifth pretreatments. All animals were tested with 5.0 g/kg at completion of pretreatment and elimination rates were recalculated. In experiment IV, animals were trained on the spatial version of the Morris Water Maze Task (MWMT) on non-treatment days. Following completion of pretreatment and training, animals were tested after receiving an ethanol (1.0, 1.5, or 2.0 g/kg), or saline. Tests for experiments II, III, and IV were repeated in the same animals following 12 ethanol-free days. RESULTS: Chronic intermittent ethanol exposure during adolescence caused differential weight gain (experiment I). Adolescent rats developed tolerance to ethanol-induced LORR (experiment II) and metabolic tolerance to ethanol (experiment III). This tolerance was seen after 12 ethanol-free days. CIE also attenuated ethanol-induced spatial memory deficits in the MWMT (experiment IV). This effect was not long-lasting. CONCLUSIONS: Following CIE pretreatment during adolescence, tolerance developed to the hypnotic and cognitive impairing effects of ethanol, along with increased metabolic rate and decreased weight gain. These results further emphasize the ability of CIE to produce a variety of effects during adolescence, some having long-lasting consequences.     

Fetal or infantile exposure to ethanol promotes ethanol ingestion in adolescence and adulthood: a theoretical review

BACKGROUND: Despite good evidence that ethanol abuse in adulthood is more likely the earlier human adolescents begin drinking, it is unclear why the early onset of drinking occurs in the first place. A review of experimental studies with animals complemented by clinical, epidemiologic and experimental studies with humans supports the idea that precipitating conditions for ethanol abuse occur well before adolescence, in terms of very early exposure to ethanol as a fetus or infant. Experimental studies with animals indicate, accordingly, that ethanol intake during adolescence or adulthood is potentiated by much earlier exposure to ethanol as a fetus or infant. METHODS: Two broad theoretical frameworks are suggested to explain the increase in affinity for ethanol that follows very early exposure to ethanol, one based on effects of mere exposure and the other on associative conditioning. Studied for 50 years or more in several areas of psychology, "effects of mere exposure" refers to enhanced preference expressed for flavors, or just about any stimuli, that are relatively familiar. An alternative framework, in terms of associative conditioning, is guided by this working hypothesis: During ethanol exposure the fetus or infant acquires an association between ethanol's orosensory (odor/taste) and pharmacological consequences, causing the animal subsequently to seek out ethanol's odor and taste. RESULTS AND CONCLUSIONS: The implication that ethanol has rewarding consequences for the fetus or young infant is supported by recent evidence with perinatal rats. Paradoxically, several studies have shown that such early exposure to ethanol may in some circumstances make the infant treat ethanol-related events as aversive, and yet enhanced intake of ethanol in adolescence is nevertheless a consequence. Alternative interpretations of this paradox are considered among the varied circumstances of early ethanol exposure that lead subsequently to increased affinity for ethanol.     

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.