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

Behavioral, neurobiological, and neurochemical mechanisms of ethanol self-administration: A translational review

Alcohol use disorder has multiple characteristics including excessive ethanol consumption, impaired control over drinking behaviors, craving and withdrawal symptoms, compulsive seeking behaviors, and is considered a chronic condition. Relapse is common. Determining the neurobiological targets of ethanol and the adaptations induced by chronic ethanol exposure is critical to understanding the clinical manifestation of alcohol use disorders, the mechanisms underlying the various features of the disorder, and for informing medication development. In the present review, we discuss ethanol's interactions with a variety of neurotransmitter systems, summarizing findings from preclinical and translational studies to highlight recent progress in the field. We then describe animal models of ethanol self-administration, emphasizing the value, limitations, and validity of commonly used models. Lastly, we summarize the behavioral changes induced by chronic ethanol self-administration, with an emphasis on cue-elicited behavior, the role of ethanol-related memories, and the emergence of habitual ethanol seeking behavior.

The effects of multiple early life stressors on adolescent alcohol consumption

Alcohol use disorder is pervasive and effects the health of millions. Identifying factors such as early life stress that contribute to the development of alcohol use disorder is therefore critical, especially those that contribute to adolescent drinking, a strong predictor of AUD development. The majority of prior studies have examined early life effects on adult drinking, but have not studied intake during adolescence, and no prior studies have examined how the effects of multiple stressors may be additive. Therefore, this study determined if experiencing individual or multiple stressors increases adolescent alcohol intake. Male Long Evans rats underwent either early or late maternal separation (postnatal day 2-9 or 13-20), early adolescent social defeat (PND 30-40), both, or neither. All rats were then given two-hour access to alcohol, and voluntary intake assessed daily in late adolescence (PND 41-51). In adulthood, sensitivity to alcohol's sedative effects was assessed using loss and regain of righting reflex tests. Results indicate that experiencing maternal separation (at either time point) or social defeat increased adolescent alcohol consumption, but experiencing the combined stressors did not, and that no stressor significantly affected body weight during adolescence or loss and regain of righting reflex in adulthood. Overall, this pattern of effects suggests that experiencing any individual early life stressor may increase adolescent alcohol intake, in agreement with prior literature, but that the combined effects of multiple early life stressors may be more complicated.

Voluntary elevated ethanol consumption in adolescent Sprague-Dawley rats: Procedural contributors and age-specificity

Alcohol consumption is typically initiated during adolescence, with the incidence of binge drinking (production of blood ethanol concentrations [BECs] > 80 mg/dL) peaking during this stage of development. Studies in outbred rats investigating the consequences of adolescent ethanol exposure have typically employed intragastric, vapor, or intraperitoneal administration to attain BECs in this range. While these procedures have yielded valuable data regarding the consequences of adolescent exposure, they are varyingly stressful, administer the full dose at once, and/or bypass digestion. Consequently, we have worked to develop a model of voluntary elevated ethanol consumption in outbred adolescent Sprague-Dawley males and females, building on our previous work (see Hosová & Spear, 2017). This model utilizes daily 30-min access to 10% ethanol (v/v) in chocolate Boost® from postnatal day (P)28-41. Experiment 1 compared intake levels between (1a) animals given either ball-bearing or open-ended sipper tube tips for solution access, (1b) animals separated from their cage mate by wire mesh or isolated to a separate cage during solution access, (1c) animals given solution access with or without simultaneous access to banana-flavored sugar pellets, and (1d) animals that were either moderately food-restricted or fed ad libitum. Experiment 2 compared intake levels between animals given daily solution access and animals given access only on a "Monday-Wednesday-Friday" intermittent schedule. Experiment 3 compared adolescent and adult (P70-83) consumption using the finalized procedure as based on the results of Experiments 1 and 2. As in our previous work, consumptions well within the binge range were produced on some days, with high-consumption days typically followed by several days of lower consumption before increasing again. Sipper tube type (1a) and simultaneous pellet access (1c) did not affect consumption, while intake was significantly higher in non-isolated (1b), food-restricted (1d), daily-access (2), and adolescent (3) animals. However, although ethanol intake was higher in food-restricted animals, the resulting BECs were equivalent or higher in non-restricted animals, likely due to a hepatoprotective effect of moderate food restriction. Post-consumption intoxication ratings correlated with BECs and were notably higher in adults than adolescents, despite the lower voluntary consumption levels of adults, confirming prior reports of the attenuated sensitivity of adolescents to ethanol intoxication relative to adults. The final model utilized ball-bearing sipper tube tips to provide daily access to 10% ethanol in chocolate Boost® to free-feeding adolescent animals separated from their cage mate by wire mesh, with no food provided during solution access. This easy-to-implement model is effective in producing elevated voluntary ethanol consumption in adolescent, but not adult, Sprague-Dawley rats.

Impact of a novel environment on alcohol-induced locomotor activity in Wistar rats

Clinical studies have shown a positive correlation between novelty-seeking behavior and the susceptibility to consume drugs of abuse. Although several animal studies have demonstrated this correlation with psychostimulants or morphine, studies with alcohol have shown conflicting results. The aim of this work was to investigate alcohol-induced motor effects in Wistar rats with different responses to novelty. Animals were classified as Low- (LR) or High-Responders (HR) to novelty, depending on their horizontal activity in an automated open field. Motor activity was recorded in naïve, saline, and alcohol-administered rats at different doses (0.1, 0.25, 0.5, 1.0, or 2.5 g/kg). Horizontal movements, rearings, and stereotyped behaviors were evaluated. After the behavioral test, animals were sacrificed and blood alcohol concentrations (BACs) were measured. Low (0.1 and 0.25 g/kg) and high (2.5 g/kg) alcohol doses decreased horizontal movements in LR animals, whereas 1.0 g/kg increased this parameter in HR rats. Rearings were increased by alcohol 1.0 g/kg in LR animals. In HR rats, alcohol doses of 0.5 and 1.0 g/kg also increased this parameter. Stereotyped behaviors were decreased by an alcohol dose of 2.5 g/kg in LR animals, but were increased by an intermediate dose (1.0 g/kg) in HR rats. Differences in horizontal movements and rearings were found between LR and HR animals at certain ethanol doses. Horizontal movements (0.25 g/kg) and rearings (0.5 g/kg) were lower in LR than HR rats; however, rearings were lower in HR than LR rats at 1.0 g/kg. BACs were similar between LR and HR rats at all ethanol doses. These findings suggest that HR rats are more responsive to the stimulant effects of intermediate alcohol doses, whereas LR animals are sensitive to low/high doses of the drug. Sensitivity to alcohol motor effects may substantially depend on the initial animal's response to a novel environment. The stimulant effects of alcohol may constitute important behavioral traits significantly associated with the rewarding properties of the drug.

Chronic Nicotine Exposure Initiated in Adolescence and Unpaired to Behavioral Context Fails to Enhance Sweetened Ethanol Seeking

Nicotine use in adolescence is pervasive in the United States and, according to the Gateway Hypothesis, may lead to progression towards other addictive substances. Given the prevalence of nicotine and ethanol comorbidity, it is difficult to ascertain if nicotine is a gateway drug for ethanol. Our study investigated the relationship between adolescent exposure to nicotine and whether this exposure alters subsequent alcohol seeking behavior. We hypothesized that rats exposed to nicotine beginning in adolescence would exhibit greater alcohol seeking behavior than non-exposed siblings. To test our hypothesis, beginning at P28, female rats were initially exposed to once daily nicotine (0.4 mg/kg, SC) or saline for 5 days. Following these five initial injections, animals were trained to nose-poke for sucrose reinforcement (10%, w/v), gradually increasing to sweetened ethanol (10% sucrose; 10% ethanol, w/v) on an FR5 reinforcement schedule. Nicotine injections were administered after the behavioral sessions to minimize acute effects of nicotine on operant self-administration. We measured the effects of nicotine exposure on the following aspects of ethanol seeking: self-administration, naltrexone (NTX)-induced decreases, habit-directed behavior, motivation, extinction and reinstatement. Nicotine exposure did not alter self-administration or the effectiveness of NTX to reduce alcohol seeking. Nicotine exposure blocked habit-directed ethanol seeking. Finally, nicotine did not alter extinction learning or cue-induced reinstatement to sweetened ethanol seeking. Our findings suggest that nicotine exposure outside the behavioral context does not escalate ethanol seeking. Further, the Gateway Hypothesis likely applies to scenarios in which nicotine is either self-administered or physiologically active during the behavioral session.

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

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

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

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

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

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

Learning by subtraction: Hippocampal activity and effects of ethanol during the acquisition and performance of response sequences

Learning is believed to be reflected in the activity of the hippocampus. However, neural correlates of learning have been difficult to characterize because hippocampal activity is integrated with ongoing behavior. To address this issue, male rats (n = 5) implanted with electrodes (n = 14) in the CA1 subfield responded during two tasks within a single test session. In one task, subjects acquired a new 3-response sequence (acquisition), whereas in the other task, subjects completed a well-rehearsed 3-response sequence (performance). Both tasks though could be completed using an identical response topography and used the same sensory stimuli and schedule of reinforcement. More important, comparing neural patterns during sequence acquisition to those during sequence performance allows for a subtractive approach whereby activity associated with learning could potentially be dissociated from the activity associated with ongoing behavior. At sites where CA1 activity was closely associated with behavior, the patterns of activity were differentially modulated by key position and the serial position of a response within the schedule of reinforcement. Temporal shifts between peak activity and responding on particular keys also occurred during sequence acquisition, but not during sequence performance. Ethanol disrupted CA1 activity while producing rate-decreasing effects in both tasks and error-increasing effects that were more selective for sequence acquisition than sequence performance. Ethanol also produced alterations in the magnitude of modulations and temporal pattern of CA1 activity, although these effects were not selective for sequence acquisition. Similar to ethanol, hippocampal micro-stimulation decreased response rate in both tasks and selectively increased the percentage of errors during sequence acquisition, and provided a more direct demonstration of hippocampal involvement during sequence acquisition. Together, these results strongly support the notion that ethanol disrupts sequence acquisition by disrupting hippocampal activity and that the hippocampus is necessary for the conditioned associations required for sequence acquisition.